Basic way of heat transfer

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Heat transfer is the study of the law of heat energy transfer between objects or inside objects due to the existence of temperature differences. The objects mentioned here include solids, liquids and gases. In nature, wherever there is a temperature difference, the heat transfer process occurs.

The transfer of thermal energy can be achieved in three ways, they are: conduction, convection and radiation, these three ways are different in nature.

1. Heat conduction

Heat conduction relies on the direct contact of object particles to transfer energy. For example: in a gas, this transfer is completed when the gas molecules collide; in an insulator, this transfer is by means of the vibration of neighboring molecules; in a conductor, this transfer is mainly the thermal motion of free electrons. The characteristic of heat conduction is that in the process of heat transfer, no obvious macroscopic displacement occurs in each part of the object.

In opaque solids, thermal conduction is the only way thermal energy is transferred. When there is a temperature difference in the object, the heat energy will automatically transfer from the high temperature to the low temperature. According to Fourier’s law of heat conduction, the heat conduction rate (also known as heat flow rate) in an object is proportional to the temperature gradient and the cross-sectional area through which the heat flow passes, namely

qk=-λA(dT/dX)     (1-1)

In the formula,

qk—— heat conduction rate, W;

λ—— thermal conductivity (thermal conductivity), W/(m·K);

A—— cross-sectional area, m2;

T—— temperature, K;

X—— Distance along the heat flow direction, m.

In the direction of heat flow, since the temperature T always decreases with increasing distance X, this makes the temperature gradient always negative. Therefore, adding a negative sign to the right side of the heat conduction formula (1-1) makes the heat conduction rate qk a positive value, indicating that the heat flow is consistent with the positive direction of the distance X.

2. Convective heat transfer

Convective heat transfer can only occur in fluids (liquids and gases). When the fluid micelles change their positions in space, they act as heat carriers and transfer heat energy.

Convective heat transfer process can be divided into two categories: natural convection heat transfer and forced convection heat transfer. Natural convection heat transfer refers to the heat transfer phenomenon caused by the buoyancy force in the fluid due to different densities; forced convection heat transfer refers to the heat transfer phenomenon that occurs between the fluid and the wall with different temperatures under the action of external force. The convective heat transfer process is always accompanied by the heat conduction process in which the particles are in direct contact with the particles.

Whether it is natural convection heat transfer or forced convection heat transfer, the flow state and thermophysical properties of the fluid play a very important role in the heat transfer rate of convective heat transfer. According to Newton’s law of cooling, the heat transfer rate of convective heat transfer is proportional to the temperature difference between the surface and the fluid and the surface area in contact with the fluid, namely

qc=hcA(Ts-Tf)     (1-2)

In the formula,

qc—— Convective heat transfer rate, W;

hc——Convection heat transfer coefficient, W/(m2·K);

A—— the surface area in contact with the fluid, m2;

Ts——surface temperature, K;

Tf – fluid temperature, K.

3. Radiation heat transfer

The process of radiative heat transfer is that part of the thermal energy of an object is converted into electromagnetic waves – the radiation energy is emitted outward, and when the electromagnetic waves hit other objects, they are partially absorbed by the latter and re-converted into thermal energy.

All objects can always emit electromagnetic waves as long as their temperature is higher than absolute zero; at the same time, all objects also absorb radiant energy from the outside world. Unlike conduction and convection, the transmission of electromagnetic waves can take place even in a vacuum, such as solar radiation reaching the ground.

Usually, the radiation energy emitted by an object due to a certain temperature is called thermal radiation. The wavelength range covered by thermal radiation is approximately 0.3 to 50 μm. In this wavelength range, there are three bands: ultraviolet, visible and infrared, of which the ultraviolet band is below 0.4 μm, the visible band is 0.4~0.7 μm, and the infrared band is above 0.7 μm. The vast majority of thermal radiation is concentrated in the infrared band.

According to Stefan-Boltzmann’s law, the radiation power of an object is proportional to the 4th power of the temperature of the object and the surface area of the object, namely

qR=εσAT4     (1-3)

In the formula,

qR—— radiated power, W;

σ—— Stephen-Boltzmann constant, 5.669× 10-8W/(m2·K4);

ε—— emissivity;

A—— surface area, m2;

T——Surface temperature, K.

Emissivity is the ratio of the radiant power emitted by an object to the radiant power emitted by a black body at the same temperature. The emissivity is divided into the normal emissivity and the hemispherical emissivity. In the case of engineering applications, the hemispherical emissivity can generally be replaced by the normal emissivity.

In fact, the three heat transfer modes of conduction, convection and radiation often occur at the same time, but under certain conditions, sometimes this mode is dominant, and sometimes another mode is dominant.