Simulation Study of Porosity Effects of Porous Media on Pool Boiling Heat Transfer Performances
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摘要: 采用介观相变格子Boltzmann(lattice Boltzmann, LB)方法，在孔隙尺度下研究了多孔介质的孔隙率对池沸腾换热过程的影响，重点分析了不同孔隙率时气泡的运动过程，并对气泡在多孔介质中的典型状态进行了力平衡分析，进而探究了多孔介质孔隙率影响沸腾传热的机理.结果表明，与无多孔介质的平板表面相比，多孔材料能够有效地降低初始成核的壁面过热度，增强流体的扰动，并且能够显著提升临界热流密度(critical heat flux，CHF)值.在所研究的工况中，孔隙率ε=73.2%时，CHF值提升最大，约为平板的3.6倍，其余孔隙率的多孔介质最小也可将其CHF值提升至平板的2.3倍.研究发现，当孔隙率从97.7%开始逐渐减小时，CHF值逐渐增大，同时沸腾换热曲线向左上方移动，这是因为减小孔隙率能够增大有效换热面积，减小气泡成核的壁面过热度，从而强化沸腾换热.当孔隙率减小到ε=73.2%时，若继续减小孔隙率，热流密度将突然下降，沸腾传热性能显著降低.通过对沸腾过程中气泡的受力进行分析后发现，当孔隙率较小时，过小的孔隙直径显著增大了气泡的逸出阻力，降低了气泡的上升速度，延长了气泡脱离多孔介质的时间，且此时气泡会在蒸发动量力、接触压力以及摩擦力等的共同作用下聚集在加热器上表面，形成气膜，从而恶化沸腾传热.Abstract: The mesoscopic phase change lattice Boltzmann method was used to study the effect of the medium porosity on pool boiling heat transfer at the pore scale. The motion processes of bubbles were mainly considered for different porosities, and the force balance was analyzed in typical states of bubbles in porous media, to explore the mechanism of the influence of medium porosity on boiling heat transfer. The results show that, compared with the flat surface without a porous medium, porous materials can effectively reduce the wall superheat of initial nucleation, enhance the disturbance of fluid, and significantly improve the critical heat flux (CHF). In the simulation case, the CHF value grows the greatest with porosity ε=73.2%, which is about 3.6 times that of the flat plate case. In the cases of other porosity values, the presence of porous media can increase the CHF value for at least 2.3 times that of the flat plate case. The numerical simulation further demonstrates that, as the porosity gradually decreases from 97.7%, the CHF value will gradually increases, and the boiling heat transfer curve will shift to the upper left. This is because a decrease in the porosity can increase the effective heat transfer area, reduce the wall superheat of bubble nucleation, and strengthen boiling heat transfer. When the porosity decreases to ε=73.2%, the heat flux density will suddenly drop and the boiling heat transfer performance will significantly decrease with the reduction of the porosity. The analysis of force balance of the bubbles during the boiling process indicates that, for a low porosity, too small pore diameters would significantly increase the escape resistance of bubbles, reduce their rising speed, and lengthen the time of bubbles leaving the porous medium; at the same time, bubbles will gather on the surface of the heater under the combined actions of the evaporation momentum, the contact pressure, and the friction, thus deteriorating the boiling heat transfer performance.
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Key words:
- porous medium /
- pool boiling heat transfer /
- bubble dynamics /
- porosity
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图 1 物理问题以及多孔介质间的导热示意图
注 为了解释图中的颜色，读者可以参考本文的电子网页版本，后同.
Figure 1. Schematic diagrams of physical problems and heat conduction between porous media
图 2 孔隙率为92.6%的多孔介质在加热温度Tb=0.98Tc，1.01Tc，1.05Tc下的各个时刻气泡状态图
Figure 2. Bubble state diagrams of porous media with a porosity of 92.6% at heating temperatures Tb=0.98Tc, 1.01Tc and 1.05Tc
图 3 孔隙率为73.2%的多孔介质在加热温度Tb=0.98Tc，1.01Tc，1.05Tc下的各个时刻气泡状态图
Figure 3. Bubble state diagrams of porous media with a porosity of 73.2% at heating temperatures Tb=0.98Tc, 1.01Tc and 1.05Tc
图 4 孔隙率为61.2%的多孔介质在加热温度Tb=0.98Tc，1.01Tc，1.05Tc下的各个时刻气泡状态图
Figure 4. Bubble state diagrams of porous media with a porosity of 61.2% at heating temperatures Tb=0.98Tc, 1.01Tc and 1.05Tc
图 5 孔隙率为61.2%、接触角为73°的多孔介质在加热温度Tb=1.01Tc下的气泡状态图
Figure 5. Bubble state diagrams of porous media with a porosity of 61.2% and a contact angle of 73° under heating temperature Tb=1.01Tc
图 6 沸腾过程中单气泡在多孔介质中的力平衡
Figure 6. Force balance of a single bubble in porous medium during boiling
图 7 不同孔隙率的多孔介质以及光滑平板的沸腾曲线
Figure 7. Boiling curves of porous media with different porosities and smooth plates
图 8 不同孔隙率的多孔介质在加热温度为0.99Tc下的气泡状态及加热器温度场分布(t*=51.36)
Figure 8. Bubble state and heater temperature field distributions of porous media with different porosities at 0.99Tc heating temperature (t*=51.36)
图 9 两种加热温度下ε=73.2%和ε=67.6%的样本其沸腾过程中气泡的聚集状态
Figure 9. The aggregation states of bubbles during the boiling processes of samples with ε=73.2% and ε=67.6% at 2 heating temperatures
图 11 气泡在ε=67.6%样本中的聚集过程的受力分析
Figure 11. Force analysis of the bubble aggregation process in ε=67.6% samples
表 1 格子单位与物理单位转换
Table 1. The unit conversion from lattice units to physical units
parameter lattice unit physical unit conversion factor ρl 5.426 570.02 kg/m3 106.16 kg/m3 ρv 0.811 3 86.13 kg/m3 106.16 kg/m3 l0 16 4.72×10-6 m 2.95×10-7 m u0 0.035 8 38.56 m/s 1 077.09 m/s t0 447.8 1.224×10-7 s 2.734×10-10 s ν 0.06 1.9×10-5 m2/s 3.18×10-4 m2/s Tc 0.196 1 647.2 K 3 300.36 K pc 0.178 4 2.21×107 Pa 1.24×108 Pa cv, l 4.0 1 405.9 J/(kg· K) 351.48 J/(kg· K) hfg 0.624 7.26×105 J/kg 1.16×106 J/kg λs 32.556 390.67 W/(m· K) 12 W/(m· K) 
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表 2 不同孔隙率的多孔介质样本其气泡最大接触压力Fcpm以及平均上升速度Vave(格子单位)
Table 2. Maximum contact pressures of bubbles in porous medium samples with different porosities Fcpm and average rising speeds Vave (lattice units)
porosity ε/% t* Rr da σ Fcpm Vave 97.7 33.50 47.37 258.79 0.009 5 10.55 4.50×10-6 92.6 37.96 51.06 285.21 0.009 5 11.89 5.24×10-6 85.2 42.43 51.53 289.66 0.009 5 12.15 4.38×10-6 73.2 46.90 50.20 312.58 0.009 5 14.52 4.30×10-6 67.6 53.60 56.84 353.96 0.009 5 16.45 3.83×10-6 61.2 58.06 58.80 372.42 0.009 5 17.60 3.49×10-6 53.5 69.23 56.44 374.01 0.009 5 18.49 2.60×10-6
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