This study is motivated by thermo-acoustic instabilities, observed in liquid fuel
combustors. Our objective is to experimentally and analytically observe the effects of
acoustically perturbing a confined hollow cone spray sheet, by using a transverse excitation. This
research proposes to separate the problem into two – a cold flow problem, where we will
elucidate the breakup mechanisms of a hollow cone sheet via. hydrodynamic instabilities subject
to transverse acoustic forcing and air swirl; a hot flow problem where we will identify the
conditions needed to achieve thermo-acoustic instability, for this canonical configuration. To
achieve this, two models, one for breakup mechanisms (cold flow) and one that defines a flame
transfer function (hot flow) will be developed which can then be joined by an existing secondary
atomization model that can describe the spray distribution pattern. To achieve these dual goals,
the scope of the experimental attempt shall include high speed backlight imaging, PIV, ILIDS for
the cold flow problem and PLIF and Chemiluminescence for hot flow problem. For the
analytical treatment, a linear stability model that accounts for sheet variation thickness and sheet
curvature will be developed while the flame transfer function will be based on the fuel/air
mixture distribution.
Preliminary work has been carried out and presented at two conferences. The objective of
this work was to experimentally study the response of a hollow cone spray sheet subjected to
transverse acoustic excitation. This was quantified by measuring changes in the spray breakup
length and swirl angle, phased average images, and oscillatory behaviour of the sheet edge. We
used a pressure swirl nozzle embedded into a swirler with 60° vane angles and a geometric swirl
no. of SG = 0.981. Water is supplied from a pressure vessel, pressurized at 3 bar (gauge) to
produce a hollow cone spray sheet. For cases where swirl air was introduced, the flow rate was
fixed at 1250 slpm. For both conferences two separate experimental rigs were used such that the
spray was transversely acoustically perturbed using a symmetrical and asymmetrical
configuration. Focusing on the symmetric acoustic forcing configuration, two transverse ducts of
equal length and an internal rectangular cross section of 120 x 205 mm are attached on either
side of the spray chamber such that the duct axis is perpendicular to the spray axis. For this
configuration the resonant frequencies were found to be 135 Hz and 325 Hz; while at 135 Hz a
pressure node was present at the spray nozzle axis, at 325 Hz a pressure anti-node was present
and hence only these two were selected for this study. For each frequency, the acoustic excitation
of our spray, with and without swirl air, was carried out. Overall, we see that the spray sheet
edge responds at all frequencies, along with the spray cone angle and breakup length. Phase
averaged images shows a “flapping motion” of the spray.