CONSIDERATIONS ON OPERATION OF A TURBOJET ENGINE WITH THE BYPASS DESIGN

Transcription

1 Proceeding of the 6th International Conference on Mechanic and Material in Deign, Editor: J.F. Silva Gome & S.A. Meguid, P.Delgada/Azore, July 2015 PAPER REF: 5603 CONSIDERATIONS ON OPERATION OF A TURBOJET ENGINE WITH THE BYPASS DESIGN Miroław Kowalki () Air Force Intitute of Technology, Warzawa, Poland () mirolaw.kowalki@itwl.pl ABSTRACT The paper analyze operation of a turbojet engine with the bypa deign with derivation of formula uitable for calculation of key operational parameter, i.e. work of the thermal cycle, unit thrut, unit conumption of fuel. The invetigation included imulation tudie carried out with ue of the K-15 engine of the Polih make ince it i the engine with the air bleed of the bypa type. The tudie demontrated that light benefit can be achieved for the engine operation, in particular with regard to it cot efficiency and uch benefit increae in pace with the growth in the ma flow of air through the engine channel. Keyword: adaptation engine, parameter of the engine air. INTRODUCTION One of the mot appealing direction in development of turbojet engine i repreented by the o called adaptation engine (where the term engine with the variable thermal cycle i alo ued in relevant literature reference). That cla of engine comprie a ingle-rotor turbojet engine of the bypa type where a portion of compreed air i conveyed via a dedicated bypa channel from downtream the engine compreor to the area downtream the turbine for the maximum range of the engine operation (uch engine are deigned by both Boeing and Pratt & Whitney Fig. 1) Fig. 1 - Diagram of the bypa-type turbojet engine: 1 bypa channel for air conveyed from downtream the compreor to the area downtream the turbine; 2 adjutable compreor, 3 combution chamber with low level of toxic component emiion in exhaut ga; 4 dicharge jet with the thrut reverer. For uch an engine at the maximum range of it operation (full thrut) about 25% of the air tream that leave the compreor i bled to the bypa channel. The more the flow i throttled, the le percentage rate of air i bled from downtream of the compreor in order to maintain the contant flow intenity of exhaut ga paing through the turbine. It allow keeping the engine rpm at the deired level of 100% within the ufficient range of variation for the total and unit thrut

2 Track_G Mechanical Deign and Prototyping The peculiarity of uch a olution conit in the exitence of external bypa channel with round cro-ection and a control ytem to adjut air bleeding depending on the aircraft peed of flight. It enable improvement of the engine efficiency, lowering the noie level and emiion of toxic component in exhaut ga. Another advantage of that olution i it relatively low complexity a compared to tandard olution, which ugget the opportunity to apply it to engine that are currently in ue. ENERGY BALANCE The real thermal cycle for the bypa engine i a typical cycle of a jet engine, where the working cycle area varie a a function of air amount that i bled from downtream the compreor and delivered to the area downtream the turbine Fig. 2. Fig. 2 - The real thermal cycle of a bypa turbojet engine Overall work for the thermal cycle of that engine can be calculated in the following form: where: ' B l = l 1+ l c c η l 2 ' p p m νϕ c T obi ob 1 D p H ' c ηη p m m& up ν = - relative amount of air (air rate) that i bled from downtream of the m& compreor ( m& - portion of air that i bled from downtream of the compreor to the area downtream the turbine); ϕ D - factor of velocity lo in the dicharge jet; c, - pecific heat of air and exhaut ga, repectively; p c p T H m,η - overall temperature of air on the intake cro-ection of the compreor; η - mechanical efficiency of the engine and the compreor, repectively; l L - degree of equivalent heating for a jet engine; = - breakdow L k 1 ( L - the the air to up ( ) wn of work for a compreor with air portion of the compreor work that i neceary to compre be bled, L - total work of the compreor); bleeding (bypa) k B=Π 1- the contant component that depend on the compreion factor Π of the compreor (k exponent of the ientropic curve, for air k=1.4). Application of the relationhip for the energy balance of the flow and the thermal balance for the combution chamber lead to formulation of other relationhip, including the one for the unit thrut and for unit conumption of fuel, where the graphic curve for thee relative -582-

