Roll Bond Evaporator, R1234ze(E), Flow Boiling, IR-Thermo-Analysis
Roll bond type evaporator is one of the most widely used technology in household refrigerators. Despite that, only few works that analyze the performance of this component are available in literature. Furthermore, no evidence is given to the impact on the heat transfer performance when substituting the original HFC refrigerant with an HFO inside the same evaporator. This paper presents an experimental study of R134a and R1234ze(E) inside an off the shelf roll bond evaporator, commonly used for small size domestic refrigerators. The evaporator was mounted inside a climate dark chamber where ambient temperature and humidity were maintained stable during the tests. To control the inlet conditions (evaporation temperature, inlet quality, refrigerant mass flow rate) it was used a water cooled miniature scale vapor cycle system with R134a and R1234ze(E) as working fluids. By means of an IR-thermo-camera, the whole roll bond temperature field was investigated under different working conditions. 15 thermocouples were collocated on the back of the evaporator to verify the temperatures reported in the images collected by the IR thermo-camera. From these pictures it was possible to delineate the super heating region and to point out the areas of the evaporator in which heat transfer is less efficient depending on the fluid and on the working conditions. During the experimental tests the refrigerant mass flow rate was varied by regulating the compressor speed, while ambient temperature and evaporation temperature were kept as constant. The data acquired from the vapor cycle system (i.e. condensation and evaporation pressure, evaporator inlet quality, vapour superheating, refrigerant mass flow rate) coupled with the IR thermo-camera images allowed to evaluate the behavior and the efficiency of the roll bond. Since the data were collected maintaining the same operating conditions in term of ambient temperature and humidity, heat flow rate and evaporation temperature, it was possible to highlight differences among the two refrigerants in term of mass flux. Furthermore, on the basis of the IR images and of the thermocouples measurements, an average heat transfer coefficient was defined and determined both on the air and the refrigerant side. The average heat transfer coefficients of the two refrigerants are compared and outlined in the paper.