SESSION 6
TRANSPORT PHENOMENA IN TWO-FLUID SYSTEMS
Chairmen: H.R. Jacobs, N. Brauner
DEVELOPMENT AND EVALUATION OF A TWO-PHASE LOOP-TYPE THERMOSYPHON FOR COOLING TELECOMMUNICATIONS SYSTEM
W.T Kim*, K.S. Song*, and Y. Lee**
* Electronic Packaging Technology Section, Electronics and Telecomunications Research
Institute, Taejon Korea, 305-350
** Department of Mechanical Engineering, University of Ottawa, Ontario, Canada, KIN 6N5
EXTENDED ABSTRACT
An experimental study on the heat transfer performance of a loop-type two-phase closed
Thermosyphon for electronic components such as multichip modules(MCM) used in many
telecommunications systems is described. The assembled loop-type Thermosyphon shown in
Fig 1 was manufactured with ultimate cooling capacity of up to 10 W/cm2 with maximum surface
temperature of the evaporator section of 75oC, FC-72, FC-87, water, R-113, and Ethanol have
been used as the working fluids. For Heating the evaporator section, the special flat type plate
heater, simulated for MCM operation, has been developed. A heater consisted of a heating
element made from BcO on a ceramic sheet plate with metal as an electrical conductor. The
amount of working fluid, inclination and numbers of the condenser section, and configuration of
the evaporator section have been used as the experimental parameters. A visualization of the
movement of liquids with booling the scattering of liquid drops and the condensing of vapor has
made the thermal mechanism in the loop-type thermosyphon. In addition, the heat transfer
coefficients and the thermal resistance mechanism shown in Fig. 2 have been obtained.
OXYGEN TRANSFER COEFFICIENTS TO EMULSIONS IN A BUBBLE COLOUMN REACTOR
S. Ben-Zvi Yona and J. C. Merchuk
Department of Chemical Engineering, Ben-Gurion University of the Negev,
P.O.Box 653, Beer-Sheva 84105 ISRAEL.
ABSTRACT
A relatively simple measurement system was set for the evaluation of oxygen transfer
coefficients to an emulsion, using the dynamic method. Emulsions of Soya Bean Oil
(SBO) and water covering the whole range of composition were tested in a bubble
column reactor. The bubble column was 5L by volume, containing 2.5L of the emulsion.
A mathematical model was written for the description of the three phase system: two
immisible liquid phases and gas phase. The model proposed for the analysis of the
experimental data assumes that the gas phase is in contact with the continuous phase
only. Simulation was performed in order to check the behavior of the model. It was
concluded from the first part of the simulation, that the balance on the gas phase could
not add information to the analysis since the oxygen concentration differences between
gas inlet and outlet were very small. This was due to the short residence time of the gas
in the liquid. The simulation also revealed that the response time of the probe can not be
ignored during the analysis of results. The second part of the simulation consisted of
estimating oxygen transfer coefficients on the basis of the simulated data in order to
check the search method that was used and the model function.
The measurements were found to be successful and oxygen transfer coefficients from gas
bubbles to the continuous phase and from the continuous phase to the droplets were
obtained via the mathematical model. The model fit was best for the description of oil
continuous emulsions and for water continuous emulsions, mainly with low fraction of
dispersed oil.
It was found that all oxygen transfer coefficients increase with the increase of continuous
phase fraction, in either SBO or water continuous emulsions, and for either the transfer
from gas to the continuous phase or from the continuous phase to droplets. Like wise,
the increase of continuous phase fraction is followed by an increase of holdup. Both
oxygen transfer coefficients and holdup increase with the increase of gas flow rate.
Variations in oxygen transfer coefficients of the emulsion system, with the emulsion
composition and gas flow rate were related to changes in the properties of the phases
(such as the spreading coefficient) and the fluid dynamics.
