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Feb 25, 2024

Influence of extruded injector nozzle on fuel mixing and mass diffusion of multi fuel jets in the supersonic cross flow: computational study

Scientific Reports volume 13, Article number: 12095 (2023) Cite this article 507 Accesses Metrics details The efficient injection system has a great role on the overall enactment of air breathing

Scientific Reports volume 13, Article number: 12095 (2023) Cite this article

507 Accesses

Metrics details

The efficient injection system has a great role on the overall enactment of air breathing propulsion systems at supersonic flow. In this work, the usage of extruded multi-injectors in the fuel distribution and mixing through the combustor is fully investigated. The usage of the extruded nozzles considerably intensifies the formation of the vortices nearby the injectors and this research has tried to visualize the role of these vortices on the diffusion of the fuel jet through the combustor of the scramjet. The influences of the jet space on the strength of produced circulations are fully discussed. The simulation of the high-speed air stream moving the combustion chamber with extruded nozzles is done via Computational Fluid dynamics. Based on our computational data, the use of extruded multi-jets enhances the penetration and diffusion of the hydrogen cross jet in supersonic airflow. Increasing the gap between injectors improves fuel mixing performance by up to 27% downstream of the jets, primarily by enhancing the lateral penetration of the fuel jet.

The most significant advance in propulsion technology for accessing space is the invention of scramjet engines1,2,3,4,5. A launch vehicle with a scramjet engine could work over part of the launcher's intra-atmospheric flight with more sophisticated effectiveness than a similar rocket via releasing mass for increased payload or reusability5,6,7. To show the high impacts of this device in real applications, a scramjet could operate in hypervelocity speeds up to Mach 12. Due to these advantages of scramjet engines, this technique has been investigated considerably in recent decades. In fact, this is the only known technique for accessing outer space. Besides, this propulsion system produced the required power and thrust for high-speed flight8,9,10.

Scramjet engine includes four main stages: Inlet (compression), diffuser; burner and exhaust nozzle. In the first stage, the pressure of the supersonic air is amplified and then, the fuel mixing happens in the diffuser and the auto-ignition process happens in the burner and high enthalpy gas is exhausted with high momentum from the outlet nozzle11,12,13. The main difference between this engine with ramjet engine is the velocity of the incoming air inside the chamber of combustion where the fuel injection and auto-ignition process happen14,15,16. In the ramjet engine, the velocity of the airflow reduces to subsonic speed although its initial speed could be up to Mach = 2. In fact, the mixing and ignition happen in the subsonic domain17. However, the supersonic flow preserved its velocity in the combustor of the scramjet engine and fuel injection and mixing progression in this section happens in the supersonic velocity. This different condition makes these processes more complex and complicated by reason of the higher velocity of the incoming air18,19,20. In fact, the high velocity of air limited the time of fuel mixing while several shocks happened because of air jet interactions with jet plume. To resolve these difficulties, few practical systems have been established to preserve the efficiency of this type of engine even in higher velocities21,22,23.

As mentioned, the distribution and combustion of fuel is done in the combustion chamber and the mechanism of the fuel injection and mixing happens via the injection methodology (active or passive)24,25. In active concept, vibration splitter, pulse jet and wavy wall are the conventional procedure for the efficient injection of fuel26,27,28. In these methods, forced excitation of a large-scale mechanism is used. On the other hand, ramps, lobe mixers, taps, cavities and vanes are used for the shot of the fuel inside the combustion chamber. Besides, the transverse and counter flow jet are the two most popular techniques in this category29,30,31.

The application of the transverse jets is extensively investigated in the context due to considerable benefits in mixing the fuel32,33. Replacement of a single jet with multiple equivalent jets also enhances the performance of this approach for fuel diffusion by the usage of the vortex within gaps of injectors34,35. Owing to these advantages, this work has tried to develop this methodology via applying extruded nozzles of multi-jets in combustion chambers32,36. In the proposed configurations, fuel nozzles are positioned from a higher level that may improve the fuel mixing and augment the vortex feature in the gap of injectors.

