Direct measurements of in-cylinder integral length scales of a transparent engine

 This paper presents a hardware technology and its accompanying software developed to measure the in-cylinder integral length scales of the flow inside a transparent four-stroke engine directly, using a fiber laser Doppler velocimeter (LDV). A traverse table was designed to combine two 1-component fiber optical probes to form an LDV system with the capability of both 2-component single-point and 1-component two-point simultaneous measurements for velocities. This paper demonstrates the evaluation of radially separated lateral and longitudinal integral length scales. The location was at a mid-plane of the top dead center (TDC) clearance height, and the engine was motored at a speed of 500 rpm. Data analysis was processed using statistical techniques and the physical meaning of the results was explained. Received: 27 August 1996/Accepted: 6 December 1996     

An experimental study to determine fractal parameters for lean premixed flames

 In this study the fractal parameters of a lean, premixed methane-air flame were determined over a range of turbulence conditions. The focus of the present work was to improve the experimental technique so as to resolve the inner cutoff scale, the outer cutoff scale, and the fractal dimension. By adjusting the flow velocity through a set of three interchangeable grids in a steady-flow combustion tunnel, a range of turbulence intensities and scales was obtained within the test section. The integral scale varied from 2.5 to 5.5 mm and the turbulence intensity varied from 0.5 to 3.8 times the laminar burning velocity, while the equivalence ratio of the fuel–air mixture was 0.60. The flame was stabilized inside a 51 mm square, open-ended test section by means of a small, centrally-located, pilot burner. A 60 mm ×45 mm cross section of the flame was visualized by means of an argon-ion laser sheet and titanium dioxide seeding, and was recorded on high-sensitivity black and white film by a 35 mm camera using a shutter speed of 1/8000 s. The film negatives were digitized at 60 pixels/mm, equivalent to a resolution of 12 pixels/mm (83 μm per pixel) on the scale of the flame. Using commercially available software, the images were analyzed to identify the position of the flame front; custom software was used to determine the fractal dimension and the inner and outer cutoff scales of the turbulent flame. In the range of conditions reported in this paper, it was observed that the fractal dimension increased with turbulence level but the values were approximately 5% lower than those reported by others. The inner cutoff scale was found to increase with decreasing turbulence, thus confirming an earlier hypothesis about the smoothing effect of flame propagation at low turbulence levels. The outer cutoff scale varied from 11 to 16 mm and its value tended to decrease with increasing turbulence level. Received: 8 August 1995 / Accepted: 1 July 1996     

Simultaneous velocity and scalar measurements in premixed recirculating flames

 The use of a laser-Doppler velocimeter has been extended to the analysis of turbulent heat transfer in a strongly sheared disc-stabilised propane-air flame through its combination with either laser Rayleigh scattering or digitally-compensated fine-wire thermocouples. The laser velocimeter was based on a conventional forward scattering system from the green light of a 5W Argon-Ion laser, while the Rayleigh signals used the blue line of the same laser. The procedure for the numeric compensation of the thermocouple signals included analysis of the effect of velocity and temperature on the time constant of the thermocouple and was optimised to allow combined velocity–temperature samples acquired by a purpose-built digital interference with a frequency up to 2000 Hz, without deterioration of the thermocouple by particle accretion. The maximum effective data rate for the combined Rayleigh/LDV system is shown to be around 130 Hz, which corresponds to a data rate of valid Doppler signals around 400 Hz and statistics based on more than 15 000 measurements is made possible. The results obtained with the two systems agree qualitatively, although the use of thermocouples attenuates the measured velocity-temperature correlations. The results are used to assess the extent to which turbulent mixing in flames is altered by the accompanying heat release and quantify the processes of non-gradient diffusion in a strongly recirculating premixed flame. Received: 15 November 1996/Accepted: 2 September 1997     

A ballistic compressor-based experiment for the visualization of liquid propellant jet combustion above 100 MPa

 This paper describes the components and operation of an experimental setup for the visualization of liquid propellant (LP) jet combustion at pressures above 100 MPa. The apparatus consists of an in-line ballistic compressor and LP injector. The ballistic compressor, based on a modified 76 mm gun, provides high-pressure (ca. 55 MPa) clear hot gas for the jet ignition. A piston (projectile) is fired toward a test chamber beyond the barrel’s end, and its rebound is arrested in a transition section that seals the test chamber to the barrel. The LP jet is injected once the piston is restrained, and combustion of the jet further elevates the pressure. At a preset pressure, a disc in the piston ruptures and the combustion gas vents sonically into the barrel. If a monopropellant is used, the jet injection-combustion process then resembles liquid rocket combustion but at very high pressures (ca. 140 MPa). This paper discusses the ballistics of the compression and compares experimental results to those predicted by a numerical model of the apparatus. Experimentally, a pressure of 70 MPa was achieved upon a 12.5 volumetric compression factor by firing a 10 kg piston into 1.04 MPa argon using a charge of 75 g of small-grain M1 propellant. Received: 16 December 1996/Accepted: 15 July 1997     

