The Numerical Simulation of Gas Turbine Inlet-Volute Flow Field

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1 World Journal of Mechanics, 013, 3, doi:10.436/wjm Published Online July 013 (htt:// The Numerical Simulation of Gas Turbine Inlet-Volute Flow Field Tao Jiang 1, Kezhen Huang 1 Military Reresentative Office at the 46 Shiyad, Dalian, China China Shi Develoment and Design Center, Wuhan, China wh.hust.wi@gmail.com Received Aril 18, 013; revised May 0, 013; acceted May 7, 013 Coyright 013 Tao Jiang, Kezhen Huang. This is an oen access article distributed under the Creative Commons Attribution License, which ermits unrestricted use, distribution, and reroduction in any medium, rovided the original work is roerly cited. ABSTRACT The structural and aerodynamic erformance of the air inlet volute has an imortant influence on the erformance of the gas turbine. On one hand, it requires the airflow flowing through inlet volute as even as ossible, in order to reduce the ressure loss, to avoid a decrease in the effective outut ower and an increase of the fuel consumtion rate of the internal combustion engine which indicate the inefficiency of the entire ower unit; On the other hand, it requires the size of the inlet volute to be as small as ossible in order to save mounting sace and roduction costs. The thesis builds the structure model and develos flow fields numerical simulation of several different sizes of the inlet volutes. Further, the unreasonable aerodynamic structure is imroved according to the flow field characteristics and thereby, a better aerodynamic erformance of the inlet volute is obtained. Keywords: Axial Flow Comressor; Inlet Volute; Numerical Simulation; Pressure Loss; Uneven Degree 1. Introduction The inlet volute structure and aerodynamic erformance is an imortant art of the gas turbine research [1]. Most researches has focused on the imeller for the thought that imeller has a greater imact on the comressor erformance so that the studies on the airflow of inlet volute are fewer. Actually, inlet volute also has an imortant imact on comressor erformance. On one hand, it requires that the airflow through the inlet volute as even as ossible, the ressure loss as small as ossible in order to avoid the reduced efficiency of the entire ower lant; On the other hand, the size of the inlet volute should be as small as ossible to make it easier to arrange under the remise of ensuring the erformance requirements []. The evenness of air intake which influences the surge line of comressor directly is one of the main factors to ensure the normal oeration of combustion engine. Generally, the flow field uneven degree value of the comressor inlet volute outlet surface should be less than 15%. The ressure loss should be as small as ossible and the common design demands the total ressure loss of entire air intake system (including the inlet volute and other resistance units) not exceeds the maximum value 75 mm H O. The thesis redicts the erformance of a certain tye inlet volute, analyzes the internal flow field characteristics to get the even degrees of inlet volute at the outlet by means of numerical simulations which rovide a reliable basis for the manufacture of high-erformance inlet volute [3]. Volute is an imortant art of owered mechanical lant which has alications in many fluid machineries, such as centrifugal ums, turbine, comressor, centrietal turbine etc. Though the volute is a stationary unit for turbomachinery, its internal flow is a kind of threedimensional and vortex flow henomenon [4]. The volute internal flow study focused on theoretical and exerimental research early and theoretical study and design mostly based on the axisymmetric assumtion of free viscous flow and the runner outlet flow [5,6]. But it is difficult to obtain a recise descrition of volute flow through traditional methods for the sticky characteristic in the actual flow and the volute inlet unevenness because of the limited number of leaves. With the develoment of comuter technology and comutational fluid dynamics, numerical methods have become an imortant tool in the study of volute flows [7]. The studies of volute flow lay a significant role for imroving the efficiency and erformance of turbomachinery. Therefore, to imrove the efficiency of the volute and the volute design theory, exerts and scholars at home and abroad Coyright 013 SciRes.

