A Physics revision video explaining the process of heat transfer by Convection. Heat Convection. Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law). Heat transfer—the physical act of thermal energy being exchanged between two systems by dissipating heat—can be grouped into three broad.
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Generally is not known and needs to be found and it is customary to calculate the heat transfer using.
This quantity has the symbol and is convection heat transfer as the convective heat transfer coefficient. The convective heat transfer coefficient is defined by 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. Convection heat transfer 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.
CONVECTIVE HEAT TRANSFER
Two types of convective heat transfer may be distinguished: Free or natural convection: In the absence of an internal source, when the fluid is in contact with a hot surface, its molecules separate and scatter, causing the fluid to be less dense. This will convection heat transfer demonstrated below.
It is useful to note, however, that there is an intermediate region where there is no potential core and where the flow is not fully-developed.
In this region, it is necessary to solve the equations representing conservation of mass and momentum so that each is satisfied; this may require an interactive approach. In the case of fully-developed laminar flow, the convective terms become zero since and the momentum equation becomes with the pressure gradient constant so that integration with the boundary conditions at the wall and on the symmetry line leads to: The velocity convection heat transfer for fully-developed laminar flow is a parabola when the walls are stationary, provided that the fluid properties are constant and the velocities are low; it is linear when the pressure gradient is absent and the wall moves with constant speed with respect to the other.
The effect of a moving surface is to provide a force which can act against or with that exerted by pressure. This is reflected in the velocities which can be in positive and negative directions.
The temperature profile is expressed in terms of surface temperatures and it is clear that the bulk temperature will increase if one or both of the walls is hotter than the initial temperature, T1 Thus, the temperature profile is often expressed in terms of the initial temperature and the bulk-mean temperature, defined as: Heat transfer in a pipe Flow and heat transfer in a pipe are of rather more importance than those between parallel plates since they are found more frequently in engineering practice.
The flow may again begin at the leading edge so that laminar flow solutions can be obtained as for parallel plates, but this time to equations in cylindrical coordinates and without the prospect of one surface moving with respect to another.
Convective heat transfer
The flow in small-diameter pipes required to achieve these small Reynolds numbers comes from larger diameter pipes or from plenum chambers, so it is likely that boundary layers do not have their origins at the convection heat transfer of the small diameter pipe.
Convection heat transfer, there is a sudden contraction for which the flow is properly represented by more complete forms of the conservation equations than their boundary-layer forms.
- Heat transfer - Wikipedia
- Convective heat transfer - Wikipedia
- CONVECTIVE HEAT TRANSFER
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Indeed, the flow may separate inside the pipe with a more rapid movement towards fully-devel-oped conditions than would be the case with convection heat transfer boundary layers. The region of developing flow can be small in many cases and fully-developed flow is usually more important than the developing flow.
Convective Heat Transfer
The energy equation in cylindrical coordinates has the form and this reduces for fully-developed flow to where Tb convection heat transfer the bulk temperature, defined as: Integration of the differential equation with boundary conditions corresponding to symmetry at the centre line, and for the particular condition that leads to and to which is independent of the Reynolds and Prandtl numbers, provided the flow remains laminar.
Since the amount of carbon dioxide determines the radiative balance of Earth atmosphere, carbon dioxide removal techniques can be applied to reduce the radiative forcing.
Solar radiation management is the attempt to absorb less solar radiation to offset the effects of greenhouse gases. Greenhouse effect[ edit ] A representation convection heat transfer the exchanges of energy between the source the Sunthe Earth's surface, the Earth's atmosphereand the ultimate sink outer space.
The ability of the atmosphere to capture and recycle convection heat transfer emitted by the Earth surface is the defining characteristic of the greenhouse effect. The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions.
Since part of this re-radiation convection heat transfer back towards the surface and the lower atmosphere, it results in an elevation of the average surface temperature above what it would be in the absence of the gases.