Solid Fuel Ramjet Research Articles

Characterization of a Cavity Flameholding Solid-Fuel Ramjet in an Optical Combustor

Three fuel grain geometries were investigated in a two-dimensional, optically accessible solid fuel ramjet combustor to improve the understanding of the internal ballistics as it pertains to flameholding and overall performance. Two-cavity flameholding-configured fuel grains demonstrated increased flameholding and similar performance to the baseline flat fuel grain. Optical diagnostics such as high-speed CH* chemiluminescence and high-speed three-color camera pyrometry were utilized to measure and analyze the emissive species to draw conclusions about the reacting flowfield. Additionally, optical access afforded the ability to determine spatially resolved regression rates within the flameholding region of each fuel grain. The measured fuel grain regression rates demonstrated similar performance between the three configurations; however, the spatial distribution of the regression rates resulting from the high local heat flux due to the cavity corner is postulated to be the driving factor in increasing the flameholding capability of the cavity fuel grains.

Characterizing and predicting bluff-body solid fuel ramjet performances via shape design and multi-objective optimization model

In this work, we propose a rapid optimization approach to examine its application potential for the design and performance prediction and optimization of a solid fuel ramjet (SFRJ) with a bluff body. For this, the shape of the bluff body is parameterized first using the non-uniform rational B-spline method. We then develop a model for predicting SFRJ performances by incorporating both levy motion-gradient descent and support vector regression methods. It is found that a faster prediction is achievable, while the average error is maintained to be less than 5%. We then develop a multi-objective optimization model by considering the full thrust and minimum total pressure loss (TPL). The optimization model is examined using the non-dominated sorting genetic algorithm. A cost parameter is also created to facilitate the tradeoffs between the thrust and TPL in the Pareto front, when different bluff-body design configurations are considered. The present results reveal that an increase in the cost parameter will elevate the turbulence intensity within the SFRJs while drawing the incoming air closer to the fuel surface, resulting in an increase in thrust and regression rate, but the TPL will also increase. When prioritizing the TPL reduction in the design stage, the optimized solution reduces TPL by 50%. Meanwhile, the net thrust is shown to be decreased by less than 3.5%. Furthermore, flow-field investigation reveals that the improved performance of the optimized SFRJ is due to more uniform flow velocity gradients around the bluff body and a reduced rear vortex, resulting in reduced momentum loss. Our proposed optimization approach's robustness has been further confirmed with consistent performances, as the ramjet inlet speed varies over a broad range. It shows that our approach has great potential to be applied for the SFRJ performance prediction and optimization, being operated under various conditions.

Solid-Fuel Ramjet Regression Rate Measurements Using X-Ray Radiography and Ultrasonic Transducers

The instantaneous fuel regression rate within a solid-fuel ramjet combustor was characterized using X-ray radiography and ultrasonic transducer measurements. Experiments were performed with cylindrical center-perforated hydroxyl-terminated polybutadiene fuel grains at three mass fluxes () with consistent inlet total temperatures and chamber pressures. Ultrasonic transducer measurements demonstrated changes of web thickness ranging from 7.50 to 9.85 mm and regression rate measurements ranging from 1.35 to . The local maxima of change in the web thickness due to flow reattachment and erosive burning were consistently measured with the ultrasonic transducers. Changes in the port radius on the order of 8–9 mm and regression rates of approximately were deduced from the X-ray radiography images. The structure of the flow reattachment region was evident in measurements from the X-ray radiography images captured near the combustor entrance, whereas images captured at the midlength of the combustor exhibited more uniform fuel regression profiles. Ultrasonic measurements of change in the web thickness were consistently greater in magnitude relative to X-ray radiography measurements. X-ray radiography imaging allowed for the more accurate measurement of fuel regression with the greatest axial spatial resolution, whereas ultrasonic transducer measurements yielded the greatest radial spatial resolution. The change in web thickness calculated with weight-based techniques yielded smaller-magnitude measurements of change in the web thickness relative to X-ray radiography. The regression rate was largely invariant with the mass flux within the investigated operating regime.

