Evolution of Compression Processes in Aero-Engine Thermal Cycles

Transcription

1 The Oen Aerosace Engineering Journal, 2008,, -7 Evolution of Comression Processes in Aero-Engine Thermal Cycles Y. Daren, T. Jingfeng * and B. Wen School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 5000, China Abstract: In order to rovide a basis for the establishment of an alicable strategy for the develoment of aero-engine thermal cycles, the evolution of comression rocesses in aero-engine thermal cycles is reviewed in this aer by analyzing the comression requirements for the injection or extraction of energy. It is therefore concluded that the injection of energy in the comression is required for alications in low seed ranges, for examle, the injection of energy in turbojet comressors; the method of adiabatic comression is desirable for alications in intermediate seed ranges, for examle, shock interactions for ramjets/scramjets; and the extraction of energy is needed for alications in high seed ranges, for examle, the extraction of energy in ramjets with energy-byass. The injection or extraction of energy in the comression heavily deends on the aero-engine erformance required for alications in different seed ranges. Keywords: Aero-engine thermal cycle, comression rocess, energy injection or extraction, turbojet, ramjet/scramjet, ramjet with energy byass. INTRODUCTION As the flight seed advanced from subsonic to suersonic, and even hyersonic, significant changes took lace in aero-engine thermal cycles, esecially in their comression rocesses. Theoretically, maximum overall engine efficiency deends on the maximum amount of work which can be added in the comression rocess of a turbojet (Riggins JPP 2004 []. Addition of work is usually ossible in a comressor designed with overall engine efficiency taken into consideration. However, the ermissible amount of work which can be added in the comression rocess decreases with the increasing flight Mach number, because the thermal limit of material at the combustor exit sets an uer limit on airflow total temerature. In addition, the use of a mechanical comressor is no longer needed for the comression at a higher seed, as airflow stagnation increases significantly with the increasing flight Mach number. Consequently, a ramjet without a work interaction device becomes a referred choice for alications with the flight seed, for examle, the Mach number Ma > 3. As it becomes more and more difficult to achieve the airflow comression required for subsonic combustion in the ramjet, esecially for hyersonic alications, a suersonic combustion ramjet (scramjet, in which the suersonic combustion reduces the airflow comression, become a referred choice for hyersonic alications. When the flight seed increase further, for examle, Ma > 9, there are inevitably some technological challenges to the ramjet/scramjet comression. In order to ensure sufficient air/fuel mixing and stable combustion, the Mach number at the combustor inlet must be ket in a ermissible range. In a hyersonic case, the comression of inlet airflow must be sufficient enough to satisfy this Mach number limit at the cost of an unaccetable comression loss and a significant *Address corresondence to this author at the School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 5000, China; tangjingfeng@hcms.hit.edu.cn increase in the static temerature at the comression exit, which deteriorate the erformance of ramjet/scramjet. If the comression is mitigated, the Mach number at the combustor inlet will be too high to achieve sufficient air/fuel mixing and stable combustion over a ractical combustor length. Therefore, a ramjet with energy-byass is roosed as a solution of above mentioned roblems (Fraishtadt Tech Phys 998 [2], and its fundamental idea is that art of flow energy is extracted from the airflow field ustream of the combustor and then returned downstream of the combustor. The combination of an ustream energy extraction and a downstream energy return rocess is the so-called energy-byass. The incoming airflow goes through the rocess of comression, energy extraction, combustion, energy return and exansion in such an engine. Deending on the ustream energy extraction, the comression is ket under a certain level with an accetable rocess loss, and the airflow field in front of the combustor can be controlled to ensure a ermissible Mach number at the combustor inlet. With the downstream energy injection, the efficient use of energy is achieved. Such oerations imrove the comression, combustion and eventually engine erformance. The ramjet with energy-byass can be realized with the hel of magneto-hydrodynamics (MHD devices (Sheikin Tech Phys 992 [3] (Sheikin Tech Phys 993 [4]. In a MHD device, the axial body forces on flow elements are induced by the erendicular electric and magnetic fields existing in the transverse lane. Deceleration or acceleration of airflow is caused by these body forces with electrical ower generated or consumed onboard. Such an engine was first roosed under the name of AJAX (Kuranov AIAA 200 [5] (Gurijanov AIAA 996 [6], and another was studied under the name of MHD-Arc-Ramjet in which an arc heating device was used to return energy (Tang AIAA 2006 [7] (Tang AIAA 2007 [8]. In order to make MHD interactions effective, the areciable electric conductivity of airflow is achieved using equilibrium ionization technologies at high enough flow temeratures (Park JPP 200 [9], or nonequilibrium ionization technologies at medium flow tem / Bentham Science Publishers Ltd.

