Instrument for the measurement of heat flux from a surface with uniform temperature

An instrument for the measurement of heat flux from a surface with a nearly uniform temperature is described. This instrument contains a thin-film electrical resistance heater embedded in a copper cone which is thermally isolated from the surrounding walls. A differential thermocouple between the copper cone and the wall is nulled such that the electrical power becomes a direct measure of the surface heat flux. The advantage of this design over earlier sensors is its modular characteristic and its ability to be flush mounted in an external surface or mounted in the wall of a duct. It has been used to measure the local time-average heat transfer coefficient inside a circular duct. The time constant in this application was 43 s. For these measurements an uncertainty analysis is presented which shows that this instrument has an uncertainty of ±3.6% for a convective heat flux of 342 W/sqm. The major source of uncertainty was the surface area.