Fountain effect

Although the compositions used in fountains are usually based on black powder propellant, the sparks that are responsible for the fountain effect originate from other substances within the composition. These substances are known as emitters and it is the physical and chemical properties of the emitters that determine the characteristics of the fountain. Various additives are also used to promote the visual effects or to cheapen the composition. 

Broadly speaking, the components of the propellant react to produce hot combustion gases which heat up the particles of the emitter and eject them from the body of the firework. On contact with the air, the hot emitter particles ignite to produce the well known fountain effect as... 

In summary, the titanium fountain effect arises because of the combustion of the metal particles, and appears in the form of radiation which comprises ... 

Having introduced the theory of emission, we can proceed to the events which occur in a typical titanium fountain (Figure 5.3). This 150g firework will burn for about 25 seconds giving a fountain effect extending some 3 to 4 metres. 

The charcoal, or rather the coated charcoal, contributes to the fountain effect as does the gunpowder and aluminium by processes such as those described above. The flitter aluminium has a rather coarser particle structure than does the fine aluminium so that sparks from the former are longer lived and can survive a greater drop-height. Antimony trisulfide is commonly used to enhance the glittering effect in a series of chemical reactions with the gunpowder and aluminium. 

Correct modeling of the flow near the front of a stream requires a rigorous solution of the hydrodynamic problem with rather complicated boundary conditions at the free surface. In computer modeling of the flow, the method of markers or cells can be used 124 however this method leads to considerable complication the model and a great expenditure of computer time. The model corresponds to the experimental data with acceptable accuracy if the front of the streamis assumed to be flat and the velocity distribution corresponds to fountain flow.125,126 The fountain effect greatly influences the distribution of residence times in a channel and consequently the properties of the reactive medium entering the mold. 

Detailed kinematic investigations of flow near the front of a stream were undertaken.284 A diagram of the experimental device is shown in Fig. 4.49. In the experimental procedure, a liquid was placed in a chamber with transparent walls above an aluminum piston, which was driven downwards by connection to a suitable drive. This resulted in the appearance of streams inside the liquid,and three different flow zones could be distinguished. The so-called "fountain effect discussed in Section 2.11 appeared near the free surface, while a reverse fountain flow was observed below the moving surface. It is interesting to note the movement of two liquids with different densities, when one liquid is used as a piston to push the other (analyzed experimentally and theoretically).285 If the boundary between the two liquids is stationary and the walls of the chamber move at constant velocity, then the pattern of flow is as shown in Fig. 4.50, where flow trajectories corresponding to front and reverse fountain effects are clearly shown. Two other flow patterns -developed flow inside the main part of the chamber and circulation near the surface of the aluminum piston - were also observed. 

The third region of flow near the front is of special interest. The important feature of this region is the fountain effect, which must be considered in modelling all types of mold filling. It is important not only for estimating the hydrodynamic flow pattern, but also because the deformation of the macromolecules near the front influences their orientation and the properties of the end product. 

One of the key parameters in reactive processing is the distribution of residence times and temperatures for all particles in the liquid, because their reactivity depends on temperature, and the degree of conversion is determined by the dwell time inside the mold. Since the fountain effect changes residence time distribution from that in the hypothetical case of steady unidimensional flow, this factor becomes especially important in chemical (reactive) processing, more so than in standard injection molding of thermoplastic materials.284... 

A complete description of model of filling a rectangular mold and the subsequent solidification of an article was developed,47 and in this case a fountain effect near the stream front was included. The authors used the model to predict pressure increase during mold filling, and the distribution of the degree of conversion and temperature at both stages (filling and solidification). The results... 

A detailed analysis of the distributions of conversion, temperature and velocities was carried out126 using a model, which included the fountain effect at the front of the stream. A comparison of the results was made for molds of different geometrical form (plane cavity, cylindrical and disk-like shapes) for the same temperature, average output and cross-sectional width of the mold. It was established that the distribution of the degree of conversion is qualitatively the same in all these cases (Fig. 4.55). 