2 2 The Oen Aerosace Engineering Journal, 2008, Volume Daren et al. eratures (Sergey AIAA J 200 [0] (Sergey JPP 2002 [] (Park JPP 2003 [2]. The characteristics of the ramjet with energy-byass is shown in Fig. ( (seeing (Tang AIAA 2006 [7] for further details. The erformance analyses in revious work have demonstrated that the erformance of the ramjet with energy-byass is better than ramjet/scramjet, esecially for the flight with Ma > 9, and such an engine exands the scoe of alication in a higher flight Mach number. (Park JPP 200 [9] (Park JPP 2003 [2] (Litchford JPP 200 [3] (Park JPP 200 [4] (Burakhanov JPP 200 [5] (Kuranov JSR 2003 [6] (Heiser JPP 2005 [7] (Sheikin AIAA 2005 [8] T Temeratur e Temeratur e material Temeratur e MHDA ' 2' 2m limit limit limit for for for ARC 3' 3'' 3"' 3m Fig. (. Temerature-entroy cycle diagram of ramjets ('-2'-3'-4'- ': conventional ramjet; '-2'-2m-3''-3m-4''-': MHD accelerator case; '-2'-2m-3'''-3s-3a-4'''-': ARC case; rocess 2' 2m: energy extraction; rocess 3'' 3m and 3s 3a: energy injection. To sum u, the turbojet cycle based on a mechanical comression with energy injected and a mechanical exansion with energy extracted, is alicable to alications with Ma = 0.5 ~ 3. The ramjet /scramjet cycle without the injection or extraction of energy in the comression, is suitable for alications with Ma = 3 ~ 9. The cycle of the ramjet with energy-byass is a variant of Brayton thermal cycle for alications with Ma > 9, in which art of flow energy is extracted from the airflow field ustream of the combustor and then returned downstream of the combustor. So, it is worthwhile to discuss when the injection or exaction of energy is necessary in the comression rocess of an aeroengine thermal cycle, because it is believed that this discussion can leads to a good understanding of the develoment trend of aero-engine thermal cycles so that an alicable strategy can be established for the develoment of aeroengine thermal cycles. The evolution of comression rocesses in aero-engine thermal cycles is reviewed in this aer by analyzing the comression requirements for the injection or extraction of energy. 3s 4' 3a 4'' 4"' S COMPRESSION REQUIREMENTS FOR INJECTION OR EXTRACTION OF ENERGY An ideal Brayton thermal cycle is made u of an isentroic comression rocess ( 2, an isobaric combustion rocess (2 3 and an isentroic exansion rocess (3 4. According to the aero-thermodynamic theory, the secific thrust of a Brayton cycle engine can be exressed as, F s = v 4! v = 2L id + v 2! v ( where F s is the secific thrust of engine; L id is the secific work rovided by the ideal Brayton cycle; and v is the flow seed. Based on the thermodynamic rincile, the following can be easily derived, " # " L id = C T (! # ( $ " # "! # (2 where! = t 2 is the ressure ratio;! = T t 3 T is the temerature ratio; C is the constant ressure secific heat;! is the secific heat ratio; and, t,t,t t are the static ressure, total ressure, static temerature and total temerature resectively. From equation 2: (A The influence of factor! = t 2! "[,+# ( : when! =, L id = 0, which can t occur at zero of cycle thermal efficiency; when! =! max, L id = 0, which means the temerature at the comression exit reaches the limit imosed by the tye of material and cooling methods, and the injection of energy is imossible in the combustion rocess. So L id aroaches its maximum when! o = ( T t 3 T " 2(" #. (A2 The influence of factor! = T t 3 T : L id benefits from the increase of!, as T t 3 increases at! under a fixed T, and L id is imroved as more heat is injected for a larger T t 3. From equations and 2, it can be known that F s benefits from the increase of!, and F s reaches its maximum in the case of! o. For the design of engine, the tye of material and cooling methods determine the value of! and the corresonding value of! o. For the function of engine, factor! can be divided into two arts using exression! = t 2 t t. One art

3 Evolution of Comression Processes in Aero-Engine Thermal Cycles The Oen Aerosace Engineering Journal, 2008, Volume 3 t = (+ k! 2 Ma k 2 k! is about the characteristics of incoming airflow, and the other art t 2 t is related to the comression mechanism. Under different incoming flow conditions, i.e. different t values, factor t 2 t must be adjusted to make! aroach! o for the otimal gain of engine secific thrust. The degree of the comression required can be measured in terms of factor value of t 2 t t 2 t. Qualitatively a larger is needed in the case of a less t, and a less value of t 2 t is desirable in the case of a larger t. It is known that the value of factor t 2 t is related to the change in total ressure exerienced in the comression rocess. With the hel of