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What are the influencing factors of thermal conductivity?

2020-02-24 12:15:34
Times

What are the influencing factors of thermal conductivity? The thermal conductivity of different substances is different; the thermal conductivity of the same substance is related to its structure, density, humidity, temperature, pressure other factors. When the moisture content temperature of the same material are low, the thermal conductivity is small.


Generally speaking, the thermal conductivity of a solid is larger than that of a liquid, while that of a liquid is larger than that of a gas. This difference is largely due to the different molecular spacing between the two states. Now the coefficient values used in engineering calculation are determined by special tests.


With the increase of temperature moisture content, the thermal conductivity of the five typical building materials is increasing. This is analyzed the micro mechanism. For porous materials, when they are damped, the liquid water will replace the original air in the micropore; under normal temperature pressure, the thermal conductivity of liquid water (about 0.59w (m · K)) is far greater than that of air (about 0.026w (m · K)), so the thermal conductivity of wet materials will be greater than that of dry materials, the higher the moisture content, the greater the thermal conductivity. If water condenses into ice at low temperature, the overall thermal conductivity of the material will increase because the thermal conductivity of ice is as high as 2.2W (m · K)).


Different the effect of dampness, the increase of temperature will accelerate the heat movement of molecules, promote the heat conduction of solid skeleton the convection heat transfer of fluid in pores. In addition, the radiation heat transfer between the pore walls will also be strengthened due to the increase of temperature. If the material is wet, the temperature gradient may also have an important impact: the temperature gradient will form a vapor pressure gradient, which will make the water vapor migrate the high temperature side to the low temperature side; under specific conditions, the water vapor may condense at the low temperature side, the liquid water formed will migrate the low temperature side to the high temperature side under the driving of capillary pressure. Such a cycle, similar to the heat pipe to strengthen the heat transfer, makes the thermal conductivity of the material significantly increased.


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