Flow Boiling in Microchannels
Courtesy- M K Moharan (IITK)
What is a Microchannel ?
Need to dissipate high heat fluxes in MEMS and electronic
Pioneering work on Microchannels:
• Investigated the upper limit of burnout conditions on small
• Tuckerman and Pease demonstrated the importance of
microchannels in cooling of integrated circuitry.
Flow Boiling in a Microchannel
• Flow boiling in a channel is nothing but boiling
occurring due the flow of a liquid with certain
velocity whose walls are subjected to heat
• Microchannel flows are driven by pumps with mean
flow velocities in the range of few millimeters per
second to many centimeters per second
• Flow in microchannels will be mainly laminar
Microchannel Vs Conventional Channel
• The 2 phase heat transfer correlations for microchannels are
different from conventional channels
• Surface tension forces are more dominant and gravity forces
are negligible in microchannels
• Separate studies need to be conducted for microchannels
Features and motivation
• Higher Surface Density
• Higher Heat Transfer Coefficients
(h=Nu.k/D, Nu = constant for laminar flow)
• Low Thermal Resistance
• Volumetric heat transfer rate depends inversely on
square of the channel diameter
• Some of the important non-dimensional numbers in flow
boiling through microchannel are given below:
Capillary number , Ca = μ V / σ
Weber number, We = Dh G2/σρ
K1 = (q/Ghfg )2 * (ρl / ρg)
K2 = (q/hfg )2 * (D/ ρg σ)
why do we need dimensionless groups ?
They are useful at arriving at key basic relationships among
system variables that are valid for different fluids under
different operating conditions.
FLOW BOILING REGIMES
Ref - Effects of channel dimension, heat flux, and mass flux on flow boiling
regimes in microchannels -Tannaz Harirchian, Suresh V. Garimella
Heat Transfer Mechanism
• Flow boiling heat transfer in microchannel is often assumed to
be the result of two different mechanisms, nucleate boiling
and convective boiling
Liquid Motion Rapid Evaporation
(convective Boiling) (Nucleate Boiling)
• The local heat transfer coefficient (h) is calculated as a sum of the
two contributions i.e., nucleate and convective
• Correlations have been developed to find out the h value for
different flow conditions
• For low heat fluxes, convective heat transfer mechanism is
dominant whereas nucleate boiling was the dominant mode of heat
transfer for high heat fluxes
• For Re < 300, flow boiling mechanism is nucleate boiling dominant .
Since most of the flows through microchannels have low Re, heat
transfer during flow boiling in microchannels is seen to be nucleate
Effect of Parameters
• Effect of Contact Angle
• Effect of gravity
• Effect of surface tension
• Effect of channel dimensions
• Effect of quality
• Effect of Heat Flux on convective heat transfer coefficient
• Effect of mass flux , Effect of molecular Mass(M)
• Effect of Buoyancy
• Effect of Tsat , Tsuper, Reynolds Number, Tsub
Difference in channel dimension
macro and micro
• Instability occurs due to rapid expansion of nucleating
• The bubble expansion in the reverse direction to the
overall flow direction, and introduction of vapor into the
• Instabilities can be characterized by flow visualizations
and pressure drop fluctuations.
Methods to reduce Instabilities
1. Introduction of artificial nucleation cavities of the right size.
2. Introduction of pressure drop elements at the entrance to
each channel is expected to reduce the reverse flow
condition. The PDEs introduce a significant increase in the
flow resistance in the reversed flow direction.
CHF in microchannels
• CHF is one of the most important thermal-hydraulic transition
phenomena in flow/pool boiling and is of significant
• It sets the upper limit of heat flux for many engineering
systems and marks the transition from a very effective heat
transfer mode to a very ineffective one.
• The occurrence of CHF must be regarded as an undesirable
condition, as it will cause overheating of an individual channel
or even the entire substrate containing the microchannels.
• The transition corresponds to dryout of the liquid film on the
• The sharp reduction in the local heat transfer coefficient
follows CHF conditions.
• Parameters influencing ChF:
» Tube Diameter, Channel length
» Inlet Subcooling
» Satuaration Temperature
» Mass Flux
• Design with the following combination of characteristics
• short channel length, (ii) a low saturation temperature,
(iii) a large mass flux, (iv) a large subcooling, and (v) a
large microchannel diameter for the chosen fluid.
• Instability and experimental uncertainties are responsible for
the low values of CHF reported in literature.
