Heat Transfer Behavior in a Square Duct with Tandem Wire Coil Element Insert

Effects of insertion of tandem wire coil elements used as turbulator on heat transfer and turbulent flow friction characteristics in a uniform heat-flux square duct are experimentally investigated in this work. The experiment is conducted for turbulent flow with the Reynolds number from 4000 to 25000. The wire coil element is inserted into the duct with a view to generating a swirl flow that assists to wash up the flow trapped in the duct corners and then increase the heat transfer rate of the test duct. Apart from the full-length coil, 1D and 2D length coil elements placed in tandem inside the duct with various free-space lengths are introduced to reduce the friction loss. The results obtained from these wire coil element inserts are also compared with those from the smooth duct. The experimental results reveal that the use of wire coil inserts for the full-length coil, 1D and 2D coil elements with a short free-space length leads to a considerable increase in heat transfer and friction loss over the smooth duct with no insert. The full-length wire coil provides higher heat transfer and friction factor than the tandem wire coil elements under the same operating conditions. Also, performance evaluation criteria to assess the real benefits in using the wire coil insert into the square duct are determined.

DIRECT NUMERICAL SIMULATION OF TURBULENT FLOW IN A STRAIGHT SQUARE DUCT AT REYNOLDS NUMBER 600

This article presents the direct numerical simulation results of the turbulent flow in a straight square duct at a Reynolds number of 600, based on the duct width and the mean wall-shear velocity. The turbulence statistics along the wall bisector is examined with the turbulent flow field properties given by streamwise velocity and vorticity fields in the duct cross section. It was found that the solutions of the turbulent duct flow obtained in a spatial resolution with 1.2×10^6 grid points are satisfactory as compared to the existing numerical and experimental results. The results indicate that it is reasonable to neglect the sub-grid scale models in this spatial resolution level for the duct flow at the particular friction Reynolds number.

LAMINAR FLOW AND HEAT TRANSFER IN THE DEVELOPING REGION OF HELICAL SQUARE DUCTS

Simultaneous development of the laminar flow and heat transfer in helicalsquare ducts was numerically studied. The governing equations were written in an orthogonal helicalcoordinate system and fully parabolized in the axial direction. Results were found out over a widerange of the governing parameters. Two axial velocity entries were taken into account. Thedevelopment of secondary flow, axial velocity and temperature distribution for the large Dean numberwere examined in detail and the effects of different governing parameters on the friction factorand- the Nusselt number were annlyzee. Many new and interesting conclusions were reached. Thepresent results reveal the nature of fluid flow and heat transfer in the developing region ofhelical square ducts.

A MULTI-SCALE APPROACH FOR THE ANALYSIS OF PROPER SAMPLED DATA SCALE IN HOT-WIRE EXPERIMENT OF SQUARE DUCT FLOW

Without rational criteria to determine the Proper Sampled Data Scale （PSDS）, it would result in the expense of the too much unnecessary processing time and storage space in turbulent experiments. A novel approach for PSDS was established herein on the basis of turbulence theory and statistics. The specific procedure was given by using wavelet tools. A case study to prove the reliability and rationality of this approach was reported, where the sampled hot-wire data were from the experiment of square duct flow and turbulence kinetic energy was selected as the concerned turbulence parameter. It is shown that 2^20 quantities of the sampled data are enough to analyze turbulence kinetic energy in the present experiment. The PSDSs of three turbulence parameters at different Reynolds numbers （Re = 4.60×10^4, 7.68×10^4 and 1.23×10^5） were studied. The results illustrate that the PSDSs increase with the increment of the Reynolds number and the order of concerned turbulence parameter.

Enhanced heat transfer in a heat exchanger square-duct with discrete V-finned tape inserts

The article presents an experimental and numerical study on thermal performance enhancement in a constant heatfluxed square-duct inserted diagonally with 45° discrete V-finned tapes(DFT).The experiments were carried out by varying the airflow rate through the tested square duct with DFT inserts for Reynolds number from 4000 to 25000.The effect of the DFT with V-tip pointing upstream at various relative fin heights and pitches on heat transfer and pressure drop characteristics was experimentally investigated.Both the heat transfer and pressure drop were presented in terms of Nusselt number and friction factor respectively.Several V-finned tape characteristics were introduced such as fin- to duct-height ratio or blockage ratio(R_B=e/H = 0.075,0.1,0.15 and 0.2),fin pitch to duct height ratio(R_P= P/H=0.5,1.0,1.5 and 2.0) and fin attack angle,α = 45°.The experimental results reveal that the heat transfer and friction factor values with DFT inserts increase with the increment of R_B but the decrease of R_P.The inserted square-duct at R_B = 0.2 and R_P = 0.5 provides the highest heat transfer and friction factor while the one with R_B= 0.1 and R_P= 1.5 yields the highest thermal performance.Also,a numerical simulation was conducted to investigate the flow structure and heat transfer mechanism inside the tested duct with DFT inserts.

