Propellant Combustion Research Articles

A Discontinuous Galerkin–Finite Element Method for the Nonlinear Unsteady Burning Rate Responses of Solid Propellants

The unsteady combustion of solid propellants under oscillating environments is the key to understanding the combustion instability inside solid rocket motors. The discontinuous Galerkin–finite element method (DG-FEM) is introduced to provide an efficient yet flexible numerical platform to investigate the combustion dynamics of solid propellants. The algorithm is developed for the classical unsteady model, the Zel’dovich–Novozhilov model. It is then validated based on a special analytical solution. The DG-FEM algorithm is then compared with the classical spectral method based on Laguerre polynomials. It is shown that the DG-FEM works more efficiently than the traditional spectral method, providing a more accurate solution with a lower computational cost.

High-order adaptive time discretisation of one-dimensional low-Mach reacting flows: A case study of solid propellant combustion

Solving the reactive low-Mach Navier–Stokes equations with high-order adaptive methods in time is still a challenging problem, in particular due to the handling of the algebraic variables involved in the mass constraint. We focus on the one-dimensional configuration, where this challenge has long existed in the combustion community. We consider a model of solid propellant combustion, which possesses the characteristic difficulties encountered in the homogeneous or spray combustion cases, with the added complication of an active interface. The system obtained after semi-discretisation in space is shown to be differential–algebraic of index 1. A numerical strategy relying on stiffly accurate Runge–Kutta methods is introduced, with a specific discretisation of the algebraic constraints and time adaptation. High order is shown to be reached on all variables, while handling the constraints properly. Three challenging test cases are investigated: ignition, limit cycle, and unsteady response with detailed gas-phase kinetics. We show that the time integration method can greatly affect the ability to predict the dynamics of the system. The proposed numerical strategy exhibits high efficiency and accuracy for all cases compared to traditional schemes used in the combustion literature.

EXPERIMENTAL STUDY OF BORON CARBIDE FORMATION IN COMBUSTION OF BORON-CONTAINING SOLID PROPELLANTS

Using model aluminum dodecaboride (AlB<sub>12</sub>)/binder mixtures imitating the matrix of boron-containing solid propellants, the process of formation of boron carbide in combustion of such propellants has been experimentally studied. To characterize the samples under study, thermogravimetric analysis and differential thermal analysis were used, which made it possible to determine the initial stage of decomposition of samples with different contents of AlB<sub>12</sub>. Experiments on the rapid thermal decomposition of samples were carried out on the original URAN-1 installation, which makes it possible to create heat flows to the sample surface in the range of 70 to 300 W/cm<sup>2</sup>. It has been shown that the condensed residue formed during the pyrolysis of the binder consists mainly of carbon, which, upon interaction with boron, forms boron carbide. The dependence of the mass of the carbonaceous residue on the boron content in the propellant matrix (mixture of binder and powdered boron-containing fuel) has been determined. The mass rate of matrix pyrolysis has been studied as a function of the incident energy flux density and the content of powdered boron-containing fuel in the matrix. It is shown that during high-speed heating of the matrix, the amount of boron carbide formed strongly depends on the rate of energy supply. The dependence of the relative mass of the resulting boron carbide on the temperature in the pyrolysis zone was determined, which can be conditionally divided into four areas: (1) up to a temperature of 1400°C, boron carbide is practically not formed; (2) in the temperature range from 1400°C to about 1700°C, the amount of boron carbide formed increases sharply with increasing temperature from zero to about 10-15%; (3) in the temperature range from 1700°C to about 2500°C, the amount of boron carbide formed is slightly dependent on temperature; (4) for temperatures above 2500°C, the amount of boron carbide formed in the condensed products of the studied samples sharply increases with temperature, and at T ≈ 2600°C reaches 50% of the mass of the entire sample residue. A theoretical analysis of the investigated process has been carried out.

