High-speed Propulsion Systems Research Articles

Novel Quasi-One-Dimensional Analysis of a Choked Transonic Reacting Flow with Finite-Rate Chemistry

A numerically accurate method for solving quasi-one-dimensional transonic flow with a system of ordinary differential equations (ODEs) has been developed to estimate the performance of a high-speed propulsion system. Specifically, ramjet engines are thermally choked in the combustor region. for which the solution processes undergo a mathematical singularity problem. This singularity incurs various numerical difficulties because the derivatives are indeterminate at the sonic condition and the solution cannot march in the streamwise direction, requiring special numerical treatment. To overcome this issue, two solution processes are augmented in the ODE system: coordinate transform, and solution extrapolation from the equivalent subsonic system. An additional variable is defined to convert the spatial coordinate into a dependent variable that characterizes the transonic flow only by the solution curve passing through the critical point. After the transformation, solution variables are extrapolated from the subsonic solutions obtained with a predicted critical (sonic) condition via a forward iterative shooting method such that the flow properties are smoothly varying both upstream and downstream of the critical point. As a result, there is a good agreement in the predicted critical point with the theoretical location. Moreover, the new method successfully yields the transonic solution without any abrupt or discontinuous changes in flow properties including species fractions, which is almost impossible with existing methods.

Research on flame temperature measurement method based on water vapor emission spectrum

Internal temperature monitoring of high-speed propulsion systems is highly important for engine performance evaluation and lifetime prediction. As a passive optical measurement method without the need for an external light source and without flow field interference, the emission spectrum measurement technique has good application prospects for harsh measurement environments. As the main combustion product, high-temperature water vapor shows a strong emission intensity that is highly suitable for temperature measurement applications. We propose use of the band integral ratio to remove the high resolution measurement requirements for the spectrum acquisition system. In addition, the temperatures of methane-oxygen flames with different equivalent ratios are measured successfully under the condition that the influence of self-absorption on the measurements is considered.

Numerical study of the effect of sidewalls on shock train behaviour

Strongly coupled sequences of shock waves, known as shock trains, are present in high-speed propulsion systems, where the presence of sidewalls substantially modifies the boundary layer thickness, skin friction and streamwise pressure distribution. In the present contribution, scale-resolved numerical simulations are performed on supersonic channel (infinite span) and square duct flows to evaluate the effect of sidewall confinement with and without shock trains. Comparable secondary flow vortices are observed in the duct case with and without the presence of the shock train. The absence of a separation region at the leading shock of the duct case results in lower flow deflection compared with the channel case, leading to a reduced shock strength. The principal effect of the sidewalls is to cause a shock train that is approximately twice as long and composed of a larger number of shocks. A modification of previous models, based on a momentum thickness-based blockage parameter, leads to an improved collapse of the channel and duct cases.

Reduced order design and investigation of intakes for high speed propulsion systems

Ramjet propulsion is commonly preferred to power supersonic and hypersonic vehicles for cruising faster than Mach 3. This is an elegant solution owing to the lean architecture which does not embody any rotating parts. Although the geometry of the engine is simple as compared to turbine based configurations, the flow physics through the engine duct is quite complex and the flow speeds modulate between the supersonic and subsonic regimes multiple times. The design and performance analysis of ramjet engines are vital to ensure that propulsion system can satisfy the flight trajectory requirements. Therefore, this study introduces a reduced order holistic approach for design and assessment of the flow development in high-speed propulsion systems composed of generic elements of ramjet/scramjet engine configurations. Accordingly, the intakes designed based on axisymmetric flow templates are used to provide the necessary freestream flow modulation prior to the isolator through which a normal shock assumption is applied. The resultant flow properties are utilized for the combustion module where the flow expansion within the combustor and nozzle components are computed based on 1D steady inviscid flow equations coupled with detailed chemistry approach and JANAF tables. The module was validated and verified with the experimental and numerical data obtained for a dual-mode ramjet/scramjet combustor. Consequently, the parameters such as thrust, fuel consumption and specific impulse are calculated to quantify the engine performance for each design. Finally, the employment of the low fidelity model is demonstrated over a family of ramjet flow paths where the design space is confined based on the requirements of a high-supersonic cruise vehicle.

