Droplet Weber Number Research Articles

Gasoline direct injection spray simulation

In this paper the problems related to mixture formation in a GDI engine are analyzed. The atomization of a hollow cone fuel spray generated by a high pressure swirl injector is studied by means of a numerical technique. The model distinguishes between primary atomization and secondary breakup. The latter was modeled, as done in a previous work on Diesel atomization, using different mechanisms as the droplet Weber number changes. At first the spray atomization in a quiescent chamber, at ambient pressure and temperature, was considered. The validation of the model was made comparing the numerical penetration and spray morphology with experimental results. Combustion simulations were also performed comparing numerical results with experimental data of a GDI (Gasoline Direct Injection), 4 stroke, 4 cylinder, 4 valves per cylinder engine. Such simulations were made to analyze and understand the mixture formation mechanism in both stoichiometric and stratified operation mode. The results show how, the interaction between the air motion and the fuel spray, leading factor in spray atomization, is fundamental to realize an efficient mixture formation and combustion locally very lean, typical of stratified charge combustion.

Experimental Study on Cooling a Hot Solid Surface with Water Mist

In this paper, a series of experiments are performed to investigate the hot solid surface cooling with water mist under different conditions, such as different initial surface temperatures and mist characteristics (droplet size, droplet velocity, etc.), while these parameters are simultaneously varied. A 150 150 10mm 3chrome-plated stainless steel plate is heated from below by a 1500-W electric cooker continuously. A downward pressure nozzle is positioned on a square steel plate, 1000mm above the surface to inject water mist, the 0.5mm K-type thermocouples are jointed on the surface to obtain its temperature, and a TVS-2000ST Thermography is used to visualize the transient thermal behavior of a water mist and obtain the temperature distribution of the surface. The characteristics of the water mist are obtained by a modified PIVS (Particle Image Velocimetry and Sizing) technique previously. Based on the experiments, the effects of mist parameters and the initial surface temperature on droplet evaporative cooling are analyzed. Further discussion is carried out by considering the relationship between these parameters and the droplet Weber number, the results show that an inverse relationship between the cooling efficiency and mist droplet Weber number existed in this study.

On the interaction of a liquid droplet with a pool of hot cooking oil

An experimental study is presented for distilled water droplets impacting on a heated pool of cooking oil. The impaction process was recorded using a high-speed digital camera at 1000 frames per second. The initial droplet diameter was fixed at 3.1±0.1 mm and all experiments were performed at room temperature (20°C). The impact Weber ( We) number of the water droplets was fixed at 200. As the water droplet impacted the hot peanut oil pool, it fragmented, and ultimately produced a vapor explosion. Experiments were also performed applying methoxy-nonafluorobutane C 4F 9OCH 3 (HFE-7100) to hot peanut oil with similar impact We number. Dramatic differences were observed when HFE-7100 droplets were used. At peanut oil temperatures above ≈180°C, HFE-7100 droplets did not result in a vapor explosion.

Modeling on Spray-Wall Interaction for Direct Gasoline Injection Engines

This study is focused to propose the new spray-wall interaction model for application of direct injection engine systems. In this paper, the experimental approaches are discussed for two kinds of droplet-spray regime. As the 1st step, small non-dispersed droplets or droplet array impinge on the heated flat wall with the variation of droplet Weber number, impingement angle and wall temperature. In the 2nd step, actual spray through the gasoline injector for D.I.S.I. Is impinged on the flat wall. Here, droplet behavior on the wall and post impingement properties are measured by high-speed photographic systems. Then, the new submodel is constructed based on droplets experimental results. This new model is incrporated into KIVA-3 code. And the results calculated by this model are compared with experimental results. It was found that the results by this model for the spray dispersion near the wall were consistent well with the actual process over all boiling state conditions.

A GENERAL HEAT TRANSFER CORRELATION FOR IMPACTING WATER SPRAYS ON HIGH-TEMPERATURE SURFACES

A method for the general correlation of heat transfer effectiveness for sprays impacting vertically downward on a high-temperature surface has been developed. A dimensional analysis showed that the mass velocity of the spray can be substituted for the droplet velocity in the droplet Reynolds and Weber numbers, greatly improving the correlation with the heat flux data in the film boiling regime. The spray Reynolds number, defined as Re s = Gd/ w , and spray Weber number, We s = G 2 d/ 𝜌 σ , were shown to correlate data from many authors covering a wide range of spray parameters. This correlation supports the results of previous parametric experiments, and is analogous to correlations developed for the critical heat flux of sprays and circular jets. Dependence of the heat transfer performance on spray mass flux and droplet diameter represents the influence of the overall heat transfer capacity of the spray as well as the contribution of droplet interactions. The Leidenfrost temperature of the spray was also shown to be dependent on the spray Weber number.

