Modelling of Spray Combustion with Doubly Conditional Moment Closure

Turbulent spray combustion is characterised by a strong coupling of evaporation, mixing and chemical reaction. This leads to a wide spectrum of combustion regimes, where self-propagating premixed flames and diffusion-controlled non-premixed flames may occur simultaneously within the same flame. The physical processes involved in spray combustion and their interaction take place over a broad range of scales, which makes their modelling in numerical simulations challenging. This thesis presents the development of Doubly Conditional Moment Closure (DCMC) for the modelling of turbulent spray combustion. This modelling approach allows us to consider the effects of finite-rate chemistry and spray evaporation on the flame. Using a parametrisation of the flame structure, based on mixture fraction and reaction progress variable permits us to resolve premixed, non-premixed and intermediate combustion modes. In the first part of this thesis, the model development is presented. With its foundation as a statistical model, DCMC does not require any strong assumption in terms of the combustion mode or regime. The DCMC equation is derived in a general form, which involves only a minimum number of modelling assumptions about the physical processes involved. Closure for the DCMC equation is discussed and a complete set of models is suggested. Since little experience exists in the modelling of doubly-conditional terms, the closure models were generalised from conventional Conditional Moment Closure (CMC) or adapted from other combustion models with similar parametrisation. In the second part, the DCMC model is validated for two test flames. The DCMC model was first applied to the Cambridge spray jet flame using the Reynolds-Averaged Navier-Stokes (RANS) approach. This flame is characterised by significant pre-vaporisation and behaves as a propagating spray flame, with similarities to premixed flames, but with small-scale inhomogeneity in the gaseous mixture and the presence of liquid droplet interacting with the flame – a problem which requires the doubly-conditional description of the flame structure employed in the DCMC model. The role of the spray terms on the flame structure and mixing field were assessed using RANS and promising results were obtained. Finally, a Large-Eddy Simulation (LES) with DCMC acting as sub-grid scale combustion model was applied to the Rouen spray jet flame. LES-DCMC was found to accurately predict the spray statistics, lift-off height and flame shape. Small-scale effects of the spray on the flame could be resolved thanks to the doubly-conditional parametrisation of the flame structure. Temporal fluctuations and spatial variations of the flame structure were investigated. Spatial gradients of the doubly-conditional flame structure were small and convective transport was found to play a minor role on the flame structure compared to the effects of micro-mixing and chemical reaction in the DCMC equation. The findings of this work suggest that, besides spray combustion, DCMC shows great potential for the modelling of partially premixed flames and extinction.PhD scholarship by the Gates Cambridge Trust. High-performance computing resources provided by the UK Consortium on Turbulent Reacting Flows (UKCTRF)

An experimental study of transition to turbulence in plane Poiseuille flow

Zsfassung in dt. SpracheDer laminar-turbulente Umschlag einer druckgetriebenen Strömung zwischen zwei parallelen Platten, auch ebene Poiseuille Strömung genannt, wird mit Hilfe einer Methode zur Strömungsvisualisierung untersucht. Dieser Übergangsprozess ist subkritisch, sodass währenddessen laminare und turbulente Bereiche in der Strömung gleichzeitige vorhanden sind. Der Fokus dieser Arbeit liegt auf der zeitlichen Entwicklung dieser räumlich beschränkten, turbulenten Bereiche in einem experimentellen Kanal großer Breite und Länge. Zu diesem Zweck werden ovale Wirbelflecken unter Zuhilfenahme von lokalen Störungen einzeln ausgelöst und deren zeitliche Entwicklung verfolgt, wobei auffällt dass diese Flecken eine Streifenform ausbilden. Deshalb wurde ein neuer Störmechanismus entworfen, mit dem die Entstehung solcher Streifen in der Strömung direkt angeregt werden können. Hierdurch konnte gezeigt werden, dass, erstens, diese Streifen die natürlich vorkommende Form lokal beschränkter Turbulenz darstellen und, zweitens, dass diese Streifen in einem deutlich niedrigeren Bereich von Reynolds-Zahlen bestehen können, als bisher geglaubt. Es konnte auch gezeigt werden, dass diese Streifen einen ausgezeichneten Winkel relativ zur Fließrichtung annehmen. Diese Winkel liegen in einem engen Bereich dessen Grenzen von der Reynolds-Zahl abhängen. Außerdem werden Zu- und Abnahme der Größe von existierenden Streifen, sowie die Keimbildung neuer Bänder detailliert beschrieben. Zum Schluss wurde der kritische Punkt für den Turbulenten Umschlag einer ebenen Poiseuilleströmung aus den durchschnittlichen Zuwachs- bzw. Abklingraten abgeleitet. Dieser Schwellwert, unterwelchem alle turbulenten Strukturen abklingen, liegt deutlich tiefer als alle bisherigen Schätzungen.The laminar-turbulent transition of a pressure driven flow between two parallel plates, called plane Poiseuille flow, is studied using a flow visualisation technique. The transition process in the subcritical regime is characterised by the co-existence of laminar and turbulent regions. The present work focuses on the evolution of localised turbulence in a channel with a large aspect ratio and length. For this purpose, individual spot-shaped structures are triggered via a localised perturbation. Their development is monitored in time, which shows that these turbulent spots grow into the shape of stripes. Consequently, a new perturbation technique is developed that directly excites localised stripes in the flow. It is shown that the stripes are the natural form of localised turbulence and that they can exists at Reynolds numbers much lower than previously believed. It is also shown that these stripes are inclined to the mean flow direction in a narrow range of angles which depends on the Reynolds number. Furthermore, the growth and decay of existing turbulent stripes and the nucleation of new stripes is described in detail. Finally, the average growth and decay rates of these stripes are used to deduce the critical point in plane Poiseuille flow, below which turbulence cannot be sustained. This critical number is lower than existing estimates in the literature.5

