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

Abstract

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 (DH) of 45.5mm and a fully developed inlet condition. Three parameters are examined to find out the optimal design, including the pitch ratio (Pi/DH=1, 2, 3, 4, and ∞), the number of slat per half louver (1≤Ns≤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/DH and ascending Ns under the present test conditions. Among all the tested cases, the case with Pi/DH=1 and Ns=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/DH. The new finding is that there exists a critical slat number of Ns=3 above which the TPF value is a weak function of Ns. In contrast, below the critical Ns the TPF value increases with decreasing Ns. From the viewpoint of heat transfer enhancement, one could apply the louvered channel as a heat exchanger with small Pi/DH and large Ns. 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.