A high-resolution laser Doppler anemometer for three-dimensional turbulent boundary layers

 A new laser Doppler anemometer optimized for high spatial resolution near a wall is described. The instrument uses short focal length optics, a mirror probe in the flow, and side-scatter collection to produce a measuring volume 35 μm in diameter by 66 μm long. Data are presented for a two-dimensional boundary layer, demonstrating the instrument’s ability to measure Reynolds shear stresses as close to the wall as 0.1 mm, or y ⁺≈3. Received: 6 October 1995/Accepted: 9 April 1996     

In situ fuel concentration measurement near a spark plug in a spray-guided direct-injection spark-ignition engine using infrared absorption method

Vaporized fuel concentration in a spray-guided direct-injection spark-ignition (SG-DISI) engine was measured using an optical sensor installed in a spark plug. A laser infrared absorption method was applied to quantify the instantaneous gasoline concentration near the spark plug. This paper discusses the feasibility of obtaining in situ air–fuel ratio measurements with this sensor installed inside an SG-DISI engine cylinder. First, the effects of the spray plume from a multi-hole injector on the vaporized fuel concentration measurements near the spark-plug sensor were examined using a visible laser. We determined the best position for the sensor in the engine, which was critical due to the spray and vapor plume formation. Then, a 3.392-μm He–Ne laser that coincided with the absorption line of the hydrocarbons was used as a light source to examine the stratified mixture found during ultra-lean engine operation. A combustible mixture existed around the spark plug during the injection period when a preset air–fuel ratio of 45.0 was used with different fuel injection timings and net mean effect pressure conditions. The effects of the orientation of the spark plug on the measured results and ignitability of the SG-DISI engine were examined. Orienting the spark plug vertically to one of the spray plumes provided more accurate results and better engine reliability. The study demonstrated that it was possible to qualify the air–fuel ratio near the spark plug during the injection period using the developed spark-plug sensor in an SG-DISI engine.     

A particle image velocimetry system for microfluidics

 A micron-resolution particle image velocimetry (micro-PIV) system has been developed to measure instantaneous and ensemble-averaged flow fields in micron-scale fluidic devices. The system utilizes an epifluorescent microscope, 100–300 nm diameter seed particles, and an intensified CCD camera to record high-resolution particle-image fields. Velocity vector fields can be measured with spatial resolutions down to 6.9×6.9×1.5 μm. The vector fields are analyzed using a double-frame cross-correlation algorithm. In this technique, the spatial resolution and the accuracy of the velocity measurements is limited by the diffraction limit of the recording optics, noise in the particle image field, and the interaction of the fluid with the finite-sized seed particles. The stochastic influence of Brownian motion plays a significant role in the accuracy of instantaneous velocity measurements. The micro-PIV technique is applied to measure velocities in a Hele–Shaw flow around a 30 μm (major diameter) elliptical cylinder, with a bulk velocity of approximately 50 μm s^-1. Received: 26 November 1997/Accepted: 26 February 1998     

PIV measurements of a microchannel flow

 A particle image velocimetry (PIV) system has been developed to measure velocity fields with order 1-μm spatial resolution. The technique uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope, and a cooled interline-transfer CCD camera to record high-resolution particle-image fields. The spatial resolution of the PIV technique is limited primarily by the diffraction-limited resolution of the recording optics. The accuracy of the PIV system was demonstrated by measuring the known flow field in a 30 μm×300 μm (nominal dimension) microchannel. The resulting velocity fields have a spatial resolution, defined by the size of the first window of the interrogation spot and out of plane resolution of 13.6 μm× 0.9 μm×1.8 μm, in the streamwise, wall-normal, and out of plane directions, respectively. By overlapping the interrogation spots by 50% to satisfy the Nyquist sampling criterion, a velocity-vector spacing of 450 nm in the wall-normal direction is achieved. These measurements are accurate to within 2% full-scale resolution, and are the highest spatially resolved PIV measurements published to date. Received: 29 October 1998/Accepted: 10 March 1999     