Modeling the process of slag formation in paste propulsion

The results of modeling the accumulation of slag residue in the combustion chamber of a propulsion system on pasty propellant obtained using a multiphase flow evolution model are presented. The influence of composition solutions on the process of slag formation is established; the significance of this process for the quality of such engines is shown. The nature of the influence of the parameters of the burning process for propellants of the type under consideration on the intensity of slag formation is determined. It is shown that the degree of involvement of metallic fuel in the agglomeration process has the greatest influence on slag formation. It has been established that the most effective "tool" for suppressing slag formation in the combustion chambers of paste propulsion is to change the regularities of the burning process, mainly associated with the formation of condensed products at the surface of the burning propellant.

Theoretical analysis on applying steam reforming to the primary combustion of the boron-based fuel-rich propellant

The primary combustion products of boron-based fuel-rich propellant contain a significant amount of condensed boron, which poses challenges for complete burnout of the primary combustion products during the secondary combustion process. To enhance the ignition and combustion characteristics of these primary combustion products, a pioneering theoretical analysis on applying steam reforming to the primary combustion of the boron-based fuel-rich propellant was conducted. The results demonstrate that steam reforming improves the ignition and combustion characteristics of the primary combustion products through three main aspects: optimizing the composition of combustion products, improving energy distribution between the primary combustion and secondary combustion process, and elevating combustion temperature, respectively. It theoretically confirms the feasibility of this innovative combustion organization method for solid rocket scramjet. Furthermore, it has been found that there is an optimal steam-fuel ratio (the value is 1.0 for B35), which facilitates the conversion of combustible components in the primary combustion products from a gas–solid state to a pure gaseous phase. Simultaneously, the temperature of combustion products after steam reforming reaches the highest. Therefore, theoretically speaking, the steam-fuel ratio of solid rocket scramjet with steam reforming should not exceed the optimal steam-fuel ratio to ensure better ignition and combustion characteristics of the combustion products after steam reforming. This analysis provides valuable theoretical support for the design of solid rocket scramjets with steam reforming.

Reduction of agglomeration effect by aluminum trihydride in solid propellant combustion

AbstractAluminum trihydride (AlH3) has gained considerable attention as a substitute fuel for aluminum in solid propellants. In this study, we conducted a systematic investigation to evaluate the effects of AlH3 content, ranging from 0 % to 18 %, on propellant ignition, combustion, and agglomeration. Experimental methods such as thermogravimetry−differential scanning calorimetry (TG‐DSC), laser ignition, high‐speed photography, and collecting condensed combustion products (CCPs) were employed. The results indicate that AlH3 significantly promotes the high‐temperature decomposition of ammonium perchlorate (AP). Meanwhile, the addition of cyclotetramethylene tetranitramine (HMX) in propellant does not affect the hydrogen release reaction of AlH3. As the AlH3 content increases from 0 % to 18 %, the spectral emission intensity of the propellants decreases, and the ignition delay time initially increases from 253 ms to 321 ms, and then decreases to 258 ms. Furthermore, the burning rate increases with increasing the AlH3 content, while the pressure exponent is reduced from 0.551 to 0.460. The inclusion of AlH3 in propellants significantly inhibits aluminum agglomeration near the burning surface. Additionally, as the AlH3 content increases, the mean particle size D43 of the CCPs decreases from 50.95 μm to 8.28 μm at 1 MPa. The agglomeration degree of aluminum is very weak at 7 MPa, especially when the AlH3 content exceeds 9 %. The conclusions drawn from this study can serve as valuable guidance for optimizing propellant formulations.

Improving the combustion efficiency and agglomeration of aluminum-water propellants via n-Al/CuO metastable intermolecular composites

