Experimental study of contacting surfaces microgeometry and gas gap impact on thermal contact conductance of metallic joints

Abstract

Thermal contact resistance is an important phenomenon in heat transfer and impacts many fields of technology. The interrelated influences of many factors—such as geometry and microgeometry of contacting surfaces, their mechanical and thermal properties, apparent contact pressure, type and properties of interstitial fluid—make prediction of this resistance problematic and necessitate both theoretical and experimental research. The paper reports an experimental study of thermal contact conductance (the reciprocal of thermal contact resistance) of joints created by flat, conforming metallic surfaces in static contact. The measurements were performed for samples made of mild steel and stainless steel with relatively low roughness (σ = 0.5 - 2.14 µm) and large slope of asperities (m = 0.4 – 1.3), resulting in values of σef/mef = 1.25 - 1.96 µm that are distinctly lower than reported in the majority of experimental research on thermal contact conductance. The apparent contact pressure was changed from 1.5 to 11 MPa and for some cases up to 19 MPa. Analysis of the results obtained showed that the correlations available in the literature, for greater values of roughness and smaller asperity slope, mostly overestimated the predicted thermal contact conductance. The elasticity indices values for samples used in this study indicated plastic deformation of asperities, but it was concluded that the smoothest surfaces of both kinds of steel underwent elastic deformation. A new experimental correlation for contact of mild steel surfaces (σ = 0.5 - 2.14 µm, σef/mef = 1.25 - 1.96) was proposed. The results of this study indicate the need for further investigation of thermal contact conductance, especially for this range of surface microgeometry parameters, which is not dealt with in the correlations available in the literature. There is also a need to study the impact of asperity curvature on contact conductance. The results of this study proved the important role that air gaps play in the transfer of heat across a joint. The gap conductance values were predicted correctly for low mechanical loads and rough surfaces.