Trajectory Analysis of a Solid Fuel Ramjet Propelling an Artillery Shell

This paper intends to estimate the possible range of a 155 mm artillery shell propelled using a ramjet in a fuel-rich propellant configuration. It also intends to obtain an optimum design of ramjet to maximize the range. A six degree of freedom trajectory simulation was developed to obtain the trajectories of the shell with a ramjet, which dynamically evaluates the off-design performance of the ramjet during the flight. A fuel-rich propellant was taken from literature for evaluating the performance of the ramjet. Various trajectories of shell were obtained at different initial thrusts and A/F ratios and with a constant combustion efficiency of 0.95. Results showed that with a 39-calibre gun, the range enhancement of around 105% compared to the current range of 24 km was obtained with a 155 mm artillery shell without any base bleed unit. This was achieved for the ramjet providing thrust equal to 1.2 times the drag at sea level with an initial A/F ratio of 11.

Oxidation reaction kinetics of HTPB-boron carbide/polytetrafluoroethylene formulations as a solid fuel

A solid-fuel ramjet engine powered by boron carbide (B4C) can generate nearly the same amount of energy as boron. However, the inherent oxide layer on the B4C surface restricts the energy released during its oxidation. In this study, we examine the effect of polytetrafluoroethylene (PTFE) on the oxidation of B4C and the removal of the oxide layer during the oxidation reaction. The B4C/PTFE binary composite powder was prepared using a ball milling process at three different concentrations (5:20, 10:20, and 15:20). Hydroxyl-terminated polybutadiene (HTPB) loaded with binary composite powder was manufactured by vacuum casting technique. The pure HTPB and HTPB/PTFE fuels were manufactured as reference formulations. The B4C/PTFE binary composite powder was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), and high-resolution scanning electron microscope (HRSEM). The thermal oxidation characteristics of prepared fuel formulations were studied using the thermogravimetric technique. The kinetics of the oxidation reaction was studied using both isothermal and non-isothermal methods. The thermogravimetric curves showed that the oxidation reaction of HTPB-based fuel occurred in four steps. When B4C/PTFE binary composite powder is added to HTPB, it decomposes faster and at a higher rate. The decomposed fluorine species of PTFE significantly improved the oxidation reaction of composite powder and increased the energy release rate of B4C. The kinetic studies of oxidation suggested that the addition of B4C/PTFE binary composite powder into HTPB lowered the activation energy (Ea: S1 > S2 > S3 > S4 > S5) required to prompt the oxidation reaction. Based on thermal and kinetic results, a potential oxidation promotion mechanism of B4C/PTFE composite powder was proposed. The initial stages of B4C oxidation resulted in a glassy layer of B2O3 oxide forming at the interface of B4C and PTFE powder. As a result of the oxidation reaction between B4C/PTFE and O2, CF4, Trifluoroboron (BF3) and CO2 were generated. At high temperatures, the oxide shell ruptured, allowing the O2 to diffuse into the core of the B4C particle and releasing a large amount of heat energy.

Regression and Flame Structure in Cavity Flameholding Solid-Fuel Ramjet Fuel Grains

Introducing cavity flameholders into a solid-fuel ramjet fuel grain demonstrated increased fuel loading with sustained combustion in previously unfavorable geometries. Volumetric fuel loading improvements of up to 26% were demonstrated to sustain combustion. Regression patterns of cavity fuel grains are presented and show that the effect of implementing a cavity flameholder is to change the location of maximum regression and the reattachment point. The addition of a cavity flameholder does not appear to have a significant effect on combustion efficiency. However, it is noteworthy that longer cavities increased the chamber pressure above what was observed for a center-perforated fuel grains as a result of the increased mass addition and higher equivalence ratio associated with the higher regression rate. Large-eddy simulation computations were performed using a fourth-order discontinuous Galerkin finite element solver with a novel flamelet and progress variable formulation. The predictions agree well with the experiments and point to the increased heat transfer for longer cavities as the main flameholder mechanism. The larger heat feedback is supported by the formation of a stronger recirculation region, which leads to increased coherent fluctuations due to the transition between local and global instabilities.

