Under certain circumstances pulse combustors have been shown to improve both heat transfer and drying rate when compared to steady flow impingement. Despite this potential, there have been few investigations into the use of pulse combustor driven impingement jets for industrial drying applications. The research presented here utilized experimental and numerical techniques to study the heat transfer characteristics of these types of oscillating jets when impinging on solid surfaces and the heat and mass transfer when drying porous media. The numerical methods were extensively validated using laboratory heat flux and drying data, as well as correlations from literature. As a result, the numerical techniques and methods that were developed and employed in this work were found to be well suited for the current application. It was found that the pulsating flows yielded elevated heat and mass transfer compared to similar steady flow jets. However, the numerical simulations were used to analyze not just the heat flux or drying, but also the details of the fluid flow in the impingement zone that resulted in said heat and mass transport. It was found that the key mechanisms of the enhanced transfer were the vortices produced by the oscillating flow. The characteristics of these vortices such as the size, strength, location, duration, and temperature, determined the extent of the improvement. The effects of five parameters were studied: the velocity amplitude ratio, oscillation frequency, the time-averaged bulk fluid velocity at the tailpipe exit, the hydraulic diameter of the tailpipe, and the impingement surface velocity. Analysis of the resulting fluid flow revealed three distinct flow types as characterized by the vortices in the impingement zone, each with unique heat transfer characteristics. These flow types were: a single strong vortex that dissipated before the start of the next oscillation cycle, a single persistent vortex that remained relatively strong at the end of the cycle, and a strong primary vortex coupled with a short-lived, weaker secondary vortex. It was found that the range over which each flow type was observed could be classified into distinct flow regimes. The secondary vortex and persistent vortex regimes were found to enhance heat transfer. Subsequently, transition criteria dividing these regimes were formed based on dimensionless parameters. The critical dimensionless parameters appeared to be the Strouhal number, a modified Strouhal number, the Reynolds number, the velocity amplitude ratio, and the H/Dh ratio. Further study would be required to determine if these parameters offer similar significance for other configurations.

A novel combustor design, referred to as the Stagnation Point Reverse-Flow (SPRF) combustor, was recently developed that is able to operate stably at very lean fuel-air mixtures and with low NOx emissions even when the fuel and air are not premixed before entering the combustor. The primary objective of this work is to elucidate the underlying physics behind the excellent stability and emissions performance of the SPRF combustor. The approach is to experimentally characterize velocities, species mixing, heat release and flame structure in an atmospheric pressure SPRF combustor with the help of various optical diagnostic techniques: OH PLIF, chemiluminescence imaging, PIV and Spontaneous Raman Scattering. Results indicate that the combustor is primarily stabilized in a region downstream of the injector that is characterized by low average velocities and high turbulence levels; this is also the region where most of the heat release occurs. High turbulence levels in the shear layer lead to increased product entrainment levels, elevating the reaction rates and thereby enhancing the combustor stability. The effect of product entrainment on chemical timescales and the flame structure is illustrated with simple reactor models. Although reactants are found to burn in a highly preheated (1300 K) and turbulent environment due to mixing with hot product gases, the residence times are sufficiently long compared to the ignition timescales such that the reactants do not autoignite. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zones regime throughout the combustor, and it tends to become more flamelet like with increasing distance from the injector. Fuel-air mixing measurements in case of non-premixed operation indicate that the fuel is shielded from hot products until it is fully mixed with air, providing nearly premixed performance without the safety issues associated with premixing. The reduction in NOx emissions in the SPRF combustor are primarily due to its ability to stably operate under ultra lean (and nearly premixed) condition within the combustor. Further, to extend the usefulness of this combustor configuration to various applications, combustor geometry scaling rules were developed with the help of simplified coaxial and opposed jet models.

