Review on the Rotating Detonation Engine and It’s Typical Problems

Detonation is a promising combustion mode to improve engine performance, increase combustion efficiency, reduce emissions, and enhance thermal cycle efficiency. Over the last decade, significant progress has been made towards the applications of detonation mode in engines, such as standing detonation engine (SDE), Pulse detonation engine (PDE) and rotating detonation engine (RDE), and the understanding of the fundamental chemistry and physics processes in detonation engines via experimental and numerical studies. This article is to provide a comprehensive overview of the progress in the knowledge of rotating detonation engine from the different countries. New observations of injection, ignition, and geometry of combustor, pressure feedback, and combustion modes of RDE have been reported. These findings and advances have provided new opportunities in the development of rotating detonation for practical applications. Finally, we point out the current gaps in knowledge to indicate which areas future research should be directed at

Propagating laminar flame characteristics for single and two phase alternative fuel mixtures

This thesis investigates enhanced methods for analysing non-linear effects in propagating laminar flames, enabling more accurate evaluation of laminar flame characteristics such as Markstein length and unstretched flame speed whilst proposing a new method for evaluating extinction stretch rate. Furthermore, a new cloud-combustor is developed and commissioned enabling laminar flame characteristics through droplet fuel mists to be explored again utilising advanced non-linear analysis. Re-analysis of previous low-ignition energy methane-water flames reveals the analytical non-linear characteristic. The analysis also demonstrates the need for a larger chamber to avoid pressurised effects during the latter stages of propagation, potentially reducing the accuracy of the adopted methodology. Non-linear analysis shows interesting trends concerning Markstein length at higher water loading in particular when it increase to 15% (by volume), and laminar burning rate decreased. The non-linear analysis technique is deployed to analyse four hydrocarbon fuels, two traditional paraffinic fuels in methane and propane, and two alternative alcohol fuels namely ethanol and methanol. It is shown that overdriven flame data can be used to predict flame extinction stretch rate, as long as a sufficient time period is disregarded to allow the effects of the early ignition-affected period to subside. The new technique proposed for evaluated critical extinction stretch rate shows good agreement with the traditional counter-flowing flame technique. Results for the four fuels reveal a common profile for extinction stretch-rate as a function of equivalence ratio, which was anticipated due to the similar fundamental combustion characteristics of the chosen fuels. Based on the non-linear analysis, it is shown analytically that this common profile may be represented by a combination of the iv unstretched laminar burning velocity, the Markstein length and the expansion ratio of the fuel. Ethanol in air is used to benchmark Cardiff University’s new, large 35Litre ‘Cloud Combustor’ for an investigation of flame propagation through fuel mists across a wide range of equivalence ratios. Non-intrusive, in-situ droplet sizing with concurrent flame propagation is achieved for the first time. The fuel mist flame data was subsequently compared to that for pure vapour mixtures at nominally identical ambient conditions in order to study the reported enhancement in flame speed exhibited in previous studies, and to compare qualitatively against conflicting published views reported in literature. It was found that with the onset of instabilities at certain droplet size an enhancement in flame speed could be shown for rich mist flames compared to those of analogous vapour flames. Based on mechanisms detailed elsewhere that provide a possible explanation for this enhancement full discussion and correlations that help to understand the nature of flame speed through droplet mists are presente

Photoacoustic tomography setup using LED illumination

Photoacoustic tomography (PAT) is a hybrid imaging modality that combines optical contrast with ultrasound resolution. Most of the PAT configurations are based on high-energy solid-state lasers such as Nd:YAG laser. In this work, a PAT system that uses light-emitting diode (LED) as a light source is introduced. The system is designed so that the imaged target can be stationary. The target is illuminated by a LED light source from one side and the pressure wave is measured using an acoustic transducer that is rotated around the target. Image reconstruction is based on Bayesian approach to illposed inverse problems. The system was tested with light absorbing targets also in limited-view and sparse angle measurement situations. The results show that LED-based instrumentation and advanced reconstruction methods can form a potential PAT system that can also be applied in limited-view and sparse angle photoacoustic tomography

