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Radiator Thermal Performance in a Power Transformer with Fans

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The present simulation validate the article “CFD Study on Thermal Performance of Radiators in a Power Transformer: Effect of Blowing Direction and Offset of Fans“.

 

This ANSYS Fluent project includes CFD simulation files and a training movie.

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Description

Paper Description

The present problem concerns the simulation of heat transfer and airflow around a radiator. The present simulation is based on the reference article”CFD Study on Thermal Performance of Radiators in a Power Transformer: Effect of Blowing Direction and Offset of Fans” using ANSYS Fluent software. Also, the results are compared and validated with the results of the reference paper. This radiator has three fans for transmitting airflow in the horizontal direction.

This radiator consists of several rows of fins and aluminum plates that increase the heat transfer rate. Hot water flows through the upper and lower pipes of the radiator and its middle fins, which is not modeled; In fact, it is assumed that all the walls of the pipes and radiator fins have determined temperature at different points. Therefore, the temperature profile depends on the height of the radiator plates. To define this temperature profile, the TEMPERATURE boundary condition applying a UDF is used.

This temperature profile of the plates, on which the UDF is written, is obtained in such a way that the temperature of the hot water entering the radiator from the upper part is initially assumed to be 366.15 K, which after flowing into the pipes and downward moving, heat exchange with the ambient air flow, and reaches its lowest temperature value of 353.15 K in the lower part. These temperature changes are mainly due to the movement of the flow in the vertical direction, therefore depends on the Y position. The mentioned UDF is written linearly in the form 346.15 + 5 * y.

Also, to apply the effect of the fans, the fan boundary condition is used in the plates located in the middle of the side of the radiator. Since the input flow to these fans is in the horizontal direction, the rotational speed around the x-axis is considered. The flow in the fans is assumed to be suction by default, but in this model, the fan flow must be blowing, so Reverse Fan Direction is activated to change the direction of air flow in the fan. Also, the amount of pressure jump for each fan is defined as a polynomial pressure function in terms of velocity; Thus, the polynomial function has two coefficients equal to 80 and -10.

Radiator and Fan Geometry & Mesh

The present modeling has been done in three dimensions using Design Modeler software. The present model consists of three main parts, including the radiator body, fans and the surrounding space. The radiator consists of four sections, each of which consists of fourteen rows of plate fans and two hot water transfer pipes at the top and bottom of the radiators. Also, three fans have been used to create air flow next to the body of the radiators; The location of the fans in the model of horizontal radiators is on the left side of the radiator and creates air flow in the horizontal direction.

The following figure shows a diagram of a radiator with horizontal fans.

Radiator

The meshing of the present model has been done using ANSYS Meshing software and the mesh type is unstructured. Also, according to the physics of the problem and the need for heat transfer between the radiator fins and the airflow, the INFLATION boundary layer mesh is used on the surfaces of the radiator walls. Also, the FACE SIZING command on the wall surfaces of the radiator fins is used to increase the mesh accuracy . The element number is equal to 4683472. the following figure shows the mesh.

radiator radiator radiator

Radiator and Fan 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 equal to -9.81 m.s-2 along the Y-axis. Because in this case, in addition to investigating the effect of the forced convection from the fans, the effect of free convection due to the buoyancy forces has also been investigated.

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

Models (radiator)
k-epsilonViscous model
Standardk-epsilon model 
RNGNear wall treatment
on Energy
Boundary conditions (radiator)
pressure inletInlet type
0 Pagauge total pressure 
323.15 Ktotal temperature
Pressure outletOutlet type
0 Pagauge pressure 
323.15 Kbackflow total temperature
wall Walls type
UDFtemperature for radiator’s walls 
0 W.m-2heat flux for fan’s walls
  Fan
x : -1zone average direction 
polynomialPressure jump 
Solution Methods (radiator)
Coupled Pressure-velocity coupling
StandardpressureSpatial discretization
First order upwindmomentum
Second order upwindenergy
First order upwindturbulent kinetic energy
First order upwindturbulent dissipation rate
Initialization (radiator)
StandardInitialization method
323.15 Ktemperature 

Paper Validation

At the end of the solution process, the net rate of heat loss from the body of the radiator plates to the airflow was calculated and compared with the reference paper. To make this comparison, Figure 5 of the article and the ONAF mode (with forced convection heat transfer) are used. It can be assumed that the rate of heat loss from the radiator body to the surroundings is equal to the air temperature difference between the inlet and outlet of the computational space for air flow.

Thus, by using the Report and selecting the Total Heat Transfer Rate option from the Flux Report section, the difference in heat transfer rate between the input and output boundaries can be calculated. This validation is shown in the following table and figure.

boundarytotal heat transfer rate (W)
Inlet161919.94436
Outlet-217923.87695
net-56003.93

radiator

Also, after the completion of the solution process, two-dimensional and three-dimensional contours related to temperature, velocity, and pressure, as well as three-dimensional path lines and velocity vectors are obtained.

 

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.

To order your ANSYS Fluent project (CFD simulation and training), contact our experts via info@mr-cfd.com, online support, and WhatsApp.

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