The growing need of mobile power sources with high energy density and the robustness to operate also in the harshest environmental surroundings lead to the idea of downscaling gas turbines to ì-scale. Classified as PowerMEMS devices, a couple of design attempts have emerged in the last decade. One of these attempts was the Belgian “PowerMEMS” design started back in 2003 and aiming towards a ì-scale gas turbine rated at 1 kW of electrical power output.<p>This PhD thesis presents the scientific evaluation and development history of different combustion chamber designs based upon the “PowerMEMS” design parameters. With hydrogen as chosen fuel, the non-premixed diffusive “micromix” concept was selected as combustion principle. Originally designed for full scale gas turbine applications in two different variants, consequently the microcombustor development had to start with the downscaling of these two principles towards ì-scale. Both principles have the advantage to be inherently safe against flashback, due to the non-premixed concept, which is an important issue even in this small scale application when burning hydrogen. By means of water analogy and CFD simulations the hydrogen injection system and the chamber geometry could be validated and optimized. Besides the specific design topics that emerged during the downscaling process of the chosen combustion concepts, the general difficulties of microcombustor design like e.g. high power density, low Reynolds numbers, short residence time, and manufacturing restrictions had to be tackled as well.<p>As full scale experimental test campaigns are still mandatory in the field of combustion research, extensive experimental testing of the different prototypes was performed. All test campaigns were conducted with a newly designed test rig in a combustion lab modified for microcombustion investigations, allowing testing of miniaturized combustors according to full engine requirements with regard to mass flow, inlet temperature, and chamber pressure. The main results regarding efficiency, equivalence ratio, and combustion temperature were obtained by evaluating the measured exhaust gas composition. Together with the performed ignition and extinction trials, the evaluation and analysis of the obtained test results leads to a full characterization of each tested prototype and delivered vital information about the possible operating regime in a later UMGT application. In addition to the stability and efficiency characteristics, another critical parameter in combustor research, the NOx emissions, was investigated and analyzed for the different combustor prototypes.<p>As an advancement of the initial downscaled micromix prototypes, the following microcombustor prototype was not only a combustion demonstrator any more, but already aimed for easy module integration into the real UMGT. With a further optimized combustion efficiency, it also featured an innovative recuperative cooling of the chamber walls and thus allowing an cost effective all stainless steel design.<p>Finally, a statement about the pros and cons of the different micromix combustion concepts and their correspondent combustor designs towards a possible ì-scale application could be given. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

For gas turbines, the demand for high-performance, more efficient and longer-life turbine blades is increasing. This is especially so, now that there is a need for high-power and low-weight aircraft gas turbines. Thus, the search for improved design methodologies for the optimisation of combustor exit temperature profiles enjoys high priority. Traditional experimental methods are found to be too time-consuming and costly, and they do not always achieve near-optimal designs. In addition to the above deficiencies, methods based on semi-empirical correlations are found to be lacking in performing three-dimensional analyses and these methods cannot be used for parametric design optimisation. Computational fluid dynamics has established itself as a viable alternative to reduce the amount of experimentation needed, resulting in a reduction in the time scales and costs of the design process. Furthermore, computational fluid dynamics provides more insight into the flow process, which is not available through experimentation only. However, the fact remains that, because of the trial-and-error nature of adjusting the parameters of the traditional optimisation techniques used in this field, the designs reached cannot be called “optimum”. The trial-and-error process depends a great deal on the skill and experience of the designer. Also, the above technologies inhibit the improvement of the gas turbine power output by limiting the highest exit temperature possible, putting more pressure on turbine blade cooling technologies. This limitation to technology can be overcome by implementing a search algorithm capable of finding optimal design parameters. Such an algorithm will perform an optimum search prior to computational fluid dynamics analysis and rig testing. In this thesis, an efficient methodology is proposed for the design optimisation of a gas turbine combustor exit temperature profile. The methodology involves the combination of computational fluid dynamics with a gradient-based mathematical optimiser, using successive objective and constraint function approximations (Dynamic-Q) to obtain the optimum design. The methodology is tested on three cases, namely: (a) The first case involves the optimisation of the combustor exit temperature profile with two design variables related to the dilution holes, which is a common procedure. The combustor exit temperature profile was optimised, and the pattern factor improved, but pressure drop was very high. (b) The second case involves the optimisation of the combustor exit temperature profile with four design variables, one equality constraint and one inequality constraint based on pressure loss. The combustor exit temperature profile was also optimised within the constraints of pressure. Both the combustor exit temperature profile and pattern factor were improved. (c) The third case involves the optimisation of the combustor exit temperature profile with five design variables. The swirler angle and primary hole parameters were included in order to allow for the effect of the central toroidal recirculation zone on the combustor exit temperature profile. Pressure loss was also constrained to a certain maximum. The three cases show that a relatively recent mathematical optimiser (Dynamic-Q), combined with computational fluid dynamics, can be considered a strong alternative to the design optimisation of a gas turbine combustor exit temperature profile. This is due to the fact that the proposed methodology provides designs that can be called near-optimal, when compared with that yielded by traditional methods and computational fluid dynamics alone. / Thesis (PhD)--University of Pretoria, 2009. / Mechanical and Aeronautical Engineering / unrestricted