Flame Dynamics in an Optically Accessible Solid Fuel Ramjet Combustor

Flow–flame interactions were investigated in an optically accessible solid fuel ramjet combustor. Experiments were performed with a single hydroxyl-terminated polybutadiene fuel slab located downstream of a backward-facing step in a rectangular chamber. To emulate flight-relevant combustor conditions, unvitiated heated air was directed through the combustion chamber with an inlet temperature of , chamber pressures of 450–690 kPa, and port Reynolds number of . To characterize the heat-release distribution and velocity field, 20 kHz -chemiluminescence and 10 kHz particle imaging velocimetry measurements were used. Comparison between the mean chemiluminescence images acquired at three flow conditions indicates reduction in flame height above the grain with increasing air mass flow rate. Dominant heat-release coherent structures in the statistically stationary flow are identified using the spectral proper orthogonal decomposition technique implemented on time series of instantaneous images. The spatial mode shapes of the chemiluminescence and velocity field measurements indicated that the flow–flame interactions were dominated by vortex shedding generated at the backward-facing step in the combustor, at Strouhal numbers of 0.06–0.10. The frequency corresponding to these modes is shown to be invariant of air mass flux, indicating that system dynamics are primarily dependent on the backward-facing step geometry and the bulk velocity in the combustor.

Performance of a Solid-Fuel Ramjet Combustor with Bypass Air Addition

The influence of bypass air addition on the performance of a solid-fuel ramjet combustor was investigated. Experiments were performed with cylindrical center-perforated hydroxyl-terminated polybutadiene fuel grains and unvitiated heated air at flight-relevant pre-ignition combustor inlet total temperatures. The core air mass flow rate was kept constant, whereas the bypass ratio was varied from 0 to 30% with consistent pre-combustion chamber pressures. The average regression rate and mass flow rate of the fuel were greatest for the 0% bypass ratio case. The regression rate decreased for the 30% bypass ratio and for cases in which carbon black was not added to the fuel grain. Increasing the mass flow rate of air bypassed to the secondary combustion zone reduced the overall chamber pressure. The thrust increased with the increasing bypass ratio. The effect of bypass air on the chamber pressure yielded lower characteristic velocities at the 30% bypass ratio than at the 0 and 15% bypass ratios. The combustion efficiency of the device was maximized for the 15% bypass ratio. The key outcomes of the bypass air addition are to lower the global equivalence ratio and to reduce the combustion chamber pressure such that there is a nonzero bypass ratio for which the combustion efficiency is maximized.

Experimental study of hexamethylenetetramine gasification at different temperatures of gas flow

The work is devoted to the simulation of the propulsion system. A low-temperature gas generator might be included in the solid-fuel ramjet to produce high-calorific gases. The gasification of a solid porous hexamethylenetetramine (urotropine) during the filtration of high-temperature gas through it was experimentally studied. It is shown that with an increase in the initial temperature of the filtered gas, the time of urotropine gasification decreases, the intensity of the fuel gasification process increases, which leads to an increase in the flux of urotropine gasification products. In the temperature range of 450–500 K, an intensive gasification of urotropine occurred, while the temperature of the outgoing gaseous products changed slightly. An increase in the inlet gas temperature from 650 to 850 K led to an increase in the mass rate of urotropine gasification from 0.23 to 0.69 g/s, as well as an increase in the flux of urotropine gasification products from 0.31 to 0.93 g per 1 g of nitrogen. At temperatures below 800 K urotropin only sublimates. At higher gasification temperatures not only sublimation occurs, but also decomposition of urotropin to simpler gaseous products (mainly hydrogen).

