Hydraulic Nozzle

a short tube for discharging fluid into the atmosphere or the overflow of fluid from one reservoir to another. Hydraulic nozzles are not only tubes but also channels, orifices in thick walls, and slits and gaps between parts in machines. The length of a hydraulic nozzle at which the entire cross section is filled with fluid and the maximum transmission is achieved for external and internal cylindrical nozzles is 3-4 times the diameter. For conical converging and diverging nozzles there are optimal taper angles. A conoidal hydraulic nozzle whose longitudinal cross section conforms to the shape of the jet flowing from the orifice has the greatest transmission capacity. Specially designed nozzles are used in atomizers for spraying fuel.

The rate of fluid flow through a hydraulic nozzle is given by the formula , where ω_out is the cross-sectional area of the nozzle’s outlet, H is the pressure head that determines the fluid flow, and μ_noz is the discharge coefficient, which is determined experimentally and which is a function of the nozzle and head design and of the physical properties of the fluid.

Because of the compression of the stream when the fluid escapes into the atmosphere, a region of reduced pressure is created in a hydraulic nozzle (down to a vacuum; h_vac= 0.75 H). When the pressure reaches a limiting value (0.1 mega-newton per sq m, or 10.33 m H₂0), so-called stalling of the nozzle’s operation occurs (the continuity of the cross section is disturbed), and μ_noz becomes equal to the discharge coefficient for an orifice. The pressure H_lim at which this phenomenon takes place depends on the nature of the fluid, its temperature, and the nozzle’s length (for example, for cold water H_lim = 0.14 meganewton per sq m, or 14 m H₂0).