When studying pressure-gain combustion and wave rotor combustors, it is vital that any experimental model accurately reflect the real world conditions/applications being studied; this not only confirms previous computational and analytical work, but also provides new insights into how these concepts and devices work in real life. However, mixture stratification can have a noticeable effect on multiple combustion properties, including flame propagation, pressure, ignition time delay, and more, and this is especially true in constant-volume combustion chambers. Because it is beneficial to model wave rotor systems using constant-volume combustors such as what is employed in the IUPUI Combustion and Propulsion Research Laboratory, these stratification effects much be taken into account and reduced if possible. This study sought to find an effective method to reduce stratification in a rectangular constant-volume combustion chamber by means of manual recirculation pump. Spark-ignited flames were first produced in the chamber itself and studied using schlieren and color videography techniques as well as quantitative pressure histories. After determining the pump's effectiveness in reducing stratification, it was next employed when a hot jet of combustion products from a separate combustion chamber was used as an ignition source instead of the spark plug - a process typically employed in real wave rotor combustors. Lastly, the pump was used to study the leakage from the system for future test cases in order to offer further recommendations on how to effectively use the recirculation system. This process found that key properties significant to wave rotor development, such as time ignition delay, were affected by these stratification effects in past studies that did not account for this detail. As such, the pump has been permanently incorporated into the wave rotor model, as stratification is a vital. Additionally, significant fuel leakage is possible during rotational pre-chamber cases, and this should be address before proceeding with such experiments in the future. To combat this, the pump system has been reduced in volume, and suggestions have been provided on how to better seal the main rectangular chamber in the future.

The combustor-diffuser system remains one of the most studied sections of the turbomachine. Most of these investigations are due to the fact that quite a bit of flow diffusion is required in this section as the high speed flow exits the compressor and must be slowed down to enter the combustor. Like any diffusion process there is the chance for the development of an unfavorable adverse pressure gradient that can lead to flow separation; a cause of drastic losses within a turbine. There are two diffusion processes in the combustor-diffuser system: The flow first exits the compressor into a pre-diffuser, or compressor discharge diffuser. This diffuser is responsible for a majority of the pressure recovery. The flow then exits the pre-diffuser by a sudden expansion into the dump diffuser. The dump diffuser comprises the majority of the losses, but is necessary to reduce the fluid velocity within acceptable limits for combustion. The topic of active flow control is gaining interest in the industry because such a technique may be able to alleviate some of the requirements of the dump diffuser. If a wider angle pre-diffuser with separation control were used the fluid velocity would be slowed more within that region without significant losses. Experiments were performed on two annular diffusers to characterize the flow separation to create a foundation for future active flow control techniques. Both diffusers had the same fully developed inlet flow condition, however, the expansion of the two diffusers differed such that one diffuser replicated a typical compressor discharge diffuser found in a real machine while the other would create a naturally separated flow along the outer wall. Both diffusers were tested at two Reynolds numbers, 5x104 and 1x105, with and without a vertical wall downstream of the exit to replicate the dump diffuser that re-directs the flow from the pre-diffuser outlet to the combustor. Static pressure measurements were obtained along the OD and ID wall of the diffusers to determine the recovered pressure throughout the diffuser. In addition to these measurements, tufts were used to visualize the flow. A turbulent CFD model was also created to compare against experimental results. In the end, the results were validated against empirical data as well as the CFD model. It was shown that the location of the vertical wall was directly related to the amount of separation as well as the separation characteristics. These findings support previous work and help guide future work for active flow control in a separated annular diffuser both computationally and experimentally.

