| Supercritical water cooled reactor(SCWR)is a new type of nuclear energy system with high efficiency and safety,which is based on advanced water-cooled reactor and innovative design of supercritical power generation.The structure of core channel in supercritical water cooled reactor is special.The rod bundle should bear heat load of MW grade.The key to SCWR thermo-hydraulic is to effectively reduce the supercritical zone temperature of fuel cladding.The research of PWR proves that the grid spacer can strengthen the heat and mass transfer in the core sub-channel.In order to accurately reveal the heat and mass transfer law of nuclear reactor core with the grid spacer,the flow and heat transfer law of the SCWR triangular reactor core with grid spacer is studied in this paper.The flow and heat transfer characteristics in the core channel of with standard grid spacer are studied experimentally;The flow blockage rate and dimensionless longitudinal distance of the grid spacer are proposed;The effects of thermal parameters,grid spacer type,grid spacer longitudinal distance and flow blockage rate on the flow and heat transfer characteristics and entropy generation behavior of the core are studied by numerical simulation.Then the effects of grid spacer on the heat transfer and flow characteristic in the triangular core sub-channel are obtained.Experimental study on the influence of supercritical water flow and heat transfer with grid spacer in triangular core channel SCWR.The fuel rod diameter is 8 mm and the pitch ratio is 1.4,in the range of heat flux 400-800 kW/m~2,pressure 23-28 MPa and mass flow velocity 700-1300 kg/(m~2.s),the influence of supercritical water flow and heat transfer law in the triangular sub-channel with grid spacer were studied by the combination of heat load method and variable heat load method.The influence of thermal parameters on the core channel was obtained.It is found that the grid spacer can reduce the wall temperature and improve the heat transfer coefficient.The turbulence intensity in the downstream part of the grid spacer is greatly increased,and the heat transfer capacity between the tube wall and the fluid is enhanced.The heat flux has a great influence on the pseudo critical zone and the high enthalpy zone.The greater the heat flux is,the worse the heat transfer capability is,and the heat transfer deterioration is likely to occur.Reducing heat flux can improve heat transfer capacity;Mass flow rate has a great effect on the whole enthalpy range,and the effect on the pseudo critical zone is the greatest.Increasing mass flow rate can not only reduce wall temperature and temperature difference but also improve heat transfer coefficient;The effect of pressure on heat transfer characteristics is small.The higher the pseudo critical zone pressure is,the summit of heat transfer coefficient is smaller,Increasing pressure can restrain the change of physical properties.The influence of supercritical water flow and heat transfer low of SCWR triangular core sub-channel was studied by numerical simulation.The numerical simulation of flow and heat transfer characteristics of supercritical water in triangular sub-channel with different types of grid spacers was carried out systematically.The effect of grid spacers and thermal parameters on the flow and heat transfer characteristics of supercritical water were studied.It is found that the local heat transfer enhancement occurs in the downstream area of the grid spacer,and the local heat transfer enhancement ability of staggered blade grid spacer is the strongest;The pressure has a greater impact on the pseudo critical region,while other regions have a smaller impact.The greater the pressure near the pseudo critical point is,the higher the inner wall temperature is;The temperature difference near the pseudo critical point is minimum,the greater the pressure is,the higher the temperature difference is,The heat transfer coefficient draws near the pseudo critical point is biggest.The larger the pressure is,the smaller the peak value is,and the difference is in other regions is not obvious;The larger the mass flow rate is,the lower the wall temperature is,and the smaller the temperature difference is;Improving the mass flow rate can enhance heat transfer and reduce the wall temperature;The heat flux has a greater influence on heat transfer,increasing the heat flux,moving upward the curve of the inner wall temperature and temperature difference.The higher the heat flux is,the higher the wall temperature and temperature difference are.The heat flux has little effect on the heat transfer coefficient in the low enthalpy region,but has significant effect on the quasi-critical region and the high enthalpy region.Increasing the heat flux,the peak of the heat transfer coefficient moves to the low enthalpy area,and reducing the heat flux can inhibit the deterioration of heat transfer to a certain extent.The influence of blocking ratio and grid spacing on flow and heat transfer of supercritical water in SCWR triangular core channel was studied.The blocking ratio of core positioners and dimensionless longitudinal spacing of core positioning grids are proposed.The effects of blocking ratio of staggered blade grid spacer and longitudinal spacing of grid in triangular sub-channel on supercritical flow and heat transfer were studied by numerical simulation.It is found that the spacing of the core grid spacers should not be too small,and the use of the grid spacers at the end of the channel should be avoided.Increasing the spacing of the grid spacer before the pseudo critical temperature region is beneficial to enhancing heat transfer.The flow blocking rate has a great influence on the flow field and secondary flow in the channel.The larger the flow blocking rate is,the stronger the disturbance of the flow field in the downstream area of the grid spacer is.An obvious eddy is produced downstream of the positioning grid,which can effectively reduce the temperature difference between the wall and the fluid.Further increase the blocking rate,and has no obvious effect on heat transfer enhancement.When fuel rod diameter is 8 mm,grid spacing ratio is 1.4 and grid spacing is750 mm,the flow blockage rate is 0.3078(blade angle is 45 degrees),the heat transfer enhancement effect is the best.Entropy generation behavior of supercritical water flow and heat transfer in SCWR triangular core channel with grid spacer was studied.In order to further reveal the mechanism of supercritical heat transfer in the core reactor and reduce irreversible losses.The effects of grid spacer type,thermal parameters and flow blockage ratio on the entropy generation behavior of supercritical flow and heat transfer in the triangular core channel were studied by numerical simulation.The results show that the entropy generation near the wall is the highest,the center position is the smallest.The positioning grid reduces the entropy production,increases the gradient of radial entropy production and decreases the average entropy production.The difference between the standard and blocking-vane grid spacers are small,the staggered-vane grid spacers have the smallest entropy production,and the Be number is the smallest,but the entropy generation caused by energy dissipation increase.Pressure has a great influence on the entropy generation in the quasi-critical region.The higher the pressure is,the greater the entropy generation is.The heat flux effects on the entropy production in the whole enthalpy range.The higher the heat flux is,the greater the entropy generation is.Increasing the mass flow rate can reduce the entropy generation and improve the heat transfer capacity.Increasing the flow blockage rate of the grid spacer can reduce the entropy generation,and the flow blockage rate is 0.3078,which has the best effect.This study provides a beneficial theoretical support for the layout and structure optimization of core grid spacers,and is benefit to the development of SCWR nuclear energy technology.This study was funded by the National Natural Fund NO.51876024. |
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