Flow boiling critical heat flux enhancement via different-mode-interacting boiling in narrow gaps

Abstract

Flow boiling heat transfer has the potential to achieve the required phase-change performance cooling in narrow gaps. To further improve the critical heat flux (CHF) and heat transfer coefficient (HTC) in flow boiling, a method of different-mode-interacting boiling (DMIB) involving the interaction between neighboring boiling of different heat flux areas or different modes of nucleate and film boiling was utilized. We employed a nonuniform heating plate, in which copper and Polytetrafluoroethylene (PTFE) with different thermal conductances were alternately arranged near the upper surface to achieve a spontaneous nonuniform temperature distribution. Experiments were performed using a square heating surface (10 mm × 10 mm) with two material widths (W) of different materials. The results were compared with a uniform plate using deionized water as working fluid under the conditions of inlet subcooling ΔTsub = 2.0, 10, 20, and 30 K, flow velocity v = 0,1, 0.2, and 0.4 m/s, and gap size h = 1.0, 2.0, and 5.0 mm. The nonuniform surfaces of W = 0.5 and 1.0 mm are found to significantly enhance the CHF at all values of v, ΔTsub, and h. The enhancement of CHF and HTC was owing to the high efficiency of the DMIB and reasonable liquid supply on the nonuniform surfaces. In addition, v and ΔTsub had a significant effect on the CHF and HTC, whereas the trends were identical. The reduction in v and ΔTsub could increase the ratios of CHF, where the highest enhancement ratio achieved was 89% at h = 1.0 mm, ΔTsub = 2.0 K, and v = 0.1 m/s. Moreover, increments in v and ΔTsub led to higher HTC enhancements, whereas the highest ratio increased by a factor of 12 at h = 5.0 mm, ΔTsub = 30 K, and v = 0.4 m/s.