CFD Report Group 3

1

CCB 3033

Advanced Transport Process

May 2015 Semester

CFD SIMULATION OF HEAT EXCHANGE

EQUIPMENT

GROUP 3

TEAM MEMBERS:

Name ID

Muhamad Asyraf Bin Mohd Aris 17670

Hani Zarith Alia Binti Zaharudin 17516

Nor Nadiah Ahmad Hamidi 17154

Syed Amirul Alwi Bin Syed Mohd Zaki 17274

Due Date : 18 August 2015

2

TABLE OF CONTENTS

No. Title Page

1 Introduction about Heat Exchanger 3

2 Governing Equations and Simulation Method 5

3 Flow Regime in Heat Exchanger 8

4 Heat Transfer Coefficient 10

5 Results 12

6 Discussion 26

6 Conclusions 27

7 References 27

3

Introduction About Heat Exchanger

A heat exchanger is a device used to transfer heat between one or more fluids. The fluids

may be separated by a solid wall to prevent mixing or they may be in direct contact. There are

three primary classifications of heat exchangers according to their flow arrangement. For

efficiency, heat exchangers are designed to maximize the surface area of the wall between the

two fluids, while minimizing resistance to fluid flow through the exchanger.

Heat exchanger consists of heat transfer elements such as a core or matrix containing the

heat transfer surface, and fluid distribution elements such as headers, manifolds, tanks, inlet and

outlet nozzles or pipes, or seals. Usually, there are no moving parts in a heat exchanger;

however, there are exceptions, such as a rotary regenerative exchanger (in which the matrix is

mechanically driven to rotate at some design speed) or a scraped surface heat exchanger.

In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and

travel in parallel to one another to the other side. In counter-flow heat exchangers the fluids enter

the exchanger from opposite ends. The counter current design is the most efficient, in that it can

transfer the most heat from the heat (transfer) medium per unit mass due to the fact that the

average temperature difference along any unit length is higher. In a cross-flow heat exchanger,

the fluids travel roughly perpendicular to one another through the exchanger.

Types of heat exchangers:

1. Shell-and-Tube Exchanger

This exchanger, shown in Fig. 1.5, is generally built of a bundle of round tubes mounted

in a cylindrical shell with the tube axis parallel to that of the shell. One fluid flows inside

the tubes, the other flows across and along the tubes.

2. Plate Heat Exchanger

These exchangers are composed of many thin, slightly separated plates that have very

large surface areas and small fluid flow passages for heat transfer. Advances

in gasket and brazing technology have made the plate-type heat exchanger increasingly

practical.

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3. Plate and Shell Heat Exchanger

It combines plate heat exchanger with shell and tube heat exchanger technologies. The

heart of the heat exchanger contains a fully welded circular plate pack made by pressing

and cutting round plates and welding them together. It does completely without gaskets,

which provides security against leakage at high pressures and temperatures.

4. Plate Fin Heat Exchanger

Plate and fin heat exchangers are usually made of aluminium alloys, which provide high

heat transfer efficiency. The material enables the system to operate at a lower temperature

difference and reduce the weight of the equipment. Plate and fin heat exchangers are

mostly used for low temperature services.

5

Governing Equations

1. Convective heat flux equation

q=h(Text-T)

2. Reynolds number

Re = 𝑣𝐷

3. Continuity Equation

𝑆𝐷𝑅= distributed resistance

𝑆𝐷𝑅 = −(𝐾𝑖 +

𝑓𝑑

) 𝜌𝑉2𝑖

2− 𝐶𝑛𝑉𝑖

6

i=u,v and w momentum equation

f= friction factor

d = hydraulic diameter

C= permeability

Note : K-factor term operates on a single momentum equation

Sω= rotating coordinates

𝑺𝝎 = −𝟐𝝆𝝎𝒊 × 𝑽𝒊 − 𝝆𝝎𝒊 × 𝝎𝒊 × 𝒓𝒊

𝜔= rotational speed

r= distance from axis of rotation

4. Energy equation

7

Simulation Method

1. Open COMSOL.

2. Add study Stationary.

3. Add Physics.

a) Non-isothermal

4. Add materials - water to all boundaries.

5. Draw the geometry according to individual values.

6. In Non-Isothermal flow:

a. Add inlet with To=298 K and v=1.11 m/min.

b. Add outlet at boundary.

c. Add heat flux at circles.

7. Compute study.

8. Add study group to build.

a. Temperature profile and velocity field streamline in 2D.

b. Temperature and Velocity profile in revolve-3D (225o).

c. Isosurface plot for temperature.

d. Temperature distribution at different z position.

9. Do a trial and error between heat transfer coefficient and exit temperature.

10. Plot graph.

a. Trial and error process to determine heat transfer coefficient.

b. Relationship between heat transfer coefficient and T2.

8

Flow Regime in Heat Exchanger

Flow regime can be determined from the Reynolds number.

Reynolds number = Inertia force/ Viscous force

(ρvL)/μ

Where ρ = density of the fluid

v = velocity of the fluid

L = Length of the fluid inlet

μ = dynamic Viscosity of the fluid

[999.9(kg/m3)*0.11*105(m/s)*0.05(m)] / [0.896*103] Pa.s

6.26, which is in the limit of Laminar flow.

Hence, the flow regime can be considered as Laminar Flow.

17274

9

17154

17516

17670

10

Heat Transfer Coefficient

By using COMSOL, the heat transfer coefficient for the heat exchanger can be determined.

