Heat Flux Definition
Heat flux is defined as the amount of heat transferred per one unit area per one unit time to or from a surface. Basically, it is given as a derived quantity since it involves the principle of two quantities, viz. the amount of heat transfer per one unit time and the area to/from which the heat transfer takes place.
The derived SI Unit of heat rate is watt or joule per second. Heat flux density defines the heat rate per one unit area. In the SI unit of the heat flux, density is measured in Watts per one meter square (W/m2). Heat flux is given as a vector quantity, which holds both direction and magnitude.
Fourier’s Law
Fourier’s law is an essential application of these concepts. For a pure solid substance, the conductive heat flux (JHc) in one dimension can be expressed by Fourier’s law which is given as:
JHC = λ\[\frac{dT}{dZ}\]
Where,
JHC= Conductive heat flux,
λ = Thermal conductivity constant and
T = Temperature
Heat Flow Rate Formula
The heat flow rate can be described as the amount of heat transferred per one unit time in the material. The heat flow rate in a rod will depend on the rod’s cross-sectional area, the temperature difference between the length of the rod and both the ends. The formula is used to calculate the heat flow rate of any material. The equation for heat flux or the formula of heat flux is given as:
Q = -k \[(\frac{A}{l})\]∆T
Where,
k = Thermal Conductivity,
Q = Heat Transfer Per Unit Time,
l = Length of the Material,
A = Cross-Sectional Area
∆T = Temperature Difference
Solved Examples
Question:
One face of the copper plate is 5 cm thick, which is maintained at 500℃, and the other face is maintained at 100℃. Calculate the heat transferred via plate.
Solution:
Given parameters are,
coefficient of the thermal conductivity of copper - λ = 385,
dT = 500 – 100= 400
dx = 5
Substitute the resultant values in the heat flux density formula or the surface heat flux equation:
JHC = λ\[\frac{dT}{dZ}\]
JHC = 385 x\[\frac{400}{5}\]
JHC=30800MW
Methods to Measure the Heat Flux
We Can Measure the Heat Flux Primarily by the Below Two Methods:
The most common but often impractical method is by measuring the temperature difference over a piece of material with a known thermal conductivity. This specific method is not proper and not easy to perform since the thermal resistance of the material being tested is often unknown.
The second most accurate method of measuring the heat flux is with the help of a heat flux transducer or heat flux sensor. It measures the amount of heat that is being transferred to or from the surface that the heat flux sensor is mounted to. The common type of heat flux sensor is given as a differential temperature thermopile. This particular method of thermal conductivity or resistance does not need a known parameter.
FAQs on Heat Flux Formula
Q1. Give the Application of Heat Flux?
Answer: The Heat flux value has several applications. It helps to evaluate the performance of heat transfer in several industrial applications, such as metal heat treatment, thermal management of electronic devices, thermal protection of space shuttles, boiler maintenance and nuclear reactors, geophysics, spray cooling and more.
Q2. Explain If There is Any Heat Transfer in a Vacuum?
Answer: Radiation is described as the energy that is emitted by matter in the form of either electromagnetic waves or photons as a result of the changes in the electronic configurations of the molecules or atoms. Unlike convection and conduction, the transfer of energy by radiation does not need the presence of an intervening medium. In fact, the energy transfer by radiation is the fastest (at the speed of light) and it also suffers no attenuation in a vacuum. This is the process of how the energy of the sun reaches the earth.
