MRF project

Objective: The objective of this project is to provide guidelines and recommendations for the Moving Reference Frame (MRF) fan modeling technique in ANSYS Icepak for the given CAD model.

Problem Description:

An Icepak 3D fan object may sometimes behave differently from real fan behavior. Typically this is due to flow features like swirl and centrifugal spread resultant from the spinning fan rotor as well as the rotor blade geometry. In an Icepak fan-based setup, this swirl is estimated and calculated off a 2D annular flow face rather than off the physical surfaces of the blade.

The MRF approach allows for a more accurate representation of the internal geometry of the fan and therefore results in better modeling of the flow features. In Icepak MRF simulations, the actual blade geometry is modeled as a CAD block object and a fluid cylinder rotates into contact with the surfaces to create a spinning flow.

To get good results we need to set up the problem properly and solution settings for an MRF model, good meshing techniques like meshing assemblies separately and multi-level meshing will give good results.

Introduction about Moving Reference Frame (MRF):

The MRF model is, perhaps, the simplest of the two approaches for multiple zones. It is a steady-state approximation in which individual cell zones can be assigned different rotational and/or translational speeds. The flow in each moving cell zone is solved using the moving reference frame equations. If the zone is stationary ( ), the equations reduce to their stationary forms. At the interfaces between cell zones, a local reference frame transformation is performed to enable flow variables in one zone to be used to calculate fluxes at the boundary of the adjacent zone.

It should be noted that the MRF approach does not account for the relative motion of a moving zone with respect to adjacent zones (which may be moving or stationary); the mesh remains fixed for the computation. This is analogous to freezing the motion of the moving part in a specific position and observing the instantaneous flowfield with the rotor in that position. Hence, the MRF is often referred to as the "frozen rotor approach."

While the MRF approach is clearly an approximation, it can provide a reasonable model of the flow for many applications. For example, the MRF model can be used for turbomachinery applications in which rotor-stator interaction is relatively weak, and the flow is relatively uncomplicated at the interface between the moving and stationary zones. In mixing tanks, for example, since the impeller-baffle interactions are relatively weak, large-scale transient effects are not present and the MRF model can be used.

Another potential use of the MRF model is to compute a flow field that can be used as an initial condition for a transient sliding mesh calculation. This eliminates the need for a startup calculation. The multiple reference frame model should not be used, however, if it is necessary to actually simulate the transients that may occur in strong rotor-stator interactions, the sliding mesh model alone should be used.

Limitations for MRF model:

• Translational and rotational velocities are assumed to be constant (time-varying , are not allowed).

• The relative velocity formulation cannot be used in combination with the MRF and mixture models. For such cases, use the absolute velocity formulation instead.

• You must not have a single interface between reference frames where part of the interface is made up of a coupled two-sided wall, while another part is not coupled (i.e., the normal interface treatment). In such cases, you must break the interface up into two interfaces: one that is a coupled interface, and the other that is a standard fluid-fluid interface.

Geometry:

• The Cabinet is created with a dimension of 730mm * 12mm * 180mm, the proper CAD model is set up in the cabinet.
• In the place of vents for the CAD model, we have replaced them with the grilles for the air intake and exhaust.
• Here we are not used the fan model which is given by the Icepak default.

• A perforated thin vent resistance type is used with 465741 of the free area ratio.
• The CAD model imported, it looked like as

Meshing:

• To obtain the non-conformal meshing, we have toggled the mesh assemblies separately option in the mesh control.

• A non-conformal mesh, when there are a partial or zero matchings of nodes at the interface. This means that one, few, or every node on one side of the interface doesn’t have a match on the other side of the interface.

• Non-conformal interface calculations are handled using a virtual polygon approach, which stores the area vector and centroid of the polygon faces.

• This approach does not involve node movement and cells are not necessarily water-tight cells. Hence gradients are corrected to take into account the missing face area.

• When the Mesh separately options for an assembly is turned on, the offset added to Assembly in respective direction is called Slack Values.

• By default, the slack values are zero in all directions. However, in order to use non-conformal mesh, the slack values should always be a positive number except it can be zero when the non-conformal boundary touches a hollow object or cabinet boundary.

• The Slack values should be specified in such a way that cells on both sides of the interface are either fluid or solid, with the same material, and it does not violate any of the "Non-Conformal Assembly Rules" given in the attachment.

• Here we generate the non-conformal mesh with the slack values, the generated elements are 1036121, and nodes are 1117613.
• A slack value of 3 and 5mm is used for the assemblies.

• Here the Max element size is taken as 4, which is the element is divided into firstly 2 by 2, then after 1 by 1 and then after finally 0.5 by 0.5.
• Multi-Level options help to capture objects of different scales with reduced mesh count.

• Here the max. levels are given as, for fan blades, it is 3 and for other components except for fan cases, the applied level is 2.
• The meshing for the CAD model in X-axis looked like:

• We can see here in this picture the non-conformal mesh is formed and at the same time, the elements in the mesh for the fan blades are closely attached which is a need parameter.

• This is a display of fan blade model mesh, shows how the meshing elements are plotted over the blades.
• The Mesh in Y-axis looks like:

Numerical setting:

Basic parameters:

• In the basic parameters where we have given whether the solution runs for temperature, flow, and setting up for the initial temperature as well as radiation temperature, etc.

• Here we are solving both the flow and temperature variables for the CAD model given.
• We are neglecting the radiation effect because we are interested in solving the Moving Reference Frame (MRF) model.
• The turbulent model we are calculating is the Realizable two-equation.

The realizable k-ɛ model differs from the standard k−ɛ model in two ways.

