The significance of beam steering on laser-induced incandescence measurements in laminar counterflow flames

Published online: 17 October 2018Beam steering is often encountered in laser diagnostic measurements, especially in flame environments, due to changes in refractive index caused by thermal and species gradients. It can negatively affect the accuracy of the results. In this work, the effects of beam steering on laser-induced incandescence (LII) measurements of pre-vaporized-liquid counterflow flames are assessed. The focus on counterflow flames is to facilitate future detailed experimental campaigns on one-dimensional nonpremixed sooty flames. It is found that the temperature and species gradients in the counterflow configuration have a much more significant impact on the beam profile than in laminar flat flames, especially for heavier fuels. As a result of the changes in the beam profile, for the same applied laser energy, the local fluence shifts markedly with fuel type, therefore, having a direct impact on the LII measurements. A procedure is developed for ensuring accurate measurements and it is shown that, for a specific fuel, it is possible to tailor the laser energy, such that the collected LII signal in the counterflow flames is nearly independent of beam-steering effects.S. Kruse, P. R. Medwell, J. Beeckmann, H. Pitsc

Characteristics of turbulent n-heptane jet flames in a hot and diluted coflow

Distinctive behaviour of turbulent n -heptane jet flames is revealed by conventional photography and laser-induced fluorescence of the hydroxyl radical (OH-LIF) performed in a Jet in Hot Coflow (JHC) burner. Prevaporised n -heptane, carried by air, issues into a hot vitiated coflow at two temperatures (1250 K and 1315 K) with three coflow oxygen levels (X O 2 = 3%, 6%%, and 9%), mimicking moderate or intense low oxygen dilution (MILD) combustion conditions. Results for n -heptane flames are compared to those for ethylene, ethanol, and natural gas flames. Two main discrepancies are revealed: firstly, the apparent liftoffheight of n -heptane flames decreases gradually with the increasing coflow oxygen level, while the appar- ent liftoff height of other fuels exhibit a non-monotonic trend; secondly, a transitional flame structure of n -heptane occurs in a coflow with X O 2 = 3%, while this structure occurs in a coflow with X O 2 >/-9% for other fuels. Calculations using a closed homogeneous reactor model support the interpretation of the experimental data, namely that the temporal profiles of OH in the n -heptane flames are similar for the 3% and 9% O 2 cases, differing from the behaviour of other fuels. A comparative analysis of n -heptane and ethanol chemistry, focused on the fuel pyrolysis and net negative heat release regions, suggests that it is more difficult for n -heptane than ethanol to meet one of the criteria of MILD combustion.Jingjing Ye, Paul R. Medwell, Michael J. Evans, Bassam B. Dall

A Theoretical Review of Rotating Detonation Engines

Rotating detonation engines are a novel device for generating thrust from combustion, in a highly efficient, yet mechanically simple form. This chapter presents a detailed literature review of rotating detonation engines. Particular focus is placed on the theoretical aspects and the fundamental operating principles of these engines. The review covers both experimental and computational studies, in order to identify gaps in current understanding. This will allow the identification of future work that is required to further develop rotating detonation engines

Temperature and reaction zone imaging in turbulent swirling dual-fuel flames

Gaseous and liquid dual-fuel flames present both a practical approach to emissions reduction and a challenge to current state-of-the-art combustion modelling. This paper uses simultaneously imaged temperature and normalised OH signal fields to investigate flame structure and provide experimental data for model validation across a range of swirl-stabilised n-heptane spray flames. These data are obtained by non-linear excitation regime two-line atomic fluorescence (NTLAF) of indium, and planar laser-induced fluorescence (OH-PLIF), respectively. Swirling gas streams are varied by flowrate (63–88% of blow-off), premixed equivalence ratio (including air-only), and by type of gaseous fuel (natural gas and hydrogen). Results are used to describe how hot combustion products interact with the fuel spray: heating and diluting the region above the apex of the spray cone at low air flowrates but drawing fuel into outer branches of the flame with increasing air flowrates. Adding natural gas to the swirling air stream, at a concentration below the lean flammability limit, gives rise to a temperature increase in the outer branches with little effect on the hot region above the apex of the spray, along the burner centreline. The size of this region is significantly reduced; however, using hydrogen. As the concentration of gaseous fuel increases towards the lean flammability limit, peak temperatures shift towards the outer branch of the flame. Exceeding the lean flammability limit, an additional reaction zone begins to form in the premixed swirling stream, adjacent to the outer branch of the swirl flame. Stable outer branches of the swirl flame, however, become less prevalent and the peak temperatures of the spray flame return to burner centreline. This study provides insight into the complex behaviour of dual-fuel flames, a complementary dataset to related, PLIF-only studies and validation data for the development of numerical modelling tools.M.J.Evans, J.A.M.Sidey, J.Ye, P.R.Medwell, B.B.Dally, E.Mastorako

Multiple line arrays for the characterization of aeroacoustic sources using a time-reversal method

This letter investigates the use of multiple line arrays (LAs) in a Time-Reversal Mirror for localizing and characterizing multipole aeroacoustic sources in a uniform subsonic mean flow using a numerical Time-Reversal (TR) method. Regardless of the original source characteristics, accuracy of predicting the source location can be significantly improved using at least two LAs. Furthermore, it is impossible to determine the source characteristics using a single LA, rather a minimum of two are required to establish either the monopole or dipole source nature, while four LAs (fully surrounding the source) are required for characterizing a lateral quadrupole source.A. Mimani, C. J. Doolan, and P. R. Medwel

