Particle image velocimetry and infrared thermography measurements in a two-pass 90-deg ribbed parallelogram channel

Infrared thermography and pressure measurements are presented of the detailed local Nusselt number (Nu₀) distributions over top-endwall and the associated pressure drop coefficients (f₀) of a stationary two-pass 90-deg ribbed parallelogram channel. The parallelogram cross-section has equal adjacent sides of 45.5 mm in length (aspect ratio = 1) and channel hydraulic diameter of 32.17 mm. The rib pitch-to-height ratio, rib width-to-height ratio and rib-height to channel-height ratio are 10, 1 and 0.1, respectively. It is aimed at exploring the effects of Reynolds number, Re = 5000-20,000, on Nu₀ and regionally averaged (Nu¯₀) Nusselt numbers along the ribbed inlet/outlet legs and over the bend. To gain insight into the heat transfer augmentation mechanisms, the PIV measurements were also performed for correlating the fluid flow and heat transfer characteristics in the two legs and inside the bend at Re = 10,000 in terms of the streamwise main flow and cross-sectional secondary flow, turbulent kinetic energy as well as spanwise and streamwise Nu₀ and Nu¯₀. It is found that the flow dynamic mechanisms responsible for the rib-top and mid-rib heat transfer enhancement are different for the inlet and outlet passes. The presences of a skewed high Nu₀ streak between the last inlet-leg rib and the bend as well as two high Nu₀ zones inside the bend are the new found features lacking in the corresponding two-pass 90-deg ribbed square channel. In addition, simple correlations of Nu₀ and f₀ with Re are acquired. Thermal performance factors are about 66% and 28% higher than the previous reported smooth-walled counterpart at Re = 5000 and 20,000, respectively. They are further compared with those of the corresponding square channels available from the literature.