VWE {:9 g] {53;}: - , ."""“ _b ‘ ‘VJJ. ‘. ABSTRACT HEAT AND MASS TRANSFER IN ONIONS BY John R. Rosenau The response of the product to its environment must be known if the processing of a biological product is to be Optimized. The overall purpose of this study was to model the heat and moisture transfer response of onion bulbs to any given boundary condition. The "Alternating Direction Explicit Procedure" for the solution of the transfer equations was adapted to a special finite difference grid system. This grid system was constructed orthogonal to the principle trans- fer directions within the onion bulb. The model was designed to be easily adapted to any axially symmetric body. The model parameters required to model heat and moisture transfer in onions were obtained. The heat transfer portion of the model simulated the actual process very well as indicated by small RMS and maximum differences between predicted and measured center temperatures. The model showed that for heat John R. Rosenau transfer considerations, the bulb may be modeled as a sphere with thermal conductivity equal to the radial thermal conductivity of the bulb. The mass transfer portion of the model was in only fair agreement with experimental weight loss measurements conducted as a test of the model. The differences are attributed mainly to the effect of respiration and the effect of the cracking and loosening of the outer scales during the drying process. Approved V4 , Major Professor Approved epartment Chairman HEAT AND MASS TRANSFER IN ONIONS By Y\ (M? John R? Rosenau A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Agricultural Engineering 1970 027/01 ACKNOWLEDGMENTS The interest, counsel, and time generously extended by the author's major professor Dr. F. W. Bakker-Arkema (Agricultural Engineering) is gratefully acknowledged. The author appreciates the help and guidance offered by Dr. J. V. Beck (Mechanical Engineering), Dr. W. G. Bickert (Agricultural Engineering), Dr. A. M. Dhanak (Mechanical Engineering), and Dr. R. Hamelink (Mathematics). The author thanks Dr. G. E. Merva (Agricultural Engineering) and Dr. B. F. Cargill (Agricultural Engineer- ing) for their interest and help in defining the research problem. The financial assistance given the author by Michigan State University, the National Science Foundation, and the Agricultural Engineering Department is acknowledged. Special thanks are extended by the author to his wife, Marion, for her help and encouragement during this study. ii TABLE OF CONTENTS Page LIST OF TABLES . . . . . . . . . . . . . v LIST OF FIGURES. . . . . . . . . . . . . vi SYMBOLS . . . . . . . . . . . . . . . Vii Chapter I. INTRODUCTION . . . . . . . . . . . 1 II. LITERATURE REVIEW. . . . . . . . . . 5 2.1 Introduction. . . . . . . . . . 5 2.2 Specific Heat . . . . . . . . . 7 2.3 Density . . . . . . . . . . . 10 2.4 Thermal Conductivity . . . . . . . 11 2.5 Moisture Diffusion. . . . . . . . 13 2.6 Mathematical Modeling. . . . . . . 25 III. OBJECTIVES . . . . . . . . . . . . 33 IV. MATHEMATICAL MODEL . . . . . . . . . 34 4.1 Introduction. . . . . . . . . . 34 4.2 Finite Difference Grid . . . . . . 35 4.3 Solution of Node Equations . . . . . 46 V. DENSITY . . . . . . . . . . . . . 55 VI. HEAT TRANSFER . . . . . . . . . . . 57 6.1 Introduction. . . . . . . . . . 57 6.2 Axial Conductivity. . . . . . . . 60 6.3 Radial Conductivity . . . . . . . 63 6.4 Heat Transfer Simulation. . . . . . 65 iii Chapter Page VII. MOISTURE TRANSFER . . . . . . . . . 67 7.1 Introduction . . . . . . . . . 67 7.2 Determination of Moisture Diffusivity. 68 7.3 Mass Transfer Model Testing . . . . 76 VI I I . CONCLUSIONS . . . . . . . . . . . 7 9 Suggestions for Further Study . . . . . 80 BIBLIOGRAPHY . . . . . . . . . . . . . 81 APPENDIX . . . . . . . . . . . . . . 85 1. Tables . . . . . . . . . . . 85 2. HTRAN. . . . . . . . . . . . 91 3. MTRAN. . . . . . . . . . . . 103 4. HMTRAM . . . . . . . . . . . 116 iv LIST OF TABLES Table Page 1. Experimental Values and Results of the Onion Density Determination Data . . . . . . 85 2. The Fourier Series Approximating the Onion Shape I O O O O O O O O O O O O 86 3. Results of the Axial Thermal Conductivity Experiments . . . . . . . . . . . 87 4. Results of the Radial Thermal Conductivity Experiments . . . . . . . . . . . 88 5. Results of the Radial Moisture Diffusivity Experiments . . . . . . . . . . . 89 6. Data Points for the Onion Moisture Isotherm at 86 F (Saravacos, 1960) . . . . . . 90 Figure 2.6.1. LIST OF FIGURES A Portion of a Two Dimensional Finite Difference Grid in Cartesian Coordinates Relationship Between PERIM (z), DPERIM (z), and z 0 O O O O I O O O O O 0 Node Points for a Two-Inch Diameter Sphere . . . . . . . . . . . Node Points for a Two-Inch Diameter Onion A General Surface Node. . . . . . . A General Central Axis Node . . . . . An End Node . . . . . . . . . . Test Case Model Error Using the ADEP on a Two-Inch Diameter Sphere . . . . . Temperature Sensitivity Coefficients at the Center of a Two-Inch Diameter Onion Cross Sectional View of the Sample Holder Used in the Axial Conductivity Tests . Heat Transfer to Onion Bulbs Subjected to a Step Change in Ambient Temperature Cross Sectional View of the Denny Osmometer. Onion Moisture Isotherm . . . . . . Predicted Moisture Diffusivity Dir. . . Moisture Loss from Onion Bulbs at 70 F and 31% Relative Humidity . . . . . . vi Page 27 36 39 40 42 44 44 50 59 61 66 69 71 74 77 0 0 OJ C Gfap Gfrp ml Le Nu Pr Re Sc Sh SYMBOLS Permeability (lbmft/lbfhr) Concentration (moles/ft3) Heat Capacity (Btu/F) Diffusion Coefficient (ftz/hr) Internal Energy (Btu) Activation Energy (1bfft/mole) Axial Geometric Factor (ft) Radial Geometric Factor (ft) Enthalpy (Btu) Molar Enthalpy (Btu/mole) Molar Mass Flux (moles/ftz) Lewis Number (dimensionless) Molecular Weight of Water Nusselt Number (dimensionless) Pressure (lbf/ftz) Prandtl Number (dimensionless) Universal Gas Constant (1bfft/mole°R) Reynolds Number (dimensionless) Schmidt Number (dimensionless) Sherwood Number (dimensionless) vii :3‘ 5‘ Q: OI w u. :3 '0 .0 Temperature (°R) Volume (ft3) Partial Molar Volume (ft3/mole) Volume of the Node i,j (ft3) Mole Fraction (dimensionless) Specific Heat (Btu/lbmF) Molar Heat Capacity (Btu/mole F) Diameter (ft) Convective Heat Transfer Coefficient (Btu/hr ftzF) Convective Mass Transfer Coefficient (ft/hr) Colburn j-factor (dimensionless) Conductivity (But/hr ftF) Moisture Content (d.b.) (dimensionless) Moles (moles) Partial Pressure (lbf/ftz) Heat (Btu) Heat flux (Btu/hrz) Radius (ft) Time (hr) Distance (ft) Distance (ft) Distance (ft) Velocity (ft/hr) Water Potential (atm) Mobility (moles ft/hr lbf) viii 7 Specific Gravity (dimensionless) Y Activity Coefficient (dimensionless) U Chemical Potential (Btu/mole) 0 Density (1bm/ft3) pdm Density of the Solids (lbm/ft3) w Mass Fraction (dimensionless) Subscripts I End Node Index J Surface Node Index a Axial Direction a Ambient c Based on Concentration dm Dry Matter m Based on Moisture Content r Radial Direction i Node Index 1 ith Species j Node Index w Water 3 Solids 5 Surface wv Water Vapor x Direction y Direction 2 Direction ix Based on Chemical Potential Fixed Coordinate System Coordinate System Moving with the "Medium" Time Index Coordinate System Moving with the "Solids" Coordinate System Moving with the "Volume“ Saturated Pure I . INTRODUCTION The remarkable efficiency of the agricultural production system in the United States has been achieved through advances in three basic areas: (i) the development of improved growing practices and genetic varieties, (ii) the mechanization of crop and animal production, and (iii) the evolution of new processing techniques to handle the resulting production increases. The development of these processing methods is extremely important as the benefits of increased production are wasted if the system for handling, processing, and distributing this production is inadequate. Inherent to the methods incorporated for the pro- cessing of agricultural products is the response of the products to their total processing environment. If the effect of all of