Literature
review

I.  
Introduction

A turbine emission probe is usually a rig with multiple-holes,
to test the components of emission from a gas turbine. Over the last 150 years,
significant evidence shows that the greenhouse gases from human activity causes
climate change.1 There is 27% of total amount
of emission caused by transportation.1
Monitoring the vehicle engine emission is an effective way to know about the
change in the amount of greenhouse gases. A Probe is a critical tool for
sampling gas, because not only does the sampling gas go through it, but also
its structure may affect the measured result. If there are flaws on a probe
design, the sampled gas may fail to be representative of the gas in the tested
chamber. So, designing a probe which can get a highly accurate representative sample
is important.

II.  Challenges
in probe working conditions – previous work

As combustion engines are further optimised; the working temperature
and pressure in the inner combustion chambers of engines are also increasing. So,
the probes also work in increasingly severe conditions.  Many different conditions for combustion
engine operation have been studied. For example, some probes work in a high
temperature chamber. At K11, which is a famous combustion facility around the word,
the temperature on its combustion chambers can reach to 2573 K.2 The velocity of flow passing
the probe may also be faster than normal flow. A sampling probe in supersonic
flow has been studied by Zhu.3 F. Kock considered the two-phase
flow. So, these areas of investigation will lead the probe study to optimize
probes for these extreme workplaces.

A.  High
temperatures

For the last three decades, thermophoretic sampling techniques
have been widely utilized to directly observe particles generated in flames and
internal combustion engines.4, 5 As mentioned in section 2,
combustion chambers can now reach extremely (and increasingly) high temperatures.
Probes need to work at a high temperature, so the studies on probes at high
temperature environment are essential. Because of the high environmental
temperature, the probes require thermal protection; the main method of this high
temperature protection for probe is water-cooling, which was explored by Grey
in the late 1940s.6 In the present day, the
water-cooled probe has now been well-developed. Swan also invented a high
temperature probe which can withstand 3273.15K.7  The water-cooled method does not just offer a
protection for the probe, but can stand the pressure in the probe.6 In order to study the properties
of water-cooled probe, there are two main measures, namely experiment and
simulation. Duong did experimental work on the two types of water-cooled
probes, and collected the data about the temperature distribution of some main
points on the probes. Then he analysed the model of the flowing sampling gases
in the probes. This method is a good way to get visual data. However, Duong
noted that the sampling gas in the probe with a high temperature will likely
reflect an overestimation of PCDD levels, which is a component that needed to
be measured in this paper. This means the accuracy of the data depends on the
probe and test equipment. The reference 8 uses shadowgraph method to
observe the fluid, as the  probe closes
the perturbation of fuel flow during sampling. In this reference, a high-speed
camera is utilized to capture the image of probes. The motion of probes can be
calculated by the Z-shape shadowgraph optics system. This reference compared
three samples from three different probes. It offers a theory to analyse a ‘vibrated’
probe. However, it does not mention how the fluid changes in a vibrated probe
and it does not theoretically analyse the rule of this change.

Simulating the fluid in a probe is a common way to research the
properties of probe. The Computational Fluid Dynamic (CFD), see also section 3
of this review) is used to simulate the fluid in a probe by setting the
environment temperature, the velocity of fluids and the model of fluid. For a
high temperature probe, heat transfer should be considered.9 The calculation of CFD can
show the properties of the probe and efficiency of cooling system. The authors
of reference 9 used the distribution of
temperature on probes to analyse the relation between heat transfer
coefficient  and the velocity of fluid.
The experimental data also showed the same result, which verified the simulated
result. Reference 2 suggests that CFD can be used
to calculate the temperature and pressure on the probes to analyse the pattern of
the distribution of the temperature and the maximum stress point on the probe.
Also, the calculation result is proven by the experimental data. It compared
two different probes, and offers a better design of probe in an extremely high
temperature. However, it does not study the accuracy of this probe. For the
study on high temperature, it shows that CFD simulation can offer a numerical
result which can lead to specific theoretical analysis. The experimental data
can verify the numerical result and offer enough data to build a reasonable
model for the fluid.

 

B.  Problems
with two-phase flow

In many industrial processes, two-phase flow involving sprays
is utilised for mass or heat transfer processes, in which process there are turbulent
fluid and relatively high heat transfer coefficients; so it is necessary to
study this flow pattern in CFD simulation.10For a two-phase flow probe,
measuring the temperature and humidity of fluid is achieved by measuring the
presence of particles. However, because of the droplets present in the flow,
these measured particles may also deposit on the wall of probe. The loss of
particles will cause measurement error, and may even cause damage to devices
such as dew-point hygrometers.10

There are many studies on two-phase flow. Ropelato et. al.11 built a multiphase model, considering
dissipation of turbulent kinetic energy of the gas phase, based on the
assumption that there is no shear stress on the solid phase. Their model
predicts the fluid dynamics of inlet change, of a downer reactor, with the
support of experimental data. Liu et. al.
12 improved the model by
considering the particles’ collision frictional stress. He applied a Eulerian-Eulerian
approach to solve the 3D Reynolds equation, and simulated dense gas-particle
flows in a downer reactor. Besides the Eulerian approach, in which the solid
phase was considered as a continuum; the Lagrangian approach was also popularly
adopted, which combined Computational Fluid Dynamics (CFD) and Discrete Element
Method (DEM) to study various multiphase systems.12–14 Zhang et. al.15 utilized this method to solve
the motion of particles in a flow, and the result shows the change tendency of the
velocity of particles.  Zhao et. al. 16applied the CFD-DEM approach to
calculate the velocity of particles, the distribution of particles in the flow,
and the gas velocity under different gas-particle conditions.