Analysis of impact of droplets on horizontal surfaces

Abstract This paper presents the results of an experimental investigation of droplets impacting on horizontal surfaces. The effects of the impact parameters on the droplet impingement are studied. The results are presented for different droplet Weber numbers, ranging from 50 to 1080 and for three liquids: water, isopropanol and glycerin. Four kinds of surfaces were used with characteristic wettability (given in terms of the contact angle): smooth glass, PVC, wax and rough glass. We studied in some detail the kinematics of the moving contact line during the spreading process. Particularly we are interested in the effects of the wettability of the wall by the liquid. The surface wettability has been observed to have a strong influence on the spreading of droplet in the later stages of the process. The results are presented in the form of charts describing the spreading diameter and apex height of droplets in terms of time.

Experiments on the dynamics of droplet collisions in a vacuum

Highly controlled experiments of binary droplet collisions in a vacuum environment are performed in order to study the collision dynamics devoid of aerodynamic effects that could otherwise obstruct the experimental observations by causing distortion or even disintegration of the coalesced mass. Pre-collision droplets are generated from capillary stream break-up at wavelengths much larger than those generated with the typical Rayleigh droplet formation in order to reduce the interactions among the collision products. Experimental results show that the range of droplet Weber number necessary to describe the boundaries between permanent coalescence and coalescence followed by separation is several orders of magnitude higher than has been reported in experiments conducted at standard atmospheric pressures with lower viscosity liquids (i.e. hydrocarbon fuels and water). Additionally, the time periods of both the oblate and prolate portions of the coalesced droplet oscillation have been measured and it is reported for the first time that the time period for the prolate portion of the oscillation grows exponentially with the Weber number. Finally, new pictorial results are presented for droplet collisions between non-spherical droplets.

Spray Cooling Under Reduced Gravity Condition

The present paper reports on the results of a series of authors’ parabolic flight experiments on spray cooling in addition to ground-based experiments in which the influence of heater orientation and the behavior of rebounded droplets were especially studied in detail. Water and FC-72 (perfluorocarbon) were employed alternatively as a test liquid sprayed from a single full-cone nozzle onto a Cr-plated surface of an electrically heated copper block for transient cooling experiments or onto a transparent ITO (indium tin oxide) coated surface of a glass block for steady state experiments in a relatively low superheat region. Each experimental run was accomplished within some 15 sec through which a stable reduced gravity condition (0.01 times the terrestrial gravity) was maintained in the aircraft. Cooling curves were obtained over a wide range of each of the following parameters: the wall superheat, the spray volume flux and the Weber number for the spray droplets. It is demonstrated that the gravity dependency of the spray cooling characteristics varies with the spray volume flux and the droplet Weber number. Qualitative interpretations of the observed gravity dependency are provided.

IMPROVING DROPLET BREAKUP AND VAPORIZATION MODELS BY INCLUDING HIGH PRESSURE AND TURBULENCE EFFECTS

This article reviews recent experimental work conducted at the Laboratoire de Combustion et Systemes Reactifs (LCSR), Orleans, France, on single droplet breakup and vaporization, Emphasis is essentially put on high pressure and turbulence effects. The experimental facilities developed and the diagnostics used are first presented. Droplet breakup studies are conducted with cryogenic and non-cryogenic droplets subjected to aerodynamic shear forces under high-pressure conditions. The transition criteria between droplet breakup regimes, characteristic breakup times, and secondly droplet distributions are obtained for uniquely low values of the density ratio between liquid and gas phases and systematically varied values of droplet Weber and Reynolds numbers. Combined effects of high pressure and temperature 011 droplet vaporization are also systematically explored. The variation pat/ems of average vaporization rates with reduced pressure and temperature are conclusively established and compared to estimates from the quasi-steady model. The influence of turbulence on droplet vaporization rates is explored in detail. It is demonstrated that droplet vaporization rates increase significantly with turbulent Reynolds number; even when the droplet size is smaller than the turbulence integral length scale. Comprehensive correlations are established to take into account these various effects. Suggestions are made for ways of including these correlations as sub-models into spray combustion numerical prediction codes and for future work to further improve them.

