Enhanced heat transfer in pin fin heat sink working with nitrogen gas–water two-phase flow: variable pin length and longitudinal pitch

In the current study, thermal–hydraulic characteristics of nitrogen gas–water two-phase flow through a plate–pin fin heat sink are investigated experimentally. Water flow through the smooth case, i.e., heat sink without pin fin, is considered as baseline. Four new models of the pin fin with variable longitudinal pitch and pin length having low-to-high and high-to-low arrangements are proposed. They are named for short as LP–LH, LP–HL, PL–LH, and PL–HL. The results indicate that in all heat sinks, the Nusselt number values of the two-phase flow in comparison with those of the single-phase flow are higher, but the friction factor values of the two-phase flow in comparison with those of the single-phase flow are lower. Also, the friction factor and the Nusselt number of both the single-phase flow and the two-phase flow in the heat sinks with pin fin are greater than those in the heat sink without pin fin. The highest values of the Nusselt number are recorded for PL–HL at water mass flow rate of 0.0093 kg s−1 and gas volume flow rate of 0.8 L min−1. At these flow rates in PL–HL, the Nusselt number of the two-phase flow is increased about 45.6% relative to the single-phase flow.