Experimental study of heat transfer characteristics on post departure from nucleate boiling in narrow rectangular channel

<bold>Background</bold>Nuclear energy, with its low carbon emissions, high energy density, and operational stability, is an important part of the modern energy system. Plate-type fuel elements, which feature narrow rectangular channels, are commonly adopted in research reactors for their compactness. However, during accident transients like Loss of Coolant Accident (LOCA), post Departure from Nucleate Boiling (post-DNB) occurs in these channels, leading to severe heat transfer deterioration. Existing studies on post-DNB heat transfer have mostly focused on circular channels, and research on narrow rectangular channels is insufficient, limiting the accuracy of safety analysis for research reactors.<bold>Purpose</bold>This study aims to investigate the post-DNB heat transfer characteristics in narrow rectangular channels under low-pressure conditions, clarify the effects of key thermal-hydraulic parameters on stable film boiling, and provide experimental basis for establishing post-DNB heat transfer models for such channels.<bold>Methods</bold>Firstly, an experimental section with indirect hot patch was designed to conduct stable film boiling heat transfer experiments and avoid the uncertainties and computational difficulties caused by transient experiments. The narrow rectangular channel was formed by welding two L-shaped Inconel plates, with two copper hot patches (covering the full width of the channel) fixed on the heating plates <italic>via</italic> silver brazing. Then, the transient response of the wall temperature was obtained by properly arranging thermocouple temperature measurement points, and experiments on stable film boiling were conducted under low-pressure conditions, with mass flow rates ranging from 80.9 kg·(m²·s)^-1 to 206.1 kg·(m²·s)^-1, inlet subcooling from 27.2 K to 53.2 K, and system pressure controlled from 0.11 MPa to 0.26 MPa. Finally, repeatability tests were performed to verify the reliability of the experimental data, with the maximum average deviation of heat transfer coefficients at the same axial position.<bold>Results</bold>Similar to the circular tube test results of other scholars, the heat transfer coefficient measured in post-DNB stable film boiling shows a U-shaped trend with equilibrium quality. With the increase of mass flow rate, the heat transfer coefficient increases significantly, from 22.5 W·(m²·s)^-1 to 69.2 W·(m²·s)^-1 within the experimental range. When the system pressure rises from 0.11 MPa to 0.25 MPa, the heat transfer coefficient increases by 11%. The effect of inlet subcooling depends on the mass flow rate: at low mass flow rates (around 80 kg·(m²·s)^-1), increasing inlet subcooling reduces the heat transfer coefficient by 21.3%; at high mass flow rates (around 150 kg·(m²·s)^-1), increasing inlet subcooling slightly increases the heat transfer coefficient. Compared with circular channels under similar conditions, the post-DNB heat transfer coefficient in narrow rectangular channels is significantly lower due to their geometric characteristics with the maximum average deviation of heat transfer coefficients being 8.30% at the same axial position.<bold>Conclusions</bold>The study reveals the post-DNB heat transfer characteristics in narrow rectangular channels under low-pressure conditions. The effects of mass flow rate, system pressure, and inlet subcooling on heat transfer performance are clarified, and the relationships between these parameters and the post-DNB heat transfer coefficient are obtained. These results provide important experimental data for the development of post-DNB heat transfer models for narrow rectangular channels, which is of great significance for improving the safety analysis of research reactors during accident transients.