- Email: [email protected]
Experimental study on the performance of an aircraft environmental control system
Applied Thermal Engineering 29 (2009) 3284–3288
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on the performance of an aircraft environmental control system Hongli Zhao a, Yu Hou a,*, Yongfeng Zhu b, Liang Chen a, Shuangtao Chen a a b
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China The First Aircraft Institute of AVIC1, Xi’an 710089, PR China
a r t i c l e
i n f o
Article history: Received 4 July 2008 Accepted 1 May 2009 Available online 10 May 2009 Keywords: Environmental control system Off-design performance Dynamic response Brayton cycle
a b s t r a c t In this paper, the experimental study on the off-design performance and dynamic response of an aircraft environmental control system (ECS) was presented. A bootstrap air cycle refrigeration system with high pressure water separation was employed in the ECS. Both the static test and the dynamic test were conducted on the ECS. The performance of the key components including precooler, recuperator, compressor, and turbine was investigated. The test results show the performance of the tested ECS can meet the requirements of design. However, some parameters including outlet pressure, turbine efficiency, and rotational speed might vary obviously when the operation conditions changed. The outlet humidity ratio and outlet temperature might also fluctuate largely during the start-up process. It is necessary to take into account the off-design performance and the transient performance in the design of ECS. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Reverse Brayton air refrigeration cycle used in the ECS for the aircraft was first advanced by J. Herschel in 1834 [1]. Now the most common refrigerator employed in ECS for aircraft is based on the reverse Brayton refrigeration cycle. Despite the fact that systems based on this air cycle have low efficiency, and their cooling rate capacity at sea level is very low, their main advantages, namely, their low mass, compact size and high reliability, make them to be the most appropriate choice [2,3]. With the development of aircraft, the number of electronic devices used in the aircraft keeps on increasing, which no doubt requires the ECS to increase its cooling capacity. Moreover, the requirement for higher reliability and better performance is placed on the ECS. So far much work has been carried out on the ECS. Vargas and Bejan [4] proposed an integrative approach to the thermodynamic optimization of components for energy systems to maximize the global performance of the system, and they used the cross-flow heat exchanger of the ECS for aircraft as an example to illustrate this approach. Pérez-Grande and Leo [5] adopted two optimization criteria (minimum weight and minimum entropy generation) simultaneously to optimize two finned cross-flow heat exchangers used in the ECS of commercial aircraft. They also employed an integrative approach of optimization. Ordonez and Bejan [6] took into account two basic questions in the thermodynamic optimization of the ECS for aircraft: the minimum power requirement and the features that could be optimized to secure operation at minimum power. They analyzed the robustness of the optimized models as * Corresponding author. Tel./fax: +86 29 82664921. E-mail address: [email protected] (Y. Hou). 1359-4311/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2009.05.002
well. Eichler presented a dynamic response simulation of the ECS, which was intended to study stability, study sensitivity, identify possible problem areas in the design, and serve as a tool to evaluate the dynamic effects of any future design changes [7]. These numerical studies provide effective tools for the design and optimization of ECS. However, due to the complex flight conditions, the operation profiles of ECS will differ from the design. Correspondingly, the chilled air from ECS may not meet the requirements for the flight safety because of the variations of operation conditions, including pressure, temperature, and humidity ratio of the supply air or ram air. Moreover, the temperature, humidity, or pressure of the chilled air may also change greatly during the start-up process and the changing process of flight tasks. Therefore, it is necessary to investigate the variations of thermodynamic parameters during the off-design operation and the dynamic response of ECS. In this paper, both static and dynamic experiments were conducted on a bootstrap air cycle ECS with high pressure water separation. The off-design performance of the system was investigated under operation conditions at different flight altitude. The transient performance of the system was also studied during the start-up process. 2. System description The ECS in the experiments was a bootstrap air cycle refrigeration system with high pressure water separation. Compared with basic refrigeration system, bootstrap air cycle refrigeration system can reach higher efficiency because the latter can recover the work delivered by the turbine to further pressurize the air stream [8]. The pressurization to the air stream may increase temperature
H. Zhao et al. / Applied Thermal Engineering 29 (2009) 3284–3288
drop in the turbine and the flow rate of air stream, thereby increasing the cooling capacity of the ECS. The advantages of the bootstrap air cycle refrigeration system are more prominent in the following situations: at the high altitude when pressure of the air extracted from the engine core is insufficient to meet the requirements of ECS, and the second situation when the aero engine is idled during descending. The use of bootstrap air cycle refrigeration system in ECS can help to avoid the problems caused by the insufficient cooling capacity. To ensure the comfortableness of the cabin and the operation of the electronic devices, the conditioned air should meet the requirement of humidity ratio besides temperature and pressure. And it is necessary to select a proper method for water separation which can keep the effectiveness of water separation as well as the overall performance [9]. The advantages of high pressure water separation are as following: (1) Vapor contained in air is easier to be condensed at high pressure because the dew temperature for given air stream with certain humidity ratio increases with the increase of pressure. (2) The efficiency of water separator in the high pressure water separation system can be improved by 10–15%. In the high pressure water separation system, the vapor is condensed before the turbine, where the speed of the air stream is low. So the size of the condensed droplets is big. However, in low pressure water separation system, the condensed droplets may break into mist because of the high speed of the air stream behind the turbine, and special measures should be taken to collect the water mist and discharge it.
