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Fan Effect on Radiator Heat Transfer CFD Simulation

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Radiator is a device used to raise air temperature in an environment.

This product includes a CFD simulation and training files using ANSYS Fluent software.

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Description

Introduction to Radiator Performance

The radiator is a device to raise the air temperature in an environment. The working mechanism of these radiators is that the outlet hot water or outlet hot steam flows of the boiler, flow through the pipes and transfer heat to the radiator fins, increasing the temperature of these plates, and consequently increasing the temperature of the room temperatures. Radiators are usually made of aluminum, cast iron, and steel because they have very good thermal conductivity. We can divide radiators into two horizontal and vertical groups according to the location of the fans.

Problem Description of the Radiator

The present problem is to simulate the heat transfer and airflow around a radiator. There are three horizontal fans for forced convection. It consists of several rows of fins and aluminum plates that increase the rate of heat transfer. Inside the top and bottom tubes of the radiator and its middle fins pass the hot water flow, which is not included in the modeling; in fact, it is assumed that all the walls of the radiator tubes and fins have specified constant temperature as a profile. Therefore, the temperature profile is the function of the height of the plates.

The Assumption for Radiator CFD Simulation

There are several assumptions for the present simulation:

The simulation is PRESSURE_BASED.

We use STEADY STATE solver.

The Earth’s gravity effect in the Y direction is considered to be 9.81 ms-2. Because in this problem, the effect of the buoyancy flow also been investigated in addition to the force convection.

Geometry

The present 3-D model is done by design Modeler software. The present model consists of three main parts including radiator body, fans and ambient space. The radiator consists of four sections, each consisting of fourteen rows of fin plates and two hot water tubes above and below the radiators. Also, three horizontal fans are used to create airflow next to the radiator body, so that the fan locates in the horizontal direction on the left side and generates airflow for force convection.

Mesh

The unstructured mesh of the present model was performed by ANSYS Meshing software. Also, due to the physic of the problem and the need for heat transfer between the fins and the fluid flow, the boundary layer (inflation) mesh has been applied on the surfaces of all model walls, especially the radiator fin walls. Face sizing is also applied to the surfaces of radiator fin walls to increase mesh accuracy. The number of cells produced is 4683472.

radiator CFD Simulation

Summary of the steps to define and solve the problem are presented in the table.

Models
k-epsilon Viscous model
Standard k-epsilon model
RNG Near wall tratment
on Energy
Boundry conditions for radiator simulation
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
wall Walls type
udf temperature for radiator’s walls
0 W.m-2 heat flux for fan’s walls
Fan
x : -1 zone average direction
polynomial Pressure jump
Solution Methods for radiator simulation
Coupled   Pressure-velocity coupling
Satndard pressure Spatial discretization
First order upwind momentum
Second order upwind energy
First order upwind turbulent kinetic energy
First order upwind turbulent dissipation rate
Initialization
Standard Initialization method
323.15 K temperature

Boundary Condition

We define a plate in the middle of the cylinders by the name of the Fan. The middle panels having a fan boundary condition in all three fans. Since the input flow to these fans is horizontal, we assume the rotational speed around the x-axis. By the way, since the fan flow is suctioned by default, but in this model, the fan flow must be blown, the reverse fan direction is activated to change the direction of airflow in the fan. Also, the pressure jump value for each fan is defined as a polynomial pressure function in terms of velocity, so that the polynomial function has two coefficients of 80 and -10.

We define a temperature boundary condition using a UDF function for the walls of radiator plates or fins. The temperature profile of the plates on which we write the UDF function  is as follows:
The temperature of the hot water entering the radiator in the initial state is about 366.15 K which we assume to pass the plates and heat transfer with the ambient airflow around it reaches the lowest temperature of 353.15 K. These temperature changes are mainly due to the movement of the flow in a vertical direction and are therefore dependent on changes in the value of Y-direction. We write the UDF function linearly in the form of “346.15 + 5 * Y”.

However, the UDF function is as a temperature profile with respect to the height of the radiator plates and the maximum and minimum temperatures in the radiator are on top and bottom tubes, respectively.

Define desired outputs

One of the objectives of this problem is to calculate the net rate of heat dissipation from the body of the radiator plates to the ambient air, using the Report Definition and selecting the Total Heat Transfer Rate from the Flux Report section. It is possible to calculate the difference in heat transfer rate at the inlet and outlet boundaries. Also, we define the maximum velocity value in the total flow in space using the Report Definition and Volume Report option.

Reference Value

Since the purpose of the problem is to study the fluid behavior and heat transfer and to calculate the parameters defined only in the specific space of the airflow, we select the airflow around the radiator as the reference zone.

 

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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