Proper orthogonal and dynamic mode decomposition analyses of nonlinear combustion instabilities in a solid-fuel ramjet combustor

In ramjet propulsion systems, syelf-excited combustion instability is highly undesirable. It may lead to violent structural vibration and even mission failure. For this, self-excited combustion instabilities in such SFRJ are investigated numerically by 2D unsteady Reynolds-Average Navier-Stokes (URANS) model and varying the inlet mass flow rate ṁair. When combustion instability occurs, large-amplitude temperature, velocity, and reacting species wavy motions are observed along the axial flow direction. To shed light on the flow features and the energy distribution among the unstable eigenmodes, proper orthogonal (POD) and dynamic mode decomposition (DMD) analyses of nonlinear combustion instabilities in a solid-fuel Ramjet (SFRJ) combustor are conducted. The POD studies reveal that the fluctuation energy of the first six modes contributes to 99% of the total fluctuation energy of all modes. It means that the main features of the SFRJ combustors could be captured by using the first six modes. Unlike POD studies, DMD analysis captures the frequency information and spatial structures in the entire flow field. Examining the DMD growth rate reveals that all dominant DMD eigenmodes are marginally stable (growth rate = 0) or unstable (positive growth rate). And the frequency of each dominant mode could be enhanced by 3–10 Hz as theṁair is increased by 0.2 kg/s. The mode energy decreases with the increase of the mode order, with the decrease up to around 90%. The DMD analysis on velocity field indicated that the flow separation could be observed near the backward facing step. And the DMD mode structure of the temperature field can significantly affect the mode structures of Mach number and YCO2 field. In general, the present work provides detailed analyses of the SFRJ flow field in the presence of self-excited combustion instability by using POD and DMD approaches.

Effects of new solid propellant configurations on the combustion characteristics of a ramjet

The solid-fuel ramjet (SFRJ) motor is the most favorable air-breathing propulsive system and a suitable option for extending the speed and range of some military applications as in ramjet-powered missiles. However, SFRJ encountered severe challenges associated with low regression rate, as this latter plays an important role on the performance of the solid-fuel ramjet motor. To improve the solid-fuel ramjet's regression rate, this paper proposes new configuration which employs two solid fuel tubular grains with keeping overall simplicity of classic SFRJ motor, then the new design is numerically investigated by a developed in-house CFD code. The developed code is used to estimate Reynolds-averaged Navier–Stokes equations of reacting, unsteady, turbulent, and swirling flow which is validated and verified for several test cases. The numerical results for proposed designs are compared with classic SFRJ. The results reveal significant increase in solid fuel regression rate and heat transferred into grain's interior surface. Moreover, the thrust, specific impulse, and characteristics velocity are increased as well.

Numerical Investigation of Inlet Thermodynamic Conditions on Solid Fuel Ramjet Performances

In this work, 2D numerical RANS (Reynolds Average Navier-Stokes) simulations were carried out to investigate the thermodynamic performance of a solid fuel ramjet (SFRJ) with different inlet conditions. This is achieved by using an in-house FORTRAN code to simulate a 2D turbulent, reacting, unsteady flow in the ramjet engine. The inlet conditions are characterized by three key parameters: (1) swirl number ( S N ), (2) mass flow rate ( m ̇ air ), and (3) inlet temperature ( T in ). With the code numerically validated by benchmarking with a number of computed cases, it is applied to perform systematic studies on the turbulent flow recirculation, combustion, and heat transfer characteristics. It is found that increasing S N , m ̇ air , or T in can dramatically enhance the combustion heat release rate, regression rate, and combustor average temperature. Furthermore, the analysis on the chemical reaction intermediate (CO) reveals that the chemical reaction is more sufficient with increased m ̇ air , but S N = 0 . In addition, a secondary vortex is generated at the corner of the backward facing step in the presence of a swirl flow resulting from the instability of the shear layer. Finally, the nonlinear correlations between the heat transfer, combustion characteristics, and flow field characteristics and the corresponding inlet thermodynamic parameters are identified.

