Experimental Investigation On The Characteristics Of Falling Film Evaporation On The Horizontal Enhanced Tubes In Refrigerating System

Substitute of HCFC refrigerants is a big issue because of serious global environmental problem. The amount of production and consumption of the conventional refrigerants, HCFCs, is very huge in our country. It is impractical to replace HCFC refrigerants in a short time, particularly the development of new refrigerants has not yet be utilized well in practice, Therefore, the decreasing & technology can be as one of the substitute technologies to decrease the consumption of conventional refrigerants in current, while other substitute technologies can be developed.According to the investigation, the falling film evaporation applied in large-sized heat pump systems reduces refrigerant charge by 30% compared with the system widely using flooded evaporation. So the development of falling film evaporator is a major aspect of the decreasing & technology.Based on the decreasing & technology, theoretical analysis and experimental investigation on the flooded evaporation and falling film evaporation of horizontal tubes has been conducted in this study. The heat transfer models of flooded evaporation were presented to predict the heat transfer coefficients and the prediction results showed that the heat transfer coefficient of flooded evaporation on tubes increases with the increasing saturation temperature and heat flux; Ribatski correlations predicted the experimental data of ours well in this study. Moreover, the liquid film thickness models of the falling film evaporation of horizontal tubes by polar coordinate were used and as the calculation results, the liquid film thickness of refrigerants on tubes decrease firstly and then increase with the angle and increase more and more slowly with the liquid film Reynolds.Experiments of flooded evaporation and falling film evaporation were conducted in three types of enhanced single tubes: TJX-2,TDR and Serrated tubes. It is noted that the heat transfer coefficient of flooded evaporation on enhanced tubes is high at a high heat flux and at a high density of porous structure. The heat transfer coefficient of falling film evaporation on enhanced tubes depends on spray mass flow rate besides heat flux. There is an optimal spray mass flow rate which is appropriate for the system at a range of heat flux. The heat transfer coefficient of falling film evaporation depends on both the density of porous structure and the density of enhanced structure and increases with them.Finally, a horizontal tube bundle falling film evaporator was designed for the experiment apparatus in this study. And the liquid distributor of tube bundle style was analyzed and calculated. Results showed that when the liquid distributor size was D=20mm and H=25mm, the spray heterogeneous degree and the mean outer velocity of the spray hole were relatively smaller and the comprehensive performance was better. The static pressure in the spray tube increased firstly and then decreased with the increasing number of spray hole.