CAO Xiao-yu

【Abstract】This paper summarizes the principle，characteristics，classification，research status of heat pipe system in waste heat recovery，and the development trend of new heat pipe.Compared with other heat exchanger systems，heat pipe system has obvious advantages and will be more widely used in the future.

【Key words】Heat pipe；New heat pipe；Waste heat recovery

中圖分類號： TB65 文獻(xiàn)標(biāo)識碼： A 文章編號： 2095-2457（2018）24-0187-003

DOI：10.19694/j.cnki.issn2095-2457.2018.24.091

0 Introduction

At present，China's energy utilization still has the main problems of low utilization efficiency，poor economic efficiency，and great pressure on the ecological environment. Energy conservation and emission reduction，reduction of energy consumption，and improvement of comprehensive energy utilization rate are the important contents of energy development strategic planning.They are the fundamental ways to solve the energy problems in China and are in the priority of development.

Heat pipe is a kind of heat transfer element with strong thermal conductivity，which relies on the phase change of its internal working medium.It originated from the successful application of the U.S.in the 1960s on the Earth satellite.The heat pipe is made of steel，copper and aluminum tubes filled with heat conducting medium and sealed by vacuum.The working substance in the tube is composed of a variety of inorganic active metals and their compounds.It has extraordinary thermal activity and thermal sensitivity.This thermal superconducting working fluid is activated at a certain temperature and transfers heat in the form of molecular oscillatory phase transition.The conducting temperature does not decay and can be transferred at a very fast speed（sound speed）.Because of its unique structure and phase change heat transfer mechanism， the heat pipe has the following characteristics： （1）high safety and reliability. There is no overpressure in the heat pipe，not afraid of dry burning， because the internal pressure of the heat pipe does not change with temperature after vaporization of liquid working fluid，and heat recovery of heat pipe is a secondary wall heat transfer，which is generally different from conventional heat exchangers；（2）heat conductivity is strong The heat conduction speed is fast，the intensity is big，the efficiency is high（the heat transfer efficiency reaches 98% above）， the energy saving effect is obvious；（3）the isothermal property is good.The heat transfer resistance is small，and a great heat flux is transmitted under very small temperature difference；（4）heat flux can be changed.The heat pipe can alter the area of evaporation section or cooling section independently；（5）the environment is adaptable.The temperature of the heat pipe wall can be adjusted above the dew point of the low temperature fluid by changing heat flow，so dew point corrosion can be prevented.At the same time，the heat pipe will produce self-vibration when conducting heat，so that ash is not easy to adhere to the wall and fins，not easy to plug ash；heat pipe can be designed according to the needs of the environment；（6）long service life，wide application fields.The service life is over 10 years.The single heat pipe can be removed and replaced.The maintenance is simple and the cost is low.[1]

1 Principle and classification of heat pipes

1.1 Working principle of heat pipe

The inner part of the heat pipe is pumped into a negative pressure state and filled with a suitable liquid， which has a low boiling point and is easy to volatilize. The wall has a wick，which is composed of capillary porous material.One part of the heat pipe is evaporative end and the other is condensation terminal，when one end of the heat pipe is heated，the liquid in the capillary evaporates rapidly，the steam flows to the other end under a small pressure difference，and releases heat，condenses again into a liquid，and the liquid flows back to the evaporation section along the action of the porous material by capillary force.The heat is transferred from one end of the heat pipe to the other end.This cycle is fast， and heat can be transmitted continuously.

1.2 Classification of heat pipes

Because there are many kinds，forms and uses of heat pipes，and the material，process and working principle of heat pipes are different，there are many classifications of heat pipes.According to the working temperature of the heat pipe，it can be divided into high-temperature heat pipe（450-1000℃），normal-temperature heat pipe（0-250℃），medium-temperature heat pipe（250-450℃），low-temperature heat pipe（-273-0℃）and so on.According to the recirculation mode of working liquid，it can be divided into capillary heat pipe，gravity heat pipe，rotating heat pipe， point hydrodynamic heat pipe，magnetic fluid heat pipe， osmotic heat pipe and so on； according to the use，it can also be divided into heat transfer pipe，heat switch，heat diode，refrigeration heat pipe and so on.

