Paul R. Medwell

Development of temperature imaging using two-line atomic fluorescence

This work aims to advance understanding of the coupling between temperature and soot. The ability to image temperature using the two-line atomic fluorescence (TLAF) technique is demonstrated. Previous TLAF theory is extended from linear excitation into the nonlinear fluence regime. Nonlinear regime two-line atomic fluorescence (NTLAF) provides superior signal and reduces single-shot uncertainty from 250 K for conventional TLAF down to 100 K. NTLAF is shown to resolve the temperature profile across the stoichiometric envelope for hydrogen, ethylene, and natural gas flames, with deviation from thermocouple measurements not exceeding 100 K, and typically ≲30 K. Measurements in flames containing soot demonstrate good capacity of NTLAF to exclude interferences that hamper most two-dimensional thermometry techniques.

Modeling Lifted Jet Flames in a Heated Coflow Using an Optimized Eddy Dissipation Concept Model

Moderate or intense low oxygen dilution (MILD) combustion has been established as a combustion regime with improved thermal efficiency and decreased pollutant emissions, including NOx and soot. MILD combustion has been the subject of numerous experimental studies, and presents a challenge for computational modeling due to the strong turbulence–chemistry coupling within the homogeneous reaction zone. Models of flames in the jet in hot coflow (JHC) burner have typically had limited success using the eddy dissipation concept (EDC) combustion model, which incorporates finite-rate kinetics at low computational expense. A modified EDC model is presented, which successfully simulates an ethylene-nitrogen flame in a 9% O2 coflow. It is found by means of a systematic study in which adjusting the parameters and from the default 0.4082 and 2.1377 to 3.0 and 1.0 gives significantly improved performance of the EDC model under these conditions. This modified EDC model has subsequently been applied to other ethylene- and methane-based fuel jets in a range of coflow oxidant stream conditions. The modified EDC offers results comparable to the more sophisticated, and computationally expensive, transport probability density function (PDF) approach. The optimized EDC models give better agreement with experimental measurements of temperature, hydroxyl (OH), and formaldehyde (CH2O) profiles. The visual boundary of a chosen flame is subsequently defined using a kinetic mechanism for OH* and CH*, showing good agreement with experimental observations. This model also appears more robust to variations in the fuel jet inlet temperature and turbulence intensity than the standard EDC model trialed in previous studies. The sensitivity of the newly modified model to the chemical composition of the heated coflow boundary also demonstrates robustness and qualitative agreement with previous works. The presented modified EDC model offers improved agreement with experimental data profiles than has been achieved previously, and offers a viable alternative to significantly more computationally expensive modeling methods for lifted flames in a heated and vitiated coflow. Finally, the visually lifted flame behavior observed experimentally in this configuration is replicated, a phenomenon that has not been successfully reproduced using the EDC model in the past.

Reaction Zone Weakening Effects under Hot and Diluted Oxidant Stream Conditions

This paper reports computational results, to complement experimental observations, on the turbulence–chemistry interaction of nonpremixed jet flames issuing into a heated and highly diluted oxidant stream. It is found experimentally that large-scale vortices and flame stretch can lead to spatial thinning and a decrease in OH concentration. This reduction in OH is described as a weakening of the reaction zone. Accompanying reaction zone weakening is also an increase in H2CO levels. The reduction in reaction rates is most noticeable at low oxidant stream O2 levels. The heated and low oxygen oxidant conditions typify those of moderate or intense low oxygen dilution (MILD) combustion. The computational results indicate that the effects of the low oxygen levels of MILD combustion leads to both a reduction in reaction rates and an increase in transport of O2 across the reaction zone. The relationship between the reaction rate and level of O2 permeation suggests that a form of partial premixing can occur under MILD combustion conditions. This partial premixing leads to the formation of flame intermediates, which contribute to the stabilization of the flames. The permeation effects are most pronounced at high strain rates, which are commonly encountered in practical MILD combustors.

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.