In this work, we investigated the combustion and agglomeration characteristics of aluminum-water propellants by replacing the original Al with binary n-Al/CuO metastable intermolecular composites (MICs). Through laser ignition tests and thermogravimetric-differential scanning calorimetry, we evaluated the oxidation reactivity, ignition delay, burning rate, agglomeration properties, and condensed combustion products of aluminum-water propellants containing different CuO loadings. Compared with conventional aluminum-water propellants, the introduction of binary MICs is found to lower the initial temperature of Al and improve its oxidation activity. Both the burning and heating rates scale linearly with the CuO loading, increasing by a factor of 5 and 6, respectively, when the CuO loading reaches 5 wt.%. The combustion efficiency of the modified propellants is found to improve by 5−12 %. The mean size of the condensed combustion products drops from over 400 μm to around 200 μm due to MICs addition, indicating weakened agglomeration. The ignition delay time is slightly shortened, and the combustion intensity first increases but then decreases as the CuO loading increases. In summary, this work indicates that replacing Al with binary Al/CuO-MICs can significantly alter the combustion and agglomeration properties of aluminum-water propellants. The experimental data and insight from this work could help guide the development of advanced aluminum-water propellants for various propulsion and energy applications.

Polyphenols-inspired interface modification and reduced agglomeration of solid propellants via a functionalized fluorine-containing organic substance coating layer

Interface modification of aluminum particles plays a crucial part in reducing the agglomeration during the combustion of solid propellants. In this study, the aluminum (Al) particles are pretreated with gallic acid (GA) to get Al@GA with an adhesion-favorable surface, which facilitates the further coating procedure. One new fluorine-containing organic substance is prepared, namely p-HFBA. The p-HFBA further coats the Al@GA to produce Al@GA/p-HFBA. The interface between Al@GA and the coating layer p-HFBA forms AlF bonds and induces a pre-ignition reaction with an aluminum oxide shell. Hence, the agglomerate size of propellants containing Al@GA/p-HFBA significantly decreases from 176.30 μm to 84.72 μm. The agglomerates volume is reduced by approximately 97 %. Additionally, more gaseous products investigated by TG-MS provide the Al particles with greater impetus (aerodynamic forces, Fae), enabling them to leave the burning surface and reduce the residence time. The rise in Fae and drop in adhesion force (Fad) effectively reduce the agglomeration intensity according to the skeleton layer model. This work proves that interface modification can effectively enhance the combustion performance of aluminized solid propellants.

Role of Oxides of Iron on the Combustion of Composite Solid Propellants: A Review

The effects of iron oxides (α-Fe2O3, γ-Fe2O3, Fe3O4 in the different size ranges from micron to nano) on the combustion of composite solid propellants, also known as hydroxyl terminated polybutadiene/aluminium/ammonium perchlorate (HTPB/Al/AP) systems, are thoroughly reviewed in this work. The effect of the oxides of iron in (i) condensed phase; (ii) sub-surface and surface; (iii) premix flame (gas phase); and (iv) final flame (gas phase) in deflagration zone are also critically reviewed. The effect of catalyst on the lower pressure limit (LPL), upper pressure limit (UPL) and ignition delay is also studied as a part of the combustion phases. It is understood that unlike during the combustion of pure ammonium perchlorate, the role of iron oxides starts its influence on binder system degradation, binder-ammonium perchlorate reactions at surface and HClO4 decomposition in gas phase. The role of aluminium is to contribute for larger energies in both primary and diffusion flame zone, the heat generated accelerates the surface reactions further. Another interesting topic is nano-oxides of iron, since nanosize contribute for definite increase in burn rate, but the disadvantage of agglomerations both at mixing level and combustion level needs to be addressed. Several strategies are suggested, with the most significant ones being co-precipitation with ammonium perchlorate and the utilization of dispersion agents.

Effect of grain length on GOx-paraffin hybrid rocket engines performance and regression rate

An experimental campaign was carried out on a hybrid rocket engine, using gaseous oxygen and paraffin wax-based fuel, to investigate the effect of fuel grain length on motor performance and internal ballistics. The results of the current work offer the reader a comprehensive overview of the pros and cons of reducing the fuel grain length and the difficulties in optimizing combustion efficiency. Steady-state numerical analyses were also performed to support the experimental findings. The motivation for the current work originates from a previous explorative test campaign performed with a 220 mm fuel grain length where only fuel-rich conditions were experienced. A trade-off analysis was carried out by introducing two additional configurations with grain lengths of 130 mm and 70 mm, since the grain length directly affects the mixture ratio and engine performance. Propellant mixing, combustion instability, entrainment, and regression rate are the main topics discussed in the paper. The results show that the reduction of the grain length increases the mixture ratio close to the optimum value, but mixing and combustion instability issues are promoted. No significant effects of the grain length on the regression rate have been found, leading to a discussion on the physics of the mass transfer mechanism of paraffin wax.

