Modelling and CFD Analysis of Single Stage IP Steam Turbine

Transcription

1 International Journal of Mechanical Engineering, ISSN: , Vol.42, Issue Modelling and CFD Analysis of Single Stage IP Steam Turbine C RAJESH BABU Mechanical Engineering Department, Gitam School Of Technology, GITAM UNIVERSITY, Rudraram, Hyderabad , India crb.rajesh@gmail.com ABSTRACT A steam prime mover with rotary motion of the driving element, or rotor, and continuous operation. It converts the thermal energy of steam into mechanical work. The steam flow proceeds through directing devices and impinges on curved blades mounted along the periphery of the rotor. By exerting a force on the blades, the steam flow causes the rotor to rotate. Unlike the reciprocating steam engine, the steam turbine makes use of the kinetic rather than the potential energy of steam. The performance of the turbine depends on the efficient conversion of steam enthalpy into mechanical power with minimum flow losses. In order to reduce the leakage loss hub/shroud sealing between the stages is considered. The hub/shroud sealing improves the performance ofthe turbine and higher power is generated from the stages. To simulate the hub/shroud sealing, blades & seals are modelled and meshed separately. The bladed region and hub/shroud seal region are attached by General Grid Interface. The flow domain and mesh generation for seal area needs to be accurate to get the correct interface with blades. Keywords-Intermediate pressure; Steam turbine; Single stage; Seals; Cylindrical blades; ANSYS-CFX 1. INTRODUCTION The motive power in a steam turbine is obtained by the rate of change in momentum of a high velocity jet of steam impinging on a curved blade which is free to rotate. The performance of the turbine depends on the efficient conversion of steam enthalpy into mechanical power with minimum flow losses. To simulate the Turbine flow path, it is necessary to understand the exact flow domain properly. The flow domain in the case of Steam Turbine consists of blade path (both Stationary and Rotating blades fixed to casing and rotor respectively), Labyrinth seals. It is therefore required that before going ahead with 3D modelling and grid generation, the common interfaces should be clearly defined between each blade and between each stage. The software s that is used for generating the geometry and meshing is decided based on nature and complexity of geometry. Using Computational Fluid Dynamics (CFD) modelling techniques, we can quickly simulate the flow path. 2. METHODOLOGY To analyze the flow and to evaluate the performance, the basic steps involved in solving any CFD problem are as follows Identification of flow domain. Geometry Modelling. Grid generation. Specification of boundary Conditions. Selection of solver parameters and Convergence criteria. Results and post processing. The common interface points are defined between each blade & seals and between each stage which are obtained from 2D drawings. The blade section data is given in the form of point s i.e. suction side points & pressure side points. As per axis of rotation, and moving blade rotation profile points data is prepared. The flow inside a Steam turbine passes through the bladed and seal passages, which can be described as periodic passages. Geometrically these passages are rotationally periodic about its axis of rotation. For the purpose of flow domain discretization, one blade passage is considered for grid generation. The tool used for grid generation is ANSYS TURBO-GRID software package for the stator and rotor blade passages. Input to turbo grid is given by three data files namely hub. Curve, shroud.curve and profile.curve. Volume inside the flow passage. After getting good quality mesh, the mesh is saved as *.grdfile. In Steam Turbine Labyrinth or strip type of seals are invariably used because whenever there is a clearance between a moving and a stationary part with a pressure difference across the clearance then sealing must be provided to minimize leakage. The flow domain of the seals is modelled in IDEAS from the 2D drawings and exported into ICEM-CFD to generate the mesh. Before generating the Hexa-mesh the geometry should be repaired in order not to get negative volumes and to get the better quality of the mesh. The mesh files with *.grd extension is imported separately for both the Guide blades, Moving blades, and the Labyrinth seals which are imported from ICEM-CFD without an extension file. Once the grids were imported, suitable transformations were made to align the mesh assemblies as per 2D drawings. A stage consists of Stator and Rotor blades along with the seals. Defining the

2 International Journal of Mechanical Engineering, ISSN: , Vol.42, Issue domains and boundaries - Guide blade as stationary domain and moving blade as rotating domain, Inlet, outlet, blade periodicity, hub rotating wall, counter rotating wall, blade hub interface, and blade shroud interface. The interface between the Stationary and Moving blades is declared as stage interface. The boundary conditions were applied at inlet and outlet. Based on the thermodynamic data, Mass flow rate and static temperature boundary conditions are defined on guide blade inlet and average static pressure condition as defined for moving blade outlet. Fig.1.Sections of first stage Guide Blade Fig.2. Solid model of the first stage Guide Blade Fig.3.Fluid model of the Seals (both hub and shroud) Shroud.curve Profile Curve Single fluid passage Fig.4.Single Passage after using profile. Curve Hub.Curve Fig.5. Template of 3D Blade in Turbo-Grid

3 International Journal of Mechanical Engineering, ISSN: , Vol.42, Issue Fig.6.Hub and Shroud surfaces of the GuideBlade Fig.7.G1 Guide Blades periodically arranged throughout the circumference 3. SIMULATION The combined analysis consists of large number of elements 26, 65,545 with many general grid interfaces and stage interfaces with multiple frames of reference. The Computational requirement of solving the 26, 65,545 node problems with so many interfaces are enormous and could not be done on the single processor machine. For this purpose Cluster computing facility with IBM P615 processors is used to obtain the solution using 4 processors with 2GB RAM is used to obtain the solution. After defining the boundary conditions for all combined seven Fig.8. Single stagefluid flow path stages with mass flow and static temperature at inlet, and average static temperature at outlet. After specifying all conditions definition file is written using write solver file Command. The governing equations of the simulation are solved in CFX-Solver. The solution is converged with 1e- 5 RMS residuals with high resolution. For simulation purpose two equation turbulent (K-ε) models is used. ANSYS CFX-Solver solves the solution variables of the simulation for the problem specification generated in ANSYS CFX-Pre.