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(3) The cooling capacity may be increased. Under the same cooling condition, more water can be condensed at high pressure than that at low pressure. Considering the high efficiency of water separation as well, we could further lower the temperature at the turbine exit because it can be free from the limitation of freezing point, which may reduce the fraction of the air bleed from the engine core as well as the fuel penalty of cooling pack. The ECS consists mainly of two subsystems: the bleed system and the air cycle machine, Fig. 1a. The high-temperature and pressurized air stream which is drawn from the engine is precooled by a primary radiator. A compressor driven by the expander is located at the end of the radiator, in which the pressure and temperature of the air stream is raised. The air stream is then cooled by the bleed air from out-of-cabin in the precooler. After the precooler the air stream enters a recuperator, successively passing through the warm side of heat exchanger, condenser, water separator, and the cold side of heat exchanger. After the recuperator, the air stream expands in the turbine in which its pressure and temperature are lowered. Finally it is discharged into the cabin. High-pressure water separation was employed. In the condenser, the air stream is cooled down to the dew-point, and therefore the vapor contained in the air stream is condensed to water droplets. The condensed water droplets in company with the air stream flow into the separator where the water is discharged. The t–s diagram of the thermodynamic cycle of the ECS was shown in Fig. 1b. In the design of the ECS, optimization of the parameters was conducted under the design condition. The minimum fuel penalty is selected as the objective function, and the parameters chosen as
Fig. 1. (a) Schematic diagram of the ECS. (b) t–s diagram of the thermodynamic cycle.
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H. Zhao et al. / Applied Thermal Engineering 29 (2009) 3284–3288
An uncertainty analysis was performed on the dependent results including efficiency of turbomachineries, and effectiveness of the heat exchangers. The Kline and McClintock method [10] was used which sums the square of errors:
wA ¼
" j X @A i¼1
@zi
wz
2 #1=2
where wA, zi and wz are, respectively, the total uncertainty with the dependent variable A, the independent variable which affects the dependent variable A, and the uncertainty of variable zi. The uncertainties of calculated results are summarized in Table 3.
4. Results and discussions 4.1. Results of static tests
Fig. 2. Outlet parameters under tested conditions.
variables to obtain the objective functions are the expansion ratio, the turbine efficiency, the compressor efficiency, the precooler effectiveness, the recuperator effectiveness, and the condenser effectiveness. 3. Experimental method In the experiments, two air supply systems were employed to provide the two streams in the ECS for aircraft: the main stream to be conditioned and the ram stream. A high-pressure compressor system was used to supply the air stream that has the same pressure, temperature and humidity ratio with air stream extracted from the engine core of the aircraft. The ram stream was supplied by a low-pressure compressor system. The operation profiles were varied to simulate the flight conditions at a series of altitude. The operation parameters are listed in Table 1. The design flight altitude was 300 m, and the outlet temperature was required to be lower than 10 °C. The design of humidity ratio required there was no liquid water condensed at the outlet. As shown in Fig. 1, thermodynamic parameters were measured at eleven positions in the system. In the tests, the measured data included flow rate, temperature, relative humidity ratio, pressure, and pressure drop. The sensors used and uncertainties of the measurements are given in Table 2.
