Numerical simulation of the wind shadow effect on the convective heat transfer to glass bulb sprinklers

Abstract

A theoretical analysis of the wind shadow effect caused by the arms of the sprinkler frame has been performed using a finite volume computer code. The calculations show that the influence of the arms of the frame upon the convective heat transfer to the bulb is small when the arms of the sprinkler frame and the bulb are orientated perpendicular to the flow. The arms of the frame cause a slight acceleration of the flow around the bulb. Hence, the heat transfer to the bulb in this orientation is somewhat greater than for free flow around a cylinder. In the case when the arms of the sprinkler frame are aligned with the flow, the heat transfer is decreased by approximately a factor of two for the particular geometry considered. This is in agreement with results from wind-tunnel tests for the same geometry. The Nusselt number for this case is still roughly proportional to the square root of the Reynolds number. This finding is of practical importance, as the lumped mass theory for calculating the time to operation of sprinklers relies heavily on the assumption of a square root dependence of the Reynolds number. Good agreement is found between calculated and measured velocity profiles in the region between the glass bulb and the upstream arm of the frame, when the arms are aligned with the airflow.