Effect of turbulence-driven secondary flow on particle preferential concentration and clustering in turbulent square duct flows

In this study,the preferential concentration and clustering of inertial particles in fully developed turbulent square duct flows are studied using large eddy simulations combined with Lagrangian approach,where the Reynolds number is equal to Reτ=600(based on the mean friction velocity and duct full height),and the particle Stokes number ranges from 0.0007 to 1.16.The results obtained for duct flows are compared with those for channel flows under the same working conditions.Then,the effect of the secondary flow on the particle concentration in duct flows is investigated.The equation of particle motion is governed by the drag force,lift force,added mass force,pressure gradient force,and gravity.The inter-phase interaction that was considered includes one-way and two-way coupling.The simulations of a single phase are verified and in good agreement with the available literature data.For the discrete phase,particles in the duct flow are found to be more dispersed in the vertical direction compared with the channel flow.In near-wall regions,a small fraction of particles tends to accumulate in duct corners,forming stable particle streaks under the effect of the secondary flow.Meanwhile,most particles are likely to reside preferentially in the low-speed flow regions and form elongated particle streaks steadily in the middle region of duct or channel floors.The Voronoi diagram analysis shows that the near-wall secondary flows in the square duct could cause particle clusters to transfer from regions of high to low concentration,and this trend increases with particle size.In addition,two-way coupling is found to enhance the near-wall particle accumulation and to promote particles to form more elongated streaks than one-way coupling.Finally,the mechanism responsible for the particle preferential concentration in turbulent square duct flows is determined.

Heat Transfer Augmentation in Developing Flow Through a Ribbed Square Duct

An experimental study is conducted to investigate the heat transfer augmentation in developing turbulent flow through a ribbed square duct. The duct is made of 16ram thick bakelite sheet. The bottom surface of the ribbed wall having rib pitch to height ratio of 10 is heated by passing a c current to the heater placed under it. The uniform heating is controlled using a digital temperature controller and a variac. The results of ribbed duct are compared with the results of a smooth duct under the same experimental conditions. It is observed that the heat transfer augmentation in ribbed duct is better than that of the smooth duct. At Re=5.0× 10^4 , the mean temperature of air flowing through the ribbed duct increases by 2.45 percent over the smooth duct, whereas in the fibbed duct Nusselt number increases by 15.14 percent than that of the smooth duct with a 6 percent increase in pressure drop.

VISUALIZATION OF FLOW THROUGH A ROTATING U-BEND DUCT

The flow through a rotating U bend duct is investigated by means of visualization. The U bend duct has a cross section of 50 mm×50 mm and a ratio of bend mean radius R c to hydraulic diameter of the duct D of 0.65. The rotation axis is parallel with the bend axis. Three cases with rotation number of Ro=-0.2, 0 and 0.2, respectively, are studied at a Reynolds number of 100 000. The results show that the combined effect of rotation and bend curvature strongly influences the flow field, especially in the downstream region of the bend. The evident difference among the flow patterns with different rotation number shows that the secondary flow induced by Coriolis force takes an important role in determining the flow structure.

Simulation of Air Flow along Human Lung for Cylindrical Channel of Porous Medium

The steady flow behavior in terminal bronchus of human lung for cylindrical channel of porous medium has been studied. The governing equations have been solved analytically and numerically for cylindrical channel. Finite difference method is incorporated to simulate the problem. The numerical results are compared with square duct channel for different parametric effect. It is observed that the flow rate is increased in cylindrical channel compared to square duct channel for the increasing value of pressure gradient, porosity and permeability. On the contrary, the flow rate is decreased in square duct channel compared to cylindrical channel for increasing value of viscosity. Flow rate in both channels is analyzed and compared for non-porous medium also. It is observed that flow rate is increased very high in cylindrical channel compared to square duct channel for both medium.