Oxygen Mass Flux Research Articles

Effects of Ti and Mg particles on combustion characteristics of boron–HTPB-based solid fuels for hybrid gas generator in ducted rocket applications

In the field of propellants and explosives, boron has been considered as an attractive fuel due to its calibre to produce high energy density. Therefore, boron as a fuel has great prospect for application in solid fuel ducted rocket (SFDR) with hydroxyl-terminated polybutadiene (HTPB) which is widely used solid fuel. Experimentally, it has been observed that ignition and combustion of boron is problematic due to boron oxide (B2O3) coating around active boron which inhibits its ignition/combustion. In the present study, titanium and magnesium have been used with boron-HTPB based solid fuel as burning promoters. Various combustion parameters have been evaluated for nine different fuel compositions. An opposed flow burner (OFB) system has been used for fuel evaluation with the help of gaseous oxygen (GOX) impingement where oxygen mass flux (Gox) is maintained between 20 and 57 kg/m2-s. Favorable result has been observed for boron-magnesium combinations as compared to boron-titanium. Around 18% enhancement in regression rate has been achieved for boron-magnesium-HTPB sample (at max. Mg %) compared to that of boron-HTPB sample. In all the boron-based samples, greenish appearance has been observed during combustion which corresponds to BO2 emission. Spectroscopy analysis confirmed the same. Burning events have been analysed using color and high-speed camera. Condensed combustion products (ejected from the burning surface) have been analysed by various material characterization techniques such as: FE-SEM, EDX, XRD, and TGA. Through TGA, the active residual boron content in the condensed product of magnesium-based sample is found to be 33%. This value is found to be the lowest among the tested fuel compositions. The present investigation has been mainly focused on the formulation of new composition of boron-based solid fuel having high energy density.

Net carbon-di-oxide conversion and other novel features of packed bed biomass gasification with O2/CO2 mixtures

Biomass thermo-chemical conversion under O2/CO2 conditions is experimentally investigated to establish its CO2 conversion potential. Packed bed configuration with counter-current flame propagation is chosen for this purpose and three different fuels are used (agro-residue pellets, coconut shells and wood pellets). For oxidizer streams containing 20, 25, 30, 40 and 50% O2 (v/v) (rest CO2) steady flame propagation is established at different superficial velocities and the following quantities are measured – temperature along the reactor, mass consumption rate and exit gas composition. Corresponding reference experiments with O2/N2 mixtures are performed for comparing the net CO2 conversion. The propagation regimes, similar to earlier studies with air, fall under the following two categories – (1) gasification – volatile oxidation accompanied by char reduction and (2) char oxidation dominated – simultaneous char and volatile combustion. Invoking equilibration of volatile oxidation under gasification conditions, a new procedure is established to estimate the net CO2 conversion (NCC). In general, NCC is significant around the volatile stoichiometric point (ϕv=1) and falls off on either side as the oxygen mass flux is decreased or increased. NCC is zero in very rich regime i.e. when ϕv≫1 due to low bed temperatures and also when ϕv<1 due to char oxidation. Maximum NCC of 627 g/kg of biomass is observed with 30% O2/70% CO2 (v/v) case at ϕv=0.96 for agro residue pellets. Cold gas efficiency (ηg) is as high as 85% for the maximum NCC case. Enhanced gasification efficiencies of O2/CO2 cases as compared to corresponding air cases is due to the additional conversion of CO2 to CO (predominantly C + CO2 → 2CO). The difference in the fraction of char left over with CO2 and N2 cases is consistent with this observation. Corrected fuel flux (CFF), proposed to address these variations in fuel heating value, oxygen fraction in the oxidizer etc., is shown to exhibit universality with respect to oxygen mass flux and fuel type.

