Effect of Pressure Drop and Nozzle Size

Effect of Pressure Drop and Nozzle Size For a nozzle with a developed pattern, the average drop size can be estimated to fall with rising AP (pressure drop) by Eq. (14-196) ... 

In this study we use the geometry variable mold and equipped with hot nozzle device to control the mold pressure, to reduce the pressure drops of the melt We investigate the effects of pressure control as well as using a hot nozzle device on the optical properly and cell stracture (cell size and cell density) of the parts, which were injected with the PC material. Modification of cell stracture and the reflection effect of the products ate the targets of this study. 

Droplet size, particularly at high velocities, is controlled primarily by the relative velocity between liquid and air and in part by fuel viscosity and density (7). Surface tension has a minor effect. Minimum droplet size is achieved when the nozzle is designed to provide maximum physical contact between air and fuel. Hence primary air is introduced within the nozzle to provide both swid and shearing forces. Vaporization time is characteristically related to the square of droplet diameter and is inversely proportional to pressure drop across the atomizer (7). 

The decrease in the mean droplet size with increasing liquid injection pressure may be attributed to two effects. First, the high pressure-drop across the exit orifice makes the process more like a pressure atomization at high pressure. Second, the liquid is squeezed into fine ligaments as it flows through the injector orifice, and the ligaments are shattered into small droplets by the explosion downstream of the nozzle exit. 

With this atomiser, the drop size is effectively independent of viscosity, and the size spectrum is narrower than with other types of pressure nozzle. 

For any spraying operation, it is necessary to specify the amount of toxicant required per unit area, the volume of liquid per unit area, and the drop size. To control these, the operator has three main variables at his command—operating pressure, nozzle size and type, and phase ratio of the emulsion. Each has some effect on all the factors specified. In addition, in aerial spraying the orientation of the nozzles in the air stream may also be varied to control drop size. 

Spray nozzle spray angles and droplet size formation are functions of the pressure drop across a spray header. Effective pressure drop range for a spray nozzle distributor varies from 5 to 20psi (from 34 to 138 kPa) for good design practice. Specific designs may operate outside this range. 

The pressure drop allowed through the inlet and discharge lines is unlimited as long as the capacity of the line is adequate for the relief requirements. That is, at the required flow rate the vessel pressure must not exceed the maximum allowable accumulated pressure. In sizing a safety disc, it is usually assumed that the entrance loss at the nozzle is the governing restriction insofar as capacity is concerned. Thus the effective orifice area is considerably larger than the effective orifice area of a safety valve of the same pipe size. Consequently,... 

The air core diameter and its turbulence characteristics determine the spray characteristics, such as drop size distribution. The larger the diameter of the air core, the thinner is the liquid film (sheet thickness), and, therefore, the smaller the droplet sizes. The air core reduces the effective flow area at the discharge orifice and causes a reduction in the volumetric flow rate through the nozzle for a given pressure drop. Therefore, the larger the air core diameter, the smaller is the discharge coefficient. 

The most successful technique for the stable species has been microprobe sampling followed by mass spectral analysis (alternatively by gas chromatography or other microanalytical techniques). The most successful microprobes have been pencil-like quartz tubes drawn down to orifices a thousandth of an inch in diameter or less. Because of the strong pressure drop (samples are pumped off at 10 atm), the probes act as miniature supersonic nozzles and the gas residence time in the probe is very short. The reactions are quenched by the rapid pressure drop (10- second and temperature drop (lO °K/second) due to adiabatic expansion in the nozzle. Reactions with half-lives as short as a few microseconds will be quenched. It is not necessary to cool the probes because of the short residence times. The effect which such a probe has on a flame is minor because of the small size of sample withdrawn (1-2 jag/sec). The aerodsmamic disturbance is minimized because the probe sucks off its own bow wave. The thermal disturbance is small because the probe wall temperature is only slightly below that of the flame. With such probes, it has been possible in favorable cases to make reproducible and reliable composition measurements with a precision of 2% and a least count of 10 mole fraction with a spatial reproducibility of 10 inches and resoution of 2 X 10 inches (see Figs. 3 and 5a). The scope and limitations of such composition measurements are discussed in Chapter IX of Fristom and Westenberg (1965). 

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