Abstract:
The main
objective of the investigation is to provide a physically realistic model
describing the movement of liquid phase in an industrial axial compressor during online
washing. ln order to achieve this objective, CFD simulations were developed for
predicting the water particle trajectories inside an industrial axial compressor, the rate of
evaporation of water droplets on axial compressor blades by inertial impaction, turbulent
diffusion, pressure and temperature increments. Simulation of water droplet trajectories
and
evaporation of water droplet content in designed 260MW gas turbine engine were
undertaken.
Two
types of boundary conditions have been considered for a droplet. The first is
the inlet
boundary condition, which describes the properties of a droplet at the entry/inlet
position. It was assumed that the droplets are evenly distributed at the inlet face. For
the
droplet inlet boundary, there were 4 main factors considered, namely droplet initial
temperature, droplet flow rate, droplet axial slip velocity and droplet average initial
diameter. Each factor has at least two levels, namely the base line level and some
deviation or deviations to the base-line level.
The second
boundary condition is the wall boundary condition, which models what
happens when a droplet interacts with a solid wall. For this second type boundary
condition, all simulations in this study, the coefficient of restitution of all solid walls was
set at
very low number, coefficient of restitution = 0.0005. It is assumed that a droplet
that hits a solid wall will undergo a non-perfect rebound.
Other than mechanical effects due to the impact of particulates during online
washing, the main aero thermal effects are due to the flow changes occurring on account
of the two
phase nature of the working fluid, heat and mass transfer among the two
phases, modification in thermal and transport properties, and changes in chemical action
and combustion.
Practically, it is not possible, in general, to assure that all of the washing fluid, which
entered the
engine during online washing, would always be converted to the gas phase
by the time the working fluid leaves the high pressure compressor, or even the
combustor. Thus, both the state of the fluid and the cross-sectional distribution of
washing fluid are of concern throughout the engine flow path, especially the first 3 front
stages of the axial compressor.
It must be noted that the preceding statements take no account of the presence and
effects of bypass doors, vents, and valves. This is a set of devices for which there are
no reliable data for air-fluid mixture operation, and thus, no guidance in design or
installation.
They can introduce major changes in the amount and distribution of the
discrete
phases of fluid along the flow path in compressors.
Finally, however there is a need for at least one set of definitive tests on selected
components and engines. At the moment, there is neither clear view nor experimental
data available to validate those said objectives. On one hand, there is no validated
computational model available to assist with understanding the complex relationship
between the
injection parameters and the resultant flow pattern and trajectories of the
droplet as well as there is no information regarding the droplet build up on the blade
surface in a
multistage axial compressor during online washing. Nevertheless, even
though without experimental validation, this investigation is very important as advancing
tools to further understand the axial compressor online washing phenomenon which is
still not
fully understood even today. Therefore, post-axial-compressor fluid motion is
important in understanding the role of air/fluid mixing and fluid evaporations and will be
analysed and presented in this study.
More
precisely, this investigation tries to quantify the effect of injected droplet
parameters on generic multistage axial compressor. As this work is focused on the
model
(droplet parameter) sensitivities rather than the CFD model itself, presented in
this
investigation herewith are only the main features of the generic droplet trajectories
flow
pattern in a multistage industrial axial compressor.