Combustion Characteristics of Boron-HTPB-Based Solid Fuels for Hybrid Gas Generator in Ducted Rocket Applications

ABSTRACT Metal/metalloid particles as additives in polymeric fuels have been investigated since several decades, among which boron has been remaining the most attractive one due to its high energy density. In the present study, a low-cost screening instrument called opposed flow burner (OFB) is used to investigate the salient features of various boron-HTPB-based solid fuels to understand the major performance parameters such as burning efficiency and regression rate. Pure-HTPB (baseline solid fuel) and HTPB loaded with boron nanoparticles (nB) at various concentrations (5–40% by wt.) have been tested. In the OFB system, single gaseous oxygen (GOX) jet has been applied on the surface of the solid fuel pallet at oxygen mass flux (Gox) range of 20–57 kg/m2-s in order to compare the performance of various solid fuels. High-speed videography and UV–VIS spectroscopy have been employed to realize the burning process of fuel samples. Emission spectra obtained from the experiments clearly identify the gas-phase intermediate species (BO and BO2) of boron ignition/combustion. Several material characterization techniques such as field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), bomb calorimetry, and thermogravimetric analysis (TGA) have also been applied on pre- and post-burn samples to identify the physiochemical changes. The residual active boron content is found out to be almost negligible in fine-condensed combustion products whereas significant amount of un-burnt boron is present in the coarse ejected agglomerates.

Carbon Nanotubes' Effect on Mitochondrial Oxygen Flux Dynamics: Polarography Experimental Study and Machine Learning Models using Star Graph Trace Invariants of Raman Spectra.

This study presents the impact of carbon nanotubes (CNTs) on mitochondrial oxygen mass flux (Jm) under three experimental conditions. New experimental results and a new methodology are reported for the first time and they are based on CNT Raman spectra star graph transform (spectral moments) and perturbation theory. The experimental measures of Jm showed that no tested CNT family can inhibit the oxygen consumption profiles of mitochondria. The best model for the prediction of Jm for other CNTs was provided by random forest using eight features, obtaining test R-squared (R2) of 0.863 and test root-mean-square error (RMSE) of 0.0461. The results demonstrate the capability of encoding CNT information into spectral moments of the Raman star graphs (SG) transform with a potential applicability as predictive tools in nanotechnology and material risk assessments.

Modeling aerobic biodegradation in the capillary fringe.

Vapor intrusion from volatile subsurface contaminants can be mitigated by aerobic biodegradation. Laboratory column studies with contaminant sources of chlorobenzene and a mixture of chlorobenzene, 1,2-dichlorobenzene, and 1,4-dichlorobenzene showed that contaminants were rapidly degraded in thin reactive zones with high biomass and low substrate concentrations in the vicinity of the capillary fringe. Such behavior was well characterized by a model that includes oxygen-, substrate-, and biomass-dependent biodegradation kinetics along with diffusive transport processes. An analytical solution was derived to provide theoretical support for the simplification of reaction kinetics and the approximation of reactive zone location and mass flux relationships at steady state. Results demonstrate the potential of aerobic natural attenuation in the capillary fringe for preventing contaminant migration in the unsaturated zone. The solution indicates that increasing contaminant mass flux into the column creates a thinner reactive zone and pushes it toward the oxygen boundary, resulting in a shorter distance to the oxygen source and a larger oxygen mass flux that balances the contaminant mass flux. As a consequence, the aerobic biodegradation can reduce high contaminant concentrations to low levels within the capillary fringe and unsaturated zone. The results are consistent with the observations of thin reactive layers at the interface in unsaturated zones. The model considers biomass while including biodegradation in the capillary fringe and unsaturated zone and clearly demonstrates that microbial communities capable of using the contaminants as electron donors may lead to instantaneous degradation kinetics in the capillary fringe and unsaturated zone.

酸化剤流旋回型ハイブリッドロケットエンジン性能 パラメータの時刻変化予測

For hybrid rocket engines, there are some unique relationships between the parameters, such as the fuel regression rate, oxidizer mass flow rate, equivalence ratio and so on, which determine the burning properties. These relationships are strongly related to the engine performance. Our final goal of this study is to establish the optimum design method which considers the engine performance of hybrid rocket engines for practical hybrid rockets varying with time. In this paper, we focused on the time variation of ballistic parameters of the swirling-oxidizer-flow-type hybrid rocket engine. We establish the new simulation method, the Two-Step combustion model, in which at the fuel grain head the fuel regression rate is independent of the oxygen mass flux and in the rear region depends on the flux according to the ordinary hybrid combustion formula. The model succeeded to predict the time variation of the burning properties and ballistics parameters, which coincided with the experimental burning values and time behavior. These results show the possibility that there are two different combustion mechanisms in the burning process of the swirling-oxidizer-flow-type hybrid rocket engine.

