The dynamics of a high heat flux thermal bubble is constrained by the thermal energy carried on the bubble surface right after the bubble formation because of thermal isolation of vapor. This article proposes a way by assigning time delays between dual bubbles to transfer effectively energy from one bubble into the other, thus, breaks energy limitation that one single bubble can usually carry. Experiment result has demonstrated that the useful work as large as 40% can be transferred from one bubble into the other for the ignition time delay set between 2 and 3 ls in a dual bubble system. At the same time, the total extractable useful work in a dual bubble system is 20% higher than twice that of a single-bubble system with the same input heat energy. This phenomenon opens up a new way to transfer or concentrate energies from distributed energy sources with limit energy density into a much higher one for higher power application. Keywords Bubble dynamics Á Bubble interaction Á Energy transfer Á Energy efficiency Á Microbubble Á Microfluidics List of symbols S Cavity spacing, mm D b Average bubble departure diameter, mm D Separation distance between dual heaters, m D s Half of the maximum bubble size for single heater case, m MEMS Micro-electro-mechanical-systems DT Ignition time difference of two independently growing bubbles, ls I Electrical current, A r Resistance of platinum heater, X Area Heating surface area of the platinum heater, m 2 q 00 Heat flux, w/m 2 P b Bubble pressure, N/m 2 P ? Ambient pressure, N/m 2 R Bubble radius, m q l Density of liquid, kg/m 3 l l Viscosity of liquid, N s/m 2 r Surface tension of liquid, N/m V Bubble volume, m 3 t 0 Bubble initiating time t ?1/2max The time when bubble grows to half of the maximum bubble volume t maxThe time when volume of the bubble reaches the maximum bubble volume t -1/2max The time when bubble collapses to half of the maximum bubble volume A(t)Interfacial surface area, m 2 W(t)Useful mechanical work, J
This paper proposes a novel passive micromixer design for mixing enhancement by forming a large three-dimensional (3-D) flow vortex in a counterflow microfluidic system. The counterflow fluids are self-driven by surface tension to perform mixing in an open chamber. The chamber design consists of two rectangular bars to house the chamber and to form two opening inlets from opposite directions. The best design is selected from various versions of mixing chambers. The mixing effectiveness is tremendously increased by folds of contacting surface between two fluids induced and enhanced due to the stretching of two fluid contacting interfaces by the formation of a 3-D large size vortex structure inside the mixing chamber itself with unaccountable numbers of fluid layers. Both numerical simulations and experiments are performed and compared to identify the design parameters for maximum utilization in this microfluidic system, such as the length of rectangular bar, microchannel wall height, and mixing chamber size. Compared to traditional micromixers operated by two-dimensional (2-D) vortex, this passive mixer can greatly enhance mixing efficiency and reduce mixing time by tenfold from around 10 s to less than 10 ms by 3-D effective chaotic flow structures in a more compact size. This mixing chamber is also suitable for an H-shape digital fluidic system for parallel mixing process in different mixing ratio simultaneously as a lab-on-a-chip system.[1509]
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