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.
