What is a heat pipe radiator? How it works, products.

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Resume

Basic Work of Heat Pipe

Basic Characteristics of Heat Pipe

Heat Pipe Compatibility and Lifetime

Heat Pipe Manufacturing

Manufacturing Process

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Heat Pipe Technology is a heat transfer element called "heat pipe" invented by G.M. Grover of the LosAlamos National Laboratory of the U.S. in 1963. G.M. Grover of LosAlamos National Laboratory invented a kind of heat transfer element called "heat pipe", which makes full use of the principle of heat conduction and the rapid heat transfer properties of the cooling medium, and rapidly transfers the heat from the heated object through the heat pipe to the outside of the heat source, whose thermal conductivity exceeds the thermal conductivity of any known metal. Heat pipe technology used to be widely used in aerospace, military and other industries, since it was introduced into the radiator manufacturing industry, making people change the traditional radiator design ideas, get rid of simply rely on a high air volume motor to get a better cooling effect of a single cooling model, the use of heat pipe technology makes the radiator even if the use of low-speed, low-volume motors, the same can be satisfied with the results, so that troubled by the air-cooled heat dissipation of noise issues The heat pipe technology enables the heat sink to get satisfactory results even with low speed and low airflow motors, which makes the noise problem plaguing the air-cooled heat dissipation well solved and opens up new horizons in the cooling industry.

From a thermodynamic point of view, why does a heat pipe have such a good thermal conductivity? Objects of heat absorption, exothermic is relative, where there is a temperature difference exists, there is bound to be heat from the high temperature to the low temperature transfer phenomenon. From the heat transfer of the three ways: radiation, convection, conduction, which is the fastest heat conduction. Heat pipe is the use of evaporation refrigeration, so that the temperature difference between the two ends of the heat pipe is very large, so that the heat is rapidly conducted. The general heat pipe consists of a tube shell, a liquid-absorbing core and an end cap. The inside of the heat pipe is pumped into a negative pressure state, filled with the appropriate liquid, which has a low boiling point and is easily evaporated. The wall of the tube has an absorbent core, which consists of a capillary porous material. A section of the heat pipe for the evaporating end, another section for the condensing end, when a section of the heat pipe is heated, the liquid in the capillary tube evaporates rapidly, the vapor flows to the other end under the tiny pressure difference, and releases heat, and re-condenses into liquid, the liquid is then flowed back to the evaporating section along the porous material by the action of the capillary force, and so on more than the cycle of heat from one end of the heat pipe to the other end. This cycle is rapid, the heat can be a continuous conduction.

Edit the basic work of the heat pipe

Typical heat pipe by the shell, suction core and end cap composition, the tube will be pumped into 1.3 × (10 negative 1 - 10 negative 4) Pa negative pressure after filling with the appropriate amount of working liquid, so that the suction core capillary tightly attached to the tube wall. The porous material is filled with liquid and sealed. One end of the tube for the evaporation section (heating section), the other end of the condensation section (cooling section), according to the application needs in the middle of the two sections can be arranged adiabatic section. When one end of the heat pipe is heated when the liquid in the capillary core evaporates and vaporizes, the steam flows to the other end under a slight pressure difference and releases heat to condense into liquid, and the liquid flows back to the evaporation section along the porous material by the action of capillary force. So the cycle is not their own, the heat from one end of the heat pipe to the other - end. Heat pipe in the realization of this heat transfer process, contains the following six interrelated major processes:

(1) heat from the heat source through the heat pipe wall and full of working liquid suction core transfer to the (liquid - vapor) interface;

(2) liquid in the evaporation section of the (liquid - vapor) interface;

(2) the liquid in the evaporation section of the (liquid - vapor) interface. section;

(3) the vapor in the vapor chamber flows from the evaporation section to the condensation section;

(4) the vapor condenses on the vapor-liquid dividing surface in the condensation section:

(5) the vapor is transferred to the condensation section through the wall of the heat pipe and the suction core filled with the working liquid. Liquid dividing surface condensation:

(5) heat from the (vapor - liquid) interface through the suction core, liquid and tube wall to the cold source:

(6) in the suction core due to the capillary effect of the condensed working liquid back to the evaporation section.

Edit the basic characteristics of the heat pipe

The heat pipe is a heat transfer element that relies on the phase change of the working liquid inside itself to realize heat transfer, with the following basic characteristics.