As shown in Fig. 2, the conditioned air stream with the temperature of 7.5 °C was obtained under the design condition in the tests. The humidity ratio of the air stream conditioned was 5.02 g/kg which was lower than the humidity ratio of the saturated air (6.67 g/kg). Thus, the ECS could well meet the design requirements in terms of temperature and humidity. Fig. 2 also shows the outlet parameters of the ECS under the other five off-design conditions from 0 m to 10,000 m. With the increase of flight altitude, the pressure at the outlet changed little until 7000 m. Afterward, the outlet pressure dropped dramatically, and reached 85 kPa at 10,000 m. The decrease of the pressure at the outlet might result from the decrease of inlet pressure (Table 1). As shown in Fig. 2, the outlet temperature increased from 0 m
Table 3 Uncertainties of calculated results. Variables
Maximum uncertainty (%)
Expansion ratio Compression ratio Precooler effectiveness Recuperator effectiveness Condenser effectiveness Turbine efficiency Compressor efficiency
±4.0 ±2.1 ±1.0 ±11.5 ±12.2 ±5.9 ±8.0
Table 1 Operation parameters under tested conditions. H (m)
d1 (g/kg)
p1 (kPa)
t1 (°C)
_ (kg/h) m
t10 (°C)
p10 (kPa)
_ 10 (kg/h) m
0 300 1000 7000 9000 10,000
21.4 21.3 20.9 0.5 1.8 1.0
585.95 571.05 583.96 469.42 383.92 367.5
118.8 118.3 118.8 95.3 92.9 88.7
746.03 735.89 746.6 683.01 583.6 582.47
50.3 78.0 58.9 62.8 7.1 7.0
109.95 130.81 116.7 120.5 101.05 101.03
1429.04 2888.56 2019.58 2289.57 496.99 497.11
Table 2 Uncertainties of measurements. Variables
Sensors
Range
Uncertainty value
Temperature (t1–t11) Pressure (p1, p2, p8, and p10) Pressure drop (Dp2,3, Dp3,4, Dp4,5, Dp5,6, Dp6,7, Dp8,9, and Dp10,11) _ 10 Þ _ 1 and m Mass flow rate ðm Relative humidity ratio u1 ; u4 ; u5 ; and u9
Platinum resistance thermometer (°C) Pressure transducer (MPa) Differential pressure transducers (kPa) Flow meter (kg/h) Humidity transducer
50 to 250 0–0.2; 0–1.0; 0–1.5 0–50 100–2000; 400–4000 0–100%
±0.5 °C ±0.5% ±0.5% ±1% ±2%
H. Zhao et al. / Applied Thermal Engineering 29 (2009) 3284–3288
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Fig. 5. Pressure drop in the heat exchangers and water separator. Fig. 3. Efficiencies of the components.
to 300 m, which might be due to the increase of the temperature of ram air (Table 1). Thereafter, the outlet temperature decreased steadily until the altitude reached 9000 m because of the decrease in the temperature of ram air. However, the outlet temperature increased from 9000 m to 10,000 m, which might result from the decrease of inlet pressure. The outlet humidity ratio kept a low level throughout of the region in the tests, and its variation was less pronounced than that of pressure and temperature. This might be attributed to the high-pressure water separation. It is necessary to keep the high effectiveness of the precooler because a large fraction of the cooling of the main air stream takes place in it. Moreover, the condensed water should be removed as much as possible in the water separator. In other words, it is also necessary to keep the high effectiveness of the water separator. Referring to Fig. 3, it is observed that the effectiveness of precooler and water separator was high and varied little in the test region. As shown in Fig. 3, both the recuperator effectiveness and the condenser effectiveness remained almost constant from 0 m to 1000 m. But afterward they increased obviously, which might result from the decrease of the humidity ratio and flow rate of the inlet air. The variation of turbine efficiency and compressor efficiency are also shown in Fig. 3. The efficiency of turbine varies little until
Fig. 4. Expansion ratio, compression ratio, and rotational speed.
