Enhanced flow boiling heat transfer in narrow gap with different widths using different-mode-interacting boiling

Abstract

The concept of different-mode-interacting boiling (DMIB) utilizes the areas of two different thermal conductance materials by expanding the wetted area over the regions of their dry areas. This enhances the critical heat flux (CHF) and heat transfer coefficient (HTC) in a wide channel (e.g., Wch = 80 mm) to provide adequate cooling in previous study. In this study, the effectiveness of DMIB in enhancing the CHF and HTC was investigated when using a channel width (Wch = 10 mm) equal to that of the heating surface, aiming to provide better efficiency for both uniform surface (US) and nonuniform surfaces (NUSs) with material widths (W = 0.5 and 1.0 mm) in narrow gaps. Experiments were performed with a heating surface of 10 mm2 using subcooled deionized water at different temperatures ΔTsub = 2.0, 10, 20, and 30 K, inlet velocities v = 0.1, 0.2, and 0.4 m/s (corresponding to mass fluxes: 100, 200, and 400 kg/m2s), and gap sizes h = 1.0, 2.0, and 5.0 mm. The results revealed that the heat transfer performance of the NUSs was significantly enhanced compared to the US for both Wch. However, both the CHF and HTC values were higher with Wch = 80 mm than with Wch = 10 mm owing to the expansion of coalesced bubbles in the flow direction, sustained nucleate boiling regime, and effective liquid rewetting during the intermittent nucleate boiling-partial dryout near the CHF owing to the DMIB. The maximum CHF and CHF enhancement ratios for Wch = 10 mm were 3.59 MW/m2 and 52.2 %, respectively, whereas for Wch = 80 mm, they were 3.66 MW/m2 and 89 %, respectively. In addition, the HTC and HTC enhancement ratio for Wch = 10 mm were 1261 kW/m2K and a factor of 7, respectively, whereas those for Wch = 80 mm were 2026.9 kW/m2K and a factor of 12, respectively. In contrast, only a minimal difference existed in the heat transfer performance of Wch = 10 mm compared to that at Wch = 80 mm; therefore, Wch = 10 mm can be considered as having a better cooling efficiency per channel width due to the minimal pumping power and significant CHF and HTC enhancement.