Polymeric Fuels Research Articles

Expandable graphite effect on solid fuel and propellant combustion

This paper presents an investigation on a novel method for manipulating and enhancing the burning of solid fuels and propellants by expandable graphite additive. Expandable graphite (EG) is a form of intercalated graphite. At elevated temperature it undergoes an increase in volume, forming elongated strings/fibers many folds longer than the original particles/flakes. It was found experimentally that the addition of 1-5% of expandable graphite (original particle size 100-150 micrometer, onset of expansion at 200-230°C) to a polymeric fuel matrix (polyester) in hybrid combustion, increased the fuel regression rate by 2 fold and more. It was hypothesized that the EG strings forming near the burning surface protrude into the hot gas environment increasing heat transfer into the fuel via conduction. Furthermore, the swelling effect at the surface layer might increase the effective surface area, hence further increasing the burning rate. High-speed photography of a surface subjected to flame in oxidizing atmosphere showed EG fibers protruding and growing on the surface, supporting the increased heat transfer hypothesis. On the other hand, the addition of the same EG to an ammonium perchlorate-based propellant showed a tendency to reduce burning rate and even extinguish the flame at EG fractions higher than about 3-5%. It is concluded that in the case of hybrid or solid fuel ramjet combustion, EG can serve as a novel and effective burning rate enhancer while employing polymeric fuels of good mechanical properties in contrast to paraffin fuels which have inferior mechanical properties. The influence of EG on the combustion of solid propellants needs further investigation, looking for combinations of EG and propellant types that may exhibit a similar effect.

Review on the regression rate-improvement techniques and mechanical performance of hybrid rocket fuels

Human spaceflight, space tourism, and the launch of microsatellites are all expected to grow in the future. In fact, with the recent increase of the satellite market, almost 1000 smallsats per year are foreseen to be launched over the next decade. Hybrid rocket propulsion has received significant attention for military and commercial applications due to its potential safety, throttleable, and restart ability when compared with solid rockets, economicity, simplicity, and compactness features when compared to liquid rockets. However, in order to make this new technology the future of the next generation of rockets, some drawbacks of the heterogeneous combustion in hybrid rockets such as low fuel regression rate and varying oxidizer-to-fuel ratio during the combustion process must be well addressed. The diffusion-limited combustion in hybrid rocket motor is responsible for the low regression and poor combustion efficiency of fuels such as Hydroxyl-terminated polybutadiene (HTPB), Poly methylmethacrylate (PMMA), and other polymeric binder-fuels. The paraffin-based solid fuel represents a potential solution to the slow regression rate of current solid polymeric fuels. However, paraffin-based fuels suffer from poor mechanical properties and rapid volatilization, preventing their full development and applications for a space mission. In this work, a review of various techniques to improve hybrid rocket fuel's ballistic and mechanical performance is presented.

Performance Evaluation of Boron/Hydroxyl-Terminated Polybutadiene-Based Solid Fuels Containing Activated Charcoal

Numerous researches have been conducted on boron-based fuels and propellants due to the attractive theoretical energy potential of boron. However, sustained energy production during combustion of a boron-based fuel/propellant is still uncertain because efficient burning of boron is challenging. The present paper deals with various boron-loaded polymeric fuels, where activated charcoal (AC) is introduced as a burning enhancer for application in hybrid fuel ducted rocket. The solid fuel samples are tested with the help of a classical screening tool, which is known as opposed flow burner (OFB) or stagnation flow burner. Opposed flow burner is run with oxygen mass flux between 20 and . The fuel is evaluated on the basis of major performance parameters, such as regression rate, species formation during combustion using a spectrometer, residual active ingredients in postcombustion product using standard material characterization techniques, etc. The major outcome is evaluated from thermogravimetric analysis (TGA), where active residual boron is found to be around 6% only in postcombustion product for the fuel sample containing B and AC in equal proportion. Opposed flow burner helps in reducing the time and cost of research involving expensive fine fuel ingredients, and also facilitates to identify optimum fuel formulations for hybrid gas generator in ducted rocket applications.