Experimental investigation of the impaction of water droplets on cylindrical objects

The impaction of water droplets on isothermal cylindrical wires has been investigated experimentally in the present study. Mono-size droplets of 110, 350 and 680 μm in diameter were generated using piezoelectric droplet generators. The effects of droplet velocity and wire size were varied parametrically to reveal the impacting phenomena. Typical modes of the impaction outcome are disintegration and dripping. For droplets impacting on small wires, finer drops are disintegrated if the impacting droplet velocity is high, and larger dripping drops are observed if the velocity is low. For droplets impacting on large wires, bigger pendent drops are gradually formed which would eventually detach from the wires under the influence of gravity. In addition, droplets impacting on wires with waxy surface generate smaller dripping drops than that of the non-waxed wires. A non-dimensional regime map and new formulations in terms of the droplet Weber number, the wire Bond number and the size ratio of the wire diameter to incoming droplet diameter have been established to identify the regime for each mode of outcome and to predict the size of the dripping drops within the experimental limits.

Heat Transfer of Sprays of Large Water Drops Impacting on High Temperature Surfaces

Experiments were performed to evaluate the heat transfer of monodisperse sprays of large droplet diameters, ranging from 3 to 25 mm, on high temperature surfaces. This range of drop sizes has not previously been studied, and it was of interest to determine their heat transfer characteristics and how they relate to sprays of smaller drops. Parametric tests showed that the spray heat flux depends on mass flux with a power-law relationship, and that spray effectiveness, which relates the actual spray heat transfer to the maximum heat transfer potential, varies with d−1/2. There was no discernible relationship between the heat transfer and droplet velocity. These results agreed favorably with published results for smaller droplets. The current data was compared to previous tests with smaller droplets using the droplet Reynolds and Weber numbers. This analysis showed some grouping, with a marked separation at We = 80, where the dynamic behavior of droplets has been shown to change for nonwetting impaction.

Sensitivity Of The Spreading Rate Of A Liquid Droplet On A Surface To The Approximation Of The Contact Angle

This paper presents a numerical study of the impaction of a liquid droplet on a flat, solid surface. In particular, the sensitivity of the spreading rate of the droplet to the approximation of the contact angle is investigated. The full Navier-Stokes equations, subject to free surface boundary conditions and a dynamically inactive ambient, are solved using the Galerkin finite element technique. The free surface is determined by the spineflux method. Simulation of the spreading rate requires information on the dynamic contact angle at the solid-liquid-gas interface. A wetting parameter, η is introduced to determine the contact angle. Results show that the spreading rate is sensitive to η when the Weber number of droplet upon impact is small (We = 1.4 in this study). Here, the inertial forces and surface tension forces are comparable. The sensitivity decreases as the Weber number increases. At large Weber numbers, inertial effects dominate the surface tension forces and, hence, the spreading rate. Thus, the contact angle approximation has little influence. Larger Reynolds numbers also result in greater inertial effects. For such cases, there is also reduced sensitivity to the wetting parameter.

Mapping of impact and heat transfer regimes of water drops impinging on a polished surface

Still and high-speed photographic techniques were used to record the impact behavior of water droplets on a hot aluminum surface. Drop velocity and surface temperature were two important parameters governing both the impact behavior and ensuing heat transfer. Droplet Weber numbers of 20, 60 and 220 identified three major classes of impact behavior, while surface temperatures ranging from 280 to 100°C were used to define heat transfer regimes corresponding to film boiling, transition boiling, nucleate boiling, and film evaporation. Temperatures corresponding to the critical heat flux and the Leidenfrost point showed little sensitivity to both droplet velocity and impact frequency. The photographic results and heat transfer measurements were used to construct droplet impact regime maps which identify the various boiling regimes for each of the three Weber numbers. These maps serve as a new useful foundation for understanding droplet impact behavior as well as future analytical or numerical modeling of droplet and spray heat transfer.

Effects of surface roughness on water droplet impact history and heat transfer regimes

Still and high speed photographic techniques and heat transfer measurements were used to study the impact of water droplets on heated surfaces with different roughness. The study encompassed droplet Weber numbers of 20, 60 and 220 and surface temperatures of 100–280°C, covering droplet stability and heat transfer regimes established previously by the authors for polished surfaces. Three different surface finishes, polished, particle blasted and rough sanded, with respective arithmetic average surface roughness values of 97, 970 and 2960 nm, were applied to the test surfaces. While the temperature corresponding to critical heat flux (CHF) was fairly independent of surface roughness, the Leidenfrost point (LFP) temperature was especially sensitive to surface finish. The parametric effects of Weber number and surface temperature were consolidated into droplet impact regime maps for each of the three surface finishes. Aside from depicting the commonly known boiling curve regimes of film, transition and nucleate boiling, and thin film evaporation, these maps illustrate the complex liquid-solid interactions which occur during the lifetime of the impacting droplet within each of the boiling curve regimes, thus serving as an effective reference for future modeling of droplet heat transfer. Copyright © 1996 Elsevier Science Ltd.

