Cfd analysis of turbulence in a gas turbine combustor with reference to the context


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Cfd analysis of turbulence in a gas turbine combustor with reference to the context

  1. 1. INTERNATIONAL JOURNALEngineering and TechnologyRESEARCH IN International Journal of Advanced Research in OF ADVANCED (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME ENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print)ISSN 0976 - 6499 (Online) IJARETVolume 4, Issue 2 March – April 2013, pp. 01-07© IAEME: ©IAEMEJournal Impact Factor (2013): 5.8376 (Calculated by GISI) CFD ANALYSIS OF TURBULENCE IN A GAS TURBINE COMBUSTOR WITH REFERENCE TO THE CONTEXT OF EXIT PHENOMENON a b P.S. Jeyalaxmi , Dr.G.Kalivarathan a Research Scholar, CMJ University, Meghalaya, Shillong. b Principal/ PSN Institute of Technology and Science, Tirunelveli, Tamilnadu, Supervisor, CMJ University, Shillong. ABSTRACT The main objective of this investigation is based on the interaction between the combustor and turbine with respect to the increased temperatures and heat transfer related aspects. All classic secondary flow models involved a key assumption flux the flow at the inlet to the turbine was either a well-behaved, two-dimensional turbulent boundary layer with a uniform mean field at low turbulence, or a constant total pressure field. While these studies have led to great insight into the nature and driving forces of secondary flows, they neglected to consider that the flow exiting the combustor may be complicated. Experimental and computational studies have also been done to attempt for quantifing the effects of a variety of inlet conditions on the secondary flows, heat transfer, and aerodynamic losses in the turbine. Some of the conditions considered include Reynolds number, boundary layer thickness, endwall and vane geometry, and temperature field. These studies have greatly improved the understanding of secondary flows. Many experimental and computational studies have been completed to model combustor core flow and exit profiles independently of the turbine. It is observed that non-uniformities in the flow field exist in the span wise and pitch wise directions in temperature, pressure and velocity. Keywords: CFD, Combustor, Turbulence, Endwall, Vane Geometry 1
  2. 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME1.0 INTRODUCTION It is important to reiterate that a key assumption made in the flow models presented isthat the inlet flow was a two-dimensional turbulent boundary layer. This is a criticalassumption for an engine where the flow exiting the combustor is complex. In addition, thedata and measurements for these studies were taken on a variety of both stator and rotorvanes with varying turning angle and geometries, which could account for differences in theevolution of the suction side vortex. By relating the measured surface heat transfer to themeasured flow fields, it was shown that in general high Stanton numbers occurred wheremainstream fluid was brought down to the endwall surface as a result of the downward legsof the vortices. As the passage vortex turned upwards, the Stanton number is decreased. Forthe Reynolds number comparison, a higher peak Stanton number occurred at the leading edgefor the lower Reynolds number. Another area of interest has been the effect of temperaturegradients in the incoming flow on the flow patterns through the passage and heat transfer onthe vane surface and endwall. This effect is critical to understand given the large temperaturevariations which would be expected exiting a combustor where 50% or more of the flow maybe cooler fluid from dilution or other forms of cooling. The radial profile caused decreasedheat transfer on the stator and a slight increase on the rotor. The increase was due again to thesegregation of hot and cold fluid with the hot migrating towards the pressure surface. Thepitchwise varying profile was found to cause a large heat load increase, but only if the hotspot was directly aligned in front of the vane. The geometry used did not have the all theelements necessary to completely represent a realistic combustor liner; however, it wasclearly identified from this study that changes occurred in the total pressure. Details are notavailable for comparing any effects that the two different total pressure profiles had on thesecondary flow field in the vane section. The cascade was located first in a low turbulenceuniform hot gas stream, second in a high-turbulence uniform hot gas stream, and finally in ahigh-turbulence, non-uniform hot gas stream downstream of a combustor exit. The effect ofhigh turbulence was to reduce the cooling effectiveness on the suction surface by 10%, andthe high turbulence and non-uniform flow decreased the effectiveness by 21%.2.0 NO VANE CASES COMPUTATIONAL DOMAINS The three combustor model cases with no downstream turbine vane differed only inthe type of cooling schemes employed in the combustor. The baseline case, which directlymodeled the baseline experimental case, utilized the complete cooling scheme including axialfilm cooling holes over four liner panels, two rows of dilution jets, and a coolant slot at thecombustor-turbine interface. The second case included the film cooling and exit slot butneglected to add the dilution flow thus enabling the examination of the effects of the dilutionjets on the combustor flow field. The third case considered film cooling and dilution, buteliminated the exit slot allowing us to see the effects of the slot flow on the exit profiles. Thethree cases with no vane all had approximately the same computational domain with theexception of the angle of contraction for the slot versus the no slot cases (17.15° versus15.47°). It was shown computationally that the effect on the flow field of changing the anglewas negligible. 2
