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NASA NTRS2016

Computational Flow Field in Energy Efficient Engine (EEE)

In this paper, preliminary results for the recently-updated Open National Combustion Code (Open NCC) as applied to the EEE are presented. The comparison between two different numerical schemes, the standard Jameson-Schmidt-Turkel (JST) scheme and the advection upstream splitting method (AUSM), is performed for the cold flow and the reacting flow calculations using the RANS. In the cold flow calculation, the AUSM scheme predicts a much stronger reverse flow in the central recirculation zone. In the reacting flow calculation, we test two cases: gaseous fuel injection and liquid spray injection. In the gaseous fuel injection case, the overall flame structures of the two schemes are similar to one another, in the sense that the flame is attached to the main nozzle, but is detached from the pilot nozzle. However, in the exit temperature profile, the AUSM scheme shows a more uniform profile than that of the JST scheme, which is close to the experimental data. In the liquid spray injection case, we expect different flame structures in this scenario. We will give a brief discussion on how two numerical schemes predict the flame structures inside the EEE using different ways to introduce the fuel injection.

CFD↗

NASA NTRS1982

Symposium (International) on Combustion, 19th, Technion Israel Institute of Technology, Haifa, Israel, August 8-13, 1982, Proceedings

Topics discussed are related to elementary reactions, reaction mechanisms and modeling, laminar flames, flame chemistry, turbulent reacting shear flows, turbulent premixed flames, turbulent combustion measurements, continuous combustors, detonation, detonation and explosion, heterogeneous detonation, propellant combustion, fire-ignition and thermal degradation, fire-flame spread and burning, fire-modeling, spray combustion, and droplet combustion. Coal combustion kinetics and mechanisms are considered along with coal combustion mechanisms and pyrolysis, coal combustion techniques, NOx in coal combustion, gaseous pollutants, soot and PAH, soot and inorganic pollutants, I.C. engine combustion, and ignition and extinction. Attention is given to intricate paths and simple steps in chemical kinetics and combustion, the formation of polycyclic aromatic hydrocarbons by combustion, turbulent flame structure and speed in spark ignition engines, and unresolved problems in SOx, NOx, and soot control in combustion.

Source record↗

NASA NTRS1985

The structure of dilute combusting sprays

An experimental and theoretical study of drop processes in a turbulent flame is described. The experiments involved a monodisperse (105 and 180 micro m initial diameter) stream of methanol drops injected at the base of a turbulent methane-fueled diffusion flame burning in still air. The following measurements were made: mean and fluctuating phase velocities, mean drop number flux, drop-size distributions and mean gas-phase temperatures. Measurements were compared with predictions of two separated flow models: (1) deterministic separated flow, where drop-turbulence interactions are ignored; and (2) stochastic separated flow, where drop-turbulence interactions are considered using random-walk computations. The stochastic separated flow analysis yielded best agreement with measurements, since it provides for turbulent dispersion of drops which was important for present test conditions (and probably for most combusting sprays as well). Distinguishing the presence or absence of envelope flames around the drops, however, was relatively unimportant for present test conditions, since the drops spent most of their lifetime in fuel-rich regions of the flow where this distinction is irrelevant.

Shuen, J. S.↗

NASA NTRS1995

Cluster Combustion of Binary-Fuel Drops

Most fuels used in practical combustion liquid-fuel sprays devices are blends of several components. In many cases, the composition of the fuel is considered critical for the operation of the device due to efficiency and pollution production considerations. This investiga- tion focuses on cluster flames for binary-fuel drops. Studied are spherical clusters of relatively cold drops exposed to an axial flow.

cluster flames liquid-fuel sprays spray systems co↗

NASA NTRS1993

Secondary atomization in the combustion of electrostatic sprays

The combustion of electrosprays in a laminar counterflow diffusion flame has been experimentally studied by measuring droplet size and velocity distributions and gas-phase temperature. Detailed examination of the evolution of droplet size distribution as droplets approach the flame shows that, if substantial evaporation occurs before droplets 'interact' with the flame, the size distribution becomes bimodal. A secondary, sharp peak, in fact, develops in correspondence of diameters about one order of magnitude smaller than the mean. No evaporation mechanism can account for the development of such bimodality, that can be explained only in terms of a disintegration of droplets into finer fragments of size much smaller than the parent ones. This fission is of electric nature and it occurs when the repulsion of electric charges overcomes the surface tension cohesive force ultimately leading to a disintegration into finer fragments at or about the so-called Rayleigh limit. We here report on the first observation in combustion environments of such 'explosions'. If, on the other hand, droplets enter the very high temperature region before exploding, there appears to be no evidence of bimodality in their size distribution. In this case, in fact, flame chemi-ions may neutralize the charge on the droplets and thus prevent disruption.

