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A novel method for flame stabilization in a strut-based scramjet combustor
Combustion and Flame 210 (2019) 292–301
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Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
A novel method for flame stabilization in a strut-based scramjet combustor Qiongyao Qin a,b, Ramesh Agarwal b,∗, Xiaobing Zhang a,∗ a b
School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, MO 63130, USA
a r t i c l e
i n f o
Article history: Received 12 August 2019 Revised 28 August 2019 Accepted 28 August 2019
Keywords: Supersonic combustion Novel strut Total pressure loss Combustion efficiency Flame stabilization
a b s t r a c t For successful operation of propulsion system of an air-breathing hypersonic aircraft, flame stabilization in its scramjet combustor is very critical. In this paper, a novel strut is proposed for flame stabilization in a scramjet combustor. The novel strut is a modification of the traditional strut with the angles of its rear part enlarged. Employing the two-dimensional compressible multi-species Navier–Stokes equations and a one-step hydrogen–air reaction model, six simulation cases are performed. An analysis of the simulations is conducted to explain the stabilization of the flame by the proposed novel strut. The shock waves induced by the rear part of the novel strut cause a high-temperature and high-pressure region for combustion and an effective total pressure loss. The strong shock waves affect only the nearby region of the strut for the reason that only a small part of the wedge is wedge angle enlarged. The novel strut is thus able to stabilize the flame behind it and can be used as a flame holder in a scramjet combustor. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Investigation of flame stabilization of a scramjet combustor is a subject of considerable current interest since the scramjet engine is one of the key propulsion technologies for an air-breathing hypersonic aircraft [1,2]. In past decades, many investigations have been conducted and considerable effort has been devoted towards improving the combustion efficiency of a scramjet combustor [3–9]. It is very challenging to achieve sufficiently high combustion efficiency and stability in supersonic combustion [10]. In this paper, a novel strut is proposed to stabilize the flame in a scramjet combustor. Over fifty years, many theoretical, experimental and numerical investigations have been reported in the literature about the design and performance of a scramjet combustor. Ferri was one of the first to lead the development of a hydrogen fueled scramjet engine in the United States in the early seventies [11]. He analyzed the mixing controlled supersonic combustion [12,13]. Waidmann et al. conducted experimental investigations on the combustion process in a supersonic combustion ramjet [14]. A great deal of experimental data was obtained which provided impetus for subsequent research. Sunami et al. conducted experiment on
∗
Corresponding authors. E-mail addresses: [email protected] (R. Agarwal), [email protected] (X. Zhang).
a new scramjet, aiming to obtain a better engine performance as well as working characteristics and operability for a wide range of flight Mach numbers [15]. Guerra et al. experimentally investigated the combustion in a two-dimensional hydrogen jet in a supersonic air stream. OH spectra intensity and wall static pressure were measured and Schlieren photographs were obtained [16]. Fureby et al. studied supersonic combustion stabilized behind conventional and alternating-wedge injection struts using experimental data and computational results [10]. In the area of numerical investigations, Choubey and Pandey have reported a series of excellent simulations. They evaluated the performance of a typical two-strut scramjet combustor using ANSYS FLUENT [3,5,6]. Choubey and Pandey studied the effects of different injection schemes on the flow field [17]. They also conducted valuable researches on cavity Scramjet combustor [18–21]. Pandey et al. studied the effect of variation in hydrogen injection pressure and inlet air temperature on the flow-field of a typical double cavity scramjet combustor [22]. Nordin-Bates et al. conducted a numerical study of HyShot II scramjet combustor using Large Eddy Simulation (LES); the simulation results were able to capture the experimental wall-pressure and heat-flux quite well compared to the experimental data [23]. Santana and Weigand investigated the inlet-combustor interactions for a scramjet hydrogen-fueled engine at a flight Mach number of eight with three different angles of attack [24]. Edwards simulated the supersonic reacting wall jet with a hybrid Large-Eddy Simulation/Reynolds-averaged Navier–Stokes (LES/RANS) solver [25]; the computed results agreed well with the
https://doi.org/10.1016/j.combustflame.2019.08.038 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Q. Qin, R. Agarwal and X. Zhang / Combustion and Flame 210 (2019) 292–301
experimental data. Oevermann [26] numerically investigated the turbulent hydrogen combustion in a scramjet using flamelet modeling and compared the numerical results with the experimental data. Although there are many investigations reported in the literature about the strut-based scramjet combustor [27–31], however the half angles of the struts used in these investigations are relatively small. In this paper, a novel strut with the angle of the rear wedge enlarged is proposed. Since an oblique detonation wave can be induced behind a shock wave [32–34], the novel strut which induces strong shock waves is expected to stabilize of the flame. To evaluate the usefulness and performance of the novel strut in flame stabilization, a numerical simulation employing the two-dimensional compressible multi-species ReynoldsAveraged Navier–Stokes (RANS) equations with the standard k − ε model is conducted. Six cases are simulated to evaluate the performance of the proposed novel strut for flame stabilization. The combustion efficiency and total pressure loss of the combustors with the novel strut and the traditional strut are evaluated. A qualitative analysis is conducted to explain the stabilization of the flame by the novel strut. 2. Computational details 2.1. Governing equations In simulation of the reacting flow in scramjet combustor, the multispecies model needs to be taken into consideration. The twodimensional compressible multi-species Navier–Stokes equations can be expressed in conservation law form as:
∂ Q ∂ E ∂ F ∂ Eυ ∂ Fυ + + = + +S ∂t ∂x ∂y ∂x ∂y
(1)
Q = [ρ1 , . . . , ρN , ρ u, ρv, ρ E ]T
(2)
E= F=
T ρ1 u, . . . , ρN u, ρ u2 + p, ρ uv, u(ρ E + p)
(3)
T ρ1 v, . . . , ρN v, ρ uv, ρv2 + p, v(ρ E + p)
(4)
T ∂ c1 ∂ cN Eυ = ρ D1 , . . . , ρ DN , τ , τ , uτ + vτxy + qx ∂x ∂ x xx xy xx T ∂ cN ∂ c1 Fυ = ρ D1 , . . . , ρ DN , τxy , τyy , uτxy + vτyy + qy ∂y ∂y S = [ω˙ 1 , . . . , ω˙ N , 0, 0, 0]
T
(5)
(6) (7)
In Eqs. (1)–(7), Q is the conservation variables vector, E and F are convective flux vectors, Ev and Fv are viscous flux vectors, and S is the chemical reaction source terms vector. u and v are velocity component in x-direction and y-direction, respectively, p is the pressure, E is the total energy per unit mass, ci is the mass fraction of species i, and T is the temperature. Single step, laminar finite rate kinetics is been utilized to model the chemistry [29,30,35,36]. A single step chemical reaction of H2 (see Table 1) is considered. The single step model has been widely
293
Table 1 Single step chemical reaction of H2 . Reaction
A
n
E
H2 + 1/2O2 = H2 O
9.87e8
0.0
3.1e7
used in strut based scramjet simulations and the results obtained have been found to be in reasonable agreement with the available experiment data. The reaction rate is computed using the Arrhenius equation:
k = AT n e−E/RT
(8)
The Finite Volume Method (FVM) is used to solve the governing equations. The AUSM+ scheme [37–40] is employed to discretize the inviscid fluxes and k − ε model is chosen as the turbulence model. 2.2. Description of the novel strut The strut, shown in Fig. 1(a), is a fuel injector and a flame holder. The incoming flow is compressed on the surface of the strut, which has a half angle of θ 1 . If the half angle of the strut is enlarged, as shown in Fig. 1(b), stronger shock waves will be induced on the surface of the strut. Stronger shock waves will induce higher temperature and higher backpressure behind the strut, which are more desirable for combustion. However, the strut with larger half angle shown in Fig. 1(b) will cause larger total pressure loss which is not desirable for a propulsion system, although it promotes combustion. Considering these two factors, the proposed novel strut can be regarded as an appropriate combination of the struts shown in Fig. 1(a) and (b). Only the angle of the rear part of the strut is enlarged as shown in Fig. 1(c). The novel strut can be divided into three parts: a leading wedge, a step and a rear wedge with angle enlarged. The heights of the three struts are kept the same; this is to guarantee the same blockage ratio. The lengths of the traditional strut and the novel strut are the same. In the simulation, the half angle of the leading part θ 1 is 6◦ . The half angle of the rear part θ 2 is 30◦ . The length of the strut L is 32 mm. Hydrogen is injected parallel to the air stream through a hole whose height is D. The hole is located at the center of the strut base and the height is 1 mm. The height of rear wedge h is 0.8 mm. Given that this investigation is aimed at evaluation of the feasibility and performance of the novel strut, the effect of the rear wedge height on flame stabilization is not studied here. In applications, the wedge height can be different in different scramjet combustors depending on the requirements. The schematic of a typical scramjet combustor is shown in Fig. 2. The tip of the strut is located at x = 35 mm and y = 25 mm. The upper wall of the combustor diverges at x = 58 mm and the divergence angle is 3◦ . Air enters the combustor at the inlet with a static pressure of 10 0,0 0 0 Pa and static temperature of 300 K at different Mach numbers. The hydrogen is injected into the air stream through a mass flow inlet boundary. The total temperature of the hydrogen is 300 K. A pressure outlet boundary condition is applied at the exit of the combustor. No-slip condition is employed on all wall boundaries including the combustor walls and the surface of the strut. All the wall boundaries are regarded as adiabatic. Figure 3 shows the
Fig. 1. Schematic of the strut: (a) the traditional strut, (b) the strut with half angle enlarged and (c) the novel proposed strut.
