Liquid is overheated and evaporated quickly when it enters into the environment with lower saturation pressure than that corresponding to its initial temperature. This phenomenon is known as the flash evaporation. A natural low-pressure environment and flash evaporation have unique characteristics and superiority in high altitude and outer space. Therefore, flash evaporation is widely used in aerospace. In this paper, spray flash evaporation and jet flash evaporation which are two different forms were introduced. Later, key attentions were paid to applications of flash evaporation in aerospace. For example, the flash evaporation has been used in the thermal control system of an aircraft and the propelling system of a microsatellite and oil supply system of a rocket motor. Finally, the latest progresses in the calculation model and numerical simulation of flash evaporation were elaborated.
Liquid will be overheated and thereby evaporated when it enters into the environment with lower saturation pressure than that corresponding to its initial temperature. This phenomenon is called the flash evaporation [
Recently, aerospace technology develops quickly. Aircrafts and spacecrafts have extensive applications and important significances in civil purpose, military purpose, and scientific exploration. They have become the research hotspot in the world. The former one includes planes, helicopters, and supersonic bomber. The latter one covers space shuttle, man-made earth satellite, manned spacecraft, space probe, and so on. Air vehicles often serve in high altitude or outer space. These spaces are tough environments with low temperature and low pressure. In these environments, air vehicles will experience high temperature and high-speed frictions at running through the atmospheric layers. These hostile environments influence the safe and stable running of air vehicles significantly [ Air vehicles are equipped with tremendous electronic devices for the accurate control and management. Short-term high-power-consumption running of these electronic devices often will cause sharp temperature rises in local regions [ A microsatellite has characteristics of high functional density, high flexibility, short R&D period, low R&D cost, and distributed combination into “virtual macrosatellite.” It has become one of three research hotspot and development frontiers in the space technology field at present [ Large spacecrafts, such as manned spacecrafts and space probes, have to be launched into the outer space at the essential speed by the thrust of the carrier rocket. Later, they make orbital motion based on inertia. The carrier rocket acquires the thrust by the fast full combustion of propellant and the release of high-temperature vapor. However, coking of fuel oil and delayed ignition may occur due to the low spray quality and long evaporation time of fuel oils. These problems restrict the fast high-efficiency combustion of fuel oils. Hence, how to gain high-quality spray and shorten the ignition time becomes key problems that have to be solved urgently in the rocket motor field.
The boiling point of liquid working medium is increased with the growth of pressure. On the contrary, the boiling point decreases with the reduction of pressure. For example, the boiling point on the sea level is 100°C, but it decreases to 90°C at high mountains with an altitude of about 3 km. When the ambient pressure of high-temperature and high-pressure liquid drops sharply to lower than the saturated pressure corresponding to its initial temperature, the initial temperature of the liquid is higher than the boiling point under the corresponding pressure. In this case, the liquid is overheated and enters into the thermal imbalance. The liquid is evaporated quickly. This process is called the flash evaporation.
Flash evaporation of single-component liquid is an isenthalpic process. It is generally processed into heat insulation. Therefore, the evaporation capacity in the flash evaporation of the single-component liquid is
If the enthalpies of liquid and vapor are unknown, the evaporation capacity can be calculated by the following formula:
According to applications of flash evaporation in aerospace, flash evaporation is divided into two forms: spray flash evaporation and jet flash evaporation. The spray flash evaporation has sheltering of the hot surface in the spraying process. The jet flash evaporation has no sheltering of the hot surface. These two forms of flash evaporation are based on the principle of the same pressure drop. The spray flash evaporation is often used in aerospace electron cooling and spacecraft thermal control field. The jet flash evaporation is often used in the propelling system of spacecrafts and the fuel injection system of a rocket motor. Therefore, the spray flash evaporation has one additional process of arriving at the hot surface compared to the jet flash evaporation.
