Spray Flash Research Articles

Experimental Investigation on Flash Evaporation Related to Pipe Leakage

Abstract Pipelines such as the surge line and main pipe are easily subjected to thermal stratification and thermal fatigue as a result of the nonuniform temperature distribution in the nuclear power plants. When the surge line or main pipe subjected to thermal stratification and thermal fatigue keeps operating for long time, the pipe leakage may happen due to the existence of pipeline crack. When the fluids with high temperature and pressure leak in the crack, the water will evaporate quickly, which means this process belongs to spray flash evaporation process. The flash evaporation related to pipe leak was experimentally studied in the paper. The experiment was carried out under high temperature and high pressure with low spray rate. The temperature and relative humidity (T&H) variations over time were monitored in the experiment with installing T&H detectors. The T&H variations at different measurement positions and with different spray rates were analyzed, respectively. In addition, the effect of the dimensionless parameters including the Weber number and Jakob number was also investigated. Results indicated that the response speed increased with the increase of the spray flow rate. Higher Weber number and higher Jakob number led to higher evaporation rate. The slight pipe leakage can be predicted by using the (T&H) in the hazardous areas.

Performance improvement of spray flash evaporation desalination systems using multiple nozzle arrangement

This research aims to improve the performance of the spray flash evaporation as a key component of Discharge Thermal Energy Combined Desalination (DTECD) systems using a multi-nozzle head in various arrangements for the first time. Two novel nozzle arrangements were proposed and compared with the conventional single nozzle. The injection of saline water inside the vacuum chamber was performed under various operating conditions including inlet flow rate, pressure injection, superheat degree, and salinity. Furthermore, the droplet sizes and distribution were observed and analysed using shadowgraph imaging. A similar outcome was reached between the droplets measurement analysis and measured evaporation rate and gain output ratio which implied the most efficient arrangement. The proposed arrangement in which five nozzles are located in the farthest distance totally improved the efficiency of the system under various conditions up to a maximum 28% compared to the conventional single nozzle for the same flow rate. In addition, it was found that the number of nozzles plays a more significant role than their arrangements for a certain pressure injection. Moreover, the optimised maximum superheat degree for the most efficient arrangement was found to be 19 °C. These results provide new fundamental understanding in the area of spray flash evaporation and reveal that increasing the number of nozzles and placing them in the farthest distance apart can improve the efficiency of the system.

An experimental study on explosive boiling of superheated droplets in vacuum spray flash evaporation

Spray flash evaporation is an effective desalination method, which increases specific surface area of salty water by liquid atomization, thereby improving desalination performance and maximising low-grade heat source utilization. During evaporation, explosive boiling phenomenon occurs inside superheated droplets on a heated surface. In order to understand the mechanism of explosive boiling, spray flash evaporation of distilled water and 3.5 wt% salty water in a high vacuum vessel was observed visually. Meanwhile, a parametric study was carried out to scrutinize the impacts of the variation of ambient pressure, heat flux, and surface superheat degree. The experiment data indicates that nucleate site is located in the upper layer of a droplet due to internal superheated liquid and Marangoni convection. In different operating conditions, bubble fragmentation process or crown fragmentation process happens at nucleate site. The fragmentation time of pure water, which is mainly influenced by heat flux and surface superheat degree, shrinks with higher heat flux and higher surface superheat degree. The fragmentation time of 3.5 wt% salty water decreases with ambient pressure drops and superheat degree increments.

Heat transfer performance and optimization of a close-loop R410A flash evaporation spray cooling

Flash spray cooling has been subject to increased attention because of its high heat dissipation capacity at low surface temperature in the application of high power technologies. In this study, experiment was conducted to study the effects of spray distance and nozzle diameter on heat transfer performance in a closed-loop R410A flash spray cooling system for the first time. Five spray distance from 10 mm to 30 mm and three nozzles with same internal structure but different diameters of 0.51, 0.56 and 0.69 mm were employed. The experiment results indicated the critical heat flux (CHF) value firstly increased and then deceased with the increase of spray distance, which is consistent with previous research of spray cooling with FC-72 and FC-87. The highest CHF value reached 264 W/cm2 while maintaining surface temperature below 30 °C and heat transfer coefficient (HTC) was about 210 kW/(m2·K) at 25 mm, which were 60% higher than those at 10 mm spray distance. The nozzle with medium orifice diameter of 0.56 mm showed a superior cooling performance, instead of larger nozzle with higher refrigerant flow rate. Therefore, there existed a counterbalance between the mass flow and outlet velocity in determining the optimum nozzle orifice diameter.

