Francisco J. Diez

Primary Breakup of Turbulent Round Liquid Jets in Uniform Crossflows

An experimental investigation of the deformation and breakup properties of turbulent round liquid jets in uniform gaseous crossflows is described. Pulsed shadowgraph and holograph observations were obtained for turbulent round liquid jets injected normal to air crossflow in a shock tube. Crossflow velocities of the air behind the shock wave relative to the liquid jet were subsonic (36-90 m/s) and the air in this region was at normal temperature and pressure. Liquid injection was done by a pressure feed system through round tubes having inside diameters of 1 and 2 mm and length-to-diameter ratios greater than 100 to provide fully developed turbulent pipe flow at the jet exit. Test conditions were as follows: water and ethyl alcohol as test liquids, crossflow Weber numbers based on gas properties of 0-282, streamwise Weber numbers based on liquid properties of 1400-32,200, liquid/gas density ratios of 683 and 845, and jet exit Reynolds numbers based on liquid properties of 7100-48,200, all at conditions in which direct effects of liquid viscosity were small (Ohnesorge numbers were less than 0.12). Measurements were carried out to determine conditions required for the onset of breakup, ligament and drop sizes along the liquid surface, drop velocities after breakup, liquid column breakup as whole, rates of turbulent primary breakup, and liquid column trajectories. Phenomenological theories proved to be quite successful in interpreting and correlating the measurements.

Self-Preserving Properties of Unsteady Round Buoyant Turbulent Plumes and Thermals in Still Fluids

The self-preserving properties of round buoyant turbulent starting plumes and starting jets in unstratified environments. The experiments involved dye-containing salt water sources injected vertically downward into still fresh water within a windowed tank. Time-resolved images of the flows were obtained using a CCD camera. Experimental conditions were as follows: source diameters of 3.2 and 6.4 mm, source/ambient density ratios of 1.070 and 1,150, source Reynolds numbers of 4,000-11,000, source Froude numbers of 10-82, volume of source fluid for thermals comprising cylinders having the same cross-sectional areas as the source exit and lengths of 50-382 source diameters, and stream-wise flow penetration lengths up to 110 source diameters and 5.05 Morton length scales from the source. Near-source flow properties varied significantly with source properties but the flows generally became turbulent and then became self-preserving within 5 and 20-30 source diameters from the source, respectively. Within the self-preserving region, both normalized streamwise penetration distances and normalised maximum radial penetration distances as functions of time were in agreement with the scaling relationships for the behavior of self-preserving round buoyant turbulent flows to the following powers: time to the 3/4 power for starting plumes and to the 1/2 power for thermals. Finally, the virtual origins of thermals were independent of source fluid volume for the present test conditions.

Round Turbulent Thermals, Puffs, Starting Plumes and Starting Jets in Uniform Crossflow

The self-preserving properties of round turbulent thermals, puffs, starting plumes and starting jets, in unstratified and uniform crossflow, were investigated experimentally. The experiments involved dye-containing fresh water (for nonbuoyant flows) and salt water (for buoyant flows) sources injected vertically downward into crosflowing fresh water within a water channel. Time-resolved video images of the flows were obtained using CCD cameras. Experimental conditions were as follows: source exit diameters of 3.2 and 6.4 mm, source Reynolds numbers of 2,500-16,000, source/ambient velocity ratios of 4-35, source/ambient density ratios (for buoyant flows) of 1.073 and 1.150, volumes of injected source fluid (for thermals and puffs) comprising 16-318 source diameters, streamwise (vertical) penetration distances of 0-200 source diameters and 0-13 Morton length scales (for buoyant flows) and crosstream (horizontal) penetration distances of 0-620 source diameters

Effects of heat release on turbulent shear flows. Part 3. Buoyancy effects due to heat release in jets and plumes

