Atomization and Sprays
Spray dynamics simulations (RANS) for pulsatile injection at different ambient pressure and temperature conditions
The emphasis is on understanding the effect of higher ambient pressure and temperature as well as the initial fuel temperature on the spray dispersion and evaporation processes. The spray characteristics like penetration length, SMD, droplet and gas-phase velocities, and the mixture distribution are investigated.
Effects of orifice divergence on hollow cone spray at low injection pressures
In this work, the effects of orifice divergence on spray characteristics have been reported. Parameters such as spray cone angle, liquid sheet thickness, coefficient of discharge, break-up length, and Sauter mean diameter are greatly affected by the half divergence angle at orifice exit. An experimental investigation is carried out in which water sprays from five atomizers having half divergence angle values of 0 degrees, 5 degrees, 10 degrees, 15 degrees, and 20 degrees are studied at different injection pressures. Image processing techniques are used to measure spray cone angle and break-up length from spray images, whereas the sheet thickness outside the orifice exit is obtained using the scattered light from a thin Nd-YAG Laser beam. Phase Doppler interferometry is also used to obtain the Sauter mean diameter at different axial locations. A few numerical simulations based on the volume of fluid method are included to obtain physical insight of the liquid film development and air core flow inside the atomizer. It is observed that the liquid sheet thickness as well as tangential and radial components of velocity at orifice exit are modified drastically with a change in half divergence angle. As a consequence, the droplet size distribution is also altered by variation in the nozzle divergence angle. The mechanism responsible for such variations in the spray behavior is identified as the formation of an air core or air cone inside the liquid injector as a result of the swirl imparted to the liquid flow. (Read More...)
Experimental study of spray breakup phenomena in small-scale simplex atomizers with and without air swirl
Spray breakup processes of small-scale simplex atomizers have been characterized. With tangential entry of fuel, the cases of no air flow and concentric co-swirl air flow are considered. Ten simplex atomizers with different values of dimensionless tangential port area K have been employed. The breakup mode changes from film breakup to jet breakup, with increase in K value. For small K values, small port area causes high liquid swirl, which aids in the formation of an air core and a hollow cone spray with large cone angle. At high K values, jet breakup gives rise to a full cone spray with a small cone angle. The SauterMean Diameter (SMD) for the spray exhibits bimodal variation in the radial direction close to the atomizer. Based on a large volume of data for kerosene and water sprays, accurate correlations (covering wide operating conditions) have been developed for the spray cone angle, Cd, and axial variation of SMD for small-scale simplex atomizers. Swirl air interaction with the liquid film causes a transition from closed tulip shape to an open spray at a critical air flow rate. The critical air flow rate for transition is initial condition-dependent and it exhibits hysteresis. (Read More...)
Two-phase modeling of evaporation characteristics of blended methanol-ethanol droplets
The paper presents a two-phase numerical model to simulate transient vaporization of a spherical two component liquid fuel droplet. The model considers variation of thermo-physical properties in both liquid and vapor-phases, as functions of temperature and species concentrations. Multi-component diffusion and surface tension effects are also considered. The model has been validated using the experimental data available in literature. The validated model is used to study the vaporization characteristics of both suspended and moving methanol–ethanol blended droplets in an atmospheric pressure environment. Relative strengths of forced convection and Marangoni convection are studied and compared for both suspended and moving droplets. Results in terms of streamlines, isotherms and isopleths at different time instants are reported and discussed. For low relative velocities, solutal Marangoni effects are seen to be important.
S. Raghuram et al., International Journal of Multiphase Flow, 52 (2013) 46-59
Numerical study of transient evaporation of moving two-component fuel droplets
This paper presents the numerical simulation of evaporation of a moving two-component liquid fuel spherical droplet under atmospheric pressure. The transient two-phase numerical model includes variations of thermo-physical properties as functions of temperature and species concentration in liquid and vapor phases, multi-component diffusion, and surface tension effects. The model has been validated using the experimental data available in the literature for suspended heptane-decane-blended droplets evaporating under a forced convective air environment. The validated model is used to study the vaporization characteristics of moving binary droplets. The blends considered in this study are isooctane blended with ethanol and decane blended with methyl-butyrate. The temporal variations of the evaporation constant, droplet Reynolds number, and drag coefficients are presented. Variations of integrated quantities, such as the time-averaged evaporation constant, droplet lifetime, and droplet final penetration distance as a function of blend composition, are also presented. The behavior of isooctane-ethanol blends is seen to be quite different from that of methyl-butyrate-decane blends.
S. Raghuram & Vasudevan Raghavan, Atomization and Sprays, 22(6)(2012)493–513
Numerical study of Marangoni convection during transient evaporation of two-component droplet under forced convective environment
Numerical simulations of the evaporation of stationary, spherical, two-component liquid droplets in a laminar, atmospheric pressure, forced convective hot-air environment are presented. The transient two-phase numerical model includes multi-component diffusion, a comprehensive method to deal with the interface including the surface tension effects and variation of thermo-physical properties as a function of temperature and species concentration in both liquid- and vapor-phases. The model has been validated using the experimental data available in literature for suspended heptane-decane blended droplets evaporating under a forced convective air environment. The validated model is used to study the vaporization characteristics of heptane-decane droplets under different convective conditions. For an initial composition having 75% by volume of more volatile fuel component, the evaporation transients are presented in terms of variations in interface quantities. Flow, species and temperature fields are presented at several time instants to show the relative strengths of forced convection and Marangoni convection. Results show that at low initial Reynolds numbers, the solutal Marangoni effects induce a flow-field within the liquid droplet, which opposes the flow of the external convective field. The strength of this liquid-phase flow field increases with the consumption of the more volatile fuel component.
S. Raghuram et al., International Journal of Heat and Mass Transfer, 55 (2012) 7949-7957
Two-phase transient simulations of evaporation characteristics of two-component liquid fuel droplets at high pressures
This paper presents comprehensive numerical simulations of evaporation of droplets constituted of two liquid fuels in high pressure nitrogen ambient under normal gravity condition. A transient, two-phase and axisymmetric numerical model has been used for the simulations. Transport processes in liquid- and vapor-phases have been solved along with interface coupling conditions. Gas-phase non-idealities, solubility of ambient gas in liquid-phase, and pressure and temperature based variable thermo-physical properties in both liquid- and vapor-phases are considered in the numerical model. Phase equilibrium has been estimated using fugacity coefficients of all species in both phases. The range of Weber number has been chosen such that droplet remains almost spherical throughout its lifetime. Simulations have been carried out until the droplet surface regresses to one-tenth of its initial value or when the critical state for the mixture is reached. The numerical model has been quantitatively validated against the experimental data available in literature. The validated model is used to systematically study the evaporation characteristics of suspended n-heptane-hexadecane droplets in nitrogen ambient. The effects of the pressure, temperature, initial liquid-phase composition and forced convection velocity on evaporation characteristics have been discussed in detail.
Saroj Ray et al., International Journal of Multiphase Flow, [in press]
Droplet clustering and local spray unsteadiness in air-assisted sprays
The paper has two goals. First, to experimentally characterize the droplet clusters in air-assist sprays, and second, to study the consequence of droplet clustering on the local spray unsteadiness by measurement of turbulent number flux of droplets. Unlike clustering of droplets in spray-laden turbulent flows, the entrained air flow around a spray plays a very important role in the dispersion of droplets in the spray. The present work also examines the influence of local liquid mass fraction on clustering of droplet in the spray, which has not been considered in detail in the earlier works. (Read More...)
M. Manish and S. Sahu, Experimental Thermal and Fluid Science, 100 (2019), 89--103