Effect of primary nozzle exit diameter on underexpanded coaxial jet characteristics
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Chandra Bose Gurusamy
and
Ganesan Sudalaimuthu
Abstract
The effect of Primary Nozzle Exit Diameter (PNED) on the decay characteristics and flow structure of circular underexpanded sonic coaxial jets has been studied numerically. Primary nozzles with exit diameters of 6 mm, 9 mm, and 12 mm were examined while maintaining a constant secondary nozzle width of 6 mm. The analysis was conducted at nozzle pressure ratios (NPRs) of 3, 5, and 7, with the 6 mm PNED configuration serving as the baseline for comparison. Axial Pitot pressure measurements and numerical Schlieren techniques were employed to evaluate pressure decay trends, shock-cell formation, and supersonic core behaviour. The study also focused on shear layer interactions between the primary and secondary jets and their subsequent interaction with the ambient environment. Results demonstrate that PNED has a significant influence on shock structure, jet coherence, and mixing efficiency. The 6 mm PNED exhibited both the shortest potential core and the most rapid Pitot pressure decay, highlighting its superior mixing performance across all NPRs. In contrast, jets with 9 mm and 12 mm PNEDs displayed extended supersonic cores, more persistent shock-cell structures, and reduced entrainment. Results show that the Potential Core Length (PCL) increases by about 28 % as PNED enlarges from 6 mm to 12 mm, indicating delayed mixing due to weaker shear-layer interaction. The results emphasize the importance of PNED in jet structure and mixing in coaxial configurations. The insights gained offer valuable guidance for optimizing nozzle design in propulsion, flow control, and other high-speed jet applications.
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Research ethics: This study did not involve human participants or animals; therefore, Ethical Approval was not required in accordance with institutional and international guidelines.
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Informed consent: Not applicable, as the research did not involve human participants.
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Author contributions: All authors contributed substantially to the conception, design, data acquisition, analysis, and interpretation of the study. All authors reviewed and approved the final version of the manuscript before submission.
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Use of Large Language Models, AI and Machine Learning Tools: No AI/ML tools were used in the preparation of this manuscript.
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Conflict of interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Research funding: This work received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
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Data availability: The data supporting the findings of this study are available from the corresponding author upon reasonable request.
Nomenclature (SI Units)
- AP
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Primary Nozzle exit Area (mm2)
- AS
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Secondary Nozzle Exit Area (mm2)
- AR
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Area Ratio
- BR
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Bypass Ratio
- Dh
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Hydraulic Diameter (mm)
- DP
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Primary Nozzle Exit Diameter (mm)
- DS
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Secondary Nozzle Exit Diameter (mm)
- DR
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Diameter Ratio
- FMZ
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Fully Merged Zone
- IMZ
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Initial Merging Zone
- IZ
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Intermediate Zone
- LT
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Lip Thickness
- MR
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Mach Ratio
- MP
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Mass Flow of the Primary Nozzle (kg/mm2)
- MS
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Mass Flow of the Secondary Nozzle (kg/mm2)
- NPR
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Nozzle Pressure Ratio
- PCL
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Potential Core Length
- PNED
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Primary Nozzle Exit Diameter (mm)
- SCL
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Supersonic Core Length
- SNED
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Secondary Nozzle Exit Diameter (mm)
- X
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Coordinate along the jet axis (mm)
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