Theoretical Analysis And Experimental Study Of Falling Film Evaporation Outside Horizontal Tubes In The Refrigerating System

In the current severe environment pressure, refrigerant substitute and thedevelopment of new refrigerant is urgent, and the reduction&technology can beused as the current form complement and transition for the work of refrigerantsubstitute. According to the statistics and investigation, under the precondition of thesame refrigerating capacity, the amount of refrigerant consumed by a falling filmevaporation is lower than the flooded evaporator by25%.Based on the reduction and the substitute&technology of refrigerant,theoretical analysis and conclusion on the flow mechanism, heat transfercharacteristics, the schedule of reinforcement of the falling film evaporation onhorizontal tubes has been carried on in this study. Besides, the application of thefalling film evaporation in the refrigeration system and the influence of the oil on theheat transfer coefficient is also summarized and explained. Under different operatingconditions, the heat transfer characteristics of falling film evaporation presentsdifferent change rules, and lubricating oil contained in the system gives certaininfluence on the heat transfer characteristics. The existence of the lubricating oil cansignificantly reduce the heat transfer coefficient. A refrigeration cycle system usingR134a as a working fluid with falling film evaporation is detailed and set up, thusthe experimental study of horizontal tube falling film evaporation can be conductedwith the research variables about the amount of spray with refrigerant, heat flux andevaporation temperature to summarize the experiment rule.In addition, according to the heat transfer mechanism of horizontal tube fallingfilm evaporation, different kinds of the pipe outside surface structure can improvedifferent level outside the convective heat transfer coefficient, therefore in theprocess of experiment, the TJX tube, the EX tube, test1,2tubes are tested in thisexperiment. Analysis show that The performance of the enhanced tube in the fallingfilm evaporation mainly depends the spray quantity and heat flux density, workingmedium and different strengthening features of tube type. With the increase of heatflux, the heat transfer coefficient outside the tube is increasing before the occurrenceof the critical heat flux The performances of enhanced tubes are better than that of asmooth tube, and the heat transfer coefficient of different enhancement tube is alsodifferent. The surface structure makes a certain effect on the experimental results. Finally, using experimental data of the smooth tubes, a new heat transfercorrelation is used to predict the heat transfer coefficient of the smooth tube basedon the Ribatski and Thome’s model. The fitting error between the results with theexperimental data is within the plus or minus20%, and correction factors for theheat transfer prediction of enhanced tubes are proposed, which can provide thereference for the research of heat transfer on enhanced tube.