Nitrogen Droplets Research Articles

Experimental study on the dynamic behaviors and spreading characteristics of a liquid nitrogen droplet impacting superheated wall

Liquid nitrogen droplets impacting superheated wall is a fundamental phenomenon in liquid nitrogen spray cooling. Its dynamic characteristics directly affect the cooling performance. In this paper, we design and build an experimental setup to investigate a cryogenic liquid nitrogen droplet impacting superheated wall. Using a high-speed camera, we capture the dynamic behaviors of the liquid nitrogen droplet upon impact. The effects of wall temperature and Weber number on the impacting dynamics and spreading characteristics of droplets are studied. The experimental results show that the droplets exhibit six regimes of dynamic behaviors under different wall temperatures and Weber numbers, namely, contact boiling, atomization boiling, film boiling with complete rebound, film boiling with retraction and breakup, film boiling with instantaneous splashing, and film boiling with crown splashing. The Leidenfrost temperature at which film boiling occurs is found to be independent of the Weber number. The spreading characteristics of droplet under different boiling regimes are summarized. During contact boiling, droplet spreading is affected by both wall temperature and Weber number. In contrast, for atomization boiling and film boiling, the spreading is solely affected by the Weber number and is insensitive to the wall temperature. But the underlying mechanisms are different. The former is due to the efficient boiling heat transfer making it insensitive to the wall temperature, while the latter is due to the near thermal isolation caused by the vapor film. Lastly, dimensionless fitting correlations for the maximum spreading coefficient of droplets under different boiling regimes are established. This study can provide guidelines for the design and development of liquid nitrogen spray cooling.

Numerical investigation on the tracer followability of nitrogen droplets over the airfoil in cryogenic wind tunnel

Cryogenic wind tunnels (CWT) achieve high Reynolds number aerodynamic performance measurements by cooling the medium with liquid nitrogen. Utilization of naturally-occurring liquid nitrogen droplets as tracer particles for non-intrusive measurement applications in CWT shows great potential in the previous work. However, the droplet motion characteristics over the airfoil remain to be investigated, and the followability evaluation approach is lacking. Accordingly, an Euler-Lagrange model for transonic particle flow over the NACA0012–64 airfoil was established. The effect of flow characteristics on the particle motion was explored. The results reveal that the flow separation and shock wave phenomenon over the airfoil causing velocity deviation between airflow and droplet. The shock wave is the main deviation driver, and flow separation causing trace blind zones. Droplets larger than 10 μm fail to effectively follow the variation of airflow. The blind zone enlarges with angle of attack (AoA), particularly when AoA exceeding 10 degrees. Moreover, Stokes number is found to be inadequate for characterizing the followability of tracer particles in turbulent flow. Consequently, an evaluation method based on the Basset-Boussinesq-Oseen equation was developed. This research provides theoretical and technical support for advancing the use of nitrogen droplets as tracer particles in CWT.

Experimental study on the dynamics of a liquid nitrogen droplet impacting a surface under Leidenfrost conditions

Liquid nitrogen droplet impacting a superheated surface is a fundamental phenomenon of liquid nitrogen spray cooling, whereas the mechanisms behind which are still unclear. We designed and developed a visual experimental system to investigate the dynamics of a liquid nitrogen droplet impacting a superheated surface under cryogenic conditions. The impact dynamics of the liquid nitrogen droplet at the Leidenfrost state are captured, and the effects of Weber number (We) and surface temperature on the spreading and rebound characteristics of the droplet are analyzed. The findings show that the droplet exhibits spreading, retraction and rebound at a low We. Droplet spreading and rebound characteristics are mainly affected by We while insensitive to surface temperature. The maximum spreading coefficient exhibits a power-law increase with We, while the maximum rebound coefficient shows an upward and then downward trend with We. The dimensionless maximum spreading time, dimensionless residence time, and dimensionless maximum rebound time show power-law increase with We. Corresponding fitting correlations for these factors for liquid nitrogen droplets are also proposed. This study contributes to an in-depth understanding of the impact dynamics of cryogenic liquid droplet under cryogenic conditions.

