..... Click the link for more information. (since a given volume of the fluid contains less matter at a higher temperature than at the original, lower temperature). As a result, the warmer, less dense portion of the fluid will tend to rise through the surrounding cooler fluid, in accordance with Archimedes' principle Archimedes' principle, principle that states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The principle applies to both floating and submerged bodies and to all fluids, i.e., liquids and gases.
..... Click the link for more information. . If heat continues to be supplied, the cooler fluid that flows in to replace the rising warmer fluid will also become heated and also rise. Thus, a current, called a convection current, becomes established in the fluid, with warmer, less dense fluid continually rising from the point of application of heat and cooler, denser portions of the fluid flowing outward and downward to replace the warmer fluid. In this manner, heat eventually may be transferred to the entire fluid. Convection currents are widely observed in both liquids and gases. Many aspects of weather are connected with convection currents. For example, when a portion of the atmosphere becomes heated by contact with a warm area of land, it rises into the cooler, higher altitudes, with the result that some of the moisture carried with it may be condensed to form clouds and precipitation. Convection currents have been used for heating and ventilation since ancient times. Both hot-air and hot-water heating systems use convection to transfer heat through the entire structure being heated. Convection currents also assist in the ventilation of mines.
convection
Process by which heat is transferred by movement of a heated fluid such as air or water. Most fluids expand when heated. They become less dense and more buoyant, and so rise. The heated molecules eventually cool, become more dense, and sink. This repeated process sets up convection currents that account for the uniform heating of the air in a room or water in a kettle. Air convection can be forced by a fan, and water convection by a pump. Atmospheric convection currents can be set up by local heating effects such as solar radiation or contact with cold surfaces. Such currents are usually vertical and account for atmospheric phenomena such as clouds and thunderstorms.
convection
Convection (heat)
The transfer of thermal energy by actual physical movement from one location to another of a substance in which thermal energy is stored. A familiar example is the free or forced movement of warm air throughout a room to provide heating. Technically, convection denotes the nonradiant heat exchange between a surface and a fluid flowing over it. Although heat flow by conduction also occurs in this process, the controlling feature is the energy transfer by flow of the fluid—hence the name convection. Convection is one of the three basic methods of heat transfer, the other two being conduction and radiation. See Conduction (heat), Heat radiation, Heat transfer
Natural convection is exemplified by the cooling of a vertical surface in a large quiescent body of air of temperature t∞. The lower-density air next to a hot vertical surface moves upward because of the buoyant force of the higher-density cool air farther away from the surface. At any arbitrary vertical location x, the actual variation of velocity u with distance y from the surface will be similar to that in illus. b, increasing from zero at the surface to a maximum, and then decreasing to zero as ambient surrounding conditions are reached. In contrast, the temperature t of the air decreases from the heated wall value t's to the surrounding air temperature. These temperature and velocity distributions are clearly interrelated, and the distances from the wall through which they exist are coincident because, when the temperature approaches that of the surrounding air, the density difference causing the upward flow approaches zero.
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The region in which these velocity and temperature changes occur is called the boundary layer. Because velocity and temperature gradients both approach zero at the outer edge, there will be no heat flow out of the boundary layer by conduction or convection. See Boundary-layer flow
When air is blown across a heated surface, forced convection results. Although the natural convection forces are still present in this latter case, they are clearly negligible compared with the imposed forces. The process of energy transfer from the heated surface to the air is not, however, different from that described for natural convection. The major distinguishing feature is that the maximum fluid velocity is at the outer edge of the boundary layer. This difference in velocity profile and the higher velocities provide more fluid near the surface to carry along the heat conducted normal to the surface. Consequently, boundary layers are very thin.
Heat convection in turbulent flow is interpreted similarly to that in laminar flow. Rates of heat transfer are higher for comparable velocities, however, because the fluctuating velocity components of the fluid in a turbulent flow stream provide a macroscopic exchange mechanism which greatly increases the transport of energy normal to the main flow direction. Because of the complexity of this type of flow, most of the information regarding heat transfer has been obtained experimentally. See Laminar flow, Turbulent flow
Convection heat transfer which occurs during high-speed flight or high-velocity flow over a surface is known as aerodynamic heating. This heating effect results from the conversion of the kinetic energy of the fluid as it approaches a body to internal energy as it is slowed down next to the surface. In the case of a gas, its temperature increases, first, because of compression as it passes through a shock and approaches the stagnation region, and second, because of frictional dissipation of kinetic energy in the boundary layer along the surface.
The phenomena of condensation and boiling are important phase-change processes involving heat release or absorption. Because vapor and liquid movement are present, the energy transfer is basically by convection. Local and average heat-transfer coefficients are determined and used in the Newton cooling-law equation for calculating heat rates which include the effects of the latent heat of vaporization.
Convection (heat)
The transfer of thermal energy by actual physical movement from one location to another of a substance in which thermal energy is stored. A familiar example is the free or forced movement of warm air throughout a room to provide heating. Technically, convection denotes the nonradiant heat exchange between a surface and a fluid flowing over it. Although heat flow by conduction also occurs in this process, the controlling feature is the energy transfer by flow of the fluid—hence the name convection. Convection is one of the three basic methods of heat transfer, the other two being conduction and radiation. See Conduction (heat), Heat transfer
Natural convection is exemplified by the cooling of a vertical surface in a large quiescent body of air of temperature t∞. The lower-density air next to a hot vertical surface moves upward because of the buoyant force of the higher-density cool air farther away from the surface. At any arbitrary vertical location x, the actual variation of velocity u with distance y from the surface will be similar to that in the illustration b, increasing from zero at the surface to a maximum, and then decreasing to zero as ambient surrounding conditions are reached. In contrast, the temperature t of the air decreases from the heated wall value t's to the surrounding air temperature. These temperature and velocity distributions are clearly interrelated, and the distances from the wall through which they exist are coincident because, when the temperature approaches that of the surrounding air, the density difference causing the upward flow approaches zero.
)
The region in which these velocity and temperature changes occur is called the boundary layer. Because velocity and temperature gradients both approach zero at the outer edge, there will be no heat flow out of the boundary layer by conduction or convection.
When air is blown across a heated surface, forced convection results. Although the natural convection forces are still present in this latter case, they are clearly negligible compared with the imposed forces. The process of energy transfer from the heated surface to the air is not, however, different from that described for natural convection. The major distinguishing feature is that the maximum fluid velocity is at the outer edge of the boundary layer. This difference in velocity profile and the higher velocities provide more fluid near the surface to carry along the heat conducted normal to the surface. Consequently, boundary layers are very thin.
Heat convection in turbulent flow is interpreted similarly to that in laminar flow. Rates of heat transfer are higher for comparable velocities, however, because the fluctuating velocity components of the fluid in a turbulent flow stream provide a macroscopic exchange mechanism which greatly increases the transport of energy normal to the main flow direction. Because of the complexity of this type of flow, most of the information regarding heat transfer has been obtained experimentally.
Convection heat transfer which occurs during high-speed flight or high-velocity flow over a surface is known as aerodynamic heating. This heating effect results from the conversion of the kinetic energy of the fluid as it approaches a body to internal energy as it is slowed down next to the surface. In the case of a gas, its temperature increases, first, because of compression as it passes through a shock and approaches the stagnation region, and second, because of frictional dissipation of kinetic energy in the boundary layer along the surface.
The phenomena of condensation and boiling are important phase-change processes involving heat release or absorption. Because vapor and liquid movement are present, the energy transfer is basically by convection. Local and average heat-transfer coefficients are determined and used in the Newton cooling-law equation for calculating heat rates which include the effects of the latent heat of vaporization.
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