Louver/Grille characterisation

Loss Coefficient for a Hexa-Grille:

Objective: To design and analyze a flow domain that includes a part of Hexa-Grille inside it. Here we are set to study the parametric study, trials of the parameter, and primary and compound functions that we need to report.

Introduction:

Simulations of electrical equipment, enclosed electronics components, and electrical systems are typical applications where grilles are used.

The grille loss coefficient is a parameter whose value is a function not just of the free area ratio, but also of other parameters such as grille design, Reynolds number, grille thickness and flow regime (laminar or turbulent).

Icepak has an automatic mode that calculates the loss coefficient as a function of the free area ratio. However, this calculation is based on empirical correlations in the fully turbulent flow regime.

Since most applications are for natural convection in the transitional flow regime it is important to develop a methodology to take into account the appropriate flow regime.

Hexa-Grille problem description:

The model includes a cabinet that is 160 mm in length with inlet and outlet openings at the two ends (with a cross-sectional area of 7.363 mm x 12.7 mm), and four symmetry walls at the other sides.

The model also includes a part of the Hexa-grille placed at the center of the channel in the streamwise direction.

The grille has one full hexagonal hole at the center and four quarter hexagonal holes placed around it. This pattern was selected because it forms a periodic region and is sufficient to calculate the loss coefficient.

• So from the above figure, we are considering only a single part of the whole Hexa-grille structure into a flow domain.

Geometry:

• Firstly we need to create a cabinet after creating a new project in the Icepak.
• By creating a new project in the Icepak, it defaults gives us a cabinet to perform the simulation. To that cabinet, we need to do the necessary changes as per our requirements.
• The cabinet or flow domain in our case has to be a length of 240 mm. The height and width of the cabinet are the same as the Hexa-grille we are creating.
• The actual dimensions for the model of Hexa-grille is 7.363 mm x 12.7 mm x 1 mm.

• In the above figure, it shows the openings we have given to the cabinet and the vertices of the Hexa-grille.
• The Hexa-grille looks like:

• After giving openings and the symmetric walls for all the four sides of the cabinet, we need to provide the parametric function to the flow.
• We need to give a parametric function for the flow in the Min-Z direction.

Defining Reynolds number:

• Define a velocity parameter at the inlet opening in terms of the Reynolds number Re.
• The velocity at the inlet opening in terms of the Reynolds number Re, which is customarily
  used in loss coefficient plots in lieu of velocity, is calculated as `Re = (rho*v*l)/mu`

Where Re = Reynolds number

Rho = density of air

v = velocity of air

L or l = characteristic length

mu = Dynamic viscosity

Here, L = D_h = 4A/P

D_h = (4*(12.7*7.363)*1e-6) / ((2*12.7+2*7.363)*1e-3)

D_h = 0.009321 m

D_h = 9.322e-3 mm.

`Re = (rho*v*l)/mu = (vl)/nu`

`Re = (vl)/nu`

l = D_h

`Re = (vD_h)/nu`

`(Re*nu)/D_h = v`

• In the Cabinet, we need to give the parametric function to one of the openings, i.e., cabinet_default_side_minz in the model manager.
• Select the Z-velocity as we kept the opening in the Z-direction, and set the value to "$Re*1.84e-5/9.322e-3"

• After clicking done, a dialogue box will appear to ask the initial value of Re, and we need to give an initial value of Re as 10.
• Now we need to give another 8 Re variables in the Run optimization -> Setup -> select parametric trials -> go to -> Design variables -> give the required values in the Discrete values box as " 10 50 100 500 1000 2000 3000 4000"
• Note: The gap between the numbers must be maintained.
• So the changes we made looks like:

• The above figure represents the total number of trials we are going to run in the simulation.
• It initially start with the Reynolds number 10, and keeps on calculating all the variables and end up to the variable 4000.

Defining the report that includes pressures and velocities:

• We need to first create the variables in the summary report to calculate the pressure and velocity which are also visible in the domain of the primary and compound function.