the mechanical, chemical, thermal, and biological forces acting on a product can be predicted, the deve10pment of Optimal processing systems for that product is made possible. This research project examines a part of the total problem outlined above--the development of a mathematical model simulating the response of yellow globe onions (var. Abbott and Cobb 192) to a convective heat and moisture transfer boundary condition. There are two main reasons for choosing this thesis topic. First, the onion production industry in Michigan is in the process of rapid change. The harvesting and material handling operations have been totally mechanized as have the packaging and shipping operations. However, problems still remain. The first occurs just after harvest when the onions are "cured." Normally, this is done by leaving the covered pallets of freshly harvested and topped onions in the field. During the curing period, the onion necks dry, helping to prevent disease organisms from entering the bulb through the place where-the green top was attached. In addition, the skins develop the traditional golden color, while the outermost skin drys, cracks, and falls off ("shucks"), N taking with it any attached field dirt. With warm dry weather, curing takes about a week; with cool rainy weather, up to three weeks. The author has found that the curing process can be shortened to about three days by passing air at 100 F and 90% relative humidity over the bulbs. More work is needed, however, to Optimize this artificial curing process. The processing step following curing is storage. While storage conditions of 32 F and 75% relative humidity have been recommended (Franklin et_§l., 1966), more work needs to be done to identify the various effects of respiration, moisture transfer, and spoilage on the weight loss of marketable onions in storage. Another problem area in the processing sequence occurs when the bulbs are brought in from storage. If run through the packaging line at storage temperatures (about 35 F), water condenses on the bulbs. In the some- what dusty atmosphere of the packing room, this moisture picks up dirt causing an inferior product. The onions thus need to be warmed before processing. The Optimal design of curing, storage, and heating equipment depends on the prediction of the response of the Onion bulb to its environment. Thus, a mathematical model of the bulb is needed. When the environment is significantly affected by interaction with the product (such as in the case of a deep bin), the model of the bulb may be included within larger models (Bakker-Arkema gt 31., 1969), to predict the response of the entire process. One problem associ- ated with such nested models, however, is the large amount of computer storage and time required for their solution. The second reason for the research was the hope that studying the heat and moisture transfer processes in onion bulbs would lead to a better understanding of these processes in other high moisture products. The onion has a relatively large size and a structure which allows sections to be removed with a minimum of cell damage. Thus, experiments may be performed on sections of the product to determine its transfer properties without changing these properties through the sectioning process. II . LITERATURE REVIEW 2.1 Introduction The investigation of the processes of heat and moisture transfer can be divided into two parts--the trans- fer processes occurring within the product, and the inter- action of the product surface with the environment. Con- cerning moisture migration, Van Arsdel (1963) writes: The drying of a moist substance always involves the movement of a quantity of water away from a dry sur- face. The separation is usually regarded for purposes of analysis as the result of two successive phenomena: (1) migration of water within the moist body to its surface; and (2) conveyance of the vaporized water away from the body . . . the factors that determine the rate of movement of water within the body can be regarded as independent of the external conditions. A useful analysis of the process can be made on the basis of this simplified picture, even though in some cases it may become evident that vaporization is in fact occurring in an ill-defined zone within the moist body instead of only at its geometric surface. The surface-environment interaction can be described by two convective transfer coefficients. The first, h, is defined by the equation 4" = h (TS - Ta) (2.1-1) and the second, hD’ by the equation J8 = h (c — c ). (2.1—2) In general, these coefficients are determined by the nature of the air flow pattern around the product. When radiation heat transfer is significant, h can be modified to include its effects. Much research has been performed investigating these coefficients. Since onions are usually processed in deep bins and pallet boxes, the work of Barker (1965) in reviewing the subject of heat transfer in packed beds should be mentioned. By reference to Barker's article, a "Colburn j-factor" can be obtained for any given Reynolds number and product shape. The heat transfer coefficient h is then obtained by the relation hd 1/3 k = Nu = j Re Pr (2.1-3) air and the mass transfer coefficient hD by h d DD = Sh = j Re Sol/3 (2.1-4) air where the Reynolds number Re is given by the onion bulb diameter times the mass flow rate of air per square foot of bed area divided by the absolute viscosity. The Prandtl number Pr is given by the kinematic viscosity divided by the thermal diffusivity ka. , and the Schmidt 1r number So by the kinematic viscosity divided by the mutual diffusivity of water in air D . . air While the above formulas can only be considered approximate, they do characterize the interaction between the product and its environment. The transport processes within the product are outlined in the following sections. 2.2 Specific Heat If heat is added to a simple closed constant pres- sure system in which no work other than that used to change the system volume is performed, the change in internal energy of that system is given by AB = q - PAV. (2.2—1) The quantity H is defined by the equation H = E + PV. (2.2-2) Thus, in this process, AH = q. (2.2-3) During the heat addition process the temperature of the system rises. This then is the basis for the definition Of heat capacity, namely, C = (——d . (2.2-4) If the system is homogeneous, the specific heat is given by 32 and the molar heat capacity by _ C c : _r12° (2.2.6) When the system can be considered a homogeneous mixture, the enthalpy Of the system is the sum of the partial enthalpies, i.e., H = Z n. H. (2.2-7) i where ni is the number of moles of species i present and Hi, the partial molal enthalpy, is defined by the equation _ 3H _ Likewise, c = z n. 6.. (2.2—9) i In general, 5: is not a constant for a particular species but varies with changes in temperature as well as in the relative mole fractions making up the mixture. Nevertheless, it is customary as a first (and usually quite accurate) approximation to consider the 53's for a food system as being constant. Siebel (1892) considered food as composed of solids and water for which he used the respective specific heats of 0.2 and 1.0 Btu/1bmF. Charm (1963) used this approach with the equation C = 0.5 w + 0.3 w + 1.0 w (2.2-10) S m f wherein wf, ms, and ww refer to the mass fractions of fat, solids, and moisture. If this formula is applied to onions of 90% moisture content (w.b.), 9.8% solids non- fat, and 0.2% fat, c is calculated as 0.93 Btu/lbmF. If the formula were applied to 80% moisture content onions (19.6% solids nonfat and 0.4% fat), c would be 0.86 Btu/1bmF. Ordinanz (1946) gives identical values of c for onions at 80 and 90% moisture content (w.b.). Reidel (1951) separated the solids portion of fruits and vegetables into XO "soluble" and Xu "insoluble" fractions by examining the refractive index of the juice. He tested onions of 85.5% moisture content (w.b.) and used weight fractions of l3.