Propagating laminar flame characteristics for single and two phase alternative fuel mixtures

Doctoral thesis English OPEN
De La Rosa Urbalejo, Daniel
  • Subject: TJ

This thesis investigates enhanced methods for analysing non-linear effects in propagating laminar flames, enabling more accurate evaluation of laminar flame characteristics such as Markstein length and unstretched flame speed whilst proposing a new method for evaluating extinction stretch rate. Furthermore, a new cloud-combustor is developed and commissioned enabling laminar flame characteristics through droplet fuel mists to be explored again utilising advanced non-linear analysis.\ud Re-analysis of previous low-ignition energy methane-water flames reveals the analytical non-linear characteristic. The analysis also demonstrates the need for a larger chamber to avoid pressurised effects during the latter stages of propagation, potentially reducing the accuracy of the adopted methodology. Non-linear analysis shows interesting trends concerning Markstein length at higher water loading in particular when it increase to 15% (by volume), and laminar burning rate decreased.\ud The non-linear analysis technique is deployed to analyse four hydrocarbon fuels, two traditional paraffinic fuels in methane and propane, and two alternative alcohol fuels namely ethanol and methanol.\ud It is shown that overdriven flame data can be used to predict flame extinction stretch rate, as long as a sufficient time period is disregarded to allow the effects of the early ignition-affected period to subside. The new technique proposed for evaluated critical extinction stretch rate shows good agreement with the traditional counter-flowing flame technique. Results for the four fuels reveal a common profile for extinction stretch-rate as a function of equivalence ratio, which was anticipated due to the similar fundamental combustion characteristics of the chosen fuels. Based on the non-linear analysis, it is shown analytically that this common profile may be represented by a combination of the\ud iv\ud unstretched laminar burning velocity, the Markstein length and the expansion ratio of the fuel.\ud Ethanol in air is used to benchmark Cardiff University’s new, large 35Litre ‘Cloud Combustor’ for an investigation of flame propagation through fuel mists across a wide range of equivalence ratios. Non-intrusive, in-situ droplet sizing with concurrent flame propagation is achieved for the first time. The fuel mist flame data was subsequently compared to that for pure vapour mixtures at nominally identical ambient conditions in order to study the reported enhancement in flame speed exhibited in previous studies, and to compare qualitatively against conflicting published views reported in literature. It was found that with the onset of instabilities at certain droplet size an enhancement in flame speed could be shown for rich mist flames compared to those of analogous vapour flames.\ud Based on mechanisms detailed elsewhere that provide a possible explanation for this enhancement full discussion and correlations that help to understand the nature of flame speed through droplet mists are presented
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