Numerische und experimentelle Untersuchungen zum Wärmetransport in einem Automobilscheinwerfer
In this work the heat transfer in an automotive headlamp is investigated both numerically and experimentally. In this case the heat transport is affected by interaction between natural convection, heat conduction and radiation. In the headlamp, the dominant role in the heat transfer belongs to the radiation. However, due the simultaneous action of all other basic heat transport mechanisms, there are overheatings (so called Hot Spots) on the housing and on the optical lens. To get an insight into Hot Spot formation and behaviour is the main subject of the presented work. To model the coupled heat transport, both two-dimensional and three- dimensional numerical models were developed and solved with the help of the commercial CFD code STAR-CD with the DTRM method to calculate radiation. The DTRM method takes into account the diffuse and neglects specular radiation.At the beginning, the effect of the heat transport was studied by means of a two-dimensional model for the following cases: pure convection, pure radiation, and convection coupled with the radiation. As a result, the heat transfer correlations were clarified, and the measures to reduce the hot spot temperature were elaborated, such as an increase of the convective force (increase of Ra number), an increase of emissivity of the lamp, and an increase of conductivity of the lens (decrease of thermal conductivity ratio). Also, 2D simulation models were extended to the 3D geometry. In addition to the numerical simulation, the arising flow-mechanical phenomena were studied experimentally and the corresponding physical models for the flow and temperature analysis were prepared. At the beginning, the flow was analysed with the help of the Particle Image Velocimetry technique to study the natural convection and the flow near hot spots. Then, the behavior of the Hot Spots as a function of the internal pressure was investigated. By means of the internal pressure, the relationship between the natural convection and radiation can be varied, which allowed us to study the parameter range (10 < Ra < 1000000), which was not available earlier. As well, to analyze the general heat transfer mechanisms, a new measuring method was developed, which is based on the application of the luminance camera and thermochromic liquid crystals. This method is used for the contactless determination of the temperature distribution inside of the transparent model. Using the infrared camera, the temperature field can be measured on the outside surface of the model. The comparison of this method with standard ones shows a difference of +/-0.35. The results obtained both from numerical and experimental models were used to analyze a real headlamp. It turned out that the diffuse radiation mechanism in the DTRM method used in aforesaid numerical models does not properly describe the heat transfer under real conditions. Therefore, another optimized method was elaborated featured by a simultaneous use of the CFD code and the selfdeveloped software to calculate the specular radiation (socalled Ray tracer). With the help of the optimized method, the correct Hot Spot position and its temperature level in the headlamp were determined.