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1. Passive Directional Motion of Fluid During Boiling Driven by Surface Asymmetry in a Dielectric Fluid

   Bhavnani, S. H. (July 2019, Journal of enhanced heat transfer)

   Passive thermal management is of interest in cooling of electronics and avionics in terrestrial and reduced gravity environments. This paper describes the use of microscale asymmetric surface patterns, or ratchets, to generate preferential fluid motion during phase change. The asymmetric patterns take the form of an array of ratchet structures. Preferentially directed bubble growth is demonstrated for boiling on surfaces with such ratchets augmented with re-entrant cavities to produce nucleation at preferred sites. During pool boiling in FC-72, the asymmetric geometry of microstructures causes bubbles to grow normal to the sloped surface rather than in a vertical direction, resulting in a net motion in a preferential direction. Bubble growth from the re-entrant cavities is studied using high-speed photography and image processing techniques. The concept of self-propulsion is extended to an open-ended channel configuration, wherein high-speed videos that document preferential motion of vapor slugs with velocities in the range of several mm/s are presented. Liquid motion is explained using a semi-empirical force balance. 

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2. Assessment of Thermally Actuated Pumping in an Open-ended Channel with Multi-Scale Surface Asymmetry

   Safarkoolan, R. (July 2020, 18th International Conference on Nanochannels, Microchannels, and Minichannels, virtual, July 12-15, 2020)

   Passive fluid pumping during boiling using the concept of asymmetry in the geometry of the heated surface is studied experimentally. The geometry consists of a channel that is located within a chamber filled with dielectric fluid. The channel ends (inlet and outlet) are exposed to the chamber, such that the ends of the channel have the same pressure prior to the addition of heat. Two types of asymmetry are introduced on the heated surface, and their effect is assessed on the net bubble growth and motion within the open-ended channel. The first is a millimeter-scale asymmetry caused by contouring the vertical walls of the channel into repeating 60-30-degree ratchets. The second asymmetry consists of microscale reentrant cavities, located periodically on the shallow ratchet face of the ratchet. To assess the motion of two-phase flow within the channel, visualization at various heat fluxes ranging from 0.8 – 2.6 watts per square centimeter and subcooling from 2.7 - 11.5 degrees C is performed. Videos of bubble ebullition, mergers and slug transport are analyzed to obtain growth rates, velocities, and frequency counts of slugs emanating from either end of the open-ended channel. 

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3. “Assessment of Thermally Actuated Pumping in an Open-ended Channel with Multi-Scale Surface Asymmetry,” virtual, July 12-15, 2020

   Safarkoolan, R. (July 2020, 18th International Conference on Nanochannels, Microchannels, and Minichannels)

   Passive fluid pumping during boiling using the concept of asymmetry in the geometry of the heated surface is studied experimentally. The geometry consists of a channel that is located within a chamber filled with dielectric fluid. The channel ends (inlet and outlet) are exposed to the chamber, such that the ends of the channel have the same pressure prior to the addition of heat. Two types of asymmetry are introduced on the heated surface, and their effect is assessed on the net bubble growth and motion within the open-ended channel. The first is a millimeterscale asymmetry caused by contouring the vertical walls of the channel into repeating 60-30-degree ratchets. The second asymmetry consists of microscale reentrant cavities, located periodically on the shallow ratchet face of the ratchet. To assess the motion of two-phase flow within the channel, visualization at various heat fluxes ranging from 0.8 – 2.6 W/cm2 and subcooling from 2.7 - 11.5 oC is performed. Videos of bubble ebullition, mergers and slug transport are analyzed to obtain growth rates, velocities, and frequency counts of slugs emanating from either end of the open-ended channel. 

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4. Investigating the effect of the fluid properties on bubble dynamics and heat transfer in a tapered microgap with multiphase flow modeling

   https://doi.org/10.1016/j.applthermaleng.2023.121825  

   Pal, Divyprakash; Shukla, Maharshi Y; Kandlikar, Satish G; Perez-Raya, Isaac (January 2024, Applied Thermal Engineering)

   Nucleation and bubble dynamics on a heater surface contribute to high heat transfer rate in pool boiling. Introducing two-phase flow in narrow channels further improves heat transfer. Use of expanding taper microgap geometry further enhances heat transfer, and proper balancing of taper angles and flow lengths leads to self-sustained flow boiling in tapered microgap geometries. This paper focuses on understanding the underlying enhancement mechanism by studying the bubble behavior as they expand and accelerate in the direction of increased taper. The present study conducts a 2D simulation analysis of bubble growth in tapered microgaps with numerical simulations to identify the effect of the fluid properties and tapered angle in the bubble and fluid dynamics behavior. Ansys-Fluent is customized with user-defined-functions (UDFs) accounting for the interfacial heat and mass transport, including a sharp interface and direct calculation of mass transfer with temperature gradients. The study was conducted using air injection and boiling simulation from the conception to the departure of a bubble. The tapered angles were 5°, 10°, and 15°, with flowrates between 3 ml/min to 30 ml/min, 1 mm air inlet, and at 1 mm distance from the convergent end. The departure time of 10 subsequent bubbles was recorded to check the configuration with the quickest bubble removal. A critical flowrate and surface tension region was established for the escape direction of the bubble. In addition, the numerical simulation considered the tapered microgap with a nucleating bubble at atmospheric conditions with a wall superheats of 5 K. The results show that the bubble growing over the heated surface creates fluid circulations and interfacial conditions that suppress the thermal boundary layer leading to an increased local heat transfer coefficient within a range of 1 mm from the interface. 

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5. Boiling Heat Transfer Using Spatially-Variant and Uniform Microporous Coatings

   https://doi.org/https://doi.org/10.1115/IPACK2019-6307  

   Quang N. Pham, Youngjoon Suh (October 2019, ASME 2019 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems)

   Two-phase thermal management offers cooling performance enhancement by an order of magnitude higher than single-phase flow due to the latent heat associated with phase change. Among the modes of phase-change, boiling can effectively remove massive amounts of heat flux from the surface by employing structured or 3D microporous coatings to significantly enlarge the interfacial surface area for improved heat transfer rate as well as increase the number of potential sites for bubble nucleation and departure. The bubble dynamics during pool boiling are often considered to be essential in predicting heat transfer performance, causing it to be a field of significant interest. While prior investigations seek to modulate the bubble dynamics through either active (e.g., surfactants, electricity) or passive means (e.g., surface wettability, microstructures), the utilization of an ordered microporous architecture to instigate desirable liquid and vapor flow field has been limited. Here, we investigate the bubble dynamics using various spatial patterns of inverse opal channels to induce preferential heat and mass flow site in highly-interconnected microporous media. A fully-coated inverse opal surface demonstrates the intrinsic boiling effects of a uniform microporous coating, which exhibits 156% enhancement in heat transfer coefficient in comparison to the polished silicon surface. The boiling heat transfer performances of spatially-variant inverse opal channels significantly differ based on the pitch spacings between the microporous channels, which dictate the bubble coalescent behaviors and bubble departure characteristics. The elucidated boiling heat transfer performances will provide engineering guidance toward designing optimal two-phase thermal management devices. 

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