% (of the total weight) as soluble solids, and 1.5% as insoluble solids in the equation c = (l - Xu)(1 — 0.57 X0) + 0.29 Xu. (2.2-11) In this case c is 0.916 Btu/lbmF. For 90% moisture con- tent onions (w.b.), assuming the soluble and insoluble 10 fractions are in the same proportion to each other, c would be 0.942 Btu/lbmF. In light of the above, it was concluded that further work on determining the specific heat of onions is not needed at the present time. Charm's equation (eqn. 2.2-10) was used to determine c in the calculations involving specific heat in this research. 2.3 Density The importance of a product's specific heat has been described in the previous section. Density merits consideration in that it, when multiplied by the specific heat, expresses the system's heat capacity on a per unit volume basis. It is often easier to determine a food product's specific gravity y (by measuring its buoyancy in water) than its density. The density is then determined by multiplying the specific gravity by the density Of water. While the density of water is slightly dependent upon its 3 (at 70 F) temperature, it does not vary from 62.27 lbmft by more than 0.4% in the range of 40 to 100 F (Holman, 1963). Thus, in this temperature range the following equation is sufficiently accurate. 0 = 62.27 Y (2.3-l) 11 The author was unable to find literature values for the density of onion flesh. It thus became necessary to determine this property eXperimentally. 2.4 Thermal Conductivity In the one dimensional case, Fourier's equation of heat conduction defines a material's thermal conductivity k. Consideration of the law of the conservation of energy in conjunction with the above leads to the following familiar differential equation for the temperature history of a body in which there is no heat generation: 8T _ 3 8T _ no 3-5 — 32 (k 82). (2.4 2) This simple definition of k, however, must be expanded when the multi-dimensional case is studied since thermal conductivity is a tensor and not a scalar property. Thus, in three dimensions (Arpaci, 1966), .. 22 61x kll k12 k13 ax " _ _ a—'I‘ qy — k21 k22 k23 ' 3y - .. 3_T_ _ qz k31 k32 k33 32 . (2.4 3) 12 If a material is orthorombic, and if its principle directions are coincident with the x, y, and 2 directions, the following equations are equivalent to equation (2.4-3): .u _ 32 - . 8T ‘ II = k _ 204-5 qy y 3y . ( ) and _ 22 .. qz - k2 32 - (2.4 6) The law of conservation of energy can be applied to these equations, along with pertinent initial and bound- ary conditions, to generate a model for the temperature- time history of a product. (See Section 2.7 of this chap- ter and all of Chapter IV.) Since this history is dependent upon the thermal conductivity of the product, these values must be determined and included in the mathematical model. Van Arsdel (1963) gives a general description of the values of thermal conductivity which might be expected in biological products: In fresh fruits and vegetables, whose moisture content is very high, the conductivity is not far from that of pure water. As drying takes place, however, con- ductivity falls. If shrinkage is complete, so that the dry product is free from internal voids, the de- crease in conductivity is only minor, but if the body becomes highly porous as it dries the low conductivity of the air in the open spaces reduces the overall con- ductivity markedly. 13 Since the onion is a high moisture content vege- table, one would expect its thermal conductivity to be close to that of pure water which is 0.349 Btu per hr ft F at 70 F (Holman, 1963). Since the thermal conductivity of water varies from the value at 70 F by a maximum of 4.9%, in the temperature range of 40-100 F, one would also ex- pect the thermal conductivity of onion flesh to be quite constant within this range. Reidy (1968) has conducted an extensive review of experimental techniques used to determine thermal proper- ties. He noted that steady state techniques could not be expected to give accurate results with high moisture bio- logical products due to the problem of moisture migration within the product during the test. He concluded that numerical methods should be used with transient type experiments involving these materials. As the author has been unable to find values for thermal conductivity of onion flesh in the literature, one of the objectives of this research (see Chapter III and Chapter VI) was to determine values for the thermal con- ductivity of onion flesh. 2.5 Moisture Diffusion GOrling (1958) outlined the process of moisture movement within a biological product as a combination of five different mechanisms: (i) liquid movement caused by capillary forces, (ii) diffusion of liquids caused by 14 concentration differences, (iii) surface diffusion, (iv) water vapor diffusion, and (v) vapor flow caused by differ- ences in total pressure. Before proceeding with the subject of diffusion, the distinction between the movement of moisture via dif- fusion and via bulk flow should be mentioned. Bulk flow is flow caused by the action of a pressure gradient or gravity on water existing within a relatively large channel such as a pipe. Poiseuille flow and Knudsen flow are familiar examples of bulk flow. While such flows exist in the xylem vessels of the larger plants during the grow- ing process, the movement of water within the flesh of a biological product can be considered a diffusion process, i.e., the movement is caused by a chemical potential gradient (as discussed later in this section) and not by a total pressure gradient or gravity directly (Saravacos, 1962). The traditional literature on the subject of dry- ing used the theory and mathematics associated with heat transfer. Thus, the diffusion equation 3C J=-D‘ w w 0 -§;' (2.5-l) was written in obvious analogy to the heat conduction equation and all of the mass transfer mechanisms were grouped together under one overall diffusion coefficient D5 and one driving force sew/32. 15 Babbit (1940) pointed out that the true driving force for moisture movement is not the concentration gradient. He showed this by noting that water can be made to diffuse against a concentration gradient. He con- cluded that the driving force for diffusion was the gradient of the equilibrium water vapor pressure. Kramer (1969) showed that the true driving force for diffusion of water through a medium is the gradient of the chemical potential of water Buw/Bz. Kramer, how- ever, followed the soil physics literature convention and used the chemical potential in another form. By dividing the chemical potential by the molar volume of pure liquid water, the chemical potential is transformed into WW, the "water potential,‘ which is dimensionally equivalent to pressure. Kramer mentioned that the transformation is employed simply for convenience. By using the chemical potential gradient instead of the concentration gradient, a diffusion equation analogous to equation (2.5-l) can be developed. If JS is the molar flux of water with respect to a reference frame moving at the same rate as the "medium," then C 0 3p m _ w w w _ Jw - 32 (2.5 2) where 0w may be thought of as a mobility.