Numerical Study Of Turbulent Flow And Heat Transfer In The Noncircular Ducts

Ducts with noncircular cross section are widely used in the heat exchangers and other devices,for example,in the cooling system of gas turbine and the cooling ducts for the combustor and the nuclear reactor.Prediction of the flow and heat transfer performances for noncircular ducts plays a key role on the design and manufacture of the engineering equipments.Turbulent forced-convection heat transfer in a noncircular duct is often encountered.Nevertheless,a survey of available literature indicates a shortage of enough information for turbulent flow and heat transfer characteristics in the noncircular ducts,meanwhile it has been a common practice to utilize the hydraulic diameter Dh as a characteristic length to obtain heat transfer coefficient in the noncircular ducts by using correlation for circular tube.However,it is questionable that whether this application is suitable or not.Up to now,a lot of investigations have been conducted on the turbulent flow and heat transfer characteristics in the noncircular ducts,and the poor performance on the prediction with the use of hydraulic diameter were found.In addition,some correlations were proposed for the calculation of turbulent heat transfer coefficient in noncircular ducts,while most of the correlations are appropriate for duct of a single or several special geometries,a general correction that can be applied to most cross-sectional shapes is lacking.Conducting the experiments on the turbulent flow and heat transfer performances in the noncircular ducts is difficult for it is uneasy to fabricate the noncircular ducts precisely and the shapes of the noncircular cross-section are various,but the method of numerical simulation is competent.In this paper,the k-? turbulence model in conjunction with the wall functions method are applied to the simulation on the turbulent flow and heat transfer in a set of noncircular ducts.Heat transfer during turbulent air flow in the equilateral triangular,square,regular hexagonal,regular octagonal,regular dodecagonal ducts and circular tube were numerically simulated.A large number of comparisons and analysis were made on the simulation results,and the following conclusions can be drawn:1.For the circular tube,good agreement can be found between the simulation results and the experimental data,and this also shows the credibility and rationality of the method of simulation.2.Some features can be concluded on the basis of the flow and heat transfer performances in the circular tube:?1?Along the duct,there is a crest value for the axial velocity,and it decreases as the Reynolds number increases.?2?The thermal enty length is not strongly sensitive to the Reynolds number in the turbulent flow.?3?The flow and heat transfer characteristics are independent on the distribution of the inlet velocity when the turbulent flow is fully developed,on the other hand,the heat transfer performance is affected by the inlet velocity distribution only when the flow is not fully developed.3.The flow and heat transfer performances in the noncircular ducts indicate that under the same Reynolds number,both the velocity entry length and thermal entry length reduce when the number of the sides of regular polygon increases.Comparing with the other parts of the walls,the axial velocity and the wall shear stress as well as the heat transfer coefficient are relatively low in the corner region,this is mainly caused by the blocking effect of the corner walls.Along the duct,the part of the wall which is influenced by the corner's blocking effect enlarges gradually.For the regular polygonal ducts,the local heat transfer coefficient along circumferential direction is not uniform;there is an appreciable reduction in the corner region and the smaller the angle of the corner region,the more appreciable deterioration it causes.Meanwhile,it can be seen that the maldistribution characteristic of the circumferential local heat transfer coefficient becomes more apparent as the angle of the corner region gets smaller.4.The differences of friction factor f and heat transfer Nusselt number between the regular polygonal ducts and the circular tube decrease as the number of the sides of regular polygon increases.The use of hydraulic diameter for regular polygonal ducts such as equilateral triangular duct leads to unacceptably large errors,in the order of 36.9%,in turbulent heat transfer determined from the circular tube correlations.5.A circularity C? is proposed to correct the existing correlations,and it is to be noted that the deviations between the numerical simuation results and the new heat transfer correlation N_u=C_?Nu_circle are within 6.6%.