The Effect of Tip Clearance on Aerodynamic Performance of an Axial Compressor

International Journal of Material and Mechanical Engineering (IJMME) Volume 3 Issue 4, November 2014 DOI: 10.14355/ijmme.2014.0304.03

www.ijm-me.org

The Effect of Tip Clearance on Aerodynamic Performance of an Axial Compressor PI Jun *1, LIN Liang 2 Sino-European Institute of Aviation Engineering, Civil Aviation University of China, Dongli, Tianjin 300300, China pijun8@163.com; 2ll119gtxy@163.com

*1

Abstract The performance of an axial compressor rotor for different tip clearance sizes is investigated by a numerical simulation with CFD software NUMECA. The variable quantity of the simulation is the relative tip clearance. Three different sizes are 1%, 3%, 5% of the tip chord length. The results of numerical analysis show that with the increase of tip clearance size, the performance of the compressor changes obviously, the performance parameters such as efficiency, pressure ratio tend to decrease obviously.

Keywords Axial Compressor; Tip Clearance; Numerical Simulation

Introduction The compressor is one of the core components of gas turbine engines, its design, manufacturing and material selection not only determine the performance of overall engine, but also directly affect the security and stability of the engine. The tip clearance is designed to avoid the friction and collision of the compressor blade and the wall surface, the size of the tip clearance is about 1% of the blade height. Under the pressure differential of the tip blade pressure surface and the suction surface, part of the fluid flows over the tip clearance and causes the tip leakage flow (or called tip clearance flow). It is a very complex three-dimensional viscous flow and has a very significant impact on the performance and stability of the axial flow compressor. Early research focused on the simplified model of the tip clearance flow, for example the Rain model and the Chen model [1, 2], while recent researches have focused on the study of the mechanism and the stability of the tip clearance flow. Studying and understanding of the mechanism is of great significance for improving the aerodynamic performance and stability of aircraft engines and other gas turbine engine.

Due to the tip clearance, the three-dimensional flow

1

Fund Project: Central Universities Fundamental Research special for Civil Aviation University of China (3122013H001)

on the top of the axial compressor’s blade is very complex. The movement of the rotor relative to the shroud and the small size of the tip clearance makes the measurement of the internal flow in the experiment very difficult. But the study of tip clearance flow has a long history, many experts have dedicated to the research of the tip clearance flow field and obtained certain results through the theoretical studies and experimental researches. Yamada K, Furukawa M[3] , who studied the tip clearance flow leakage vortex of the cross-sound stage compressor, pointed out that the leakage vortex in the near stall condition would break, resulting the blocking effect in the channel and causing the rotating stall; Boristav T, Choon S[4] studied the influence of the tip clearance leakage flow of the trail in the front vane, when the flow characteristics change, it will lead to reduction of losses and blockage and pressure increases; MA Wensheng, GU Chunwei[5] who used the CFD numerical simulation of a multi-stage axial compressor in a two-stage, pointing out the characteristics of the tip clearance loss, and the simulation results match with the design parameters of the interstage of this compressor; Wang Xiangfeng, WANG Songtao, HAN Wanjin[6] simulated the tip clearance flow of a multi-stage axial compressor and studied the influence on the overall performance of the compressor; DENG Xiangyang[7] studied that the changes in working conditions would affect the frequency of leakage flow, the load of blade increases, the leakage flow fluctuation frequency decreases; WU Yanhui, CHU Wuli[8,9] studied the structure of the tip clearance flow, the different leakage when the compressor is in different working conditions and the changes of the aerodynamic performance in the main flow path.

The author takes a single-stage axial compressor Rotor37 for the study, through numerical simulation and the analysis of the characteristics of tip clearance flow under different size of the clearance, finds that the gap size has a great influence in the aerodynamic

77

www.ijm-me.org

International Journal of Material and Mechanical Engineering (IJMME) Volume 3 Issue 4, November 2014

performance of the compressor.

TABLE1 AXIAL FLOW COMPRESSOR AERODYNAMIC PERFORMANCE IN 3 GAP SIZES

Calculation Methods and Grid Models The software of numerical simulation is NUMECA. The discrete grid of cascade channel is generated by using NUMECA software pre-processing module Autogrid5, with cascade using O-structured grid, import and export extensions using H-type structured grid. The three-dimensional steady flow simulation is calculated by N-S equation solver module FINE. Turbulence model selects the Spalart-Allamaras model.

The number of CFL is 3.

Calculation boundary conditions given below: At the inlet with given total temperature and total pressure, the direction of air intake is along the axial direction; at the outlet with given static pressure, the shroud is adiabatic and the rotor is in constant rotational speed, while the shroud is in stationary. The model and grid of computing are shown in Figure 1 and 2 below:

Gap size

Efficiency

Pressure ratio

0.333mm 0.999mm 1.665mm

0.8448 0.8360 0.8231

1.900 1.856 1.802

The effect of tip clearance flow on the aerodynamic performance of the compressor is mainly reflected in obstruction leakage losses. According to the calculation results in the table, we can see that with the increase in tip clearance, the total efficiency of the compressor and the pressure ratio has declined.

