Implication of Diamond Shaped Dual Strut On Combustion Characteristics in A Cavity-Based Scramjet Combustor

international journal of hydrogen energy xxx (xxxx) xxx
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Implication of diamond shaped dual strut on

combustion characteristics in a cavity-based
scramjet combustor
Lakka Suneetha, Pitambar Randive*, K.M. Pandey

Department of Mechanical Eng., National Institute of Technology, Silchar, Assam, 788010, India
highlights
Combustion performance in scramjet using single and dual strut is compared.

Spacing in diamond-shaped dual strut strongly dictates the combustor performance.
Optimal combustion performance is found at spacing (D) ¼ 1 mm in dual strut.
article info abstract
Article history: The present study deals with the implication of the novel diamond-shaped dual strut with
Received 27 February 2020 a backward-facing step on the combustion characteristics of a cavity-based scramjet
Received in revised form combustor. The dual strut with diamond shape is considered for the investigation since it
17 April 2020 triggers the flow separation leading to the disturbances in the flow especially at the top and
Accepted 24 April 2020 bottom wall thus serving the flame holding purpose. Firstly, the combustion characteristics
Available online xxx of a combustor with a single strut are compared and evaluated with a dual strut based
combustor to portray the effect of a dual strut. A separate study is carried out to investigate
Keywords: the influence of spacing in a dual strut on the performance of the scramjet combustor. Our
Dual strut study reveals that the dual strut with a cavity greatly affects the formation of vortices,
Cavity separation region, and recirculation region which is evident from an increase in the mixing
Scramjet and combustion efficiency. It can be observed that the formation of vorticity and the
Hydrogen recirculation region is found to be larger due to the strong and multiple reflections of a
Diamond-shaped strut shock in the case of the diamond-shaped dual strut with a backward-facing step injection
as compared to the single diamond-shaped strut. Further, it is observed that the spacing(D)
in the dual strut also affects the mixing and combustion performance. It is found that the
value of mixing and combustion efficiency decreases with an increase in spacing inde-
pendent of Mach number due to the presence of larger separation regions. It can also be
observed that the length of the recirculation region occupies entire the cavity making flame
stable when the spacing is the least. This is desirable as far as engine performance is
concerned.
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding authors.
E-mail addresses: kp691975@gmail.com, pitambar6975@gmail.com (P. Randive).
https://doi.org/10.1016/j.ijhydene.2020.04.217
0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: Suneetha L et al., Implication of diamond shaped dual strut on combustion characteristics in a cavity-based
scramjet combustor, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.04.217
2 international journal of hydrogen energy xxx (xxxx) xxx
combustion performance [13] since the profile of the strut

