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International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 8, August 2018, pp. 447 461, Article ID: IJMET_09_08_049 –Available online at http://iaeme.com/Home/issue/IJMET?Volume=9&Issue=8 ISSN Print: 0976-6340 and ISSN Online: 0976-6359

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A NUMERICAL -EXPERIMENTAL STUDY OF TURBULENT HEAT TRANSFER FLOW A

CROSS SQUARE CYLINDER IN A CHANNEL Dr. Kareem Khalaf Ali and Hatem A. Hassan

Baquba Technical Institute- Middle Technical University, Baghdad, Iraq

ABSTRACT The three-dimensional (3D) of turbulent heat transfer flow structure a cross

square cylinder in a Channel had been investigated numerically and experimentally. The research examines the flow structure and heat transfer rate by solving the governing equations (continuity equation ,Navier -Stokes, and energy) using the very

known program (FLUENT) version (12.1).The governing equation was solved for three dimensions ,turbulent flow, incompressible with an appropriate turbulent

modeling a large eddy simulation (LES).The numerical solution was carried out for changes the heat transfer rate from the walls of the square cylinder and channel at

rang of Reynolds number 4200 ,5500 and 6500.The distribution obtained for parameters velocity ,pressure ,vorticity ,secondary flow . The experimental work has

been made by measurement heat transfer rate ,constant heat flux from wall of channel, and calculation Nusselt number distribution along the Channel .The results obtained heat transfer (Nusselt number Nu, heat flux) distribution along the wall of

the channel at Re=6500. The main results showed the existence of square cylinder makes wall boundary layer separate, secondary flow vorticity downstream of the

cylinder and two large vorticities in wake of the cylinder. The result showed an increase in Nusslet number from the wall of channel reach to the maximum value

36%. Both the parameters insert of the square cylinder and increasing of Reynolds number have a significant influence on the flow field and heat transfer from the walls.

The results showed good agreement between the theoretical and experimental and close agreement with results in the previous literature. Keywords: Square Cylinder, Turbulent Flow, Heat transfer, three dimension flow, Nusselt number and constant heat flux.

Cite this Article: Kareem Khalaf Ali and Hatem A. Hassan, A Numerical -Experimental Study of Turbulent Heat Transfer Flow a Cross Square Cylinder In A

Channel, International Journal of Mechanical Engineering and Technology, 9(8), 2018, pp. 447 461. –http://iaeme.com/Home/issue/IJMET?Volume=9&Issue=8

A Numerical -Experimental Study of Turbulent Heat Transfer Flow a Cross Square Cylinder in a Channel


NOMENCLATURE Cp Specific heat (J/kg K) H Height of square cylinder (mm) h Convection heat transfer coefficient (-) kf Fluid thermal conductivity (-) Nu Nusselt number (-) P Pressure (N/m2) qw Wall heat flux (W). Re Reynolds number (- ) St Stanton number (-) T Temperature (K) T∞ Free stream temperature (K) Tu Turbulent intensity (-) Tw Wall temperature (K). u Velocity component in X- direction(m/s). u,v,w Velocity component in x,y,z direction (m/s) U∞ Stream velocity (m/s) k Solid thermal conductivity

GREEK SYMBOLS ρ Density kg/m3

ν kinematic viscosity N.s/ kg μ Laminar viscosity N.s/m2

τij Reynolds stress tenser N/m2

1. INTRODUCTION The separation and recirculation zone appear when the bluff body is placed near a wall,

separated flows appear around any object in turbulent flow, which appears in engineering applications such as High-rise buildings, chimneys, flows over bridges, tube banks in heat exchangers, cooling towers, cooling of electronic equipment, mixing flow and similar flow

structures are important examples of fluid flow around the object with flow separation. In these cases the separation and wake of the bluff object regions very important. The separated

region around a square cylinder installed in the channel. The flow is turbulent, incompressible, three dimension. In this case the flow structures and heat transfer change by the separated flow[1,2]. Also Heat very affected the flow structure near a surface, the rate byof heat transfer increase from the walls and affected by the Reynolds number and cylinder aspect ratio, [1-3].The flow system with an array of square cylinder represents an idealization

of a certain feature of turbulent flow heat transfer. First, it is necessary to understand the reason for the formation of the vortex, separation and secondary flow. The number of

research's had investigated the effects of geometrical parameters and Reynolds number such as the cylinder aspect ratio]. 1] was investigated the effect of channel confinement on the [two-dimension (2D) laminar flow and heat transfer across the square cylinder. They used the model of turbulent large eddy simulation (LES) was performed to simulate they found the effect of Reynolds number and cylinder aspect ratio, . ]was investigated Turbulent flow in a [2