Two-Hundred-Newton Laboratory-Scale Hybrid Rocket Testing for Paraffin Fuel-Performance Characterization

A series of firing tests have been performed on a laboratory-scale hybrid rocket engine of 200 N class, fed with gaseous oxygen through a converging nozzle injector, to assess the mechanical feasibility and regression rate of a newly developed paraffin-based fuel. Such an injector configuration, by producing recirculation at the motor head hand, has been already demonstrated to influence the standard fuels regression rate, which yields an increase with the port diameter at given mass flux. In this study, paraffin-fuel regression rate dependence on the mass flux and grain port diameter in the form of a power function is determined to be similar to that established with polymeric fuels, despite the different mechanism of consumption that involves the fuel surface liquid-layer instability other than the vaporization typical of classical polymers. Comparison with some data in the literature is presented. Data retrieved from the testing campaign are compared with numerical results obtained by adopting a simple but efficient modeling strategy and a commercial solver. The numerical solution gives evidence of the recirculating flow at the injector exit, which is also responsible for the paraffin contamination observed in the motor prechamber. A good agreement is found with chamber pressure experimentally measured.

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.

Development of EVA-SEBS based wax fuel for hybrid rocket applications

An attempt has been made in the present study to obtain the suitable wax based fuel that could be used in the practical hybrid rocket applications. It is known that wax has high regression rate compare to other polymeric fuels and is a most suitable hybrid fuel but suffers with poor mechanical properties. Thus, additives such as EVA (Ethylene Vinyl Acetate) and SEBS (Styrene-Ethylene-Butylene-Styrene Copolymer Grafted with 2% Maleic Anhydride) have been utilized to enhance its mechanical properties. Apart from the mechanical properties, regression rate, combustion efficiency and sliver loss studies have also been conducted. Various combinations of wax with EVA and SEBS samples have been prepared. Among these fuels developed, wax mixed with the combination of 10% SEBS and 5% EVA had shown significant improvement in the mechanical properties (maximum tensile strength was 4.26 MPa and percentage elongation was 24.6%). These properties are within the range to be utilized as a solid fuel in practical applications. Regression rate studies on the fuels developed in the present study had shown that the wax with SEBS based fuels obtained higher regression rate than wax with EVA based fuels. The wax fuel with combination of 10% SEBS and 5% EVA had shown the best regression rate characteristics along with mechanical properties that required for practical applications. The increment in combustion efficiency was observed with addition of polymeric additives to wax. With the addition of 20% EVA to wax the combustion efficiency improved from 63% to 72%. The burnt profiles from injector end to the nozzle end were more uniform with wax-SEBS fuel grains than pure wax fuel grains. Hence, the sliver loss is expected to be lower with wax-SEBS fuel.

Toxic Combustion Product Yields as a Function of Equivalence Ratio and Flame Retardants in Under-Ventilated Fires: Bench-Large-Scale Comparisons.

In large-scale compartment fires; combustion product yields vary with combustion conditions mainly in relation to the fuel:air equivalence ratio (Φ) and the effects of gas-phase flame retardants. Yields of products of inefficient combustion; including the major toxic products CO; HCN and organic irritants; increase considerably as combustion changes from well-ventilated (Φ < 1) to under-ventilated (Φ = 1–3). It is therefore essential that bench-scale toxicity tests reproduce this behaviour across the Φ range. Yield data from repeat compartment fire tests for any specific fuel show some variation on either side of a best-fit curve for CO yield as a function of Φ. In order to quantify the extent to which data from the steady state tube furnace (SSTF [1]; ISO TS19700 [2]) represents compartment fire yields; the range and average deviations of SSTF data for CO yields from the compartment fire best-fit curve were compared to those for direct compartment fire measurements for six different polymeric fuels with textile and non-textile applications and for generic post-flashover fire CO yield data. The average yields; range and standard deviations of the SSTF data around the best-fit compartment fire curves were found to be close to those for the compartment fire data. It is concluded that SSTF data are as good a predictor of compartment fire yields as are repeat compartment fire test data.

Studies on EVA‐Based Wax Fuel for Launch Vehicle Applications

AbstractA fuel for the hybrid rocket system was developed. The attempts made to improve the mechanical properties of wax by adding EVA (ethylene vinyl acetate) are illustrated. It was observed that the mechanical properties obtained by adding EVA to wax were dependent on the process of curing the fuel specimen. A proprietary method of curing the fuel specimen was established. The percentage elongation obtained with 20 % EVA and 80 % of wax was around 17 %, which was much higher than the values obtained with pure wax (4 %). It was observed from the study that the higher the percentage of EVA content in wax was, the better the mechanical properties were. Regression rate studies with EVA and wax combination were carried out. It was observed that the regression rate decreased upon addition of EVA in wax. This reduction was compensated by using a bluff body at the head end. The regression rate obtained with 20 % EVA and 80 % wax even with the use of bluff body is lower than that obtained with pure wax, but is around 3.5 times the regression rate obtained with polymeric fuels.

