Radiator Thermal Performance in a Power Transformer with Fans
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“.
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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.
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 and Fan CFD Simulation
To simulate the present model, we consider several assumptions:
- 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, we also investigate the effect of free convection due to the buoyancy forces.
The following table presents a summary of the defining steps of the problem and its solution:
|RNG||Near wall treatment|
|Boundary conditions (radiator)
|pressure inlet||Inlet type|
|0 Pa||gauge total pressure|
|323.15 K||total temperature|
|Pressure outlet||Outlet type|
|0 Pa||gauge pressure|
|323.15 K||backflow total temperature|
|UDF||temperature for radiator’s walls|
|0 W.m-2||heat flux for fan’s walls|
|x : -1||zone average direction|
|Solution Methods (radiator)
|First order upwind||momentum|
|Second order upwind||energy|
|First order upwind||turbulent kinetic energy|
|First order upwind||turbulent dissipation rate|
At the end of the solution process, we calculate the net rate of heat loss from the body of the radiator plates to the airflow and compare it with the reference paper. To make this comparison, we use Figure 5 of the article and the ONAF mode (with forced convection heat transfer). We can assume 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, we can calculate the difference in heat transfer rate between the input and output boundaries. The following table and figure show the validation.
|boundary||total heat transfer rate (W)|
Also, after the completion of the solution process, we obtain two-dimensional and three-dimensional contours of temperature, velocity, and pressure, as well as three-dimensional path lines and velocity vectors.
There is a mesh file in this product. By the way, the Training File presents how to solve the problem and extract all desired results.