A nitrogen Leidenfrost droplet on a water pool: Experiments, theory and simulations of droplet shrinkage and ice formation

The cooling capabilities of liquid nitrogen are exploited in various fields, where the liquid is often in apparent contact with a soft or even liquid partner. Even though the heat transfer between the partners is mostly of actual concern for the given application, it is not yet fully described. In the present work, the shrinkage of a nitrogen Leidenfrost droplet on a water pool and the resulting formation of ice inside the pool are examined experimentally, theoretically and numerically. Experiments are performed using nitrogen droplets of varying size, deposited onto a water pool which is initially always at its melting temperature. Droplet shrinkage and ice formation are captured using a high-speed video camera providing a synchronized top and side-view on the scene. An existing analytical model for the interface shape of the droplet and the pool in the given situation is extended to enable theoretical prediction of the temporal evolution of the droplet size and the volume of ice formed inside the pool. While only heat transfer at the droplet bottom is considered in the original model, in the present work also the major contributions to the heat transfer at the droplet top and at the pool meniscus are accounted for. Additionally, numerical simulations of droplet shrinkage are performed using a commercial finite-element simulation software (COMSOL Multiphysics). For both the theoretical model and the numerical simulations, the droplet and the pool are assumed isothermal and heat transfer is only considered in the gaseous ambient. Finally, both the theoretical predictions and numerical results generally well resemble the experimental findings for droplet shrinkage, where the numerical simulations show a slightly better agreement. Also the theoretical predictions for the ice volume forming inside the pool are in good agreement with the experimental results, confirming the good predictive capabilities of the theoretical model for the present situation.

Feasibility investigation of non-equilibrium condensing nitrogen droplets as tracer particles in cryogenic wind tunnels

Application of high-precision non-intrusive flow field measurement techniques in transonic cryogenic wind tunnels (CWT) is confined to the lack of suitable tracer particles. The present study explored the feasibility of utilizing non-equilibrium condensing nitrogen droplets as tracer particles. A numerical model of the nitrogen non-equilibrium condensation was established. Accordingly, the non-equilibrium condensation process of nitrogen droplets and feasibility analysis of tracing were investigated. The results reveal that the occurrence of nitrogen droplets as well as the droplet concentration and mean radius can be regulated in the nucleation zone and droplet growth zone by varying gas temperature or pressure. Furthermore, the feasibility of employing nitrogen droplets as tracer particles in the working conditions of CWT is confirmed through the analysis of tracking capability, response time, concentration, and duration of droplet traceability. Formulas correlating used to calculate duration of droplet traceability have been proposed. This study proved the potential application of condensing nitrogen droplets as tracer particles in CWT.

Development of a lab-device for evaporation-free supply of pure liquid nitrogen for droplet- and jet-generation

Cryogenic liquids such as liquid nitrogen are of relevance for numerous processes in engineering, and the food and pharmaceutical industries. However, as a result of its strong evaporation at ambient conditions, its handling for laboratory purposes and experimentation is so far cumbersome. In the present work an original design approach for a supply device for liquid nitrogen is developed and characterized in detail. With the device pure liquid nitrogen is supplied from a pressurized dewar flask to a hypodermic needle without contamination of the liquid with its own vapor or frost, finally enabling to generate a free liquid jet or single droplets in a way comparable to the handling of non-cryogenic liquids using a syringe and a hypodermic needle. Compared to previous approaches for the generation of liquid nitrogen droplets in scientific studies which mostly rely on a reservoir for liquid nitrogen from which droplets form at a bottom outlet due to gravity, the present design allows generation of droplets and free liquid jets in a significantly better controlled and more flexible way. The device is experimentally characterized for varying operational conditions during generation of a free liquid jet, and its versatility for laboratory research purposes is further briefly demonstrated.

Molecular dynamics simulation of droplet evaporation in a one-dimensional standing wave acoustic field

In this paper, a molecular dynamics (MD) simulation has been adopted to investigate impacts of one-dimensional standing wave acoustic field on the droplet evaporation. Evaporation processes of single and multi-component droplets in nitrogen are simulated by considering acoustic wave amplitude and frequency impacts. Results reveal that the droplet evaporation rate is apparently enhanced in a pressure oscillation environment. The larger oscillation amplitude and acoustic field frequency would reduce the droplet lifetime. Besides, the instantaneous evaporation rate and droplet temperature show a periodic oscillation characteristic when the oscillation amplitude is relatively large. These simulation results indicate that the instant droplet evaporation rate would be increased by the larger pressure oscillation frequency. Moreover, water molecules in the droplet would display a stronger response to the acoustic field by comparing with the ethanol case.

