Atomization and Sprays

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This unique system allows us to directly determine the correlation between the spray characteristics and the combustion products. Drop collision studies are relevant to a broad spectrum of fields ranging from large scales such as astrophysics and meteorology, to much smaller scales as seen in aerosols and nuclear physics. We have been conducting experimental research on the collision dynamics of liquid droplets. Our research has focused on the determination of the collision outcomes in terms of various controlling parameters, such as the Weber and the Reynolds numbers.

Experimental observations suggest that there are five major types of outcome when two liquid masses collide in a gaseous environment. They are bouncing, partial coalescence, coalescence, separation, and shattering. In a bouncing collision contact of drop surfaces is prevented by the intervening gas film, and drops bounce apart without any mass exchange.

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A partial coalescence collision happens only for drops with very large size difference and very small velocities. The small drop attaches to the big drop and flows into the big drop because of the pressure difference between them. Before the small drop is completely absorbed by the large drop, the surface tension cuts off the bridge and a secondary drop is generated. Coalescence collisions refer to the collisions in which two drops permanently combine and generate one single drop. A separation collision is one in which drops coalesce temporarily and later separate into one single string of two or more drops of various sizes.

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And shattering collision, which is the characteristic of high relative velocity collision, is one in which the collided drops disintegrate into a cluster of many liquid fragments shortly after the collision. These five types are only the most general ones, more sub-categories within them can be defined if number of drops generated, mass transfer rate, and shape relaxation rate, etc. Two different analytical models are developed for the separating collisions, namely reflexive and stretching separation models.

Reflexive separation is found to occur for near head-on collisions, while stretching separation occurs for large-impact-parameter collisions. We have conducted three-dimensional numerical simulations of drop collisions with and without the effect of the surrounding environment. Our numerical models are based on an Eulerian, finite-difference, Volume-of-Fluid method.

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Surface tension is included using the Continuum Surface Force method. Both head-on and off-axis collisions are modeled numerically. A separation criterion based upon deformation data is found. The effect if Reynolds number investigated and regions of permanent coalescence and separation are plotted in the Weber-Reynolds number plane.

The role of viscosity and its effect on dissipation is also investigated. Finally, the validity of the assumptions made in some of the collision models is assessed. The problems of droplet impacts on a porous substrate have been encountered in the daily life. A hot water drop stream passes through the filtered ground coffee is what many people normally do the first thing in the morning.

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In a rainy day, raindrops fall on the ground and soils act just as a filter to allow water to penetrate through into the underground channel. In industry, droplets are widely chosen to increase the total contact surfaces of the flows and help improve the efficiency of designs significantly. For examples, fuel injecter injects small droplets to increase the combustion capability. Spray humidifier uses droplets to raise the evaporation rate.

One of the most direct application of droplet impacts on porous media, however, is in inkjet printing on papers.

This technology has been proven and adopted to become one of the most home-used printing techniques at the least cost. With the development of digital camera, people can even print photographs directly using an inkjet photo printer at home very efficiently. It is the demand of the technologies that motivates the current study.

Droplet impacts on a porous substrate is a problem involving with very complicated and intriguing physics. When a droplet impacts on a solid surface, it either spreads or rebounds. A similar situation occurs as a droplet impacts on a porous substrate. In addition to spreading and rebounding, a capillary penetration of the liquid droplet into the porous substrate may be resulted as well depending on the flow conditions of the incoming mother droplet interacted with the contact surfaces. Droplets may penetrate into the substrate either wholly or partially. In some cases, they may not even penetrate at all.

These phenomenon are critical to the final equilibrium state of the droplets on porous substrates and are crucial for distinguishing the final performance of the spreading especially as inkjet printing is concerned. The spreading of an inkjet droplet on papers determines the resolution of the printing significantly. To improve the process requires knowledge among droplet impacts, droplet spreading and droplet penetration altogether. Since the problems involve impacts and penetration of droplets on a porous substrate, it is beneficial to understand the physics involved.

A series of literature research has been included. Hopefully, they can provide some understanding toward the physics of these problems and serve as a starting point of the present study. The aim of this research is to better understand the effects of a planar shock wave impinging on a fluid droplet.

The procedure is started by the creation of vaccuum in a shock tube, and then rupturing the membrane placed at one end of the shock tube. The shock wave that is produced travels the length of the shock tube passing through two pressure transducers, which are mounted flush with the inner tube walls. Each transducer sends a signal to a charge amplifier where the signal is amplified and then sent to the addition and compatibility circuits.