Heat and mass transfer analysis for paraffin/nitrous oxide burning rate in hybrid propulsion

This research presents a physical-mathematical model for the combustion of liquefying fuels in hybrid combustors, accounting for blowing effect on the heat transfer. A particular attention is given to a paraffin/nitrous oxide hybrid system. The use of a paraffin fuel in hybrid propulsion has been considered because of its much higher regression rate enabling significantly higher thrust compared to that of common polymeric fuels. The model predicts the overall regression rate (melting rate) of the fuel and the different mechanisms involved, including evaporation, entrainment of droplets of molten material, and mass loss due to melt flow on the condensed fuel surface. Prediction of the thickness and velocity of the liquid (melt) layer formed at the surface during combustion was done as well. Applying the model for an oxidizer mass flux of 45kg/(sm2) as an example representing experimental range, it was found that 21% of the molten liquid undergoes evaporation, 30% enters the gas flow by the entrainment mechanism, and 49% reaches the end of the combustion chamber as a flowing liquid layer. When increasing the oxidizer mass flux in the port, the effect of entrainment increases while that of the flowing liquid layer along the surface shows a relatively lower contribution. Yet, the latter is predicted to have a significant contribution to the overall mass loss. In practical applications it may cause reduced combustion efficiency and should be taken into account in the motor design, e.g., by reinforcing the paraffin fuel with different additives. The model predictions have been compared to experimental results revealing good agreement.

Polymer Combustion as a Basis for Hybrid Propulsion: A Comprehensive Review and New Numerical Approaches

Hybrid Propulsion is an attractive alternative to conventional liquid and solid rocket motors. This is an active area of research and technological developments. Potential wide application of Hybrid Engines opens the possibility for safer and more flexible space vehicle launching and manoeuvring. The present paper discusses fundamental combustion issues related to further development of Hybrid Rockets. The emphasis is made on the two aspects: (1) properties of potential polymeric fuels, and their modification, and (2) implementation of comprehensive CFD models for combustion in Hybrid Engines. Fundamentals of polymeric fuel combustion are discussed. Further, steps necessary to accurately describe their burning behaviour by means of CFD models are investigated. Final part of the paper presents results of preliminary CFD simulations of fuel burning process in Hybrid Engine using a simplified set-up.

INVESTIGATION OF PARAFFIN-BASED FUELS IN HYBRID COMBUSTORS

Hybrid propulsion is a promising alternative for space propulsion; however, due to the low regression rate and hence low thrust levels of the classic polymeric fuels, new alternatives are sought. In recent years, liquefying, paraffin-based fuels have been investigated for their much higher regression rate compared to that of polymeric fuels. This paper presents experimental and theoretical research conducted at the Technion. Both plain paraffin fuel and paraffin-polymer mixtures have been tested. Plain paraffin exhibits a high regression rate but poor mechanical properties. The addition of polymer improves the mechanical properties but lowers the regression rate compared to plain paraffin fuel. Nevertheless, it indicates a promising direction to pursue a relatively high burning rate fuel of good mechanical properties. A model accounting for heat and mass transfer in the presence of liquefying fuel has been developed to predict the regression rate of the paraffin-based fuel. The model developed includes an additional mass loss mechanism, i.e., the liquid melt flowing along the grain. It is shown that this mechanism might have a noticeable effect on the overall regression rate and a potential for decreasing system performance. The test results exhibit good correlation with the model predictions.

RECENT ADVANCES IN HYBRID PROPULSION

The idea of the hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, producing a design that minimizes the chance of a chemical explosion. While the hybrid enjoys many safety and environmental advantages over conventional systems, large hybrids have not been commercially viable. The reason is that traditional systems use polymeric fuels that evaporate too slowly, making it difficult to produce the high thrust needed for most applications. Research at Stanford University and Space Propulsion Group (SPG) has led to the development of paraffin-based fuels that burn at regression rates 3-4 times that of polymeric fuels. Under the action of the oxidizer flow, the new fuels form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid-gas interface can substantially increase the rate of fuel mass transfer, leading to a much higher surface regression rate than can be achieved with a conventional fuel. To demonstrate the use of these fuels, a series of scale-up tests using several oxidizers has been carried out on intermediate-scale motors. The data from these tests are in agreement with small-scale, low- pressure, and low-mass-flux laboratory tests and confirm the high regression rate behavior of the fuels at chamber pressures and mass fluxes representative of commercial applications. Recently, SPG has developed a new class of oxidizers based on refrigerated mixtures of N2O and oxygen. The mixtures combine the high vapor pressure of dissolved oxygen with the high density of refrigerated N2O to produce a self-pressurizing oxidizer with high density and good performance. The combination of these technologies leads to a hybrid rocket design with reduced system size and mass.

