Convection

The three forms of heat transfer are: conduction, convection and radiation, through which heat is transported between areas with different temperatures. Convection occurs only by means of fluid materials, the evaporation of water or fluids. Convection itself is the transport of heat through the movement of fluid. For example, when heating water in a saucepan, the water that comes into contact with the bottom of the saucepan rises as it heats, while the water on the surface runs down the sides as it cools, taking the place left by the hot portion.. Similar to driving, it requires a material for transfer. Unlike radiation, which does not need a medium for the transfer to occur.

Heat transfer involves the transport of heat in a volume and the mixing of macroscopic elements from hot and cold portions of a gas or liquid. It also includes the exchange of energy between a solid surface and a fluid or by means of a pump, fan or other mechanical device (mechanical, forced or assisted convection). This is characterized by the Nusselt number (Nu) which is a function of the Reynolds (Re) and Prandtl (Pr) numbers.. In the case of laminar flow within the pipe, the Sieder-Tate equation is used. For turbulent flow inside a pipe, the Boelter-Dittus equation is used.

In free or natural heat transfer, a fluid is hotter or colder. Contact with a solid surface causes a circulation due to the density differences that result from the temperature gradient in the fluid. Circulation is caused by buoyant forces and viscous forces. The relationship between both forces is the Grashof number (Gr) which is a function of the Reynolds and Prandtl number.

Convection can be external or internal. When it is external then the fluid moves over the surfaces and if it is internal then it moves inside the surfaces.

As well as the hydrodynamic layer in momentum transfer, the thermal boundary layer in heat transfer serves to contrast the thicknesses of the layers. The relationship between the property transfer layers is used to know which transfer is greater at the molecular level, such a relationship is the Prandtl number. The number of Pr is greater than 1, less than 1 or equal to 1. It serves to know how they are linked to each other. Lubricants have high Pr numbers. The Pr of gases is 0.70.

The heat transfer by convection is expressed with Newton's Law of cooling:

dQdt=hAs(Ts− − Tinf){displaystyle {frac {dQ}{dt}}}=hA_{s}(T_{s}-T_{inf })}

Symbol	Name
h{displaystyle h}	Convection coefficient
As{displaystyle A_{s}}	Body area in contact with the fluid
Ts{displaystyle T_{s}}	Temperature on the surface of the body
Tinf{displaystyle T_{inf }}	Fluid temperature away from the body

The convective coefficient, that is, the constant for conduction, is the thermal conductivity. It depends on the fluid properties, system geometry, flow velocity, temperature distribution, and temperature variation. The dimensional analysis allows to determine an equation that relates the convection coefficient with other variables which can be quantified, this occurs for forced convection as well as for free convection. If an exact analysis of the boundary layer is carried out then, from the Navier-Stokes Equation, a final equation for the momentum balance is obtained depending on the circumstances. The same procedure is carried out for the energy balance and another final equation is obtained. The relationship that allows determining the convection coefficient results from linking the previous ones. For fluids with Pr=1 it happens that the number of Nu depends only on the number of Re.

Atmospheric convection

Convection in Earth's atmosphere involves the transfer of enormous amounts of heat absorbed by water. It forms clouds of great vertical development (for example, cumulus congestus and, above all, cumulonimbus, which are the types of clouds that reach greater vertical development). These clouds are the typical carriers of thunderstorms and heavy rainfall. When they reach a very high altitude (for example, about 12 or 14 km) and cool down abruptly due to the low atmospheric temperature at that height, they can produce electrical storms, hailstorms and heavy rains, since the raindrops increase in size as they rise. violently and then rush to the ground either in a liquid or a solid state. They can be shaped like a large asymmetrical mushroom; and sometimes a trail that resembles a kind of anvil is formed in this type of cloud.

The process that causes convection within the Earth's atmosphere is extremely important and generates a series of fundamental phenomena in the explanation of winds and the formation of clouds, troughs, cyclones, anticyclones, precipitation, etc. All atmospheric heat convection processes and mechanisms obey the physical laws of thermodynamics. Of these processes, the one that explains the water cycle in nature or the hydrological cycle is fundamental. Almost all the aforementioned phenomena have to do with this last mechanism. Subsidence is the inverse phenomenon of convection, whereby the air at high altitude cools considerably and forms an anticyclonic zone that descends due to its greater density, bringing cold and dry air to the earth's surface, which can give rise to eddies of dust and even tornadoes when they come into contact with a convection zone.

The path of water in the atmosphere is also called the hydrological cycle (or water cycle) due to the ability of water to absorb heat and release it thanks to its ability to transform from one physical state to another. Broadly speaking, the hydrological cycle works as follows: the sun's rays heat the surfaces of marine and terrestrial waters which, by absorbing this heat, go from the liquid to the gaseous state in the form of water vapor. The vapor rises to a certain height and in doing so, it loses heat, condenses and forms clouds, which are made up of very small water droplets that remain in suspension at a certain height. When this condensation is accelerated, due to the rise of the cloud mass itself (convection), clouds of greater vertical development are formed, with which the drops increase in size and form precipitation, which can be both solid (snow, hail) as watery (rain), depending on the temperature. This precipitation can fall both in the sea and in the emerged lands. Finally, part of the water that precipitates on the continents and islands passes back into the atmosphere by evaporation or produces fluvial currents that carry a large part of the terrestrial waters back to the seas and oceans, thus closing the cycle, which repeats itself.

Behavior of any fluid in heat transfer

When a fluid gives up heat, its molecules slow down, so its temperature decreases and its density increases, its molecules being attracted by the Earth's gravity.

When the fluid absorbs heat, its molecules accelerate, so its temperature increases and its density decreases, which makes it lighter.

The coldest fluid tends to go down and occupies the lowest level of the vertical and the hottest fluids are displaced to the highest level, thus creating the earth's winds.

Convective heat transfer consists of the contact of a fluid with an initial temperature with another element or material with a different temperature. Depending on the variation in temperatures, the molecular energy charges of the fluid will vary, and the interacting elements of the system will carry out work, where the one with the highest energy or temperature will transfer it to the one with the lowest temperature. This thermal transfer will be carried out until both have the same temperature; while the process is being carried out, the molecules with less density will tend to rise and those of greater density will decrease in level. The molecules that are in the lower layers increase their temperature.

Heat exchangers

A heat exchanger is a device built to efficiently exchange heat from one fluid to another, whether the fluids are separated by a solid wall to prevent mixing, or in direct contact. Heat exchangers are widely used in refrigeration, air conditioning, heating, power generation, and chemical processing. A basic example of a heat exchanger is a car radiator, in which the hot radiator fluid is cooled by the flow of air over the radiator surface.

The most common heat exchanger arrangements are co-flow, counter-flow, and cross-flow. In parallel flow, both fluids move in the same direction during heat transfer; in countercurrent, the fluids move in the opposite direction and in crossflow the fluids move at a right angle to each other. The most common types of heat exchangers are shell and tube, double tube, extruded finned tube, spiral fin tube, U-tube, and plate. More information on heat exchanger flows and configurations can be found in the heat exchanger article.

When engineers calculate the theoretical heat transfer in an exchanger, they must deal with the fact that the temperature gradient between the two fluids varies with position. To solve the problem in simple systems, the logarithmic mean temperature difference (DTML) is often used to statistically determine a mean temperature value. In more complex systems, direct knowledge of the DTML is not possible and the Number of Transfer Units (NUT) method can be used instead.