Gibbs equation (Bejan Wiley988 [9] the definitions of total ressure and total temerature, the change in total ressure in the comression rocess can be exressed as, ds irr = C! d(lnt " R!d(ln = C!d(lnT t " R!d(ln t where ds irr is the differential of entroy; R is the gas constant; and d( ln, d( ln t, d( lnt, d( lnt t are the differentials of corresonding factors. (3 From equation 3, the anticiated change in total ressure in the comression rocess, i.e. the needed t 2 t, can be achieved as the following indications, (B For an ideal or isentroic comression, ds irr = 0, so C! d(lnt t = R! d(ln t, which means the total ressure increases as energy is injected, and the total ressure decreases as energy is extracted; (B2 For a real comression, ds irr = C! d(lnt t " R! d(ln t > 0, which means more energy than what is needed for the ideal comression must be injected at the same total ressure increment, and less energy will be extracted at the same decrease in total ressure; (B3 For an adiabatic comression, d ln(t t = 0, so ds irr =!R " d ln( t, which means the decrease in total ressure will be caused by the increase in entroy for rocess losses. DEMONSTRATION OF COMPRESSION REQUIRE- MENTS FOR INJECTION OR EXTRACTION OF EN- ERGY Comression Required for Turbojets The temerature at the turbine inlet must be ket within a ermissible range for the safe oeration of a turbojet engine. Table lists the secifications of some 3rd-generation turbofan engines characterized with T t3 in the range of 600 ~ 800 K. Corresonding to T t3, factor! o is 20. ~ 2.4 at sea level, 3.4 ~ 33.5 at an altitude of 0 km and 33.~35.3 at an altitude of 20 km resectively. It can be seen from Fig. (2 that for the alications listed in Table! is always located within! o range when Ma = 0.5 ~ 3, excet M53-P2 engine of Mirage aircraft. It can also be seen that! is always greater than t. In Table. Secifications of Turbojet Engine Alications Country Engine Tye Aircraft Alication! T t3 /K Aircraft Max (v /km h - Aircraft Max (Ma F00-PW-220 F F0-GE-00 F ~ USA F404-FD2 F F404-GE-402 F/A F0-GE-02 B-B F0-GE AL-3F Su Russia or USSR RD-33 MIG D-30F6 MIG France M53-P2 Mirage China PRC Taihang Taihang growth versions

4 4 The Oen Aerosace Engineering Journal, 2008, Volume Daren et al. order to obtain!, the degree of the comression required for turbojets is formulated as inequality t 2 t >. t 2 t π o π o π o t Fig. (2. Relationshi between ressure ratio and flight Mach number for turbojet alications. According to the indication (B of equation 3, the comression required can be obtained by the imlementing the increase in total ressure induced by the injection of energy. The injection of energy is achieved in turbojet engines by a comressor in which the incoming flow is comressed simultaneously when energy is injected into the flow field, and the energy for the comressor comes from the energy extracted from a turbine. It can also be seen from Fig. 2 that if comression loss is considered, more energy is needed by the comressor design. Comression Required for Ramjets/Scramjets Ma For a ramjet/scramjet engine, the determination of the total temerature at the combustor exit requires a combination of elaborate comutations and exerienced judgments, because such a temerature deends on many interrelated t variables such as engine configuration, combustion detail, heat transfer and cooling method. Although there are many considerations, T t3 must be ket within a ermissible range for an accetable engine design. The T t 3 range used in this aer is 2 500~8 500 K. (Heiser AIAA ES 994 [20] The corresonding! o is 64.6 ~ at sea level, 05.6 ~ 8.8 at an altitude of 0 km and.9 ~ 85.5 at an altitude of 20 km resectively. The military nature of the alications of the ramjet/scramjet makes it difficult to have access to the detailed descritions of the ramjet/scramjet. Data concerning the secific comression issues in this aer come from the ublished information about comression configurations and working conditions. Listed in Table 2 are some comression details, as the results of CFD analysis done with Fluent tools. (Kuranov JSR 2003 [6] (Rozario AIAA 2007 [2] (Emami NASA 995 [22] (Patrick JPP 996 [23] (Ding AIAA 200 [24] (Rodriguez JPP 2003 [25] (Voland AIAA 999 [26] (Ault JPP 994 [27] (Van JPP 996 [28] (Chan JSR 995 [29] t It can be seen from Fig. 3 that! is always less than in the range of Ma = 3 ~ 9. In order to obtain!, the degree of the comression required for ramjet/scramjet engines is formulated as inequality t 2 t <. In ramjet/scramjet engines, incoming flow is comressed by adiabatic shock interactions without the injection or extraction of energy, and the decrease in total ressure is caused by the increase in entroy induced by losses in the comression rocess. As shown in Fig. (3 that! are located within! o range when Ma = 3 ~ 9, the decrease in total ressure resulting from adiabatic shock interactions rovides the comression required for the ramjet/scramjet alications in this seed range. Moreover, it can be seen from Fig. (4 that adiabatic shock interactions rovide ram- Table 2. Comression Details of Ramjet/Scramjet Engines Data Label Ma /Pa T /K t t 2 t ( x0-2 t 2 Data Source Ref. [2, 22] Ref. [23] Ref. [24] Ref. [25, 26] Ref. [27, 28] Ref. [29] Ref. [6]

