Mixed convection from heated room surfaces

Abstract

In most building thermal load calculations natural convective heat transfer is assumed for internal room surfaces. When mechanical ventilation is used, the natural convective heat transfer coefficient (CHTC) will underestimate the convective heat transfer from the room surface. In this paper, measured local and surface-mean convective heat transfer data are presented for convection from a heated surface partially covered by an air jet. Experiments were conducted in a small office-sized, well-insulated environmental chamber with heating plates fixed to the internal surfaces. A fan box with an adjustable nozzle was placed at one end of the surfaces within the chamber to create the convective flow. Cooling was provided at one wall to represent a heat sink. Equations are presented for predicting mixed convection heat transfer as a combination of natural and forced convection for walls, floor and ceiling. When the jet acted on a heated surface the measured CHTC was greater than the equivalent natural convection CHTC, although the increase for the ceiling was much greater than for the floor or wall.