An integral method is presented for determining effects of buoyancy due to heat release on the properties of reacting jets and plumes. This method avoids the Morton entrainment hypothesis entirely, and thus removes the ad hoc ‘entrainment modelling’ required in most other integral approaches. We develop the integral equation for the local centreline velocity u c ( x ), which allows modelling in terms of the local flow width δ ( x ). In both the momentum-dominated jet limit and buoyancy-dominated plume limit, dimensional arguments show δ ( x ) ≈ x , and experimental data show the proportionality factor c δ to remain constant between these limits. The entrainment modelling required in traditional integral methods is thus replaced by the observed constant c δ value in the present method. In non-reacting buoyant jets, this new integral approach provides an exact solution for u c ( x ) that shows excellent agreement with experimental data, and gives simple expressions for the virtual origins of jets, plumes and buoyant jets. In the exothermically reacting case, the constant c δ value gives an expression for the buoyancy flux B ( x ) that allows the integral equation for u c ( x ) to be solved for arbitrary exit conditions. The resulting u c ( x ) determines the local mass, momentum and buoyancy fluxes throughout the flow, as well as the centreline mixture fraction ζ c ( x ) and thus the flame length L . The latter provides the proper parameters Ω andΛ that determine buoyancy effects on the flame, and provides power-law scalings in the momentum-dominated and buoyancy-dominated limits. Comparisons with buoyant flame data show excellent agreement over a wide range of conditions.

Self-Preserving Mixing Properties of Steady Round Nonbuoyant Turbulent Jets in Uniform Crossflows

The self-preserving mixing properties of steady round nonbuoyant turbulent jets in uniform crossflows were investigated experimentally. The experiments involved steady round nonbuoyant fresh water jet sources injected into uniform and steady fresh water crossflows within the windowed test section of a water channel facility. Mean and fluctuating concentrations of source fluid were measured over cross sections of the flow using planar-laser-induced-fluorescence (PLIF). The self-preserving penetration properties of the flow were correlated successfully similar to Diez [ASME J. Heat Transfer, 125, pp. 1046–1057 (2003)] whereas the self-preserving structure properties of the flow were correlated successfully based on scaling analysis due to Fischer [Academic Press, New York, pp. 315–389 (1979)]; both approaches involve assumptions of no-slip convection in the cross stream direction (parallel to the crossflow) and a self-preserving nonbuoyant line puff having a conserved momentum force per unit length that moves in the streamwise direction (parallel to the initial source flow). The self-preserving flow structure consisted of two counter-rotating vortices, with their axes nearly aligned with the crossflow (horizontal) direction, that move away from the source in the streamwise direction due to the action of source momentum. Present measurements extended up to 260 and 440 source diameters from the source in the streamwise and cross stream directions, respectively, and yielded the following results: jet motion in the cross stream direction satisfied the no-slip convection approximation; geometrical features, such as the penetration of flow boundaries and the trajectories of the axes of the counter-rotating vortices, reached self-preserving behavior at streamwise distances greater than 40–50 source diameters from the source; and parameters associated with the structure of the flow, e.g., contours and profiles of mean and fluctuating concentrations of source fluid, reached self-preserving behavior at streamwise (vertical) distances from the source greater than 80 source diameters from the source. The counter-rotating vortex structure of the self-preserving flow was responsible for substantial increases in the rate of mixing of the source fluid with the ambient fluid compared to corresponding axisymmetric flows in still environments, e.g., transverse dimensions in the presence of the self-preserving counter-rotating vortex structure were 2–3 times larger than transverse dimensions in self-preserving axisymmetric flows at comparable conditions.

Electrokinetic Microactuator Arrays and System Architecture for Active Sublayer Control of Turbulent Boundary Layers