Correlation between burning surface temperature and regression rate for polymethylmethacrylate

Prediction of the regression rate of polymeric fuels is important, especially for the optimal design of solid fuel ramjets or hybrid rocket motors. The burning surface temperature and the surface activation energy in the Arrhenius pyrolysis equation are required for prediction of the regression rate. As a typical polymeric fuel, polymethylmethacrylate (PMMA) was chosen for the present study. The burning surface temperature of PMMA was measured for both end-burning and sandwich-burning experimental devices. In the end-burning experiment, the oxygen flow impinged on the top of the specimen to burn the PMMA. In the sandwich-burning experiment, PMMA sheets were sandwiched between AP pellets. Temperature profiles near the burning surface were measured with a thermocouple embedded in the specimen. Simultaneously, the emergence of the thermocouple from the burning surface was determined with a video camera. The burning surface temperature obtained in the present study was found to be almost independent of the regression rate. The surface activation energy of PMMA was more than 1000 kJ/mol. Our measured surface temperature or the activation energy differs from the values measured with a thermocouple in previous studies. This difference probably is a result of mistaken identification of the burning surface temperature in the previous studies, because it was found that the sudden change or tiny plateau of the slope, in the temperature profile of PMMA, did not signal the burning surface.

Scale-Up Tests of High Regression Rate Paraffin-Based Hybrid Rocket Fuels

Recent research at Stanford University has led to the identification of a class of paraffin-based fuels that burn at surface regression rates that are three to four times that of conventional hybrid fuels. The approach involves the use of materials that form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid-gas interface substantially increases the rate of fuel mass transfer, leading to much higher surface regression rates than can be achieved with conventional polymeric fuels. Thus, high regression rate is a natural attribute of the fuel material, and the use of oxidizing additives or other regression rate enhancement schemes is not required. The high regression rate hybrid removes the need for a complex multiport grain, and most applications up to large boosters can be designed with a single port configuration. The fuel contains no toxic or hazardous components and can be shipped by commercial freight as a nonhazardous commodity. At the present time, grains up to 0.19 m [19.1 cm (7.5 in.)] in diameter and 1.14 m [114.8 cm (45.2 in.)] long are fabricated in a general-purpose laboratory at Stanford University. To further demonstrate the feasibility of this approach, a series of scale-up tests with gaseous oxygen have been carried out using a new Hybrid Combustion Facility (HCF) at NASA Ames Research Center. Data from these tests are in agreement with the small-scale, low-pressure, and low mass flux laboratory tests at Stanford University and confirm the high regression rate behavior of the fuels at chamber pressures and mass fluxes representative of commercial applications.

Theoretical prediction of piloted ignition of polymeric fuels in microgravity at low velocity flows

A numerical model developed for the prediction of the piloted ignition delay of solid polymeric materials exposed to an external radiant heat flux is used to predict the ignition delay and critical heat flux for ignition of solid fuels in microgravity at low velocity flows. The model considers the coupled thermochemical processes that take place in the condensed phase, including oxidative and thermal pyrolysis, phase change, radiation absorption, and heat and mass transfer in a multi-phase and multi-composition medium. Ignition is considered to occur when a critical pyrolysate mass flow rate is reached at the sample surface. Microgravity experimental surface temperature and ignition delay data previously obtained in a KC-135 aircraft are used to infer, in conjunction with the theoretical analysis, the critical mass flow rate for ignition. This value is then used to predict the ignition delay as a function of the external radiant heat flux, and the critical heat flux for ignition. Calculations are made for Polymethylmethacrylate (PMMA) and a Polypropylene/Fiberglass composite at airflows of 0.09 and 0.15 m/s under microgravity conditions and at 1.0, 1.75 and 2.5 m/s under normal gravity. The experiments and theoretical predictions show that the ignition delay and critical heat flux for ignition decrease as the forced airflow velocity decreases. It is predicted that at the tested lower velocities, the critical heat flux for ignition is close to half the value measured in normal gravity. The results have important implications since they indicate that materials could ignite easier under the conditions expected in spacecraft, and consequently stricter design specifications may be needed for fire safety.