A novel structure for improving the service life of a propulsion system by controlling two-phase reactive flow

Methods to control two-phase reactive flow induced by the combustion of propellant particles are important for improving the service life of mechanical systems. A novel groove structure was proposed to control the two-phase reactive flow. A two-phase flow model was utilized to predict propellant combustion in fluid fields and coupled with the transient heat transfer process. The numerical model was validated by comparing numerical and experimental results. Numerical results indicate that the groove structure effectively decreases the scouring velocity of two phases and the accumulation of particles near the barrel surface. The groove also reduces the peak temperature of the barrel surface, which contributes to less thermal erosion. Furthermore, shooting tests prove that the groove structure greatly extends the service life of the barrel. The findings of the study will be applied to improve the service life of propulsion systems.

The effects of Al (111) and Al2O3/Al (111) surfaces on the decomposition of ammonium perchlorate: A DFT-D study

Ammonium perchlorate (AP) is a crucial oxidizing agent in composite solid propellants, where aluminum (Al) is used as an additive to increase the energy density of composite solid propellants. The interaction between AP and Al significantly improves the ignition and combustion performance of propellants, but the mechanism of their interactions still lacks essential understanding, especially the interaction between AP and Al, as well as the Al surface containing aluminum oxide (Al2O3). To solve this problem, the adsorption and decomposition processes of AP and its decomposition products such as perchloric acid (HClO4) and ammonia (NH3) on the pure-Al (111) and Al2O3/Al (111) surfaces were studied by using the density functional theory (DFT). The effects of temperature and pressure on the adsorption and reactions were discussed. AP tends to adsorb on the pure-Al (111) surface with O-termination, while under the influence of Al2O3, AP is more inclined to adsorb on the surface of Al2O3/Al (111). At the low-temperature stage, the Al2O3 molecules on the surface of Al (111) promote the disengagement H of AP decomposition and the adsorption and decomposition of NH3, while at the high-temperature stage, the pure Al surface is more conducive to the AP disengagement H and NH3 decomposition. The increase in the pressure is in favor of NH3 adsorption, but the effect gradually decreases. HClO4 cannot adsorb on the pure-Al (111) and AP (001) surfaces, but it can adsorb on the surface of Al2O3/Al (111). In the gas phase, the decomposition of HClO4 priors to NH3. At the high temperature and pressure, the pure-Al (111) surface can contribute significantly to the adsorption and activation of NH3. The findings are very valuable for regulating the combustion of solid propellants.

Mussel-inspired PTW@PDA composites for developing high-energy gun propellants with reduced erosion and enhanced mechanical strength

The severe erosion and inadequate mechanical strength are prominent challenges for high-energy gun propellants. To address it, novel PTW@PDA composites was prepared by polydopamine(PDA)-modifying onto potassium titanate whisker(PTW, K2Ti6O13), and after was incorporated into gun propellant as erosion-reducing and mechanical-reinforcing fillers. The interfacial characterizations results indicated that as-prepared PTW@PDA composites exhibits an enhanced surface compatible with propellant matrix, thereby facilitating their dispersion into propellants more effectively than raw PTW materials. Compared to original propellants, PTW@PDA-modified propellants exhibited significant less erosion, with a Ti–K-based protective coating being detected on the eroded steel. And 0.5 wt% and 1.0 wt% addition of PTW@PDA significantly improved impact, compressive and tensile strength of propellants. Despite the inevitably reduction in relative force, PTW@PDA slightly increase propellant burning rate while exerting little adverse impact on propellant dynamic activity. This strategy can provide a promising alternative to develop high-energy gun propellant with less erosion and more mechanical strength.