Cyclic variability of pulsed spray vapor fields

 A two wavelength extinction technique is used to quantitatively measure the temporal fuel vapor field produced by pulsed automotive fuel injectors. Line-of-sight measurements are spatially deconvoluted to yield a picture of the overall, cycle-averaged, radial vapor concentration profiles. Vapor measurements are also given for single, consecutive, injection pulses, showing cycle-to-cycle variations in the vapor field. Methods to characterize the cyclic variability in the vapor field of a pulsed spray are discussed. As an illustrative example, the differences between an air assist and a non-air assist injector are shown as a function of duty cycle and axial location. Received: 11 August 1997/Accepted: 10 February 1998     

A backlighted imaging technique for particle size measurements in two-phase flows

 An imaging technique that uses backlighting has been developed to measure drop sizes in annular two-phase flows with small concentrations of drops in the gas phase. Advantages over conventional photography are realized in that data collection and analysis times are shortened considerably, and consistent unbiased results can be expected. A magnification of 1.9 was used to measure drops above 50 μm. A drop size distribution was obtained for an air–water system as a superficial gas velocity of 30 m/s and a liquid flow of 20 g/s. The data are used to substantiate a theory for the rate of deposition. Received: 6 February 1997/Accepted: 3 February 1998     

Stability evaluation of an accurately controllable multiple injection system

Aiming to further improve diesel engine fuel efficiency and exhaust emissions, we conducted tests of a newly devised rapidly operating secondary actuator that provides controllable multiple injection. Up to six shots per cycle were implemented under high-pressure injection. A high-speed camera and a flow rate metering system were used to obtain quantitative data regarding diesel spray and fuel flow dynamics. The advantages of the applied technique are very stable phasing, dwelling, duration, and dosing of multiple injection shots proved from data analysis.Abbreviations ARB Arbitrary waveform generator - CRIS Common rail injection system - DC Direct current - ECU Electronic control unit - EDU Electronic injection driving unit - fps Frames per second - HP High-pressure - LDA Laser Doppler anemometer (velocimeter) - ROSA Rapidly operating secondary actuator - RPM Revolutions per minute - SOI Start of injection - TDC Top dead center - TTL Transistor–transistor logic     

A high-resolution laser Doppler anemometer: design, qualification, and uncertainty

A new high-resolution laser Doppler anemometer (LDA) has been developed with a working distance of 350 mm, allowing operation in lab-scale wind tunnels. The measurement volume size is 35 μm in diameter by 60 μm in length, allowing resolution of the smallest turbulence scales even at fairly high Reynolds numbers. The controversial question of velocity and validation bias in LDA data is resolved with an experimental method for measuring and removing those effects. Uncertainty estimates are also derived for all the mean and Reynolds stress measurements. Received: 27 June 1999/Accepted: 30 August 2000     

Three-dimensional flow visualization in the wake of a miniature axial-flow hydrokinetic turbine

This paper examines the velocity profile of fuel issuing from a high-pressure single-orifice diesel injector. Velocities of liquid structures were determined from time-resolved ultrafast shadow images, formed by an amplified two-pulse laser source coupled to a double-frame camera. A statistical analysis of the data over many injection events was undertaken to map velocities related to spray formation near the nozzle outlet as a function of time after start of injection. These results reveal a strong asymmetry in the liquid profile of the test injector, with distinct fast and slow regions on opposite sides of the orifice. Differences of ～100 m/s can be observed between the ‘fast’ and ‘slow’ sides of the jet, resulting in different atomization conditions across the spray. On average, droplets are dispersed at a greater distance from the nozzle on the ‘fast’ side of the flow, and distinct macrostructure can be observed under the asymmetric velocity conditions. The changes in structural velocity and atomization behavior resemble flow structures which are often observed in the presence of string cavitation produced under controlled conditions in scaled, transparent test nozzles. These observations suggest that widely used common-rail supply configurations and modern injectors can potentially generate asymmetric interior flows which strongly influence diesel spray morphology. The velocimetry measurements presented in this work represent an effective and relatively straightforward approach to identify deviant flow behavior in real diesel sprays, providing new spatially resolved information on fluid structure and flow characteristics within the shear layers on the jet periphery.     

