Phase Doppler Particle Analyzer Research Articles

An Experimental Study of Mist/Air Film Cooling With Fan-Shaped Holes on an Extended Flat Plate—Part 1: Heat Transfer

Motivated by the need to further improve film cooling in terms of both cooling effectiveness and coolant coverage area, the mist/air film cooling scheme is investigated through experiments using fan-shaped holes over an extended downstream length in this study. Both an existing wind tunnel and test facility, used in previous work, have been retrofitted. The first modification was extending the length of the flat plate test section to cover longer distances downstream of the injection holes, up to X/D = 100, in order to investigate whether mist cooling can be harnessed farther downstream where single-phase film cooling is not effective. X represents the axial distance downstream of the cooling hole of diameter D. The second modification was to incorporate fan-shaped (diffusion) holes which are proven to have a higher film cooling efficiency, than cylindrical holes. The objective is to investigate whether mist can further enhance the film cooling performance of the already highly effective fan-shaped holes. A phase Doppler particle analyzer (PDPA) system is employed to measure the droplet size, velocity, and turbulence information. An infrared camera and thermocouples are both used for temperature measurements. Part I is focused on the heat transfer result on the wall. The results show that, at low blowing ratios when the film is attached to the surface, the enhancement of the mist film cooling effectiveness, compared to the air-only case, on the centerline of the hole ranges from 40% in the near hole region to over 170% at X/D = 100. Due to the diffusive nature of the fan-shaped hole, the laterally averaged enhancement is on par with that on the centerline. The significant enhancement over the extended downstream distance from X/D = 40–100 is attributed to the evaporation time needed to evaporate all of the droplets. Each droplet acts as a cooling sink and flies over a distance before it completely vaporizes. This “distributed cooling” characteristic allows the water droplets to extend the cooling effects farther downstream from the injection location. At higher blowing ratios, when the cooling film is lifted off from the surface, the cooling enhancement drops below 40%. Although the enhancement in the near hole region X/D < 40 is about 20% lower than that achieved by using the cylindrical holes, the magnitudes of the mist adiabatic film cooling effectiveness using fan-shaped holes are still much higher than those of the cylindrical holes. Part II of this study is focused on analyzing the two-phase droplet multiphase flow behavior to explain the fundamental physics involved in the mist film cooling.

Understanding the condensation process of turbulent steam jet using the PDPA system

In the present study, we investigated condensation characteristics of water droplets in a turbulent steam jet with the Reynolds number from 78,400 to 140,700, such as Particle Size Distribution (PSD), Total Number Concentration (TNC), and Liquid Water Content (LWC) obtained from the Phase Doppler Particle Analyzer (PDPA) system and measured the temperature profile. The temperature decay rate and the temperature half width (rT, 0.5) were obtained for steam jets. The increased latent heat release due to condensation was found to have an effect on the decreased temperature decay rate and the increased spread rate with an increase of the steam jet's Reynolds number. Along the radial direction at z/d = 10, TNC and LWC increased up to the range within r / rT, 0.5 = 1.2, but the turbulence intensity continued to increase beyond the range. These results suggest that the evaporation effect started to occur due to the mixing with the surrounding air. Along the axial direction, we used the effective diameter in order to confirm the Reynolds number effect. As the Reynolds number became larger, particle formation started to occur further downstream along the axial direction. At the downstream of the peak position, the values of TNC and LWC were found to decrease due to the mixing process.

The effect of the Reynolds number on the velocity and temperature distributions of a turbulent condensing jet

In this study, we investigated the effect of the Reynolds number (Re) on the velocity and temperature distributions of a turbulent condensing jet from a long pipe nozzle. The Re of the condensing jet ranged from 78,000 to 170,000. Jet velocity was measured using a Phase Doppler Particle Analyzer (PDPA) system and temperature was measured with a K-type thermocouple. We used the effective radius to investigate the effect of Re on the velocity and temperature distributions and compared our results with previous results on air (single phases) jet. From the experimental results, it was found that distributions of velocity and temperature show a self-similarity state in the far-field. The condensation more occurred at low Re. Thus, from momentum conservation, the velocity decay rate decreases by 18.2% as the Re increases from 78,000 to 170,000. Compared to air jet issuing from the pipe nozzle, velocity of condensing jet decays more rapidly due to the condensation of water vapor. The velocity spread rate also falls by 8.1% as the Re increases from 78,000 to 170,000. The temperature decay rate decreases by 11.1% and temperature spread rate increases by 7.4% as the Re increases from 78,000 to 170,000 since jet entrainment decreases at higher Re. The latent heat was released at the condensing jet by condensation of vapor. Thus, the temperature decay rate of condensing jet is smaller than that of air jet while the temperature half width of condensing jet is larger than that of air jet.

