The main research objective is to determine the effect of coal-water
fuel (CWF) treatment on atomization quality when applied to an ultrafine
coal water fuel (solids loading - 50%) and at elevated pressures. The fuel
treatment techniques are expected to produce secondary atomization, i.e.,
disruptive shattering of CWF droplets subsequent to their leaving the
atomizing nozzle. Upon combustion, the finer fuel droplets would then yield
better burnout and finer fly ash size distribution, which in turn could
reduce problems of turbine blade erosion. The parallel objective was to
present quantitative information on the spray characteristics of CWF
(average droplet size and spray shape and angle) with and without fuel
treatment for purposes of application to the design of CWF-burning gas
turbine combustors.
The experiments include laser diffraction droplet size measurements and
high speed photographic studies of CWF sprays in the MIT Spray Test Facility
to determine mean droplet size (mass median diameter), droplet size
distribution, and spray shape and angle. For the spray tests at elevated
pressures, pressure vessels were constructed and installed in the spray test
rig. For support of data analyses, a capillary tube viscometer was used to
measure the CWF viscosity at the high shear rate that occurs in an atomizer
(> 104 sec' ).
A semi-empirical relationship was developed giving the CWF spray
droplet size as a function of the characteristic dimensionless parameters of
twin-fluid atomization, including the Weber number, the Reynolds number, and
the air-to-fuel mass flow ratio. The correlation was tested experimentally
and good agreement was found between calculated and measured drop sizes when
the high shear viscosity of the CWF was used in the semi-empirical equation.
Water and CWF spray tests at elevated pressure were made. Average
droplet sizes measured as a function of atomizing air-to-fuel ratios (AFRs)
at various chamber pressures show that the droplet mass median diameter
(MMD) decreases with increasing AFR at a given chamber pressure and
increases with increasing chamber pressure at a given AFR. In particular,
the results show that droplet sizes of CWF sprays decrease with increasing
chamber pressure if the atomizing air velocity is held constant.
Of the fuel treatment techniques investigated, the heating of CWF
(flash-atomization) was found to be very effective in reducing droplet size,
not only at atmospheric pressure but also at elevated pressure. Secondary
atomization by C02 absorption (used in a previous study) had given favorable
results on CWF combustion, but in this present case this fuel treatment did
not seem to have any observable effect on the drop size distribution of the
CWF spray at room temperature.
The spray angle was observed to reduce with increasing chamber pressure
for given atomizing conditions (AFR, fuel flow rate, fuel temperature). The
decreasing entrainment rate per unit length of spray with increasing chamber
pressure was mainly responsible for the reduction of the spray angle. The
heating of the CWF increased the spray angle, both at atmospheric and
elevated pressures. A model was developed to predict spray angle change for
the effects of the flash-atomization as a function of AFR, fuel flow rate,
and the superheat of the water