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Fig. 2. Schematic of the scramjet combustor with the novel strut.
Fig. 3. Schematic of the mesh in the scramjet combustor with the novel strut (the entire mesh with 350,0 0 0 cells is not shown to keep clarity in the figure).
Fig. 4. Comparison between the computed density contours and the experimental shadowgraph: (a) and (b) are computational density contours; (c) experimental shadowgraph.
mesh inside the scramjet combustor. Structured mesh is used for the main region except in the region surrounding the strut, where an unstructured mesh is used. The size of the structured mesh is around 0.3 mm × 0.3 mm. The height of the first point in the boundary layer is 0.001 mm. The edge of the cell in the unstructured mesh has a length which ranges from 0.005 mm to 0.3 mm. The total number of cells is around 350,0 0 0. The mesh is solution independent and in generating the mesh, it is ensured that y + < 1 for the grid point next to the boundary.
3. Results and discussions 3.1. CFD validation CFD validation is done against the experimental data given in the paper of Oevermann [26]. The comparison between the experimental shadowgraph and the computed density contours is shown in Fig. 4. The mass flow rate is computed from the boundary conditions used in the work of Oevermann [26]. The reflections of the
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Fig. 5. Comparison between the computed and experimental velocity profiles at (a) x = 78 mm; (b) x = 125 mm; (c) x = 207 mm; and (d) at y = 25 mm.
Fig. 6. Pressure contours in the combustor at Mach 4.5 with (a) novel strut and (b) traditional strut.
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Fig. 7. Temperature and H2 O mass fraction contours of various flow properties in the combustor at Mach 4.5 (a) temperature contours with novel strut, (b) temperature contours with traditional strut; (c) H2 O contours with novel strut and (d) H2 O contours with traditional strut.
shock waves induced by the tip and tail of the strut agree very well with the experimental shadowgraph. The computed global flow field shows reasonable agreement with the experimental work. The velocity profiles along the centerline and at three different stream wise cross-sections are shown in Fig. 5. Although the computation is conducted under a two-dimensional assumption, the profiles show acceptable agreement with the experimental profiles. To get a better agreement, a three-dimensional model is really necessary since the hydrogen is injected through a row of 15 holes in the experiment. Nevertheless, the two-dimensional numerical model utilized in this investigation is sufficient to obtain the global flow features in the scramjet combustor and to evaluate the performance characteristics of the proposed novel strut.
Table 2 Computational cases. Strut type
Traditional strut
Cases Mach number
1 4.5
2 5
Novel strut 3 5.5
1 4.5
2 5
3 5.5
3.2. Evaluation of the performance of the proposed novel strut To evaluate the performance characteristics of the proposed novel strut at different Mach numbers of the air stream, three Mach numbers are considered at the inlet for both the traditional strut and the novel strut as shown in Table 2. To compare the difference between the combustion efficiencies in the combustors
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297
Fig. 8. Temperature contours inside the combustor with (a) novel strut at Mach 5, (b) traditional strut at Mach 5, (c) novel strut at Mach 5.5 and (d) traditional strut at Mach 5.5.
Fig. 9. Total pressure loss and combustion efficiency in the scramjet combustors with novel strut and the traditional strut.
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with the traditional strut and the novel strut, the mass flow rate of the hydrogen injection is set as a constant at 0.15 Kg/s for both struts. Figure 6 shows the pressure contours in the scramjet combustors with the novel strut and the traditional strut at Mach 4.5. The shock waves in Fig. 6(a) are mainly induced by the strut tip and from the reflections of these shock waves from the combustor walls. However, in the combustor with traditional strut shown in Fig. 6(b), a series of new shock waves are observed which are
induced by the combustion front. The combustion front can also be seen in Fig. 7(b) and (d) which provide the temperature contours and the H2 O mass fraction contours, respectively. The reason why these shock waves are not observed for the novel strut is that the combustion front is held right behind the strut as shown in Fig. 7(a) and (c). Figure 8 shows the temperature contours at higher Mach numbers. With the Mach number of the air stream being increased from 4.5 to 5.0 and 5.5, the combustor with the
Fig. 10. Pressure and temperature profiles near the strut base: (a) pressure profiles along x = 0.0678 mm for the cold flow without injection; (b) temperature profiles along x = 0.0678 mm for the cold flow without injection; (c) pressure profiles along x = 0.0678 mm for the cold flow with injection; (d) temperature profiles along x = 0.0678 mm for the cold flow with injection; (e) pressure profiles along y = 0.0284 mm for the cold flow with injection; and (f) temperature profiles along y = 0.0284 mm for the cold flow with injection.
