Experimental Investigations on Laminar Burning Velocities of n-Heptane + Air Mixtures at Higher Mixture Temperatures Using Externally Heated Diverging Channel Method

Abstract

Experimental measurements of laminar burning velocity are presented for mixture temperatures above the autoignition temperature of premixed n-heptane + air mixtures using an externally heated mesoscale diverging channel method. Direct measurements of laminar burning velocity are carried out at 1 atm of pressure for an unburnt mixture temperature range (350−600 K) with the mixture equivalence ratio varying from ϕ = 0.6 to 1.5. Nonlinear power-law correlation is proposed to delineate the effect of change in the mixture temperature on the burning velocity variation at various equivalence ratios. A minimum value for the temperature exponent is observed for stoichiometric n-heptane + air mixtures. The maximum value of laminar burning velocity is measured for ϕ ≈ 1.1 at all mixture temperatures. The reported data sets are compared with the available experimental results and the predictions of various detailed kinetic models, i.e., LLNL V3.1 (2011), JetSurF 2.0 (2010), and Poli Mi (2014). Good agreement of the present data is observed with the predictions of the LLNL V3.1 reaction mechanism at all unburnt mixture temperatures. The predictions of other kinetic models show slight under-prediction at higher mixture temperatures. Sensitivity analysis using the LLNL V3.1 mechanism is reported to highlight the contribution of key reactions enhancing/reducing the laminar burning velocity at 470 and 600 K mixture temperatures. With an increase in the mixture temperature from 470 to 600 K, the influence of the chain branching reaction (H + O 2 ↔ O + OH) on laminar burning velocity increases nearly 63.5%. Formation of a vinyloxy radical (CH 2 CHO) from the oxidation of a vinyl radical (C 2 H 3 ) enhances the burning velocity at a 470 K temperature. Oxidation of C 2 H 3 (at 600 K) becomes crucial in governing engine knocking in low-temperature oxidation of n-heptane. It is deduced from the reaction pathway diagram that the production of a highly reactive compound, C 2 H 2 , at 600 K plays a significant role and governs the overall reaction rate of the mixture.

n‐Heptane oxidation in a high‐pressure flow reactor

“…No data are available for pressures above 65 bar. 26 Other results used in model validation were all obtained at pressures well below those of engines; they include jet-stirred reactor experiments 8,9 at pressures of 1-10 bar and laminar flame speed data, 27,28 obtained at atmospheric pressure.…”

Laminar Burning Velocity Measurements of Toluene + Air Mixtures and Ternary Surrogate Formulation at Elevated Temperatures

“…At the stabilized flame location, the flame area ( A f ) and unburned mixture temperature ( T f ) were measured. With the prior measurement of inlet conditions, such as the inlet channel area ( A in ), temperature ( T in ) at the channel inlet and the inlet velocity of the mixture ( U in ), the laminar burning velocity of the toluene + air mixture was determined using the modified form of the mass conservation equation applied between the channel inlet and the flame stabilization location as Similar to previous studies, − various flame dynamics are observed in the channel by changing the inlet mixture velocity, mixture strength, and rate of heating across the channel. A planar flame captured at ϕ = 1.2, U in = 48 cm/s using a DSLR camera is shown in Figure .…”

“…Measurement of the laminar burning velocity of the toluene + air mixture was carried out using an externally heated diverging channel in the EHDC setup at atmospheric pressure (0.1 MPa). This method has been successfully employed earlier to measure the LBV of distinct premixed fuel–air mixtures for both gaseous fuels − and liquid fuels − accurately at different operating conditions. A schematic diagram of the experimental setup is shown in Figure .…”

“…The uncertainty associated with the measurement of laminar burning velocity is due to the uncertainties in the mass flow rate, temperature, flame area, deformation in flame shape, and flame position. In the present work, all experiments are repeated at least three times, and all of the uncertainties are determined using the error propagation rule reported in Bosschaart et al In the present study, the uncertainties are quantified within a limit of ±5% of the measured value. , …”

Spray and Combustion Characteristics of Gasoline-like Fuel under Compression-Ignition Conditions

“…Among the few fundamental GCI studies that have been performed to date, binary blends of iso -octane/ n -heptane named as primary reference fuel (PRF) were often used as fuel surrogates to emulate the combustion characteristics of commercial fuels that have high compositional complexity and variability. − Iso -octane and n -heptane represent both ends of the octane rating scale, which makes them commonly used as surrogates of gasoline and diesel for fundamental studies . For example, Pastor et al have performed a fundamental investigation on the ignition and combustion behavior of PRF-blends under CI engine-relevant conditions, simulated using a constant-pressure combustion vessel.…”