Observation of bubble dynamics in enhanced flow boiling via different-mode-interacting boiling in narrow gaps

Different-mode-interacting boiling (DMIB) method utilizes unsteady movement of liquid and vapor interface near the boundary of different thermal conductance materials in nonuniform surface (NUS) geometries by expanding the wetted area over the dry area. Its effectiveness has been proven to enhance both critical heat flux (CHF) and heat transfer coefficient (HTC) in microchannel narrow gaps in previous studies. In this study, a visual photographic experiment is conducted to understand the bubble dynamics and flow patterns using DMIB in a single microchannel narrow gap. The test section is a square heating surface of 10 x 10 mm2 and experiments were conducted at inlet subcooling Delta Tsub = 2-30 K, inlet velocity v = 0.1-0.4 m/s, gap size h = 1.0-5.0 mm, channel widths Wch = 10 & 80 mm, and NUS W = 0.5 & 1.0 mm. Different bubble behaviors such as bubble growth, coalescence, departure, rewetting, and partial dryout was described. Based on qualitative analysis of the solid-liquid-vapor layer at high heat flux close to CHF, a DMIB phenomenon capable of delaying the occurrence of CHF is presented. The flow pattern observed can be divided into three: single phase regime, fully developed nucleate boiling (bubbly flow), and INB-PD (intermittent nucleate boiling-partial dryout). It was revealed that DMIB has a direct effect on boiling crisis with the intermittent behavior of large coalesced vapor bubble and returned nucleate boiling are key factors in CHF boiling crisis mechanism.