Experimental investigation on vertical heat transfer characteristics of light metallic layer with low aspect ratio

Highlights: • New correlation of vertical heat transfer for low aspect ratio is recommended. • New correlation is more suitable than G-D for extremely low aspect ratio. • Using ΔT_b,t to calculate Nu has much less error. • Decreasing the height leads to enhancing the thermal focusing effect. In-Vessel Retention (IVR) strategy can ensure the nuclear safety and the integrity of the Reactor Pressure Vessel (RPV) during the severe nuclear accident. The HEat transfer behavior of Light Metallic layer experiment for the Low aspect Ratio (Height/Diameter less than 0.16) (HELM-LR) has been built to investigate the heat transfer characteristics of the light metallic layer. Water was used as the simulant and the test section was heated from below and cooled from both top and side. A new fitting correlation is recommended by HELM-LR experiment to simulate the vertical heat transfer of the light metallic layer when the aspect ratio is below 0.16. The experimental results indicated that the new fitting correlations could better predict the calculated results than Globe-Dropkin correlation under the low aspect ratio condition (1.72 × 10⁷ < Ra < 4.05 × 10⁹). The correlation Nu = 0.1995Ra^0.284 with the temperature difference between the bottom and the top boundary is recommended.