Near-nozzle region

Recently, there has been an increasing number of numerical studies on the gas delivery stage 161 163 324 325 496 608 and some experimental measurements in the near-nozzle region. 160 162 169 170 [177][327][608]-[6io] Extensive theoretical, 611 numerical, 161 and experi-mentah170 175 studies on high-speed gas jet flows in the near-nozzle region have been conducted to investigate velocity profiles, pressure distributions, shock waves and flow structures. 

Figure 5.2. Calculated gas flow fields in the near-nozzle region of free-fall atomizers. Primary gas pressure 0.140 MPa secondary gas pressure 0.189 MPa angle of secondary gas nozzle relative to the spray centerline 10° angle of primary gas nozzle relative to the spray centerline (a) 0°, (b) 22.5°, and (c) 30° designed for minimizing recirculation gas flow. (Reprinted from Ref. 612.)...

The molten wax method requires that the properties of a simulant are very close to those of the liquid of interest. Thus, the choice of suitable materials is limited. The method also suffers from some practical problems in preheating the wax and errors incurred by changes in physical properties of the wax during cooling after leaving the injector. Since the properties of the wax (notably surface tension and viscosity) critically influence the process of droplet formation, it may not be accurately reproduced due to the changes in these properties with temperature. Therefore, it may be required that the air in the near-nozzle region, where the key process of droplet formation occurs, be heated to the same temperature as that of the molten wax. 

Yue, Y, Powell, C. R, Poola, R., Wang, J., and Schaller, J. K. "Quantitative Measurements of Diesel Fuel Spray Characteristics in the Near-Nozzle Region Using X-Ray Absorption." Atomization and Sprays 11, no. 4 (2001) 471-89. 

The validation of the CAB model has been performed by means of experimental data for non-evaporating, evaporating and reacting sprays under cmitroUed conditions in either a constant-volume or a constant-pressure combustion vessel. Particular attention has been given to the spray structure in the near-nozzle region by comparing the mass distribution with data from X-ray measurements reported in Ref. [19]. 

Y. Yue, C.F. Powell, R. Poola, J. Wang, and J.K. Schaller. Quantitative measurements of diesel fuel spray characteristics in the near-nozzle region using X-ray absorption. Atomization and Sprays, 11(4) 471 90, 2001. 

F.X. Tanner, K.A. Feigl, S.A. Ciatti, C.F. Powell, S.-K. Cheong, J. Liu, and J. Wang, Structure of high-velocity dense sprays in the near-nozzle region. Atomization and Sprays, 16 579-597, 2006. 

Lounnaci, K., Idlahcen, S., Sedarsky, D., Roze, C., Blaisot, J.-B., Demoulin, F.-X. (2015). Image processing techniques for velocity, interface complexity, and droplet production measurement in the near-nozzle region of a diesel spray. Atomization arui Sprays, 25,9. 

An enclosiu e for spray ejected into ambient conditions is shown in Fig. 16.3. It is used to analyze the effect of entrainment on the resulting spray. Entrainment of air occurs since liquid, exiting the nozzle, accelerates the surrounding gas which is then mixed into the spray. By applying the shown enclosure, entrainment is lowered due to the lid-like structure that hampers air flow in the near nozzle region. In order to allow a visualization of the spray inside the enclosure, it is made of Perspex. In the middle of the enclosure, there is an inlet for the atomizer, which is connected to the test facility described in Fig. 16.1. The enclosure is open at the bottom. 

Ertl,M., Weigand, B. (2015). Direct numerical simulations of surface waves on shear thinning Praestol jets in the near nozzle region. In ICLASS, Tainan, Taiwan. 

The observed flame features indicated that changing the atomization gas (normal or preheated air) to steam has a dramatic effect on the entire spray characteristics, including the near-nozzle exit region. Results were obtained for the droplet Sauter mean diameter (D32), number density, and velocity as a function of the radial position (from the burner centerline) with steam as the atomization fluid, under burning conditions, and are shown in Figs. 16.3 and 16.4, respectively, at axial positions of z = 10 mm, 20, 30, 40, 50, and 60 mm downstream of the nozzle exit. Results are also included for preheated and normal air at z = 10 and 50 mm to determine the effect of enthalpy associated with the preheated air on fuel atomization in near and far regions of the nozzle exit. Smaller droplet sizes were obtained with steam than with both air cases, near to the nozzle exit at all radial positions see Fig. 16.3. Droplet mean size with steam at z = 10 mm on the central axis of the spray was found to be about 58 /xm as compared to 81 pm with preheated air and 96 pm with normal unheated air. Near the spray boundary the mean droplet sizes were 42, 53, and 73 pm for steam, preheated air, and normal air, respectively. The enthalpy associated with preheated air, therefore, provides smaller droplet sizes as compared to the normal (unheated) air case near the nozzle exit. Smallest droplet mean size (with steam) is attributed to decreased viscosity of the fuel and increased viscosity of the gas. 

Figure 10 shows the nozzle in place in the liquid. As air under pressure is applied to the atomizer nozzle, a high velocity air stream emerges from the orifice. The bulk liquid is aspirated through the liquid feed line hole, and the liquid column near the region of the jet is atomized. 

Flaw detection from the nozzle bore is performed with 2MHz 7 L, 17 L and 45 S contact pulse-echo probes. 49 S and 30 L probes are applied from the nozzle cone and shell course on the nozzle side of the weld. Detection scans of the nozzle weld from the shell course are performed with 2MHz 49 S, 60 S, 0 L and 17 L probes. The near-surface region above the weld and the nozzle corner region are examined with 2MHz O L and 70° L twin crystal probes. Scans for radial weld flaws are made with 49-60°S and 70°L probes (see Fig 1). 

An irregular trough of metal loss is apparent along the circumference of the ring (Fig. 16.4). Metal loss is severe near the nozzle holes (Fig. 16.5). The corroded zone is covered with light and dark corrosion products and deposits. Analysis of these revealed substantial quantities of copper and zinc. Microscopic examinations revealed exfoliation of the aluminum ring in corroded regions. 

In order to ensure that most of the counterflow is entrained from the ambient medium, PIV pictures are taken of the near field as shown in Fig. 18.9. These figures clearly show that the flow in the vicinity of the nozzle exit is significantly altered by the presence of the suction flow. The velocity field and the associated streamline pattern show a region of reversed flow clearly suggesting the presence of a countercurrent shear layer. It is also clear that most of the reverse flow is the entrained ambient air. 

In the shear layer region, sufficient air is available for complete combustion, and this region is associated with the highest temperatures. In case 2, the total air is kept the same (at 8.8 scfm) as in case 1, but now primary air is introduced at the expense of the secondary air. Except for the region in the near vicinity of the nozzle, the highest temperatures are now obtained in the middle of the conical preburner. This is presumably associated with the greater availability of air in the midregions in this case. 

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