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Table 3 The total pressure loss and the combustion efficiency at the exit of the combustor. Strut type
Novel strut
Mach number Total pressure loss (%) Combustion efficiency (%)
4.5 27.1 76.4
5 27.6 75.2
Traditional strut 5.5 28.3 73.6
4.5 31.1 51.9
5 30.8 70.0
5.5 30.7 68.5
novel strut still performs quite well and the flame is held behind the strut. The flame is below downstream in the combustor with traditional strut as shown in Fig. 8(b) and (d). The combustion efficiency and the total pressure loss of the combustors with the novel strut and the traditional strut are evaluated in Fig. 9. The total pressure loss and the combustion efficiency are calculated using the method introduced by Choubey and Pandey [3]. The total pressure losses for all the cases are the same before x = 0.0595 m. The branch point of the novel strut and the traditional strut is located at x = 0.0595 m and that is where the step on the novel strut begins. Attention needs to be paid to the phrase “the novel strut and the traditional strut” which refers to “the combustor with the novel strut and the combustor with the traditional strut”. The total pressure losses at the exit of the combustor with the novel strut are lower than that with the traditional strut at different Mach numbers, although there is a region where the total pressure losses of the novel strut are larger than that for the traditional strut. The lower total pressure losses of the novel strut can be explained by the earlier initiation of combustion which causes more energy release. Especially, the novel strut performs very well at Mach number of 4.5. All the total pressure losses and the combustion efficiencies at the exit are shown in Table 3. The total pressure loss is decreased by 4% compared to that for the traditional strut at Mach number of 4.5. In Fig. 9(b), the combustion efficiencies along the stream in the combustor with the novel strut increase smoothly, while the combustion efficiencies in the combustor with traditional strut undergo a sharp increase at x = 0.11 m, x = 0.15 m, and x = 0.14 m for Mach numbers of 4.5, 5.0 and 5.5, respectively. The combustion efficiencies at the exit of the combustor with the novel strut are higher than that with the traditional strut at different Mach numbers. Especially, the combustion efficiency is increased by 24.5% at Mach number of 4.5. Although the rear part of the novel strut causes strong shock wave and large total pressure loss, it stabilizes the flame and the total pressure loss can be regard as the effective total pressure loss. 3.4. Mechanism of the flame stabilization of the novel strut To figure out the mechanism of the flame stabilization of the novel strut, the cold flow simulations in the combustors with the novel strut and traditional strut at Mach number of 4.5 are conducted. Both the cold flow with and without hydrogen injection are simulated. The mass flow rate is the same as with the reacting flow. The pressure and temperature profiles along the line x = 0.0678 m, which is the vertical line behind the strut at a distance of 0.8 mm, are shown in Fig. 10(a) and (b). The strut base is located at x = 0.067 m. The most obvious difference between the pressure profiles of two struts is the peak pressure caused by the strong shock wave. Besides, another difference is the pressure behind the strut base, i.e., the pressure profiles between 0.02164 m − −0.02836 m. From the magnified picture in Fig. 10(a), it can be seen that the backpressure of the novel strut is 90 0 0 Pa higher than that for the traditional strut. The backpressure of the novel strut is 14,500 Pa and that of the traditional strut is 5500 Pa. In Fig. 10(b), the maximum temperature difference between the two struts is obtained around y = 0.02164 m and y = 0.02836 m. The temperature difference is around 100 K. Figure 10(c) and (d) shows the pressure and temperature profiles in the cold flow of the two
Fig. 11. Velocity contours around the strut base: (a) and (b) velocity contours for the cold flow with injection; (c) and (d) velocity contours for the reactive flow with injection.
struts with injection. The locations of the maximum differences are the same as that for the cold flow with no injection. The pressure difference behind the strut is about 50 0 0 Pa and the temperature difference is around 100 K. Figure. 10(e) and (f) shows the pressure and temperature profiles along y = 0.0284 mm. It can be seen that the pressure and the temperature of the novel strut is higher than that of the traditional strut. Based on the analysis of the flow field parameters of the two struts, it is found that the temperature and pressure difference in the region near the strut base is as expected. Attention should be paid to the global reaction model of the hydrogen–air combustion. The one-step model is not sufficient to predict the ignition delay period or the detailed flame structure. Thus, there is no confidence to relate the ignition delay period with the flame stabilization. The only conclusion can be drawn here is that the flame stabilization is related to the temperature and pressure near the strut base. A detailed reaction mechanism which can predict the ignition delay period and flame feature correctly is really needed for revealing the detailed mechanism of the flame stabilization. However, this is not the purpose of this research. This research is focused on designing a new strut for stabilizing the flame behind the strut. In the cold flow with no combustion, the injection flow fields of the novel strut and the traditional strut are similar and both show the bottle-like shock waves as shown in Fig. 11(a) and (b). The injection flow fields are affected by the backpressure at the strut base as shown in Fig. 10(c) and (d). In the reacting flow of the novel strut, the flame is held behind the strut and energy release increases the backpressure near the strut base. This leads to a narrow injection flow field and larger low-velocity zone, as shown in Fig. 11(c). However, the flame is blown far away from the strut in the traditional strut and a similar injection is observed as for the cold flow with no combustion. To verify the effect of temperature and pressure on the flame stabilization, the case 1 (Mach number of 4.5) is simulated with temperature and pressure of the inlet increased by 10 0 K and 20,0 0 0 Pa, respectively. The combustion front is observed to move back towards the strut and the flame is held by the strut as shown in Fig. 12(a). Figure 12(b) and (c) shows the injection flow field of the cold flow and the reactive flow. The
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Fig. 12. Temperature and velocity contours in the combustor with the increased temperature and pressure at the inlet: (a) temperature contours for the reacting flow; (b) velocity contours for the cold flow with injection; (c) velocity contours for the reacting flow.
Fig. 13. Temperature profiles near the strut base: (a) pressure profiles along y = 0.0284 mm for the cold flow with injection; (b) temperature profiles along y = 0.0284 mm for the cold flow with injection.
