Engineering

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Transfer of heat from one fluid to another is an important operation for most of the chemical industries. The most common application of heat transfer is in designing of heat transfer equipment for exchanging heat from one fluid to another fluid. Such devices for efficient transfer of heat are generally called Heat Exchanger.

Main picture

 

Problem Description:

Simulation of shell & tube Heat Exchanger with  parallel flow arrangement. Shell diameter 50 mm & tube diameter 10mm. A Simplified model with shell length 300 mm and 3 tubes was used. Shell sides fluid is heating oil Shell Thermia Oil B & tube side is cold water.

 

Methodology:

 

Method

 

Geometry:

Shell & Tube Heat Exchanger was modeled in Catia using Surface Workbench .Further imported and meshed in ICEM CFD.

geometry

    Figure 1: Heat Exchanger Model

Meshing:

Heat exchanger model is meshed using ICEM-CFD. Initially a relatively coarser mesh is generated. This mesh contains mixed cells (Tetra and Hexahedral cells) having both triangular and quadrilateral faces at the boundaries.

Care is taken to use structured hexahedral cells as much as possible. Later on, a fine mesh is generated. Finer mesh is generated by refining parameters like scale factor, Maximum element  near wall

When meshing for 3D heat transfer problems such as this we have to strike a balance between setup time, computational expense & numerical diffusion.

In this case the mesh is refined to an extent for downsizing the effect of Numerical Diffusion ( False Diffusion ) phenomenon. Numerical Diffusion is a dominant source of error in multidimensional problems as it is not a real diffusion. Case like that under discussion which is more convection dominated is highly susceptible to this error. It is also noted that Numerical Diffusion is inversely proportional to  resolution of mesh. Hence care is taken in our case to appropriately refine the mesh.

Meshing

    Figure 2: Meshed Model In ICEM CFD

Solution:

Problem Setup

The mesh is checked and quality is obtained. The analysis type is changed to Pressure Based type. The velocity formulation is changed to absolute and time to steady state.

Models

Energy is set to ON position. Viscous model is selected as “k-ε model (2 equations)."

Materials

The create/edit option is clicked to add water-liquid and copper to the list of fluid and solid respectively from the fluent database.

Material properties of shell and tube heatexchanger

 

Cell zone conditions

The parts are assigned as water and copper as per fluid/solid parts.

Boundary Conditions

Boundary conditions are used according to the need of the model. The inlet and outlet conditions are defined as mass flow inlet and outflow outlet. The walls are separately specified with respective boundary conditions. No slip condition is considered for each wall. The details about all boundary conditions can be seen in the table  as given below.

Boundary Conditions of shell and tube heatexchanger

 

Solution Methods
The solution methods are specified as follows:
• Scheme = Simple
• Gradient = Least Square Cell Based
• Pressure = Standard
• Momentum = Second Order Upwind
• Turbulent Kinetic Energy = Second Order Upwind
• Turbulent Dissipation Rate = Second Order Upwind


Solution Control and Initialization
• Under relaxation factors the parameters are
• Pressure = 0.3 Pascal
• Density = 1 kg/m3
• Body forces = 1 kg/m2s2
• Momentum = 0.7 kg-m/s
• Turbulent kinetic energy = 0.8 m2/s2
Then the solution initialization method is set to Standard Initialization whereas the reference frame is set to Relative cell zone.

Result & Discussion:

Contours

The temperature variations for parallel flow condition are obtained and are shown in figures below.  Temperature variation is quite evident at the inlet , outlet and various sections of the Shell & Tube.

Contour Plot of Shell

contour plot of shell

 

    Figure 3: Figure Shows temperature variation in shell.  It is observed that the temperature at inlet is around 523K which then drops

    to around 450k at outlet.

 

Contour Plot of Tube

contour plot of tube

Figure 4:
• It is observed that cold water enters the tube, heats up gradually and flows out in hot condition.
• It is seen that the temperature variation in tube 1 ( on the top), tube 2( in the middle), tube 3( at the bottom) is not the same.
• An important observation is the presence of a high temperature zone just below the hot oil inlet on the top surface of the tube 1.
• The temperature obtained at outlets is in the range of 375Kfor the tube1, 357K for tube 2 & 351 for tube 3.

 

Contour Plot of Mid Plane

Mid Plane

Figure 5: Figure Shows temperature variation in mid plane of shell and tube heat Exchanger.

 

Discussion:

The shell temperature decrease from the inlet boundary to the outlet boundary. It indicates the loss of heat which is used up to heat the water in the tubes.

In figure 4, the top tube can be seen with a high temperature area just below the inlet. It relates to the physical phenomenon where the hot oil stream will be impinging upon the top tube. Also the tubes below in the same vicinity does not show this area which indicates the flow field being uniformly distributed.

The heating could be further enhanced by utilizing baffles for directing and holding the oil flow at appropriate locations.