Heat Flux Density

[Millicent]

Inphysics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density[1], heat-flow density or heat-flow rate intensity, is a flow of energy per unit area per unit time. Its SI units are watts per square metre (W/m2). It has both a direction and a magnitude, and so it is a vector quantity. To define the heat flux at a certain point in space, one takes the limiting case where the size of the surface becomes infinitesimally small.

A commonly known, but often impractical, method is performed by measuring a temperature difference over a piece of material with a well-known thermal conductivity. This method is analogous to a standard way to measure an electric current, where one measures the voltage drop over a known resistor. Usually this method is difficult to perform since the thermal resistance of the material being tested is often not known. Accurate values for the material's thickness and thermal conductivity would be required in order to determine thermal resistance. Using the thermal resistance, along with temperature measurements on either side of the material, heat flux can then be indirectly calculated.

One of the tools in a scientist's or engineer's toolbox is the energy balance. Such a balance can be set up for any physical system, from chemical reactors to living organisms, and generally takes the following form

Heat flux or thermal flux, also known as heat flow density, is the rate at which heat is transferred per unit area, per unit time, through an object or substance, usually from a hotter to a colder area. It is typically measured in watts per square meter (W/m) and is a measure of the energy flow through a material or system. Understanding heat flux is important in a wide range of applications, including thermal management, energy efficiency, and heat transfer.

Heat transfer is the transfer of thermal energy from one body to another. This can happen through three primary mechanisms: conduction, convection, and radiation. The rate of heat transfer, represented by the symbol Q, is the amount of heat that flows per unit of time. This flow of heat can be measured in various units, but one common unit is Btu/hr. This unit is often used in industrial applications where heat transfer rates are critical to the operation of machinery and systems.

The rate of heat transfer, represented by the symbol Q, is the amount of heat that flows per unit of time. This flow of heat can be measured in various units. This unit is often used in industrial applications where heat transfer rates are critical to the operation of machinery and systems.

Heat flux is a measure of the rate of heat transfer per unit area, while convection is a mode of heat transfer that occurs when a fluid is heated and rises, and as it cools, it sinks, leading to a continuous transfer of heat. In other words, heat flux is a scalar quantity that describes the flow of heat through a material, while convection refers to the movement of heat in a fluid, typically caused by differences in density.

Flux units refer to the units used to measure the rate at which heat energy is transferred through a surface per unit area. The most common units of heat flux include watts per square meter (W/m2) and British thermal units per square foot per hour (Btu/ft2-hr).

A flow of energy per unit of area per unit of time is known as heat flux or thermal flux. It is also known as heat flux density, heat-flow density, or heat flow rate intensity. Watts per square meter (W/m2) are the SI units. It is a vector quantity since it has both a direction and a magnitude.

Heat flux is the rate of thermal energy flow per unit surface area of the heat transfer surface, e.g, in a heat exchanger. The main parameter while calculating heat transfer is heat flux. There are 3 types of generalized classification is there that helps to distinguish between heat fluxes by convection, heat conduction, and radiation. We will further study the types of Heat Flux and the heat flux formula.

Heat flux also named as thermal flux, is referred to as heat flux density, heat-flow density is a flow of energy per unit of area per unit of time. In SI its units are watts per square meter \(\left (\fracWm^2 \right)\). As heat flux has both a direction and a magnitude, and so it is a vector quantity.

Heat flux by the convection process is directly proportional to the temperature difference between solid, liquid, or gaseous media participating in heat transfer. Under the conduction process, the heat flux vector is directly proportional to and usually parallel to the temperature gradient vector. The heat flux formation due to radiation is a flux of electromagnetic radiation. In contrast to convection and heat conduction, it may occur without any intervening medium.

The Heat flux value has many applications. It helps to evaluate heat transfer performance in many industrial applications, such as thermal protection of space shuttles, thermal management of electronic devices, metal heat treatment, maintenance of boilers, and nuclear reactors, spray cooling, geophysics, etc.