ОПРЕДЕЛЕНИЕ СРЫВНЫХ ХАРАКТЕРИСТИК В ФОРКАМЕРЕ КОМБИНИРОВАННОГО ПРЯМОТОЧНОГО ВОЗДУШНО-РЕАКТИВНОГО ДВИГАТЕЛЯ НА ПОРОШКООБРАЗНОМ АЛЮМИНИЕВОМ ГОРЮЧЕМ С УЧЕТОМ ТОРМОЖЕНИЯ ВОЗДУХА В ВОЗДУХОЗАБОРНОМ УСТРОЙСТВЕ

The paper considers a combined ramjet engine powered by powdered aluminum fuel . The prototype was a combined solid-fuel ramjet engine. The advantages of the engine under consideration are given. An example of the combustion of finely dispersed aluminum powder is considered, from which it follows that the initial temperature of the aluminum powder will affect the stall characteristics in the pre-chamber. The following characteristics of the PAF ramjet were determined: the temperature of air stagnation in the air intake device, the temperature of the mixture of aluminum powder and the stalled air flow, the excess air ratio, and the stall characteristics in the pre-chamber taking into account the air stagnation temperatures. All parameters are de-termined for engine operating altitudes equal to 0.5, 10 and 18 km. A comparison is made of the limiting flame propagation ve-locities under standard conditions and with an incident air flow. Based on the obtained values of the characteristics, the interval of values of the coefficient of air sampling from the inlet to the pre-chamber of the ramjet engine on the PAF was determined, corresponding to the maximum possible areas of the engine operating parameters. Using the obtained values of characteristics, the combustion process of powdered aluminum in the pre-chamber of a ramjet engine takes place without flame blowout, which excludes unstable operation of the propulsion system as a whole. The use of powdered aluminum makes it possible to regulate the thrust in a wide range of values, and a high initial temperature of the air entering the pre-chamber for repeated switching on and off of the engine. Based on the available data, the type of engines under consideration is suitable for combat missiles of various classes, but the most suitable for aircraft-based missiles.

Swirling effect on thermodynamic performance in a solid fueled ramjet with paraffin-polyethylene

In this work, systematic firing tests have been conducted to evaluate the thermodynamic performances and combustion behaviors of Solid Fuel Ramjet (SFRJ) fueled by paraffin-polyethylene blends fuel. The experiments are carried out via connected pipe facility. And three key parameters are identified, i.e. 1) fuel paraffin concentration, 2) swirl intensity (denoted by swirling number), and 3) solid fuel ramjet combustor geometry. These parameters are experimentally investigated one at a time. To characterize the propulsion system performance, the static temperature, pressure, and engine thrust data are logged in real-time. The time-averaged regression rate is obtained via a three dimensional scanner. A high-speed camera is employed to capture the flame shapes. The acquired pressure data from the engine is further analyzed by using Fast Fourier Transform to obtain its frequency spectrum and acoustical energy distribution over the frequency range of interest. The present results show that increasing the swirl number can increase the regression rate, combustion efficiency, and total thrust. However, increasing the paraffin concentration has a positive effect on the regression rate, but negative effect on the combustion efficiency. In addition, the regression rate is found to be inversely proportional to the inlet diameter and port diameter. In general, the present work shed lights on the thermodynamic performances of a SFRJ fueled by paraffin-polyethylene blends.

An Accurate Performance Prediction of Solid Fuel Ramjets Using Coupled Intake-Combustor-Nozzle Simulation

ABSTRACTThe performance of the solid fuel ramjet is accurately predicted using full part simulation of this propulsion system, where the flow fields of the intake, combustion chamber, and the nozzle are numerically studied all together. The conjugate heat transfer is considered between the solid phase and the gas phase to directly compute the regression rate of the fuel. The finite volume solver of the compressible turbulent reacting flow is utilized to study the axisymmetric three dimensional flow fields, and two blocks are used to discretize the computational domain. It is shown that the combustion chamber's pressure is changed due to the fuel flow rate's increment which must be taken into account in predictions. The results demonstrate that omitting the pressure dependence of the regression rate and also the effect of the combustor's inlet profile on the regression rate, which specially exists when simulating the combustion chamber individually, under-predicts the solid fuel burning rate when the regression rate augmentation technique is applied to improve the performance of the solid fuel ramjets. It is also illustrated that using the inlet swirl to increase the regression rate of the solid fuel augments considerably the thrust level of the considered SFRJ, while the predictions without considering all parts of the ramjet is not accurate.