1. In the Define summary report panel, click New
2. In the Objects drop-down list, select cabinet_default_side_minz and click Accept.
3. In the Value drop-down list, select UZ
4. Now we need to create new parameters for the other opening also: for that
5. Repeat steps (1) and (2), then select Pressure in the Value drop-down list.
6. Then repeat the steps 1 and 4 for the Cabinet_default_side_maxz
7. Click the Close button to accept the settings and close the panel.
8. The summary report we set up looks like as:

• Now go to Run optimization -> Functions. Here we need to provide the inputs for the primary functions, those we have set up in the summary report.
• Here in the primary function, we need to create four definitions. Which are: Static pressure in and out, and velocity in and out.
• We add the definitions by clicking the New button there.

• For the Compound functions, the same as primary function click the New button to create the definitions.
• Here in the compound function, we need to create and find the Dynamic pressure in and out (Pdyn_in, Pdyn_out) and the total pressure in and out (Ptot_in, and Ptot_out), and as well as the K loss factor.

For Dynamic pressure inlet: (`(1/2*rho*v^2)`)

• 0.5*1.16*$Vel_in*$Vel_in

For Dynamic pressure outlet: (`(1/2*rho*v^2)`)

• 0.5*1.16*$Vel_out*$Vel_out

For Total pressure inlet:

• $Pstat_in+$Pdyn_in

For Total pressure outlet:

• $Pstat_out+$Pdyn_out

For K_loss:

• ($Ptot_in-$Ptot_out)/$Pdyn_out

• Now, click Done to save and close the parameters and optimization dialogue box.
• After completing the mesh settings, click the Run button in the above to calculate or start the solution.

Generate Mesh:

• Here we generated a Non-conformal mesh with the mesh assemblies seperatley option.
• As we created the assembly for the blocks, means for the single Hexa-grille, we have to give the slack values in the Z-direction.
• Particularly in Z-direction for is, our interest in finding flow domain is in Z-direction and if we provide the slack values to X and Y-directions the assembly can go outside of the cabinet.
• So, we only give the slack values in the Z-direction.

• For the even better refinement near the walls of the Hexa-grille, we have incorporated the object parameter values also.

• Now using these settings in the meshing parameters, we can generate the mesh using the Mesher-HD type with the Normal mesh.

• The mesh generates 628448 number of elements and 686315 number of nodes to form an acceptable mesh.
• The below figure is the formation of the Mesh in the Z-direction, which is the flow direction in the flow domain.

• The mesh in the Y-direction look like:

• The solid type of mesh object Hexa-grille is, in this, we can see the mesh lines on the top side is matching with another side of the solid body.

Numerical Settings:

• Keep all the settings like basic parameters, basic settings, iteration numbers, converge criteria, and advance settings as it is.
• So, the flow regime can automatically change form laminar when the Reynolds number is calculated until 2000 to Turbulence when the Reynolds number is calculated for 3000 and 4000.

Residuals:

• Here the each and every oscillation in the residual plot describes that the solution for Re 10, 50, 100 .... is converged and started calculated for other Re's.

Results:

• The results for the parametric study for the variable Reynolds number is given in a table. The table contains results for all the functions that we have created.

• Here from the above table, we can clearly see that the increase in the Reynolds number increases the value of the velocity and as well as the pressure also.
• The above table is called as the summary report of the optimized data of the variables.
• The summary report of the primary functions are:

• Here in this table, we can see the max and min velocity and pressures at bothy openings of the flow domain or cabinet.
• The min and max velocity at the inlet opening is 7.89 m/s, and the pressure is 23.36 N/m2.
• The min. velocity at outlet opening is 4.99 m/s and max. velocity is 9.82 m/s.
• The Max. pressure at outlet is almost equal to 0 N/m2. (actual is 0.0058N/m2) means we have a normal or ambient pressure is located in that region.
• If we see the pressure, there will be a massive drop in the pressure. Any type of obstruction, restriction, or roughness in the system will cause resistance to airflow and cause a pressure drop. ... The maximum pressure drop from the supply side to the points-of-use will occur when the compressed air flow rate and temperature are highest.