Results are presented from development of microactuator arrays that function on the electrokinetic principle to provide active control of streamwise sublayer vortical structures in turbulent boundary layers. The electrokinetic microactuatorarraysinducevolumedisplacementsinthesublayerbyelectrokineticpumpingunderan impulsively applied electric e eld. These microactuator arrays consist of individual microchannels formed in a substrate and e lled with a 1- πm-scale doped porous polymer matrix that provides the required ‡ ‡-potential when wetted by the corresponding electrolyte. A system architecture is presented for large dense arrays of such microactuators that provides for greatly reduced control processing requirements within individual unit-cells containing a relatively small number of sensors and actuators. The resulting microactuator arrays have characteristics that make them potentially suited for practical sublayer control on full-scale aeronautical and hydronautical vehicles. Essentially loss-less frequency response of the electrokinetic microactuators has been demonstrated to 10 kHz. Several such microelectrokinetic actuator (MEKA) arrays have been fabricated from a basic three-layer design. A MEKA-5 full-scale hydronautical array, composed of 25,600 individual electrokinetic microactuators with 350- πm centerto-center spacings, arranged in a 40 £40 pattern of unit-cells, each composed of a 4 £ £4 matrix of actuators, was successfully fabricated in a 7 £7 cm 2 tile in 250-πm-thick Mylar substrate material. Microelectromechanical system design and fabrication processes were used to produce a top layer for the MEKA-5 hydronautical-scale array. DC performance tests indicate that the MEKA-5 array achieves the required e ow rates for active sublayer control on hydronautical vehicles with applied voltages of no more than 15‐ 20 V.

Design and fabrication of unsteady electrokinetic microactuator arrays for turbulent boundary layer control

The development of a micro-electro-kinetic-actuator (MEKA) array for the study and control of the viscous sublayer of a turbulent boundary layer is presented. Several generations of such MEKA arrays have been fabricated that explore the use of non-conventional MEMS materials such as quartz, polymers or mylar and are combined where needed with conventional MEMS design and fabrication processes. The present study is the first attempt to exploit the potential advantages of using the electrokinetic principle as the basis for a new class of microscale actuators suitable for active sublayer control. Among such advantages are that these actuators have no moving parts, and that they achieve the flow rates required for this type of flow control. Moreover, the inherent problem of matching the length and time scales between microactuators and the physical system being controlled makes the viscous sublayer a natural choice for these types of actuators. The electrokinetic drivers are fabricated inside microchannels, 250 mm to 2 mm in diameter, using a liquid-phase polymerization process that generates 1 µm doped pores. This process greatly simplifies the fabrication of a large number of actuators, and using this technique we are able to fill 100% of the 25 600 microchannels that form the typical MEKA 5 array. The array is fabricated using a novel three-layer design that contains (i) a top layer with the actuator nozzles, electrodes and leadouts, (ii) a center layer containing the individually addressable electrokinetic driver channels in which electrolyte pumping occurs in response to a time-varying electric field that will induce volume displacement in the sublayer and (iii) a bottom layer containing an electrolyte reservoir and common electrode. The functionality incorporated in this three-layer design with independent unit cells demonstrate all the elements needed for turbulent boundary layer control.

Self-Preserving Mixing Properties of Steady Round Buoyant Turbulent Plumes in Uniform Crossflows

The self-preserving mixing properties of steady round buoyant turbulent plumes in uniform crossflows were investigated experimentally. The experiments involved salt water sources injected into fresh water crossflows within the windowed test section of a water channel. Mean and fluctuating concentrations of source fluid were measured over cross sections of the flow using Planar-Laser-Induced-Fluorescence (PLIF) which involved seeding the source fluid with Rhodamine 6G dye and adding small concentrations of ethanol to the crossflowing fluid in order to match the refractive indices of the source flow and the crossflow. The self-preserving penetration properties of the flow were correlated successfully based on the scaling analysis of Diez et al. (2003) whereas the self-preserving structure properties of the flow were correlated successfully based on the scaling analysis of Fischer et al. (1979); both approaches involved assumptions of no-slip convection in the cross stream (horizontal) direction (parallel to the crossflow) and a self-preserving line thermal having a conserved source specific buoyancy flux per unit length that moves in the streamwise (vertical) direction (parallel to the direction of both the initial source flow and the gravity vector). The resulting self-preserving structure consisted of two counter-rotating vortices having their axes nearly aligned with the crossflow direction that move away from the source in the streamwise (vertical) direction due to the action of buoyancy. Present measurements extended up to 202 and 620 source diameters from the source in the streamwise and cross stream directions, respectively. The onset of self-preserving behavior required that the axes of the counter-rotating vortex system be nearly aligned with the crossflow direction. This alignment, in turn, was a strong function of the source/crossflow velocity ratio, uo /v∞ . The net result was that the onset of self-preserving behavior was observed at streamwise distances of 10–20 source diameters from the source for uo /v∞ = 4 (the smallest value of uo /v∞ considered), increasing to streamwise distances of 160–170 source diameters from the source for uo /v∞ = 100 (the largest value of uo /v∞ considered). Finally, the counter-rotating vortex system was responsible for substantial increases in the rate of mixing of the source fluid with the ambient fluid compared to axisymmetric round buoyant turbulent plumes in still environments, e.g., transverse dimensions in the presence of the self-preserving counter-rotating vortex system were 2–3 times larger than the transverse dimensions of self-preserving axisymmetric plumes at similar streamwise distances from the source.© 2005 ASME

