Step-transient method for measurement of the heat transfer coefficient at surfaces exposed to simulated building outdoor environments using the sol-air thermometer

Abstract

The environmental boundary conditions of the building exterior surface could be expressed in terms of the sol-air temperature To and the heat transfer coefficient ho. This has previously been derived by applying Thevenin’s theorem to the linear thermal network model of the convective and radiative environment. Here, the sol-air thermometer, previously used only for measurement of To, was applied for accurate measurement of ho. The step-transient method was used, where the temperature of the sol-air thermometer was initially raised to above To and then monitored during its transient return to thermal equilibrium. This method was validated by (1) comparison of ho results against values obtained with a steady-state method and (2) comparison of predicted heat flux against the electrical heater power, supplied for validation purpose. Accurate results were obtained, with 7.3% measurement uncertainty. The present sol-air thermometer time constant τ was around 1 h. Based on predictions from dynamic modelling, the τ could be reduced 10-fold, with only small effects on the accuracy from heat loss through the insulation layer.