Droplet Relative Velocity Research Articles

Model for Fuel Droplet Evaporation in Combustion Chamber of Liquid Propellant Rocket Engines

Complete burning of liquid propellants droplets is very important to avoid combustion instability and to get higher specific impulse from liquid rocket engines. Combustion takes place in gaseous phase and reaction is fast. So, the time consumed by a droplet for complete burning is the time taken by a droplet to get evaporated. An analytical mono component fuel droplet evaporation model is developed and proposed for droplet evaporation in liquid rocket engine based on heat and mass transfer. Effect of the fuel droplet curvature on vapor pressure is incorporated in the model. The model is applied on a small-scale combustion chamber of bi-propellant liquid rocket engine where fuel concentration away from droplet is not zero. A code is generated and applied. The results of the model show that fuel droplet diameter, combustion chamber pressure, chamber diameter, mass flow rate and droplet relative velocity with hot gas has significant effects on the life of droplet and characteristic length. Effects of chamber pressure and diameter are more significant for larger droplet size.

Optical manipulation for studies of collisional dynamics of micron-sized droplets under gravity.

A new experimental technique for creating and imaging collisions of micron-sized droplets settling under gravity is presented. A pair of glycerol droplets is suspended in air by means of two optical traps. The droplet relative velocities are determined by the droplet sizes. The impact parameter is precisely controlled by positioning the droplets using the two optical traps. The droplets are released by turning off the trapping light using electro-optical modulators. The motion of the sedimenting droplets is then captured by two synchronized high-speed cameras, at a frame rate of up to 63 kHz. The method allows the direct imaging of the collision of droplets without the influence of the optical confinement imposed by the trapping force. The method will facilitate efficient studies of the microphysics of neutral, as well as charged, liquid droplets and their interactions with light, electric field and thermodynamic environment, such as temperature or vapor concentration.

A numerical study of evaporation characteristics of spherical n-dodecane droplets in high pressure nitrogen environment

Evaporation of stagnant (zero relative velocity) as well as moving spherical droplets of n-dodecane in a zero-gravity and high pressure nitrogen environment is modeled. The non-ideal effects, solubility of ambient gas into the liquid-phase, variable thermo-physical properties, and gas- and liquid-phase transients are included in the model. The model is quantitatively validated using published experimental data. Numerical predictions show that, for stagnant droplets at sub-critical ambient temperatures, the droplet lifetime continuously increases with pressure, while at critical temperature, the lifetime initially increases and thereafter remains almost constant. At super-critical temperatures, the lifetime decreases continuously with increasing ambient pressure and the average evaporation constant shows a local maximum at a particular ambient pressure. In the case of moving droplets, at super-critical ambient temperature, the rate of increase of average evaporation constant with ambient pressure becomes significant as the initial droplet relative velocity increases. For low initial velocities (<1 m/s), the average evaporation constant gradually increases with ambient pressure and subsequently levels-off with further increase in ambient pressure. The droplet lifetime decreases with increase in ambient pressure or initial velocity. Penetration distance of the moving droplets decreases with ambient pressure and increases with initial droplet relative velocity. The mechanisms influencing the differences in evaporation under varying conditions of pressure and temperature are discussed.

Influence of gravity on collisions of monodispersed droplets in homogeneous isotropic turbulence

This paper studies the gravity influence on collisions of monodispersed droplets in homogeneous isotropic turbulence by means of direct numerical simulations (DNSs). The DNS results show that, in certain Stokes and Reynolds regimes, collision frequencies are significantly reduced in the presence of gravity. Those decreases are mainly attributable to the decrease in the droplet relative velocity, since the change in radial distribution function—often referred to preferential concentration—is small. Further analysis of the results reveals that droplet sedimentation due to gravity shortens the droplet-fluid interaction time, consequently weakening the relative motions between droplets. These observations lead to an analytical model that can be used to estimate the velocity fluctuations of sedimenting particles under gravity. Utilizing this model, we constructed a further analytical model for estimating the gravitational influence on collisions. Given flow and particle parameters, the model calculates the ratio of collision frequencies with and without the effect of gravity. Past studies simply noted that the gravitational influence is negligible when the droplet sedimenting velocity is much smaller than the flow velocity fluctuations. Our analytical model further suggests that the gravitational influence on collisions of monodispersed cloud droplets with non-negligible sedimentation rates stays negligibly small even in high Reynolds number flows, such as those typically found in convective clouds.

On the Combined Effects of Turbulence and Gravity on Droplet Collisions in Clouds: A Numerical Study

Abstract This paper examines the combined influences of turbulence and gravity on droplet collision statistics in turbulent clouds by means of direct numerical simulation (DNS). The essential microphysical mechanisms that determine the geometric collision kernel are explored by studying how gravity affects droplet relative velocities and preferential concentration of both monodisperse and bidisperse droplet distributions. To this end, collision statistics of large amounts of droplets with radii ranging from 10 to 90 μm, driven by a turbulent flow field and gravity, are calculated. The flow is homogeneous and isotropic and has a dissipation rate of ε = 4.25 × 10−2 m2 s−3. The results show that in the calculation of collision statistics, the interplay between gravity and turbulence is an essential element and not merely an addition of separate phenomena. For example, the presence of gravity leads to clustering of large droplets interacting with the larger scales of turbulence in the DNS. The collision statistics of a bidisperse droplet distribution, even with a very small radius difference, shows profoundly different behavior than the monodisperse case.

