Alon Gany

Investigation of a Solid Fuel Scramjet Combustor

The combustion of a solid fuel under supersonic crosse ow conditions in a scramjet cone guration has been studied experimentally. Self-ignition and sustained combustion of poly-methyl-methacrylate with no external aid (such as reactive gas injection or a pilot e ame ) were demonstrated in static tests simulating a e ight Mach number of about 5 at high altitude. The appropriate inlet conditions, i.e., stagnation temperature and pressure in excess of 1200 K and 16 atm, respectively, were provided by a vitiated air heater. The diverging combustion chamber included a fore-end e ame stabilization zone, whose e ameholding limits were determined experimentally. Flow and combustion phenomena were studied both by pressure measurements along the fuel grain and by video recording, taking advantage of the fuel transparency. Comparison between theoretical results of a one-dimensional e ow model with test data showed fair agreement, indicating the existence of a supersonic e ow regime within the combustor. The video pictures provided temporal and spatial fuel regression rate data, using a computerized image analysis system. Nomenclature

Agglomeration and ignition mechanism of aluminum particles in solid propellants

Aluminum powders added to conventional rocket propellants burn either as single particles or agglomerates which contain hundreds or even thousands of the original particles. Combustion efficiency and acoustic stability characteristics are very dependent on the final Al/Al 2 O 3 particle size injected into the chamber flowfield. High-speed photographs of burning homogeneous propellants provided data on agglomeration size and visualization of the flow processes as a function of pressure (1 to 10 MPa), initial particle size (5 to 100 μm), and aluminum mass fraction (0.1 to 13%). Photomicrographs of extinguished surfaces revealed the importance of particle accumulation in a thin mobile reaction layer adjacent to the burning surface. A model was developed that interpreted data and observations from several sources. The model accounts for accumulation of aluminum particles in the mobile reaction layer, retention of particles by surface tension forces, melting, and ignition at the surface. The following agglomeration and particle behavior items are categorized: decreasing agglomerate size with increasing pressure, minimum mass loading required for agglomeration, prominent agglomeration for particles with diameters less than the reaction layer thickness, and sharply reduced agglomeration for larger particles. The model provides an approach for controlling and interpreting agglomerate size behavior.

Ignition and Combustion of Boron Particles in the Flowfield of a Solid Fuel Ramjet

Theoretical investigation on the behavior of individual boron particles in the flowfield of a solid fuel ramjet (SFRJ) combustor is presented. The study was motivated by the observed difficulties in achieving good combustion efficiencies of boron required to exploit its remarkable theoretical energetic performance. The equations describing the gas flowfield and the particle behavior are solved numerically. The solution presents the trajectory, temperature, and history of the boron particles due to the interactions with the surrounding gas, as well as the ignition envelope and combustion time. The results demonstrate the limited ranges of particle size and ejection velocity which enable ignition and sustained combustion, reveal why practical systems often exhibit poor combustion efficiencies, and predict the conditions where ignition and efficient combustion of boron are feasible. Nomenclature

Geometric effects on the combustion in solid fuel ramjets

The paper summarizes an experimental investigation concerning the geometric and size effects on the cumbustion in solid fuel ramjets (SFRJs). Polymethylmethacrylate (PMMA) solid fuel was used, and the combustor simulated conditions resulting from flight at sea level and Mach 3, Instantaneous and local fuel regression measurements indicated the following conclusions: the local regression rate is closely related to the local convective heat flux; the honuniformity of fuel regression rate has an attenuating effect on the dependence of the mean regression rate on the mass flux in extended burn-time tests; the regression pattern is not affected by downstream conditions; and mean regression rate decreases with increasing port diameter. Nondimensional scales normalized by the port diameter were found to give generalized expressions for different motors.

Investigation of a small solid fuel ramjet combustor

Experimental and analytical investigations of a small solid-fuel ramjet (SFRJ) combustor were conducted. A static test system with a 25-kW electrical air heater simulated the air temperature and pressure encountered in flight at a Mach number of 3 at sea level. The transparent polymethylmethacrylate fuel used in the tests permitted continuous video photography, revealing the local fuel-regression-rate behavior and the instantaneous ignition and combustion phenomena. The results demonstrated high combustion efficiency and indicated peculiar local and average fuel-regression-rate correlations. The analysis indicated that the specific conditions resulting from the low Reynolds number range in small SFRJ motors, in contrast to large combustors, enhance the effect of the sudden-expansion heat-transfer regime relative to the boundary-layer regime. 14 refs.

