High-power solid-state laser cooling technology
1 Introduction
Laser diode pumping solid-state lasers (DPLs) have attracted great interest due to their high efficiency, high beam quality, compact structure and long life. In recent years, with the successful development of high-power diode lasers, the development of DPL and its application in military, industrial, medical, scientific research and other fields have been promoted.
The heat loss of the laser diode (LD) accounts for more than 50% of the total power consumption during normal operation, and the instability of the working temperature of the laser diode caused by the heat loss will change its output wavelength, which will affect the efficient and stable output of the DPL. In addition, heat is generated during the light pumping of the laser crystal of a solid-state laser, which also needs to be cooled. With the increase of the power of the solid-state laser pumped by the laser diode, the heat load generated by the device is increasing, and the heat dissipation density is getting higher and higher, and the DPL cooling problem has become a technical difficulty in the current DPL research.
In order to solve the problem of high-power DPL heat dissipation, many scholars at home and abroad have carried out a lot of research work in recent years, and proposed a variety of cooling methods such as microchannel liquid convection heat exchange, solid cooling, spray cooling and micro heat pipe cooling. In this paper, the research status of these technologies is reviewed and analyzed, and on this basis, microchannel boiling heat exchange cooling and liquid nitrogen cooling technologies are proposed.
2 Technical Principles
There are different types of high-power solid-state lasers, such as solid-state heat-capacitance lasers, new thin-slice lasers, fiber lasers, end-face pumping lasers, etc., although the shape and heat dissipation of each laser heat dissipation device are different, but its main heat dissipation devices are the pumping source and gain medium. The cooling principle of the pumping source and gain medium can be illustrated in Figure 1. According to the theory of heat transfer, laser cooling can be expressed as follows:
In the formula, the heat dissipation is the heat dissipation capacity of the heat dissipation device, the α is the convective heat transfer coefficient of the cooling working fluid in the heat sink channel, the heat exchange area of the heat sink channel, the wf is the temperature of the inner wall of the heat sink channel, and the f is the temperature of the cooling working fluid. 狋h is scattered
Thermal device temperature. The purpose of laser cooling is to take away the heat dissipation of the heat dissipation device and ensure a certain temperature of the heat dissipation device.
It can be seen from equation (1) that in order to improve the heat dissipation, the convective heat transfer coefficient of the cooling working fluid in the heat sink channel should be increased as much as possible, the heat exchange area of the heat sink channel should be increased, and the temperature of the cooling working fluid should be reduced. At the same time, the thermal conductivity of the heat sink is reduced, so that the temperature of the inner wall of the heat sink channel is uniform and as close to the temperature of the heat sink device as possible.
3 Research status
The research status of four cooling technologies, namely microchannel liquid convection heat transfer, solid cooling, spray cooling and micro heat pipe cooling, is reviewed and analyzed.
3.1 Microchannel liquid convection heat exchange cooling
According to the definition of Bowers and Mudawar [1], a channel with a hydraulic diameter between 0.01~1mm is called a microchannel. Microchannel liquid convection heat transfer cooling is that the liquid flows through the internal microchannel of the heat sink, and the heat of the heat dissipation device is taken away by convection heat exchange with the inner wall of the microchannel (see Figure 2). The heat sink of the microchannel greatly increases the heat transfer area, and the convective heat transfer coefficient of the liquid in the microchannel is much higher than that of the convective heat transfer coefficient in the conventional channel due to the microscale effect. Therefore, in terms of cooling principle, microchannel liquid convection heat transfer cooling increases the convective heat transfer coefficient of the cooling working fluid in the heat sink channel and the heat transfer area of the heat sink channel
Improve heat dissipation and cooling capacity.
The report of microchannels was first published in the literature of American scholars Tuckerman and Pease in the early eighties [2], who introduced a water-cooled ribbed radiator with silicon substrate, which has a rib and channel width of only 50 μm, a rib height of 300 μm, and a thermal resistance of only 0.09 k/W.
The heat flux is up to 790 W/cm2.
Since the 90s of the 20th century, famous universities such as Stanford University, the University of California, and the University of Maryland in the United States have carried out related projects and cooperated with companies such as Intel and HP. According to the needs of military technology, the Lawrence Livermore National Laboratory (LLNL) in the United States also started the research of microchannel heat dissipation technology earlier.
Silicon-based material etching technology was originally used by some researchers to cool high-power laser diode arrays due to its unique advantages in microchannel processing and small microchannel size error [34]. Their studies have shown that microchannel heat sink cooling is superior to traditional cooling methods.
