Phase Doppler Technique Research Articles

Gas-phase velocity estimation in practical sprays by Phase-Doppler technique

Practical sprays are characterized by a two-way coupling between droplets and the surrounding gas. The effect of sprays on process performance is critical in numerous applications; hence, the gas-phase velocity is often estimated via two methods, using Phase Doppler measurement data. The first one is using a rule of thumb, i.e., estimating the gas-phase velocity by analyzing droplet sizes below a few micrometers. The second way of estimating the Stokes number, Stk, is using a threshold well below unity, such as 0.1. There are numerous definitions available in the literature for Stk, resulting in several magnitudes difference. These complex problems are resolved in this paper in the following way. A new definition for Stk is provided, which is sufficiently robust for, e.g., pressure and twin-fluid atomizers. According to the results, the Stk < 0.1 threshold means droplet sizes between 2 and 10 micrometers above 10 m/s gas-phase velocities. Filtering for too small droplets could lead to biased characteristics, especially in estimating the turbulent properties. Hence, the gas-phase velocity estimation is a function of the measurement setup and has a significant spatial dependence. The reader can find the software code online and the algorithm in Appendix A.

Novel Phase Doppler Profile Sensor For Simultaneous Measurement Of Size, Velocity, And Position Of Droplets

In numerous technical flow applications knowing size, velocity, and position of dispersed structures simultaneously enables the comprehensive characterization of the underlying dynamics. A suitable interferometric measurement technique capable of measuring simultaneously the three droplet characteristics is introduced for the first time. This novel measurement technique called phase Doppler profile sensor (PDPS) combines the measuring principles of the laser Doppler profile sensor (LDV-PS) for velocity measurement along a measuring line with the phase Doppler technique for particle size determination. A proof-of-concept of this technique was successfully performed by comparison to another established measurement technique on a controlled use case of a displaceable mono-disperse droplet stream. In this first optical setup, a conventional phase-Doppler sensor is used as receiving unit with a conventional pinhole aperture cutting the measurement volume. Hence, the resulting performance for simultaneously measuring the droplet diameter, velocity and position along a measuring line mark the preliminary status reached and are supposed to improve in future measurements with a removed pinhole aperture. So far, the average relative deviation of PDPS compared to the reference technique simultaneously measuring diameter, position, and velocity is 1.14% (diameter), 2.18% (position), and 0.4% (velocity) when using a general laboratory setup.

Gas-Phase Velocity Estimation in Practical Sprays by Phase-Doppler Technique

Gas-Phase Velocity Estimation in Practical Sprays by Phase-Doppler Technique

Evaluation of the atomization characteristics of aviation fuels with different viscosities using a pressure swirl atomizer

With advances in the fuel spray field, much attention has been paid to the atomization characteristics of aviation fuels. The physical properties of fuels play a vital role in their resulting spray structure and spatial distribution of droplets. These characteristics are becoming more important for choosing alternative fuels for optimal performance in next-generation gas turbines. The current study experimentally investigated the atomization quality and spray structure of aviation fuels with different viscosities sprayed through a pressure swirl-type nozzle by employing laser diagnostics. A set of CCD cameras synchronized to the Nd:YAG laser characterized the spray structure, and the size and velocity of the drops were quantified by the phase Doppler technique. The fuels were tested under various temperature and injection pressure conditions. An alteration in both the injection pressure and the physical properties of the fuel were found to significantly dominate the spray structure and atomization quality. The transitional development of hollow-conical sprays was shown to require a sufficient amount of swirl intensity, characterized by their physical properties and injection pressure. Fuels with a lower viscosity were experimentally found to generate well-developed hollow-conical sprays with finer drops, wide distribution in size, and higher velocity components, which are recognized as ideal spray properties in the combustion field.

Air–liquid interactions in a pressure-swirl spray

The energy transfer between a liquid hollow cone spray and the surrounding air has been studied using both imaging and phase-Doppler techniques. The spray was produced by a pressure-swirl atomizer discharging Jet A-1 fuel at inlet over pressures of Δp=0.5, 1.0 and 1.5 MPa into quiescent ambient air.The liquid exits the nozzle as a conical film which thins as it spreads and develops long- and short-wave sinusoidal instabilities with breakup occurring, at the length smaller than that predicted by the inviscid model, to form film fragments and ultimately droplets downstream the spray.The single shot imaging characterised the spray regions of near-nozzle flow, the breakup processes and the developed spray. The phase-Doppler system resolved the three components of velocity and size for the droplet flow as measured on radial profiles for four axial distances from the nozzle exit.A Stokes number, Stk, analysis of the droplets’ response times to the airflow time-scales showed that droplets < 5 μm followed the airflow faithfully and so were used to estimate the local airflow velocity. This allowed a comparison of both the droplet and airflow fields in terms of their mean and fluctuating velocity components to be made.The formation of the hollow cone spray and the interaction of the fragments and droplets with the air, through viscous drag, induce complex entrained airflows. The airflow was found to be highly anisotropic, fluctuating preferentially in the downstream direction, and spatially varying within three distinct spray regions. The air drag establishes a positive size–velocity correlation of droplets; their Stk reduces with axial distance and increases with droplet size and Δp; so that Stk≈1 for 20–40 μm droplets and the largest droplets (80–160 μm, Stk > 10) move ballistically.The spatially resolved mean and turbulent kinetic energies of the air and spectra of the droplet velocity fluctuations are detailed in the paper. These findings are relevant to scientists and engineers modelling the complex two-phase flows.

Droplet dynamics and size characterization of high-velocity airblast atomization

Airblast atomizers are especially useful and commonplace in liquid fuel combustion applications. However, the spray formation processes, the droplet dynamics and the final drop size distributions are still not sufficiently understood due to the coupled gas-liquid interactions and turbulence generation. Therefore, empirical and semi-empirical approaches are typically used to estimate the global spray parameters. To develop a physical understanding of the spray evolution, a plain-jet airblast atomizer was investigated in an atmospheric spray rig using the phase-Doppler technique. The simultaneous drop size and axial and radial velocity components were measured on radial traverses across the spray at various axial distances from the nozzle for a range of atomizing pressures. The droplet turbulent and mean kinetic energies were found to be proportional to the atomizing pressure. Hence, the scatter of the radial motion of the droplets increased with the atomizing pressure. A droplet stability analysis was performed to locate the regions characterized by ongoing secondary atomization. The volume-to-surface diameter, D32, of the fully developed spray was compared with estimates provided by five published formulae. The role of liquid viscosity, hence the Ohnesorge number, was found to be negligible in the investigated regime. Three commonly used size distribution functions were fitted to the measured data to analyze their dependence on the atomizing pressure. The Gamma distribution function was found to give the best approximation to the atomization process.

Hydrodynamic Nuclei Concentration Technique in Cavitation Research and Comparison to Phase-Doppler Measurements

Small particles, especially bubbles in the micro-meter range, influence the cavitation of the propellers. The prediction of cavitation inception and water quality measurements are important in cavitation research. The Hydrodynamic Nuclei Concentration (HDNC) technique can be used for reliable bubble concentration measurements in fluid flows. The HDNC technique bases on the analysis of scattered light from the cavitation nuclei in the water. The HDNC technique can distinguish between bubbles and solid particles. The particle type classification is important, because the number concentration of solid particles is often much higher than the nuclei concentration in cavitation tunnels and in seawater. Verification experiments show, that the HDNC technique reaches similar capabilities in number concentration estimation as Phase Doppler (PD) technique in much shorter acquisition time.

Comparative study of the cooling of a hot temperature surface using sprays and liquid jets

This experimental work aims at investigating the cooling of hot surfaces by using full cone sprays; comparison with the use of a liquid jet is also considered. The wall is a 175mm diameter nickel disk and 5mm thickness heated by electromagnetic induction up to about 800°C. In the case of the sprays, the goal is to link the spray properties with the heat flux removed from the heated surface. For the spray, the influence of the mass flux distribution as well as the droplets properties on the cooling are studied by using three different spray nozzles. The mass flux distribution for each of the sprays is measured with the help of a patternator. The heat removed during the cooling phase is investigated with the use of infrared thermography while the droplet properties are characterized simultaneously by a Phase Doppler system. The Phase Doppler technique is mainly applied to study the statistical properties of the outcoming droplets in the vicinity of the heated surface. A decrease of the outcoming droplets size compared to the incoming one is well observed. Moreover, Phase Doppler device has also highlighted that the presence of the surface may have a significant influence on the upstream spray flow. Compared to the liquid jet, heat flux measurements have clearly demonstrated that the sprays lead a more spatial uniformity of the extracted heat flux and a better cooling efficiency. When spray cooling is considered, the influence of the mass flux on the heat flux and the cooling efficiency is in qualitative agreement with previous studies performed in similar conditions. In addition, a comparative study of performances, in term of liquid consumption and cooling duration, is performed by considering a similar surface temperature decrease for the three sprays and the liquid jet. Among the four experiments, the liquid jet has the longest cooling time as well as the highest liquid consumption. Furthermore, the best performances are reached for the spray having the highest mass flux value.

EXPERIMENTAL INVESTIGATIONS ON A PIEZO-ACTIVATED HOLLOW CONE INJECTOR - PART II: THE INFLUENCE OF NEEDLE LIFT ON DROPLET SIZE DISTRIBUTIONS AND VORTEX FORMATION

The outward-opening, piezo-driven pintle injector, designed for spray guided direct injection gasoline engines, produces a hollow cone spray formed by a dense array of distinct streaks of fuel systematically ordered around the circumference of the cone. The most significant advantage of the piezo drive is its ability for precision control of the injector needle. In this work, different aspects of the dynamics of the hollow cone spray produced by the injector have been studied for short injection durations as a function of the needle lift. The time-varying structure of the hollow cone and, in particular, the entrained flow field have been evaluated with planar Mie imaging. The Phase Doppler technique (PDA) has allowed the axial and radial droplet velocities and sizes to be quantified in a vertical plane containing a single fuel streak. During the fuel injection period the main droplet velocity field generally aligns with the imaged spray half cone angle. The PDA sample count has allowed estimates to be made of the location of the dense streak as a function of time. In the early phase of the injection, the axis of the streak is unstable, as it moves toward the outside of the cone until a stable position is reached. The droplet size distribution at a given distance from the nozzle is also influenced by the needle lift as the breakup length is also a function of the needle lift. As the main spray propagates, droplets in the shear layers transfer momentum to the entrained air and results in two counter-rotating vortices being generated, clockwise on the inside and counter-clockwise on the outside of the cone. The outer vortex, which dominates the axial flow field, has been tracked with time to determine the radial and axial displacements of its centre with respect to the injector nozzle.

Application of femtosecond lasers to the polarization ratio technique for droplet sizing

Ultra-short laser pulses (tpulse = 220 fs) were applied to a monodisperse droplet stream, demonstrating the feasibility of the polarization ratio technique for instantaneous droplet size measurements with high spatial and temporal resolution. The polarization ratio technique is based on the signal ratio of perpendicularly and parallel polarized components of Mie scattered light. When applied to sprays this technique yields signal ratios proportional to the diameter D21. Using a highly reproducible monodisperse droplet stream the drop sizing of individual droplets allows the comparison to well accepted methods such as the phase Doppler technique or direct imaging. Droplet sizes were varied over a wide range, including droplet sizes relevant for practical combustion devices. The application of ultra-short laser pulses avoids oscillations in the scattered signal caused by interference between different scattering orders. This effect is limited to diameters larger than a cut-off diameter that depends on the coherence length of the light source. These oscillations are a major drawback when using CW laser sources operating at long coherence lengths, as they lead to an ambiguous relation between droplet diameter and polarization ratio and necessitate spatial or temporal averaging. It is shown that the application of femtosecond lasers significantly reduces the number and magnitude of oscillations in the polarization ratio and therefore improves the precision of the polarization ratio technique for droplet sizing by a factor of 3 on average depending on the mean droplet diameter.

Assessment of measurement efficiency in laser- and phase-Doppler techniques: an information theory approach

Laser- and phase-Doppler diagnostic techniques provide information on particle characteristics in the form of discrete probability distribution functions. Most methods assess the amount of information required for an accurate measurement through the first- and second-order moments of these distributions. However, considering that a measurement is the distribution and not its moments, a different approach is developed based on information theory (IT) concepts in order to assess if the information provided to the experimentalist is enough to ensure an accurate statistical analysis. The methodology and stopping criteria are presented and used in previously reported measurements obtained with laser- and phase-Doppler techniques. Results show that using an IT approach to assess the reliability of data provided by a measurement means evaluating the degree of stabilization of a discrete probability distribution, where more information acquired does not necessarily imply a more accurate measurement. The statistical analysis performed using the number of samples indicated by the IT method, compared to the total sample size previously measured, shows similar results. Moreover, measurement time is substantially reduced if the IT method is used, thus improving measurement efficiency.

An analysis of spray development with iso-octane, n-pentane, gasoline, ethanol and n-butanol from a multi-hole injector under hot fuel conditions

High-pressure multi-hole injectors for direct-injection spark-ignition engines offer some great benefits in terms of fuel atomisation, as well as flexibility in fuel targeting by selection of the number and angle of the nozzle’s holes. However, very few data exist for injector-body temperatures representative of engine operation with various fuels, especially at low-load conditions with early injection strategies that can also lead to phase change due to fuel flash boiling upon injection. The challenge is further complicated by the predicted fuel stocks which will include a significant bio-derived component presenting the requirement to manage fuel flexibility. The physical/chemical properties of bio-components, like various types of alcohols, can differ markedly from gasoline and it is important to study their effects in direct comparison to liquid hydrocarbons. This work outlines results from an optical investigation (high-speed imaging and droplet sizing) into the effects of fuel properties, temperature and pressure conditions on the extent of spray formation. Specifically, gasoline, iso-octane, n-pentane, ethanol and n-butanol were tested at 20, 50, 90 and 120°C injector body temperatures for ambient pressures of 0.5bar and 1.0bar in order to simulate early homogeneous injection strategies for part-load and wide open throttle engine operation; some test were also carried out at 180°C, 0.3bar. Droplet sizing was also performed for gasoline, iso-octane and n-pentane using Phase Doppler and Laser Diffraction techniques in order to understand the effects of low- and high-volatility components on the atomisation of the multi-component gasoline. The boiling points and distillation curves of all fuels, their vapour pressures and bubble points, as well as density, viscosity and surface tension were obtained and the Reynolds, Weber and Ohnesorge numbers were considered in the analysis.

Measurement of Pulse Width from a Bubble Cloud under Multibubble Sonoluminescence Conditions

The pulse width from a bubble cloud under multibubble sonoluminescence (MBSL) conditions was measured for the first time using a time-correlated single photon counting technique (TC/SPC). The measured pulse width from several thousand bubbles in water was approximately 250.9 ps, with scattered pulses occurring 1.5 ns before and after the maximum light pulse intensity. The observed pulse width from a bubble cloud, which appears to be comparable to that of the single bubble sonoluminescence, indicates that the clouds of bubbles collapse simultaneous to emitting a light that is synchronized with the applied ultrasound. Also, pulse widths from clouds of multibubbles in water–surfactant and water–alcohol solutions were measured to investigate the surfactant and alcohol effect on the sonoluminescence intensity and pulse width. Size distribution of the bubble cloud at the multibubble conditions was also measured by phase-Doppler technique.

Spray Generated by an Airblast Atomizer Under Elevated Ambient Pressures

The objective of this work is to study a spray generated by an airblast atomizer at elevated ambient pressures. The study includes the measurement of a typical spray's integral parameters (average drop diameter and average three-dimensional velocity vector of liquid and gas phases), numerical simulation of spray propagation, and models validation. The numerical study of the spray evolution was carried out using an accurate numerical spray module based on an Euler-Lagrange method accounting for drops evaporation. The experimental data were collected using the phase Doppler and particle image velocimetry techniques. The drop diameter and two velocity components were measured simultaneously using two-dimensional phase Doppler system. These data were used as initial conditions for the numerical simulations (data in the neighborhood of the atomizer exit) and for the validation of the numerical method (data on the spray parameters measured at various distances from the atomizer). The droplet diameter and spray velocity are shown to be influenced by the enhancing of the ambient chamber pressure.

G102 ディーゼル機関燃料噴霧における渦構造の研究(GS1-流体一般,一般セッション)

A phase doppler technique was used for the measurement of vortex scale in diesel sprays after the end of fuel injection. The data sampling rate of the phase doppler system was 250kHz. Investigated were diesel fuel sprays injected intermittently into the atmosphere from an injector with a orifice 0.113 mm in diameter. The rail pressure was set at 40 MPa by using a common rail system. Measurement position was located at 40 and 80 mm from the nozzle exit. It was found that the time scale of turbulence was nearly constant in the spray, and that the spatial scale of turbulence depended on the mean velocity. The movement of the vortex in the direction of spray axis was observed.

Methods for measuring refractive index and absorption coefficient of a moving particle using polarized-type phase-Doppler technique

Two methods are proposed for measuring the refractive index or the absorption coefficient simultaneously with the diameter and velocity of a moving spherical particle by using a polarized-type phase Doppler method. A particle refractive index is determined from phases of reflected and refracted rays detected individually by using different polarization angles of scattered light, whereas an absorption coefficient is obtained by applying the Lambert–Beer law to intensity of a dual-burst Doppler signal generated by the reflected and refracted rays. The optimum conditions of an optical system were estimated theoretically. Experiments were performed with polystyrene particles in water, water droplets, and water-black ink droplets to confirm usefulness of the proposed methods.

Identification of droplet burning modes in lean, partially prevaporized swirl-stabilized spray flames

Experimental and numerical investigations of single droplet burning modes in a lean, partially prevaporized swirl-stabilized spray flame are reported. In the experiment single droplet flames have been visualized by CH-PLIF and simultaneous recording of the Mie signal. Two single droplet burning modes were identified: the envelope flame is a spherical diffusion flame burning at near-stoichiometric conditions. The wake flame is a potentially lean, partially premixed flame located downstream of the droplet. The droplet burning mode is of practical relevance, since it has significant impact on NO formation due to incomplete prevaporization. The droplet burning mode is determined by the ratio of chemical and convective time scales. The convective time scale is related to the droplet slip velocity. The impact of turbulent gas phase velocity fluctuations on droplet mechanics and droplet burning is discussed, based on a previous numerical investigation. In the present study the droplet slip velocity was measured with the 3D Phase Doppler (3D-PD) technique. For the measured slip velocities and ambient conditions in the hot gas region of the spray flame, simulations of single droplet burning were performed utilizing detailed models for chemical reaction, diffusive transport and vaporization. An agreement between the droplet burning modes predicted by the simulation and the droplet burning modes observed in the experiments was found.

102 液滴の非球形性が位相ドップラー法による滴径計測に及ぼす影響の計算電磁気学に基づく検討(GS-6 熱流体計測・試験)

102 液滴の非球形性が位相ドップラー法による滴径計測に及ぼす影響の計算電磁気学に基づく検討(GS-6 熱流体計測・試験)

Phase Doppler measurements of spray impact onto rigid walls

In this work, an experimental study of spray impact onto a horizontal flat and rigid surface is presented. The phase Doppler technique has been used to characterize both the impacting and the secondary spray in terms of mass and number flux, size distribution and velocities of the droplets above the target. A high-resolution CCD camera has been used to measure the average liquid film thickness formed due to spray impact, whereas a high-speed CMOS camera has been used to characterize the splashing droplets from the wall. This visualization of the splashing phenomenon and the knowledge about the liquid film thickness are used to formulate a new physical model of the crown evolution. Furthermore, information about the incident-to-ejected mass fraction and number fraction are novel contributions of this study. Considerable data are provided comparing the impact of single drops onto a liquid film to impact of drops in a spray, and the significance of the observed differences for modelling efforts is discussed. The measurements of this study are also shown to be rather sensitive to the placement of the phase Doppler measurement volume above the surface and to the operating parameters of the instrument. These effects have been documented and discussed for this particular measurement situation.

Multi-dimensional particle sizing techniques

Two techniques for multi-dimensional sizing of spherical particles are discussed and compared with one another: interferometric particle imaging (IPI) and a novel technique known as global phase Doppler (GPD). Whereas the IPI technique is known from various previous studies and uses a laser light sheet illumination of the particle field, the GPD technique is a new method and employs two intersecting laser light sheets. The resulting far-field interference pattern arises from the interference of like scattering orders from the particle, similar to the phase Doppler technique. A description of this far-field interference is given for both techniques. Both multi-dimensional particle sizing techniques sample the scattered light in the far-field by means of a defocused imaging system. The diameter of each droplet illuminated by the laser light sheet(s) is determined by measuring the angular frequency of the interference fringes in the defocused images. Combined with a pulsed laser, the technique also allows the velocity of the particle to be determined, similar to particle tracking velocimetry (PTV). However, the size of the defocused image of each particle also depends on the position of the particle perpendicular to the laser sheet, hence, with appropriate calibration, the third component of velocity is also accessible. The two techniques, IPI and GPD, are compared to one another in terms of implementation and expected accuracy. Possibilities of combining the two techniques are also discussed. Some novel approaches for the signal processing have been introduced and demonstrated with simulated and real signals.