Title CFD Simulation of Heat Exchanger Equipment Heat Exchanger Heat Transfer Heat Fluid Dynamics Computational Fluid Dynamics 987.8 KB 15
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1. In the Model Builder window>Geometry I, the geometry is built using 2 rectangles, 2

squares and 4 circles. (Take note of the coordinates of each geometry)

MATERIALS

1. In the Material Browser window, Built-In>Water is added to the entire model.

NON-ISOTHERMAL FLOW

1. In the Model Builder window, Model I>Non-Isothermal Flow is expanded.

 Inlet 1 assigned to boundary 2 (𝑈0 = 0.1𝑚/𝑠)

 Outlet 1 assigned to boundary 9 (𝑃0 = 0𝑃𝑎)

 Temperature 1 assigned to boundary 2 (𝑇0 = 293.15𝐾)

 Outflow assigned to boundary 9

 Heat flux assigned to boundaries 19 to 30. Inward heat flux is selected

(𝑞0 = ℎ(𝑇𝑒𝑥𝑡 − 𝑇))

2. The value of heat transfer coefficient is specified.

MESH

1. In the Model Builder window, Model I>Mesh>Free Triangular with the following setting:

 Maximum element size = 0.0137m

 Minimum element size= 3.3e-4

 Maximum element growth rate=1.3

 Resolution of curvature=0.3

 Resolution of narrow region=1

2. The boundary selected is the entire geometry excluding the 2 circles and 2 semi-circles.

RESULT

1. In the Model Builder window, a cut line 2D is constructed with the following setting:

𝑥 𝑦

Point 1 0 1.05
Point 2 0.05 1.05

STUDY

1. In the Model Builder window, Compute button is clicked.

RESULT

1. Back to Result again, Derived Values>Line Average>Evaluate

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2. This step is repeated for trial and error in order to get the desired value of heat transfer

coefficient.

4.0 Results and Discussions

4.1 Velocity field streamline in 2D

In both of the graphs above, Figure (a1) and Figure (a2) there are differences in terms of velocity flow

line as the dimensions for both the heat exchanger varies respectively. In figure (a1), the length of y (0.1)

which is much shorter compared to the y length (0.14) in figure (a2) causes the velocity stream of water

in both heat exchanger varies respectively. As observed, there are more dead zones in figure (a1) as the

differences in dimensions causes more dead zones towards the output nozzle as compared to figure (a2).

The reason behind the existence of the dead zones are due to more stagnation of water being formed in

figure (a1) compared to figure (a2). Apart from that, there are also more dead zones in figure (a1)

compared to figure (a2) at the input zones respectively. The heating coils in figure (a1) which is slightly

placed higher in the y axis compared to figure (a2) causes the streamline of the water to be obstructed.

The phenomenon of Vena Contracta actually occurs in the flows of water for both heat exchangers. This

causes more water to be stagnant; forming more dead zones as there will be a pocket or region where it

is protected from the flow of water. In short, the velocity flow of water in heat exchanger, figure (a2) is

much more efficient as there are less dead zones and the function of heat transfer would be more

efficient. This is due to the dimension of y which is shorter allowing more spaces for water to flow more

efficiently.

Figure

(a2)

X=0.1

Y=0.14

Figure

(a1)

X=0.14

Y=0.1

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Using the trial and error, we have finally found the heat transfer coefficient needed to operate both the

heat exchanger that has its own respective dimensions as stated in the graph. For Nelson’s results the

optimum heat transfer coefficient is found to be 540 W/(m
2
K) for the heat exchanger to operate while

for Vanessa’s results, the optimum heat transfer coefficient is found to be 647 W/(m
2
K).

5.0 Conclusion

In this project, we learned that by changing the dimensions of the heat exchanger, the heat transfer

coefficient changes. This shows that the heat transfer coefficient is directly affected by the width of the

slit and the diameter of the tube. The heat transfer coefficient needed for a process can be estimated

when the outlet fluid temperature is given. This can be done by the simulation of Non-Isothermal

Laminar Flow package in COMSOL Multiphysics with trial and error method. The outcome of this project

is that we are able to compare the velocity and temperature profile of heat exchange with different

dimensions (Nelson’s x=0.1, y=0.14; Vennesa’s x=0.14, y=0.1) at an outlet fluid temperature of 58℃.

In the comparison for velocity field streamline in 2D, it is found that the velocity flow of water in the

heat exchanger is more efficient when the length of y is longer as there are less dead zones which then

allow more spaces for water to flow more. As for the velocity profile in 3D, the phenomena Vena

Contracta is observed to have occurred in both heat exchangers. In this case, smaller diameter of the

heat exchanger will oozes out the water at a higher velocity.

Then, the comparison for temperature profile in revolved 2D and 3D also shows that the longer length

of Y enables more heat to be supplied to the flow through the output. In iso-surface comparison, it can

be seen that the heat flux for both heat exchangers are the same. However, a slight change can be

observed from the middle of the heat exchanger onwards. The distribution of heat is observed to be

longer and more concentrated towards the output nozzle. Here, it is again shown that the width of the

slit y affects the heat distribution in the heat exchanger.

A line graph of temperature plotted using COMSOL Multiphysics enables us to study the heat exchanger

from its top view. It is observed that the outer layer of the heat exchanger is mostly in blue colour while

the inner part towards the core is red in colour. This is due to heat loss to the surrounding atmosphere

from the core of the heat exchanger itself. The graph also shows that as arc length of the reactor

increases, heat decreases.