Influence of heat spread in the measurement layer on the accuracy in measuring local heat fluxes by the gradient method

The errors due to tangential heat transfer in a layer of material of low thermal conductivity covering a wind tunnel model around which heat flux is measured by the steady gradient or auxilliary wall method are derived as functions of layer and heat flux parameters. A consideration of heat conduction in a three-layered composite covering deposited on a plane or curved isothermal surface is used to determine the thicknesses of protective, measurement and glue layers of the composite corresponding to error bounds. A comparison of error estimates with the exact analytical solution for heat conduction in uniform flat plates shows a convergence between exact and estimated errors with a decrease in plate thickness. Wind tunnel investigations confirm the calculations, showing a decrease in error with a decrease in measurement layer thickness. Discontinuities in heat flux distribution are found to cause a practically insignificant error.