Sand particle lift-off velocity measurements and numerical simulation of mass flux distributions in a wind tunnel

Lift-off velocity of saltating sand particles in wind-blown sand located at 1.0 mm above the sand bed surface was measured using a phase Doppler particle analyzer in a wind tunnel. The results show that the probability distribution of lift-off velocity can be expressed as a lognormal function, while that of lift-off angle follows an exponential function. The probability distribution of lift-off angle conditioned for each lift-off velocity also follows an exponential function, with a slope that becomes steeper with increasing lift-off velocity. This implies that the probability distribution of lift-off velocity is strongly dependent on the lift-off angle. However, these lift-off parameters are generally treated as an independent joint probability distribution in the literature. Numerical simulations were carried out to investigate the effects of conditional versus independent joint probability distributions on the vertical sand mass flux distribution. The simulation results derived from the conditional joint probability distribution agree much better with experimental data than those from the independent ones. Thus, it is better to describe the lift-off velocity of saltating sand particles using the conditional joint probability distribution. These results improve our understanding of saltation processes in wind-blown sand.

Spray breakup and structure of spray flames for low-volatility wet fuels

Studies of high-water-content fuels (a.k.a., wet fuels) have demonstrated that, under proper conditions, stable combustion can be achieved at very high water concentrations. Stable spray flames of wet fuels have been attained with fuel/water mixtures having stoichiometric adiabatic flame temperatures as low as 251°C. In this study, we investigate low-volatility wet fuels, using glycerol as the fuel and ethanol as a stabilization additive. This study expands on previous work by determining the minimum amount of ethanol that needs to be added to a glycerol/water mixture to produce a stable flame and by investigating the spray dynamics and structure for these fuels, to delineate the mechanism of ignition and to understand how ethanol alters the vaporization behavior, droplet breakup, and spray dynamics. Detailed 2-D velocity, Sauter mean diameter (SMD), 2-D flux, and number concentration measurements were performed with a Phase Doppler Particle Analyzer (PDPA) in sprays of three fuel/water mixtures: (a) 30% glycerol/70% water, (b) 30% glycerol/10% ethanol/60% water, and (c) the same mixture as (b) but in a combusting spray. All percentages are by weight. Results show that the addition of ethanol to the glycerol/water mixture turns the hollow-cone spray pattern into a narrow full-cone pattern, leading to recirculation of fine droplets in the region just downstream of the nozzle, which is essential to ignition. The high concentration of fine droplets, along with the high vapor pressure and high activity coefficient of ethanol, lead to extremely rapid vaporization of ethanol in the inner recirculation zone. The combustion of the ethanol raises the temperature in this region, while the swirling flow brings heat upstream towards the nozzle, further enhancing stability. These results explain why the addition of 10% ethanol can lead to robust flames of glycerol/water mixtures that might not be expected to yield stable combustion.

Experimental investigation on the macroscopic and microscopic spray characteristics of dieseline fuel

The application of dieseline fuel (blends of diesel and gasoline) into compression ignition engines has been intensively investigated in recent years due to its good combustion performance and low smoke emissions. In this study, the main objective was to investigate the spray characteristics of dieseline fuel. Macroscopic spray characteristics in terms of spray morphology and penetration were investigated in a constant volume vessel under elevated ambient pressures using high-speed imaging system, while the microscopic spray characteristics including droplet size and velocity distribution were measured at atmospheric condition using Phase Doppler Particle Analyzer (PDPA) system. The results have shown that the gasoline/diesel blending ratio played an important role in affecting the spray atomization process. With the increase of the gasoline/diesel blending ratio, spray penetration length, droplet mean velocity, and droplet Sauter Mean Diameter (SMD) decreased gradually, especially at low injection pressure and low ambient pressure conditions. Both the results of macroscopic and microscopic studies indicate a better atomization quality can be achieved by increasing the proportion of gasoline in the dieseline.

