Study On The Cooling Performance Of Supercritical CO₂ In The Helically Coiled Tube

At present,energy shortage and environmental pollution is two basic problems that global development should face and making full use of existing residual heat and energy is an important means to solve the problem.From the view of the situation of our country's energy and environment,strengthening energy-saving and emission reduction as well as utilization of residual heat and energy are very necessary.Our country has rich residual heat and energy,the use of trans-critical CO₂ industrial heat pump will convert them to power,which is very favorable for our country on improving the energy utilization rate and improving the environment.At the same time,as CO₂ gradually replace CFCs and HCFCs,transcritical CO₂ refrigeration system is widely used in refrigeration system,which plays an important role.for environmental protection and the prevention of global warming.The existing researches on supercritical CO₂ is mainly the convection heat transfer under heating or cooling conditon using experimental or numerical simulation method in the straight tube,and there is relatively very few study on convection heat transfer of supercritical CO₂ in helically coiled tube using numerical simulation method.The research about the effect of helically coiled tube parameters and section shape on the convection heat transfer of supercritical CO₂ is also relatively very few.This paper uses the Fluent software and studys the convection heat transfer of supercritical CO₂ in straight tube considering the wall thickness,then the effect of helically coiled tube parameters and section shape on the convection heat transfer of supercritical CO₂ is also studied.The main contents are as follows:?1?Cooling heat transfer of supercritical CO₂ in horizontal straight tubes with wall is numerically investigated by using Fluent software.The results show that almost all models are able to present the trend of heat transfer qualitatively,and the stand k-?with enhanced wall treatment model shows the best agreement with the experimental data.Then further studies are discussed on velocity,temperature and turbulence distributions.The parameters which are defined as the criterion of buoyancy effect on convection heat transfer are introduced to judge the condition of the fluid.The relationships among the inlet temperature,outlet temperature,the mass flow rate,the heat flux and the diameter are discussed and the difference between the cooling and heating of CO₂ are compared.?2?The cooling heat transfer of supercritical CO₂ is simulated numerically in the horizontal helically coiled tube using RNG k-?turbulence model.On this basis,the effect of the pitch p,tube diameter of helically coiled tube d,radius of helically coiled tube R on the flow and heat transfer is studied.The results show that:p has little effect on heat transfer,the change of d and R cause the change of the curvature and the Reynolds number and so have an impact on the flow and heat transfer of the fluid.The change condition of the pressure is different with the p,d,R increasing.By using the numerical simulation values obtained,the Nusselt number correlation with the curvature parameter is put forward under the condition of cooling heat transfer in the circular section horizontal helically coiled tube,with high precision.At the same time the research shows that under the setting conditions,the wall temperature and heat transfer coefficient close to the inner part of horizontal helically coiled tube are lower than that of other regions.?3?The cooling heat transfer and pressure drop characteristics of supercritical CO₂in the horizontal helically coiled tube are numerically simulated using RNG k-?turbulent model when the section is square.The parameters of the tube are:inner diameter of 4mm,a length of 2000mm,a pitch of 10mm,radius of helically coiled tube of 20mm,curvature of 0.1.The results show that the heat transfer coefficient and pressure drop in the helically coiled tube are larger than that in the straight tube because of the secondary flow.The heat transfer coefficient and pressure drop increase with the increase of mass flux and the peak value of heat transfer coefficient is shifted to high temperature region with the increase of pressure.The gravitational Richardson number and the centrifugal Richardson number decrease with the increase of the mass flux and they vary in parabola with the temperature.The gravitational buoyancy affects the fluctuation of pressure distribution.At the same time,the pressure distribution increases with the fluid temperature increasing.Because the effect of buoyancy gets the maximum at the critical temperature,the static pressure difference has the minimum values in the pseudo-critical temperature region.?4?At present,many studies only focus on the flow in circular tubes,and little on the flow in non-circular tubes.Now the compact heat exchangers are widely used,which are characterized with the high ratio of heat transfer area to core volume.In order to increase the compactness,flow channels with other cross-section shapes have attracted wide attention and been adopted.The RNG k-?with enhanced wall treatment mode is used to study the the cooling flow,heat transfer and pressure drop characteristics of supercritical CO₂ in the non-circular helically coiled tube.The parameters of the tube are:a length of 2000mm,a pitch of 10mm,radius of helically coiled tube of 20mm.It is found that the heat transfer coefficient fluctuates up and down on the critical temperature region along the flow direction of the fluid.On the basis,the comparisons of heat transfer coefficient and pressure drop characteristics are operated when the cross section is triangular,square and circular respectively.The results show that:the heat transfer coefficient of the circular is larger than that of the square and the heat transfer coefficient of the square is greater than that of triangle;the pressure drop of the circular is smaller than that of the trianglar and the pressure drop of the trianglar is smaller than that of square.At the same time the contours show that the effect of the buoyancy force on h is weaker than that of the centrifugal force when the cross section is triangular,square and ellipse respectively.A quantitative analysis is carried on to study the effect of gravity based on the setting conditions and the results shows that the effect of gravity can be ignorable on the heat transfer coefficient.The change condition of Ri_g is consistent with that of Gr/Re^2.7 in judging the scope of the buoyancy force.?5?The effect of the pitch p,tube diameter of helically coiled tube d,radius of helically coiled tube R on the flow and cooling heat transfer are studied by using the RNG k-?turbulence model in the horizontal helically coiled tube when the cross section is triangular,square and ellipse respectively.The results show that:p has little effect on heat transfer,the change of d and R cause the change of the curvature and the Reynolds number and so have an impact on the flow and heat transfer of the fluid.The change condition of the pressure is different with the p,d,R increasing.By using the numerical simulation values obtained,the Nusselt number correlation with the curvature parameter is put forward under the condition of cooling heat transfer in the triangular,square and ellipse section horizontal helically coiled tube,with high precision.The Nu correlation calculation values of helically coiled tube with different cross sections are compared,at the same time,a modified empirical correlation with roundness is proposed.