Welcome to solsarin, In this post o we’re mentioning “heat transfer that involves mass movement is”
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion
and exchange of thermal energy (heat) between physical systems.
Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation
and transfer of energy by phase changes.
Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer.
While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.
Heat conduction, also called diffusion, is the direct microscopic exchange of kinetic energy of particles through the boundary between two systems.
When an object is at a different temperature from another body or its surroundings
heat flows so that the body and the surroundings reach the same temperature, at which point they are in thermal equilibrium.
Such spontaneous heat transfer always occurs from a region of high temperature to another region of lower temperature
as described in the second law of thermodynamics.
Heat convection occurs when bulk flow of a fluid (gas or liquid) carries its heat through the fluid.
All convective processes also move heat partly by diffusion, as well. The flow of fluid may be forced by external processes
or sometimes (in gravitational fields) by buoyancy forces caused when thermal energy expands the fluid (for example in a fire plume)
thus influencing its own transfer. The latter process is often called “natural convection”. The former process is often called “forced convection.”
In this case the fluid is forced to flow by use of a pump, fan or other mechanical means.
Physical Medicine and Rehabilitation
G. Ding, … K. Cheng, in Comprehensive Biomedical Physics, 2014
10.15.3.3 Moxibustion Heat Transfer in APs
Heat transfer into the tissue during moxibustion plays a key role in the curative effect of moxibustion.
The results suggest that the moxibustion heat can reach deeply into biological tissue, such as the IOM between the tibia and fibula.
The numerical simulation also showed that heat is first absorbed and then emitted by the tissue, which forms heat oscillation (Cheng et al., 2008).
The numerical simulation shows that the heat from the warm needle moxibustion transfers into a deeper part of the tissue than indirect moxibustion.
The simulation also proves that because silver is highly conductive,
the tissue temperature 7 mm below the skin during moxibustion is 320 K when using silver needles
while gold and stainless‐steel needles only generate 300 and 290 K, respectively.
This may be why ancient Chinese acupuncturists used warm silver needle moxibustion in deficiency syndromes for the best curative effect (Cheng et al., 2007).
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EVAPORATION | Basic Principles
Heat Transfer by Convection
Heat transfer by convection occurs as a result of the movement of fluid on a macroscopic scale, in the form of eddy and circulation currents.
This convective movement can take two forms:
Natural convection: these currents arise from the heating process itself
because of the differences in density between the heated fluid and the surrounding colder fluid, which causes the former to rise
and the latter to sink to take the place of the former.
Forced convection: in this type of convection
the currents are created by an external device, e.g., a circulation pump causing turbulent flow in a pipe.
When heat transfer occurs from a surface into the body of a fluid, natural convection currents are weakest at the surface
Consequently heat transfer across this film can only occur by conduction, and, as mentioned above, thermal conductivity in fluids is low.
Hence, the main resistance to heat transfer into fluid in a pipe is this film adjacent to the pipe wall.
An increase in the velocity of the fluid moving through the pipe will reduce the thickness of this static film and give rise to an overall increase in the heat transfer into the fluid.
In theory, the heat load transferred across this film is defined as in eqn (1). However, in practice, it is difficult to calculate the film thickness, X, and so the following relationship is used:
where h is the heat-transfer coefficient.
Thermal resistance is therefore the reciprocal of the heat-transfer coefficient, i.e., 1⧸h.
Forced convection is of more importance than natural convection in industrial food processes and equipment, where the fluids are under turbulent flow conditions.
Whenever possible, streamline flow conditions should be avoided or at least minimized, as the heat transfer coefficients of fluids (in convective heat transfer) are much greater than thermal conductivity factors (in conductive heat transfer).
In a tubular heat exchanger (e.g., an evaporator), where perhaps one fluid is flowing inside a pipe and being heated (or cooled) by another outside, the consideration must be of heat transfer both inside and outside the tube. In the latter case, flow can be either lengthwise along the tube (in either direction) or at right angles to the single tube or tube bundle.
Also, those fluids passing along the length of the heat exchange tube (either inside or outside)
will experience either an increase or decrease in temperature, obviously because of heat transfer.
This means that in order to correctly quantify this heat transfer
it is first necessary to define the difference in temperature between the fluids, given that it will not only vary at different points along the length of the tube,
It is necessary to calculate an ‘average’ value for the temperature difference, and the factor normally employed is the logarithmic mean temperature difference
Sleep Disorders Part I
Heat transfers between the body and the environment occur via four channels:
(1) conduction – transfer between the skin surface area and any materials with which the body is in contact;
(2) convection – transfer induced by air movements around the body;
(3) radiation – transfer between the skin surface and the surrounding surfaces in the form of invisible, electromagnetic energy;
and (4) evaporation – transfer via respiratory and transepidermal water losses and from sweating.
With the exception of evaporation, which always represents body heat loss
the other three transfers (referred to as dry heat transfers)
can be either negative (heat loss from the body) or positive (heat gain by the body).
Heat exchanges depend not only on the ambient parameters but also on morphological and anatomical parameters. From the latter point of view, neonates are at a disadvantage when compared with adults, since a neonate’s high skin surface area to body volume ratio increases heat losses to the environment.
The neonate’s low weight (body mass acts as a heat buffer)
and the poorly insulating body shell are responsible for greater fluctuations in internal temperature than in adults.
Moreover, high skin permeability in premature neonates enhances evaporative water loss, particularly during the first weeks of life.
Thus, the risk of hypothermia increases and neonates have a greater need for additional heat than adults.
However, they also have less insulating subcutaneous tissue, which thus leads to greater heat losses.
Finally, the heat reservoir in the elderly is lower than in young adults (Van Someren et al., 2002).
Medtronic CryoCath Technology
Jeffrey Silver BA, … Dan Wittenberger BS, in Cryoablation of Cardiac Arrhythmias, 2011
Thermal Conduction and Convection
Heat transfer is the process by which energy in one body is communicated to another body with which it is in contact.
The transfer of heat always occurs from a body with greater potential and kinetic energy to a body of lower potential and kinetic energy, in other words, from high to low temperature.
heat flows from your hand to the object, warming the object up and creating a sensation of cold in your hand.
The phenomenon described earlier (heat flow from high-energy to low-energy molecules) is consistent with the second law of thermodynamics. Heat does not spontaneously flow from a body at lower temperature to a body at higher temperature. Thermal conduction occurs in all forms of media, including solids, liquids, and gases.
Although the principles of heat conduction are based on the transfer of energy in materials by vibrations at the molecular level
heat convection is a macroscopic and observable mechanism of thermal transfer that involves bulk motion of fluids.
Heat convection occurs only in liquids and gases, or in mixtures of both. In forced convection
For example, it is well-known that a hot plate of metal will cool faster when exposed to a blowing fan than when exposed to still air.
Another mechanism of heat convection is natural convection, which occurs when temperature gradients across a fluid create density gradients.