What is Heat Transfer?

Heat transfer is the movement of thermal energy, in the form of heat, between physical systems with different temperatures using four distinct mechanisms — advection, conduction, convection, and radiation. Heat transfer impacts almost every aspect of modern life, from cooking food or cooling a laptop to generating electricity in a power plant. Every day, the heating of the Earth by solar radiation from the sun is a process of heat transfer, just as storms are caused by convection currents in the earth's atmosphere.

Heat transfer occurs as a natural phenomenon in any situation where there are temperature differences between two systems, as described in the second law of thermodynamics. Engineers also use heat transfer devices to move energy in the form of heat from one place to another, delivering heat where needed and removing it where it can cause problems. The safety, efficiency, and performance of products are driven by keeping the temperature of each part of a product within a desired temperature range.

Heat transfer is fundamentally about moving energy in the form of heat from one system to another. The discipline looks at the amount of energy that is stored or transferred in the form of heat and at the different ways that the energy is moved through the systems.

Here are some important terms used in heat transfer and their meaning.

Convection Currents

Convection currents refer to a fluid that sees a local temperature increase and corresponding density drop due to heating. The density drop creates a buoyancy force that causes the warmer fluid to rise until it cools. Then, as regions cool, the density decreases, and the fluid region is pulled down by negative buoyancy. These currents can easily be seen in a pot of boiling spaghetti, where the convective currents cause the pasta to twist and turn in the water.

Heat Capacity or Thermal Mass

The heat capacity of a system is the amount of heat needed to raise the temperature of the system by one degree. The higher the heat capacity, the more energy needed to increase its temperature. It is an important value in heat transfer because it reveals how materials respond to heat and how fast the object can heat up or cool down.

Heat Flux

Heat flux measures the amount of energy transferring through a unit area. It is the total heat divided by the area of a surface where it is being measured. It is a critical value in heat transfer because it tells engineers the amount of energy traveling from one object to another or from an object to a fluid.

Heat Generation

Heat generation is the creation of thermal energy through multiple processes, including chemical reactions, combustion, nuclear fusion, nuclear fission, thermoelectrics, electrical resistance, mechanical friction, or pressure changes in a fluid.

Heat Transfer Coefficient

The heat transfer coefficient (h) of a surface measures the relationship between the heat flux between a surface and fluid touching the surface and the temperature difference between the surface and the fluid. The value can be calculated if you know the velocity and the thermal properties of the fluid. It is sometimes referred to as the film coefficient.

Mass Transfer

Just as heat transfer deals with the movement of heat from one system to another, mass transfer deals with the movement of mass. This is important in heat transfer because any mass moving between systems has an internal thermal energy that moves with the mass. Air moving over a heat sink is both mass transfer and heat transfer.

Phase Change

When matter transitions from one state to another, it undergoes a phase change. Strictly speaking, phase change is not heat transfer, but it is used with heat transfer to control temperatures in systems. The changes in molecular bonds inside the material changing phase absorb energy if going from solid to liquid (melting) or liquid to gas (boiling or vaporizing), and release energy if transitioning from gas to liquid (condensation) or liquid to solid (freezing). Engineers often use phase change to remove or add heat to a system.

Temperature

Temperature is a measure of the kinetic energy of the vibrating and colliding atoms and molecules in an object.

Thermal Conductivity

The thermal conductivity (k) of a material is a measure of the ability of that material to conduct heat. The inverse of thermal conductivity is thermal resistivity.

Thermal Energy or Heat

Heat is the internal energy of matter that takes the form of the microscopic motion of subatomic particles, atoms, and molecules. The higher the temperature, the higher the energy. Strictly speaking, heat only refers to the amount of thermal energy being transferred. But most people use heat and thermal energy interchangeably.

Thermal Equilibrium

Thermal equilibrium is the state when two objects in contact are at the same temperature. When thermal equilibrium is achieved, heat transfer stops between the two objects.

Thermodynamics

Thermodynamics is a branch of physics focused on heat, work, and temperature and how they affect the energy and entropy of systems and the physical properties of matter. It is described by the four laws of thermodynamics. Heat transfer physics is the part of thermodynamics that deals with the flow of heat between objects.