Multi-bubble scheme and structural analysis of a hypersonic stratospheric flight vehicle

The STRATOFLY MR3 vehicle is the main objective of the STRATOFLY project, which aims to develop a hypersonic air breathing concept capable of covering antipodal routes in less than three hours. The aircraft architecture features a waverider configuration, internally supported by multi-bubble integral cryogenic tanks hosting LH2 propellant, being one of the major challenges the integration of lightweight structures with the high-speed propulsion system. The objective of this research is to completely define an efficient structural scheme of the multi-bubble structures. To do so, a multidisciplinary analysis of the full-scale aircraft model is carried out to assess the viability of the vehicle prototype. Once the flaws of the initial structural layout are identified, a set of stiffener elements were developed to generate a scheme which can withstand the loads that hypersonic flight entails. In the multi-bubble structures, a topology optimization strategy was applied to obtain a set of tension rods connecting the top and bottom parts of the bubbles to support the pressure loads. The proposed configuration was sized and analyzed for multiple points of the aircraft mission, obtaining stress levels below the failure criteria adopted for each material. In addition, the results show low displacements that guarantee an adequate aerodynamic behavior and engine performance, while mantaining global natural frequencies in the range of commercial airplanes.

Propulsive and combustion behavior of hydrocarbon fuels containing boron nanoparticles in a liquid rocket combustor

Nano-sized energetic particle as fuel additives is of great significance for liquid hydrocarbon fuels that will exhibit high-density and high-calorific value in high-speed propulsion systems. An experimental investigation has been conducted to determine the propulsive and combustion behavior of hydrocarbon fuels containing boron nanoparticles. In this study, nano-sized boron particles with average diameter of 20 nm are added into the basic fuel JP-10 and quadricyclane, respectively. They are then referred as slurry fuels and burned in the rocket combustor using pure oxygen as the oxidizer. With a wide range of excess oxidizer coefficients, three parameters characterizing the propulsion performance are employed to evaluate the effect of boron nanoparticles on hydrocarbon fuels. It is found that the boron-based slurry fuel showed superior density specific impulse. It is increased by 1.18% and 1.44% when the addition of boron particles with 10% mass fraction are added into the basic fuel JP-10 and quadricyclane, respectively. Combustion depositions of the boron-based slurry fuel located at different positions are then collected for deep analysis by means of the energy dispersive spectrometer, X-ray diffractometer, and scanning electron microscope. Comprehensive microanalysis results demonstrate that boron particles first combine with the C-element in the hydrocarbon fuel to form the boron carbide with a reticular structure, and then the boron carbide oxidizes to form block-shaped boron oxide. However, the surface boron oxide hindered the further reaction of the internal boron carbide, which limits the energy release of the boron particles and ultimately leads to the unsatisfactory combustion efficiency of the slurry fuel.

Experimental validation of liquid hydrocarbon based fuel rich gas generator for high speed propulsion systems

