Dynamic Heat Transfer Coefficient, Two-Phase Flow, Refrigerants
The German automotive industry is trying to achieve full electric mobility in Germany. Therefore the research focus is on the battery as the new primary energy storage. Not only the shorter range and higher costs of this technology are a problem. In addition to that, the battery is a highly dynamic and demanding consumer from a thermodynamic point of view. The working temperature is limited to the range between 20 and 40 °C. Below 0 °C and over 60 °C the life expectancy and the capacity would take damage. Thus the air conditioning cycle of the automobile needs to be adapted to the new challenge. Since the industry wants to simulate every state of the dynamic air conditioning system in the car, numerically robust models are required. A dynamic system model was created and tested in Modelica. Experience shows that in particular low heat loads and mass flows close to zero are causing problems to the solvability of such dynamic cooling cycles. One aspect of these numerical problems is the dynamic heat transfer coefficient at these marginal loads. Hence the aim of the study at the Institute of Thermo-Fluid Dynamics is to investigate the dynamic behavior of the heat transfer coefficient under these conditions for the refrigerant R-134a. This paper presents a test rig to measure the heat transfer coefficient of the refrigerant R-134a in a small diameter horizontal pipe. The experiment layout is basically a cooling cycle. The basic components are a receiver tank filled with R-134a, a pump, an evaporator pipe and a condenser. The evaporator pipe made out of copper is the main element. Thermocouples, pressure sensors and a mass flow sensor provide the information necessary to determine the heat transfer coefficient of the horizontal pipe. In addition to the cycle the natural convection of R-134a in the pipe is of interest. Because there are working points of the automobile, where a small heat load is applied and the air conditioning is switched off. Besides the experimental investigations a FEM model is build. With COMSOL Multiphysics the two-phase-flow is simulated and compared with the results of the experiments. The Validation is an important step to build a numerically robust model for the system simulation with Modelica.