STUDY OF HEAT TRANSFER BETWEEN CONCENTRIC TUBFS AT HIGH TEMPERATURE

Abstract

Study of heat transfer between two concentric tubes has wide industrial applications including nuclear reactor design, cooling of electronic equipment, aircraft cabin insulation and thermal storage systems. In nuclear reactor heat transfer takes place between pressure tube and calendria tube through a gas medium. Under coolant loss accidental conditions, the pressure tube reaches high temperature and it may cause damage to the structural integrity of the system. The heat transfer phenomena at high temperature is different from that at low temperature. The fraction of convective and radiative heat transfer rates vary with temperature. Prediction of convection mode heat transfer is quite complex as it depends on convection currents and these convection currents further depends on many parameters such as system geometry, velocity profiles, fluid properties In the present study, heat transfer between two concentric tubes of specified dimensions used in Indian Pressurized Heavy Water Reactor (PHWR) at high temperature has been investigated. Annular space between the inner tube and outer tube is filled with air. The inner tube is subjected to heat flux boundary condition and the outer tube out wall is subjected to free stream boundary condition. The heat transfer characteristics are investigated by conducting experiments on physical model as well as numerical simulation using FLUENT software package. Both experimental and simulation results reveals that temperature varies over the inner and outer surfaces of annular air space circumferentially but the variation is small in magnitude. Velocity magnitudes also varied over the inner and outer surfaces of annular air space circumferentially. The fluid plume ascends from bottom to top over the inner cylinder outer surface and then it descends from top to bottom over the outer cylinder inner surface. These velocity magnitudes are very small in magnitude. The heat transfer coefficients are almost uniform over the two surfaces of annular air space. The heat transfer by convection mode is of 5.6% of the total heat. transfer rate. High temperatures are obtained at top position of the tubes and it is recommended that the designer should, consider this high temperature, but for this problem the temperature variations are small because of low annular space width. Nearly 94% of heat transfer occurs by radiation mode and remaining 6% by convection mode. The simulation results deviated from the experimental results by 6%. The simulation results are in close agreement with the experimental results.