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Nicholls, J. A.

Publications and source records attributed to Nicholls, J. A..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

The effect of fuel sprays on emissions from a gas turbine combustor

A research gas turbine combustor which allows for independent control of drop size, fuel/air ratio, air inlet temperature, pressure, reference velocity, and residence time has been used to study the influence of fuel spray characteristics on primary zone emissions. Local concentrations of oxides of nitrogen, carbon monoxide and unburned hydrocarbons at various locations in the flow, local gas temperature, gas velocity, and the turbulence intensity (measured with laser velocimetry) are obtained. A comprehensive data analysis program to compute the local fuel/air ratio and the sum of the mole fractions of the species present confirms the validity of the emissions data. Increasing drop size is correlated with an increase in unburned hydrocarbons, while increasing residence time is associated with sharp decreases in hydrocarbons and carbon monoxide and an increase in oxides of nitrogen.

Nicholls, J. A.↗
NASA NTRS1979

Effect of fuel sprays on emissions

A research gas turbine combustor was operated under realistic conditions such that the influence of individual variables (in particular, fuel spray characteristics) on emissions could be determined. The special combustor allows independent control over drop size, fuel-air ratio, air inlet temperature, pressure, reference velocity, and residence time. Also, it lends itself to theoretical modeling and turbulent intensity measurements through use of laser velocimetry. Emission results for a range of operations are presented. A number of graphs show which show the variations of emissions levels with one variable at a time are included. In every case, the fuel is jet A, the pressure is atmospheric, and combustion is limited to a primary zone.

Nicholls, J. A.↗
NASA NTRS1978

Calculation of CO concentration for liquid fueled gas turbine combustor

The extensive computation time required for the numerical integration of the differential equations obtained in studies of the concentrations of pollutants emitted by gas turbine combustors, can be reduced significantly by assuming the heat releasing hydrocarbon reactions to be in local equilibrium. In determining the CO and NO concentrations produced in spray combustion, it is, therefore, tempting to use the same local equilibrium assumption in order to simplify the kinetic calculations. An investigation of the validity of the local equilibrium assumption is conducted by using a simple analytical model, and then by actually carrying out the kinetic and local equilibrium calculations for typical case.

Patil, P. B.↗
NASA NTRS1978

Shock wave ignition of magnesium powders

The paper is concerned with magnesium powder ignition caused by the interaction of powder with incident shock wave-induced flows. Experimentally observed ignition delays are correlated with predicted times for particle melting and the onset of thermal erosion, and a good agreement is found between experiment and theory. Ignition delays were measured as functions of particle screening, cloud concentration, incident shock strength, initial pressure, and oxygen concentration (diluted with nitrogen). An expression for the Nusselt number in the Reynolds number range 0-10,000 and the Mach number range 0-6 is proposed. The expression, when applied to predict particle behavior, provides results in agreement with experimental data for M/Re less than 0.5.

Fox, T. W.↗
NASA NTRS1977

Ignition of magnesium powders in shock wave induced flows

Ignition of magnesium powders, resulting from an interaction with incident shock wave-induced flows, has been studied. A numerical analysis of particle dynamics and heating is presented and compared with experimental measurements. The induction period is strongly dependent on convective heat transfer and not on chemical reaction rate. Experimentally observed ignition delays were on the order of 100 microns and were inversely dependent on shock strength and initial pressure, directly dependent on particle size and cloud concentration, and independent of oxygen concentration down to 2.3 percent oxygen. Good agreement between experiment and theory was realized.

Fox, T. W.↗
NASA NTRS1976

Shock wave ignition of metal powders

The interaction between isolated metal particles and shock-induced flowfields is analyzed to characterize physical events and determine experimental feasibility for shock wave ignition studies. Emphasis is placed on incident shock wave phenomenon as opposed to reflected shock wave. A pre-ignition particle-flow interaction is hypothesized which presumes a vapor phase ignition reaction and an absence of significant vaporization prior to particle melting and erosion. Rapid vaporization of eroded mass is expected, with ignition reaction in the particle wakes. Experimental observation of the 6438-A Cd line and the 3721-A MgO band support the ignition mechanism hypothesized. Scattered cloud data are believed to substantiate the hypothesis that ignition delays could be correlated with predicted melting times.

Fox, T. W.↗
NASA NTRS1975

Mechanism of ignition in shock wave interactions with reactive liquid droplets

A detailed qualitative analysis of the processes leading to the explosive ignition of a reactive liquid droplet that is suspended in a gas-phase oxidizer and subjected to the passage of a shock wave, is presented. The interval of time between shock wave passage and ignition is described by identifying a two-stage process which consists of a period of relative reactive dormancy that is followed by a chemical induction period leading to the thermal explosion of reactant that has been stripped from the liquid drop, vaporized, and mixed with the gas-phase oxidizer. The results of first-order calculations based on this model are presented and compared with experimental data for diethlcyclohexane drops in oxygen.

