| Aiming at the drawbacks of the traditional uniform cross section ducts,an unusual section duct was proposed in this paper.The unusual section duct was used to improve the temperature distribution nonuniformity,high pressure drop and the accumulative condensed water and other shortcomings.The theoretical and mathematical models,heat transfer mechanism,characteristic studies and others were systemically investigated using experimental method,numerical simulation and theoretical analysis.In the paper,the unusual section duct includes trapezoid,cosine type,parabolic form and circular arc ducts.First of all,the temperature distributions in ducts can be derived by use of boundary layer theory,and it can be seen that the temperature difference is inversely proportional with x1/2from the theoretical equation in uniform section duct.The temperature difference distribution in trapezoidal duct and temperature distribution in trapezoidal fins with different slope angle can be obtained by using the simulation method.In the meanwhile,the corresponding heat transfer coefficient and pressure drop are also calculated.With the increasing ofβ,temperature and temperature difference distributions in fins and ducts become more uniform when 0°≤β≤25°.The overall heat transfer performance first increases and then decreases whenβincreases from 0°to 25°,and it is clear that the best heat transfer performance of the duct occurs atβ=20°.Also the minimum f factor in trapezoidal duct（β=25°）decreases by 22.6%when comparing with a rectangular duct（β=0°）at the same Reynolds number.By using the fluid mechanics theory and similarity solution,the flow characteristics were analyzed theoretically,and the simulation method was used to obtain the flow characteristics and field in the unusual duct.Secondly,the numerical simulation model of unusual duct on heat transfer,pressure drop characteristics and field synergy for the unique structure parameters is established.The heat transfer enhancement mechanism in unusual duct with different structure parameters was evaluated by using average field synergy angle based on the field principle.It concluded that,the higher the slope angle（0°≤β≤20°）,the smaller the amplitude,the greater the focal length are,the greater the overall heat transfer performance for the corresponding trapezoidal,cosine shape and parabolic flow ducts are.We can see that the heat transfer perfoemanfe in circular arc duct first and then decreases with the increasing R.Actually,forβ=20°,theηohstdαvalue of the trapezoidal duct is about 25%higher than that of rectangle duct by using the core volume goodness factor.And the order of overall heat transfer perfoemance is cosine type,parabolic form,trapezoid,circular arc and uniform cross section ductsThirdly,heat transfer and pressure drop characteristics on unusual heat transfer surface duct were experimentally studied.The time of experiment can be reduced about one time by using equipower heating method.Experimental correlations of five types of unusual ducts for j and f factors were fitted by using multiple linear regression method,which can predict 90%of the data.The results show that the h increases as hf increases.And the h is increases,decreases and increases as hf increases for the three ducts respectively.As for circular arc,the h first increases and then decreases when R increases,and the best heat transfer performance occurs at R=250 mm.In order to obtain the temperature and flow fields and the temperature distribution in fins,the law of conservation of mass,momentum and energy,and combined with lame coefficient are used to general multidimensional model of thermal conductivity and heat convection.The integrated mathematical description and deduction method in general curvilinear coordinate were imitated.Take trapezoid and cosine type ducts as an example,the correctmess of multidimensional models were analyzed by comparing with the experimental and simulation data,based on MATLAB platform and the finite difference method,and the maximum error are 4.89%and 5.15%for temperature distribution and heat transfer quantity.In order to analyze the film condensation process,thichness and heat conduction in liquid film,multidimensional film condensation and dual zone models on surfaces of unusual duct is established.The model of filmwise condensation was built based on Nusselt theory,lame coefficient,heat transfer theory and hydromechanics.The liquid film thickness equation was duduced and calculated according to the theory of boundary layer equation.Take cosine type ducts as an example,condensation heat transfer coefficient and the rate of condensate water are sloved and analyzed.It concluded that the main heat transfer areas are in crest and interzonal regions;temperature gradient on liquid film surface is gradually decreases along the flow direction;thichkness of liquid film thin progressively with the increase of the steam flow rate. |
