Combustor

Abstract: Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. TBCs, which comprise metal and ceramic multilayers, insulate turbine and combustor engine components from the hot gas stream, and improve the durability and energy efficiency of these engines. Improvements in TBCs will require a better understanding of the complex changes in their structure and properties that occur under operating conditions that lead to their failure. The structure, properties, and failure mechanisms of TBCs are herein reviewed, together with a discussion of current limitations and future opportunities.

Abstract: Combustion instability remains a critical issue limiting the development of low-emission, lean-premixed (LPM) gas turbine combustion systems. The present work provides a comprehensive review of the advances made over the past two decades in this area. Recent developments in industrial dry-low-emission (DLE) swirl-stabilized combustors are first summarized. Various swirl injector configurations and related flow characteristics, including vortex breakdown, precessing vortex core, large-scale coherent structures, and liquid fuel atomization and spray formation, are discussed. Nonlinear behaviors of combustion processes observed in combustors are described. The influence of fuel preparation, combustor geometry, and operating conditions on combustion characteristics in swirl-stabilized combustors is examined. The mechanisms driving combustion instabilities, including hydrodynamic instabilities, equivalence ratio fluctuations, flame surface variations, and oscillatory liquid fuel atomization and evaporation are investigated. Instability stabilization methods, including both passive and active control techniques, are also reviewed. Finally, recent progress in both analytical modeling and numerical simulation of swirl-stabilized combustion are surveyed.

Abstract: Higher operating efficiencies, fewer pollutant emissions, and low capital investment have made gas turbines a dominant technology for a new power generating capacity in the US and worldwide. This book offers gas turbine users and manufacturers a valuable resource to help them sort through issues associated with combustion instabilities. In the last ten years, substantial efforts have been made in the industrial, governmental, and academic communities to understand the unique issues associated with combustion instabilities in low-emission gas turbines. The objective of this book is to compile these results into a series of chapters that address the various facets of the problem. The Case Studies section speaks to specific manufacturer and user experiences with combustion instabilities in the development stage and in fielded turbine engines. The book then goes on to examine The Fundamental Mechanisms, The Combustor Modeling, and Control Approaches.

