Benveniste Natan

Combustion Characteristics of Gel Fuels

An experimental investigation of the combustion phenomena of gel fuel droplets was conducted using a computerized video system. The ignition and combustion processes were viewed and recorded while the droplet and flame temperatures were measured simultaneously. A parametric investigation was conducted to evaluate the effect of the gellant content in the fuel, the ambient pressure and the oxygen mass fraction on the droplet burning rate. The heat of vaporization of gel fuels was measured and was found to increase with increasing the gellant content. The experimental results correlate well with the d-law and indicate that gels burn similar to liquids, however at lower burning rates. Ignition of gels was more difficult in comparison to liquids, especially at relatively high pressures.

EXPERIMENTAL INVESTIGATION OF THE COMBUSTION OF ORGANIC-GELLANT-BASED GEL FUEL DROPLETS

The phenomena involved in the combustion of two organic-gellant-based gelled fuels, one non-metallized and one metallized, were investigated. The non-metal part consisted by 15% organic gellant and 85% JP-8. A high-speed (up to 2,000 frames/sec) digital video camera was employed in the present study. The combustion characteristics of the organic-gellant-based gel fuels were found to be different from those of inorganic-gellant-based gels. In both the metallized and the non-metallized gel cases, a non-permeable elastic film, consisted mainly by the gellant, was formed around the droplet and prevented fuel evaporation. This produced an expanding vapor bubble in the droplet interior that resulted in significant swelling of the droplet. At a certain stage, the film was ruptured, allowing the fuel vapors to escape and it collapsed on the droplet surface. This process repeated itself until all combustible material was consumed.

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

EXPERIMENTAL INVESTIGATION OF A SUPERSONIC COMBUSTION SOLID FUEL RAMJET

An experimental parametric investigation of a solid-fuel supersonic combustion chamber in a scramjet configuration was conducted. A hydrogen-burning vitiated air heater was designed and built to simulate flight conditions of Mach 5.5 in high and medium altitudes in a static test facility (stagnation temperatures and pressures of up to 1500 K and 50 atm, respectively). Flow and combustion phenomena were studied by pressure measurements along the fuel grain and by analyzing video images, digitized from the video recording of each test. Self-ignition and flameholding characteristics were consistent with results from previous works. Flow and combustion characteristics changed as inlet conditions extended, exhibiting initially choked or unchoked flow patterns. In the parametric investigation, incoming airflow conditions (total pressure, total temperature, and mass flow rate) defined the parametric space, yielding a regression rate power law of these parameters. Combustion efficiency was found to decrease with the increase of each of the inlet parameters. Increasing the diversion semiangle of the diverging section of the solid grain resulted in lower regression rate values.

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

Flow of Gel Fuels in Tapered Injectors

The governing equations of the steady e ow of gel propellants and fuels in a tapered tube injector have been formulated assuming a power-law rheological model. A parametric investigation was conducted to evaluate the effect of the injector geometry and pressure gradient on the e ow rate and the mean apparent viscosity of the gel for various fuels and metal loadings. The theoretical results indicate that the e ow rate increases signie cantly with decreasing thepower-law index and with increasing the pressuredrop in the injector. Theapparent viscosity is not uniformineachcrosssection;itismaximalattheaxisandreachesminimumatthewall.Themeanapparentviscosity of the gel exhibits a signie cant decrease with increasing the convergence angle of the injector. This implies that to obtain betteratomizationofthegel,high convergenceanglesarerequired.AcomparisonbetweenRP-1/Algelswith various aluminum mass fractions was conducted. The results indicate the existence of an optimal metal loading.

Numerical solution of the turbulent supersonic flow over a backward facing step

The steady two-dimensional viscous supersonic turbulent flow over a backward facing step was calculated using the PHOENICS CFD code. The two-equation k– turbulence model was employed for the turbulent flow simulation. The effects of the incoming boundary layer, Reynolds number and inlet Mach number on the flow were investigated. The maximum free stream Mach number was 3.5. The PHOENICS code was found to be adequate for supersonic flow simulations with M ≤ 3.5, however, at hypersonic turbulent flow conditions convergence was not obtained. The numerical results indicate that the separation point is positioned on the step face, below the corner. The calculations also show that a lip shock is formed to match the flow conditions at the step corner. The results exhibit favorable agreement with data from both experiments and other numerical simulations.

Atomization characteristics of gel fuels

In the present research an effort is made to relate the rheological properties of gels propellants with their atomization behavior. The experimental investigation demonstrates that pseudoplastic, viscoinelastic water gels exhibit similar spray pattern with Newtonian liquids, however, they are more difficult to atomize. The experimental results indicate that atomization is significantly affected by the gellant content in the fuel and the injector geometry. The characteristic Sauter Mean Diameter of the spray increases with increasing the gellant content due to the respective increase of the shear viscosity. Wide angles of convergent flow injectors require less upstream pressure to achieve the same atomization performance with regular triplet atomizers. It is demonstrated that gelling agents may be combined to achieve desired rheological characteristics. Nomenclature d Exit diameter D Droplet diameter f Mass fraction of gellant A F fuel FN Flow Number K Power-law consistency index rh Mass flow rate n Power-law rate index Ni Droplet number O Oxidizer P Pressure SMD Sauter Mean Diameter CC Convergence angle [3 Impingement angle * Graduate Student t Senior Lecturer, Senior Member AIAA. Copyright

Ignition of boron particles coated by a thin titanium film

A theoretical model for the ignition of titanium-coated boron particles in dry air has been developed. In general, the results indicate reduction in ignition time due to a high heating rate of the particle resulting from the reactions of titanium with both boron and oxygen and the deterioration of the protective properties of the coating layers due to mechanical stresses. However, ignition strongly depends on the titanium coating thickness and the ambient temperature.

Theoretical Model of the Transient Combustion of Organic-Gellant-Based Gel Fuel Droplets

Experimental evidence of the combustion process of an all-organic gel fuel droplet indicates that at a certain time after ignition, evaporation of the liquid fuel results in the formation of an elastic layer of high-viscosity gellant around the droplet, which prevents further vaporization. As a result, constantly expanding vapor bubbles are produced within the droplet. Eventually, the layer ruptures and jets of fuel vapor are released. A theoretical, time-dependent model of organic-gellant-based gel droplet combustion has been developed and numerically solved. The results indicate that the evaporation rate of the liquid fuel from the droplet surface depends on droplet size and strongly affects the thickness of the gellant layer. The tensile stress, applied to the gellant layer during the formation of the bubbles, reaches high levels in short periods of time and causes the droplet to rupture when it exceeds the layer material rupture stress. The stage during which the gellant layer is formed is almost three orders of magnitude longer than the stage of bubble formation and layer rupture.