injection flow fields are similar to those of the novel strut which are shown in Fig. 11(a) and (c). The pressure and temperature profiles along y = 0.0284 mm are shown in Fig. 13. The pressure profile of the traditional strut with changed inlet condition is slightly higher than the original profile, but is much lower than that of the novel strut. The temperature profile is nearly the same with the novel strut but slightly higher in the rear part. It is difficult to confirm whether the pressure or the temperature dominates the most in flame stabilization by considering just one case. A theoretical analysis and a parametric study are really necessary and these will be completed in our future investigation. One thing has been
established in this study is that the flame is stabilized behind the novel strut and the flame stabilization can be explained by the increased pressure and temperature near the strut base induced by the enlarged angle in the rear part of the novel strut. 4. Conclusions A novel strut for a scramjet combustor is proposed. The angle of the rear part of the novel strut is enlarged. A numerical model based on the Navier–Stokes equations is established to simulate the combustion in the strut-based scramjet combustor and to
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verify the superior performance of the proposed novel strut compared to the traditional strut. It is found that the flame is stabilized behind the novel strut and the flame stabilization can be explained by the increased pressure and temperature near the strut base induced by the enlarged angle in the rear part of the novel strut. The shock waves induced by the rear part of the novel strut cause a high-pressure and high-temperature region for combustion. The strong shock waves affect only the nearby region of the strut for the reason that only a small part of the wedge is wedge angle enlarged. Thus, the total pressure loss caused by the rear wedge can be regard as an effective total pressure loss. The novel strut can be used as a flame holder in the scramjet combustor. Acknowledgment The first author is grateful for the hospitality and the resources provided by the CFD laboratory of the mechanical engineering department of Washington University in St. Louis, USA References [1] F.J. Förster, N.C. Dröske, M.N. Bühler, J. Von Wolfersdorf, B. Weigand, Analysis of flame characteristics in a scramjet combustor with staged fuel injection using common path focusing Schlieren and flame visualization, Combust. Flame 168 (2016) 204–215. [2] B. Liu, G. He, F. Qin, Q. Lei, J. An, Z. Huang, Flame stabilization of supersonic ethylene jet in fuel-rich hot coflow, Combust. Flame 204 (2019) 142–151. [3] G. Choubey, K.M. Pandey, Effect of parametric variation of strut layout and position on the performance of a typical two-strut based scramjet combustor, Int. J. Hydrog. Energy 42 (2017) 10485–10500. [4] G. Choubey, K.M. Pandey, Numerical studies on the performance of scramjet combustor with alternating wedge-shaped strut injector, Int. J. Turbo Jet Engines 34 (2015) 11–12. [5] G. Choubey, K.M. Pandey, Investigation on the effects of operating variables on the performance of two-strut scramjet combustor, Int. J. Hydrog. Energy 41 (2016) 20753–20770. [6] G. Choubey, K.M. Pandey, Effect of different strut + wall injection techniques on the performance of two-strut scramjet combustor, Int. J. Hydrog. Energy 42 (2017) 13259–13275. [7] G. Choubey, K.M. Pandey, Effect of variation of angle of attack on the performance of two-strut scramjet combustor, Int. J. Hydrog. Energy 41 (2016) 11455–11470. [8] J.P. Drummond, Enhancement of mixing and reaction in high-speed combustor flowfields, International Colloquium on Advanced Computation and Analysis of Combustion, Moscow, Russia, 1997, pp. 1–14. [9] G.B. Northam, C.S. Byington, Evaluation of parallel injector configurations for supersonic combustion, 25th Joint Propulsion Conference (1989). [10] C. Fureby, K. Nordin-Bates, K. Petterson, A. Bresson, V. Sabelnikov, A computational study of supersonic combustion in strut injector and hypermixer flow fields, Proc. Combust. Inst. 35 (2015) 2127–2135. [11] E.T. Curran, Scramjet engines: the first forty years, J. Propul. Power 17 (2001) 1138–1148. [12] A. Ferri, H. Fox, Analysis of fluid dynamics of supersonic combustion process controlled by mixing, Sympos. (Int.) Combust. 12 (1) (1969) 1105–1113. [13] A. Ferri, Mixing controlled supersonic combustion, Ann. Rev. Fluid Mech. 5 (1973) 301–338. [14] W. Waidmann, F. Alff, U. Brummund, M. Bohm, W. Clauss, M. Oschwald, Experimental investigation of the combustion process in a supersonic combustion ramjet (SCRAMJET) combustion chamber, DGLR-Jahrestagung 1994. [15] T. Sunami, A. Murakami, K. Kudo, M. Kodera, M. Nishioka, Mixing and combustion control strategies for efficient scramjet operation in wide range of flight Mach numbers, 11th AIAA/AAAF International Space Planes and Hypersonic Systems and Technologies Conference (2002).
301
[16] R. Guerra, W. Waidmann, C. Laible, An experimental investigation of the combustion of a hydrogen jet injected parallel in a supersonic air stream, 3rd International Aerospace Planes Conference (1991). [17] G. Choubey, K.M. Pandey, Effect of different wall injection schemes on the flow-field of hydrogen fuelled strut-based scramjet combustor, Acta Astronaut. 145 (2018) 93–104. [18] N. Kumar, G. Choubey, L. Suneetha, K.M. Pandey, Effect of variation of length– to-depth ratio and Mach number on the performance of a typical double cavity scramjet combustor, Acta Astronaut. 128 (2016) 540–550. [19] G. Choubey, K.M. Pandey, Effect of variation of inlet boundary conditions on the combustion flow-field of a typical double cavity scramjet combustor, Int. J. Hydrog. Energy 43 (2018) 8139–8151. [20] G. Choubey, Y. Devarajan, W. Huang, Recent advances in cavity-based scramjet engine – a brief review, Int. J. Hydrog. Energy 44 (2019) 13895–13909. [21] C. Paper, K.M. Pandey, A. Maji, T. Deshamukhya, A brief review on the recent advances in scramjet engine, AIP Conf. Proc. 1859 (020036) (2017) 1–6. [22] K.M. Pandey, G. Choubey, F. Ahmed, D.H. Laskar, P. Ramnani, Effect of variation of hydrogen injection pressure and inlet air temperature on the flow-field of a typical double cavity scramjet combustor, Int. J. Hydrog. Energy 42 (2017) 20824–20834. [23] K. Nordin-Bates, C. Fureby, S. Karl, K. Hannemann, Understanding scramjet combustion using les of the HyShot II combustor, Proc. Combust. Inst. 36 (2017) 2893–2900. [24] E.R. Santana, B. Weigand, Numerical investigation of inlet-combustor interactions for a scramjet hydrogen-fueled engine at a flight Mach number of 8, AIAA J. (2012) 1–13. [25] J.R. Edwards, J.A. Boles, R.A. Baurle, Large-eddy/Reynolds-averaged Navier–Stokes simulation of a supersonic reacting wall jet, Combust. Flame 159 (2012) 1127–1138. [26] M. Oevermann, Numerical investigation of turbulent hydrogen combustion in a scramjet using flamelet modeling, Aerosp. Sci. Technol. 4 (20 0 0) 463–480. [27] F. Génin, S. Menon, Simulation of turbulent mixing bBehind a strut injector in supersonic flow, AIAA J. 48 (2010) 526–539. [28] W. Huang, Z.G. Wang, S.B. Luo, J. Liu, Parametric effects on the combustion flow field of a typical strut-based scramjet combustor, Chin. Sci. Bull. 56 (2011) 3871–3877. [29] K. Kumaran, V. Babu, Investigation of the effect of chemistry models on the numerical predictions of the supersonic combustion of hydrogen, Combust. Flame. 156 (2009) 826–841. [30] K.M. Pandey, S. Roga, G. Choubey, Computational analysis of hypersonic combustor using strut injector at flight Mach 7, Combust. Sci. Technol. 187 (2015) 1392–1407. [31] P. Gerlinger, P. Stoll, M. Kindler, F. Schneider, M. Aigner, Numerical investigation of mixing and combustion enhancement in supersonic combustors by strut induced streamwise vorticity, Aerosp. Sci. Technol. 12 (2008) 159–168. [32] C.A. Nordeen, D. Schwer, F. Schauer, J. Hoke, T. Barber, B.M. Cetegen, Role of inlet reactant mixedness on the thermodynamic performance of a rotating detonation engine, Shock Waves 26 (2015) 1–12. [33] H. Teng, H. Dick, Z. Jiang, Initiation characteristics of wedge-induced oblique detonation waves in a stoichiometric hydrogen–air mixture, Proc. Combust. Inst. 36 (2017) 2735–2742. [34] Q. Qin, X. Zhang, A novel method for trigger location control of the oblique detonation wave by a modified wedge, Combust. Flame 197 (2018) 65–77. [35] K. Kumaran, V. Babu, Mixing and combustion characteristics of kerosene in a model supersonic combustor, J. Propul. Power 25 (2009) 583–592. [36] K. Kumaran, P.R. Behera, V. Babu, Numerical investigation of the supersonic combustion of kerosene in a strut-based combustor, J. Propul. Power 26 (2010) 1084–1091. [37] M. Liou, Mass flux schemes and connection to shock instability, J. Comput. Phys. 160 (20 0 0) 623–648. [38] M. Liou, A sequel to AUSM, part II: AUSM + −up for all speeds, J. Comput. Phys. 214 (2006) 137–170. [39] M. Liou, The evolution of AUSM schemes, Def. Sci. J. 60 (2010) 606–613. [40] M. Liou, A sequel to AUSM: AUSM+, J. Comput. Phys. 129 (1996) 364–382.