For long duration hypersonic flight vehicles, research on high speed air-breathing propulsion systems has been progressing across the world for the past few decades. Ramjets/Scramjets fueled with hydrogen/liquid hydrocarbon fuels are the propulsion systems considered for different regimes of flight envelope. Dual Combustion Ramjet (DCR) engine is a prospective candidate for one of such kind of applications, where the engine is expected to work for a wider range of flight Mach number regime. It has a fuel rich subsonic dump combustor and a supersonic combustor, which work in tandem. In the present work, extensive experimental and analytical work has been carried out to develop a fuel rich gas generator often called as dump combustor suitable for operation from 20 to 28 km altitude and in the range of 4–6 flight Mach number. Connect pipe mode testing has been carried out by varying the throat area of dump combustor for Mach 4 conditions. Tests have been carried out with Jet-A fuel at different equivalence ratio and with fuel injection in different combination and stages. Fuel injection scheme used here is of shower head type in intake arm and a single simplex swirler with film cooling holes in the dump combustor. Study has been also carried out by varying the location of swirler along the combustor length. Combustor air entry conditions are simulated using H2–O2 based air heater and a test bed nozzle. An exhaustive amount of tests have been carried out to establish condition suitable for consistent ignition & sustained combustion in this highly fuel rich condition. A One dimensional (1D) mathematical model with NASA CEA interface has been developed to predict & compare the test performance. Sustained ignition is achieved for fuel flow rate of 0.8 kg/s and at an equivalence ratio of 0.8–0.9.

Compressibility Effects on the Scalar Dissipation Rate

ABSTRACT Scalar dissipation rate plays an important role in combustion, especially for fast chemistry in non-premixed flames. Even for finite-rate chemistry, it is a parameter of major importance in models based on flamelets, probability density function methods, and conditional moment closures. Compressibility can influence the behavior of scalar dissipation but existing models do not consider them in the form of large dilatational effects, which are present in combustion in high-speed propulsion systems such as scramjets in hypersonic flight. This paper considers these effects on the scalar dissipation rate, in particular its mean value, spectral density, probability density function, as well the alignment of the scalar gradient with the strain field of the flow. The major result of this study is that the majority of the compressibility effects on inertial-type scales can be taken into account if the classical scaling laws of incompressible turbulence are rescaled by taking only the solenoidal contributions. However, additional effects of compressibility need to be considered at smaller scales.

± 180° Discontinuous PWM for Single-Phase PWM Converter of High-Speed Railway Propulsion System

As high-capacity alternating current/direct current (ac/dc) power conversion systems, single-phase pulse-width modulation (PWM) converters used in high-speed railway propulsion systems adopt high-voltage Insulated-Gate Bipolar Transistors (IGBTs) as switching elements. Due to their high breakdown voltage characteristics, the switching dynamics are inferior to those of low-voltage IGBTs and switching losses are more dominant than conduction losses despite operating at relatively low switching frequencies of hundreds to several kHz. To solve this problem, this paper proposes ± 180° discontinuous PWM (DPWM) suitable for a single-phase circuit. With the simple addition of offset voltages, the proposed DPWM method can be implemented easily and switching losses can be reduced by half by clamping the switching legs of the H-bridge converter to the positive or negative dc rail during every half cycle. In addition, temperature deviation between the power stacks can be minimized by using selective application of clamping modes. The validity and effectiveness of the proposed DPWM are verified through simulations and experiments of a prototype converter.

Multifunctional Graphene-Based Additives for Enhanced Combustion of Cracked Hydrocarbon Fuels under Supercritical Conditions

ABSTRACT As a particulate fuel additive, functionalized graphene sheets (FGS), with and without the decoration of nanoparticles, provide a means to form stable colloids with liquid hydrocarbons, act as in situ catalysts, and prevent attached nanoparticles from agglomerating or sintering during heating. Recent pyrolysis experiments and simulations have shown the synergetic effect of Pt and FGS structures accelerating fuel conversion and hydrogen formation. In this paper, the role of graphene-based additives (with and without platinum (Pt) nanoparticles) on enhancing the ignition and combustion characteristics of cracked n-dodecane hydrocarbon fuels under supercritical conditions is examined. Supercritical ignition and combustion experiments were conducted using a high pressure and high temperature windowed combustion chamber coupled to a supercritical fuel pyrolysis reactor. The combustion experiments indicated that the presence of small amounts of particulate additives (100 ppmw) to the fuel can reduce ignition times and increase subsequent combustion rates. In particular, the addition of FGS-supported Pt nanoparticles reduced ignition delay times by nearly a factor of 3 (12.4 to 4.2 ms), increased spray spreading angles by approximately 32.0% (15.4 to 20.3°), reduced the flame liftoff length by 54.0% (1.74 to 0.8 mm), and demonstrated an increase in fuel conversion for a fixed reaction time of 35.0% relative to the pure fuel baseline. These results support the notion that nanostructured fuel additives have the potential to enhance both the heat sink capacity and combustion performance of liquid hydrocarbon fuels for their use in advanced high speed propulsion systems.

