Review of aerospace-oriented spray cooling technology
Introduction
The concept of the green energy air/space plane, which emphasizes the use of electric-driven equipment [1,2], has been in the spotlight recently [3]. Both the National Aeronautics and Space Administration (NASA) and the China Aerospace Science and Technology Corporation (CASC) have started programs to develop more-electric and full-electric air/space vehicles as the first priority. With the advance of micro-electro-mechanical systems, the development of the on-board electronic/electrical equipment, for example airborne micro-semiconductor radars, laser diode arrays, electro-driven actuators, and satellite electronics as shown in Fig. 1, presents the trend of miniaturization, modularization, and high-integration [4,5] which facilitates the rapid development of the more-electric and full-electric aerospace vehicle [6,7]. The advanced electronic/electrical equipment will definitely enhance the performance of the aerospace planes due to their compact structure, light mass, and high energy conversion efficiency. However, the extensive deployment of the electronic/electrical equipment will bring a clear rise in the power load where a huge amount of waste heat will be generated [9] due to the incomplete energy conversion [8,10,11]. The total thermal load could be up to 103 W [12]. Even worse, the aggressive micro-miniaturization of the electronic component would greatly shrink the effective surface area for the heat dissipation. That means the required dissipated heat flux will increase exponentially in order to keep the temperature of the equipment within an acceptable range, otherwise overheating-induced destruction would occur in the electronic/electrical equipment which overshadows the security, reliability, and operational capability of flight missions. Murshed et al. [13] suggested that the operating temperature of the majority of the power-electronics should be below 85 °C. Another article unveiled that most insulated gate bipolar transistors (IGBTs) are not recommended to be operated above 125 °C [14]. Simultaneously, the required heat flux to be removed has reached an order of 102 W/cm2 [15]. Hence, both cooling capability and temperature need to be satisfied [16].
In addition to the on-board high-power equipment, the severe thermal environment outside the aerospace vehicle also negatively affects the heat removal of the aerospace equipment. For example, the near-space hypersonic vehicle usually operates in an airspace where its altitude is 20–100 km [17]. The atmosphere of such high flight altitude is extremely thin which causes the quantity of the introduced air to be severely limited. Even worse, the temperature of the introduced air is pretty high due to the aerodynamic heating caused by the large Mach number (12-25 Ma). Therefore, the introduced air as a traditional major air-based heat sink should be abandoned in the thermal control system (TCS) of the near-space flight plane. Similarly, the thermal control strategy for the space vehicles is also faced with serious challenges as the space environment is more severe than the atmospheric environment. Even worse, high orbital maneuverability in near-earth orbital operation, Mars/lunar exploration, interplanetary deep space exploration will cause strong fluctuations of external heat flows, accompanied by periodical switches of high power devices. That determines that the heat exchange between external and in-cabin environments will experience a dynamic process with drastic changes. Therefore, thermal control strategies independent of the external environment should be promoted to satisfy the high adaptability and high reliability of the thermal control requirements for future aerospace flight missions.
Due to the rapid development of near-space flight technology, the boundary between the traditional spacecraft and the traditional aircraft is being eliminated, which would promote the development of subsystems of the flight vehicles including the TCS. As far as the TCS of the traditional spacecraft is concerned, the heat pipe [18,20] and single-phase mechanically pumped fluid loop (MPFL) [21,22] are two major liquid-based cooling schemes extensively adopted in the TCS of space vehicles. The reason why these two cooling methods are widely used is that they are both gravity-immune which means their effective operations are less affected by the gravitational field, so they could function under the space micro-gravity. As far as the TCS of the traditional aircraft is concerned, two main cooling media are introduced, namely the air and the fuel. Coming from the aero-engine, the introduced air will be transported to the cabin to cool the on-board equipment and passengers/crews. The fuel is used to be the cold end of the heat exchanger to absorb a part of waste heat generated from the environmental control system and equipment cabins. However, these thermal control strategies mentioned above fail to handle heat flux of an order of 102 W/cm2. In addition, the thermal control performance of the fuel system depends on the amount of the remaining fuel, determining that such strategy will become unreliable in the late flight stage [23]. In summary, it is imperative to propose novel, durable, and environment-independent aerospace-oriented thermal control strategies to ensure an effective and reliable operation of the on-board high power density equipment.
As shown in Fig. 2 [26], the two-phase cooling technology can cover a range of 102–103 W/cm2 with a small temperature difference, which suggests a promising solution [[27], [28], [29]]. Microchannel cooling, jet impingement, and spray cooling technology are the top three active two-phase cooling schemes proposed by Visaria and Mudawar [19]. Among these three methods, the spray cooling was highly recommended to be the most promising cooling method [24] as it possesses several remarkable advantages: lower specific area, higher heat flux dissipation capability (up to 103 W/cm2), lower temperature difference between the surface area and the working medium, and lower coolant flow rates in the given condition. Spray cooling can be achieved when a bulk of working fluid passing through a nozzle is atomized either by high hydraulic pressure or high pressure gas into numerous fine droplets. The generated droplets subsequently impinge on a heated surface with high momentum, generating a layer of thin liquid film. Simultaneously, the heat is dissipated using both sensitive and latent heat where droplet-impinging and nucleate boiling effects are responsible for the high heat flux dissipation ability. Paris et al. [25] concluded that the maximum dissipated heat flux could be up to 1200 W/cm2 with a surface temperature of 100 °C. Therefore, adopting spray cooling is expected to satisfy both requirements of adequate cooling capability and surface temperature.
