Droplet Combustion Model Research Articles

Microgravity combustion of methanol and methanol/water droplets: Drop tower experiments and model predictions

To experimentally validate single and bicomponent droplet combustion models, microgravity methanol and methanol/water droplet combustion experiments were conducted in the 2.2-s drop tower facility at NASA Lewis Research Center. The experiments were then simulated using a transient, bicomponent droplet combustion model developed earlier. Tests were performed in oxidizing environments of 18%–35% O2/N2 with initial liquid water contents of 0–20%. Instantaneous droplet diameter measurements were made using back-lit, high-speed photography. The instantaneous flame position was determined by monitoring the chemiluminescence from electronically excited hydroxyl radicals (OH*). Analysis of the flame and droplet diameter data yielded burning rates and flame standoff ratios for a wide array of methanol and methanol/water droplet combustion conditions. For initially pure methanol droplets, the available burn time (≈1.5 s) was not, in general, sufficient to observe extinction or significant nonlinearity in the regression of diameter squared with respect to time. For each oxygen content, the numerical model predicted the burning rate to within 10% and the flame position to within one normalized diameter without any independent parameter adjustment. In the case of 10% and 20% initial water content, substantial nonlinearity in diameter squared was observed. The numerical model, which accurately accounts for changes in liquid transport properties caused by variable liquid water content, did not predict the nonlinearity to the extent that it was observed. A possible explanation is that with the addition of initial water content to the droplet, the internal mass and thermal transport is enhanced as a result of increased internal mixing. The sources of this internal mixing are likely multifold but are not due to relative gas/liquid convection effects at the droplet surface.

Comparison of droplet combustion models in spray combustion

This study compares the spray combustion characteristics predicted by the above three models with experimental observations resulting from various inlet mean droplet sizes. A simplified combustor, similar to that used for the spray BurkeSchumann diffusion flame, is assumed. The combustor possesses coaxial cylindrical tubes with fuel spray originating from the inner tube and an airstream between the inner and outer tubes. The tube rim is slightly enlarged for flame holding. Poly disperse sprays are employed, with three inlet volumemean droplet sizes, 40, 60, and 100 /am in diameter, being selected to serve as bases for the computations in order to investigate the spray combustion characteristics of fine- and large-droplet sprays. The spray flame configurations and the axial fuel-consumption ratios predicted by the three models reveal detailed spray characteristics and overall combustion performance; information, which when compared with experimental observations, will determine the most accurate model.

Vaporization behavior of fuel droplets in a hot air stream

Tm GASIFICATION behavior of a liquid droplet provides a fundamental input for the modeling of many spray systems, and has been studied extensively [l-4]. Theoretical analysis for the multicomponent fuel droplet presents several complexities absent in a similar analysis for the single component droplet. First, the phase-change process at the droplet surface and the transport of a fuel vapor mixture in the gas phase need to be properly described. Second, the evaporation process is inherently time varying due to the continuous change in the composition and temperature of the droplet as vaporization proceeds. Another difference between the two cases is due to the phenomenon of microexplosion [2]. The earlier viewpoint [2] of multicomponent droplet vaporization assumes that the composition and temperature within the droplet are spatially uniform but time varying. Such theories predict that the gasification process is similar to batch distillation in that the sequence of gasification is controlled by the volatility differentials among the different components. However, Sirignano [4] showed that even in the limit of high vortex strength, the internal liquid circulation can only reduce the characteristic length scale for diffusion by a factor of three. Landis and Mills [S] investigated the vaporization of a bi-component fuel droplet in a stagnant atmosphere. A quasi-steady gas-phase model was used and the equations governing the unsteady mass and heat diffusion within the droplet were solved numerically. Results when compared with rapid mixing behavior showed significant differences. A unique feature of the diffusion-dominated droplet gasification mechanism is the possible attainment of approximately steady-state temperature and concentration profiles within the droplet, which then leads to a steady-state gasification rate. Based on this concept, Law and Law [6] formulated a d2-law model for multicomponent droplet vaporization and combustion. As indicated above, the literature on the gasification behavior of an isolated droplet is extensive. However, most of these studies deal with droplet combustion or evaporation under high-temperature conditions. Not much information is available on the behavior of evaporating droplets in relatively low-temperature air streams. Under such conditions, the possibility of an envelope flame is precluded and the droplet gasification rate is low. The droplet heat-up time may not be negligibly small compared to its lifetime, although the latter is relatively large and the liquid-phase transient processes may still be important. In this paper, the vaporization behavior of pure and multicomponent fuel droplets flowing in a well-characterized laminar flow is studied. The predictions of three vaporization models are compared with the experimental data.