Kareem Khalaf Ali and Hatem A. Hassan


channel with a Built- Square cylinder and study heat transfe .They used best turbulent in model was a large eddy simulation (LES). They discuss flow structure in a channel for

Reynolds number change from 1000 to 15000 and find Nusselt number distribution. It was observed that install of a square cylinder makes the attached wall boundary layer separate

with subsequent recirculation zone downstream the cylinder. 3] was investigated Turbulent [flow structure in a long channel with a Built- rectangular cylinder ,shows the structure of invorticity around the rectangular cylinder and wake region . 4] was investigated low Reynolds [

number and unsteady incompressible flows over two circular cylinders in tandem are numerically simulated for a range of Reynolds number with varying gap size. The governing

equations were solved using a second-order implicit finite volume method. Both the parameters have a significant influence on the flow field. ref[5] Investigated the effect of

blockage ratio on the combined free and forced convection from a long heated square obstacle confined in a horizontal change .The numerical computations for low Reynolds number and Richardson number for blockage ratio 0.125 and 0.25 they found temperature field decrease with increase value of blockage ratio and the total drag coefficient increases with increasing

of the blockage ratio for fixed values of the Reynolds and Richardson number. From this review, we can show that studies of heat transfer for a square cylinder have not been carried to the same extent and those for fluid flow. Also, most studies focused on two-dimensional,

laminar flows. There is still less recently published work about heat transfer and three- dimensional (3D) turbulent flow across a square cylinder and turbulent Reynolds number

flow. The present work focus on investigate numerically and experimentally the flow past square cylinder arranged in the channel over wide ranges of Reynolds number (Re) and find

distribution of heat transfer rate. Also, Build up a test rig, some experimental tests were carried out in the (low-speed) wind tunnel to determine heat transfer from the wall of square

cylinder and channel. By measuring temperature distribution and heat flux. Reported and discuses effects of Reynolds number on flow structure and heat transfer from small square cylinder and floor walls of the channel. In numerical use the turbulent flow would appear around a square cylinder the turbulent model was used large eddy simulation (LES) ,three-

dimensional, turbulent motion and incompressible for the computational domain. Reported and discusses the distribution of Nusselt number (Nu).The Reynolds number was 4200, 5500 and 6500.

2. THEORETICAL MODEL

2.1. NUMERICAL METHOD In order to examine turbulent, secondary flow the governing equations (continuity ,Navier -Stokes, and energy equations) will be solved by using the known package FLUENT version

(12.1) .The governing equations are solved in the turbulent region with an appropriate turbulent model (LES) in three dimensions second order accuracy for time and spatial

discretization of the equations. The convective and diffusion fluxes in the momentum and energy equations. A third order Runge-Kutta algorithm is used for time integration .The

continuity and the pressure gradient terms in the momentum are treated implicitly .while the convective and diffusive terms are treated explicitly .The linear system of pressure is solved by an efficient conjugate gradient method with preconditioning.

2.2. COMPUTATIONAL METHOD AND BOUNDARY CONDITION The computational domain consists of a square cylinder in a channel .Which extended in a spanwise direction Fig (1, A, B) presents the Geometric configuration of the channel within square cylinder. All the dimensions are dependent on the cylinder side length. For one square cylinder height (H) , the length of the channel was selected as (20H), in order to reduce the



effect of inlet and outlet boundary condition on the computational flow field .The width of the channel was selected as (5H) ,these dimensions were selected dependent on previously

published works [ 2 ,11].Uniform flow is assumed in the inlet and outlet channel boundary condition is of the convective type with Uc equal 75% of the mean velocity .It is well known that such a convective boundary condition is capable of predicting unsteady flow behavior at the exit ,with very good accuracy. The span wise direction boundary condition was selected as periodic, thermal boundary conditions were selected as constant heat flux for square cylinder and one of channel wall, while the other walls and cylinder surface were insulated. A uniform temperature and convective temperature boundary condition were selected for the inlet and outlet respectively.