Harpole, George M, et al. [5] optimized the microchannel heat exchanger. They built a complete model of a 2D flow/heat transfer microchannel heat exchanger with design parameters of a heat flux of 1kW/cm2 and a maximum surface temperature of no higher than 25°C. Pure water was used as the coolant, or the temperature was cooled below freezing point with an 8% aqueous solution of methanol (freezing point of -5 °C). The effective heat transfer coefficient is 100W/(cm2·K), and
The total pressure drop is about 2×105Pa.
Huang Zhe [6] mentioned the water-cooled copper microchannel heat dissipation technology in the research results of the high-reliability, high-power 808nm laser developed by nLightPhotonics. A microchannel heat sink is composed of a thin layer of copper processed by electrical discharge machining (EDM) and then welded to one
blocks, forming microchannel arrays. The laser strip is welded to the edge of the microchannel, and the turbulent flow of deionized water in the microchannel produces a high thermal conductivity, resulting in a very low thermal resistance.
Li Qifeng et al. [7] reported the structure and main manufacturing process of the V-groove silicon microchannel cooler, and developed a sample of the cooler, and its cooling capacity was in good agreement with the numerical simulation results.
Lv Wenqiang et al. [8] simulated heat dissipation calculations for microchannel coolers with different structures. The structural parameters of the cooler are optimized, and the modular microchannel cooler can meet the heat dissipation requirements of high-power diode laser slats with continuous 50W or pulse power of 120W (20% duty cycle), and the stacked two-dimensional stacked array DL can be used as a pumping source for high-average power DPL.
Yun Liu et al. [9,10] from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences prepared an oxygen-free copper heat sink with an overall size of 15 mm long, 12 mm wide, and 2.2 mm thick, with a microchannel depth of 300 μm and a width of 400 μm.
There are many experimental studies on microchannel liquid cooling by scholars at home and abroad, and microchannel liquid cooling and heat sink have been widely used at home and abroad.
However, there is no consistent conclusion on the flow pattern transition, the influencing factors of frictional resistance coefficient, and the influencing factors of heat transfer coefficient, and the heat exchange correlation and flow equation with universal laws like those in conventional channels have not yet been established. For the application of microchannel liquid convection heat transfer technology to the cooling of high-power solid-state lasers, it is necessary to solve many problems such as system optimization, reducing channel flow resistance, improving system stability, and reducing channel corrosion and clogging.
3.2 Solids cooling
Diamond, which has a very high thermal conductivity, is mainly used as a solid cooling material. One side of the pumping surface is pasted with a diamond sheet, and the sheet is outside and not
The pumping surface is directly water-cooled. The heat generated by the operation of the laser crystal is quickly transferred to the diamond surface and then removed by convection into the surrounding space or with other cooling devices such as liquid-cooled microchannels. In this way, it not only ensures the rapid dissipation of heat, but also makes the temperature distribution on the surface of the laser crystal uniform, and effectively improves the thermal lensing effect of the laser crystal. Therefore, in terms of cooling principle, solid cooling is to reduce thermal conductivity and thermal resistance to improve heat dissipation capacity.
Israeli scholar Yitshak Tzuk et al. [11] experimentally verified the feasibility of using diamond foil to cool high-power solid-state lasers. In the experiment, 2.3mm×4mm×24mm Nd∶YVO4 plate-like laser crystals were sandwiched between a 0.3mm thick optical diamond foil and a liquid copper heat sink. Under the condition that the output light wavelength is 808nm, the output power obtained when the pumping energy is 600W is 200W. According to the literature, the maximum output power using traditional cooling methods is only about 100W. California, USA
H.P. Chou et al. [12] of TextronSystem also reported the performance of a compact diode-pumped solid-state laser based on diamond cooling technology, and based on their experimental data, a 100kW laser was conceptually designed.
There are no reports of solid-state cooling of lasers in China.
3.3 Spray cooling
Spray cooling is to atomize the coolant through an atomization device (the main device is a nozzle) under the action of applied energy and then spray it to the heat exchanger meter
surface, through the phase transformation of the atomized droplets, the heat is taken away by the heat to achieve cooling purposes (see Figure 3). According to a foreign report [13], the heat flux density of spray-enhanced heat exchange can be as high as 1000W/cm2 using water as the coolant; With Freon as the coolant, the heat flux density also exceeds 100W/cm2. Therefore, from the cooling principle, spray cooling increases the cooling working fluid
The convective heat transfer coefficient on the heat exchange surface to improve the heat dissipation and cooling capacity.