Solvent effects on two-line atomic fluorescence of indium

We aim to investigate the potential of four different organic solvents, namely, acetone, ethanol, methanol, and isopropanol, and the organic-solvent-water mixtures as a seeding medium for the two-line atomic fluorescence technique. Water is used as the reference case. Indium, which has been previously shown to have suitable spectroscopic attributes, is chosen as the thermometry species in the present study. Acetone and methanol are shown to enhance the fluorescence signal intensity the most (approximately threefold to fivefold at stoichiometric conditions) when used. Acetone and methanol are shown to improve the fluorescence emission over the entire stoichiometric envelope of flame, most significantly in the rich combustion region, as well as a twofold enhancement in the signal-to-noise ratio.

Instantaneous Temperature Imaging of Diffusion Flames Using Two-Line Atomic Fluorescence

This work investigates the first demonstration of nonlinear regime two-line atomic fluorescence (NTLAF) thermometry in laminar non-premixed flames. The results show the expediency of the technique in the study of the reaction zone and reveals interesting findings about the indium atomization process. Indium fluorescence is observed to be strongest at the flame-front, where the temperature exceeds 1000 K. The uncertainty in the deduced temperature measurement is ∼6%. The temperature profile across the reaction zone shows good agreement with laminar flame calculations. The advantages and inherent limitations of the technique are discussed.

Enhancing the focal-resolution of aeroacoustic time-reversal using a point sponge-layer damping technique

This letter presents the Point-Time-Reversal-Sponge-Layer (PTRSL) technique to enhance the focal-resolution of aeroacoustic Time-Reversal (TR). A PTRSL is implemented on a square domain centered at the predicted source location and is based on damping the radial components of the incoming and outgoing fluxes propagating toward and away from the source, respectively. A PTRSL is shown to overcome the conventional half-wavelength diffraction-limit; its implementation significantly reduces the focal spot size to one-fifth of a wavelength for a monopole source. Furthermore, PTRSL reduces the focal spots of a dipole source to three-tenths of a wavelength, as compared to three-fifths without its implementation.

New Seeding Methodology for Gas Concentration Measurements

This paper presents the first demonstration of the pulsed laser ablation technique to seed a laminar non-reacting gaseous jet at atmospheric pressure. The focused, second harmonic from a pulsed Nd:YAG laser is used to ablate a neutral indium rod at atmospheric pressure and temperature. The ablation products generated with the new seeding method are used to seed the jet, as a marker of the scalar field. The neutral indium atoms so generated are found to be stable and survive a convection time of the order of tens of seconds before entering the interrogation region. The measurements of planar laser-induced fluorescence (PLIF) with indium and laser nephelometry measurements with the ablation products are both reported. The resulting average and root mean square (RMS) of the measurements are found to agree reasonably well although some differences are found. The results show that the pulsed laser ablation method has potential to provide scalar measurement for mixing studies.

The role of primary and secondary air on wood combustion in cookstoves

ABSTRACT A two-stage solid fuel research furnace was used to examine the claim that through forced draught greater mixing and more complete combustion could be achieved. By varying the primary air (PA) and secondary air (SA) flow the influence on the combustion process was investigated. In the first part of the combustion, when the release of volatile compounds predominates, the variation of neither PA nor SA had a significant influence. In the second part when mainly char is oxidised an increase in both PA and SA lead to a rising nominal combustion efficiency ()), with a greater impact observed with SA. Furthermore higher air flows caused the heat transfer, to a pot above the furnace, to decline. Therefore forced draught does lead to greater mixing and mitigation of emissions, but in the presented configuration a trade-off between a higher NCE and a lower heat transfer needs consideration.

Ignition characteristics in spatially zero-, one- and two-dimensional laminar ethylene flames

In the continual effort to reduce emissions and improve efficiency, moderate or intense low-oxygen dilution combustion has been suggested for aeroengine applications. This new application of moderate or intense low-oxygen dilution combustion requires further insight in applying the knowledge from conventional analyses of well-mixed systems to non-premixed flames. The ignition of ethylene, a key species in hydrocarbon oxidation, is simulated in simplified combustion systems with three different hot oxidants using detailed chemical kinetics. Zero-dimensional batch reactors, one-dimensional opposed-flow flame simulations, and planar two-dimensional laminar coflowing slot flame simulations are used to compare different ignition metrics across the autoignitive and moderate or intense low-oxygen dilution combustion regimes. It is found that the autoignition of ethylene with hot air may be described in two dimensions as the intersection of a critical hydroxyl fraction and the most reactive mixture fraction. Alth...