Quasi-3D CFD algorithm for the flow and heat transfer process in steam condensers
[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.[Methods]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.[Results]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.[Conclusions]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.
NETL
NSTL
万方数据