Spray-enhanced heat transfer is a complex flow heat transfer process, which is controlled by many factors such as droplet particle size distribution, droplet velocity, spray angle, physical properties, heat exchange surface roughness, heat flow, etc., and many scholars have proposed different heat exchange mechanisms [14]. Under the condition of low superheat, the single convective heat transfer plays a major role, and the impact of the droplets will increase the disturbance to the liquid film, thereby strengthening the heat exchange. Under the condition of high superheat, phase transfer heat occupies a dominant position in intensified heat transfer. At present, there are two widely recognized heat exchange theories: one is the thin liquid film evaporation theory [15], which holds that the fog droplets form a thin liquid film on the heat exchange surface, and the liquid film conducts heat through the heat exchange; The temperature of the top layer of the liquid film is saturated, and the efficient heat exchange of the thin liquid film is realized due to the increase of the temperature gradient of the thin layer of the liquid film. The other is the theory of secondary nucleation [13|~16], which holds that the bubbles carried by the droplets impact on the liquid film layer create nucleation conditions and cause violent boiling on the liquid film, thereby enhancing heat exchange.
Choi and Yao [17] experimentally investigated the effect of the impact angle of the droplet on the heat transfer surface on the heat transfer efficiency. It is found that for vertical injection, the heat transfer efficiency is higher in the nuclear boiling section. For horizontal spraying
The heat exchange efficiency is higher in the transition boiling section. Sodtke and Stephan [18] experimented with different surface roughnesses and found that the heat transfer effect of the surface with fine roughness was significantly higher than that of the smooth surface, and the increase in the length of the three-phase contact line due to the small roughness also led to the rapid evaporation of the thin liquid film. Silk [19] studied the effects of straight fins, tapered fins, etc., on the heat transfer performance, and the critical heat flux density of straight fins compared to smooth heat transfer surfaces
(CHF) can be up to 50% higher. Gao Shan, Qu Wei [20] and others exploited
In the Volumeoffluid (VOF) method, the physical and mathematical models of a single water droplet impacting the thermostatic plate are established, and then the numerical simulation and analysis results are carried out to study its kinetic and heat transfer characteristics.
Chen Wei et al. [21] applied a closed microjet cooling system to light-emitting diode (LED) heat dissipation in experiments, and the results showed that when the ambient temperature was 25.6°C, the temperature of the LED chipset rose to 72°C after 1 min without heat dissipation measures, and the temperature dropped to 34.81°C after the system started to work. When the input power of the LED chip is 9.3W, its heat flux density is 14.53W/cm2, and the cooling system can still quickly reduce the temperature of the LED chip to 54.34°C. The high-efficiency diode pumping source technology studied by Raytheon in the United Kingdom is to use spray cooling to improve the heat transfer coefficient, and the experimental heat flux density is greater than that
５００Ｗ／ｃｍ２。 Dr. Rini of Rinitechnologies, Inc., USA, simulated the heat dissipation of a high-power diode array for a high-power (100~300kW) laser system that is expected to be used in the future. Contrast microcommunication
Spray cooling heat dissipation system with ammonia as coolant requires less coolant flow than 1/12 of the former, and the volume and weight of the system are greatly reduced. Therefore, spray cooling is highly likely
An important way to cool high-power lasers.
Although spray cooling has the advantages of large heat transfer coefficient, high critical heat flux density and low coolant flow, it affects the heat exchange
Multiple parameters (injection angle, injection velocity, droplet size, heat exchange surface, gravity factor, etc.) are not independent of each other, and it is difficult to analyze the influence of a single parameter on heat transfer through experiments. The heat exchange itself contains gas-liquid mutuals
At present, only some limited related tests can be done, such as the dynamic characteristics of bubble droplets, pool boiling, droplet impact, smooth heat exchange surface and some simple spray cooling simulations, but there is no complete theory of the system. In the cooling device of high-power solid-state lasers, heat exchange is generally carried out in a limited space, and some even need to be exchanged in a closed space, and the heat exchange mechanism and heat transfer law under these conditions need to be further studied.
3.4 Micro heat pipe cooling
The concept of micro heat pipes was proposed in 1984 by T.P. Cotter at the Fifth International Conference on Heat Pipes held in Japan [22].
