A STUDY OF FLASHING PHENOMENA UNDER RAPID DEORESSURIZATION

本项目的目的是阐明与液体汽化开始相关的热力学判据，即所谓的闪蒸现象，并预测快速减压下容器内汽液相压力和温度的时间历程。对液体沸腾过程的行为进行了摄影观测，并利用热电偶和压电式换能器的定量测量系统记录了液体沸腾过程的温度和压力瞬变过程。(1)快速开启阀门后的大部分汽液压力瞬变的特征是突然的指数型压力下降，以及适当的欠冲下液体的爆炸性汽化引起的连续压力上升，这与过热液体的下限和初始饱和条件之间的压力差相对应。尽管许多闪蒸过程是从靠近液面的上部到储罐底部开始的，根据气液界面和固液界面对其热物性的依赖程度，发现存在三种不同的压力瞬变模式。(2)改进了Alamgir和Lienhard提出的非均相成核理论，使之适用于本研究中降压率相对较低的情况。在所发展的理论中，初始的凹陷过程用指数函数来近似，该指数函数的特征是非平衡压力松弛时间。结果表明，该弛豫时间对气泡开始产生的热力学条件有很大影响，据估计，气泡产生的热力学条件为国标34-40。

The purpose of this project is to clarify the thermodynamical criterion associeted with the inception of vaporization of liquid, so-called the flashing phenomena , and to predict the time history in pressure and temperature of vapor-liquid phase within a vessel under the rapid depressurization.Experiments have been performed with a R-113 by the quick openning of solenoid valve. Behaviors of boiling process of liquid have been made use of photographic observation , and each of their temperature and pressure transients have been recorded by quantative measuring systems using thermocouples and piezoelectric tranducer.The results are summarized as follows;(1) Most of vapor-liquid pressure transients after the rapid openning the valve has been characterized by the sudden exponential-like pressure drops, and the successive pressure rise caused by the explosive vaporization of liquid under the appropiriate undershoot,Hich correspods to the difference in pressure between the lower limit and initial saturated condition of superheated liquid.Although many of flashing process has been initiated from the upper region near a liquid surface to the bottom of reservoir, they have been observed there exists three type patterns of the pressure transients according to the effectiveness of the dependence of both vapor-liquid and solid-liquid interfaces on their thermophysical properties.(2) The heterogeneous nucleation theory that Alamgir and Lienhard have succesfully dealed with, is improved for the case of relatively reduced depressurization rate of this study. In the developed theory,that is, an initial depression process is approximated by an exponential function which is caracterized by non-equilibirium pressure relaxation time <gamma> . It has been found that this relaxation time affects strongly on the thermodynamical condition associeted with the inception of bubble generation, which is estimated Gb Number 34-40.