10-10-2012, 02:05 PM
Heat transfer at the phase interface of bubbles collapsing in subcooled liquids and during subcooled boiling
Heat transfer at the phase.pdf (Size: 525.59 KB / Downloads: 31)
Abstract
Heat transfer phenomena with bubble condensation
in subcooled liquids and during subcooled
boiling are discussed, which were studied by using
holographic interferometry. This inertialess and nonintrusive
measuring technique allows interesting insights
into fluid dynamics and thermodynamics of
phase change processes. The resistance for the heat
transport through the phase boundaries is mainly on
the liquid side and therefore the heat transfer coefficient
can expressed by simple correlations following
the procedure in forced convective flow. For the sake
of simple boundary conditions first bubble condensation
in subcooled liquids with constant and homogenous
temperature are studied and than the more
complex situation with large temperature gradients in
the wall near boundary during subcooled boiling are
discussed.
Introduction
The operation of highly powered boiling equipment
can be seriously affected by subcooled boiling. Despite
of the fact, that the bulk temperature of the liquid is
below the saturation temperature, bubbles exist due to
the superheating of the boundary layer near the wall.
Each bubble has an extremely short lifetime between
nucleation and collapse, but the average void fraction
may be quite high and influences the pressure drop of
flow strongly.
Nucleation and bubble formation are well understood
and frequently studied in the literature. In contrast
to this, much less is known about the temporal
course of the heat transfer through the phase interface,
in spite of the fact, that bubble condensation was already
early studied in the literature [1, 11, 14, 16].
Voloshko (1973). In subcooled boiling the bubbles
produced in the superheated boundary layer adjacent
to the wall, have a lifetime of a few milliseconds up to a
few hundreds of a second only, after they enter into the
subcooled bulk. So inertialess and non-invasive measuring
techniques are needed to monitor the phenomena
at the phase interface and to give information on
the heat transfer there.
Measuring technique
Two series of measurements are presented here, one
with homogenous temperature of the liquid in which
the bubbles condense and an other one in which conditions
of subcooled boiling existed, i. e. the bubbles
where produced at a heated wall along which liquid
was flowing having a bulk temperature below the
boiling point. In both conditions the temperature profile
in the liquid near the phase interface was measured
by using the holographic interferometry together with
a high-speed camera.
Under the first conditions saturated vapor was
blown out of a capillary, which was carefully guardheated,
to avoid condensation in the narrow path. The
liquid in which the vapor entered was slowly flowing
down, to guaranty constant temperature during the
condensation period. Details of the experimental set
up are reported in [12]. Under the conditions of subcooled
boiling with a temperature gradient at the wall
the experimental set up was much more complicated.
Details are presented in [5]. Here only the design of
the test section will be briefly described. As shown in
Fig. 1. Water was flowing upward through a rectangular
channel which had two opposite walls made of
copper and one of them was homogenously heated.
The other both opposite walls were made of glass to
allow optical observation by the holographic interferometry.
Conclusion
An attempt was made to analyse heat transfer phenomena
which occur when vapour bubbles condense in
liquids having a temperature below the saturation
temperature of the vapor in the bubble. The aim was
mainly to contribute to the understanding of the very
complicated phenomena influencing this process. For
this the holographic interferometry was a very valuable
help.