Collision dynamics of two liquid nitrogen droplets under a low-temperature condition

Collision of nitrogen droplets is a basic phenomenon in an array nitrogen spray cooling system, while the understanding on which is still lacking. We conduct a numerical simulation to investigate the collision dynamics of two nitrogen droplets in a low-temperature environment under various Weber numbers, Reynolds numbers and impact parameters, and simulation of collision of two water droplets is conducted for comparison. Incompressible Navier-Stokes equations are solved to simulate the colliding process, and Volume of Fluid (VOF) method and adaptive mesh refinement technique are used to capture gas-liquid interface. Three regimes of collision are found for nitrogen droplet collision, including coalescence, reflection separation and stretching separation. Upon collision, a lower surface tension and lower viscous dissipation consume less initial kinetic energy, which is conducive to the separation of droplets. Compared with water droplets, collision of nitrogen droplets easily enables breakage due to the lower surface tension and viscous dissipation. Separation of nitrogen droplets leads to the increase of secondary droplet number and enlarges heat transfer area of droplets and gas, which would enhance droplet vaporization and temperature descendence of the environment.

On the trajectory and reach of fire-suppressant liquid nitrogen droplets released from a spray nozzle

Liquid nitrogen (LN2) can be used to supplement or replace currently used fire extinguishers in challenging fire situations. This environmentally-benign cryogenic fluid can be produced readily from deep refrigeration of air. This manuscript addresses the application of liquid nitrogen to fires through a nozzle. Its aim is to investigate the survivability of LN2 droplets, following breakup of the jet exiting the nozzle, and the amount of liquid that reaches a target. Numerical simulations were carried out to explore the effects of relevant parameters, such as the initial droplet size and velocity, the droplet injection angle, and the thermal radiation on the droplet flight distance and the liquid droplet mass. The results indicate that all these parameters influence the fate of the LN2 droplets. Coarse atomization, high initial velocities, short nozzle-to-target distances, and shallow jet angles maximize the amount of the cryogen delivered to the target. A small-scale experiment was performed to test the results of the calculations. The comparison of the experimental results and the theoretical calculations was deemed satisfactory. In both cases the reach of the liquid droplets was in the order of 1 m from the nozzle, for an initial droplet velocity in the neighborhood of 20 m/s as measured in the experiments.

Investigation of Dynamic Characteristics of Liquid Nitrogen Droplet Impact on Solid Surface

Liquid nitrogen spray cooling technology exhibits excellent heat transfer efficiency and environmental protection performance. The promotion of this technology plays an important role in improving the sustainable development of the refrigeration industry. In order to clarify its complex microscale behavior, the coupled Level Set-VOF method was adopted to study the dynamic characteristics of liquid nitrogen droplet impact on solid surface in this paper. The spreading behaviors under various factors (initial velocity, initial diameter, wall temperature, and We number) were systematically analyzed. The results show that the spreading behaviors of liquid nitrogen droplet share the same process with the normal medium, which are rebound, retraction, and splashing. For the droplet with smaller velocity and diameter, Rebound is the common phenomenon due to the smaller kinetic energy. With the increase of droplet diameter (0.2 mm to 0.5 mm) and velocity (0.1 m/s to 5 m/s), the spreading factor increases rapidly and the spreading behaviors evolve into retraction and splashing. The increase of wall temperature accelerates the droplets spreading, and the spreading factor increases accordingly. For the liquid nitrogen droplets hit the wall, the dynamic behaviors of rebound (We < 0.2), retraction (0.2 < We < 4.9), and splashing (We > 4.9) will occur with the droplet weber number increased, which are consistent with the common medium. However, due to liquid nitrogen having lower viscosity and surface tension, the conditions of morphological transformations are different from the common media. The maximum spreading diameter has a power correlation with We, the power index of We is 0.306 for liquid nitrogen, lager than common medium (0.25). The reasons are: (1) the better wettability of liquid nitrogen, and (2) the vapor generated by the violent phase change ejects along the axial direction. The article will provide a certain theoretical basis for liquid nitrogen spray cooling technology, and can also enrich the flow dynamics of cryogenic fluids.