(1) very high thermal conductivity heat pipe inside the main working liquid vapor, liquid phase change heat transfer, thermal resistance is very small, so has a very high thermal conductivity. Compared with silver, copper, aluminum and other metals, the unit weight of the heat pipe can transfer several orders of magnitude more heat. Of course, high thermal conductivity is also relative, the temperature difference always exists, may violate the second law of thermodynamics, and the heat transfer capacity of the heat pipe is limited by a variety of factors, there are a number of heat transfer limit; heat pipe axial thermal conductivity is very strong, radial and not much improvement (except radial heat pipe).

(2) excellent isothermal heat pipe cavity of the steam is in a saturated state, saturated steam pressure is determined by the saturation temperature, saturated steam from the evaporation section of the condensing section of the pressure drop is very small, according to the thermodynamics in the equation can be seen, the temperature drop is also very small, so the heat pipe has excellent isothermal.

(3) variable heat flow density heat pipe can independently change the evaporation section or the cooling section of the heating area, that is, a smaller heating area to input heat, and a larger cooling area output heat, or heat pipe can be a larger area of heat transfer heat input, and a smaller cooling area output heat, so that you can change the density of the heat flow, to solve the problem of heat transfer of some of the other methods are difficult to solve.

(4) reversibility of the direction of heat flow discretion A horizontally placed cored heat pipe, because of its internal circulation power is capillary force, so any one end of the heat can be used as an evaporation section, and the other end of the outward heat dissipation becomes the condensation section. This feature can be used for spacecraft and artificial satellites in space temperature leveling, can also be used for the first exothermic after heat absorption of chemical reactors and other devices.

(5) thermal diode and thermal switch performance heat pipe can be made into a thermal diode or thermal switch, the so-called thermal diode is to allow the heat flow in one direction only, and does not allow the flow in the opposite direction; thermal switch is when the temperature of the heat source is higher than a certain temperature, the heat pipe begins to work, when the temperature of the heat source is lower than the temperature, the heat pipe does not transfer heat.

(6) thermostatic characteristics (controllable heat pipe) Ordinary heat pipe of the parts of the thermal resistance basically does not change with the change in the amount of heat, so when the amount of heat changes, the heat pipe to prepare for the part of the temperature also changes. But people developed another kind of heat pipe - variable heat pipe, so that the condensing section of the thermal resistance with the increase in the amount of heat and reduce, with the decrease in the amount of heat and increase, so that the heat pipe in the case of a substantial change in the amount of heat, the steam temperature change is very small, to achieve the control of the temperature, which is the thermostatic properties of the heat pipe.

(7) the adaptability of the environment The shape of the heat pipe can be changed with the conditions of the heat source and cold source, the heat pipe can be made into a motor shaft, gas turbine blades, drills, scalpels, etc., the heat pipe can also be made into a separate type, in order to adapt to the heat transfer over long distances or impulse heat fluid can not be mixed in the case of heat transfer; heat pipe can be used both on the ground (gravitational field), but also can be used in space (gravity field).

The figure above shows the shape of the vapor-liquid interface in the heat pipe, the vapor mass flow rate, the pressure, and the trend of the wall temperature T w and the vapor temperature T v along the length of the pipe. Along the entire length of the heat pipe, the static pressure difference between the vapor and liquid phases at the vapor-liquid interface is in equilibrium with the local capillary pressure difference there.

△ Pc (capillary pressure head - is the working fluid circulation within the heat pipe to overcome the vapor from the evaporating section to the condensing section of the pressure drop

△ Pv, condensing liquid from the condensing section back to the evaporating section of the pressure drop

△ Pl and the gravitational field on the flow of the pressure drop of the liquid (△ Pg can be a positive value, a negative value or zero). depending on the position of the heat pipe in the gravity field).

Therefore, Δ Pc ≥ ΔPl + Δ P v + Δ Pg is a necessary spare part for the normal operation of the heat pipe.

Due to the use of heat pipe, type and type more, coupled with the heat pipe in the structure, material and working fluid and other aspects of the different, so the classification of the heat pipe is also a lot of commonly used classification methods are the following.

(1) according to the working temperature of the heat pipe tube to distinguish between the heat pipe can be divided into low-temperature heat pipe (-273 - 0 ℃), room temperature heat pipe (0-250 ℃), medium temperature heat pipe [250 ---450 ℃), high-temperature heat pipe (450 a 1000 ℃) and so on.