7000 m, and then it decreases obviously. On the contrary, the efficiency of compressor was larger than that of the turbine, and varied little throughout out of the test region. The explanation can be found by considering Figs. 3 and 4 together. By inspection, one can see that the variation of turbine efficiency and compressor efficiency were similar with that of expansion ratio and compression ratio, respectively. The decrease of expansion ratio might lead to that of the turbine efficiency. The compressor and the turbine are installed on the same shaft, and therefore have the same rotational speed. As shown in Fig. 4, the rotational speed has similar trend with the expansion ratio. Fig. 5 shows the variation of the pressure drop of the heat exchangers and the water separator as a function of flow rate. As shown in Fig. 5, the pressure drop in all of these four components increased with the flow rate. Moreover, the pressure drop in the precooler was the largest, and the pressure drop in water separator took the second place. The recuperator and the condenser had similar pressure drop. 4.2. Results of dynamic tests As the flight condition varies, the inlet condition of the cooling pack changes correspondingly. To ensure the safety of the flight, the ECS must supply proper temperature air to the cabin and to the electronic devices, within the entire performance envelope of
Fig. 6. Transient inlet temperature.
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Fig. 7. Transient inlet pressure.
Fig. 9. Transient outlet temperature.
5. Conclusions In this paper, the bootstrap air cycle and high pressure water separation were discussed, and the off-design performance and dynamic response of a bootstrap air cycle refrigeration system with high pressure water separation were experimentally investigated. The variations of outlet pressure, turbine efficiency, and rotational speed with operation conditions were pronounced. Moreover, there existed fluctuations of outlet humidity ratio and outlet temperature during the start-up process. The results indicate the offdesign performance and dynamic response should be taken into account in the design of the ECS to enhance its general performance and reliability. Acknowledgements
Fig. 8. Transient outlet humidity.
the aircraft and within the entire scale of external conditions of temperature and pressure. So there are specifications on the dynamic behavior which require the system to stabilize quickly and to not exceed specific inlet temperatures to the cabin and equipment which would be harmful to the passengers and to the electronic devices. Moreover, proper flow levels must be maintained and it is necessary that no severe pressure changes reach the cabin which would be uncomfortable to passengers. The dynamic tests were performed to examine the transient performance of the cooling pack. The variations of temperature and pressure at inlet are shown in Figs. 6 and 7, respectively. Figs. 8 and 9, respectively, show the corresponding trend of outlet humidity ratio and outlet temperature. After the inlet temperature and inlet pressure became steady, the outlet humidity ratio and outlet temperature tended to be stable. However, as shown in the four figures, the outlet humidity ratio and outlet temperature fluctuate largely as the inlet temperature and inlet pressure increased during the start-up process.
This project was supported by Program for Changjiang Scholars and Innovative Research Team in University (IRT0746) and Program for New Century Excellent Talents at the University of China (NCET-05–0834). References [1] G. Walker, Cryocoolers, Pt. 1 Fundamentals, Plenum Press, New York, 1983. [2] Y. Hou, H.L. Zhao, C.Z. Chen, Developments in reverse Brayton cycle cryocooler in China, Cryogenics 46 (5) (2006) 403–407. [3] F.L. Rosenbush, ECS Schemes for All Electric Airlines. 2nd International Congress on Environmental Systems, No. 820870 in SAE Technical Paper Series, Society of Automotive Engineers, Warrendale, PA, 1982. [4] V.C. Vargas Jose, A. Bejan, Integrative thermodynamic optimization of the environmental control system of an aircraft, International Journal of Heat and Mass Transfer 44 (20) (2001) 3907–3917. [5] I. Pérez-Grande, T.J. Leo, Optimization of a commercial aircraft environmental control system, Applied Thermal Engineering 22 (17) (2002) 1885–1904. [6] J.C. Ordonez, A. Bejan, Minimum power requirement for environmental control of aircraft, Energy 28 (12) (2003) 1183–1202. [7] J. Eichler, Simulation study of an aircraft’s environmental control system dynamic response, Journal of Aircraft 12 (10) (1975) 757–778. [8] R.M. Grabow, T.W. Kreter, G.E. Limberg, Ram air driven air cycle cooling system for avionics pods, in: Proceedings – Society of Automotive Engineers, 1986, pp. 27–34. [9] D.S. Matulich, Aircraft fog control systems, in: Proceedings – Society of Automotive Engineers, 1986, pp. 41–50. [10] S.J. Kline, F.A. McClintock, Describing uncertainties in single sample experiments, Mechanical Engineering 75 (1) (1953) 3–8.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on the performance of an aircraft environmental control system Hongli Zhao a, Yu Hou a,*, Yongfeng Zhu b, Liang Chen a, Shuangtao Chen a a b
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China The First Aircraft Institute of AVIC1, Xi’an 710089, PR China
a r t i c l e
i n f o
Article history: Received 4 July 2008 Accepted 1 May 2009 Available online 10 May 2009 Keywords: Environmental control system Off-design performance Dynamic response Brayton cycle