Spray flash evaporation is mainly used in flash evaporation spray cooling in aerospace. It is the development of spray cooling under the evaporation mode and is used to gain the maximum heat exchange with unit volume of liquid working medium. Under vacuum conditions, flash evaporation spray cooling has unique advantages. The flash evaporation of liquid working medium can remove residual heats of spacecrafts without using other media. Generally, flash evaporation spray cooling is divided into two flash evaporation processes. One is the flash evaporation in the process when liquid drops fly before arriving at the cooled surface. It is called the droplet flash evaporation [
(a) Physical images of flash evaporation spray cooling [
High-pressure liquid is sprayed onto the low-pressure environment, developing flash evaporation in the low-pressure environment after being overheated. This phenomenon is called jet flash evaporation in this paper and is also called the decompression flash evaporation by many scholars [
Flash evaporation of single droplet in vacuum is a complicated coupling process of heat and material transfer. However, the calculation model and numerical simulation of this process are not perfect yet. Gao et al. [
Single-droplet evaporation model.
Wu et al. [
Wu et al. studied the influences of droplet ambient temperature, humidity, and droplet radius on droplet evaporation according to this model.
Wang et al. [
Droplet flash evaporation model proposed by Wang et al. [
Xi et al. [
Chen et al. [
Film flash evaporation model.
There are few studies on the film flash evaporation model. However, many scholars have carried out numerical simulations on the complete process of spray flash evaporation. Based on a previous study [
The complete vacuum flash evaporation spray cooling model.
Since the system can diffuse heat to the vacuum environment through radiation,
Jet flash evaporation generally occurs in waste emission of a manned spacecraft, starting blowdown of the rocket engine and fuel injection system of the engine. It influences the safe and effective running of space activities. An experimental study and numerical simulation on characteristics of jet flash evaporation have important significance to prevent its disadvantages and make full use of its advantages.
It is difficult to realize the complicated experimental conditions, such as vacuum environment and flash evaporation of fuel oil. Therefore, many scholars have carried out numerical studies on jet flash evaporation. Fuel oil flash evaporation in the combustor of aeroengine is a complicated nonequilibrium and multicomponent two-phase process. Lee et al. [
Heat management of electronic elements and heat removal in local high-temperature places in aerospace are important to stable and safe running of air vehicles [
The A-B process is the traditional spray cooling. The A-C process is the flash evaporation spray cooling which is realized by pressure drop. The D-E process is the flash evaporation spray cooling which is realized by temperature rise.
There are few studies on the vacuum-based flash evaporation spray cooling. Aoki [
(a) Droplet diameter (
In the same time, many scholars have studied by using water as the working medium. Chen et al. [
(a) Experimental system of the vacuum flash evaporation spray cooling [
A microsatellite attracts wide attentions in the world due to the short R&D period, low mass, and low cost. It has become an important development direction in the space technology. The propelling system of a microsatellite is mainly restricted by the energy consumption, volume, and weight. A cold air propelling system is used mostly. Although the cold air propelling system has simple structure, the applied gas working medium has low density. Therefore, the gas bottle has large volume and low safety performance. Subsequently, the liquefied propelling system is used more and more in order to increase the storage density. The liquefied propelling system stores gaseous working medium in the liquid state. However, the liquid working medium needs tremendous latent heat of vaporization for revaporization, which has to be supplied by electric energy on the satellite. This restricts the application of the liquefied propelling system significantly.
