Steam turbines play a vital role in power generation as a prime mover which converts kineticenergy of steam into mechanical energy. The turbine normally consists of several stages witheach stage consisting of a stationary blade and a rotating blade. Many of the utility steamturbines are of three cylinder constructions, followed by first the high pressure (HP), intermediatepressure (IP) and last low pressure (LP) stage respectively, where high, intermediate and lowdescribe the pressure of the steam. A typical intermediate pressure cylinder module is chosento carry out the project work. To achieve the above objectives we need to model separately thebladed region and attach the hub/shroud seal region to it by General Grid Interface (GGI).IDEAS software is used for geometric modeling, CFX TURBO-GRID software is used for meshingthe bladed region, ICEM-CFD software is used for meshing the hub/shroud region of the sealsand CFX Software is used for physics definition, solving and analyzing the problem. Analysishas been carried out for the 8th stage with and without seal. The results are compared andfound to be in close comparison with two dimensional (2D) experimental calculations.
1 Assistant Professor, Department of Mechanical Engineering, LAEC, Bidar, Karnataka, India.
INTRODUCTION
BHEL is manufacturing a wide variety ofturbines over the last 50 years to meet India?sgrowing need for power. Steam turbine playsa vital role in power generation as a primemover, which converts kinetic energy ofsteam to mechanical energy.
Many of the utility steam turbines are threecylinder constructions i.e. high pressure
cylinder in which pressure is maximum withminimum specific volume and so blade heightis minimum, intermediate pressure cylinderin which pressure is intermediate so the bladeheight is intermediate and low pressurecylinder which has a minimum pressure leveland maximum specific volume and hence LPcylinder blade height is maximum.
utility steam turbine is chosen to carry outthe CFD analysis. The analysis requiressolving of fluid problem in bladed region. Thiscan be done in three approaches, analytical,experimental and numerical.
Analytical methods which assume acontinuum hypothesis are more suited forsimple problems and are not suited forcomplex fluid flow problems. Experimentalmethods are suited for complex fluid flowproblems but the expenditure for carrying outthe analysis is high. The other limitation isthat the determination of the fluidcharacteristics in the interiors becomescomplex and difficult. Hence, numericalapproach is more feasible approach foranalysis of a particular design because itovercomes the limitations of the two methodsand it gives a close approximate for complexform of fluid problems too. Numericalapproach involves discretization of thegoverning mathematical equations givesnumerical solutions for the flow problems.
The analysis is carried out by identifyingthe flow domain. The domain is modeleddiscretized and governing equations aresolved using commercially availablesoftware. The results are post processed andcompared results which were obtained using2 dimensional program developed andavailable at BHEL.
ELEMENTS OF STEAM TURBINE
The bladed region of steam turbine consistsof the following as shown in Figure 1.
1. Stationary Blades
2. Moving Blades
3. Labyrinth Seals
Aerofoil Blades
An aerofoil blade is a streamlined bodyhaving a thick, rounded leading edge and athin trailing edge in order to achieve a highlift-drag ratio. Its maximum thickness occurssomewhere near the midpoint of the chord.Both the stationary and Rotating bladesshould be designed such that it should becapable of obtaining the desired pressuredrop and turning towards the tangentialdirection between the driving surface andtrailing surface of the vane passage, so thatthe flow comes out of the stationary bladewith a desired velocity both in magnitude anddirection. The exit flow will have high velocitywith a high tangential component. Thus theflow enters axially in the stationary as wellas the moving blades and both the tangentialforce and torque exerted by the fluid jet onthe following rotating blade row depends onthe change in the tangential velocity of thefluid. The blade with respect to axis and bladenomenclature is shown in Figure 2 and Figure3 respectively.
provided at the two turbine ends where theshaft is taken out of the casing, at theclearance between the diaphragm and therotor of an impulse stage and on the bladetips when provided with shrouds. Mostly thelabyrinth and strip type of seals are used inthe turbo machines. The number of stripsused and their arrangement depends uponthe pressure difference across the clearanceand the basic construction arrangementsused for sealing the diaphragm are shown inFigure 4 and these are generally known asLabyrinth seals. The flexible type of labyrinthseals used on diaphragms of the highpressure stages are as shown. Tip seals tothe turbine stage in the CFD models are usedfor the more accurate stage performancepredictions.
