(230a) Jet Dynamics in Supercritical Fluid Flow

Abstract for AIChE annual
meeting, 2019

Jet dynamics in supercritical fluid
flow

H.T. Fabich, M.S. Conradi, S.A.
Altobelli, D.O. Kuethe, E. Fukushima

Jet flow, or fluid flow through a
small orifice into a larger vessel, is relevant from hot tubs to diesel
injectors.  In some types of reactors, including diesel injectors, it would be
desirable for the fluid to be in the supercritical region as there is no
surface tension and therefor no droplet formation.  The lack of droplet
formation will increase the amount of surface area and therefore the efficiency
of the reaction.   The flow properties of a fluid in the near-critical and
supercritical region are known to differ from gas or liquid and are difficult
to study due to the elevated temperatures and pressures.

The critical point of a fluid is
defined by the pressure and temperature (P_c, T_c) where a
liquid and its vapor become indistinguishable; it is the terminus of the
vapor-liquid coexistence curve. Supercritical fluids, with P > P_c
and T > T_c, are known to have gas-like viscosity, liquid-like
density, and no surface tension.

Magnetic resonance Imaging (MRI) is
used to acquire spatially resolved information about the velocity in gas,
liquid, near-critical and supercritical fluids.   Preliminary 2D velocity
encoded images for jet flow of hexafluoroethane (C₂F₆), P_c
= 3.05 MPa, T_c = 19.9°C, are
presented as depicted in Figure 1a.  In Figures 1b-c, the temperature was
controlled to 20 ± 2°C, near
T_c.    Figure 1b is a map of longitudinal velocity at 2.07 MPa, subcritical,
and Figure 1c at 3.05 MPa, critical.  Pressure is measured at the recirculating
pump inlet.  For a constant mass flow, the jet velocities are 400 mm s^-1
(Re = 8000) and 130 mm s^-1 (Re = 4000) for Figure 1b and 1c,
respectively, due to a fluid expansion ratio of approximately 3 between the two
pressures.  The jet in Figure 1b, below the critical point, dissipates more
quickly.  Even at a lower jet velocity, near the critical point, higher
velocities are measured in the vessel as the moving fluid retains a relatively
narrow profile throughout the imaging region, as shown in Figure 1c.  In
addition, Figure 1c shows the jet falling in the gravitational field indicating
a density variation between the flowing and non-flowing fluid.