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Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame
A novel method for flame stabilization in a strut-based scramjet combustor Qiongyao Qin a,b, Ramesh Agarwal b,∗, Xiaobing Zhang a,∗ a b
School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, MO 63130, USA
a r t i c l e
i n f o
Article history: Received 12 August 2019 Revised 28 August 2019 Accepted 28 August 2019
Keywords: Supersonic combustion Novel strut Total pressure loss Combustion efficiency Flame stabilization
a b s t r a c t For successful operation of propulsion system of an air-breathing hypersonic aircraft, flame stabilization in its scramjet combustor is very critical. In this paper, a novel strut is proposed for flame stabilization in a scramjet combustor. The novel strut is a modification of the traditional strut with the angles of its rear part enlarged. Employing the two-dimensional compressible multi-species Navier–Stokes equations and a one-step hydrogen–air reaction model, six simulation cases are performed. An analysis of the simulations is conducted to explain the stabilization of the flame by the proposed novel strut. The shock waves induced by the rear part of the novel strut cause a high-temperature and high-pressure region for combustion and an effective total pressure loss. The strong shock waves affect only the nearby region of the strut for the reason that only a small part of the wedge is wedge angle enlarged. The novel strut is thus able to stabilize the flame behind it and can be used as a flame holder in a scramjet combustor. © 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
1. Introduction Investigation of flame stabilization of a scramjet combustor is a subject of considerable current interest since the scramjet engine is one of the key propulsion technologies for an air-breathing hypersonic aircraft [1,2]. In past decades, many investigations have been conducted and considerable effort has been devoted towards improving the combustion efficiency of a scramjet combustor [3–9]. It is very challenging to achieve sufficiently high combustion efficiency and stability in supersonic combustion [10]. In this paper, a novel strut is proposed to stabilize the flame in a scramjet combustor. Over fifty years, many theoretical, experimental and numerical investigations have been reported in the literature about the design and performance of a scramjet combustor. Ferri was one of the first to lead the development of a hydrogen fueled scramjet engine in the United States in the early seventies [11]. He analyzed the mixing controlled supersonic combustion [12,13]. Waidmann et al. conducted experimental investigations on the combustion process in a supersonic combustion ramjet [14]. A great deal of experimental data was obtained which provided impetus for subsequent research. Sunami et al. conducted experiment on
∗
Corresponding authors. E-mail addresses: [email protected] (R. Agarwal), [email protected] (X. Zhang).
a new scramjet, aiming to obtain a better engine performance as well as working characteristics and operability for a wide range of flight Mach numbers [15]. Guerra et al. experimentally investigated the combustion in a two-dimensional hydrogen jet in a supersonic air stream. OH spectra intensity and wall static pressure were measured and Schlieren photographs were obtained [16]. Fureby et al. studied supersonic combustion stabilized behind conventional and alternating-wedge injection struts using experimental data and computational results [10]. In the area of numerical investigations, Choubey and Pandey have reported a series of excellent simulations. They evaluated the performance of a typical two-strut scramjet combustor using ANSYS FLUENT [3,5,6]. Choubey and Pandey studied the effects of different injection schemes on the flow field [17]. They also conducted valuable researches on cavity Scramjet combustor [18–21]. Pandey et al. studied the effect of variation in hydrogen injection pressure and inlet air temperature on the flow-field of a typical double cavity scramjet combustor [22]. Nordin-Bates et al. conducted a numerical study of HyShot II scramjet combustor using Large Eddy Simulation (LES); the simulation results were able to capture the experimental wall-pressure and heat-flux quite well compared to the experimental data [23]. Santana and Weigand investigated the inlet-combustor interactions for a scramjet hydrogen-fueled engine at a flight Mach number of eight with three different angles of attack [24]. Edwards simulated the supersonic reacting wall jet with a hybrid Large-Eddy Simulation/Reynolds-averaged Navier–Stokes (LES/RANS) solver [25]; the computed results agreed well with the
https://doi.org/10.1016/j.combustflame.2019.08.038 0010-2180/© 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Q. Qin, R. Agarwal and X. Zhang / Combustion and Flame 210 (2019) 292–301
experimental data. Oevermann [26] numerically investigated the turbulent hydrogen combustion in a scramjet using flamelet modeling and compared the numerical results with the experimental data. Although there are many investigations reported in the literature about the strut-based scramjet combustor [27–31], however the half angles of the struts used in these investigations are relatively small. In this paper, a novel strut with the angle of the rear wedge enlarged is proposed. Since an oblique detonation wave can be induced behind a shock wave [32–34], the novel strut which induces strong shock waves is expected to stabilize of the flame. To evaluate the usefulness and performance of the novel strut in flame stabilization, a numerical simulation employing the two-dimensional compressible multi-species ReynoldsAveraged Navier–Stokes (RANS) equations with the standard k − ε model is conducted. Six cases are simulated to evaluate the performance of the proposed novel strut for flame stabilization. The combustion efficiency and total pressure loss of the combustors with the novel strut and the traditional strut are evaluated. A qualitative analysis is conducted to explain the stabilization of the flame by the novel strut. 2. Computational details 2.1. Governing equations In simulation of the reacting flow in scramjet combustor, the multispecies model needs to be taken into consideration. The twodimensional compressible multi-species Navier–Stokes equations can be expressed in conservation law form as:
∂ Q ∂ E ∂ F ∂ Eυ ∂ Fυ + + = + +S ∂t ∂x ∂y ∂x ∂y
(1)
Q = [ρ1 , . . . , ρN , ρ u, ρv, ρ E ]T
(2)
E= F=
T ρ1 u, . . . , ρN u, ρ u2 + p, ρ uv, u(ρ E + p)
(3)
T ρ1 v, . . . , ρN v, ρ uv, ρv2 + p, v(ρ E + p)
(4)
T ∂ c1 ∂ cN Eυ = ρ D1 , . . . , ρ DN , τ , τ , uτ + vτxy + qx ∂x ∂ x xx xy xx T ∂ cN ∂ c1 Fυ = ρ D1 , . . . , ρ DN , τxy , τyy , uτxy + vτyy + qy ∂y ∂y S = [ω˙ 1 , . . . , ω˙ N , 0, 0, 0]
T
(5)
(6) (7)
In Eqs. (1)–(7), Q is the conservation variables vector, E and F are convective flux vectors, Ev and Fv are viscous flux vectors, and S is the chemical reaction source terms vector. u and v are velocity component in x-direction and y-direction, respectively, p is the pressure, E is the total energy per unit mass, ci is the mass fraction of species i, and T is the temperature. Single step, laminar finite rate kinetics is been utilized to model the chemistry [29,30,35,36]. A single step chemical reaction of H2 (see Table 1) is considered. The single step model has been widely
293
Table 1 Single step chemical reaction of H2 . Reaction
A
n
E
H2 + 1/2O2 = H2 O
9.87e8
0.0
3.1e7
used in strut based scramjet simulations and the results obtained have been found to be in reasonable agreement with the available experiment data. The reaction rate is computed using the Arrhenius equation:
k = AT n e−E/RT
(8)
The Finite Volume Method (FVM) is used to solve the governing equations. The AUSM+ scheme [37–40] is employed to discretize the inviscid fluxes and k − ε model is chosen as the turbulence model. 2.2. Description of the novel strut The strut, shown in Fig. 1(a), is a fuel injector and a flame holder. The incoming flow is compressed on the surface of the strut, which has a half angle of θ 1 . If the half angle of the strut is enlarged, as shown in Fig. 1(b), stronger shock waves will be induced on the surface of the strut. Stronger shock waves will induce higher temperature and higher backpressure behind the strut, which are more desirable for combustion. However, the strut with larger half angle shown in Fig. 1(b) will cause larger total pressure loss which is not desirable for a propulsion system, although it promotes combustion. Considering these two factors, the proposed novel strut can be regarded as an appropriate combination of the struts shown in Fig. 1(a) and (b). Only the angle of the rear part of the strut is enlarged as shown in Fig. 1(c). The novel strut can be divided into three parts: a leading wedge, a step and a rear wedge with angle enlarged. The heights of the three struts are kept the same; this is to guarantee the same blockage ratio. The lengths of the traditional strut and the novel strut are the same. In the simulation, the half angle of the leading part θ 1 is 6◦ . The half angle of the rear part θ 2 is 30◦ . The length of the strut L is 32 mm. Hydrogen is injected parallel to the air stream through a hole whose height is D. The hole is located at the center of the strut base and the height is 1 mm. The height of rear wedge h is 0.8 mm. Given that this investigation is aimed at evaluation of the feasibility and performance of the novel strut, the effect of the rear wedge height on flame stabilization is not studied here. In applications, the wedge height can be different in different scramjet combustors depending on the requirements. The schematic of a typical scramjet combustor is shown in Fig. 2. The tip of the strut is located at x = 35 mm and y = 25 mm. The upper wall of the combustor diverges at x = 58 mm and the divergence angle is 3◦ . Air enters the combustor at the inlet with a static pressure of 10 0,0 0 0 Pa and static temperature of 300 K at different Mach numbers. The hydrogen is injected into the air stream through a mass flow inlet boundary. The total temperature of the hydrogen is 300 K. A pressure outlet boundary condition is applied at the exit of the combustor. No-slip condition is employed on all wall boundaries including the combustor walls and the surface of the strut. All the wall boundaries are regarded as adiabatic. Figure 3 shows the
Fig. 1. Schematic of the strut: (a) the traditional strut, (b) the strut with half angle enlarged and (c) the novel proposed strut.
