GT2017-63790

Effect of Stage Axial Distances on the Aerodynamic Performance of Three stage Axial Turbine using Experimental Measurements and Numerical Simulations Yang CHEN1 2 Zhuhai ZHONG2 Jun LI1 Weijiu ZHOU2 Gangyun ZHONG2 Qi SUN2 Yan PING2 Shan WANG2 1 Institute of Turbomachinery Xi an Jiaotong University Xi an 710049 China 2 Dongfang Steam Turbine Co Ltd Deyang 618000 China Corresponding author junli ABSTRACT The stage axial distance significantly influences the aerodynamic performance of turbines under some constraints Experimental measurements and numerical simulations are used to analyze the effect of stage axial distances on the aerodynamic performance of three stage axial turbine in this work The aerodynamic performance of three stage axial turbine with three different stage axial distances is experimentally measured at the air turbine test rig of Dongfang Steam Turbine Co LTD Experimental results show that efficiency increases when the stage axial distance decreases for the geometry under study with relative stage distance ranged from 0 14 to 0 35 and the effect of stage axial distance on the optimization velocity ratio here is very limited In addition unsteady Reynolds Averaged Navier Stokes RANS simulations were carried out with nonlinear harmonic method to analyze the detailed flow field of the experimental three stage axial turbine The numerical aerodynamic efficiency of three stage axial turbine is in good agreement with the experimental data Furthermore the small stage axial distance is preferred for the higher efficiency The detailed flow field and aerodynamic parameters of three stage axial turbine are also illustrated and discussed NOMENCLATURE 0 c isentropic expansion velocity at nozzle outlet x C axial chord D stage axial distance rotor hub diameter 0 H isentropic enthalpy drop k isentropic exponent n rotational speed 0 P total pressure at turbine inlet 2 P static pressure at the turbine exit R gas constant 0 T total temperature u linear velocity at the rotor hub aerodynamic efficiency PS pressure side SS suction side INTRODUCTION Turbines are widely used all over the world nowadays and fine design is performed under many constraints for an optimal modern turbine The stage axial distance between the stator and the rotor blade could not only influence the size and the weight of steam turbine but also influence the aerodynamic performance For these reasons the stage axial distance becomes a hot topic in turbomachinery research Many research works have been conducted on the effect of the stage axial distance on the aerodynamic performance of steam turbines Hushmandi et al 1 made a research about the impact of multi block and stage axial distance on a two stage partial admission axial turbine with low reaction blades Numerical results showed that the first stage efficiency of the two stage partial admission turbine was higher under smaller axial distance while the second stage efficiency deteriorated They found that the variation of the stage axial distance would influence the axial force of the first stage rotor Gr nman et al 2 studied a low reaction supersonic axial flow turbine at designed and off designed conditions with variable stator rotor axial distances The obtained results suggested that the efficiency of the turbine decreased when the axial distance increased and this trend was more obvious under off designed conditions They ascribed the efficiency drop to the increased total pressure losses and thus recommended the smaller axial distance for the turbine designing Yamada et al 3 tested the effect of the axial distance on the turbine performance with three axial distances at two different conditions The study revealed that the turbine efficiency showed an upward trend linearly by reducing the distance at the off designed condition Similar experimental and the corresponding numerical researches were carried out by Restemeier et al 4 with variable blade row distances on a 1 5 stage axial flow turbine Proceedings of ASME Turbo Expo 2017 Turbomachinery Technical Conference and Exposition GT2017 June 26 30 2017 Charlotte NC USA GT2017 63790 1Copyright 2017 ASME test rig They concluded that the smaller distance would be preferred for low aspect ratio turbines Cizmas et al 5 provided a numerical study in a 1 5 stage turbine with four kinds of stage axial distances and the result suggested that the efficiency of the turbine increased as the axial distance increased Venable et al 6 gave a similar conclusion that the total pressure drop increased when the axial distance decreased and the efficiency of the turbine increased slightly with larger axial distance Gr nman et al 7 reviewed many literatures and pointed out that the axial distance influenced the aerodynamic performance of turbines positively or negatively which could differ between turbines They summarized five main loss sources that would be influenced by the axial distance and presented four design parameters that correlated with the different efficiency curve shapes for the first time Despite their work had cast a new sight on this confusing topic more data should be available and the item should be studied further When it came to understanding the mechanisms that the axial distance imposed on the turbine the inherent unsteady effect induced by the relative motion of the rotor and stator rows in turbine must be mentioned as a key point And many numerical works studied the effect of the axial distance in the view of unsteady flow in which a dual time stepping method or the phase lag time marching method and domain scaling method were mainly used Kikuchi et al 8 carried out experimental and numerical investigations on aerodynamic performance and unsteady flow effect in a single stage with three different axial distances by moving the stator blade axially The total pressure loss and time averaged flow field upstream and downstream of the rotor blade were captured by experimental measurements and the interaction of