Investigation of R32 spot-evaporators.

Within technical applications limited space for heat exchange is a widespread problem. The design of the part is given by technical functionality and often the areas for heat transfer are quite small. On the other hand productivity of i.e. cutting tools is directly proportional to the thermal load caused by the process. Both parameters lead to heat fluxes up to 107 W/m2.
To face this problem a new kind of evaporator has been designed at the Institute of Materials and Processes (IMP) of the UAS Karlsruhe. Spot evaporators are small cylindrical objects with an outer diameter of 2 – 5 mm. The refrigerant (R32) is lead into the evaporator through a concentric capillary tube, sprayed and redirected on the facing surface of the evaporator and lead out in an evaporated or partly evaporated state. The paper focusses on the investigation of these new evaporators. A first model to describe the behavior of the refrigerant inside the evaporator will be presented. Within the model well known equations for heat -transfer, spray-cooling and pipe-flow are combined to describe the behavior of the spot-evaporator. To investigate the accuracy of the model, several tests have been carried out at a test-loop with an oil-free R32 circuit. The paper also discusses the differences between the model and the measurement. It is shown, that spot-evaporators are adequate evaporators for cooling high heat fluxes within technical applications.