(358j) Influence of Thermophysical Properties of Cryogenic Fluids on Growth and Collapse of Cavitating Bubbles

Influence of Thermophysical Properties
of Cryogenic Fluids on Growth and Collapse of Cavitating Bubbles

A distinctive feature of the dynamics of liquids is
the possibility of the coexistence of a vapor phase with the liquid phase. Such
formation and disappearance of a vapor phase in liquid are known as cavitating
flow [Rayleigh, 1917]. Cavitating flow has been significantly governed by the
physical-chemical properties of the liquid. The investigation needs to be
performed to study the influence of thermo-physical properties of cryogenic
fluid on the cavitating flows.

In Cavitating flow, plenty of nuclei available for
the inception of cavitation. The fluid is in a precarious imbalance due to repetitive
growth and collapse of cavitating bubbles until an equilibrium condition is
achieved. Thus, the primary focus is on an exhaustive investigation of bubble
growth and collapse. This event rate is dominated by two mechanisms: a momentum
for the impulsion/expulsion of ambient liquid, and a heat transfer to initiate
phase change at the interface. These mechanisms are known as the inertia
(momentum) control and the heat transfer control, respectively. As the
cavitating bubble grows, latent heat is extracted from the surrounding liquid
to the interface, creating a thermal boundary layer. The consequence is a small
local decrease of the liquid temperature, which results in a slight drop in the
vapor pressure. This pressure depression suppresses the further development of
the bubble because a greater pressure drop is needed to maintain the process.
This phenomenon is known as ‘thermal suppression of cavitation,' as it plays a
moderating role in the development of cavitation. The influence of thermodynamic
effects can usually be neglected in fluids for which the critical-point
temperature is much higher than the working temperature. But these effects
become significant when the critical-point temperature is close to the
temperature of the fluid, as in the case of cryogenic fluids [Ruggeri &
Moore, 1968].

Cryogenic fluids are characterized by steep density
variation and a small latent heat of vaporization as compared to room
temperature fluids. The vapor pressure of cryogenic liquid is highly sensitive
to temperature change as shown in figure 1(b). As a consequence, the liquid
surrounding the vapor cavity undergoes substantial evaporative cooling. [Balje, 1981].

Figure 1 Variation in thermophysical properties for Liquid Nitrogen (NIST:
REFPROP)

(a)    
Liquid to vapor
density ration vs Temperature and (b) Vapor pressure vs. Temperature

Previous studies have concentrated on the
consequences of the thermodynamic effect rather than on the investigation of
the mechanism itself. However, on the local scale, each bubble undergoes a
considerable rise of temperature during the collapse while significant local
cooling of the liquid is also expected within the cavitation areas. The
behavior of bubble breakdown process is governed by different thermophysical properties of fluids viz. vapor pressure,
viscosity, and surface tension. The thermodynamic effects associated with the
dynamically changing interfacial structures occurs due to coalescence and
break-up of bubbles, significant discontinuities of the fluid properties and
complicated flow field near the interface results in due to the prevailing
phase change process. The repeating transient collapses of cavitating bubbles
in high-pressure regions influences the shape and stability of the bubbles,
which can respond with the formation of a micro-jet. These re-entrant jets
penetrate the bubble induce the process of cavitation erosion.

Therefore, for modeling inertial and heat transfer
controlled bubble growth in cryogenic fluids due to multiphase malevolent
cavitation process and to investigate the effect of thermo-physical properties
of cryogenic fluids, demands a computational approach which can simultaneously
capture different elementary physical phenomenon (i.e., nucleation,
evaporation/condensation, coalescence/fission, bubble-turbulence interaction,
and so on). In this paper, a multiphase formulation which solves energy
equation in conjunction with the mass and momentum conservation that accounts
for variable thermodynamic properties of the cryogenic fluid and nucleation
transport equation has been used to investigate the fundamental dynamics of
growing or collapsing spherical and non-spherical bubbles in an infinite domain
of cryogenic liquid subjected to large pressure variations.