Experimental investigation of the effect of heat sink orientation on subcooled flow boiling performance in a rectangular microgap channel

•Flow boiling study in a microgap channel with different spatial orientations.•Orientation effects on CHF, heat transfer coefficient and pressure drop were reported.•Orientation effects were negligible on flow boiling performances in microgap channel. Investigation on the effect of heat sink orientation on the flow boiling performance in a rectangular microgap channel of width (w) 20 mm, length (L) 30 mm and depth (d) 220 µm with a hydraulic diameter (Dh) of 435 µm were performed. Experiments were conducted in different orientations such as horizontal downward facing (HD), horizontal upward facing (HU), horizontal with heating area vertically aligned (HV), vertical up flow orientation (VUF) and vertical downflow orientation (VDF) with mass fluxes (G) ranging from 193 to 911 kg/m2 s using deionized water as the working fluid. The experimental results were compared with the results of 31 parallel ‘U’ shaped microchannel study conducted by the authors earlier. The experimental results in microgap channel showed more or less similar critical heat flux (CHF) and heat transfer coefficients across all the orientations tested, indicating that the effect of gravity is negligible on the flow boiling performance in microgap channel. Even though the CHF and heat transfer coefficients for different orientations are comparable, the total pressure drop for vertical downflow orientation (VDF) was higher compared to other orientations. Significant pressure fluctuations were observed for the VDF orientation during flow boiling at low mass fluxes in the microgap channel. Comparison of flow boiling in the parallel microchannels with microgap channel depicts that the bubble growth pattern is entirely different. The bubble grows or elongates in both spanwise and axial directions in the case of the microgap channel which reduces the interaction between the flowing liquid and the vapour enabling the microgap channel to perform consistently at all orientations.