2 T. JIANG, K. Z. HUANG 31 have done a lot of work. They mainly concentrate on three asects: 1) Exerimental test of the volute flow field; ) Numerical simulation of the volute flow field; 3) Study on volute geometric characteristics.. Mathematical Calculation Model Based on the inlet volute flow field aerodynamic characteristics, it can be viewed as comressible viscous flow. The conservative mass, momentum, and energy equations of full gas ignoring mass force and with constant heat transfer coefficient C and C F are: * u 0 (1) t t t I u u u () E E I u u q (3) For comressible gas, state equation connecting density and ressure (static ressure) should also be regarded as a art of control equation. In the above equation, I ij is unit tensor; is viscous stress tensor. For the Newton fulid: ij u i u u i u n u (4) 3 xi xj x i q q i is heat flux vector. Assuming the fluid comly with Fourier heat transfer law: q KT (5) 1 E e uu is the total energy of unit fluid and e is the internal energy of unit mass of fluid. This is Navier-Stokes equation of comressible viscous gas ignoring mass force. For comressible gas, state equation should also be regarded as a art of control equation. Therefore, RT (6) in the equation: density; u velocity vector; ressure; e internal energy of unit mass of fluid; K thermal conductivity; T temerature; dynamic viscosity coefficient. The dynamic viscosity coefficient varies with the temerature change. Its value can be obtained through the Sutherland equation generally used in engineering: ut T T0 TS (7) u0 T0 T TS In the equation, T 0 = K; T s is the Sutherland constant. T s = K in the air; μ 0 is the dynamic viscosity coefficient at an atmosheric ressure and temerature of K. The thesis emloys the Favre average because the flow simulating is comressible. In fact, the Favre average is time average for instantaneous ressure and density but mass weighted average for other variables. Firstly, the definition of Renault average is 3 t l * x, t x, d t t t t (8) t In the equation, t is the time eriod which is large enough comared with ulsation eriod of seed but small enough as comared with the flowing size of the macroscoic time. In the rocessing of the exeriment data, the exact selection of t is very imortant, but there is no need to care about its value due to the absence of t in the turbulence model. The definition of mass weighted average is: t l * xt, xt, tdt t t t l * * x, t x, d t t t t t * In the equation, is the average velocity according to the definition of Renault average. Time average the control equations based on the Favre average method, then we get the following form: (9) * v 0 (10) t t I u iuj u u u (11) EuE t,, I uiu j q (1) RT (13) As indicated of the mass weighted averaged control equations, they become not closed, because of a new unknown quantity which is a kind of stress caused by the turbulent fluctuation called Renault stress. To make the equations closed, a certain assumtion must be made, namely the establishment of the exression of the stress (or the introduction of new turbulence model). The value Coyright 013 SciRes.

3 3 T. JIANG, K. Z. HUANG of turbulent fluctuation and the average time can be linked through these exressions or turbulence model equations. There is no secific laws of hysics can be used to create turbulence model, therefore, the current turbulence model can only be based on exerimental observations. According to Boussinesq and theoretical assumtions of molecular motion: u i u u uu ui ij ui ij (14) 3 xj xj xi 3 In the equation, u i different to u is the eddy viscosity coefficient which is a function of the satial coordinates, deending on the flow state rather than a hysi- 1 cal arameter. k u i uj u k is the fuctuating kinetic energy of unit mass fluid turbulence. 1 Now, define t ui u j uk k and sub- 3 3 stitute (.14) into the control equations: * u 0 (15) t t eff u u u (16) EuE t eff I u q (17) RT (18) In the equations, eff is a combined effective ressure of and t. eff t k (19) 3 Further, for the ij u k u u ij uui ij uui 3 xk xj x i (0) Thereby, if the effective viscosity coefficient u uu, substitute it into the above equation, we get: eff t u k u u ij ueff ij ueff 3 xk xj x i (1) So, the control equations of (15)-(18) are identical to the formulas of (1)-(3), (6). 3. Numerical Modeling and Simulation Two volute models are designed according to the turbine shae and the inlet volute whose intake ort is designed in a circular shae is named Tye A. The geometric model in Figure 1. Due to the irregularity of the model structure, it is difficult to generate the overall structure of the mesh. So it is necessary to do the segmentation rocessing, then the regular structures generate the structural mesh and the irregular structures generate non-structural mesh. Note that we must act from inut to outut, outut to inut or from the middle to both sides to generate the mesh. The mesh number is between 0 to 30 million. The results are shown in Figure. According to the model characteristics and erformance requirements of an ideal gas as working fluid, the given boundary conditions are as follows: Inut: mass flow inlet, the total temerature of 300 K; Outut: ressure on exorts, the total temerature of 300 K, to adjust the inlet mass flow rate so that the exit velocity can reach 100 m/s; Solid wall: adiabatic, no-sli. Make the numerical simulation of volute flow field according to the control equations of the Favre averaged Figure 1. Tye A turbin volute. Figure. Tye A model mesh. Coyright 013 SciRes.

4 T. JIANG, K. Z. HUANG 33 N-S equations with the given conditions. Aly the standard k model as turbulence model, uwind to the discrete convection term as uwind, central difference scheme to the dissiative term, the SIM- PLE algorithm to the ressure-seed iteration of the equation. And the solution rocess emloys the underrelaxation factor. As for adjustment of the inlet mass flow to achieve that the exit velocity may reach 100 m/s, the residual lots can be set to detect the seed, the ressure and the flow of the exit face in the calculation. For the A-1 tye model when the inlet mass flow rate is 90 kg/s and the exit velocity is 100 m/s. It can be divided into six conditions: the inlet flow of 15, 30, 45, 60, 75, 90 kg/s; while the A-model of tye is divided into six conditions: the inlet flow 0, 40, 60, 80, 100, 10 kg/s. Figures 3-8 are the rofile of the conditions of exorts face velocity. Table 1 indicates the uneven degree of the Tye A model exort face velocity field δ > 15%, namely, the model designed is unreasonable. Figure 5. Profile of exorts face velocity when Q = 45 kg/s. Figure 6. Profile of exorts face velocity when Q = 60 kg/s. Figure 3. Profile of exorts face velocity when Q = 15 kg/s. Figure 7. Profile of exorts face velocity when Q = 75 kg/s. Figure 4. Profile of exorts face velocity when Q = 30 kg/s. 4. Structural Otimization The volute structural imrovements in this thesis mainly have two uroses: first, to make the outlet section more uniform and stable in order to imrove the comressor inlet conditions and erformance; second, to reduce the volute ressure loss, thereby reducing the entire comressor ressure losses and imroving the efficiency of Coyright 013 SciRes.

