The transformation of the CFM 56-7 turbofan into a triple flow jet engine; a way of reducing noise pollution.

Cuciumita, Cleopatra ; Petcu, Romulus ; Vilag, Valeriu 等

1. INTRODUCTION

Nowadays, air transportation becomes more and more restrictive, implying low chemical and noise pollution, low fuel consumption and high thrust force. In order to keep up with these restrictions, engineers must come up with new, improved solutions. The main objective of this paper is to investigate the characteristics and the performances of a triple flow jet engine in order to determine the eventual possibilities of improving the performances of jet engines to keep up with these restrictions. The idea is to transform the existing CFM 56-7 turbofan into a triple flow jet engine and to prove that this is a way of reduce noise. Experimental studies using sound absorbing materials on this type of engine resulted in a more silent engine (Ispas, 1991). But antipollution standards evolution implies further reduction of turbojet engines noise. The triple flow jet engine studied here implies the separation of the by-pass flow of a turbofan by adding a new fan stage down-stream of the existing one, as shown in figure no. 1. The calculus is based on the hypothesis that the turbine produces a maximum specific mechanical work at sea level static (SLS). Because the starting point is an existing engine, some of the engine's parameters remain the same. For this new type of engine are then calculated the performances and the outlet velocity for each flow which, combined, give the noise level of the engine, in order to compare them with the existing solution.

[FIGURE 1 OMITTED]

2. CONTENT

The following conditions have to be satisfied for the engine to work properly and to assure the best performances for its configuration (Bejan, 1996; Pimsner, 1983):

* The operating at stationary regime at SLS:

[P.sub.T1] = [P'.sub.C1] (1)

[P.sub.T2] = [P.sub.V2] (2)

[P.sub.T3] = [P.sub.V3] (3)

* The power produced by the turbine has a maximum known value:

[P.sub.Tmax] = [P.sub.T1] + [P.sub.T2] + [P.sub.T3] (4)

* The compression specific work from the primary air flow is known:

[1.sub.C1] = [1'.sub.C1] + [1.sub.V2] + [1.sub.V3] (5)

* According to the mass flow characteristics of the two fans:

[K.sub.2] x [l.sub.C2] [approximately equal to] [K.sub.3] x [l.sub.C3] (6)

* At these equations are added the expressions which connect the power to the related specific work

* The existence of the triple flow jet engine:

[1.sub.C2] > [1.sub.C3] (7)

* Lowest level of noise possible obtained through the mixing of the three flows. Experimental noise studies made on triple flow coaxial cylindrical cells offer us the image of an optimal outlet velocity distribution for this type of engine compared with a double flow one, as shown in figure no. 2.

[FIGURE 2 OMITTED]

Because in this study is intended a comparison with the existing solutions, the new engine also has to check the following hypotheses:

* The exterior fan must produce the same specific mechanical work

[l.sub.C3] = [l.sub.V3] = const (8)

* The maximum diameter of the engine should remain the same:

[K.sub.2] + [K.sub.3] = K (9)

* The transformed engine must provide the same pressure ration on the main flow.

* The new system must develop a maximum specific thrust force. Its optimization leads to:

[K.sub.3] = 4 x [K.sub.3] (10)

According to these hypotheses, the parameters of the CFM 56-7 turbofan from table no. 1 remain constant after its transformation into a triple flow jet engine (Daly, 1997).

In table no.2 are given the results of the thermodynamic cycles' analysis for the main interest parameters in order to obtain a reasonable comparison between an existing double flow jet engine (CFM 56-7) and a theoretical triple flow one (Pimsner, 1983; Berbente et al., 1997)

Based on these results, it can be noticed in figure no. 3 and no.4 that the outlet velocity distribution is similar to the optimum experimentally established.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

3. CONCLUSION

By analysing the results obtained, some conclusions can be withdrawn with relation to the advantages and disadvantages implied by the new type of engine obtained by adding a new fan stage. The main advantage shown by this study is the reduction of the noise pollution by:

* An optimal distribution of the outlet velocity according to the experimental studies, as shown earlier.

* The decrease of the outlet velocity from the main flow with 5.6 %

* The decrease of outlet velocity medium gradient from 45 % to approximately 31 %

* The decrease of velocity gradient between the main flow, consisting of hot burnt gases and the second flow, which can be considered cold air, down to 21 %

Hence, it can be said that the triple flow jet engine is a way of reducing the noise as compared to the double flow existing solution. This research is, though, limited to a theoretical approach in which the efficiency of different components have been approximated based on other experimental data. For a complete image of the problem, the authors consider that experimental tests must be done in the future, to support this theory. The calculus also shows an increase in specific thrust and a decrease in specific fuel consumption with approximately 4%. Furthermore, these advantages are obtained through minimum constructive changes which assume adding a new, intermediate, fan stage. The small improvement given to the engine's performances is lost though, because the engine's weight also rises, which is a first disadvantage that occurs. But the biggest problem, yet to be solve, is that the new system requires a specific work developed by the turbine with approximately 90 kJ/kg more than the existing one. Thus, the real challenge is, as further research, to improve the turbine, by adding a new stage for example, so that it can provide the required specific work for the compressor and both fans.

4. REFERENCES

Bejan, A. (1996). Termodinamica tehnica avansata (Advanced engineering thermodynamics), Editura Tehnica, 73-31-0994-0, Bucuresti

Berbente, C.; Mitran, S & Zancu, S. (1997). Metode numerice (Numerical methods), Editura Tehnica, 73-31-1135-X, Bucuresti

Daly, M. (1997). Aero-Engines, Jane's, Coulsdon, UK

Pimsner, V. (1983). Motoare aeroreactoare (Aero-jet engines), Editura Didactica si Pedagogica, Bucuresti

Ispas, S. (1991). Motorul turboreactor (Turbojet engine), Editura Tehnica, 973-31-0273-3, Bucuresti

CUCIUMITA, C[leopatra]; PETCU, R[omulus]; VILAG, V[aleriu] & STANCIU, V[irgil] *

* Supervisor, Mentor

Tab. 1. Constant parameters for both types of engine

[[pi].sub.C] = 32.7

[1.sub.C1] = 700 kJ/kg

[M.sub.al] = 59 kg/s

K = 5.1

[i.sub.3] = 1725 kJ/kg

[l.sub.V3] = 29.93 kJ/kg

Tab. 2. Comparative parameters results

Double flow jet engine Triple flow jet engine

[1.sub.T]=853 kJ/kg [1.sub.T]=944 kJ/kg

[C.sub.51]=402.9 m/s [C.sub.51]=301.6 m/s

[C.sub.52]=222.6 m/s [C.sub.52]=380.4 m/s
 [C.sub.53]=222.6 m/s

[F.sub.sp]=1538 m/s [F.sub.sp]=1597.6 m/s
F=90737 N F=94260 N

[C.sub.sp]=0.0435 kg/Nh [C.sub.sp]=0.0419 kg/Nh