2 Research status of heat pipe in waste heat recovery

Heat pipe is a phase change energy storage heat exchanger which combines heat exchange with heat storage device.It has excellent thermal conductivity，the heat transfer coefficient is nearly one order of magnitude higher than the traditional metal heat exchanger.It also has a series of advantages，such as good isothermal property， controllable temperature，strong heat transfer capacity， arbitrary change of heat transfer area on both sides of the cold and hot，long-distance heat transfer，no additional auxiliary power equipment and so on.The heat pipe needs to select the material and working fluid according to the different usage temperature[2].The carbon steel-water gravity heat pipe has the advantages of simple structure， low price，convenient manufacture and easy popularization， which makes it widely used.In practical application，the heat pipe used for industrial waste heat recovery is between 50-400℃.It is used for heat recovery of drying furnace，solidifying furnace and drying furnace，waste steam recovery，and air preheater of boiler or kiln.[3]

3 Research status of new heat pipes

3.1 Pulsating heat pipe

Pulsating heat pipe is a new type of high efficiency heat pipe developed over the past twenty years.Akachi，a Japanese scholar，listed the different types of loop heat pipes for the first time in its patents.They are closed loops in structure，and at least one flow control valve ensures the flow of fluid in the loop along a one-way flow.[4]

Pulsating heat pipe is formed by capillary bending and divided into evaporation heating section，adiabatic section and condensation section.Heat is absorbed in the evaporation heating section，and heat is transferred to the condensing section through the adiabatic section.The oscillating flow heat pipe is first vacuum pumped，then filled with heat transfer working fluid such as distilled water，ethanol，acetone and so on，and finally sealed.Due to the high degree of vacuum in the tube，the working temperature is very low and the vaporization speed is fast. In the process of liquid filling，due to the influence of capillary force，wall friction and other factors，the state between the steam column and the liquid column is formed in the tube.During heating，the vapor column expands due to the heat absorption in the evaporation heating section，and new bubbles are formed.The expansion of bubbles promotes the flow of the vapor column and liquid column in the tube.

When the steam column and liquid column pass through the condensation section，the heat is released and the steam column contracts.In this way，the temperature difference between the heating section and the condensing section promotes the expansion of the bubble in the evaporation heating section and the contraction of the condensing end，and the pressure difference between the cold and the hot ends promotes the reciprocating flow of the working medium in the tube，thus achieving the purpose of heat transfer.

Because of the high vacuum，high vaporization speed， small expansion resistance，reciprocating flow velocity of working fluid is also very fast，which makes the heat transfer performance of pulsating heat pipe very high.

Pulsating heat pipe is usually made of copper， aluminum，stainless steel and other capillaries bent into serpentine structure，which is divided into three parts： evaporation heating section，adiabatic section and condensation section.Compared with traditional heat pipes with capillary cores，this new type of heat pipe has the following outstanding advantages：（1）The diameter and volume of the pipe are very small，and it is easy to realize miniaturization.（2）Simple structure，no need for capillary core，and low cost.（3）Reliable operation，strong adaptability of liquid reflux and not easy to dry.[5]

3.2 Flat heat pipe

Flat-plate heat pipe[6]is a kind of flat-plate heat pipe，belonging to a type of heat pipe，and flat-plate heat pipe has more prominent advantages than ordinary heat pipe，its shape is very conducive to heat diffusion of concentrated heat source. Because of its light weight，good start-up and uniform temperature，flat heat pipe has become a hot research topic in the field of heat dissipation of electronic components，and has been applied abroad.