<p dir="ltr">A fast model of a Rotating Detonation Combustor (RDC) is developed based on the Method of Characteristics (MOC). The model provides a CFD-like solution of an unwrapped 2D RDC flow field in under 10 seconds with similar fidelity as 2D Reacting URANS simulations. Parametric studies are conducted using the simplified model, and the trends are analyzed to gain insight into the underlying physics of rotating detonation combustors. A methodology to assess the performance of operation with multiple waves is presented. The main effect of increasing waves is found to be the increase in the exit Mach number of the combustion chamber. The design process of a mixer component is also presented. The mixer lies downstream of a channel-cooled RDC with subsonic exit and upstream of a Rolls-Royce M250 helicopter engine in open-loop configuration. The mixer dilutes the RDC exhaust with approximately 250% air to condition the flow for the M250 turbine at steady state operation, while also acting as an isolator with a choked throat to prevent back propagation of pressure waves. The mixer aerodynamic design was completed using 2D axisymmetric RANS simulations, and the mechanical design was evaluated using Ansys Mechanical FEA and was found to be able to survive the high thermal stresses present both during the transient heating and steady state operating condition.</p>

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<p dir="ltr">This study contributes to research involving wave rotor combustors by studying the</p><p dir="ltr">development of a hot jet issuing from a cylindrical pre-combustion chamber. The pre-chamber was</p><p dir="ltr">developed to provide a hot fuel-air mixture as an ignition source to a rectangular combustion</p><p dir="ltr">chamber, which models the properties of a wave rotor channel. The pre-combustion chamber in</p><p dir="ltr">this study was rebuilt for study and placed in a new housing so that buoyancy effects could be</p><p dir="ltr">studied in tandem with other characteristics. The effectiveness of this hot jet is estimated by using</p><p dir="ltr">devices and instrumentation to measure properties inside the pre-chamber under many different</p><p dir="ltr">conditions. The properties tracked in this study include maximum pressure, the pressure and time</p><p dir="ltr">at which an aluminum diaphragm ruptures, and the moment a developed flame reaches a precise</p><p dir="ltr">location within the chamber. The pressure is tracked through use of a high-frequency pressure</p><p dir="ltr">transducer, the diaphragm rupture moment is captured with a high-speed video camera, and the</p><p dir="ltr">flame within the pre-chamber is detected by a custom-built ionization probe. The experimental</p><p dir="ltr">apparatus was used in three configurations to study any potential buoyancy effects and utilized</p><p dir="ltr">three different gaseous fuels, including a 50%-50% methane-hydrogen blend, pure methane, and</p><p dir="ltr">pure hydrogen. Additionally, the equivalence ratio within the pre-chamber was varied from values</p><p dir="ltr">of 0.9 to 1.2, and the initial pressure was set to either 1.0, 1.5, or 1.75 atm. In all cases, combustion</p><p dir="ltr">was initiated from a spark plug, causing a flame to develop until the diaphragm breaks, releasing</p><p dir="ltr">a hot jet of fuel and air from the nozzle inserted into the pre-chamber. In the pressure transducer</p><p dir="ltr">tests, it was found that hydrogen produced the highest pressures and fastest rupture times, and</p><p dir="ltr">methane produced the lowest pressures and slowest rupture times. The methane-hydrogen blend</p><p dir="ltr">provided a middle ground between the two pure fuels. An equivalence ratio of 1.1 consistently</p><p dir="ltr">provided the highest pressure values and fastest rupture out of all tested values. It was also found</p><p dir="ltr">that the orientation has a noticeable impact on both the pressure development and rupture moment</p><p dir="ltr">as higher maximum pressures were achieved when the chamber was laid flat in the “vertical jet”</p><p dir="ltr">orientation as compared to when it was stood upright in the “horizontal jet” orientation.</p><p dir="ltr">Additionally, increasing the initial pressure strongly increased the maximum developed pressure</p><p dir="ltr">but had minimal impact on the rupture moment. The tests done with the ion probe demonstrated</p><p dir="ltr">that an equivalence ratio of 1.1 produces a flame that reaches the ion probe faster than an</p><p dir="ltr">equivalence ratio of 1.0 for the methane-hydrogen blend. In its current form, the ion probe setup</p><p>18</p><p dir="ltr">has significant limitations and should continue to be developed for future studies. The properties</p><p dir="ltr">analyzed in this study deepen the understanding of the processes that occur within the pre-chamber</p><p dir="ltr">and aid in understanding the conditions that may exist in the hot jet produced by it as the nozzle</p><p dir="ltr">ruptures. The knowledge gained in the study can also be applied to develop models that can predict</p><p dir="ltr">other parameters that are difficult to physically measure.</p>