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Burning Rates and Instabilities in the Combustion of Droplet and Vapour Mixtures

Sulaiman, Shaharin Anwar (2006) Burning Rates and Instabilities in the Combustion of Droplet and Vapour Mixtures. PhD thesis, The University of Leeds.

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It is well established that the laminar burning rate plays an important role in turbulent combustion and previous work at Leeds has suggested that the laminar burning velocity of an aerosol mixture is little different from that of a gaseous mixture at similar conditions. However, it has been shown that flames within well defined droplet suspensions (aerosols) more readily become unstable than for gaseous ones. Flame instabilities, characterised by wrinkling and cellular surface structure, increase the burning rate due to the associated increase in surface area. For gaseous mixtures, the effect has been shown theoretically and experimentally to be a function of Markstein number and critical Peclet number, which marks the flame radius at which cellularity is first observed. In aerosol combustion, the presence of liquid droplets has been shown to influence instabilities by causing earlier onset of cellularity than for gaseous flames. Therefore it is imperative to conduct a fundamental study to understand the complex interactions between droplets and combustion. In the present work, spherically expanding flames were employed to quantify the burning rates in gaseous and aerosol flames and to determine their differences. !so-octane-air aerosols were generated by expansion of the gaseous pre-mixture, based on the Wilson cloud chamber principle of expansion cooling, to produce a homogeneously distributed suspension of fuel droplets. Flames were centrally ignited for quiescent aerosols at near atmospheric pressures, drop sizes of up to 30 ~m and overall equivalence ratios between 0.8 to 2.0. The flame progress was monitored using high-speed schlieren photography, from which burning rates were determined. In turbulent studies, measurements were made for stoichiometric aerosols at root mean square turbulence velocities of between 1.0 and 4.0 m/s. For companson, gaseous combustion at conditions similar to those of aerosols were studied. From the laminar study, it was shown that the burning rate of lean mixtures is independent of droplet diameter. However, at higher equivalence ratios, the burning rate became a strong function of droplet diameter and equivalence ratio. This was associated with the onset of instabilities, which were, in tum, related to measured values of Markstein number and critical Peclet number for aerosol flames in a similar manner to those for gaseous flames. Heat loss from the flame due to droplet evaporation is probably the main reason for instabilities in aerosol flames. Interestingly, droplets which were assumed by previous workers to be fully evaporated at the flame front, were shown, at certain conditions in the present work,to survive behind the flame front. Thus other possible mechanisms for instabilities in aerosol flames could be important. For very rich mixtures, gaseous flames were shown to be partially smooth, slow, and were strongly affected by natural convection. Conversely, with the presence of droplets at similar mixture conditions, flames were found to be fully cellular and faster, with little sign of the effect of natural convection. This was suggested to be due to early instabilities caused by the presence of droplets, which, in tum, increased the burning rates. Oscillating flames, in which the flame speed and flame structure alternated between low and high values and smooth and cellular respectively, during flame development, were observed for some experimental conditions. These oscillations were most probably caused by aerodynamic interaction between droplets and gas motion ahead of the flame. This was examined using simultaneous laser sheet imaging and PIV analysis, with a simple model proposed by Atzler et al. (2001) which simulated aerodynamic interaction between droplets and gas phase motion ahead of the flame front. A dimensionless comparison between turbulent flames of aerosol and gaseous mixtures showed similar burning rates. The measurements were compared with existing turbulent burning velocity expressions and correlations. In general, these expressions are in quite good agreement with the present results, particularly at low stretch rate.

Item Type: Thesis (PhD)
Academic Subject : Academic Department - Mechanical Engineering - Materials - Engineering materials - Metals alloys - Phase transformation
Subject: T Technology > TS Manufactures
Divisions: Engineering > Mechanical
Depositing User: Users 2053 not found.
Date Deposited: 27 Sep 2013 11:03
Last Modified: 25 Jan 2017 09:46
URI: http://utpedia.utp.edu.my/id/eprint/6962

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