 

III.Computational Fluid Dynamic

CFD is a branch of fluid mechanics that uses
numerical analysis and data structures to solve and analyse problems that
involve fluid flows. Computers are used to perform the calculations
required to simulate the interaction of liquids and gases with surfaces,
defined by some boundary conditions. With high-speed supercomputers,
better solutions can be achieved. Ongoing research yields software that
improves the accuracy and speed of complex simulation scenarios, such as
transonic or turbulent flows. Initial experimental validation of such software
is performed using a wind tunnel with the final validation coming in full-scale
testing, e.g. flight tests. 17 Almost every theoretical
analysis of probes, needs fluid to simulate. For example, for high temperature
probes, Brouckaert et.6 used a CFD model to analyse
the temperature on probes. The result accurately shows the distribution of heat
on the probes, and it is also agreement with the experimental data. The authors
of reference 2 also used CFD to simulate the
probe in the working environment, to analyse the efficiency of cooling systems
in the probe. Also, the result offers a better understanding on the allowable
temperature on the walls of probe, which can be an index to assess the design
of the probe.

For a probe to measure two-phase flow or multiphase flow, Lui
et. al. 12 predicted hydro-dynamics in
CFD by using the Eulerian–Eulerian two-fluid approach. This method accurately
predicts the motion of particles (see also section 2.2). Recently, two-phase
flow studies are largely based on this approach. For example, reference 14 used this method to study the
behave of particle in a reactor bed. A combination of experimental study using
particle image velocimetry (PIV) and numerical investigation using CFD and the
discrete element method (DEM) was applied, to analyse the liquid-solid flow.
The particle of motion was described by the CFD model. Ropelato et. al. 11 use the CFD to calculate the
distribution of velocity of the two-phase flow. The theoretical result well
matched with the experimental data, which proves the CFD is powerful tool for
fluid analysis.

Alought the CFD is an excellent tool for fluid analysis, it
relies on building the right model of the fluid under different working
environment. This requires a good understanding of the fluid in the probe at
different situations, which usually is based on an abundant of experimental
data and previous studies. Also, the experimental data can assess the models.
So, the theoretical analysis of probes is usually conducted in tandem with
experimental work, for best results.

IV.Analysis

The papers cited all studied the properties of probes in
different working environments, by calculating the velocity of fluid or the
distribution of heat on the probes or experimental observation. These studies
offer accurate models of fluid in probes, theoretical analysis and effective
approaches about numerical methods for further studies.

These studies delivered a specific understanding about the
probes’ work on extreme situations, but some problems are excluded from mention.
These papers do not study the accuracy
of the probe itself, which is important for measuring sampling gas. The need
for accuracy of measuring the emission gases from combustion turbine is
increasing; especially considering the increasingly proven connection between
the greenhouse gas and climate change; 1 emission control and restrictions
are severer than in the past. For example, legislation that affects gas turbine
emissions directly have also been enacted. In 2001 the E.U. enacted the “Large
Combustion Plant Directive (LCPD)” which was part of the wider policy directive
named “Directive on Integrated Pollution Prevention and Control (IPPC)”. This
piece of legislation affects Gas turbines with a power output of greater than
50MW. 18   So, a higher working accuracy of probes for emission
gas should be studied.

According to authors of reference 19, for example, the structure
of a probe may cause sampling gas blockages in the probe. In this paper, there
are two probes in design, and the gas collection process is simulated by CFD.
The velocity of flow in different holes showed that the gas from the hole which
is furthest away from the outlet may cost more time than expected, because the
gas from the hole which is closer to outlet blocks this gas. Also, the gas from
further hole has turbulent flow, which causes it to circulate in the probe.
That not only decreases the velocity of emission coming out of the probe, but
also causes some particles and gas dwelling in the probe. So, the accuracy of
the sampling from the probe is decreased. That means the sampling is not
representative. So, further work on decreasing the effect on turbulence within
probe is needed.

V.  Conclusions & future work

It has been shown that both computational and experimental
observation can provide useful assessment of the distribution of heat and
pressure about emission probes. In particular, the tandem use of CFD and
experimental work can give reliable and consistent data on the properties of
probes in a working environment.

However, it is also shown that the internal structure of these
probes is problematic for the accuracy of their measurement. Holes and their
positioning on the probes can lead to turbulent fluid flow.

Improving the accuracy of probes, by reducing the effect on
turbulence of gases should be studied. So, a new design of probe should be
made. CFD can perhaps offer a situational assessment of these new designs.Literature
review