Experimental study of the flow regimes resulting from the impact of an intermittent gasoline spray

The present paper reports a complete set of measurements made with a two-component phase Doppler anemometer of the two-phase flow generated at the impact of a transient gasoline spray onto a flat surface. The spray is generated by a pintle injector and the fuel used was gasoline. The measurements of droplet size–velocity were processed to provide time fluxes of number, mass, normal momentum, and energy of the poly-dispersion of droplets ejected at impact, and analyzed based on predictive tools available in the literature. The results show that splash is the dominant mechanism by which secondary droplets are ejected from the surface, either in the stagnation region or in the core region of the spray. In the stagnation region, a large fraction of each incident droplet adheres to the surface and the axial incident momentum contributes with a larger parcel than tangential momentum. As a result, the normal velocity of ejected droplets is much smaller than that of the original incident droplets, while tangential velocity is enhanced. The region near the stagnation point is immediately flooded upon impact of the leading front of the spray, forming a liquid film that is forced to move radially outwards as droplets continue to impinge during the steady period. Spray/wall interaction in the core region thus occurs in the presence of a moving thin liquid film, which enhances transfer of tangential momentum. As a result, film spreading and dynamics as a result of impingement forces are crucial to accurate model spray/wall interaction. The outer region of the spray is dominated by the vortical structure induced by shear forces, which entrains small responsive secondary droplets to re-impinge. Furthermore, prediction of the outcome of spray impact requires a precise knowledge of the two-phase flow in the presence of the target.     

Direct velocity gradient measurements using a dual-point laser—Doppler anemometer

The present paper describes a modified laser—Doppler anemometer set up to permit local velocity measurements at two separated points in space. In this way direct measurements of local shear and elongational rates become possible, providing information relevant in many flow studies of non-Newtonian fluids. Details of the optical and electronic sub-units are described and applications of the velocity gradient anemometer in channel flow are given to demonstrate its potential for measurement of rheological fluid properties and to provide insight into flows of non-Newtonian fluids.     

Numerical and experimental investigation on droplet dynamics and dispersion of a jet engine injector

In the case of turbine combustors operating with liquid fuel the combustion process is governed by the liquid fuel atomization and its dispersion in the combustion chamber. By highly unsteady flow field conditions the transient interaction between the liquid and the gaseous phase is of interest, because it results in a temporal variation of air–fuel ratio which leads to a fluctuating temperature distribution. The objective of this research was the investigation of transient flow field phenomena (e.g. large coherent structures) on droplet dynamics and dispersion of an isothermal flow (of inert water droplets) as a necessary first step towards a full analysis of spray combustion in real-life devices. The advanced injector system for lean jet engine combustors PERM (Partial Evaporated Rapid Mixing) was applied, generating a dilute polydispersed spray in a swirled flow field. Experiments were performed using Phase Doppler Anemometry (PDA) and a patternator to determine the droplet polydispersity, concentration maps, and velocity profiles in the flow. An important finding is the effect of large-scale coherent structures due mainly to the precessing of the vortex core (PVC) of the swirling air jet on the particle dispersion patterns. The experimental results then serve as reference data to assess the accuracy of the Eulerian–Lagrangian computations using a Large Eddy Simulation (LES), a Unsteady Reynolds-Average Navier–Stokes Simulation (URANS) and two simplified (steady-state) simulations. There, a simplified droplet injection model was used and the required boundary conditions of injected droplet sizes were obtained from measurements. Important transient effects of deterministic droplet separation observed during experiments, could be perfectly replicated with this injection model. It is convincingly shown, through extensive computations, that the resolution of instantaneous vortical structures is indeed crucial; hence the LES, or a reasonably-well resolved URANS are preferred over the steady-state solutions with additional, stochastic-type, turbulent dispersion models.     

Imaging of an underexpanded nozzle flow by UV laser Rayleigh scattering

 Rayleigh scattering of ultra-violet laser light is applied as a diagnostic tool to record gas density distributions in a supersonic nozzle flow. The output beam of a pulsed ArF excimer laser (λ=193.4 nm) is focussed into a thin light sheet radially intersecting a dry air flow emanating from a circular nozzle. An intensified CCD camera is used to record the Rayleigh scattered light in a direction perpendicular to the light sheet. Since the Rayleigh scattering intensity is directly proportional to the local gas density, this results in two-dimensional gas density distribution maps of radial slices through the flow. Images of the flow density are presented for stagnation pressures between 0.2 and 0.7 MPa (0.1 MPa ≡1 bar), showing the transition from subsonic to supersonic flow and, at higher pressures, the formation of a Mach disk. Density maps can be recorded with single laser pulses, effectively freezing the flow structure on a 20 ns time scale. The diamond pattern, characteristic for underexpanded supersonic nozzle flows, is quantitatively monitored, with the experimental results being in reasonable agreement with predictions from a simplified theoretical model. Received: 25 September 1996/Accepted: 19 May 1997     