Abstract: This paper reviews the occurrence of the precessing vortex core (PVC) and other instabilities, which occur in, swirl combustion systems whilst identifying mechanisms, which allows coupling between the acoustics, combustion and swirling flow dynamics to occur. Initially, the occurrence of the PVC in free and confined isothermal flows is reviewed by describing its occurrence in terms of a Strouhal number and geometric swirl number. Phase locked particle image velocimetry and laser doppler anemometry is then used to describe the three-dimensional flow fields, which are generated when swirling flow is discharged into an open environment. This shows the presence of a rotating and precessing off centred vortex and associated central recirculation zone (CRZ), extending up to one burner exit diameter. The presence of axial radial eddies close to the burner mouth, in and around the CRZ, is clearly shown. Typically one large dominant PV is found, although many harmonics can be present of lower amplitude. The occurrence of these phenomena is very much a function of swirl number and burner geometry. Under combustion conditions the behaviour is more complex, the PVC occurrence and amplitude are also strong functions of mode of fuel entry, equivalence ratio and level of confinement. Axial fuel entry, except at exceptionally weak mixture ratios, often suppresses the vortex core precession. A strong double PVC structure is also found under certain circumstances. Premixed or partially premixed combustion can produce large PVC, similar in structure to that found isothermally: this is attributed to the radial location of the flame front at the swirl burner exit. Provided the flame is prevented from flashing back to the inlets values of Strouhal number for the PVC were excited by ∼2 compared to the isothermal condition at equivalence ratios around 0.7. Confinement caused this parameter to drop by a factor of three for very weak combustion. Separate work on unconfined swirling flames shows that even when the vortex core precession is suppressed the resulting swirling flames are unstable and tend to wobble in response to minor perturbations in the flow, most importantly close to the burner exit. Another form of instability is shown to be associated with jet precession, often starting at very low or zero swirl numbers. Jet precession is normally associated with special shapes of nozzles, large expansions or bluff bodies and is a different phenomenon to the PVC. Strouhal numbers are shown to be at least an order of magnitude less than those generated by the PVC generated after vortex breakdown. Oscillations and instabilities in swirl combustion systems are illustrated and analysed by consideration of several cases of stable oscillations produced in swirl burner/furnace systems and two where the PVC is suppressed by combustion. The first cases is a low frequency 24 Hz oscillation produced in a 2 MW system whereby the PVC frequency is excited to nearly six times that for the isothermal case due to interaction with system acoustics. Phase locked velocity and temperature measurements show that the flame is initiated close to the burner exit, surrounding the CRZ, but is located inside a ring of higher velocity flow. Downstream the flame has expanded radially past the high velocity region, but does not properly occupy the whole furnace. This allows the flame and swirling flow to wobble, exciting instability. The next family of oscillations reviewed occur in a 100 kW swirl burner/furnace systems whereby oscillations in the ∼40 Hz range are excited with flow fields akin to those found in pulsating combustors where the flow is periodically stopped in the limit cycle of oscillation. The phase locked velocity and temperature measurements show a number of mechanisms that can excite oscillation including substantial variations in shape and size of the CRZ during the limit cycle of oscillation, and wobble of the whole flame and flow as shown by negative tangential velocities close to the centre line. Analysis is then made of a high frequency ∼240 Hz oscillation in the same 100 kW swirl burner/furnace system, this oscillation being caused by minor geometry changes. The flame was shown to not fully occupy the furnace, allowing irregular wobble and precession of the flow and flame to develop, being especially noticeable close to the outer wall. The addition of an exit quarl to the swirl burner is shown to substantially reduce the amplitude of oscillation by eliminating the external recirculation zone (ERZ), reducing flow/flame wobble and variations in the size and shape of the CRZ. The quarl used was designed to largely occupy the space normally taken up by the ERZ. Two gas turbine combustor units firing into chambers are then considered, strong PVCs are developed under isothermal conditions, these are suppressed with premixing in the equivalence number range 0.5–0.75. PVC suppression is attributed to the equivalence ratios used, the burner configuration, location of the flame front and associated combustion aerodynamics. Other work on an industrial premixed gas turbine swirl burner and can showed the formation of strong helical coherent structures for equivalence ratios greater than 0.75. LES studies showed the PVC contributed to instability by triggering the formation of radial axial eddies, generating alternating patterns of rich and lean combustion sufficient to reinforce combustion oscillations via the Rayleigh criteria. Finally, it was concluded that coupling between the acoustics and flame/flow dynamics occurs through a number of mechanisms including wobble/precession of the flow and flame coupled with variations in the size and shape of the CRZ arising from changes in swirl number throughout the limit cycle. Remedial measures are proposed.

Abstract: Swirling flows have been commonly used for a number of years for the stabilization of high-intensity combustion processes. In general these swirling flows are poorly understood because of their compelexity. This paper describes the recent progress in understanding and using these swirling flows. The main effects of swirl are to improve flame stability as a result of the formation of toroidal recirculation zones and to reduce combustion lengths by producing high rates of entrainment of the ambient fluid and fast mixing, particularly near to the boundaries of recirculation zones. Two main types of swirl combustor can be identified as follows: The Swirl Burner. Here swirling flow exhausts into a furnace or cavity combustion occurs in and just outside the burner exit. The Cyclone Combustion Chamber. Here air is injected tangentially into a large, usually, cylindrical chamber and exhausts through a centrally located exit hole in one end. Combustion mostly occurs inside the cyclone chamber. Initially the isothermal performance of swirl combustors is considered, and it is demonstrated that, contrary to many previous assumptions, the flow is often not axisymmetric but three-dimensional time-dependent. Under most normal nonpremixed combustion conditions, the swirling flow returns to axisymmetry, although there is still a residual presence of the three-dimensionality, particularly on the boundary of the reverse flow zone. Swirl increases considerably the stability limits of most flames; in fact with certain swirl burners, the blow-off limits are virtually infinite. Cyclone combustion chambers have large internal reverse flow zones which provide very long residence times for the fuel/air mixture. They are typically used for the combustion of difficult materials such as poor quality coal or vegetable refuse. In contrast to the swirl burner which usually has one central toroidal, recirculation zone, the cyclone combustor often has up to three concentric toroidal recirculation zones. Sufficient information is also available to indicate that stratified or staged fuel or air entry may be used to minimize noise, hydrocarbon, and NOx emissions from swirl combustors.