Fluent Heat Flux

[Michele Firmasyah]

Ijust finished a thermal analysis and obtained nice plots of temperature profile and heat flux profile. Since the heat flux at one contacting surface is not uniform, it is difficult to calculate the heat flow power through that surface.

Is there a way now in Simulate to get the average heat flow power at a surface. I know Fluent has it, because I have been using it in another simulation job (for a different company. Note: I am working concurrently for different employers).

It looks like that I need to add one more dummy part to form the "interface" and assign a thermal contact resistance value to it. The prescribed temperature will then be assigned to the dummy part. I am to setup and have a test run.

Answer - For coupled wall, the general usage scenario of UDF macro DEFINE_HEAT_FLUX is to impose the heat flux from fluid side and then let Ansys Fluent do the thermal coupling between fluid and solid zones.

Important: This function allows you to modify the heat flux at walls adjacent to a solid. Note, however, that for solids since only heat conduction is occurring, any extra heat flux that you add in a heat flux UDF can have a detrimental effect on the solution of the energy equation. These effects will likely show up in conjugate heat transfer problems. To avoid this, you will need to make sure that your heat flux UDF excludes the walls adjacent to solids, or includes only the necessary walls adjacent to fluid zones.

The present problem simulates the heat transfer and cooling of a wall from a model with a semi-cylinder shape, using ANSYS Fluent software. The model rotates around a particular axis (model z-axis) at a speed equivalent to 400 rpm. Therefore, to define this rotational motion in the model, the frame motion technique with a rotational speed of 400 rpm has been used. The exterior sectional wall of the model under constant heat flux is equal to 1000 W.m-2, and on the outer surface of this sectional wall, there are five ducts for airflow.

Cooling airflow with a velocity of 29.215 ms-1 (assuming Reynolds 10000 for the inlet airflow to the model) and a temperature of 300 K enter the model through five inlet ducts the outlet section located at the top of the model at equivalent pressure Atmospheric pressure is released.

The present model is designed in three dimensions using Design Modeler software. The model consists of an semi-cylinder with a diameter of 50 mm and a height of 200 mm, on the lateral surface of which are five air inlets for a diameter of 5 mm.

At the end of the solution process, two-dimensional and three-dimensional contours related to pressure, speed, and temperature are obtained. Also, two-dimensional velocity vectors and two-dimensional flow lines have been obtained in different sections of the model. Two-dimensional sections are created on pages parallel to the X-Z plane of the model; So that they include the first (lowest), third (middle), and fifth (highest) inputs of the model. The contours show that the temperature is high near the heat flux wall, and when the airflow from the inlets hits this heat flux wall at high speed, it reduces the temperature.

I found the initialization settings particularly insightful. Could you clarify if initializing the z-velocity to -29.215 m.s-1 takes into account the rotational motion of the semi-cylinder or is it solely for the airflow within the ducts?

The initialization setting for the z-velocity at -29.215 m.s-1 is specifically for the airflow being introduced into the model through the ducts. This value is set to match the inlet condition of the cooling airflow. The rotational motion of the semi-cylinder is accounted for separately in the frame motion technique which sets the rotational speed of the model to 400 rpm. The negative sign indicates the direction of the airflow relative to the coordinate system defined in the simulation.

The tutorial on Rotary Cooling using ANSYS Fluent was exceptional! The explanations, especially on how to set up the rotational motion and the details concerning boundary conditions were clear and easy to follow. The included figures and graphs provided excellent visual support to the theoretical aspects. My understanding of heat transfer phenomena in a dynamic case like this has improved significantly.

The gravity effect has been ignored in this simulation. This is a common assumption in CFD analysis when the gravity effect is negligible compared to other forces in the system, such as when the force of the airflow and rotation are significantly more influential on the heat transfer process.

Glad to hear that our course meets your needs! The focus of this training is primarily on the simulation of heat transfer and cooling, rather than on predicting equipment lifespan and efficiency loss over time. That said, with the knowledge gained from this simulation process, one may apply additional simulations or methodologies to predict equipment lifespan, although that would typically require more advanced analysis incorporating fatigue, material degradation, and possibly coupling with other simulation disciplines.

