convection

[duì liú]

Main heat transfer mode of fluid

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Convection refers to the relative flow caused by different temperatures of various parts of the fluid, that is, the process of heat transfer of fluid (gas or liquid) through the macro flow of its own parts. liquid or Gas In the middle, the hotter part rises and the colder part drops, circulating and mixing with each other, finally making the temperature tend to be uniform. Since the thermal conductivity of the fluid is very small, there is little heat transferred through heat conduction, and convection is the main heat transfer mode of the fluid.^[1]

Convective separability natural convection and Forced convection Two. Natural convection often occurs naturally, which is caused by density change due to concentration difference or temperature difference. The temperature gradient in the fluid will cause the density gradient change. If the low-density fluid is at the bottom and the high-density fluid is at the top, natural convection will be formed under the action of gravity. Forced convection is the convection caused by external forces. Convective heat transfer can be accelerated by increasing the flow speed of liquid or gas.^[1]

Chinese name: convection
Foreign name: convection
Pinyin: duì liú

Role: fluid Main points of conduct heat mode
Interpretation: The relative flow in the fluid due to different temperatures of various parts
Classification: Natural convection, forced convection

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seven Engineering convection

definition

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convection

liquid phase or Gas phase Of each part of relative motion 。 because Concentration difference Or convection scale caused by density change caused by temperature difference natural convection ； Due to external force (such as stir ）And the generated convection scale Forced convection 。 about electrolyte For example, solute Will move with the convection of the liquid phase Mass transfer A type of process.

The process by which a fluid (gas or liquid) transfers heat through the macroscopic flow of its parts. Fluidic Thermal conductivity Very small, via heat conduction There is little heat transferred, and convection is the main heat transfer mode of fluid. Convection can be divided into natural convection and Forced convection 。 Temperature in fluid gradient Will cause density gradient If the low-density fluid is below and the high-density fluid is above, natural convection will occur under the action of gravity. In winter, indoor heating is conducted by natural convection of indoor air, which also exists in the atmosphere and ocean. Make fluid by external action Circulating flow Therefore, the heat transfer is forced convection.^[2]

Atmospheric convection

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Atmospheric convection

atmospheric convection

The vertical upward movement of a mass of air in the atmosphere under the action of heat or power. On the one hand, the exchange of heat, momentum and water vapor between the lower and upper levels of the atmosphere can be generated through atmospheric convection; on the other hand, the water vapor caused by convection coagulation Precipitation may occur. Atmospheric convection under thermal action mainly refers to that in the atmosphere with unstable stratification, the density of a mass of air is less than that of the ambient air, so it is subject to buoyancy greater than gravity Archimedes buoyancy The upward movement formed under the action of. Small scale, short-term, sudden and Cumulonimbus cloud The precipitation is often caused by atmospheric convection under thermal action. Under dynamic action Atmospheric convection Mainly refers to the air flow level convergence Or the ascending movement formed under the condition of terrain. Large scale precipitation in the atmosphere is often frontal surface And the associated horizontal convergence and uplift of the air flow, while the precipitation generated in the fixed area near the mountains is often caused by the forced uplift of the terrain. The atmospheric convection formed by some special terrain (such as trumpet shaped terrain) has both the role of terrain uplift and the role of terrain in horizontal convergence of air flow.^[1]

On the one hand, thermal and dynamic effects can form atmospheric convection, on the other hand, atmospheric convection can affect the thermal and dynamic structure of the atmosphere, which is the feedback effect of atmospheric convection. This feedback is particularly important in the tropical region where the atmosphere is located steam Condensation heating is usually applied to a large area Atmospheric motion Important energy.

Convective gas reservoir

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troposphere

Located in the lowest layer of the atmosphere, it concentrates about 75% of the air quality and more than 90% of the water and gas quality. Its lower boundary is connected with the ground, upper bound Height varies with geographical latitude and season. The average altitude in low latitude areas is 17~18 kilometers Mid latitude The average area is 10-12 kilometers, and the average polar area is 8-9 kilometers. Summer is higher than winter.

In the troposphere, the temperature decreases with the increase of altitude, and the average temperature decreases by 0.65 ℃ for every 100 meters. Due to the great impact of the surface, Meteorological elements (air temperature, humidity, etc.) is unevenly distributed horizontally. Air regular Vertical movement And irregular turbulence. The moisture, dust and heat in the upper and lower layers exchange and mix. Since more than 90% of the moisture is concentrated in the troposphere, many weather phenomena such as clouds, fog, rain, snow, etc. occur in the troposphere.

The troposphere layer from the ground to 1 to 2 kilometers is greatly affected by the ground fluctuation, dry and wet, cold and warm, called Friction layer (or atmospheric boundary layer ）。 The friction layer above is less affected by the ground conditions, which is called Free atmosphere 。 Troposphere and its upper stratosphere There is a transition layer between them, called Tropopause The thickness is about several hundred meters to two kilometers. The temperature near the tropopause changes abruptly with the increase of height, or the temperature decreases slightly with the increase of height, or the temperature remains unchanged with the increase of height, or the temperature increases slightly with the increase of height. It has a strong blocking effect on vertical movement.

