Accepted Manuscript CFD analysis of a scramjet combustor with cavity based flame holders Obula Reddy Kummitha, K.M. Pandey, Rajat Gupta PII:
S0094-5765(17)31305-X
DOI:
10.1016/j.actaastro.2018.01.005
Reference:
AA 6634
To appear in:
Acta Astronautica
Received Date: 17 September 2017 Revised Date:
6 December 2017
Accepted Date: 2 January 2018
Please cite this article as: O.R. Kummitha, K.M. Pandey, R. Gupta, CFD analysis of a scramjet combustor with cavity based flame holders, Acta Astronautica (2018), doi: 10.1016/ j.actaastro.2018.01.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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CFD analysis of a scramjet combustor with cavity based flame holders Obula Reddy Kummithaa , K.M. Pandeyb , Rajat Guptab a Research
Scholar,Department of Mechanical Eng., NIT Silchar, Assam - 788010, India of Mechanical Eng., NIT Silchar, Assam - 788010, India
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b Professor,Department
Abstract
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Numerical analysis of a scramjet combustor with different cavity flame holders has been carried out using ANSYS 16 - FLUENT tool. In this research article the internal fluid flow behaviour of the scramjet combustor with different cavity based flame holders have been discussed in detail. Two dimensional Reynolds-Averaged Navier-Stokes governing(RANS) equations and shear stress turbulence (SST) k − ω model along with finite rate/eddy dissipation chemistry turbulence have been considered for modelling chemical reacting flows. Due to
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the advantage of less computational time, global one step reaction mechanism has been used for combustion modelling of hydrogen and air. The performance of the scramjet combustor with two different cavities namely spherical and step cavity has been compared with the standard DLR scramjet. From the comparison of numerical results, it is found that the development of recirculation
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regions and additional shock waves from the edge of cavity flame holder is increased. And also it is observed that with the cavity flame holder the residence
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time of air in the scramjet combustor is also increased and achieved stabilized combustion. From this research analysis, it has been found that the mixing and combustion efficiency of scramjet combustor with step cavity design is optimum as compared to other models. Keywords: scramjet, SST k − ω model, cavity flame holder, mixing ∗ Corresponding
author Email address:
[email protected] (Obula Reddy Kummitha)
Preprint submitted to Acta Astronautica
January 3, 2018
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efficiency, combustion efficiency, bow shock, recirculation zones.
1. Introduction
The scramjet is a variant of ramjet where combustion is going to take place
at supersonic speed, unlike ramjet where it is going to take place in sonic speed. In the 21s t century there is an increase space launch and there is a need for reusable launch vehicles. Scramjet may fulfil this need as it can be used as a
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reusable launch vehicle and a potential cruise missile which can reach and sustain hypersonic speed. NASA has tested X51-A scramjet which has successfully flown
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for 200 seconds and at Mach 5 speed. Indian space research organization (ISRO) has also tested a scramjet in 2016 which has achieved a speed of Mach 6 for 10
about 5 seconds. Unlike a typical turbojet engine which needs a device like a compressor for achieving high speeds, the scramjet does not have any moving parts and thus reducing the weight of the jet and increasing the thrust to weight ratio. The scramjet has a converging inlet where the air is compressed and a
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combustor, where fuel is going to mix with compressed air and a diverging nozzle where the heated air is accelerated to produce thrust. The scramjet is accelerated to supersonic speeds by using some external mechanisms such as rockets or planes. (NASA has used B-52 Star fortresses and a solid rocket booster for the project X51-A)
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In scramjet technology the main challenging task in order to design a new scramjet is the mixing of fuel and air and make sure that stabilized combustion
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has been taken place to burn enough fuel to generate a huge amount of energy to overcome the tremendous drag forces experienced at high speeds. In order to design a new scramjet, researchers must choose a fuel which can burn rapidly and generates a large enormous amount of thrust. In order to achieve the
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above criteria hydrogen is the most suitable fuel, since it has a great feature of highest lower heating value (LHV) as compared to the other hydrocarbon fuels; generally it describes the amount of energy released when fuel is burned. In the design of scramjet combustor fuel injection techniques have been de-
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signed based on two important parameters which are defined as ignition process and flame holding. The ignition process and flame holding mechanism charac-
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teristics in scramjet combustor has been investigated by many researchers by
considering various fuel injection techniques [1, 2] and with different flame holder cavities [3, 4]. The performance of scramjet combustor is greatly depends on the