Effect of Enclosed Flame on Spray Characteristics and Emissions from Preheated Bio-oil Using an Air-blast Atomizer

Abstract Global atmosphere pollution has become a serious problem for today. The emissions from the combustion of fossil fuels contribute a notable part to this pollution. Given a wide spread of different biofuels available for combustion applications, the present study concentrates on atomization spray characteristics of vegetable oils. In this study preheated vegetable oil (VO) is used to reduce the kinematic viscosity, and thus, improve atomization. A commercial air-blast atomizer operated at ambient conditions of temperature and pressure is used to atomize the VO. Flame spray characteristics are measured using a laser sheet visualization system and a Phase Doppler Particle Analyzer system. Experiments are conducted for unheated and preheated VO at 100 °C for a given ALR of 2.0 in enclosed flame conditions simulate realistic gas turbine conditions. VO is combusted in an atmospheric pressure burner with air blast atomizer and swirling combustion air around. The mean axial and RMS velocities, SMD and drop size distribution data are acquired. The measurements are taken for preheated VO at 100 °C and 150 °C respectively. The transverse profiles of mean axial velocity showed peaks at the center while they showed a decreasing trend at the outer edges of the spray. RMS axial velocity increases for flame spray compared to cold spray. Higher VO inlet temperatures led to smaller droplets and higher mean axial velocity for a given ALR. Results also suggest that insulated enclosure provide additional heat feedback from the flame to improve the overall spray characteristics, an effect that has been quantified in the present study.

Water aerosol formation: Transient process induced by shock waves

The formation of aerosols artificially is an interesting topic nowadays. This paper studied the interaction between water sheets and shock waves experimentally to seek the interaction mechanism and simulate the formation processes of water aerosol. Optical instruments like normal high speed camera and multiple-spark high speed camera were employed in the experiments and the data obtained was solved with common statistics software Statistical Product and Service Solutions (SPSS). Phase Doppler Particle Analyzer (PDPA) was also used to give a description of the water aerosol shown with diameter and velocity parameters. After the analysis of typical photos and data obtained, results showed that once past through by shock waves, the water sheet faced a surface stripping and breaking progress. The shock-wave induced in the layer of water is a compressive elastic-wave, whose magnitude is related to the ratio of acoustic impedance of the two media. Shock propagates through the thickness of the water-wall and when it reaches the next water/air interface is nearly totally reflected as a tensile wave. Such tensile wave will induce cavitation in the water-wall. Subsequent to cavitation, water keeps moving with uniform velocity and undergoes tensile strain, with small vapor bubbles forming while the water keeps moving in the direction of the blast. Finally, based on the dispersal mechanism, dimensionless parameters calculated were brought in to build the water dispersal model.

A Review of Experimental Techniques for Measuring Micro- to Nano-Particle-Laden Gas Flows

Dispersed micro- to nano-particle-laden gas flows are common in many engineering and environmental applications. Characterizing both their dispersed and carrier phase using experimental methods is very important for determining their properties and behavior. This paper reviews techniques for measuring the carrier phase, as well as the dispersed particles ranging from the micro- to the nano-scale. We focus not only on the developments of specific techniques over the last 20 years, but also on relationships and comparisons among these techniques. In addition to a systematic description and classification of these methods, we discuss the parameters they measure, such as particle velocity, size, composition and concentration. A more detailed review is provided for several important measurement techniques, including particle image velocimetry, the phase-Doppler particle analyzer and light-scattering intensity measurements for microparticles, as well as the scanning mobility particle sizer, the fast mobility particle sizer and the electrical low pressure impactor for nano-particles. The founding principles, development histories, various applications of these techniques and where they are going are summarized. This article provides a resource for investigators that plan to study micro- or nano-particle-laden gas flows in various contexts.

