Measurements of triggering and transient growth in a model lean-premixed gas turbine combustor

Instability triggering and transient growth of thermoacoustic oscillations were experimentally investigated in combination with linear/nonlinear flame transfer function (FTF) methodology in a model lean-premixed gas turbine combustor operated with CH 4 and air at atmospheric pressure. A fully premixed flame with 10kW thermal power and an equivalence ratio of 0.60 was chosen for detailed characterization of the nonlinear transient behaviors. Flame transfer functions were experimentally determined by simultaneous measurements of inlet velocity fluctuations and heat release rate oscillations using a constant temperature anemometer and OH */CH * chemiluminescence emissions, respectively. The phase-resolved variation of the local flame structure at a limit cycle was measured by planar laser-induced fluorescence of OH. Simultaneous measurements of inlet velocity, OH */CH * emission, and acoustic pressure were performed to investigate the temporal evolution of the system from a stable to a limit cycle operation. This measurement allows us to describe an unsteady instability triggering event in terms of several distinct stages: (i) initiation of a small perturbation, (ii) exponential amplification, (iii) saturation, (iv) nonlinear evolution of the perturbations towards a new unstable periodic state, (v) quasi-steady low-amplitude periodic oscillation, and (vi) fully-developed high-amplitude limit cycle oscillation. Phase-plane portraits of instantaneous inlet velocity and heat release rate clearly show the presence of two different attractors. Depending on its initial position in phase space at infinitesimally small amplitude, the system evolves towards either a high-amplitude oscillatory state or a low-amplitude oscillatory state. This transient phenomenon was analyzed using frequency- and amplitude-dependent damping mechanisms, and compared to subcritical and supercritical bifurcation theories. The results presented in this paper experimentally demonstrate the hypothesis proposed by Preetham et al. based on analytical and computational solutions of the nonlinear G-equation [J. Propul. Power 24 (2008) 1390-1402]. Good quantitative agreement was obtained between measurements and predictions in terms of the conditions for the onset of triggering and the amplitude of triggered combustion instabilities. © 2011 The Combustion Institute.

Re-conceptualising value in the engineering design process: The value cycle map

Introducing a "Cheaper, Faster, Better" product in today's highly competitive market is a challenging target. Therefore, for organizations to improve their performance in this area, they need to adopt methods such as process modelling, risk mitigation and lean principles. Recently, several industries and researchers focused efforts on transferring the value orientation concept to other phases of the Product Life Cycle (PLC) such as Product Development (PD), after its evident success in manufacturing. In PD, value maximization, which is the main objective of lean theory, has been of particular interest as an improvement concept that can enhance process flow logistics and support decision-making. This paper presents an ongoing study of the current understanding of value thinking in PD (VPD) with a focus on value dimensions and implementation benefits. The purpose of this study is to consider the current state of knowledge regarding value thinking in PD, and to propose a definition of value and a framework for analyzing value delivery. The framework-named the Value Cycle Map (VCM)- intends to facilitate understanding of value and its delivery mechanism in the context of the PLC. We suggest the VCM could be used as a foundation for future research in value modelling and measurement in PD.

System-of-system Approaches and Challenges for Multi-Site Manufacturing

In the multi-site manufacturing domain, systems-of-systems (SoS) are rarely called so. However, there exist a number of collaborative manufacturing paradigms which closely relate to system-of-system principles. These include distributed manufacturing, dispersed network manufacturing, virtual enterprises and cloud manufacturing/manufacturing-as-a-service. This paper provides an overview of these terms and paradigms, exploring their characteristics, overlaps and differences. These manufacturing paradigms are then considered in relation to five key system-of-systems characteristics: autonomy, belonging, connectivity, diversity and emergence. Data collected from two surveys of academic and industry experts is presented and discussed, with key challenges and barriers to multi-site manufacturing SoS identified.

Effects of nonuniform reactant stoichiometry on thermoacoustic instability in a lean-premixed gas turbine combustor

