Keywords: Liquefied Petroleum Gas; non-premixed flame; coflow burner; crossflow burner; backward facing step; kinetic mechanism; stability maps

Liquefied Petroleum Gas (LPG) is a mixture of several hydrocarbons. Both saturated and unsaturated hydrocarbons are present in varying proportions. It is obtained as a by product during fractional distillation of crude oil. It is also obtained as a co-product during natural gas
processing. LPG is compressed to pressure in the range of 4 bar to 6 bar, stored as liquid in cylinders and transported to several places. Due to low diffusivity and presence of higher
order hydrocarbons, LPG diffusion flames are longer for the same volumetric flow rate, when
compared to the flames of methane and biogas. They are luminous, due to the formation of
several intermediate hydrocarbon radicals as well as radiation by soot. Since LPG is heavier
than air and is a multi-component fuel, proper understanding of its burning characteristics is
necessary to employ LPG efficiently in several types of burners.
Flame characteristics and its stability in burners are studied through experiments and
numerical analyses. Both these methodologies complement each other to reveal the physical
insights in various flames. The characteristics of flames from single component fuels such as
methane, propane and biogas have been studied systematically in the past. However, studies
on multi-component fuels such as LPG, especially through numerical simulations, are scarce.
Non-premixed mode of combustion is used in domestic and industrial applications due to its
wide operating range and stability. Coflow and crossflow configurations are used in these
quite generally. Crossflow flames have wider operating range and provide better control,
when obstacles such as backward facing steps and bluff bodies are used to increase the
stability of flames. However, studies involving crossflow flames of LPG are scarce.
Similarly, studies on soot production in LPG flames are also scanty.
The scarcity on numerical studies involving LPG is because of the fact that a kinetic
mechanism used to model its oxidation is quite complex. Reliable numerical simulations of
flames involving LPG as a fuel are most essential to study the efficient use of LPG in
domestic and industrial burners. Such simulations require a suitable reaction mechanism
containing the necessary species and reactions to predict key combustion characteristics in
flames accurately. These gaps in literature form the motivation of the present thesis.
The global objective or scope of this thesis is to carry out a combined experimental
and numerical study to understand the structure, stability and emissions in laminar LPG

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flames. The specific objectives of the work are to obtain experimental temperature data in
coflow flames for validating the numerical model, to propose stability maps and to establish
stable operating regimes in crossflow flames without and with obstacles through systematic
experiments, to develop and validate a short (or compact) chemical kinetic mechanism to
simulate LPG flames using comprehensive numerical model and to numerically study the
flow, species and temperature fields to understand the flame characteristics and soot
formation in coflow and crossflow flames.
To accomplish the above objectives, first, coflow non-premixed flames are studied
experimentally. The fuel flow rate is increased progressively from 10 g/h to 17.9 g/h, such
that a jet diffusion flame sustains at the burner exit. Keeping the fuel flow rate at 10 g/h,
coflow air is increased progressively from 0% to 400% of the stoichiometric value, to obtain
coflow diffusion flames. Subsequently, for the same fuel flow rate, in the absence of coflow,
primary air is supplied amounting to 0% to 50% of the stoichiometric value, to form partially
premixed flames. Finally, the primary air is varied as above in the presence of coflow to get
partially premixed flames in coflowing air stream. Direct flame photographs and
shadowgraph images are captured for measuring the flame characteristics such as flame
height and plume width. Plume temperatures (both in axial and radial directions) are
measured for a particular case in each configuration to provide data for validating the
numerical model.
Next, LPG-air flames in non-premixed crossflow burner, where air flows
perpendicular to the fuel stream, are analysed experimentally. In the absence of obstacles,
flames are unstable at higher air velocities. The presence of obstacles improves flame
stability. The flame stability is assessed through careful experiments for cases without and
with obstacles such as bluff bodies and backward facing steps. The power rating is varied
from 250 W to 1200 W. The air velocity is varied in the range 0.2 to 3 m/s. For cases without
obstacles, as the air velocity is increased, the flame transitions from plate stabilized flame to
separated flames (both symmetric and asymmetric). In the presence of obstacles, increasing
the air velocity results in two more stable operating regimes such as lifted flame and obstacle
stabilized (bluff body stabilized or step stabilized) flame regimes. Stability maps are plotted
for all cases and compared with the stability maps for methane and biogas obtained on a
similar burner.
A short or compact chemical kinetic mechanism for LPG - air, consisting of 43
species and 392 reactions, is derived and validated for propane and n-butane diffusion flames,
as well as premixed flames of propane-n-butane mixtures.

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Selected experimental cases are simulated using commercial CFD software, Ansys
FLUENT 16.1. Full multi-component diffusion along with thermal (Soret) diffusion is
considered for calculating diffusional velocity of species. Suitable sub-models are included
for enthalpy transport by diffusing species, soot formation and its oxidation and radiation
losses contributed by gas and soot.
Temperature, flow and species fields from numerical simulations of coflow flames
are systematically analysed. The distributions of soot volume fraction and soot oxidation rate
have been presented for several configurations. The net amount of soot formed per unit mass
flow of the fuel gradually increases as the coflow air is increased from zero to 150% of
stoichiometric air. Then, it rapidly decreases as the coflow air is increased to 200%. After
this, it does not change much. The amount of soot emitted in partially premixed flames
decreases sharply with increase in primary air. With coflow air, the primary air is able to
enhance the reaction rate, temperature, as well as reduce the soot formation. Negligible
amount of soot is emitted on increasing the primary air to around 40%. Details of flame structure and soot formation characteristics in crossflow flames are
analysed through numerical simulations. For cases without obstacles, effect of air velocity on
flame characteristics and soot formation is studied. In the presence of backward facing steps,
effects of step height and location of rear face of the step upstream of the fuel injector are
also studied systematically. Numerical simulations complement the experimental results by
providing valuable information on flow, temperature and species fields, which are very
difficult to measure even using sophisticated instrumentation techniques.
In summary, characteristics of LPG-air non-premixed flames in coflow and crossflow
configuration have been studied experimentally and numerically to understand the flame
structure, stability and soot formation aspects in these flames. This work is first of its kind as
there are no studies available on stability and soot forming characteristics of coflow and
crossflow flames involving multi-component fuels such as LPG. Stability maps clearly depict
the stable operating regimes and are essential for burner design. Numerical model with the
compact mechanism is well validated and comprehensive enough to explain the physics of
the flames in various configurations. Therefore, this thesis can provide useful data and model
for efficient design of domestic and industrial LPG gas burners.