What is Convection Heat Transfer?
The process of heat transfer through the bulk movement of molecules inside fluids such as gases and liquids is known as convection heat transfer. The initial heat transfer between the body and the fluid occurs by conduction, but the bulk of the heat transfer occurs owing to fluid motion.
The process of heat transfer in fluids through the actual motion of materials is known as convection heat transfer.
- It occurs in both liquids and gases.
- It could be natural or forced.
- It involves the mass transfer of fluid portions.
Thermal expansion occurs when a fluid is heated from below. The fluid's density decreases as it cools in the lower layers, which are hotter. Colder fluid is denser, as we all know. The less-dense, hotter section of the fluid rises due to buoyancy. It is replaced by a colder, denser fluid. When this component heats up and rises to be replaced by the colder upper layer, the process is repeated. Convection is used to transfer heat in this way.
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Types of Convection Heat Transfer
Convection heat transfer occurs when the fluid is heated at a lower level. As a result of the buoyancy mechanism, the fluid below becomes less dense and the hotter component rises. The heated fluid takes the place of the denser, cooler fluid. When that section heats up and rises, the process repeats. As a result, it shifts back to the colder upper layer. Heat is transferred through convection in this manner. Convection can be divided into two types:
- Natural or free convection
- Forced convection
Natural or Free Convection
when the fluid moves due to buoyancy forces induced by density fluctuations caused by temperature variations in the fluid When a fluid comes into touch with a heated surface in the absence of an internal source, its molecules separate and scatter, making the fluid less dense. As a result, the fluid is displaced, and the cooler fluid becomes denser, sinking the fluid. As a result, the hotter volume of the fluid transfers heat to the cooler volume. The upward flow of air caused by a fire or a hot object, as well as the circulation of water in a heated pot from below, are both well-known instances.
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Convection is created by external systems such as pumps and fans, and this is referred to as forced convection. Forced convection can be seen in the use of a geyser or water heater to heat water, as well as the use of a fan in the summer.
Newton's Law of Cooling
Newton's law of cooling defines the rate equation for convective heat transfer between a surface and an adjacent fluid. Mathematically,
Convection Heat Transfer Formula
dQ/dt = hA(ts-tf)
- dQ/dt = Heat transfer rate
- A = Area exposed to heat transfer
- ts= Surface temperature
- tf= Fluid temperature
- h = Coefficient of convective heat transfer or convection heat transfer coefficient
Convection Heat TransferCoefficient
The value of the coefficient of convective heat transfer or convection heat transfer coefficient ‘h’ depends on the following factors
- Thermodynamic and transport properties (e.g. viscosity, density, specific heat, etc.)
- Nature fluid flow
- The geometry of the surface
- Prevailing thermal conditions
Convection Heat Transfer Example
We can see the various example of heat transfer by convection in our day-to-day life. A few common convective heat transfer examples are given below:
- Boiling Water
- Hot Air Popper
- Blood Circulation in Warm-Blooded Mammals
- Hot Air Balloon
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