The movement of thermal energy between systems can take place in one of four modes, where energy is transferred as heat flows from the higher-temperature object to the lower-temperature object. In most cases, two or more types of heat transfer are active at the same time, depending on whether the objects involved are touching, surrounded by a fluid, or visible to one another. 

Here is a brief definition of each mode.

Advection

Thermal advection is the mechanism of thermal energy transfer where heat is transported from one location to another through the motion and momentum of a fluid. Advection is sometimes referred to as forced convection to differentiate it from the strict definition of convection because the fluid flow in advection is not caused by buoyancy forces but instead is imparted by adding energy to the system.

A fan cooling a computer motherboard is an example of advective heat transfer.

Thermal Conduction

Thermal conduction describes the transfer of heat between two objects in direct contact or within an object with a temperature difference across the object. It describes the transfer of energy through thermal diffusion as described in Fourier's law for heat conduction. The speed of energy transfer is driven by the thermal conductivity of the material and the temperature gradient in the object or objects. For two objects in physical contact, the pressure and fit between the two surfaces determines the thermal contact resistance.

An example of thermal conduction is the handle on a pot on a stove. Heat moves from the pot's base up its walls and into the handle.

Convection

Convection, or convective heat transfer, is the transfer of thermal energy due to the motion of a fluid driven by buoyancy caused by temperature differences in the fluid. Engineers generally refer to it as free convection or natural convection to differentiate it from advection or forced convection.

A common example of convection is simply leaving a mug of hot coffee or tea sitting out. The warm drink transfers heat into the air, and buoyancy forces carry the heat away.

Radiation

Radiation heat transfer is a mechanism that transfers heat energy as electromagnetic waves/photons. Thermal energy causes the atoms in any form of matter to move, and the movement of the charged particles in those atoms (protons and electrons) results in the emission of electromagnetic radiation. Heat transfer due to thermal radiation only occurs in a vacuum or through a medium that is transparent to the infrared wavelengths emitted due to an object's temperature. 

Energy in the form of heat can be used to do work, or it can have a negative impact on any system due to temperature differences. Heat transfer fundamentals are applied by engineers to control the amount of thermal energy entering systems, move heat energy as desired within systems, and remove thermal energy from systems using one or more transfer mechanisms.

Here is a list of a few of the more common applications of heat transfer.

Cooking

The preparation of food through the application of heat to make it safe and edible. Fire was originally used as the heat source, where a combination of convection and radiation heat transfer moves heat from the burning fuel to the food.

Over time, ovens were developed to create a high-temperature enclosure where convection of the heated air and radiation from the walls cooked the food. Modern cooking can use electrical resistance or electrical induction as the heat source to transfer energy into cooking containers. Air fryers and convection ovens are examples of using advection in the form of fans blowing high-temperature air over the food to increase the heat flux into the food to cook it faster.

Electronics Thermal Management

Another very common application of heat transfer in modern life is cooling electronic devices. The resistance in electronic components generates heat, and various heat transfer methods are used to move the heat away from the components.

At its most basic, as in a mobile phone, conduction is used to move the thermal energy from components to the case and screen, where convection moves the heat to the surrounding air. For higher-temperature devices like computers, heat sinks are designed to create a large surface area for convection, and, if needed, advection is used by placing fans in the device to increase the momentum of the air, moving the heat away and increasing the heat transfer coefficient.

Heating and Cooling

Humans have also used heat transfer to design devices and materials that maintain a comfortable temperature inside structures:

• Buildings use insulation to reduce the heat transfer from inside the structure to the outside environment.
• Coatings on windows reduce the amount of infrared radiation that passes through windows, and air conditioning systems use phase change to extract heat from air and return low-temperature air to rooms.
• For heating, combustion or electricity is used to heat air or steam that transports the warmed fluid to where it is needed.

Materials Processing

From making metal alloys to extracting petroleum products from crude oil, heat transfer plays an essential part in most examples of materials processing.

In each case, heat transfer is used to obtain and maintain a desired temperature in the raw material to create a desired phase change, chemical reaction, or metallurgical modification. Heat transfer is studied and used to optimize the efficiency of materials processing to minimize the amount of energy needed.

Automotive Engines

Internal combustion engines (ICEs) create a significant amount of heat. Although a few engines rely on air cooling, most use pumped water to pull heat from the engine block, which is made from steel or aluminum — both conductors of heat.