  3. 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME3.0 DOWNSTREAM VANE CASES COMPUTATIONAL DOMAINS The combustor geometry of the first vane case was equivalent to the baseline case; filmcooling, dilution and an exit slot with the addition of the vane. The second case contained filmcooling and dilution, but no exit slot. The last case modeled film cooling, dilution and the exitslot, but the film cooling holes were oriented with a 45° compound angle in addition to the 30°stream wise angle. This compound angle model was computed to compare with the axial holemodel in order to determine the differences produced in the combustor exit profile. The threevane cases had approximately 2.3 million cells each and modeled 640 cooling holes and threedilution jets. The slot case modeled seven slot feed holes and 52 slot pin fins. The vane stagnationwas located 3.6 cooling slot step heights downstream of the trailing edge of panel four. Theoutflow boundary was 1.5 vane chord lengths downstream of the vane trailing edge. Anadditional 0.1 chord lengths was added to the boundary in the streamwise direction to avoidhighly skewed cells at the outflow. The compound angle hole case with the turbine vane wasidentical to the axial hole case except that the periodic side boundaries, rather than being setthrough the centerline of a row of cooling holes, are offset halfway between rows of holes toaccount for the 45° angle. The number of holes and width of the domain at one pitch stillremained the same for the compound angle case4.0 GOVERNING EQUATIONS AND SOLUTION METHODS The numerical method used in all the solutions was a segregated solution algorithm with acontrol-volume based technique. All solutions were computed using Fluent Version 5.0 (Fluent,Inc., 1998). Fluent also offers a coupled solver which also uses a control-volume based technique.This general segregated solution technique consists of an integration of the governing equationsof mass, momentum, energy and turbulence on the individual cells within the computationaldomain to construct algebraic equations for each unknown dependent variable. The discretizedequations are then linearized and a solution of the resulting system of linear equations givesupdated values of the dependent variables.5.0 INITIALIZATION In judging the convergence of the solution, the residuals of several quantities weremonitored. The normalized residuals for continuity, x-momentum, y-momentum, z-momentum,energy, k and ε were monitored after each iteration. For the RSM turbulence model, the residualsfor the Reynolds stresses were also monitored. As the solution proceeded, the residuals decayedto some small value and then leveled out. The un-scaled residual was calculated as the imbalancein the conservation equation of a particular variable over all the computational cells. The residualwas then scaled using a factor representative of the flow rate of the variable through the domain.For the continuity equation the scaling factor was the largest absolute value of the continuityresidual in the first five iterations. There is no universal law for judging convergence of asolution. The Fluent default setting requires that the scaled residuals decrease to 10-3 for allequations except energy, for which the criterion is 10-6. A second approach for judgingconvergence is to require that the un-scaled residuals drop by three orders of magnitude. For allcases computed, the convergence criteria were set at 10-4 for all equations except energy, whichwas set at 10-7. Each computation was continued 50-100 iterations beyond convergence to insurethat the residuals continued to decrease steadily and that the solution was actually converged. 3
  4. 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME6.0 TURBULENCE MODELING AND NEAR WALL TREATMENT Based on these findings, all six models were initially computed using the RNG k-εmodel with non-equilibrium wall functions for the near-wall treatment. For one case, a studyof the turbulence modeling was conducted. The no vane, slot, dilution case was computedwith the original RNG k-ε boundary conditions, with the RNG k-ε model with a differentinlet turbulence boundary condition, and with the Reynolds Stress Model (RSM). Fluentallows solution-based grid adaption, which is an extremely valuable tool. It provides theability to adapt the grid based on specific values or gradients of important flow characteristicssuch as velocity, temperature or turbulence. It also allows adaption based on a range ofdesired wall unit values, or on specific flow boundaries. A third alternative for adaption is totarget cells with a certain volume or certain volume change between cells since large volumechange between adjacent cells may cause convergence problems within the mesh. Figure 1. a, b Global and local coordinate systems and reference lengths for combustor and vane sections. 4