S192022 極低温に冷却されたCAMUI型固体燃料の点火特性

During development of a large size CAMUI-type hybrid rocket motor in 2006, anomalous combustion associated with large pressure spike frequently occurred. After troubleshooting, low temperature of the fuel was found to have caused anomalous combustion. Because a cryogenic liquid oxygen tank is around the motor, polyethylene fuel grain is cooled to liquid oxygen temperature. The anomalous combustion was duplicated with a downsized motor. The size of the fuel grain was 2/5 of large size motor. Because Damkohler number is proportional to the ratio of pressure and oxygen mass flux, the ratio for the subscale motor was conformed to that for the full-scale motor. In a reproductive experiment, pressure spike also appeared. This result suggests that blow-off occurs before the pressure spike. After the blow-off, gas mixture in the chamber may ignite to cause the pressure spike.

GLYCIDYL AZIDE POLYMER AND POLYETHYLENE GLYCOL MIXTURES AS HYBRID ROCKET FUELS

Two types of hybrid rocket motors that use mixtures of glycidyl azide polymer (GAP) and polyethelene glycol (PEG) as the solid fuel are proposed and the combustion characteristics are presented. GAP and PEG mixtures in which the PEG concentrations are lower than a critical value between 40 and 50 mass% can sustain self-combustibility and can be used as a solid fuel for gas hybrid rocket systems, while mixtures with higher PEG concentrations and without self-combustibility can be employed as a solid fuel for traditional hybrid rockets. Thrust control and tailoring using changes in both the oxidizer/fuel (O/F) ratio and grain composition were investigated for the gas hybrid system, and successful control was accomplished in experiments. Firing tests were conducted of a traditional hybrid rocket motor as functions of the oxygen mass flux and burning pressure. The effect of GAP is obvious, and GAP enhances the regression rates of the solid fuel grain, and an effect of pressure was also found. The distributed flame model is discussed briefly here to explain the regression rate increase by GAP.

Use of numerical modeling in design for co-firing biomass in wall-fired burners

Co-firing biomass with coal or gas in the existing units has gained increasing interest in the recent past to increase the production of environmentally friendly, renewable green power. This paper presents design considerations for co-firing biomass with natural gas in wall-fired burners by use of numerical modeling. The models currently used to predict solid fuel combustion rely on a spherical particle shape assumption, which may deviate a lot from reality for big biomass particles. A sphere gives a minimum in terms of the surface-area-to-volume ratio, which impacts significantly both motion and reaction of a particle. To better understand the biomass combustion and thus improve the design for co-firing biomass in wall-fired burners, non-sphericity of biomass particles is considered. To ease comparison, two cases are numerically studied in a 10 m long gas/biomass co-fired burner model. (1) The biomass particles are assumed as solid or hollow cylinders in shape, depending on the particle group. To model accurately the motion of biomass particles, the forces that could be important are all considered in the particle force balance, which includes a drag for non-spherical particles, an additional lift due to particle non-sphericity, and a “virtual-mass” force due to relatively light biomass particles, as well as gravity and a pressure-gradient force. Since the drag and lift forces are both shape factor- and orientation-dependent, coupled particle rotation equations are resolved to update particle orientation. To better model the reaction of biomass particles, the actual particle surface area available and the average oxygen mass flux at particle surface are considered, both of which are shape factor-dependent. (2) The non-spherical biomass particles are simplified as equal-volume spheres, without any modification to the motion and reaction due to their non-sphericity. The simulation results show a big difference between the two cases and indicate it is very significant to take into account the non-sphericity of biomass particles in order to model biomass combustion more accurately. Methods to improve the design for co-firing biomass in wall-fired burners are finally suggested.

