Plain-orifice applications

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.,... 

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. 

Although the results of the above thermodynamic calculations are independent of the choice of scalable variables, such as nozzle length and diameter, residence time, and mass flow rate (as long as the initial conditions of Po and To are the same), the aerosol dynamic processes (discussed in the next section) are substantially affected by these choices. It is therefore of interest to compare the plain orifice to the long capillary on the basis of these scalable variables (Table 2). For the capillary, L, D, ti 2 (residence time in the cylindrical portion of the nozzle between points 1 and 2), and m (mass flow rate) were held constant (column 2), while for the plain orifice each of these variables were alternately set to the corresponding value for the capillary. For all cases, Po and To, as well as L/D for both the orifice and the capillary, were kept constant thus, the calculated thermodynamic properties were independent of the scalable variables (Table 2) and are also applicable to the results shown in Figure 8. 

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