Improvement of waterjet and abrasive waterjet nozzle

This investigation is concerned with the improvement of the nozzle design for water and abrasive water jet machining. The mechanism of formation and characteristics of pure water and abrasive water jets are investigated in order to determine quasi-optimal process conditions. To improve the pure water jet machining, a pulsed water jet nozzle, which employs the principle of the Helmholtz type resonator, is investigated experimentally and numerically. The experiments show the advantages of this nozzle over the commercial nozzle in cutting and cleaning. A numerical solution of the differential equations of continuity, momentum conservation, turbulent kinetic energy and dissipation for two dimensional axi-symmetric flow by employing the FIDAP package is developed and used for the numerical prediction of pulsed turbulent flow inside the nozzle. The determination of the optimal nozzle parameters aided by numerical simulation is carried out and the best ratios of the parameters are: h (cavity length) / d1 (diameter of upstream nozzle) = 3.0 and d2 (diameter of downstream nozzle) / d1 (diameter of upstream nozzle) = 1.3. The results of simulation agree well with the experiments. The numerical prediction of the velocity at the exit of the pulsed nozzle is validated by the velocity measurement by a laser transit anemometer. The obtained velocity changes periodically and ranges from 190 m/s to 230 m/s. A numerical analysis enables us to evaluate nozzle design and the effectiveness of the numerical prediction is validated experimentally. The numerical solutions and experimental results present the improvement on the pure water cutting and cleaning and provide a technological basis for the improvement of pulsed water jet machining and technology. To increase the efficiency of abrasive water jet machining, an improved abrasive water jet nozzle is developed and experimentally investigated. The performance of the abrasive water jet is improved by the modification of the abrasive particles path prior to the collision with the water jet. This modification is obtained by control of the angle (a) between the top-shaped surface of the focusing tube and the water flow direction and change of distance (H) between the water nozzle and focusing tube. The improvement of water-particles mixing increases the rate of material removal and simplifies the alignment procedure. It is found that the optimal parameters for the nozzle design are: a = 45° and H = 1.587 mm. The experimental results and analysis show the potential of this modified nozzle for applications in abrasive water jet machining

Characterization of a Pulsating Drill Bit Blaster

The drill bit blaster (DBB) studied in this paper aims to maximize the drilling rate of penetration (ROP) by using a flow interrupting mechanism to create drilling fluid pulsation. The fluctuating fluid pressure gradient generated during operation of the DBB could lead to more efficient bit cutting efficiency due to substrate depressurization and increased cutting removal efficiency and the vibrations created could reduce the drill string friction allowing a greater weight on bit (WOB) to be achieved. In order to maximize these mechanisms the effect of several different DBB design changes and operating conditions were studied in above ground testing. An analytical model was created to predict the influence of various aspects of the drill bit blaster design, operating conditions and fluid properties on the bit pressure characteristics and compared against experimental results. The results indicate that internal tool design has a significant effect on the pulsation frequency and amplitude, which can be accurately modeled as a function of flowrate and internal geometry. Using this model an optimization study was conducted to determine the sensitivity of the fluid pulsation power on various design and operating conditions. Application of this technology in future designs could allow the bit pressure oscillation frequency and amplitude to be optimized with regard to the lithology of the formations being drilled which could lead to faster, more efficient drilling, potentially cutting drilling costs and leading to a larger number of oil and natural gas plays being profitable.Mechanical & Aerospace Engineerin

Experimental Characterization of Combustion Instabilities and Flow-Flame Dynamics in a Partially-Premixed Gas Turbine Model Combustor.