Figure 2: Blade with Respect to Turbine Axis
Figure 3: Blade Nomenclature
Labyrinth Seals
The provision of seals is necessary tominimize the leakage whenever there is aclearance between a moving and a stationarypart with pressure difference across theclearance. In a steam turbine seals are
Figure 4: Flow Domain at Labyrinth Seal
METHODOLOGY
Geometrical Model
In order to analyze the flow and to evaluatethe performance, basically three steps arerequired as follows:
As the flow domain consists of blade andseal passages, the modeling is carried outas described below.
Geometrical Model of Blades
The blade of the IP steam turbine is ofcylindrical type and blade extends betweenhub and shroud surfaces. The geometry ofblade is extracted from blade profile co-ordinates are shown in Figure 5a and Figure5b for 8th stage guide blade and moving bladerespectively, given in the form of suction sideand pressure side points, which are locatedalong the radial positions of the blade. Grid Generation of Blades Using
CFX-Turbo Grid
The flow inside a Steam turbine passesthrough the bladed and seal passages, whichcan be described as periodic passages.Geometrically these passages arerotationally periodic about its axis of rotation.For the CFD analysis, it is assumed that theflow is also rotationally periodic in thesepassages. Therefore, the flow computationcan be made in one of the periodic passagewhile applying periodic boundary conditionsat periodic interfaces. For the purpose of flowdomain discretization, one blade passage isconsidered for 3D-grid generation. The toolused for grid generation is CFX-TURBOGRIDsoftware package for the stator and rotorblade passages. Input to this software isgiven by three data files namely, hub.curve,shroud.curve, and profile.curve. These filescontain the hub, shroud and profile curve datafiles in global cartesian coordinates orcylindrical form.
Geometrical Model of Seals
Labyrinth seals are attached at the hub andshroud surface of the blades to reduce theleakage flow. Modeling of seals has beendone in IDEAS by extracting the data fromthe AutoCAD drawing in Figure 6 by extrudingthe seals in either of the directions then thesolid model of the seal with the requiredlength is obtained.
The hub curve runs upstream to downstreamand must extend of the blade leading edge.The hub data file contains the hub curve datapoints in cartesian form and downstream ofthe blade trailing edge. The profile points arelisted, line-by-line, in free format ASCII stylein order from upstream to downstream. Thesedata points are used to place the nodes onthe hub surface, which is defined as thesurface of revolution of a curve joined bythese points.
Shroud Data File
The shroud data file contains the shroudcurve data points in cartesian or cylindricalform the shroud curve runs upstream todownstream and must extend upstream ofthe blade leading edge and downstream ofthe blade trailing edge the points are listed,line by line in free format ASCII style in orderfrom upstream to downstream. These datapoints are used to place the nodes on theshroud surface, which is defined as thesurface of revolution of a curve joined bythese points.
Considering XZ plane with ‘X’ as axis ofrotation, the Figure 7 shows, hub curve andshroud curve. Shroud. curve
Shroud Curve
14.725 0 639.574
12 0 640.5
-12.2 0 645.5
-15.125 0 646.1158
Hub Curve
14.725 0 518.8
11.8 0 519.6
-11.8 0 522.4
-15.125 0 522.8
Figure 7: Hub Curve and Shroud Curve
Profile Data File
The “profile” data file contains the blade“profile” curves in cartesian or cylindricalform. The profile points are listed, line-by-linein free format ASCII style in a closed loopsurrounding the blade. The blade profilesshould lie on a surface of revolution tofacilitate transformation to conformal space.A minimum of two blade profiles are required,one which lies exactly on the hub surface andone which lies exactly on the shroud surface.The profiles must be listed in the file in orderfrom hub to shroud. Multi bladed geometriesare handled by placing multiple blade profiledefinitions in the same profile.