efficient mixing of fuel and air and combustion stability. These two parameters
are greatly affected by the fuel injection technique and flame holding mecha-
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nism. Therefore the improvement of scramjet performance needs to be enriched fuel injection technique and cavity flame holder. In past research studies many
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researchers have been studied to enhance the scramjet performance with different fuel injection techniques using different struts, but it has been identified 40
that strut injection techniques has a major disadvantage of high pressure losses. Fuel injection technique along with cavity based flame holder is the combined technique which attracts more attention for the purpose of fuel injection and flame stabilization. Many researchers [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] have been studied the cavity flame holder concepts with different parameters to visualize the cavity flow field and to find out the effect of cavity on ignition,
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mixing and combustion efficiency of scramjet combustor. The key parameter which is discussed by many researchers is residence time of supersonic air because all the performance parameters are the function of residence time, and
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found that it has been increased by the addition of cavity flame holder to the scramjet combustor.
Wei Huang et al. [16] discussed on overview of scramjet combustor with
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different fuel injection techniques like strut injection, cantilever ramp injection and normal injection along with cavity flame holder. All the fuel injection techniques were summarized and comparison was made and proposed a promising
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fuel injection technique with the combination of two or more fuel injection techniques and with the combination of fuel injection plus cavity flame holder. In supersonic flows, the value of cavity is more to enhance the mixing and resident time of air in the combustion chamber. F.Xing et al. [17] studied the effect of cavity flow on scramjet combustor performance. The primary goal of their 3
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research was to investigate the effect of corner plate which was located at the
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entry of cavity holder. For the case of non-reacting flow, the boundary layer thickness of downstream flow of the rear wall cavity increases with increase in distance of corner plate from the side wall of the combustor. For the case of
reacting flow analysis, they found that the distance affects greatly the location 65
of fuel rich regions and achieves flame stabilization in cavity flow field and en-
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hances the early ignition and reduces the ignition delay and also decreases the weight of the scramjet by reducing the length of the scramjet.
P.Manna et al. [18] investigated a flight-worthy scramjet combustor through
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optimization process using CFD. Numerical simulations were carried out for reacting flow field of scramjet of the combustor with different strut arrangements and fuel injection schemes to enhance the performance of scramjet and to obtain an optimized scramjet model. Three dimensional Reynolds Averaged NavierStokes (RANS) governing equations were solved for more approximations of flow parameters visualizations along with two equations k-epsilon turbulence 75
model. Lagrangian particle tracking approach and eddy dissipation model along
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with fast rate chemical kinetics were used to simulate the trajectory of kerosene droplets and combustion modelling. From the numerical results they found that at the exit plane of the combustor, unused oxygen stagnated at the sides of the combustor walls and a considerable amount of unburnt kerosene vapours were present in the core regions of the combustor. From the optimization process,
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they found modified strut locations and fuel injection scheme and improved the combustion efficiency and thrust by 18.6% and 18.3% respectively as compared
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to the baseline configuration. MSR Chandra Murty and D Chakraborty [19] explored numerically the an-
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gular injection of hydrogen fuel into a scramjet combustion chamber. Numerical modelling was done with three dimensional Navier-Stokes governing equations and two equations turbulence models along with finite rate chemistry turbulence model. The main objective of this research paper was the investigation of the influence of chemical kinetics on the internal flow field of scramjet combustor.
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From the comparison of two turbulence models, it was found that k − ω tur4
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bulence model performs better than the k − model. And also it was observed
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that global one step finite rate chemistry reaction model performs well in predicting the flow structure in the combustor. Finally, authors were concluded
that simple chemistry can describe the hydrogen - air reaction in combustion 95
chamber reasonably well.
