【作者】 李艳；

【导师】 李金海； 李春辉；

【作者基本信息】 河北大学 ， 测试计量技术及仪器， 2014， 硕士

【摘要】 由于高精度、无可动部件等特点，国内外普遍采用音速喷嘴作为标准表，用以对其他类型的气体流量计进行量值传递。音速喷嘴使用的前提是临界流的实现及保持，临界背压比作为音速喷嘴的关键参数，直接关系到装置的可用性。基于此，ISO9300建议对于喉部雷诺数低于2×105的音速喷嘴需保持0.25的临界背压比，或进行临界背压比的实际测量测试。本文以中国计量科学研究院的pVTt装置为主要实验平台，建立了一套音速喷嘴临界背压比测试系统，并按照ISO9300要求，加工了相同喉径不同扩散段长度、相同喉径不同扩散段角度及相同扩散角不同喉径，等3组，共18个音速喷嘴，喉部雷诺数（5.25×104～1.81×105）范围内，重点对上游安装条件及几何参数对喉部雷诺数2×105以下音速喷嘴临界背压比影响进行了实验研究，结果发现：1）实验用音速喷嘴的临界背压比均大于0.45；2）对于扩散角为4.0°的音速喷嘴，喉部雷诺数超过1.1×105，其临界背压比可达到ISO9300的设计值，且不受上游安装条件的影响；否则上游安装条件的变化对临界背压比存在显著影响。在实验结果的基础上，对上游安装条件及几何参数对喷嘴临界背压比的影响进行了分析：1）上游安装条件影响：在保持临界流的情况下，音速喷嘴对上游安装条件的要求低。提前非壅塞现象的出现会带来音速喷嘴临界背压比的降低，该现象本身呈现出多样性及不稳定性。对于相同喉径的音速喷嘴，该现象容易出现在大扩散角的音速喷嘴，为避免该现象发生而带来临界背压比的降低，应尽量选择小扩散角的音速喷嘴。2）几何参数的影响：相同喉径音速喷嘴，较短扩散段长度、或较大扩散角带来的音速喷嘴扩散段的流动分离及激波更为靠近音速喷嘴喉部导致其临界背压比降低。层流阶段，对于相似几何结构的喷嘴，随着音速喷嘴喉径的增加，其边壁区厚度逐渐降低，从而造成激波与边壁区间的相互作用减弱，带来临界背压比的增加。更多还原

【Abstract】 Due to the characteristics of high precision, no movable parts, the sonic nozzle iscommonly used as standard meter to calibrate other types of gas flowmeters. The realizationand maintaining of critical flow is the precondition for the application of sonic nozzle. So, thecritical back pressure ratio is the key parameter for the sonic nozzle, which is directly relatedto the application of the device. Therefore, ISO9300suggests that the back pressure ratio ofthe sonic nozzle as the Reynolds number of throat part less than2×105should be no biggerthan0.25, or is subjected to actual test.Based on the pVTt device of National Institute of Metrology, the system of sonic nozzlecritical back pressure ratio test was established. In accordance with the requirements of ISO9300on the sonic nozzle geometrical parameters, three groups of sonic nozzles, such as, thesame throat diameters with different diffusion length, the same throat diameter with differentdiffuser angle and different throat diameters with the same diffuser angle, total of18nozzles,were manufactured, which the Reynolds number was within (5.25×104~1.81×105). Theupstream installation conditions and geometrical parameters on the critical back pressure ratioof sonic nozzle were investigated as the Reynolds number of throat part less than2×105. Theexperimental results showed that and results showed that,1) The critical back pressure ratio for all sonic nozzles within this paper were larger than0.45;2) For the sonic nozzles with diffuser angle of4°, the critical back pressure ratio could reachthe designed value of ISO9300, which was not influenced by the upstream installationconditions, when the Reynolds number of throat part was larger than1.1×105; Otherwise,the upstream installation condition had significant effects on critical back pressure ratio.On the basis of the experimental results, the effects of upstream installation conditionsand geometrical parameter of sonic nozzle to critical back pressure ratio were analyzed:1) Effects of upstream installation conditions: when the critical flow condition is reached, therequirement of upstream installation conditions for sonic nozzle is low. The premature unchoking phenomenon will take the critical back pressure ratio of sonic nozzle decreased,which shows diversity and instability. For the sonic nozzles with the same throat diameter,this phenomenon easily happens for the sonic nozzle with larger diffuser angle. To avoidthis phenomenon, the nozzle with small diffuser angle is recommended.2) Effects of the geometrical parameters: for the sonic nozzle with the same throat diameter,the stronger flow separation and shock closer to the throat section will lead to its criticalback pressure ratio decreasing due to the shorter diffuser length or larger diffuser angle.Within the laminar flow, for the sonic nozzles with the similar geometrical parameters, theboundary layer thickness will gradually be decreased with the sonic nozzle throat diameterincreasing, which results in weak interaction between shock and boundary layer. Finally,the critical back pressure ratio is increased.更多还原