Graham J. Nathan

Mixing characteristics of axisymmetric free jets from a contoured nozzle, an orifice plate and a pipe

The differences in mixing performance between axisymmetric turbulent jets issuing from three common types of nozzle, viz. a contoured (or smooth contraction) nozzle, a sharpedged orifice and a long pipe, are investigated. The investigation is carried out using both qualitative flow visualizations and quantitative measurements of the centerline passive temperature. It is revealed that the jet issuing from an orifice plate provides the greatest rate of mixing with ambient fluid, while the pipe jet has the lowest rate. Physical insight into the differences is explored using a planar imaging technique and measurements of power spectra of the fluctuating velocity

The influence of Reynolds number on a plane jet

The present study systematically investigates through experiments the influence of Reynolds number on a plane jet issuing from a radially contoured, rectangular slot nozzle of large aspect ratio. Detailed velocity measurements were performed for a jet exit Reynolds number spanning the range 1500≤Reh≤16 500, where Reh≡Ubh/υ with Ub as the momentum-averaged exit mean velocity, h as the slot height, and υ as the kinematic viscosity. Additional centerline measurements were also performed for jets from two different nozzles in the same facility to achieve Reh=57 500. All measurements were conducted using single hot-wire anemometry to an axial distance (x) of x≤160h. These measurements revealed a significant dependence of the exit and the downstream flows on Reh despite all exit velocity profiles closely approximating a “top-hat” shape. The effect of Reh on both the mean and turbulent fields is substantial for Reh<10 000 but becomes weaker with increasing Reh. The length of the jet’s potential core, initial pri...

Development of temperature imaging using two-line atomic fluorescence

This work aims to advance understanding of the coupling between temperature and soot. The ability to image temperature using the two-line atomic fluorescence (TLAF) technique is demonstrated. Previous TLAF theory is extended from linear excitation into the nonlinear fluence regime. Nonlinear regime two-line atomic fluorescence (NTLAF) provides superior signal and reduces single-shot uncertainty from 250 K for conventional TLAF down to 100 K. NTLAF is shown to resolve the temperature profile across the stoichiometric envelope for hydrogen, ethylene, and natural gas flames, with deviation from thermocouple measurements not exceeding 100 K, and typically ≲30 K. Measurements in flames containing soot demonstrate good capacity of NTLAF to exclude interferences that hamper most two-dimensional thermometry techniques.

Mixing Characteristics of a Flapping Jet from a Self-Exciting Nozzle

Experimental results of the mixing characteristics of a low-frequency flapping jet from a self-exciting nozzle are presented. The simple fluidic device used to generate the flapping motion is also described. The nozzle contains no external trigger and, unlike the flip-flop nozzle of Viets, contains no external feedback path. Both conventional and conditional averaging schemes are employed to characterise the turbulent mixing characteristics of the jet using data obtained from hot-wire anemometry. Flow-visualisation is used to characterise the flapping motion. It is revealed that the dynamic flapping motion enhances the large-scale mixing of the jet while concurrently suppressing the generation of the fine-scale turbulence. The results also indicate that high turbulence intensities, initiated by the flapping motion, are sustained even in the far-field flow region.

Phase-averaged velocity in a fluidic precessing jet nozzle and in its near external field

Abstract Phase averaged laser-Doppler measurements of the axial velocity components made within and in the near exit field of a precessing-jet nozzle have verified a number of flow features reported in the research literature. The nozzle is a short cylindrical tube with an axisymmetric inlet at one end, and with a centrebody and a small exit lip at the other end. The diameter ratio of the abrupt expansion at the inlet is 1:5. The measurements of the internal flow field reveal a radially deflected internal jet which reattaches asymmetrically and precesses around the wall of the chamber. The phase-averaged flow inside the chamber can be divided into regions of forward flow and regions of reverse flow. The distribution of these regions inside the chamber implies the presence of large-scale recirculation. Representative reverse mean flow speeds of recirculation are about 30% of the forward flow speed. Measurements inside the chamber suggest that the effect of reversed flow on the velocity decay of the inlet-jet flow is similar to that of an ambient counter flow. Measurements in the external jet suggest that the initial entrainment rate of the external precessing jet is between six and seven times that of an equivalent free turbulent jet. The phase-averaged deflection angle of the present emerging jet is 50° but this decreases to about 30° within 0.4 chamber diameters of the exit plane.

Measurement and prediction of NOx emissions from unconfined propane flames from turbulent-jet, bluff-body, swirl, and precessing jet burners

The relationships among flame radiation, NOx emissions, and residence time are explored for unconfined propane turbulent diffusion flames with widely dissimilar mixing characteristics. Variations in mixing of a turbulent-jet were achieved at constant initial conditions using coaxial air through swirl and bluff-body burners and using a precessing jet burner to allow the separate effects of jet Froude number and flame temperature to be explored. A wide range of flame types were produced, from the strongly radiating and sooting flame of the precessing jet burner, which is buoyancy dominated, through recirculation-zone-dominated bluff-body and swirl flames, for which partial quenching of the reactions occurs. It was found that the radiant fraction scales with residence time for all the flames, suggesting a relationship between Froude number and global flame temperature. The importance of flame temperature, and hence thermal NOx, in all flames is demonstrated. The NOx emission indices from these flames are well correlated with the residence time and non-adiabatic flame temperature, demonstrating that the role of flame radiation is significant. Departure from purely thermal NOx predictions for high-temperature recirculation-zone-dominated flames is attributed to the poor correlation of the reaction-zone volume with the flame volume of this class of flames. Departure for the most radiant, and hence lowest temperature, flames is attributed to increased relative significance of the prompt mechanism.