Laboratory measurement of suspended sediment concentration using an Acoustic Concentration Profiler (ACP)

Vertical profiles of suspended sediment concentration have been gathered in a laboratory flume using a 2.25 MHz acoustic transducer. The acoustic concentration profiler (ACP) was calibrated in a vertical duct for homogeneous concentrations of two uniformly sized sediments. The transducer was then transferred to a 6 m horizontal flume where concentration profiles were measured in steady and unsteady flows. For the steady flow tests, concentration measurements made with the ACP and with suction samplers are compared. The results demonstrate that the ACP provides an accurate method of non-intrusively measuring sediment concentrations of more than 2.5% by volume. Received: 24 September 1998/Accepted: 12 April 1999     

High speed rotor wake total temperature measurements using a hot-film anemometer

 To develop a quantitative understanding of unsteady and interacting turbomachine flow fields, it is necessary to quantify the instantaneous efficiency of high speed turbomachines. This requires the measurement of both the unsteady velocity and total temperature variation in the exit flow of a high speed rotor. In this paper, techniques to utilize a single slant-film anemometer to measure unsteady total temperature are developed and evaluated. Then a series of preliminary experiments are performed in a high speed axial fan facility to quantify the instantaneous rotor efficiency. This is accomplished by utilizing these single slant-film methods to measure the total temperature in the rotor wakes. Results show that measurements at multiple overheats and several probe orientations are required. The simplest method proves to be useful for determining parameters used in other methods. An analysis based on King’s law gives good results even when measurements are outside the calibration range. Within the calibration range, a polynomial representation of the wire response to mass flux and total temperature yields good total temperature fluctuation results. A model analysis technique is also assessed. Received: 13 November 1997/Accepted: 16 February 1998     

Spray characteristics of a gasoline direct injector injector with short durations of injection

The sprays into atmosphere from a GDI injector were visualised and the velocity and droplet characteristics measured at an injection pressure of 50 bar and at different injection durations, with emphasis on short injection periods. The images show that the initial and closing delay times were 0.225 and 0.2 ms, respectively, and that the cone angle increased with injection duration to a constant value of 62° at 0.5 ms. They also revealed large droplets ahead of the main spray with its smaller droplets. An injection duration of 0.15 ms led to fuel leaving the injector with little atomisation, but a 30% increase led to the formation of the cone, which was present for times greater than 0.5 ms. The poor atomisation associated with short injection durations and the initial phase of longer injections, was due to low swirl velocities. The droplet velocities were higher in the initial phase of injection than in the main phase, with values up to 50 m/s. The Sauter mean diameters of the initial and main-spray droplets were approximately 55 and 35 μm respectively and with a tendency to decrease with time from the start of injection. Received: 20 October 2000/Accepted: 30 March 2001     

Fluorescent solution microcapsules as ideal tracer particles in long-term tracking of 3-D Lagrangian trajectories

 Fluorescent solution microcapsules (diameter <100 μm) have been invented as tracer particles suitable for the long-term tracking of 3-D Lagrangian trajectories in the rotating annulus fluid experiments. In more than 10% of the tracking experiments, the microcapsule was observed to follow the flow passively for more than 10 h, at their maximum 18 h. Received: 21 May 1997/Accepted: 7 January 1998     

Flow seeding with an air nebulizer

 In the framework of studies on anemometric measurements or tomographic visualizations, the seeding of a reactive flow by a nebulizer has been analyzed. In order to determine the performances of this apparatus, different liquids have been tested to evaluate the influence of their physical properties on the droplets size distribution. Measurements have been made using a Phase Doppler Anemometer. It is shown that the mean D ₁₀ diameter lies between 2 and 3 μm, whatever the liquid may be, except pure water, and that, within a certain range, viscosity and surface tension have little influence on the size distribution. The ability of these liquids to seed reactive mixture is also discussed and especially their interaction in premixed flames. Received: 4 May 1998/Accepted: 15 February 1999