EMULSION PROBLEMS IN LIQUID-LIQUID SEPARATION UNITS OF CHEMICAL PROCESSES
Dr. M. Holzbecher
F. Sauer
Liquid-liquid separation normally belongs to the very simple and
economical unit operations in chemical engineering. In practice, of
course, there are many exceptions to this rule. The most complicated
systems are emulsions. (There are different definitions for the term
emulsion. In this paper only systems with strong coalescence obstruction
are called that way.)
Concern of this lecture is a description of the stepwise procedure in
treating emulsion difficulties in separation units of chemical processes.
Most of the investigations on the field of emulsion breaking are concerned
with a special, well defined kind of emulsion (petroleum industry, food
industry etc.). In chemical processes we find both types of emulsions, w/o
and o/w.
Moreover, there is a great diversity of stabilization mechanisms,
electrical, mechanical, rheological. In many cases the =
liquid-liquid-system contains solids, usually very fine dispersed playing
a surface active rule.
Therefore, the first step to solve emulsion problems is a conscientious
characterization of the system. On this score belongs the question whether
the system actually shows significant coalescence obstruction. In many
separation problems the system is sedimentation controlled. For an
unexperienced observer the behaviour can be very similiar to real
emulsion.
Another important point is the classification in the above mentioned o/w
and w/o types. W/o emulsions are often much more difficult to break than
o/w type. The most difficult point of the characterization is the
identification of all effective stabilization factors.
If solids are playing a dominant role it is often detectable by
photomicrography. Electrical barriers can be estimated by measurement of
zeta potentials. Mechanical and rheological factors, however, are
especially in industrial practice difficult to access.
Here is still a great requirement of relative simple and stable
measurement systems and much more experience.
After characterization of the system the next step is the avoidance of
enulsification by changing the process. We often can influence the
concentrations, the energy inputs, temperature, pressure gradients and
residence time. A widespread problem in chemical processes is the addition
of aids, stabilisators, inhibitors etc. Beside their positive effects,
they often show unwelcome surfactant behaviour.
If emulsification is unavoidable demulsification methods must be brought in.
In chemical industry we use different methods like temperature, pH,
mechanical methods, adsorption, chemical demulsification and electrical
fields. Especially the field of chemical demulsification is a rapidly
growing field. In contrast to petroleum industry not only the effect of
demulsification but also the change of product quality is a very important
criterion. The identification of the optimal demulsifier is purely
empirical. There are only a few crude rules of heuristic quality.
However, the success in process improvement is often in spite of this
simle approaches striking. This will be shown with the help of some daily
work examples.
AN EULERIAN FINITE ELEMENT METHOD FOR INTERFACE TRACKING PROBLEMS
Nancy Lock, Marc Medale, Marc Jaeger
Institut Universitaire des Systèmes Thermiques Industriels, UMR C.N.R.S 6595
Technopole de Château-Gombert, 5 rue Enrico Fermi
13453 Marseille Cedex 13, France
We present in our paper a numerical model mainly devoted to interface flow problems.
Furthermore, we are specially concerned in viscous incompressible flows where interface(s)
separate(s) non-miscible fluids and where interfacial effects could be dominant.
In our finite element model, we have chosen the Eulerian approach where fluids move
through a fixed mesh. The interface is tracked with a pseudo-concentration based method.
Although this method is generally less accurate than the Lagrangian one, it allows the treatment
of interface subject to severe deformation. Moreover in order to improve the accuracy, we have
supplemented the model with a local mesh adaptation algorithm. From a reference mesh in
which the interface is identified, it consists in dividing crossed interface elements such that
element boundaries coincide with the interface(s). This procedure allows us :
- to represent accurately the material discontinuity across the interface instead of smearing it
over some finite region;
- to include interfacial phenomena such as surface tension by employing specific boundary
elements at the interface.
The velocity field within each fluid is calculated by solving the Navier-Stokes and continuity
equations using discontinuous pressure approximation elements.
The model efficiency is demonstrated on numerical examples like Rayleigh-Taylor
instability and bubble evolution in a gravity field. Good agreement has been obtained in the
comparison of our results with theory or available numerical results.
A sketch of the computed interface pattern in the late stage of Rayleigh-Taylor instability
(Atwood number=0.05) is shown below.
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