The effects of non-uniform temperature distribution on the ignition of a lean homogeneous hydrogen–air mixture

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Abstract

To characterize the ignition process in homogeneous charge compression ignition engines, high fidelity simulations are performed to study the effects of different initial temperature distributions on the autoignition of a turbulent homogeneous mixture at high pressure. The effects of the initial temperature distribution on the ignition and subsequent heat release are studied by comparison of simulations with three initial random temperature fields having different skewness. It is found that the scalar mixing and turbulence have a significant influence on the initial location and further evolution of the ignition kernels. A comparison of the integrated heat release rates shows that the presence of a hot core leads to early ignition and increased duration of burning, while a cold core leads to a dormant end gas, which is consumed by slow combustion. The extent of flame fronts is quantified by a temperature gradient cut-off, revealing distinct behavior in the appearance of flame fronts for the three cases. Finally, two distinct ignition regimes, namely the spontaneous propagation and the deflagration regimes, are identified, and a predictive criterion is defined based on the spontaneous propagation speed and deflagration speed at the local mixture conditions. The predictions are found to be consistent with the observed results, suggesting a potential strategy in the modeling of HCCI combustion process.

Introduction

The concept of homogeneous charge compression ignition (HCCI) engines has attracted significant research interests in recent years as an alternative IC engine design with promises for high efficiency and low emissions. In HCCI engines, the reactant charges are very lean and nearly homogeneous, hence the ignition and combustion processes are believed to be primarily controlled by the kinetically driven processes. As such, one of the key challenges in the development of HCCI engines is to control the start of combustion and to ensure smooth heat generation during the cycle under a wide range of load conditions.

To overcome the technical challenges and to assist the development of the HCCI engines, accurate predictive simulations of the ignition and subsequent combustion processes in an HCCI-like environment are important. However, a full three-dimensional simulation that includes detailed chemical kinetic models and turbulence is beyond the capabilities of the computational resources in the foreseeable future [1]. Consequently, most of the previous modeling work incorporating detailed chemistry employed either zero-dimensional or multi-zone models [2] that divide the combustion chamber into a finite number of isolated zones. These models are based on the assumption that turbulence and mixing have little effect during the rapid ignition process in HCCI engines.

Despite the conceptual framework that HCCI combustion is anticipated to be nearly homogeneous, however, there are experimental observations suggesting that ignition and combustion processes in HCCI engines are far from homogeneous [3]. Non-uniform ignition events arising from local “hot spots” have also been reported [4]. Partial mixture stratification in an HCCI engine can also be achieved using exhaust gas recirculation, which is considered a feasible means to control the start of combustion and prolong the burning duration [5]. Therefore, it is of practical importance to understand and to characterize the autoignition process of a nearly homogeneous lean mixture subjected to fluctuations in flow and scalar fields, such that the overall impact of the turbulence and mixing on the ignition behavior is fully accounted for.

Another notable issue in recent experimental studies is the observation of a flame-like wave propagation in HCCI engines [3], [6]. Although these experiments strongly support the presence of reaction front propagation, it is unclear whether the observed front indeed represents a deflagration wave. In an earlier work, Zeldovich [7] identified three distinct regimes of a reaction front propagating through a non-uniform mixture: spontaneous propagation, normal detonation, and deflagration. The ignition front propagation arising from the hot spots in HCCI engines can also be described using a similar approach, as in Gu et al. [8]. Such a classification of different ignition regimes under various engine operating conditions will provide valuable insights into the modeling of HCCI ignition and combustion.

Therefore, the objective of the present study was to perform high-fidelity simulations of the ignition of a homogeneous lean mixture in the presence of turbulence and non-uniform temperature distribution, as a canonical problem to unravel some fundamental issues related to ignition in an HCCI-like environment. Using two-dimensional calculations with detailed hydrogen–air chemistry under high-pressure conditions, the effects of different temperature distributions on the overall ignition behavior are investigated. A criterion to distinguish two ignition regimes, namely the spontaneous propagation and deflagration, is proposed and applied to the simulation data to identify the ignition characteristics in HCCI applications.

Section snippets

Numerical method and initial conditions

The autoignition of two-dimensional turbulent lean hydrogen–air mixture is computed using direct numerical simulation in a closed volume at high pressure conditions to represent the compression ignition process in an HCCI engine. The full compressible Navier–Stokes, species, and energy equations for a reacting gas mixture are solved using a fourth-order Runge–Kutta method for time integration and an eighth-order explicit spatial differencing scheme [9], [10]. The chemical mechanism for

Reference case behavior

We first compute two reference cases of ignition of an identical mixture to represent some idealized system response. These reference cases are then compared with the turbulent ignition results to assess the effects of turbulent mixing on autoignition. The first reference case is called the “isolated cells,” where the two-dimensional turbulent field is assumed to be an ensemble of constant volume cells, which have absolutely no interaction with the neighboring cells, such that their ignition

Conclusions

High fidelity simulations were performed to study the effects of different temperature distributions on the autoignition of a turbulent homogeneous mixture. In particular, the effects of hot and cold core gases on the heat release of the system were examined. The results showed that the temperature distribution and mixing rate have a major influence on the location of the first ignition sites and the subsequent combustion and heat release. It was found that the presence of a hot core gas leads

Acknowledgments

The work at UM was supported by the Consortium on HCCI Engine Research directed by the UM and funded by the Department of Energy (DOE), and also by DOE, Office of Basic Energy Sciences, SciDAC Computational Chemistry Program. The work at SNL was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the DOE. Calculations were performed at the DOE’s National Energy Research Computational Facility. The authors thank Dr. Scott Mason of

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