In the design of scramjet combustor fuel injection, ignition and flame holding
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plays an important role because of the efficient combustion with in a manageable length of the combustor is possible only with rapid mixing of fuel with supersonic air stream and this is achieved with a successful fuel injection and flame holding phenomenon. M.R Gruber et al. [20] studied the fundamental concepts of cavity
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based flame holder supersonic combustors with experimental and computational investigations. The scramjet computational domain was equipped with several open type cavity flame holders. The investigation was carried out based on aft ramp angles and offset ratios. From the numerical analysis, it was identified 105
that aft ramp angle plays an efficient role in the study of a shear layer that spans over the cavity wall. For the combination of ramp angle = 90o and offset
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ratio = 1, it is observed that a strong compression wave was developed as the flow separates from cavity upstream wall. Shorter resident time and higher drag coefficients were found in cavities with shallower ramp angles. 110
Peng gao et al. [21] worked on numerical research of scramjet combustor
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with different structure cavities. From the numerical analysis, they found that the effect of structure cavity on the mixing of fuel with supersonic air stream and stabilized fire burning at supersonic speed were quite different. And also
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they identified that with optimization of different structure cavities the flame 115
stabilization can be improved and better scramjet combustor design can be developed.
The combined use of strut and cavity flame holder is the most attractive
technique for mixing enhancement in supersonic scramjet combustors. Chenlin Zhang et al. [22] investigated the combined configuration of strut and cavity
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flame holder along with multi-staged injection of fuel with a numerical solver and found that multi-staged injection of fuel increases the performance of com5
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bustor as compared to the single stage and also found that the combination of
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strut along with cavity flame holder greatly increases the performance of combustor. In combustion phenomenon the mole fraction of species influence the 125
rate of combustion, therefore it is more important to consider the proper predefined species mole fractions for combustion modelling. Wei Huang et al. [23] investigated the influence of H2 O species mass fraction and chemical reaction
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mechanism on H2 - O2 combustion in supersonic flows. Two chemical reaction mechanisms have been considered namely, two step and seven step reaction 130
mechanisms. From the numerical analysis of two different chemical reaction
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mechanisms and with different mass fraction of H2 O species found that the chemical reaction mechanism has only a slight impact on combustion process. The mass fractions of H2 O greatly influence the intensity of turbulent combustion and found an optimum value of 0.15 for the considered range in this 135
paper.
The achievement of flame stability in supersonic combustor is the difficulty and most interesting task. Yixin Yang et al.[24] conducted both the experimen-
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tal and numerical study to investigate the effect of cavity on flame stability in supersonic combustion, for this two combustor configurations have been con140
sidered and are defined as parallel dual-cavity and tandem dual-cavity configurations. From the study of experimental and numerical analysis they found
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that, for the both combustor configurations the flame stability has been attained with the co-existence of three regions which were identified as a reacting reservoir which is existed within the cavity recirculation zone, premixed and hydrogen-rich combustion region existed in the jet mixing region and the third
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one is downstream combustion zone which is supported by the pre-mixed and combustion regions. The length to depth ratio of cavity flame holder also influences the combustion phenomena which are investigated with more details by Navin Kumar Matho [25]. The mixing and combustion processes greatly
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influences by the transverse fuel injection which is deeply investigated by Wei Huang [26] by considering the single and multiport injection schemes. The combustion modes of hydrogen with different fuel injection schemes 6
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along with the cavity flame holder were analyzed by Hongbo et al.[27]. From
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the numerical analysis, they found that the stability of combustion was not obtained without a cavity flame holder. Three modes of combustion were observed with the cavity assisted hydrogen jet combustion which were classified as cavity
shear layer stabilized combustion, cavity assisted jet wake combustion and the combined of cavity assisted and recirculation region stabilized combustion. The
