Pulsating nozzle

A general appreciation of some of the effect of gas pulsation on the performance and impact on the compressor pulsation drums, their nozzles, the piping system, and the cylinder valve performance, as well as possible other effects in some unique systems or items of equipment, can be discovered by examining the cylinder performance using an indicator card. This examination (see Chapter 12) can reveal acceptable and unacceptable performance in terms of pressure variations within the cylinder as the piston passes through its cycle. Hicks presents a helpful analysis see Figures 13-3 and 13M. 

The pulsations can cause the use of excess horsepower when compared to the ideal or a system design that reduces pulsations and thereby improves cylinder performance and efficiency. The pulsation shaking forces in the suction and discharge dampeners (bottles) can be evaluated by computer analysis, and the magnitude and frequency in hertz can be reduced to an acceptable level by adjusting the dimensions (size) of the dampeners. The magnitude of the internal forces directly affects the mechanical stress on the nozzles of the cylinder and of the dampeners. Compressor... 

Experience has proven that pulsation drums/bottles/ dampeners must be designed and fabricated using exceptionally heavy construction and good welding techniques and that the addition of support gusset plates to reinforce nozzles and other attachments is essential. 

In the second configuration (moderate swirl) tested (see Fig. 20.2a), only the air stream was forced and no liquid-fuel pulsations were imposed. The experiments were performed with a Parker-Hannifan Research Simplex Atomizer. The atomizing nozzle consisted of a primary liquid ethanol feed with a coaxial primary air stream. The air stream passed through a set honeycomb, flow-straightener, and swirl vanes to provide the necessary level of swirl. Three loudspeakers were used to excite the primary air. 

The effects of liquid fuel pulsation without air forcing were visualized at four instances of time (images not included). At time 0, a high concentration of fuel became visible at the nozzle exit. At time 7t/2, the fuel droplets became evenly dispersed through the quarter cycle. Times tt and 37t/2 showed similar droplet distributions, homogeneous throughout the flow. 

In the presence of flow pulsations, the indications of head meters such as orifices, nozzles, and venturis will often be undependable for several reasons. First, the measured pressure differential will tend to be high, since the pressure differential is proportional to the square of flow rate for a head meter, and the square root of the mean differential pressure is always greater than the mean of the square roots of the differential pressures. Second, there is a phase shift as the wave passes through... 

Pressure pulsations caused by sharp-edged nozzle entrances... 

For a low liquid viscosity d/dj 1.9 applies. If liquid output pulsates, uniformly spaced droplets are obtained here d/dj = 1. Another possibility to produce monosized droplets consists in using pneumatic extension nozzles [317]. 

HFJV High-frequency jet ventilation small volumes of air are released in a pulsating fashion through a jet nozzle, and directed down the airway in a patient simultaneously receiving conventional ventilation. Can provide from 240 to 660 breaths per minute. Inspiration is active, but expiration is passive, predisposing to air trapping. 

An alternative to spray drying of solids that are suspended in a liquid using high pressure single and two phase or rotary nozzles (see Section 7.4.3) is the break-up of a low pressure stream of slurry in gas dynamic atomization. In this process, the fluid is pumped to an orifice where it is released into a pulsating flow of hot gas (Fig. 7.81) and atomized. 

Longer nozzles tend to produce more degrees of freedom as indicated in results for an LjD = 6 case depicted in Fig. 27.12. Massflow pulsations are more complex and therefore yield a richer frequency spectrum. Increases in Reynolds number yield similar effects but it becomes challenging to compute these cases without incorporation of a turbulence model. The laminar-to-turbulent characteristics of this flowfield present substantial challenges for this flowfield as most two-equation turbulence models rely on turbulent inflow conditions to close the problem. Clearly this is an area ripe for future research. 

The amplitude of the fluctuations summarized in Fig. 27.13 are significant (relative to linear theory) the sharper inlets show pulsations greater than 1% for aU conditions assessed. These large scale pulsations can be further amplified by either boundary layer instabilities or aerodynamic interactions outside the nozzle. This unsteadiness will lead to finite-amplitude waves on the free-surface immediately downstream of the orifice exit, i.e., as a small-amplitude Klystron effect. Depending on the capillary length scale, these waves could be amplified to the point... 

Fig. 27.13 Summary of massflow pulsation amplitudes for various nozzle designs [33]...

Fig. 33.7. In this figure the atomizing gas enters fi om the top while the liquid enters firom a circumferential slot. As both fluids reach the core opening, the liquid is pushed toward the nozzle exit by the gas pressure. At an arbitrary time (tj), the liquid flow is redirected by the gas pressure and a thin film is formed at the nozzle wall. The hquid partially blocks the gas flow, building a pressure. As the pressure builds to a critical value, a hquid chunk is removed. This process causes an oscillatory spray formation. The frequency of this oscillation depends on the liquid and gas flow rates. The frequency increases with increasing the velocity of the liquid or the gas. Two separate variables are important for the pulsation (a) shear stresses at the liquid/gas interface, and (b) fluid momentum.

By increasing the gas velocity, the shear stress at the interface is augmented consequently breakup is accelerated and the frequency is increased. By increasing the liquid velocity, the liquid momentum is increased which accelerates the rush of the liquid toward the axis of the nozzle, hence increasing the gas-liquid interaction and accelerating the liquid breakup and increasing the fi-equency. This self-induced pulsation is one of the major sources of noise in twin-fluid nozzles. 

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