In the present research study, the computational fluid dynamic is selected for the investigation of the flow around the extruded multiple jets at the supersonic free stream. Flow analyses are presented to disclose the impacts of vortices on the mechanism of fuel distribution encounter supersonic cross-air stream. The effects of jet spaces on jet diffusion and mixing are entirely investigated in this research. A comparison of the fuel mixing index and circulation strength is also done for achieving the optimum condition for hydrogen injection.

The computational visualization of the high-speed flow is mainly done by means of resolving RANS equations with SST turbulence model37,38. The simulation of the hydrogen jet in the air stream is done by coupling the species transport equation with the main governing equations39. Besides, the compressible effects and jet interactions result in the shock waves and this is achieved by the coupling of the energy equation with primary equations. The ideal gas assumption is a reasonable choice for the calculation of the density in our problem. The main governing equations of our problem were fully presented in the prior published works40,41. A fully implicit approach is used for solving our main governing equations42.

The boundary conditions for our suggested configurations are displayed in Fig. 1. The pressure far-field is applied for the compressible air flow with Mach = 4 and static temperature of 1000 K at atmospheric pressure43,44. A wall with constant temperature is considered on the bottom and top of the domain. The hydrogen is our fuel type and it is injected through four injectors at different heights of 0, 0.25 mm, 0.5 mm and 0.75 mm from the bottom of the domain. The fuel jets with a total pressure of 10% of the main free stream is applied. As displayed in Fig. 2, three jet gaps of 4Dj, 7Dj and 10Dj are investigated since jet spaces have great impacts on the role of the circulation in the multi-jet configurations. The dimension of the domain along the x, z and y is 100 mm, 5 mm and 8 mm, respectively.

Boundary conditions and model description.

Applied grid for the chosen model.

The produced grid is also demonstrated in Fig. 2. As the fuel jet injection in cross-flow produced shock interactions, the resolution of produced grid near the tip of the jet nozzle is increased for shock capturing. The top view of the grid in Fig. 3 shows that the structured grids are generated in this study. It is also found that the applied grid is uniform in the whole domain to avoid errors due to grid non-uniformity. A grid study is also done in this investigation for the grid independency examination as presented in Table 1. In this table, the fuel mass concentration for the four produced grids on the definite surface positioned downstream of the injectors are compared. The presented data indicate that the fine grid is satisfactory for the coming simulations.

Mach contour on a jet plane.

Assessment of the numerical data with practical work is also done for the valuation of the correctness of attained results45,46,47. In this work, the value of the penetration height downstream of a single injector with a diameter of 2 mm is equated with further scientific works. Our data is equated with other works. The trend of the results and deviation from other investigations indicate that the obtained results are rational48,49.

The interface between jets is crucial for fuel mixing in a multi-jet system. Figure 3 demonstrates the impact of three distinct jet gaps on the interaction of fuel jets within the combustion chamber, where fuel injectors with similar heights are extruded. The first important factor related to these configurations is the angle of the bow shack. It seems that the bow shock angle of the model with jet space = 3Dj is higher than other arrangements. The variation of the shear layer is also highlighted in the figure to trail the jet feature is the symmetry plane where the main interactions happen. It is observed how jet spaces and upstream jet influence the deflection of barrel shocks. Increasing the jet spaces strengthen the deflection of the barrel shocks.

The stream and mass concentration of these configurations are displayed in Fig. 4. The concentration of the hydrogen jets is high near the configuration with less jet space while jet configurations with high gap spaces result in less fuel concentration in the gap of extruded injectors. The figure also displays that the vortex structure has more pronounced effects in high jet gaps. In the jet space of 7Dj, the altitude of the mixing area is more uniform than other configurations while fuel distribution is done at a wider distance.

Evaluation of the mixing area on the jet plane.