The total number of nodes, number of hexaelement and aspect ratio obtained fromICEM-CFD In a steam turbine labyrinth orstrip type of seals are invariably used. Theflow domain of the seals is modeled in IDEASfrom the 2D drawings and exported into ICEMCFD to generate the mesh. Before generatingthe hexahedral-mesh the geometry shouldbe repaired in order to get no negativevolumes and to get the better quality of themesh are given in Table 2.
Profile Curve
The blade profile must lie on the surface ofrevolution of hub and shroud as shown inFigure 8.
S.No.
1
2
Component
Stator Blade(G8)
Rotor Blade(M8)
Number ofNodes
1207444
100380
Number ofHexa Elements
112176
93024
Table 1: Mesh Data for Components of 8th
Stage Blades
Ansys CFX
CFD analysis is carried out to understand theflow through the turbine, predict the pressuredistribution and velocity profiles on the bladesand predict the various losses. Ansys CFX-11 software tool is used for analysis purpose.The Analysis is carried out using CFX-Pre,CFX-Solver and CFX-Post modules.
RESULTS AND DISCUSSION
The analysis is carried out in two stages. First,individual stage analysis is done and latercombined analysis for all the 5 stages has
S.No.
1
2
Component
G8 Seals
M8 Seals
Number ofNodes
68,096
71,864
Number ofHexa Elements
58,860
60,925
Table 2: Mesh Data for Data for8th Stage Seals (Hub and Shroud)
Figure 8: Single Bladed Passage After Using
The mesh is generated for the stator androtor blades with the total number of nodes,maximum and aspect ratio obtained fromTURBOGRID are given in Table 1.
been carried out. The stage analysis hasbeen carried out for the turbine stages withthe constant mass flow and it consists ofstator, rotor, and seals. The variousperformance parameters like pressure,temperature distribution and velocity profileson the blades, isentropic efficiencies, powerhave been computed using the CFX Macrosand with the help of mollier chart. As the eightstages consisting of guide blade, movingblade with a stage interface between theblades is simulated, and the solution isobtained with high resolution convergence upto 1e-5.The analysis is carried out with andwithout seals for 8th stage shown in Figures9 and 10 respectively. The results obtainedare discussed below.
In the pre processing the following fluiddomains and boundary conditions arespecified for the eighth stage analysis.
The variation of pressure across the stage isseen in Figure 11, the pressure contour plotwhich is a series of lines linking points withequal values of a given variable pressure.The variable values can quickly be associatedwith the colored regions of the plot. It is shownin the Figure 11 that the pressure goes ondecreasing from entry to exit of the stage. Atthe entrance the maximum of 18.56 bar isobserved and a minimum of 15.5 bar isobtained at the exit is obtained.
� Option: Specified pitch angle� Pitch angle side 1: 2.465753� Pitch angle side 2: 3.130435
For the present simulation solver parametersare specified as follows
� Advection scheme: Upwind (1st
order) and High resolution (2nd order)� Time scale control: Auto time scale� Maximum iterations: 300� Residual convergence criteria: RMS� Residual convergence target: 1E-5� Computational time : 8 hours and 22
min.Run the Solver Monitor
The solver is allowed to run till the requiredconvergence is obtained.
Figure 12: Stage Pressure Contour Plots
POST PROCESSING
Results which are obtained from the CFXmacro for the eighth stage with seal.
The variation of pressure across the stage isseen in Figure 12, it is a pressure contourplot which is a series of lines linking pointswith equal values of a given variablepressure. It is shown in the Figure that thepressure goes on decreasing from entry toexit of the stage. At the entrance themaximum of 18.42 bar is observed and aminimum of 15.84 bar is obtained at the exitis obtained.
