Transient convection heat transfer for helium gas at various flow decay times

•The surface temperature difference (ΔT) increased during the flow decay process.•The increase rate for the ΔT was higher for a shorter flow decay time (tdecay).•The increment of the ΔT was almost proportional to the heat generation rate.•Heat transfer coefficient linearly decreased to a constant value at each tdecay. Transient convection heat transfer was experimentally studied for a horizontal cylinder in helium gas under flow decay conditions. The experiment was conducted by using helium gas as the coolant, and a platinum cylinder as the test heater. A uniform heat generation rate was applied to the cylinder. The cylinder temperature was maintained at a design value under a specific initial flow rate and heat generation rate. Then, the flow rate of the helium gas started to decrease according to the designed linear functions, with different flow decay times. The surface temperature of the cylinder and the heat flux were measured during the flow decay transient process for various flow decay times, initial flow velocities, and heat generation rates. It was found that the temperature of the cylinder increased rapidly for a shorter flow decay time during the flow decay process. The increment of the surface temperature difference was higher for a higher heat generation rate. The transient heat transfer coefficient was also obtained during the flow decay process. It was clarified that the heat transfer coefficient decreased to a constant value for each flow decay time for a definite heat generation rate and a definite initial flow velocity, and the decrease rate was higher for a shorter flow decay time.