Experimental And Fluid Scaling Studies On The Convective Heat Transfer Of Supercritical Fluid

Supercritical water-cooled reactor(SCWR) is one of the Gen-IV reactors, which shows higher economy and compact system. In-depth understanding and accurate prediction are crucial for core design of the SCWR. Reliable knowledge on the heat transfer characteristics of supercritical fluid is not adequate until now. Experimental study on steady-state and transient heat transfer test of supercritical fluid in tubes and theoreticcal study on fluid scaling of supercritical convective heat transfer are carried out in this thesis.Steady-state and transient experiments have been carried out at the supercritical model fluid thermal hydraulics test facility(SMOTH) with working fluid of Freon R134 a. The pressure of steady-state experiment ranges from 4.3 to 4.7 MPa, mass flux ranges from 400 to 2500 kg/m2 s, and heat flux ranges from 10 to 180 k W/m2. The pressure of transient experiment varies from 3.9 to 4.7 MPa, the pressure transient rate is up to 7.3 k Pa/s.The results of steady-state test show that heat transfer under supercritical pressure is characterized as three phenomena: normal heat transfer, heat transfer enhancement, heat transfer deterioration(HTD). In the region far from the pseudo-critical point, the fluid shows normal heat transfer in the whole range of test parameters. The heat transfer coefficient(HTC) is affected by heat flux, mass flux, pressure and tube diameter in the near-critical region. The heat transfer is enhanced due to the large specific heat of the fluid near heated wall in the case of low heat flux, and is deteriored because of the change of flow structure in the case of high heat flux. The mass flux shows significant influence on HTC in the near-critical region. The effect of pressure is clear in the near-critical region, when the pressure is close to the critical value. The HTC in the tube of smaller diameter is bigger, and the difference grows with the heat flux. The acceleration parameter shows regular relationship on the HTC ratio. A heat transfer correlation is proposed based on the acceleration parameter, which excludes irratation of wall temperature and direct correction of fluid propertied and shows excellent prediction accuracy in the validation. The occurrence of heat transfer deterioration(HTD) is affected by heat flux, mass flux, pressure and tube diameter. A correlation of onset heat flux of HTD is developed taking the above factors into account.The entrance effect is significant in supercritical convective heat transfer, which could radically change the HTC profile and postpone the HTD. The length of affected region is more than 100 hydraulic diameters in the case of HTD. By inserting an non-heated section in the test section, the HTC of fluid at upstream and downstream of the non-heated section shows 4 times difference at most. The sensitiveness of HTC to the change of flow structure indicates that upstream flow condition should be carefully considered in the heat transfer under supercritical pressures.The variation of wall temperature in the transient test is determined by the initial parameter under supercritical pressure. In the process of depressurization, wall temperature decreases under supercritical pressure but increases abruptly under sub-critical pressure. Heat transfer characteristics in the transition of HTD under supercritical pressure to CHF under sub-critical pressure are experimentally investigated. The variation of HTC lags behind the transition of pressure during the pressure rising process.The scalings of pressure, temperature, heat flux and mass flux are developed based on critical pressure, the combination of fluid temperature and pseudo-critical temperature, Nusselt number and dimensionless temperature, heat transfer correlation, respectively. The new scaling law achieves satisfactory result in the validation by a look-up table of experimental data.