natural convection. Types of convection, and how they differ

Thermal conductivity coefficient at room temperature.

The order of magnitude of the thermal conductivity coefficient for various substances.

Convection This is the 2nd way of heat transfer in space.

Convection- this is the transfer of heat in liquids and gases with an uneven temperature distribution due to the movement of macroparticles.

The transfer of heat together with macroscopic volumes of matter is called convective heat transfer, or simply convection.

Heat transfer between liquid and solid surface. This process has a special name. convective heat transfer(heat is transferred from the liquid to the surface or vice versa)

But convection in its pure form does not exist; it is always accompanied by heat conduction, such a joint heat transfer is called convective heat transfer.

The process of heat exchange between the surface of a solid body and a liquid is called heat dissipation, and the surface of the body through which heat is transferred - heat transfer surface or heat transfer surface.

Heat transfer is the transfer of heat from one fluid to another through a solid wall separating them.

Types of fluid movement. Distinguish between forced and natural convection. The movement is called forced if it occurs due to external forces not related to the heat transfer process. For example, due to the communication of energy to it by a pump or a fan. The movement is called free, if it is determined by the heat transfer process and occurs due to the difference in the densities of heated and cold fluid macroparticles.

Movement.modes, liquids. Fluid motion can be steady and unsteady. established called such a movement in which the speed at all points in the space occupied by the fluid does not change with time. If the flow velocity changes in time (in magnitude or direction), then the movement will be transient.

Experimentally established two modes of fluid motion: laminar and turbulent. At laminar flow all fluid particles move parallel to each other and to the enclosing surfaces. At turbulent mode particles of a liquid move randomly, disorderly. Along with directed motion along the flow, the particles can move across and towards the flow. In this case, the velocity of the liquid continuously changes both in magnitude and in direction.

The selection of laminar and turbulent regimes has great importance, since the mechanism of heat transfer in liquids will be different depending on the mode. In the laminar regime, heat in the transverse direction of the flow is transferred only by heat conduction, and in the direction of the flow it is transferred only by heat conduction, and in turbulent, in addition, due to turbulent vortices, or convection.

The concept of the boundary layer. Studies have shown that in the flow of a viscous fluid washing a body, as it approaches its surface, the speed decreases and becomes equal to zero on the surface itself. The conclusion that the velocity of a fluid lying on the surface of a body is zero is called the sticking hypothesis. It is valid as long as the liquid can be considered as a continuous medium.

Let an unbounded fluid flow move along a flat surface (Fig.). The fluid velocity far from it is equal to w0, and on the surface itself, according to the no-slip hypothesis, it is equal to zero. Therefore, near the surface there is a layer of frozen liquid called dynamic boundary layer, in which the speed varies from 0 to ...... Since the speed in the boundary layer approaches w 0 asymptotically, the following definition of its thickness is introduced: thickness dynamic boundary layer is the distance from the surface at which the speed differs from w0 by a certain amount, usually 1%.

As one moves along the surface, the thickness of the boundary layer increases. First, a laminar boundary layer is formed, which becomes unstable with increasing thickness and collapses, turning into a turbulent boundary layer. However, even here, near the surface, a thin laminar sublayer is preserved……., in which the liquid moves laminarly. On fig. shows the change in velocity within the laminar (section I) and turbulent (section II) along

Convection- transfer of heat by moving particles of matter. Convection takes place only in liquid and gaseous substances, as well as between a liquid or gaseous medium and the surface of a solid body. In this case, there is a transfer of heat and thermal conductivity. The combined effect of convection and heat conduction in the boundary region near the surface is called convective heat transfer.

Convection takes place on the outer and inner surfaces of the building fences. Convection plays a significant role in the heat exchange of the internal surfaces of the room. At different values temperature of the surface and the air adjacent to it, there is a transition of heat towards a lower temperature. The heat flux transmitted by convection depends on the mode of motion of the liquid or gas washing the surface, on the temperature, density and viscosity of the moving medium, on the surface roughness, on the difference between the temperatures of the surface and the surrounding medium.

The process of heat exchange between the surface and the gas (or liquid) proceeds differently depending on the nature of the occurrence of gas motion. Distinguish natural and forced convection. In the first case, the movement of gas occurs due to the temperature difference between the surface and the gas, in the second - due to forces external to this process (fan operation, wind).

Forced convection in the general case can be accompanied by the process of natural convection, but since the intensity of forced convection noticeably exceeds the intensity of natural convection, when considering forced convection, natural convection is often neglected.