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transition mechanisms of combustion from ramjet to scramjet were explored numerically by Wei Huang and Li Yan [28]. They found that both the jet-tocrossflow pressure ratio and inlet boundary conditions have significant impact
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on transition mechanism. The effects of single strut and double strut fuel injection on the performance of scramjet combustor along with the effect of operating variables and also the strut plus wall injection techniques were investigated by 165
Gautam Choubey and K.M.Pandey [29, 30, 31]. From the numerical analysis, they found that the performance two strut injections have been increased. And also found that the scramjet combustor performance has been increased with high temperature of vitiation air. The creation of shockwaves and its shock train
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is greatly depends on the strut configuration and its position in the flow field of the combustor, which is more clearly investigated by Wei Huang [32]. The concept of wall-mounted cavity is the most attractive technique for mixing augmentation for supersonic flows. Wei-Huang et al.[33, 34] investigated the effect
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of wall-mounted cavity on mixing process and the variation of inlet boundary conditions on combustion performance in supersonic flows and identified that 175
the mixing efficiency of the configuration with wall-mounted cavity was more
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than the conventional physical model. The combustion also depends on the mechanism of the fuel insertion. For
efficient combustion, the fuel insertion should be in such a way where the air stream has suffered a considerable pressure loss. In this experiment, we have
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used a parallel strut injection because there is the probability of an increase in shock wave formation due to considerable change in the cross section of the combustor. As the scramjet is accelerated to supersonic speeds there is a high probability of sweeping of the fuel before the total combustion and this affects 7
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the efficiency and flight time of the scramjet to reduce this effect and to increase the time of combustion we have introduced cavity based flame holders and the
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eddies generated in this cavities due to sudden change in pressure is going to prolong the residence time of the mixture in the supersonic flow and thereby increasing its flight time and efficiency.
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2. Numerical modeling
In every numerically solved problems, the selection and modelling of governing equations play an important role because of the internal flow physics are
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greatly affected by the flow governing equations. In reacting flow problems the combustion phenomenon is mainly influenced by flow variables. The flow in a scramjet combustor is considered to be a compressible and turbulent case, hence 195
in this problem, the flow governing equations are defined as compressible and turbulent flow governing equations. These governing equations are described mathematically as Reynolds averaged Navier-Stokes equations (RANS). By em-
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ploying Reynolds Averaged Navier-Stoke (RANS) equations we can investigate the behaviour of the scramjet with a wide range of operating conditions and 200
geometries. With RANS equations we can precisely determine the position of formation of shock waves and their characteristics. The flow governing equations and species transport equations are considered as defined below [35, 36, 37, 9].
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Density based solver with SST (Shear stress transport) K − ω turbulence model [38, 39] has been used. Mixture materials are hydrogen and air, reaction is considered as finite-rate/eddy dissipation (volumetric reaction) and density is taken as an ideal gas. Finite-rate/eddy dissipation chemistry turbulence model
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avoids Arrhenius calculations and reduces the computation time and reaction rates are controlled by turbulence. Continuity Equation ∂ρ ∂ + (ρui ) = 0 ∂t ∂xi
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(1)
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Momentum Equation ∂ ∂ ∂P ∂ (ρui ) + (ρui uj ) = − + (τij ) ∂t ∂xi ∂xi ∂xi Energy Equation ∂ ∂ ∂ (ρet ) + (ρht uj ) = (τij ui − qi ) ∂t ∂xi ∂xi
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Species transport equation
(2)
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∂(ρYi ) ∂ ∂ + (ρuj Yi ) = − (ρu˘i Yi ) + ωi ∂t ∂xj ∂xi
(3)
(4)
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where, τij is the stress tensor and Yi is the mass fraction of chemical species. And ωi is the chemical source term of species i. The heat flux vector qi due to 215
conduction and convection is given as follows, qi = −λ
∂T + ρΣk=1 hk Yk u˘j,k ∂xj
(5)
The diffusion velocities u˘j,k are calculated by using ficks law which is given
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below
Yk u˘j,k = −Dk,m
∂ Yk ∂xj
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The stress tensor has been calculated by using Boussinesq hypothesis which relates the Reynolds stresses to the mean strain tensor and defined as follows.