5 Evolution of Comression Processes in Aero-Engine Thermal Cycles The Oen Aerosace Engineering Journal, 2008, Volume 5 jet/scramjet engines with accetable changes in total ressure in the comression rocesses when Ma = 3 ~ 9, which are also significantly demanded for the design of ramjet/scramjet engines. t 2 log( t log( t π ο δ =8 π δ = 22 energy and the increase in entroy can all cause the decrease in total ressure. Thus the comression required for alications with Ma > 9 can be rovided in a way that the decrease in total ressure is mainly caused by the extraction of energy, but not by the increase in entroy. This means that the dee decrease in total ressure can be mainly caused by the extraction of energy, and most imortantly the rocess entroy increment would also be ket in a ermissible level t 2 t choice for δ =8case π o 6 Fig. (3. Relationshi between ressure ratio and flight Mach number for ramjet/scramjet alications. Comression Required for Ramjets with Energy-Byass It can be seen from Fig. (3 that when Ma > 9,! is always less than t, and! locates beyond! o range. This means that although the decrease in total ressure is caused by the increase in entroy induced by shock comression losses, the decrease in total ressure can not rovide the comression required for alications with Ma > 9. In order to make! aroach! o, more decrease in total ressure than that listed in Table 2 is needed. Thus when Ma >9, inequality t 2 t < can be exlained as a dee decrease in total ressure. M a Ma M a This comression required can be simly rovided by a dee comression. More losses in the dee comression rocess result in more decrease in total ressure. More decrease in total ressure rovide the comression required so that the dee comression makes! aroach! o. Simultaneously the corresonding t 2 t of the dee comression will be the order of magnitude of 0 - or even less, as shown in Fig. (4. For adiabatic shock interactions, such t 2 t results in a significant entroy increase which is detrimental to engine erformance. Therefore, the dee comression can not rovide a comrehensive solution to the airflow comression when Ma > 9. It can be seen from the above analyses that both the dee decrease in total ressure and an accetable rocess entroy increment are needed when Ma > 9. In this case, the comression required can be re-exlained as the combination of the dee decrease in total ressure and the accetable rocess entroy increment. Based on equation 3, the extraction of 2 7 t2 choice for δ = 22 case t Ma Fig. (4. Relationshi between total ressure ratio and flight Mach number for ramjet/scramjet alications. The ramjet with energy-byass, which has the combination of the adiabatic shock comression rocess and the energy extraction rocess ustream of the combustor, can rovide the comression required for alications with Ma > 9. Moreover, the return of the extracted energy downstream of the combustor is achieved for the efficient use of energy, since the extracted energy is sourced from the movement of engine/aircraft. It should be noted that Ma = 9 is used here as the terminal of the alication range. In fact, the determination of this value requires a combination of detailed comutations and comarative analyses of engine erformance for with or without energy-byass. The use of this secific value in this aer is only because as shown in Fig. (3,! locates in! o range when Ma = 3 ~ 9 and! locates beyond! o range when Ma > 9. It is assumed for intuitive demonstration that the incoming flow is comressed through four rams with equal shock angles!, and then rocessed with or without the energy extraction rocess under different conditions. It can be seen from Fig. (3 that the achievement of the extraction of energy would be desirable for the comression required when Ma is greater than a secific value in the different! case. DEVELOPMENT TREND OF COMPRESSION PROCESSES IN AERO-ENGINE THERMAL CYCLES As discussed above, the injection of energy is required for the turbojet comression; the method of adiabatic shock interactions is desirable for the ramjet/scramjet comression; and the extraction of energy is needed in the ramjet with energy-byass.

