On-design Cycle Analysis of a Separate-flow Turbofan Engine with an Interstage Turbine Burner

Kyaw Shwin School of Jet Propulsion

Beihang University (BUAA) Beijing, China

kshwin2004@gmail.com

Abstract—Engine design starts with parametric cycle analysis. This paper focus on design cycle analysis of a turbofan engine with Interstage Turbine Burner (ITB) . The ITB, which is located between the high and low pressure turbine, i.e., transition duct [3]. The objective of parametric cycle analysis (On-Design) is to compare the engine performance parameters (specific thrust and thrust specific fuel consumption), to design choices (compressor pressure ratio, fan pressure ratio, bypass ratio) between with and without ITB. The ITB engines are shown to provide significantly higher specific thrust with small increase in specific fuel consumption compared to conventional base engines. And then, the ITB engines also widen the operational range of flight Mach number and compressor pressure ratio. To simulate the on design cycle analysis for this paper, visual C++ language program will be used.

Keywords- turbofan engine, interstage turbine burner, parametric cycle analysis, NOx emission

I. INTRODUCTION

Throughout aero-vehicle evaluation, scientists and engineers are trying to improve engine efficiencies, to make it smaller, less fuel consumption, and yet more powerful. Scientists have proposed solutions of how to achieve these goals and one of them is introducing an Interstage Turbine Burner (ITB) into the engines. The major advantages associated with the use of ITB are an increase in thrust and reduction in NOx emission. Another advantage is the safety improvement, where flameout of the either main burner or the ITB will not shut down the engine [2].

One expects to see an increase in specific thrust (Fs) and a decrease in specific fuel consumption (sfc) in the final design of an engine. Unfortunately, Fs and sfc compromise to each other, so an increase in specific thrust always results in an increase of specific fuel consumption for constant thermal efficiency. Nevertheless, Turbofan engines with ITB increase its engine performance further by varying engine design parameters, such as compressor pressure ratio (CPR), fan pressure ratio (FPR), and bypass ratio (BPR). Besides, it can sustain the higher pressure and temperature by adding a new material. That’s the reason why we should

introduce ITB to base turbofan engine. The results also will show that the ITB provided more performance gain at higher speed when compared to a baseline turbofan.

II. ASSUMPTION

This parametric cycle analysis idealized the engine components and assumed that the working fluid behaved as a perfect gas with constant specific heats. These idealizations and assumptions permitted the basic parametric analysis of several types of engine cycles and the assumptions are:

The working fluid is air (calorically perfect gas) . The area at each station is constant

All components are adiabatic except main combustor and ITB.

Constant polytropic efficiencies of compressor, turbine and fan.

III. PARAMETRIC CYCLE ANALYSIS

Designation starts at station 0 denoting undisturbed flow well ahead of the inlet, and ends at station 9181 denoting exhaust flow condition of the core engine at nozzle exit. The fig. 1 is the station numbering of a separate flow turbofan engine with ITB.

The fig. 2 (a) and (b) are the major advantages associated with the use of ITB. Fig. 2 (a) shows the thrust increase and (b) illustrates the reduction in NOx emission. In fig. 2a, the inlet temperature of the high-pressure turbine remains unchanged. When the flow goes through secondary combustor, higher specific thrust results as shown in fig a. In fig. 2b, the temperature of the primary combustor is decreased; therefore the amount of thermal NOx production can be reduced. By lowering the temperature of the main combustor and the high-pressure turbine, a smaller amount of cooling air is required.

2010 Second International Conference on Computer Modeling and Simulation

978-0-7695-3941-6/10 $26.00 © 2010 IEEE

DOI 10.1109/ICCMS.2010.41

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2010 Second International Conference on Computer Modeling and Simulation

978-0-7695-3941-6/10 $26.00 © 2010 IEEE

DOI 10.1109/ICCMS.2010.41

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Figure 1 Station numbering of a turbofan engine with ITB

(a)

(b) Figure 2. Thermodynamic cycle of a turbofan engine with ITB (a) Higher Thrust (b) NOx Reduced

IV. THERMODYNAMIC CYCLE ANLAYSIS

In this analysis, two engine design choices, namely (1) Fan Pressure Ratio (FPR) and (2) Bypass Ratio (BPR). And then the effect of turbine inlet temperature Tt4 will also study to obtain specific thrust and specific fuel consumption on the conventional base engine and ITB. The goal of this study is to develop the engine thrust and reduce the NOx emission by using ITB in turbofan engine. Furthermore, this analysis is the concept of a commercial jet using ITB; such an aircraft would have several advantages:

Increased power during critical mission pointsReduced peak combustion temperature for increased turbine life/reduced cooling Possible reduction in specific fuel consumption comparatively increase in specific thrust Decreased NOx emissions

V. RESULTS AND DISCUSSION

A. Fan Pressure Ratio

In fig. 3 (a) and (b) is a varying of fan pressure ratio with specific thrust and specific fuel consumption. Fig. c is the performance comparison of specific thrust and specific fuel consumption. The specified conditions are (H=0, Ma0=0, Tt4=1800, B=8, Pi_CL=2, Pi_CH=15, Wa0=2000). Increasing fan pressure ratio is a way to supply more energy to bypass flow. As shown in figures, ITB engine gains the benefit of increasing FPR, where specific thrust increases and specific fuel consumption decreases gradually. Start from FPR=1.6, specific thrust gradually decrease in base engine but ITB engine obviously increase. Why conventional base engine’s specific thrust lower than ITB; because more work is extracted from LPT to fan in order to achieve the specified FPR. Lower energy of low pressure turbine exhaust stream give lower average velocity of the fan and the engine core stream, this is the reason of lower thrust. Fortunately, we can fill this gap by using ITB to supply more energy to low pressure turbine, LPT to drive the fan with only slight increase in specific fuel consumption. In K.H. Liew, E. Urip, S.L. Yang, and Y.K. Siow [1] said specific fuel consumption of ITB is becoming lower than that of the turbofan base engine at FPR beyond 2.7; furthermore ITB engine operates more efficiently. But that is the supersonic flight condition, so the supersonic condition will not be shown here because this paper intends for high bypass ratio civil aircraft.