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     

Generating arbitrarily sized interrogation windows for correlation-based analysis of particle image velocimetry recordings

 We describe a technique that allows an arbitrary size of the interrogation window when using the traditional FFT algorithm in analysing PIV recordings by either cross- or auto-correlation methods. The length and width of the effective interrogation window are no longer required to be composed of a number of pixels making a power of 2 (16, 32, 64 etc). This gives a higher flexibility in selecting the appropriate window size. Received: 28 January 1997/Accepted: 11 August 1997     

Particle image velocimetry with optical flow

 An optical Flow technique based on the use of Dynamic Programming has been applied to Particle Image Velocimetry thus yielding a significant increase in the accuracy and spatial resolution of the velocity field. Results are presented for calibrated synthetic sequences of images and for sequences of real images taken for a thermally driven flow of water with a freezing front. The accuracy remains better than 0.5 pixel/frame for tested two-image sequences and 0.2 pixel/frame for four-image sequences, even with a 10% added noise level and allowing 10% of particles of appear or disappear. A velocity vector is obtained for every pixel of the image. Received: 18 July 1997/Accepted: 5 December 1997     

High speed optical tomography system for quantitative measurement and visualization of dynamic features in a round jet

An optical tomography system that is capable of operating at frame rates of up to 5 kHz has been used to obtain spatially resolved cross-sectional temperature images of a heated round jet. These tomographic images show dynamic details in the evolving vortical flow structures found in the near field of the jet that are consistent with previous studies of low speed jet flow. Reconstructions produced by the system are quantitative temperature distributions of a planar cross section of the jet measuring temperature differences with a spatial resolution of 1.4 mm. Received: 31 July 1997/Accepted: 18 December 1997     

Concentration measurements during mass transfer across liquid-phase boundaries using planar laser induced fluorescence (PLIF)

A non-intrusive optical measuring technique, the planar laser induced fluorescence is described to investigate mass transfer of a fluorescent dye (rhodamine B) across an interface between two partial immiscible liquids (1-butanol and water). The spatial resolution of the measurement technique used for the mass transfer investigations was 31.6 μm. The measured concentration profiles do not correspond with those, which were expected on the basis of usual theories. A solution equilibrium at the phase boundary could not be found. Received: 21 January 1999/Accepted: 4 May 1999     

Radar imaging of water surface flow fields

 We describe the capabilities of coherent high resolution radar to observe remotely the effects of an upwelling subsurface flow on the water surface. This observation is possible because the radar radiation backscatters very strongly from surface features with dimensions similar to its wavelength, in this case X-band at 0.03 m. This technique provides imaging capability with relatively high spatial resolution (∼0.3 m) and fast time sampling (∼0.006 s) over a large surface area. The processed data reveal both the line-of-sight velocity spectrum of moving water surface features, and their water surface radar backscatter cross-section. We believe that the surface features are generated by subsurface vortices oriented normal to the surface. The vortices are advected with the bulk flow of the jet. Our radar observations of the down-stream flow from a submerged waterjet that is directed parallel to the surface are consistent with those previously measured by laser velocimetry. Received: 25 February 1994/Accepted: 16 May 1996     

A pulse-expansion wave tube for nucleation studies at high pressures

 The design and performance of a new pulse-expansion wave tube for nucleation studies at high pressures are described. The pulse-expansion wave tube is a special shock tube in which a nucleation pulse is formed at the endwall of the high pressure section. The nucleation pulse is due to reflections of the initial shock wave at a local widening situated in the low pressure section at a short distance from the diaphragm. The nucleation pulse has a duration of the order of 200 μs, while nucleation pressures that can be achieved range from 1 to 50 bar total pressure. Droplet size and droplet number density can accurately be determined by a 90°-Mie light scattering method and a light extinction method. The range of nucleation rates that can be measured is 10⁸ cm^-3 s^-1<J<10¹¹ cm^-3 s^-1. We will illustrate the functioning and possibilities of the new pulse-expansion wave tube by nucleation rate measurements in the gas-vapour mixture nitrogen/water in the temperature range 200–260 K, and in the mixture methane/n-nonane as a function of supersaturation S at various total pressures up to 40 bar and temperatures around 240 K. Received: 5 June 1996/Accepted: 9 December 1996     

