Abstract—An overall performance simulation model of the intercooled recuperative aero-engine was built, and an overall performance simulation program was developed. Then, analysis of the effects of intercooling and recuperation on the thermodynamic cycle parameters and performance parameters of the turbofan engine can be carried out. The result indicates that the turbofan engine performance can be improved with the proper use of intercooling and recuperation. With the same thermodynamic cycle parameters, the thermal efficiency and the net thrust is larger, while the specific fuel consumption is lower. To gain the same performance, lower overall pressure ratio and lower bypass ratio can be chosen for the intercooled recuperative aero-engine.
intermediate pressure compressor (IPC) and the high pressure compressor (HPC), and the recuperator is placed between the low pressure turbine (LPT) and the exhaust. Figure 1 displays the IRA stations; Figure 2 displays the h-s diagram of the engine cycle; Figure 3 displays the flow chart of the engine performance simulation program [3].
I. INTRODUCTION
Recently, the aero-engine performance level is very high and the further improvements of the known concepts seem increasingly difficult. At the same time, the environment protection level of the engine has been the new research driver since the environment problems have been widely concerned. To achieve higher overall performance and meet the environmental requirements, many new technologies should be developed. In the aspect of the thermodynamic cycle revolution, one of the potential means is making use of the intercooled recuperative cycle technology which has already been utilized in the industry and the marine engines. Currently, the intercooled recuperative aero-engine (IRA) is being investigated in some European Research Programmes such as AC ARE and NEWAC [1].
II. THERMODYNAMIC CYCLE AND NUMERICAL MODEL
Figure 2. h-s diagram for an intercooled recuperative cycle
To study the effects of intercooling and recuperation on turbofan engine performance, a simulation model of the turbofan using the intercooled recuperative cycle technology has been built based on the conventional two spool unmixed flow turbofan [2]. The intercooler is placed between the Figure 3. Flow chart for solution of IRA
TABLE I. DESIGN POINT DATA FOR PERFORMANCE CALCULATION
Item
Altitude (m)
Mach Number
TET (K)
Mass Flow (kg/s)
BPR
OPR
ICE
HXE
Value
11000
0.82
1750
700
10
30.5
0.6
0.6
Figure 4 shows the IRA performance for different Altitude and Mach Number. The Altitude changes from the sea level to 14000m, and the Mach Number changes from 0 to 1. The black spot represents the cycle design point.
OPR is not very high. When the OPR is high enough, the air temperature is still very high although after intercooling, the combustor temperature rise is limited, so the SFC can be kept at a normal level. Besides, the necessary compression work in the HPC is reduced, so the core efficiency of the engine with intercooling is higher than that of non-intercooled engine.
The core efficiency of the engine with recuperation is higher than that of non-recuperative engine. That is because some of exhaust heat from the engine core exhaust gases can be recovered by adding recuperation progress to the engine cycle and the TET is higher as a result. However, the airflow overall temperature in the HPC exit increases with increasing OPR and the impression of the heat-exchange process in the recuperator gets worse; the core efficiency lines of the engine with and without recuperation intersect at a higher OPR. The core efficiency of the engine with recuperation can be increased by adding recuperation process, because the airflow overall temperature in the HPC exit decreases and the impression of heat-exchange process in the recuperator gets improved [4].
Figure 4. IRA performance for different Altitude and Mach Number
Figure 5. Core efficiency of different engine concepts
B. Effect of ICE and HXE on Cycle Efficiency Aero-engine Cycle Efficiency is defined by Overall
Pressure Ratio (OPR) and Turbine Entry Temperature (TET), and a higher TET leads to a higher Cycle Efficiency. Keeping a constant Fan Pressure Ratio (FPR) and TET, the effect of the different cycle innovations is shown in Figure 5, where the core efficiency is calculated for different but consistent cycles (consistent technology level and the same simulation tool). Obviously, the IRA core efficiency is the highest when OPR is not very high, and there exists an optimum OPR.
The core efficiency of the conventional cycle engine increases with increasing OPR. But with higher temperature, the movement of the air molecules becomes much more intense, and it is increasingly difficult to compress the air. By using the intercooled recuperative cycle technology, the air flowing into HPC can be cooled down throughout the intercooling process and a higher OPR can be achieved. However, to keep TET constant, the specific fuel consumption (SFC) increases, so the core efficiency of the IRA is lower than that of the conventional cycle engine when
Figure 6. Effect of ICE and HXE on Cycle Efficiency for different OPR
For different combinations of intercooler effectiveness (ICE) and recuperator effectiveness (HXE), larger ICE means better impression of the intercooling process and less necessary compression work to achieve the same pressure ratio; and larger HXE means better impression of the recuperation process and lower SFC to achieve the same TET. As is shown in Figure 6, without a recuperator, the core efficiency decreases with increasing ICE. While if there is a recuperator, keeping a constant HXE, the core efficiency increases with increasing ICE; and keeping a constant ICE,
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the core efficiency increases with increasing HXE when OPR is not very high.
C Effect of ICE and HXE on SFC and Net Thrust Keeping FPR and TET constant, the SFC and the Net
Thrust of different engine concepts for different OPRs are shown in Figure 7 and Figure 8.