Pierce, T. H.↗
NASA NTRS1974

Two phase detonation studies

An experimental study of the passage of a shock wave over a burning fuel drop is described. This includes high speed framing photographs of the interaction taken at 500,000 frames per second. A theoretical prediction of the ignition of a fuel drop by a shock wave is presented and the results compared with earlier experimental work. Experimental attempts to generate a detonation in a liquid fuel drop (kerosene)-liquid oxidizer drop (hydrogen peroxide)-inert gas-environment are described. An appendix is included which gives the analytical prediction of power requirements for the drop generator to produce certain size drops at a certain mass rate. A bibliography is also included which lists all of the publications resulting from this research grant.

Nicholls, J. A.↗
NASA NTRS1974

Fuel droplet combustion

An analytical model is developed that demonstrates the behavior of fuel drop ignition, and experimental results are presented that describe the effect of aerodynamic shattering on the process of fuel drop ignition. It is shown that the explosive ignition of a fuel drop which interacts with a shock wave in an oxidizing atmosphere depends upon the production of a fuel-oxidizer mixture in the wake of the original drop. The creation of this mixture is dependent on the formation of fine spray as a result of the aerodynamic shattering of the original fuel drop, the evaporation of the fine spray, and the mixing of the fuel vapor with the surrounding hot oxidizer. Only then does the explosive ignition occur as a result of the chemical reaction between the gaseous fuel and oxidizer.

Kauffman, C. W.↗
NASA NTRS1974

Ignition of partially shattered liquid fuel drops in a reflected shock wave environment

An experimental investigation of the ignition of individual fuel drops after their interaction with an incident and a reflected shock wave near the end wall of a shock tube has been carried out. The influence of the aerodynamic shattering of the fuel drop by the convective flow on the ignition characteristics has been examined by varying the drop-end wall separation distance. Data are presented which show the ignition delay times to be a function of the various experimental conditions encountered in this study. A comparison is made with previous investigations concerning the ignition of a liquid fuel drop due only to the interaction with an incident shock wave.

Wierzba, A. S.↗
NASA NTRS1973

Time variation in the reaction-zone structure of two-phase spray detonations.

A detailed theoretical analysis of the time-varying detonation structure in a monodisperse spray is presented. The theory identifies experimentally observed reaction-zone overpressures as deriving from blast waves formed therein by the explosive ignition of the spray droplets, and follows in time the motion, change in strength, and interactions of these blast waves with one another, and with the leading shock. The results are compared with experimental data by modeling the motion of a finite-size circular pressure transducer through the theoretical data field in an x-t space.

Pierce, T. H.↗
NASA NTRS1972

Atomization of liquid droplets in a convective gas stream.

New experimental results are reported for the atomization of liquid droplets by a high-speed air stream. A series of experiments are described in which water droplets, ranging in diameter from 1000 to 2700 microns, were introduced into the test section of a shock tube and allowed to interact with the convective flow established by the passage of a propagating shock front. A rotating drum camera system was employed to obtain a collection of streak photographs of the atomization process from which the breakup time, the breakup distance, and the ultimate atomized particle size were determined.

Ranger, A. A.↗
NASA NTRS1972

Two Phase Detonation Studies Conducted in 1971

The research covered by this third annual progress report represents a continuation of our efforts devoted to the study of detonation waves in liquid-gas systems. The motivation for the work is associated with liquid propellant rocket motor combustion instability although certainly the studies are also applicable to internal combustion engines, jet propulsion engines, safety aspects of spilled liquid fuel, coal mine explosions, and weaponry. The research has been divided into 5 phases, although all of them are intimately related. For the most part these phases are briefly summarized and the reader is referred to other publications for a more complete treatment. The exception to this is where the material herein represents the only printed information available on the particular facet of the problem. Phase A has been primarily concerned with the breakup and ignition of fuel drops by shock waves. The experimental portion of this study as well as a theoretical treatment of the ignition behavior was completed in the past year. The research is now concentrating on the passage of a shock wave over a burning drop. Phase B has been devoted to the assessment of the approximate energy release pattern in two phase detonations insofar as they affect the significant overpressures observed.

Nicholls, J. A.↗
NASA NTRS1972

A two dimensional theory for two phase detonation of liquid films.

A theory for the propagation of detonations through tubes coated with a thin fuel film is developed. Vaporization is assumed as the rate limiting process dominating the detonation structure. Inclusion of the boundary layer displacement effect resulted in better agreement between computed and measured propagation speed, pressure ratio, and reaction zone length than was obtained in an earlier theory in which this effect was neglected. New film detonation data is presented covering a wide range of fuel air ratios. A general Chapman-Jouguet condition is formulated for film detonations, and use of the plane of complete film vaporization as the Chapman-Jouguet plane is justified in the case of thin films.

Rao, C. S. R.↗