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Fig. 2. Schematic of the scramjet combustor with the novel strut.
Fig. 3. Schematic of the mesh in the scramjet combustor with the novel strut (the entire mesh with 350,0 0 0 cells is not shown to keep clarity in the figure).
Fig. 4. Comparison between the computed density contours and the experimental shadowgraph: (a) and (b) are computational density contours; (c) experimental shadowgraph.
mesh inside the scramjet combustor. Structured mesh is used for the main region except in the region surrounding the strut, where an unstructured mesh is used. The size of the structured mesh is around 0.3 mm × 0.3 mm. The height of the first point in the boundary layer is 0.001 mm. The edge of the cell in the unstructured mesh has a length which ranges from 0.005 mm to 0.3 mm. The total number of cells is around 350,0 0 0. The mesh is solution independent and in generating the mesh, it is ensured that y + < 1 for the grid point next to the boundary.
3. Results and discussions 3.1. CFD validation CFD validation is done against the experimental data given in the paper of Oevermann [26]. The comparison between the experimental shadowgraph and the computed density contours is shown in Fig. 4. The mass flow rate is computed from the boundary conditions used in the work of Oevermann [26]. The reflections of the
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Fig. 5. Comparison between the computed and experimental velocity profiles at (a) x = 78 mm; (b) x = 125 mm; (c) x = 207 mm; and (d) at y = 25 mm.
Fig. 6. Pressure contours in the combustor at Mach 4.5 with (a) novel strut and (b) traditional strut.
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Fig. 7. Temperature and H2 O mass fraction contours of various flow properties in the combustor at Mach 4.5 (a) temperature contours with novel strut, (b) temperature contours with traditional strut; (c) H2 O contours with novel strut and (d) H2 O contours with traditional strut.
shock waves induced by the tip and tail of the strut agree very well with the experimental shadowgraph. The computed global flow field shows reasonable agreement with the experimental work. The velocity profiles along the centerline and at three different stream wise cross-sections are shown in Fig. 5. Although the computation is conducted under a two-dimensional assumption, the profiles show acceptable agreement with the experimental profiles. To get a better agreement, a three-dimensional model is really necessary since the hydrogen is injected through a row of 15 holes in the experiment. Nevertheless, the two-dimensional numerical model utilized in this investigation is sufficient to obtain the global flow features in the scramjet combustor and to evaluate the performance characteristics of the proposed novel strut.
Table 2 Computational cases. Strut type
Traditional strut
Cases Mach number
1 4.5
2 5
Novel strut 3 5.5
1 4.5
2 5
3 5.5
3.2. Evaluation of the performance of the proposed novel strut To evaluate the performance characteristics of the proposed novel strut at different Mach numbers of the air stream, three Mach numbers are considered at the inlet for both the traditional strut and the novel strut as shown in Table 2. To compare the difference between the combustion efficiencies in the combustors
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297
Fig. 8. Temperature contours inside the combustor with (a) novel strut at Mach 5, (b) traditional strut at Mach 5, (c) novel strut at Mach 5.5 and (d) traditional strut at Mach 5.5.
Fig. 9. Total pressure loss and combustion efficiency in the scramjet combustors with novel strut and the traditional strut.
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with the traditional strut and the novel strut, the mass flow rate of the hydrogen injection is set as a constant at 0.15 Kg/s for both struts. Figure 6 shows the pressure contours in the scramjet combustors with the novel strut and the traditional strut at Mach 4.5. The shock waves in Fig. 6(a) are mainly induced by the strut tip and from the reflections of these shock waves from the combustor walls. However, in the combustor with traditional strut shown in Fig. 6(b), a series of new shock waves are observed which are
induced by the combustion front. The combustion front can also be seen in Fig. 7(b) and (d) which provide the temperature contours and the H2 O mass fraction contours, respectively. The reason why these shock waves are not observed for the novel strut is that the combustion front is held right behind the strut as shown in Fig. 7(a) and (c). Figure 8 shows the temperature contours at higher Mach numbers. With the Mach number of the air stream being increased from 4.5 to 5.0 and 5.5, the combustor with the
Fig. 10. Pressure and temperature profiles near the strut base: (a) pressure profiles along x = 0.0678 mm for the cold flow without injection; (b) temperature profiles along x = 0.0678 mm for the cold flow without injection; (c) pressure profiles along x = 0.0678 mm for the cold flow with injection; (d) temperature profiles along x = 0.0678 mm for the cold flow with injection; (e) pressure profiles along y = 0.0284 mm for the cold flow with injection; and (f) temperature profiles along y = 0.0284 mm for the cold flow with injection.
