San-Mou Jeng

Efficient Statistical Transport Model for Turbulent Particle Dispersion in Sprays

A statistical transport model for turbulent particle dispersion is formulated having significantly improved computational efficiency in comparison to the conventional stochastic discrete-particle methodology. In the proposed model, a computational parcel representing a group of physical particles is characterized by a normal (Gaussian) probability density function (pdf) in space. The mean of each pdf is determined by Lagrangian tracking of each computational parcel, either deterministically or stochastically

Exciplex liquid-phase thermometer using time-resolved laser-induced fluorescence.

Pulsed photoexcitation of hydrocarbon fuels doped with organic molecules exhibits a temperature-dependent fluorescence spectrum that is used as the basis for a weakly intrusive optical thermometer. By use of pulsed excitation from a 308-nm 8-ns XeCl excimer laser with gated detection of the fluorescence emissions from doped n -heptane, we demonstrate that time-resolved measurement of the excited monomer and the redshifted excited-state complex (exciplex) fluorescence emissions can yield sub-1 degrees accuracy for temperatures ranging from 440 K to the vicinity of the critical temperature (540 K). The experiments also show that the exciplex fluorescence spectrum is pressure independent below and above supercritical pressure.

LOX vaporization in high-pressure, hydrogen-rich gas

LOX droplet vaporization in high-pressure hydrogen-rich gas is analyzed, with special attention to thermodynamic effects which compel the surface to heat to the critical state and to supercritical vaporization processes on heating to criticality. Subcritical vaporization is modeled using a quasi-steady diffusion-controlled gas-phase transport formulation coupled to an effective-conductivity internal-energy-transport model accounting for circulation effects. It is demonstrated how the droplet surface might heat to the critical state, for ambient pressures slightly greater than the critical pressure of oxygen, such that the bulk of propellant within the droplet remains substantially below the critical mixing temperature.

Probability density function shape sensitivity in the statistical modeling of turbulent particle dispersion

The performance of a recently introduced statistical transport model for turbulent particle dispersion is studied here for rigid particles injected into a round turbulent jet. Both uniform and isosceles triangle pdfs are used. The statistical sensitivity to parcel pdf shape is demonstrated.

Liquid rocket spray combustion stability analysis

A computational approach to the analysis of spray combustion stability in liquid rocket combustors is proposed which is based on the unsteady quasi-two-dimensional Euler equations with interphase source terms derived from a Lagrangian treatment of the combusting spray. Based on a preliminary evaluation, the computational methodology presented here is a promising research tool and a potential design/development aid for investigating the stability characteristics of liquid rocket engines. The method is characterized by low numerical noise; the Lagrangian treatment of the spray offers improved flexibility for the direct modeling of spray combustion.

Supercritical droplet gasification experiments with forced convection

Preliminary results of a comprehensive experimental program are presented which offer the first direct observations of suspended n-heptane droplet gasifications in pure nitrogen with forced convection without the interference to optical probing associated with a flame. Measurements show attainment of a wet-bulb temperature until reduced pressures exceed about 1.0 under supercritical gas temperatures. Thereafter, temperature measurements indicate fully transient heat-up through the critical temperature. The surface is found to regress in a continuous manner with the measured temperature approaching the critical value at the end of the droplet lifetime under supercritical conditions with very mild level of convection. At increased level of convection for the same ambient conditions, similar sized droplets will undergo significant deformation during the gasification process until partially convected away as a dense vapor cloud as the critical temperature is approached.

Nonlinear analysis of longitudinal-mode liquid propellant rocket combustion instability

A computational technique for the nonlinear analysis of longitudinal-mode liquid propellant rocket combustion instability is examined based on the unsteady, quasi-one-dimensional Euler equations with appropriate source terms introduced to account for interphase transport coupling with the spray. The method is first assessed for unsteady, nonreacting, isentropic duct flow with specified admittances at the outflow and inflow boundaries. For small amplitude disturbances, numerical results based on a sufficient number of grid points compare favorably with predictions from a small-disturbance linearized analysis.

Evaluation of an efficient statistical transport model for turbulent droplet dispersion in dilute combusting sprays

Evaluation of an efficient statistical transport model for turbulent droplet dispersion is made for a dilute spray of methanol droplets injected into a turbulent, axisymmnetric methane-fuelled diffusion flame burning in stagnant air. In the dispersion model, a computational parcel representing a group of physical particles (droplets) is considered to have a normal probability density function (pdf) in space. The mean is determined by Lagrangian tracking through a sequence of stochastically generated turbulent eddies and the variance is evaluated from a statistical formulation based on the linearized particle equations of motion. The basic validity of this model is established through comparison with available experimental data and with theoretical predictions using a conventional stochastic direct modeling approach. The conclusion of the evaluation is that the proposed dispersion model compares favorably with experimental data and provides a valid technique for simulating turbulent combusting sprays with significant computational savings over conventional methods.

An experimental study of high-pressure droplet combustion

The results are presented of an experimental study on suspended n-heptane droplet combustion in air for reduced pressures up to P(r) = 2.305. Transition to fully transient heat-up through the critical state is demonstrated above a threshold pressure corresponding to P(r) of roughly 1.4. A silhouette imaging technique resolves the droplet surface for reduced pressures up to about P(r) roughly 0.63, but soot formation conceals the surface at higher pressures. Images of the soot plumes do not show any sudden change in behavior indicative of critical transition. Mean burning rate constants are computed from the d-squared variation law using measured effective droplet diameters at ignition and measured burn times, and corrected burning times are computed for an effective initial droplet diameter. The results show that the burning rates increase as the fuel critical pressure is approached and decrease as the pressure exceeds the fuel critical pressure. Corrected burning times show inverse behavior.

Hypergolic bipropellant spray combustion and flow modelling in rocket engines

A predictive tool for hypergolic bipropellant spray combustion and flow evolution in small rocket combustion chambers is described. It encompasses a computational technique for the gas-phase governing equations, a discrete particle method for liquid bipropellant sprays, and constitutive models for combustion chemistry, interphase exchanges, and unlike impinging hypergolic spray interactions. Emphasis is placed on the phenomenological modeling of the hypergolic liquid bipropellant gasification processes. Sample computations with the N2H4-N2O4 propellant system are given in order to show some of the capabilities and inadequacies of this tool.