Quantity Inlet Outlet Ratio(Out/In)
Tempe-rature
688.801 K 664.746 K 0.96118
TotalTempe-rature
691.248 K 675.846 K 0.93458
Pressure 1.84207e+
006 kg m^-
1s^-2
1.58472e+
006 kg
m^-1 s^-2
0.85235
Total
Pressure
1.96454e+
006 kg m^-
1 s^-2
1.61431 e+
006 kg
m^-1 s^-2
0.89595
Comparison of CFD Values and 2D Values
The CFD analysis results are compared with2D program output. The program output isverified experimentally. The comparison chartof 2D values and CFD values for 8th stageare shown in the table. The values obtainedshow that the CFD values are closer to 2Dprogram and are within the acceptable limits.
CONCLUSION
CFD study was carried out for evaluating theperformance of a utility Steam Turbine IPModule. The flow in a turbine blade passageis complex and involves understanding ofenergy conversion in three dimensionalgeometries.
• The performance of turbine depends onefficient energy conversion andanalyzing the flow path behavior in thevarious components IP Steam Turbine.
• The CFD analysis of the turbine flowpath helps in analyzing the flow andperformance parameters and theireffects on performance parameters liketemperature, pressure and Poweroutput.
The Intermediate Pressure turbineconsisting cylindrical profiles used forstationary and moving blades. The blades arealso designed with sealing strips betweenstationary parts and rotating parts to reduceleakage losses. The flow path of the turbinewith blades and seals is modeled andmeshed using different software’s like IDEAS,
ANSYS-ICEMCFD, ANSYS-TURBO-GRID,etc. The mesh for the blade region isgenerated separately with ANSYS-TURBO-GRID and mesh for the seals are generatedfrom ANSYS-ICEM-CFD and attached byGeneral Grid Interface. The analysis iscarried out for a single stage initially andsubsequently for all the combined 5 stages.The combined analysis consists of largenumber of element nodes with many GeneralGrid Interfaces and stage interfaces betweenmultiple frames of reference. IBM Clustercomputing server with P615 processor isused to obtain the solution using 4 processorswith 2GB RAM each. The solution isconverged with 1e-5 with high resolution.
The results are analyzed for mass flowrates, temperature and pressure distributionson
REFERENCES
1. Ali Chaibakhsh et al., (2008), “SteamTurbine Model”.
2. Brendon et al., (1996), “Advance SteamTurbine Technology for ImprovedOperating Efficiency”.
3. K A Thole (2005), “Adiabatic Effective-ness Measurements of Endwall Film-cooling for a First Stage Vane”, Journalof Turbomachinery by ASME, Vol. 127,No. 297.
4. Knot et al., (2005), “Adiabatic Effective-ness Measurements of Endwall Film-cooling for a First Stage Vane”, Journal
of Turbomachinery by ASME, Vol. 127,No. 297.
5. Lethander et al., (2004), “Optimizing theVane-endwall Junction to ReduceAdiabatic Wall Temperatures in a TurbineVane Passage”, by ASME.
6. Michael W et al., (2005), “Steam TurbineModernization Solutions Provide a WideSpectrum of Options to ImprovePerformance”, Siemens PowerGeneration (pg), Germany, Siemens.
7. Rainer Quinkertz et al., (2010), “HighEfficiency Stem Turbine Packages forConcentrated Solar Power Plants”.
8. S P Harasgama et al., (1991),“Measurements and Computations ofExternal Heat Transfer and Film Coolingin Turbines”, Technical Memorandum P1223, (ISABE), pp. 1-6.
10. Sundranayagam et al., (2001), “TheEffect of the Variation of Axial VelocityIncompressible Flow Through a Cascadeof Aerofoils”, Vol. 130, No. 02, pp. 1019.
11. Vivek Asthana (2008), “Performance ofPower Plants with High TemperatureConditions at Sub-critical Pressures”, 5thEuropean Thermal-Sciences Confe-rence, The Netherlands.