The spray characteristic of gas-liquid coaxial swirl injector by experiment

Using the laser phase Doppler particle analyzer (PDPA), the spray characteristics of gas-liquid coaxial swirl injector were studied. The Sauter mean diameter (SMD), axial velocity and size data rate were measured under different gas injecting pressure drop and liquid injecting pressure drop. Comparing to a single liquid injection, SMD with gas presence is obviously improved. So the gas presence has a significant effect on the atomization of the swirl injector. What’s more, the atomization effect of gas-liquid is enhanced with the increasing of the gas pressure drop. Under the constant gas pressure drop, the injector has an optimal liquid pressure drop under which the atomization performance is best.

Effect of fuel temperature on the methanol spray and nozzle internal flow

The increasing of fuel temperature can reduce the droplet size and have an advantage of improving spray atomization, while investigations of the effect of temperature on the methanol injector internal flow and external spray is rare. Firstly, a detailed three dimensional numerical simulations of nozzle internal flow have been conducted to probe into the cavitation in methanol injector nozzles, and then an experimental study has been carried out to investigate the droplet size and velocity of methanol spray at various temperatures using the Phase Doppler Particle Analyzer (PDPA) detecting system. And results show that the region of cavitations in nozzle orifice enlarges as methanol temperature and injection pressure increases, and the temperature for 'super-cavitation' occurring decreases gradually with the increasing of injection pressure. Moreover, the nozzle exit velocity, discharge coefficient and cavitations number were also analyzed. However, the discharge coefficient reduces nearly equal under various pressure when the methanol temperature is higher than 60°C. In addition, the Sauter Mean Diameter (SMD) and velocity of methanol droplet were also analyzed, and found that the droplet velocity reaches the maximum value when the methanol temperature is 60°C.

The influence of flash boiling conditions on spray characteristics with closely coupled split injection strategy

The isooctane spray characteristics under various flash boiling conditions were studied with high speed imaging technique and Phase Doppler particle Analyser (PDPA) technique. The potential of closely coupled split injection strategy to alleviate fuel impingement and the corresponding spray characteristics were also investigated. It was found that flash boiling led to obvious collapse of spray and improved atomization. The increasing strength of flash boiling (excluding the transitional stage) resulted in increasing penetration and potential impingement. Stronger flash boiling caused smaller droplets and higher possibility for the droplets to further breakup. Closely coupled split injection strategy effectively reduced the overall spray penetration and the potential impingement. The flash boiling considerably weakened the collision between split injections and the interaction in liquid phase but strengthened the interaction in gas phase. The dwell interval exerted marginal effects on the penetration rate for each split injection event under flash boiling conditions due to the weakened interaction in liquid phase. By contrast, under non-flash boiling condition, the second split injection observably affected by the variation of dwell interval. Besides, split injection strategy resulted in larger droplet sizes than single injection strategy and the increasing flash boiling strength decreased the size difference between injection strategies.

Spray development and droplet characteristics of high temperature single-hole gasoline spray

As increasing fuel injection pressure in GDI (Gasoline Direct Injection) engines has been used for improving atomization quality, raising fuel temperature is also receiving more and more attention as another promising method to improve mixing quality and combustion process. In order to understand atomization process with increased fuel temperatures, spray development and droplet characteristics of high fuel temperature spray were investigated by using back-illumination imaging technique and Phase Doppler Particle Analyzer (PDPA) with fuel temperature up to 376°C as supercritical state. Results show that sprays dispersed more widely and became more transparent with increasing fuel temperature. Spray penetrations had some reduction and spray cone angle had obvious increase with increased fuel temperature not more than critical temperature. While droplet arithmetic mean diameter (D10) and Sauter Mean Diameter (SMD) was reduced from about 10μm to 1.5μm and from 30μm to 3μm as fuel temperature rise from 25°C to 261°C, high temperature more than 261°C resulted in droplets became too small to be measured by the PDPA instrument.