Semiempirical Model for Homogeneous Nitrogen Condensation in Hypersonic Wind Tunnels

A semiempirical density gradient theory (DGT) is used to model nanoscale nitrogen droplets formed by homogeneous nucleation. The constant of the inhomogeneous DGT term is found using planar surface tension values and recent nucleation rate measurements from available literature. Resulting values for the free energy barrier, surface tension, and mean density of critical droplets are used in nucleation and droplet growth rate models to evaluate the condensation process. Good agreement is found between this model and published values for condensation onset in hypersonic wind tunnels, with an average deviation of 2.2 K in static temperature.

Leidenfrost effect studied by video analysis

When liquid droplets touch a sufficiently hot surface the Leidenfrost effect leads to levitation of the droplets on a steam layer. At the same time the heat flux from hot surface to droplet is strongly reduced and the droplet lifetime is increased. The understanding of this effect, which can readily be observed on a standard stove, requires knowledge of fluid dynamics, basic mechanics and thermodynamics. It therefore combines appealing experiments with interdisciplinary physics and is well-suited for teaching physical principles, experimentation, modelling and data analysis in schools and in the early stages of university physics. The aim of this work is to present some guidance for projects in this area. Video analysis was used to quantitatively measure droplet sizes and lifetimes of water and liquid nitrogen droplets placed on a hot plate. A simple model for a levitating droplet was developed and compared to the data. The findings indicate that video analysis is a suitable way to examine the Leidenfrost effect for droplet diameters between about 3 and 10 mm. From the analysis the effective thermal conductivities of the steam layers were determined and were found to be two to three orders of magnitude smaller than the thermal conductivities of the corresponding gases at rest. By measuring several different droplets under varying parameters it was concluded that the precision of the experimental setup is sufficient to provide a practical tool for pupils and undergraduate students to explore this fascinating everyday phenomenon.

Numerical Study of the Effects of Injection Fluctuations on Liquid Nitrogen Spray Cooling

Spray cooling with liquid nitrogen is increasingly utilized as an efficient approach to achieve cryogenic cooling. Effects of injection mass flow rate fluctuations on the evaporation, temperature distribution, and droplet distribution of a spray field were examined by employing a validated Computational Fluid Dynamics (CFD) numerical model. The numerical results indicated that injection fluctuations enhanced the volume-averaging turbulent kinetic energy and promoted the evaporation of the whole spray field. The strengthened mass and heat transfer between the liquid nitrogen droplets and the surrounding vapor created by the fluctuating injection led to a lower temperature of the whole volume. A relatively smaller droplet size and a more inhomogeneous droplet distribution were obtained under the unsteady inlet. The changes of the frequency and the amplitude of the fluctuations had little effects on the overall spray development. The results could enrich the knowledge of the relation between the inevitable fluctuations and the overall spray development and the cooling performance in a practical spray cooling system with cryogenic fluids.

Evaporation of liquid nitrogen droplets in superheated immiscible liquids

Liquid nitrogen or other cryogenic liquids have the potential to replace or augment current energy sources in cooling and power applications. This can be done by the rapid evaporation and expansion processes that occur when liquid nitrogen is injected into hotter fluids in mechanical expander systems. In this study, the evaporation process of single liquid nitrogen droplets when submerged into n-propanol, methanol, n-hexane, and n-pentane maintained at 294 K has been investigated experimentally and numerically. The evaporation process is quantified by tracking the growth rate of the resulting nitrogen vapour bubble that has an interface with the bulk liquid. The experimental data suggest that the bubble volume growth is proportional to the time and the bubble growth rate is mainly determined by the initial droplet size. A comparison between the four different bulk liquids indicates that the evaporation rate in n-pentane is the highest, possibly due to its low surface tension. A scaling law based on the pure diffusion-controlled evaporation of droplet in open air environment has been successfully implemented to scale the experimental data. The deviation between the scaling law predictions and the experimental data for 2-propanol, methanol and n-hexane vary between 4 and 30% and the deviation for n-pentane was between 24 and 65%. The more detailed bubble growth rates have been modelled by a heuristic one-dimensional, spherically symmetric quasi-steady-state confined model, which can predict the growth trend well but consistently underestimate the growth rate. A fixed effective thermal conductivity is then introduced to account for the complex dynamics of the droplet inside the bubble and the subsequent convective processes in the surrounding vapour, which leads to a satisfactory quantitative prediction of the growth rate.