Structural differences of ethanol and DME jet flames in a hot diluted coflow

This study compares the flame structure of ethanol and dimethyl ether (DME) in a hot and diluted ox- idiser experimentally and computationally. Experiments were conducted on a Jet in Hot Coflow (JHC) burner, with the fuel jet issuing into a 1250-K coflow at three oxygen levels. Planar measurements using OH-LIF, CH 2 O-LIF, and Rayleigh scattering images reveal that the overall spatial distribution and evolution of OH, CH 2 O, and temperature were quite similar for the two fuels. For both the ethanol and the DME flames, a transitional flame structure occurred as the coflow oxygen level increased from 3% to 9%. This indicates that the flames shift away from the MILD combustion regime. Reaction flux analyses of ethanol and DME were performed with the OPPDIF code, and ethane (C 2 H 6 ) was also included in the analyses for comparison. These analyses reveal that the H 2 /O 2 pathways are very important for both ethanol and DME in the 3% O 2 cases. In contrast, the importance of fuel-specific reactions overtakes that of H 2 /O 2 reactions when fuels are burnt in the cold air or in the vitiated oxidant stream with 9% O 2 . Unsteady laminar flamelet analyses were also performed to investigate the ignition processes and help interpret experimental results. Flamelet equations were solved in time and mixture fraction field, which was pro- vided by non-reactive Large-Eddy Simulation (LES).Jingjing Ye, Paul R. Medwell, Konstantin Kleinheinz, Michael J. Evans, Bassam B. Dally, Heinz G. Pitsc

Downstream evolution of n-heptane/toluene flames in hot and vitiated coflows

The scenario of fuel injected into hot surrounds is found in a range of practical combustion applica- tions. These flame conditions have been emulated using a jet-in-hot-coflow-burner using prevaporised n -heptane and mixtures of n -heptane and toluene, relevant to gasoline and diesel fuel surrogates. This paper reports measurements of six lifted, turbulent flames, with a constant jet flow of a prevaporised fuel/N 2 mixture at 380 K into various hot and vitiated coflow conditions. Five of these flames issued into coflows generated by the combustion of different mixtures of ethylene/air and one had a coflow from a natural gas/air flame. Two n -heptane/toluene fuel blends were also measured to study the effect of soot propensity. Gas sampling, non-linear excitation regime two-line atomic fluorescence (NTLAF) and laser- induced incandescence (LII) were used to characterise the flames, investigate the mixing between the hot coflow and the surrounding air, and measure the flame temperature for the different coflow config- urations. A comparison of results of the flames issuing into hot coflows is presented, indicating that the hottest flame is not associated with the coflow containing the highest concentration of O 2 , but with the minimum soot loading and, consequently, the minimum radiative heat loss. Subsequent numerical simu- lations of canonical opposed-flow flames demonstrate that the soot loading in the downstream region of the flames is strongly dependent on PAH formation in the hot coflow region and further analyses reveal the chemical pathways which are most impacted by small variations in hot coflow composition.Michael J. Evans, Paul R. Medwell, Zhiwei Sun, Alfonso Chinnici, Jingjing Ye, Qing N. Chan, Bassam B. Dall

Narrative writing, reading and cognitive processes in middle childhood: what are the links?

This study investigated the relationship between measures of reading and writing, and explored whether cognitive measures known to be related to reading ability were also associated with writing performance in middle childhood. Sixty-Four children, aged between 8 years 9 months and 11 years 9 months, took part in a battery of writing, reading, and cognitive ability tasks. Reading fluency emerged as having a strong relationship to written language performance, after controlling for age and verbal reasoning. While children with reading difficulties were weak at spelling accuracy, they were otherwise found to produce written compositions of similar quality to typical readers. Boys produced less written text than girls, but did not demonstrate weaker written language abilities. Collectively the results demonstrate that writing skills can be separated into transcription and composition processes, and highlight the need for further research on the relationship between reading fluency and children’s writing

An analysis of combustion from a top-lit up-draft (TLUD) cookstove

It is estimated that three billion people worldwide cook with traditional stoves. These stoves generaly consist of three stones and an open fire, and produce harmful emissions that are known to cause fatal illnesses. Traditional stoves claim the lives of 1.6 million people every year and cause the death of more children under the age of five than any other single cause. In addition, widespread land degradation and deforestation have resulted from inefficient fuel consumption of traditional stoves. This paper presents some results from an extensive investigation into design and experimental evalation of top-lit up-draft (TLUD) microgasifier cookstoves aimed at addressing problems associated with traditional cooking approaches. Results indicate that geometric variations of the stove and fuel size influence burn rates and emissions production, and must therefore be considered for any design and implementation.Cristian Birzer, Paul Medwell, Josh Wilkey, Tomas West, Matthew Higgins, Gregory MacFarlane and Matthew Rea

Whose writing is it anyway? Issues of control in the teaching of writing

This is a postprint of an article whose final and definitive form has been published in the Cambridge Journal of Education© 2006 Copyright University of Cambridge, Faculty of Education; Cambridge Journal of Education is available online at http://www.informaworld.comIn the UK, teachers have moved from a process approach to the teaching of writing to a more didactic and objectives led programme. This has given rise to concerns about the suppression of creativity and enjoyment. Writing is a convention bound activity where spelling, punctuation and expectations about different text types imply a right and wrong way of writing. On the other hand, the best writers are able to use and subvert conventions in creative and individual ways. Teachers of young writers are faced with the difficulty of teaching the correct conventions at the same time as encouraging individual responses. This paper considers evidence from a small-scale study that may shed some light on how teachers cope with these potentially opposing demands. Evidence points to teachers giving very clear guidance to pupils about what is expected of them and carefully scaffolding pupils' learning. However, scaffolding implies a stage where control is handed over to the learners and in this study there was little evidence of these teachers handing over the control. It is argued that for children to learn the conventions at the same time as developing confidence to use these conventions in individual and creative ways, this handover of control is essential