Salimzhan A. Gafurov,

Leonid V. Rodionov

Samara State Aerospace University

Moskovskoe shosse, 34, Samara, 443123, Russian Federation

gafurov@ssau.ru

Maxim G. Mikheev

OAO Kuznetsov

Zavodskoe shosse, 29, Samara, 443009, Russian Federation

mecmod@mail.ru

NUMERICAL SIMULATIONS OF DYNAMIC LOADS IN AVIATION COMBINED PUMPS

Previous research has shown that aviation fuel combined pumps, which consist of a screw centrifugal stage and a gear stage, are the most loaded units of the jet turbine engines. Thus a combined pump is the key component that limits the reliability and endurance of the fuel system and, as a result, of the whole engine. The main purpose of the article is to present the results of the research aimed at developing measures that help to reduce dynamic loads of aviation fuel combined pumps. The CFD analysis has been used to calculate an unsteady three-dimensional viscous flow multi-component fluid in the screw centrifugal and gear stages. The calculations have been used to determine unsteady loads of combined pumps in their different operating regimes. To examine the effectiveness of the CFD analysis, we conducted a series of experiments. The experimental results proved the accuracy of the numerical model. The results illustrate how the proposed measures reduce the flow unsteadiness of multi-component fluid in the screw centrifugal and gear stages. We can predict that the suggested measures will enhance the reliability and endurance of aviation fuel pumps.

Key words: Fuel system; gas turbine engine; combined pump; cavitation; turbulence; combined air; pressure oscillation; reliability

Introduction

The elements of an aircraft fuel pumping system are under a significant vibration load [1, 2].

An aircraft engine fuel system contains a large number of functionally related hydro-mechanical components, each of which can be a source of vibration, pressure, flow, increased vibration and noise [3]. One of the most significant sources of increased loads is combined air which can enter the pump. All fuels are able to dissolve a significant amount of gas. Vortex disturbances occurring in the supply pipeline contribute to the bubble formation. This leads to breakdown of the dynamic equilibrium in the fuel-air system, accompanied by the liquid-vapor transition. As a result cavitation properties are decrease significantly [4].

Paper [5] presents an experimental dependence to estimate the cavitation performance change on account of the free air. This work shows that even a small amount of free gas in fluid significantly decreases pump cavitation performance.

Paper [6] describes the mathematical model which can determine the boundaries of the stability region of the pump-pipe system in case of presence of free air. This model takes into account the capacity and inertia of this system as well as its hydraulic friction. The authors conclusively establish the ambiguousness of free air influences [7].

Screw centrifugal pump

The aviation screw centrifugal pump (Figure 1) considered in this study of a commercial type. This pump consist of an open type impeller with 11 straight blades and double-lead screw. The single volute casing is unvaned. The shape of the single volute casing is designed according to the theory of a constant average velocity for all sections of the volute. We examined the angular contact bearing (Figure 1). The working fluid is kerosene.

Figure 1. A view of CP

with angular contact bearing

Test condition

The test conditions for this study are presented in Table 1. Volume fractions of combined air were chosen based on operating conditions of fuel system. Maximum volume fraction of combined air is 7% in real operating conditions.

Table 1. Test conditions

№	Npump, rev/min	G, kg/h	Т,0С	% combined air
1	6400	16740	25	0
2	6400	16740	25	4
3	6400	16740	25	5.3
4	6400	16740	25	6.5

Mathematical model

The adequate description of pump working processes requires numerical solutions of unsteady three-dimensional Navier-Stokes equations in the whole computational domain.

4.1 Governing equations

The change of momentum, heat and mass transfer in fluid described by the Navier-Stokes equations. It is necessary to use additional equations of cavitation and turbulence for modeling working processes in pump. These equations can also be solved with the Navier-Stokes equations.

In pump realized multicomponent flow in case when combined air hit into the supply line. Therefore the flow consist of liquid and gaseous phases of kerosene and combined air. Sum of each volume fractions should be:

(1)

As a result the Navier-Stokes equations (transport and state) as well as the equation of cavitation and turbulence are given for the case of multicomponent viscous flow.

5 Validation of the numerical method

In order to validate the present method, steady two phases flows at different operating regimes were simulated. The verification of numerical model has been based on comparison of the head and cavitation characteristics in cases of working without combined air obtained by calculation and experimental ways. In Figures 2, variation of head, cavitation parameter and efficiency versus flow rate have been shown.

The numerical results of the problem show very good agreement with the experimental results. Evaluation by Fisher showed the adequacy of the model. Calculation error is not more than 4%, taking into account the error gauges.

Thus, the numerical model can accurately predict the pump head and cavitation characteristics variations in a wide range of operating regimes.

a
b
Figure 2: Comparison of experimental and calculated values. a - head characteristic; b - cavitation performance

Conclusion

The present study is mainly focused on the modeling of the two-phase multi-component flow in the screw centrifugal pumps.

Proposed mathematical model is able to determine loading state of bearings in operating regimes with combined air.

According to results provided it may be concluded that proposed mathematical model can be used to calculate energy and cavitation characteristics of the pump.

The numerical simulation was possible to reproduce the pump test, being able to characterize the behavior of the flow under two-phase and multi-component flow conditions.

References

(use Harvard style)

Ning K., Lovell, M.R., (2002) On the Sliding Friction Characteristics of Unidirectional Continuous FRP Composites. ASME Journal of Tribology. 124 (1), pp. 5-13. DOI: 10.1115/1.40224567

Igolkin, A.A. (2014) Vibroacoustic loads reduction in pipe systems of gas distribution stations. Journal of Dynamics and Vibroacoustics. [Online] 1 (1). p. 3-10 Available from www.dynvibro.ru.

Singhal, A. K., Athavale, M. M., Li., H. Y., Jiang, Yu, (2002) Mathematical basis and validation of the full cavitation model. ASME J. Fluids Engineering. 122, pp. 617–624

Tung, C. Y. (1982) Evaporative Heat Transfer in the Contact Line of a Mixture. A Thesis Submitted in partial fulfilment of the Requirements of Rensselaer Polytechnic Institute for the Degree of Doctor of Philosophy. Troy, NY: Rensselaer Polytechnic Institute

Zagurenko, A.G., Korotovskikh, V.A., Kolesnikov, A.A., Timonov, A.V., Kardymon, D.V., (2008) Tekhnicheskaya i ekonomicheskaya optimizatsiya processa proektirovaniya gidravlicheskogo razriva plasta [Technical and economic optimization of hydrofracturing design]. Neftyanoe khozyaistvo – Oil Industry. 11, pp. 54-57 (in Russian)

Astakhov, M.V., Tagantsev, T.V. (2006) Eksperimental'noe issledovanie prochnosti soedinenii «stal'-kompozit» [Experimental study of the strength of joints "steel-composite"]. Trudy MGTU «Matematicheskoe modelirovanie slozhnykh tekhnicheskikh sistem» [Proc. of the Bauman MSTU “Mathematical Modeling of Complex Technical Systems”]. 593, pp. 125-130 (in Russian)

The Second International Conference on Dynamics and Vibroacoustics of Machines

September 15-17, 2014, Samara, Russia

2

Nomenclature

Salimzhan

2014-06-18

kirill blyumin