Gomez, Alessandro↗

NASA NTRS2001

Combustion of Interacting Droplet Arrays in a Microgravity Environment

Investigations into droplet interactions date back to Rex et al. Annamalai and Ryan and Annamalai published extensive reviews of droplet array and cloud combustion studies. In the majority of the reviewed studies, the authors examined the change in the burning rate constant, k, (relative to that of the single droplet) that results from interactions. More recently, Niioka and co-workers have examined ignition and flame propagation along arrays of interacting droplets with the goal of relating these phenomena in this simplified geometry to the more practical spray configuration. Our work has focussed on droplet interactions under conditions where flame extinction occurs at a finite droplet diameter. In our previous work, we reported that in normal gravity, reduced pressure conditions, droplet interactions improved flame stability and extended flammability limits (by inference). In our recent work, we examine droplet interactions under conditions where the flame extinguishes at a finite droplet diameter in microgravity. The microgravity experiments were in the NASA GRC 2.2 and 5.2 second drop towers, and the JAMIC (Japan Microgravity Center) 10 second drop tower. We also present progress on a numerical model of single droplet combustion that is in the process of being extended to model a binary droplet array.

Dietrich, D. L.↗

NASA NTRS2001

Detailed Studies on Flame Extinction by Inert Particles in Normal- and Micro-gravity

The combustion of dusty flows has been studied to lesser extent than pure gas phase flows and sprays. Particles can have a strong effect by modifying the dynamic response and detailed structure of flames through the dynamic, thermal, and chemical couplings between the two phases. A rigorous understanding of the dynamics and structure of two-phase flows can be attained in stagnation flow configurations, which have been used by others to study spray combustion as well as reacting dusty flows. In earlier studies on reacting dusty flows, the thermal coupling between the two phases as well as the effect of gravity on the flame response were not considered. However, in Ref. 6, the thermal coupling between chemically inert particles and the gas was addressed in premixed flames. The effects of gravity was also studied showing that it can substantially affect the profiles of the particle velocity, number density, mass flux, and temperature. The results showed a strong dynamic and thermal dependence of reacting dusty flows to particle number density. However, the work was only numerical and limited to twin-flames, stagnation, premixed flames. In Ref. 7 the effects of chemically inert particle clouds on the extinction of strained premixed and non-premixed flames were studied both experimentally and numerically at 1-g. It was shown and explained that large particles can cause more effective flame cooling compared to smaller particles. The effects of flame configuration and particle injection orientation were also addressed. The complexity of the coupling between the various parameters in such flows was demonstrated and it was shown that it was impossible to obtain a simple and still meaningful scaling that captured all the pertinent physics.

Andac, M. G.↗

NASA NTRS2019

Pilot Injector Redesign to Reduce N+3 Cycle Emissions for a Gas-Turbine Combustor

An overview is given of an effort for the use of CFD analysis to complement design and configuration definition of third generation Lean-Direct Injection combustion concepts (LDI-3) for NASA’s N+3 program. The National Combustion Code (OpenNCC) was used to perform non-reacting and two-phase reacting flow computations for a three-cup, nineteen-element flame tube array with redesigned pilot injectors to improve spray and emissions characteristics when compared to a previous LDI-3 design. All computations were performed with a consistent approach to mesh-generation, spray modeling, ignition and kinetics modeling for a ‘medium-power’ cycle condition. Computational predictions of the aerodynamics of a new pre-filming pilot injector were used to arrive at an optimized aerothermal design that meets effective area and fuel-air mixing criteria. The newly designed pilot injectors were shown to provide considerable improvements in aerodynamic stability, flame-tube pattern factor and NOx emissions, when compared to the original design.

nitrogen oxides injectors↗

NASA NTRS1984

Combustion characteristics in the transition region of liquid fuel sprays

A monodisperse aerosol generator was modified to study ignition requirements, flammability limits, and flame speeds in the transition region. An ignition system was developed and tested. The fabrication of an optical drop sizing system is nearly complete. Preliminary measurements of droplet size effects on the minimum ignition energy for n-heptane sprays performed. Parameteric studies of droplet size effects on minimum ignition energies of various fuels including alcohols are in progress.