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Table 3 The total pressure loss and the combustion efficiency at the exit of the combustor. Strut type
Novel strut
Mach number Total pressure loss (%) Combustion efficiency (%)
4.5 27.1 76.4
5 27.6 75.2
Traditional strut 5.5 28.3 73.6
4.5 31.1 51.9
5 30.8 70.0
5.5 30.7 68.5
novel strut still performs quite well and the flame is held behind the strut. The flame is below downstream in the combustor with traditional strut as shown in Fig. 8(b) and (d). The combustion efficiency and the total pressure loss of the combustors with the novel strut and the traditional strut are evaluated in Fig. 9. The total pressure loss and the combustion efficiency are calculated using the method introduced by Choubey and Pandey [3]. The total pressure losses for all the cases are the same before x = 0.0595 m. The branch point of the novel strut and the traditional strut is located at x = 0.0595 m and that is where the step on the novel strut begins. Attention needs to be paid to the phrase “the novel strut and the traditional strut” which refers to “the combustor with the novel strut and the combustor with the traditional strut”. The total pressure losses at the exit of the combustor with the novel strut are lower than that with the traditional strut at different Mach numbers, although there is a region where the total pressure losses of the novel strut are larger than that for the traditional strut. The lower total pressure losses of the novel strut can be explained by the earlier initiation of combustion which causes more energy release. Especially, the novel strut performs very well at Mach number of 4.5. All the total pressure losses and the combustion efficiencies at the exit are shown in Table 3. The total pressure loss is decreased by 4% compared to that for the traditional strut at Mach number of 4.5. In Fig. 9(b), the combustion efficiencies along the stream in the combustor with the novel strut increase smoothly, while the combustion efficiencies in the combustor with traditional strut undergo a sharp increase at x = 0.11 m, x = 0.15 m, and x = 0.14 m for Mach numbers of 4.5, 5.0 and 5.5, respectively. The combustion efficiencies at the exit of the combustor with the novel strut are higher than that with the traditional strut at different Mach numbers. Especially, the combustion efficiency is increased by 24.5% at Mach number of 4.5. Although the rear part of the novel strut causes strong shock wave and large total pressure loss, it stabilizes the flame and the total pressure loss can be regard as the effective total pressure loss. 3.4. Mechanism of the flame stabilization of the novel strut To figure out the mechanism of the flame stabilization of the novel strut, the cold flow simulations in the combustors with the novel strut and traditional strut at Mach number of 4.5 are conducted. Both the cold flow with and without hydrogen injection are simulated. The mass flow rate is the same as with the reacting flow. The pressure and temperature profiles along the line x = 0.0678 m, which is the vertical line behind the strut at a distance of 0.8 mm, are shown in Fig. 10(a) and (b). The strut base is located at x = 0.067 m. The most obvious difference between the pressure profiles of two struts is the peak pressure caused by the strong shock wave. Besides, another difference is the pressure behind the strut base, i.e., the pressure profiles between 0.02164 m − −0.02836 m. From the magnified picture in Fig. 10(a), it can be seen that the backpressure of the novel strut is 90 0 0 Pa higher than that for the traditional strut. The backpressure of the novel strut is 14,500 Pa and that of the traditional strut is 5500 Pa. In Fig. 10(b), the maximum temperature difference between the two struts is obtained around y = 0.02164 m and y = 0.02836 m. The temperature difference is around 100 K. Figure 10(c) and (d) shows the pressure and temperature profiles in the cold flow of the two
Fig. 11. Velocity contours around the strut base: (a) and (b) velocity contours for the cold flow with injection; (c) and (d) velocity contours for the reactive flow with injection.
struts with injection. The locations of the maximum differences are the same as that for the cold flow with no injection. The pressure difference behind the strut is about 50 0 0 Pa and the temperature difference is around 100 K. Figure. 10(e) and (f) shows the pressure and temperature profiles along y = 0.0284 mm. It can be seen that the pressure and the temperature of the novel strut is higher than that of the traditional strut. Based on the analysis of the flow field parameters of the two struts, it is found that the temperature and pressure difference in the region near the strut base is as expected. Attention should be paid to the global reaction model of the hydrogen–air combustion. The one-step model is not sufficient to predict the ignition delay period or the detailed flame structure. Thus, there is no confidence to relate the ignition delay period with the flame stabilization. The only conclusion can be drawn here is that the flame stabilization is related to the temperature and pressure near the strut base. A detailed reaction mechanism which can predict the ignition delay period and flame feature correctly is really needed for revealing the detailed mechanism of the flame stabilization. However, this is not the purpose of this research. This research is focused on designing a new strut for stabilizing the flame behind the strut. In the cold flow with no combustion, the injection flow fields of the novel strut and the traditional strut are similar and both show the bottle-like shock waves as shown in Fig. 11(a) and (b). The injection flow fields are affected by the backpressure at the strut base as shown in Fig. 10(c) and (d). In the reacting flow of the novel strut, the flame is held behind the strut and energy release increases the backpressure near the strut base. This leads to a narrow injection flow field and larger low-velocity zone, as shown in Fig. 11(c). However, the flame is blown far away from the strut in the traditional strut and a similar injection is observed as for the cold flow with no combustion. To verify the effect of temperature and pressure on the flame stabilization, the case 1 (Mach number of 4.5) is simulated with temperature and pressure of the inlet increased by 10 0 K and 20,0 0 0 Pa, respectively. The combustion front is observed to move back towards the strut and the flame is held by the strut as shown in Fig. 12(a). Figure 12(b) and (c) shows the injection flow field of the cold flow and the reactive flow. The
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Fig. 12. Temperature and velocity contours in the combustor with the increased temperature and pressure at the inlet: (a) temperature contours for the reacting flow; (b) velocity contours for the cold flow with injection; (c) velocity contours for the reacting flow.
Fig. 13. Temperature profiles near the strut base: (a) pressure profiles along y = 0.0284 mm for the cold flow with injection; (b) temperature profiles along y = 0.0284 mm for the cold flow with injection.
injection flow fields are similar to those of the novel strut which are shown in Fig. 11(a) and (c). The pressure and temperature profiles along y = 0.0284 mm are shown in Fig. 13. The pressure profile of the traditional strut with changed inlet condition is slightly higher than the original profile, but is much lower than that of the novel strut. The temperature profile is nearly the same with the novel strut but slightly higher in the rear part. It is difficult to confirm whether the pressure or the temperature dominates the most in flame stabilization by considering just one case. A theoretical analysis and a parametric study are really necessary and these will be completed in our future investigation. One thing has been
