Hollow-cone nozzles

The working principle of hollow cone nozzles is that the liquid throughput is subjected to rotation by a tangential inlet and is then further accelerated in the conical housing toward the orifice (see the sketch in Figure 19). A liquid film with a thickness d is thereby produced, which spreads to a hollow cone sheet and disintegrates into droplets at the discharge from the orifice. 

By exceeding a certain discharge velocity, turbulence forces increase to such an extent that film disruption takes place immediately at the orifice. Now the droplet size is independent of the film thickness. This state of atomization is described by the critical Weber number. Measuring data obtained with hollow cone nozzles of different geometry and pure liquids as well as lime-water suspensions are represented in Figure 19. Wep,crit... 

Figure 19 Liquid film atomization with hollow cone nozzles by turbulent forces. Source From Ref 21.

Dahl HD, Muschelknautz E. Atomization of liquids and suspensions with hollow cone nozzles. Chem Eng Technol 1992 15 224-231. 

CARS measurements were made in a bluff-body stabilized flame with turbulent and recirculating flow characteristics similar to those found in many practical combustors. The combustor was operated at atmospheric pressure with inlet air temperatures between 280 and 300K, an air flow rate of 0.5 kg/ sec, and an upstream Reynolds number 1.5 x 105. Gaseous propane was injected from a hollow-cone nozzle located at the center of the bluff-body combustor at a flow rate of 7.06 kg/hr. The flame consisted of a blue cone originating at the nozzle followed by a yellow-luminous tail. 

The fitting line corresponds to the analytical expression for the wave disintegration of pure liquids by hollow cone nozzles ... 

Fig. 43 Fil m disintegration by wave oscillation. Measurements with hollow cone nozzles ofdifferent geometry and with lime-water-suspensions. For an explanation of signs see the original publication [68]. The fitted line is valid for pure liquids.

Technol. 15 (1992) 224—231 Atomisation of Liquids and Suspensions with Hollow Cone Nozzles... 

Quantitative data on hollow-cone nozzles, which are also important in pesticide sprays, appear to be even more scanty than the data for fan-jet nozzles. Fraser (16) states that the mean diameter for cone nozzles is proportional to v0 2, and Knight (19) has produced an expression ... 

The sprays were formed from a series of hollow-cone nozzles (supplied by Delavale-Watson, Widnes, Lancs, England) and a series of ceramic tipped, fan-jet nozzles (supplied by E. A. Allman Ltd., Chichester, Sussex, England). Their characteristics are listed in Table I. 

Variation of Drop Size with Viscosity in Hollow-Cone Nozzles. 

Figure 1 shows the variation in volume median diameter of drops formed from a typical hollow-cone nozzle (Spraycone WG 4008) with increase in viscosity from 1 to about 100 cp. To maintain a constant emission rate, the operating pressure was increased as the viscosity was increased. The variation of drop size with viscosity is similar to that already reported using fan-jet nozzles (II), and the results can be considered in terms of three viscosity ranges ... 

Figure 2. High speed photographs of a hollow-cone nozzle in operation...

Since this expression has been derived by an empirical fit of experimental data, it is unwise to assume that it is generally applicable when the various parameters are varied beyond the limit tested. It has given satisfactory results for various sizes of hollow-cone nozzles operating under the range of conditions shown in Table II. 

Table II. Range of Parameters Used in Deriving Equation 3 for Hollow-Cone Nozzles...

Sheet Breakup. Using the same treatment as that described for hollow-cone nozzles a similar expression has been developed (II) ... 

Since Equation 3 for hollow-cone nozzles can also be reduced to Equation 5, the latter appears to be a general expression that can be applied to the sheet breakup of all types of fan-jet and hollow-cone nozzles. The function of Re appears to take the same form in all cases, but its value may vary with different designs of nozzle. 

Practical Application of the Theory. The practical difficulty in using Equations 3 and 4 is in measuring the sheet dimensions I and a for hollow-cone nozzles r and 6 for fan-jet nozzles. Unlike the remaining parameters in Equations 3 and 4, these dimensions and angles cannot be measured in the field. However, they can be measured from flash photographs in the laboratory using a nozzle design similar to that to be used in the field. From the laboratory results, a plot of the appropriate Vi function vs. 1/Re can be drawn, where Re is calculated from ... 

Commonly, the most important feature of a nozzle is the size of droplet it produces. Since the heat or mass transfer that a given dispersion can produce is often proportional to (1/D /), fine drops are usually favored. On the other extreme, drops that are too fine will not settle, and a concern is the amount of liquid that will be entrained from a given spray operation. For example, if sprays are used to contact atmospheric air flowing at 1.5 m/s, drops smaller than 350 jxm [terminal velocity =1.5 m/s (4.92 ft/s)] will be entrained. Even for the relative coarse spray of the hollow-cone nozzle shown in Fig. 14-88, 7.5 percent of the total liquid mass will be entrained. 

The three-stage freezing model has been used to simulate a freezing spray for CB. The liquid CB is initially at a temperature of 318 K and has been injected with a pressure of 6 bar into the spray tower by means of a hollow cone nozzle with an orifice of 0.5 mm and cone angles of 80° and 12.5°. The ambient gas temperature is... 

Fig. 2.26 Droplet diameter of different spray systems (based on [31]) a hollow cone nozzles (airless) and industrial two phase nozzles (air assisted), b spray systems derived from gasoline injectors (airless)...

The results of experiment No. 1 (hollow cone nozzle with attached swirl body) and No. 2 (removed swirl body) show that the swirl body does not affect the particle formation. The mean particle diameter ( io.s) as well as the Sauter mean diameter (SMD) are rather the same. The experiments No. 3 and No. 4 compare the atomization quality of a HCN without swirl body and a simple orifice. The orifice obviously can be used to generate powders which have particle size characteristics which are comparable to powders which are produced with a hoUow cone nozzle of the same diameter. In the experiments No. 5 and No. 6, the influence of the L/D ratio of a spray device on the particle size was investigated. On the first look, a higher L/D ratio might lead to bigger particles. 

---