Heat transfer enhancement due to surface modification in the close-loop R410A flash evaporation spray cooling

Flash spray cooling has been considered as one of the most promising technologies of heat dissipation for high power electronic devices because of its high cooling capacity at low surface temperature. In this study, experiments were conducted to study the heat transfer enhancement due to surface modification in the close-loop R410A flash spray cooling system. The test surfaces including the macro-structured surfaces with pyramid and square fins with two orders of roughness, and the nano-porous surfaces with different pore diameters were examined. The experimental results indicated that the surface with macro fins could tremendously enhance the heat transfer due to the increase of wetted area. However the pyramid fins with less increase in wetted area showed a better heat transfer performance than square fins, indicating fin structure played a more important role than the increase in wetted area. Higher roughness could further improve the cooling performance of macro-structured surface. The maximum CHF of 330 W/cm2 and heat transfer coefficient of 300 kW/(m2 K) were achieved by the surface with rough pyramid fins, corresponding to 60% enhancement and 5 times respectively over smooth flat surface, while the wall temperature was maintained below 10 °C. The nano-porous surface could also lead to better heat transfer performance by increasing the number of nucleation sites and improving the wettability to working fluid. The result of pore size effect showed that the CHF value at first declined and then increased with the increasing pore size. It was postulated that this phenomenon was related to the transition of dominating heat transfer mechanism from “evaporation-limited region” to “viscosity-limited regime”. These findings helped to guide further investigations of enhanced surface aiming at enhancing heat transfer performance in flash evaporation spray cooling.

Preparation of reduced sensitivity co-crystals of cyclic nitramines using spray flash evaporation

Abstract The present day weapon technology demands novel energetic materials that exhibit simultaneous high explosive yield and reduced sensitivity. This article demonstrates application of spray evaporation to prepare reduced sensitive co-crystals of high performance nitramine explosives like HMX and CL-20 with a relatively less insensitive explosive 1,1-diamino-2,2-dinitroethylene or FOX-7. Stronger intermolecular hydrogen bonding in FOX-7 is responsible for limited solubility in most of organic solvents. Large solubility differences of FOX-7 with HMX and CL-20 restricts it's co-crystallization through classical methods that yields thermodynamically favorable product. Spray flash evaporation, a kinetic crystallization method, has been therefore adopted and could successfully produce CL-20/FOX-7 (2:1) and HMX/FOX-7 (4:1) co-crystals. The fine powdered materials obtained were characterized by SEM, powder XRD, Raman spectroscopy, DSC-TGA etc. Multipoint Raman spectra showed consistent occurrence of spectral features indicating stoichiometric co-existence of ingredients in the crystal lattices. DSC analysis showed absence of all thermally assisted solid-solid phase transformation in the co-crystals as they were observed in pristine materials. The thermal stability calculated in terms of activation barrier for decomposition, revealed the CL-20/FOX-7 co-crystal to be intermediately stable on comparison to their constituents while, the HMX/FOX-7 co-crystal is more stable. Compared to pure HMX and CL-20, both the co-crystals have shown higher insensitivity to impact force, suggesting them to be suitable for future generation insensitive munitions.

Mathematical study of spray flash evaporation in a spray-assisted seawater desalination chamber

Spray flash evaporation is a method for the new desalination technology driven by renewable energy. This paper studies the flash evaporation phenomenon of the seawater jet sprayed into a low-pressure chamber. A droplet flash evaporation model has been developed to enable an in-depth understanding of the flash evaporation. This model is capable of predicting the spray flash speed, the evaporated mass rate and the flashing efficiency under different working conditions. Applying this model, the effects of different operating variables on the flash evaporation process are investigated. Results from the developed model reveal a larger Jakob number, Reynolds number and Weber number all lead to the increase of the production rate. The flashing efficiency can attain values larger than 0.9 with the nozzle diameter ranging 5–14 mm when the flash chamber is 150 mm height. The flashing efficiency with nozzle diameter of 5 mm is larger than 0.9 when the flash chamber is 25 mm height. The flow velocity of 10 m/s is recommended for the current case to obtain a large production rate. A flash chamber larger than 500 mm height is needed for complete evaporation considering the presence of dissolved salt in sea water.

Energetic Nanoparticles and Nanomaterials for Future Defense Applications

The integration of nanostructured materials in defense systems is expected to improve their performance in terms of power, safety, and reliability. That is why considerable research effort has been undertaken by major military powers worldwide in this domain. The first important step was to develop the production capacities of organic explosives in the state of fine powders with submicron to nanosized particle size distributions. The Spray Flash Evaporation (SFE) process, which is a unique method for producing such materials, was developed at industrial scale. Explosive nanopowders obtained by this process were subsequently mixed with nanosized pyrotechnic compositions such as nanothermites, to prepare hybrid detonating materials able to replace lead-based primary explosives. Composite propellants can also be prepared by SFE which allows mixing their components in a single step with better homogeneity. The ultimate challenge is to move from powder to object, in order to integrate energetic nanomaterials in operational systems. Although the research in this last domain is still in its infancy, several ways of preparation of objects from nanothermites have been recently reported in scientific literature. The focus will be on two examples studied in our laboratory. The first one is the preparation of nanothermites in the state of solid, porous foams; the second one is the use of nanothermites for coating grains of propulsive powder to change their combustion properties.