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τij = (µ + µt )[
∂uj 2 ∂uk 2 ∂ui + − δij ] − ρkδij ∂xj ∂xi 3 ∂xk 3
(7)
Turbulence modelling is the key part in numerical modelling problems for
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the analysis of dissipation and development of internal thermal energy in a flow stream. The accuracy of all the physical flow parameters is dependent on the accuracy of turbulence modelling. Hence, the widely used shear stress turbulence (SST) model has been considered for more accurate prediction of
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boundary layer separation and vortices development region. SST turbulence model has an important feature which combines the both k − and k − ω turbulence model such that k − model is used in free shear flow and switches to the k − ω model used in the inner region of the boundary layer. The turbulent 9
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kinetic energy k and the specific dissipation rate ω can be obtained by the following turbulence governing equations [38, 40].
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∂ ∂ ∂ ∂k ˜ k − Yk + Sk (ρk) + (ρkui ) = (Γk )+G ∂t ∂xi ∂xj ∂xj
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∂ ∂ ∂ ∂ω ˜ ω − Yω + Dω + Sω (ρω) + (ρωui ) = (Γω )+G ∂t ∂xi ∂xj ∂xj
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˜ k and Gω represents the development of turbulent kinetic energy due where G to gradients of mean velocity and generation of ω. Γω and Γk represents the diffusivity of ω and k respectively. Yk and Yω shows the dissipation of k and ω
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due to turbulence. Dω represents the cross diffusion term and it is written as follows,
Dω = 2(1 − Ft )ρσω ,2
1 ∂k ∂ω ω ∂xj ∂xj
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where Ft is the blending function and is defined as
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where
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Ft = tanh(arg14 )
√
4ρσω ,2 k 500ϑ k arg14 = min[max( Cµ ωy 0 , y 2 ω ), CD 2] kω y
where CDkω is the positive portion of the cross diffusion term and y repre-
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sents the normal distance to the wall. 2.1. Combustion modelling
When we model a scramjet combustor, combustion process is the most im-
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portant task and it is greatly influenced by turbulence. Combustion can be defined as a rapid chemical reaction with high turbulence. Huang et al.[41] studied the effect of reaction mechanism on a strut based scramjet combustor internal flow physics and found that the hydrogen-air reaction mechanism makes a slight
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difference to the static pressure on internal walls of the combustor and Mach number. And also Gerlinger et al.[42] done a research on the effect of several hydrogen reaction mechanism on scramjet performance and flow visualization. 10
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From the open literature [41, 42, 43, 44] on hydrogen -air reaction mechanism
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it has been identified that a reaction mechanism has slight effect to predict the internal flow structure like a shock waves and recirculation regions which effect the mixing phenomenon. And also it was reported that with less computational
cost the performance parameters can be predicted with single step chemistry
considered and defined as follows:
3. Geometry modelling
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2H2 + O2 Þ 2H2 O
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model. In this research paper also global one step reaction mechanism has been
The new computational geometries are developed with the pre design modular of ANSYS - FLUENT and all the dimensions are taken from the DLR experimental setup [45, 46, 47] except the dimensions of the flame holder. The 265
scramjet has a total length of 340 mm and a height of 50 mm at the inlet of the compressor section and 62 mm at the outlet of diffuser section. The par-
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allel strut is placed at a distance of x = 77 mm and y = 25 mm, it has a half angle of 6o and has a hydrogen fuel injector in it. The cavity flame holders are placed at a distance of 120 mm which has been selected as there is a chance of 270
oblique shock formation which may be directly impacted into the cavity which is desirable as it may increase the pressure and thus increasing the mixing rate.
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Here we have modelled two types of cavity flame holders which are step cavity with a ramp and a spherical cavity both of them are shown below. The
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cavity flame holders have an l/d < 10. Both the cavities have a depth of 10 mm 275
and a length of 30 mm. Each step in the stepped cavity has a length of 5 mm and depth of 5 mm. The radius of spherical cavity is 15 mm. Fig. 1 shows the computational domains of the scramjet with and without cavities.
4. Boundary conditions Here we have considered Dirichilet boundary conditions at the inlet as we
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know the exact inlet parameters which are given below and the Neumann bound11
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Figure 1: Geometry modeling of scramjet (a) DLR Experimental setup model (b) scramjet with spherical cavity (c) scramjet with step cavity
ary conditions are considered at the exit. Numerical analysis has been carried
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out at steady state and the computation is initialized from the inlet. No slip condition is considered at the walls of the isolator, combustion chamber and the diffuser section. The boundary conditions are defined from the experimental setup [48] and shown in Table 1.
