The Effect of Square Tube Location in a Vertical Array of Square Tubes on Natural Convection Heat Transfer

摘要

Experimental investigation is reported on natural convection heat transfer from the outer surface of a vertical array of horizontal square tubes in air. Five tubes equally spaced are used with cross section 0.02 × 0.02 m~2. The tubes are subject to constant heat flux boundary condition using internal constant heat flux heating elements in the range 46-510 W/m~2. Experiment is done for arrays of 2-5 square tubes and for four center-to-center separation distance to hydraulic diameter ratios. Study is concentrated on the effect of tube location in the array and on the geometry of the array. Results show that the downstream tubes exhibit reduced Nusselt numbers than that of a single tube for small center-to-center separation ratio of 2.5. This reduction depends on the location of the tube in the array and the number of tubes in each array. Results also show that as the ratio increases, enhancement in heat transfer over that of a single tube is observed and critical ratio is obtained at a specified value of the modified Rayleigh number for the upper (downward) tubes in each array. Local circumference averaged correlations are proposed for the upper tubes in each array and for any other individual tube in each array geometry. An overall general averaged correlation is also reported for each tube in the array.