established in this study is that the flame is stabilized behind the novel strut and the flame stabilization can be explained by the increased pressure and temperature near the strut base induced by the enlarged angle in the rear part of the novel strut. 4. Conclusions A novel strut for a scramjet combustor is proposed. The angle of the rear part of the novel strut is enlarged. A numerical model based on the Navier–Stokes equations is established to simulate the combustion in the strut-based scramjet combustor and to
Q. Qin, R. Agarwal and X. Zhang / Combustion and Flame 210 (2019) 292–301
verify the superior performance of the proposed novel strut compared to the traditional strut. It is found that the flame is stabilized behind the novel strut and the flame stabilization can be explained by the increased pressure and temperature near the strut base induced by the enlarged angle in the rear part of the novel strut. The shock waves induced by the rear part of the novel strut cause a high-pressure and high-temperature region for combustion. The strong shock waves affect only the nearby region of the strut for the reason that only a small part of the wedge is wedge angle enlarged. Thus, the total pressure loss caused by the rear wedge can be regard as an effective total pressure loss. The novel strut can be used as a flame holder in the scramjet combustor. Acknowledgment The first author is grateful for the hospitality and the resources provided by the CFD laboratory of the mechanical engineering department of Washington University in St. Louis, USA References [1] F.J. Förster, N.C. Dröske, M.N. Bühler, J. Von Wolfersdorf, B. Weigand, Analysis of flame characteristics in a scramjet combustor with staged fuel injection using common path focusing Schlieren and flame visualization, Combust. Flame 168 (2016) 204–215. [2] B. Liu, G. He, F. Qin, Q. Lei, J. An, Z. Huang, Flame stabilization of supersonic ethylene jet in fuel-rich hot coflow, Combust. Flame 204 (2019) 142–151. [3] G. Choubey, K.M. Pandey, Effect of parametric variation of strut layout and position on the performance of a typical two-strut based scramjet combustor, Int. J. Hydrog. Energy 42 (2017) 10485–10500. [4] G. Choubey, K.M. Pandey, Numerical studies on the performance of scramjet combustor with alternating wedge-shaped strut injector, Int. J. Turbo Jet Engines 34 (2015) 11–12. [5] G. Choubey, K.M. Pandey, Investigation on the effects of operating variables on the performance of two-strut scramjet combustor, Int. J. Hydrog. Energy 41 (2016) 20753–20770. [6] G. Choubey, K.M. Pandey, Effect of different strut + wall injection techniques on the performance of two-strut scramjet combustor, Int. J. Hydrog. Energy 42 (2017) 13259–13275. [7] G. Choubey, K.M. Pandey, Effect of variation of angle of attack on the performance of two-strut scramjet combustor, Int. J. Hydrog. Energy 41 (2016) 11455–11470. [8] J.P. Drummond, Enhancement of mixing and reaction in high-speed combustor flowfields, International Colloquium on Advanced Computation and Analysis of Combustion, Moscow, Russia, 1997, pp. 1–14. [9] G.B. Northam, C.S. Byington, Evaluation of parallel injector configurations for supersonic combustion, 25th Joint Propulsion Conference (1989). [10] C. Fureby, K. Nordin-Bates, K. Petterson, A. Bresson, V. Sabelnikov, A computational study of supersonic combustion in strut injector and hypermixer flow fields, Proc. Combust. Inst. 35 (2015) 2127–2135. [11] E.T. Curran, Scramjet engines: the first forty years, J. Propul. Power 17 (2001) 1138–1148. [12] A. Ferri, H. Fox, Analysis of fluid dynamics of supersonic combustion process controlled by mixing, Sympos. (Int.) Combust. 12 (1) (1969) 1105–1113. [13] A. Ferri, Mixing controlled supersonic combustion, Ann. Rev. Fluid Mech. 5 (1973) 301–338. [14] W. Waidmann, F. Alff, U. Brummund, M. Bohm, W. Clauss, M. Oschwald, Experimental investigation of the combustion process in a supersonic combustion ramjet (SCRAMJET) combustion chamber, DGLR-Jahrestagung 1994. [15] T. Sunami, A. Murakami, K. Kudo, M. Kodera, M. Nishioka, Mixing and combustion control strategies for efficient scramjet operation in wide range of flight Mach numbers, 11th AIAA/AAAF International Space Planes and Hypersonic Systems and Technologies Conference (2002).
301
[16] R. Guerra, W. Waidmann, C. Laible, An experimental investigation of the combustion of a hydrogen jet injected parallel in a supersonic air stream, 3rd International Aerospace Planes Conference (1991). [17] G. Choubey, K.M. Pandey, Effect of different wall injection schemes on the flow-field of hydrogen fuelled strut-based scramjet combustor, Acta Astronaut. 145 (2018) 93–104. [18] N. Kumar, G. Choubey, L. Suneetha, K.M. Pandey, Effect of variation of length– to-depth ratio and Mach number on the performance of a typical double cavity scramjet combustor, Acta Astronaut. 128 (2016) 540–550. [19] G. Choubey, K.M. Pandey, Effect of variation of inlet boundary conditions on the combustion flow-field of a typical double cavity scramjet combustor, Int. J. Hydrog. Energy 43 (2018) 8139–8151. [20] G. Choubey, Y. Devarajan, W. Huang, Recent advances in cavity-based scramjet engine – a brief review, Int. J. Hydrog. Energy 44 (2019) 13895–13909. [21] C. Paper, K.M. Pandey, A. Maji, T. Deshamukhya, A brief review on the recent advances in scramjet engine, AIP Conf. Proc. 1859 (020036) (2017) 1–6. [22] K.M. Pandey, G. Choubey, F. Ahmed, D.H. Laskar, P. Ramnani, Effect of variation of hydrogen injection pressure and inlet air temperature on the flow-field of a typical double cavity scramjet combustor, Int. J. Hydrog. Energy 42 (2017) 20824–20834. [23] K. Nordin-Bates, C. Fureby, S. Karl, K. Hannemann, Understanding scramjet combustion using les of the HyShot II combustor, Proc. Combust. Inst. 36 (2017) 2893–2900. [24] E.R. Santana, B. Weigand, Numerical investigation of inlet-combustor interactions for a scramjet hydrogen-fueled engine at a flight Mach number of 8, AIAA J. (2012) 1–13. [25] J.R. Edwards, J.A. Boles, R.A. Baurle, Large-eddy/Reynolds-averaged Navier–Stokes simulation of a supersonic reacting wall jet, Combust. Flame 159 (2012) 1127–1138. [26] M. Oevermann, Numerical investigation of turbulent hydrogen combustion in a scramjet using flamelet modeling, Aerosp. Sci. Technol. 4 (20 0 0) 463–480. [27] F. Génin, S. Menon, Simulation of turbulent mixing bBehind a strut injector in supersonic flow, AIAA J. 48 (2010) 526–539. [28] W. Huang, Z.G. Wang, S.B. Luo, J. Liu, Parametric effects on the combustion flow field of a typical strut-based scramjet combustor, Chin. Sci. Bull. 56 (2011) 3871–3877. [29] K. Kumaran, V. Babu, Investigation of the effect of chemistry models on the numerical predictions of the supersonic combustion of hydrogen, Combust. Flame. 156 (2009) 826–841. [30] K.M. Pandey, S. Roga, G. Choubey, Computational analysis of hypersonic combustor using strut injector at flight Mach 7, Combust. Sci. Technol. 187 (2015) 1392–1407. [31] P. Gerlinger, P. Stoll, M. Kindler, F. Schneider, M. Aigner, Numerical investigation of mixing and combustion enhancement in supersonic combustors by strut induced streamwise vorticity, Aerosp. Sci. Technol. 12 (2008) 159–168. [32] C.A. Nordeen, D. Schwer, F. Schauer, J. Hoke, T. Barber, B.M. Cetegen, Role of inlet reactant mixedness on the thermodynamic performance of a rotating detonation engine, Shock Waves 26 (2015) 1–12. [33] H. Teng, H. Dick, Z. Jiang, Initiation characteristics of wedge-induced oblique detonation waves in a stoichiometric hydrogen–air mixture, Proc. Combust. Inst. 36 (2017) 2735–2742. [34] Q. Qin, X. Zhang, A novel method for trigger location control of the oblique detonation wave by a modified wedge, Combust. Flame 197 (2018) 65–77. [35] K. Kumaran, V. Babu, Mixing and combustion characteristics of kerosene in a model supersonic combustor, J. Propul. Power 25 (2009) 583–592. [36] K. Kumaran, P.R. Behera, V. Babu, Numerical investigation of the supersonic combustion of kerosene in a strut-based combustor, J. Propul. Power 26 (2010) 1084–1091. [37] M. Liou, Mass flux schemes and connection to shock instability, J. Comput. Phys. 160 (20 0 0) 623–648. [38] M. Liou, A sequel to AUSM, part II: AUSM + −up for all speeds, J. Comput. Phys. 214 (2006) 137–170. [39] M. Liou, The evolution of AUSM schemes, Def. Sci. J. 60 (2010) 606–613. [40] M. Liou, A sequel to AUSM: AUSM+, J. Comput. Phys. 129 (1996) 364–382.