Thermal management of high heat flux devices in many areas has promoted the development of advanced microchannel heat sinks. This study proposed a novel type of periodic expanded-constrained microchannels (PECM) heat sink with flow separation and convergence passages. Its convective heat transfer and pressure drop performance was explored by both experimental tests and numerical simulations, and its cooling efficiency was compared with rectangular microchannels. Forced convection tests were conducted at Reynolds number of 150-820 and two levels of heat fluxes using deionized water. Results indicated that the PECM samples showed significant heat transfer enhancement, i.e., 50% to 117%, compared to the rectangular counterpart in the test ranges. Such enhancement can be attributed to that the periodic expanded constrained configurations of PECM induced periodic interruption and redevelopment of thermal and hydraulic boundary layers, as well as good flow separation, fluid acceleration and mixing. Moreover, the heat transfer enhancement of PECM was not accompanied with the expense of pressure drop penalty, and all the PECM samples reduced the pressure drop for 10-74% compared to the rectangular counterpart. Besides, the effect of microchannel gap width in the main flow passages on the thermal and hydraulic performance was also assessed by preparing three samples with microchannel gap widths of 0.5, 0.8 and 1.0 mm. It was found that the pressure drop and thermal resistance of PECM increased monotonically with decreasing the microchannel gap widths. The sample with the largest microchannel gap width of 1.0 mm presented the best overall thermal and hydraulic performance. (C) 2019 Elsevier Ltd. All rights reserved.