K loss:

• The loss coefficient (ζ) is a dimensionless number (characteristic coefficient) to calculate the head loss (HL).
• `H_L = zeta*(v^2/(2g))`

Where v = Characteristic flow velocity in the relevant hydraulic component (usually the flow velocity in the cross-section of the connection downstream of the component)

g = Acceleration due to gravity

Zeta = loss coefficient

• Plot the loss coefficient, Kfact, against the Reynolds number, Re.
• In the Parametric trials panel, click the Plot button to open the Selection panel.
• In the Selection panel, select Re as the 􀀀 axis variable, and click Okay.
• In another Selection panel which automatically opens up, select Kfact as the y-axis variable, and
  Click Accept.

• After selecting the Re in x-axis and K_loss in the y-axis, the plot will appear as:

• From the above plot, we can say that that as the Reynolds number is increasing the loss coefficient is also decreasing.
• After it reaches the turbulence area the K_loss becomes almost stagnant which changes at the rate of 1e-3.

Velocity:

• The max. velocity recorded inside the flow domain is 12.31 m/s.
• In the figure, we can see how the velocity and flow are behaving on the walls of the Hexa-grille and in the neighboring area.
• We can see how the velocity is increasing from the walls of the Hexa-grille to the edges and then in the flow domain area.

Velocity Animation: https://youtu.be/UpPmfd4kKi4

Loss coefficient of Louver:

Louver Plate Kits are designed to provide ventilation for areas where excessive moisture or internal heat is present. They are easily installed by making a cutout of the proper size and attaching the louver plate to the enclosure.

Problem Description:

Here we need to deal with the K_loss values that are obtained from the Hex-grille simulations. We need to include the Loss curve instead of loss coefficients in the Grille parameters.

In the previous simulation, we have taken the loss coefficient a single value commonly. But in this, we have to load or edit (means give the values of loss coefficients) manually.

Geometry:

• By opening the Icepak, click on the Existing project and load the given CAD model into the Icepak.
• The dimension of the cabinet we set is 1200 mm * 500 mm * 1300 mm. In this domain, only the geometry is fitted.

• The above figure shows the openings, grilles, and heat sink, etc,. are shown.
• The louver geometry looks like:

• After giving openings and the symmetric walls for all the four sides of the cabinet, we need to provide the parametric function to the flow.
• We need to give a parametric function for the flow in the Min-Y direction.

• In the Cabinet, we need to give the parametric function to one of the openings, i.e., cabinet_default_side_minY in the model manager.
• Here we need to give the input for the velocity as a function of the Reynolds number Re.

Note:

• Giving the parameter values, primary function and compound function entries are same as the above Hexa-grille simulation.
• One of the changes for the simulation is the flow direction in this simulation is changed to Y-direction in the flow domain.
• Another change is, we are using the loss coefficient values that are obtained in the Hexa-grille simulation in this simulation by using the Text editor and the variables of the of Reynolds number are also the same as 10 50 100 .....

• We have changed the flow direction from Z to Y. So, we need to change the direction in the summary report also:

Generate Mesh:

• Here we generated a Non-conformal mesh with the mesh assemblies seperatley option.
• As we created the assembly, the slack values have been given to the each assembly separately for the components.
• The slack values given for both the assemblies are as:

• The object parameters are given for one of the assembly and for the grille. Because the assembly is non-continued and the grille is a circular grille.
• For the circular grille, we need to provide the diameter count.

• By using all these parameters we have done the non-conformal mesh with the slack values and object parameters applied to it.

• A mesher-HD type mesh is used with the fine mesh type to generate the mesh in the Cabinet and on the geometry.
• The max. elements used are 15 mm * 20 mm * 15 mm and the Min. elements used are 0.1 mm * 0.1 mm * 0.1 mm.
• The mesh generates a 1040968 number of elements and a 1095088 number of nodes to form an acceptable mesh.
• The below figure is the formation of the Mesh in the Y-direction, which is the flow direction in the flow domain.

