Numerical Study On Flow And Heat Transfer Characteristics Of Composite Finned Flat Tube Heat Exchangers

Direct air-cooling system has solved the serious water shortage problem in the construction of thermal power plants in the "three north" area where coal is rich and water is scarce.As the main heat exchange equipment of direct air cooling system,the heat transfer performance of direct air cooling condenser directly affects the economy and safety of the whole air cooling unit.Finned tube is the core heat exchanger of air-cooled condenser,the structure of the element plays an important role in the overall cooling effect of the condenser.Therefore,it is necessary to study the structure of the finned tube.Wavy finned flat tube is widely used in direct air-cooled condensers because of its good anti-freezing performance,large heat transfer area and small flow resistance.The characteristics of plain fin with less resistance and better heat transfer efficiency of wavy fin are considered comprehensively.Firstly,the flat tube structure of plain-wavy composite fins is proposed.Then,the connection between plain and wavy fins of composite fins is disconnected,which provides a new way for the further optimization of fin tubes.In this thesis,three-dimensional numerical simulations of air flow and heattransfer outside flat tubes with plain fins,wavy fins and plain-wavy composite fins were carried out by using Fluent.The effects of inlet velocity,breaking distance,fin spacing,fin height,the proportion and location of the wavy fin in the composite fin on the surface average heat transfer coefficient,the pressure drop and the comprehensive heat transfer performance factor;The internal mechanism of flow resistance and heat transfer enhancement is revealed by analyzing the local characteristics of velocity field and temperature field;By comparing the plain fin,corrugated fin and composite fin,the optimal composite fin structure is obtained.Thereafter,the heat transfer and resistance performances of continuous composite finned flat tube and discontinuous composite finned tube were compared.The results show that the surface average heat transfer coefficient and the pressure drop are the smallest for plain fins and the largest for wavy fins,and composite fins are between them.The average heat transfer coefficient h of composite fins is related not only to the proportion of wavy fins,but also to the location of wavy fins;the larger the proportion of wavy fins is,the larger the average heat transfer coefficient h is,when the proportion of wavy fins is the same,the average heat transfer coefficient h of the wavy fins placed at the front of the composite fins is larger.The proportion of wavy fins in composite fins is the main factor affecting the pressure drop,while the position of wavy fins in composite fins has little effect on the pressure drop.The surface average heat transfer coefficient and the pressure drop of discontinuous finned tube are largerthan those of continuous finned tube.Meanwhile,discontinuous composite fin has better comprehensive heat transfer performance than continuous composite fin.When the breaking distance is 8 mm,the comprehensive heat transfer performance of the discontinuous composite fin(b15p5)is the best.The comprehensive heat transfer performance of the discontinuous composite fin(b15p5)is better than that of the continuous composite fin(B15P5)in the studied speed range,and the discontinuous composite fin(b15p5)has the best comprehensive heat transfer performance when the inlet speed is 2m/s.