EXPERIMENTAL STUDY ON TRANSIENT HEAT TRANSFER ENHANCEMENT FOR HELIUM GAS FLOWING OVER A THIN TWISTED PLATE

摘要

Transient forced convection heat transfer due to exponentially increasing heat input to a heater is important as a database for safety assessment of the transient heat transfer process in a Very High Temperature Reactor (VHTR). The knowledge of heat transfer enhancement using a heater with twisted configuration is also important for the high performance design of intermediate heater exchanger (IHX) in VHTR system. In this study, forced convection transient heat transfer for helium gas at various periods of exponential increase of heat input to a short thin twisted plate with various helix angles was experimentally studied. A forced convection heat transfer experimental apparatus was used to measure the experimental data. The test heater was mounted horizontally along the center part of a circular test channel. Twisted plates were made of thin platinum plate with a thickness of 0.1 mm and width of 2 mm and 4 mm. The heat generation rates of the heater were controlled and measured by a heat input control system. The heat generation rate, Q, was raised with exponential function, Q = Q_0exp(t/τ). Where, t is time, and τ is period of heat generation rate. The mean temperature of the test heater was measured by resistance thermometry. The heat flux was obtained by the energy conservation equation. The test heater surface temperature was calculated from heat conduction equation of the heater. The transient heat transfer experimental data were measured for the periods ranged from 80 ms to 17 s and at a gas temperature of 303 K under 500 kPa. The flow velocities ranged from 4 m/s to 10 m/s. In the experiments, the twisted plates with different width were tested. The surface temperature and heat flux are increasing exponentially with the time. 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 about 1 s. The heat transfer coefficients for total length of the twisted plate were compared with the values of flat plate which has the same width and thickness with the twisted one. The local mean heat transfer coefficients have been tested as well. The heat transfer coefficients of twisted plate are about 10% for 2 mm-width one and 15% for 4 mm-width one higher than those of flat plate with same width at the quasi-steady state. And also, the heat transfer coefficients for the first half pitch are 24% higher than that for the total length of the same twisted plate. Therefore, an enhancement in the heat transfer coefficient for the twisted plate was clarified.