the stator and rotor blades was investigated with unsteady simulations The obtained result suggested that the vortices and wakes were more diffused and induced stronger interference with secondary flow and therefore the efficiency slightly reduced for the larger axial distance case Gaetani et al 9 10 performed an extensive experimental analysis and unsteady numerical simulations Steady flow measurements were carried out the blade row interaction and its relationship with axial distance were evaluated The time averaged flow field was strongly influenced by the distance and different interaction phenomena were gained with different axial distances The interaction was dominated by the vortex blade interaction in the hub region and by the rotor incidence unsteadiness With those findings an optimum axial distance would minimize the unsteady losses and achieve the maximum overall efficiency Kachel et al 11 presented experimental and numerical investigations on the unsteady flow field in a three stage model turbine and the factors that affected the unsteady pressure field were given Similar experimental investigations on unsteady studies were carried out by Gallus 12 for a 1 5 stage turbine and an axial flow compressor A new visualization method for unsteady flow was brought out by Gezork et al 13 with which the direction that unsteady waves traveled toward and transported energy in could be shown Chang et al 14 investigated the relationship between the Mach number and the axial distance by unsteady numerical simulations for a transonic gas turbine The smaller distance witnessed the decreased pressure and the increased Mach number in the core flows between the stator and the rotor blades but the trend was not followed by the flows near the two endwalls And the efficiency of the turbine with the normal distance was highest compared with the smaller or larger distance Sharma et al 15 pointed out some differences about flow parameters between the steady and unsteady flows and the hotter pressure sides and colder suction sides of the rotor blades induced by the circumferential temperature distortions would be captured only by unsteady simulations Beside the literatures listed above Pan et al 16 and Kirik et al 17 gave more studies about unsteady effect In this paper experimental and numerical studies on the aerodynamic performance of the three stage axial turbine were carried out with variable stage axial distances The geometry used in the study was the three stage axial turbine developed by Dongfang Steam turbine Co Ltd 18 which featured typical HP turbine stages in the current design practices Spacer rings were designed for each stage so that the axial distance could be altered Time averaged flow measurements were performed under different distances and different rotational speeds In addition unsteady Reynolds Averaged Navier Stokes RANS simulations were conducted with nonlinear harmonic method The simulation results were compared with the experimental data The aerodynamic efficiency of three stage axial turbine decreases with the increase of the stage axial distance The detailed flow field and aerodynamic parameters of three stage axial turbine were also illustrated and discussed by means of the entropy and harmonic perturbations EXPERIMENTAL FACILITY The experimental facility here is an air driven three stage axial impulse turbine Fig 1 provides an overview photo of the test rig The rig comprises wind regime system electric system measurement and controlling system water system and experimental body Horizontal type of the rig is used A hydraulic dynamometer is employed to measure the output power of the experimental turbine stage and an enhanced accuracy torquemeter provides additional measurements for the experiments The torquemeter is installed between the experimental shaft and hydraulic dynamometer device Fig 1 Three stage turbine experimental test rig Figure 2 gives a view of the experimental test three stage 2Copyright 2017 ASME axial turbine The stage inlet temperature and pressure are controlled by the heat exchangers and the steady flow components respectively The maximum flow rate obtained is 1800m3 min and maximum pressure ratio is 2 5 The rotational axis is designed as a rigid shaft comprising three parts and connected with each other by bolts The test section of the rotor serves as a changeable shaft for the experiments with different hub radii ranging from 660mm to 1000mm the maximum rotational speed of the shaft can be up to 5000rpm Fig 2 Experimental test three stage turbine Fig 3 Meridional plane of experimental test three stage turbine Table 1 Geometrical parameters of three stage turbine Stage Hub diameter mm Blades number Blades height mm Axial chord mm Stator 1 860 00 40 43 90 63 23 Rotor 1 860 00 94 44 33 42 52 Stator 2 860 00 40 45 70 78 07 Rotor 2 860 00 94 46 55 41 9 Stator 3 860 00 56 47 90 69 55 Rotor 3 860 00 90 48 76 45 5 Table 2 Axial distance of experimental measurements Item GAPL GAPM GAPS thickness of spacing ring mm 0 00 6 00 12 00 D1 mm 21 82 15 82 9 82 D2 mm 22 94 16 94 10 94 D3 mm 22 90 16 9 10 9 D Cx 0 35 0 29 0 33 0 25 0 22 0 24 0 16 0 14 0 16 Figure 3 shows a meridional plane of the experimental three stage axial turbine Spacer rings are designed to adjust the axial distance between the stator and rotor rows through moving the stator row in the axial direction And it can be clearly seen that the thicker spacer ring would result in the smaller axial distance The geometrical parameters of the experimental three stage axial turbine are listed in Table 1 Table 2 gives the axial distances of the experimental three stage axial turbine and D CX is the stage axial distance divided by the corresponding stator axial chord The positions where the aerodynamic parameters are measured are as follows the inlet section of the second turbine stage the section between the second stator and the second rotor blade outlet section of the second turbine stage The aerodynamic parameters