Application of Maximum Entropy Principle to Predict Droplet Size Distribution for Swirl Injectors

An existing droplets size distribution function, which was established based on the maximum entropy principle, momentum conservation law and energy conservation law, was used to predict the characteristics of spray field of a swirl injector with small geometry characteristics constant. The experimental system for spray characteristics was built, and an experimental study on the spray characteristics of swirl injectors with different geometries was conducted with phase Doppler particle analyzer (PDPA). It shows a coincidence between the experimental results and the spray characteristics predicted with the maximum entropy principle. The maximum entropy method can be a useful tool for the design of this type of injector.

Effect of flash boiling on microscopic and macroscopic spray characteristics in optical GDI engine

The flash boiling sprays in a single-cylinder optical gasoline direct injection (GDI) engine were studied using phase Doppler particle analyzer (PDPA) and high-speed photography. Initially, the factors which may influence the in-cylinder measurement reliability of PDPA were carefully examined under atmospheric conditions. Afterwards, engine experiments were conducted under idle and wide open throttle (WOT) conditions with the fuel temperature ranging from 20°C to 90°C. The results show that under idle operation, the spray collapsed towards the injector axis as the fuel temperature increased. At high fuel temperatures, the spray penetration increased so that the probability of fuel-wall impingement rises. As the fuel temperature increased from 20°C to 60°C, the AMD and SMD slumped by 43.6% and 33.2%, indicating the dramatic improvement in atomization quality. Concurrently, the droplet uniformity decreased sharply. As the temperature further increased to 90°C, the AMD and SMD only dropped slightly, but the droplet uniformity became better. Under WOT operation, no distinct evidence of spray deformation was found except for the case at 90°C. The AMD and SMD dropped steadily with the increase in fuel temperature, which is mainly attributed to the reduction in viscosity and surface tension. Besides, the droplet uniformity increased gradually along with the fuel temperature.

Influence of deposit on spray behaviour under flash boiling condition with the application of closely coupled split injection strategy

The effects of deposit on the spray macroscopic characteristics and droplet behaviour under flash boiling condition were investigated through high speed imaging and Phase Doppler Particle Analyzer (PDPA) techniques. Closely coupled split injection strategy was tried to alleviate the negative effect of deposit on spray characteristics. It was found that with single injection strategy, deposit led to smaller cone angle and longer penetration due to poorer atomization and dispersion. Deposit also resulted in larger droplet size by introducing large droplets and lowering the potential of further breakup into small ones. When split injection strategy was employed under non-flash boiling condition, the effects of deposit of on the over-penetration and the resultant high potential of impingement were alleviated, while the negative effect of deposit on droplet size was enhanced. However, when split injection was used under strong flash boiling condition, the negative influence of deposit on the spray over-penetration was significantly alleviated and the negative impact on the droplet size was almost eliminated although reduced potential for large droplets to further break up was still found. The employment of optimal split injection strategy is believed to be important for the alleviation of the adverse effect of deposit on the spray characteristics and the emissions.

Experimental research on the characteristics of methane/air explosion affected by ultrafine water mist

The inhibition effects of ultrafine water mists on 6.5%, 8%, 9.5%, 11%, and 13.5% methane explosions were experimentally studied in a sealed visual vessel. The mist (10μm) produced by a mist generation system in the vessel was measured by a phase doppler particle analyzer. A high-speed camera was used to record the explosion flame affected by spraying concentration and a high frequency pressure sensor was used to acquire the explosion pressure. Meanwhile, the relationship between flame propagation and pressure rising with time was analyzed. The appearance height of “tulip” flame was increased and appearance moment was delayed obviously with the mist amount increased. The variation trend was illustrated from the viewpoint of the interactions among the flame front, the flame-induced reverse flow and the vortices. Moreover, cellular structure appeared in the burned zone and experienced four developing stages, and its formation indicates that water vapor can cause the intrinsic flame instability and absorb heat on the burned zone further. The pressure underwent two accelerating rises, which was affected by mist amount. The accelerating rise processes were related to the accelerating propagation of combustion wave. Furthermore, methane explosion can be absolutely suppressed by the mist.

