Flows with heat transfer

Flows with heat transfer

Flows with heat transfer

Flows with heat transfer are one of the common cases of flows in nature or industry. Blood flow in a vessel or fluid flow in heat exchangers are all flows with heat transfer. Heat is a type of energy transferred from one to another due to the temperature difference between two systems. In other words, heat transfer takes place only because of the temperature difference. According to the second law of thermodynamics, heat transfer is always from a body with a higher temperature to a body with a lower temperature and never in the opposite direction.

Heat transfer in nature is done by conduction, convection, radiation, or a combination. Conductive heat transfer is similar to heat transfer in metal dishes used for cooking, where the flame is under the dish, but the handle is also heated. Conductive heat transfer is done through energy transfer between the object’s molecules. Displacement heat transfer is like heating the car’s interior with car heaters in winter, and radiation heat transfer is like heating the earth by the radiation emitted from the sun to the ground.

Due to the many applications of flows with heat transfer in industry and the need for engineers, all three methods of heat transfer have been developed in CFD software and can be simulated.

CONDUCTION

Heat transfer in solids is done by conduction. In hot air furnaces and boilers, the flame’s heat is transferred through the steel body of the combustion chamber to the fluid on the other side, which is air or water.

By receiving energy in a body, the internal energy of the molecules in it increases. And with the increase in the molecules’ energy, the molecules’ vibrancy has increased, And their average speed increases. Molecules with high energy collide with nearby molecules and make them move. In this way, heat energy is transferred to solid bodies. Due to the difference in molecular structure, conductivity is higher in solids than liquids and higher in liquids than gases. Because heat transfer is done by direct contact between molecules and the gas molecules are far apart, heat transfer by conduction is very low in gases.

CONVECTION

In fluids, heat transfer is not significant in conduction, and most heat and energy transfers occur through displacement. In displacement heat transfer, heat is transferred from one fluid to another fluid or solid body. The most important type of displacement heat transfer can be seen in the cooling of gas turbines, heating and cooling facilities, combustion, supersonic and supersonic flows, cooling of atomic reactors, and many other cases. In computational fluid dynamics, estimating the thermal boundary layer is crucial in heat transfer simulation, especially in the cooling discussion. It should also be noted that there is displacement heat transfer in compressible flows due to the change in density and enthalpy.

RADIATION

Heat transfer by radiation method is carried out through wave movement like light waves and by transferring photons with energy. It is transferred from one body to another without the intervention of a medium. Almost all the earth’s thermal energy is supplied by radiation from the sun. Heat waves may be visible or invisible based on the object’s temperature emitting the wave. For example, if a metal is heated enough, it will redden and emit visible heat waves (optical spectrum).

When visible and invisible radiation waves hit an object, they are divided into three parts, one part passes through the object, one part is reflected, and the object absorbs the other. Part of the radiation waves absorbed by the body causes the body’s temperature to rise.

 Like a mirror, objects with a bright color and a shiny surface reflect most of the radiant energy. In contrast, materials with a dark and rough surface absorb most of the radiant energy. A perfect absorbing body is called a black body. Transparent objects such as glass and air pass most of the radiation waves. The amount of radiation depends on the source’s temperature and the surface type. Black and uneven surfaces are good radiators and absorbers, but shiny, smooth, and bright surfaces are weak radiators and absorbers because they reflect most of the radiant energy.

Radiation

Application of CFD in heat transfer simulation          

As mentioned, nowadays, all heat transfer mechanisms can be simulated with the help of CFD software. Engineers need to simulate and analyze flows associated with heat transfer and heat transfer mechanisms. Because the operation of important systems and equipment in the industry, such as heat exchangers, radiators, and solar cells, is based on one or a combination of heat transfer mechanisms. In the following, we will examine some cases of CFD applications in systems whose performance is based on heat transfer.

RADIATORS

Radiators are one of the most common pieces of equipment used for heating in the house, car, hall, etc. The heating mechanism in radiators is based on the circulation of hot fluid. Generally, the fluid used inside the radiators is water. Before entering the radiator, the water receives heat and is heated in a package or engine room. It enters the radiator and moves in the spiral tubes, transferring heat to the outside air and causing air heating.

Of course, the radiator tube is heated first by the conduction heat transfer mechanism. Then the heat transfer is done with the outside air. At the radiator outlet, the water in it is cooled due to the heat transfer it has done, and it must return to the engine house for heating. Finally, after heating, this cycle is repeated.

convection

Using CFD, we can simulate the heating cycle in the radiator and analyze the heat transfer mechanisms. This work helps us to design radiators with higher efficiency and lower energy consumption.

SOLAR CELLS

The Earth receives a lot of energy from the Sun through radiation every second. Therefore, engineers considered using this clean, accessible, and inexhaustible energy. For this purpose, solar cells were invented. A solar cell is an electronic component that converts the thermal energy received from the Sun by radiation into electrical power. We know that with CFD, we can simulate the radiation heat transfer mechanism, which helps us a lot in designing solar cells and improving their efficiency.

solar cell

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