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Heat Recovery System with Counter and Cross Flow Exchangers

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This simulation is based on the data in the reference article “Numerical model and effectiveness correlations for a run-around heat recovery system with combined counter and cross flow exchangers” and the results are compared and validated with the results in the paper.

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Description

Paper Description

The present problem concerns the simulation of a plate panel of a cross-flow heat exchanger. This simulation is based on the data in the reference article “Numerical model and effectiveness correlations for a run-around heat recovery system with combined counter and cross flow exchangers” and the results are compared and validated with the results in the paper. This heat exchanger consists of two special flow channels; In this way, the air flow flows from one side of the central panel of the panel and the flow of solution, flows from the other side, but in the opposite direction to the air flow.

In general, this panel belongs to one of the two panels in the closed cycle of solution and air flow. The fluids used in the present model include air and ethylene glycol or (CH2OH) 2, although their thermophysical properties are manually defined in the fluent. Ethylene glycol is a colorless, odorless, low volatility, low viscosity material whose properties are defined as temperature-dependent polynomials. Also, since hot and cold flows are not integrated in this heat exchanger, there is no need to use a multiphase flow module, and on the other hand, a separating plate is used as an interface.

The dissolved fluid in the model has a temperature higher than the air flow and therefore the purpose of the problem is to cool the hot air flow outlet. The general purpose of the present problem is to study the fluid behavior and heat transfer of the flows and to investigate the performance efficiency of the heat exchanger.

Geometry & Mesh

The geometry of the present model is designed in three dimensions using Design Modeler software. The present model belongs to a heat exchanger panel with two flow paths; So that on the one hand, the flow of hot dissolved fluid and on the other hand, the flow of cold air move in the opposite direction. Exterior walls also act as insulation. The dimensions of the heat exchanger are 1 m * 0.5 m, from which the air flow enters and exits from a width of 0.5 m; While the flow of solution enters and exits from sections with a length of 0.1 m from the length of the heat exchanger.

Therefore, the ratio of the length of the inlet or outlet of the solution flow to the total length of the heat exchanger is 0.1. The following figure 1 shows a view of the geometry.

heat recovery

The meshing of the model has been done using ANSYS Meshing software and the mesh type is structured. The element number is 155000. The cells near the wall boundaries are smaller and more accurate. The following figure shows the mesh. heat recovery

CFD Simulation

To simulate the present model, several assumptions are considered:

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

A summary of the defining steps of the problem and its solution is given in the following table:

Models
LaminarViscous model
onEnergy
Boundary conditions
Velocity inletInlet type
2.731074 m.s-1velocityair
302.5 Ktemperature
0.026125 m.s-1velocityliquid
310 Ktemperature
Pressure outletOutlet type
0 Pagauge pressureair
0 Pagauge pressureliquid
wallWalls type
insulatedall outer walls
coupledall inner walls
Solution Methods
Simple Pressure-velocity coupling
second order upwindpressureSpatial discretization
second order upwindmomentum
second order upwindenergy
Initialization
HybridInitialization method

Paper Validation

At the end of the solution process, the efficiency of the heat exchanger is calculated and compared with the value obtained in the reference paper. This comparison and validation is based on the values in the diagram in Figure 11 of the article. This diagram shows the changes in the heat exchanger efficiency (Ԑ) in terms of the ratio xi / x0. This ratio xi / x0 is defined as the ratio of the length of the liquid flow inlet to the total length of the converter. According to the modeled geometry of the present numerical simulation, the value of this ratio in the current work is 0.1.

The relationships in the paper are used to calculate the heat exchanger efficiency. The amount of air and liquid inlet temperature was defined in the boundary conditions and the outlet temperature of the air and fluid was obtained using the software REPORT command and the Area Weighted Average method. It should be noted that the amount of heat transfer from the cold fluid to the hot fluid during the simulation process is equal to the amount of heat transfer from the hot fluid to the cold fluid.

heat recovery

 Present simulationPaper result
Effectiveness based on xi/x00.6650.716

Also, according to the relations and calculations mentioned above, the ratio of the heat capacity of the liquid to the air can be calculated and according to the diagram in Figure 19, the efficiency of the heat exchanger can be obtained in the amount of the mentioned ratio and be compared with the calculated efficiency according to the above relations.

heat recovery

 Present simulationPaper result
Effectiveness based on CL/CA0.6650.580

Results

Also, two-dimensional contours related to temperature, pressure and velocity have been obtained in two sections X-Z and Y-Z, as well as two-dimensional path lines.

All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.

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