On the effects of coating thickness in transient heat transfer experiments using thermochromic liquid crystals
On the effects of coating thickness in transient heat transfer experiments using thermochromic liquid crystals
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DOI：
10.1016/j.expthermflusci.2015.08.011
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发表时间：
2016
影响因子：
3.2
通讯作者：
S. Schulz;S. Brack;A. Terzis;J. Wolfersdorf;P. Ott
中科院分区：
文献类型：
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作者：
S. Schulz;S. Brack;A. Terzis;J. Wolfersdorf;P. Ott
瞬态热传递实验通常采用热致变色液晶来暂时映射表面温度。所需的传热系数，然后计算从傅立叶的一维瞬态热传导方程的解决方案，该方程被设置为模拟在固-液界面处的壁温。然而，实验条件并不总是证明这一假设，由于额外的涂料层的出现屏蔽实际的液晶直接暴露于工作流体。在评估过程中忽略这些附加层的厚度会产生有偏差的传热结果。为了系统地评估涂层厚度对评估传热的影响，本调查报告在瞬态实验中的三种不同的液晶类型的层中的应用。这些都进行了两个不同的流动制度，使用单独的测试设施，即一个四面体形状的涡流发生器和射流从一个低纵横比冲击通道内的一排孔线。分别研究了基于水力和射流孔直径的100，000和50，000雷诺数。在考虑测量的实际液晶涂层厚度后，研究表明，忽略层厚度可能导致对所得热传递的显著低估，特别是对于大厚度。通过考虑相应的涂层厚度，实验差异可以从14%减少到5%以下，实现高数据冗余。
Transient heat transfer experiments typically employ thermochromic liquid crystals to temporally map surface temperatures. The desired heat transfer coefficient is then calculated from the solution of Fourier’s 1D transient heat conduction equation which is set to model the wall temperature at the solid–fluid interface. However, the experimental conditions do not always justify this assumption due to occurring layers of additional paint shielding the actual liquid crystal from the immediate exposure to the working fluid. The disregard of these additional layers with respect to their thicknesses in the evaluation process produces biased heat transfer results. In order to systematically assess the effect of coating thickness on the evaluated heat transfer, the present investigation reports on the application of three different liquid crystal types in layers in transient experiments. These were conducted for two different flow regimes using separate test facilities, ie a flow over a tetrahedra-shaped vortex generator and jet flows from an in-line row of orifices within a low aspect ratio impingement channel. Reynolds numbers of 100,000 and 50,000 based on hydraulic and jet orifice diameter were investigated, respectively. Upon consideration of the actual liquid crystals’ coating thicknesses from measurements, the investigations show that disregarding the layer thicknesses can lead to a significant underestimation of the resulting heat transfer, particularly for large thicknesses. By taking into account the respective coating thicknesses the experimental discrepancies could be reduced from 14% to less than 5%, accomplishing high data redundancy.