Dimensional effects on the transition from ignition to flame spread in microgravity

Three-dimensional (3-D) and two-dimensional (2-D) simulations of the transition from radiative ignition on a solid fuel to flame spread in an imposed wind were performed in microgravity. Two-dimensional flames were found to quench (due to poor oxygen supply) more easily (i.e., at larger wind speeds) than 3-D flames. Results from the 2- and 3-D simulations were compared during the transition phase at wind speeds that ultimately lead to quenching of the 2-D flame but survival of the 3-D flame. In all locations near and in the flame, oxygen mass flux was larger in the 3-D flames and dominated by diffusion (as opposed to convection). Oxygen was supplied to the core of the 3-D flame due to diffusion from the sides of the flame (in a cross-wind direction). Diffusion in the 2-D flame was limited to directions parallel to the wind. This cross-wind diffusion was most significant at early times during transition when the flame was small and had a relatively large curvature. The 3-D flame, therefore, required less oxygen supply from an external wind to undergo transition to flame spread. Once flame spread was established there was little difference between the 3-D flames (in the centerline plane) and 2-D flames, due to the decreased curvature of the three-dimensional flame relative to the curvature during ignition and transition.

Forced cocurrent smoldering combustion

An analytic model of the propagation of smoldering combustion through a very porous solid fuel is presented. Here smoldering is initiated at the top of a long, radially insulated, uniform fuel cylinder, so that the smolder wave propagates downward, opposing an upward forced flow of oxidizer. Because the solid fuel and the gaseous oxidizer enter the reaction zone from the same direction, this configuration is referred to as cocurrent (or premixed-flame-like). It is assumed that the propagation of the smolder wave is one-dimensional and steady in a frame of reference moving with the wave. Buoyancy is included and shown to be negligible in the proposed application of a smoldering combustion experiment for use on the Space Shuttle. Radiation heat transfer is incorporated using the diffusion approximation and smoldering combustion is modeled by a finite rate, one-step reaction mechanism. Because the solid and the gas move at different velocities, both the downstream temperature, T f, and the smolder velocity, ν, are eigenvalues. The dimensionless equations are very similar to those governing the propagation of a laminar premixed flame. A straightforward extension of the activation energy asymptotics analysis presented by Williams for premixed flames yields an expression for a dimensionless eigenvalue determining T f. A global energy balance provides a relation for the smolder velocity, ν. Predictions are compared with the experimental findings of Rogers and Ohlemiller and with the numerical results of Ohlemiller, Bellan, and Rogers. Key results include (1) for a given solid fuel, T f depends only on the initial oxygen mass flux, m ̇ oi ″ , and increases logarithmically with m ̇ oi ″ ; (2) ν increases linearly with m ̇ oi ″ and at fixed m ̇ oi ″ , increasing the initial oxygen mass fraction, Y oi, increases ν; (3) steady smolder propagation is possible only for Y oi ≥ c eff (T f − T i ) Q , with extinction occurring when all of the energy released in the reaction zone is used to heat the incoming gas. General explicit expressions for T f and ν are presented.

Buoyancy effects on smoldering combustion

A theoretical and experimental study is carried out to determine the effect of buoyancy on the rate of spread of a cocurrent smolder reaction through a porous combustible material. Since buoyant forces are proportional to the product g( ϱ gi − ϱ g), they can be controlled experimentally by varying either the gravitational acceleration, g, or the density difference, ϱ gi − ϱ g. The latter approach was followed in the present work. Measurements are performed of the smolder spread rate through porous α-cellulose (0.83 void fraction) as a function of the ambient air pressure. The experiments are carried out in a pressure vessel for ambient pressures ranging from 0.5 to 1.2 atm. The rate of spread was obtained from the temperature histories of thermocouples placed at fixed intervals along the fuel centerline. The smolder velocity was found to increase as the ambient pressure was increased. Extinction was found to occur when the buoyancy forces could not overcome the drag forces, indicating that at least for the present experimental conditions transport by diffusion cannot, by itself, support the spread of a smolder reaction. This conclusion is particularly important for outer space conditions where gravity and consequently buoyancy could be negligible. In the analysis, which assumes one-dimensional processes, the transport equations are solved to give the smolder spread rate as a function of the inlet oxygen mass flux. This mass flux is then estimated by balancing buoyancy and drag forces. Assuming that the smolder chemical reaction is only weakly dependent on pressure, the analysis finally predicts a smolder velocity dependence of the form v ∼ Y oi gϱ 2 gi ∼ Pa 2, i.e. is proportional to the ambient pressure squared. Good qualitative agreement is found between the theoretical predictions and the experimental results.