Onset of Nucleate Boiling for subcooled flow through a one-side heated narrow rectangular channel

Onset of Nucleate Boiling (ONB) is an important limit in the design of nuclear reactors and most flow boiling systems. Preventing the ONB occurrence protects systems from unfavorable thermal hydraulic events, such as Onset of Flow Instability (OFI) and Critical Heat Flux (CHF). In this study, a simultaneous measurement and visualization experiment on the ONB is carried out for a narrow rectangular channel heated from one side. The rectangular channel has a thickness of 2.35 mm, width of 54 mm, and length of 560 mm. The experiment is conducted for upward flow direction under nearly atmospheric pressure. The inlet conditions are chosen to cover a wide range of operational conditions: inlet temperature (35-65 °C) and mass flow rate (0.015-0.130 kg/s). Based on the inlet flow conditions, a uniform heat flux (50-800 kW/m2) is applied in a stepwise manner to the heated surface. The slope of the wall temperature versus the heat flux curve decreases at the ONB point. Ten thermocouples (TCs) are installed into the heated block to measure the wall temperature distribution. On the other hand, a high-speed camera is used to visualize the ONB point and compare it with the wall temperature deviation point. Based on the experimental data, the influence of mass flow rate and inlet temperature on the bubble behavior along the test section is observed. The results show a new trend for the influence of inlet temperature on the superheated temperature of the wall at the ONB point. A similar trend is observed using CFX analysis for the test section. The present results are compared with other experimental studies conducted by different research institutes, with different ONB heat flux correlations such as in the studies of Jens and Lottes (1951), Bergles and Rohsenow (1964), and Thom et al. (1965). The correlations underestimate the experimental results. Therefore, a new correlation is developed to predict the ONB heat flux, which has good agreement with the experimental data within an error of ±16.5%.