Nomenclature affects both the fluid flow and heat transfer mechanism.
Further, the growth of recirculation regions [14]and the nature
Dt Turbulent diffusivity
of the shock waves propagation [15] which ultimately affects
E Energy
mixing and combustion efficiency depends on the strut pro-
H Enthalpy
file. Recently, the diamond-shaped strut is extensively used
L/D Length to depth ratio of a cavity
since it promotes a stable flame and enhances the mixing rate
Ma Mach number
of fuel-air [16e18]. Nowadays, the scientific community has
m:H2; inj Mass flow rate of hydrogen fuel, kg/s
employed dual strut with a backward-facing step to increase
m:H2; ðxÞ Mass flow rate of hydrogen at a given section
the mixing rate of fuel and air [19,20]. This is primarily because
P Static pressure, Pa
the introduction of dual strut instead of single strut promotes
P0 Stagnation pressure, Pa
more separation of fluid flow from the upper side and as well
P0;i Pressure at inlet
as the lower side thus increasing the turbulence and mixing
PW =P0;i Wall pressure ratio
intensity. This suggests that it would be very much advanta-
Q Heat flux
geous if the dual strut, that too with diamond shape is used in
Re Reynolds number
conjunction with a cavity in a scramjet combustor.However,
T0 Stagnation temperature, K
there is little knowledge available on this aspect. Hence, an in-
T0, inlet Inlet temperature, K
depth investigation of the influence of strut configuration i.e.
u, v &w Velocity components in X, Y and Z direction
diamond-shaped dual strut with a backward-facing step in
ut Friction velocity
conjunction with a cavity is needed to understand its impli-
U0 Mean velocity component
cations on combustion in greater detail.
Xi Mole fraction of ithspecies
Earlier works in the scramjet engine started in the nine-
yP Distance from the point P to the wall
teenth century when NASA [21] designed the hydrogen-fueled
Yi Mass fraction of ithspecies
scramjet engine and discussed various aspects which include
Greek symbols thermal efficiency, engine system integration, structural
a Angle of a cavity integrity [22]. They reported that hydrogen fuel can also be
aR Mass fraction of least abundant reactant employed as cryogenic fuel to allow engine cooling [23e25].
as Stoichiometric mass fraction of fuel Further, it is considered to be one of the widely used fuel for
mt Turbulent viscosity hypersonic as well as supersonic conditions because of its
m Dynamic viscosity superior features like rapid atomization, vaporization, faster
n Kinematic viscosity dissipation rate [26] and its eco-friendly nature [27e29].
r Density, kg/m3 Consequently, hydrogen becomes the most appropriate fuel to
t Shear stress use in a scramjet engine [30,31]. Also, the specific impulse for
hcomb Combustion efficiency the hydrogen-fueled scramjet engine is higher as compared to
hmix Mixing efficiency the hydrocarbon fueled scramjet engine [32]. This has trig-
gered a lot of interest in the research on the hydrogen-fueled
scramjet combustor [33e38]. In this context, Liu et al. [39]
investigated the effect of hydrogen equivalence ratio on
combustion characteristics of a scramjet combustor at higher
Introduction Mach number. Further, Ye et al. [40] studied the influence of
thermal transpiration as well as the hydrogen flow behavior in
The past few decades have witnessed extensive research ef- the microchannel with the semi-circular obstacles. Later on,
forts in the area of combustion in scramjet engines since it is Xie et al. [41] numerically investigated the effect of thermal
one of the promising ideal propulsion devices for upcoming stratification to increase the hydrogen fuel flow and heat
space vehicles and high-speed weapons [1]. It is well known transfer rate. Kahraman et al. [42] and Lee et al. [43] reported
that reliable thrust is of utmost importance for the scramjet that hydrogen is used as an additive for hydrocarbon fuel to
propulsion system over a wide flight envelope i.e., at altitudes reduce the emissions of NOX and CO2 . Moreover, it has been
of 20e55 km and flight Mach numbers of 3e25 [2,3] which observed that most of the swirl stabilized combustors [44e46]
strongly depends on the flame stability. This operational as well as the cavity stabilized combustors [47e49] in-
range of flight Mach numbers complicates the design of vestigations have employed hydrogen as a fuel to reduce the
scramjet particularly at higher Mach numbers wherein less flame blow-out problems.
residence time which is in the order of milliseconds [4] affects The geometrical configuration of a dual strut and its posi-
the performance of the combustor. These issues have been an tions plays a vital role in improving the mixing as well as
obstacle in the development of a genuinely full-grown design combustion phenomena in a cavity assisted supersonic
of a scramjet engine. To overcome these issues, the re- combustor. In this context, some of the notable works include
searchers have employed various flame holding techniques the use of novel strut design [50e52] and also the introduction
which include cavity [5,6], backward-facing step [7,8], the strut of dual strut [19,20]. For instance, Qin et al. [50] designed a
[9,10], pylon [11,12] and ramp [13,14]injection technique. strut for a scramjet combustor by enlarging the angle of the
Among these techniques, the introduction of the strut in rear part of the strut. They observed that the flame becomes
scramjet proves to be instrumental in improving the stable behind the strut as well as shockwave induces owing to
international journal of hydrogen energy xxx (xxxx) xxx 3
the presence of high temperature and the high-pressure re- comparative evaluation of the combustion performance in the
gion at the rear part of the strut. Aravind and Kumar [51] case of the combustor with a single and dual strut is per-
discussed the flow characteristics with and without reaction formed to establish the effectiveness of dual strut to enhance
owing to the improved strut injection schemes. They reported the performance. Thereafter, the effect of spacing (D) in the
that the improved strut injector increases the formation of dual strut on the combustor performance is discussed. The
vortices in the streamwise direction to enhance the rate of result delineated herein are presented in the form of shock
fuel-air mixing. Li et al. [19,20] investigated the combustion wave structure, streamline distribution and the variation of
characteristics of scramjet combustor with a dual strut. They mixing as well as combustion efficiency along the down-
reported that the combustion and mixing performance has a stream length of the combustor.
great influence on the geometrical configuration of the strut.
Also, they observed the double-strut with backward-facing
steps enhances the mixing and combustion efficiency as Problem definition and theoretical formulation
compared to single-strut. Further, Zhang et al. [52] experi-
mentally studied the combined influence of strut and wall In the current study, the implications of the dual strut on the
injection scheme in a flush-wall scramjet combustor with combustion characteristics of a cavity assisted scramjet
liquid kerosene. They found that the core flame enhances combustor is investigated. First of all, the combustion per-
with the increasing of fuel equivalence ratio. formance of combustor with dual strut is compared with that
An introduction of the cavity in the combustor has been a of the combustor with a single strut followed by the discussion
practice quiet frequently [53e57] to increase the flame stabil- on the effect of spacing(D) in a strut. Fig. 1 shows a schematic
ity. It is well known that the low-speed flow recirculation re- diagram of the cavity-based combustor with a diamond-
gion is developed in the region of the cavity which leads to an shaped dual strut injection.
increase in the fuel residence time and air-fuel mixing rate.- The isolator cross-sectional area is 358
Recently, the U.S. Air Force’s X-51 [53] has employed a cavity 25.4 mm2whereas the combustor length is 444 mm. A cavity
as a flame stabilization device for an integrated fuel injection is enclosed within the combustor whose depth and length are
approach due to its simple geometry and minimum losses. 12.7 mm and 50.8 mm, respectively. The dual strut is located
Later, Liu et al. [54] used a multiport injection system in a at X ¼ 326.5 mm, Y1 ¼ 24.9 mm and Y2 ¼ 25.9 mm from the
cavity assisted scramjet combustor. They argued that the inlet of the isolator. The length of each strut is 32 mm with a
flame stability is easier to anchor by the interaction of multi- cone angle of 12 . The hydrogen fuel injection is started from
ple jets and to reduce the thermal chocking in the case of the step of a strut of 1 mm in diameter. It may be noted that
parallel injection scheme. Moradi et al. [55] studied the the spacing (D) in the dual strut indicates the perpendicular
different shapes of a cavity in a scramjet combustor. They distance between the top and bottom portion of the strut and
observed that the cavity with the trapezoidal profile results in varied from D ¼ 1 to 4 to understand the effect of spacing on
a maximum enhancement in mixing. Further, the combustion performance.
Hassanvandet al. [56] Numerically investigated the effects of
dual micro-jets in a 3D supersonic combustor. They stated Governing equations
that the vertical hydrogen jet gives better mixing performance
than the horizontal hydrogen jets. Edalatpouret al. [57] The numerical simulations are performed to understand the
investigated the multi-port injection of hydrogen jets within influence of a diamond-shaped dual strut profile with a
the region of the cavity in a scramjet combustor. They re- backward-facing step and also the distance between the strut
ported that the jet pressure ratio has a strong influence on the profile. The finite volume method (FVM) was employed to
performance of the combustor. discretize the governing equations along with the boundary
Despite the several works cited above, the effect of dual conditions. Two dimensional compressible, Reynolds Aver-
strut configuration with the backward-facing step on com- aged Navier Stokes (RANS) equations are as given below:
bustion performance in a scramjet combustor has remained Continuity Equation:
largely overlooked. Hence, the present work is undertaken
vr v
wherein the combustion performance of scramjet with þ ðrui Þ ¼ 0: (1)
vt vxi
diamond-shaped dual strut is discussed. First of all, a
Momentum Equation:

v v vP v vui vuj 2 vui v
ðrui Þ þ rui uj ¼ þ m þ dij þ
vt vxj vxi vxj vxj vxi 3 vxi vxj

ru0i u0j : (2)
Energy Equation:
v v v
ðreÞ þ reuj ¼ tij ui qj (3)
vt vxj vxj
Where e and tij are the energy and shear stress, respectively.
Species transport equation:
Fig. 1 e Schematic: Cavity assisted supersonic combustor
with a diamond-shaped dual strut.
v ! vu vT vk vu
ðrYi Þ þ V:ðrYi !
u Þ þ V:ð Ji Þ ¼ ui (4) ¼ 0; ¼ 0; ¼ 0; ¼0: (9)
vt vx vx vx vx
!
where the mass diffusion term ð Ji Þis expressed as given below Performance parameters of a supersonic combustor

! m VT
Ji ¼ rDi;m þ t VYi DT;i (5) The performance parameters like mixing as well as combus-
Sct T
tion efficiency are significant to measure the performance of a
where Yi is the mass fraction of ith species, DT;i is the thermal scramjet combustor. The mixing (hcomb) efficiency [63] and the
diffusion coefficient of ith species, and ui is the chemical re- combustion efficiency [64] can be expressed by
action rate of ith species.
Z 0
Accordingly,turbulent Schmidt number ðSct Þ ¼ rD mt
is taken as
t m:H2;inj rgas uYH2 dA
m:H2ðxÞ
0.7. hcomb ðxÞ ¼
AðxÞ
¼1 (10)
m:H2;inj m:H2;inj
Boundary conditions
Z0
Table 1 Describes the inlet conditions of supersonic free aR rudA
stream air and hydrogen fuel. The significant boundary con- hmix ðxÞ ¼ (11)
m:H 2 ðxÞ
ditions used to the computational domain are as follows
where aR ¼ mass fraction of the least available reactant
(i) inlet: 8 9
>
< a; a as >
=

aR ¼ 1a (12)
The flow conditions of a supersonic combustor [58e60] are >
: as ; a > as >
;
1 as
given as
where a is the fuel mass fraction and as is the injectant stoi-
3 K1=2
u ¼ U0; T ¼ Tinle; k ¼ ðU0 IÞ2 ; u ¼ 1=4 : (6) chiometric mass fraction.
2 Cm 0:07d
where turbulent kinetic energy ðkÞ ¼ 3

