蒸汽冷凝器流动传热过程的准三维CFD模拟算法

Quasi-3D CFD algorithm for the flow and heat transfer process in steam condensers

管侧冷却水在流向和横向(垂直于流向)上的分布对管壳式蒸汽冷凝器内部各物理量的分布和冷凝器的设计优化都会产生影响,因此有必要对其进行研究。该文提出了一种考虑冷却水横向分布的准三维CFD模拟算法,并基于自主开发的二维CFD冷凝过程数值计算代码ConDesign-2D对已有算例进行了模拟。结果表明,冷却水沿流向的温升不容忽略,它直接降低了换热温差,进而影响到了冷凝速率等物理量的分布,冷却水的横向分布也使得冷凝速率分布更加均匀。相比二维模拟,该算法能解析出流场在冷却水流向上的第三维分布信息,又避免了全三维模拟带来的高昂计算成本,可以应用于实际冷凝器的模拟计算。

[Objective]Shell-and-tube steam condensers are widely used in power stations and ships;their performance,which requires an in-depth understanding of the internal flow and heat transfer process,ensures device efficiency and safety.The distribution of tube-side cooling water in its flow(z-direction)and transverse(x-y plane)directions influences the distributions of various physical quantities inside a condenser and its design optimization,which cannot be calculated by purely two-dimensional(2D)computational fluid dynamics(CFD)simulation.Moreover,the complexity of the two-phase flow,turbulence,phase change,and heat transfer mechanisms inside the condenser makes full 3D CFD simulations computationally expensive.Herein,a quasi-3D algorithm considering the transverse distribution of cooling water was proposed and applied to simulate the condensation process in a shell-and-tube steam condenser.The tube bundle region was simplified through porous media assumption,introducing extra resistance source terms to momentum transport equations.The condensation source term in the continuity equation was computed based on equivalent thermal resistance,referring to the summation of cooling-water convection,tube wall,condensate water,and noncondensable gas(air)thermal resistance.Considering the temperature rise along the z-direction,the quasi-3D algorithm split the condenser into several sections along the z-direction,performed 2D simulation in the midplane of each section,and used the secant iteration method to balance the steam pressure drop.The simulation was conducted based on ConDesign-2D,a self-developed 2D CFD code for the condensation process of shell-and-tube condensers,which adopted unstructured meshes and the collocated-grid-based SIMPLE algorithm.The temperature rise of cooling water along the flow direction directly reduced the heat transfer temperature difference by 10%and affected the distribution of important physical quantities such as the condensation rate.Compared with the reference condensation rates of different sections,the calculated values considering the transverse distribution of cooling water are more accurate than those that do not.Additionally,the transverse distribution of the cooling water led to a more uniform condensation rate distribution due to the negative feedback between cooling-water temperature and condensation rate.However,this variable had less influence on the distribution of noncondensable gas(air).The comparison with 2D results revealed that 2D and quasi-3D simulations gave similar results on the midplane of the condenser,which illustrates the linearity of field distributions in the z-direction.According to the computation complexity analysis,the complexity of the full 3D simulation can be 2-3 orders of magnitude higher than that of 2D simulation,whereas quasi-3D simulation can be only 1 order of magnitude higher.Compared with 2D simulation,the proposed quasi-3D algorithm can compute the 3D distribution information of the flow field in the cooling-water flow direction and avoid the high computational cost brought by full 3D simulation.Therefore,for the rapid and accurate prediction of 3D flow and the heat transfer process inside a practical shell-and-tube condenser,the proposed quasi-3D algorithm is preferred over 2D and full 3D simulations.