Convective heat transfer

Simulation of thermal convection. Red hues designate hot areas, while regions with blue hues are cold. A hot, less-dense lower boundary layer sends plumes of hot material upwards, and likewise, cold material from the top moves downwards. This illustration is taken from a model of convection in the Earth's mantle.

Convective heat transfer, often referred to simply as convection, is the transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer(convection) in liquids and gases. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of conduction (heat diffusion) and advection (heat transfer by bulk fluid flow).

Convection can be "forced" by movement of a fluid by means other than buoyancy forces (for example, a water pump in an automobile engine). Thermal expansion of fluids may also force convection. In other cases, natural buoyancy forces alone are entirely responsible for fluid motion when the fluid is heated, and this process is called "natural convection". An example is the draft in a chimney or around any fire. In natural convection, an increase in temperature produces a reduction in density, which in turn causes fluid motion due to pressures and forces when fluids of different densities are affected by gravity (or any g-force). For example, when water is heated on a stove, hot water from the bottom of the pan rises, displacing the colder denser liquid, which falls. After heating has stopped, mixing and conduction from this natural convection eventually result in a nearly homogeneous density, and even temperature. Without the presence of gravity (or conditions that cause a g-force of any type), natural convection does not occur, and only forced-convection modes operate.

The convection heat transfer mode comprises one mechanism. In addition to energy transfer due to specific molecular motion (diffusion), energy is transferred by bulk, or macroscopic, motion of the fluid. This motion is associated with the fact that, at any instant, large numbers of molecules are moving collectively or as aggregates. Such motion, in the presence of a temperature gradient, contributes to heat transfer. Because the molecules in aggregate retain their random motion, the total heat transfer is then due to the superposition of energy transport by random motion of the molecules and by the bulk motion of the fluid. It is customary to use the term convection when referring to this cumulative transport and the term advection when referring to the transport due to bulk fluid motion.[1]

Overview

This color schlieren image reveals thermal convection from a human hand (in silhouette form) to the surrounding still atmosphere. Photographed using schlieren equipment.
Papers lifted on rising convective air current from warm radiator

Convection is the transfer of thermal energy from one place to another by the movement of fluids. Although often discussed as a distinct method of heat transfer, convection describes the combined effects of conduction and fluid flow or mass exchange.

Two types of convective heat transfer may be distinguished:

Internal and external flow can also classify convection. Internal flow occurs when a fluid is enclosed by a solid boundary such when flowing through a pipe. An external flow occurs when a fluid extends indefinitely without encountering a solid surface. Both of these types of convection, either natural or forced, can be internal or external because they are independent of each other. The bulk temperature, or the average fluid temperature, is a convenient reference point for evaluating properties related to convective heat transfer, particularly in applications related to flow in pipes and ducts.

Further classification can be made depending on the smoothness and undulations of the solid surfaces. Not all surfaces are smooth, though a bulk of the available information deals with smooth surfaces. Wavy irregular surfaces are commonly encountered in heat transfer devices which include solar collectors, regenerative heat exchangers and underground energy storage systems. They have a significant role to play in the heat transfer processes in these applications. Since they bring in an added complexity due to the undulations in the surfaces, they need to be tackled with mathematical finesse through elegant simplification techniques. Also they do affect the flow and heat transfer characteristics, thereby behaving differently from straight smooth surfaces.[4]

For a visual experience of natural convection, a glass filled with hot water and some red food dye may be placed inside a fish tank with cold, clear water. The convection currents of the red liquid may be seen to rise and fall in different regions, then eventually settle, illustrating the process as heat gradients are dissipated.

Newton's law of cooling

Convection-cooling is sometimes loosely assumed to be described by Newton's law of cooling.[5]

Newton's law states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings while under the effects of a breeze. The constant of proportionality is the heat transfer coefficient.[6] The law applies when the coefficient is independent, or relatively independent, of the temperature difference between object and environment.

In classical natural convective heat transfer, the heat transfer coefficient is dependent on the temperature. However, Newton's law does approximate reality when the temperature changes are relatively small.

Convective heat transfer

The basic relationship for heat transfer by convection is:

where is the heat transferred per unit time measured in Watts, A is the area of the object measured in metres squared, h is the heat transfer coefficient measured in watts/metres square and kelvin , Ta is the object's surface temperature in kelvin and Tb is the fluid temperature in kelvin .[7]

The convective heat transfer coefficient is dependent upon the physical properties of the fluid and the physical situation. Values of h have been measured and tabulated for commonly encountered fluids and flow of situations.

References

  1. ↑ Incropera DeWitt VBergham Lavine 2007, Introduction to Heat Transfer, 5th ed., pg. 6 ISBN 978-0-471-45727-5
  2. ↑ http://biocab.org/Heat_Transfer.html Biology Cabinet organization, April 2006, “Heat Transfer”, Accessed 20/04/09
  3. ↑ http://www.engineersedge.com/heat_transfer/convection.htm Engineers Edge, 2009, “Convection Heat Transfer”,Accessed 20/04/09
  4. ↑ Aroon Shenoy, Mikhail Sheremet, Ioan Pop, 2016, Convective Flow and Heat Transfer from Wavy Surfaces: Viscous Fluids, Porous Media, and Nanofluids, CRC Press, Taylor & Francis Group, Florida ISBN 978-1-498-76090-4
  5. ↑ Based on a work by Newton published anonymously as "Scala graduum Caloris. Calorum Descriptiones & signa." in Philosophical Transactions, 1701, 824–829; ed. Joannes Nichols, Isaaci Newtoni Opera quae exstant omnia, vol. 4 (1782), 403–407.
  6. ↑ "Heat Transfer Mechanisms". Colorado State University. The College of Engineering at Colorado State University. Retrieved 14 September 2015.
  7. ↑ "Convective Heat Transfer Convection Equation and Calculator". Engineers Edge. Retrieved 14 September 2015.

See also

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