3 Proceeding of the 6th International Conference on Mechanic and Material in Deign, Editor: J.F. Silva Gome & S.A. Meguid, P.Delgada/Azore, July 2015 parameter are exhibited in Fig. 1. It wa found out that both parameter are ubject to gradual drop but the decreae rate i higher for the unit conumption of fuel, which confirm economical propertie of the bypa engine. A a conequence, higher efficiency factor are achieved for engine with higher bypa ratio, which i hown in Fig. 4. Fig. 3 - Relative drop of the unit thrut and the unit conumption of fuel for bypa engine Fig. 4 - Curve for the overall relative efficiency of a bypa engine IMPLEMENTATION OF THE BYPASS AIR BLEEDING TO THE K-15 ENGINE Deign olution for the air bleeding channel uitable to bypa engine ugget that any of the relevant engineering mean can be applied to upgrade the K-15 turbojet engine. However, imulating computation have demontrated that the condition for collaboration between the engine compreor and the turbine impoe ome contrain, both to the amount of bypaed air and the tarting point when the bypa channel i activated. A typical graphic curve for acceleration of the K-15 engine, plotted with the ue of the dedicated imulation model, i hown in Fig. 5. In turn, Fig. 6 explain how application of the air bypa of that kind ha led to change in the acceleration curve for that engine. The reult revealed in thi paper were achieved for 10% of the amount of air being bled to the bypa channel for the maximum and teady range of the engine operation (i.e. 94.5% of n max ). It wa found out that the air bleeding lead to lowering of the line for collaboration between the turbine and the compreor with imultaneou increae of the margin for teady operation of the engine. It reult from the fact that le power of the turbine i tranmitted to the compreor, which, in conequence, reduce the compreion factor achievable for the compreor. Thu, the maximum amount of air to be bled in that way i ubject to certain limitation and, in the cae of the K-15 engine, it reache 10 15% of the total flux of air that i dicharged from the compreor. When the ratio of air being bled to the bypa channel keep growing and reache more than 20%, the collaboration condition are no longer maintained, which i hown in Fig. 7. Similar effect can be achieved when the ame and already recommended amount of air bleeding i preerved with imultaneou decreae of the engine operation range (engine rpm) for which the bypa channel i being opened. Fig. 8 correpond to the ituation in which the collaborating condition are dirupted when 10% of the total amount of air dicharged from the compreor i being bled to the bypa channel but the bleeding valve i being opened at the rpm range below 90% of n max

4 Track_G Mechanical Deign and Prototyping Fig. 5 - Graphic interpretation for acceleration of the K-15 engine (without air bleeding); 1 threhold for teady operation, 2 acceleration, 3 line of collaboration in the teady tate. Fig. 6 - Graphic interpretation for acceleration of the K-15 engine where 10% of air i bled into the bypa channel at the maximum teady operation range (i.e. ca. 94.5% n max ) Fig. 7 - Graphic interpretation for acceleration of the K-15 engine where 20% of air i bled into the bypa channel at the maximum teady operation range (i.e. ca. 94.5% n max )