Influence of structure of polymeric fuels on the combustion behaviour of composite solid propellants

This is a first report in which the influence of the structure of polymeric binders, such as molecular weight ( M), end-group functionality and cis-trans proportion, on the combustion behaviour of composite solid propellants (CSP) has been examined. In admixture with ammonium perchlorate (AP), dicarboxylic acids were used to simulate the commonly used telechelic polymers such as carboxy-terminated polybutadiene (CTPB) fuels. The burning rate ( r ̇ ) varies exponentially with the M of the acids and embraces the values for CTPB. The end-group effect diminishes at higher M due to dilution of the acid groups. The dependence of r ̇ on M is expressed as a newly defined molecular weight sensitivity of burning rate ( σ m). The σ m values can be divided into three regimes: I (118 < M < 150), II (150 < M < 200) and III ( M > 200) for polymeric fuels. The σ m values in regimes I and II are explained by the selective tendency for anhydride formation; in regime III, fragmentation of the polymeric fuel occurs. In regimes I and II, volatilization of the acids occurs at lower temperatures than for their neat form and in regime II this volatilization is limited by the phase transition of AP. The volatilization rate decreases with increase in M, but the ratio of the condensed-phase to gas-phase heat release increases with M.

Fuel Regression Mechanism in a Solid Fuel Ramjet

AbstractA model relating the combustion characteristics, particularly the regression rate, to the fuel properties in a solid fuel ramjet (SFRJ), is presented. The analysis is based on inspecting the different phenomena (e.g., chemical kinetics and heat transfer) involved in the fuel decomposition process in order to determine the rate controlling steps, and modeling the important mechanisms. Model predictions were compared to experimental findings for four polymeric fuels, showing excellent qualitative agreement in classifying different fuels, as well as good quantitative predictions of the fuel regression rates. The model can serve as a useful tool for the selection of appropriate fuels, as well as for preliminary design of the engine and mission profile prior to static firings of the final SFRJ prototype.

Soot From Fires: II. Mechanisms of Soot Formation

The mechanisms described for soot formation have been reviewed. It is clear that no one simple mechanism can yet totally describe the complex processes in volved. It has been well established that ions have an important role in the nucleation stage, but controversy still surrounds the extent of their importance as precursors of soot. In the case of polymeric fuels, the gaseous fuels produced by thermal breakdown of the polymer are small organic molecules, which will then, generally, produce soot by mechanisms similar to those for fuels originally gaseous.

Recycling of polyolefinic plastics by partial oxidation

A process is described by which polyolefinic wastes are converted into a gas mixture containing hydrocarbons (mainly methane, ethylene and ethyne) in relatively high concentrations. The process consists in the partial oxidation of the polyolefins; the molten polyolefinic material is atomized with a plain-jet atomizer and converted in a spray flame with an amount of oxygen not sufficient for complete combustion. In this way part of the polyolefin is pyrolysed mainly to methane, ethylene and ethyne. The process of partial oxidation was investigated for several model fuels: a mixture of C 14-C 17 n-alkanes, atactic polypropylene, isotactic polypropylene and polyethylene. Measurements of the composition of the flame gases with variations in the C : O ratio of the feed and the atomizing conditions are reported. The results show that with optimal reaction conditions up to 43% of the carbon contained in the feed can be converted into C 2 hydrocarbons in the case of n-alkanes, whereas up to 18% is converted into methane. For the polymeric fuels about 30% of the carbon is converted into C 2 hydrocarbons.

Quantification of threat from a rapidly growing fire in terms of relative material properties

AbstractEquations have been developed which give the time available for escape or rescue, i.e., the time interval between detection and blockage of the escape route by smoke, heat or toxic gases. Alternative assumptions are explored concerning exponential vs power law fire growth and an extended fire plume vs uniform filing of the building. The equations are developed in such a form that the threat variable by which the fire is detected is not necessarily the same threat variable which first blocks the escape route. A number of interesting results have been obtained, and numerical values of key parameters measured in various test fires at Factory Mutual Research are tabulated. It is shown that for many polymeric fuels smoke will block the escape route well before temperature or toxicity becomes excessive. In such cases, if the fire, assumed to be growing exponentially, is detected by its smoke, the detector being located in the escape route, then the escape time, surprisingly, is independent of the smokiness of the material as well as the size and shape of the building. It is determined only by the growth rate constant (doubling time) of the fire and the sensitivity of the detector.