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5. Grid independence study Grid independence study is done to determine that the numerical solution
which has been obtained is independent of the type of grid taken. It means that the solution which is obtained will not differ or have negligible fluctuations even
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if the number of mesh elements has been changed. Fig.2 shows the grid inde-
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Air
H2
Ma
2.0
1.0
u(m/s)
730
1200
T(K)
340
250
P(Pa)
100000
100000
ρ(kg/m3 )
1.002
0.097
YO2
0.232
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YN2
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Table 1: Variable values for air and fuel inlets
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pendent study of all the models and it is observed that from the optimized grid size point the solution is not changing even with an increase in mesh elements. 2400
DLR Numerical Spherical cavity
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Step cavity
1800 1600
Converged grid size point and begining of stable solution
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Temperature (K)
2000
1200
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40000
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Number of elements
Figure 2: grid independence study
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6. Results and discussion
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The numerical analysis of scramjet combustor with two different cavities has
been performed with same numerical modelling and boundary conditions and
their respective results have been obtained and they have been compared with
each other to see the effect of the geometrical shape of the cavity which has been attached to the scramjet combustor.
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In supersonic scramjet combustors recirculation zone plays an important role to stabilize the flame and enhances the mixing and combustion of air-fuel. As
the air flowing with supersonic speed in combustion chamber, it results in a less
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resident time of the order of 1 millisecond. So the available time for mixing of air-fuel, atomization, and vaporization and for ignition and combustion is very less and it causes for wastage of fuel as it is flushed out with the supersonic air 305
stream. For all these complications somewhat the solution will be the properly designed flame holder cavity. In this paper, such type of two cavities are designed and numerical simulation has been carried out.
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The scramjet combustion chamber flow field with cavity flame holder is anal-
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ysed by the following features as specified in Fig.3 The development of boundary
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Figure 3: Cavity flow field [49, 50]
layer, shear layer, shock waves and recirculation region are the most important characteristics to discuss the flow field of scramjet combustor with cavity flame holder. In the current study all the flow field characteristics have been discussed by using the pressure and density based flow structure of the scramjet as shown in Fig.4 and Fig.5.
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Figure 4: Pressure contours (a) Standard scramjet model (b) Scramjet with spherical cavity (c) Scramjet with step cavity
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Figure 5: Density contours (a) Standard scramjet model (b) Scramjet with spherical cavity (c) Scramjet with step cavity
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From the pressure and density contours, it has been observed that the oblique
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shock waves are developed at the leading edge of the strut and undergoes into multiple reflections in-between combustion chamber walls and fuel stream and
develops a shock train and also entertain the mixing of fuel with the supersonic airstream. Expansion shock waves are generated at the trailing edge of the 320
strut and undergo into multiple reflections, but not that much more efficient in
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mixing of air-fuel as compared with oblique shock waves.
From the comparison of the standard model with the cavity attached models, it has been identified that additional shock waves and recirculation zones are
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developed in the cavity based model. Recirculation zones are the major role player for the mixing enhancement because it increases the residence time of supersonic air stream and the vortices developed from the recirculation zones are entered into fuel stream and carry the fuel into this recirculation zone and enhances the mixing of air-fuel.
In step cavity and semi sphere cavity models, it is observed that expansion 330
shock waves are developed at trailing and leading edge of the cavity and these
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are helpful for multiple interactions of supersonic air stream and fuel stream. The development of vortices is more in the case of a step cavity flame holder as compared to the other two models due to the presence of step design in the cavity flame holder and it produces double recirculation zones and they merge together and entered into the mainstream.