Enhancing the Ignition and Combustion Performances of Solid Propellants Incorporating Al Particles Inside Oxidizers.

The combustion efficiency of Al plays a critical role in the combustion of high-energy aluminum-based solid propellants. For traditional formulations, the Al powders are dispersed in the binder matrix, leading to limited contact with the oxidizers and hence usually insufficient combustion and higher values of the pressure exponent. In this paper, various core-shell structural Al/oxidizer composites such as Al@HMX, Al@AP, and AP@Al have been prepared by a spray-drying technique based on which solid propellants with precise interfacial control between Al particles and oxidizers were realized. Compared to the control sample, the modified propellants have a greater heat of explosion of 5890 J g-1 (15% higher) and a reduced ignition delay time of 58 ms (65% decrease). Without changing the content of components, the burn rates of propellants can be easily modulated by tuning the interfacial contact of Al and oxidizers, where it varies in a wide range of 4.56-5.79 mm s-1 at the same pressure of 1 MPa. After introducing Al/oxidizer composites, the lowest pressure exponent of 0.19 within 1-15 MPa could be achieved by using Al@HMX and AP@Al composites. The agglomeration of Al was also inhibited by using Al/oxidizer composites, and the mechanism can be interpreted by using a classical "pocket" model. Moreover, the improved combustion efficiency of the solid propellants was verified by a noticeable reduction in the unreacted Al content.

3D trajectory, morphology and phase discrimination of burning Al/AP propellant particles with time-resolved astigmatic dual-beam interferometric particle imaging

This work implements astigmatic dual-beam interferometric particle imaging (ADIPI) in propellant combustion diagnostics to enable the measurement of the three-dimensional (3D) spatial position and particle morphology of opaque particles. The ADIPI allows the discrimination of different substances, such as metal droplets, flames, and pyrolysis residues. A general theoretical model based on the generalized Huygens-Fresnel integral and the ray transfer matrix model is developed to describe the identification of ADIPI signals for regular and irregular particles. An experimental system combining a 13 kHz time-resolved ADIPI with digital in-line holography (DIH) is built to enable simultaneous measurements of the propellant combustion field. The typical ADIPI signals presented by different substances in the combustion field are analyzed by comparing the experimental results of DIH. The time-resolved 3D trajectory reconstruction of burning particles and the measurement of irregular particle size and morphology are realized. Due to the effect of the flame, the average deviation of the depth locating of the particles is 1.4 mm with a standard deviation of 1.0 mm. The average deviation value of the depth position relative to the measured distance is 1%. In propellant combustion measurements, the motion of burning particles can be accurately determined, proving its feasibility and practicality.

Aluminum particle agglomeration characteristics and suppression method during the combustion of aluminum‐based solid propellants: A review

AbstractAluminum is widely used in solid rocket propellants to improve the energy performance of the propellant. However, during propellant combustion, aluminum particles may agglomerate, resulting in slag deposition, incomplete combustion of metal particles, increased two‐phase flow losses, and reduced motor specific impulse. Therefore, it is of great scientific significance and engineering value to understand the agglomeration mechanism of aluminum particles and to explore the method of agglomeration suppression. Based on this, this paper firstly briefly introduced the agglomeration process of aluminum particles during propellant combustion. Subsequently, factors influencing the agglomeration of aluminium particles were analysed. The influencing factors can be roughly divided into two aspects: propellant‘s own formulation and external environment. Immediately afterwards, the existing agglomeration suppression methods were emphasized and summarized and analyzed. Presently, agglomeration suppression methods can be broadly classified into four categories, namely, conventional formulation optimization, use of aluminum nanoparticles, and modification of aluminum particles using fluorine‐containing compounds or alloy particles. Finally, several directions that need to be focused on in the future are proposed to address the problems in the study of aluminum particle agglomeration suppression methods.