* The solids *Hartley and Crank (1949) presented a similar development but with emphasis on liquid diffusion. Their expression 1/0AnN is the same as 0w. 16 may also move with respect to this reference frame and the flux relative to the solids, J3, is the flux needed to determine drying rates. ms If v is the velocity of the medium with respect to the solids, s _ m ms _ Jw - Jw + v Cw (2.5 3) and J8 = o = J‘“ + vms c . (2.5-4) S S S . (2.5-5) The rate at which the solids move with respect to the medium defines another mobility 03 by the equation m 3“5 JS — ' CS {25 w. (2.5'6) From the Gibbs-Duhem equation (Moore, 1962), C dp = lap . (2.5—7) S C w 5 Therefore, 311 Jm= c {2 ——w (2.5-8) U) and then n W 2 3p C 8n — _ __E. w __E. - — Cw Qw 82 C s 32 ' (2'5 9) Cw auw = - (Cs 9w + Cw as) C; 32 " (2.5-10) defined by Cw Buw = — ' - RT D 32 , (2.5 11) _. Bil”. _ — Cs (CS Qw + CW 08). (2.5 12) Since 0 pw - “w - RTlnaw — RT1n(waw) — RTln 5:. (2.5-13) the equation for the molar flux of water may be stated in a number of equivalent ways. The most traditional involves the concentration of water. Thus, an alna Blna alnx w _ w = w w _ ‘52— ‘ RT 32 RT 231an 32 ' (2'5 14) But, Cw Expanding de yields BXW BXw de = 36— de + EC- dC W S or CS CW dXW = '7 de " —'2' dCs C C S 18 From the Gibbs-Duhem relations, dC V __§ = _ .2 de V 5 Therefore, Csvs + vaw l de = 2_ dC = 2_ de C V V s s and alnx 8C w = 1 w 32 C C V. 8w w s yielding, J5 = - (CS 9w + Cw QS)RT Elnaw BCW w CCV SS 31an 32 ' (2.5-16) (2.5-17) (2.5-18) (2.5-l9) (2.5-20) (2.5-21) This development shows that Dé as used in equation (2.5-1) is given by 19 RT Elnaw D8 = (Cs Qw + Cw as) alnX C C V w s s Blna = D11 i alnx‘”. (2.5—22) CVs w At this point the distinction between D; and DC should be explained. Dé as defined in equation (2.5-l) relates the molar flux with respect to the solid to the concentration gradient. DC as used in many texts on mass transfer (especially Bird, Steward, and Lightfoot, 1960) is the mutual diffusion coefficient which relates the molar flux with respect to the volume fixed reference plane to the concentration gradient. Thus, The relation of Dé to DC can now be determined. Since v _ mv _ Jw — J3 + v Cw’ (2.5 24) vmv = - v'm = — V' Jm - V Jm , (2.5-25) w w s s and JV = Jm - c (\7 Jm + \7 Jm) , (2.5-26) w w w w w s s substituting for J? and J: into equation (2.5-26) yields 20 V—_ — _. — Jw — (CS Qw + CW 98) VS Cw . (2.5 27) Substituting for Buw/Bz gives finally BE Blnaw BCW C :31an 82 J = - (C Q + C Q ) S W W S w (2.5-28) which is identical with the result given by Hartley and Crank (1949). Thus, D! 59. = , (2.5-29) C When shrinkage upon drying is negligible, the partial molar volume of water V; is zero and the term 1/C8Vé is unity. In this case, Dé and DC are identical. Fish (1958) found that in potato starch gel, Dé varied with temperature and the variation could be de- scribed by Arrhenius' equation as -E l _ I __a_ .. DC — DO exp ( RT) . (2.5 30) The constant D6 varied little with moisture content but the activation energy Ea increased with decreasing moisture content. Jason (1958) obtained good agreement between the experimental and theoretical results in the drying of fish muscle by using two values for Dé--the first during the 21 initial falling rate drying period and the second during the later stages of drying. He postulated that the trans- ition from the first to the second was associated with the uncovering of the unimolecular water layer normally bound to the portein molecules. Jason (like Fish) noted an Arrhenius type of relationship between the diffusion coefficient and temper- ature. His results agree with those of Fish in that D5 was quite constant but Ea increased with decreasing moisture content. Wang (1958) used a different equation to model moisture diffusion. While he solved a problem in simul- taneous heat and mass transfer, the equations he used degenerate for a one dimensional isothermal case to J = W (2.5-31) where pw is the equilibrium vapor pressure. Since 3p 3lnp 3p __W==RT W = 53 W , (2.5-32) 32 32 p 32 w equation (2.5-10) can be rewritten as C 3p 8:- 1&4: - Jw (Cs 9w + CW 98) C p 32 . (2.5 33) 22 Thus, MWRT cw D' M.w cw = (CS 9w + CW 525) C— = —“——— . (2.5-34) W S pWV B Since Cw includes all of the moisture existing per unit volume in the material, the gas law does not provide a simple relationship between pw and Cw' One cannot say a priori that B' is a constant. Young (1968) used a related approach to model simultaneous heat and mass transfer in spherical, porous, hygroscopic solids. He assumed that the gradient of the density of water vapor in the pore spaces pwv is the driv- ing force for mass transfer, thus making a distinction between pgm, the overall moisture density and pwv' the density of the water vapor in the void fractions of the material. Two further assumptions were that the overall moisture content (d.b.) was related to the vapor concen- tration in the void fraction and the temperature by m = a + B pwv - y T (2.5-35) where a, 8, and y are constants and that the diffusion coefficient associated with the vapor concentration gradient driving force could be modeled as I _ — Dwv — D1 + Dzm + D3T (2.5 36) and D are constants. D2' 3 where D1, 23 It can be shown that D&v is related to D; by the equation D' m p D¢V = _£LEV_JBE . (2.5-37) WV While Young did not attempt to verify his model experi- mentally, he did solve the model numerically for many different input conditions. One important result was the illustration of the fact that the heat and mass trans- fer equations may be solved separately whenever temper- ature equilibrium is reached much faster than moisture equilibrium. Young developed a modified Lewis number as a criterion for this condition. (See Chapter VIII.) In spite of the fact that Dév is not a constant, extremely good fits between experimental data and the corresponding mathematical models have been obtained in some cases by using constant values of D&V., For example, Roa and Bakker-Arkema (1969) found that in freeze-dried meat cubes, a model using a constant Dév and one using a second order polynomial D¢v fitted the adsorption data almost equally well. Another possibility for the driving force should be mentioned--the moisture content (d.b.), m. Since 3uw 3lnaw 32 3m. 35 ' (2°5’38) 24 equation (2.5—10) becomes CWRT 3lnaw C 3m 3 J 8-- 2E - w - (CS 0w + CW 98) 32 . (2.5 39) The mass flux of water with respect to the solids J: is given by = M J . (2.5-40) w Therefore, C 31na .s _ _ w w 3m _ 3w — M.W RT (CS 0w + CW 05) Csm 31nm 32 (2.5 41) defining Dfi in the equation .5 _ . 3m 3W " - pdm Dm E (2.5-’42) as 31na . _ 52_ W _ D — C5 (C5 0w + CW 98) 31nm . (2.5 43) It follows that " 31naw D' = D’ C V = D = D' . (2.5-44) s s c u 31nm Fish (1958) found that in potato starch gel the term 3lnaw/3lnm varied from approximately 1.50 at moisture contents between 0.02 and 0.18 (d.b.) to approximately 0.02 near saturation. He also found that, at 35 C, Di was 25 7 constant (2.4 x 10— cmz/sec) for samples over 0.30 mois- ture content (d.b.). For samples below this moisture con- 10 cmZ/sec. at 0.8% tent, DA decreased rapidly (l x 10- moisture content). The mass of water included in an incremental volume of material dV is pdmmdv. The law of the conser- vation of mass when combined with equation (2.5-42) yields in the one dimensional case _ . 3m) 3 —3_z(m32 . (2.5 45) OJIO) H'B Thus, the use of the moisture content as the driving force for moisture transfer leads to an equation that is rela- tively simple to solve in that the driving force for mois- ture transfer and the measure of water existing within the material are the same. For this reason, the moisture con- tent was used as the driving force in the mathematical model as described in Chapters IV and VI. 2.6 Mathematical Modeling As shown in Sections 2.4 and 2.5, heat and moisture transfer within a material can be described by parabolic partial differential equations such as (2.4-2) and (2.5-45). This section outlines in general terms how these equations can be put into finite difference form, and describes a particular method--the "alternating direction explicit procedure" for solving the finite difference equations. 26 For the purpose of illustration, constant property, isotropic, two dimensional heat conduction shall be con- sidered. The modifications needed to extend this to the heat and moisture transfer problem under consideration in this research are straightforward and are outlined in Chapter IV. The partial differential equation for the constant property, isotropic, two dimensional case is 2 2 pc 22-: k (§—%-+ g—g) . (2.6-l) 3x 3y In order to develop the finite difference approxi- mations to this equation, a rectangular grid is super- imposed on the material as shown in Figure 2.6.1. To make the notation easier, the node points are referred to by subscripts i, j, and n, with i and j referring to the increment in the x and y directions, and n to the increment in time. By the Taylor series expansion theorem, 2 T. . - 2 T. . + T. . 2.2 = 1-1.3 1'1 1+1'3 + o (sz) (2 6-2) 2 2 . . 3x Ax Similarly, 2 T. . - 2 T. . + T. . 