Expertise

Research	Functional nanomaterials
Interests	Synthesis methods
	Process characterisation
	Laser diagnostics

Overview

The current focus of this lab is to synthesize nanomaterials in the gas-phase. The PI originally researched into development of laser diagnostic techniques to accurately measure species and temperature within flames. Using this as a foundation, emphasis has been transferred from energy generation using gas-phase combustion towards finely-tuned nanomaterials production.

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University of Duisburg-Essen ˗ Germany

Absolute Measurement of SiO
At Duisburg, a campaign to quantify and understand flame chemistry for gas-phase synthesis of nanomaterials required the use of laser diagnostics. This project centered around Laser Induced Fluorescence (LIF) to spatially resolve the extent of key intermediate species within the synthesis flame. Flames were seeded with organosilicon compounds, to produce nanoparticles of silica (SiO₂). The intermediate pivotal to silica generation is silicon monoxide (SiO) – and fortuitously is relatively straightforward to measure using LIF.

Achieved
For a selected range of synthesis conditions, 2-D profiles of laser induced fluorescence of SiO were generated by directing a sheet of UV laser light through the seeded H₂/O₂ flames. Together with LIF-based measurements of flame temperature, profiles of SiO concentration can be inferred. A concerted effort was made to render such profiles on an absolute (mole %) scale through rigorous spectroscopic considerations.

Modelling Chemistry
The aim was to systematically compare measured SiO concentration and temperature distributions for the well controlled premixed flat flames doped with two different precursors (HMDSO and TMS) and to connect the experimental data with current mechanistic understanding of the underlying chemistry.

Achieved
We developed a new reaction mechanism that incorporated clustering reactions. Such early production of silica clusters was hypothesized from observing the presence of a double-peak structure in SiO profiles, which may have originated from such generation of clusters – and their subsequent sublimation downstream to release a second burst of SiO. By surveying a range of flame conditions, we discovered that a double-peak structure in the SiO profiles is common to both precursors used – lending credence to our mechanistic understanding regarding clusters.

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Publications

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KAUST ˗ Saudi Arabia

A New Frontier
At KAUST, my research centered around the use of a shock tube, involving rapid shock-heating of gaseous reactive test mixtures and measuring subsequent species-concentration and temperature time profiles. Laser absorption spectroscopy is commonly employed for such measurements; however, they need to be highly time-resolved owing to the rapid speed of the reactions involved (μs-ms). I developed a novel laser-spectroscopic technique that is fully time resolved for use with shock tubes, and yet does not compromise on the integrity of data acquisition that otherwise occurs in competing approaches.

Achieved
An initial attempt of our strategy only managed to quasi-continuously measure at repetition rates of 250 kHz, due to spectral distortion problems. However, in the second attempt we managed to prove it possible to extend measurement rates in excess of 1 MHz. This translates into instantaneous full spectral measurements that span over a timescale of only nanoseconds! This had not hitherto been demonstrated in shock tube research before. The findings were published in Optics Letters and was granted a US patent.

Dynamic Flame Response
On a similar theme of understanding and controlling flame instability in combustion systems studied earlier at Cambridge, this project aimed at characterizing the interaction of buoyancy-induced instabilities with forced excitation of the fuel supply. Laminar coflow diffusion flames were acoustically forced, whose frequency responses were recorded as a function of excitation frequency and amplitude. The evolving structure of such flames was also examined through use of video analysis and particle imaging velocimetry (PIV).

Achieved
For specific combinations of excitation frequency and amplitude, the frequency response of the flames was found to couple to that of the forcing, where the contribution of natural puffing frequency disappears. Such instances of coupling exhibited many harmonics of the excitation frequency, related indirectly to the natural puffing frequency. We showed how such harmonics form, through application of PIV, and furthermore unveiled insight into the physics of how the flame couples to the forcing under certain conditions. Our frequency response characterization provides quantitative results, which are of utility for both modelling studies and active-control strategies.

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Publications

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Patent

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University of Cambridge ˗ England

Precise Thermometry
At Cambridge, an accurate technique for measuring temperature in sooting flames was developed. Sooting flames, observed in diesel engines  – or in the synthesis of carbon black for ink, tyres and plastics – possess complex hydrocarbon chemistry. To understand and quantify such fuel-rich flames requires improved temperature data – achieved through Two-Line Atomic Fluorescence of indium nanoparticles seeded to test flames. Another project, based instead on air-rich flames, involved application of Coherent anti-Stokes Raman Scattering (CARS) to accurately measure flame temperature towards understanding the physics of thermoacoustic instability in jet-engines.

Achieved
For sooting flames, the new LIF-based technique using indium was an advancement towards generating more reliable temperature datasets for validating CFD codes of turbulent structures. Our technique is more cost-effective than previous implementations, which rely on bulky dye laser systems. For lean thermoacoustic flames, CARS offers stronger signals with excellent time resolution capability. Using the more accurate results from this work, more up-to-date CFD models of a test burner was validated, with the aim of improved understanding and prediction of thermoacoustic instability in new jet-engine technology.

Flamefront Curvature Effects
Turbulent premixed flames, as seen in furnaces and engines, exhibit a flamefront that is wrinkled. The flamefront is where the exothermic reactions take place, separating the incoming fuel-air mixture from the hot burnt gases. Wrinkling results in strong curvature variation along the flamefront, and it has been shown that such curvature affects the local rate of reaction, and consequent combustion behaviour.

Achieved
Flamefront curvature was measured from contours that outline the flamefront, which were generated from laser-induced fluorescence images of the OH radical. LIF images of CH₂O were also generated, and toegther with OH can be used to locally resolve the heat release rate (HRR) at the flamefront. By correlating the local HRR with the local curvature of the flamefront it was shown that there is an enhancement in local flame speed at sections of the flamefront with a non-zero curvature, and this agrees with numerical models.

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Publications

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Education

2005 – 09	University of Cambridge Ph.D. Chemical Engineering
2001 – 05	University of Cambridge B.A. M.Eng. Chemical Engineering

^{Note: The Cambridge curriculum also includes modules on Mechanical, Electrical and Civil Engineering in the first year.}