Experimental Study On Flash Evaporation Atfer Pressure Drop

Flash evaporation is widely applied to seawater desalination, ocean thermal energyconversion power plants, steam accumulator and cooling of high temperature part of the spaceshuttle and nuclear reactor. It is far more violent than the static evaporation, and differentfrom common boiling phenomena with internal heating surface which is widely studied on. Itsinternal heat transfer and flow mechanism is complex, with various involving factors. Thereare significant differences in the current analysis results.Based on the previous studies, a new experimental Flash evaporation device is designedin this article, serving for the flash evaporation investigation under vacuum, atmospheric,medium, and high pressure. Large size window and high-speed CCD camera are applied forobservation. Accurate, rapidly responded, micro Fiber Bragg Grating (FBG) serves tomeasure dramatic changes of temperature, pressure in phase transition. The seal device isdesigned to seal and potect fibers and wires strenching out of the flash chamber.The experiment under1atm and vaccum pressure is performed in the distilled water. Theinitial conditions is following: the initial temperature is45℃~100℃, the superheat degree is0~13℃, the initial height is390mm,440mm,490mm, and540mm, the maximum vacuum ofthe Flash evaporator is70kPa, pressure difference is24kPa~40kPa in differentdepressurization rates. Flashing phenomena is divided into three stages in this thesisaccording to the record and data analysis, and flow and heat transfer principles are served toexplain different phenomena. The swelling height, the maximum of penetration depth andpenetration velocity are defined to describe flash evaporation, the impact of initialtemperature, degree of superheat, the initial liquid level height on the above parameters arequalitatively and quantitatively analyzed, and the relevant formulas are fitted. Flashingphenomena is classified as fully，un-fully and critical flash evaporation. The first and secondcritical superheats are defined based on three types of flash evaporation.Temperature, non-equilibrium temperature faction NEF, flashing time, quantity andpressure are discussed, qualitatively and quantitatively. It is found that the initial liquidtemperature, superheat, initial level of height, resistance coefficient, and depressurization ratehave significant influence on the parameters mentioned above. Finally, relevant formulas are well fitted to predict the relationship and influence.The flow and heat transfer mechanism of flash evaporation is indicated explicitly,quantitative study on the influencing factors can provide reference and basis for the industrialapplication. This study is of great importance on theoretical research and practical applicationof flash evaporation.