A well-defined methodology to extract laminar flame speeds at engine-relevant conditions

Abstract

This paper presents a new method for accurate laminar flame speed measurements relevant to engine operating temperature and pressure conditions, where literature data are scarce or non-existent. The experimental setup consists of a high-pressure, high-temperature spherical chamber that combines optical and pressure-rise data collection techniques. The optical method is based on high-speed recordings of flame images using a schlieren setup, whereas the pressure-rise approach requires pressure measurements of isentropically compressed unburned gases taken during flame propagation. The range of usable data in the pressure-rise method is limited by stretch effects at lower pressures and the onset of instabilities at higher pressures. Here, the lower pressure limit was extended using optical data measured at isentropic conditions. A criterion to define the upper-pressure limit is proposed by the onset of instabilities during combustion that is especially useful for pressure readings taken in spherical chambers without optical access. Experimental laminar flame speeds are compared to those obtained with simulations using two detailed kinetic models, where very good agreement is found. Finally, a well-defined procedure is proposed showing how to obtain an extensive range of experimentally accurate laminar flame speed data that can be used to validate kinetic schemes by using only few measurements.

Novelty and Significance statement

The novelty of this paper is condensed in a clear and straightforward methodological approach to extracting high-fidelity data from a small number of experiments. While the optical and pressure-rise methods are not new, it is useful to know how to combine the two techniques and cross-validate them to ensure extracted data are accurate and free of effects such as instabilities. A new criterion is proposed to help detect the onset of instabilities regardless of their type. This can be valuable for research groups using only spherical chambers without optical access. A guideline is given at the end of the paper based on profound evidence for our claims using optical data. With the proposed methodology, accurate data can be obtained using only the pressure-rise method. Its application allows for obtaining engine-relevant data without replacing the bath gas with helium or argon.