Numerical simulations of premixed cool flames of dimethyl ether/oxygen mixtures

With the goal of providing a more detailed fundamental understanding of ignition processes in diesel engines, this study reports analysis of a direct numerical simulation (DNS) database. In the DNS, a pseudo turbulent mixing layer of dimethyl ether (DME) at 400 K and air at 900 K is simulated at a pressure of 40 atmospheres. At these conditions, DME exhibits a two-stage ignition and resides within the negative temperature coefficient (NTC) regime of ignition delay times, similar to diesel fuel. The analysis reveals a complex ignition process with several novel features. Autoignition occurs as a distributed, two-stage event. The high-temperature stage of ignition establishes edge flames that have a hybrid premixed/autoignition flame structure similar to that previously observed for lifted laminar flames at similar thermochemical conditions. A combustion mode analysis based on key radical species illustrates the multi-stage and multi-mode nature of the ignition process and highlights the substantial modelling challenge presented by diesel combustion.

Section: Qualitative Descriptionsupporting

confidence: 87%

Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation

Krisman

Hawkes

Talei

et al. 2016

“…As discussed in previous papers [13,14,75], one of the primary difficulties in establishing stable cool flames is finding conditions which sufficiently enhance the lowtemperature reactions that sustain the cool flame without also overly favoring the reactions that promote the transition to a hot flame. Ozone is particularly useful in this regard due to its tendency to release O radicals when it begins thermal decomposition near 450 K. These O radicals have a strong effect on radical-starved cool flames, which typically have maximum OH concentrations of only a few ppm, but only slightly strengthen hot flames, which can possess radical pools of the order of a few percent.…”

Section: Initiation Of Cool Diffusion Flames With and Without Ozonementioning

confidence: 99%

“…Ignition in such systems is often a complex, two-stage process [9] in which the heat release by the first stage, the cool flame, strongly affects the ignition timing of the second stage [10,11]. These low-temperature cool flames have also been shown to be an important influence on engine knock [12] and can extend the lean flammability limit [13][14][15]. Even in continuous combustion systems such as gas turbines, the low-temperature chemistry can significantly modify the fuel composition, which in turn may affect the turbulent burning velocity [16,17] or stabilization mechanism [18,19] of the main flame and possibly contribute to phenomena such as lean blow off [20].…”

Section: Introductionmentioning

confidence: 99%

Study of the low-temperature reactivity of large n-alkanes through cool diffusion flame extinction

Reuter

Lee

Won

et al. 2017

Self Cite

The low-temperature oxidation of hydrocarbon fuels has received increasing attention as advanced engines seek to operate in less conventional combustion regimes. Large n-alkanes are a notable component of many real transportation fuels and possess strong reactivity in this important low-temperature range. These n-alkanes have been studied extensively in various canonical kinetic experiments but seldom in systems with strong coupling between lowtemperature chemistry, transport, and heat release. To address this issue, the present study investigates self-sustaining n-alkane cool diffusion flames in a counterflow burner. The extinction limits of both hot diffusion flames and cool diffusion flames are measured at atmospheric pressure for a range of n-alkanes from n-heptane to n-tetradecane. It is observed that while these fuels behave similarly for hot flames, the larger n-alkanes are substantially more reactive in the low-temperature cool flame regime. Moreover, ozone addition strongly enhances the low-temperature chemistry to the point where the differences in fuel reactivity are nearly suppressed.The experimental measurements are compared with numerical simulations employing both detailed and reduced chemical kinetic models of various sizes. Although the different kinetic models adequately predict the extinction limits of the hot flames, a large scatter is present in the model results for cool flames, and a general overprediction of the measured cool flame extinction limit is observed for all of the fuels studied. This implies that the cool flame heat release is not well agreed upon by the current chemical kinetic models, despite their capability to reproduce many homogeneous reactor experiments at low temperatures. Furthermore, it is observed that the cool flame heat release is spread over a substantial number of reactions involving large molecules, a trait that makes it particularly difficult to create reduced kinetic models that can accurately describe cool flame behavior. The results of this study suggest that the cool flame platform can provide crucial validation of the coupling between chemistry, transport, and heat release in flames at low temperatures. 2 3 4 5 6 7 8

“…The study of cool flames has received renewed interest due to the need for knocking control in advanced engines [1,2] and the recent observations of cool flames in microgravity [3][4][5] and plasma assisted low temperature combustion [5][6][7][8]. Although cool flames were discovered two centuries ago [9,10], many fundamental flame properties such as the flammability limit and flame speeds of cool flames, especially at high pressures and with mixture dilution, remain unknown.…”

Section: Introductionmentioning

confidence: 99%

On the propagation limits and speeds of premixed cool flames at elevated pressures

2017

Self Cite

The flame speeds and propagation limits of premixed cool flames at elevated pressures are numerically modelled using dimethyl ether mixtures. The primary focus is paid on the effects of pressure, mixture dilution, computation domain, and heat loss on cool flame propagation. The results showed that cool flames exist on both fuel lean and fuel rich sides and dramatically extend the lean and rich flammability limits of conventional hot flames. There exist three different flame regimes: the hot flames, lean and rich cool flames, and double flames. A new flame flammability diagram including both cool flames and hot flames at elevated pressures is obtained. The results show that pressure significantly changes cool flame propagation and burning limits. It is found that the increase of pressure affects the propagation speeds of lean and rich cool flames differently due to the negative temperature coefficient effect. On the lean side, the increase of pressure accelerates the cool flame chemistry and shifts the transition limit of cool flame to hot flame to a lower equivalence ratio. At lower pressure, there is an extinction transition from hot flame to cool flame. However, above a critical pressure, the hot flame directly transfers to a cool flame without hot flame extinction. Moreover, increases in dilution reduce the heat release of the hot flame and promote cool flame formation. Furthermore, the results show that a smaller downstream computation domain and higher heat loss also extend the cool flame transition limit and promote cool flame formation.