6 6 The Oen Aerosace Engineering Journal, 2008, Volume Daren et al. Thus, with the increase in the flight Mach number, the comression rocesses of aero-engine thermal cycles consist of the comression with energy injection in the case of low seed ranges, the adiabatic comression in the case of intermediate seed ranges, and the comression with energy extraction in the case of high seed ranges. The injection or extraction of energy in the comression rocess aims at the realization of the engine erformance required for alications in different seed ranges. It should be noted that when extraction of energy is required in the comression rocess for alications with Ma > 9, ramjet with energy-byass is one of the ossible solutions, and a new tye of engine can be develoed when a new way of energy extraction is available for hyersonic alications. DISCUSSION There are many data sources about turbojet alications in the oen literature as well as very limited descritions about ramjet/scramjet alications. Due to commercial cometitions or military secrets, it is no doubt that there are differences between the data adoted in this aer and the real engine attributes. We cannot guarantee the accuracy of ublic data. However, these data differences have little effect on the viewoint of this aer, because this work objective is to analyze the ublic data and demonstrate the general trends, but not to discuss any articular design. Furthermore, these data differences may cause the imrecise determination of the terminal of those engine alication ranges. The recise determination of the terminal of engine alication ranges is not rational in real life, since there are always common alication ranges for two tyes of engines. Just like the range of Ma =.5 ~ 3 as the common range of turbojet and ramjet/scramjet alications, there must also be a common flight seed range, in which ramjet/scramjet with or without energy-byass are develoed for different alications. Furthermore, just like the range of Ma < 3 as the dominant range of turbojet alications, there must also be a secific range for the alications of ramjet with energybyass, which can be exressed as Ma > 9 or a faster seed range. In this aer, the uses of the secific values as the terminal of those engine alication ranges are only due to the data locations shown in Figs. (2-4. CONCLUSIONS The evolution of the comression rocesses in aeroengine thermal cycles is reviewed. The theoretical analyses about the comression rocesses show that the injection or extraction of energy in the comression rocess heavily deends on the aero-engine erformance required for alications in different seed ranges. The injection of energy in the comression rocess is required for alications in low seed ranges, for examle, the injection of energy in turbojet comressors; the method of adiabatic comression is desirable for alications in intermediate seed ranges, for examle, shock interactions for ramjets/scramjets; and the extraction of energy is needed for alications in high seed ranges, for examle, the extraction of energy in ramjets with energy-byass. ACKNOWLEDGEMENTS This work is funded by National Natural Science Foundation of China under contract No , Hi-Tech Research and Develoment Program of China under contract No. 2002AA72205, and Aeronautics Technology Suort Foundation under contract No. Hagong07. The areciation will also be given to Dr. E. G. Sheikin of hyersonic systems research institute of Russia for the academic communication, and rofessor Y. Jialu of Harbin institute of technology of China PRC for the instructions. REFERENCES [] D.W. 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7 Evolution of Comression Processes in Aero-Engine Thermal Cycles The Oen Aerosace Engineering Journal, 2008, Volume 7 [2] D. Rozario, and Z. Zouaoui, Comutational fluid dynamic analysis of scramjet inlet, AIAA Paer, [22] S. Emami, and C.A. Trexler, Exerimental investigation of inletcombustor isolators for a dual-mode scramjet at a Mach number of 4, NASA Technical Paer, [23] E.R. Patrick, E. Saied, and A.T. Carl, Unsteady ressure behavior in a ramjet/scramjet inlet, J. Pro. Power, vol. 2(3, , 996. [24] M. Ding, L. Hua, and F. Xiao, Two-dimensional viscous simulation of a inlet-isolator flows at mach number 4, AIAA Paer, [25] C.G. Rodriguez, Comutational fluid dynamics analysis of the central institute of aviation motors/nasa scramjet, J. Pro. Power, vol. 9(4, , [26] R.T. Voland, A.H. Auslender, M.K. Smart, A.S. Roudakov, V.L. Semenov, and V. Kochenov, CIAM/NASA mach 6.5 scramjet flight and ground test, AIAA Paer, [27] D.A. Ault, and D.M. Van Wie, Exerimental and comutational results for the external flowfield of a scramjet inlet, J. Pro. Power, vol. 0(4, , 994. [28] D.M. Van Wie, and D.A. Aultt, Internal flowfield characteristics of a scramjet inlet at mach 0, J. Pro. Power, vol. 2(, , 96. [29] H.C. Chan, C.K. Suk, J.D.W. Kenneth, and T.N. Henry, Numerical analysis of hyersonic low-density scramjet inlet flow, J. Sacecra. Rocke., vol. 32(, , 995. Received: October 3, 2007 Revised: January 5, 2008 Acceted: January 23, 2008