Numerical Assessments on Flow Topology and Heat Transfer Behavior in a Round Tube Inserted with Three Sets of V-Ribs

Simulation of fluid-flow topology and thermal behavior in a round tube heat exchanger(RTHX)installed by three V-rib sets is reported.The expected phenomena for the rib installation are the generated vortex flow,impinging flow,greater fluid blending and thermal boundary layer disturbance(TBLD).These phenomena are key causes of the augmentation of heat transfer potentiality and thermal efficiency of the RTHX.Effects of rib height(b1=0.05D–0.25D and b2=0.05D–0.25D),rib pitch or rib spacing(P=D,1.5D and 2D)and fluid directions(positive x(+x flow direction)and negative x(–x flow direction))on fluid-flow behavior and thermo-hydraulic characteristic are considered.The laminar air flow under Reynolds numbers between 100 to 2000 calculated by the inlet condition is focused.The current numerical problem of the RTHX fitted with V-ribs can be solved by a commercial code/program(the finite volume analysis).Firstly,the tested-tube model is carefully validated.The preliminary results of the validation show that the numerical model has great consistency for fluid flow and thermal structure prediction.The simulated outcomes are plotted in features of streamlines flow,local Nusselt number contours and temperature contours which explain the mechanism within the RTHX.The thermal assessments within the RTHX are performed with dimensionless variables,which include the Nusselt number,the friction factor and the thermal enhancement factor.The important mechanisms:vortex flow,impinging flow,better fluid blending and TBLD,are observed when the RTHX are installed with ribs.The maximum heat transfer potentiality is 19 times upper than that of the RTHX without ribs and the optimum thermal enhancement factor is around 4.10.

Numerical Investigations of Laminar Air Flow and Heat Transfer Characteristics in a Square Channel Inserted with Discrete X-V Baffles (XVB)

Thermal performance enhancement in a square channel heat exchanger(HX)using a passive technique is presented.Vortex turbulator insertion in a square channel HX as a passive technique is selected for thermal improvement.The vortex turbulator of interest is discrete X-V baffles(XVB).The discrete XVBs are inserted in the square channel with the main aim of generating vortex flow.The vortex flow generated can support the enhanced convective heat transfer coefficient and also enhance HX performance.Effects of baffle configuration(type A and B),baffle size(w/H=0.05,0.10,0.15 and 0.20),baffle distance(e/H=1,1.5 and 2)and flow direction(±x air flow paths)on fluid flow and thermal topologies are numerically investigated by using a commercial code.As shown by the numerical results,the predicted flow configuration with the discrete XVB insertions,which include impinging and vortex streams,is found through the HX channel.The perturbing thermal boundary layer and greater air blending are also found through the HX channel inserted with the discrete XVB.These mechanisms promote and augment the convection heat transfer coefficient,heat transfer rate and rise thermal potentiality.The maximum Nusselt number of the channel with the baffles inserted is 11.01 times upper than that of the smooth channel,while the greatest thermal performance factor(TPF)is observed to be around 3.45.

3D numerical study on flow structure and heat transfer in a circular tube with V-baffles

A 3D numerical investigation has been carried out to examine periodic laminar flow and heat transfer characteristics in a circular tube with 45°V-baffles with isothermal wall.The computations are based on the finite volume method(FVM),and the SIMPLE algorithm has been implemented.The fluid flow and heat transfer characteristics are presented for Reynolds numbers ranging from 100 to 2000.To generate main longitudinal vortex flows through the tested section,V-baffles with an attack angle of 45°are mounted in tandem and in-line arrangement on the opposite positions of the circular tube.Effects of tube blockage ratio,flow direction on heat transfer and pressure drop in the tube are studied.It is apparent that a pair of longitudinal twisted vortices(P-vortex)created by a V-baffle can induce impingement on a wall of the inter-baffle cavity and lead a drastic increase in heat transfer rate at tube wall.In addition,the larger blockage ratio results in the higher Nusselt number and friction factor values.The computational results show that the optimum thermal enhancement factor is around 3.20 at baffle height of B=0.20 and B=0.25 times of the tube diameter for the V-upstream and V-downstream,respectively.

Turbulent forced convection in a heat exchanger square channel with wavy-ribs vortex generator

Turbulent forced convective heat transfer and flow con figurations in a square channel with wavy-ribs inserted diagonally are examined numerically. The in fluences of the 30° and 45° flow attack angles for wavy-ribs, blockage ratio, R B= b/H = 0.05–0.25 with single pitch ratio, R P= P/H = 1 are investigated for the Reynolds number based on the hydraulic diameter of the square channel, Re = 3000–20000. The use of the wavy-ribs, which inserted diagonal in the square channel, is aimed to help to improve the thermal performance in heat exchange systems.The finite volume method and SIMPLE algorithm are applied to the present numerical simulation. The results are presented on the periodic flow and heat transfer pro files, flow con figurations, heat transfer characteristics and the performance evaluations. The mathematical results reveal that the use of wavy-ribs leads to a higher heat transfer rate and friction loss over the smooth channel. The heat transfer enhancements are around 1.97–5.14 and 2.04–5.27 times over the smooth channel for 30° and 45° attack angles, respectively. However, the corresponding friction loss values for 30° and 45° are around 4.26–86.55 and 5.03–97.98 times higher than the smooth square channel, respectively. The optimum thermal enhancement factor on both cases is found at R B= 0.10 and the lowest Reynolds number, Re = 3000, to be about 1.47 and 1.52, respectively, for 30° and 45° wavy-ribs.