Influence of Taylor waves on standing windows of oblique detonation wave

An oblique detonation wave stabilized over a body has been studied due to the ongoing development of high-speed propulsion systems, such as oblique detonation wave engines and ram accelerators. Standing window of oblique detonation wave reflects the degree of standing difficulty. Firstly, Shock relations coupled with chemical equilibrium are solved to draw detonation polar diagrams between oblique detonation angle and wedge angle by iterative algorithm. In these diagrams, the wedge angle detaching from the wedge nose is referred as the maximum wedge angle θιηॉκ and forming CJ detonation in the direction normal to the oblique detonation wave is referred as the minimum wedge angle θख़τ. The region between the two wedge angles is regarded as the standing windows. G. Emanuel holds that Taylor waves follow the oblique detonation wave as the wedge angle is smaller than θख़τ. Based on reactive Euler equation, wedge-induced oblique detonation is simulated in this paper. For the case of the wedge angle below θख़τ, Taylor waves change post-detonation wave flow field to maintain standing oblique detonation wave. Therefore, the standing window becomes wider.

Development of a robust solver to model the flow inside the engines for high-speed propulsion

The demand for discovering new commercial routes as well as the possibility to shortening civilian long-haul flights boosted the interest of civil hypersonic vehicle designs. Among all the multiple projects started by the various nations, the European community funded project STRATOFLY aims at refining the baseline LAPCAT II-MR2.4 design for further improvements. The new aircraft would enable a flight shorter that 3 hours from Brussels to Sydney, carrying 300-passengers above the already crowed atmosphere. The wide Mach range operability, up to Mach 8, demands the use of multiple engines, leading to a highly integrated propulsion system. The current study is focused on the development of new CFD platform to estimate the performance of the combined propulsion system during the supersonic to hypersonic transition. In order to control the complex flow physics, highfidelity CFD simulations remain the fundamental tools for the preliminary investigations. On the current framework, an advanced robust compressible solver has been develop d in order to handle the different flow regimes. The new tool solves Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations by employing cell-centered Finite Volume Method constructed on openFoam toolbox. Two innovative high-order discretization schemes, with different abilities, based on approximated Riemann solvers were developed for capturing the flow physics within high-speed propulsion systems. Advanced time discretization has been taken into account to increase the temporal accuracy. At the end, the whole implementation has been validated in multiple test cases, ranging from incompressible to hypersonic regimes, confirming its excellent stability, robustness and accuracy.

Numerical investigation of flame–vortex interactions in laminar cross-flow non-premixed flames in the presence of bluff bodies

