Experimental study of flow boiling cooling in a novel variable density pin–fin device

• Increasing variable density pin–fin heatsink was experimentally studied. • For an inlet subcooling of 30 K, three flow rates (100–150–200 ml/min) were evaluated. • Boiling curves, heat transfer coefficient and pressure drop fluctuations were investigated. • Upstream compressible volume instability and reversal flow were observed. • The maximum critical heat flux achieved was 58.11 W/cm 2 for the 200 ml/min test. Flow boiling is an effective cooling technique for microelectronic systems. However, it presents flow instability issues, most of them associated with critical heat flux situations. In recent years, decreasing variable density microstructured heatsinks have been successfully tested, proving that with a suitable pathway design, the flow boiling instabilities can be mitigated. However, increasing variable density design remains largely unexplored, despite obtaining promising results in single-phase applications. The present work is an experimental study that analyzes the flow boiling performance of a novel increasing density pin–fin array with jet impingement technology. Working with DI water at atmospheric pressure, for an inlet temperature of 75 °C (inlet subcooling of 30 K), 3 flow rates (100–150–200 ml/min) were performed under heat fluxes up to 55 W/cm 2 . Focusing on the cooling device design, thermofluidic studies were carried out, supported by high-speed flow visualization. The results demonstrated that this unique cooling device reduces bubble blockage while enhancing bubble breakage and departure. In terms of flow patterns, bubbly, plug, slug and annular flow were observed. The main heat transfer mechanisms detected were single-phase convection, saturated boiling, nucleated boiling and film evaporation. The highest heat transfer coefficient (h th ) was obtained for the 200 ml/min test and had a value of 9323 W/°C·m 2 . The maximum critical heat flux (CHF) achieved was 58.11 W/cm 2 for the 200 ml/min test. A flow boiling performance evaluation was carried out using the dimensionless Boiling utilization (Bu) number. Compared to existing literature, this novel cooling device emerges as one promising solution.