Aluminized Solid Propellants Burning in a Rocket Motor Flowfield

Combustion and agglomeration processes of aluminum particles emitted from the surface of an aluminized double-base propellant (NC/TMETN) were studied under rocket motor, crossflow conditions. High-speed color photographs (-2000 frames/s) were taken of burning AI/AI2O3 agglomerates forming on the surface, moving along the surface, and entering the flowfield. As an example, a propellant containing 6-^m Al burning at 7 MPa and 6 m/s crossflow produced a mean agglomerate size of about 250 ^m. Analysis of size distributions of the agglomerates leaving the surface revealed that the following parameters decrease with increasing pressure: collision frequency on the surface, the agglomerate stay time on the surface, and mean agglomerate size. Increasing the crossflow velocity decreased the mean agglomerate size. The propellants which contained the large aluminum particles (50 /*m vs 6 j*m) burned without the aluminum igniting or agglomerating on the surface.

Advantages and drawbacks of boron-fueled propulsion

Abstract The theoretical energetic advantages and the practical problems and characteristics associated with three modes of ramjet-type propulsion systems are discussed. It is shown that the use of boron may result in a remarkable theoretical energetic gain compared with hydrocarbon fuels, especially in volume limited systems. However, the poor combustion characteristics of boron and the high ignition temperature required (1900 K), impose special constraints on the combustor system in order to obtain high efficiencies. Solid fuels and propellants containing a high percentage of boron tend to exhibit irregular combustion, which may cause thrust modulation, while agglomeration of boron particles may result in incomplete combustion due to the insufficient residence time in the combustor.

Internal Ballistics Considerations of Nozzleless Rocket Motors

Thefundamentalsoftheinternalballisticsofsolid-propellantnozzlelessrocketmotorshavebeenstudiedtheoreticallyandexperimentally.Aimedatthemajorbasicaspects,thetheoreticalanalysisconsistedofaquasi-steady-state one-dimensionale ow ofa perfect gas in a constant-area port, assuming a uniform burning rate along the grain asa resultofthecompensating effectsoferosiveand pressure-dependentburning contributions.Thesimplie edanalysis has beenfound to be an excellent tool to exhibit fundamental characteristics. Itrevealed that thetheoreticalachievable specie c impulse in a nozzleless rocket cannot exceed approximately 86% of its value in an adapted-nozzle rocket operating at the same average chamber pressure. An experimental parametric study using cylindrical propellant grains has demonstrated good agreement with the major parameters addressed in the theoretical analysis, indicating that the assumption of a uniform burning rate is a realistic approximation.

Burning and flameholding characteristics of a miniature solid fuel ramjet combustor

The objective of this research was to characterize the combustion and flameholding limits in solid fuel ramjet combustors of particularly small size (10mm i.d.). The main combustion characteristics, as well as the specific effects resulting from the special size range, are pointed out. Several hundred static tests were performed with three different fuel types, polymethylmethacrylate (plexiglass), polyethylene, and polybutadiene. Air inlet temperatures of 800 K, 520 K, and room temperature were tested to simulate flight Mach numbers of 3, 2, and low subsonic, respectively. All three fuels exhibited very good flameholding capability at the higher air temperature (800 K). The flammability limits decreased at 520 K and became very narrow at room temperature. The flameholding capability also deteriorated with decreasing combustor size. The flow-reattachment distance, coinciding with the maximum fuel regression rate zone, was shown to be proportional to the inlet step height and was independent of fuel type and port-flow Reynolds number. Nomenclature A — area d = diameter F = thrust G = mass flux H = step height k = thermal conductivity M = Mach number m = exponent m = mass flow rate n = exponent p = pressure Q = heat capacity (sensible enthalpy) Q = heat loss rate q = heat flux Re = Reynolds number r = fuel regression rate T — temperature V = volume xr = reattachment distance Subscripts

Two-phase flow effects on hybrid combustion☆

Abstract Two-phase flow effects on the combustion and performance of a cylindrical hybrid rocket motor are investigated, using a modification of an advanced model for reactive flow which has given good agreement with experiments performed with a gaseous oxidizer. A spray distribution function describing the local number density and size distribution of the droplets in the motor is introduced. Numerical methods enable the prediction of fuel regression rate, temperature, velocity and concentration profiles, local number and size distribution of the droplets, percent of mass evaporated and of oxidizer burned. The effect of different liquid oxidizers is investigated showing that those having a high heat of vaporization do not evaporate completely even in relatively long motors. Due to the heat consumed in the vaporization process, the central section of the motor remains comparatively cool, and, therefore, a longer motor than that used for liquid rockets is needed for completion of droplet evaporation. Although appropriate injectors directing most of the liquid towards the intensive combustion zone improve the efficiency, the use of mixing diaphragms and an aft-mixing-chamber is recommended.