Plain-orifice

Pressure Atomization Plain- Orifice 25-250 Diesel engines, Jet engine afterburners, Ramjets Simple, Rugged, Cheap Narrow spray angle, Solid spray cone... 

Plain-orifice atomizers are widely used for injecting liquids into a flow stream of air or gas. The injection may occur in a co-flow, a contra-flow, or a cross-flow stream. The best known application of plain-orifice atomizers is perhaps diesel injectors. This type of injectors is designed to provide a pulsed or intermittent supply of fuel to the combustion zone for each power stroke of the piston. As the air in the combustion zone is compressed by the piston to a high pressure, a very high pressure (83-103 MPa) is required to allow the fuel to penetrate into the combustion zone and disintegrate into a well-atomized spray. 

Another important application of plain-orifice atomizers is jet engine afterburner injectors. The fuel injection system typically consists of one or more circular manifolds supported by struts in a jet pipe. The fuel is supplied to the manifold by feed pipes in the support struts and sprayed into the combustion zone through the orifices in the manifold. Increasing the number of orifices and/or using a ringlike manifold may promote uniform distribution of liquid. To reduce the risk of blockage of orifices, a minimum orifice size of 0.5 mm is usually regarded as practical for kerosene-type fuels. 

One of the limitations of plain-orifice atomizers is the narrow spray cone generated. For most practical applications, large spray cone angles are desired. To achieve a wide spray cone, a simplex, i.e.,... 

Table 4.3. Correlations for Mean, Minimum and Maximum Droplet Sizes Generated in Pressure Jet Atomization by Plain-Orifice Atomizers... 

The main applicatiOTi for plain orifice nozzles is in fuel combustion [1]. Therefore most correlations have been developed for this application [46—51]. Tanasawa and Toyoda [46] derived 24.5.i for diesel sprays in still air. Harmon [47] derived a correlation 24.5.ii that considered many different properties for both the liquid and the gas. In (24.52), liquid viscosity has very little impact on the SMD. Also, it predicts that an increase in surface tensitm will lead to finer atomization. This is contrary to the findings of most other experiments. Figure 24.31 shows a graph with both equations plotted plus (24.5.iii) by Merrington and Richardson [48], where do was set to 0.2 mm and (7l to 50 m/s. The diesel properties were taken from [50] as follows p = 826 kg/m, p = 2.744 mPa s and a = 0.0286 N/m. 

Miesse [49] predicted a formula for the D0.999, also known as the D ax for plain orifice nozzles. The equation was derived using second hand data and best-fitting it. The Dmax is related to both the Weber s number and the Reynolds number, as well as the exit orifice diameter. Weber s number is said to be less significant than Reynolds number. To observe this, the equation has been plotted in Fig. 24.32 as a function of both Weber s and Reynolds number. 

At the end, Table 24.5 contains a series of similar equations in which the variable A constantly appears. As explained in the table, A represents the radial spatial integral scale of turbulence. In practice, the five formulas are very hard to apply because of the recurring presence of the A variable. However, when put together, they offer a very complete way of theoretically determining the SMD of plain orifice nozzles. Equation 24.5.x by Dumouchel s [52] formula applies only for cylindrical liquid jets of a high Weber number. Equation 24.5.xi by Sallam et al. [53] applies for all liquid jets injected into still air. Sallam and... 

Cleary et al. [56] came up with another expression for plain orifice nozzles. What is interesting about their formula is that it works only when the liquid has been subcooled (i.e., it is incompressible). It considers many relevant liquid properties, and also the nozzle s geometrical attributes. The velocity of the Uquid is also considered in Reynolds and Webers numbers. 

Table 24.6 shows correlations for various flat fan hydraulic nozzles that do not quite fall into the plain orifice category. The first three equations [58-60] are fairly old, and contain conditions and variables in them that render them incomparable to each other. Equation 24.6.i assumes that the air is still, 24.6.ii has a very small range of viscosity, and 24.6.iii completely neglects viscosity, rendering it useless for all liquids except water and oil. Equation 24.6.i, as quoted from Kreith and Goswami [58] is plotted in Fig. 24.36 as a function of injection pressure. 

Fig. 24.35 Comparison of three recent equations for plain orifice nozzles by Lee et al. [55], Sallam and Faeth [54], and Sallam et al. [53], Properties used were that of water, and L = 40 m/s, D = 5 mm...

Abstract Plain orifice, or pressure atomizers are the most commonly used atomizers due primarily to their simplicity and ease of manufacture. This chapter provides background on the characteristics of these devices in terms of spray production and general behavior. Classical linear theories are reviewed to provide a basis for theoretical droplet size predictions. More recent developments assessing the unsteadiness within these devices, and its role in spray production, is also provided in subsequent discussion. The chapter closes with modem nonlinear simulations of spray production using modem numerical techniques. 

---