Flow Boiling in Microchannels

1. Flow Boiling in Microchannels Abhiishek Velichala Anand Vijaykumar Eniola Eniket Nellie Rajarova Courtesy- M K Moharan (IITK)

2. 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 :

3. 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

4. MicrochannelVs 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

5. 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

6. 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/ ρgσ) 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.

7. FLOW BOILING REGIMES Ref - Effects of channel dimension, heat flux, and mass flux on flow boiling regimes in microchannels -Tannaz Harirchian, Suresh V. Garimella

8. 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 (convective Boiling) Rapid Evaporation (Nucleate Boiling) Courtesy- Kandlikar

9. …Continued • The local heat transfer coefficient (h) is calculated as a sum of the two contributionsi.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

10. 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

11. Difference in channel dimension macro and micro

12. 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.

13. Methods to reduce Instabilities • Introduction of artificial nucleation cavities of the right size. • 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.

14. 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.

15. 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. 

16. Variation of heat transfer coefficient and wall temperature with wall heat flux in the 400 µm x 400 µm microchannels, G = 630 kg/m2s (Harirchian and Garimella 2008a). Effect of channel size on heat transfer coefficient for G = 630 kg/m2s;arrows denote the heat flux at which suppression of nucleate boiling occurs

17. Effect of mass flux on local heat transfer coefficient in the 400 lm 400 lm microchannels; the arrows mark the heat fluxes at which suppression of nucleate boiling is observed Boiling curve for R-134a

18. Effect of heat flux and mass flux on the heat transfer coefficient for R-134a Effect of thermodynamic vapor quality on the heat transfer coefficient for R-134a

19. Effect of saturation pressure on the heat transfer coefficient for R-134a Effect of hydraulic diameter on the heat transfer coefficient for R-134a

20. 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.

21. 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.

22. Applications Chemical-vapor-deposited (CVD) diamond Micro-channel Heat Sinking for Laser-Diode Arrays A typical micro-channel heat sink used for Mocroprocessor cooling

23. 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.

24. 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.

25. 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

26. 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.

27. 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

28. 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;

29. Dimensionless group Confinement Number = Bo-0.5 Considers the ratio of surface tension to buoyancy forces on a bubble 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.

30. 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.

31. Thank you • Questions ??