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The present study aims to propose innovative louver-type turbulators to enhance the heat transfer rate in three major ways, i.e. core flow disturbance, jet impingement, and extended heat transfer surface. These louvers are installed in the twin-pass square channel with a hydraulic diameter ( D H ) of 45.5 mm and a fully developed inlet condition. Three parameters are examined to find out the optimal design, including the pitch ratio ( Pi / D H = 1, 2, 3, 4, and ∞), the number of slat per half louver (1 ≤ N s ≤ 4), and Reynolds number (5000 ≤ Re ≤ 20000). Particle Image Velocimetry (PIV) and Infrared Thermometry (IT) are respectively employed to measure the detailed velocity maps and wall temperature distributions. With acquired Nusselt number ( Nu ) ratio, the pressure measurements are also performed to estimate the Fanning friction factor ( f ) and further evaluate the thermal performance factor (TPF). The results show that both Nu ‾ / Nu ∞ and f ¯ / f ∞ ratios rise with descending Pi / D H and ascending N s under the present test conditions. Among all the tested cases, the case with Pi / D H = 1 and N s = 4 provides the highest Nu ‾ / Nu ∞ , almost twice the value of smooth reference; nevertheless, it suffers from high f ¯ / f ∞ penalty. It is also found that the TPF level is a relatively weak function of Pi / D H . The new finding is that there exists a critical slat number of N s = 3 above which the TPF value is a weak function of N s . In contrast, below the critical N s the TPF value increases with decreasing N s . From the viewpoint of heat transfer enhancement, one could apply the louvered channel as a heat exchanger with small Pi / D H and large N s . The boundary layer disturbance, on the other hand, is more cost-effective than core flow disturbance as a mechanism to augment heat transfer from the viewpoint of thermal performance. [ABSTRACT FROM AUTHOR]

Infrared thermography and pressure measurements are presented of the detailed local Nusselt number ( Nu 0 ) distributions over top-endwall and the associated pressure drop coefficients ( f 0 ) 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 Nu 0 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 Nu 0 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 Nu 0 streak between the last inlet-leg rib and the bend as well as two high Nu 0 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 Nu 0 and f 0 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. [ABSTRACT FROM AUTHOR]