Velocity reconstruction in puffing pool fires with physics-informed neural networks

Pool fires are canonical representations of many accidental fires which can exhibit an unstable unsteady behavior, known as puffing, which involves a strong coupling between the temperature and velocity fields. Despite their practical relevance to fire research, their experimental study can be limited due to the complexity of measuring relevant quantities in parallel. In this work, we analyze the use of a recent physics-informed machine learning approach, called hidden fluid mechanics (HFM), to reconstruct unmeasured quantities in a puffing pool fire from measured quantities. The HFM framework relies on a physics-informed neural network (PINN) for this task. A PINN is a neural network that uses both the available data, here the measured quantities, and the physical equations governing the system, here the reacting Navier-Stokes equations, to infer the full fluid dynamic state. This framework is used to infer the velocity field in a puffing pool fire from measurements of density, pressure, and temperature. In this work, the dataset used for this test was generated from numerical simulations. It is shown that the PINN is able to reconstruct the velocity field accurately and to infer most features of the velocity field. In addition, it is shown that the reconstruction accuracy is robust with respect to noisy data, and a reduction in the number of measured quantities is explored and discussed. This study opens up the possibility of using PINNs for the reconstruction of unmeasured quantities from measured ones, providing the potential groundwork for their use in experiments for fire research.Aerodynamic

A laser-induced breakdown spectroscopy method to assess the stochasticity of plasma-flame transition in sprays

Abstract: An experimental approach is presented to evaluate the impact of plasma composition arising from pulse-to-pulse energy and mixture fluctuations on the non-resonant laser-induced ignition of sprays. This allows for spark events to be conditioned on the successful or failed establishment of a flame kernel, a phase dominated by plasma decomposition and recombination reactions and the on-set of combustion reactions, that is, independent of the subsequent flame growth phase controlled by propagation phenomena only, such as fuel availability and turbulent strain. For that, laser-induced breakdown spectroscopy of the spark-generated plasma is carried out, followed by OH ∗ high-speed imaging of the kernel. Exploratory experiments in spatially uniform and polydisperse kerosene droplet distributions in a jet suggest that the hydrogen concentration in the plasma deriving from the fuel dissociated by the spark is closely related to the generated OH ∗ radicals levels and, in turn, with the success of establishing a flame kernel. This suggests that the ignition process is heavily controlled by mixture fluctuations at the spark, inherent of spray flows. The instantaneous mixture at the spark is estimated with a stochastic model, with the probability density function of the equivalence ratio exhibiting values higher than twice the mean value, while the highest probability occurs at lean conditions between the gaseous equivalence ratio and the overall equivalence ratio. The findings corroborate insights on the early-phase ignition obtained from direct numerical simulations, and the framework paves the way for the development of smart online engine-health tools to assess relight capability in future aeroengines

Simulations and experiments on the ignition probability in turbulent premixed bluff-body flames

The ignition characteristics of a premixed bluff-body burner under lean conditions were investigated experimentally and numerically with a physical model focusing on ignition probability. Visualisation of the flame with a 5 kHz OH* chemiluminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work in non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability P_ign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. Flows corresponding to flames close to the blow-off limit could not be ignited, although such flames were stable if reached from a richer already ignited condition. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high P_ign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. A modelling effort that takes convection and localised flame quenching into account by tracking stochastic virtual flame particles, previously validated for non-premixed and spray ignition, was used to estimate the ignition probability. The applicability of this approach to premixed flows was first evaluated by investigating the model’s flame propagation mechanism in a uniform turbulence field, which showed that the model reproduces the bending behaviour of the S_T -versus-u' curve. Then ignition simulations of the bluff-body burner were carried out. The ignition probability map was computed and it was found that the model reproduces all main trends found in the experimental study.M.P. Sitte gratefully acknowledges financial support from the Gates Cambridge Trust. The experiments were carried out by E. Bach who was a Masters student from Karlsruhe Institute of Technology visiting the University of Cambridge in 2011.This is the final version of the article. It first appeared from Taylor & Francis via http://dx.doi.org/10.1080/13647830.2016.115575