An experimental investigation on the collision behaviour of hydrocarbon droplets

The collisional dynamics of equal-sized water and normal-alkane droplets, in the 150 μm radius range, have been experimentally studied for situations involving 0(1) droplet Weber numbers and head-on to grazing impact parameters. Results show that in the parametric range investigated the behaviour of hydrocarbon droplets is significantly more complex than that of water droplets. For head-on collisions, while permanent coalescence always results for water droplets, the outcome is quite nonmonotonic for the hydrocarbon droplets in that, with increasing droplet Weber number, the collision can result in permanent coalescence, bouncing, permanent coalescence again, and coalescence followed by separation with or without production of satellite droplets. Similar complexities exist for off-centre collisions. Phenomenological explanations are offered for these observations based on the material properties of the fluids, the relative influences of the normal and shearing aspects of the collision, and the nature and extent of energy dissipation due to droplet deformation during collision.

Dynamics of Droplets Impacting on Thin Heated Strips

Experiments were conducted with water droplets impacting on the edge of thin steel strips that were heated to beyond the Leidenfrost temperature. High-speed movies were taken and analyzed and showed that the shattered droplets were generally bimodal in size distribution. The volume ratio of these two size groups of generated droplets, the mean diameter of droplets, and the ejection angles and velocities of shattered droplets are shown as a function of incoming droplet Weber number, the ratio of incoming droplet diameter to strip thickness, and the offset of the droplet relative to the strip. The data are presented in nondimensional form and correlations are provided for the mean diameter of the shattered droplets. The theoretical limiting conditions of a droplet impacting normally to a large plate and cutting by a strip of zero thickness are analyzed. The present results are compared with those of the limiting conditions. The application to a nuclear reactor spacer grid behavior during two-phase dispersed flow is discussed.

Mechanisms of film boiling heat transfer of normally impacting spray

The heat transfer mechanisms of horizontally impacting sprays were studied experimentally. An impulse-jet liquid spray system and a solid particle spray system were used. The liquid spray system is capable of producing uniform droplets with the independent variables of droplet size, velocity, liquid flow rate, and air velocity. The horizontally impacting sprays give a lower heat transfer at film boiling than the corresponding vertically impacting spray. The film boiling heat transfer is mainly controlled by the liquid mass flux. At low liquid mass flux and low droplet Weber number, the heat transfer increases with the droplet Weber number. At high droplet Weber number or high liquid mass flux, the heat transfer is not significantly affected by the droplet Weber number.

An Assessment of TRAC-PD2 Refill Calculations Based on Creare Countercurrent Flow Tests

An important step in computer code development is the assessment of code capabilities through comparison of calculated results with experimental data A number of Creare countercurrent flow tests were simulated with the Transient Reactor Analysis Code (TRAC)-PD2 code to assess the emergency core coolant (ECC) lower plenum penetration and refill predictive capabilities. The tests examined in this study indicate a prediction of complete bypass and delivery at countercurrent steam flows where these phenomena occurred experimentally. Steam flows leading to partial delivery experimentally did not always lead to partial delivery in the calculations, however. A number of parameters can potentially effect TRAC refill calculations. Sensitivity studies indicate the TRAC results are most sensitive to droplet Weber number variations that affect interfacial shear and heat transfer rates. The condensation model also affects calculations with subcooled ECC liquid.

Heat Transfer Characteristics and Dynamic Behavior of Saturated Droplets Impinging on a Heated Vertical Surface

Experiments were made of impinging droplets on a heated vertical surface in a range of low droplet Weber numbers mostly from 10 to 200. Data of the heat transfer effectiveness of individual droplets measured by means of a transient technique are presented. When the wall superheat is higher than a certain value, the droplet rebounds away from the surface with a very low heat transfer effectiveness. Behavior of the droplets on the surface was observed and heat transfer characteristics during the residence time of a droplet on the high temperature surface were examined based on the time-dependent variation of the wall temperature measured near the surface.