are measured along the blade span and pitch with a multi coordinate displacement device And the probe installed in this displacement device moves along the radial circumferential and axial direction as well as rotational direction The precision of the displacement device is 0 03mm along all direction thanks to the precision guide rail guide screw and encoder device The mass flow rates in the experiment are measured by the cannon type Venturi meter with considering the temperature and pressure compensation The uncertainty of the measurement of mass flow rate is merely 0 5 A pressure and temperature coupling five hole probe integrated with A class platinum thermocouple Pt100 is designed to gain the pressure the temperature and the flow angle at the interstage The precision of the pressure measurement is 0 05 FS Repeated experiments have been carried out and the results show high repeatability for those experiments NUMERICAL METHOD Unsteady simulations were carried out to analyze the detailed flow field of the experimental three stage axial turbine at different axial distances using CFD software NUMECA The three dimensional Reynolds Averaged Navier Stokes RANS and the nonlinear harmonic method with S A turbulent model were employed For an unsteady simulation phase lagged method domain scaling method and harmonic method are provided in NUMECA Modeling with high number of blade passages or scaling the geometry will be needed if the numbers of the rotor blades and stator blades are not equal when using the domain scaling method For the phase lagged method the solution is just a succession of instantaneous solutions for each increment of the rotor position and the method consumes a lot of memories On the other hand the harmonic method can be used for multi stage unsteady simulations with less constrains and less calculation costs only one inter blade channel per blade row is needed Harmonic method deals with the flow perturbations as a time averaged value and a sum of periodic perturbations for each time frequency by casting the unsteady RANS system into the frequency domain with Fourier series The validity of this method with S A turbulent model could be found by Green et al 19 and Xiang 20 and the harmonic method was selected for the unsteady simulations in this paper The computational domain of the experimental three stage axial turbine is shown in Fig 4 The computational domain consists of three stage cascades without any purge flow cavities modeled which guarantees that the simulation results are only affected by the stage axial distance All computational domains employed multi block structured grids The cascade channel adopted HOH type grids which assured 3Copyright 2017 ASME the ortho skew final blade and grid mor comp was comp Fig Ab inlet was expe were The to ac RES Aero Th spee spee 50rp were Th ratio dista Fig u Th turbi 0 c wher turbi grids at blad ogonality an wed The grid l grids were c e to blade at t 73 points rad points was re than 3 800 putational cas set to gain putational grid g 4 Computat Fig 5 Com bsolute total p t boundary co given as an erimental mea e used as init solver was p ccelerate the c SULT AND D odynamic P he experiment ds under the ds ranged fro pm and some e repeatedly co he relationshi o of the three ances is shown 6 is obtained 60Dn he averaged id ine is calculate 32 00 H re 0 H is the ine The 0 H de leading a d avoided th independence constructed fo the inlet 65 p dially along t about 640 00 0 000 grid po ses And the fi a y value d of the secon ional domain of mputational grid pressure and to onditions and outlet bounda asurements T tial solutions parallelized ba convergence DISCUSSION Performance ts were carrie same bound om 1400rpm t experiments onducted ip between th e stage axial t n in Fig 6 Th at the hub wit deal nozzle ou ed by equation 3 e isentropic en is obtained u and trailing e he periodic b e study was c for every blad points blade to the blade heig 00 for a sing oints for all th first cell width e near one F nd stage as an f experimental th of the second tu otal temperatu d the average ary condition The steady s for the unst ased on doma N e d out with a s dary condition to 2400rpm w under certain he efficiency turbine at thr he velocity ra th equations utlet velocity n 2 nthalpy drop using Eq 3 edges with g boundary me conducted and de with 49 po o blade at the ght and the gle blade pas hree stage in h from all surf Fig 5 shows example hree stage turbi urbine stage ure were set a ed static pres n according to simulation re eady simulati ain decompos series of rotati ns The rotati with an interva n rotational sp and the velo ree different a atio here 0 cu 1 and 2 of the three s of the three s good eshes d the oints e exit total ssage n all faces s the ine s the ssure o the sults ions ition ional ional al of peeds ocity axial 0 in 1 stage 2 stage H F velo the dist diff ratio dist velo Fig N thre abo betw betw 199 coo dist the The vari to th T wou not Furt the diff exp T one Bes the actu they fact reas find k k RH0 1 For one thing ocity ratio at t optimum velo tances for ano ferent axial d o kept the sam tance decrease ocity ratio 6 Aerodynam Numerical sim ee stage axial ut 0 54 and ween the sim ween them as 90rpm were i rdinate here tances of all th GAPS GAP e vertical coo iation w he efficiency f The unsteady uld lead to th linear whi thermore the experiments ference in m erimental resu The errors be s as shown in sides the expe main differen ual experimen y are not mod t that the leaka sons are just a d out the re P P T 0 2 0 1 the trends fo three axial di ocity ratio wa other thing th distances at e me too The e es but the im mic efficiency of different ax mulations were turbine at 19 d aerodynamic mulations and s to the effici illustrated in was labeled hree stages th PM GAPL re ordinate in Fig which was def for the GAPM simulations he higher effi ich agrees w e efficiency di is only 1 mass flow r ults tween the ex n Figs 7 and 8 erimental u