Yasir M. Al-Abdeli

Swirling turbulent non-premixed flames of methane: Flow field and compositional structure

This paper introduces a new swirl burner which has simple, well-defined boundary conditions and which stabilizes complex, turbulent, unconfined flames that are not unlike those found in practical combustors. Two flames with identical swirl numbers but differing bulk jet velocities Uj are selected for further inves- tigations. Flow field measurements reveal that a second recirculation zone may exist on the centerline of the flames further downstream of the primary recirculation zone. This is attributed to vortex breakdown. The measurements also show the presence of highly rotating collar-like flow features present between the primary and secondary recirculation zones. These regions of the flow are characterized by high tangential shear stresses uw� . The compositional structures of these methane flames are measured using the simultaneous Raman- Rayleigh laser-induced fluorescence (LIF) technique. The LIF technique is used to measure concentra- tions of OH, CO, and NO. Results are presented as scatter plots and radial Favre mean profiles of tem- perature, mixing, and composition fields. As the fuel jet velocity increases and the flame approaches blowoff, a higher proportion of fluid samples shifts away from fully burned conditions and closer to a mixing asymptote. An interesting feature of these flames is that these locally extinguished samples originate mostly from regions of the flow near the high-shear, collar-like region, which is not found in similar bluff- body flames.

Recirculation and Flowfield Regimes of Unconfined Non-Reacting Swirling Flows

Abstract This paper focuses on unconfined swirling flows of air surrounding a bluff-body having a central jet of air. Only one co-flowing primary annular air stream is swirled. The flow conditions investigated here cover a range of swirl numbers and streamwise annular velocities. Emphasis is placed on discerning typical flow structures and on discovering the influence of controlling parameters on the downstream regions of the jet and the recirculation within. The paper is part of a larger program aimed at providing an improved understanding of swirling flows. Laser doppler velocimetry is used to map three components of velocity 〈 u 〉, 〈 v 〉 and 〈 w 〉. In the flows investigated, an upstream recirculation zone is established above the burner’s exit plane. This zone is typical of that found behind bluff-bodies placed in strong axial streams. Additionally, it is shown that the formation of a downstream recirculation zone, hence the onset of vortex breakdown, depends not only on the swirl number but on other flow parameters such as the axial velocity of the primary swirling air, or its Reynolds number. Together, these two parameters appear to control the radial spread of the flow. Spatially, the progression towards downstream recirculation with changes in flow parameters is seen to occur around a fixed location in the streamwise direction. Beyond this axial position, the flowfield seems to be less affected by increases in swirl number. Non-recirculating flow structures are resolved in these flows and are found to cause elevated 〈 u ′ w ′ 〉 shear stresses.

Stability characteristics and flowfields of turbulent non-premixed swirling flames

A simple, yet representative, burner geometry is used for the investigation of highly swirling turbulent unconfined, non-premixed, flames of natural gas. The burner configuration comprises a ceramic faced bluff-body with a central fuel jet. The bluff-body is surrounded by an annulus that delivers a swirling primary flow of air. The entire burner assembly is housed in a wind tunnel providing a secondary co-flowing stream of air. This hybrid bluff-body/swirl burner configuration stabilizes complex turbulent flames not unlike those found in practical combustors, yet is amenable to modelling because of its well-defined boundary conditions. Full stability characteristics including blow-off limits and comprehensive maps of flame shapes are presented for swirling flames of three different fuel mixtures: compressed natural gas (CNG), CNG–air (1:2 by volume) and CNG–H2 (1:1 by volume). It is found that with increased fuel flow, flame blow-off mode may change with swirl number, Sg. At low swirl, the flame remains stable at the base but blows off in the neck region further downstream. At higher swirl numbers, the flames peel off completely from the burners base. Swirling CNG–air flames are distinct in that they only undergo base blow-off. In the low range of swirl number, increasing Sg causes limited improvement in the blow-off limits of the flames investigated and (for a few cases) can even lead to some deterioration over a small intermediate range of Sg. It is only above a certain threshold of swirl that significant improvements in blow-off limits appear. Six flames are selected for further detailed flowfield and composition measurements and these differ in the combination of swirl number, primary axial velocity through the annulus, Us, and bulk fuel jet velocity, Uj. Only velocity field measurements are presented in this paper. A number of flow features are resolved in these flames, which resemble those already associated with non-reacting swirling flows of equivalent swirl obtained with the present burner configuration. Additionally, asymmetric flowfields inherent to some flames are revealed where the fluidic centreline of the flow (defined in the two-dimensional (U–W velocity pair) velocity field by the ⟨ω⟩ = 0 tangential velocity contour), meanders strongly on either side of the geometric centreline downstream by about one bluff-body diameter. Flow structures revealed by the velocity data are correlated to flame shapes to yield a better understanding of how the velocity field influences the flames physical characteristics.