Wei and Guo [
Propulsive performances of cold gas propulsion and flash evaporation jet propulsion [
Cold gas propulsion | Flash evaporation jet propulsion | Remarks | |
---|---|---|---|
Pattern | Phase change occurs in the tank | Phase change occurs in the nozzle throat | |
Thrust | 585 mN | 860 mN | 0.86 MPa |
Special impulse | 1108 Ns/kg | 343 Ns/kg | 0.86 MPa |
Density special impulse | 7501 Ns/m3 | 209271 Ns/m3 | 0.86 MPa |
Characteristic | The liquid-gas conversion is sufficient, the pressure in the tank drops rapidly, and the thrust is unstable | The liquid-gas conversion is incomplete, the pressure in the tank is basically stable, and the thrust is constant | |
Heat control demand | Larger power thermal control is required | Larger power thermal control is not required |
Flight verification data for the propulsion system [
Order number | Velocity increment (m/s) | Operation time (s) | Initial tank pressure (MPa) | Propellant remainder (g) | Propulsion efficiency | Remarks |
---|---|---|---|---|---|---|
1 | — | 15.36 | 0.657 | 977.44 | — | Test spray |
2 | 0.4624 | 28.67 | 0.707 | 917.74 | 0.9467 | |
3 | 0.5092 | 32.77 | 0.667 | 852.00 | 0.9664 | |
4 | 0.1001 | 7.17 | 0.685 | 839.08 | 0.8494 | |
5 | 0.1281 | 9.22 | 0.682 | 822.54 | 0.8497 | |
6 | 0.3169 | 20.48 | 0.675 | 781.63 | 0.9554 | |
7 | 0.1898 | 13.31 | 0.669 | 757.13 | 0.8500 | |
8 | 0.0949 | 7.17 | 0.653 | 744.88 | 0.8451 | |
9 | 0.5217 | 34.82 | 0.656 | 681.00 | 0.9534 | |
10 | 0.5115 | 33.79 | 0.645 | 618.37 | 0.9796 | |
11 | 0.5131 | 33.79 | 0.638 | 555.74 | 0.9935 | |
12 | 0.5170 | 33.79 | 0.648 | 492.43 | 0.9856 | |
13 | 0.5077 | 33.77 | 0.648 | 430.38 | 0.9684 | |
14 | 0.4941 | 32.77 | 0.647 | 369.88 | 0.9728 | |
15 | 0.5002 | 32.77 | 0.657 | 308.63 | 0.9698 | |
16 | 0.4978 | 31.74 | 0.673 | 247.67 | 0.9728 | |
17 | 0.4092 | 29.70 | 0.722 | 203.17 | 0.7965 | |
18 | 0.4187 | 29.70 | 0.727 | 156.42 | 0.8094 | |
19 | 0.4060 | 29.70 | 0.717 | 111.92 | 0.7958 | |
20 | 0.4187 | 29.70 | 0.727 | 65.17 | 0.8094 |
Working principle of the flash evaporation jet propulsion program.
A hot-water rocket engine uses the liquid water as the propellant. The working process is mainly divided into two stages: heating and release. Water in the pressure vessel is heated firstly until reaching the preset pressure and temperature. The nozzle jet is blocked against release, and the propellant is sprayed to produce the thrust. Flow in the nozzle jet can be viewed as the process that high-temperature and high-pressure water forms the gaseous flow and liquid flow through decompression flash evaporation [
Basic working principle of the hot-water rocket.
The thrust of the hot-water rocket engine, superheated steam rocket engine, and conventional rocket engine [
Initial pressure (MPa) | Hot-water rocket engine (N) | Superheated steam rocket engine (N) | Conventional rocket engine (N) |
---|---|---|---|
13.6 | 1616.2 | 957.9 | 1120.8 |
7.15 | 670.3 | 484.4 | 548.2 |
Different from the propelling system of the microsatellite, the rocket engine provides power to large spacecrafts or spaceships. It requires large thrust and has more complicated structure and heavier weight compared with a microsatellite. The basic principle of a liquid rocket engine is introduced as follows. The propellant is injected by the injector into the combustor. Next, it is atomized, evaporated, mixed, and burned to generate combustion products. The combustion products are ejected at a high speed, which gives a thrust to the rocket engine. Atomization and mixing of liquid fuels are vital to the successful ignition and complete combustion of the rocket engine. Many scholars and universities have carried out many studies according to the fast evaporation in liquid flash evaporation.
Flash phenomenon may be developed at the spraying of fuel oil from the high-pressure environment to the low-pressure environment or at the heating of fuel oil in the rocket engine. Lecourt et al. [
Pulsed detonation engine (PDE) is a new conceptual engine that generates thrust by intermittent or pulsed detonation waves. Pulse detonation engines are generally available in two forms. One form of PDE is the air-breathing PDE. It absorbs oxygen from the air as the oxidizing agent. Another form is the pulsed detonation rock engine (PDRE) with an oxidant. The air-breathing PDE has same basic working principle with PDE. Compared with traditional engine, PDE has simpler structure, lower cost, smaller weight and higher flight Mach number [
The United States Air Force (USAF) Laboratory designed a flash evaporation system of fuel oil and carried out abundant studies to address this problem. Tucker et al. [
The DDT time of the heptane and isooctane [
The Northwestern Polytechnical University also made associated studies. The research team of Fanwei constructed the testbed for the flash evaporation system of fuel oil to address the influences of fuel oil droplets against the PDE performance. They made a series of preliminary experiments by using water as the working medium. Li et al. [
(a) The DDT time of the heptane and isooctane. (b) The variation detonation initiation time with operation time [
Although there are many studies on PDRE and PDE, they are just in the laboratory test and there are still many key problems that have to be studied.
