(221m) Investigation of Two-Phase Flow Characteristics in a Fractal-Branching Microchannel

Investigation of two-phase flow characteristics in a
fractal-branching microchannel

Inspiration from nature to solve advanced
engineering problems has attracted the interests of engineers, designers and
scientists. Biomimetics is to imitate and apply the elements, systems and mechanisms
from nature to solve technological challenges as stated by Gleich et al. (2009).

They also added that one of
nature’s solution which is being explored is fractal shapes. Fractal shapes
appeared in a variety of cases such as snowflakes, blood vessels and plant root
systems in nature. Fractal shapes consistently appear in situations which
require mass or heat transfer throughout a large space. The optimal spreading
and transfer throughout the space characteristics of fractal shapes, making them
a practical solution to design more efficient heat and mass transfer devices. Fractal
shapes were first employed to improve fluid mechanics designs by West et al.
(1997) to minimise the workflow for bulk fluid transportation through a network
of branching tubes.

On the other hand, two-phase flow
in microscale channels has great applicability due to its diverse range of applications.
As expressed by Serizawa et al. (2002), modern and advanced technologies such
as micro-electro-mechanical systems, chemical process engineering, medical engineering
and electronic cooling utilise multiphase flow in microchannels.

This work aims to investigate the
application of nature-inspired fractal geometries as multiphase microscale flow
passage using CFD analysis. ANSYS Fluent software has been utilised to investigate
the flow characteristics numerically in order to improve the pressure drop and
heat transfer. Also, this question will be raised whether two-phase flow
patterns in fractal microchannels are different from straight channels or not.

The application of fractal microchannels
increases the specific gas-liquid contact area as mentioned by Sebastia-Saez et
al. (2018). Consequently, the heat duty and the amount of required fluid will
be reduced due to improved features of fractal shapes.

Fractal shapes are expected to provide
less pressure drop and better thermal mixing according to secondary flow motion
created at bifurcations as Pence et al. (2003), Alharbi et al. (2004) and Senn
et al. (2004) investigated. Also, better space filling and highly packed structure
will be obtained.

As demonstrated by Serizawa et
al. (2002), two-phase flows in microchannels mainly influenced by surface
tension, viscosity and inertia forces. .Based on superficial velocities of each
phase, different flow patterns can be observed such as bubbly flow, slug flow,
stratified flow, annular flow and dispersed flow. By increasing the velocity of
the gas phase, each pattern will be obtained respectively.

For instance, the simulation of air-water
flow in branching microchannel with specific superficial velocities results in separated
two-phase flow. The flow pattern in microchannel has been observed to be
unstable and tended to change with time, based on a collaboration of gas
velocity and liquid surface tension. When the air velocity is stronger than
water surface tension, the air passes through the water and separate it to
smaller parts, whereas smaller air velocities can not break the water surface
tension which generates water slugs. Hence, as time passes, the air-water
contact area increases by a generation of liquid annular flow and reaches a
constant value after a while.

In conclusion, using fractal-like
branching geometries as two-phase flow channel in the scale of micrometres,
increase the contact area of two phases. Higher two-phase interface area will
result in enhanced heat  transfer, and fractal shape will result in less
pressure drop.

Alharbi,
A.Y., Pence, D.V. and Cullion, R.N., 2004. Thermal characteristics of
microscale fractal-like branching channels. Journal of Heat Transfer, 126(5), pp.744-752.

Gleich,
A., Pade, C., Petschow, U. and Pissarskoi, E., 2010. Potentials and trends in
biomimetics. Springer Science
& Business Media.

Huang,
Z., Hwang, Y., Aute, V. and Radermacher, R., 2016. Review of fractal heat
exchangers.

Pence,
D., 2003. Reduced pumping power and wall temperature in microchannel heat sinks
with fractal-like branching channel networks. Microscale Thermophysical Engineering, 6(4), pp.319-330.

Sebastia-Saez,
D. and Arellano-Garcia, H., 2018. A model-based approach to design miniaturised
structured packings for highly efficient mass transfer in gas/liquid multiphase
flows. In Computer Aided Chemical
Engineering (Vol. 43, pp.
821-826). Elsevier.

Senn,
S.M. and Poulikakos, D., 2004. Laminar mixing, heat transfer and pressure drop
in tree-like microchannel nets and their application for thermal management in
polymer electrolyte fuel cells. Journal of Power Sources, 130(1-2), pp.178-191.

Serizawa,
A., Feng, Z. and Kawara, Z., 2002. Two-phase flow in microchannels. Experimental Thermal and Fluid
Science, 26(6-7), pp.703-714.

West,
G.B., Brown, J.H. and Enquist, B.J., 1997. A general model for the origin of
allometric scaling laws in biology. Science, 276(5309), pp.122-126.