a b s t r a c t In this paper, the experimental study on the off-design performance and dynamic response of an aircraft environmental control system (ECS) was presented. A bootstrap air cycle refrigeration system with high pressure water separation was employed in the ECS. Both the static test and the dynamic test were conducted on the ECS. The performance of the key components including precooler, recuperator, compressor, and turbine was investigated. The test results show the performance of the tested ECS can meet the requirements of design. However, some parameters including outlet pressure, turbine efficiency, and rotational speed might vary obviously when the operation conditions changed. The outlet humidity ratio and outlet temperature might also fluctuate largely during the start-up process. It is necessary to take into account the off-design performance and the transient performance in the design of ECS. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Reverse Brayton air refrigeration cycle used in the ECS for the aircraft was first advanced by J. Herschel in 1834 [1]. Now the most common refrigerator employed in ECS for aircraft is based on the reverse Brayton refrigeration cycle. Despite the fact that systems based on this air cycle have low efficiency, and their cooling rate capacity at sea level is very low, their main advantages, namely, their low mass, compact size and high reliability, make them to be the most appropriate choice [2,3]. With the development of aircraft, the number of electronic devices used in the aircraft keeps on increasing, which no doubt requires the ECS to increase its cooling capacity. Moreover, the requirement for higher reliability and better performance is placed on the ECS. So far much work has been carried out on the ECS. Vargas and Bejan [4] proposed an integrative approach to the thermodynamic optimization of components for energy systems to maximize the global performance of the system, and they used the cross-flow heat exchanger of the ECS for aircraft as an example to illustrate this approach. Pérez-Grande and Leo [5] adopted two optimization criteria (minimum weight and minimum entropy generation) simultaneously to optimize two finned cross-flow heat exchangers used in the ECS of commercial aircraft. They also employed an integrative approach of optimization. Ordonez and Bejan [6] took into account two basic questions in the thermodynamic optimization of the ECS for aircraft: the minimum power requirement and the features that could be optimized to secure operation at minimum power. They analyzed the robustness of the optimized models as * Corresponding author. Tel./fax: +86 29 82664921. E-mail address: [email protected] (Y. Hou). 1359-4311/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2009.05.002
well. Eichler presented a dynamic response simulation of the ECS, which was intended to study stability, study sensitivity, identify possible problem areas in the design, and serve as a tool to evaluate the dynamic effects of any future design changes [7]. These numerical studies provide effective tools for the design and optimization of ECS. However, due to the complex flight conditions, the operation profiles of ECS will differ from the design. Correspondingly, the chilled air from ECS may not meet the requirements for the flight safety because of the variations of operation conditions, including pressure, temperature, and humidity ratio of the supply air or ram air. Moreover, the temperature, humidity, or pressure of the chilled air may also change greatly during the start-up process and the changing process of flight tasks. Therefore, it is necessary to investigate the variations of thermodynamic parameters during the off-design operation and the dynamic response of ECS. In this paper, both static and dynamic experiments were conducted on a bootstrap air cycle ECS with high pressure water separation. The off-design performance of the system was investigated under operation conditions at different flight altitude. The transient performance of the system was also studied during the start-up process. 2. System description The ECS in the experiments was a bootstrap air cycle refrigeration system with high pressure water separation. Compared with basic refrigeration system, bootstrap air cycle refrigeration system can reach higher efficiency because the latter can recover the work delivered by the turbine to further pressurize the air stream [8]. The pressurization to the air stream may increase temperature
H. Zhao et al. / Applied Thermal Engineering 29 (2009) 3284–3288
drop in the turbine and the flow rate of air stream, thereby increasing the cooling capacity of the ECS. The advantages of the bootstrap air cycle refrigeration system are more prominent in the following situations: at the high altitude when pressure of the air extracted from the engine core is insufficient to meet the requirements of ECS, and the second situation when the aero engine is idled during descending. The use of bootstrap air cycle refrigeration system in ECS can help to avoid the problems caused by the insufficient cooling capacity. To ensure the comfortableness of the cabin and the operation of the electronic devices, the conditioned air should meet the requirement of humidity ratio besides temperature and pressure. And it is necessary to select a proper method for water separation which can keep the effectiveness of water separation as well as the overall performance [9]. The advantages of high pressure water separation are as following: (1) Vapor contained in air is easier to be condensed at high pressure because the dew temperature for given air stream with certain humidity ratio increases with the increase of pressure. (2) The efficiency of water separator in the high pressure water separation system can be improved by 10–15%. In the high pressure water separation system, the vapor is condensed before the turbine, where the speed of the air stream is low. So the size of the condensed droplets is big. However, in low pressure water separation system, the condensed droplets may break into mist because of the high speed of the air stream behind the turbine, and special measures should be taken to collect the water mist and discharge it.