An interesting feature of the conversion profile is the existence of a maximum located between the wall and the axis of the mold. This effect is explained by the superposition of the influence of temperature and residence time on the kinetics of the reaction a liquid moves faster in the central zone than at the wall, therefore the residence time is longer near the walls, but temperature is higher in the center therefore the reaction rate of the material near the walls is lower than in the center. As a result, there is a point between the center and a wall where the degree of conversion is maximum. The results in Fig. 4.55, also answer the question about the role of the fountain effect ... 

Figure 4.55. Conversion distribution at die finish of mold filling. Figures on the curves show the relative values ofx

A simple calculation method, which takes into account the fountain effect, was proposed.289 In this approach the flow is assumed to be laminar and unidimensional. The front of the stream is regarded as straight (plane), perpendicular to the walls of the mold and moving with a constant average velocity vav. Then the following dimensionless variables are introduced ... 

A coordinate of a point, containing liquid moving with velocity vav is designated Zav- As the front line is assumed to be straight, the fountain effect is described by the assumption that transverse flow occurs along this line from the part of the cross-section where the velocity is high... 

The system of equations with initial and boundary conditions formulated above allows us to find the velocity distributions and pressure drop for the filled part of the mold. In order to incorporate effects related to the movement of the stream front and the fountain effect, it is possible to use the velocity distribution obtained285 for isothermal flow of a Newtonian liquid in a semi-infinite plane channel, when the flow is initiated by a piston moving along the channel with velocity uo (it is evident that uo equals the average velocity of the liquid in the channel). An approximate quasi-stationary solution can be found. Introduction of the function v /, transforms the momentum balance equation into a biharmonic equation. Then, after some approximations, the following solution for the function jt was obtained 285... 

This system of equations allows us to take account of the flow in the frontal zone and the influence of the fountain effect on the distributions of variables in the main stream zone. The equations for this rather complicated model can be solved numerically by computer. Comparison of the calculations with experimental data shows that the maximum deviations of the predicted values from the experimental points do not exceed 15 % (Fig. 4.60). 

Fig. 13.7 Schematic representation of the flow patterns during the filling of an end-gated rectangular mold whose width is much greater than its thickness, (a) Width direction flow fronts at various times, (b) Velocity profiles in the fully developed region, and schematic representation of the fountain effect in the front region.

The term fountain effect ox fountain flow was coined and discussed by Rose (18), and it is essentially the reverse of the flow observed near a plunger emptying a fluid out of a channel of the same cross section. The two-dimensional flow in the... 

Most glitter effects are seen as fireworks stars or comets. The drossy residues produced by burning glitter compositions makes fountain effects difficult, but not impossible to produce. 

It has been found by Allen S and Jones that at low temperatures in helium, when a temperature difference is produced, a pressure difference arises. This phenomenon is known as the fountain effect, or thermomechanical effect [30]. The superfluid is believed to carry no entropy. 

The charcoal, or rather the coated charcoal, contributes to the fountain effect as does the gunpowder and aluminium by processes such as those described above. The flitter aluminium has a rather coarser... 

Here n = cq/cq is the ratio of the wave speeds in the two media. They also provide more general expressions for the case of sound beams (rather than plane waves) that are approximated by a one-term Fourier-Bessel series, having both a radial and an axial wave number. The radiation pressure difference (32) at the interface is said to explain the acoustic fountain effect where the directed sound beam creates a liquid jet. 

Haase [17] has reported some observations on thermo-osmosis of water through cellophane membrane with and without deposition of copper ferrocyanide in the pores. A well-authenticated instance of thermal migration of a liquid against a hydrostatic pressure through a permeable barrier is the fountain effect in liquid He II. Like thermoosmosis, this process gives rise to a well-defined stationary pressure difference. 

For two-liquid flow in capillaries, the behavior of the interface has to be taken into account, too. According to Huh and Scriven (1), the rate profile is, even for laminar flow, not a parabolic one. In the vicinity of the interface, the flow field has, according to Dussan (2) the shape as shown in Figure 3, allowing for phase A to penetrate into phase jB. This will occur at point 1 of Figure 3 where the flow has a stagnation point. This is the fountain effect which occurs if the ratio of viscosity of phase A to that of phase is either too high or too low. The... 

At the receding side of the moving water front, one can almost always observe small oil droplets dispersed in the water plug as a result of the fountain effect. This effect, which is a consequence of the flow pattern demonstrated in Figure 3, is shown in Figure 7. 

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