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ScienceDirectProcedia Engineering 00 (2014) 000–000

www.elsevier.com/locate/procedia

“APISAT2014”, 2014 Asia-Pacific International Symposium on Aerospace Technology, APISAT2014

Numerical Studies of Multi-cycle Acetylene-air Detonation Induced by Shock Focusing

Zhihong Zhang*, Zhiqiang Li and Gan Dong National National Key Laboratory of Science and Technology on Aero-Engine Aero-thermodynamics, School of Energy and Power Engineering,

Beijing University of Aeronautics and Astronautics, Beijing, 100083, P. R. China

Abstract

Shock focusing techniques can avoid deflagration-to-detonation transition (DDT), which make PDE more efficiency. Numerical simulations of an idealized pulse detonation engine consisting of axial inlet and circumferential inlet are presented in this paper. Using detailed acetylene-air chemical kinetic model, investigation on detonation direct initiation by shock focusing is done. Studies indicate that in static flow field, the regions of high temperature and pressure created by shock focusing can produce detonation at the condition of circumferential inlet Mach 2.4. But in dynamic flow field, Mach number should increase to 3.5 to achieve detonation© 2014 The Authors. Published by Elsevier Ltd.Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA).

Keywords: Multi-cycle detonation; Shock focusing; Numerical simulation

1. Introduction

The pulse detonation engine (PDE) is expected to be a new type of power plant for aerospace vehicles and electric generators [1]. An ideal PDE design can have a thermodynamic efficiency higher than other designs like turbojets and turbofans because a detonation wave rapidly compresses the mixture and adds heat at constant volume[2,3]. Shock focusing techniques use geometric designs to focus a shock wave or multiple shock waves to create regions of high temperatures and pressures which results in the local ignition of a reactive mixture. Research conducted by Channel found that multiple methods can be used to achieve focusing of these waves such as combustor geometry, turbulence devices or injection jets. In a reactive mixture, these regions of high temperature

* * Corresponding author. Tel.: +86- 13910610401;.E-mail address: zhangzhihong@sjp.buaa.edu.cn

1877-7058 © 2014 The Authors. Published by Elsevier Ltd.Peer-review under responsibility of Chinese Society of Aeronautics and Astronautics (CSAA).

2 Zhihong Zhang / Procedia Engineering 00 (2014) 000–000

can produce violent explosions, and eventually detonation [4,5]. In this study, shock focusing in both static and dynamic flow field is considered, since past literature that reporting on the complicated flow field of shock wave focusing is limited.

2. Problem description

Fig. 1. Schematic of detonation chamber

The geometric size of the detonation chamber is shown in Fig. 1, with the radius=5mm and length=17mm. The simple configuration of PDE model is a two-dimensional (axis-symmetric) detonation chamber with cone in the head and barrier on the wall. The detonation chamber is fully filled with the same premixed stoichiometric acetylene-air mixture at 1 atm and 300K.

The shock wave is generated, at first, from the circumferential toroidal inlet, and will diffract into the chamber. Then generate the diffracting shock wave traveling to-ward the axis of symmetry. Finally, the diffracting shock waves will focus on the top of cone and generate high temperature and pressure region.

3. Chemical kinetic model

This paper use detailed chemical kinetic model include 16 species (C2H2、CO2、CO、CH3、CHO、CH2O、CH2CO、HCCO、H2、O2、O、H、OH、HO2、H2O、N2) and 25 reactions.

4. Numerical methods

In the numerical simulation, unsteady two-dimensional axisymmetric Navier-Stokes equations for a perfect gas are solved. The standard k-ε turbulence model is used. Numerically, PISO (pressure implicit split-operator) is used for finite volume method. Time step method is adaptive . The min time step is 1e-08s and the max is 1e-06s.

5. Boundary conditions

Boundary conditions for the inflow are pressure inlet conditions and no-reflect pressure outlet for the outflow. No-slip wall boundary conditions are used for the duct wall.