Firstly it contains a new formulation for the turbulent viscosity: Cμ is not a constant like in the standard model but a variable

The second difference is a new transport equation for the dissipation rate, ɛ, that is derived from an exact equation for the transport of the mean-square vorticity fluctuation.

As a result, it mainly gives improved predictions for the spreading rate of jets, a superior ability to capture the mean flow of complex structures and for flows involving rotation, boundary layers under strong adverse pressure gradients, separation, and recirculation.

• We are also less interested in the natural convection property because we are using the fans to circulate the air inside the CAD model which directly comes under the forced convection.

• Here in the fluid part in the CAD model which consists of the blades, we have to give the MRF rotation.
• We need to give the rotation speed of 6000 RPM for the MRF.

Basic Settings:

• Here in the basic settings panel, we need to give the number of iterations to 1000.
• The convergence criteria for the flow is 1e-5, and for energy, turbulent kinetic energy and dissipation rate the criteria is 1e-7.

Parallel settings:

• Here we have used the serial processors to calculate the solution in the simulation.

Advanced settings:

• Here in this panel, we give the inputs as the discretization schemes for the pressure, momentum, and temperature are of the first order.
• The single-precision is given. Single precision means that the floating-point numbers will be represented in a 32-bit system whereas double-precision means that they will be represented in a 64-bit system. So Calculation in double precision will be more accurate.

Residual plot:

• From the above residual plot, we have seen that the solution gets converged at nearly the iteration no.600. This can be understood by seeing the monitor points set for temperature and velocity.
• After the iteration no.600, the solution remains the same and we observe no instability in the residual plot.
• At the same time the velocity, continuity, and energy equation are well below the ranges we applied in the basic setting of iterations. Which means the solution gets very well converged.
• The disturbances in the epsilon curve represent the swirl and the turbulence phenomenon inside the CAD model because we have set the fan rotation to 6000 rpm and the free air ratio for the grille is 0.4 million.

Monitor points:

• Here we can see that the solution remains constant and unchanged after the iteration number 600.
• The max. temperature recorded is nearly 60 degrees centigrade and the least is 0.

• Here in the monitor points plot, we have seen that the velocity at the monitor points we set remains unchanged after the 600 iterations.
• The max. velocity recorded is 21 m/s inside the CAD model, but here in monitor points, it is below 8 m/s because it shows the velocity only at the monitor points.

Examine and post-process the results:

Temperature:

• Filled colors, give a good visualization for the temperature, pressure, and mass flow rate, etc.
• The color legends are used to show the temperature, velocity, pressure, thermal conductivity, etc in different directions.

• Here we can see that the max. temperature is obtained at the bottom PCB components, which have a power input of 10W.

• The above figure shows the behavior of temperature inside the CAD model with the type of forced convection because of the usage of fans to suck the air inside.
• Due to the forced convection inside the model, the heat is transferred from one place to another place.

• By using the object face option, we can visualize how much the heat is generated from the sources and at the same time how the other components gain heat through convection inside the model.
• From the above figure, we can see the main hotspot of the model which generates the heat and dissipating it into the free air inside the model.

• The main objective of using the MRF model is to apply and visualize the actual swirl motion inside the CAD model that occurred due to the rotation of the fans.
• From the above figure, we can say that the flow is coming inside the model through the grilles and hitting the fans. The fans have to be rotated in the clockwise direction. For that, we can clarify this case by seeing the flow direction of air passing into the model. The flow is also clockwise.
• Finally, we can see the swirl inside the model, and also can see how the swirl is carrying out the temperature from one place to other places inside the model and finally out of the model through the other grille.

Temperature Animation: https://youtu.be/UyUvCOUawVU

Velocity:

• As we have created a couple of grilles to the enclosure, the circulation of the air has happened through these grilles only with the help of the fans.
• To circulate the air inside of the enclosure we kept the free area ratio to 0.4 million a non-dimensional number.
• The forced convection phenomena happen due to the air circulated over the heated components and PCB's.

• The above figure is the cut-plane of the fan-1 at the center of the cross-section of the fan. It shows how the flow is sucking inside the model and how it is distributed inside the CAD model.
• The max. velocity recorded is 21.35 m/s. The max. velocity has appeared at the very close adjacent on the top of the blade. Means max .velocity recorded in the blade surface.

• In the cut plane, we cannot see the max. velocity, so, moving to the object face will give us the location where the max. velocity appears.

• Here we can see the max. velocity inside the fluid volume we have created around the fans.
• At the boundaries of the fluid volume or on the top of the impeller blades the velocity is decreased, this is because it is the phenomenon of the boundary layer theory.
• A boundary layer is formed due to the viscous anchoring arising from the no-slip stagnation at the wall.

• Increasing the velocity increases the inertial forces inside the boundary layer. The absolute speed at which the fluid molecules travel. For constant viscosity(keeping the same fluid and heat addition), the viscous forces will not be able to drag many layers of moving fluid.

• From the above particle trace, we can see the swirling phenomenon which is in the clockwise direction which is a fan rotational direction.
• In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object.

Velocity Animation: https://youtu.be/GXkgE5W8gFA

Velocity-Vector-Animation: https://youtu.be/T7GJQ3Log5A

Conclusion:

• Moving Reference Frame (MRF) modeling enables more accurate predictions of the flow.
• MRF reduces errors in predicting component temperatures because it directly throws the flow on to the components. This phenomenon we can't get through the normal modeling of the fan.
• MRF modeling is very accurate in predicting the flow, turbulence, and system noise levels.
• This model also gives accurate results of flow, temperature, and some other parameters even in high-density electronic models.