The time delay holds the signal for the preset time, and then triggers a light to flash which takes a picture of the shock wave impinging on a fluid droplet. The fluid drop generator is located above the window of the shock tube, and produces liquid droplets as the shock wave passes by. Camera and flash mechanizms are placed in front of this window, enabling the shock wave and droplet interaction to be captured on film when triggered by the delay timer. The analysis of the resulting pictures will advance the understanding of the fuel injection systems, and advanced material coating technologies.


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Issue 4 DOI: Volume 28, Issue 1 DOI: Issue 5 DOI: Issue 6 DOI: Issue 7 DOI: Issue 8 DOI: Issue 9 DOI: Issue 10 DOI: Issue 11 DOI: Issue 12 DOI: Volume 27, Issue 1 DOI: Volume 26, Issue 1 DOI: Volume 25, Issue 1 DOI: Volume 24, Issue 1 DOI: Volume 23, Issue 1 DOI: Volume 22, Issue 1 DOI: Volume 21, Issue 1 DOI: These phenomenon are critical to the final equilibrium state of the droplets on porous substrates and are crucial for distinguishing the final performance of the spreading especially as inkjet printing is concerned.

The spreading of an inkjet droplet on papers determines the resolution of the printing significantly. To improve the process requires knowledge among droplet impacts, droplet spreading and droplet penetration altogether. Since the problems involve impacts and penetration of droplets on a porous substrate, it is beneficial to understand the physics involved.

A series of literature research has been included. Hopefully, they can provide some understanding toward the physics of these problems and serve as a starting point of the present study. The aim of this research is to better understand the effects of a planar shock wave impinging on a fluid droplet. The procedure is started by the creation of vaccuum in a shock tube, and then rupturing the membrane placed at one end of the shock tube.

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The shock wave that is produced travels the length of the shock tube passing through two pressure transducers, which are mounted flush with the inner tube walls. Each transducer sends a signal to a charge amplifier where the signal is amplified and then sent to the addition and compatibility circuits. The time delay holds the signal for the preset time, and then triggers a light to flash which takes a picture of the shock wave impinging on a fluid droplet. The fluid drop generator is located above the window of the shock tube, and produces liquid droplets as the shock wave passes by.

Camera and flash mechanizms are placed in front of this window, enabling the shock wave and droplet interaction to be captured on film when triggered by the delay timer. The analysis of the resulting pictures will advance the understanding of the fuel injection systems, and advanced material coating technologies. We have investigated the combustion of single droplets of fuel in various environments. Both suspended and free flowing droplets are experimentally studies.

We have two different combustion chambers: One is an electrically heated chamber and the other is a combustion combustion chamber which produces hot combustion gases using a flat flame burner. The electrically heated furnace is used for a controlled environment studies.

Spray Theory - Introduction

This chamber can be free from water vapor. Droplets are injected at the top of these chambers and their combustion during their flight is observed using direct imaging. A liquid jet issuing from a nozzle may breakup into small drops when it is subjected to even minute disturbances.

These disturbances can be in the form of surface displacement, pressure or velocity fluctuations in the supply system or on the jet surface, as well as fluctuations in liquid properties such as temperature, viscosity, or surface tension coefficient. In order to characterize the instability of a capillary jet, a harmonic disturbance is imposed on the surface of the jet and its growth rate is investigated. Such investigations have revealed that the jet is unstable for axial disturbances with wavenumbers less than a cut-off wavenumber k c , and stable otherwise.

For each wavelength of an unstable disturbance one main drop and one or more usually smaller drop s , referred to as the satellite or spherous drop s , are formed. The main objectives of the studies on the liquid jet instability have been to obtain the growth rates of the initial disturbances as a function of the disturbance wavenumber , the cut-off wavenumber, the drop sizes after the breakup, the breakup length, and the breakup time; and to determine the drop behavior after the breakup e. We have been invovlved in conducting experiments on the instability of liquid jets of various forms, such as intability of jets with non-circular cross sections, multi-fluid jets, large amplitude oscillatory jets, etc.

Theoretical investigation of the instability of a liquid jet have been mainly through either perturbation type analysis or one-dimensional models. These studies can be divided into two major categories, namely, the temporal and spatial analysis. In the temporal analysis, an infinite jet, stationary relative to a moving observer is considered and the growth rate of the disturbance amplitude along the jet is determined.

In the spatial analysis, the growth rate of the disturbance amplitude along a semi-infinite jet is considered with the nozzle conditions fixed. Linear and nonlinear perturbation analysis or direct numerical methods are used in each category.


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  • We have conducted both temporal and spacial instability of a liquid jet numerically. The Galerkin finite element method is used along with the height-flux method for the advection and reconstruction of the free surface of the jet. We have found that the satellite drops are persistently formed after the breakup. Only for very small Reynolds number Re the satellite drops are not observed. Both the initial disturbance wavenumber and amplitude determine the Re below which no satellite is formed.

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