2.2 = lLlfl 1'1 1L3+1 + o (Ayz) . (2.6-3) 27 i-l, 1+: i, j+l i+l,j+l i-l,j i,j i+l,j JUJ-I i+l,j-l Figure 2.6.1. A Portion Of a Two Dimensional Finite Difference Grid in Cartesian Coordi- nates. 28 Combining (2.6-2), (2.6-3), and (2.6-1), dT. . T. . - T. . T. . - T. . _1_'_l = i 1-113 1'1 + 1+1d 1:3 dt pc AX2 AX2 2 T. . — T. . T. . - T. . + 1.3-1 1,1.+ 1.1+1 1.] Ay Ayz + 0(AX2) + 0(Ay2) . (2.6-4) Equation (2.6-4) is the basic equation upon which the various finite difference schemes are built. Given the initial temperatures at every node point, the numerical methods attempt to approximate the term (dTi,j/dt)ave in the equation (n+1) (n) d Ti ' To I = To 0 + ——’_1 At (2.6-5) i,j i,j dt ave in order to Obtain the temperature-time history of the material. The evaluation of the right hand side of equation (2.6-4) by the inclusion of the node temperatures at different times gives rise to the various numerical methods for solution of the conduction problem. The "forward difference explicit method" uses only node temperatures at time n in order to calculate the right hand side of equation (2.6-4). Thus, 29 T(n+1) _ T(n) T(n) T(n) T(n) T(n) TiI j 1Ij = j; (i-le1Ij + i+le1Ii At pc sz Ax2 T_(n) T(n) T(n) T(n) +1Ij--11Ij + iIi+l Tin Ay2 Ay2 (2.6—6) and rearranging, Tm) T(n) T(n) _ T(n) T(n+1) = T(n) + Atk i-le1,j + i+1,j i,j Ti,j i,j pc Ax2 sz T(n) T(n) T(n) Tm) +1Ij-11Ij +irj+1ltj . (2.6-7) Ay2 Ay2 Thus, the method is "explicit, meaning that the calcu- (n+1) lation of the temperature Ti' uses only one equation I and not a set of simultaneous equations. Unfortunately, the method is unstable if either Of the ratios kAt/pch2 or kAt/pcAy2 becomes larger than about 1/4. This requires that the time step At be kept so small that the method is impractical. The "backward difference method" uses the node temperatures as evaluated at time n+1 for evaluation of the right hand side of equation (2.6-4). Since this yields a system Of simultaneous algebraic equations giving the node temperatures at the time n+1, the method is an "implicit" one. While the method is stable for any At, 30 the requirement of solving the simultaneous equations renders the method impractical. The "Crank-Nicolson" method uses the average of the node temperatures at times n and n+1 for the right hand side of equation (2.6-4). The method is stable but has the same limitation as the backward difference method in that the requirement of solving the system of simul- taneous algebraic equations created by using the node temperatures at time n+1 makes the method impractical. A fourth method is known as the "alternating direction implicit procedure" (ADIP) or the "Peaceman- Rachford" method. This method uses node temperatures at time n in the first two terms within the brackets on the right hand side of equation (2.6-4) and temperatures at time n+1 in the second two terms. In calculating the temperatures at time n+2, temperatures at n+2 in the first two terms within the brackets and at n+1 in the second two are used. The systems of resulting simultaneous algebraic equations differ from those Obtained in the backward difference or the Crank-Nicolson methods in that the un- known node temperatures appearing in any equation are all from the same nodal row or column. This makes the solution much easier since it is easier to solve I algebraic simul- taneous equations J times than to solve I times J simul- taneous equations once. This method is stable and practical but the "alternating direction explicit pro- cedure" (ADEP), as described next, is faster and easier 31 to program and was therefore used in this research (Allada and Quon, 1967). The alternating direction explicit procedure (ADEP), like the ADIP, is a two pass system. On the for- ward pass, the first and third terms within the brackets on the right hand side of equation (2.6-4) are evaluated at time n+1, and the second and fourth terms are evalu- ated at time n. On the return pass the first and third terms of the right hand side of equation (2.6-4) are evaluated at time n+1 and the second and fourth at time n+2. The method thus progresses through two time steps during the full forward and return passes. Barakat and Clark (1966) described a variation of the ADEP in which two separate solutions are generated. In one, the first and third terms within the brackets on the right hand side of equation (2.6-4) are evaluated at time n+1 while the second and fourth are evaluated at time n. In the other solution, the second and fourth terms are evaluated at time n+1 and the first and third at time n. The final solution is obtained by averaging the two solutions wherever desired. Thus, for a given At, this variation requires twice as much computer time and storage as the first ADEP method. Barakat and Clark, however, argued that the averaging ADEP is better than the non-averaging. They pointed out that the truncation At 32T error in the first pass includes the terms - A; 3E3; (n+1) 32 A; _32T Ay 3t3y (n+1). convergence, the ratios At/Ax and At/Ay must go to zero. and - This would indicate that, to Obtain The truncation errors on the return pass, however, have the opposite sign of the ones on the forward pass. This indicated to Barakat and Clark that these errors cancel in the averaging ADEP, relaxing the requirement that the ratios At/Ax and At/Ay go to zero. Barakat and Clark com- mented that this condition needs further examination. The tOpic is pursued further in Chapter IV. III. OBJECTIVES The objectives selected for this research are as follows: 1. 2. To determine the density of onion bulbs. To determine the thermal conductivity of onion flesh in the axial and radial directions. To determine the moisture permeability of onion skins. To develop a mathematical model that simulates the response of an onion bulb to its environ- ment. 33 IV. MATHEMATICAL MODEL 4.1 Introduction The partial differential equations governing the heat and moisture transfer in an orthorombic material were developed in Sections 2.4 and 2.6. It was also mentioned that, by consideration of initial and boundary conditions and of the laws of the conservation of mass and energy, a mathematical model suitable for computer solution could be developed to simulate the response of a biological product to its environment. Examination of the onion bulb, how- ever, reveals that there are two considerations that should be mentioned before undertaking the development of such a model. First, examination reveals that the bulb can be considered axially symmetric. This is the basis for the decision to use the cylindrical coordinate system. A reduction from a three dimensional problem to a two dimensional one is easily accomplished in this case by including only radial and axial dimensions. The second consideration is that the principal directions of the flesh (i.e., "axia1"--coplanar with the 34 35 central axis and tangential to the onion rings, and "radial"--perpendicu1ar to the rings) are dependent upon position. A grid system that is orthogonal to these principal directions is desirable because the node equations describing heat and mass transfer will then be simplified. Under such a system, transfer from one node to another is independent of the potential gradient existing at right angles to a line connecting the two nodal points. The following section describes the generation of such an orthogonal grid system. The grid system is the basis for three computer programs. The first of these, HTRAN, was written to simu- late the temperature response of a biological product under the assumption of no mass transfer. The second program, HMTRAN, simulates both the heat and moisture responses using the moisture content (d.b.) as the mois- ture transfer driving force. The third program, MTRAN, is identical with the second except that only moisture transfer is considered. 4.2 Finite Difference Grid In the development of the finite grid system, two computer subroutines are required. The first, PERIM (z), associates with any point (2,r) the length of the radius passing through (2,r) to the surface. The second DPERIM (z), associates with (2,r) the slope of the tangent line to the surface with respect to the axis. (See Figure 4.2.1.) 