Influence of scalar dissipation on flame success in turbulent sprays with spark ignition

Wandel, Andrew P. (2014) Influence of scalar dissipation on flame success in turbulent sprays with spark ignition. Combustion and Flame, 161 (10). pp. 2579-2600. ISSN 0010-2080

Abstract

A Direct Numerical Simulation (DNS) study is performed to determine a quantitative indicator of imminent global extinction in spray flames ignited by a spark. The cases under consideration have Group Combustion numbers sufficiently small that each droplet has an individual flame form around it, which subsequently merge. The structure of the flames is examined, including identification of non-premixed behaviour in the core of the flame and premixed flame fronts except in the presence of droplets, which cause strong non-premixed behaviour. The reaction progress variable c is studied and its dissipation rate is identified as being a key indicator of whether a flame will globally extinguish after being ignited by the spark. Specifically, immediately after the spark is deactivated, the volume containing the end of the flame front and hot products is studied in detail with respect to c. For successful flames, it is observed that regions of zero dissipation of c were predominantly restricted to the highest reaction progress variable (c>0.98), with zero probability within the range 0.95<c<0.98 and low probability within 0.9<c<0.95. In contrast, cases which subsequently extinguished had substantial probability of zero dissipation for 0.95<c<0.98. This region was a secondary structure separate from the main flame kernel that was unable to evaporate sufficient liquid to create a self-sustaining flame and therefore contributed to the subsequent quenching of the flame. In the successfully-burning case under consideration, this region was part of the main flame structure. The low reaction rate contributed to a thickened flame structure near the hot core, which reduced the heat transfer to the flame front and prevented effective evaporation and preheating of the fluid ahead of the flame front. Calculation of the conditional probability of c for its dissipation rate being zero could provide a quantitative measure to determine whether a flame is likely to extinguish within a relatively short timeframe. This is equivalent to detecting that, for every value of 0.9<c<1, there are volumes of significant size where the value of c is uniform. Note that a successful flame must have a volume of substantial size with c=1. From a practical perspective, if each individual flame kernel is monitored, then extinction is imminent if secondary structures of incomplete reactions are present when the spark ceases adding energy.


Statistics for USQ ePrint 25894
Statistics for this ePrint Item
Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: Published Version restricted in accordance with publisher copyright policy (Elsevier)
Faculty / Department / School: Current - Faculty of Health, Engineering and Sciences - School of Mechanical and Electrical Engineering
Date Deposited: 12 Oct 2014 03:04
Last Modified: 04 Jul 2016 23:36
Uncontrolled Keywords: spray; direct numerical simulations; spark ignition; extinction predictor; scalar dissipation rate; reaction progress variable
Fields of Research : 09 Engineering > 0915 Interdisciplinary Engineering > 091508 Turbulent Flows
09 Engineering > 0915 Interdisciplinary Engineering > 091501 Computational Fluid Dynamics
09 Engineering > 0902 Automotive Engineering > 090201 Automotive Combustion and Fuel Engineering (incl. Alternative/Renewable Fuels)
Socio-Economic Objective: D Environment > 96 Environment > 9603 Climate and Climate Change > 960302 Climate Change Mitigation Strategies
B Economic Development > 85 Energy > 8507 Energy Conservation and Efficiency > 850702 Energy Conservation and Efficiency in Transport
Identification Number or DOI: 10.1016/j.combustflame.2014.04.006
URI: http://eprints.usq.edu.au/id/eprint/25894

Actions (login required)

View Item Archive Repository Staff Only