TRANSIENT HEAT TRANSFER FROM SINGLE HORIZONTAL HEATERS IN FORCED FLOW OF HELIUM GAS AT EXPONENTIALLY INCREASING HEAT INPUTS

摘要

Forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a horizontal cylinder and a plate (ribbon) was experimentally and theoretically studied. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. The dependence of transient heat transfer on the gas flowing velocity becomes weaker when the period becomes very shorter. However, the gas temperature in this study shows little influence on the heat transfer coefficient. Empirical correlations for quasi-steady-state heat transfer and transient heat transfer were obtained based on the experimental data. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. The values of numerical solution for surface temperature and heat flux were compared and discussed with authors' experimental data. It was clarified that the surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. The temperature distribution near the heater becomes larger as the surface temperature increases. The values of numerical solution for surface temperature and heat flux agree well with the experimental data for the cylinder diameter of 1 mm. However, the heat fluxes show some differences from the experimental values for the cylinder diameters of 0.7 mm and 2.0 mm. And for the numerical solution for a plate, the values of numerical solutions for surface temperature and heat flux at the velocity of 6 m/s agree well with the experimental data, though they show some differences at other velocities.