ONB, OSV, and OFI for subcooled flow boiling through a narrow rectangular channel heated on one-side

This paper presents an experimental investigation of the thermal hydraulic thresholds of the subcooled boiling instability in forced convective flow, such as the onset of nucleate boiling (ONB), the onset of significant void (OSV), and the onset of flow instability (OFI). The experiment was constrained to water flows in the upward direction under atmospheric pressure through a narrow rectangular channel heated on one-side having a gap of 2.35 mm, a width of 54 mm, and length of 566 mm. The heated length and width were 300 mm and 50 mm, respectively. The experiment was performed over a wide range of inlet temperature (35–65 °C), mass fluxes (118–1400 kg/(m²s)), and heat fluxes (50–650 kW/m²). Two experimental methods were adopted to achieve and identify the ONB, OSV, and OFI: (1) the constant mass flow rate approach and (2) the constant heat flux approach. The results showed consistency between the two methods. The ONB was identified through visualized monitoring using a high-speed camera and by using the slope of the wall temperature deviation method. The ONB was predicted by Jens and Lottes’ correlation. The OSV was detected using the high-speed camera and the wall temperature–heat flux curve. Based on the experimental method, the OFI was identified using pressure drop and inlet pressure fluctuation. The data showed that the OSV and OFI could occur at similar points based on the experimental conditions. Furthermore, the OSV models, such as the modified Bowring model and Saha and Zuber's model, can be used to predict the OFI. Additionally, some OFI correlations showed good agreement with the present data. However, many other correlations underestimated the OFI results, which might have been a result of differences in the experimental parameters. Therefore, a new empirical correlation that predicted the present data and other experimental data within very good accuracy was suggested. The present study describes and discusses the influence of the experimental parameters; inlet subcooling, mass flow rate, and the imposed heat flux on the ONB, OSV, and OFI incipience.