(A) (B)

Figure 1 A, B Geometric configuration of channel within square cylinder

2.3. MESH TOPOLOGY An accurate simulation of flow over square cylinder required fine mesh, in addition, to

accurately simulate analysis .For unstructured mesh, FLUENT uses unstructured solver with an internal data structure to assign in order to the cells, faces and grids points in a mesh and to

maintain contact between adjacent cells .This gives the flexibility to use, the best grid topology for complex geometry. The solver does not force an overall structure or topology on

the mesh (i.e ,it does not require (i,j,k) indexing to locate each neighboring cells .The FLUENT code uses different element types for mesh topology. The type of element specifies the number of mesh nodes and the node pattern associated with element shapes. Building the

mesh required fine cells in the area near the square cylinder, as shown in Fig (2). The minimum grad spacing used for both x and y-direction is 0.1H, with a grid expansion ratio of

1.08.A uniform grid distribution was used in the spanwise direction .The number of grids used in the present computations was (702862) cell. So that it is convenient for turbulent flow characterized by wake region and separation .Therefore, the mesh should be manipulated and controlled manually to keep smooth mesh transition and maintain accurate mesh for a

three-dimension (3D) model with a minimum computational expense. All this parameter istaken into consideration in the simulation process. This was achieved by applying the size function. Size function used to adjacent and control the size of mesh intervals for edge and ismesh element for all faces or volumes and thus to keep smooth transition of mesh from fine mesh near square cylinder surface to coarse mesh far away at the undisturbed boundaries as shown in Fig(2). It should be mentioned that the number of grid points has an important effect on computed flow and temperature fields.



Figure 2 shown the mesh of model

2.4. MATHEMATICAL FORMULATION The turbulent flow around the bluff bodies in the channel at the high Reynolds number can be modeled three dimensional, unsteady, turbulent motion and incompressible .The equations for

the evaluation of velocity field are derived from (continuity, Napier -Stokes, and energy equations) The view of above hypotheses lead to the governing equations can be written in . the coordinates ( i,j,k,t) as:

2.4.1. FLOW EQUATIONS 1-CONTINUITY EQUATION

1

2- Navier -Stokes equations

2

3-Energy equation.

3

4- Turbulent kinetic energy.

4

5- Turbulent intensity

(Tu). 5

6- Reynolds number (Re).Calculate base on uniform inlet velocity and cylinder height as

6

2.4.2. HEAT TRANSFER CHARACTERISTIC

7

2-Stanton number (St) Calculate based on constant heat flux, inlet velocity and different between wall surface

temperature and inlet fluid.

8



3. EXPERIMENTAL WORK In order to assess the degree of correspondence exists between theoretical work and the real flow; some experimental tests were carried out in the open circuit, a (low-speed wind tunnel) . The wind tunnel had a big fan with maximum power of 5.2 , Air speed varied from 3 to 25 Kwm/s in test section. The manufactured of test rig (320mm wide, 320mm height and 600mm long).The general arrangement and direction of flow in test section as shown in Fig(3) .The sides and upper surfaces of test section were made of Perspex (plexiglass) with a thickness of (5mm) in order to give full visibility ,the dimension of the test section was (320mm wide ,320mm height and 600mm long). The small square cylinder of dimension (15mm × 15 mm)

and arrangement at equal distance m the wall. The lower surface was made from froAluminum of (10mm) thickness. To measure the surface temperature at the hot wall, it was

drilled with the number of holes (6 holes) with dimension (2mm) and depth (9mm) .Thermocouple junction of type (k) was inserted into holes and connected using an epoxy that can withstand and bear high temperature.

Figure 3 wind tunnels, Test section, and some part of the test section and the mechanism

3.1. PROCEDURE OF MEASUREMENT The number of measurements was performed to investigate the effect of the square cylinder

on heat transfer rate characteristic. This was done through measuring the temperature distribution along the test section. The heat transfer distribution measured by six

thermocouples at the wall of test section and heater to keep constant heat flux. To heat up the lower surface of test section an electric heater consisting of thin strip manufactured from Nick Schrom (nickel- chromium alloy) with (3mm) width and thickness (0.15 mm) was used .The

heater resistance was 3.2Ω/m with a power 678 W/m2 . These strips were longitudinally

wrapped on a layer of mica with a thickness of (0.4mm). To reduce energy losses, the heater was insulated with fiber glass and wood layer. The hot wall surface was fixed to the assembly

by bolts. The heater was powered with electrical supply (AC) and a variac to control the temperature required heating up the heat exchange surface as shown in figure (4). The heater starts at location front leading edge. Heater area represents a constant heat flux was supply the floor of channel surface. The heater supplies floor of the channel by a constant heat flux of 678W/m2.



Figure 4 Control of power supply to the heater

4. RESULTS AND DISCUSSION The results of numerical and experimental investigations are presented and discussed in the following sections. For each case testing first the base case flow without square cylinder and then square cylinder installed The main results are given in comparative form (in percent) . ofthe flow structure and heat transfer.