Vapor pressure relations for droplet combustion models

An important component of any model of liquid fuel combustion or ignition is a relationship between the temperature and vapor pressure of the fuel at the liquid surface. This communication examines the effects of the choice of vapor pressure equation on the predictions of a model of liquid droplet ignition. The most often used vapor pressure relation is the Clausius-Clapeyron equation of classical thermodynamics:

Flow-tube reactor studies of devolatilization of pulverized coal in an oxidizing environment

Coal oxidation studies have been performed in a flow tube reactor at gas temepratures between 1300 and 2000 K and oxygen concentrations between 7 and 16%. At high gas temperatures or high O2 concentration, particle luminosity was apparent in less than 5 msec, and 70% carbon loss was measured prior to 30 msec of residence time. For a given gas temeprature, devolatilization rates increased with increasing oxygen concentration. Energy from the oxidizing volatiles appears to be fed back to the coal particles, increasing their temperature and thereby the volatile release rate and yield. A flame sheet theory, analogous to the single droplet combustion model, predicts devolatilization rates that agree well with experimentally measured rates. The model predicts that particle size inhomogeneity plays an important role in determining the overall devolatilization rate even for relatively narrow size distributions.

Les moyens necessaires a l'etude fondamentale des instabilites de combustion d'un moteur biliquide

(Necessary means for a fundamental study of combustion instabilities in a liquid propellant engine).—Within the framework of Ariane launch vehicle studies, ONERA has entered upon a fundamental investigation of combustion instabilities, that should lead to a digital program simulating the unsteady behaviour of a liquid propellant engine with radial injection, the nominal working pressure of which is near the critical pressure of propellant. The first step of this study was to define the necessary methods and means that must be worked out to characterize the various physical processes taking place in the engine, in order to achieve this object. The droplet size and velocity distributions in space and time (characteristic of injection-atomization) is the upper boundary condition of the problem. It will be empirically described from parametric test results, so a flexible and valid measurement method must be found, the exploitation of which has to be automatized. The droplet evolution within the chamber (characteristic of combustion) depends on environmental conditions, especially forced convection due to radial injection geometry. It will be analyzed with the help of a two-dimenstional single droplet combustion model, that must stay valid near the propellant critical pressure; thus the propellant state law, and its thermodynamic and transport properties, have to be known in this area. Gas flow description, droplet trajectography in a turbulent-environment and mass, momentum and energy transport consideration (characteristic of aerodynamics) will be handled in a progressive way. Each of intermediate models will be checked, then validated, with the help of suitable test results, some of them being fit to be later used as a theoretical support for the exploitation of complementary atomization or combustion experiments. The final model that is reached constitutes the synthesis of the whole study. ONERA has taken advantage of the helpful assistance of external research centers, in particular amid the French University.

A study on sodium spray combustion

Sodium spray combustion was studied through experiments and analysis, in order to clarify the burning rate, pressure and temperature transients in a sodium spray fire. In the experiments, about 400 g sodium was sprayed in a closed vessel of 2 m 3, containing nitrogen and 0–21 vol% oxygen. Pressure, temperature and oxygen concentration were measured during and after sodium injection. The experimental results revealed that the temperature in the spray outer region was higher than that of inner region and observed oxygen consumption was not more than 80% of that expected for complete combustion of sodium. To analyze the experiments, a computer program SOFIA-II was developed based on an analytical single droplet combustion model and a two-dimensional temperature and oxygen concentration distribution model in the vessel. The calculated pressure agreed with the experimental pressure on the whole and the peak pressure difference was within 10% error.

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.

Quasi-Steady Diffusion Flame Theory with Variable Specific Heats and Transport Coefficients

Abstract A theory is developed to account for the variable nature of the specific heats C and the transport coefficients of thermal and mass diffusion, λ and D, in quasi-steady one-dimensional diffusion flames, using bipropellant droplet combustion as an illustration. In the theory C, λ and D are all temperature dependent whereas C and λ are further concentration-weighted; consequently all the existing variable-property droplet combustion models are special cases of the present generalized model. By allowing these coefficients to assume realistic functional forms, explicit expressions are derived for the concentration-weighted coefficients, the temperature and species profiles, the mass burning rate, and the flame-front standoff ratio. Predicted results yield improved, yet much less ambiguous, agreement with experimental observations. Finally it is shown that such good agreement can also be achieved by utilizing a simplified, temperature-independent, model and by evaluating the concentration-weighted coef...

Experimental study of the combustion of single aluminum particles in O 2/Ar

Single aluminum particles of 50 μ diam have been ignited by focused laser flux and observed by cinephotomicrography throughout their entire burning history in a controlled O2/Ar environment. The ignition stimulus, ambient atmosphere, and particle size have been judiciously selected and closely controlled to make the laboratory phenomenon conform to anticipated assumptions common to a droplet-combustion model: spherical symmetry, negligible convection, negligible radiation, and quasi-steady burning. The results indicate that the burning rate is constant throughout the history of a given droplet. The burning-rate law, dn=d0n-βt, was fitted to the present data by nonlinear, least-squares analysis. The results are not definitive with regard to the burning-rate exponent (n=1.5±0.5) because the measurement of droplet diameter at f/20 was diffraction-limited to ±2 μ. Given n, the instantaneous burning rate of aluminum in O2/Ar is determined to 5% by the least-squares fit. The burning rates of several droplets under identical conditions are numerically most consistent under the assumption that n=2. A survey of previous work over a restricted (X0∞, P) range shows that the average burning rate of aluminum based on initial diameter and time-to-burnout is roughly correlated by d0n ∼ τ, with 1.5