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From the flow structures of the cavity based models it is observed that shear layer has been initiated from the leading edge of the cavity and spans
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the entire length of the cavity, then it is called as an open cavity. Due to the existence of recirculation zones and vortices, open cavities are far better than
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the closed cavities for mixing enhancement of fuel and air. As the scramjet flow field involved different shock waves the pressure variation gains more interest to study how its various along the length of the combustor. The pressure variation of all the models is shown in Fig.6. As the presence of many oscillations in cavity flow the shear layer above the
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cavity is a paramount mechanism and it results in the addition and removal of 17
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(a)
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Standard-DLR
(b)
Standard-DLR 180000
Spherical cavity
Spherical cavity Step cavity
Step cavity
Pressure (Pa)
shock waves developed locations in scramjet
180000
150000
Pressure (Pa)
160000 210000
140000
120000
120000
100000 90000
0.00
0.05
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0.00
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0.30
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Length (m)
Figure 6: Pressure variation at (a) lower wall of the combustor (Y = 0) and (b) middle of the
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combustor (Y = 25 mm)
mass from the cavity through trailing edge. The impingement of shear layer at the rear wall of the cavity causes free stream flow to enter the cavity and results in an increase in pressure (Fig.6). This pressure rise develops an acoustic wave and propagates along the upstream of the cavity and impact at the front wall of 350
the cavity and develops vortices, which grow as the flow towards trailing edge
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of the cavity.
From the pressure profiles, it is observed that the rate of conversion of pressure energy into kinetic energy is more for the case of standard DLR scramjet combustor due to less drag coefficient as compared to the other two models. 355
The crest points in the pressure profiles indicate the impinging of shock waves
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at the walls of the combustion chamber. The investigation of the cavity flame holder effect on the performance of scramjet combustor has been extended by analysing velocity profiles at various
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locations along the length of the scramjet and the same plotted in Fig.7
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From the observation of velocity profiles at various locations along the length
of the combustor it has been identified that the kinetic energy has been increased from the entry of the isolator to the end of the diffuser section. But from the Fig.7 (a) and (b) it is observed that the variation of kinetic energy is not that much differentiable because combustion process has been initiated in
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the combustion chamber. As the cavity flame holder is attached the combus-
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0.06
0.06
(a)
Standard - DLR
(b)
Spherical cavity 0.05
Standard - DLR Spherical cavity
0.05
Step cavity
Step cavity
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Y (m)
Y (m)
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0.00 0.01
Velocity has been reduced and
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tion chamber at a distance of x = 120 mm, we can observe the effect of this on flow structure (Fig.7 (b)) it reduces the velocity of the main stream which is entered into the cavity and increase the resident time of air. As compared to
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the DLR scramjet combustor, cavity attached model has the more resident time of supersonic air stream and also early ignition has been occurred in the cavity
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flame holder models. Temperature is the main parameter to be considered for the analysis of combustion process and the effect of a cavity on flame stabilization. As the cavities are the open type the flame stabilization is not achieved at considerable level but it is increased as compared to the standard scramjet
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model.
Fig.9 shows the temperature profiles of all the models at different locations
along the length of the scramjet combustor. From the observation of temperature and velocity profiles it is identified that the velocity is less and temperature
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0.06
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Figure 8: Cross stream temperature profiles at different locations (a) x = 120 mm (b) x =
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136 mm (c) x = 167 mm and (d) x = 275 mm
is high at the middle of the combustor as compared to the nearest locations of 380
the combustor walls, it is due to the existence of drag force in between combustor walls and supersonic air stream and the presence of combustion process at
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the middle of the combustor respectively. If, we observe the temperature profile it is found that the temperature of the flow stream has been increased from x =
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120 mm to x = 275 mm due to the enhancement of turbulent mixing along the 385
flow direction in the combustion chamber. From the comparison of standard and cavity models, it is differentiated that the temperature of the main stream which is entered into the cavity has been increased due to the development of acoustic wave (compression wave) and multiple reflections in between molecules and cavity walls.
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The combustion stabilization can be classified as cavity stabilized combustion and jet-wake stabilized combustion [51, 27]. With the absence of cavity
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flame holder in standard DLR scramjet model, the combustion stabilization
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with jet-wake stabilized combustion cannot be achieved at the considerable level. From the temperature profile at the location of x =136 mm it is identified that 395
cavity stabilized combustion has been initiated at the leading edge of the two
different cavities and it causes for the increase in temperature at that location.