Sub-nanomol per second flow measurement near atmospheric pressure

 We present a measurement technique designed to accurately measure small flow rates near atmospheric pressure, and demonstrate the ability to measure flows on the order of 10^-10 mol/sec. The technique is based upon a modification of a constant-volume mass accumulation scheme where the flow rate is measured by monitoring the change in pressure of a known volume of gas. We identify two phenomena, thermally-induced fluctuations and thermodynamically-induced fluctuations, which will affect the resolution and dynamic range of the instrumentation and discuss how the problems associated with them can be mitigated. We estimate the resolution to be approximately 2.0×10^-13 mol/s, and the maximum measurable flow rate to be to the order 2×10^-9 mol/s. Received: 16 July 1996/Accepted: 2 June 1997     

Accuracy limits for planar measurements of flow field velocity, temperature and pressure using Filtered Rayleigh Scattering

 We present experimental results using Filtered Rayleigh Scattering to make planar measurements of velocity, temperature and pressure in ambient air and in a Mach 2 free jet. The ambient air measurements are used to identify and calibrate experimental uncertainties. The Mach 2 measurements demonstrate the usefulness of the technique for making accurate planar measurements in a high speed flow. The measured values for velocity, temperature and pressure in the Mach 2 jet ranged, through a shock system, from 205 to 235 m/s, 150 to 170 K and 700 to 1000 torr, with estimated uncertainties of ±5.4 m/s, ±3.2 K and ±38 torr (±2 to 3%, ±2% and ±4–5%, respectively). Received: 10 December 1996/Accepted: 14 July 1997     

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     

Visualization and measurements of void fraction in a gas-molten tin multiphase system by X-ray absorption

 A technique has been developed to measure void fraction using X-rays in a 10 cm thick pool of molten tin with gas injection at various flow rates. Visualization of the multiphase mixture using high energy X-rays can be performed at imaging rates of 220 fps with 256×256 pixel resolution or at 30 fps with 480×1128 pixel resolution. The images are subsequently processed to obtain two dimensional distributions of the chordal average void fraction in the mixture. The estimated relative uncertainty of the measurement is discussed in detail and shown to be of the order of 10% of the reported value. Received: 6 June 1997/Accepted: 2 December 1997     

Particle tracking velocimetry beneath water waves. Part I: visualization and tracking algorithms

 A novel particle tracking system working with a high particle concentration for the measurement of flow fields beneath water waves is described. It features a 1–4 cm thick light sheet parallel to the main wave propagation direction so that the seeding particles stay long enough in the illuminated area to enable tracking over several wave periods. An area of up to 14.0×10.0 cm² is observed by a CCD camera with up to 200 fields/s. The polychromatic scattering theory of small particles in a light sheet illumination is investigated, enabling the segmentation of individual particles at high particle concentration. Received: 12 July 1995/Accepted: 18 April 1997     

LIF measurements of concentration profiles in the aqueous mass boundary layer

 A laser-induced fluorescence (LIF) technique is described to measure vertical concentration profiles of gases in the aqueous mass boundary layer at a free water surface. The technique uses an acid-base reaction of the fluorescence indicator fluorescein at the water surface to visualize the concentration profiles. The technique is capable of measuring two-dimensional vertical concentration profiles at a rate of 200 frames/s and a spatial resolution of 16 μm. The mass boundary layer at a free surface is characterized by significant fluctuations. Direct surface renewal is observed. The mean profiles also support rather surface renewal models than turbulent diffusion models. Received: 21 May 1997/Accepted: 18 December 1997     

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     

Interferometric spark calorimetry

 We present a technique based on interferometry for the measurement of the quantity of energy transferred from a spark to the gases surrounding it. The primary advantage of this new technique is that it can be used to make accurate measurements not only in stagnant, but also in flowing gases. It has been used to make measurements with an estimated accuracy of better than ±5% in gases flowing at up to 6 m/s. We present the interferometric measurement technique, a simple phase-stepping holographic interferometer for making the measurement, and measurements made using a standard automotive ignition system. Measurements in flowing air at 1 bar show that the amount of energy transferred from a spark to the air doubles when the flow speed is increased from 0 to 2.2 m/s. Received: 3 March 1998/Accepted: 5 April 1999     

Time-series PIV measurements in a very large rotating tank

 We describe a non-intrusive PIV system developed for performing high-resolution measurements in a field of view of 2 m×3 m, as required on the LEGI-Coriolis 13 m diameter rotating platform. Particle preparation, laser illumination, photographic digitization, and cross-correlation analysis techniques are explained. Some results on the wake behind a cylinder illustrate the possibilities of this PIV system. Received: 29 October 1996/Accepted: 5 April 1997