6. Results of studies

6.1. Shock focusing in initial Static flow field

In static flow field of acetylene-air mixture, the regions of high temperature and pressure created by shock wave focusing for an incident Mach number of 2.4 can produce denotation. The contours of temperature and pressure are

Zhihong Zhang / Procedia Engineering 00 (2014) 000–000 3

shown in Fig. 2 and Fig. 3. The contours of mass fraction of HO2 and CHO showing in Fig. 4 indicate that denotation wave is coupled with the chemical reaction.

Fig. 2. Contours of temperature during denotation. (a) shock wave traveling; (b) first focus; (c)detonation in cone; (d)detonation in chamber.

Fig. 3. Contours of pressure during denotation. (a) shock wave traveling; (b) first focus; (c)detonation in cone; (d)detonation in chamber.

Fig. 4. (a)Contours of mass fraction of CHO during denotation; (b)Contours of mass fraction of HO2 during denotation.

a b

c d

ab

c d

a b

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6.2. Refilled by axial inlet

After the ignition in static flow field is finished, close the circumferential toroidal inlet and fresh air will flow into the chamber through axial inlet because of the pressure difference. Refill acetylene-air mixture into the chamber when the chamber is cooled enough. And the finished state is shown in Fig. 5.

Fig. 5.(a) Contours of mass fraction of C2H2 after refill; (b)Contours of mass fraction of O2 after refill.

6.3. Shock focusing in dynamic flow field

Unlike the static flow field, in dynamic flow field of acetylene-air mixture, the regions of high temperature and pressure created by shock wave focusing for an incident Mach number of 2.4 can not produce denotation. Fig. 6 shows the contours of temperature and pressure. The temperature and pressure of the focusing region is nearly 1027K and 4.5MPa. Increasing the incident Mach number to 3, the contours of temperature and pressure of focusing region are show in Fig. 7. The transient temperature and pressure of the focusing region is nearly 1428K and 10MPa. When the incident Mach number increases to 3.5, the transient temperature and pressure of the focusing region is nearly 3000K and 30MPa as the Fig. 8 shows, which produce denotation.

Fig. 6. Contours of temperature and pressure of focusing region with Incident Mach 2.4. (a)temperature; (b)pressure.

Fig. 7. Contours of temperature and pressure of focusing region with Incident Mach 3. (a)temperature; (b)pressure.

a b

a b

ba

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Fig. 8. Contours of temperature and pressure of focusing region with Incident Mach 3.5. (a)temperature; (b)pressure.

7. Conclusions

Numerical investigation of shock focusing in both static and dynamic flow field of acetylene-air mixture is done. The toroidal shock wave focusing is an effective way to ignite detonation with the region of high pressure and temperature. In static flow field of acetylene-air mixture, the regions of high temperature and pressure created by shock wave focusing for an incident Mach number of 2.4 can produce denotation. But the dynamic flow field will weaken the shock wave when it diffracting in the chamber. In this condition, increasing the incident Mach number play a key role. In dynamic flow field, the temperature of the focusing region with incident Mach number of 2.4 is 1027K and pressure is 4.5MPa. When the incident Mach number change from 2.4 to 3.5, the temperature and pressure of the focusing region increase to 3000K and 30MPa .

Acknowledgements

The authors wish to thank Zhiming Li and Lijing Liu at BEIHANG University for many useful discussions during the course of this study.

References

[1] Roy, G.D., Frolov, S.M., Borisov, A.A., Netzer, D.W. Pulse detonation propulsion: challenges, current status, and future perspective. Prog. Energy Combust. Sci. 30 (6) 546-672 (2004)

[2] Subhash Chander, Dr. TK Jindal, Integration challenges in design and development of pulse detonation test rig. IJAREEIE, Oct 2012, Vol. 1,pp. 291-304.

[3] Wintenberg, E., Shepherd, J.E. Thermodynamic cycle analysis for propagating detonation. J. Propuls. Power. 22(3), 694-697 (2005)[4] B.T. Channell, Evaluation and selection of an efficient fuel/air initiation strategy for pulse detonation engines. Master’s Thesis, Naval

Postgraduate School, Monterey, California, Sep 2005.[5] M.A. Nettleton, Recent work on gaseous detonations, Shock Waves 12 (2002) 3-12.

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