Partially-premixed, swirl combustion is applied in gas turbine combustors to achieve flame stabilization and reduced emission production. However, this method is also inherently sensitive to combustion instabilities which can cause large pressure, velocity, and heat release fluctuations. This thesis investigates thermoacoustic coupling created by flow-flame dynamics in a gas turbine model combustor (GTMC) for a variety of fuels and operating flow rates. Several naturally occurring instability modes were identified to control the acoustic response of the system, including Helmholtz resonances from the plenum and convective-acoustic effects which cause equivalence ratio oscillations. Laser Doppler velocimetry was used to measure radial flow in the GTMC, which can set up flow-fields which create loudly resonating flat-shaped flames, in comparison to quiet V-shaped flames. Flame location and shape altered convective time delays which determine the relative phases of pressure and heat release oscillations. Simultaneous pressure and chemiluminescence imaging showed that the heat release, pressure fluctuations, and flame motion are all coupled at the same instability frequency. Videos of the flame motion also revealed that the precessing vortex core (PVC), created by the swirling flow, influences the rocking behavior of the flame. Acetone was added to the fuel to act as a tracer in fluorescence measurements which indicated the localization of unburned fuel. It was discovered that fuel was distributed in lobes which corresponded to locations surrounding the shear layer outside of the central recirculation zone, and that the relative distribution of the lobes adjusted to forcing by the flow. Finally, high-speed formaldehyde planar laser-induced fluorescence was applied to study the motion of preheat zone surfaces in response to the oscillations of the instability. The flame surface density and wrinkling fluctuated at the acoustic frequency and displayed dampened motions correlated with the PVC precession. In non-resonating flames, the behavior of the formaldehyde structure and marked flame surfaces were dominated by the PVC motion, but the degree of surface area fluctuations was reduced compared to unstable flames. Instabilities in the GTMC are driven by a complex combination of thermoacoustic and flow-field couplings which are influenced by the operational conditions, fueling, mixing, and convective time delays.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/102385/1/pallison_1.pd

Acoustic absorption and the unsteady flow associated with circular apertures in a gas turbine environment

This work is concerned with the fluid dynamic processes and the associated loss of acoustic energy produced by circular apertures within noise absorbing perforated walls. Although applicable to a wide range of engineering applications particular emphasis in this work is placed on the use of such features within a gas turbine combustion system. The primary aim for noise absorbers in gas turbine combustion systems is the elimination of thermo-acoustic instabilities, which are characterised by rapidly rising pressure amplitudes which are potentially damaging to the combustion system components. By increasing the amount of acoustic energy being absorbed the occurrence of thermo-acoustic instabilities can be avoided. The fundamental acoustic characteristics relating to linear acoustic absorption are presented. It is shown that changes in orifice geometry, in terms of gas turbine combustion system representative length-to-diameter ratios, result in changes in the measured Rayleigh Conductivity. Furthermore in the linear regime the maximum possible acoustic energy absorption for a given cooling mass flow budget of a conventional combustor wall will be identified. An investigation into current Rayleigh Conductivity and aperture impedance (1D) modelling techniques are assessed and the ranges of validity for these modelling techniques will be identified. Moreover possible improvements to the modelling techniques are discussed. Within a gas turbine system absorption can also occur in the non-linear operating regime. Hence the influence of the orifice geometry upon the optimum non-linear acoustic absorption is also investigated. Furthermore the performance of non-linear acoustic absorption modelling techniques is evaluated against the conducted measurements. As the amplitudes within the combustion system increase the acoustic absorption will transition from the linear to the non-linear regime. This is important for the design of absorbers or cooling geometries for gas turbine combustion systems as the propensity for hot gas ingestion increases. Hence the relevant parameters and phenomena are investigated during the transition process from linear to non-linear acoustic absorption. The unsteady velocity field during linear and non-linear acoustic absorption is captured using particle image velocimetry. A novel analysis technique is developed which enables the identification of the unsteady flow field associated with the acoustic absorption. In this way an investigation into the relevant mechanisms within the unsteady flow fields to describe the acoustic absorption behaviour of the investigated orifice plates is conducted. This methodology will also help in the development and optimisation of future damping systems and provide validation for more sophisticated 3D numerical modelling methods. Finally a set of design tools developed during this work will be discussed which enable a comprehensive preliminary design of non-resonant and resonant acoustic absorbers with multiple perforated liners within a gas turbine combustion system. The tool set is applied to assess the impact of the gas turbine combustion system space envelope, complex swirling flow fields and the propensity to hot gas ingestion in the preliminary design stages

Investigation of Partially Premixed Combustion Instabilities through Experimental, Theoretical, and Computational Methods.

Partially premixed combustion has the merits of lower emission as well as higher efficiency. However, its practical application has been hindered by its inherent instabilities. This work is a study of instabilities in partially premixed combustion, through a combination of numerical simulation, theoretical modeling, and experimental investigation, with the hope of furthering our understanding of the underlying physics. Specifically, a Flamelet/Progress Variable (FPV) combustion model in the context of Large Eddy Simulation (LES) is extended to simulate a piloted (partially) premixed jet burner (PPJB). The ability and shortcomings of this state-of-the-art high fidelity combustion model are assessed. Furthermore, a Modular Reduced-order Model Framework (MRMF) is developed to integrate a range of elementary models to describe the instabilities that may occur in combustors utilizing partially premixed combustion technologies. A multi-chamber Helmholtz analysis is implemented, which is shown to be an improvement over previous single-chamber analyses. The assumptions and predictions of the proposed model are assessed by pressure and simultaneous Particle Image Velocimetry (PIV)–formaldehyde (CH2O) Planar Laser Induced Fluorescence (PLIF) measurements on a Gas Turbine Model Combustor (GTMC) at a sustained rate of 4 kHz. The proposed model is shown to be able to predict the instability frequency at experimental conditions. It also explains the trends of the variation of instability frequency as mass flow rates and burner geometry are changed, as well as the measured phase shift between different chambers of the burner. Finally, under the current framework an explanation of the dependence of the existence of combustion instability on equivalence ratio is provided.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111342/1/yuntaoc_1.pd

