Mechanical Engineering

This paper presents the underlying theory, associated mathematical modelling and analysis of a sponge-layer damping technique, termed the Time-Reversal-Sponge-Layer (TRSL), that significantly improves the performance of aeroacoustic Time-Reversal (TR). The TR technique requires the use of multiple Line Arrays (LAs) in a Time-Reversal Mirror (TRM) to accurately predict the source location and its characteristics. However, it is shown that when using multiple LAs, the interference between the opposite propagating fluxes near the LA boundaries results in the formation of spurious local maxima regions throughout the computational domain, thereby reducing the capacity of TR to resolve acoustic sources. The novel TRSL technique proposed in this work minimises this unwanted interference by damping the flux normally incident on a LA boundary and is implemented using the Pseudo-Characteristic Formulation (PCF) of the two-dimensional Linearised Euler Equations (LEE). The performance of TRSL is assessed by simulating a number of test cases such as an idealised time-harmonic monopole, dipole and lateral quadrupole sources as well as multiple (two) dipole sources of different strengths located in a nonuniform mean shear flow. The use of TRSL suppresses the formation of spurious maxima and significantly improves the source map, thereby demonstrating the effectiveness of this damping technique. The performance of TRSL is compared with two other methods: a TR superposition technique and Conventional Beamforming (CB). The TR superposition technique prevents the flux interference problem near the LA boundaries by
superposing the instantaneous time-reversed acoustic pressure fields computed from individual LAs. The source map obtained using the superposition technique was found to be identical to that obtained using the TRSL damping technique, however, the computational cost was much higher. A comparison with CB indicated that although CB accurately predicts the aeroacoustic source location, the relative magnitudes of the side-lobes in the CB source map(s) are substantially higher in comparison to the corresponding TR source map(s). It was also shown that TR with use of the TRSL is better suited for resolving multiple coherent sources in a domain than the CB method, especially when one source is much weaker than other sources in the domain.

ABSTRACT This paper seeks to address unusual flame stabilisation behaviour observed in experimental jet flames which issue into a hot coflow. It has been observed that increasing the temperature and/or oxygen concentration in the coflow can lead to an increase in flame liftoff height. The paper isolates the role of chemistry, and in particular flame intermediates, on the observed phenomenon with a view to better understand how the behaviour changes over a range of conditions. A descriptive theory for this behaviour is proposed, which is based on the well-established theory that a build-up of radicals and intermediate species is responsible for autoignition of these flames. This paper systematically examines the role of these precursors with a view to better understanding of the chemical kinetics and to assess if the observed behaviour is chemistry-dominated. To this end, laminar flame calculations and ignition delay curves are presented, and the findings are validated with experiments. The results indicate that chemical effects alone are insufficient to fully explain the observations, but the calculations support the general trends noted in the experiments and highlight the importance and relative effects of some key precursors. In particular, the production and consumption of formaldehyde in a low oxygen environment supports the unusual flame behaviour observed experimentally.

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.

ABSTRACT Lifted flames have attracted significant research over many years due to the importance of fundamental understanding of the stabilization mechanism. More recently, lifted flames in a hot coflow have been used to investigate autoignition properties of jet flames. Several experimental and numerical studies to predict the autoignitive liftoff characteristics of a jet in a vitiated coflow have recently appeared in the literature. The configuration of a jet issuing into a heated and diluted oxidant stream can also emulate the fundamental operating conditions of MILD combustion. This paper investigates similarities and differences of the liftoff behavior of jet flames under a wide variety of coflow oxidant conditions, ranging from autoignitive conditions to MILD combustion. The liftoff behavior is observed to not be monotonic with either coflow temperature or oxygen content. The results clearly indicate that there is a fundamental transition in the stabilization mechanism, depending on the oxidant stream properties.

ABSTRACT The role of hydrogen addition on the structure of the Moderate or Intense Low oxygen Dilution (MILD) combustion regime is examined using a combination of experimental techniques and laminar flame calculations. Laser diagnostic imaging is used to simultaneously reveal the in situ distribution of the hydroxyl radical (OH), formaldehyde (H2CO), and temperature using the Jet in Hot Coflow (JHC) burner. The fuels considered are natural gas, ethylene, and LPG (each diluted with hydrogen 1:1 by volume). Hydrogen addition to the primary fuel was found necessary to stabilise the flames. Further to the role of hydrogen in the stabilisation of the flames, hydrogen addition also leads to the reaction zone exhibiting similar structure for different primary fuel types. The independence of the reaction zone structure with hydrogen addition suggests that a wide variety of fuels may be usable for achieving MILD combustion.