Forced convection is a

mechanism Mechanism may refer to: *Mechanism (economics), a set of rules for a game designed to achieve a certain outcome **Mechanism design, the study of such mechanisms *Mechanism (engineering), rigid bodies connected by joints in order to accomplish a ...

, or type of transport, in which fluid motion is generated by an external source (like a pump, fan, suction device, etc.). Alongside

natural convection Convection is single or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...

thermal radiation Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electro ...

, and

thermal conduction Thermal conduction is the diffusion of thermal energy (heat) within one material or between materials in contact. The higher temperature object has molecules with more kinetic energy; collisions between molecules distributes this kinetic energy ...

it is one of the methods of heat transfer and allows significant amounts of heat energy to be transported very efficiently.

Applications

This mechanism is found very commonly in everyday life, including

central heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. A central heating system has a Furnace (central heating), furnace that converts fuel or electricity to heat through processes. The he ...

and

air conditioning Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...

and in many other machines. Forced convection is often encountered by engineers designing or analyzing

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

s, pipe flow, and flow over a plate at a different temperature than the stream (the case of a shuttle wing during re-entry, for example).

Mixed convection

In any forced convection situation, some amount of natural convection is always present whenever there are

gravitational force Newton's law of universal gravitation describes gravity as a force by stating that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the sq ...

s present (i.e., unless the system is in an inertial frame or free-fall). When the

is not negligible, such flows are typically referred to as

mixed convection In fluid thermodynamics, combined forced convection and natural convection, or mixed convection, occurs when natural convection and forced convection mechanisms act together to transfer heat. This is also defined as situations where both pressu ...

Mathematical analysis

When analyzing potentially mixed convection, a parameter called the

Archimedes number In viscous fluid dynamics, the Archimedes number (Ar), is a dimensionless number used to determine the motion of fluids due to density differences, named after the ancient Greek scientist and mathematician Archimedes. It is the ratio of gravita ...

(Ar) parametrizes the relative strength of free and forced convection. The Archimedes number is the ratio of

Grashof number In fluid mechanics (especially fluid thermodynamics), the Grashof number (, after Franz Grashof) is a dimensionless number which approximates the ratio of the buoyancy to viscous forces acting on a fluid. It frequently arises in the study of si ...

and the square of

Reynolds number In fluid dynamics, the Reynolds number () is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between Inertia, inertial and viscous forces. At low Reynolds numbers, flows tend to ...

, which represents the ratio of buoyancy force and inertia force, and which stands in for the contribution of natural convection. When Ar ≫ 1, natural convection dominates and when Ar ≪ 1, forced convection dominates. :

Ar= \frac

When

isn't a significant factor, mathematical analysis with forced convection theories typically yields accurate results. The parameter of importance in forced convection is the Péclet number, which is the ratio of advection (movement by currents) and diffusion (movement from high to low concentrations) of heat. :

Pe=\frac

When the Peclet number is much greater than unity (1), advection dominates diffusion. Similarly, much smaller ratios indicate a higher rate of diffusion relative to advection.

References

External links

{{DEFAULTSORT:Forced convection Thermodynamics Heat transfer

Applications

Mixed convection

Mathematical analysis

See also

References

External links