Augmented Heat Transfer of Internal Blade Tip Wall by Pin-fin Arrays

Transcription

1 Group Seminar,, Division of Heat Transfer, LTH, Nov 13, 2008 Augmented Heat Transfer of Internal Blade Tip Wall by Pin-fin Arrays Gongnan Xie Supervised by Prof. Bengt Sundén, Division of Heat Transfer, Lund University Guided by Dr. LieleWang, Dr.Esa Utriainen,SIEMENS Industrial Turbomachinery, 11/13/2008 1

2 Outline Introduction Description of physical problem Computational details Overview Seclection of turbulence model Governing equations Boundary conditions Grid indenpendence Results and Discussion Parameter definitions Model Validation Velocity and temperature fields Pressure loss and heat transfer Overall performance Concluding remarks /19

3 1. Introduction The heat transferred to the turbine blade is substantially increased as the turbine inlet temperature is increased. Cooling methods are therefore much needed for the turbine blades to ensure a long durability and safe operation. A very common way to cool the blade tip is to adopt internal cooling by designing serpentine (two-pass, threepass or multi-pass) channels with a 180- deg turn/bend inside a blade. Many investigations have proven that pin-fins can improve the cooling in low aspect ratio channels for gas turbines, typically at the trailing edges /19

4 1. Introduction Dr Bunker presented a method to provide substantially increased convective heat flux on the internal cooled tip cap of a turbine blade, where arrays of discrete shaped pins were fabricated and placed. Heat transfer augmentation is up to a factor of 2.5. negligible increase in tip turn pressure drop /19

5 1. Introduction Motivation Reasons: o Limited details of the heat transfer and flow on the pin-fins and endwall are available. o Most previous studies were concerned about the heat transfer on the leading and trailing walls of two-pass channels, and thus very limited information is available for tip walls. Aim: o To present detailed flow field and heat transfer enhancement over tip walls, and to facilitate better understanding of three dimensional flow and heat transfer for pin-finned tips. o To compare the overall performances of pin-finned tip twopass channel and smooth tip two-pass channel /19

6 2. Description of Physical Problem Channel: Aspect Ratio (AR) = 1:2 Pins: Circular, staggered H/D=2 X/D=2.6 S/D=3. Number of pin-fins: 184 Area enhancement is 1.74 made of aluminium /19

7 3. Computational Details 3.1 Overview: o FLUENT (64-bit) o GAMBIT (32-bit) o RNG k-ε turbulence model o SIMPLEC (coupling velocity and pressure) o 3D, Incompressible, Steady-state o Stationary o Constant thermal property: dry air =30 o C (ΔT in all cases are around 5C) /19

8 3. Computational Details 3.2 Selection Turbulence model: o k-ε standard o k-ε RNG o k-ε Realizable o k-ω SST o RSM o v 2 f Nu ave k-ε-std v 2 f RSM exp.data k-ε-rng k-ε-realizable k-ω-sst Turbulence Models (smooth-tip two-pass channel, Re=200,000) /19

9 3. Computational Details 3.3 Boundary Conditions: o Inlet: uniform u and T, i.e., T in =300K o Outlet: outflow o Tip-wall: const. heat flux, i.e., 1000 W/m 2 o The remaining walls: adiabatic o Standard wall function o Convergence, 10-4 for u,v,w,k,ε, 10-7 for T o Steady-state, incompressible, Stationary /19

10 3. Computational Details 3.4 Grid inpendence: o GAMBIT (32-bit) o Multi-Block o 90% cells for near the tip o one case needs about 24 hrs (64-bit Windows-XP, Core CPU, 8 G of RAM) Table 1 Grid independence study (pin-finned-tip channel, Re=200K) Grid 1 (1,253,980) Grid 2 (1,361,608) Grid 3 (1,525,033) Nu Δp Difference,% 1.35%, 2.9% baseline 0.54%, 0.9% 90% cells 1 kb is for 1 cell, 1 million needs about 1GB of RAM /19

