Research Of Flow And Heat Transfer In The 200MW Direct Air-cooled Condenser Tube

Because of the uneven distribution of water resources in China and the introduction of a series of policies in recent years,water saving has also become one of the most important indicators for the development of the electric power industry.Air cooling technology has a significant advantage in water saving.However,when the direct air cooling unit is running in winter,the exhaust flow rate often fails to achieve the design value under the influence of heating and peak shaving.Condenser may freeze,which seriously affects the economy and safety of the unit.Whether the freezing is occurring is closely related to the heat transfer of the steam in the fin tube of the direct air-cooled condenser.Therefore,the study of the flow and heat transfer characteristics of the two phase flow in the tube can open avenues to stiplate the anti freezing measures.The main work of this article is as follows:Based on the principle of two phase flow,a model for calculating steam condensation using VOF method is established.The mass and energy equation for condensation of pure steam in the direct air cooled fin tube was established by using a custom function UDF.In addition,considering the relatively high content of non condensable gas in the counterflow unit,the existence of non condensable gas will greatly affect steam condensation.By defining the mass,energy and component transport equations by UDF,a two component two-phase flow condensing model containing non condensable gas is established.Taking the condenser of a 200 MW direct air cooling unit as the research object,three dimensional steady numerical simulation of a pipe in a CW counterflow unit is carried out,and the flow and heat transfer characteristics in a single pipe are solved by the interphase interface tracing algorithm.By studying the steam condensation in a single pipe in downstream area,the influence of condensate flow characteristics,steam condensation position,freezing position and heat transfer intensity is obtained by changing the load and wall temperature of the unit.The results show that the condensate flow in the tube,the larger the inlet steam flow,the more uniform the upper and lower distribution.The volume fraction of the steam in the pipe increases along the flow direction first and then is small,and finally tends to be stable.The condensation position and the distance between the freezing position and the entrance increase with the increase of the inlet steam flow,but with the decrease of the external temperature,the trend gradually slows down.The increase of steam flow will reduce the number of Nu in the tube and increase the flow resistance.In the counter flow area,the influence of the inlet steam flow rate and the non condensable gas flow on the condensation position and the freezing position of the single pipe is analyzed.The results show that the concentration of non condensable gas at the critical position of freezing is certain,and is not affected by the flow of imported steam.The flow of non condensable gas at the entrance is linear with the concentration of the non condensable gas at the critical position of the freezing point.In summary,the freezing phenomenon is more likely to occur in the counter current than in the flow area,and the freezing can be effectively prevented by taking the concentration of non condensable gas at the outlet of the counter flow tube as a freeze proof index.