The 3-D visualization of the jet stream in the selected configurations are illustrated in Fig. 5. The feature of the jet and stream indicates that the flow vortex is more visible in the model with small jet spaces. The jet interactions are higher in the low jet spaces as the barrel shock is closer. Figure 6 illustrates the flow hydrodynamics with a focus on circulations within the jet gap. This structure is dominant in the configuration with a jet gap of 7Dj. Increasing the jet space does not confine the flow within the extruded nozzles. In addition, a lower gap limited the mass flow rate within the extruded nozzles and this restricted its impacts on the jet stream and spreading in the combustion chamber. Multi-jet configurations are known for producing a horseshoe vortex upstream of the first nozzle. The size and strength of this vortex are influenced by the interface between the first jet and the mainstream. As previously mentioned, when the lower jet spacing is reduced, the jets combine to act as an extended jet, enhancing the interaction with the free stream and causing the horseshoe vortex to extend further into the domain. This results in improved fuel mixing near the injectors.

association of the fuel jet interactions for unlike jet spaces.

Evaluation of the flow stream and its impact on the fuel mixing mechanism.

Figure 7 displays the mixing zone and flow structure near the injector (x/Djet = 5) and far distance (x/Djet = 15). In the vicinity of injectors, there are two distinctive vortices that are more noticeable in the low jet space of 3Dj. The upper and the lower vortex are made by the communication of the jets with the mainstream. As described in the previous paragraph, the lower one is owing to the horseshoe vortex and its strength is related to the connections of the supersonic air stream with first jets. The upper vortex is related to the velocity difference due to the production of the vortex in the gap of extruded nozzles. In far distances, the impacts of the jet arrangements are not noticeable (Fig. 7).

Estimation and comparison of the mixing zone and flow on planes downstream of the extruded nozzles.

Figure 8 compares the change of circulation factor near the injectors as well as downstream extruder nozzles. Achieved results indicate that the maximum circulation strength was noticed in the model with a jet gap of 7 Dj. In fact, decreasing the jet space to less than 7Dj limited vortex effects while increasing the gap of injectors would not allow the formation of the vortex in jet gaps as noticed in the trend of circulation factor in the gap of the last two jets. Besides, the circulation strength is diminished in far distances for all cases.

Difference of the circulation factor along the jets.

Figure 9 shows the disparity of the fuel mixing downstream of the different gaps of extruded nozzles along with simple multi-jet configuration without extruded nozzles. Increasing the jet gap would increase the mixing in the far distance while all jet has similar mixing efficiency near the first nozzle. Comparison of extruded nozzle configuration with a simple nozzle show that the application of the extruded nozzle and the same jet space would increase the fuel mixing about 42% in the distance of 25 mm to 40 mm behind the first nozzle. This shows the influences of vortex formation within the gap of the injector. The normal mixing performance of the model with jet space of 7 Dj and 10 Dj in distances of 30 mm to 40 mm is almost equal. However, mixing enactment of the case with a gap of 7Dj is about 12% more than 10 Dj.

Role of jet spaces on the mixing performance of extruded nozzles.

The impacts of the extruded nozzle on fuel mixing of multi-circular jets are fully investigated at supersonic cross flow. The impacts of jet spaces on the size of the mixing area are revealed and the mechanism of the fuel distribution by the extruded nozzle are presented. CFD is applied for the visualization of the high compressible flow with cross multi-jets. The influences of jet spaces on the strength of the mixing and circulation efficiency are examined and the mixing effects of multi-extruded nozzles with simple ones are compared with full details. A comparison of the extruded nozzles with simple ones confirms the higher mixing efficiency of extruded nozzles. According to our findings, the lower jet spacing restricts the formation of vortices within the gap. Conversely, increasing the nozzle gap reduces the strength of the vortex, even when using extruded nozzles. We have observed that the mixing mechanism is closely tied to the presence of strong vortices at the jet distance.

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Li, Z. et al. Influence of lateral injection from the circular strut on mixing performance of hydrogen jet at supersonic combustion chamber. Int. Commun. Heat Mass Transf. 140, 106543 (2023).

Article CAS Google Scholar

Gerdroodbary, M. B. Aerodynamic Heating in Supersonic and Hypersonic Flows: Advanced Techniques for Drag and Aero-Heating Reduction (Elsevier, 2022).

Google Scholar

Li, Z. et al. Effect of inner strut on mass diffusion and mixing efficiency of cross fuel jet inside the scramjet combustor. Int. Commun. Heat Mass Transf. 140, 106514 (2023).

