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 infinity), 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)over-bar/Nu(infinity), and (f)over-bar/f(infinity) 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)over-bar/Nu(infinity), almost twice the value of smooth reference; nevertheless, it suffers from high (f)over-bar/f(infinity) 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. (C) 2017 Elsevier Ltd. All rights reserved.