2
ðU0 IÞ2 . Further,
U0 indicates the mean flow velocity whereas I refer to the Numerical procedure
turbulent intensity which is assumed to be less than 5%.
The specific dissipation rate (u) is given as The current investigation employs a two-dimensional den-
sity-based solver of FLUENT 14.0 [65] with an implicit second
K1=2 ordered upwind scheme for the quick convergence. The SST k-
u¼ (7)
C1=4
m 0:07d u turbulence model is used to capture the intricate physics
such as the formation of a shock wave and separation region.
where cm ¼ 0:09 and d refer to the diameter of the combustor.
The SST k-u model is considered in the present study which
employs the standard k-ε model in the detached regions
(ii) Top and Bottom wall:
whereas the Wilcox k-u model in the near-wall region [61]. It
can be noticed that the SST k-u turbulence model is more
The adiabatic, as well as a no-slip boundary condition, is
precise than the k-ε turbulence model which gives an accurate
used at the top and bottom wall. Further, all the parameters
prediction of jet flows and mixing layers [66]. The detailed
except u are set to zero [61] and are given as below
formulation of the SST k-u turbulence model can be found in Li
6n et al. [67] and Roy et al. [68].
u ¼ 0; k ¼ 0; u ¼ (8)
b1 ðDyÞ2
Turbulence-chemistry modeling
where Dy refers to the distance between the wall and the next
near-wall grid point. The general form of the chemical reaction for the hydrocarbon
is.
(iii) Outlet:

y 79 y 79 y
C x Hy þ x þ O2 þ N2 / xCO2 þ H2 O þ xþ N2
The variables including pressure and temperature have 4 21 2 21 4
been extrapolated from the internal cells [62]. (13)
The stoichiometric ratio of a hydrocarbon is given as,
WCx Hy 36x þ 3y
Table 1 e Parametric values taken for the current study. fst ¼ ¼
103ð4x þ yÞ
(14)
y
Fluid Ma P0 T0 Mole fraction (Xi) xþ 4
WO2 þ 21WN2
79
(kPa) (K) O2 H2 H2O N2 The equivalence ratio of a hydrocarbon is given as,

Hydrogen 2.52 820 330 0 1.0 0 0
Air 1 610 1350 0.21 0.0 0.21 0.58
the entire flow. Accordingly, the distance between the wall

f m_ f m_ ∞
f¼ ¼ (15) and the first-row cell is fixed at a distance of 0.001 mm [74].
fst fst
Herein, Wi is the molecular weight of ith species, m_ ∞ is the