36 DPERIM (z)- fan 0 0 -PERIM(:H :fi-r Figure 4.2.1. Relationship Between PERIM (z), DPERIM (z), and z. 37 Expressions for PERIM (z) and DPERIM (2) were determined for onion bulbs by fitting a ten term Fourier sine series to the onion shape. This was done by taking 35mm slides of ten individual onions. From the projected images, the length to maximum diameter ratio was found to be 3/2. The diameters corresponding to various points along the central axis of the bulbs were recorded. The ratios of the diameters to the maximum diameter of each bulb gave about 200 points outlining the shape of an average onion with unit diameter. The ten term series was fitted to these points by the least squares technique as performed by GAUSHAUS--a computer program supplied by the Michigan State University Computer Laboratory. Table 2 of the Appendix gives the resulting series. DEPERIM (z) is obtained by term by term differentiation of PERIM (z) with respect to 2. To develop the grid proper, node points were placed on the central axis. Since the base and the tip of the onion were thought to play a relatively important role in the mass transfer process, the node points were placed progressively closer together near the base and near the tip. When, for the purposes of model testing, the grid was developed for a Sphere, eleven node points were placed on the central axis at points corresponding to 0.16, 1/16, 2/16, 4/16, 6/16, 8/16, 10/16, 12/16, 14/16, 15/16, and 16/16 Of the diameter. When developed on the onion bulb, an additional point was placed at the length ratio of 9/16 38 so that the total grid would follow the bulb shape more closely (see Figures 4.2.2 and 4.2.3). From each of the central axis node points, a line was projected outward along the principal material direc- tions. This was done in the following manner: at each central axis node point, an incremental distance was formed as one hundredth of the distance between the two adjacent axial node points. Using this incremental dis- tance, a grid line was projected outward from the node point adjusting itself to remain along the radial princi- pal direction, which was always assumed at right angles to the axial principal direction. The axial principal direction was determined by assuming the tangent of the angle between the principal direction and the central axis is given by DPERIM (z) multiplied by the ratio r/PERIM (2). When the grid line so generated reached the surface of the bulb, it was sectioned to give nodes at points corresponding to 0/32, 8/32, 16/32, 20/32, 24/32, 26/32, 28/32, 30/32, 31/32, and 32/32 of the line length. The total grid is shown in Figures 4.2.2 and 4.2.3 for the two-inch sphere and the two-inch diameter onion bulb, respectively. When the grid was developed for HMTRAN and MTRAN, two additional rows of points were added near the surface at the ratios (as described above) of 61/64 and 63/64. This was done because HMTRAN and MTRAN had to be designed for cases in which the modeling time would be much less than the time constant for mass transfer. 39 .mumcmm Hmpofimfia cocHIOBE a How mucflom mpoz .N.N.v musmflm 3: 1.5sz 38 9.6 «.6 ad .Im) q 4 . . . . u m4 . m .. ... m m m .H .. f. .. .. m x. .5 mod I 3.0 . 1 mod .. u I mod I o 8 (‘41)) SfllOVH .COHcO HODOEMHQ SOCHIOBB o How mucflom mooz 3: 1:0sz 36 cud .m.m.w musmflm 0.0 N_.0 00.0 V0.0 «0.0 0 _ L a a _ 1 q _ .2. _ m4 _ q a M. A u :1 0 r. .. . .. m m n .. o . coo an n n no on. on .L o a ... n m ... «00 V O o ooh an m o m 4 . m n . l . n .. .u H. .. coo s .... 1 mod mm 1 00.0 10.0 41 After the grid was developed, the nodal volumes were determined. The volume of a general interior node (i,j) was calculated by the following equation (see Figure 4.2.4): Vol = firiLi (Bl + Bz)ri+1.j + (A1 + A3”in 1I3 8 ri+l,j + ri,j + (B5 + B6)ri-l,j + (Al + A3)ri,{] r. . + r. . 1-1,] 1’] + H [(87 + B8)ri,j+1 + (A2 + A4)riLj ri,j+1 i,j (B + B )r. ._ .+ 3 4 1,3 l. (4.2-1) 4. P N + 3’ a; H p. L____ . . + . . rl'J-l 1’] The volume of a surface node was calculated in the same manner. Referring to Figure 4.2.5, _ "ri,J B2ri+lLJ + A3ri,J BSri-1,J + A3ri,J Vol. 4 + . . . + . r1+1,J + r1,J r1-1,J r1,J + r +r BBri,J-l + A2ri,J B4ri,J+l + A4ri,J i,J-1 i,J ri,J+1 + ri,J (4.2-2) The volume of a general central axis node is given by (see Figure 4.2.6) _ fl 2 2 _ Voli’l — —§§-[A2(3Al + Bl) + A4(3Al + B6) 1 . (4.2 3) 42 Ba Figure 4.2.4. A General Interior Node. ) " I I ...—.1- .1 B 5 Figure 4.2.5. A General Surface Node. 43 The volumes of the two end nodes are simply (see Figure 4.2.7) 3 Vol = Vol = -— A2 . (4.2-4) The transfer of heat or moisture from one nodal volume to another is directly proportional to what Dusinberre (1961) calls the "geometric factor." This geometric factor is defined as the effective tranSport area divided by the distance between the two node points. Referring again to Figure 4.2.4 for a general interior node, the geometric factor between node (i,j) and node (i+1,j) is given by (B + B )r. . + (A + A )r. . Gfapi . = 4n 1 2 l+lLJ 1 3 1L3 (4.2-5) I j 6A2 + B3 + B8 The geometric factor between node (i,j) and (i,j+l) is given similarly as 6A + B + B ° Gfrpi . = 4n '3 1 1 6 The geometric factor between two surface nodes (i,J) and (i+l,J) as shown in Figure 4.2.5, is Gfa = 2“ Bz ri+1,J + A3 ri,J pi,J 3A2 + B3 (4.2-7) The geometric factor between a surface node and the ambient surroundings is somewhat different from the above 44 B. . l+l,l H-l,2 A2 ' 90 A; L l,l (,2 A4 3 B7 I-I,l i-I,ZJ 94 I“ Figure 4.2.6. A General Central Axis Node. Figure 4.2.7. An End Node. 45 in that transfer between the two is proportional to a heat or mass transfer coefficient, instead of a con- ductivity or a mass diffusivity. Referring to Figure 4.2.5, Gfrpi = W I J ri,J (A2 + A4) . (4.2-8) With reference to Figure 4.2.6, the geometric factor between a central axis node and its axial neighbor is given by 2 1r (Bl + Al) Gfapi’l = 4 (3A2 + BB) (4.2-9) The geometric factor between a central axis node and its radial neighbor is given by (4.2-10) Gfrpi’l = A2 + A4 + B7 + 138 . The end nodes present a somewhat different case in that heat and moisture transfer takes place between them and all of the adjacent nodes as well as with the ambient. Thus, referring to Figure 4.2.7, the geometric factor between the end node (1,1) and the node (2,j) is given by 2’1 (4.2-11) 64m Gfapl'j = 46 The geometric factor between (1,1) and the surface node (2,J) is (4.2-12) wIN O Gfapl J = The geometric factor between (1,1) and (2,1) is given by 2 2 Bl Gfapl’l - '3' T . (4.2‘13) 2 The geometric factor between (1,1) and the ambient is Gfrpl,J = __4_ . (4.2-14) After having determined the geometric factors, the transfer parameters are assigned to each node. In HTRAN these consist of ka, kr’ c, p, as well as h for the surface and end nodes. In addition to the heat transfer parameters, the initial values for D$a, Dfir, in HMTRAN. In MTRAN, only D' , D' ma mr and hD have to be assigned , and hD are assigned. This completes the development of the grid system. The next section describes the solution of the system of resulting node equations. 4.3 Solution of Node Equations The Alternating Direction Explicit Procedure (ADEP) for the solution of the conduction equation was reviewed in Section 2.7. The present section describes how this method was utilized to solve the node equations 47 resulting from the finite difference grid outlined in the previous section. The case of pure heat conduction is developed first, after which the modifications necessary to include moisture transfer are described. As was mentioned in Section 2.7, the ADEP is a two pass method. However, when attempting to model a homogeneous, isotropic sphere for the purposes of model testing, it was found that, for the present grid system, the two pass system was not truly symmetrical. Instead, a four pass system was used which proved satisfactory. The first pass starts at the bottom center of the product and progresses row by row (moving outward upon each) until it reaches the top. If this progression is designated as northeast, the other three passes may be simply described by the directions southwest, southeast, and northwest. The following paragraphs outline the equations used for the first pass. The equations used on the other passes are all similar. For an interior node, as shown in Figure 4.2.4, the equations are the same as described in Section 2.7. Thus, for the first pass, (n) (n+1) Vol T ./At + kaGfapi—l,jTi-l,j . .C. . .. . T(n+1) = ‘31.] 