Second-Order Conditional Moment Closure Simulations of Autoignition of an n-heptane Plume in a Turbulent Coflow of Heated Air

Abstract

Autoignition of an n-heptane plume in a turbulent coflow of heated air has been studied using the conditional moment closure (CMC) method with a secondorder closure for the conditional chemical source term. Two different methodologies have been considered: (i) the Taylor expansion method, in which the second order correction was based on the solution of the full covariance matrix for the 31 reactive species in the chemical mechanism and hence was not limited to a few selected reactions, and (ii) the conditional PDF method, in which only the temperature conditional variance equation has been solved and its PDF assumed to be a β-function. The results compare favorably with experiment in terms of autoignition location. The structure of the reaction zone in mixture fraction space has been explored. The relative performance of the two methodologies is discussed.

Experiments and Simulations of n-Heptane Spray Auto-Ignition in a Closed Combustion Chamber at Diesel Engine Conditions

“…Investigations employing second order closure have been presented in [30,44]; due to the strong increase in computational cost conditional fluctuations have been neglected in the present investigation to enable the multitude of runs required for the sensitivity analyses. As discussed in more detail in [30,45], first-order closure has been shown to be sufficient for auto-ignition in cases where the scalar dissipation rate rapidly decays to sub-critical values. Nonetheless, future, more detailed investigations should assess the importance also for the present conditions as conditional fluctuations could-in principle-play a role for this spray.…”

“…The simulation of non-premixed systems by means of CMC has seen successful application to turbulent piloted, lifted jet, bluff-body, swirl-stabilised and counterflow flames [22][23][24][25][26][27][28] as well as auto-ignition of gaseous jets [29][30][31]. Concerning autoigniting liquid fuel sprays at diesel engine relevant conditions, investigations by means of CMC have been documented in [32] for the set-up of [5] containing spray penetration and ignition delay data of n-heptane sprays at various temperatures.…”

Numerical Study of n-Heptane Auto-ignition Using LES-PDF Methods

“…The most numerous examples are in lifted flames with vitiated co-flow, where RANS [20][21][22] and LES [19,23,24] amongst others, where able to obtain good agreement with measured data. However, in non-stationary auto-ignition there are fewer RANS examples [25,26] as RANS-type models are incapable of providing the transient information required to reproduce auto-ignition phenomena. However, to date there appear to have been few LES investigations of auto-ignition [27,28] and none for complex hydrocarbons.…”

LES-CMC Simulations of Different Auto-ignition Regimes of Hydrogen in a Hot Turbulent Air Co-flow

“…5 and 6), explains why first-order CMC, which is in general expected to be less accurate than second-order CMC, is adequate for the problem at hand. In [38], it was shown for n-heptane auto-ignition and the present configuration that second-order closure is important only when the mean scalar dissipation rate is high and conditional fluctuations are large. In the case under study the region of high scalar dissipation rate is not close to the region where auto-ignition occurs, so that second-order effects are less important.…”

“…In the CMC code, developed [38] and validated [12] at Cambridge University, equations (4) and (5) are solved, using the velocity and mixing field from the flow field solver. The mean density, required for the flow calculations, is obtained from the conditionally averaged values of the CMC calculations, weighted by the mixture fraction probability density function (β-PDF) (Eq.…”