Influence of surface roughness and waviness upon thermal contact resistance

This work deals with the phenomenon of thermal resistance between contacting solids. Attention is directed towards contiguous solids possessing both surface roughness and waviness. When two such surfaces are brought together under load, they actually touch at isolated microcontacts, and the resulting real area is the sum of these microcontacts. Because of the waviness the microcontacts are confined to a region called the contour area which may occupy some fraction of the total available area. The non-uniform pressure distribution over the contour area results in microcontacts which vary in size and density. In the absence of an interstitial fluid and negligible radiation heat transfer, all the heat crossing the interface must flow through the microcontacts. A thermal analysis, based on size and spatial distribution, results in a thermal resistance equation which differs from previously developed theories. The equation is verified by liquid analog tests which show that the size and spatial distribution are very significant.(cont.) A surface deformation analysis considers the influence of surface roughness upon the elastic deformation of a rough hemisphere. An equation is developed which shows the extent of the contour area as a function of the surface geometry, the material properties, and the applied load. The equation is compared with existing theories and qualitatively checked against experimental results. Experimental heat transfer data were obtained to verify the thermal and deformation theories. The agreement between theory and test is quite good over a large range of surface geometry and applied loads.Sponsored by the National Aeronautics and Space Administration DS

Thermal contact conductance in a vacuum

The object of this work is to develop analytically equations by which one could predict the thermal contact conductance between contiguous surfaces operating in a vacuum environment. In this work the solution to the problem is obtained by considering that any surface can be modelled as being either: 1) nominally-flat but rough, 2) a smooth surface having cylindrical waviness, 3) a smooth surface having spherical waviness, or 4) a surface having either cylindrical or spherical waviness plus roughness. Since the radiative heat transfer and the conduction through the interstitial fluid are negligible, the conduction of heat across the metal contact spots is the dominant mechanism. It is considered that the prediction of thermal contact conductance must be approached by: 1) examining the surface geometry, 2) proposing mathematical models for the solution of the heat transfer problem, 3) determining the surface parameters from deformation analysis, and 4) obtaining experimental data to substantiate the proposed models. The surface analysis is actually a critical examination of profiles of real surfaces as obtained by profilometers. From such profiles it is proposed that real surfaces can be idealized by assuming that any surface is a combination of a wavy and rough component. The thermal analysis is based upon the models proposed and the solutions for the steady-state condition are obtained for the various models and the appropriate boundary conditions. Certain surface parameters appear in the thermal contact conductance equations, which require that an analysis of the deformation of the surface under load be undertaken. The deformation analysis is separated into two regimes: 1) purely elastic and 2) purely plastic. The surface parameters are then determined as functions of the applied load for the proposed models under the restrictions of pure elastic or pure plastic deformation.Sponsored by the National Aeronautics and Space Administration DS

The effect of surface roughness and waviness upon the overall thermal contact resistance

A thermal contact conductance equation was developed which considers both the effect of surface roughness and waviness. It was shown that the overall thermal contact conductance is determined by the roughness at large contact pressures or rough surfaces. It is also shown that surface roughness increases the contour radius over that predicted by the theory of Hertz. The surface roughness influences the magnitude of the waviness resistances by spreading the load at the contact over a larger region. The theory was seen to be in very good agreement with experimental data.Sponsored by the National Aeronautics and Space Administratio

HT-FED2004-56633 MODELS AND EXPERIMENTS FOR LAMINAR NATURAL CONVECTION FROM HEATED BODIES IN ENCLOSURES HT-FED04-56633

ABSTRAC

Culham, “Effect of surface asperity truncation on thermal contact conductance

Abstract-This paper presents studies on thermal contact conductance at light contact loads. Surface profilometry measurements are presented which show that actual surface asperity height distributions are not perfectly Gaussian. The highest asperities are truncated, causing existing thermal contact conductance models to underpredict experimental data. These observations have been incorporated into modifications of existing contact conductance models. The truncation leads to an enhancement of thermal contact conductance at light contact pressures. The preliminary model has been compared against thermal contact conductance data presented in the open literature, and good agreement is observed. The results show that the truncation is a function of the roughness level: the rougher the surface, the more truncated the surface height distribution