Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (similar to 10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (similar to 100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000-2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.

Preparation, structure and luminescence of BaMAl10O17 : Eu2+ (M = Be, Mg, Ca, Zn, Cd, Mn, Co, Li) system

The BaMA(10)O(17) (M = Be, Mg, Ca, Zn, Cd, Mn, Co, Li) system has been synthesized by solid state method and characterized by XRD. The results show that when M is Mg, Zn, Mn, Co, Li, there exists the structure of beta-Al2O3 for BaMAl10O17 system, and when M indicates Cd, beta-Al2O3 structure is formed accompanying alpha-Al2O3 phase, and when M represents Be and Ca, beta-Al2O3 structure cannot be formed. This demonstrates that the condition forming beta-Al2O3 structure compounds for the system BaMAl10O17 is 0.05nm < R-M < 0.09nm (R-M represents the radius of M). The thought that if a M ion can form a stable spinel structure there exsits a corresponding magnetoplumbite and beta-alumina structure is proposed for BaMAl10O17 system according to the experimental results. When M is Li, Be, Zn, Eu2+ activator produces an emission of nearly 450 nm with half height width about 50 nm, when M is Mn, there are simultaneously the emissions of Eu2+ and Mn2+ and the excitation energy of Eu2+ can transfer to Mn2+ in the host, when M is Cd, Eu2+ displays a double-emission band, which can be explained by the Jahn-Teller's effect. It is possible for the system BaMAl10O17 with M being Li, Be, Zn to become blue-emitting component in three colour lamp through further study.