In the future, only stationary processes of convective heat transfer will be considered, assuming that the speed and temperature are constant in time at any point in the air. But since the temperature of the elements of the room changes rather slowly, the dependences obtained for stationary conditions can be extended to the process non-stationary thermal conditions of the room, at which at each considered moment the process of convective heat transfer on the inner surfaces of the fences is considered to be stationary. The dependences obtained for stationary conditions can also be extended to the case of a sudden change in the nature of convection from natural to forced, for example, when a recirculation device for heating a room (fan coil or split system in heat pump mode) is turned on in a room. Firstly, the new air movement mode is established quickly and, secondly, the required accuracy of the engineering assessment of the heat transfer process is lower than possible inaccuracies from the lack of correction. heat flow during the transition state.

For engineering practice of calculations for heating and ventilation, convective heat transfer between the surface of the building envelope or pipe and air (or liquid) is important. In practical calculations, to estimate the convective heat flux (Fig. 3), Newton's equations are used:

where q to- heat flux, W, transferred by convection from the moving medium to the surface or vice versa;

ta- temperature of the air washing the surface of the wall, o C;

τ - temperature of the wall surface, o C;

α to- coefficient of convective heat transfer on the wall surface, W / m 2. o C.

Fig.3 Convective heat exchange of the wall with air

Convection heat transfer coefficient, a to - physical quantity, numerically equal to the amount of heat transferred from air to the surface of a solid body by convective heat transfer at a difference between air temperature and body surface temperature equal to 1 o C.

With this approach, the entire complexity of the physical process of convective heat transfer lies in the heat transfer coefficient, a to. Naturally, the value of this coefficient is a function of many arguments. For practical use, very approximate values ​​are accepted a to.

Equation (2.5) can be conveniently rewritten as:

where R to - resistance to convective heat transfer on the surface of the enclosing structure, m 2. o C / W, equal to the temperature difference on the surface of the fence and the air temperature during the passage of a heat flux with a surface density of 1 W / m 2 from the surface to the air or vice versa. Resistance R to is the reciprocal of the convective heat transfer coefficient a to.

If you stretch your hand over a hot stove or over a burning electric light bulb, you can feel how jets of warm air rise above these objects. A sheet of paper suspended over a burning candle or electric light bulb begins to rotate under the influence of rising warm air.

This phenomenon can be explained as follows. The air comes into contact with the hot lamp, heats up, expands and becomes less dense than the surrounding cold air. The force of Archimedes, which acts on warm air from the side of cold air from the bottom up, exceeds the force of gravity, which acts on warm air. Thus, warm air rises, thereby giving way to cold air.

We can observe similar phenomena when a liquid is heated from below. Warm layers of liquid - less dense, and therefore lighter - are displaced upwards by denser and heavier, cold layers. Cold layers of liquid, having dropped down, are heated by a heat source and are again displaced by a less heated liquid. Thus, such a movement evenly warms up all the water. This can be seen more clearly if you put a few crystals of potassium permanganate on the bottom of the vessel, which colors the water in purple. In such experiments, we can observe another type of heat transfer - convection(Latin word "convectio"- transfer).

It should be noted that during the process of convection, energy is moved by the jets of gas or liquid themselves. For example, in a room with heating, due to the phenomenon of convection, the flow of heated air rises to the ceiling, and cold air falls to the floor. Thus, the air at the top is much warmer than near the floor.

There are two types of convection: natural(or in other words free) and forced. The examples of heating fluid and air in a room are examples of natural convection. We can observe forced convection when we stir the liquid with a spoon, a stirrer, a pump.

Substances such as liquids and gases must be heated from below. If you do the opposite - heat them from above, there will be no convection. Warm layers cannot physically sink below cold, denser and heavier layers. Thus, for the convection process to proceed, it is necessary to heat gases and liquids from below.

AT solids convection cannot occur. We already know that in solids, particles oscillate around a certain point, because they are held together by mutual attraction. Therefore, when solids are heated, no substance can form in them. In solids, energy can be transferred by conduction.

Convection is widespread in nature: in the lower layers earth's atmosphere, seas, oceans, in the bowels of our planet, on the Sun (in layers up to a depth of ~ 20-30% of the radius of the Sun from its surface). With the help of the phenomenon of convection, gases and liquids are heated in various technical devices.

A simple example of convection can also be the cooling of food in a refrigerator. Freon gas circulating through the pipes of the refrigerator cools the layers of air at the top of the refrigerator. The cooled air, having gone down, cools all the products, and then goes up again. When we lay out food in the refrigerator, do not impede air circulation in it. The grate, located behind the refrigerator, serves to remove warm air, which is formed in the compressor during gas compression. The cooling mechanism of the grate is also convective, so you should leave free space behind the refrigerator so that convection can take place without difficulty.

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