Chemically Reacting Flows Division

Autoignition in Turbulent Flows


Autoignition of Liquid Fuels in Turbulent Inhomogeneous Flows

 

Autoignition of fuels in turbulent environments is relevant to diesel engines, low-emission Homogeneous Charge Compression Ignition (HCCI) engines, new-generation Gasoline Compression Ignition (GCI) engines, and liquid fuel lean-burn gas turbines.

The investigation of turbulent non-premixed autoignition of liquid fuels in this work has been motivated by recent experimental and computational findings for gaseous fuels in which turbulence was found to have a delaying effect on the occurrence of autoignition. These studies also suggest that autoignition occurs usually away from stoichiometry at a “most reactive mixture fraction” and at locations in the turbulent flow with low scalar dissipation. Hence, experiments for realistic and future liquid fuels like diesel, biodiesel and even lighter gasoline must be performed to examine more fully the effects of the scalar dissipation rate and its fluctuations on liquid fuel droplet formation, evaporation, the slow pre-ignition chemical reactions, and ultimately autoignition itself.

This work is an experimental effort aimed at understanding the fundamentals of the autoignition of liquid fuels in turbulent flows. We have an experimental arrangement for the quantitative observation of autoignition in a well-defined flow, in which a liquid fuel is injected axisymmetrically from a finite-sized circular nozzle into a co-flow of high temperature turbulent air.

Original phenomena are observed concerning the emergence and observation of the various regimes in the autoignition behaviour. Quantitative information pertaining to the effects of turbulence intensity on the location of autoignition is recorded. Turbulence intensity is an important parameter identified in the earlier work on the autoignition of gaseous fuels in a similar arrangement. The data from such investigations can be used for the validation of advanced turbulent combustion models for liquid fuels.

Autoignition Phenomenon

Autoignition Phenomenon

Autoignition Occurence
Autoignition Occurence

 

Micro-combustion


Flame Dynamics and Flow Characterisation in a Micro-Channelled Combustor

The increasing use of Micro-Electro-Mechanical Systems (MEMS) has generated a significant interest in combustion-based power generation technologies, as a replacement of traditional electrochemical batteries which are plagued by low energy densities, short operational lives and low power-to-size and power-to-weight ratios. Moreover, the versatility of integrated combustion-based systems provides added scope for combined heat and power generation.

In this study it is observed that the flame enters channel and propagates towards the injection manifold as a planar flame for a short distance (and time). Following this stage, the flame passes through the middle region of the channel in a chaotic fashion, and finally develops a characteristic trident shape and quenches upon reaching the injection slots. It is found that an increase in the flow Reynolds number results in an increase in the length (and associated time) over which the planar flame travels once it has entered a micro-channel, and a significant decrease in the time between its conversion into a chaotic flame and its extinction.

The results indicate that the flame propagation is strongly influenced by the flow conditions within the channel although a definite conclusion about the dependence of the flame behaviour on air-fuel equivalence ratio cannot be made.

Flow characterisation experiments involving high speed imaging (using a rheoscopic fluid) and PIV (Particle Image Velocimetry) in a geometrically similar channel record the vortices and fluid recirculation patterns helping correlate the dependence of flame propagation behaviour with the nature of flow.

The data and results from this study will not only help the development of new micro-power generation devices, but they will also serve as a validation case for combustion models capable of predicting flame behaviour in the presence of strong thermal and flow boundary layers, a situation common to many industrial applications.

Modes of Operation
Modes of Operation