3285
(3) The cooling capacity may be increased. Under the same cooling condition, more water can be condensed at high pressure than that at low pressure. Considering the high efficiency of water separation as well, we could further lower the temperature at the turbine exit because it can be free from the limitation of freezing point, which may reduce the fraction of the air bleed from the engine core as well as the fuel penalty of cooling pack. The ECS consists mainly of two subsystems: the bleed system and the air cycle machine, Fig. 1a. The high-temperature and pressurized air stream which is drawn from the engine is precooled by a primary radiator. A compressor driven by the expander is located at the end of the radiator, in which the pressure and temperature of the air stream is raised. The air stream is then cooled by the bleed air from out-of-cabin in the precooler. After the precooler the air stream enters a recuperator, successively passing through the warm side of heat exchanger, condenser, water separator, and the cold side of heat exchanger. After the recuperator, the air stream expands in the turbine in which its pressure and temperature are lowered. Finally it is discharged into the cabin. High-pressure water separation was employed. In the condenser, the air stream is cooled down to the dew-point, and therefore the vapor contained in the air stream is condensed to water droplets. The condensed water droplets in company with the air stream flow into the separator where the water is discharged. The t–s diagram of the thermodynamic cycle of the ECS was shown in Fig. 1b. In the design of the ECS, optimization of the parameters was conducted under the design condition. The minimum fuel penalty is selected as the objective function, and the parameters chosen as
Fig. 1. (a) Schematic diagram of the ECS. (b) t–s diagram of the thermodynamic cycle.
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H. Zhao et al. / Applied Thermal Engineering 29 (2009) 3284–3288
An uncertainty analysis was performed on the dependent results including efficiency of turbomachineries, and effectiveness of the heat exchangers. The Kline and McClintock method [10] was used which sums the square of errors:
wA ¼
" j X @A i¼1
@zi
wz
2 #1=2
where wA, zi and wz are, respectively, the total uncertainty with the dependent variable A, the independent variable which affects the dependent variable A, and the uncertainty of variable zi. The uncertainties of calculated results are summarized in Table 3.
4. Results and discussions 4.1. Results of static tests
Fig. 2. Outlet parameters under tested conditions.
variables to obtain the objective functions are the expansion ratio, the turbine efficiency, the compressor efficiency, the precooler effectiveness, the recuperator effectiveness, and the condenser effectiveness. 3. Experimental method In the experiments, two air supply systems were employed to provide the two streams in the ECS for aircraft: the main stream to be conditioned and the ram stream. A high-pressure compressor system was used to supply the air stream that has the same pressure, temperature and humidity ratio with air stream extracted from the engine core of the aircraft. The ram stream was supplied by a low-pressure compressor system. The operation profiles were varied to simulate the flight conditions at a series of altitude. The operation parameters are listed in Table 1. The design flight altitude was 300 m, and the outlet temperature was required to be lower than 10 °C. The design of humidity ratio required there was no liquid water condensed at the outlet. As shown in Fig. 1, thermodynamic parameters were measured at eleven positions in the system. In the tests, the measured data included flow rate, temperature, relative humidity ratio, pressure, and pressure drop. The sensors used and uncertainties of the measurements are given in Table 2.