In MR CFD simulations, we ensure accuracy and validation by comparing results with theoretical predictions, empirical data, or experimental results when available. We also conduct sensitivity analyses, mesh independence studies, and monitor convergence of results to establish confidence in the simulation outputs.

This course was really effective for understanding rotary cooling simulation. The connection between heat flux conditions and the impact of inlet flow velocity was particular insightful. Are future updates planned to include simulations with varying material properties, or different shapes other than the semi-cylinder?

We appreciate your feedback on the Rotary Cooling of an object with a Constant Heat Flux tutorial. As of now, we do continually strive to improve and extend our training products, which may include simulations with varying material properties or different geometric configurations depending on the demands and interests of our users. Keep an eye out for announcements regarding new simulation tutorials.

Thank you! We appreciate your positive feedback and are delighted to hear that you are satisfied with the thoroughness and reliability of the results from our rotary cooling simulation using ANSYS Fluent. If you have further questions or need assistance with similar simulations, feel free to reach out.

This tutorial shows you how to simulate forced convection in a pipe using ANSYS FLUENT. The simulation corresponds to the forced convection experiment in MAE 4272 at Cornell University. The diagram shows a pipe with a heated section in the middle where constant heat flux is added at the wall. The ambient air is flowing into the pipe from the left with a uniform velocity. We'll use FLUENT to solve the relevant boundary-value problem and obtain the velocity, temperature, pressure and density distribution in the pipe. Inputs necessary for the simulation, such as the velocity at the pipe inlet and heat flux added at the wall, are obtained from one particular experimental run. Results from the simulation will be compared with corresponding experimental values.

Heat flux, sometimes referred to as thermal flux, is a flow of energy per unit area and per unit time. It is typically measured through a solid surface, as in the case of conduction heat transfer. It can also be measured at a fluid-solid boundary; this is typical of a convective heat transfer analysis. In other words, it quantifies the amount of heat transferred through a surface in a specific direction. Engineers and product designers often encounter heat flux considerations in the design and optimization of systems where heat transfer is a critical factor, such as electronic devices, HVAC systems, and industrial processes.

Understanding the temperature gradient is pivotal in grasping the nuances of heat flux. The temperature gradient (\(\fracdTdx)\) represents how the temperature changes concerning the distance traveled. A steep gradient implies a rapid change in temperature over a short distance, intensifying the heat flux. Conversely, a shallow gradient corresponds to a gradual change in temperature, resulting in a lower heat flux.

Similarly, the thermal conductivity (\(k)\) of a material plays a pivotal role in determining how well it conducts heat. Materials with high thermal conductivity facilitate efficient heat transfer, making them suitable for applications where minimizing thermal resistance is crucial. In contrast, materials with low thermal conductivity may be chosen to insulate against heat transfer.

However, heat flux can be estimated and analyzed much earlier in the design cycle of a product or part. For this, engineers can leverage the power of engineering simulation, particularly cloud-native simulation.

Heat Flux is available as a boundary condition, global result field, and surface result control within SimScale. It is available in both Conjugate Heat Transfer/IBM (CFD) and Heat Transfer (FEA) analysis types.

Heat flux can be defined as an input parameter to a simulation, also known as a boundary condition. Within the Conjugate Heat Transfer (CHT) analysis type, it can be defined via a Wall boundary condition. Note that a thermal flux wall boundary condition should only be applied on an external surface of the domain, and it is incorrect to define it on an internal surface. If a heat load on a body inside the domain needs to be defined, this should be done via a Power source.

The default temperature definition on a wall boundary condition is Fixed value; however, this can be changed to External wall heat flux. From there, there are 3 options for heat flux definition. A Fixed heat flux definition has units of power per unit area (W/m2). Fixed power allows the user to define a value in absolute Watts; it is not a function of the area. A Derived heat flux allows the user to define a Heat transfer coefficient or film coefficient. A Derived heat flux allows the user to define a heat transfer coefficient and reference temperature. The heat transfer units are Power/Temperature delta*Area \((W/(K.m^2\)).

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