Numerical study on flow and heat transfer of supercritical carbon dioxide under non-uniform heat flux influences
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摘要:
针对部分极端工况下表面热流密度高、分布不均匀的特点，采用数值计算分析方法研究非均匀热流对超临界二氧化碳流动与换热特性的影响。针对内径为4 mm水平圆管内超临界压力二氧化碳的流动传热，依次在相同总热流不同变化斜率、不同总热流相同变化斜率的2组线性非均匀热流工况下进行数值模拟，得到线性非均匀热流工况下热物性分布、浮升力影响对流换热特性规律。仿真结果表明：相较于均匀热流，非均匀热流工况下加热段入口处附近上壁面传热恶化更严重，线性非均匀热流分布下该处管壁温度可达均匀热流工况下的1.85倍；对给定的实际表面热流边界条件下的超临界二氧化碳流动换热进行数值仿真，加热段入口处上壁面最大温度达出口温度的3.41倍，改变流动方向后这一比值降低到1.50，据此对采取超临界二氧化碳为工质的相关冷却方案给出了建议。
Abstract:In some extreme conditions, heat flux with high density is non-uniformly distributed on the surface. To address this issue, this paper analyzed the influence of non-uniform heat flux on the flow and heat transfer characteristics of supercritical carbon dioxide by employing numerical calculation and analysis methods. Numerical simulations were sequentially performed to analyze the flow and heat transfer of supercritical carbon dioxide in a horizontal circular tube with a diameter of 4 mm under two sets of linear non-uniform heat flux conditions including the same total heat flux with different slopes, as well as different total heat fluxes with the same slope. The influence of the distribution of thermophysical properties and buoyancy lift on the convective heat transfer characteristics under linear non-uniform heat flux conditions was obtained. The simulation results show that the heat transfer of the upper wall near the entrance of the heating section under non-uniform heat flux conditions severely deteriorates compared with that under uniform heat flux. The maximum tube wall temperature under linear non-uniform heat flux is 1.66 times that under uniform heat flux. Furthermore, numerical simulations of flow and heat flux of supercritical carbon dioxide on a given actual surface heat flux boundary are carried out, and the maximum tube wall temperature at the entrance of the heating section is 3.41 times that at the outlet, while the ratio decreases to 1.50 when the flow direction changes. Relevant suggestions are offered for cooling solutions using supercritical carbon dioxide as a working medium.
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Key words:
- non-uniform heat flux /
- supercritical fluids /
- carbon dioxide /
- convective heat transfer /
- buoyancy lift
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图 2 压力为8 MPa时热物性随温度分布
Figure 2. Variation of thermophysical properties with temperature under pressure is 8 MPa
图 6 验证网格无关性的加热段壁温分布
Figure 6. Tube wall temperature distribution in heating section for grid independence verification
图 7 验证网格无关性的对流换热系数
Figure 7. Convective heat transfer coefficient verification for grid independence verification
图 8 工况1～工况5加热段热流密度分布
Figure 8. Heat flux density distribution in heating section in condition 1～condition 5
图 9 工况1～工况5流向温度分布
Figure 9. Temperature distribution along flow direction in condition 1～condition 5
图 10 工况1～工况5温度沿程分布
Figure 10. Axial temperature distribution in condition 1～condition 5
图 11 工况1～工况5对流换热系数沿程分布
Figure 11. Axial convective heat transfer coefficient distribution in condition 1～condition 5
图 12 对流换热系数随主流温度变化趋势
Figure 12. Variation of convective heat transfer coefficient with main stream temperature
图 14 工况3及拓展工况热流沿程分布
Figure 14. Axial heat flux distribution in condition 3 and its expanded conditions
图 15 工况3及拓展工况流向温度分布
Figure 15. Temperature distribution in condition 3 and its expanded conditions
图 17 工况3及拓展工况对流换热系数
Figure 17. Convective heat transfer coefficient of condition 3 and its expanded conditions
图 18 工况3及拓展工况$Nu$随$Re$变化趋势
Figure 18. Variation of $Nu$ with $Re$ in condition 3 and its expanded conditions
图 19 工况3及拓展工况$ Bu $分布
Figure 19. Distribution of $ Bu $ in condition 3 and its expanded conditions
图 21 实际非均匀热流下温度场分布
Figure 21. Temperature distribution under actual non-uniform heat flux
图 23 实际非均匀热流对流换热系数曲线
Figure 23. Convective heat transfer coefficient curve of actual non-uniform heat flux
图 24 实际非均匀热流$Nu$随$Re$变化趋势
Figure 24. Variation of $Nu$ with $Re$ under actual non-uniform heat flux
图 25 实际非均匀热流边界沿程$ Bu $分布
Figure 25. Distribution of $ Bu $ under actual non-uniform heat flux boundary
表 1 非均匀度研究工况设置
Table 1. Conditions setting on non-uniformity
工况编号 q沿程分布/(kW·m−2) q平均值/(kW·m−2) 1 q=32.5 32.5 2 q=−8.125x+40.625 32.5 3 q=−16.25x+48.75 32.5 4 q=−24.375x+56.875 32.5 5 q=−32.5x+65 32.5 
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表 2 热流密度平均值工况设置
Table 2. Condition setting on average value of heat flux density
工况编号 q沿程分布/(kW·m−2) q平均值/(kW·m−2) 3 q=−16.25x+48.75 32.5 3-1 q=−16.25x+56.875 40.625 3-2 q=−16.25x+65 48.75 3-3 q=−16.25x+73.125 56.875 3-4 q=−16.25x+81.25 65
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