ABSTRACT
This paper provides heat transfer enhancement and flow friction data for swirl flows
entering tangentially to an annulus roughened with helical-ribs. The effects of the
roughness intensity, tangential to total momentum flux ratio and Reynolds number
were studied. The tested annulus has a radius ratio of 0.25 with the inner surface
insulated and the outer one was heated with a uniform heat flux. The inner surface of
the outer tube was roughened with helically coiled wires which act as turbulence
promoters breaking up the viscous sublayer formed in the fluid region nearest to the
wall. Roughness was made by using helical-wires at two different diameter to pitch
ratios (d/p) of 0.025 and 0.050 within a range of Reynolds number from 5x103 to
25x103, and at values of the tangential to total momentum flux ratios of 1.73, 2.94,
3.46 and 5.88. It was found that there is a significant enhancement in the heat
transfer due to inlet injection swirl compared to that gained by helical roughness.
Also, the efficiency index is critically dependent on the roughness Reynolds number,
momentum flux ratio and the roughness intensity and it is almost less than that for
the rough plain annulus. New correlations for Nusselt number enhancement ratio
and the Fanning friction factor ratio were obtained as functions of Reynolds number,
momentum flux and d/p ratios. Also, a maximum Nusselt number enhancement ratio
of about 2.0 was found corresponding to 8.7 fold increase in the flow friction factor
for a single entry injector with Mt/MT of 5.88 at relatively dense roughness with d/p of
0.05.
KEYWORDS: Heat transfer augmentation, flow friction, forced |