[2) According to the working fluid reflux power distinction Heat pipe can be divided into core heat pipe, two-phase closed heat siphon (also known as gravity heat pipe), gravity-assisted heat pipe, rotary heat pipe, electrofluidic power heat pipe, magnetic fluid power heat pipe, infiltration heat pipe and so on.

(3) according to the combination of shell and working fluid (this is a customary method of division) can be divided into copper - water heat pipe, carbon steel. Water heat pipe, copper-steel composite - water heat pipe, aluminum - acetone heat pipe, carbon steel - Rong heat pipe, stainless steel. Sodium heat pipe and so on.

(4) Distinguished by the form of structure can be divided into ordinary heat pipe, separated heat pipe, hair thread pump circuit heat pipe, miniature heat pipe, flat plate heat pipe, radial heat pipe and so on.

(5) according to the function of the heat pipe can be divided into heat pipe to transmit heat, heat diode, heat switch, heat control heat pipe, simulation heat pipe, refrigeration heat pipe and so on.

Edit the compatibility of the heat pipe and life

The compatibility of the heat pipe means that the heat pipe in the expected design life, the working fluid inside the tube with the shell does not undergo a significant chemical reaction or physical changes, or changes but not enough to affect the performance of the heat pipe. Compatibility has important significance in the application of heat pipe. Only a heat pipe with good long-term compatibility can ensure stable heat transfer performance, long-term working life and the possibility of industrial applications. Carbon steel - water heat pipe is through the chemical treatment method, effectively solve the problem of chemical reaction between carbon steel and water, which makes the carbon steel - water heat pipe such as high-performance, long life, low-cost heat pipe to be in the industry to promote the use of large-scale.

There are many factors that affect the life of the heat pipe, in summary, the main form of incompatibility caused by the effect of the pipe are the following three aspects, namely: the production of non-condensable gases: the deterioration of the working fluid thermal properties: corrosion of the pipe shell material, dissolution.

(1) produce non-condensable gases due to the working fluid and tube finish material chemical reaction or electrochemical reaction, the production of non-condensable gases, in the heat pipe work, the gas is vapor flow blowing to the condensation section to gather up to form a gas plug, so that the effective condensing area is reduced, the thermal resistance increases, the deterioration of the heat transfer performance, the heat transfer capacity is reduced or even failure.

(2) deterioration of the physical properties of the working fluid Organic working medium at a certain temperature, will gradually decompose, which is mainly due to the nature of the organic working fluid is unstable, or chemical reaction with the shell material, so that the working medium to change its physical properties, such as toluene, alkanes, hydrocarbons and other organic working fluids are prone to this type of incompatibility phenomenon.

(3) corrosion of the shell material, dissolution, the working fluid in the shell continuous flow, while the existence of temperature differences, impurities and other factors, so that the shell material dissolution and corrosion, flow resistance increases, so that the heat pipe heat transfer performance is reduced. When the tube shell is corroded, causing a decline in strength, and even cause corrosion perforation of the shell, so that the heat pipe completely failed. This type of phenomenon often occurs in the alkali metal high temperature heat pipe.

Edit this section of the heat pipe manufacturing

1 . Heat pipe parts and their processing

The main parts of the heat pipe for the shell, end cap (head), suction core, waist plate (connecting seals) four parts. Different types of heat pipes have different requirements for these parts.

2 . Shell

Most of the shell of the heat pipe is metal seamless steel tube, according to different needs can be used in different materials, such as copper, aluminum, carbon steel, stainless steel, alloy steel and so on. The tubes can be standard round or shaped, such as oval, square, rectangular, flat, corrugated, etc.. Tube diameter can be from 2mm to 200mm or even larger. The length can be from a few millimeters to more than l00 meters. Low-temperature heat pipe heat exchanger tubes in foreign countries mostly use copper, aluminum as raw materials. The use of non-ferrous metals for the pipe is mainly to meet the requirements of compatibility with the working fluid.

3 . End cap

The end cap of the heat pipe has a variety of structural forms, it is connected to the heat pipe hublot due to structural forms vary. The size of the outer circle of the end cap can be slightly smaller than the shell. After matching, the protruding part of the tube shell can be used as the welding part of the argon arc welding, no need to fill in the welding rod, the weld is smooth and the quality of the whole is easy to ensure.