5 Proceeding of the 6th International Conference on Mechanic and Material in Deign, Editor: J.F. Silva Gome & S.A. Meguid, P.Delgada/Azore, July 2015 Fig. 8 - Graphic interpretation for acceleration of the K-15 engine where 10% of air i bled into the bypa channel at the operation range of about 88% n max ). The foregoing phenomenon i caued by the fact that the output power of the turbine decreae and become inufficient to drive the engine compreor, it auxiliary power pack and to overcome mechanical reitance, which finally lead to diruption of the condition for the compreor and turbine collaboration and to engine hutdown. The parameter that make it poible to evaluate quality of the engine acceleration i the duration time of the acceleration proce. The impact of the air bleeding rate onto the relative time of the engine acceleration proce achieved for the bypa engine i hown in Fig. 9. Fig. 9 -Variation of the relative time of the engine acceleration a a function of a relative amount of air being bled from downtream the compreor to the area downtream the turbine Due to intrinic characteritic parameter pecific for individual deign ubaemblie of the engine, the acceleration time could be meaured from the minimum engine peed of about 10,500 rpm, which i ca. 66% of n max. It i the real range that can be monitored when the acceleration time of the K-15 engine i meaured during it regular operation (except for full-range acceleration). According to the technical documentation of the engine, that time period meaured for the range from 66% to 95% of n max hould not exceed 4 econd. However, due to the need to reveal the actual impact of air bleeding to the bypa channel onto the acceleration time of the engine, the imulation computation were pread onto the range from 66% to 99.5%. It wa caued by the fact that bleeding at the maximum rpm for teady operation of the engine (n 94.5% n max ) ha nearly no impact onto the acceleration time, regardle the bypa rate. The actual impact wa oberved only for the range above -585-

6 Track_G Mechanical Deign and Prototyping 95% of n max, which i the range that cannot be meaured during regular operation of the engine. In addition, the acceleration time wa meaured for the amount of air being bled to the bypa channel ranging from 0 to 15% of the total air amount dicharged by the compreor. Exceeding of the threhold of 15% led to diruption of the condition of collaboration between the compreor and the turbine, which diabled meaurement of the acceleration time. The analyi of Fig. 9 make it poible to find out that the acceleration time increae in pace with the amount of air being bled into the bypa channel. The impact i lightly more ignificant (up to nearly 2%) when the rate of bypaed air i le than 5%. Finally no ignificant prolongation of the acceleration time wa revealed due to implementation of uch air bleeding method and no ubtantial retardation for the engine to reach it maximum operation range wa detected. RESULTS AND CONCLUSIONS The imulation modelling demontrated that implementation of air bleeding facilitie i poible for exiting turbine jet engine. It wa confirmed that air bleeding to the bypa channel can tart at the engine operation range (engine rpm) even lightly le than the maximum rpm, which ugget the opportunity to achieve even more benefit, chiefly due to reduction of the fuel conumption per power unit. Unfortunately, it i till aociated with ome contrain that directly depend on the amount of air that can be bypaed via external channel from downtream the compreor to the area downtream the turbine. Such amount can be increaed in pace with the total flux of air through the main flow channel of a jet engine. It mean that more ignificant benefit from air bleeding olution of that kind can be achieved for large-ized jet engine. It i why application of thee deign approache i deemed uitable to power uperonic paenger and cargo aircraft. There are alo attempt to ue uch engine to power uperonic trategic bomber or trike fighter. There alo idea to apply the operation principle of adaptation and tream jet engine to power hyperonic aircraft capable of reaching the flight velocity Ma>4. REFERENCES [1]-Kowalki M.: Analiza termogazodynamiczna parametrów użytkowych turbinowego ilnika odrzutowego typu bypa. Rozprawa doktorka, Warzawa (Thermodynamic analyi of operational parameter attributable to a bypa turbojet engine. PhD thei, Waraw 1995). [2]-Kowalki M., Orkiz M., Szczecińki S.: Napęd lotniczy typu bypa. (Avionic drive of the bypa type). WPT 3/1993. [3]-Kowalki M., Orkiz M.: Wrażliwość pracy obiegu turbinowych ilników adaptacyjnych. III Sympozjum naukowe - WSOSP, Dęblin (Senitivity factor for work of thermal cycle in adaptation turbojet engine. The 3 rd Scientific Sympoium Air Force Officer High School, Dęblin 1996). [4]-Muzyńki M, Orkiz M.: Modelowanie turbinowych ilników odrzutowych. Biblioteka Naukowa Intytutu Lotnictwa, Warzawa (Modelling of turbojet engine. Scientific library of the Air Force Intitute of Technology, Waraw, 1997). [5]-Orkiz M.: Charakterytyki użytkowe turbinowych ilników odrzutowych, a tateczność pracy prężarki. (Operational characteritic of turbojet engine and operation tability of their compreor), WOSL, Dęblin