Article CAS Google Scholar

Yang, C., Zhang, J. & Huang, Z. Numerical study on cavitation–vortex–noise correlation mechanism and dynamic mode decomposition of a hydrofoil. Phys. Fluids 34, 125105 (2022).

Article ADS CAS Google Scholar

Dong, M.-Z., Liao, J., Choubey, G. & Huang, W. Influence of the secondary flow control on the transverse gaseous injection flow field properties in a supersonic flow. Acta Astronaut. 165, 150–157 (2019).

Article ADS CAS Google Scholar

Li, L.-Q. et al. Investigation on three mixing enhancement strategies in transverse gaseous injection flow fields: A numerical study. Int. J. Heat Mass Transf. 132, 484–497 (2019).

Article CAS Google Scholar

Huang, W., Du, Z.-B., Yan, L. & Xia, Z.-X. Supersonic mixing in airbreathing propulsion systems for hypersonic flights. Prog. Aerosp. Sci. 109, 100545 (2019).

Article Google Scholar

Gerdroodbary, M. B. Scramjets: Fuel Mixing and Injection Systems (Butterworth-Heinemann, 2020).

Google Scholar

Li, L.-Q., Huang, W., Yan, L., Li, S.-B. & Liao, L. Mixing improvement induced by the combination of a micro-ramp with an air porthole in the transverse gaseous injection flow field. Int. J. Heat Mass Transf. 124, 109–123 (2018).

Article Google Scholar

Huang, W. Mixing enhancement strategies and their mechanisms in supersonic flows: A brief review. Acta Astronaut. 145, 492–500 (2018).

Article ADS Google Scholar

Li, Z., Abu-Hamdeh, N. H., Bantan, R. A. & Musa, A. Effects of downstream ramp on the fuel diffusion of single cross jet at supersonic flow: Computational study. Int. J. Heat Mass Transf. 195, 123183 (2022).

Article Google Scholar

Hassanvand, A., Saei Moghaddam, M. & Barzegar Gerdroodbary, M. Analytical study of heat and mass transfer in axisymmetric unsteady flow by ADM. J. Comput. Appl. Res. Mech. Eng. (JCARME) 11, 151–163 (2021).

Google Scholar

Jiang, Y., Hajivand, M., Sadeghi, H., Gerdroodbary, M. B. & Li, Z. Influence of trapezoidal lobe strut on fuel mixing and combustion in supersonic combustion chamber. Aerosp. Sci. Technol. 116, 106841 (2021).

Article Google Scholar

Jiang, Y. et al. Effect of cavity back height on mixing efficiency of hydrogen multi-jets at supersonic combustion chamber. Int. J. Hydrogen Energy 45, 27828–27836 (2020).

Article CAS Google Scholar

Huang, W., Du, Z.-B., Yan, L. & Moradi, R. Flame propagation and stabilization in dual-mode scramjet combustors: A survey. Prog. Aerosp. Sci. 101, 13–30 (2018).

Article Google Scholar

Huang, W. Effect of jet-to-crossflow pressure ratio arrangement on turbulent mixing in a flowpath with square staged injectors. Fuel 144, 164–170 (2015).

Article CAS Google Scholar

Sun, C., Gerdroodbary, M. B., Abazari, A., Hosseini, S. & Li, Z. Mixing efficiency of hydrogen multijet through backward-facing steps at supersonic flow. Int. J. Hydrogen Energy 46, 16075–16085 (2021).

Article CAS Google Scholar

Liu, X. et al. Numerical simulation of the hydrogen mixing in downstream of lobe strut at supersonic flow. Int. J. Hydrogen Energy 45, 25438–25451 (2020).

Article CAS Google Scholar

Huang, W. & Yan, L. Progress in research on mixing techniques for transverse injection flow fields in supersonic crossflows. J. Zhejiang Univ. Sci. A 14, 554–564 (2013).

Article Google Scholar

Shi, J. et al. Tribology and dynamic characteristics of textured journal-thrust coupled bearing considering thermal and pressure coupled effects. Tribol. Int. 180, 108292 (2023).