Monodisperse droplet heating and evaporation: Experimental study and modelling

Results of experimental studies and the modelling of heating and evaporation of monodisperse ethanol and acetone droplets in two regimes are presented. Firstly, pure heating and evaporation of droplets in a flow of air of prescribed temperature are considered. Secondly, droplet heating and evaporation in a flame produced by previously injected combusting droplets are studied. The phase Doppler anemometry technique is used for droplet velocity and size measurements. Two-colour laser induced fluorescence thermometry is used to estimate droplet temperatures. The experiments have been performed for various distances between droplets and various initial droplet radii and velocities. The experimental data have been compared with the results of modelling, based on given gas temperatures, measured by coherent anti-stokes Raman spectroscopy, and Nusselt and Sherwood numbers calculated using measured values of droplet relative velocities. When estimating the latter numbers the finite distance between droplets was taken into account. The model is based on the assumption that droplets are spherically symmetrical, but takes into account the radial distribution of temperature inside droplets. It is pointed out that for relatively small droplets (initial radii about 65μm) the experimentally measured droplet temperatures are close to the predicted average droplet temperatures, while for larger droplets (initial radii about 120μm) the experimentally measured droplet temperatures are close to the temperatures predicted at the centre of the droplets.

Acceleration intermittency and enhanced collision kernels in turbulent clouds

Scaling arguments suggest that in turbulent clouds, the droplet collision kernel is a fluctuating quantity with peak values at least an order of magnitude greater than the typically used kernel corresponding to droplet sedimentation in still air. These peaks are expected to occur in regions of intense fluid acceleration that are encountered in the fine scales of atmospheric turbulence, characterized by exceedingly large Reynolds numbers and strong energy-dissipation intermittency. The increase in the collision kernel is due to enhanced droplet relative velocities resulting from fluid accelerations and likely is further enhanced by increases in the droplet collision efficiency, which is a function of relative velocity. Velocity data obtained from a turbulent atmospheric flow are presented in order to support the general picture of acceleration intermittency and to estimate its peak values. Several independent estimates all point to the collision kernel as having peak magnitudes several times larger than the traditional kernel that accounts for gravitational acceleration alone.

Observations of flame structure in the combustion of monodispersed droplet streams

To clarify the understanding of flame structure in practical droplet spray combustors, some unique experimental techniques have been applied to an investigation of flame structure development. A free-droplet combustion experiment provided a well-controlled simulation of spray combustion flames through the production of parallel, monodispersed fuel droplet streams which travel vertically upwards on a precise trajectory in a uniform and stable combustion environment. An electro-optical image intensifier camera was developed to produce high-speed, time-resolved photographs of flame structure development. Measurements of droplet flame temperature profiles were made by the optical reversal technique known as the Kurlbaum method in which naturally occurring soot particles serve as the reversing medium. Group or collective burning of entire droplet streams occurred whenever the streamwise droplet spacing was of the order of twenty to twenty-five droplet diameters, depending primarily on the ambient oxygen concentration and to a lesser extent on the ambient temperature and droplet relative velocity. Streamwise flame spread was found to be affected greatly by self-induced buoyancy resulting from the heat of combustion. However, transverse (horizontal) merging of flames of parallel droplet streams was not observed at stream spacings as small as six droplet diameters. Measured temperature profiles for free-droplet flames are somewhat higher than comparable measurements reported for simulated droplet flames supported on liquid-fueled porous spheres, and the maxima (approximately 2100 K) are located further from the droplet. The observations of the present investigation indicate that available droplet combustion models tend to be oversimplified representations of practical spray combustion flames since important effects of droplet/droplet interaction, droplet/gas aerodynamics and self-induced convection are usually neglected.

A Theoretical and Experimental Investigation of the Ignition of Fuel Droplets

Abstract The influence on the self-ignition of single fuel droplets of ambient temperature and oxygen concentration, droplet relative velocity, droplet size, and fuel type was investigated both experimentally and theoretically. Using a unique droplet combustion experimental system, measurements of droplet ignition delay times were obtained for two pure fuels having widely different properties (furfuryl alcohol and butyl alcohol) and two different droplet diameters (200μ and 300μ nominal diameter) for representative droplet combustion conditions. A mathematical model describing the development of profiles for fuel vapor, temperature, and oxygen in the boundary layer surrounding a single fuel droplet moving relative to a hot oxidizing atmosphere was used to correlate the observed ignition delay times. Ignition delay times are found to increase substantially with decreasing ambient temperature and to a somewhat lesser degree with increasing droplet diameter and decreasing fuel volatility. A modest tendency f...