Tham khảo tài liệu 'air and abrasive acceleration_4', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 82 3 Air and Abrasive Acceleration Fig. Flow situation near the exit of a convergent-divergent nozzle Oosthuizen and Carscallen 1997 a Overexpanded flow with shock front leaving the nozzle exit b Overexpanded flow with disappearing normal central shock front outside the nozzle c Overexpanded with oblique shocks outside the nozzle d Almost isentropic nozzle flow e Underexpanded flow with severe expansion waves outside the nozzle p inlet p w co 05 Q_ rô p ambient exit plane shock front Fig. Operating conditions of a convergent-divergent nozzle see text for 1 to 5 adapted from Oertel 2001 Jet Structure 83 Ãm was sensitive to the nozzle exit pressure. For all loading ratios exit pressure increased with decreasing particle size. Exit pressure also increased with an increase in the mass flow ratio abrasive air. Although the nozzle in Fokke s 1999 work was designed in a way that the air exit pressure would meet the atmospheric pressure the exit pressures were all well below the atmospheric pressure as the abrasive particles were added. Effects of abrasive particle on the gas flow properties in the nozzle vanished for small mass flow rates abrasive air Am 1 . It is recommended to run a Laval nozzle at least at the design pressure for the nozzle being used and at higher pressure if possible. When the nozzle is operated below the design pressure the flow forms shock waves that slow down air flow as well as abrasive particles. Schlieren optics has been utilised by several authors to reveal such shock wave structures Settles and Garg 1995 Mohamed et al. 2003 Kendall et al. 2004 . Some images are provided in Figs. and . Figure summarises possible pressure decay modes along a convergent-divergent nozzle operating between nozzle inlet pressure and ambient pressure. Up to the throat in the divergent section the nozzle pressure decreases for all flow conditions. If the nozzle is being designed in a way that the nozzle exit pressure meets the .