abstractFlame stabilisation in a combustor having vortices generated by flame holding devices constitutes an interesting fundamental problem. The presence of vortices in many practical combustors ranging from industrial burners to high speed propulsion systems induces vortex–flame interactions and complex stabilisation conditions. The scenario becomes more complex if the flame sustains after separating itself from the flame holder. In a recent study [P.K. Shijin, S.S. Sundaram, V. Raghavan, and V. Babu, Numerical investigation of laminar cross-flow non-premixed flames in the presence of a bluff-body, Combust. Theory Model. 18, 2014, pp. 692–710], the authors reported details of the regimes of flame stabilisation of non-premixed laminar flames established in a cross-flow combustor in the presence of a square cylinder. In that, the separated flame has been shown to be three dimensional and highly unsteady. Such separated flames are investigated further in the present study. Flame–vortex interactions in separated methane–air cross flow flames established behind three bluff bodies, namely a square cylinder, an isosceles triangular cylinder and a half V-gutter, have been analysed in detail. The mixing process in the reactive flow has been explained using streamlines of species velocities of CH4 and O2. The time histories of z-vorticity, net heat release rate and temperature are analysed to reveal the close relationship between z-vorticity and net heat release rate spectra. Two distinct fluctuating layers are visible in the proper orthogonal decomposition and discrete Fourier transform of OH mass fraction data. The upper fluctuating layer observed in the OH field correlates well with that of temperature. A detailed investigation of the characteristics of OH transport has also been carried out to show the interactions between factors affecting fluid dynamics and chemical kinetics that cause multiple fluctuating layers in the OH.

Multi-angular Flame Measurements and Analysis in a Supersonic Wind Tunnel Using Fiber-Based Endoscopes

This paper reports new measurements and analysis made in the Research Cell 19 supersonic wind-tunnel facility housed at the Air Force Research Laboratory. The measurements include planar chemiluminescence from multiple angular positions obtained using fiber-based endoscopes (FBEs) and the accompanying velocity fields obtained using particle image velocimetry (PIV). The measurements capture the flame dynamics from different angles (e.g., the top and both sides) simultaneously. The analysis of such data by proper orthogonal decomposition (POD) will also be reported. Nonintrusive and full-field imaging measurements provide a wealth of information for model validation and design optimization of propulsion systems. However, it is challenging to obtain such measurements due to various implementation difficulties such as optical access, thermal management, and equipment cost. This work therefore explores the application of the FBEs for nonintrusive imaging measurements in the supersonic propulsion systems. The FBEs used in this work are demonstrated to overcome many of the practical difficulties and significantly facilitate the measurements. The FBEs are bendable and have relatively small footprints (compared to high-speed cameras), which facilitates line-of-sight optical access. Also, the FBEs can tolerate higher temperatures than high-speed cameras, ameliorating the thermal management issues. Finally, the FBEs, after customization, can enable the capture of multiple images (e.g., images of the flow fields at multi-angles) onto the same camera chip, greatly reducing the equipment cost of the measurements. The multi-angle data sets, enabled by the FBEs as discussed above, were analyzed by POD to extract the dominating flame modes when examined from various angular positions. Similar analysis was performed on the accompanying PIV data to examine the corresponding modes of the flow fields. The POD analysis provides a quantitative measure of the dominating spatial modes of the flame and flow structures, and is an effective mathematical tool to extract key physics from large data sets as the high-speed measurements collected in this study. However, the past POD analysis has been limited to data obtained from one orientation only. The availability of data at multiple angles in this study is expected to provide further insights into the flame and flow structures in high-speed propulsion systems.

Flow Diagnostics in Shock Wave-Boundary Layer Interaction Experiments in Hypersonic Flow

Shock Wave-Boundary Layer Interaction (SBLI) is a phenomenon occurring in high-speed propulsion systems that is highly undesirable. Numerous methods have been tested to manipulate and control SBLI which includes both active and passive flow control techniques. To determine the improvements brought by the flow control techniques, advanced and state-of the-art flow diagnostics and experimental techniques are required, especially when it involves high-speed flows. In this study, a number of advanced flow diagnostics were employed to investigate the effect of micro-vortex generators in controlling SBLI in Mach 5 such as Pressure Sensitive Paints (PSP), Particle Image Velocimetry (PIV), schlieren photography and oil-flow visualization. The flow diagnostics successfully visualized the boundary layer separation and also the improvements brought by the micro-vortex generators.

