Visualization of heat transfer characteristics using thermochromic liquid crystal temperature measurements in channels with inclined and transverse twisted-baffles

Abstract

Channels with ribs/baffles are widely applied in thermal engineering applications such as heat exchangers, solar water/air heaters and gas turbine cooling. In the present work, inclined and transverse twisted-baffles (I-TBs/T-TBs) are developed for heat transfer enhancement in a channel. The effects of Reynolds number (Re = 4000, 8000, 12,000, 16,000 and 20,000) roughness pitch ratios (p/w = 4.0, 6.0, 8.0, 10.0 and 12.0) and baffle twist ratios (y/w = 2.0, 3.0, 4.0 and 5.0 corresponding to twisted-baffle loop numbers (N) of 5, 7, 8 and 9) were investigated using air (Prandtl number, Pr = 0.7) as a working fluid. Heat transfer behavior was examined using thermochromic liquid crystal temperature measurements. Typical transverse and inclined baffles were also tested for comparison. Heat transfer, flow friction and thermal performance are reported in terms of Nusselt numbers (Nu), Nusselt number ratios (Nu/Nus), friction factors (f), friction factor ratios (f/fs) and thermal performance indices (η). Experimental results reveal that under most conditions examined, I-TBs show better heat transfer, lower frictional losses and higher thermal performance than TBs, IBs and T-TBs. For I-TBs, maximum heat transfer and thermal performance are obtained at a moderate pitch ratio (p/w = 6.0). However, in cases of T-TBs, heat transfer and thermal performance monotonically increase with a decreasing pitch ratio. Friction losses caused by both I-TBs and T-TBs decrease considerably with increasing pitch ratio. For I-TBs, heat transfer and thermal performance monotonically increase with increasing twist ratios. However, for T-TBs, maximum heat transfer and thermal performance are obtained at y/w = 3.0. Over the present studied range, a channel with I-TBs having optimal geometry (p/w = 6.0 and y/w = 5.0) yields maximum thermal performance indices, as high as 1.98, which is greater than the maximum values yielded by channels with T-TBs, TBs, IBs, and smooth channels, by around 74.1%, 98%, 52.5% and 98.3%.