Master's Thesis Michael Sietmann
Simulation of the influence of frosting and defrosting on the flow properties and the thermodynamic behavior of a fin heat exchanger of an air-water heat pumpCopyright: EBC
Air-Water heat pumps play an important role in the regenerative heat supply of buildings. In the chourse of the energy transition, its importance for heat supply will continue to increase in the future. Thereforemeasures should be aimed at increasing the efficiency of the heat pump. The air-water heat pump uses the ambient air as its heat source. The ambient conditions like temperature and humidity have therefore a high influence on the performance of the heat pump. During operation of the heat pump, the ambient air is cooled down in the finned heat exchanger. If the air temperatur falls below the freezing point of water and at the same time the humidity of the air is high enough, a frost layer can form on the surfaces of the heat exchanger. This frost layer has a negative impact on the performance of the heat pump due to several effects. The frost layer leads to an increased pressure loss, because the frost layer can block parts of the flow cross section. At the same time the frost layer acts as a thermally insulating layer on the surfaces of the heat exchanger and decreases the heat transport from the air to the heat exchanger. In order to be able to develop measures against the icing of the finned tube heat exchanger, models are needed to predict the growth of the frost layer and to simulate the influence of the frost layer on the air flow and the heat transfer in the heat exchanger. In this thesis a simulation model for the icing of a finned tube heat exchanger with the CFD-Software Fluent is developed. The basis is a multi-phase model that is already integrated in the software. That model is extended by a user-defined function. With this expanding function, the phase change from water vapor of the moist air to ice is simulated. With the developed simulation model it is possible to qualitatively represent the formation and the temporal development of the frost layer in the finned tube heat exchanger. The effects of the frost layer on the flow through the heat exchanger and the termodynamic properties of the heat exchanger can be investigated with the simulation. A comparison with a reference work shows that plausible results can be generated for the pressure loss due to icing. Due to a short simulation time the influence of the frost layer on the heat transfer can not be predicted well.