Article Google Scholar

Jiang, Y. et al. Effect of free stream angle on mixing performance of hydrogen multi-jets in supersonic combustion chamber. Int. J. Hydrogen Energy 45, 25426–25437 (2020).

Article CAS Google Scholar

Rong, G. et al. Investigation of counter-rotating shock wave and wave direction control of hollow rotating detonation engine with Laval nozzle. Phys. Fluids 34, 056104 (2022).

Article ADS CAS Google Scholar

Shen, D., Cheng, M., Wu, K., Sheng, Z. & Wang, J. Effects of supersonic nozzle guide vanes on the performance and flow structures of a rotating detonation combustor. Acta Astronaut. 193, 90–99 (2022).

Article ADS Google Scholar

Billig, F., Waltrup, P. & Stockbridge, R. Integral-rocket dual-combustion ramjets: A new propulsion concept. J. Spacecr. Rockets 17, 416–424 (1980).

Article ADS Google Scholar

Wu, H., Zhang, Z., Zhang, F. & Roberts, W. L. Time-resolved low-pressure air-assisted spray performance and unsteadiness evaluation. Phys. Fluids https://doi.org/10.1063/5.0145761 (2023).

Article Google Scholar

Edalatpour, A., Hassanvand, A., Gerdroodbary, M. B. & Moradi, R. Injection of multi hydrogen jets within cavity flameholder at supersonic flow. Int. J. Hydrogen Energy 44, 13923–13931 (2019).

Article CAS Google Scholar

Liu, L., Mei, Q. & Jia, W. A flexible diesel spray model for advanced injection strategy. Fuel 314, 122784 (2022).

Article CAS Google Scholar

Liu, L., Peng, Y., Zhang, W. & Ma, X. Concept of rapid and controllable combustion for high power-density diesel engines. Energy Convers. Manag. 276, 116529 (2023).

Article CAS Google Scholar

Zhang, Y., Gerdroodbary, M. B., Hosseini, S., Abazari, A. & Li, Z. Effect of hybrid coaxial air and hydrogen jets on fuel mixing at supersonic crossflow. Int. J. Hydrogen Energy 46, 16048–16062 (2021).

Article CAS Google Scholar

Wang, Y., Zhou, X. & Liu, L. Feasibility study of hydrogen jet flame ignition of ammonia fuel in marine low speed engine. Int. J. Hydrogen Energy 48, 327–336 (2023).

Article CAS Google Scholar

Xia, C. et al. Simulation study on transient performance of a marine engine matched with high-pressure SCR system. Int. J. Engine Res. 24, 1327–1345 (2023).

Article CAS Google Scholar

Li, Y., Gerdroodbary, M. B., Moradi, R. & Babazadeh, H. The influence of the sinusoidal shock generator on the mixing rate of multi hydrogen jets at supersonic flow. Aerosp. Sci. Technol. 96, 105579 (2020).

Article Google Scholar

Wang, Y., Zhai, W., Zhang, H., Cheng, S. & Li, J. Injectable polyzwitterionic lubricant for complete prevention of cardiac adhesion. Macromol. Biosci. 23, 2200554 (2023).

Article CAS Google Scholar

Gerdroodbary, M. B., Fallah, K. & Pourmirzaagha, H. Characteristics of transverse hydrogen jet in presence of multi air jets within scramjet combustor. Acta Astronaut. 132, 25–32 (2017).

Article ADS Google Scholar

Moradi, R., Mosavat, M., Gerdroodbary, M. B. & Abdollahi, A. The influence of coolant jet direction on heat reduction on the nose cone with Aerodome at supersonic flow. Acta Astronaut. 151, 487–493 (2018).

Article ADS CAS Google Scholar

Li, Z. et al. Mixing enhancement of multi hydrogen jets through the cavity flameholder with extended pylon. Acta Astronaut. 175, 300–307 (2020).

Article ADS CAS Google Scholar

Evans, J. S. & Schexnayder, C. J. Jr. Influence of chemical kinetics and unmixedness on burning in supersonic hydrogen flames. AIAA J. 18, 188–193 (1980).

