Experimental Study On Flow And Heat Transfer Characteristics Of Gas-liquid Two-phase Flow In Square Channels

The blanket is a significant component in fusion reactor for tritium breeding,neutron shielding and heat transfer.Thermal hydraulic design of the blanket directly determines heat transfer efficiency and safe operation of the fusion reactor.Under normal operating conditions,the nuclear energy of fusion reactor will be converted into heat energy and transferred to the steam turbine depending on the forced circulation of blanket heat transfer system;When the main pump of the blanket system is not available under accident conditions,the natural circulation will be built in the fusion blanket heat transfer system to remove the residual and decay heat of the fusion reactor and to mitigate accident consequences.Therefore,systemic experimental researches are conducted in this study for the flow and heat transfer characteristics of the square channel under natural circulation and forced circulation circumstances.The purpose of this study is to accumulate experiences and provide supportive data for the thermal-hydraulic design of the water-cooled fusion blanket.First,the models and technical parameters of the relevant components for the experimental test loop are determined according to the theoretical design.Then the experimental loop is built and tuned.It can be found that the turbulent flow in the square channel occurs in advance compared with in the circular channel.And the transition region becomes narrow with the Reynolds number range of 1800-3000 in the square channel.The reasons are:the edge effect of the square channel causes the formation of vortex and secondary flow easily,leads to the destruction of the laminar layer and promotes transition to turbulent flow.The experimental results of laminar friction coefficient agree well with the Shah-London empirical correlation,theoretical analysis result.In turbulent region,the smaller the geometrical dimension is,the bigger the friction coefficient is.With the increase of the Reynolds number,the influence of the local loss gradually improves in the square channel,which resultes in the friction coefficient of the square channel exceeding the friction coefficient of the circular channel.For the heat transfer law,the single-phase heat transfer coefficient of the square tube is greater than that of the circular channel in laminar region and turbulent region.The natural circulation heat transfer intensity is lower than that of the forced circulation under same conditions.The explanation is that:in the vertical upflow heated test channel,the cross-section velocity distribution is flatter due to the effect of buoyancy,leading to a smaller velocity gradient between the boundary layer and the core area,which weakens the turbulent transport effect and the heat transfer intensity.In subcooled boiling experiment of square channel,the location where the wall temperature begins to deviate from linear regularities is the ONB location.The higher heat flux is favorable to the occurrence of ONB while the higher of mass flow and system pressure are inhibitory to the occurrence of ONB.The heat transfer coefficient of subcooled boiling is larger than that of single-phase convection,because the disturbance of bubbles enhances the heat transfer effect.Some new experimental correlations are developed for the prediction of ONB heat flux,ONB wall superheat and ONB location in square channel under natural circulation and forced circulation based on experimental data.Besides,according to the temperature distribution law of subcooled boiling,a mathematical model is established and a new calculation method of subcooled boiling heat transfer coefficient is proposed.It can be found that the lower heat flux triggers ONB in natural circulation,which indicates the heat transfer intensity is lower in natural circulation.In saturation boiling experiment of square channel,it can be found that the heat flux and system pressure have a great effect on saturation boiling heat transfer while the mass quality has a little effect on it.So nucleate boiling heat transfer mechanism plays a dominate role in saturation boiling region of square channel.With the increase of heat flux,the number of nucleation sites increases and the frequency of bubble detaching from the heated wall also ascends;with the increase of system pressure,the diameter of bubble detaching from heated wall decreases;so the boiling heat transfer can be enhanced when the system pressure and heat flux are improved.In the saturated boiling region of natural circulation,flow instability occurs.Due to the lower heat transfer intensity of natural circulation and heat transfer deterioration induced by flow pulsation,the saturated heat transfer coefficient of natural circulation is also be lower than that of forced circulation.In the annular flow with high mass quality,the second kind of heat transfer deterioration(dry-out)occurs with the liquid film evaporation.It can be found that with the increase of mass flow and the decrease of inlet vapor quality,the occurrence of dry-out phenomenon is delayed.The influence of entrance condition and the influence of local thermal parameters of dry-out point should be considered simultaneously.And the experimental correlation for the prediction of occurrence of dry-out point in square channel is developed.For natural circulation system,flow instability occurs due to the strong coupling relationship among pressure drop,mass flow and vapor quality.With the increase of system pressure and inlet subcooling degree,the onset point of flow instability(OFI)is delayed;the critical mass flow increases;the critical heat flux increases;the fluctuation amplitude decreases;and the critical vapor quality increases.With the increase of inlet resistance,the critical mass flow decreases;the critical heat flux decreases;the fluctuation amplitude decreases;and the critical vapor quality increases.The pulsation period can be calculated through Fourier series fitting in time domain and spectrum analysis in frequency domain,and it can be found that pulsation period decreases with the increase of system pressure.The instability boundary graphs are obtained based on experimental data.Finally,The mathematical model is developed to predict the onset of flow instability(OFI)according to the similarity principle and the law of ?.According to the previous description,the heat transfer intensity of natural circulation is lower in single-phase region,subcooled boiling region and two-phase boiling region compared with that of forced circulation.And flow instability occurs in advance in natural circulation compared with in forced circulation.