Mantle convection

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mantle convection hypothesis

Mantle convection

A description of the interior of the earth Material movement And interpreting the crust or lithosphere Mechanistic hypothesis 。 It believes that mantle There is convection of matter in circulation 。 In the heating center of the mantle, the material becomes lighter and slowly rises to form Upflow , to the asthenosphere advection After a certain distance, the advection meets with another opposite advection to form a downflow, and then flows back to the bottom of the upflow at the depth to supplement the upflow, thus forming an annular pair of fluids. Carried on the upper advection of the fluid Lithospheric plate Make large-scale and slow Horizontal movement 。 Formed at upflow Mid ocean ridge , downflow causes subduction and Continental collision 。 In 1928, British geologist A. Holmes believed that the crust at the upwelling place split to form a new ocean floor, and the crust at the downwelling place of convection squeezed to form mountains. 1939 D T. Griggs proposed that due to poor heat conduction of rocks, Radiant heat The aggregation of causes convection. Late 1960s Plate tectonics After establishment, mantle convection movement is generally considered as Plate movement Driving force.

Sublithospheric Asthenosphere 10% Melt 。 The solid mantle below the lithosphere behaves like viscous liquid due to high temperature and pressure toughness And can generate flow. Mantle mesogene radio isotope The transformation generates heat and heats up, and the density decreases, so Light substances The upward movement and downward movement of heavy materials in order to reach the stable state of the lowest energy level are called mantle convection, which is very slow. Its upwelling can last for tens of millions to hundreds of millions of years.

earthquake wave velocity The discovery of anisotropy of Mantle convection The hypothesis that causes the directional arrangement of crystals strongly supports mantle convection hypothesis 。 J. Morgan proposed a uniaxial plume mantle convection model in the 1970s. The fluid rises from the bottom of the mantle at a rate of several centimeters per year, forming the upward current as the axis, Downflow The outer cylinder is opposite to the fluid. The region of the crust facing the upwelling is the hot spot.

Thermal convection

Thermal convection refers to the phenomenon that heat propagates from one place to another in space through flowing media. The larger the ventilation hole area in the fire field, the faster the thermal convection speed; The higher the position of the ventilation hole, the faster the thermal convection speed. Heat convection is an important way of heat transmission, and is the most important factor affecting the development of initial fire. The main factors affecting heat conduction are: temperature difference thermal conductivity And the thickness and cross-sectional area of the conductive object. The greater the thermal conductivity, the smaller the thickness, and the more heat transferred.

fluid mechanics

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In fluid mechanics, the convection process refers to the change of the total amount of c (physical quantities carried in the unit volume of fluid, such as density, heat, pollution concentration, etc.) in the control volume V due to convection when the fluid is in the macro flow state. It can be considered as composed of two parts: one is that c changes with time in V, and the other is that c changes in fluid due to fluid flow and position changes in region V. The intensity of convection is related to the flow velocity.

Difference from diffusion

The diffusion process in fluid mechanics includes molecular diffusion formed by molecular Brownian motion and turbulent diffusion formed by fluid turbulent motion. This diffusion makes the physical quantity c flow field Medium moves from high value to low value. Diffusion rate Is proportional to the gradient of c.

In hydrodynamic momentum governing equation The convection term in is nonlinear, which brings great inconvenience to the general finite element numerical method.

heat transfer

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(1) The process of defining or explaining the heat transfer within a substance (system) is called heat transfer 。

(2) It indicates that heat transfer is realized through heat conduction, convection and thermal radiation In three ways. In actual Heat transfer process These three methods are often accompanied by.

① Heat conduction: Heat transfer from one part of the system to another or from one system to another is called heat conduction. Heat conduction is the main mode of heat transfer in solids. In gas or liquid, heat conduction and convection often occur simultaneously. Various substances have different thermal conductivity. Generally, metals are good conductors of heat, while glass, wood, cotton wool products, feathers, furs, liquids and gases are bad conductors of heat. Asbestos has extremely poor thermal conductivity, and is often used as Thermal insulation material 。

② Convection: The process of circulating flow between the hotter part and the cooler part of a liquid or gas to make the temperature uniform. Convection is a unique way of heat transfer in liquid and gas Convection phenomenon It is more obvious than liquid. Convection can be divided into natural convection and forced convection. Natural convection often Spontaneous occurrence , caused by uneven temperature. Forced convection is formed due to the external influence on fluid agitation. Convective heat transfer can be accelerated by increasing the flow speed of liquid or gas.

③ Thermal radiation: objects have the ability to emit energy due to their own temperature. This way of heat transfer is called thermal radiation. Although heat radiation is also a way of heat transfer, it is different from heat conduction and convection. It can transfer heat directly from one system to another without depending on the medium. Thermal radiation to electromagnetic radiation The higher the temperature, the stronger the radiation. The wavelength distribution of radiation also varies with the temperature. For example, when the temperature is low, it is mainly invisible infrared light When the temperature is 500 ℃ or higher, visible light and even ultraviolet radiation will be emitted successively. Thermal radiation is the main way of long-distance heat transfer. For example, the heat of the sun is transmitted to the earth through space in the form of thermal radiation.

Engineering convection

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In earthwork construction, it is often encountered that earthwork convection means borrowing from each other and causing waste.

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