Literature probe working conditions – previous work As

LiteraturereviewI.  IntroductionA 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 causesclimate change.

1 There is 27% of total amountof emission caused by transportation.1Monitoring the vehicle engine emission is an effective way to know about thechange in the amount of greenhouse gases. A Probe is a critical tool forsampling gas, because not only does the sampling gas go through it, but alsoits structure may affect the measured result. If there are flaws on a probedesign, the sampled gas may fail to be representative of the gas in the testedchamber. So, designing a probe which can get a highly accurate representative sampleis important.

II.  Challengesin probe working conditions – previous workAs combustion engines are further optimised; the working temperatureand pressure in the inner combustion chambers of engines are also increasing. So,the probes also work in increasingly severe conditions.  Many different conditions for combustionengine operation have been studied. For example, some probes work in a hightemperature 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 passingthe probe may also be faster than normal flow. A sampling probe in supersonicflow has been studied by Zhu.3 F.

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Kock considered the two-phaseflow. So, these areas of investigation will lead the probe study to optimizeprobes for these extreme workplaces.A.  HightemperaturesFor the last three decades, thermophoretic sampling techniqueshave been widely utilized to directly observe particles generated in flames andinternal 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 hightemperature environment are essential. Because of the high environmentaltemperature, the probes require thermal protection; the main method of this hightemperature protection for probe is water-cooling, which was explored by Greyin the late 1940s.6 In the present day, thewater-cooled probe has now been well-developed.

Swan also invented a hightemperature probe which can withstand 3273.15K.7  The water-cooled method does not just offer aprotection for the probe, but can stand the pressure in the probe.6 In order to study the propertiesof water-cooled probe, there are two main measures, namely experiment andsimulation.

Duong did experimental work on the two types of water-cooledprobes, and collected the data about the temperature distribution of some mainpoints on the probes. Then he analysed the model of the flowing sampling gasesin the probes. This method is a good way to get visual data. However, Duongnoted that the sampling gas in the probe with a high temperature will likelyreflect an overestimation of PCDD levels, which is a component that needed tobe measured in this paper. This means the accuracy of the data depends on theprobe and test equipment. The reference 8 uses shadowgraph method toobserve the fluid, as the  probe closesthe perturbation of fuel flow during sampling. In this reference, a high-speedcamera is utilized to capture the image of probes. The motion of probes can becalculated by the Z-shape shadowgraph optics system.

This reference comparedthree 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 probeand it does not theoretically analyse the rule of this change.Simulating the fluid in a probe is a common way to research theproperties of probe. The Computational Fluid Dynamic (CFD), see also section 3of this review) is used to simulate the fluid in a probe by setting theenvironment temperature, the velocity of fluids and the model of fluid. For ahigh temperature probe, heat transfer should be considered.

9 The calculation of CFD canshow the properties of the probe and efficiency of cooling system. The authorsof reference 9 used the distribution oftemperature on probes to analyse the relation between heat transfercoefficient  and the velocity of fluid.The experimental data also showed the same result, which verified the simulatedresult. Reference 2 suggests that CFD can be usedto calculate the temperature and pressure on the probes to analyse the pattern ofthe distribution of the temperature and the maximum stress point on the probe.Also, the calculation result is proven by the experimental data.

It comparedtwo different probes, and offers a better design of probe in an extremely hightemperature. However, it does not study the accuracy of this probe. For thestudy on high temperature, it shows that CFD simulation can offer a numericalresult which can lead to specific theoretical analysis. The experimental datacan verify the numerical result and offer enough data to build a reasonablemodel for the fluid. B.

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

10There are many studies on two-phase flow. Ropelato et. al.11 built a multiphase model, consideringdissipation of turbulent kinetic energy of the gas phase, based on theassumption that there is no shear stress on the solid phase.