• Here we can see the Non-conformal mesh that applicable to the Louvers, heat sink and other components in the geometry.
• The non-conformal meshing reduces the meshing time in the simulation and enhances the convergence criteria of the solution.
• The solid type of mesh object Hexa-grille is, in this, we can see the mesh lines on the top side is matching with another side of the solid body.

Numerical Settings:

• Keep all the settings like basic parameters, basic settings, iteration numbers, converge criteria, and advance settings as it is.
• So, the flow regime can automatically change form laminar when the Reynolds number is calculated until 2000 to Turbulence when the Reynolds number is calculated for 3000 and 4000.

Loss Coefficient:

• The loss coefficient (ζ) is a dimensionless number (characteristic coefficient) to calculate the head loss (HL).
• `H_L = zeta*(v^2/(2g))
• Where v = Characteristic flow velocity in the relevant hydraulic component (usually the flow velocity in the cross-section of the connection downstream of the component)

  g = Acceleration due to gravity

  Zeta = loss coefficient

• In the previous simulation, we have taken a single loss coefficient value to calculate the pressure loss specifications.

• But for now, we are using the loss curve for the pressure loss specifications. So that we can specify a set of inputs to calculate the loss coefficient.
• The loss curve can be implemented in two ways, we have to had a set of data and the curve of that data.
• The first method is by graphical, and the second method is by the text editor. Here I choose the text editor method.

• A total of 8 values as a set we have given for the 8 different Reynolds number variables.

Residuals:

• Here the each and every oscillation in the residual plot describes that the solution for Re 10, 50, 100 .... is converged and started calculated for other Re's.

Results:

• The results for the parametric study for the variable Reynolds number is given in a table. The table contains results for all the functions that we have created.

• Here from the above table, we can clearly see that the increase in the Reynolds number increases the value of the velocity and as well as the pressure also.
• The above table is called as the summary report of the optimized data of the variables.
• The summary report of the primary functions are:

• The pressure for the outlet shows the negative and Zero because the size of the cabinet is big as compared to the geometry.
• At the inlet the pressure is situated, this pressure is developed because of the flow which is coming into the cabinet.
• Any type of obstruction, restriction, or roughness in the system will cause resistance to airflow and cause a pressure drop. ... The maximum pressure drop from the supply side to the points-of-use will occur when the compressed air flow rate and temperature are highest.
• The K_loss vs Re number graph looks like:

• Here if we can see the k_loss, it is almost the same with the error of +/- 0.1. The highest loss coefficient is recorded at the Reynolds number 2000 and almost the same for the Reynolds number 3000 with a slight change.
• This close phenomenon is because of loading the different loss curve values in the pressure loss specifications instead of giving a single free air ratio value.

Velocity:

• The max. velocity recorded inside the flow domain is 9.46 m/s and the min. velocity is 0 m/s.
• In the figure, we can see how the velocity and flow are behaving on the walls of the louvers and in the neighboring area.
• We can see from the above figure how the flow is divided and the how the wake region is forming behind the geometry.
• The reason for the formation of the wake region in the cabinet behind the geometry is because of the formation of the negative pressure in that region.

Velocity Animation: https://youtu.be/JgDSFMvLYvs

• The contour of the pressure looks like:

• That negative pressure behind the geometry is the main reason for the formation of the wake region.
• The max. pressure is obtained is on geometry, which means on the components of the geometry which is a natural phenomenon.
• The max. pressure attained in the cabinet is 41.98 N/m2.

Conclusion:

In this challenge, we used the parameterization tool to calculate the loss coefficient of a grille and louver for different values of Reynolds number (Re). We also defined other functions (e.g., static pressure and velocities at the inlet and outlet) that were reported for different Reynolds numbers. The results show that as Re increases, the loss coefficient decreases.

In the case of Louver, we also learned how to give the loss coefficient values manually in two types; one is a text editor and the other is graphical. In this, as the Reynolds number is increasing the difference in loss coefficient is too small (as +/- 0.1).