Self-Preserving Properties of Steady Round Nonbuoyant Turbulent Jets in Uniform Crossflows

The properties of steady round nonbuoyant turbulent jets in uniform crossflows were studied, motivated by applications to the dispersion of heat and potentially harmful substances from steady exhaust flows. Emphasis was placed on self-preserving conditions far from the source where source disturbances have been lost and where jet properties are largely controlled by the conserved properties of the flow. The experiments involved steady round nonbuoyant fresh water jet sources injected into uniform and steady fresh water crossflows within the windowed test section of a water channel facility. Flow visualization was carried out by photographing dye-containing source jets. Mean and fluctuating concentrations of source fluid were measured over cross sections of the flow using Planar Laser-Induced Fluorescence (PLIF). The self-preserving properties of the flow were correlated successfully based on scaling analysis due to Fischer et al. (1979) which involves assumptions of no-slip convection in the cross stream direction (parallel to the crossflow) and a self-preserving nonbuoyant turbulent line puff having a conserved momentum force per unit length that moves in the streamwise direction (parallel to the initial source flow). The flow structure consisted of two counterrotating vortices, with their axes nearly aligned with the crossflow direction, that move away from the source in the streamwise direction due to the action of source momentum. Present measurements extended up to 160 source diameters from the source in the streamwise direction and yielded the following results: jet motion in the cross stream direction satisfied the no-slip convection approximation; geometrical features, such as the penetration of flow boundaries and the trajectories of the axes of the counter-rotating vortices, reached self-preserving behavior at streamwise distances greater than 40–50 source diameters from the source; parameters associated with the structure of the flow, e.g., contours and profiles of mean and fluctuating concentrations of source fluid, however, did not reach self-preserving behavior prior to reaching streamwise (vertical) distances greater than 70–80 source diameters from the source.Copyright

INVESTIGATION OF NONBUOYANT LAMINAR JET DIFFUSION FLAMES: A PARADIGM FOR SOOT PROCESSES IN TURBULENT FLAMES

The structure and soot properties of steady nonbuoyant round laminar jet diffusion flames at microgravity were studied based on measurements obtained on orbit during three flights of the Space Shuttle Columbia (Flights STS-83, 94 and 107). The test conditions included ethylene- and propane-fueled flames burning in still air at ambient temperature of 300 K and ambient pressures of 35-130 kPa, for jet exit diameters of 0.40- 2.70 mm and jet exit Reynolds numbers of 46-1186, to yield steady nonbuoyant round laminar jet diffusion flames with most of the flames near the laminar smoke- point. The first phase of the study involved evaluation of the classical analysis of the structure of steady nonbuoyant round laminar jet diffusion flames due to Spalding (1979), after empirically extending it to account for the presence of luminosity due to the presence of soot within the flames. It was found that the extended Spalding (1979) analysis provided excellent predictions of the flame shape properties of the test flames when radiative heat losses were small so that quenching and flame-tip opening were avoided. This analysis also shows that flame properties are identical functions of time for nonbuoyant laminar *