An instrument for measuring heat flux from an isothermal surface

This paper describes an instrument for measuring heat flux distribution on an isothermal surface. The isothermal surface consisted of walls made from thick-walled copper block with high thermal conductivity and heated by cartridge heaters. The heaters were inserted inside the copper block and were separately controlled by a proportional–integral–derivative (PID) control system to create a uniform wall temperature boundary condition on the surface to within ±0.2 °C. A specially designed balance type heat flux sensor was positioned flush with the surface of the heated copper block to enable measurement of the local time-average heat flux at various positions along an isothermal surface. This isothermal heat flux sensor consisted of a thin and narrow nickel/chromium ribbon embedded in, but thermally and electrically insulated from, the heated copper block walls on all sides. Differential thermocouples were mounted on the ribbon to measure differences in temperature between the ribbon and the wall at various positions along the ribbon. An uncertainty analysis revealed that the sensor had a heat flux uncertainty of 6.62% for a convective heat flux of 120 W/m2. The instrument was tested for a laminar slot-jet impinging on the isothermal wall and the experimental results were in a good agreement with the numerical data.