Cernansky, N. P.↗

NASA NTRS1966

A study of heterogeneous detonations.

Heterogeneous detonations, discussing polydisperse and monodisperse spray detonations and liquid fuel film shock-induced combustion

COMBUSTION INSTABILITY↗

NASA NTRS1983

Review of alternative fuels data bases

Based on an analysis of the interaction of fuel physical and chemical properties with combustion characteristics and indicators, a ranking of the importance of various fuel properties with respect to the combustion process was established. This ranking was used to define a suite of specific experiments whose objective is the development of an alternative fuels design data base. Combustion characteristics and indicators examined include droplet and spray formation, droplet vaporization and burning, ignition and flame stabilization, flame temperature, laminar flame speed, combustion completion, soot emissions, NOx and SOx emissions, and the fuels' thermal and oxidative stability and fouling and corrosion characteristics. Key fuel property data is found to include composition, thermochemical data, chemical kinetic rate information, and certain physical properties.

Harsha, P. T.↗

NASA NTRS1995

Japan's research on particle clouds and sprays

Most of energy used by us is generated by combustion of liquid and solid fuels. These fuels are burned in combustors mainly as liquid sprays and pulverized solids, respectively. A knowledge of the combustion processes in combustors is needed to achieve proper designs that have stable operation, high efficiency, and low emission levels. However, current understanding of liquid and solid particle cloud combustion is far from complete. If combustion experiments for these fuels are performed under a normal gravity field, some experimental difficulties are encountered. These difficulties encountered include, that since the particles fall by the force of gravity it is impossible to stop the particles in the air, the falling speeds of particles are different from each other, and are depend on the particle size, the flame is lifted up and deformed by the buoyancy force, and natural convection makes the flow field more complex. Since these experimental difficulties are attributable to the gravity force, a microgravity field can eliminate the above problems. This means that the flame propagation experiments in static homogeneous liquid and solid particle clouds can be carried out under a microgravity field. This will provide much information for the basic questions related to combustion processes of particle clouds and sprays. In Japan, flame propagation processes in the combustible liquid and solid particle clouds have been studied experimentally by using a microgravity field generated by a 4.5 s dropshaft, a 10 s dropshaft, and by parabolic flight. Described in this presentation are the recent results of flame propagations studies in a homogeneous liquid particle cloud, in a mixture of liquid particles/gas fuel/air, in a PMMA particle cloud, and in a pulverized coal particle cloud.

Sato, Jun'ichi↗

NASA NTRS1988

Plasma Spraying Of Dense, Rough Bond Coats

Simple modification of plasma torch facilitates spraying of coarse powders. Shape of nozzle changed to obtain decrease in velocity of gas and consequent increase in time particles spend in flame before impact on substrate. Increased residence time allows melting of coarser powders, spraying of which results in rougher bond surfaces.

Miller, Robert A.↗

NASA NTRS1986

Combustion characteristics in the transition region of liquid fuel sprays

A number of important effects have been observed in the droplet size transition region in spray combustion systems. In this region, where the mechanism of flame propagation is transformed from diffusive to premixed dominated combustion, the following effects have been observed: (1) maxima in burning velocity; (2) extension of flammability limits; (3) minima in ignition energy; and (4) minima in NOx formation. A monodisperse aerosol generator has been used to form and deliver a well controlled liquid fuel spray to the combustion test section where measurements of ignition energy have been made. The ignition studies were performed on monodisperse n-heptane sprays at atmospheric pressure over a range of equivalence ratios and droplet diameters. A capacitive discharge spark ignition system was used as the ignition source, providing independent control of spark energy and duration. Preliminary measurements were made to optimize spark duration and spark gap, optimum conditions being those at which the maximum frequency or probability of ignition was observed. Using the optimum electrode spacing and spark duration, the frequency of ignition was determined as a function of spark energy for three overall equivalence ratios (0.6, 0.8, and 1.0) and for initial droplet diameters of 25, 40, 50, 60, and 70 micro m.

