Experimental Investigation of Heating Orientation Effects on Flow Boiling in Manifold Microchannel Heat Sinks

Two-phase cooling systems have emerged as effective solutions for managing the power dissipation in high heat flux electronic devices. Among them, flow boiling in manifold microchannels (MMC) has garnered attention due to its high heat transfer performance and low pressure drop. Heating orientation plays a critical role in influencing the flow boiling behavior and results in different heat transfer performances. However, there is limited work available on the effect of heating orientations in manifold microchannel heat sinks. In this study, we experimentally investigate the effect of heating orientations on heat transfer performance of a copper manifold microchannel heat sink. R1233zd(E) refrigerant is used as working fluid. Two heating orientations are evaluated: downward heating (DH), and horizontal heating with vertical manifolds (HVMF). Experiments are performed at flow rates of $2.5 \mathrm{~g} / \mathrm{s}$ and $7.5 \mathrm{~g} / \mathrm{s}$, with the inlet subcooling temperature maintained at 5 K. The results show that horizontal heating with vertical manifolds (HVMF) configuration achieves up to approximately 16% higher heat transfer coefficient compared to the downward heating (DH) configuration. Additionally, a hysteresis phenomenon is observed. The hysteresis phenomenon decreases as the mass flow rate increases.