TAILIEUCHUNG - Turbo Machinery Dynamics Part 2

Tham khảo tài liệu 'turbo machinery dynamics part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | AIRCRAFT POWER PLANT 23 increasing the bypass ratio from 0 to 5. Ideally the engine must have a fan pressure ratio of about 3 but this would call for two or more fan stages. In practice only one fan stage is used to lower weight and fan noise causing the core jet velocity to be much higher than the fan duct discharge velocity. Military engines such as TF-39 for C5A transport planes do not have such restrictions have one-and-a-half fan stages and hence a bypass ratio of 8 is used. Note that the best value for the bypass ratio of a given aircraft also depends on the engine and fuel weight noise and installation drag. Turbofan engines are ideal for subsonic applications. Supersonic conditions call for an afterburner where the combustion of the mixed fan and gas generator exhausts occurs. The fan pressure ratio will then be subject to the conditions pt6 pt3 or Pf PcPt and tf tctt. If aft combustion raises the mixed streams temperature to da the velocity at exhaust also is the same. Thrust expression then becomes F l20a 0 Tf_1 _ M dm dt a 1 a e Tf Y _ 1 M Equation is modified for tf 0t 00 1 a- Tc Tf 0t Tc 000 Specific impulse takes the form I 29g Í - _1 T Y a0h gCpTo da-00 Ml Engine performance for a 1 and Pc of 24 at M0 0 is shown in Fig. . For comparison the non-after-burning performance from Eqs. and is also shown. Note that Pf varies with M0 for the given matching items and a 1 but in reality a also varies with M0. The thrust ratio of afterburning or thrust-augmenting to non-after-burning case is large which has merit if the requirement is for a subsonic cruise followed by a supersonic burst in speed. Turboprop engines have similarities with the turbofan version but have a considerably higher bypass ratio and consequent propulsive efficiency. A number of qualitative differences exist between them. The propeller does not have a diffuser so the tips are exposed to the airflow that combines the velocity of aircraft and the blade s own .

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