B. Fan Bypass Ratio

Fig. 4 shows the single performance parameters vs. the bypass ratios at a compression ratio of 45. The specified conditions are (H=0, Ma0=0, Tt4=1800, Pi_CF=1.5, Pi_CL=2, Pi_CH=15, Wa0=2000). As shown in figures, the specific thrust gain by the ITB engine over the base engine widens significantly as the bypass ratio increases fig. 4a, while the specific fuel consumption rate decrease to approach the level of base engine over bypass ratio 12. This is the clearly indication that the ITB engines benefit more from the increase bypass ratio than the base engine. In addition, we notice that the base engines stop producing positive thrust for bypass ratio over 13. Besides, the ITB engines are capable of operating with a much larger-bypass ratio with decreasing fuel consumption rate and no sign of decreased specific thrust. So using ITB engines are very suitable and have good advantages for ultra-high bypass ratio turbine engines.

C. High Pressure Turbine Inlet Temperature

The following fig. 5 (a) and (b) is the variable of turbine inlet temperature Tt4 with specific thrust and specific fuel consumption. The specified conditions are (H=0, Ma0=0, B=8, Pi_CF=1.5, Pi_CL=2, Pi_CH=15, Wa0=2000). As

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shown in figures, the minimum Tt4 has to be at least 1650K in turbofan base engine. Meanwhile, ITB engine can be as low as about 1500K with more specific thrust. It is required over 2000K to get 350N-sec/Kg specific thrust in turbofan base engine. However, in ITB engine, 1700K can produce that amount of thrust with light increase of specific fuel consumption. Although the specific fuel consumption is increase according with high Tt4 in turbofan base engine, it is different in ITB. The higher the turbine inlet temperature, we gain the low specific fuel consumption. But we cannot use very high turbine inlet temperature in order to keep the small frontal area; it is still requires advanced turbine technology for a very high turbine inlet temperature. So I think it is so interesting to investigate for everyone who wants to apply the ITB’s advantage to keep the same turbine inlet temperature Tt4 while maintaining a small frontal area.

VI. CONCLUSION

As a conclusion, a parametric cycle analysis studies of turbofan engine with ITB which is presented in this paper can be summarized as followings:

Through the addition of ITB, high pressure turbine inlet temperature Tt4 can be lower while producing more specific thrust with less specific fuel consumption compare with conventional base engine. ITB engine can support ultra-high bypass ratio with higher Fs and lower sfc than conventional base engine. ITB engine gains the benefit of increasing FPR, where specific thrust increases and gradually decreases specific fuel consumption.

As I introduced above, the major advantages associated with the use of ITB are an increase in thrust and reduction in

NOx emission. Another advantage is the safety improvement, where flameout of the either main burner or the ITB will not shut down the engine.

ABBREVIATIONS

ITB Interstage Turbine Burner FPR Fan Pressure Ratio Pi_CF Fan Pressure Ratio BPR Fan Bypass Ratio Fs Specific Thrust SFC Specific Fuel Consumption Tt4 High Pressure Turbine Inlet Temperature Tt48 Low Pressure Turbine Inlet Temperature

REFERENCES

[1] K.H. Liew, E. Urip, S.L. Yang, and Y.K. Siow, “A complete parametric cycle analysis of a turbofan with interstage turbine burner,” AIAA 2003-685.

[2] K.H. Liew, E. Urip, S.L. Yang, “Performance Cycle Analysis of a Two-spool, Separate exhaust Turbofan with InterstageTurbine Burner”, AIAA 2004-3311.

[3] Y.K. Siow and S.L.Yang, “Numerical Study and Design of Interstage Turbine Burner,” 38th AIAA, ASME, SAE, ASEE Joint Propulsion Conference & Exhibit , 7-10 July 2002, Indianapolis, Indiana.

[4] Jack D. Mattingly, William H. Heiser , David. T. Pratt., Aircraft Engine Design , AIAA Education Series, Second Edition.

[5] Jack D. Mattingly, William H. Heiser , David. T. Pratt., “Aircraft Engine Design ,” AIAA Education Series, Second Edition.

[6] Oates, G.C., “The Aerothermodynamics of Gas Turbine and Rocket Propulsion,” 3rd ed., AIAA Education Series, AIAA, Reston, VA, 1997.

(a) (b)

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(c)

Figure 3. Performance of turbofan engine vs. Pi_CF at H=0, Ma0=0, Tt4=1800, B=8, Pi_CL=2, Pi_CH=15, Wa0=2000

(a)

(b)

(c) Figure 4. Performance of turbofan engine vs. BPR at H=0, Ma0=0,Tt4=1800, Pi_CF=1.5, Pi_CL=2, Pi_CH=15, Wa0=2000

(a)

(b) Figure 5. Performance of turbofan engine vs. H=0, Ma0=0, B=8, Pi_CF=1.5, Pi_CL=2, Pi_CH=15, Wa0=2000

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