Influence of nickel powders on burning behaviors of single-based propellant in variable-pressure and constant-pressure combustion conditions

Single-based propellants containing 0.4 wt%, 0.8 wt% and 1.6 wt% nickel powders(NPs) were prepared and investigated in closed bomb tester and transparent combustion chamber, which can provide variable-pressure and constant-pressure conditions for propellant burning, respectively. In closed bomb tester, propellants burned to a pressure above 5–220 MPa. The results showed that burning rate of propellants was increased by 19.0 %, 12.5 % and 11.9 % in 5–8 MPa, 8–16 MPa and 20–200 MPa, when 1.6 wt% NPs used in propellant. Meanwhile, propellants burning in combustion chamber were with a constant pressure of 1.0 MPa, 1.5 MPa, 2.0 MPa, 3.0 MPa and 5.0 MPa. The burning rate of propellants can be increased by NPs in 1.0–5.0 MPa. While the catalytic efficiency(Z) had high dependence on NPs contents and burning pressure. At 5.0 MPa, 0.4 wt%, 0.8 wt% and 1.6 wt% of NPs enhanced burning rate of propellants by 42 %, 44 % and 68 % than that of original propellants. In pressure below 3.0 MPa, burning rate of propellants cannot be increased with 0.4 wt% and 0.8 wt% NPs, but that can be enhanced with 1.6 wt% NPs, suggested that higher NPs content and higher pressure were conducive to catalytic effect, which was significant with the pressure above 5 MPa. Furthermore, after NPs incorporating into propellant, the distance from flame zone to propellant burning surface was significantly reduced. And the potential catalytic mechanism was proposed.

A Metal Inorganic Framework Designed as a Propellant Burn Rate Modifier

A Metal Inorganic Framework Designed as a Propellant Burn Rate Modifier

Modeling transient combustion and regression behavior of NEPE propellant based on random particle packing

A sophisticated numerical framework has been proposed as a powerful tool to comprehend the thermal behavior of NEPE propellant during rapid pressure decay. This framework consists of two crucial components: a 2D propellant pack generation module based on the sequential algorithm and a refined combustion model code. To track the regression of the propellant surface, we employ a straightforward single-value equation of the Hamilton-Jacobi type. This equation enables us to model the movement and evolution of the propellant surface as it undergoes combustion. To validate the credibility of the simulations, comparisons between the simulated burning rate and predicted average surface temperature are made with experimental studies. Encouragingly, reasonable agreement has been achieved between the numerical predictions and experimental data, further affirming the reliability and effectiveness of the proposed numerical framework. Simulation results suggest that the flame structure transforms from a complex diffusion flame in the initial stage of this transient burning to a premixed flame with a heat core altering with time in the final stage of fast depressurization. The surface cores of coarse AP and HMX particles regress most slowly, while the interface between AP and HMX powders moves most fast. Regarding the regression of the propellant surface, it is observed that the surface core of coarse AP and HMX particles exhibits a slower regression rate, while the interface between AP and HMX powders experiences a faster movement. This discrepancy in regression rates underscores the heterogeneity of NEPE propellant combustion, where different components contribute differently to the overall regression process. Furthermore, the entire surface morphology undergoes significant changes during rapid depressurization and combustion in the atmospheric environment. This suggests that pressure changes alone are not the sole factors influencing the shape of the propellant surface. The presence of oxidizing particles within the propellant pack emerges as an important factor that influences the regression mechanism of the propellant surface. This investigation provides valuable insights into the dynamic behavior of NEPE propellant, shedding light on the underlying physical phenomena at play.

Field recovery from digital inline holographic images of composite propellant combustion base on denoising diffusion model.

Digital inline holography has gained extensive application in the optical diagnosis of solid propellant combustion. However, this method confronts several challenges. Firstly, the calculation time required for reconstruction and depth of field extension is excessively long. Secondly, the excessive smoke, airflow, and flame during combustion cause significant interference and poor reconstruction quality, which reduces the accuracy of particle identification. To address these issues, we have developed a holographic image reconstruction technique for aluminum particle combustion based on the Attention Mechanism, U-net, and Diffusion models. This approach enables end-to-end reconstruction of aluminum particle combustion holographic images, while effectively circumventing the interference of airflow combustion and flame.