Rotary head is a form often used at home and abroad, rotary head is directly rotary pressed in rotary press machine, this end cap form has beautiful appearance, good strength, save material and labor, it is a good end cap form.

4. Liquid-absorbing core structure

Liquid-absorbing core is an important part of heat pipe. Liquid-absorbing core structure will directly affect the performance of the heat pipe and heat pipe heat exchanger. In recent years, with the development of heat pipe technology, researchers in various countries in the structure of the liquid absorption core and theoretical research has done a lot of work, the following is a typical structure to make a brief introduction.

1. Core type

A good performance of the core should have:

(1) large enough capillary suction pressure, or smaller core effective aperture

(2) smaller resistance to liquid flow, that is, there is a high permeability

(3) good heat transfer characteristics, that is, there is a small radial thermal resistance.

(4) good process repeatability and reliability, simple manufacturing, inexpensive.

The structure of the core can be broadly divided into the following categories:

(1) close to the wall of the single-layer and multi-layer cores of such cores

Multi-layer net layers should be as close as possible to the net, the net and the pipe wall should be a good fit between the number of layers of the net l to 4 or more, the number of layers of the net mesh can be the same or different. If the net layer more, the liquid flow cross-section, resistance is small, but the radial thermal resistance; with a fine mesh when the capillary suction but also increase the resistance to flow. Such as the number of layers in the near-wall factors with coarse pore mesh, the surface layer with fine pore mesh, so that the surface fine pore mesh to provide a greater capillary suction pressure, the channel inside the coarse pore mesh so that the flow resistance is smaller, but does not improve the shortcomings of the radial heat gall. Mesh core structure of the core can get a higher capillary force and more sued capillary lift height, but because of the lower permeability, liquid reflux resistance is larger, the axial heat transfer capacity of the heat pipe is limited. In addition, its radial thermal resistance is larger, poor process repeatability and can not adapt to the pipeline bending, so in the slender heat pipe is gradually replaced by other cores.

(2) sintered powder core by a certain number of mesh metal powder sintered on the inner wall of the tube to form a sintered powder core with the wall of the integration of the sintered powder core, there is also a metal wire mesh sintered on the inner wall of the tube core. This core has a high capillary suction, and greater improvement in radial thermal resistance, to overcome the shortcomings of poor repeatability of the core process, but because of its poor permeability, so the axial heat transfer capacity is still smaller than the axial groove channel core and dry channel type core.

(3) axial channel type core in the shell wall to provide axial fine groove capillary pressure head and the liquid state flow channels, groove cross-section shape can be rectangular, trapezoidal, circular and variable cross-section groove, groove type core, although the capillary pressure head is small, but the liquid flow resistance is very small, so you can achieve a higher axial heat transfer capacity, radial thermal resistance is small, process repeatability is good, you can get accurate early childhood He parameters, so you can more correctly calculate the capillary pressure head, but the permeability is poor, so it is still small than the dry channel type core. Thus, the capillary limit can be calculated more correctly, this tube bending performance is basically unchanged, but because of its anti-gravity working ability is very poor, not suitable for tilt (hot end on) work for the zero gravity conditions in space is very applicable, so widely used in space vehicles.

(4) combined core General core often can not take into account both capillary suction and permeability. In order to have a high capillary suction, it is necessary to choose a finer network into the metal powder, but it is still poor permeability, the combination of multi-layer network although there is an improvement in this regard, but its radial heat Yin big. Combination of core can take into account the capillary force and permeability, so as to obtain high axial heat transfer capacity, and most of the core of the radial thermal resistance is very small. It is basically divided into two parts of the core. Part of the capillary suction, the other part of the liquid return channel role.

Edited manufacturing process

As mentioned earlier, the three main components of the heat pipe is the shell, core and mass. During the design process, the fabrication can begin after the materials of the answer shell and core are reasonably selected. Usually the manufacturing process of the heat pipe includes the following process operations and is carried out according to certain procedures.

1, machining --- 2, cleaning --- 3, tube core production --- --- 4, cleaning --- 5, welding --- 6, leak detection --- ---7, Degassing ---8, Leakage detection ---9, Filling ---10, Sealing --- 10, sealing --- 11, baking --- 12, inspection

The actual manufacture of the The actual manufacture of the time can often reach 20, or even hundreds of processes. Here are just some of the simplest necessary processes.