Ionic liquid propellant has high stability, environmental-friendly and equivalent energy with hydrazine propellant. As an alternative to the hydrazine propellant in the orbital thrusters, it has attracted the attention of many researchers [
Besides, spacecrafts and air vehicles will discharge waste liquid and blowdown of propellants to vacuum during the flight or mission execution. In this process, liquid materials develop flash evaporation upon the sharp pressure drop. This will cause large-scaled freezing of liquid, blockage of the discharge channels, and freezing of the air vehicle surface. These will influence the safe running of air vehicles significantly. The liquid flash evaporation has been used in the America-Russian space station. Hence, many scholars have carried out abundant studies in order to protect the safe stable running of air vehicles [
This paper focuses on the application of flash evaporation technology in aerospace. Although there are many studies concerning the application of flash evaporation in aerospace, it is still in the development stage. Some conclusions and prospects are proposed.
Flash evaporation spray cooling is a complicated coupling of heat transfer process and mass transfer process. The existing mathematical model is not mature to describe the flash evaporation spray cooling. Many scholars studied the droplet flash evaporation and film flash evaporation independently. These two processes influence mutually. Droplet size and speed influence the film thickness significantly. Therefore, establishing a complete and simple mathematical model of flash evaporation spray cooling has important guidance to future studies Due to the complexity of droplet flash evaporation, scholars prefer to make many hypotheses to droplet flash evaporation. Everyone hypothesized that droplets keep unchanged spherical shape in the flash evaporation. However, flash evaporation is a strong process during which maintaining the spherical shape of droplets is difficult. Scholars can make further studies on this problem Nowadays, the vacuum jet flash evaporation has different classifications. There is no public accepted and universal model to describe the complete process of vacuum jet flash evaporation. In future, scholars can attempt to make uniform complete description of the vacuum jet flash evaporation
The flash evaporation spray cooling has high coefficient of heat transfer and good environmental adaptability. It is applicable to heat management of spacecrafts. However, the studies of flash evaporation spray cooling are not comprehensive compared with the conventional spray cooling. Many researchers proposed the theoretical models to study the process of the flash evaporation spray cooling. At the meantime, the flash evaporation spray cooling with water and volatile cryogens as working medium has been studied. Due to the complexity of flash evaporation spray cooling, it still stays in the experimental stage. Studies on flash evaporation spray cooling only focus on droplet size, impact speed, nozzle type, and spray angle. In future, scholars should study the flash evaporation spray cooling with reference to conventional spray cooing. Influences of roughness of heating surface, different scales and geometric structure of the heating surface, multiscale geometric structure of the heating surface, multinozzle array form, cooling medium, and mixed cooling media on heat transfer in flash evaporation spray cooling are discussed The propelling systems of a microsatellite, including flash evaporation jet propulsion program and hot-water rocket engine, have been studied for a long time. They have been successfully applied to practical engineering. But the working medium is single. In the future, mixed working medium can be considered to adapt to complex situations. Adding the flash evaporation system of fuel oil to the PDRE can improve combustion efficiency. Domestic and foreign scholars have conducted a large number of experimental studies on different fuels. And some researchers used the residual working heats of PDRE to improve the flash evaporation system of fuel oil. They all achieved good results. But the flash evaporation system of fuel oil in the PDRE is only in the experimental stage, and there are still many key problems that have to be studied
The authors declare that they have no conflicts of interest.
Wei Ma and Siping Zhai contributed equally to this work.
This work was supported by the Guangxi Colleges and Universities Program of Innovative Research Team and Outstanding Talent, the National Natural Science Foundation of China (Project no. 51506033), the Innovation Project of GUET Graduate Education (Project no. 2016YJCX18), and the Guangxi Natural Science Foundation (Grant no. 2017JJA160108).