  5. 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME7.0 COMPUTATIONAL RESULTS The effect of the dilution jets on the flow patterns in the combustor core is clearlyseen by looking at the in-plane velocity vectors without and with dilution in the stagnationplane cutting through the first row of dilution jets. The disruptions from the dilution injectionto the uniformity of the flow field resulted in a strong secondary flow pattern at thecombustor exit. In addition to the non-uniform velocity field, the dilution jets created non-uniformities in the temperature and pressure fields at the combustor exit. The normalizedtemperatures are calculated based on a mass averaged temperature for each case at thecombustor exit. Negative values of θ are seen where the temperature in the combustor is lessthan the mass average temperature of the mainstream flow and the coolant flow. For the nodilution case, the temperature at the center of the combustor was the same as the mainstreaminlet temperature, and the effect of the film on the liner can be clearly seen. Once dilutionwas added with the secondary flows induced by the dilution jet injection caused a reductionin the film effectiveness close to the wall with hot spots near the vane stagnation regions. Figure 2. Comparison of Cp contours in plane CS (x/L = 1.02) for case 4 (slot, no vane, dilution) using the RNG k-and RSM turbulence models with experimental data. 5
  6. 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME Figure 3. Comparison of pitchwise spatially averaged Cp across the span for case 4 (no vane,slot, dilution) for the RNG k-and RSM models and experimental data in plane CS (x/L = 1.02).8.0 CONCLUSION It is seen that the combustor exit profiles affects the development of the secondaryflow field through the turbine vane passage for the downstream vane cases. The secondaryflows were driven by the incoming total pressure profiles created as a result of the variouscombinations of dilution, slot injection, and film-cooling hole orientation. The no slot caseshows a relatively uniform total pressure profile approaching the vane passage in the near-wall region and thus no leading edge vortex was formed. Some vertical motion was presentabove 20% of the span due to the increase in total pressure at the midspan. The addition ofthe slot produced a significant change in the total pressure profile creating a low pressure spotin the near-wall region and additional pitchwise non-uniformities. The slot case shows thedevelopment of the leading edge vortex, but still no distinct passage vortex was detected.Additional vortical motion above 20% of the span was present as in the no slot case due tothe increase in total pressure at the midspan. The addition of the slot also led to increase theaverage effectiveness levels on the endwall.REFERENCES1. Anand, M. S., Zhu, J., Connor, C., and Razdan, M. K. (1999) “Combustor Flow Analysis Using and Advanced Finite-Volume Design System,” ASME Paper No. 99-GT-273.2. Barringer, M.D., Richard, O.T., Stitzel, S.M., Walter, J.P. and Thole, K.A. (2001) “Flow Field Simulations of a Gas Turbine Combustor,” to be presented at IGTI 2001, New Orleans, LA.3. Barringer, M.D (2001), “Flow Field Simulations of a Gas Turbine Combustor,” thesis, Mechanical Engineering Department, Virginia Polytechnic Institute and State University, to be completed May 2001.4. Bicen, A. F., Tse, D. and Whitelaw, J. H. (1988) “Flow and Combustion Characteristics of an Annular Combustor,” Combustion and Flame 72: 175-192. 6
  7. 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 2, March – April (2013), © IAEME5. Boyle, R. J. and Giel, P. W. (1997) “Prediction of Non uniform Inlet Temperature Effects on Vane and Rotor Heat Transfer,” ASME Paper No. 97-GT-133.6. Burd, S. W. and Simon, T. W. (2000) “Effects of Slot Bleed Injection Over a Contoured Endwall on Nozzle Guide Vane Cooling Performance: Part I – Flow Field Measurements,” ASME Paper No. 2000-GT-199.7. Burd, S. W., Satterness, C. J. and Simon, T. W. (2000) “Effects of Slot Bleed Injection Over a Contoured Endwall on Nozzle Guide Vane Cooling Performance: Part II – Thermal Measurements,” ASME Paper No. 2000-GT-200.8. Holdeman, V. D. (1993) “Mixing of Multiple Jets with a Confined Subsonic Crossflow,” Prog.Energy Combust. Sci, Vol. 19, pp. 31-70.9. Kang, M., Kohli, A., and Thole, K. A., 1999, “Heat Transfer and Flow field Measurements In The Leading Edge Region of A Stator Vane Endwall,” Journal of Turbomachinery, Vol.121, No. 3, pp. 558-568 (also presented as ASME Paper 98-GT- 173).10. Kang, M., and Thole, K. A., 2000, “Flowfield Measurements in the Endwall Region of a Stator Vane,” accepted to the Journal of Turbomachinery, Vol. 122, pp. 458-466.11. K. V. Chaudhari , D. B. Kulshreshtha and S. A. Channiwala, “Design and Experimental Investigations of Pressure Swirl Atomizer of Annular Type Combustion Chamber for 20 KW Gas Turbine Engine” International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 3, Issue 2, 2012, pp. 311 - 321, ISSN Print: 0976-6480, ISSN Online: 0976-6499.12. Tarun Singh Tanwar, Dharmendra Hariyani and Manish Dadhich, “Flow Simulation (CFD) & Static Structural Analysis (FEA) of a Radial Turbine” International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 252 - 269, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.13. Ashok Tukaram Pise and Umesh Vandeorao Awasarmol, “Investigation of Enhancement of Natural Convection Heat Transfer from Engine Cylinder with Permeable Fins” International Journal of Mechanical Engineering & Technology (IJMET), Volume 1, Issue 1, 2010, pp. 238 - 247, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.14. Cherian Paul and Parvathy Venugopal, “Modelling of Interfacial Heat Transfer Coefficient and Experimental Verification for Gravity Die Casting of Aluminium Alloys” International Journal of Mechanical Engineering & Technology (IJMET), Volume 1, Issue 1, 2010, pp. 253 - 274, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.15. A.Saravanapandi Solairajan and Dr.G.Kalivarathan, “Investigation of Heat Transfer Through CNT Composites-Focusing on Conduction Mode” International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 1, 2013, pp. 66 - 73, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 7