4.1. THE STRUCTURE OF FLOW FIELD Results were obtained of a turbulent, three-dimensional (3D) flow across square cylinder

installed in a channel, Reynolds number (Re) based on uniform inlet velocity (U∞) and cylinder height (H) ,in order to compare the numerical and experimental. The flow structure consists of velocity, pressure, velocity vector, vortices and particle path line, was obtain to give a clear view. The velocity distribution around the square cylinder as shown in Fig(5) for

the Re=6500 and turbulent intensity Tu =0.053 the position of flow separation at the front leading edge of the square cylinder, the stagnation point in the front of cylinder, but very high speed separation in sides of cylinder and very clear the wake reign which have high vorticity.

It is observed for single Reynolds number (Re), there are three types of separations, separation from leading edges with reattachment, separation from trailing edges, and

separation from leading edges without reattachment at trailing edges. The velocity fluctuating get a uniform shape on upper and lower surface because the shear layers approach the to

channel. The effect of square cylinder reach to the walls of channel and lead to changing in flow direction with very high velocity fluctuating. The contour of pressure distribution on the surface of the cylinder gives the clear view as shown in Fig(6)the very high-pressure region in front of the cylinder ,but low pressure in sides ,wake reigns and the effect extends to the wall of the channel .the change since shear layer flows separated in leading edge of the square

cylinder reattach to the side wall of the cylinder and separated again at the trailing edge. Fig(7) show the(x,y) drowning of pressure distribution in the channel at Re=6500 and

turbulent intensity Tu=0.053 . The very high increase in pressure caused by square cylinder .It can be show that the maximum values of pressure in leading edge of square cylinder. There are maximum value for pressure in centerline of channel because of the interaction between the two shear layers of the upper and lower square cylinder surface and extension to the wake

region. The mean streamwise velocities component at Re=4200 and turbulent intensity Tu=0.053 in centerline for the symmetric plane Z=0 and downstream of the cylinder as shown in Fig(8). At the Re=4200 and Tu=0.056 the separation at leading edge lead and stagnation point in the front of the cylinder very closed to square cylinder, As leading edge separation occurred, the wake region width increased as a result of the flow behavior changes .but very high speed in sides of the cylinder, the effect extension in long distance in the channel. Figs



(9) show the velocity fluctuating get a uniform shape on upper and lower surface. The clear view effect of cylinder on velocity reach the channel wall that leads to an effect on heat

transfer rate from channel walls and square cylinder. The velocity vector distribution around the square cylinder Re=6500 and Tat u=0.053 as shown in fig(10),the change of streamline

direction toward wall channel. The separated shear layers of the upstream square cylinder form two counter-rotating vortices in wake region. A high value of turbulent intensity (Tu) is explained due to vortex shedding. The particle pathline around the square cylinder as shown in Figs (11, 12) it is clear the separation of flow in front of the cylinder and we can see the two large vortices in the wake region behind of square cylinder, the shear layers separate and tend to merge at a downstream point on the centerline. It is also observed when increasing Re

=4200 to Re=6500, that wake vortices change from elongated vortices to shorter vortices. From velocity, pressure distribution and particle pathline around the square cylinder it is clear the flow structure very effectively by the square cylinder .

4.2. STRUCTURE OF HEAT TRANSFER RATE THE The effect of install square cylinder on the heat transfer can show by Nusselt number

distribution the result with and without square cylinder: First when constant heat flux from wall of channel this part experimental results. Fig (13)

shows the experimental results of Nusselt number (Nu) distribution on wall floor of the channel without a square cylinder at Reynolds number Re=4200 and Tu=0.056 .It should be know that the definition of the (Nu) was based on constant heat flux of the wall floor and

cylinder height (H),

and (Tw -T∞ ) is the different between the wall surface

temperature and inlet fluid. The higher Nu in the inlet region, then decreasing to the lower value after distance inlet .This show heat transfer exchange between flow and channel wall. The effect of square cylinder illustrates in Fig (14) show Nusselt number (Nu) distribution on the wall floor of a channel with install small square cylinder at Re=4200 and turbulent intensity Tu=0.056. There are increasing in heat transfer across the wall of channel .The are

increase in velocity in two sides of cylinder lead to increasing in heat transfer convection ,Also voracity system very increasing mixing shear layer with mean flow. The comparison between two cases with and without square cylinder illustrates in Fig (15) the heat transfer increase in (Nu) along the channel wall,It can be seen that the maximum increase value 36 % in wake region which effected by square cylinder in the test section. Also, the effect of turbulent intensity on heat transfer, A high value of turbulent intensity (Tu) increasing heat

transfer rate as a result increase effect of vortex shedding. Fig (16) shows the contour of Surface heat flux on heat wall the effect of square cylinder extension to the heat flux wall.