Flat-plate heat pipe is a type of heat pipe，and flat-plate heat pipe has more prominent advantages than ordinary heat pipe，its shape is very conducive to heat diffusion of concentrated heat source.Its working principle is that the flat heat pipe is a vacuum cavity with capillary structure on its inner wall.The cavity is pumped into vacuum and filled with working fluid.When heat is transferred from heat source to evaporation zone，the working fluid in the cavity will start to produce liquid phase gasification in the low vacuum environment.At this time，the working fluid absorbs heat energy and expands rapidly.The vapor phase will fill the whole cavity very quickly.When the vapor phase working medium touches a relatively cold area，it will produce condensation，thus releasing the heat accumulated during evaporation.The condensed liquid working medium will return to the heat source of evaporation due to the capillary adsorption of the capillary structure.So that cycling can take away the heat generated by the module.

Because of its own structural characteristics，flat heat pipe has shown good working characteristics in heat transfer，it is mainly reflected in the following aspects：efficient thermal conductivity，variable heat flux density，excellent isothermal，reversible heat flow direction，structural adaptability.

3.3 Micro heat pipe

Micro heat pipe is a new technology developed along with the development of microelectronic technology. Miniature heat pipe is widely used to cool electronic equipment in many fields，such as aerospace，aviation，military weapons，vehicles，computer and so on.It is one of the main ways to effectively cool high heat flux electronic devices，and has become an important development direction and research hotspot of modern heat pipe technology.

Micro heat pipe is widely used in the cooling of electronic equipment.The practical applications are tubular Micro heat pipe，flat Micro heat pipe and Micro heat pipe array.Different structural types of MHP have different applications，[7].

4 Summary and Prospect

Heat pipe technology is widely used in aerospace，military，petroleum，chemical，metallurgy，machinery，power，electronics，coal，railway，communications，textile and other fields.

Heat pipe technology was first used in spacecraft， because spacecraft parts facing the sun and back to the sun temperature difference is large，easy to damage，the use of heat pipe to achieve thermal balance can solve this problem.The use of heat pipes can prevent the frozen soil from being damaged by laying oil pipelines or railways on the plateau.Using heat pipe to heat exchanger to recover waste heat from industrial production can save a lot of energy.

In the power electronics industry，because the heat pipe self-cooling cooling cooling system does not need fans，no noise，maintenance-free，safe and reliable，long service life，heat pipe air-cooling or even self-cooling can replace the water-cooling system，saving water resources and related auxiliary equipment investment.In addition， heat pipe cooling can also concentrate or even seal the heating parts，and transfer the cooling part to the outside or far away，which can prevent dust，moisture，explosion， improve the safety and reliability of electrical equipment and application.Therefore，it is widely used in power semiconductor discrete devices，modules and components，such as industrial converter technology，soft-start technology，frequency conversion technology，reactive power compensation technology.

With the development of science and technology，the research and application fields of heat pipes are expanding，especially the emergence of micro-heat pipe technology，which makes heat pipes used in medical operation，electronic device chip，notebook computer CPU cooling，circuit control panel cooling，solar water heater， solar power plant，nuclear power project and has been greatly developed.

【Reference】

[1]Rongshui Rao.Research progress of heat pipe technology and its application in metallurgical industry[J].Industrial heating， 2001（3）.

[2]Yefeng Tan.Research on the recovery and utilization of waste heat from industrial furnaces[D].Ji'nan：Shandong University，2006.

[3]Bin Zhao，Zibing Wang，Application of Heat Pipe and Heat Exchanger in Waste Heat Recovery of Iron and Steel Industry [J].Metallurgical Power，2005，（3）.

[4]Chuanbao Chen.Research on oscillating flow heat pipe and its application in solar collector [D].North China Electric Power University（Beijing），2009.

[5]Tang Xiaofeng.Heat transfer law of oscillating flow heat pipe and its application in LED radiator[D].North China Electric Power University，2014.

[6]Yaping Zhang，Xiaoling Yu，Quanke Feng.Research status and latest progress of flat heat pipe[J].Vacuum and low temperature， 2006，12（4）：221-225.

[7]Yibing Liu，Xinmin Huang，Guohua Liu，et al.Advances in the study of miniature heat pipes[J].Low Temperature and Superconductivity，2008，36 （2）： 39-43.