When sonic nozzles of significantly smaller diameter are used as standard flow meters, the critical back pressure ratio is affected by the boundary layer at the nozzle throat. However, the effect of the boundary layer on choking criteria is still controversial. Then, the choking phenomenon of a convergent nozzle flow has been experimentally investigated using four convergent nozzles with the same diameter followed by a straight pipe of a variable length.As a result, it is shown that the critical back pressure ratio is smaller than that for the steady one-dimensional isentropic flow and decreases as the boundary layer thickness increases. Moreover, the main flow Mach number at the nozzle exit is supersonic when the back pressure ratio is equivalent to the choking condition, and the Mach number increases as the boundary layer thickness increases.
The experimentaI results 〔 }f the ch 〔, king pressure ratio and the choked 日ow rate in u c nverging nezzle are con / pared with three theoretical values calculated by the Fanno 且 w theory , the compound fioLv mode1 , and thc average now rn 〔 〕deL The experiment . atvalues are in qualita 亡 ive agreenler ユt with the calculated va ] ues in any of these cases , and agree best with the 、 ・ alucs by し he average 飾 w m σde1 . Based upon thcse compar 孟 sons , the mechanism of the choking of the flow where し he velocity distTibutic 〕ll is n { 〕t uniform due to the beundary 】 ayer is rnade dear ,
The effect of the boundary laycr し hickness 〔 m the chokinR phenomenon of a c り nvergent nozzle flc )w has heen experilneiitttlly invest 正 gated using four nozzles with thc same diameter but dif 「 ercnt bounclaryユ ay じ r thickness at the exit of the nozzle 、 、 Vhen the fiow is just choked , the cross − section where the main 日ow Mach number is unity is located ups し rea1 皿 of the nozzlc exi し, and the mainHow is accelerated し o supersonic froln the section to the Il〔〕zzle exit . The fine structure f st互ch a choked How in a convergent llozzle is 〔 三 xplained taking the boundary laycr d量 splacemcnt thickness into consideration ,
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