Characterization of turbulent jets from high-aspect-ratio rectangular nozzles

Turbulent free jets issuing from rectangular slots with various high aspect ratios (15–120) are characterized. The centerline mean and rms velocities are measured using hot-wire anemometry over a downstream distance of up to 160 slot heights at a slot-height-based Reynolds number of 10 000. Experimental results suggest that a rectangular jet with sufficiently high aspect ratio (>15) may be distinguished between three flow zones: an initial quasi-plane-jet zone, a transition zone, and a final quasi-axisymmetric-jet zone. In the quasi-plane-jet zone, the turbulent velocity field is statistically similar, but not identical, to those of a plane jet.

The Influence of Fuel Jet Precession on the Global Properties and Emissions of Unconfined Turbulent Flames

Abstract An investigation was conducted to characterize the shape and size, the global radiation, and the CO and NOx, emissions of precessing jet flames and to compare these results with conventional, free, turbulent jet diffusion flames. Jet precession was accomplished using a mechanically-rotated nozzle. Parameters varied include nozzle diameter, jet deflection angle, initial jet velocity (Reynolds number), dimensionless precession frequency (Strouhal number), and fuel type. Measured quantities included visible flame dimensions; NOx, NO2, and CO emissions; and radiant heat flux. Results of these experiments showed that two broad regimes of flame type exist for precessing jet flames. For precession Strouhal numbers (St p ) less than about 0.01, flames are non-luminous and, in comparison with conventional flames, are short and broad. For St p > 0.03, flames are highly luminous, and of a shape which is between that of a conventional jet flame and the low-St p flame. In general, radiant fractions increase w...

Corrections to facilitate planar imaging of particle concentration in particle-laden flows using Mie scattering. Part 2: Diverging laser sheets

Part 1 describes a model to account for the effect of particles on laser sheet attenuation in flows where particles are heterogeneously distributed and where particles are small compared with the imaged volume. Here we extend the model to account for the effect of a strongly diverging light sheet, which is desirable when investigating many turbulent flows, e.g., in two-phase combustion problems. A calibration constant, C(kappa), is derived to account for the attenuation of the incident laser sheet due to extinction of the laser beam through a seeded medium. This is shown to be effective in correcting both the effect of in-plane laser sheet attenuation and out-of-plane signal trapping due to particles in a jet flow heavily seeded with 5 g/s of 25-40 microm spherical particles. In the uncorrected case, attenuation causes up to 15% error in the mean concentration and 35% error on the rms fluctuations. Selecting an appropriate C(kappa) was found to remove the error in the mean concentration and reduce error on the rms fluctuation by half. Methods to estimate or measure an appropriate value of C(kappa) are also presented.Planar nephelometry is a laser-based technique of imaging the light scattered from particles to provide information about the local number density of these particles. In many seeded flows of practical interest, such as pulverized coal flames, particle loadings are sufficiently high for the incident laser beam to be severely attenuated. Measurements in these flows are therefore difficult, and limited data are available under these conditions. Laser attenuation experiments were conducted in suspensions of spherical particles in water at various concentrations. This is used to formulate a calibration for the effects of diffuse scattering and laser sheet extinction. A model for the distribution of light through a heavily seeded, light-scattering medium is also developed and is compared with experimental results. It is demonstrated that the scattered signal may be considered proportional to the local particle concentration multiplied by the incident laser power. The incident laser power varies as a function of the attenuation by obscurement. This correction for planar nephelometry images thus extends the technique to provide pseudoquantitative data for instantaneous particle concentration measurements.

Sodium and Potassium Released from Burning Particles of Brown Coal and Pine Wood in a Laminar Premixed Methane Flame Using Quantitative Laser-Induced Breakdown Spectroscopy:

A quantitative point measurement of total sodium ([Na]total) and potassium ([K]total) in the plume of a burning particle of Australian Loy Yang brown coal (23 ± 3 mg) and of pine wood pellets (63 ± 3 mg) was performed using laser-induced breakdown spectroscopy (LIBS) in a laminar premixed methane flame at equivalence ratios (Φ) of 1.149 and 1.336. Calibration was performed using atomic sodium or potassium generated by evaporation of droplets of sodium sulfite (Na2SO3) or potassium sulfate (K2SO4) solutions seeded into the flame. The calibration compensated for the absorption by atomic alkalis in the seeded flame, which is significant at high concentrations of solution. This allowed quantitative measurements of sodium (Na) and potassium (K) released into the flame during the three phases of combustion, namely devolatilization, char, and ash cooking. The [Na]total in the plume released from the combustion of pine wood pellets during the devolatilization was found to reach up to 13 ppm. The maximum concentration of total sodium ([Na]maxtotal) and potassium ([K]maxtotal) released during the char phase of burning coal particles for Φ = 1.149 was found to be 9.27 and 5.90 ppm, respectively. The [Na]maxtotal and [K]maxtotal released during the char phase of burning wood particles for Φ = 1.149 was found to be 15.1 and 45.3 ppm, respectively. For the case of Φ = 1.336, the [Na]maxtotal and [K]maxtotal were found to be 13.9 and 6.67 ppm during the char phase from burning coal particles, respectively, and 21.1 and 39.7 ppm, respectively, from burning wood particles. The concentration of alkali species was higher during the ash phase. The limit of detection (LOD) of sodium and potassium with LIBS in the present arrangement was estimated to be 29 and 72 ppb, respectively.