Experimental study on convective heat transfer with turbulence promoters

Abstract

An experimental investigation on forced convective heat transfer in square ducts has been carried out. An experimental setup equipped with measuring instruments and a personal computer, has been used for investigation. The duct in the set up is a straight horizontal square duct of 50mmx50mm. The test section consists of a heated zone preceded by an unheated zone. There are one smooth and two ribbed test specimens. The ribs with cross section 1mmx1mm are machined out from a 12.5 mm thick Aluminum plate at two different pitch to rib height ratios namely 8 and 12 and are used as turbulence promoters. In doing so resistance to heat transfer from the heated bottom wall to the flowing air is reduced. Measurements of the velocity and temperature in the duct cross section at x/D = 26.5 downstream from the leading edge of heated part are taken in the region of -1 < (zlB) < 1 and 0 <S (y/B) <S 0.92 at ten different Reynolds Numbers varying from 4.36x104to 8.71x104 To examine the similarity between the velocity and temperature fields, the normalized mean values of GIGe and u/ue have been plotted. The velocity profiles show clearly the effect of both heating and turbulent promoters on their shapes. In the smooth duct the velocity maxima tend to shift towards the heated wall from the centre due to the change in viscosity with temperature while in the ribbed ducts those shift towards the opposite smooth wall due to the increased roughness of the ribbed wall. Temperature profiles of smooth duct show that there is hardly any heat transfer beyond the centre of the duct while in the ribbed ducts the heat transfer continues almost up to the top smooth wall. The universal velocity distribution of the smooth duct fall on a straight line in the turbulent part of the wall region 1..85 < (z/B) < 6 where the law of the wall is generally valid. The universal velocity distribution for the ribbed duct fall below those of the smooth duct showing higher frictional loss. The Nusselt Number and Stanton Number found for the smooth and the ribbed ducts in the present experimental studies compared well with some of the well known published data. The ribbed wall provides higher Nusselt Number as well as Stanton Number than those of the smooth wall over the range of Reynolds Number studied. In general, the ribbed duct pIe = 8 and pIe = 12 provides 26% and 20% augmentation of heat transfer and 58% and . 56% higher friction factor than those of plane surface respectively. Friction factors decrease with increase of pIe ratio and the results lie close to the published data.