The Distribution of Static Pressure at Tip Region

a) 1%Gap size

FIGURE1 A SINGLE-STAGE AXIAL COMPRESSOR ROTOR MODEL

b) 3%Gap size

FIGURE2 THE COMPUTING DOMAIN OF A SINGLE ROTOR GRID MODEL

Calculation Results and Analysis The Influence of Different Gap Size The different gap sizes are selected as 1%, 3%, 5%, of the tip chord length; they are 0.333mm, 0.999mm, 1.665mm. We get the results of different compressor aerodynamic performance in 3 gap sizes in Table 1 below:

78

c) 5% Gap size FIGURE3 STATIC PRESSURE DISTRIBUTION AT 99% BLADE HEIGHT

International Journal of Material and Mechanical Engineering (IJMME) Volume 3 Issue 4, November 2014

The different gap size leads to a complex flow at the tip region, causing the distribution of static pressure near the shroud changes obviously. In order to observe clearly the effect on the compressor performance in different gap size, the author takes a section of the flow field about 99% of the compressor rotor blade height, so that we can observe the changes in certain parameters. Figure 3 shows the distribution of the static pressure in this section with three kinds of gap size case.

www.ijm-me.org

on the flow field; when the gap increases, the leakage vortex and its range of influence are increasing, while a separation vortex in contrast with the main flow direction is formed at the pressure surface, , thus causing a greater energy loss.

Because of the tip clearance, there will be a leakage flow, we can see a low pressure groove at the suction side of the blade, when the tip clearance is 1% of the tip chord length, and the low-pressure groove is narrow. But when the tip clearance increased from 1% to 5% of the tip chord length, the low-pressure groove increases significantly, the range increases and the tip leakage vortex is enhanced. With increasing tip clearance, the effects on static pressure near the shroud in different gap size has become more obvious, the intensity and scope of the tip leakage vortex increases and the performance of the flow field near the tip gets worse.

a) 1% Gap size

The Analysis of Flow Field at Tip Region To further understand the impact of different gap size on compressor performance, the author takes the same section with the parameter velocity, as the velocity is a vector in three-dimensional space; the author selects the absolute value of the velocity as a reference, in order to obtain the distribution of the flow field contours shown in Figure 4.

Because of the pressure difference between the suction and pressure surfaces of the blades, the fluid passes through the tip clearance, forming a leakage vortex at the suction surface near the shroud, and also forming a separation vortex at the pressure surface, after flowing through the gap, it will be influenced by the mainstream and the pressure gradient. As can be seen from the figure, the gap size increases from 1% to 5% of the tip chord length, the size of the tip leakage vortex begins to become larger, the position of the core of vortex begins to move downstream, while the intensity and the range also increase, the increase of obstruction leads to the mainstream flow loss increases. This is because when the gap is small because the frictional resistance of the shroud is relatively large, it leads to a relatively small amount of leakage, the energy loss is relatively small, after the main fluid flowing by, it can be quickly returned to the mainstream in the same direction with small impact

b) 3% Gap size

c) 5% Gap size FIGURE4 VELOCITY DISTRIBUTION AT 99% BLADE HEIGHT

Conclusions In this paper the author uses the NUMECA software to simulate and analyze a single-stage axial compressor rotor in 3 different tip clearances, conclusions are as follows: (1) The existence of the tip clearance causes a loss of energy, with the growing tip clearance, the compressor pressure ratio, efficiency and other performance parameters have shown a downward

79

www.ijm-me.org

International Journal of Material and Mechanical Engineering (IJMME) Volume 3 Issue 4, November 2014

trend.

Investigation on unsteady tip clearance flow in a

(2) With the increase of the gap size, tip clearance leakage vortex flow continues to strengthen, the impact near the shroud on static pressure distribution becomes more obvious, the smaller the gap can improve the performance, but the larger gap will cause worse performance in the flow field.

compressor by numerical simulation [J]. Journal of

(3) The pressure difference between the pressure side and the suction side at the tip region causes the formation of the tip clearance flow. With the increase of tip clearance, the tip clearance flow near the shroud area is enhanced, resulting in energy loss.

(4)The size and strength of the tip clearance leakage vortex increase with the gap size increases, when the gap is small, after passing by the mainstream, the leakage vortex will return to the same direction soon; while the gap is large, there will be a separation vortex at the pressure surface side.

engineering thermophysics, 2006.

MA Wensheng, GU Chunwei. The effect of tip clearance on compressor

performance

[J].

Journal

of

Power

Engineering, 2007.

Rain,D.A.Tip Clearance Flow in Axial Flow Compressor and Pumps[R], 1954, California Institute of Technology, Hydrodynamics

and

Mechanical

Engineering

Laboratories Report No.5.

WANG

Xiangfeng,

WANG

Songtao,

HAN

Wanjin.

Numerical simulation study on multistage compressor performance with changing tip clearance [J]. Science technology and engineering, 2009.

WU Yanhui, CHU Wuli, LU Xingen. The effect of tip leakage flow structure on compressor aerodynamics [J]. Journal of engineering thermophysics, 2006.

REFERENCES

Boristav T,Choon S,Tan.Effect of Upstream Unsteady flow Conditions on Rotor Tip Leakage Flow[D]. ASME Paper, 2005-GT-0358,2005.

CHEN G.T. Similarity analysis of compressor tip clearance flow structure[J].ASME Journal of Turbomachinery, 1991, 113( 2) : 260- 271.

DENG Xiangyang, ZHANG Hongwu, ZHU Junqiang.

80

WU Yanhui, CHU Wuli, ZHANG Yanfeng. Effects of tip clearance size on tip leakage flow field [J]. Journal of Northwestern Polytechnical University,2007.

Yamada

K,Furukawa

M,Nakano

K.Unsteady

Three-

dimensional Flow Phenomena Due to Breakdown of Tip Leakage Vortex in a Transonic Axial Compressor Rotor[D]. ASME Paper,2004-GT-53745,2004.