Enhancement of Cavitation Intensity in Co-Flow and Ultrasonic Cavitation Peening

Water cavitation peening is a surface treatment process used to generate beneficial compressive residual stresses while being environmentally sustainable. Compressive residual stresses generated by the collapse of the cavitation cloud at the workpiece surface result in enhanced high cycle fatigue and wear performance. Co-flow water cavitation peening, a variant of cavitation peening involves injection of a high-speed jet into a low-speed jet of water, which makes the process amenable to automation and imparts the variant with the ability to process large structural components. Ultrasonic cavitation peening, another variant of cavitation peening, is used for peening small areas. However, an increase in cavitation intensity is needed to reduce the processing time for practical applications and to enhance process capabilities for a wide range of materials in both these variants. An experimental investigation along with numerical modelling is presented to demonstrate cavitation intensity enhancement through suitable modifications to the inner jet nozzle design in co-flow water cavitation peening. Particularly, the effects of upstream inner jet organ pipe nozzle geometry, inner jet nozzle orifice taper, and inner jet nozzle orifice length are studied to show enhanced cavitation intensity, measured via extended mass loss tests, strip curvature and residual stress measurements, high-speed videography, and impulse pressure measurements. It is found that the optimum inner jet organ pipe nozzle design, which generates enhanced pressure fluctuations through the introduction of a resonating chamber in the upstream section of the inner jet nozzle, generates 61% greater mass loss compared to the unexcited inner jet nozzle. Strip curvature, high speed imaging, and impulse pressure measurements support the mass loss results. Finally, residual stresses generated with the optimum organ pipe nozzle are shown to be deeper and more compressive than those generated with the unexcited nozzle design. The inner jet nozzle variants with diverging, zero and converging tapers are investigated experimentally and numerically to understand their influence on cavitation intensity. It is shown that the converging taper nozzle generates greater cavitation intensity, measured via mass loss and strip curvature measurements, than the zero and diverging taper nozzles. Impulse pressure measurements show the greater frequency of high-intensity events generated by the converging taper nozzle compared to the zero and diverging taper nozzles. Computational fluid dynamics (CFD) simulations help explain the experimental findings. Four nozzle variants with varying inner jet nozzle orifice length to orifice diameter ratios of 1,2,5 and 10 are investigated experimentally and numerically. The inner jet nozzle with an orifice length to orifice diameter ratio of 2 is shown to generate greater cavitation intensity than the other inner jet nozzles. A PEO aqueous solution (cavitation media) with 1000 parts per million by weight (wppm) polymer concentration is shown to enhance cavitation intensity by 69% over cavitation media with only water. High speed videography, impulse force, and surface roughness measurements confirm the greater cavitation activity in the 1000 wppm PEO aqueous solution. This demonstrates that suitable modifications can be engineered in the cavitation media to enhance cavitation intensity in ultrasonic cavitation peening. Thus, this thesis presents experimental and numerical investigations leading to superior inner jet nozzle design in co-flow cavitation peening and an experimental investigation of the role of polymer additives for suitable modification of cavitation media to enhance cavitation intensity in ultrasonic cavitation peening.Ph.D

The pulsations and energy transfers in a double-orifice combustor

This work examines the effect of longitudinal oscillations on the heat transfer in a naturally-aspirating, propane-fuelled combustor. Previous investigations in the field have been predominantly experimental in nature, although theoretical studies of the effect of oscillations on local heat transfer coefficients have been made. In this work, a linearised wave equation, which governs the propagation of sound waves in a gas confined by a straight tube and exhibiting an axial temperature variation, is used to correlate local heat transfer coefficients by a quasi-steady-state method. An apparatus was . constructed, and measurements of the gas, wall and water temperatures and of the gas pressure amplitudes were taken in a concentric tube heat exchanger, which formed part of the resonating section