Boiling curve of subcooled flow boiling of DI water from forced convection region until reaching CHF under different operation parameters in a vertical tube

Subcooled flow boiling has important applications in cooling systems with high heat fluxes. Additionally, it has higher heat transfer efficiency and better performance in critical heat flux compared to saturated flow boiling. In this paper, subcooled flow boiling heat transfer of deionized (DI) water under varying heat fluxes was investigated experimentally, in a vertical tube with inner and outer diameters of 4.3 and 6.3 mm, respectively. The heat transfer coefficient (HTC) behavior was studied as the flow transitioned from forced convection in a single-phase state to subcooled flow boiling in a two-phase state, ultimately reaching the critical heat flux (CHF). The curve of subcooled flow boiling of deionized water was presented by explaining the heat transfer mechanism in various boiling regions. Investigation of the effects of various parameters, such as mass flux, pressure, subcooled temperature, the length of the heating tube, and output equilibrium vapor quality (Xe), indicated that CHF increased by 42% and 42.7% with increasing mass flux from 689 to 1148 kg m⁻² s⁻¹ and pressure from 1 to 3 bar, respectively. Meanwhile, the critical heat flux decreased by 5% and 24.2% with the increase in subcooling temperature and heating tube length, respectively. Moreover, increasing the mass flux, absolute pressure, and subcooling temperature enhanced the behavior of the heat transfer coefficient. However, as the length of the test section tube increased, the HTC decreased.