As shown in Fig. 2, the conditioned air stream with the temperature of 7.5 °C was obtained under the design condition in the tests. The humidity ratio of the air stream conditioned was 5.02 g/kg which was lower than the humidity ratio of the saturated air (6.67 g/kg). Thus, the ECS could well meet the design requirements in terms of temperature and humidity. Fig. 2 also shows the outlet parameters of the ECS under the other five off-design conditions from 0 m to 10,000 m. With the increase of flight altitude, the pressure at the outlet changed little until 7000 m. Afterward, the outlet pressure dropped dramatically, and reached 85 kPa at 10,000 m. The decrease of the pressure at the outlet might result from the decrease of inlet pressure (Table 1). As shown in Fig. 2, the outlet temperature increased from 0 m
Table 3 Uncertainties of calculated results. Variables
Maximum uncertainty (%)
Expansion ratio Compression ratio Precooler effectiveness Recuperator effectiveness Condenser effectiveness Turbine efficiency Compressor efficiency
±4.0 ±2.1 ±1.0 ±11.5 ±12.2 ±5.9 ±8.0
Table 1 Operation parameters under tested conditions. H (m)
d1 (g/kg)
p1 (kPa)
t1 (°C)
_ (kg/h) m
t10 (°C)
p10 (kPa)
_ 10 (kg/h) m
0 300 1000 7000 9000 10,000
21.4 21.3 20.9 0.5 1.8 1.0
585.95 571.05 583.96 469.42 383.92 367.5
118.8 118.3 118.8 95.3 92.9 88.7
746.03 735.89 746.6 683.01 583.6 582.47
50.3 78.0 58.9 62.8 7.1 7.0
109.95 130.81 116.7 120.5 101.05 101.03
1429.04 2888.56 2019.58 2289.57 496.99 497.11
Table 2 Uncertainties of measurements. Variables
Sensors
Range
Uncertainty value
Temperature (t1–t11) Pressure (p1, p2, p8, and p10) Pressure drop (Dp2,3, Dp3,4, Dp4,5, Dp5,6, Dp6,7, Dp8,9, and Dp10,11) _ 10 Þ _ 1 and m Mass flow rate ðm Relative humidity ratio u1 ; u4 ; u5 ; and u9
Platinum resistance thermometer (°C) Pressure transducer (MPa) Differential pressure transducers (kPa) Flow meter (kg/h) Humidity transducer
50 to 250 0–0.2; 0–1.0; 0–1.5 0–50 100–2000; 400–4000 0–100%
±0.5 °C ±0.5% ±0.5% ±1% ±2%
H. Zhao et al. / Applied Thermal Engineering 29 (2009) 3284–3288
3287
Fig. 5. Pressure drop in the heat exchangers and water separator. Fig. 3. Efficiencies of the components.
to 300 m, which might be due to the increase of the temperature of ram air (Table 1). Thereafter, the outlet temperature decreased steadily until the altitude reached 9000 m because of the decrease in the temperature of ram air. However, the outlet temperature increased from 9000 m to 10,000 m, which might result from the decrease of inlet pressure. The outlet humidity ratio kept a low level throughout of the region in the tests, and its variation was less pronounced than that of pressure and temperature. This might be attributed to the high-pressure water separation. It is necessary to keep the high effectiveness of the precooler because a large fraction of the cooling of the main air stream takes place in it. Moreover, the condensed water should be removed as much as possible in the water separator. In other words, it is also necessary to keep the high effectiveness of the water separator. Referring to Fig. 3, it is observed that the effectiveness of precooler and water separator was high and varied little in the test region. As shown in Fig. 3, both the recuperator effectiveness and the condenser effectiveness remained almost constant from 0 m to 1000 m. But afterward they increased obviously, which might result from the decrease of the humidity ratio and flow rate of the inlet air. The variation of turbine efficiency and compressor efficiency are also shown in Fig. 3. The efficiency of turbine varies little until
Fig. 4. Expansion ratio, compression ratio, and rotational speed.