1I3 1I15 1IJ 1,] pi,j Ci’j Voli’j/At + kaGfapi-l,j + krGfrpi,j-l (n+1) [ (n) _ (n>J + krGfrpi,j-1T + kaGfapi,j Ti+1gi Ti,j (n) _ (n)] + krGfrpi.j [ 1.1+1 Tin . (4.3-l) 48 Heat transfer between the product surface and the ambient is proportional to the surface heat transfer coef- ficient. Thus, the above equations are modified for a surface node, as shown in Figure 4.2.5, to yield for the first pass, T(n) (n+1) T(n+1) = pLJ cLJ V011, J Ti, J/At + k aGfapi- -L JTi- 1, J 1,J pi,J ci,J Vo J/At + k aiGfap -l J + k rGfrpi j- -l (n+1) [ (n) T(n) krGfrp1,J-l T ,J-l + kaGfapi,JTi+l,J Ti, J m] + h Gfrp ,J [Ta T1,J . (4.3-2) Since a central axis node, as shown in Figure 4.2.6, has only three faces, the first pass equation is T(n) (n+1) T(n+1) _ 01,1 1,1 V011,1T1/At + k aGfapi- 1, lTi-l, 1 T1,1 pi l c. VO l/At + k aGfapi_ 1 1 (n) T(n) + k aGfapL 1 [TTi+l,l T1,1) + krGfrpi'l [T in) - T(n)] . The end nodes are unique in that each has more than four adjacent nodes as shown in Figure 4.2.7. The first pass equation for the node (1,1) is 49 J (n) p c Vol T /At + Z k Gfap . T(n+l)= 1,1 1,1 1,1 1,1 i?1 a 1,3 1,1 01,1 Cl,l Voll,1/At + hl Gfrpl’J (n) _ (n) T2,j T1,j + h1 GfrpLJ Ta . (4.3-4) The heat transfer model was tested by comparing the analytical and model temperature histories for the center of a two-inch diameter sphere initially at uniform temperature and subjected to a step change in ambient temperature. The surface heat transfer coefficient was set at 500 Btu/hr ftZF, the initial temperature was uni- form at 40 F, the ambient temperature was 100 F, the density was 58.8 lbm/ft3, the specific heat was 0.93 Btu/ lbmF, and the axial and radial thermal conductivities were 0.3 Btu/hr ft F. The results, as shown in Figure 4.3.1, indicate that the model agrees reasonably well with the analytical solution which was calculated by the following equation (Grigull, 1964): B 2 sin Vn - vn cos v --v2 kt T = 100 - 60 Z v - sin v cos v ech———7§—). m=l n n n pc r (4.3—5) The eigenvalues vn in the above equation are the solutions to the following transcendental equation: 50 0.3 0.4 0.5 0.6 0.7 TIME Um) 0.2 0.l _ _ 2.0 " _ _ o o o. o. o. In In 9. av NEH—RGNQENP ...zm_m2< 02¢ JdEz. NI... zumxrrum mozmmmuua NIP no m¢ Hmcofluomm mmouo .H.~.n whomflm no.0 u to CO... Gm>ozmm 20.5.5» m¢HH>HmDMMHQ ensemfloz pmuoflpmum .m.m.> wusmflm A... .3 hzmkzoo map—.902 m t n N _ o 1 ‘9 ‘3 C“ C) I l l I J J I 1 JV ‘f 0| C! a) “I Q’ GI C, I! _1_ ‘9 0' (“I *3») ,0I I: do All/\Isnddlo 75 of Table 5 of the Appendix. The average mass flux was 2 2 1.80 x 10’ lbm/ft hr. Comparing this result with 3 4.35 x 10' 1bm/ft2hr, the mass flux obtained from equation (7.2-4) under the same conditions yields D' = 4.14 x 102 0' . (7.2-12) ua ur Before concluding this section, the validity of a tacit assumption made earlier should be established. The above method rests upon the assumption that the resistance to mass transfer through the specimen lies wholly within the sample proper and the surface resistance to the right of the sample is negligible. (See Figure 7.2.1.) (The surface resistance to the left of the sample is obviously zero since the fluid is pure water.) Reynolds analogy states that the convective heat transfer coefficient and the convective mass transfer coefficient are related by hD = C 2/3 (7.2-13) where pai is the air density, c . the specific heat, and r air Le the ratio of the thermal diffusivity of air to the mutual diffusion coefficient of the air-water vapor mix- ture. If h is assumed to be 5 Btu/hr ft2 F; pair’ 0.075 lbm/ft3; c , 0.24 Btu/lbm F and Le, 0.845, then air hD is 312 ft/hr. Since 76 h M .s _ D w _ _ Jw - RT ( wvs Pwva) ’ (7'2 l4) the maximum water vapor pressure difference from the sur- face to the ambient (using the mass flux of 1.4 x 10-3 1bm/ft2hr from run 4 of Table 5 of the Appendix) is 1.425 x 10-3 psi. Since the minimum water pressure drop from the pure water chamber to the ambient is 0.036 psi (run 5), the drop in pressure from the surface to the ambient is negligible. 7.3 Mass Transfer Model Testing To obtain data with which to test the mathematical model outlined in the previous sections and Chapter IV, five two-inch diameter onion bulbs were placed in the air stream of an “Aminco-Aire" unit for twenty-four days. The temperature was maintained at 70 F and the relative humidity at 31%. One of the onions sprouted after a week; the average weight loss from the other four versus time is shown in Figure 7.3.1. Predicted moisture losses for the same conditions were obtained by solution of the mathematical model and are included in Figure 7.3.1. The convective mass transfer coefficient hD was set to 300 ft/hr. The initial moisture content was assumed to be 9.00. As Figure 7.3.1 shows, there is only fair agreement between the predicted and experimental values. The differ- ence between the two is believed due to three factors: WEIGHT L083 1: I03 (lb...) 2<,+ 77 '9 I7- I6- l5” l4- l3- l0 *- 9 '" ‘91 _. 0“ 8 9‘“ c Q(’ 7 " (j' o 6 )- 5 I- ‘ PR ' ' A 4 3 . 2 I 11 i J 1 L 200 300 400 500 600 TIME (ha) Figure 7.3.1. Moisture Loss From Onion Bulbs at 70 F and 31% Relative Humidity. i) ii) iii) 78 The predicted values are based on the assumption that the initial moisture content distribution is uniform. This accounts for the predicted moisture loss values being greater than the experimental values at small time values. Respiration losses are not accounted for in the predicted weight losses. As the onion dries, the outer scales crack and loosen uncovering additional surface area of higher moisture content. This accounts for the experimental moisture losses being higher at the longer times. VIII . CONCLUSIONS The first objective for this research as given in Chapter III was to determine the density of onion bulbs. The final value obtained was 58.8 1bm/ft3. The second objective listed was to obtain values of thermal conductivity in the axial and radial directions. The final values obtained were 0.30 Btu/hr ft F and 0.217 Btu/hr ft F for the axial and radial thermal conductivity respectively. The third objective was to obtain values for the moisture diffusivity. The predicted values for the radial diffusivity DLr and the axial diffusivity Dfia are given by the equations 0hr = RT[4.553 x 10’18 m6°023 + 1.779 x 10'14(T-530)J (8-1) and 0' = 414 0' . (8—2) 113 Llr The final objective was to develop a mathematical model to simulate the reSponse of the onion bulb to its 79 80 environment. To achieve this objective, the Alternating Direction Explicit Procedure (ADEP) for solving the trans- fer equation in cartesian coordinates was adapted to a non- uniform grid system based on the onion shape. The heat transfer portion of the model simulated the actual process very well as indicated by the small RMS and maximum differences between the predicted and experimental center temperatures observed during the tests for radial thermal conductivity (see Table 4). The mass transfer portion of the model was in only fair agreement with the experimental weight losses observed as a test of the model. Suggestions for Further Study The author concludes that further work is needed: 1) to determine the magnitude of the weight losses due to respiration compared to those due to moisture loss, ii) to measure the effect of the cracking and loosening of the outer layers of the bulb, and iii) to include this last effect in the modeling of the mass transfer process. BIBLIOGRAPHY BIBLIOGRAPHY Allada, S. R. and Quon, D. (1966) A stable, explicit and numerical solution of the conduction equation for multi-dimensional non-homogeneous media. Chemical Engineering Progress Symposium Series. Heat Transfer. Los Angeles. 