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7.21.2 User Inputs for Radiators

Once the radiator zone has been identified (in the Boundary Conditions panel), you will set all modeling inputs for the radiator in the Radiator panel (Figure  7.21.1), which is opened from the Boundary Conditions panel (as described in Section  7.1.4).

Figure 7.21.1: The Radiator Panel
figure

The inputs for a radiator are as follows:

1.   Identify the radiator zone.

2.   Define the pressure loss coefficient.

3.   Define either the heat flux or the heat transfer coefficient and radiator temperature.

4.   Define the discrete phase boundary condition for the radiator (for discrete phase calculations).



Identifying the Radiator Zone


Since the radiator is considered to be infinitely thin, it must be modeled as the interface between cells, rather than a cell zone. Thus the radiator zone is a type of internal face zone (where the faces are line segments in 2D or triangles/quadrilaterals in 3D). If, when you read your grid into FLUENT, the radiator zone is identified as an interior zone, use the Boundary Conditions panel (as described in Section  7.1.3) to change the appropriate interior zone to a radiator zone.

Define $\rightarrow$ Boundary Conditions...

Once the interior zone has been changed to a radiator zone, you can open the Radiator panel and specify the loss coefficient and heat flux information.



Defining the Pressure Loss Coefficient Function


To define the pressure loss coefficient $k_L$ you can specify a polynomial, piecewise-linear, or piecewise-polynomial function of velocity, or a constant value.

Polynomial, Piecewise-Linear, or Piecewise-Polynomial Function

Follow these steps to set a polynomial, piecewise-linear, or piecewise-polynomial function for the pressure loss coefficient:

1.   Choose polynomial, piecewise-linear, or piecewise-polynomial in the drop-down list to the right of Loss-Coefficient. (If the function type you want is already selected, you can click on the Edit... button to open the panel where you will define the function.)

2.   In the panel that appears for the definition of the Loss-Coefficient function (e.g., Figure  7.21.2), enter the appropriate values. These profile input panels are used the same way as the profile input panels for temperature-dependent properties. See Section  8.2 to find out how to use them.

Figure 7.21.2: Polynomial Profile Panel for Loss-Coefficient Definition
figure

Constant Value

To define a constant loss coefficient, follow these steps:

1.   Choose constant in the Loss-Coefficient drop-down list.

2.   Enter the value for $k_L$ in the Loss-Coefficient field.

Example: Calculating the Loss Coefficient

This example shows you how to determine the loss coefficient function. Consider the simple two-dimensional duct flow of air through a water-cooled radiator, shown in Figure  7.21.3.

Figure 7.21.3: A Simple Duct with a Radiator
figure

The radiator characteristics must be known empirically. For this case, assume that the radiator to be modeled yields the test data shown in Table  7.21.1, which was taken with a waterside flow rate of 7 kg/min and an inlet water temperature of 400.0 K. To compute the loss coefficient, it is helpful to construct a table with values of the dynamic head, $\frac{1}{2}\rho v^2$, as a function of pressure drop, $\Delta p$, and the ratio of these two values, $k_L$ (from Equation  7.21-1). (The air density, defined in Figure  7.21.3, is 1.0 kg/m $^3$.) The reduced data are shown in Table  7.21.2 below.


Table 7.21.1: Airside Radiator Data
Velocity (m/s) Upstream
Temp (K)
Downstream
Temp (K)
Pressure
Drop (Pa)
5.0 300.0 330.0 75.0
10.0 300.0 322.5 250.0
15.0 300.0 320.0 450.0


Table 7.21.2: Reduced Radiator Data
v (m/s) $\frac{1}{2}\rho v^2$ (Pa) $\Delta p$ (Pa) $k_L$
5.0 12.5 75.0 6.0
10.0 50.0 250.0 5.0
15.0 112.5 450.0 4.0

The loss coefficient is a linear function of the velocity, decreasing as the velocity increases. The form of this relationship is


 k_L = 7.0 - 0.2 v (7.21-9)

where $v$ is now the absolute value of the velocity through the radiator.



Defining the Heat Flux Parameters


As mentioned in Section  7.21.1, you can either define the actual heat flux $(q)$ in the Heat Flux field, or set the heat transfer coefficient and radiator temperature $(h, T_{\rm ext})$. All inputs are in the Radiator panel.

To define the actual heat flux, specify a Temperature of 0, and set the constant Heat Flux value.

To define the radiator temperature, enter the value for $T_{\rm ext}$ in the Temperature field. To define the heat transfer coefficient, you can specify a polynomial, piecewise-linear, or piecewise-polynomial function of velocity, or a constant value.

Polynomial, Piecewise-Linear, or Piecewise-Polynomial Function

Follow these steps to set a polynomial, piecewise-linear, or piecewise-polynomial function for the heat transfer coefficient:

1.   Choose polynomial, piecewise-linear, or piecewise-polynomial in the drop-down list to the right of Heat-Transfer-Coefficient. (If the function type you want is already selected, you can click on the Edit... button to open the panel where you will define the function.)

2.   In the panel that appears for the definition of the Heat-Transfer-Coefficient function, enter the appropriate values. These profile input panels are used the same way as the profile input panels for temperature-dependent properties. See Section  8.2 to find out how to use them.

Constant Value

To define a constant heat transfer coefficient, follow these steps:

1.   Choose constant in the Heat-Transfer-Coefficient drop-down list.

2.   Enter the value for $h$ in the Heat-Transfer-Coefficient field.

Example: Determining the Heat Transfer Coefficient Function

This example shows you how to determine the function for the heat transfer coefficient. Consider the simple two-dimensional duct flow of air through a water-cooled radiator, shown in Figure  7.21.3.

The data supplied in Table  7.21.1 along with values for the air density (1.0 kg/m $^3$) and specific heat (1000 J/kg-K) can be used to obtain the following values for the heat transfer coefficient  $h$:


Velocity (m/s) $h$ (W/m $^{2}$-K)
5.0 2142.9
10.0 2903.2
15.0 3750.0

The heat transfer coefficient obeys a second-order polynomial relationship (fit to the points in the table above) with the velocity, which is of the form


 h = 1469.1 + 126.11 v + 1.73 v^2 (7.21-10)

Note that the velocity $v$ is assumed to be the absolute value of the velocity passing through the radiator.



Defining Discrete Phase Boundary Conditions for the Radiator


If you are modeling a discrete phase of particles, you can set the fate of particle trajectories at the radiator. See Section  22.13 for details.


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