| Condensers are an important part of condensing steam turbines in modern large-scale power plants.Maintaining the condenser vacuum is of great significance for the stable operation of the unit.The flow field of steam in the tube bundle area of the condenser is mainly determined by the tube bundle arrangement.The design of the tube bundle arrangement greatly affects the heat transfer performance of the condenser.Therefore,the optimization of the flow field of steam in the tube bundle area has an important role in maintaining the ideal vacuum of the condenser.The optimization of steam flow field in the condenser tube bundle area mainly includes changing the contour line of the tube bundle layout,adding baffles or partitions in the tube bundle area,or changing the arrangement of the tube bundle.Previous research and engineering experience show that the steam flow field and heat load distribution are different in different tube bundle arrangements,but there is no conclusion that one type of tube bundle is superior to another.However,for the numerical study of the arrangement of tube bundles in the inner area of large condenser tube bundles,there has not been a general guideline and a suitable model so far.The main goals of this article are to improve the arrangement of condenser tube bundles and to provide a theoretical basis for the optimization of condenser tube bundle layout.The following aspects of research have been carried out:The first part introduces the steam flow field and heat transfer numerical model of the condenser shell.The steam condensation amount derived from the condensation phase transformation heat equation is used as the mass source term,which is added into the continuity equation,and the corresponding momentum source term is added into the momentum equation.The complex problem of fluid flow and condensation heat transfer in the shell of the condenser is transformed into a single mathematical two-dimensional flow of the shell-side steamair mixture in the tube bundle in each main cold area of the condenser.The governing equations of the gas phase on the shell side of the condenser are established,and the influence of the condensate on the gas phase flow and heat transfer is considered to simulate the thermal performance of the condenser.In the second part,the 1000 MW Condenser of a power plant steam turbine was taken as the research object,and the model proposed in the first part was verified by numerical simulation.According to the equipment geometric parameters and operating parameters provided by the manufacturer,a tube bundle model is established on a two-dimensional plane.In order to study the heat transfer performance in each tube bundle area,the uniform tube bundle layout model is divided into 12 areas from inlet to exhaust port,and reasonable boundary conditions are set.The obtained numerical simulation results are compared with the manufacturer's designed operating values to verify the model.In the third part,the model of uniform and non-uniform tube spacing arrangement is taken as the research object,and small tube bundle layout models are established respectively.Comparing the flow field of steam in the arrangement of uniform and non-uniform pipe spacing,it is found that under the same inlet flow rate,the vapor resistance of the steam arranged in the non-uniform pipe bundle is lower,the velocity distribution is more uniform,and the velocity fluctuation is smaller.Under the conditions of different inlet flow rates,it can be found that the location of steam accumulation in the small model of the tube bundle is related to the flow velocity of the steam.In the conditions of different inlet flow rates,it can be found that the closer the air accumulation location is to the outlet,the better the heat transfer performance of steam in the tube bundle,which is a basis for improving the overall tube bundle layout.In the fourth part,the problem that the steam velocity fluctuates greatly and the steam resistance is relatively high in the uniform tube bundle arrangement is needed to solve to achieve the optimization of the cold end of the 1000 MW unit.Based on the uniform tube bundle arrangement,a non-uniform tube bundle arrangement method is proposed.Under the same boundary conditions,the flow field and heat transfer performance of steam in these two tube bundle arrangements are compared and analyzed.When non-condensing steam flows inside the condenser,the air is concentrated in the air-cooled area,and the vapor resistance in the tube bundle area is large,which will seriously affect the performance of the condenser.Because the pressure drop at the condenser inlet changes greatly,the steam condensation situation is more severe,so the number of tube rows per row of the tube bundle in the steam inlet area is reduced.Under the condition that the overall heat exchange area is unchanged,the reduced condenser cooling tubes are moved to the bottom of the tube bundle to reduce the accumulation of air.Under the condition of rated air leakage,the improved non-uniform tube bundle model has a pressure drop of 62.40% for steam inlet and outlet,a steam condensation rate of 0.64% and a heat transfer coefficient of 2.78%.And the improved non-uniform tube bundle arrangement scheme has less vapor resistance than the uniform tube bundle arrangement.In the fifth part,Because the main characteristics of the double-peaks multilayer tube bundle arrangement are the uneven distribution of the tube spacing,and the outer and inner dense.Therefore,the porous medium model is not suitable.Based on the model proposed in the second part,the flow state of the steam and heat transfer characteristics of the arrangement are analyzed.Compared with the uniform tube bundle arrangement,the doublepeaks multilayer tube bundle arrangement has lower heat transfer performance and larger vapor resistance.In order to ensure that steam passes from the tube bundle inlet to the suction port,the number of tubes passing through in the direction of the steam flow line is basically the same,and a lower pressure drop is obtained.Therefore,the height of the air-cooled zone from the arrangement of the bimodal multilayer tube bundles is increased from the bottom.Compared with results before improvement,the improved double-peaks multilayer tube bundle has a pressure drop of 51.37%,a condensation rate of 3.88%,and a heat transfer coefficient of 11.14%... |
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