mass flow rates of supersonic freestream and m_ f is the mass Validation
flow rate of fuel, respectively.
If y ¼ 2 andx ¼ 0, the stoichiometric ratio of a fuel is. The validation study is performed to know the accuracy of the
numerical solver. Two different test cases have been consid-
36x þ 3y 3
fH2 ;st ¼ ¼ ¼ 0:0291 (16) ered and compared with the experimental studies to check
103ð4x þ yÞ 103
the accuracy. Firstly, the experimental work of Micka et al.
In this study, the finite rate eddy dissipation model is used [58e60] for flow through cavity assisted supersonic combustor
since it gives a better prediction than the eddy dissipation is considered for the comparison of the results. The second
model [69]. A single-step kinetic mechanism [70] is considered validation test case is taken from an experimental and nu-
for the present investigation to model the interaction of the merical work of a diamond-shaped scramjet combustor by
hydrogen and oxygen as shown in Table 2. Gerlinger and Brüggemann [75].
Arrhenius equation describes the reaction rate which is Fig. 2 shows the comparison of the present results with the
written as. results of Micka et al. [58e60]. It can be noticed that the
pressure reaches the peak values at the location of a fuel in-
K ¼ ATn expðE = RTÞ (17)
jection part owing to the strong interaction of shock waves. It
Where K indicates the reaction rate and n is the reaction order. is seen that the SST k-u model shows a close agreement with
Further, T, E and A refer to the static temperature, activation the results of Micka et al. [58e60] as compared to the other
energy, and the pre-exponential collision frequency factor, turbulence models. However, the small discrepancy is
respectively. Further, it is well established that the single-step observed at the trailing edge of the cavity which is seen in
chemistry model is appropriate for the estimation of the Fig. 2. It may be noticed that the error arises out of differences
performance parameters with the impressively less compu- in implementation of in boundary conditions and settings in
tational amount [71]. experimental work. From Fig. 2, the error is found less than 5%
and agrees well with the results of Micka et al. [58e60] thus
Mesh independence study validating the methodology.
Another validation is carried out for the flow through a
In this study, a mesh quality evaluation is performed to know strut based supersonic combustor with the work of Gerlinger
the optimum number of meshing elements that would cap- and Brüggemann [75] wherein diamond-shaped strut is used.
ture the physics accurately. To do that, the numerical simu- The hydrogen fuel is injected at sonic condition whereas the
lations are performed with different grids to ascertain the air at Mach 2 enters through the entrance of the combustor
optimum mesh which is resolved sufficiently to capture the section. The pressure distribution is an important parameter
important flow features like flow separation, shock wave in- to analyze the structure of shockwaves in the flow field of a
teractions, and recirculation region. In this context, the grid combustor which is shown in Fig. 3. Accordingly, the experi-
sensitivity test is performed for a single and dual strut profile. mental shadowgraph image and pressure distribution are
Three different grid sizes are considered for each geometry. It shown in Fig. 3(a) and (b), respectively. It can be observed that
can be seen that the distribution of temperature is almost the shock waves appear right at the leading edge of the strut
invariant with the increase of several elements from M2 to M3. which reflects symmetrically from the bottom and top walls
Hence, a Mesh M2 is chosen for all the numerical simulations while it intersects at X ¼ 0.14 m. Fig. 3 depicts the close
in the present study. Accordingly, the single strut and dual agreement of the structure of shock waves with the experi-
strut profiles with mesh sizes and 1395 215 and 1400 mental results of Gerlinger and Brüggemann [75]. Further, the
220 elements, respectively are chosen for the entire analysis distribution of hydrogen mole fractions at various streamwise
(refer to Table 3). It may be noted that the accumulated error is locations of a combustor as shown in Fig. 4. It can be observed
observed in the numerical analysis [72,73] for various mesh that the hydrogen mole fraction reaches one when the dis-
system which is as shown in Table 3. Further, the term yþ is tance X ¼ 178 mm whereas the hydrogen mole fraction is 0.55
employed to check the accuracy. Hence, when the distance X ¼ 275 mm. Hence, it can be noticed that
the streamwise locations at X ¼ 178 mm and X ¼ 275 mm show
rut yP
yþ ¼ (18) reasonably good agreement with the results of Gerlinger and
m
Brüggemann [75] as shown in Fig. 4.
It may be noticed that the value of yþ is lesser than 1.0 for
Results and discussion
Table 2 e Reaction rates for the hydrogen-air mechanism The present work discusses the influence of diamond-shaped
[71]. dual strut on combustion characteristics in a cavity-based
scramjet combustor. The diamond-shaped strut is consid-
Mechanism Reaction A (m3/mol/s) E (J/mol) n
ered for the present investigation since the combustor per-
One-step 2Н 2 þ Ο2 /2Н 2 Ο 9:87 10 8
3:1 10 7
0
formance is found to be better owing to its peculiar diamond
Table 3 e Estimation of accumulated errors for the various mesh systems.

Mesh Single strut Dual strut
Grid T(K) Error (%) Grid T (K) Error (%)
M1 600 110 2450 6.8 615 115 2385 8.26
M2 1395 215 2615 0.57 1400 220 2585 0.57
M3 1715 250 2630 0 1700 252 2600 0
and thermal transport would be greatly affected when dual

strut that too, with diamond shape is employed in an attempt
to needed to enhance the mixing and combustion efficiency.
Hence, the influence of a diamond-shaped dual strut based
scramjet combustor with a cavity flame holder on mixing and
combustion phenomenon has been studied and discussed in
detail in the following sections. The investigations are carried
out to know the effect of spacing (D) in the diamond-shaped
dual strut with a backward-facing step dual strut in the
Fig. 2 e Model validation: the mean wall pressure ratio range from D ¼ 1 mm to 4 mm. The results are described in the
along the downstream length of a combustor. form of shock wave structure, streamlines distribution, mix-
ing and combustion efficiency in the subsequent sections.
shape of the strut as reported in our previous work [76]. Also,
there are studies by Manna et al. [77] and Deepu et al. [17] who Comparative assessment of the effect of the dual strut over
reported that the attachment of a shock owing to the sharp single strut
leading and the blunt trailing edges of a diamond-shaped strut
increases the stability of a flame. In addition to this, the Although most of the studies on scramjet combustors have
introduction of dual strut promotes the flow separation lead- employed single strut, it would be interesting to know the
ing to the disturbances in the flow especially at the top and influence of dual strut on combustion in performance. This is
bottom wall thus serving the flame holding purpose particularly because dual strut offers greater opportunity for
[19,20,50e52]. This allows one to think that the flow behavior the upstream flow to separate both from the upper side as well
Fig. 3 e Model validation: pressure distributions along the length (a) experimental [75] and (b) present work.
Fig. 4 e Model validation: the variation of H2 mole fraction

