Flash evaporation or flashing is an evaporation phenomenon caused by a sudden pressure drop sufficiently below the saturation pressure. Due to this sudden drop in pressure, the liquid undergoes a quick phase transition and the sensible heat of the liquid converts into latent heat of evaporation. An accidental release of pressurized liquid from supply pipelines or liquid storage tanks can generate a flashing jet with violent phase change. This fast phase change can cause rapid mixture with an oxidant like air, and if the liquid is flammable, disastrous effects such as explosions, fires, and damage to industrial equipment can occur. On the other side, flash evaporation can be beneficially employed in industrial applications. It can have a significant effect on the performance of many devices (e.g. injectors and reactors). The understanding of the fluid behavior of the flash evaporation and the resultant shock waves can help to prevent disastrous consequences that may occur due to accidental fluid release as well as can be useful to further develop the phenomena’s industrial and technological applications such as utilize resulting shock waves in creating useful compression.This work investigates spray flashing evaporation phenomena and resulting shock waves experimentally and numerically, considering pure water as working fluid. 2D transient simulation was conducted by using Ansys Fluent to simulate the multiphase flow field in the evaporation zone. The Mixture model was applied using the concept of slip velocities to model multiphase flows as non-homogeneous multiphase model where the phases (liquid water and water vapor) move at different velocities. In addition, a rectangular vacuum chamber connected to a circular nozzle was used in the experiments. A z-type Schlieren technique with a Photron high-speed camera was used to observe the propagation of the moving shock wave and the flow. The numerical simulation results show that once the superheated water is injected in the low-pressure environment, the flashing phenomenon occurs and a shock wave is induced. This shock wave is a moving shock wave and it travels forward into the vacuum chamber. The two-phase mixture expands behind the moving shock wave that travels faster into the water vapor. The entire flow characteristics such as pressure, velocity, Mach number, speed of sound, density, and volume fraction are discontinuous across the shock wave. After the moving shock wave strikes the end of the vacuum chamber, it reflects from the wall as reflected shock wave that propagates back into the chamber, further increasing the pressure behind it. An experimental work was carried out with injected liquid water having an initial temperature ranging between 40 and 100°C and vacuum pressure ranging between 4000 Pa and 10000 Pa. To show the shock wave structure, the Schlieren technique was used and Schlieren photographs were mathematically filtered using the Image Processing and Analysis in Java (ImageJ). The experimental results were in good agreement with the numerical results, confirming that once the superheated water is injected in the vacuum chamber, it flashes directly and can induce a shock wave. The results confirm that with increasing superheat of the injected water, the flash evaporation accelerates. At low vacuum pressure, full flashing occurs, and a shock wave is induced especially with a high initial liquid temperature.
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