Extreme temperature exchangers capable of operating between 800°C and 1100°C and pressuresgreater than 80 bar are considered a critical component for ultra-high efficiency power generation and a range of next-generation industrial processes. A promising application for this research thrust is the use of carbon dioxide as a working fluid, whose critical point is at 73.8 bar and 31°C. As compared to traditional steam or air-based power cycles, a supercritical carbon dioxide (sCO2) cycle has less compression work near the critical point and higher cycle efficiencies which enables a smaller plant footprint. Given the extreme temperatures and pressures required for heat exchange, however, this poses a significant materials and system design challenge. This research seeks to develop an efficient and cost-effective test facility to enable the rapid testing and verification of heat exchangers within this temperature and pressure range while utilizing nitrogen as a surrogate fluid for carbon dioxide. A bench-scale test facility was first developed for moderate temperatures and pressures (100°C, 100 psi) for the purpose of developing friction factor and Nusselt number correlations for twisted S-shaped fins and for validating computational fluid dynamics (CFD) models of various fin configurations. A polyimide thermofoil heater was compressed between a mirrored system of additively manufactured heat exchanger plates fitted into a set of aluminum headers. A set of flat aluminum plates were used to compare against the twisted S-shaped finned plates made from titanium. Compared to other results within the literature, the correlations developed here for flat plates and finned surfaces are enhanced by the inlet impingement and outlet transition effects. The friction factor is up to 20.1 times larger for the flat plate correlations while the twisted S-shaped fins are up to 7.2 times greater than the literature would suggest. For the Nusselt number correlations the flat plate correlation are 6 times larger while the twisted S-shaped fins are up to 2.5 times larger than the literature would suggest. As compared to the experimental results, the CFD errors for friction factor are within -21.63% for the flat plate and -16.74% for the twisted S-fins. The maximum error in the Nusselt number for the flat plate is within +20.87%, while the twisted S-shaped fins have a maximum error on the order of -54.14%. The differences here between experiment and CFD areattributable to contact resistance effects between the heater and plate surfaces and the roughness of the printed fins. A 5 kW test facility was developed for heat exchanger characterization capable of operating at 250 bar, 300°C on the cold side and 80 bar, 1100°C on the hot side. The primary research within this work related to this facility is the development of process heat at high flow rates with a high inlet temperature, the management of the high temperature throttling process between the cold side and the hot side, and the optimization of the headers for integration with the heat exchanger. The development of process heat was achieved by a U-shaped graphite heating element with internal hexagonal channels that allow for prediction of heat transfer properties. Self-cooled nickel 200 alloy conductors are used which allow for the extreme inlet temperatures expected in sCO2 recuperative flows. The inlet conditions to the heater were as high as 450°C due to losses while the outlet flow was generally limited to less than 1100°C for the duration of the experiments at 80 bar. A thick sharp-edged orifice plate was used for high temperature compressible flow control at 7 g/s of N2 from 250 bar to 80 bar. A subset of research here attempted to develop the compressibility factors required for determining the flow rate and pressure drop relationship within a range of orifice diameters from 0.50 mm to 0.70 mm. Finally, a set of headers were developed with internal cooling channels and temperature monitoring to accommodate the extreme temperature and pressure conditions seen within the heat exchanger. A careful energy balance was performed to determine the best approach for optimizing the design and mitigating heat losses for more accurate heat exchanger characterization in future iterations of the design
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