Microheat pipes consist of a closed vessel, a capillary structure, and a working fluid. Once one end of the container is heated, the working fluid absorbs heat and vaporizes, and the resulting vapor flows to the other end of the container to exothermic condensation, and the condensate will flow back to the original heating position under capillary force or gravity. Since the working fluid in the heat pipe transfers heat through the phase change, a very high conductivity coefficient can be obtained, and the effect of uniform temperature can be achieved. Petros, Mulugetat, et al. [23] designed a laser that conducts heat through heat pipes, and conducts 150 W of heat from diodes and laser bars through three heat pipes. XieBoping et al. [24] also did some research on heat pipe heat dissipation techniques and gave rough guidelines on how and when to use heat pipes to solve thermal management problems.
At present, there is little research on the application of micro heat pipes in laser cooling in China, and it is necessary to strengthen the research work in this area.
4 Future developments
According to the cooling principle and cooling formula of laser, two technologies for high-power solid-state lasers to achieve higher heat flux density cooling are proposed, namely microchannel boiling heat exchange cooling and liquid nitrogen cooling.
4.1 Microchannel boiling heat exchange cooling
Microchannel boiling heat exchange cooling is the evaporation and boiling of the working fluid when it flows through the internal microchannel of the heat sink, and the heat is transformed by the boiling of the inner wall of the microchannel
Remove heat from the heat sink (see Figure 4). In terms of cooling principle,Microchannel boiling heat transfer cooling is the same as microchannel liquid convection heat transfer cooling, and it is also to improve the heat dissipation and cooling capacity of the working fluid by increasing the convective heat transfer coefficient and heat transfer area of the heat sink channel in the heat sink channel, but because the microchannel boiling heat transfer cooling has the dual role of microscale effect and boiling heat exchange, the convective heat transfer coefficient is greatly improved compared with the microchannel liquid convection heat transfer cooling, so its heat dissipation capacity has been greatly improved.
Scholars at home and abroad have conducted extensive research on the phenomenon of microchannel boiling heat transfer. Studies have shown that the flow within the microchannel boils for a change
The heat law is obviously different from the flow boiling heat transfer law in the conventional scale, and the heat transfer correlation and flow equation with universal law suitable for large channels are not applicable to micro channels. So far, the research on the two-phase flow and boiling heat transfer law in the microchannel refrigeration evaporator mainly includes the manifold study, the heat transfer characteristics and the nucleation of the bubble in the channel
Under microscale conditions, due to the narrow flow channel, the flow pattern changes are generally drastic, which is easy to cause changes in flow resistance, flow stability changes, and heat transfer deterioration [25~32], so the study of flow patterns is extremely important. In the study of heat transfer characteristics and the nucleation mechanism of bubbles in microchannels, there are two forms of heat transfer in microchannels: nuclear boiling and convective boiling. Nuclear boiling heat transfer coefficient and heat flux density, fluid
Parameters such as characteristics, fluid pressure and channel size are related, while the convective boiling heat transfer coefficient is determined by mass flow rate, mass gas content, fluid characteristics, channel structure and size. The formation of the vaporization core is related to the local wall superheat and the degree of fluid supercooling [33], while the channel size has an effect on the nucleation temperature [34,35].
For the micro-refrigeration system using the micro-channel evaporator, the unstable two-phase flow of boiling heat transfer in the evaporator will cause the bubbles to periodically fill the microchannel, resulting in channel air blockage, resulting in a sharp increase in wall temperature [36], resulting in the heat dissipation device being unable to be effectively cooled or even burned. Therefore, the unstable two-phase flow law of refrigerant in the microchannel evaporator is the premise to ensure the normal operation of the micro-channel evaporation system, and it is also the insurmountable theoretical basis for the widespread application of micro-channel evaporator and micro-refrigeration system.
The application of microchannel boiling heat transfer to the cooling of high-power solid-state lasers requires a series of research work, including channel optimization gauge to ensure the stability of boiling heat exchange in the evaporator; The refrigeration system is optimized to make it compact and meet the current trend of miniaturization of high-power solid-state lasers. In terms of temperature control, we strive to be precise so that the temperature distribution inside the laser is uniform to ensure its luminous quality.
4.2 Liquid nitrogen cooling
Liquid nitrogen cooling is to pass liquid nitrogen into the internal channel of the heat sink (conventional channel or microchannel), so that the liquid nitrogen evaporates and boils during the flow of the channel, and the heat of the heat dissipation device is taken away by the phase change heat of the boiling on the inner wall of the heat sink channel (see Figure 5), and the nitrogen after gasification is discharged into the atmosphere or cryogenic unit for recovery. As an excellent cryogenic coolant, liquid nitrogen is abundantly available
Rich, low price, low boiling point, colorless, non-toxic, tasteless, non-combustible and other characteristics. The boiling point of liquid nitrogen is 196°C, so in terms of cooling principle, liquid nitrogen cooling improves the heat dissipation capacity by increasing the temperature difference between the cooling working fluid and the heat dissipation surface.