at different streamwise locations: (a) X ¼ 178 mm and (b)
X ¼ 275 mm.
as the lower side. Hence, it is expected that the fluid flow

features may be distinctly different in comparison to the one
Fig. 5 e Shadowgraph image of a shock for : (a) single strut
observed when a single strut is used. This can be visualized in
and (b) dual strut in a cavity-based combustor.
terms of the nature of recirculation zones, shock waves, and
Eddy formation. To add this, geometrical factors like cavity as
well as flow parameters like Mach number would also affect Several researchers [16e18] reported that strong l-type
the flow pattern and ultimately the combustion performance. shock waves are observed in combustor when the diamond-
Therefore, an attempt is made to evaluate the implication of shaped strut is used which helps to increase the mixing per-
dual strut on combustion performance by comparing it with formance of fuel and air. Fig. 5 also shows the l-type strong
combustion performance when the single strut is used as shock waves both for single strut and dual strut. Further, it is
shown in Table 4. To do so, the variation of combustion effi- observed that the flow disturbances are developed by the
ciency, mixing efficiency is plotted and discussed to under- interaction of shock and shear layer near the region of a
stand the effects of the dual strut. It may be noted that the cavity. It can also be observed that the flow separation takes
total length, cone angles, and the injection diameters are kept place at the top and bottom wall of the dual strut by the
constant for both dual and single strut used in the scramjet interaction of the shock train. Due to this reason, the intensity
combustor. of flow disturbances is found to be greater particularly when
the dual strut is used as compared to the single strut (refer
Shock waves and streamlines Fig. 5).
It may be noted that several studies on the influence of various However, these flow disturbances help to increase the
strut profiles on the combustion performance of a scramjet formation of vorticity which is seen in Fig. 6. It is well known
combustor have been carried out [30e37]. Most of them have that the vorticity has a great influence on the mixing process
employed diamond-shaped strut which gives better combus- and the rate of burning in high-speed supersonic flows. Hence,
tion performance. Hence, a diamond-shaped single and dual vorticity contours have been plotted for the flow-through
strut is considered for the present investigation. combustor with a single strut and dual strut as shown in
Further, the detailed flow field analysis is discussed with Fig. 6. The structure of the vorticity contour is the conse-
the help of shock waves and stream-line distribution to cap- quence of the interaction of reflected shock and shear layer
ture the intricate physics associated with the transport of flow (refer Fig. 5). Two zones of vorticity structures, as well as shear
through the dual strut based supersonic combustor. It may be layers, can be seen in the case of the dual strut (refer Fig. 6(b)).
noted that several studies on various strut profiles have been On the contrary, a single zone of vorticity structure and a
carried out in a scramjet combustor [19,20,50e52]. Fig. 5 shows single shear layer is observed when single strut used (refer
the shadowgraph image of a shock for single strut and dual Fig. 6(a)). Further, it is observed that the mass exchange be-
strut in a cavity-based combustor. It can be seen that the tween the free stream and the recirculation region increases
incoming air stream gets compressed by the oblique shock owing to the formation of vorticity (refer Fig. 6). The strength
wave as seen at the leading edge of a strut. Also, the flow slows of vorticity is found to be higher when the dual strut is used as
down to the subsonic level by the impingement of a shock compared to the single strut.
which stabilizes the flame as shown in Fig. 5.
Table 4 e Diamond-shaped single and double strut used for the comparison.
Single strut Dual strut
H2O mass fraction at X ¼ 368 mm in the case of dual strut

configuration (refer Fig. 8(a)). However, the higher values of H2
mass fraction in the case of the combustor with a single strut
as compared to the dual strut are attributed to spacing in the
dual strut (refer Fig. 8(a)). It is interesting to note that the H2O
mass fraction in the case of a combustor with dual strut in-
creases gradually as shown in Figs. 8(b)e(c) which is desirable
since it increases the combustion zone [78]. The value of
hydrogen mass fraction is found to be minimum at the
centerline of a combustor due to the existence of spacing in
the dual strut (refer Fig. 8(d)). It is imperative to note that the
fuel is injected at the location of step along the streamwise
direction. It is then mixed with the high-speed air stream
passing through the gap in the dual strut. As a result, the
chemical reaction rate becomes very slow at Y ¼ 25.4 mm as
evident from a variation of Mass fraction in the case of a dual
Fig. 6 e Vorticity contour for (a) single strut and (b) dual
strut (refer Fig. 8(d)).
strut in a cavity-based combustor.
Mixing efficiency and Combustion efficiency

