Development of an optically accessible apparatus to characterize the evolution of spherically expanding flames under constant volume conditions

Abstract

Flame speed is extremely important as it affects the performances of many industrial systems. Moreover, its significance makes it a major target for the validation of kinetic mechanisms, which explains the necessity to provide ever more accurate data. Flame speed dependence on pressure and temperature conditions is interestingly assessed using, among others, spherically expanding flame in constant volume chambers. In these conditions, the flame speed derivation, based solely on the pressure evolution in the chamber, requires empirical models. The current study describes a perfectly spherical chamber with full optical access allowing simultaneous recording of the pressure inside the chamber and, fully innovative, of the flame radius evolution until the flame vanishes at wall. A careful description of the new set-up and of the accuracy of the measurements, in particular of the flame radius, are presented here. In parallel with experiments, one-dimensional transient simulations were carried out to identify the limits of the proposed new method. Then, the simultaneous use of pressure and flame radius information is compared to the traditional constant volume method based on empirical models. A first advantage relies in the direct detection of the development of instabilities during the flame propagation. In addition, although the flame speed is extremely sensitive to the flame radius determination, the actual experimental accuracy allows significant improvements in terms of accuracy, notably as initial pressure and temperature are elevated. This new set-up will allow major advances in the measurement of laminar flame velocity under extreme thermodynamic conditions.

Laminar flame speed determination at high pressure and temperature conditions for kinetic schemes assessment

“…A sketch of the experimental rig is presented in the supplementary materials (Figure a). More details about the description of the experimental set-up and the accuracy of the instruments are available in [22].…”

“…For the current study, direct flame chemiluminescence visualization was adopted. The accurate flame front position determination using chemiluminescence through the ring was assessed in [22]. The acquisition rate of the CMOS camera (PHANTOM V1611) was set at 8000 fps for the fuel lean and rich conditions and 12 000 fps for the near-stoichiometric condition.…”

“…For this condition, the flame remains perfectly spherical during the whole process, without the detection of any instability. The accuracy of the flame radius determination has been carefully assessed in [22]. The contributions of the different sources of inaccuracy were considered and precision on the flame radius of less than 0.5 % is attributed, as the pressure increases ( 0 > 2).…”

“…As explained in Omari et al [17] and Burrell et al [13], the time derivative of the pressure trace is a good indicator of the heat losses to the wall. In [22], a criterion of 90% of ( ) was introduced to define the maximum experimental pressure to be considered in order to ensure that no wall effect can occur during the propagation investigated.…”

“…This was limited by introducing radiation heat loss while keeping an inexpensive computational cost [4,13,21]. Meanwhile, we have developed an alternative method minimizing uncertainties using the isochoric method without resorting to any physical model [22]: A novel perfectly spherical isochoric combustion chamber with full optical access (OPTIPRIME), which allows the simultaneous recording of the pressure inside the chamber and of the flame radius evolution until the flame vanishes at the wall, making it fully innovative.…”