The analysis process is similar to that in the core efficiency part. For the IRA, the overall temperature of the airflow delivery into HPC decreases throughout the intercooling process, and the HPC necessary compression work decreases as well. At the same time, the overall temperature difference between it and the core flow at the LPT exit increases, and the impression of heat-exchange process in the recuperator gets improved. Throughout the recuperation process, the overall temperature of the airflow delivery into the combustor increases and the SFC to achieve the same TET is lower than that of the conventional cycle engine. In the respect of Net Thrust, the Net Thrust increased by intercooling is much higher than that decreased by the pressure loss; therefore the Net Thrust of the IRA is higher than that of the conventional cycle engine.
Figure 7. SFC of different engine concepts
Figure 8. Net Thrust of different engine concepts
Figure 9 shows the variation of SFC and Net Thrust for different combinations of ICE and HXE. Without an intercooler, the SFC and the Net Thrust both decrease with increasing HXE. Without a recuperator, the SFC decreases while the Net Thrust increases with increasing ICE. With increasing ICE, the work capacity of the bypass airflow increases due to the increase of the heat exchanging in the
intercooler. At the same time, the necessary compression work to achieve the same pressure ratio decreases. Both of them result in higher Net Thrust, and the latter also has an effect on SFC. If there is a recuperator, the waste heat recovered out of the core exhaust increases with increasing HXE. Keeping a constant TET, the SFC is lower due to the lower combustor temperature rise.
Figure 9. Effect of ICE and HXE on Engine Performance
Figure 10. Effect of ICE and HXE on SFC
Figure 11. Effect of ICE and HXE on Net Thrust
Figure 10 displays the SFC for different OPRs of the specific cycles. For the IRA, the SFC is lower than that of the non-recuperative engine with the same ICE when the OPR is low. While with a high OPR, the recuperator does not affect much on reducing the SFC. As is shown in Figure 11, there exists an optimum OPR for every constant-ICE-line or constant-HXE-line to achieve the highest Net Thrust, and the optimum OPR increases with increasing ICE [5].
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D. Effect of ICE and HXE on Bypass Ratio
Figure 12. Effect of ICE and HXE on Bypass Ratio
In Figure 12, the BPR varies from 6 to 12 and ICE varies from 0 to 0.9. It can be seen that without a recuperator, a higher BPR should be chosen, since the fuel saving is of a much bigger relevance compared to the thrust loss that will come with it. If there is a recuperator, changing the BPR will mainly influence the Net Thrust, and the change in SFC is very low. With higher HXE, it is advantageous to choose a lower BPR to gain additional thrust, while the SFC in this case is almost constant. Keeping BPR and the HXE constant, engine performance is improved with increasing ICE.
E. Effect of Pressure Loss on Engine Performance
Figure 13. Effectiveness over Pressure Loss Correlation
Although intercooled recuperative cycle technology can make a contribution to the engine cycle and performance parameters, there also exists some drawbacks. Besides cost and weight issues, the biggest one is that the intercooler and
recuperator pressure losses will counteract the performance benefits to a specific amount. Figure 13 displays the effects of the intercooler pressure loss to SFC and Net Thrust. It can be seen that a higher intercooler performance will always be beneficial for the IRA. Keeping a constant ICE, lower pressure loss results in lower SFC and higher Net Thrust; while keeping a constant intercooler pressure loss, higher ICE leads to lower SFC and higher Net Thrust. For the recuperator, a similar conclusion can be reached.
IV. CONCLUSION
The numerical simulation results indicate that intercooling and recuperation have a significant influence on aero-engine cycle and performance.
(1) For the IRA with the same thermodynamic cycle parameters, the Cycle Efficiency is higher and the Net Thrust is larger, while the SFC is lower. And there exists an optimum OPR which is lower than that of the conventional turbofan engine to achieve the best Cycle Efficiency.
(2) To gain the same performance, a lower OPR and lower BPR can be chosen for the IRA. And with higher HXE, it is much more proper to choose a lower BPR to increase the Net Thrust, while the SFC is almost constant.
(3) The weight and the pressure loss of the intercooler and recuperator will be larger with higher heat exchange effectiveness. So a moderate heat exchanger effectiveness should be chosen. Furthermore, the higher pressure loss can lead to a serious decrease in the engine performance, so it should be strictly controlled.
REFERENCES
[1] G. Wilfert, J. Sieber, A. Rolt, "New Environmental Friendly Aero Engine Core Concepts," ISABE-2007-1120.
[2] Boggia S, Rud K, "Intercooled recuperated aero engine," MTU Aero Engines, 2004.
[3] LIAN Xiao-chun, WU Hu, Elements of Aero-engine. Xi'an: Northwestern Polytechnical University Press, 2008.
[4] CAO Meng-yuan, TANG Hai-long, CHEN Min, "Preliminary analysis of thermodynamic cycle of an intercooled recuperated turbofan engine," Journal of Aerospace Power, 2009, 24(11), pp.65-70.
[5] Martin. Marx, "Investigation and Optimisation of Intercooling in an Intercooled Recuperative Aero Engine," U.K.:School of Engineering, Cranfield University, 2007.