7000 m, and then it decreases obviously. On the contrary, the efficiency of compressor was larger than that of the turbine, and varied little throughout out of the test region. The explanation can be found by considering Figs. 3 and 4 together. By inspection, one can see that the variation of turbine efficiency and compressor efficiency were similar with that of expansion ratio and compression ratio, respectively. The decrease of expansion ratio might lead to that of the turbine efficiency. The compressor and the turbine are installed on the same shaft, and therefore have the same rotational speed. As shown in Fig. 4, the rotational speed has similar trend with the expansion ratio. Fig. 5 shows the variation of the pressure drop of the heat exchangers and the water separator as a function of flow rate. As shown in Fig. 5, the pressure drop in all of these four components increased with the flow rate. Moreover, the pressure drop in the precooler was the largest, and the pressure drop in water separator took the second place. The recuperator and the condenser had similar pressure drop. 4.2. Results of dynamic tests As the flight condition varies, the inlet condition of the cooling pack changes correspondingly. To ensure the safety of the flight, the ECS must supply proper temperature air to the cabin and to the electronic devices, within the entire performance envelope of
Fig. 6. Transient inlet temperature.
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H. Zhao et al. / Applied Thermal Engineering 29 (2009) 3284–3288
Fig. 7. Transient inlet pressure.
Fig. 9. Transient outlet temperature.
5. Conclusions In this paper, the bootstrap air cycle and high pressure water separation were discussed, and the off-design performance and dynamic response of a bootstrap air cycle refrigeration system with high pressure water separation were experimentally investigated. The variations of outlet pressure, turbine efficiency, and rotational speed with operation conditions were pronounced. Moreover, there existed fluctuations of outlet humidity ratio and outlet temperature during the start-up process. The results indicate the offdesign performance and dynamic response should be taken into account in the design of the ECS to enhance its general performance and reliability. Acknowledgements
Fig. 8. Transient outlet humidity.
the aircraft and within the entire scale of external conditions of temperature and pressure. So there are specifications on the dynamic behavior which require the system to stabilize quickly and to not exceed specific inlet temperatures to the cabin and equipment which would be harmful to the passengers and to the electronic devices. Moreover, proper flow levels must be maintained and it is necessary that no severe pressure changes reach the cabin which would be uncomfortable to passengers. The dynamic tests were performed to examine the transient performance of the cooling pack. The variations of temperature and pressure at inlet are shown in Figs. 6 and 7, respectively. Figs. 8 and 9, respectively, show the corresponding trend of outlet humidity ratio and outlet temperature. After the inlet temperature and inlet pressure became steady, the outlet humidity ratio and outlet temperature tended to be stable. However, as shown in the four figures, the outlet humidity ratio and outlet temperature fluctuate largely as the inlet temperature and inlet pressure increased during the start-up process.
This project was supported by Program for Changjiang Scholars and Innovative Research Team in University (IRT0746) and Program for New Century Excellent Talents at the University of China (NCET-05–0834). References [1] G. Walker, Cryocoolers, Pt. 1 Fundamentals, Plenum Press, New York, 1983. [2] Y. Hou, H.L. Zhao, C.Z. Chen, Developments in reverse Brayton cycle cryocooler in China, Cryogenics 46 (5) (2006) 403–407. [3] F.L. Rosenbush, ECS Schemes for All Electric Airlines. 2nd International Congress on Environmental Systems, No. 820870 in SAE Technical Paper Series, Society of Automotive Engineers, Warrendale, PA, 1982. [4] V.C. Vargas Jose, A. Bejan, Integrative thermodynamic optimization of the environmental control system of an aircraft, International Journal of Heat and Mass Transfer 44 (20) (2001) 3907–3917. [5] I. Pérez-Grande, T.J. Leo, Optimization of a commercial aircraft environmental control system, Applied Thermal Engineering 22 (17) (2002) 1885–1904. [6] J.C. Ordonez, A. Bejan, Minimum power requirement for environmental control of aircraft, Energy 28 (12) (2003) 1183–1202. [7] J. Eichler, Simulation study of an aircraft’s environmental control system dynamic response, Journal of Aircraft 12 (10) (1975) 757–778. [8] R.M. Grabow, T.W. Kreter, G.E. Limberg, Ram air driven air cycle cooling system for avionics pods, in: Proceedings – Society of Automotive Engineers, 1986, pp. 27–34. [9] D.S. Matulich, Aircraft fog control systems, in: Proceedings – Society of Automotive Engineers, 1986, pp. 41–50. [10] S.J. Kline, F.A. McClintock, Describing uncertainties in single sample experiments, Mechanical Engineering 75 (1) (1953) 3–8.