Flow Boiling in Microchannels

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					Flow Boiling in Microchannels




                                      Abhiishek Velichala
                                      Anand Vijaykumar
                                      Eniola Eniket
                                      Nellie Rajarova
       Courtesy- M K Moharan (IITK)
                  INTRODUCTION
What is a Microchannel ?
   Need to dissipate high heat fluxes in MEMS and electronic
   cooling devices.
 Pioneering work on Microchannels:
• Investigated the upper limit of burnout conditions on small
   diameter tubes[1964].
• Tuckerman and Pease demonstrated the importance of
   microchannels in cooling of integrated circuitry[1981].
Classification :
         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
           Why microchannel?
                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
         Non-dimensional groups
• 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)




                       Courtesy- Kandlikar
…Continued
• 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
  boiling dominant
               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
                Flow Instability




• Instability occurs due to rapid expansion of nucleating
  bubble.
• The bubble expansion in the reverse direction to the
  overall flow direction, and introduction of vapor into the
  inlet manifold.
• 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
  engineering importance.
• 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
  tube wall.
• 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.
                     Applications
Heat Removal system (Heat Sinks, Heat exchangers)

 •Microprocessors
   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.
                            Applications
Chemical-vapor-deposited (CVD)           A typical micro-channel
diamond Micro-channel Heat Sinking for   heat sink used for
Laser-Diode Arrays                       Mocroprocessor cooling
                        Applications
•Spacecrafts
High heat flux removal technology is
perhaps the most critical component
of effective micro-spacecraft thermal
control.

Micro-channel heat sinks are the most
compatible with the thermal control
architecture shown.

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
subsystem.
                    Applications
• Biological/Chemical Applications
       For front end sample preparation (purification,
  separation, and concentration), cell sorting, micro-chemical
  reactors and even more fundamental studies of
  mechanotransduction.
• Refrigeration
       Micro-miniature refrigerators
       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
  removed.
                                             Experimental Setup

     Similarity between Pool Boiling and
        Flow Boiling in Microchannel

                   Thin Liquid Film



                   Thin Liquid Film

                       Dryout



                    Thin Liquid Film

Bubble departure           Rewetting   Slug passage
        Challenges faced in Flow Boiling
                 Applications
• 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
  in instabilities
• The dryout observed in two-phase microchannel heat
  exchangers is an obstacle to its usage in cooling applications.
• Challenging to develop and manufacture.
• Fouling
• 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
  rates
    Recommendations and Challenges for
                Future
• 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;
          Dimensionless group
Confinement Number = Bo-0.5
Considers the ratio of surface tension to buoyancy forces
 on a bubble
                                        1/ 2
                                    
               Co  
                    g  L  G Dh2 
                                      
In microchannel flows, the influence of gravity is
negligible, and use of Confinement number needs to be
reevaluated.

New criteria based on relevant forces in microchannel
flows need to be considered.
                         Summary
• 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
  transfer.
• macroscale models are not realistic for predicting flowing
  boiling coefficients in microchannels.
• threshold to confined bubble flow as the interim criterion.
                 Thank you
• Questions ??

				
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posted:11/9/2012
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