A Study On Fluid Flow And Heat Transfer Enhancement In Triangular Helical Ducts

In the light of a utility engineering equipment which is arranged at the inside or outsideof the rotary type shell, a jacket of triangular flow channel, fluid flow and heat transfercharacteristics of triangular helical ducts were investigated deeply. Due to differentgeometry of the duct, the performances of flow and heat transfer in a triangular helicalduct are certainly not equal to those in a circle helical duct, a rectangular helical duct, asemicircle helical duct and so on. For few literatures have pay attention to them so far,the relevant theories are not sufficient to guide their engineering application now.The mathematical models on an orthogonal helical coordinate system for a steady,fully developed, laminar flow through a helical duct were set up based on computationalfluid dynamics, numerical heat transfer and tensor analysis. The laminar flow andtemperature fields in two types of triangular helcial ducts were solved with the numericalmethods. The effects of Reynolds number, curvature, torsion and Prandtl number on fluidflow and heat transfer in two types of triangular helical ducts were discussed and thecomparison of laminar flow and heat transfer characteristics between them were given.The results show that the structure of secondary flow in the cross section of the triangularhelical ducts with inner flat wall is steady two vortices, while it is found that thesecondary flow in the cross section of the ducts with outer flat wall changes to fourvortices from two vortices with the increase of Dean number. The coefficient of heattransfer increases as Reynolds number, Dean number and Prandtl number increase.Compared with the triangular helical duct with inner flat wall, the triangular helical ductwith outer flat wall has far higher heat transfer coefficient at the same condition when theinner walls are heated.Fully developed fluid flow characteristics of two types of triangular helical ducts wereinvestigated by measuring the flow fields with Reynolds number varied from low to high.The coupling heat transfer coefficient between the fluid and the reactor wall was obtainedexperimentally. The results indicate that the maximum of circumferential speed in thecylindrical coordinate system shifts to the outer wall(s). The intensity of secondary flowis enhanced and the heat transfer coefficient of the duct with inner flat wall increases asReynolds number increases, while the fluid has a lesser rate in higher Reynolds number. The turbulent flow and heat transfer performances in the two types of triangular helicalducts were investigatied numerically and the effect of Reynolds number, the structureparameters of ducts and the temperature-dependent viscosity on fluid flow and heattransfer were analyzed. In two types of heat boundary conditions, the turbulent flow andheat transfer performances between them were compared. The flow resistance obtainedwith temperature-dependent viscosity is lower than the results with the constant viscosity,while the mean Nusselt number is higher than that of constant viscosity. The correlationsof turbulent flow resistance and mean Nusselt number were developed for the two typesof triangular helical ducts, which provide theory evidences for their commercialapplication.By the means of installing pins at the center of the inner flat wall, the heat transfer inthe triangular helical duct with inner flat wall was promoted. The flow fields in the crosssection and the disturbed zone after a pin were analyzed. The effects of the size, thenumber and the shape of pins on heat transfer enhancement were compared for thetriangular helical ducts with different curvature. In view of the simulated results, the flowfields after a pin change obviously and the heat transfer performance of the duct withpins is promoted significantly compared with the duct without pins. The superior heattransfer performance can be obtained when the size and the number of pins increase, butthe flow resistance increases as well. The triangular helical duct with square pins has thehighest heat transfer coefficient at the same height, cross section area of a pin and thenumber of pins.