Fig. 7 shows the streamline contours for the flow-through The disturbances in the fluid flow due to the introduction of a
combustor with a single and dual strut in a scramjet strut in the passage of supersonic flow in the combustor af-
combustor. As discussed, the flow disturbance increases fects strongly both mixing and combustion performance. This
when dual strut is employed which helps in the development can be demonstrated by plotting and comparing the variation
of eddies and vortex (refer Fig. 6(b)). Further, it is observed that of combustion and efficiency along the combustor length for
the deceleration of flow is seen in the region of the cavity and single and dual strut configuration.
also behind the strut profile (refer Fig. 7). It can also be It is seen that the mixing takes place because of the
observed that the strength of vorticity is found to be higher chemical reaction between air and fuel during the residence
behind the strut position which helps to increase the growth time. In this context, Li et al. [19,20] reported that effective
of recirculation regions. Moreover, it is seen that the size of mixing takes place at the shortest distance of a combustor.
the recirculation region increases in the case of the combustor Fig. 9(a) shows the comparison of variation of mixing effi-
with the dual strut as compared to the combustor with a ciency for a single and dual strut. As discussed in the previous
single strut (refer Fig. 7(b)). section, the probability of the H2O mass fraction decreases in
To gain a detailed insight on the implication of dual strut, the neighborhood of a cavity in a combustor with a single strut
the mass fraction of H2 and H2O has been plotted at various (refer Fig. 8(c)) and the complete mixing taking place at a
downstream locations (X ¼ 368 mm, 402 mm, 453 mm and relatively larger downstream distance of a combustor as seen
Y ¼ 25.4 mm) as shown in Fig. 8. in Fig. 9(a). On the other hand, the mixing efficiency reaches
It is well known that the gaseous fuels are injected into the one at relatively lesser distance i.e., X ¼ 0.49 m in the case of
free stream to mix with the oxidizer so that the molecular dual strut showing that mixing rate and intensity is higher.
collisions take place through the chemical reaction releasing The combustion process has a strong dependence on the
heat. Owing to this, there is a difference in the value of H2 and mixing rate of fuel-air and also flame stability. Fig. 9 (b) shows
H2O mass fraction as shown in Figs. 8(a)e(c). The distribution the variation of combustion efficiency along the combustor
of H2and H2O mass fraction shows the higher values of H2 and length. It is observed that the combustion efficiency increases
rapidly from the combustor inlet (X ¼ 0.36 m) to the cavity
region (X ¼ 0.45 m) both for single and dual strut configura-
tion. Further, the value of combustion efficiency is almost
invariant after the region of a cavity (refer Fig. 9(b)) for both the
cases considered. As already shown in Fig. 9(a), the mixing is
relatively at a slower rate taking larger downstream distance
for complete combustion in the case of the combustor with a
single strut. This leads to a relatively lesser value of com-
bustion efficiency in the case of the combustor with a single
strut as compared to the dual strut. Further, it is noticed that
the 80% of combustion efficiency is achieved when a dual strut
profile is used in the combustor due to the formation of strong
oblique shock waves along with more number of vortices.
Effect of spacing (D) in the dual strut
Fig. 7 e Distribution of streamlines for (a) single strut and When the dual strut is placed in the centerline of a cavity
(b) dual strut in a cavity-based combustor. assisted scramjet combustor, the flame stabilizes and
Fig. 8 e Mass fraction comparison at various downstream locations of a combustor: (a) X ¼ 368 mm, (b) X ¼ 402 mm, (c)
X ¼ 453 mm and (d) Y ¼ 25.4 mm (at the centerline of the combustor).
Fig. 9 e (a) Comparison of variation in mixing efficiency and (b) Comparison of variation in combustion efficiency.
enhances the mixing probability of fuel and air [19,20]. The through the distribution of streamlines which ultimately af-
spacing (D) represents the perpendicular distance between fects the mixing characteristics. Fig. 10 shows the black ribbon
struts (refer Fig. 1) which strongly influences the fluid flow type of streamlines pattern where velocity contour along X-
pattern inside the combustor section. The effect of spacing (D) direction is shown for different spacing between the dual strut
can be best described with the help of discussion on hydro- profiles. It also depicts the separation bubble, recirculation
dynamic behavior in terms of separation regions, the devel- region and its effect on shock. The effect of a dual strut on flow
opment of vortices and recirculation regions and given in patterns can be seen very prominently from the formation of
upcoming sections. the separation region. It may be noted that the separation
zone develops owing to the interaction of.
Streamlines The shock train and the boundary layer [78]. However,
The spacing (D) between the dual strut scramjet combustor there is a limit for the separation region up to which
has a great impact on the flow physics which can be seen
increases i.e., D ¼ 2 mm, the intensity and the size of the

