Experimental study of heat transfer during the flow of a gas coolant in a heated quasi-triangular channel

•Heat transfer was studied during gas flow in a heated quasi-triangular tube.•Comparison of the flow of helium-xenon mixture and air.•The distribution of the tube wall temperature is shown.•A strong non-uniformity of the heat transfer coefficient along the tube was obtained.•A method is proposed for calculating local heat transfer coefficients for unsteady flow. The paper considers experimentally the process of heat transfer during the flow of gas mixtures in a heated thin-walled quasi-triangular tube. The cross-section of the tube corresponds to the shape of the channel formed by the dense packing of the reactor rods for generating heat at a power plant. The working fluids were a helium-xenon gas mixture with a Prandtl number of 0.23 as a promising coolant for a nuclear installation generating electricity, and air with a Prandtl number of 0.71. Temperature distributions of the heated wall are obtained in a wide range of flow velocities, and the effect of laminar-turbulent transition at the initial section of the tube and high-velocity turbulent flow at the outlet on these distributions is analyzed. An approach for calculating local heat transfer coefficients for an unsteady flow, based on determining the local velocities of the coolant in the channel, is proposed. Significant non-uniformity of the heat transfer coefficient along the channel length was obtained with a decrease in its value at the inlet area and an increase towards the outlet due to flow acceleration. An increase in the heat transfer coefficient during the flow of helium-xenon mixture is more intense than in the air flow. It is shown that this circumstance should be taken into account when designing compact power plants.