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COMPREHENSIVE STUDY OF INTERNAL FLOW FIELD AND LINEAR AND NONLINEAR INSTABILITY OF AN ANNULAR LIQUID SHEET EMANATING FROM AN ATOMIZER

IBRAHIM, ASHRAF
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1154536582

Abstract Details

2006, PhD, University of Cincinnati, Engineering : Mechanical Engineering.
Performance of fuel injectors affects the combustion efficiency, pollutant emissions and combustion instability in gas turbine engines, internal combustion engines and industrial furnaces. In these combustion systems, either pressure swirl (simplex) atomizers, or prefilming airblast atomizers, or plain orifice pressure atomizers are used for fuel atomization. In this dissertation, a comprehensive model for pressure-swirl atomization is developed that includes computational treatment of the internal flow field and the nonlinear liquid sheet instability analysis for primary breakup. For a prefilming airblast atomizer and a plain orifice atomizer, nonlinear breakup processes for an annular liquid sheet and a liquid jet are analyzed using a perturbation method. Two-dimensional axi-symmetric numerical simulations have been carried out to study the unsteady, turbulent, swirling two-phase flow field inside pressure swirl atomizers with the volume of fluid (VOF) method. Internal flow field simulation results are validated using available experimental data for velocity measurements inside a large-scale prototype atomizer, the film thickness at orifice exit, the spray angle, and the discharge coefficient. The effect of air pressure and liquid viscosity on flow field inside the atomizer is investigated. The relationship between the internal flow characteristics and discharge parameters confirms that the internal flow structure plays a very important role in determining the atomizer performance. Linear and nonlinear asymmetric instability analyses are carried out to study the primary atomization of annular liquid sheets and liquid jets emanating from the pressure swirl (simplex) atomizer, prefilming airblast atomizer, and plain orifice pressure atomizer using a perturbation method with the initial amplitude of the disturbance as the perturbation parameter. For a coaxial liquid jet subjected to a swirling gas stream, the axisymmetric disturbance mode (n = 0) is the most dominant only when the gas swirl number is very small. However at higher swirl strength the helical (asymmetric) disturbance modes (n > 0) become dominant compared to the axisymmetric mode. The liquid jet breaks up over a shorter distance at higher gas swirl number. The gas swirl number for transition to a highly asymmetric breakup with a high circumferential wave number (n = 5) is found to vary as the inverse of the square root of the gas-to-liquid momentum ratio when the gas-to-liquid momentum ratio is less than 1. For annular liquid sheets, the breakup length is reduced by an increase in the liquid Weber number, initial disturbance amplitude and the inner and outer gas-liquid velocity ratios. The inner gas stream is found to be more effective in disintegrating and enhancing the instability of annular liquid sheets than the outer gas stream. Air swirl not only promotes the instability of the annular liquid sheet, but also switches the dominant mode from the axisymmetric mode to a helical mode (n > 0). As outer air swirl strength increases, the circumferential wave number (n) increases and the ligament shapes at the breakup time become highly asymmetric. Using the atomizer exit conditions as input, a non-linear sheet instability and breakup analysis has been carried out to predict the breakup length and the primary breakup for a simplex atomizer. The predictions of breakup length are compared with available experimental measurements which show good agreement. The coupled internal flow simulation and nonlinear sheet instability analysis provides a comprehensive approach to modeling atomization from a pressure-swirl atomizer.
Milind Jog (Advisor)
270 p.
Engineering, Mechanical
Atomization; Pressure Swirl atomizer; Jet; Annular sheet; Nonlinear Instability

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Citations

  • IBRAHIM, A. (2006). COMPREHENSIVE STUDY OF INTERNAL FLOW FIELD AND LINEAR AND NONLINEAR INSTABILITY OF AN ANNULAR LIQUID SHEET EMANATING FROM AN ATOMIZER [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1154536582

    APA Style (7th edition)

  • IBRAHIM, ASHRAF. COMPREHENSIVE STUDY OF INTERNAL FLOW FIELD AND LINEAR AND NONLINEAR INSTABILITY OF AN ANNULAR LIQUID SHEET EMANATING FROM AN ATOMIZER. 2006. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1154536582.

    MLA Style (8th edition)

  • IBRAHIM, ASHRAF. "COMPREHENSIVE STUDY OF INTERNAL FLOW FIELD AND LINEAR AND NONLINEAR INSTABILITY OF AN ANNULAR LIQUID SHEET EMANATING FROM AN ATOMIZER." Doctoral dissertation, University of Cincinnati, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1154536582

    Chicago Manual of Style (17th edition)

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This open access ETD is published by University of Cincinnati and OhioLINK.