Nanodiamond-based energetic core-shell composites: The route towards safer materials

This work proposes to determine how diamond has an influence on the structure (core-shell morphology or not) as well as on the desensitization of ND-RDX (Nanodiamond – cyclotrimethylenetrinitramine) mixtures. RDX is one of the most used high performance explosives worldwide for military and civil applications. ND-RDX composite core-shell particles are obtained by Spray Flash Evaporation (SFE), a patented process developed in the laboratory. The characteristics and pyrotechnical properties of the resulting core-shell compounds are then compared to those of composite materials prepared by other means like slow evaporation and physical mixing. Further investigation of the sample morphology and the core-shell architecture are also discussed in the light of XPS and BET measurements. Finally, the interest of the nanodiamond properties to elaborate an energetic material that combines high energetic performance with desensitization to impact and friction in order to ensure safer handling is outlined hereinafter.

Research Progresses of Flash Evaporation in Aerospace Applications

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.

Energy analysis of spray flash evaporation from superheated upward jets

Flash evaporation is widely used in desalination technologies. The flash evaporation from upward and downward jets shows different characteristics. To study the upward flash evaporation, a model based upon droplet analysis is developed. The present model takes the droplet motion, droplet size variation and droplet temperature into consideration. The model is validated against the corresponding experimental results from previous literature and the calculated results from the model coincide with the experimental results. The dimensionless temperature profiles of the upward and downward flash evaporation are compared. Employing this model, the influences of the injection pressure, evaporation chamber pressure and flow velocity on the energy utilization efficiency and spray flash speed are investigated. The critical time and critical distance are also compared and analyzed.

Experimental investigation on high-pressure high-temperature spray flash evaporation and the characteristic Jakob number

Experiments on high-pressure high-temperature spray flash evaporation in saturated vapor environment are conducted on a novel large-scale experimental system. The influence of initial liquid temperature and evaporation pressure on flash efficiency is investigated. These two parameters are thought to affect each stage of the spray flash evaporation process, including nucleation, bubble growth, atomization and droplet flashing. The dimensionless Jakob number (Ja), defined as the ratio between the amount of sensible heat available to the amount of energy required for vaporization, is generally acknowledged to be the characteristic number of flash evaporation, which is a typical finite-rate thermal non-equilibrium process. The larger Ja is, the higher flash efficiency is, and the more violent flash phenomenon is. Meanwhile it is found that flash efficiency is not a single-valued function of Ja. Multiple regression is then carried out and empirical equations of flash efficiency vs. Ja and dimensionless pressure are proposed, which establish coupling between system performance, initial and final states and operating conditions.

Construction of Water Level Model for After Condenser in Spray Flash Desalination System via Stochastic Process

Construction of Water Level Model for After Condenser in Spray Flash Desalination System via Stochastic Process

Nanocrystallisation of Ammonium DiNitramide (ADN) by Spray Flash Evaporation (SFE)

AbstractAmmonium DiNitramide (ADN) is an interesting oxidizer to replace ammonium perchlorate in the composition of solid propellants. In this study, Spray Flash Evaporation (SFE) is presented as a new technology to enhance stability of ADN by crystallization of nanoparticles. The crystallinity and purity of the compound was confirmed by X‐Ray Diffraction and Raman analysis. An average diameter of spherical particles around 32 nm (solution of ethyl acetate) and 34 nm (solution of methyl acetate) was measured by Scanning Electronic Microscopy (SEM). Furthermore, the downsizing to nanoscale induces a slower moisture absorption kinetic under 55 % of relative humidity, a reduction of the critical diameter of detonation and the desensitization to impact.

Experimental investigation and analysis of a new single-stage vacuum spray flash desalinator utilising a gas-liquid ejector

Abstract The aim of this research is to investigate the performance and the dynamic thermo-fluid behaviour of a new vacuum spray flash desalinator. This is the key component of the open water cycle in the discharge thermal energy combined desalination (DTECD) system utilising a gas-liquid ejector (eductor). A down-flow eductor using saline water as a motive fluid is proposed for this new single-stage vacuum desalinator. The effects of the temperature and the salinity of motive fluid on the performance of eductor are investigated. The exergy efficiency of the system and its components are evaluated. Experimental results indicate that the performance of the proposed desalinator aligns well with the evaporation model. The proposed eductor is also reliable and easy to operate for generating a vacuum as required close to 6 kPa. This pressure is lower than the corresponding saturation pressures of the operating temperature range between 55 °C and 75 °C. The results show that lower vacuum pressure is obtained when the temperature of the motive fluid is lower (about 30 °C or less). The eductor was operated using 3% and 3.5% by weight of saline water and the results show that the salinity of the motive fluid does not significantly affect the performance of the system. Thus, utilising seawater can be an alternative and cheap option for operating the eductor.

