Contoured orifice

Figure 6.4 Types of valve units, (a) An uncaged, mind valve unit. (6) A caged, round valve unit, (c) A caged, round valve unit featuring contoured orifice, fd) A rectangular valve unit it) A futed, round valve unit, if) A fixed, triangular v hre unit, (forts a and e courtesy of Gtiisch Inej parts b, c and f courtesy of Koch Engineering Company, Inc. pari d courtesy of Nutter Engineering.)...

Consider the ideal case of a single smooth sphere present in the center of a cylindrical orifice. It is possible to deduce a simple relationship relating resistance changes in the orifice to the dimensions of the fineparticle and the orifice. Early workers with resistazone counters established that this relationship held for spherical fineparticles provided that the diameter of the fineparticle did not exceed 40 % of the diameter of the orifice. Other experimental studies show that, in general, using the simple orifices available in the early instruments, fineparticles with dimensions of less than 3 % of the diameter of the orifice could not be characterized easily. This lower limit has been reduced by modern techniques such as hydrodynamic focusing and the use of contoured orifice equipped with flow straighteners of the type discussed later in this section [6-8]. 

Flow nozzles are commonly used in the measurement of steam and other high velocity fluids where erosion can occur. Nozzle flow coefficients are insensitive to small contour changes and reasonable accuracy can be maintained for long periods under difficult measurement conditions that would create unacceptable errors using an orifice installation. 

The most usual forms of platinum or, more frequently, of the platinum alloys containing 5 or 10% of rhodium, are thin protective sheaths contoured to conform with the shape of the underlying refractory. Sheaths of this type, generally 0-25 mm to 0-65 mm thick, are widely employed to protect tank lips, skimmer blocks, stirrers, thermocouple pockets, etc. More substantial —thicknesses-up to 2-5 mm thick —are used to protect orifices whose dimensional accuracy must be maintained to a high degree. 

An extruder die s orifice is usually made oversize to the shape and size of the required contour. The final shape develops downstream from the die as the plastic expands, warps and/or shrinks. The best designs are based on past experience with a particular profile shape. As usual experience or being trained by one with experience has always been a real plus. 

In 1969 Coulter s patented a conical entrance and exit but in 1970 they filed another patent which indicated a preference for cylindrical apertures over the contoured variety, possibly because of plugging difficulties. They also considered channeling the particles down the center of the aperture but rejected this approach as impractical. In 1973 they were issued a patent for a rounded orifice. 

The aerodynamic valves have a specially designed inlet (e.g., a profiled orifice in the inlet pipe, contoured diffuser, or a shrouding duct) in which the fluid flow characteristics act as a physical barrier (fluid diode) to the backflow of combustion products. Such pulse combustors are termed the valveless combustors. Example of valveless pulse combustor is shown in Figure 23.4 (Putnam et al., 1986). 

Fig. 30.12 (a) Diameter and (b) velocity contours for two impinging jets of a rocket injector, where x is in the plane of the sheet and y is in the plane of impinging jets. The orifice diameters are 0.3 mm located 6.5 mm apart, making a 90 degrees angle with each other, with jet flow rate of 3.18ml/s [20] (Courtesy of American Institute of Physics)... 

Fig. 3 Measured vapor cloud contour (LEL = 2 vol %) after jet re lease of propane (5o = 12) out of an orifice of diameter do A gaussian concentration profile at x/do - 100 is added.

An alternative design of a pulse combustor is a so-called valveless combustor in which the mechanical valves are replaced with an aerodynamic diode in the form of a profiled orifice in the inlet pipe, contoured diffuser, or a shrouding duct [28] (Figure 20.15). Similarly to the combustor with mechanical valves, the high-temperature gases from a combustion chamber start to flow just after a... 

Draw resonance, or surging, is defined as the nonuniformity in the diameter of the extrudate when a polymer melt is stretched at different take-up speeds as it comes out of an orifice. This phenomenon is shown schematically in Fig. 2.10. When the take up speed is small or when there is no stretching, only die swell is observed, as can be seen from Fig. 2.10a. When take-up speed is higher and the stretched extrudate is solidified by quenching, the contour appears as shown in Fig. 2.10b. Now the draw ratio is defined as the ratio of the linear velocity V of the extrudate settled in the quenching bath to the smallest linear velodly Vo in the die swell region. When the draw ratio (DR) )es beyond a oitical value DRc, then the resulting phenomenon is draw resonance as shown in Fig. 

---