Conjugate heat transfer analysis of helium-xenon gas mixture in tight rod bundles

Helium-xenon gas mixture (He-Xe) and tight rod bundles are widely used in gas-cooled space nuclear reactors. However, few studies have been conducted on the effects of pitch-to-diameter ratio (P/D) on thermal-hydraulic characteristics while considering conjugate heat transfer. This paper numerically investigated He-Xe heat transfer in a fuel-gas gap-cladding-coolant structure of tight rod bundles. The numerical model is validated using experimental data and the maximum error of wall temperature is less than 5 %. The results show that conjugate heat transfer significantly affects the heat transfer. The heat transfer coefficient without conjugate heat transfer is only 8.85 % of that with conjugate heat transfer in the quasi-triangular pipe. The decrease in P/D enhances the convective heat transfer and causes larger pressure drops. A new performance evaluation criterion (PEC) is presented considering the effects of heat transfer, pressure drop and volume. PEC first increases and then decreases with the increase in P/D and the best comprehensive performance occurs at P/D =1.113. The analysis of the temperature difference in the viscous sublayer (y(+) < 5) shows that the decrease in P/D enhances the turbulent effect of He-Xe and reduces the thickness of the viscous sublayer, resulting in the heat transfer enhancement. This study contributes to the optimal design of advanced space reactors.