Heat flux, also known as thermal flux, is a term used to express an amount of energy transferred from one place to the other in the form of heat. Heat, the energy associated with the temperature of an object, travels from high temperature to low temperature. Measuring this heat flux with heat flux sensors can be useful as these measurements can tell us whether the temperature of an object will go up, down or stay the same. It also tells why and how fast the temperature change is. More on this later.

The thermal conductivity of the materials of which the wall is made. Different kinds of materials conduct heat at different rates. For instance, an aluminum wall has a very high conductivity and so it will spread heat quickly. This is one of the reasons aluminum alloys are a good option for pots or pans to cook with.

The term is called the temperature gradient. We are looking for the heat flux between two sides of a wall, a cold side and a hot side. The distance between these sides, or the thickness of the wall, is . The difference of temperatures of the hot and cold side is . When divided, we attain the temperature gradient. This gradient dictates which way the heat flows and partially how fast it flows.

As before, convection is one of the three ingredients determining the total heat flux of an object. What characterizes convection is that it is transferred by a fluid or gas. This fluid can be either a liquid or a gas. This phenomenon can be separated into two kinds: natural convection and forced convection.

The first one, natural convection, occurs when fluids or gasses heat up or cool down. With their temperature, their volume and thus density change. This leads to an often circular motion in a system. A fluid or gas heats up, expands, floats up, cools down, shrinks, sinks and heats up again. For instance, think about the oceanic currents, wind currents or hot air rising above a fire.

Forced convection is a process in which gasses or fluids are deliberately put in motion. This may be convection induced by a power source such as an air conditioner, pumps, fans, boilers, using your oven or boiling a pot of water.

Furthermore, we can make a distinction between internal and external convection. Internal convection concerns systems with the fluid or gas at the center like oil pipes. External convection on the other hand, regards the fluid or gas on the outside like air around a hot car in traffic.

With:

q = Convective heat flux per unit of area. [W/m2]

hc = convective heat transfer coefficient; A constant for rate of convection, different for every particular system. [W/m2K]

= The difference in temperature of the solid and the fluid. [K]

The most difficult factor regarding convective heat flux is always the measuring or calculating of the heat transfer coefficient hc. This variable is dependent on temperature, geometry of the system, velocity of the fluid or gas, dimensions of the solid, viscosity, densities and much more.

For instance: UV-radiation, emitted by the sun, carries a lot of energy with it per quantum. So much so, that it can bump into molecules of your skin and alter the charge of the molecule, irritating your skin and leaving you burned. On the other hand, infrared radiation is also emitted by the sun, carrying relatively less energy per quantum. This, more tame, quantum bumps into your molecules with no other effect than vibrating them a bit, not capable of knocking electrons out of orbit. This radiative heat flux has much more total energy in [W] than UV and is experienced as a heat.

So, the sun, fire, lamps, televisions, radio signals and even people emit the same kind of energy: Radiation. Different frequencies result in different energies with different experiences and uses. The radiation most often referred to when it comes to heat flux sensors is far infrared radiation. This is longwave, invisible radiation. This radiation can be absorbed, reflected and emitted. Radiation is an interaction between two objects, often referred to as bodies.

With:

= radiative heat flux from body 1 to 2. [W]

= The transfer factor from body 1 to 2. [-]

= The radiative surface of body 1. [m2]

= Stefan-Boltzmann constant of 5.67 * 10-8. [W/m2K4]

T1 = Temperature of body 1 at the surface. [K]

T2 = Temperature of body 2 at the surface. [K]

The transfer factor depends on both bodies. It is the relation between the sizes and shapes of the two surfaces, but is also dependent on the emissivity and absorption of bodies 1 and 2, the emitter and receiver. The emissivity of a surface represents its ease of emitting energy and is dependent of material properties like color and smoothness. Generally speaking, the transfer factor is different for every situation and can vary between 0 and 1.

The surface area of body 1, A1, is very straightforward. The more surface the body has, the more radiation it can emit. The Stefan-Boltzmann constant, , is independent of the situation and always has the same value. Lastly, the temperatures of both bodies influence the radiation hugely. As we can see, the temperatures are raised to the fourth power, making the radiative heat flux an extremely exponential process. In other words, at higher absolute temperatures, the same temperature difference can result in a higher exchange of radiative heat.

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