Their modelpredicts the fluid dynamics of inlet change, of a downer reactor, with thesupport of experimental data. Liu et. al.12 improved the model byconsidering the particles’ collision frictional stress. He applied a Eulerian-Eulerianapproach to solve the 3D Reynolds equation, and simulated dense gas-particleflows in a downer reactor. Besides the Eulerian approach, in which the solidphase was considered as a continuum; the Lagrangian approach was also popularlyadopted, which combined Computational Fluid Dynamics (CFD) and Discrete ElementMethod (DEM) to study various multiphase systems.12–14 Zhang et.

al.15 utilized this method to solvethe motion of particles in a flow, and the result shows the change tendency of thevelocity of particles.  Zhao et.

al. 16applied the CFD-DEM approach tocalculate the velocity of particles, the distribution of particles in the flow,and the gas velocity under different gas-particle conditions.  III.Computational Fluid DynamicCFD is a branch of fluid mechanics that usesnumerical analysis and data structures to solve and analyse problems thatinvolve fluid flows. Computers are used to perform the calculationsrequired 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 thatimproves the accuracy and speed of complex simulation scenarios, such astransonic or turbulent flows. Initial experimental validation of such softwareis performed using a wind tunnel with the final validation coming in full-scaletesting, e.

g. flight tests. 17 Almost every theoreticalanalysis of probes, needs fluid to simulate. For example, for high temperatureprobes, Brouckaert et.6 used a CFD model to analysethe temperature on probes. The result accurately shows the distribution of heaton the probes, and it is also agreement with the experimental data. The authorsof reference 2 also used CFD to simulate theprobe in the working environment, to analyse the efficiency of cooling systemsin the probe.

Also, the result offers a better understanding on the allowabletemperature on the walls of probe, which can be an index to assess the designof the probe. For a probe to measure two-phase flow or multiphase flow, Luiet. al. 12 predicted hydro-dynamics inCFD by using the Eulerian–Eulerian two-fluid approach. This method accuratelypredicts the motion of particles (see also section 2.2). Recently, two-phaseflow studies are largely based on this approach.

For example, reference 14 used this method to study thebehave of particle in a reactor bed. A combination of experimental study usingparticle image velocimetry (PIV) and numerical investigation using CFD and thediscrete 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 thedistribution of velocity of the two-phase flow. The theoretical result wellmatched with the experimental data, which proves the CFD is powerful tool forfluid analysis.

Alought the CFD is an excellent tool for fluid analysis, itrelies on building the right model of the fluid under different workingenvironment. This requires a good understanding of the fluid in the probe atdifferent situations, which usually is based on an abundant of experimentaldata and previous studies. Also, the experimental data can assess the models.So, the theoretical analysis of probes is usually conducted in tandem withexperimental work, for best results. IV.AnalysisThe papers cited all studied the properties of probes indifferent working environments, by calculating the velocity of fluid or thedistribution of heat on the probes or experimental observation. These studiesoffer accurate models of fluid in probes, theoretical analysis and effectiveapproaches about numerical methods for further studies.

These studies delivered a specific understanding about theprobes’ work on extreme situations, but some problems are excluded from mention.These papers do not study the accuracyof the probe itself, which is important for measuring sampling gas. The needfor accuracy of measuring the emission gases from combustion turbine isincreasing; especially considering the increasingly proven connection betweenthe greenhouse gas and climate change; 1 emission control and restrictionsare severer than in the past. For example, legislation that affects gas turbineemissions directly have also been enacted. In 2001 the E.U. enacted the “LargeCombustion Plant Directive (LCPD)” which was part of the wider policy directivenamed “Directive on Integrated Pollution Prevention and Control (IPPC)”.

Thispiece of legislation affects Gas turbines with a power output of greater than50MW. 18   So, a higher working accuracy of probes for emissiongas should be studied. According to authors of reference 19, for example, the structureof a probe may cause sampling gas blockages in the probe. In this paper, thereare 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 whichis furthest away from the outlet may cost more time than expected, because thegas from the hole which is closer to outlet blocks this gas. Also, the gas fromfurther 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, butalso causes some particles and gas dwelling in the probe. So, the accuracy ofthe sampling from the probe is decreased.

That means the sampling is notrepresentative. So, further work on decreasing the effect on turbulence withinprobe is needed. V.  Conclusions & future workIt has been shown that both computational and experimentalobservation can provide useful assessment of the distribution of heat andpressure about emission probes. In particular, the tandem use of CFD andexperimental work can give reliable and consistent data on the properties ofprobes in a working environment.However, it is also shown that the internal structure of theseprobes is problematic for the accuracy of their measurement. Holes and theirpositioning on the probes can lead to turbulent fluid flow.Improving the accuracy of probes, by reducing the effect onturbulence of gases should be studied.

So, a new design of probe should bemade. CFD can perhaps offer a situational assessment of these new designs.VI.Reference1  EPA, “Sources of Greenhouse Gas Emissions | Greenhouse Gas (GHG)Emissions | US EPA.

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