Numerical study on phase change behavior of liquid nitrogen droplets impinging on solid surface

The distinct physical properties of liquid nitrogen make liquid nitrogen spray cooling a promising technique in aerospace engineering, the electronic industry, superconductor cooling, cryobiology, etc. In-depth study of the dynamics and thermodynamic behavior of liquid nitrogen droplets impinging on the wall surface is helpful to understand the heat transfer mechanism of spray cooling technology with liquid nitrogen. Therefore, the mathematical model of single-liquid nitrogen droplet impacted solid surface is developed by Level Set-VOF method. The effects of wall wettability (30°－150°), initial velocity (0.1, 1.6 m/s) and wall temperature (300－500 K) on the phase change behavior during the evolution of droplets are investigated, and the mathematical model of film thickness is established. The results show that enhancing the wall wettability and increasing the impact speed facilitate the spreading of the droplets in the radial direction, thereby increasing the heat exchange area and reducing the thermal resistance. Ultimately, the heat exchange performance is significantly improved. Increasing the wall temperature results in an increase in the difference between temperatures of the solid surface and the liquid, thereby significantly increasing the wall heat flux density. The lower thermal resistance at the three-phase contact line results in a higher heat flux density at the edge than in the center; the difference among the heat flux distributions on different wetted walls decreases due to the increase of initial velocity, showing a significant velocity effect. In the film boiling region, the heat transfer process is mainly concentrated in the initial stage of impact, and the gas film is the main heat transfer resistance. Based on conservation of mass and energy, a numerical model of film thickness is developed in this paper. The model predictions are in good agreement with the simulation results of this paper and others.

Preparation of ice microspheres and their application in the preparation of porous poly(l-lactic acid) (PLLA) scaffolds

Pure ice particles have been used as porogens to prepare bone tissue engineering scaffolds for several years. However, it is difficult to prepare an ideal spherical ice porogen to improve the pore structure of the porous scaffolds. In this work, sublimable ice-based microspheres were prepared by spraying the 1,4-dioxane/water (70/30, v/v) solution through a syringe needle into liquid nitrogen. Ice-based microspheres have suitable melting points (− 4 °C) and high sublimation rate. The sieved ice-based microspheres with a size of 380–500 μm were used as a porogen to prepare porous PLLA scaffold. The calculation results show that the surface tension of the solution and the interfacial tension between the solution and liquid nitrogen are small when the volume ratio of the 1,4-dioxane/water is 70/30. The 1,4-dioxane/water solution can be effectively dispersed into small droplets in liquid nitrogen and rapidly frozen into microspheres. Scanning electron microscope images show that the porous PLLA scaffold had good pore connectivity. Macropores and micropores can be observed in the cross section of the PLLA scaffold. The pore wall has a special rough structure, and many micro-/nano-PLLA fibers can be found on the surface of the pore wall.

Examination of Extinguishment Method with Liquid Nitrogen Packed in a Spherical Ice Capsule