Detailed experimental investigations of the amplitude dependence of flame describing functions (FDF) were performed using a stratified swirl-stabilized combustor, in order to understand the combustion-acoustic interactions of CH4/air flames propagating into nonhomogeneous reactant stoichiometry. Phase-synchronized OH planar laser induced fluorescence (OH PLIF) measurements were used to investigate local reaction zone structures of forced flames. To determine the amplitude-and frequency-dependent forced flame response, simultaneous measurements of inlet velocity and heat release rate oscillations were made using a constant temperature anemometer and photomultiplier tubes with narrow-band OH*/CH* interference filters. The measurements were made over a wide range of stratification ratios, including inner stream enrichment ( θ o>θ i) and outer stream enrichment ( θ o>θ i)) conditions, and compared to the baseline condition of spatially and temporally homogeneous cases ( θ o=θ i)). Results show that for the inlet conditions investigated, fuel stratification has a significant influence on local and global flame structures of unforced and forced flames. Under stratified conditions, length scales of local contours were found to be much larger than the homogeneous case due to high kinematic viscosities associated with high temperature. Stratification has a remarkable effect on flame-vortex interactions when the flame is subjected to high-amplitude acoustic forcing, leading to different evolution patterns of FDF (amplitude and disturbance convective time) in response to the amplitude of the imposed inlet velocity oscillation. The present experimental investigation reveals that intentional stratification has the potential to eliminate or suppress the occurrence of detrimental combustion instability problems in lean-premixed gas turbine combustion systems. © 2012 Copyright Taylor and Francis Group, LLC.

Effects of preferential transport in turbulent bluff-body-stabilized lean premixed CH 4/air flames

Preferential species diffusion is known to have important effects on local flame structure in turbulent premixed flames, and differential diffusion of heat and mass can have significant effects on both local flame structure and global flame parameters, such as turbulent flame speed. However, models for turbulent premixed combustion normally assume that atomic mass fractions are conserved from reactants to fully burnt products. Experiments reported here indicate that this basic assumption may be incorrect for an important class of turbulent flames. Measurements of major species and temperature in the near field of turbulent, bluff-body stabilized, lean premixed methane-air flames (Le=0.98) reveal significant departures from expected conditional mean compositional structure in the combustion products as well as within the flame. Net increases exceeding 10% in the equivalence ratio and the carbon-to-hydrogen atom ratio are observed across the turbulent flame brush. Corresponding measurements across an unstrained laminar flame at similar equivalence ratio are in close agreement with calculations performed using Chemkin with the GRI 3.0 mechanism and multi-component transport, confirming accuracy of experimental techniques. Results suggest that the large effects observed in the turbulent bluff-body burner are cause by preferential transport of H 2 and H 2O through the preheat zone ahead of CO 2 and CO, followed by convective transport downstream and away from the local flame brush. This preferential transport effect increases with increasing velocity of reactants past the bluff body and is apparently amplified by the presence of a strong recirculation zone where excess CO 2 is accumulated. © 2011 The Combustion Institute.

The foundations of a new approach to additive manufacturing: Characteristics of free space metal deposition

In this paper, we report on the realisation of a free space deposition process (FSD). For the first time the use of a moving support structure to deposit tracks of metal starting from a substrate and extending into free space is characterised. The ability to write metal shapes in free space has wide ranging applications in additive manufacturing and rapid prototyping where the tracks can be layered to build overhanging features without the use of fixed support structures (such as is used in selective laser melting (SLM) and stereo lithography (SLA)). We demonstrate and perform a preliminary characterisation of the process in which a soldering iron was used to deposit lead free solder tracks. The factors affecting the stability of tracks and the effect of operating parameters, temperature, velocity, initial track starting diameter and starting volume were measured. A series of 10 tracks at each setting were compared with a control group of tracks; the track width, taper and variation between tracks were compared. Notable results in free space track deposition were that the initial track diameter and volume affected the repeatability and quality of tracks. The standard deviation of mean track width of tracks from the constrained initial diameter group were half that of the unconstrained group. The amount of material fed to the soldering iron before commencing deposition affected the taper of tracks. At an initial volume of 7 mm3 and an initial track diameter of 0.8 mm, none of the ten tracks deposited broke or showed taper > ∼1°. The maximum deposition velocity for free space track deposition using lead-free solder was limited to 1.5 mm s-1. © 2011 Elsevier B.V. All rights reserved.

A Computer integrated unified modelling approach to responsive manufacturing

Computer modelling approaches have significant potential to enable decision-making about various aspects of responsive manufacturing. In order to understand the system prior to the selection of any responsiveness strategy, multiple process segments of organisations need to be modelled. The article presents a novel systematic approach for creating coherent sets of unified enterprise, simulation and other supporting models that collectively facilitate responsiveness. In this approach, enterprise models are used to explicitly define relatively enduring relationships between (i) production planning and control (PPC) processes, that implement a particular strategy and (ii) process-oriented elements of production systems, that are work loaded by the PPC processes. Coherent simulation models, can in part be derived from the enterprise models, so that they computer execute production system behaviours. In this way, time-based performance outcomes can be simulated; so that the impacts of alternative PPC strategies on the planning and controlling historical or forecasted patterns of workflow, through (current and possible future) production system models, can be analysed. The article describes the unified modelling approach conceived and its application in a furniture industry case study small and medium enterprise (SME). Copyright © 2010 Inderscience Enterprises Ltd.