Three-Dimensional Thermo-Hydraulic Numerical Simulation Of Steam Generator Based On Two Fluid-Turbulence Model

Steam generator(SG)is the key equipment in pressurized water reactors(PWR),which transfers heat from primary circuit to secondary circuit and whose flow and heat transfer characteristics are related to the safe,reliable and economical operation of the nuclear power plant(NPP).However,since it is involved with complicated steam-water(two-phase)flow in high temperature and high pressure,it is difficult to measure the key parameters such as pressure,temperature and void fraction,especially to carry out the visualization observation,the actual detailed working information of SG such as void fraction distribution and flow pattern at different feedwater supply schemes and operation parameters is still unknown,while these information are very significant for the improvement of SG performance.In addition,the flow characteristics of two-phase flow is directly related to the vibration,heat loss,stress corrosion and surface sag of the heat transfer tubes and other components during its service lifetime.According to the operation experience of NPPs worldwide,the SG tube rupture(SGTR)accident is one of the most frequent accidents.Therefore,it is necessary to study the mechanism of the thermo-hydraulic characteristics of SG and then to provide basic information for further study of mechanical design,hydrochemistry,material technology,etc.Above researches can contribute to structural optimization design and improve the heat transfer efficiency and safety.At present,experimental research and numerical simulation are two main methods for thermal hydraulics researches of SG.Experimental studies are mostly engineering experiments,in which the local or scaled-down model device is usually utilized to verify the rationality of the engineering design.Due to the huge size,numerous heat transfer tubes and high intensity operation parameters,it is difficult to obtain the precise parameters of internal three-dimensional flow field and temperature field through experiments,but these parameters are very significant for structural optimization of SG.In terms of numerical simulation,commercial computational fluid dynamic(CFD)software relatively has well developed to been mature,and has complete functions.But the CFD puts emphasis on universality and is lack of specificity,which leads to its application to SG numerical simulation(about SG design researches)is not widely used.Therefore,most of SG professional organizations worldwide are still committed to researching and developing the special three-dimensional thermo-hydraulic program.In the existing researches of special program development,the homogeneous or drift flow model is usually used to describe the two-phase flow.In recent years,in order to describe the flow behavior of the flow field more accurately,some scholars had adopted the two fluid model to build conversation equations for steam-water two phases respectively.However,the turbulence model is not considered in program.The influence of turbulence on flow behavior and heat transfer cannot be ignored.On the other hand,most of programs adopt porous media model for improving the computational efficiency,but the computational accuracy of porous media still needs to further improve.In addition,the results of above numerical simulation also need to further verify by experimental data.To sum up,developing the three-dimensional thermo-hydraulic numerical simulation program for SG based on two fluid-turbulence model,optimizing method of calculating porous media,and verifying the validity of program have become the development trend in the future.This work adopts two fluid model to describe the flow field at secondary side of SG.Considering the complexity of the two-phase flow behavior,an anisotropic algebraic turbulence model which both takes the influences of liquid flow and the interface momentum exchange into account is utilized.The coupling heat transfer model of primary and secondary side,flow resistance model and two-phase interface exchange model are added.The porous media model is used to simulate the complex structure of SG,and a new modified method M-GTG(Modified method based on grid combined with tube geometry)is proposed,which can accurately and efficiently calculate porous media coefficient.And the latest industrial formulas standard IAPWS-IF97 released by International Association for Properties of Water and Steam is applied to realize the real-time update of physical parameters.Based on Fortran language,then a special three-dimensional transient thermo-hydraulic characteristics analysis program 2T-THAP(Thermo-Hydraulic Analysis Program based on Two fluid-Turbulence model)is developed and is used to carry out real modeling and simulation.A small visualization scaled-down mock-up experimental bench is selected for validation,whose prototype is the SG in Guangdong Daya Bay Nuclear Power Station(GNPS).After completion of rational validation,the program is applied to SG analysis in GNPS.The analysis focuses on the thermo-hydraulic characteristics at different feedwater supply schemes and different power loads.The computational key parameters such as void fraction,temperature,pressure and heat transfer coefficients are compared with similar programs and the design parameters of prototype,the compared results show that the variation trends of above parameters are consistent and the calculation values agree well with the design parameters,which preliminarily verifies the effectiveness of the porous media model and the accuracy of the program.The analysis results show that vapor velocity is greater than liquid velocity and the variation trends of two phases are generally same.The velocity reaches the maximum value at the end of the straight tubes.After entering the upper inverted conical annular cavity,the flow velocity decreases because of the enlarged flow area and the blocking effect by tubes.In the straight tube section,the fluid appears a slight deflection from the cold side to the hot side driven by the density difference.In the U-bend section,the fluid tends to rotate from the hot side to the cold side due to the influence of the tubes structure.The force on straight tubes produced by the vapor-liquid mixture fluid flowing transversely is very small.The average fluid energy is less then 10J/m3.For the U-bend section,the maximum fluid energy appears at about 40 degrees(at cold side)and 140 degrees(at hot side),and the fluid energy is larger at cold side.The void fraction distribution at hot and cold sides is asymmetric.The influence of changing feedwater supply scheme on the void fraction distribution is mainly in the inlet section of tube bundle.In the case of nonuniform feedwater supply scheme,the mass flow rate at the hot side increases,and the temperature rise of unit volume fluid decreases,so the preheating period is prolonged,but the heat absorption capacity per unit time is more,which leads to the wall temperature drop faster,then in this case,the risks of the heat transfer deterioration caused by bubbles aggregation and thermal fatigue damage of tubes reduce.The thermal resistance of heat transfer tubes is the largest and accounts for more than 50%of the total values.The appearance of deposits and fouls would reduce the heat transfer coefficient and then leads to heat transfer efficiency decrease.Therefore,in the case of deposits and tubes blocking accumulating with the increasement of operating years,it is necessary to optimize the operation scheme to ensure the design power output.