Effects of Hydrocarbon Fuel Structure on Experimental Laminar & Turbulent Burn Rates A.A. Burluka 1 , R.G. Gaughan 2 , J.F. Griffiths 3 , C. Mandilas *.1 , C.G.W. Sheppard 1 , R. Woolley 4 1 School of Mechanical Engineering, The University of Leeds, Leeds LS2 9JT, UK 2 ExxonMobil Research and Engineering Company, Paulsboro Technical Center, Paulsboro, NJ 08066, U.S.A 3 School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK 4 The University of Sheffield, Department of Mechanical Engineering, Mappin Street, S1 3JD, UK * Corresponding Author’s Current Address, Aerosol and Particle Technology Laboratory, Thermi, 57001, Greece Abstract Experimental measurements of laminar and turbulent burn rates have been made for premixed fuel-air flames of hydrocarbons (HC) with six carbon atoms including unsaturated, cyclic and branched molecules. Measurements were performed at 0.5 MPa, 360 K and turbulent rms velocities of 2 and 6 m/s for a range of equivalence ratios. The laminar burning velocities were used to interpret the turbulent data. The measured laminar burning velocities showed dependence on the molecular structure. The burn rate ranking of fuels observed in the laminar measurements was to a degree retained in the turbulent experiments. The equivalence ratio of peak turbulent burn rates was different to that under laminar conditions and was found to be a function of the rms turbulent velocity (u’). * Corresponding author: mandilas@cperi.certh.gr Proceedings of the European Combustion Meeting 2015 Introduction Burning velocity has been the subject of numerous experimental and theoretical investigations spanning many decades as it affects engine performance, efficiency and cycle-to-cycle variability. Understanding the factors that influence the burn rate enables better control of engine combustion quality and emissions. The combustion rate in an engine is a function of the turbulent burning rate, which is itself a function of those physico-chemical features of a fuel-air mixture encapsulated in its laminar burning velocity and the turbulence characteristics of the flow field within the engine. The influence of fuel structure on the laminar burning velocity is well studied [e.g. 1-5]. However, published data on the influence of HC molecular structure on burn rate under turbulent conditions is very sparse and recent [6]. The primary aim of the current work was to investigate fuel structure and equivalence ratio effects on the laminar and turbulent burning rates of deflagrations in a spherical combustion vessel. Presented in this paper are experimentally determined laminar and turbulent burn rates for a set of HCs of varied structure, but common carbon number, C6. The fuels examined were 2,2-dimethyl butane, 2- methyl pentane (isohexane), n-hexane, cyclohexane, 1- hexene, cyclohexene and 1-hexyne. With the exception of 1-hexyne, all other fuels are representative components of gasoline blends. Experimental and Results Processing The Leeds MkII spherical bomb was employed for the studies. For reasons of brevity, description of the equipment and experimental procedure is omitted here. Nonetheless, the interested reader is referred to published work describing equipment and procedure in detail [7-8]. The results reported here refer to schlieren based measurements. All deflagrations were initiated at initial temperature and pressure of 360 K and 0.5 MPa, where published experimental data are relatively sparse. This relatively high initial temperature ensured complete fuel vapourisation and contributed to the avoidance of condensation on the walls and windows after ignition, while the elevated initial pressure was adopted to provide conditions relevant to combustion in internal combustion engines. In the early stages of combustion, for flames of mean flame radius less than the window diameter, pressure and associated unburned gas temperature remained close to the initial values. Experiments were conducted for 0.78 ≤ f ≤ 1.67. At least two laminar and five turbulent deflagrations were performed at each condition. Imaging data analysis to determine laminar and turbulent burning characteristics followed established techniques, detailed elsewhere [9- 10] and widely used [11-16]. Results and Discussion Flames for f  1.1 for the C6 fuels examined showed signs of hydrodynamic instabilities (cellularity) as early as a mean flame radius of 10-15 mm. Consequently, too few data points were available to determine the Markstein length, Lb and thus fit the data accordingly for determination of the 1D stretch free burn rate, ul. Laminar burning velocity results for the C6 group of HC are displayed in Fig. 1. Solid lines refer to results obtained by extrapolating the measured flame speeds to zero stretch then dividing the flame speed by the density ratio [10]. Normal lines refer to data obtained via application of traditional ul theory [9], while dotted lines correspond to ul values computed using ul = un,min, where, un is the stretched entrainment burning velocity. Burning velocities obtained in this way cannot be considered to be accurately defined but represent a pragmatic approach to obtaining laminar burning velocity data to aid the analysis of subsequent turbulent burning measurements. The ul of the C6 fuels peaked close to f = 1.1 and demonstrated a dependence on molecular structure that