Detection of Lint by Using Machine Vision

This thesis, commissioned by Häme University of Applied Sciences, researches the possibility of detecting lint by using machine vision. Due to the small particle size and high movement speed of the lint, various issues occur. Firstly, to detect the small lint particles a sufficient resolution is required. Secondly, since the lint has a high movement speed a high framerate is required to fully represent all the lint passing by. Lastly, a short exposure time is required to prevent inaccuracy due to motion blur. The goals of this thesis are to research the most optimal machine vision components, if the hardware currently available can detect the small particles with a sufficient framerate and a method to prevent motion blur. The most optimal components were found by performing a literature study. Calculations were made to test if the currently available hardware can fulfil the goals. A colleague created a short duration strobe light to prevent motion blur. Lastly, a practical test setup and MATLAB program were created to verify the theoretical conclusions and detect the lint. The strobe light uses four high power white LEDs with a flash duration of one microsecond. The calculations have concluded that the currently available hardware is capable of fully representing the lint passing by at a minimum particle size of 45 microns. Analyses of the MATLAB program verified that the theoretical calculations were correct.Deze thesis, uitgevaardigd door Häme University of Applied Sciences, onderzoekt de mogelijkheid tot het detecteren van stofdeeltjes door gebruik te maken van machine visie. Door de kleine dimensies en hoge voortbewegingssnelheden van de deeltjes treden er allerlei problemen op. Zo is een hoge resolutie noodzakelijk om de deeltjes te detecteren. Ook moet de framerate van de camera voldoende snel zijn om alle deeltjes die voorbij bewegen te detecteren. Ten laatste, is een korte sluitertijd van de sensor noodzakelijk om motion blur te voorkomen. Het doel van deze thesis is om de meest optimale machine visie onderdelen te onderzoeken, de mogelijkheid om het lint te detecteren met de hardware die beschikbaar is te onderzoeken en om een oplossing te zoeken voor motion blur. De meest optimale machine visie setup werd gevonden met een literatuurstudie. Berekeningen zijn gemaakt om de beschikbare hardware te testen. Een flits van zeer korte duur is door een collega student gemaakt om motion blur te voorkomen. Ten laatste, is er een praktische opstelling en een MATLAB-programma gemaakt om de theoretische conclusies te verifiëren en het stof te detecteren. De flits gebruikt vier hoogvermogen witte leds met een flitsduur van één microseconde. De berekeningen toonden aan dat de beschikbare hardware in staat is om alle deeltjes te filmen met een minimum grote van 45 micrometer. Het Matlab programma verifieerde dat de theoretische berekeningen correct waren

Experimental studies on shock wave interactions with flexible surfaces and development of flow diagnostic tools

Nowadays, light-weight composite materials have increasingly used for high-speed flight vehicles to improve their performance and efficiency. At supersonic speed, sonic fatigue, panel flutter, severe instabilities, and even catastrophic structural failure would occur due to the shock wave impingement on several flexible components of a given structural system either internally or externally. Therefore, investigation on shock wave interaction with flexible surfaces is crucial for the safety and performance of high-speed flight vehicles. This work aims to investigate the mechanism of shock wave interaction with flexible surfaces with and without the presence of the boundary layer. The first part involves the shock wave generated by supersonic starting jets interaction with flexible surfaces and the other one focuses on shock wave and boundary layer interaction (SBLI) over flexible surfaces. A novel miniature and cost-effective shock tube driven by detonation transmission tubing was designed and manufactured to simulate the supersonic starting jet and investigate the interaction of a supersonic starting jet with flexible surfaces. To investigate the characterization of this novel type shock tube, the pressure-time measurement in the driven section and the time-resolved shadowgraph were performed. The result shows that the flow structure from the open end of the shock tube driven by detonation transmission tubing agrees with that of conventional compressed-gas driven shock tubes. Moreover, this novel type of shock tube has good repeatability of less than 3% with a Mach number range of 1.29-1.58 when the weight of the NONEL explosive mixture varies from 3.6mg to 12.6mg. An unsteady background oriented schlieren (BOS) measurement system and a sprayable Polymer-Ceramic unsteady pressure sensitive paint (PC-PSP) system were developed. The preliminary BOS result in a supersonic wind tunnel shows that the sensitivity of the BOS system is good enough to visualize weak density variations caused by expansion waves, boundary layer, and weak oblique shocks. Additionally, compared with the commercial PC-PSP from Innovative Scientific Solutions Incorporated (ISSI), the in-house developed unsteady PSP system has higher pressure sensitivity, lower temperature sensitivity, and photo-degradation rate. To identify the shock movement, distortion and unsteadiness during the processes of the supersonic starting jet impingement and shock wave boundary layer interaction (SBLI) over flexible surfaces, an image processing scheme involving background subtraction in the frequency domain, filtering, resampling, edge detection, adaptive threshold, contour detection, feature extraction, and fitting was proposed and applied to process shadowgraph and schlieren sequences automatically. A large shadowgraph data set characterized by low signal to noise ratio (SNR) and small spatial resolution (312×260-pixel), was used to validate the proposed scheme. The result proves that the aforementioned image processing scheme can detect, track, localize, and fit shock waves in a subpixel accuracy. The mechanism of the interaction between the initial shock wave from a supersonic starting jet and flexible surfaces was investigated based on a square shock tube driven by detonation transmitting tube. Compared with that of the solid plate case, flexible surfaces can delay the shock reflection process because of the flexible panel deformation generated by the pressure difference between the top and the bottom. The delay time is around 8µs in the case of 0.1mm thick flexible surface, whereas it declines to around 4µs in the case of 0.3mm thick flexible surface because of the lower flexibility and deformation magnitude. However, interestingly, the propagation velocity of the reflected shock wave is basically the same for the solid plate and flexible panels, which means the flexible surface doesn’t reduce the strength of the reflection wave, although it delays its propagation. Also, there is not an apparent difference in the velocity of the reflected shock wave in the case of different incident shock Mach numbers when Ms varying from 1.22 to 1.54. These experimental results from this study are useful for validating numerical codes that are used for understanding fluid-structure interaction processes

