Experimental study on thermal flow characteristics in square serpentine heat exchangers mounted with louver-type turbulators.

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]