For the study of the gas-liquid two-phase flow characteristics and boiling heat transfer of liquid nitrogen in the tube, Steiner et al. [37,38] conducted a systematic experimental study on the boiling heat transfer and pressure drop of liquid nitrogen flow in a horizontal tube with a diameter of 14 mm. The results show that most of the experimental flow patterns are wavy flow and elastic flow, and the heat transfer coefficient is mainly determined by the heat flux density, and the dryness has an important effect on the heat transfer coefficient only when the heat flux density and pressure are high and the flow rate is very low. The dominant mechanism of heat transfer is nuclear boiling. Klimenko et al. [39] conducted an experimental study on liquid nitrogen flow boiling in horizontal stainless steel tubes with an inner diameter of 10 mm and a length of 1850 mm. It is found that the boiling heat transfer coefficient increases with the increase of heat flux density and increases with pressure
Big. Ren et al. [40] conducted an experimental study on the boiling of liquid nitrogen on the heating wire in the capillary, and used a phosphor bronze wire with a diameter of 50 μm to simulate the linear superconducting element. The results show that the capillary pairs the nuclear state
The boiling heat transfer has a significant strengthening effect, and there is an optimal pipe diameter of 1.2mm. The main reason is that the presence of a capillary causes the bubbles to detach along the tube wall parallel to the heating wire, creating a bubble at the bottom of the capillary
It can slide over the entire heating wire and has a strong disturbance effect on the liquid near the heating wire. Qi Shouliang [41] studied the flow and heat transfer characteristics of liquid nitrogen in microchannels. The boiling starting point, two-phase flow pressure drop, boiling heat transfer coefficient and criticality of liquid nitrogen flow boiling in four microchannels with inner diameters of 0.531mm, 0.834mm, 1.042mm and 1.931mm were treated
The heat flux (CHF) was studied separately, and the critical diameter of the microchannel under experimental conditions was determined according to the heat transfer characteristics. The results show that the boiling initiation point of liquid nitrogen in the microchannel is different from that of the conventional channel in the mass flow. Significant characteristics are shown in volume, pressure drop and wall temperature.
Liquid nitrogen boiling heat transfer is applied to the cooling of high-power solid-state lasers, and it is necessary to master the flow boiling heat transfer law of liquid nitrogen in the heat exchanger
The heat exchange channel is optimized to ensure the stability of the heat exchange process and avoid the occurrence of heat exchange deterioration such as local evaporation drying or channel air blockage. In addition, due to the large temperature difference between liquid nitrogen and the surrounding environment, frost is prone to occur on the surface of liquid nitrogen transmission pipes and heat exchangers, and this problem needs to be solved in practical applications.
5 Conclusions
With the increasing power of lasers and the increasing miniaturization of device size, the cooling problem of high-power solid-state lasers has become a constraint
Its output power further increases the bottleneck problem of its cooling technology
Research is imperative.
Semiconductor refrigeration, gas cooling, and conventional channel liquid cooling are all difficult to dissipate a large amount of heat in a limited space. Due to its high thermal conductivity, diamond can be used for laser crystal cooling, improving the temperature distribution and thermal stress distribution in the crystal, and quickly dissipating the heat in the crystal, but the heat is still taken away by other cooling methods. Microchannel liquid convection heat transfer cooling has been widely used, but there are still many problems that need to be solved, such as system optimization, reducing channel flow resistance, improving system stability, and reducing channel corrosion and blockage. Spray cooling is a very effective cooling method, but so far people have a fairly limited understanding of the flow and heat transfer mechanism of spray cooling
The experimental results of similar problems are even contradictory, and the heat transfer mechanism and enhanced heat transfer law in the enclosed space need to be further studied when applied to the cooling device of high-power solid-state lasers.
Microchannel boiling cooling and liquid nitrogen cooling have stronger cooling capacity and have a broad range in the development of higher power solid-state lasers
Application prospects. The microchannel boiling heat transfer mechanism is more complex than the single-phase liquid-cooled heat transfer mechanism, and it is necessary to solve the problems of heat sink channel optimization design, two-phase flow stability, refrigeration system optimization, temperature control, etc. Liquid nitrogen cooling needs to further study the law of flow boiling heat exchange, the optimal design of heat sink channel, stability, frosting and other issues.
