Optimization Principle for Natural Convection Pulsating Heating

Abstract

This is a theoretical, numerical, and experimental study of natural convection heat transfer from an immersed surface with intermittent uniform heat flux. Scale analysis predicts that the “on” time interval can be optimized such that the overall Nusselt number is maximized. Furthermore, Nu increases monotonically as the “off” time interval decreases. These features are verified and determined more accurately by complete numerical simulations for a vertical flat surface in the range 103 ≤ Ra ≤ 105 and Pr = 7, where Ra is based on heat flux and wall height. They are further verified by means of water experiments in the range 1010 ≤ Ra ≤ 1011. The pulsating heater configurations tested experimentally are the vertical cylinder, which approximates boundary layer flow on a plane surface, and the horizontal cylinder. The optimal square-wave heat pulse for a natural convection immersion heater is reported in general (scaling correct) terms.