Effects of heating orientation on flow boiling in copper manifold microchannel heat sinks

Recently, two-phase cooling configurations are being proposed to meet the power dissipation requirements of high heat flux electronic devices. Flow boiling in Manifold Microchannel (MMC) offers high heat transfer coefficients with low pressure drops, making it a popular choice. While many studies have explored flow boiling in microchannels, the influence of heating orientation in MMCs with complex 3D flow paths has not been investigated thoroughly. In this study, experiments are conducted to investigate the effects of heating orientations and mass flow rates on heat transfer performance, pressure drop, and the hysteresis phenomenon during flow boiling in a copper manifold microchannel heat sink, using the environmentally friendly refrigerant R1233zd(E) as the working fluid. Four heating orientations are studies: Upward Heating (UH), Downward Heating (DH), Horizontal Heating with Vertical Microchannels (HVMC), and Horizontal Heating with Vertical Manifolds (HVMF). The experiments are carried out with mass flow rates between 2.5 and 12.5 g/s, and the inlet subcooling temperature is set to 5 K. The results show that heating orientation significantly affects heat transfer performance, especially at high flow rates and higher heat fluxes. The Horizontal Heating with Vertical Manifolds (HVMF) configuration achieves the best heat transfer performance, while Downward Heating (DH) configuration exhibits the lowest performance. In contrast, heating orientation has a minimal effect on pressure drop performance. An increase in mass flow rate improves the heat transfer performance and raises the pressure drop in manifold microchannel heat sinks. Additionally, hysteresis phenomena are observed in both wall temperature and pressure drop between the heating and cooling curves. A notable wall temperature overshoot occurs before the Onset of Nucleate Boiling (ONB), which decreases with increasing mass flow rate. In the pressure drop curves, hysteresis is also evident, with higher pressure drops during the cooling process compared to the heating process at the same heat flux near ONB. The hysteresis in pressure drop becomes more pronounced at higher mass flow rates.