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Heat transfer by convection

Convection is the movement of a fluid that transfers heat from one positionto another. Convective transfer of heat is typically described as a separate mode of heat transmission, however it really combines the integrated processes of heat diffusion (conduction) and transfer of heat by the mass flow of fluid (advection). Convection is the most common mode of heat transmission in gases and liquids.

Convection may be "driven" by fluid movement other than buoyant forces. For example, a pump of water in an automobile engine. Convection may also be caused by the thermal expansion of fluids. When a fluid is heated, natural buoyant forces are fully responsible for motion of liquid. This process of convection is known as a "natural.” When fluids of various densities are influenced by gravity, there is a rise in temperature which induces a decrease in density. This causes liquid motion owing to forces and pressure.

Example of convection

When water is boiled on gas, hot water from the bottom of the pan is replaced by the cold and dense liquid that descends. After the heating is turned off, the mixing and conduction from natural convection result in a uniform temperature and density. Natural convection does not occur in the absence of gravity and only forced-convectionfunction takes place.
One mechanism is used in the convection heat transfer. In addition to energy transfer caused by particular motion of molecule, energy is transferred through the fluid's mass, or macroscopic motion. This motion is related with the fact that huge numbers of molecules are moving collectively or as lumps at a given time. In the presence of a temperature gradient, such motion leads to heat transmission. Because the molecules in groups retain their random motion, total heat transfer results from the superposition of energy transmission via random motion of the molecules and mass motion of the fluid.

Types of convective heat transfer

  1. Natural or free convection: When fluid motion is produced by buoyant forces induced by density fluctuations owing to thermal temperature variations in the fluid, this is referred to as free or natural convection. When a fluid comes into touch with a heated surface in the absence of an internal source, its molecules split and disperse, causing the fluid to be less dense. As a result, the fluid is displaced, while the colder fluid becomes denser and sinks. As a result, the hotter volume transfers heat to the cooler volume of the fluid. The upward movement of air caused by a fire or hot item is a well-known example, as is the circulation of water in a pot heated from below.
  2. Forced convection: When a fluid is pushed to flow over the surface by an internal source such as fans, churning, or pumps, an artificially induced convection current is created.

Natural and induced convection occur concurrently in many real-world situations (for example, heat losses at solar central receivers or cooling of photovoltaic panels) (mixed convection)

Law of cooling by Newton

Newton's law says that the rate of heat loss of a body in the presence of a wind is proportional to the difference in temperatures between the body and its surroundings. The heat transfer coefficient is the proportionality constant. The law applies when the coefficient is independent, or nearly so, of the temperature differential between the item and its surroundings.

Heat transmission via convection

The fundamental connection for convectional heat transport is:

Q = hA(T - Tբ)ᵇ

where
Q = amount of heat transmitted per unit time
A = object's area
h = heat transfer coefficient
T = surface temperature
Tf = fluid temperature
b = scaling exponent

The convective heat transfer coefficient is affected by the fluid's physical characteristics and the physical circumstances. For commonly encountered flow conditions and fluids, h values have been measured and tabulated.

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