The fluid is then run through a radiator — a large heat exchanger at the front of the car — to transfer thermal energy through advection into air using the velocity of the car when it is moving or fans when it is stationary or moving slowly.

Electric Vehicle Drive Trains

Motors and batteries in electric vehicles (EVs) can generate considerable heat that needs to be transferred out of the vehicle. Some systems are designed to use conduction to move heat away from the thermal source and then use cooling loops of liquid to transport the heat to radiators.

Spacecraft

The cooling and heating of electronics in space, and especially sensors like CMOS cameras on spacecrafts, is a unique heat transfer problem because there is no surrounding air to transfer thermal energy to. The design must balance heat generated by the electronics, infrared radiation from the environment, heat absorbed from the sun, and heat radiated to space. To keep the temperature of components in space within the proper operating range, engineers use a combination of conduction, radiation, advection, heat generation, or phase change.

Engineers designing thermal management systems rely heavily on simulation to understand the systems they are developing and to drive their designs. Simulation can be done much faster, carried out earlier in the design process, and explore far more scenarios than physical testing.

In some cases, simulation can take the form of a few equations. As systems become more complex, engineers employ finite element analysis (FEA), finite difference analysis, computational fluid dynamics (CFD), and ray tracing to model conduction, convection/advection, and radiation modes of heat transfer.

Here are some suggestions for conducting accurate and efficient simulations for heat transfer.

1. Determine If and When a Steady State or Transient Solution Is Needed

Heat transfer is a time-dependent phenomenon because it takes time for thermal energy to move between objects until thermal equilibrium is achieved. Before beginning a simulation, you should understand if you need to capture the transient behavior of the system or if you are only interested in the heat fluxes and temperatures once steady-state conditions are met.

2. Know Your Material Properties

Accurate heat transfer calculations depend heavily on material properties such as thermal conductivity, heat capacity, and emissivity. Using a tool like Ansys Granta to obtain and manage thermal material properties can be very useful.

3. Understand Your Boundary Conditions and Control Volumes

One of the most common sources of error in heat transfer simulation is applying the wrong boundary conditions or not defining the proper control volume around objects. Care should be taken to sketch out each region in the system you are modeling, and make sure you know the heat flux and heat generation in each one before you build your model.

4. Accurately Capture Fluid Velocities

Natural and forced convection are the two most common modes of heat transfer used to manage heat flow in systems, and the thermal flux from a surface to a fluid is highly dependent on velocity.

An FEA program like Ansys Mechanical structural finite element analysis software or a finite difference program like Ansys Thermal Desktop thermal-centric modeling software can represent convective heat transfer to a fluid using a heat transfer coefficient if the velocity is well known. When needed, the fluid can be simplified as a 1-D thermal-fluid network to more accurately calculate the heat transfer and velocity of the fluids in the system. This is common practice in automotive engines, jet engines, nuclear power plants, and spacecraft.

However, in complex systems where the fluid flow cannot be estimated or simplified, a full CFD tool like Ansys Fluent software is used by engineers to accurately predict fluid flow, including turbulence, convective currents, and the mixing of different fluids. Simulations combining accurate thermal conductivity in solids and fluid behavior are called conjugate heat transfer simulations.

5. Simplify Models When Possible

Very experienced thermal engineers are experts at simplifying systems for heat transfer simulation. In many cases, a 1-D network model can be used to accurately capture some or all of the thermal behavior of a system. Or 3-D geometry can be greatly simplified because small features don't impact the heat transfer response.

A tool such as Thermal Desktop can create simple models for early design studies before CAD geometry is available and maintain simplified simplification as CAD geometry becomes available with TD Direct in Ansys SpaceClaim.

A tool like TD Direct in Ansys SpaceClaim can greatly speed up the conversion of complex CAD geometry into a simplified model ideal for transient thermal analysis.

6. Leverage the Power of Vertical Applications for Electronics

Heat transfer modeling in electronics is its own specialty, so tools like Ansys Icepak electronics cooling simulation software have been developed to focus on this important area. Vertical applications understand the terminology, geometry, and boundary conditions unique to the industry, and they include ways to automate the model-building and post-processing steps. Using a vertical application not only saves time, but it also allows non-experts to more easily use simulation.

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