Variation of heat transfer coefficient and wall temperature Effect of channel size on heat transfer coefficient for G =
with wall heat flux in the 400 µm x 400 µm microchannels, 630 kg/m2s;arrows denote the heat flux at which
G = 630 kg/m2s (Harirchian and Garimella 2008a). suppression of nucleate boiling occurs
Effect of mass flux on local heat transfer coefficient in the
400 lm 400 lm microchannels; the arrows mark the heat Boiling curve for R-134a
fluxes at which suppression of nucleate boiling is observed
Effect of thermodynamic vapor quality on Effect of heat flux and mass flux on the heat
the heat transfer coefficient for R-134a transfer coefficient for R-134a
Effect of saturation pressure on the heat Effect of hydraulic diameter on the heat
transfer coefficient for R-134a transfer coefficient for R-134a
Microchannels in Nature
• Looking at the biological systems, such as a human body, Chen
and Helmes found that the blood vessels that are largely
responsible for thermal exchange known as thermally
significant blood vessels) have sizes on the order of hundreds
of micrometers, with 175 µm diameter being typical.
• The capillaries, where most of the mass transfer processes
occur, are only 4 µm in diameter
*African elephants have larger ears than those in Asia—the
higher temperature in the desert environment in Africa
requires a larger surface area for the ears, which are the main
heat dissipation devices for elephants.
Heat Removal system (Heat Sinks, Heat exchangers)
Cooling of microprocessors using flow boiling of low pressure
refrigerants in multi-micro-channel evaporator cooling elements is a
promising technique for dissipation of heat fluxes of over 300 W/cm2
while maintaining the new generation of microprocessor chips safely
below their maximum working temperature of about 85°C.
•Integrated Circuits and Laser diodes
Forced convection boiling in micro-channels is recognized as an
enabling heat transfer mode that can be effectively exploited to
dissipate heat for future ultra-high-power density electronic
components, such as integrated circuits and laser diodes.
Chemical-vapor-deposited (CVD) A typical micro-channel
diamond Micro-channel Heat Sinking for heat sink used for
Laser-Diode Arrays Mocroprocessor cooling
High heat flux removal technology is
perhaps the most critical component
of effective micro-spacecraft thermal
Micro-channel heat sinks are the most
compatible with the thermal control
Micro-channel heat sinks may be
bonded directly onto the high power
density electronic components of the
micro-spacecraft and then integrated
into the existing pumped cooling loop.
The compact size of the micro-channel
heat sink would contribute little
additional mass to the thermal control
• Biological/Chemical Applications
For front end sample preparation (purification,
separation, and concentration), cell sorting, micro-chemical
reactors and even more fundamental studies of
Unlike conventional cooling systems, which use a fan to
circulate air through finned devices called heat sinks
attached to computer chips, miniature refrigeration
would dramatically increase how much heat could be
Similarity between Pool Boiling and
Flow Boiling in Microchannel
Thin Liquid Film
Thin Liquid Film
Thin Liquid Film
Bubble departure Rewetting Slug passage
Challenges faced in Flow Boiling
• The formation of bubbles with diameters sufficient to fill the
entire channel can cause blockage of the flow, diversion of
fluid into parallel pathways.
• The added pressure drop causes increase in pumping power
and rise in saturation and junction temperatures also results
• The dryout observed in two-phase microchannel heat
exchangers is an obstacle to its usage in cooling applications.
• Challenging to develop and manufacture.
• Need for clean working fluid.
Future Scope and New Frontiers:
• Nanofluid Technology - Nanofluids are this new class of heat
transfer fluids and are engineered by suspending nanometer-
sized particles in conventional heat transfer fluids
• Use of Dielectric liquid in microchannels enables direct
contact of working liquid with electronics.
• Lab-on-a-chip devices- network of microchannels ,sensor,
electrodes and electrical circuits.
• Challenge is to get to 300 W/Cm2 of heat flux dissipation
Recommendations and Challenges for
• a more refined method for distinguishing between
macrochannel and microchannel two-phase flow and heat
transfer is required;
• more critical heat flux data are required for single and
Multichannel elements to investigate non-uniform heat flux
and non-circular channel effects, among other things;
• additional studies on the effects of non-circular channel
geometry, fluid properties, inlet header geometry, etc. on two-
phase flow pattern transitions are required;
• Flow-instability and flow-induced fluctuations need to be
better understood, and a method for delineating stable flows
from unstable flows based on system variables is needed;
Confinement Number = Bo-0.5
Considers the ratio of surface tension to buoyancy forces
on a bubble
g L G Dh2
In microchannel flows, the influence of gravity is
negligible, and use of Confinement number needs to be
New criteria based on relevant forces in microchannel
flows need to be considered.
• In summary, much research remains to be done to better
understand two-phase flow and boiling in single and multi-
channel heat transfer elements.
• recent analytical studies have shown that transient
vaporization of the thin liquid films surrounding elongated
bubbles is the dominant heat transfer mechanism.
• significant effect of mass velocity and vapor quality on heat
• macroscale models are not realistic for predicting flowing
boiling coefficients in microchannels.
• threshold to confined bubble flow as the interim criterion.
• Questions ??