竖直圆管内浮升力对超临界碳氢燃料裂解传热传质特性的影响

Buoyancy’s Effect on Heat and Mass Transfers of Supercritical Hydrocarbon Fuel with Pyrolysis in Vertical Circular Tubes

关键词:  浮升力  热裂解  结焦  超临界压力  碳氢燃料

Key words:  Buoyancy  Pyrolysis  Carbon deposition  Supercritical pressure  Hydrocarbon fuel

摘要:

为了探究再生冷却过程中，浮升力对竖直圆管内超临界碳氢燃料裂解传热传质特性的影响，基于详细裂解反应动力学模型，建立了同时考虑碳氢燃料流动传热和裂解吸热的耦合算法，在此基础上对竖直管道内，浮升力对超临界RP-3的流动、传热和裂解反应的影响展开了数值研究。计算结果表明：与不考虑浮升力的情况相比，在浮升力影响显著的条件下，浮升力增强了向下流动的碳氢燃料壁面处与中心流区域的传热传质过程，燃料温度和裂解率的径向分布更加均匀，燃料吸热能力增强，换热系数上升，同时可以有效地抑制管道壁面上结焦的生成；而对于向上流动的流体，浮升力不利于壁面处与中心流区域的传热传质，导致冷却通道内碳氢燃料温度和裂解率径向分布的不均匀性增强，燃料吸热能力降低，换热系数下降，同时增加了管道壁面上的结焦量；同时，为了更好地理解浮升力的影响，本文还对不同壁面热流密度下向上和向下冷却通道内超临界碳氢燃料的裂解传热特性进行了分析；判别式Bo^*<6.0×10^-7不能准确地预测竖直管道内浮升力对超临界碳氢燃料裂解换热的影响。

Abstract:

In order to investigate the effects of buoyancy on heat and mass transfers of supercritical hydrocarbon fuel with pyrolysis in vertical circular tubes in the regenerative cooling process， a numerical method considering convective heat transfer and endothermic pyrolytic reaction simultaneously was developed based on the detailed pyrolytic chemical reaction mechanism of China No. 3 jet fuel （RP-3）. Based on this， numerical studies were carried out to analyze the buoyancy effects on heat transfer and pyrolysis of RP-3 in vertical tubes under supercritical conditions. Results reveal that compared with the case without buoyancy， the buoyancy improves the heat and mass transfers between the near-wall region and the core flow field， which makes the radial distributions of the temperature and fuel conversion of hydrocarbon fuel more uniform in the downward flow. The fuel heat absorption capacity is enhanced， and the heat transfer coefficient increases. Moreover， the buoyancy can efficiently inhibit coke deposition in cooling channels， however， in the upward flow， the buoyancy deteriorates the heat and mass transfers between the near-wall region and the core flow field， which increases the non-uniformity of the radial distributions of fuel temperature and fuel conversion. The fuel heat absorption capacity is reduced， and the heat transfer coefficient decreases. Meanwhile， the carbon deposition increases in cooling channels. In addition， the supercritical heat transfer of hydrocarbon fuel with pyrolysis in downward and upward flows for various wall heat fluxes are compared to obtain a better understanding of the buoyancy effect. The criterion equation （Bo^*<6.0×10^-7） fails to quantitatively evaluate buoyancy effects on supercritical heat transfer of hydrocarbon fuel with pyrolysis in vertical circular tubes.