4 International Journal of Mechanical Engineering, ISSN: , Vol.42, Issue SOLVER PARAMETERS ARE SPECIFIED FOR SIMULATIONS Advection scheme: Upwind (1 st order) & High Resolution (2 nd order) Time Scale Control: Auto Time Scale or Physical Time Scale Maximum Iterations: 1000 Residual Convergence criteria: RMS Residual Convergence Target: 1E-5 The solver solves the continuity, momentum and energy equations and calculates pressure, velocity, enthalpy etc in the flow domain in each control volumes. The inlet mass flow is maintained constant throughout the stage and the outlet Pressure will be obtained by the suitable Gas turbine performance macro. Run the solver monitor the solver is allowed to run till the required convergence is obtained. Fig.10.The solver is allowed to run till the required convergence is obtained Once the solution is converged, the solver writes all the data related to grid, boundary conditions and flow parameters are stored in the result file. The result file is loaded in CFX-Post, and the results are analyzed. CFX- Post is a flexible state-of-the-art post-processor. It is designed to allow easy visualization, qualitative and quantitative post-processing of the results of CFD simulations. The performance of turbine stage is studied by using CFX MACRO. Various plots are drawn and listed in results. Using the function calculator option parameters like Mass flow rate, Velocity, Pressure, Temperature, Enthalpy, Entropy etc can be calculated in the selected regions. Plots are also available for various parameters like, Pressure contour and velocity vector, Mach number, etc, which show the variation of parameters throughout the domain. The torque and power are obtained using MACRO CALCULATOR. 5. RESULTS AND DISCUSSION The results are post processed in CFX-post for individual stages and combine seven stages and various plots are drawn. The steam parameters, power and efficiency, etc are calculated from the CFD results. It is clear from the Pressure Contour plot that the pressure drop occurs across the stage. The pressure is 39.64bar at the beginning of the stage, decreases across the stage and is 34.85bar at the exit of the stage. The Pressure Contour is useful to see the variation of pressure across the stage and modify the design if required to get uniform pressure drop. Fig St stage pressure contour plot

5 International Journal of Mechanical Engineering, ISSN: , Vol.42, Issue From the figure it is obvious that Mach number is increases from the entry to exit and minimum Mach number of the order of occurred at the entrance of the guide blade and a maximum of is occurred at the throat of the guide blade and moving blade. The velocity Streamline is useful to note the streamline motion of the Steam through the Stage. The Streamline motion is useful to study the flow pattern through the Fig st stage Mach number vector plot stage. The proper design of the stage should have the Continuous streamline motion of the Steam Without any discontinuity. 6. COMPARISION OF CFD VALUES AND 2D VALUES FOR SINGLE STAGE The CFD results are compared with two dimensional design programs. The Table 1 compares steam parameters Fig st stage velocity streamline plot obtained from 2D program and CFD analysis for 1st stage assembly. The values obtained show that the CFD values are in agreement to 2D analysis values with minor variation.

6 International Journal of Mechanical Engineering, ISSN: , Vol.42, Issue Table1. Compares steam parameters obtained from 2D program and CFD analysis for 1 st stage assembly Stage 1 with seals Description unit 2D values CFD values Temp inlet K Temp outlet K Pressure inlet Bar Pressure outlet Bar Enthalpy inlet KJ/KG Enthalpy outlet KJ/KG Power output MW CONCLUSIONS The computational fluids dynamic analysis is done using ANSYS CFX Software. The Modelling and meshing are done using I-DEAS and ICEM-CFD, ANSYS TURBO GRID tools. The energy analysis is done using K- ε model. CFD study was carried out for evaluating the performance of an Intermediate pressure utility steam turbine. The results are compared with 2D analysis validated by experimental data. The values obtained from CFD analysis are similar to 2D analysis values, the ratio of the CFD values to the 2D values are within acceptable limits. The CFD analysis of the Intermediate pressure utility steam turbine module has helped in predicting the turbine performance. REFERENCES [1]. Kerton, w.j., Steam turbine theory and practice, 7th edn, pitmen, London, [2] Yahya, S.M., turbines compressors and fans, McGraw-hill, New Delhi, [3]. Dixon, S.l, Fluid Mechanics, Thermodynamics of Turbo-machinery, 2nd edn, pergamon press, [4]. G.gopalkrishnan, and D, PrithviRaj, Treatise on Turbo machines, Scitech publications, Chennai, [5]. Shepherd, D.G., principles of Turbo machinery, 9 th printing, Macmillan, [6]. Patankar, Suhas v, Numerical heat transfer and Fluid flow. Hemisphere publications, USA [7]. H.K.versteeg and W.Malalasekera, An introduction to CFD the Finite volume method, Pearson education limited,2002. [8]. Wisclicenus, G.F., fluid mechanics of Turbo machinery, McGraw-hill, Newyork, [9]. Hurlock, J.H.,The thermodynamics efficiency of the field cycle, ASME paper no.57.a.44, [10]. Reinhard, K.et al., Experience with the world s largest Steam turbines, Brown Boveri Rev., vol.63, No.2, Feb, [11]. Salisbury, J.K.Steam turbines and cycles, Wiley s, [12]. Binder, R.C., Advanced fluid Mechanics, pretice- Hall [13]. Miller, A.J.et.al., use of steam electric power plants to provide thermal energy to urban areas, ORNL- HUD14, oak ridge national laboratory, Jan [14]. Kadambi, V and Manohar Prasad, An introduction to energy conversion, vol.3-turbomachinery, Wiley eastern ltd., Websites: