Phase Doppler System Research Articles

Droplet velocity and diameter distributions in flash boiling liquid nitrogen jets by means of phase Doppler diagnostics

Due to current and future environmental and safety issues in space propulsion, typical propellants for upper stage or satellite rocket engines such as the toxic hydrazine are going to be replaced by green propellants like the combination of liquid oxygen and hydrogen or methane. The injection of that kind of cryogenic fluids into the vacuum atmosphere of space leads to a superheated state, which results in a sudden and eruptive atomization due to flash boiling. For a detailed experimental investigation of superheated cryogenic fluids, the new cryogenic test bench M3.3 with a temperature controlled injection system was built at DLR Lampoldshausen. After a first test campaign with high-speed shadowgraphy of flash boiling liquid nitrogen sprays, a laser-based Phase Doppler system was set-up to determine the spatial distributions of droplet velocities and diameters in highly superheated sprays. The spatial distributions revealed a core region with high mean velocities close to the injector orifice. With increasing distance from the injector orifice, the sprays develop a more and more monodisperse pattern. These distributions also showed that atomization due to flash boiling generates finer sprays with growing degrees of superheat. In certain spray regions, two droplet populations varying in their direction of motion, velocity and diameter due to possible recirculation zones were observed. The experimental data of flash boiling liquid nitrogen generated within this study provide a comprehensive data base for the validation of numerical models and further numerical investigations.Graphic abstract

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.

Wind tunnel flux comparisons using a phase Doppler interferometer

Abstract It is essential to know the movement of droplets in time and space (i.e. flux) when measuring and/or predicting spray drift in agricultural application. A study was performed to assess the flux measurements of a phase Doppler system against a standard monofilament system in a wind tunnel. The primary objectives of the study were to compare flux from a new phase Doppler system against 1.7 mm cotton and 2.0 mm nylon strings at varying wind speeds (1.4, 4.2, 8.3, 12.5, and 16.7 m · s−1) and spray exposures times (5, 10, 15, 30, and 60 s) with an overarching hypothesis that the active, phase Doppler is able to accurately measure the flux regardless of exposure and spray mass whereas the static string samplers are limited to a maximum retention. The phase Doppler did measure linearly as expected, however strings did not reach a point in which they loss mass; conversely, they appeared to overload with saturation. These findings are believed to be among many variables which influence the variability of previous mass balance studies.

Combined three-color LIF-PDA measurements and infrared thermography applied to the study of the spray impingement on a heated surface above the Leidenfrost regime

This experimental study deals with the impingement of sprays on a heated wall in close to semi-industrial conditions. Therefore, an experimental set-up was specifically designed: high liquid mass flux (up to 8kg/m2/s), large droplets (up to 300μm) and high surface temperature widely above the Leidenfrost point (up to 800°C). The surface to be cooled is a 175mm diameter nickel disk with 5mm thickness heated by electromagnetic induction. The spraying devices consist in full cone sprays. Several studies, performed under these particular conditions, has allowed characterizing the heat flux removed by the spray by using techniques based on thermocouples or infrared thermography (IRT). However, to investigate more finely the heat transfer mechanism, it is necessary to measure the droplet temperature but, to date, no measurement for sprays are available in the literature. Therefore, the objective of the present work is to demonstrate the ability of the three-color Laser Induced Fluorescence (3cLIF) to meet this challenge. The combination of the 3cLIF with a Phase Doppler system allows also determining the droplet temperature per size class before and after impact. By adding the IRT measurements combined to an inverse heat conduction model, an energy balance is done in order to estimate the evaporated liquid mass. The influence of the normal Weber number on the liquid vaporization is investigated by using three sprays (Normal Weber numbers up to about 1500). Thereby, main results show that the droplets temperature after impact increases with the incident Weber number before to reach a plateau, attributed to the apparition of the splashing regime. The analysis of the energy balance shows that the mass of evaporated liquid decreases with the incident Weber number and the droplet heating. These first results are in well agreement with previous works conducted with single calibrated droplets, validating as the same time the use of combined 3cLIF-PDA measurements in the case of sprays.

Volumetric validation of mass balance using a computational phase Doppler approach for disc core nozzles

The mass balance of orchard air-blast sprayers has historically been assessed using an array of samplers to capture airborne particles. However, these methods only provide an idea of flux with no other information which is pertinent to understand the movement of droplets and their potential to drift. While droplet analysis for agricultural sprayers has always been conducted in a laboratory setting with the use of laser devices, a new phase Doppler approach is being explored to assess droplet spectra, velocity, and flux in outdoor field conditions. Therefore it is the objective of this study to develop a methodology and the potential limitations for using a phase Doppler system while in a laboratory setting. Due to the expected variability of field conditions as well as the turbulence of orchard sprayers, a computational approach was sought to assess flux from a single scan of a conical spray plume's diameter. Using a constant scanning speed of 0.0079 m/s, a disc core (D1/DC33) hollow cone nozzle was examined at 310, 410, and 520 kPa pressure at five different heights (10, 20, 30, 40, and 50 cm). Computational flux was then compared to the actual flow rate, finding a −3.3% average error with a range of −16.9% and 4.7% illustrating a small underestimation of mass with the phase Doppler which was related to distance and droplet frequency. Further, comparisons were also assessed including pattern/symmetry, droplet spectra, velocity, and the overall number of samples. The proposed methodology indicates potential for the use of phase Doppler technology for in situ measurements of spray equipment using a conical-type spray nozzle, such as that of the orchard air-blast sprayer.

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.

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.

Phase doppler measurements: system set-up optimization for characterization of a diesel nozzle

A commercial phase Doppler system was set up, optimized and used to measure the time resolved characteristics of the droplets inside a diesel spray. The purpose of this work was to understand exactly the influence of each system parameter, and to find the best setup enabling measurements in the spray zones that are densest and closest to the injector. Parametric studies were performed to gain an understanding of the particle density limits of the system and their dependence on the system parameters. Then the diesel spray produced by a single-hole injector was measured, with the fuel pressure ranging from 300 to 1300 bar and gas density in the test chamber ranging from ambient conditions to 40 kg/m3. The optic parameters (beam waist size, lenses focal length) were chosen to the best expected values allowed by the optical stand-off of the spray enclosure. The receiver slit width, which was found to have a dramatic effect on the detection of droplets during the injection main period, was tested in the range from 100 μm to 25 μm. Tests were carried out with two different slit lengths, namely 1 mm and 50 μm, with results indicating minimal effect on performance. PMT voltage (gain) was held to a moderately low value between 400 and 500 V and the laser power between 400 and 800 mW in the green line. An optimum burst threshold was found to obtain the best quality data regardless of signal background level, which varies greatly in high-density pulsed sprays. In the end, a set of results from the complete nozzle characterization in various conditions is presented in order to show the practical application of the optimization study and to provide some means of appreciating the results accuracy. The results obtained were also used to show that the gas-jet theory can be used to predict if PDPA measurement are possible in a given experimental situation.

The Potential of Phase-Doppler Anemometry

Phase-Doppler anemometry (PDA) is a well-established optical measuring technique for the nonintrusive and simultaneous determination of size and velocity of spherical particles in various engineering processes. The design of a phase-Doppler system includes simulations based on the Mie theory, which describe the scattering of a plane wave by a spherical particle. Although the greater number of particles in technical processes or in the natural environment are non-spherical, the real shape is neglected and the particles are assumed to be perfectly spherical, with smooth surfaces, and to consist of homogeneous materials. In practice, it was found that the size-response of a phase-Doppler system deviates from the theoretical prediction when these assumptions are not fulfilled. In this paper, optical arrangements and adaptations of theories of light scattering will be discussed which utilize these deviations to provide even more information on the nature of the particles being examined.

OPTICAL MEASUREMENT OF NEBULIZER SPRAYS: A QUANTITATIVE COMPARISON OF DIFFRACTION, PHASE DOPPLER INTERFEROMETRY, AND TIME OF FLIGHT TECHNIQUES

Five spray analysis instruments are compared by measurement of identical medical nebulizer sprays. The instruments tested include a phase Doppler interferometer (Aerometrics), a time-of-flight instrument (Amherst Process Instruments) and three diffraction instruments (all by Malvern). Two different jet nebulizers were used for testing. Each was tested with both water and propylene glycol. Volume distributions, mass median aerodynamic diameters (MMAD), and ninety-percent volume aerodynamic diameter measurements ( d. 90) are compared. MMAD results from the Aerosizer were consistently higher than those from the diffraction instruments. MMAD and d. 90 results from the Phase Doppler system agreed well with the diffraction instruments for water sprays, but varied from the other instruments for propylene glycol sprays. This occurred because the Phase Doppler Interferometer reported larger droplets present in the propylene glycol sprays that in some cases significantly affected the otherwise smaller sized volume distributions. These droplets were not apparent in the results from the other instruments but were noted during visualization studies. Volume distribution results demonstrate that the Malvern 2600 is inappropriate for use with sprays in the examined size range. The other volume distributions were similar with the exception of the Aerosizer, which reported higher, more normally distributed volume distributions compared to the other instruments.

Influence of Agglomerates of Conducting Spheres on the Response of a Phase-Doppler Anemometer

The influence of agglomerates of solid conducting spheres on the response of a phase-Doppler anemometer (PDA) is described for a two-sphere system by using a ray theory model. First- and second-order reflection and diffraction are considered for far-field calculations of the PDA phase difference. The numerical simulations are accompanied and supported by experimental results. Two-sphere systems of Sn63Pb37 alloy particles were captured inside an electrodynamic trap and investigated with a standard phase-Doppler system.

A Device for Phase Shift Measurement in an Advanced Phase Doppler Velocimeter

Construction and performance of a printed circuit board is described that is built for phase measurement in a state-of-the-art phase Doppler system. Special requirements in treating phase Doppler signals are highlighted and shown to be met satisfactorily by an innovative electronic design that involves signal burst detection, multi-bit sampling, frequency-band narrowing, and phase signal validation based on integrated amplitude and waveform recognition. Performance of the device is measured in terms of phase accuracy for various signal-to-noise ratios, signal frequencies and operating temperatures. The present device is also compared with some alternative techniques for phase measurement.

Sizing of Irregular Particles Using a Phase Doppler System

AbstractResponse of a phase Doppler system to irregularly shaped particles is examined and shown to deviate qualitatively as well as quantitatively from the spherical particles. Nevertheless, the measured phase distributions based on an ensemble of particles exhibit a high degree of order and simplicity. The experimental data and the stochastic modeling of the process have shown that the phase Doppler technique can be used successfully for in‐situ sizing and velocimetry of irregular particles. In the case of irregular crystalline particles, mean size and standard deviation can be deduced without requiring any assumptions regarding the functional form of the size distribution. As opposed to other optical techniques, phase Doppler can be used, in principle, near the backscattering location, so that a single optical window would be employed for transmission of laser light and collection of the scattered signals. Furthermore, size measurements can be velocity‐resolved, i.e. a size distribution can be associated with each bin of the velocity histogram.

Role of combustion on droplet transport in pressure-atomized spray flames

The transport of droplets in a pressure-atomized kerosene spray flame was examined using a twocomponent phase Doppler system to measure the droplet size and velocity distributions at several locations within the spray. The effect of combustion on droplet transport was examined by comparing the results to a nonburning spray under similar flow conditions. Directions of motion of droplets are calculated from the measured droplet velocity components to provide information on trajectories and dispersion of droplets. Results show that combustion reduces the strength of gas recirculation as evidenced by significantly fewer droplets being transported upstream toward the nozzle along the centerline for the burning spray as compared to the nonburning spray. Combustion enhances droplet vaporization and results in reduced number density and larger droplet mean diameters and velocities when compared to the nonburning spray. At the spray centerline, there is a wide range of droplet trajectories that are associated with recirculated droplets and those originating from the nozzle. There is some correlation between droplet velocity and diameter. Some larger size droplets are also found to be entrained into the recirculation pattern. At the spray boundary, few droplets deviate from the mean direction. Also, droplet velocity is well correlated with increasing diameter but appears to approach an asymptotic value in which droplet velocity becomes insensitive to diameter.

Basic Studies of Electrohydrodynamic Atomization Process using phase Doppler measurement technique

AbstractA planar phase Doppler system is used to measure submicron droplets generated by an electrospray. Measured drop dia‐meters are correlated with the liquid properties and the condition for transition of the spray from the single‐jet mode to the multi‐jet mode is introduced. In another set of measurements using a standard phase Doppler system, combined size and velocity data are employed to deduce the drag force on the drops. In a situation where the drag force is balanced primarily by the electric force, the phase Doppler measurements allow to estimate the power‐law relationship between the charge on a drop and its diameter and hence, many provide insights into the underlying atomization mechanisms.

Non-intrusive measurements of bubble size and velocity

A non-intrusive measuring technique based on video-imaging has been developed for the measurement of bubble size, velocity and frequency. Measurements carried out with this method have been compared to those obtained by an optimized phase-Doppler system in standard configuration, for a wide range of bubble sizes produced from single injectors in a quiescent environment. The two measuring techniques have yielded velocities and frequencies that are in very good agreement while the size of spherical bubbles was consistently measured by both methods. The phase-Doppler system was also used to size oblate-spheroidal bubbles moving with their equatorial plane parallel to the scattering plane, yielding measurements reasonably close to the average radius of curvature of the bubbles in the neighborhood of the equatorial plane, as calculated from the video-imaging data. Both methods were used for detailed velocity measurements of the bubble-stream in the neighborhood of the injector tip. The observed bubble-velocity variation with the distance from the injector tip does not always display the usual increasing trend leading into the terminal velocity. When injection conditions are near the transition from discrete to jet injection mode and the bubbles are small, the latter decelerate into a terminal velocity due to direct interaction of successive bubbles at the injector tip. The measured terminal velocities of bubble-chains for a variety of bubble sizes and injection frequencies, are successfully predicted by using a far-field wake approximation to account for the drafting effect which is responsible for bubble-chain velocities higher than those of single bubbles.

Particle Sizing by Planar-Phase Doppler System

Conventional phase Doppler systems are useful for sizing particles in the order of microns, but sensitive to the Gaussian beam defect which can cause sizing errors. The defect can be significant when a large size is measured. In this paper, we present a new phase Doppler system using a planar optical layout which permits large particles to be measured in a forward scattering scheme without the Gaussian beam errors. The optical system design is discussed by numerical simulation based on the Mie theory.

Phase-Doppler Diagnostic Studies of Primary and Tertiary Aerosols Produced by a High-Efficiency Nebulizer

Two phase-Doppler instruments (a two- and a four-beam laser light-scattering interferometer) are used for the diagnostic studies of aerosol produced by a pneumatically driven high-efficiency nebulizer (HEN). The new nebulizer (capillary i.d. = 79 [mu]m) is useful in plasma spectrometries for nebulization of solutions at uptake rates of 10-1200 [mu]L/min. Diagnostic studies involve measurements on primary and tertiary aerosols prior to injection into a high-temperature plasma. Simultaneous measurements are conducted on droplet size and droplet velocity distributions, size-velocity correlation, span of aerosols, droplet number density, volume flux, and percent volume of aerosol droplets under 8 [mu]m. The four beam interferometer (or the 2D phase-Doppler system) is used to assess the axial and radial velocity components of the primary aerosol from the HEN. A broad range of droplet velocities is observed near the nebulizer tip for the primary aerosol, but most tertiary droplets travel at nearly the same velocity, much slower than the primary droplets. The Sauter mean diameter (D[sub 3,2]) of the primary aerosol is found to increase both along the center line of the aerosol pathway and at the boundary of the spray. 34 refs.., 9 figs., 2 tabs.

In situ measurement of submicron droplets in electrosprays using a planar phase doppler system

Planar phase Doppler system is a promising technique for simultaneous sizing and velocimetry of particles smaller than 10 μm in diameter, in particular submicron spherical particles. Some new features of this technique are highlighted herein, including the possibility of obtaining unambiguous measurements of drop sizes in the range of 0.2-1 μm using only two signal detectors. In situ measurements of submicron electrospray droplets covering a 4:1 range of liquid viscosity and a 20:1 range of electrical conductivity are reported. The results are compared with the existing theories and empirical correlations and used to classify the capillary-type electrosprays.