5 34 T. JIANG, K. Z. HUANG Figure 8. Profile of exorts face velocity when Q = 90 kg/s. Figure 9. Tye B mode. Table 1. Profile of exorts face velocity when Q = 90 kg/s. condition Entrance flow average max min m/s The average quality comressor machine. The ressure loss is mainly from the fluid turbulence and the solid wall friction, therefore, the ressure energy is scattered in the form of heat. In order to reduce the ressure loss, strong turbulence of volute flow field should be avoided, such as adding an air guide structure in the airflow steering to slow the changes of the flow field or imroving structure comonents rone to cause strong vortex. Modifying the imort and exort is shown in Figure 9. Make the numerical simulation of volute flow field according to the control equations of the Favre averaged N-S equations with the given conditions. Aly the standard k model as turbulence model, uwind to the discrete convection term as uwind, central difference scheme to the dissiative term, the SIM- PLE algorithm to the ressure-seed iteration of the equation. And the solution rocess emloys the underrelaxation factor. As for adjustment of the inlet mass flow to achieve that the exit velocity may reach 100 m/s, it can be set to detect the seed, the ressure and the flow of the exit face and in the calculation. From the ressure loss comarison in Figure 10, the ressure loss of Tye B is the minimum. Based on the average unevenness and ressure loss of each model, the inlet volute should be designed in taering structure Figure 10. Resistance characteristic curve of Tye B model. which can reduce the average unevenness of the model. The former have required the unevenness of the volute to be less than 15%. From the ressure loss analysis, the inlet volute designed in taering structure which increases the ressure loss, however has relative little ressure loss, so the Tye B model is the rior model for the imrovement. The results could been seen in the Tables and Conclusions This thesis studies the inlet volute flow field characteristics of comressor using numerical simulation method and imroves the volute structure. The conclusions are summarized as follows: 1) Make numerical simulation on the comressor inlet volute initial model and analysis of the characteristics of about the volute structure imrovement. ) Make a variety of imrovement attemts on the volute structure and imrove numerical simulations under the same conditions for each. The numerical simulation results indicate that the taered-structure volute reduces the average unevenness of the outlet face while increases the value in the outlet channel if adding a baffle. 3) The numerical simulation results on the imroved model structure after refinement show that the uneven- Coyright 013 SciRes.

6 T. JIANG, K. Z. HUANG 35 Table. Numerical calculation results of Tye B model. condition Entrance flow Q (kg/s) average max min m/s The average quality Table 3. Pressure loss of Tye B-1 model Table 1 rofile of exorts face velocity when Q = 90 kg/s. condition Entrance flow The average quality ness of outlet face flow field has got good imrovement though the ressure loss of the imroved model increases a little bit. REFERENCES [1] X. X. Liu, Some Problems about Design of Industrial Gas Turbine Intake and Exhaust System, 1983,. -5. [] K. Q. Wu and J. Huang, Numerical Analysis of the Fan Volute Internal Vortex Flow, Engineering Thermohysics, Vol., No. 3, 001, [3] K. 1lillewaert and R. A. Vanden Braembussche, Numerical Simulation of Imeller-Volute Interaction in Centrifugal Comressors, ASME Journal of Turbomachinery, Vol. 11, No. 7, 1999, [4] E. Ayder and R. A. Van den Braembussche, Exerimental Study of the Swirling Flow in the Internal Volute of a Centrifugal Comressor, ASME Paer No GT-7. [5] E. Ayder and R. A. Van den Braembussche, Numerical Analysis of the Three-Dimensional Swirling Flow in Centrifugal Comressor Volute, ASME Journal of Turbomachinery, Vol. 116, 1994, [6] W. G. Zhang, Flow Imact on the Distribution of Centrifugal Pum Volute Pressure and Seed, Petroleum Machinery Press, 000, [7] F. Shi and H. Tsukamoto, Numerical Study of Pressure Fluctuations Caused by Imeller-Diffuser Interaction Diffuser Pum stage, ASME Journal of Fluid Engineering, Vol. 18, No. 1, 001,. 13. Coyright 013 SciRes.