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7. Performance Parameters
The mixing efficiency and combustion efficiency are the most important parameters to assess the performance of scramjet combustor. The mixing efficiency can be defined as the ratio of hydrogen mass flux that could be burned to the
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total unburned mass flux of hydrogen and the same defined mathematically as follows [52]:
Where, m ˙ H2,x
R R αρuYH2 dA αρuYH2 dA A ηmix (x) ≡ R = A (11) m ˙ H2,x ρuYH2 dA A is the total unburned mass flux of hydrogen at location x, α the
function of equivalence ratio, u the normal velocity, YH2 the mass fraction of hydrogen and ρ is the density of flow stream.
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Fig.9 (a) shows the mixing efficiency of scramjet combustor with and without cavity flame holder. It is observed that numerical and experimental results are in good agreement qualitatively and quantitatively. It is also observed that the mixing efficiency has been increased instantly in the region of cavity flame holder up to x = 145 mm from there onwards it increases gradually. By the addition of
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a cavity flame holder to the scramjet combustor, the mixing efficiency has been improved by the development of recirculation zones, additional shock waves and
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vortices. From the observation of three models, it is found that better mixing efficiency is achieved for the step cavity flame holder.
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Both the mixing and combustion phenomenon are directly proportional, bet-
ter mixing of fuel and air will give good combustion efficiency. Combustion can be defined as a rapid chemical reaction with high turbulence. The combustion equation mathematically can be written as follows [31]. R m ˙ H2,x (A(x))ρgas uYH2 dA =1− ηcomb (x) = 1 − m ˙ H2,inj m ˙ H2,inj 21
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Figure 9: (a) Mixing efficiency and (b) Combustion efficiency
Where, m ˙ H2,inj is the total mass flux of injected hydrogen and m ˙ H2,x is the 420
mass flux of hydrogen at a given location. Fig.9 (b) shows the combustion efficiency of all the models. From the comparison of all the models, it is identified that the combustion efficiency of scramjet combustor has been increased by the attachment of cavity flame holder. From the combustion profiles, it is identified that step cavity has the highest combustion efficiency because of the development of more recirculation zones and vortices in step design cavity. The same
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study can be extended with different designs of step and ramp angles.
8. Conclusion
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The numerical analysis of scramjet combustor with two different cavity flame holders has been completed by using Reynolds averaged Navier-Stokes, SST 430
k-omega turbulence model, finite rate eddy-dissipation chemistry model and
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global one step reaction mechanism. The performance of scramjet combustor is mainly measured with mixing and combustion efficiency. Combustion efficiency will be greatly influenced by mixing efficiency; hence the major objective is the enhancement of mixing efficiency of fuel and supersonic airstream. From the
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literature, it is found that the simplest way to improve the mixing efficiency and for numerical simulation is the change of scramjet combustor flow geometry called as passive technique implementation. In this paper, two different
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cavities are considered and simulation has been done at same solver and bound-
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ary conditions. For the validation of numerical model, comparison has been done in between experimental and numerical results and found that there is a good agreement qualitatively and quantitatively. The effect of cavity flame holder on scramjet combustor performance has been studied with combustor
flow structure, temperature, velocity and the both mixing and combustion ef-
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ficiency. From the visualization of scramjet flow structure, it is found that the cavity scramjet models have the more recirculation zones, shock waves and vortices and all these improved the resident time and mixing of air-fuel. By the
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addition of cavity flame holder to the basic standard model, its combustion stability has been improved. From all the contours and graphs it is observed that cavity has a great effect on the performance of a scramjet combustor, the 450
same thing has been observed in terms of increment in pressure and temperature in the vicinity of cavity flame holder and boundary layer separation. From all the results and discussion it is to be concluded that the performance of scramjet combustor with cavity flame holder has been increased as compared to
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the standard scramjet model without a cavity. Among all the models the best performance of scramjet has been found with step cavity flame holder.
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Highlights
Comparison is made for a scramjet with and without cavity flame holder
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The residence time is increased with the presence of cavity flame holder.
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Step cavity design achieved optimum performance
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