26:151. Arpaci, V. S. (1966) Conduction Heat Transfer. Addison- Wesley Publishing Company, Reading, Massachusetts. Babbitt, J. D. (1940) Observation on the permeability of hygroscopic materials to water vapor. Canadian J. of Research. 18(A):105. Barakat, H. Z. and Clark, J. A. (1966) On the solution of the diffusion equations by numerical methods. Trans. ASME, J. Heat Transfery Series C. 88:421. Bakker-Arkema, F. W., Rosenau, J. R., and Clifford, W. H. (1969) Measurements of grain surface area and its effect on the heat and mass transfer rates in fixed and moving beds of biological products. ASAE Paper No. 69-356. Barker, J. J. (1965) Heat transfer in packed bed. J. Ind. Engr. Chem. 57:43. Beck, J. V. (1964) The Optimum Analytical Design of Transient Experiments for Simultaneous Determi- nations of Thermal Conductivity and Specific Heat. Unpublished thesis, Michigan State University, East Lansing, Michigan. Beck, J. V. (1966) Analytical Determination of Optimum, Transient Experiments for Measurements of Thermal Properties. Proc. Third International Heat Transfer Conference. IV:74. Beck, J. V. (1968) Surface heat flux determination using an integral method. Nuclear Engr. and Design. 7:170. 81 83 Jason, A. C. (1958) A study of evaporation and diffusion processes in the drying of fish muscle. Fundamental Aspects of the Dehydration of Foodstuffs. Society of Chemical Industry, London. Jones, H. A., and Mann, L. K. (1963) Onions and Their Allies. Interscience, New York. Jost, W. (1960) Diffusion in Solids, Liquids, Gases. Academic Press, Inc., New York. Katchalsky, A. (1961) Membrane permeability and the thermodynamics of irreversible processes. Membrane Transport and Metabolism. Proceedings of a Sym- posium held in Prague, August 22-27, 1960. Kedem, O. (1961) Criteria of active transport. Membrane Transport and Metabolism. Proceedings of a Sym- posium held in Prague, August 22-27, 1960. Kozlowski, T. T. (ed.) (1968) Water Definits and Plant Growth. Vol. I. Development, Control and Measure- ment. Academic Press, New York. Kramer, P. J. (1969) Plant and Soil Water Relationships; A Modern Synthesis. McGraw-Hill Book Co., New York. Kuprianoff, J. (1958) Bound water in foods. Fundamental Aspects of Dehydration of Foodstuffs. Society of Chemical Industry, London. Lewis, G. N. and Randall, M. Revised by Pitzer, K. S. and Brewer, L. (1961) Thermodynamics. 2nd Edition. McGraw-Hill Book Co., New York. Lykov, A. W. (1955) Experimentalle und Theoretische Grundlagen der Trocknung. V. E. B. Verlag Technik, Berlin. Lykov, A. W. and Mikhaylow, Y. A. (1961) Theory of Energy and Mass Transfer. Prentice—Hall, Inc., Englewood Cliffs, New Jersey. Meeter, D. A. (1965) Non-Linear Least-Squares (GAUSHAUS). Michigan State University Computer Laboratory Pro- gram Documentation No. 87. Moore, W. J. (1962) Physical Chemistry. Prentice—Hall, Inc., Englewood Cliffs, New Jersey. 84 Ngoddy, P. O. (1969) A Generalized Theory of Sorption Phenomena in Biological Materials. Unpublished thesis, Michigan State University, East Lansing, Michigan. Ozisik, M. N. (1968) Boundary Value Problems of Heat Conduction. International Textbook Co., Scranton, Penn. Reidy, G. A. (1968) Thermal Properties of Foods and Methods of Their Determination. Unpublished thesis, Michigan State University, East Lansing, Michigan. Riedel, L. (1951) The refrigerating effect required to freeze fruits and vegetables. Refrigerating Engineering. 59:670. Roa, G. and Bakker—Arkema, F. W. (1969) Moisture ad- sorption by freeze—dried meat cubes. ASAE Paper No. 69-893. Rohsenow, W. M. and Choi, H. Y. (1961) Heat, Mass, and Momentum Transfer. Prentice-Hall, Inc., Englewood Cliffs, New Jersey. Saravacos, G. D. (1962) A study of the mechanism of fruit and vegetable dehydration. Food Technology. 16:78. Saravacos, G. D. (1960) Studies of the Mechanism of Fruit and Vegetable Dehydration. Unpublished thesis, Massachusetts Institute of Technology, Cambridge, Massachusetts. Siebel, E. (1892) Specific heats of various products. Ice and Refrigeration. 2:256-7. Van Arsdel, W. B. and Copley, M. J. (1963) Food Dehye dration. Vol. 1. Principles. AVI Publishing Company, Inc., Westport, Connecticut. Wang, J. K. (1958) Theory of Drying. Unpublished thesis. Michigan State University, East Lansing, Michigan. Wright, R. C., Lauritzen, J. I., and Whiteman, T. M. (1935). Influence of Storage Temperature and Humidity on Keeping Qualities ogOnions and Onion Sets. Technical Bulletin No. 475. United States Department of Agriculture. Young, J. H. (1969) Simultaneous heat and mass transfer in a porous, hygroscopic solid. Transactions of the ASAE. 12(5):720. APPENDIX 85 mumxana Ho.om Nomm. wmvm. mnmm. mma.m~ mma.m~ mma.m~ omm.mN www.mw omm.mm nmm.om 5mm.hm mmH.vm IaIm.~e uaoeuou use Audubon ovoo.o some 0:» mo uouum pMMGGMum ommao.o sawubw>ou ouupcmum hmvcm.o >ue>mum vauaoomm ammum>¢ muasmmm ammo. vmvm. mmmm. Hmvm. mma.m~ mma.m~ mmH.mN mmH.¢N onw.m~ omo.mm mmH.vN mmo.m~ mwN.o> Naa.mh ~mo.mw ovm.0h Hmmm. wmmm. mama. amam. mma.mm mmH.mN mmH.mN mma.m~ oma.v~ onm.m~ mhh.mN hao.mm Nah.mm Nom.vh wvm.mm ono.mw wufi>muo ofiuflommm Luv nevus ca unseen uo panama Amy nevus ca unmwoz Ame nan ca unmeuz HM ca m «amaum .mumn GOAuncflEumump auwmcmo :ofico ecu Ho «unseen can mosao> 30.4.4530.“qu A mamas 86 TABLE 2.--The Fourier series approximating the onion shape. d 10 . i n z PERIM (a) = 2 131 Ai Sin (1.5 d) 1 Ai 1 3 0.8322400 2 0.3689900 3 0.0462150 4 0.0074205 5 0.0822030 6 -0.0114820 7 0.0194130 8 0.0074970 9 0.0145270 10 -0.0046454 87 TABLE 3.--Results of the axial thermal conductivity experi- ments. RMS Difference Between Run ka(Btu/hr ft F) Calculated and Experimental Temperatures (F) Individual Experiments 1 0.32518 _ 1.38 2 0.251 1.44 3 0.322 0.56 4 0.324 1.36 5 0.241 2.55 Results Average Axial Conductivity 0.293 Btu/hr ft F Standard Deviation 0.042 Btu/hr ft F Standard Error of the Mean 0.019 Btu/hr ft F Weighted (by l/RMS) Average Axial Conductivity 0.30 Btu/hr ft F a an u:\sum na~.o mua>auospcoe Hoepom mouuo>< xmzm\a use evacuees 88 m uw.u£\:um mNoo.o cum: ecu no uouum vuuccmum m an uc\5um mmoo.o nodumfi>mo numcamum m on usxsum oa~.o >0a>auoapsoo apnoea omoum>< muasmmm ~h.a mh.o mn.o wam.o m mw.a hm.o cw.o va.o v so.a av.o mm.o ma~.o m vm.~ H~.H Hm.o . mo~.o N ~v.~ vm.H vH.H v-.o a mucosflummxm Hmscwbflch Ame m be un\sum . .me u uu us\sum na~.o n x eH~.o I x moan: nous» .m. nuns» mafia: mousumuomsma umucou Imummems umucmo Manama Imummeoa Houcmo Houses .m an Hn\sumv 3 six amucmewuomxm one pmumasonu Iwuwmxm can pmumasoHoo Iwuomxm can wouMH50HdU cmesumm mocwuowuwn Baeflxmz cmo3umm mucoummmwo mam :mo3umm wocmuoumwo mzm .mUQGEmemxo huw>auosoaoo Awaken» Heaven on» no nuasuumII.v mamas 89 .ooe H.eH m.mH n.m mo.o Has hobo: m mH.H oo.o m.m oe ma.o has been: a c.H0m Houascoz mo.H em.m o.m on Haoo.o em.o Hope: u oo.m mo.o m.o on oo.o baa noun: m oe.n H.ea o.m on mb.o has bonus 4 mo.o ma.o m.m on ma.o has Hobos m oe.e mo.m m.m on mo.o nae nouns m oo.o mm.m n.m on oe.o “as noun: a A be us\snae N . s Anaemoa x Ame moam 06am 1.1an 1:1 .0 m m ea an 2 madame Iuoasme up .m pagan panam oax xsam mmmz .mucmeHmmxm muH>HmDMMH© mumpmHOE Hmflpmu may no muHSmmmII.m mummy 90 86 F (Saravacos, 1960). TABLE 6.--Data points for the onion moisture isotherm at Water Activity Moisture Condent (d.b.) 0.112 0.220 0.328 0.436 0.539 0.633 0.756 0.863 0.907 0.012 0.022 0.077 0.148 0.190 0.305 0.330 0.530 0.925 91 canadoxvqxq N qaomnx V DD .m~¢«#xn 0‘13 13 rum mo pzdxl bomrmq¢domuui 41.1: new: ~1.x<:~.<—3 ZOEECU .ON.MU oxooamvN ..co=—.I ..umommvn hm oamnvl oanmommv43> ..dmommvmmm> ogmmommvldm> oxmmomfivm oa~Nomfiva oammommvx ..HNommvi oxfiuommv> ..~momflVD a oanmomnvto oanmomvaIm oammommvb ..qmommvdml oxuNonmv<<1 Zommzuzfia .mebDO.P:lzq. quwt 2419310 92 moan: n so mozfipzoe .mz.~u.~zozvx.n.mvmu nmo.n.¢.mo m.u.n.nu mm.u.w.nu quvwnaoorvxdi .uzvznxoozvdri druazq Axozowoodmoo43 .om p2~11 p oh 0c q+~zu~¥ m Op 00 ..Hszuzwlomooxdzvx. LH D G... 3.0 .Almzvxnxazvx .glazvNuxdzvN 3 Oh ow Ax o Amm+_ nm+N 0 Op 39 AAAocmoz. LA n Am+A<¢o0v\AAAxo¥v 624 m¢0bo .AzoA.Qau>uAAzoAvlrm> ocflAA¥oAVQIm> AfloNqu NA 93 Au: mn thra AAquvJO>uAAzoAVJO> AA IM\G#¢AAomux fioAqu AA 3Q o¢\AAA{o¥V 3A . 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