(342i) Hydrodynamics of Flash Evaporation of a Stagnant Liquid Column Under a Low Depressurization Rate

Hydrodynamics of flash
evaporation of a stagnant liquid column under a low depressurization rate

Kush Kumar Dewangan¹,
Prasanta Kumar Das¹

¹Department of
Mechanical Engineering, Indian Institute of
Technology, Kharagpur

Kharagpur 721302, India

kush.mech@gmail.com, pkd@mech.iitkgp.ernet.in

Abstract

Flash evaporation is a evaporation phenomenon when the liquid is
depressurized below the saturation limit. The depressurised liquid becomes
superheated and starts to evaporate rapidly. Flash evaporation is concern to a safety
purpose in chemical industry for failure of vessels contains pressurized
liquid. This rapid evaporation creates hazardous situations when the vessel
contains toxic fluid. Flash evaporation lead to explosion if the liquid is
flammable. Two types of depressurization may occur in industry; fast
depressurization when vessels break or rapture and slow depressurization when
leaks in the vessels. Higher superheat values achieved by fast depressurization
rate, while the slow depressurization creates a lower superheat liquid. The
degree of superheat depends upon the depressurization rate. Liquid flash
evaporation at high depressurization has been extensively studied. However, the
flash evaporation at low depressurization rate has been found less attention. This
paper presents experimental results of the flashing phenomena in superheated
liquid at a low depressurization rate. A quick opening ball valve is used for
depressurization of the stagnant liquid. The superheat of the test liquid ranges
from 10 °C and 90 °C. Distilled water is used as test fluid in a 10 mm diameter
borosilicate glass tube. The high-speed camera is used to analyse the propagation
process. Based on the visual analysis,
the flow in test section divides into three zones: upper zone (two-phase),
lower zone (depressurized liquid) and the middle zone (interface) that
separates the upper and lower zone as shown in Fig. 1. Evaporation front starts
from the centre or near the wall of the free surface when the degree of
superheat is sufficient. Initiation of front development and propagation is not
started immediately after the depressurization. The metastable liquid reaches
to equilibrium state by small disturbance. This disturbance is maintained by
secondary nucleation.

Fig. 1.
Three zones in test section: ejected two-phase, interface (thin evaporation
front), and depressurized liquid

Three
kinds of front shape observed by close inspection of liquid vapour interface:
concave, convex (looked from the two-phase side) and tulip. Front speed and
shapes are estimated by the image analysis. Matlab script is used for interface
detection. In addition, the exit pressure and
temperature have been measured. With the increase in liquid superheat,
evaporation front velocity increases. The experimental results have shown some
unique features of the propagation front. This study helped in understanding
the physics of the flash evaporation when vessel leaked from a small hole. The
experiment also provides the behaviour of flow in accidental condition. Details
will be provided in the full paper.