多孔孔板空化流动特性实验研究

Experimental Study on Cavitation Flow Characteristics of Porous Orifice Plate

多孔孔板常作为节流元件安装在管道中,但孔板下游的空化流动容易导致管道振动,对系统产生不利影响,本文通过实验测量不同孔隙率孔板在不同空化阶段产生的振动及对应的压差和流量。结果表明,随着空化程度的不断加剧,全频段振动加速度级显著增大,峰值逐渐也增大,振动加速度呈现宽频特性。通过空化振动曲线中拟合曲线的斜率不同,可以区分出不同空化阶段以及临界空化数,孔隙率小于1.8的孔板临界空化数随孔隙率的增大迅速减小,当孔隙率大于1.8,初生及持续空化数随孔隙率增速变缓,阻塞空化数基本不变,即适当增大孔隙率有利于延缓空化的发生。根据空化流阻曲线,未发生空化时孔板流阻系数基本不变,空化初生时流阻系数略有减小,而后随着空化不断加剧,流阻系数迅速增大,孔隙率越大,流阻越大,流阻受空化数的影响越大。综合考虑孔板的振动和流阻特性,建议孔板孔隙率应保持在1.8~2.0之间。

The installation of porous orifice plates in pipeline systems can play a role in throttling and reducing pressure,but if the pressure difference between the upstream and downstream of the orifice plate is too large,it is easy to cause cavitation in the downstream of the orifice plate.The occurrence of cavitation can cause pipeline vibration,and have adverse effects on pipeline systems.This article experimentally measures the vibration acceleration,corresponding pressure difference,and flow rate generated by porous plates with different porosity in dif⁃ferent cavitation stages.The experimental results indicate that as the degree of cavitation continues to intensify,vibration acceleration in the full frequency range significantly increases,and the peak value of vibration acceleration gradually increases.The vibration acceleration ex⁃hibits a broadband characteristic.Based on the experimental results,we plotted the vibration intensity curves of orifice plates with different porosity as a function of cavitation number.By varying the slopes of the fitted curves in the cavitation vibration curve,different cavitation stages and three critical cavitation numbers(including incipient cavitation number,constant cavitation number,and choked cavitation num⁃ber)can be distinguished.The critical cavitation number of orifice plates with a porosity less than 1.8 rapidly decreases with the increase of porosity.When the porosity is greater than 1.8,the incipient cavitation number and constant cavitation number slow down with the increase of porosity,while the choked cavitation number remains basically unchanged.This means that increasing the porosity appropriately is benefi⁃cial for delaying the occurrence of cavitation.Based on the experimental results,we plotted the variation curve of flow resistance coefficient with cavitation number for orifice plates with different porosity.From the cavitation flow resistance curve,it can be seen that:①When cavi⁃tation phenomenon has not yet occurred,the flow resistance coefficients of orifice plates remain basically unchanged.②When cavitation phenomenon just occurred,the flow resistance coefficient of the orifice plate decreases slightly,but thereafter,as the degree of cavitation in⁃tensifies,the flow resistance coefficient begins to rapidly increase.③The larger the porosity of the orifice plate,the greater the flow resis⁃tance,and the greater the influence of cavitation on the flow resistance.Taking into account the vibration and flow resistance characteristics of the orifice plate,it is recommended that the porosity of the porous orifice should be maintained between 1.8 and 2.0.