초고속 자기부상열차용 선형 동기 전동기의 추력 및 부상력 특성 개선을 위한 연구

A linear synchronous motor (Linear Synchronous Motor, under LSM) is suitable for Maglev train. This is 500㎞/h or more for running high-speed propulsion system of high-efficiency, high-output characteristics. Also, as high-speed running, it is needed solution to reduce output ripple component cause bad riding like noise and vibration. So this paper was designed 500km/h-class Maglev train and analyzed characteristics of the LSM base model using finite element analysis method. Further, improved model is designed to improve characteristics of thrust and levitation force by enforcing design parameters analysis and sensitivity analysis. And it was applied skew on field in order to reduce the ripple component still remaining. Skew interpretation of the two-dimensional is proposed and this is verified by carrying out three-dimensional finite element analysis comparing two values. It proved to be valid of skew of the two-dimensional analysis.

High Speed Propulsion System for UAQ4 Magnetic Levitating Train

This paper presents the design of a novel high speed propulsion system for UAQ4 magnetic levitating train, the feasibility of which has been successfully tested and confirmed in laboratory. A direct-current linear stepper motor that uses permanent magnets distributed on the central beam of track as the stator and the direct current power supplied coils on-board the vehicle as the rotor is proposed and analyzed. The motor performances are evaluated by varying the system parameters by carrying out a finite element numerical model refined with experimental data. The main components of a real scale motor with speed up to 580 km/h for the UAQ4 train application is measured and discussed.

Micro-Ramps for Hypersonic Flow Control

Shock/boundary layer interaction (SBLI) is an undesirable phenomenon, occurring in high-speed propulsion systems. The conventional method to manipulate and control SBLI is using a bleed system that involves the removal of a certain amount of mass of the inlet flow to control boundary layer separation. However, the system requires a larger nacelle to compensate the mass loss, larger nacelles contribute to additional weight and drag and reduce the overall performance. This study investigates a novel type of flow control device called micro-ramps, a part of the micro vortex generators (VGs) family that intends to replace the bleed technique. Micro-ramps produce pairs of counter-rotating streamwise vortices, which help to suppress SBLI and reduce the chances of flow separation. Experiments were done at Mach 5 with two micro-ramp models of different sizes. Schlieren photography, surface flow visualization and infrared thermography were used in this investigation. The results revealed the detailed flow characteristics of the micro-ramp, such as the primary and secondary vortices. This helps us to understand the overall flow physics of micro-ramps in hypersonic flow and their application for SBLI control.

Modeling of Atmospheric Turbulence as Disturbances for Control Design and Evaluation of High Speed Propulsion Systems

Atmospheric turbulence models are necessary for the design of both inlet/engine and flight controls, as well as for studying integrated couplings between the propulsion and the vehicle structural dynamics for supersonic vehicles. Models based on the Kolmogorov spectrum have been previously utilized to model atmospheric turbulence. In this paper, a more accurate model is developed in its representative fractional order form, typical of atmospheric disturbances. This is accomplished by first scaling the Kolmogorov spectral to convert them into finite energy von Karman forms. Then a generalized formulation is developed in frequency domain for these scale models that approximates the fractional order with the products of first order transfer functions. Given the parameters describing the conditions of atmospheric disturbances and utilizing the derived formulations, the objective is to directly compute the transfer functions that describe these disturbances for acoustic velocity, temperature, pressure, and density. Utilizing these computed transfer functions and choosing the disturbance frequencies of interest, time domain simulations of these representative atmospheric turbulences can be developed. These disturbance representations are then used to first develop considerations for disturbance rejection specifications for the design of the propulsion control system and then to evaluate the closed-loop performance.

Turbo graphene

Future high-speed propulsion systems will require environmentally friendly high-energy fuels. One emerging approach to this challenge is not to develop new fuels but to improve existing fuels by adding nanostructured particles. Now Richard Yetter, Ilhan Aksay and co-workers at the Pennsylvania State University and Princeton University have shown that the combustion performance of nitromethane, a potential rocket propellant, can be greatly enhanced by adding functionalized graphene sheets to the fuel.