Article ADS CAS Google Scholar

Gerdroodbary, M. B., Jafaryar, M. & Sheikholeslami, M. The efficacy of magnetic force on thermal performance of ferrofluid in a screw tube. Case Stud. Therm. Eng. 49, 103187 (2023).

Article Google Scholar

Barzegar Gerdroodbary, M. Numerical analysis on cooling performance of counterflowing jet over aerodisked blunt body. Shock Waves 24, 537–543 (2014).

Article ADS Google Scholar

Moradi, R., Mahyari, A., Gerdroodbary, M. B. & Abdollahi, A. Shape effect of cavity flameholder on mixing zone of hydrogen jet at supersonic flow. Int. J. Hydrogen Energy 43, 16364–16372 (2018).

Article CAS Google Scholar

Amini, Y. & Nasr Esfahany, M. CFD simulation of the structured packings: A review. Sep. Sci. Technol. 54, 2536–2554 (2019).

Article CAS Google Scholar

Hassanvand, A., Gerdroodbary, M. B. & Abazari, A. M. Injection of hydrogen sonic multi-jet on inclined surface at supersonic flow. Int. J. Mod. Phys. C 32, 2150043 (2021).

Article ADS CAS Google Scholar

Ghaderi, A., Kayhani, M. H., Nazari, M. & Fallah, K. Drop formation of ferrofluid at co-flowing microcahnnel under uniform magnetic field. Eur. J. Mech. B/Fluids 67, 87–96 (2018).

Article ADS MathSciNet MATH Google Scholar

Pudsey, A. S. & Boyce, R. R. Numerical investigation of transverse jets through multiport injector arrays in a supersonic crossflow. J. Propul. Power 26, 1225–1236 (2010).

Article Google Scholar

Srinivasan, R. & Bowersox, R. D. Transverse injection through diamond and circular ports into a Mach 5.0 freestream. AIAA J. 46, 1944–1962 (2008).

Article ADS Google Scholar

Zhang, Y., Rana, P., Moradi, R. & Li, Z. Mixing performance of transverse hydrogen/air multi-jet through coaxial injector arrays in supersonic crossflow. Int. J. Hydrogen Energy 46, 19645–19656 (2021).

Article CAS Google Scholar

Fuller, E., Mays, R., Thomas, R. & Schetz, J. Mixing studies of helium in air at high supersonic speeds. AIAA J. 30, 2234–2243 (1992).

Article ADS CAS Google Scholar

Ogawa, H. Mixing characteristics of inclined fuel injection via various geometries for upstream-fuel-injected scramjets. J. Propul. Power 31, 1551–1566 (2015).

Article CAS Google Scholar

McClinton, C. & Robert, A. “Ramjets and scramjets,” encyclopedia of physical science and technology (2001).

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Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran

Seyyed Amirreza Abdollahi

Department of Mechanical and Aerospace Engineering, College of Engineering and Applied Sciences, Western Michigan University, Kalamazoo, MI, 49008, USA

Ghazal Rajabikhorasani

Department of Civil Engineering, College of Engineering, Cihan University-Erbil, Erbil, Iraq

As’ad Alizadeh

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S.A.A.: wrote the main manuscript text, prepared figures and tables, reviewed the manuscript. G.R.: wrote the main manuscript text, prepared figures and tables, reviewed the manuscript. A.A.: wrote the main manuscript text, prepared figures and tables, reviewed the manuscript.

Correspondence to Seyyed Amirreza Abdollahi.

The authors declare no competing interests.

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Abdollahi, S.A., Rajabikhorasani, G. & Alizadeh, A. Influence of extruded injector nozzle on fuel mixing and mass diffusion of multi fuel jets in the supersonic cross flow: computational study. Sci Rep 13, 12095 (2023). https://doi.org/10.1038/s41598-023-39306-z

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Received: 10 June 2023

Accepted: 23 July 2023

Published: 26 July 2023

DOI: https://doi.org/10.1038/s41598-023-39306-z

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