recirculation region reduces in the cavity region as compared
to the spacing D ¼ 1 mm.
At the upstream section of a strut, the mixture tempera-
ture is not enough higher to cause the self-ignition. Hence,
there is no self-ignition in the separation region which is the
main difference between the separation and recirculation
region [79,80]. When the spacing (D) further increases to
(D) ¼ 3 mm, the size of the separation region grows from
X ¼ 0.31m-0.34 m. Also, the shock trains move in the upstream
section. As a result of this, the intensity and the size of the
recirculation region decrease (refer Fig. 10(c)). It is noteworthy
here that these observations are consistent with earlier ob-
servations reported by Huang et al. [78] and Lee et al. [79,80]. It
is observed that the size of the recirculation region is smaller
at D ¼ 4 mm as compared to D ¼ 3 mm due to the influence of
larger separation region (refer Figs. 10(ced)). Furthermore, one
more recirculation region is seen around the strut due to the
influence of back pressure (refer Figs. 10(bed)). Hence, the
combustion performance of a scramjet combustor de-
teriorates at higher spacing.
Combustion efficiency
The mixing and combustion efficiency are the two important
performance parameters for a scramjet combustor. As
described in the previous section, the complete mixing of fuel
and air is found when the mixing efficiency reaches to one
Fig. 10 e (a)e(d) Effect of spacing(D) in dual strut on the along with the shortest distance of a combustor. The variation
distribution of streamlines and (e) enlarged view of of mixing efficiency at spacing (D) in the strut is as shown in
separation region. Fig. 11. It is observed that the mixing efficiency reaches one at
X ¼ 0.5 m which implies that the complete mixing occurs with
the shortest distance of a combustor when D ¼ 1 mm.
enhancement in combustion occurs since it may reduce the
Consequently, the complete mixing of fuel and air is found
growth of the recirculation region.
at a larger distance when the spacing is D ¼ 2 mm as
When the spacing in the strut is D ¼ 1 mm, the smaller
compared to D ¼ 1 mm due to the larger separation region and
separation regions are seen at the top and bottom wall of the
the backpressure. This delays the effective mixing of fuel and
combustor. It may be noted in this context that similar ob-
air as the spacing in the dual strut increases (refer Fig. 11(a)).
servations were made by Huang [78] who stated that the
Combustor performance is quantified in terms of com-
smaller separation region aids to increase the mixing as well
bustion efficiency. It indicates the rate at which the hydrogen
as combustion performance. As a result of this, the formation
is consumed and the water vapor is produced along the
of a larger recirculation region is seen in the cavity region
streamwise direction [63]. Fig. 11(b) shows the variation of
(refer Fig. 10(a)) and intensifies the mixing of fuel-air. On the
combustion efficiency along the combustor length. It is
contrary, the larger separation region tries to move in the
observed that there is no hydrogen mass fraction from the
upstream direction (refer Figs. 10(bed)). When the spacing
inlet of an isolator (X ¼ 0) to the exit of an isolator (X ¼ 0.32 m).
Fig. 11 e Effect of spacing in dual strut on a) variation of mixing efficiency and (b) variation of combustion efficiency.
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Implication of Diamond Shaped Dual Strut On Combustion Characteristics in A Cavity-Based Scramjet Combustor

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international journal of hydrogen energy xxx (xxxx) xxx

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Implication of diamond shaped dual strut on

Lakka Suneetha, Pitambar Randive*, K.M. Pandey

Combustion performance in scramjet using single and dual strut is compared.

article info abstract

combustion performance [13] since the profile of the strut

where turbulent kinetic energy ðkÞ ¼ 3

The stoichiometric ratio of a hydrocarbon is given as,

(kPa) (K) O2 H2 H2O N2 The equivalence ratio of a hydrocarbon is given as,

 the entire flow. Accordingly, the distance between the wall

Herein, Wi is the molecular weight of ith species, m_ ∞ is the

Results and discussion

Table 3 e Estimation of accumulated errors for the various mesh systems.

and thermal transport would be greatly affected when dual

Fig. 4 e Model validation: the variation of H2 mole fraction

as the lower side. Hence, it is expected that the fluid flow

H2O mass fraction at X ¼ 368 mm in the case of dual strut

Mixing efficiency and Combustion efficiency

Effect of spacing (D) in the dual strut

increases i.e., D ¼ 2 mm, the intensity and the size of the

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