Spraying is a complex multi-parametric nonlinear process [4,30] where droplet impingement, fluid-thermal coupling, droplet-solid collision, etc. Coexist which has triggered extensive investigations across the world. Because of its excellent thermal properties, spray cooling has been or is about to be adopted in many ground-based thermal management applications in electronic cooling, energy, medical treatment, metal industry, etc. Mertens et al. [14] established a spray cooling system to thermally protect IGBT devices where a highest heat flux of 825 W/cm2 with a junction temperature below 125 °C was attained. Mudawar et al. [26] explored thermal management solutions for the hybrid electric car using spray cooling technology. Photovoltaic panels equipped with water spray cooling was investigated where a potential 17% net efficiency enhancement in electricity production was gained [31]. Besides, high power devices including metal oxide semiconductor field effect transistors [32], LED [33], and supercomputer [34] have been reported to adopt spray cooling technology to guarantee reliable operation performances. In addition to the electronic cooling, the spray cooling has also been used in the production of alloy steel [35], medical laser surgery [36], etc. It should be emphasized that all the applications mentioned above are in the normal gravitational field, suggesting spray cooling on the ground should be a relatively mature technology. These earth-based applications and researches provide the theoretical basis and practical application reference for the aerospace-oriented spray cooling (AOSC).
Admittedly, compared with the various ground applications, the aerospace spray cooling is rather rare as the investigation towards the spray cooling in the space environment or high altitude airspace is relatively limited. Such delay could be attributed to four problems: (1) The operational mechanism of the ground-based and aerospace-based spray cooling is totally different. For example, with the assistance of the normal gravity, a thin-film could be formed and effortless gas-liquid separation could be realized, which guarantee both high efficient cooling performance and durable cooling performance. In contrast, in the space micro-gravitational environment, the vapor and the liquid are difficult to be separated or recycled. This threatens the durable operations, let alone the enhanced heat transfer performance; (2) Changes in the gravitational field bring a set of changes in the heat transfer and flow dynamics of the ground spray cooling, which determines that achievements attained by the numerous investigations of the ground spray cooling technology would fail to be applied to the space applications; (3) In-orbital and air-borne experiment is costly and simulation of various aerospace environments such as micro-gravity is also complicated where the experimental condition of the on-board spray cooling is difficult to simulate; (4) as a local heat flux heat transfer technique, the two-phase spray cooling can hardly be combined in the existent overall thermal control system such as the single-phase MPFL. Prior to the practical application of the aerospace spray cooling, these four problems should be solved.
This review paper aims to review the limited up to date literature regarding the AOSC and summarize the contributions towards the fundamental research and practical application of this cooling technology. Additionally, guidance for the future development of the AOSC will be provided for the purpose of rapid deployment of the AOSC system. Note that the spray cooling for the thermal protection of on-board electronic and electrical devices is the focus of this paper, which means the temperature is within the low-temperature region. Heat transfer behaviour in this region takes place before the critical heat flux (CHF) which is a maximum heat transport ability in the low-temperature region.
Section snippets
Cooling mechanism of the spray cooling technology
In 1972, the Japanese scientist Toda first proposed the concept of mist cooling: The liquid medium is atomized into fine droplets through a certain method [37]. The atomized droplets impact the heating surface to dissipate the heat through convection, evaporation, and boiling. As shown in Fig. 3 [38] the tiny droplets are ejected from a small nozzle and then impinge the surface with a high velocity. The impinged droplets will form a sustainable liquid film where convection, evaporation, and
Effect of the gravitational field
Section 2 describes the general process of the spray cooling which takes place in the ground normal gravitational field. However, it is totally different when placing the spray cooling system in the high-altitude or space environment. Therefore, the operating and cooling mechanisms of the AOSC system is totally different compared to those of the general spray cooling system residing on earth. Fig. 4, photographed by Golliher et al. [41], demonstrates the difference between the flow
Comments, perspectives, and orientations
The AOSC is still in its infancy as few practical applications of the on-board spray cooling system were reported. Even the world-wide AOSC research is insufficient compared with the research into the ground-based spray cooling technology. The extremely high experimental cost and the required experimental conditions are primarily responsible for this dilemma. Specifically, the practical in-orbit experiment or airplane-borne experiment to create a practical space or high-altitude variable
Conclusions
Motivated by the concept of energy-optimized space/air vehicles, aerospace engineers and scientists tend to design and construct more-electric or even full-electric air-/space-planes. This trend introduces the need for advanced highly-integrated on-board electronic/electrical devices in the flight system. Correspondingly, high heat-flux dissipation methods are needed for the reliable and efficient operation of the flight system. Spray cooling has been universally recognized as an effective
Declaration of competing interest
We declare that we have no conflict of interest.
Acknowledgment
The authors would thank the editor and reviewers for their valuable efforts to polish up this manuscript. This work was co-funded the Starting Fund for early-career scientists of Yangzhou University and China Postdoctoral Science Foundation (No. 2020M671618).
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