The Study On Characteristics Of A Superheater With A New Heat Exchange Tube

The superheater is an important heat exchange equipment in the boiler.For modern high-parameter large-capacity boiler unit,the heat absorption of the superheater and the reheater accounts for about 50%of the total heat absorption of the working medium.The surface heat transfer coefficient is small and the energy utilization rate is low.Therefore,the enhanced heat transfer technology is one of the key links to improve the thermal efficiency of the boiler.Based on the viewpoint of energy saving and emission reduction of power plants,this paper proposes a new superheater tube-pitted tube,and the numerical simulation of the flow and heat transfer characteristics of the steam side and the flue side of the surface superheater are performed.It provides a reference for the application and promotion of the pitted tube in strengthening heat transfer.In this paper,the superheater tube is compared and analyzed.The results show that the dimples/pits structure can enhance the heat transfer and maintain a relatively low pressure drop at the same time comparing with the smooth tube and the fin tube.It is not easy to accumulate dirt and dust and to wear pipes,and has a certain bearing strength.Then,the flow and heat transfer characteristics of the steam side of the pitted tube were simulated with the single-tube cross-flow passage.The results show that the steam movement state changes due to the existence of periodic convex cells on the inner surface of the pitted tube.The fluid disturbance is strengthened,the boundary layer is thinned,and the synergy between the velocity field and the temperature field on the steam side is improved which enhanced heat transfer on the steam side.Compared with the smooth tube,the steam side Nu increased by 35.0%⁶0.0%,f_v increased by7.0%⁴0.0%,PEC was 1.25¹.5,the fluid field coordination angle was between80.5°⁸3.0°.As steam side Nu,f_v and PEC the objective functions,the response surface optimization results were analyzed.The optimal parameter combination was obtained:d=6.0 mm,h=1.0 mm,p=16.0 mm,u=4.8 m/s,corresponding to Nu=680.55,f_v=0.01468,PEC=1.43.Finally,the numerical simulation of the cross-flow passage filled with bundle tube is carried out,and the flow and heat transfer characteristics of the flue gas side of the bundled pitted tube are studied.The results show that the arrangement of the pits and the sunken tube bundles on the outer wall improves the flue gas convection.The scouring effect,the smoke disturbance intensified and it destroyed the flow boundary layer and shrinked the area of the wake region which deteriorates the heat exchange which enhance and the heat exchange on the flue gas side.Compared with the smooth tube bundle,the flue gas side of the tube bundle Nu increased by 8.0%⁶3.0%,PEC was 1.05¹.5,Nu increased by 12.0%⁴8.0%on the flue gas side of the bundle tube,and PEC was1.07¹.28.As the flue gas side Nu,f_g and PEC of the puff side tube bundle objective function,the best combination of parameters are obtained by optimization analysis:the in-line tube bundle smoke velocity v=7.4 m/s,s₁/D_o=2.25,s₂/D_o=2.5 corresponding to Nu=34.35,PEC=1.26 while the staggered tube bundle smoke speed v=11.0m/s,s₁/D_o=2.55,s₂/D_o=2.5 corresponding to Nu=47.02,PEC=1.31.In addition,the criterion formula of heat transfer and resistance on flue gas side and steam side are obtained respectively by the method of multiple linear regression.Based on the optimal parameters,the total heat transfer coefficient of the in-line tube bundle was 61.01W/（m²?K）,the total heat transfer coefficient of the staggered tube bundle is82.11W/（m²?K）,which is 1.31 times and 1.38 times the heat transfer coefficient of smooth tube bundle under the same conditions.The pitted tube provides references for the engineering application of the surface tube.