Spray drift assessment of different application techniques using a drift test bench and comparison with other assessment methods

As an alternative method to simplify the measurement of spray drift, the University of Turin (Department of Agricultural, Forestry and Environmental Economics and Engineering (DEIAFA)) developed a drift test bench to assess the spray drift risk generated by field crop sprayers. The method is based on the principle that the potential spray drift is directly related to the amount of initial spray that remains suspended in the air after the sprayer has passed. In this study the drift potential of 16 spray boom application techniques was assessed using the drift test bench under indoor conditions using a fully automated spray track and mineral chelate tracers. By comparison with the results from a reference spraying, the resulting drift reduction potentials were compared with the results from three other drift assessment methods (field, wind tunnel, Phase Doppler particle analyser (PDPA) laser measurements). The drift test bench trials confirmed the already known effects of nozzle type, spray pressure and boom height on drift potential but not of nozzle size and application speed. In general, the drift test bench showed similar trends as the other drift assessment methods, although large variations were present and some contradictory results were observed. The test bench is therefore considered a possible alternative for measuring potential spray drift of horizontal boom sprayers, albeit future studies should focus on reducing the sources of variation and evaluating if the methodology is able to test the effects of sprayer speed.

A canonical geometry to study wall filming and atomization in pre-filming coaxial swirl injectors

A coaxial twin counter-rotating air swirl prefilming injector with necessary optical access, is developed to visualize and quantify its internal fluid dynamics. This research injector consists of a simplex nozzle arranged at the center of two coaxial counter-rotating radial air flow swirlers, and two transparent coaxial tubes are attached, one each to the primary and secondary air swirl paths. Spray from the simplex nozzle is swirled by the primary air and impinges on the inner tube—the prefilmer, undergoes filming, and convects to the tube tip to form a liquid rim, which is sheared by the counter-rotating swirl into finer droplets. Stage-wise phase Doppler particle analyzer measurements indicate the final spray to be much finer than the simplex nozzle spray after undergoing the above processes. Time resolved laser induced fluorescence (TR-LIF) imaging techniques are applied to visualize the wall filming and primary atomization inside the injector. The simplex nozzle spray velocity under the influence of air swirl flow is measured using stereo-particle image velocimetry. The precessing vortex core from the primary swirl imparts a precessing motion to the simplex spray, which in turn induces a non-uniform filming on the prefilmer. The droplet impingement on the prefilmer leads to splashing and crater formation on the surface. The crater size distribution is obtained and compared to the droplet size of the injector spray before impingement. The film thickness variation on the prefilmer surface and the rim thickness are estimated from planar LIF experiments along with a long distance microscope. The thickness of the liquid rim is identified as a major factor in determining the final droplet size at the injector exit. These are correlated to SMD at the injector exit at different air flow rates.

Effects of turbulent intensity on nano-PMMA flame propagation behaviors

Experiments with different ignition delay times (td) corresponding to different residual turbulent intensities after dispersion were conducted to reveal the effects of turbulent intensity on nano-PMMA flame propagation behaviors. The residual vertical root-mean-square (RMS) turbulent velocities and RMS vorticities measured by a particle image velocimetry system (PIV) at td = 0.9 s, 1.0 s and 1.1 s were 0.22 m/s, 0.16 m/s, and 0.15 m/s and 50.40/s, 38.23/s, and 36.68/s, respectively. It was indicated that the residual turbulent intensity in the combustion space decayed with increasing ignition delay time. One-hundred nanometer PMMA dust flames with a nominal concentration of 450 g/m3 were approximately spherical in shape and propagated continuously after ignition at different ignition delay times. As time proceeded, the flame front of td = 0.9 s was still smooth, whereas the flame fronts of td = 1.0 s and 1.1 s were irregular due to the dominant effects of flame self-instability against the residual turbulence. The average pulsating flame propagation velocities of td = 0.9 s, 1.0 s and 1.1 s were 0.67 m/s, 0.54 m/s and 0.44 m/s, respectively. The pulsating level was enhanced by extending the ignition delay time due to the weaker residual turbulent intensity compared with the turbulence induced by the dust flame self-instability. In addition, the suspended particle size distribution was measured by a Phase Doppler Particle Analyzer (PDPA). It was found that the effective suspended particles existed as agglomerates of fine particles rather than as the primary particles themselves.