The very high velocity fluctuating increasing convection heat transfer from the wall and decreasing the thickness of boundary layer on the wall of channel. In Fig (17) showed contour distribution of Nusselt number (Nu) on the wall floor of a channel with in a square cylinder. It

is clear view the increasing in Nusselt number the near wall of channle. All of vortex shedding, turbulent intensity increasing Nusselt number (Nu) .Fig (18) show the important experimental result the effect of the Reynolds Number (Re) on heat transfer, Nusselt number (Nu) distribution with different Reynolds Number (Re) along wall floor of a channel with in a square cylinder. from figure the Nusselt number increase with an increase Reynolds Number (Re).2- Second case when constant heat flux from wall of square cylinder constant heat flux of 678W/m2 Fig(19) show the distribution of Stanton number (St) along the wall of square cylinder the higher value in upper and lower sides of small square cylinder because of the interaction between the shear layers of upper and lower surface of square cylinder and the wake region .



Figure 5 The velocity distribution around the square cylinder at Re=6500 and turbulent intensity Tu= 0.053

Figure 6 pressure distribution on the surface of the small square cylinder at Re=6500 and Tu=0.053

Figure 7 Dynamic pressure distribution in channel show the effect of square cylinder at Re=6500, Tu=0.053



Figure 8 the mean streamwise velocities compound in plane of symmetric at Z=0, Re=4200 and Tu=0.053

Figure 9 Distribution of velocity components and turbulent quantities .

Figure 10 The velocity vector distribution around square cylinder Re=6500 and Tu=0.053



Figure 11 The path line of velocity around the square cylinder. Re=6500 and Tu=0.053

Figure 12 The particle path line around the square cylinder at Re=6500 and Tu=0.053

Figure 13 Nusselt number distribution along the floor wall of a channel without a square cylinder at Re=4200 ,Tu=0.056

0

50

100

150

200

-7 3 13 23

Nu

x

Nu



Figure 14 Nusselt number distribution along wall floor of a channel with effect of a square cylinder at Re=4200,T u=0.056

Figure 15 The Nusselt number, comparison between two cases with and without a square cylinder at Re=4200 and Turbulent intensity Tu=0.053

Figure 16 Surface heat flux on heat wall with a the square cylinder at Re=4200

0

50

100

150

200

-7 3 13 23N

u

X

Nusselt number with square

cylinder

0

50

100

150

200

-7 3 13 23

Nu

X

Nu Without square Cylinder



Figure 17 The Nusselt number (Nu) couture on the floor wall of a channel with in a square cylinder at Re=4200 ,Tu=0.056

Figure 18 The Nusselt number (Nu) distribution with different Reynolds Number (Re),Re=4200,Re=5500 and Re=7000.

Figure 19 Stanton number (St) distribution on square cylinder at Re=6500 and Tu=0.053.

0

50

100

150

200

250

300

0 5 10 15 20

Nu

X

Re=4200 Re=5500

Re=7000



Figure 20 The distribution of wall sheer stress along the walls of square cylinder.

5. CONCLUSION The conclusion from Numerical and experimental results for flow field structure and the heat transfer distribution, the following conclusions were summarized:

1. Effect of a square cylinder creates a separation and recirculation zone near the wall and reattachment. The position of separation from the leading edge for all rang of Reynolds number

2. The vortex phenomenon occurred by increasing the Reynolds number, Also observed when increasing Reynolds number (Re) wake vortices change from

elongated vortices to shorter vortices 3. The install of square cylinder creates vorticity very high in wake region and the

two large vortices in the wake behind of square cylinder and decreasing the thickness of wall boundary layer of channel wall.

4. The very effect of the square cylinder on the formation of vorticity around the cylinder.

5. The effect of vorticity extension on the flow near the wall of the channel. That enhancement in heat transfer rate from the floor wall of channel.

6. Increase in Nusselt number(Nu) distribution along the floor of a channel. 7. It was found installation a cylinder in the middle of a channel lead to increasing in

Nusselt number, reach to the maximum value 36% in downstream of the reattachment point.

8. The Nusselt number (Nu) in small square cylinder increasing with increasing in the value of Reynolds number (Re).

9. Install of square cylinder increasing heat transfer higher than installing circular cylinder

10. This work can be widened in taking into account the geometric configuration such as cylinder dimension.



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