Numerical estimation of the convective coefficient for a cylindrical radiation thermometer

Outdoor thermal comfort has attracted a rising attention in recent years, also due to the general increase in summer temperatures. An important parameter to evaluate thermal comfort is the mean radiant temperature, that is typically indirectly determined through an energy balance on radiation thermometers, which requires an accurate evaluation of the convective heat exchange between such devices and the surrounding air. The conventional black-globe configuration of radiation thermometers is well suited for indoor conditions, but it can be sub-optimal in outdoor environments, as the view factors do not adequately represent a pedestrian with respect to Sun radiation. To improve this aspect, radiation thermometers shaped as slender cylinders can be designed, but in this case a mixed convection scenario, with the forced and free contributions acting along two orthogonal directions, may typically occur in outdoor conditions with moderate wind speed. The available literature seems lacking in correlations to estimate the convective coefficient fitted to such specific situation; thus, numerical simulations were performed to determine the convective coefficient. The results showed that, for the velocity and temperature conditions of interest for outdoor thermal comfort, forced convection is still largely dominant, thus literature correlations for forced convection should be reliable also in these cases.