PRECESSION AND RECIRCULATION IN TURBULENT SWIRLING ISOTHERMAL JETS

A burner with a hybrid bluff body swirl design, incorporating a central jet surrounded by a swirling annulus, is applied to the investigation of turbulent isothermal swirling jets. Nonswirling flows stabilized on this burner form only one stagnation zone on the face of the bluff body. Such a far upstream recirculation zone is typical of those formed behind solid obstructions placed in strong axial streams. The addition of swirl leads to more complex flow patterns, which may include a second recirculation zone (in more downstream axial locations), flow instabilities, and precession. Both of these effects are investigated. Laser Doppler Velocimetry is used to ascertain the presence of downstream recirculation. The seed- (alumina-) laden central jets in these flows are visualized using a combination of laser sheet forming optics and high-speed CCD camera. Postprocessing of the imaging data succeeds in resolving the precession frequency of the jet despite the variable seed density in the experiments. Precession frequency appears to be a function of swirl number as well as the Reynolds number of both the central jet and the swirling annulus.

Turbulence–chemistry interactions in non-premixed swirling flames

Swirling flames with varying departures from blow-off have recently been studied using a simple, yet representative swirl burner. Extensive measurements of flow, stability and composition fields have already been made in turbulent non-premixed swirling flames covering a range of fuel mixtures and swirl numbers. The data, which are now made available on the World Wide Web, have revealed complex flame structures involving vortex breakdown leading to swirl-induced recirculation zones, flow instability, and the occurrence of localized extinction. These effects, which are also typical of what actually occurs in many practical combustors, are most likely inter-related rather than isolated phenomena. The nature of these interactions remains poorly understood. This paper brings together flow-field, stability and composition measurements with the aim of shedding more light on the interaction between complex fluid dynamics and finite-rate chemistry. A threshold for the occurrence of localized extinction is defined and examined with regard to rates of localized flow rotation, the occurrence of flow reversals and the velocity shear stresses. It is found that local extinction occurs in regions of high shear stress that do not necessarily overlap with the mean stoichiometric contours. The formation of an elongated recirculation (bluff-body stabilized) zone or a second, downstream region of flow recirculation is largely controlled by the swirl number and the ratio of momentum in the swirling annulus and central fuel jet.

TURBULENT SWIRLING NATURAL GAS FLAMES: STABILITY CHARACTERISTICS, UNSTEADY BEHAVIOR AND VORTEX BREAKDOWN

Laser diagnostic and flow visualization techniques have previously been applied to several reacting and isothermal swirling jets. Resolved flow features include the presence of vortex breakdown and unsteady behavior. As such, turbulent flames stabilized on the present (laboratory scale) burner bear similitude to those in larger (industrial) swirl combustors. With this in mind, the impact of vortex breakdown and unsteady behavior on flame stability continues to require further study. Such understanding can be gained through applying non-intrusive laser diagnostics to flow conditions covering a broad range of flame stability characteristics. This paper presents the results of one such investigation. Laser Doppler Velocimetry (LDV) measurements are acquired (along the centreline) to ascertain the presence of time periodicity and downstream recirculation. Unsteady behavior is identified through the spectra of axial velocity data whilst negative axial velocities delineate vortex breakdown. This information is augmented with observations of visible flame length and discussed in relation to established flame stability characteristics. Findings indicate that swirl numbers, over which improvements in flame stability occur, coincide with conditions leading to unsteady behavior and shorter flames. Because downstream flow reversal is not resolved at all such conditions, information available indicates flow unsteadiness is a clearer contributing factor, compared to vortex breakdown, on the observed stability characteristics. Shorter flames coupled with improvements in flame stability have significant implications on the design and operation of swirl combustors.

Influence of neural network training parameters on short-term wind forecasting

This paper investigates factors which can affect the accuracy of short-term wind speed prediction when done over long periods spanning different seasons. Two types of neural networks (NNs) are used to forecast power generated via specific horizontal axis wind turbines. Meteorological data used are for a specific Western Australian location. Results reveal that seasonal variations affect the prediction accuracy of the wind resource, but the magnitude of this influence strongly depends on the details of the NN deployed. Factors investigated include the span of the time series needed to initially train the networks, the temporal resolution of these data, the length of training pattern within the overall span which is used to implement the predictions and whether the inclusion of solar irradiance data can appreciably affect wind speed prediction accuracy. There appears to be a relatively complex relationship between these factors and the accuracy of wind speed prediction via NNs. Predicting wind speed based on NNs trained using wind speed and solar irradiance data also increases the prediction accuracy of wind power generated, as can the type of network selected.

Flipped Classes: Drivers for Change, Transition and Implementation

Though not a commonplace teaching and learning model, much interest is being generated in flipped classes. The arguments given for parting ways with the traditional lecture, and moving into flipped classes, are well discussed in the literature and have been derived across a broad range of disciplines. However, transitioning both teaching staff and students into flipped learning and teaching (L&T) is an issue which has attracted less attention. Before implementing flipped classes, it is also necessary to identify the range of merits which students attach to this model as well as the challenges they associate with its implementation. These matters are the focus of two research questions addressed in the present work. After presenting an overview of the justifications used to introduce flipped L&T into an engineering thermodynamics unit, the processes used to transition students into this model and the particulars of how it was applied are presented. Feedback (qualitative) derived from a questionnaire conducted at the end of the teaching semester is also reported and used to shed light on the student perspective. The chapter adds to the evidence that changing student L&T styles needs to be addressed at the design stage if introducing flipped classes and that a transitional strategy is required to assist students in adapting to the new learning environment.