Combustion Characteristics of Paraffin-Polyethylene Blends Fuel for Solid Fuel Ramjet

AbstractIn this paper, a series of firing tests have been conducted via a connected pipe facility to investigate the influences of the paraffin blends fuel on combustion characteristics of solid-fu...

Internal Ballistics of a Boron-Containing Solid Fuel Ramjet

ABSTRACT The solid fuel ramjet is one of the simplest air-breathing engines, characterized by increased specific impulse. This study focuses on a parametric investigation of the internal ballistics of a boron-containing solid fuel ramjet. A 2-D axisymmetric model of a ramjet combustor flying at Mach 2.5 was developed and solved numerically utilizing a commercial computational fluid dynamics code modified to address the specific issues of the problem. The simulation results were compared to the theoretical performance calculated by the chemical equilibrium code. The solid fuel regression rate was found to depend on the port diameter and thus decreases with time. Thrust regulation of ramjet engine was demonstrated using bypass air. The specific impulse was found to decrease with boron content, probably because of low combustion efficiency caused by either insufficient oxygen supply or short residence time of the boron particles.

Numerical Investigation of Effect of Combustor Length on Combustion Characteristics of Solid Fuel Ramjet

In this paper, the effect of solid fuel ramjet (SFRJ) combustor length on combustion characteristics with swirling turbulent reacting flow is investigated via numerical approach. An in-house code written in FORTRAN is employed to simulate the 2-dimensional turbulent reacting swirling flow in the combustor of SFRJ. The results indicate that increasing the combustor length will increase the temperature near the backward-facing step and the afterburning chamber and enhance the local heat flux at the reattachment point.

Mechanism of Stabilization of the Combustion in the Duct of the Solid Fuel Grain of a Solid Fuel Ramjet

The mechanism of stabilization of the combustion in the duct of the solid fuel grain of a solid fuel ramjet with a flame holder was analyzed for the first time. The flame holder can be either a shoulder in the inlet part of the duct, or an autonomous gas generator running on solid fuel capable of self-sustained combustion. A mathematical model for evaluating the stability of the combustion in the solid fuel ramjet grain duct was developed. It was shown that there are several parameters characterizing the solid fuel grain and the solid fuel ramjet as a whole that affect the stability of the operation of the solid fuel ramjet. The main control parameters are the temperature of the combustion products in the flame holder and the flow rate of the combustion products from the flame holder (which can be controlled by varying the sizes and shape of the flame holder), and also the degree of turbulence of the air flow in the solid fuel grain duct. The stability of the combustion in the duct of the solid fuel grain of the solid fuel ramjet was studied, and the possible modes of the operation of the solid fuel ramjet while varying the control parameters were determined.

Numerical Study of Micron-Scale Aluminum Particle Combustion in an Afterburner Using Two-Way Coupling CFD–DEM Approach

Metal additives have positive effects on the flow field such as increasing the temperature and thrust, while this may decrease local gaseous velocity due to inter-phase drag forces on the other hand. An in-house computational solver was developed to study the typical multiphase flow with combustion. The solver relies on Eulerian–Lagrangian model and the two-way coupling approach. Numerical simulations were carried out on an afterburning chamber of a solid-fuel ramjet to study the impact of using aluminum particles as metal additive. For micron-scale aluminum particles, the injection area, initial temperature, diameter and mass flow rate are varied influence factors, while the afterburning chamber and inlet primary high-temperature gases are fixed for all the test cases. It was found that better particle dispersion can result in higher combustion efficiency due to increasing the gas-particle contact area as well as better heat transfer and diffusion. Injecting aluminum particles with higher initial temperature and smaller diameter tend to get ignited faster and then burn more easily, which means releasing more heat in the length-limited flow field. However, the aluminum particle massflow rate has much more complicated comprehensive influence than the changing rules of the adiabatic combustion temperature based on the NASA CEA code.