Experimental investigation on the effect of heat transfer enhancement of vacuum spray flash evaporation cooling using Al2O3–water nanofluid

Vacuum spray flash evaporation cooling (VSFEC) experiments were carried out to study the effect of heat transfer enhancement of a nanofluid comprising distilled water and Al2O3 particles with a diameter of 30 nm. The tests are performed with a flat heated surface using a nozzle with an orifice diameter of 0.5 mm and a nozzle-to-heated surface distance of 7 mm. The spray volume flux is varied in the range of 1.7 L/h-5.0 L/h, while the concentration is specified as 0ppm, 100ppm, 200ppm, 300ppm or 500ppm. The experiment results show that a more uniform surface temperature can be obtained, however, due to the difference of heat transfer mechanism, the effect of heat transfer enhancement is also different using Al2O3–water nanofluid.

Modeling of spray flash evaporation based on droplet analysis

To enable a more in-depth understanding of the flash evaporation from the downward jet and extend the research range of spray flash evaporation, a mathematical model based on the diffusion-controlled evaporation model was proposed and developed. The droplet motion, droplet size variation and temperature variation are taken into account in the present model. The model was validated against the experimental data sets from literature sources. The temperature variation against the traveled distance was obtained and analyzed. Four variables, namely, flow velocity, pressure attenuation ratio, droplet size and relative humidity, were investigated by means of this model.

Study of the Elaboration of HMX and HMX Composites by the Spray Flash Evaporation Process

AbstractThe Spray Flash Evaporation (SFE) process invented and developed at the NS3E laboratory allows obtaining different nanosized explosives (TNT, RDX, CL‐20…). This process is based on the very fast evaporation of the solvent due to the drastic modification of pressure and temperature leading to the crystallization of the molecules present in solution into nanometric or submicrometric particles. Here, we show the possibility to prepare pure HMX (Octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) or HMX based composites at the nanoscale using this process. This study mainly focuses on the size, morphology and crystallographic phases obtained for HMX and HMX/TNT composites depending on the experimental conditions (temperature, pressure, solution concentration…) used during the elaboration. For this purpose, the results obtained from scanning electron microscopy, X‐ray diffraction and Raman spectroscopy are discussed.

Experimental and mathematical study of the spray flash evaporation phenomena

Spray flash evaporation is a promising way that potentially enables various renewable energy sources to be employed for desalination. This paper specifically studies the flash evaporation phenomenon of a hot water jet sprayed into a low-pressure chamber. Special considerations are rendered to the flash evaporation process under lower temperature differences which evolves higher thermal efficiency and better low-grade heat utilization for desalination. An experimental setup has been developed and temperature profiles of the sprayed water in the axial direction are mapped. Under low flow velocity conditions, the water jet shatters into droplets due to the flash atomization effect. Such a shattering phenomenon can be captured and quantified by a mathematical model based on droplet analysis. Applying this model, the temperature profiles are translated into the mean spray droplet diameters. The effect of different variables on the flash evaporation process is further investigated. Key results revealed that the mean droplet diameters of the spray are several orders of magnitudes smaller than the nozzle diameter. Such fine droplets allow complete evaporation to be accomplished within 50 cm from the nozzle exit, enabling a compact evaporator design. Additionally, higher initial temperature differences and higher flow velocities reduce the mean droplet diameter to be smaller than 300 μm, and the corresponding vertical distance required to complete the evaporation process is shorter than 10 cm.

Experimental study on spray flash evaporation under high temperature and pressure

Most leakage problems from pipe system breaches result in the formation of spray flash evaporation. In this study, a variety of experiments were carried out in the high temperature and high pressure (HTHP) steam-water test loop to study spray flash evaporation related to tube leakage problems. The focus of our study was on the flash evaporation from the highly superheated jet (the superheat degree can be up to 200K) with a small injection rate. The temperature and relative humidity variations in the region of interest were measured in the experiment. As the transient relative humidity changes can enable better reflection of the characteristic of flash evaporation, non-dimensional relative humidity (NDRH) and the critical time of the relative humidity variation was proposed and used to analyze the flash evaporation. The effects of injection rate, injection direction, initial water temperature, and injection pressure were investigated. The experimental results showed that the increase of the injection rate and initial water temperature enhanced the flash evaporation. The corresponding critical time increased with the increase of spray angle. The injection pressure was found to result in better atomization and evaporation of the water on the premise that the injection pressure guaranteed complete flash evaporation.