Experimental study on the effect of hydraulic diameter on the flow boiling characteristics in microchannels

• Constructed a visualized experimental system for flow boiling heat transfer . • Investigated the effect of hydraulic diameter on flow boiling heat transfer. • Developed new heat transfer and pressure drop correlations. • Explored the influence of hydraulic diameter on flow boiling instability . Microchannel flow boiling heat dissipation has emerged as an effective solution for managing high heat flux in electronic devices. The hydraulic diameter of microchannels plays a crucial role in influencing flow boiling characteristics and heat sink design, yet the relationship between hydraulic diameter and flow boiling remains inadequately explored. This study employs a visualization-based experimental system with six distinct channel diameters (250 μm -1500 μm) to examine the effects of hydraulic diameters on microchannel flow boiling heat transfer characteristics. In this study, the number of microchannels was three, the working fluid was deionized water, the heat flux ranged from 203 to 880 kW/m², the system outlet pressure was 101.325 kPa, the pressure drop ranged from 0.71099 to 18.021 kPa, and the vapor quality ranged from 0.00183 to 0.37536. Results indicate that variations in microchannel hydraulic diameters lead to significant changes in flow patterns, heat transfer coefficients, and pressure drops. At a hydraulic diameter of 250 μm, annular flow forms earlier but is more prone to dry out. The heat transfer coefficient increases progressively as the hydraulic diameter is reduced. When the heat transfer coefficient enters a relatively stable change, a hydraulic diameter of 1500 μm yields a heat transfer coefficient ranging from 15 to 30 kW/m²·K. Reducing the hydraulic diameter to 750 μm increases the heat transfer coefficient to a range of 40–60 kW/m²·K, while further reducing the hydraulic diameter to 250 μm elevates the heat transfer coefficient to between 65 and 90 kW/m²·K. Pressure drop is highly sensitive to hydraulic diameter, with channels under 500 μm exhibiting the highest values and more pronounced slope variations. The pressure drop decreases as the hydraulic diameter increases. Due to inertial forces, larger hydraulic diameters induce more significant fluctuations in pressure drop and wall temperature during backflow. Through Spearman correlation analysis, this study fits heat transfer and pressure drop friction coefficients adaptable to different hydraulic diameters. This work offers theoretical insights and practical design guidance for optimizing microchannel heat sinks.