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Heat Exchanger Performance Improvement, ANSYS Fluent CFD Simulation Training Package (4 case-study)

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The present problem simulates several shell and tube heat exchangers in order to improve the performance, using ANSYS Fluent software.

This product includes Geometry & Mesh file and a comprehensive Training Movie.

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Description

Project Description

The present problem simulates several shell and tube heat exchangers in order to improve the performance, using ANSYS Fluent software. The shell and tube heat exchanger consists of a vertical cylindrical shell with seven rows of horizontal pipes and hot and cold water flows from two opposite directions. Heat transfer from the hot section to the cold section. Cold fluid with a flow rate of 0.005 kg.s-1 and a temperature of 283.15 K enters the heat exchanger tubes, and hot fluid with a flow rate of 0.01 kg.s-1 and a temperature of 343.15 K enters the heat exchanger shell. In this project, two methods have been used to enhance the rate of heat transfer.

Improvement Steps

First, changes in the geometric structure of the shell and tube heat exchanger have been applied to enhance heat transfer; In the first step of the project, a helical fin was used on the side of the heat exchanger shell, and in the second step of the project, several rows of vertical baffles were used on the side of the shell.

It should be noted that for accurate comparison between the results, the contact surface produced by the helical fin is considered equal to the sum of the contact surfaces of the vertical baffles. Then, the nanofluid material inside the shell of the heat exchanger designed in the previous steps is used instead of water flow. Thus, nanofluid material inside the heat exchanger with helical fin was used in the third step of the project. In the fourth step of the project, nanofluid material inside the heat exchanger with several rows of vertical baffles was used.

Nanofluids are fluids that carry a particular volume of solid particles on a tiny scale at the nanoscale, and these nanoparticles have high thermal conductivity. Their use enhances the heat transfer within the primary fluid. The nanofluid used in this project is Al2O3-water with 4% nanoparticles. The nanofluid used has a density equal to 1086 kg.m-3, a specific heat capacity equal to 4052 J.kg-1.K-1, a thermal conductivity equal to 0.719 Wm-1.K-1, and a viscosity equal to 0.000657 kg.m-1.s-1.

Geometry & Mesh

The present model is designed in three dimensions using Design Modeler software. The present model is related to a horizontal shell and tube heat exchanger. Its shell has a diameter of 40 cm and a length of 2.2 m, and seven rows of horizontal pipes inside this shell with a diameter of 75 mm are located. The inlet and outlet boundaries of the model are designed so that the hot and cold currents inside the heat exchanger are uneven. The structure of this heat exchanger has been changed in two ways; In the first case, a spiral barrier with four rounds inside the shell is used, and in the second case, ten rows of vertical baffles inside the shell are used. Also, the contact surfaces are equal in both cases, and therefore, the material used to design it will be the same.

Heat Exchanger

The meshing of the present model has been done using ANSYS Meshing software. The mesh type is unstructured, and the element number in the heat exchanger with the helical fin is equal to 853109 and in the heat exchanger with vertical baffles is equal to 859833.

Heat Exchanger

Heat Exchanger CFD Simulation

We consider several assumptions to simulate the present model:

  • We perform a pressure-based solver.
  • The simulation is steady.
  • The gravity effect on the fluid is ignored.

The following table represents a summary of the defining steps of the problem and its solution:

Models
Viscous k-epsilon
k-epsilon model realizable
near-wall treatment standard wall function
Energy On
Boundary conditions
Inlet (shell) Mass Flow Inlet
mass flow rate 0.01 kg.s-1
temperature 343.15 K
Inlet (tubes) Mass Flow Inlet
mass flow rate 0.005 kg.s-1
temperature 283.15 K
Outlet (shell) Pressure Outlet
gauge pressure 0 Pascal
Outlet (tubes) Pressure Outlet
gauge pressure 0 Pascal
Internal Walls Wall
wall motion stationary wall
thermal condition coupled
External Walls Wall
wall motion stationary wall
heat flux 0 W.m-2
Methods
Pressure-Velocity Coupling Coupled
pressure second order
momentum second order upwind
turbulent kinetic energy first order upwind
energy dissipation rate first order upwind
energy second order upwind
Initialization
Initialization methods Standard
gauge pressure 0 Pascal
velocity(x,y,z) 0 m.s-1
temperature (shell) 343.15 K
temperature (tubes) 283.15 K

Heat Exchanger Results

At the end of the solution process, three-dimensional contours related to changes in flow temperature inside the tube and flow temperature inside the shell, changes in velocity on the shell side, and pressure changes on the shell side are obtained. These contours are related to four modes: heat exchanger with helical fin and with water flow, heat exchanger with vertical baffles and with water flow, heat exchanger with helical fin and with the nanofluid flow, and heat exchanger with vertical baffles and with nanofluid flow.

Then the graph of Nusselt number changes and heat transfer coefficient on the side of the heat exchanger shell in all four cases are obtained. Also, the diagram of the changes of heat flux from the pipe side to the shell on the wall of the inner pipes is obtained in these four cases. Also, the average temperature of the fluid inside the shell and the inside of the heat exchanger tubes is obtained in these four cases.

The thermal analysis results show that the use of a helical fin helps strengthen the heat transfer process. Therefore, the construction of a heat exchanger with a helical fin is preferred to a heat exchanger with vertical baffles. The results also show that the use of nanofluids instead of water flow increases the rate of heat transfer; Because these nanoparticles injected into the base fluid have high thermal conductivity and, as a result, contribute more to the heat transfer between the fluid layers.

Heat ExchangerHeat ExchangerHeat ExchangerHeat ExchangerHeat Exchanger

You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.

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