Cernansky, N. P.↗

NASA NTRS1977

Current status of droplet and liquid combustion

The present understanding of spray combustion in rocket engine, gas turbine, Diesel engine and industrial furnace applications is reviewed. In some cases, spray combustion can be modeled by ignoring the details of spray evaporation and treating the system as a gaseous diffusion flame; however, in many circumstances, this simplification is not adequate and turbulent two-phase flow must be considered. The behavior of individual droplets is a necessary component of two-phase models and recent work on transient droplet evaporation, ignition and combustion is considered, along with a discussion of important simplifying assumptions involved with modeling these processes. Methods of modeling spray evaporation and combustion processes are also discussed including: one-dimensional models for rocket engine and prevaporized combustion systems, lumped zone models (utilizing well-stirred reactor and plug flow regions) for gas turbine and furnace systems, locally homogeneous turbulent models, and two-phase models. The review highlights the need for improved injector characterization methods, more information of droplet transport characteristics in turbulent flow and continued development of more complete two-phase turbulent models

Faeth, G. M.↗

NASA NTRS1982

Droplet size effects on NO/x/ formation in a one-dimensional monodisperse spray combustion system

A one-dimensional monodisperse aerosol spray combustion facility is described and experimental results of post flame NO/NO(x) emissions are presented. Four different hydrocarbon fuels were studied: isopropanol, methanol, n-heptane, and n-octane. The results indicate an optimum droplet size in the range of 48-58 microns for minimizing NO/NO(x) production for all of the test fuels. This NO(x) behavior is associated with droplet interactions and the transition from diffusive type of spray burning to that of a prevaporized and premixed case. Decreasing the droplet size results in a trend of increasing droplet interactions, which suppresses temperatures and reduces NO(x). This trend continues until prevaporization effects begin to dominate and the system tends towards the premixed limit. The occurrence of the minimum NO(x) point at different droplet diameters for the different fuels appears to be governed by the extent of prevaporization of the fuel in the spray, and is consistent with theoretical calculations based on each fuel's physical properties.

Sarv, H.↗

NASA NTRS1984

Combustion characteristics in the transition region of liquid fuel sprays

A number of important effects were observed in the droplet size transition region in spray combustion systems. In this region, where the mechanism of flame propagation is transformed from diffusive to premixed dominated combustion, the following effects have been observed: (1) maxima in burning velocity; (2) extension of flammability limits; (3) minima in ignition energy; and (4) minima in NO(x) formation. Unfortunately, because of differences in experimental facilities and limitations in the ranges of experimental data, a unified description of these transition region effects is not available at this time. Consequently, a fundamental experimental investigation was initiated to study the effect of droplet size, size distribution, and operating parameters on these transition region phenomena in a single well controlled spray combustion facility.

Cernansky, N. P.↗

NASA NTRS1998

Improved Modeling of Finite-Rate Turbulent Combustion Processes in Research Combustors

The objective of this thesis is to further develop and test a stochastic model of turbulent combustion in recirculating flows. There is a requirement to increase the accuracy of multi-dimensional combustion predictions. As turbulence affects reaction rates, this interaction must be more accurately evaluated. In this work a more physically correct way of handling the interaction of turbulence on combustion is further developed and tested. As turbulence involves randomness, stochastic modeling is used. Averaged values such as temperature and species concentration are found by integrating the probability density function (pdf) over the range of the scalar. The model in this work does not assume the pdf type, but solves for the evolution of the pdf using the Monte Carlo solution technique. The model is further developed by including a more robust reaction solver, by using accurate thermodynamics and by more accurate transport elements. The stochastic method is used with Semi-Implicit Method for Pressure-Linked Equations. The SIMPLE method is used to solve for velocity, pressure, turbulent kinetic energy and dissipation. The pdf solver solves for temperature and species concentration. Thus, the method is partially familiar to combustor engineers. The method is compared to benchmark experimental data and baseline calculations. The baseline method was tested on isothermal flows, evaporating sprays and combusting sprays. Pdf and baseline predictions were performed for three diffusion flames and one premixed flame. The pdf method predicted lower combustion rates than the baseline method in agreement with the data, except for the premixed flame. The baseline and stochastic predictions bounded the experimental data for the premixed flame. The use of a continuous mixing model or relax to mean mixing model had little effect on the prediction of average temperature. Two grids were used in a hydrogen diffusion flame simulation. Grid density did not effect the predictions except for peak temperature and tangential velocity. The hybrid pdf method did take longer and required more memory, but has a theoretical basis to extend to many reaction steps which cannot be said of current turbulent combustion models.

VanOverbeke, Thomas J.↗