Fundamental studies of premixed combustion

• Main Author: Haq, Md Zahurul
• Other Authors: Bradley, D. ; Lawes, M.
• Published: University of Leeds 1998
• Subjects: 662
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.533444

The thesis comprises a fundamental study of spherical premixed flame propagation,originating at a point under both laminar and turbulent propagation. Schlieren cine photography has been employed to study laminar flame propagation, while planar mie scattering (PMS) has elucidated important aspects of turbulent flame propagation. Thrbulent flame curvature has also been studied using planar laser induced fluorescence (PLIF) images. Spherically expanding flames propagating at constant pressure have been employed to determine the unstretched laminar burning velocity and the effect of flame stretch, quantified by the associated Markstein lengths. Methane-air mixtures at initial temperatures between 300 and 400 K, and pressures between 0.1 and 1.0 MPa have been studied at equivalence ratios of 0.8, 1.0 and 1.2. Values of unstretched laminar burning velocity are correlated as functions of pressure, temperature and equivalence ratio. Two definitions of laminar burning velocity and their response to stretch due to curvature and flow strain are explored. Experimental results are compared with two sets of modeled predictions; one model considers the propagation of a spherically expanding flame using a reduced mechanism and the second considers a one dimensional flame using a full kinetic scheme. Data from the present experiments and computations are compared with those reported elsewhere. Comparisons are made with iso-octane-air mixtures and the contrast between fuels lighter and heavier than air is emphasized. Flame instability in laminar flame propagation become more pronounced at higher pressures, especially for lean and stoichiometric methane-air mixtures. Critical Peclet numbers for the onset of cellularity have been measured and related to the appropriate Markstein number. Analyses using flame photography clearly show the flame to accelerate as the instability develops, giving rise to a cellular flame structure. The underlying laws controlling the flame speed as cellularity develops have been explored. PMS images have been analysed to obtain the distributions of burned and unburned gas in turbulent flames. These have enabled turbulent burning velocities to be derived for stoichiometric methane-air at different turbulent r.m.s. velocities and initial pressures of 0.1 MPa and 0.5 MPa. A variety of ways of defining the turbulent burning velocity have been fruitfully explored. Relationships between these different burning velocities are deduced and their relationship with the turbulent flame speed derived. The deduced relationships have also been verified experimentally. Finally, distributions of flame curvature in turbulent flames have been measured experimentally using PMS and PLIF. The variance of the distribution increases with increase in the r.m.s. turbulent velocity and decrease in the Markstein number. Reasons for these effects are suggested.