11 4. Results and Discussion 4.1 Parameter Definition: Fanning firction factor f Δp D = 2ρu L 2 i h Pin-fin wall and endwall HTC h p q q w w =, he = Tp Tf Te Tf Pin-finned tip overall Nusselt number Nu ave = A Nu + A Nu p p e e A p + A e Nu = h D / k h /19

12 4. Results and Discussion 4.2 Model validation: o Smooth-tip channel Table 2 Comparison of Nusselt numbers and pressure drop of the smooth two-pass channel Re=200K Re=310K Re=440K Nu Δp, Pa This work Exp.data Deviation,% 14.73% 7.50% 1.21% This work Exp.data Deviation,% 15.24% 15.01% 6.83% /19

13 4. Results and Discussion 4.3 Velocity and Temperature field: /19

14 4. Results and Discussion 4.4 Tip Temperature contours: A low temperaure region occurs at the pin-finned surfaces due to the cold fluid impingement and pin-fin crossflow. Especially for the pin-fins, the temperature of the leading surfaces is lower than those of the rear surfaces /19

15 4. Results and Discussion 4.5 Pressure Loss and Heat Transfer: 12.0k 1500 Δp(Pa) 10.0k 8.0k 6.0k Pin-finned tip Nu 1000 Nu e Nu p Nu ave Nu s 4.0k Smooth tip k 0.0 1x10 5 2x10 5 3x10 5 4x10 5 5x10 5 6x10 5 Re 1x10 5 2x10 5 3x10 5 4x10 5 5x10 5 6x10 5 Re The maximum HTE is about a factor of 1.64 The pin-fin heat transfer is 35%~45% higher than the endwall heat transfer. The pressure drops of the pin-finned tip channel is 32%~42% higher /19

16 4. Results and Discussion 4.6 Overall Comparison: 1.2 Criteria I Nu/Nu 0 / (f/f 0 ) Smooth tip Pin-finned tip Nu/Nu 0 / (f/f 0 ) 1/ Smooth tip Pin-finned tip x10 5 2x10 5 3x10 5 4x10 5 5x10 5 6x10 5 Re 0.0 1x10 5 2x10 5 3x10 5 4x10 5 5x10 5 6x10 5 Re Before 200K, pin-finned tip produce higher values of the parameter Nu/Nu 0 /(f/f 0 ). At all Re, higher pin-finned tip produce values of the parameter Nu/Nu 0 /(f/f 0 )1/ /19

17 4. Results and Discussion 4.6 Overall Comparison: Criteria II 4.0k Pin-finned tip Smooth tip h (W/m 2.K) 2.0k P (W) At identical required pumping power the pin-finned tip offers a higher heat transfer coefficient, and thereby higher heat transfer enhancement /19

18 5. Concluding Remarks 1) The predicted global Nusselt numbers by the RNG k-ε turbulence model for the smooth tip channel are in good agreement with exp. data 2) The pin-fins are very effective heat transfer enhancement devices for gas turbine blade tips due to combination of turn, impingement and pinfin crossflow. The pin-fins force the vortices towards the tip wall and thereby improve the turbulent mixing of the coming cold fluid and hot fluid near the tip 3) Compared to the smooth tip channel, the heat transfer enhancement of the pin-finned tip channel is a factor of up to 1.64 higher associated with around 35% higher pressure loss. The pin-fin heat transfer is 35%~45% higher than the endwall heat transfer. 4) By applying two criteria, it is found that the pin-finned tip channel provides good overall performance. It is suggested that ussage of pinfins for tip cooling is an effectvie way with a high heat transfer enhancement at a low additional pressure loss /19

19 Thanks for your attention! Questions?? Suggestions?? Acknowledgement The authors acknowledge the financial support from the TURBO POWER consortium funded by The Swedish Energy Agency (STEM), SIEMENS Industrial Turbomachinery & VOLVO AERO Corporation /19