For the value of X = 0.11m, Y = 0.17m, at T2 = 111°C and v1 = 0.11m/min, the heat transfer

coefficient, h = 104 W/m2K.

For the value of X = 0.11m, Y = 0.156m, at T2 = 111°C and v1 = 0.11m/min, the heat transfer

coefficient, h = 104.

17274

17154

11

For the value of X = 0.16m, Y = 0.128m, at T2 = 111°C and v1 = 0.11m/min, the heat transfer

coefficient, h = 71.75.

For the value of X = 0.21m, Y = 0.128m, at T2 = 111°C and v1 = 0.11m/min, the heat transfer

coefficient, h = 75

17516

17670

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Results

ID: 17154; X = 0.11m, Y = 0.156m, T2=111OC, v1=0.11m/min

Figure 1 : Temperature profile and velocity field streamline in 2D

Figure 2: Temperature and velocity profile in revolve-3D (225degrees)

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Figure 4: Isosurface plot for temperature

Figure 3: Outlet temperature surface

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Figure 5: Temperature distribution at different z position

Figure 6: Relationship between heat transfer coefficient and T2

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Figure 7: Trial and error process to determine heat transfer coefficient

ID: 17670; X = 0.21m, Y = 0.128m, T2=111OC, v1=0.11m/min

Figure 3 : Temperature profile and velocity field streamline in 2D

Optimum Point

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Figure 4: Temperature and velocity profile in revolve-3D (225degrees)

Figure 3: Outlet temperature surface

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Figure 5: Temperature distribution at different z position

Figure 4: Isosurface plot for temperature

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Figure 6: Relationship between heat transfer coefficient and T2

Figure 7: Trial and error process to determine heat transfer coefficient

ID: 17516; X = 0.16m, Y = 0.128m, T2=111OC, v1=0.11m/min

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120

Ave

rage

d O

utl

et T

2

Heat Transfer Coefficient

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120

T A

vera

ge O

utl

et T

2

Heat Transfer Coefficient

Optimum Chart

Optimum Point

19

Figure 5 : Temperature profile and velocity field streamline in 2D

Figure 6: Temperature and velocity profile in revolve-3D (225degrees)

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Figure 4: Isosurface plot for temperature

Figure 3: Outlet temperature surface

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Figure 5: Temperature distribution at different z position

Figure 6: Relationship between heat transfer coefficient and T2

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Ave

rage

d o

utl

et

T2 (

de

gre

e c

els

ius)

Heat transfer coefficient

22

Figure 7: Trial and error process to determine heat transfer coefficient

ID: 17274; X = 0.11m, Y = 0.17m, T2=111OC, v1=0.11m/min

Figure 7 : Temperature profile and velocity field streamline in 2D

0

10

20

30

40

50

60

70

80

90

100

50 55 60 65 70 75 80

T_av

g_o

utl

et-

T2

Heat transfer coefficient

Optimum Chart

Optimum Point

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Figure 8: Temperature and velocity profile in revolve-3D (225degrees)

Figure 3: Outlet temperature surface

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Figure 5: Temperature distribution at different z position

Figure 4: Isosurface plot for temperature

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Figure 6: Relationship between heat transfer coefficient and T2

Figure 7: Trial and error process to determine heat transfer coefficient

Optimum Point

26

Discussion

From the figure of temperature profile and velocity field streamline in 2D, we can observe that

the inlet temperature is approximately 300K and increases up to 500K when it passes through the

heating coil. Maximum temperature is obtained near the heat flux generation region which is the

heating coil. As a much finer mesh is generated in that region, temperature profile is accurately

predicted.

From the velocity profile in revolve 3D, we can observe that the velocity is zero when

approaching the wall due to the no slip condition principle. Maximum velocity is obtained at the

center of the inlet which is 2x10-3

m/s. Velocity is obtained near the inlet and it gradually

decreased due to the fluctuations in the geometry. After achieving a uniform flow, the velocity

raised and headed to a high velocity near the outlet.

From the figure of outlet surface temperature, it can be observed that the temperature is

maximum at the center of the outlet which is 430 K while lowest near the wall which is 370 K.

The same observation can be made from the isosurface plot for temperature.

From the graph of average outlet T2 vs Heat transfer coefficient, we can observe that the

temperature is gradually increasing with the increase of heat transfer coefficient. From the trial

and error process, we can conclude that the optimum heat transfer coefficient is 104 W/m2K.

From the calculations performed, the flow regime is said to be laminar flow. Hence, the velocity

throughout the domain is between 0 to 1 m/s. With respect to the flow, the velocity is zero near

the walls which follows the no slip condition.

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Conclusion

When we compare the three different types of x (in meter) which is 0.11, 0.16, 0.21 given

to us to build the heat exchanger, we found that increase in the length of x (in meter) results in

decrease of overall temperature of the heat exchanger. This can be seen on the surface

temperature profile in results part. Moreover, decrease in x value, makes the velocity streamline

flows easily, shown on the Streamline Velocity field. In addition, the outlet temperature surface

shows higher temperature when x value (in meter) is greater.

Furthermore, when we look at the temperature distribution curve for all the three x

values, we can see that x = 0.11 gives a more consistent curve with less oscillations compare to

the other two values. This shows that the heat exchanger with x = 0.11 has a more stable

temperature distribution compared to the other two.

References

1. Heat Exchanger (n.d). Retrieved from https://en.wikipedia.org/wiki/Heat_exchanger