Liquid nitrogen which is a cryogenic fluid can be used as a fire extinguishing agent. This is because liquid nitrogen vaporizes rapidly under atmospheric condition, which results in cooling of burning materials and its surroundings in firefighting, and also the volume of the vaporized nitrogen becomes approximately 700 times larger than that of its liquid state, which results in reduction of oxygen concentration in air or fuel concentration in combustion zone. Moreover, liquid nitrogen has no water damage and can extinguish fires more cleanly than dry chemical extinguishing agents. However, it is difficult to delivery liquid nitrogen over long distance from the extinguishing equipment to fires through the surrounding air because of its rapid vaporization. If liquid nitrogen is filled into a capsule and the capsule wall suppresses heat transfer from the surroundings to the liquid nitrogen, it will be easy to transport liquid nitrogen to the targeted fire area without rapid vaporization. Moreover, by using the capsule, it may be possible to increase the extinguishing effectiveness of liquid nitrogen. In the present study, in order to clarify the fundamental characteristics of flame extinguishment by using the capsule filled with liquid nitrogen, extinguishing experiments of a methane-air jet diffusion flame have been performed. The spherical hollow ball made of ice was used as the extinguishing capsule and the ice capsule was formed by rotating casting machine. The wall thickness and the outer diameter were 2 mm and 20 mm, respectively. The filling volume of liquid nitrogen was 20 cm3. The ice capsule was dropped freely from the height of 800 mm and impacted on the aluminum plate, in which the round burner to form a jet diffusion flame was embedded. The extinguishing probability was measured by varying the distance from the impact point of the ice capsule and the flame. The extinguishing processes were recorded with high-speed camera. Moreover, for comparison, the same extinguishing experiments have been performed using the ice capsule filled with water. As a result, liquid nitrogen shows the higher extinguishing effectiveness than water because liquid nitrogen droplets released from the ice capsule to the collided horizontal plate can travel over the longer distance due to the Leidenfrost effect and liquid nitrogen can be vaporized rapidly under room temperature condition. The extinguishable range of the ice capsule filled with liquid nitrogen is less than 200 mm.

Online Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry with In situ Mixing

Online matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is reported with in situ matrix/analyte aerosol mixing. The analyte and matrix were dissolved in separate solvents and pneumatically atomized into particles. Continuous flows of analyte particles and matrix droplets in nitrogen were mixed in a tee. The resulting particles were sampled using a nozzle, focused into a beam with an aerodynamic lens system, and vertically deposited continuously on a movable target. The matrix/analyte mixture was desorbed/ionized using a 266 nanometers pulsed laser at an incident angle of 45°. The nascent ions were analyzed using reflectron TOF MS. The performance of online MALDI-TOF MS was evaluated by the analysis of palmityl palmitate with the lithium salt of 2, 4-dihydroxybenzoic acid as the matrix. Strong and stable MALDI signals of palmityl palmitate were obtained. The matrix solvent mixture and the analyte concentration were optimized and the results demonstrate the development of an alternative for online MALDI analysis.

Computational Prediction of Cryogenic Micro-nano Solid Nitrogen Particle Production Using Laval Nozzle for Physical Photo Resist Removal-cleaning Technology

The fundamental characteristics of the cryogenic single-component micro-nano solid nitrogen (SN2) particle production using super adiabatic Laval nozzle and its application to the physical photo resist removal-cleaning technology are investigated by a new type of integrated measurement coupled computational technique. As a result of present computation, it is found that high-speed ultra-fine SN2 particles are continuously generated due to the freezing of liquid nitrogen (LN2) droplets induced by rapid adiabatic expansion of transonic subcooled two-phase nitrogen flow passing through the Laval nozzle. Furthermore, the effect of SN2 particle diameter, injection velocity, and attack angle to the wafer substrate on resist removal-cleaning performance is investigated in detail by integrated measurement coupled computational technique.

Numerical Investigation of Head-On Binary Collision of Alumina Droplets

A numerical investigation of the head-on collision of two equal-sized alumina droplets has been conducted. Direct numerical simulations were performed by employing a volume of fluid method for tracking the interface and an adaptive mesh for improving the calculation efficiency. First, simulations of the head-on collision of tetradecane droplets in nitrogen were carried out, and three different outcomes of collision were captured: bouncing, coalescence after substantial deformation, and reflexive separation. These outcomes are in good agreement with the experiments. The numerical critical Weber numbers between the different regimes are also in good agreement with their experimental values. Next, the head-on collision of alumina droplets was investigated at various Weber numbers (1–800) and at one Ohnesorge number (0.1151). In addition to the three outcomes captured earlier, coalescence after a long extension between reflexive separation without satellite and reflexive separation with one satellite is observed. Furthermore, the numerical critical Weber numbers between these distinct regimes were obtained, and the numerical critical Weber number between coalescences after substantial deformation and reflexive separation agrees well with its theoretical value. Finally, collision models of alumina droplets are presented by modifying current droplet collision models.