2-D Simulation with OH* Kinetics of a Single-Cycle Pulse Detonation Engine

Two-dimensional computational fluid dynamics (CFD) simulation with selected kinetics for H2–air mixture of a hydrogen-fuelled single-pulse detonation engine were performed through ANSYS FLUENT commercial software for diagnostic purposes. The results were compared with Chapman–Jouguet (CJ) values calculated by the CEA (Chemical Equilibrium with Applications) and ZND (Zel’dovich–Neumann–Döring) codes. The CJ velocities and pressures, as the product velocities are in agreement, however, the CJ temperatures are too higher for 2-D simulations; as a consequence, the sound velocities were overpredicted. OH* kinetics added to the reaction set allowed visualization of the propagation front with several detonation cells showing a consistent multi-headed detonation propagating in the whole tube. The detonation front was slightly perturbed at the end of the tube with inclination of front edge and fewer cell numbers, and more significantly at the nozzle entrance with velocity reduction, resulting in a weak and unstable detonation. OH* images showed the detonation reaction zone decoupled from the shock front with disappearance of cellular structure. The inclusion of OH* reaction set for CFD simulation coupled to kinetics is demonstrated to be an excellent tool to follow the detonation propagation behaviour

Development of a Detonation Diffuser

This research includes an investigation of the mechanisms of diffraction and reinitiation that enable a detonation diffuser. It describes a set of geometric parameters necessary to design a diffuser for a given detonable mixture and initial channel height. Predetonators with channel height less than the critical height are ineffective because detonations in small channels decouple into separate shock and combustion fronts when the channel height increases. A detonation diffuser allows the channel height to increase by utilizing the decoupled shock wave to reinitiate detonation. In the diffuser, a detonation initially decouples into separate shock and combustion fronts, and then the decoupled shock front reflects from an oblique surface initiating a secondary detonation that survives the expansion. This research investigated the three regions of a detonation diffuser: the initial diffraction, the reflecting surface, and the second diffraction corner. Schlieren video of two-dimensional diffracting detonations recorded the position of the detonation, decoupled shock front and flame front. Observations of the decoupled shocks reflecting from surfaces showed that a 45° reflecting surface must be placed less than 80 mm downstream of the initial diffraction corner to initiate a secondary detonation in more than 91% of repeated trials. Observations of the interaction of diffracting detonations with multiple obstacles revealed that the best performance (smallest separation, and highest Mach number) occurred when the decoupled shock reflected from four separate obstacles at approximately the same time

Three-dimensional imaging of swirled spray injection in a generic aero engine burner under realistic operating conditions

Tomographic shadowgraph imaging is applied to reconstruct the instantaneous three-dimensional spray field immediately downstream of a generic aero engine fuel injector. Within the swirl passage of the injector model, a single kerosene jet undergoes air-blast atomization in a cross-flow configuration at Weber numbers of We=360-770, air pressures of p_a=4-7 bar and air temperatures of T_a=440-570K. High-speed, high magnification shadowgraphy is used to visualize the initial fuel atomization stages within the fuel injector before the spray enters the spray chamber. The 4-camera tomographic measurement setup is described in detail and includes a depth-of-field analysis with respect to droplet size based on Mie simulations and calibration data of the point-spread function. For a volume size of 16x13x10mm³ the smallest resolvable droplet diameter is estimated to be d=10µm within the focal plane and increases to approx. 20µm towards the edges of the volume. Droplet velocities above the resolution limit were retrieved by 3-d cross-correlation of two volumetric reconstructions recorded at two consecutive time-steps. This is accompanied by an error analysis on the random error dependency on the camera viewing geometry. The results indicate increasing motion and fluctuations of the spray tail with increasing temperature and Weber number. Validation against PDA data further downstream of the burner plate revealed consistency for size classes d=10µm and d=15µm. Deviations from PDA occur in regions with strong velocity gradients due to different spatial resolutions, the presence of reconstruction ambiguities (ghost particles), uncertainties inherent to the two-frame cross-correlation of spray volumes and the finite LED pulse duration