Heat transfer is the transfer of thermal energy in the form of heat from one system to another. For example, consider a pot of water being heated by an electric stovetop. Here, heat is transferred from the hot stovetop to the pot, from the pot to the water, and from both the heat pot and the water to the surrounding air.
Heat transfer occurs in three major modes: conduction, convection, and radiation.
Conduction: This is the dominant form of heat transfer in solids. In the conductive mode of heat transfer, thermal energy transfer takes place between objects that are in direct contact with each other. No exchange of material takes place between objects. The heat transfer is dictated by the material properties of the different objects, the temperature difference between them, and their contact configurations.
Convection: This is the dominant form of heat transfer in fluids.Convective heat transfer results from the motion of fluid. Convection can be either natural convection or forced convection. Natural convection is driven by buoyancy and/or density changes in a fluid. For example, hot air rises above cold air due to a decrease in density. Forced convection occurs when an external force is applied to move the fluid. For example, a fan is used to drive cold air over hot electronic components.
Radiation: Radiation takes place by the transfer of electromagnetic waves. All matter above a temperature of absolute zero (0 kelvin) emit radiation. Radiation does not require a medium and can be transferred even through vacuum (e.g., the Earth receives the Sun’s energy in the form of radiation).
Heat transfer is omnipresent in our daily lives. From human comfort solutions such as heating and cooling of our houses and cars through heaters or air conditioning, car radiators, combustion inside jet engines, cooling down of our morning cup of coffee, etc.
What is Conjugate Heat Transfer?
Conjugate Heat Transfer (CHT) is the simultaneous heat transfer in both solid and fluid domains, including heat transfer at their interface. For example, a solid transferring heat to or from a fluid flowing over it. The heat transfer between the solid and the fluid are closely coupled. In solids, heat transfer occurs primarily due to conduction, whereas in fluids, it is primarily through convection. In cases where extremely high temperatures occur, heat transfer due to radiation must also be accounted for. Some common examples of CHT are heat exchangers, electronics cooling, convection ovens, etc.
Why is CHT Important in CFD?
Most, if not all, real-world scenarios involve coupled heat transfer processes. In a coupled system, the heat transfer in the fluid domain is highly dependent on the heat transfer in the solid domain, and vice versa. To accurately predict fluid behavior in the CFD simulation, heat transfer in both the solid and fluid must be accounted for. This is achieved by simultaneously solving governing equations for conduction in solid, and convection in fluid (and radiation if applicable), resulting in accurate predictions of the performance under thermal loads, temperature distribution of all solid and fluid components, and heat flux, and other fluid and heat transfer quantities of interest.
Some of the most common applications where CHT is employed along with CFD simulations are listed below.
- Automotive: Engine cooling, exhaust systems, heat transfer in radiators, brake system heat dissipation, battery thermal management for electric vehicles.
- Aerospace: Cooling of gas turbine blades in a jet engine, rocket nozzle and combustion chamber cooling, hypersonic vehicle heat shields and re-entry systems.
- Electronics cooling: High performance computing data center cooling, CPU and GPU heat sinks, fan cooling of electronic chips, PCB thermal analysis.
- HVAC: Heat distribution in ventilation systems, fire safety and heat transfer in structural elements, thermal comfort in residential and commercial buildings.
- Energy Systems: Heat exchangers in power plants, geothermal heat pump systems, thermal energy storage (TES) systems, Heat transfer in wind turbine gearboxes and nacelle cooling, Biomass combustion systems and thermal reactors.
Example of CHT in CFD: Electronics Cooling
Below, we look at an example simulation of CHT using the FMK CFD role in the 3DEXPERIENCE Platform. The scenario consists of an air-cooled CPU chip with a heat sink. The CPU chip generates heat during normal mode of operation. To prevent the CPU from overheating, it needs to be maintained at an appropriate temperature. To achieve this, a heat sink is placed on top of the CPU, and a velocity inlet is used to drive cold air over the heat sink. The simulation domain considered is as shown below.
Mesh
The corresponding mesh of the enclosure, the heat sink, and the CPU chip is shown below. The total mesh count is about 2.5 million elements, and 2.7 million nodes.
Materials
In a CHT simulation, it is crucial to set the different components to proper materials along with accurate material and thermal properties. The material selected for the CPU chip is silicon, whereas the heat sink and the enclosure are set to aluminum.
Boundary Conditions
A volumetric heat source of 120W is applied to the CPU chip. A velocity inlet with a volumetric flow rate of 0.001 m3/sec is set at the inlet. All walls are set as no-slip boundary condition. The outlet is set as a zero pressure outlet.
Results
The results of the steady state analysis are shown below.
Streamlines colored by temperature are shown in the plots above, clearly indicating the colder air entering the domain from the top, and warmer air around the heat sink.
The temperature inside the enclosure is shown above on the left, and the temperature of the heat sink is shown on the right, clearly indicating thermal transfer from CPU chip to heat sink, and heat sink to the convective air around it.
Final Thoughts
In this blog, we briefly described how Conjugate Heat Transfer (CHT) combined with CFD in the 3DEXPERIENCE Platform can be a powerful tool for design and analysis of a coupled fluid-thermal system. CHT allows engineers to design safer, more efficient systems across industries such as aerospace, mechanical, electronics, automotive, and energy.
As computational tools and computational resources begin to evolve, CHT simulations are becoming increasingly popular and more accessible. Designing a CHT workflow for your CFD simulations can be challenging at first. We at Fidelis can help you adopt this powerful approach, and help you build an efficient and sustainable fluid-thermal system. Contact our expert team today to learn more!
