Jovan Mitrovic

On the profile of the liquid wedge underneath a growing vapour bubble and the reversal of the wall heat flux

Abstract Combining mass and energy balances, a differential equation for the profile of a liquid wedge underneath a vapour bubble that is growing on a solid surface is derived. It connects the spatial and temporal changes of the film (wedge) thickness with the spatial temperature changes and velocity of liquid at the interface. Specifying particular conditions, the equation reduces to those from the literature. The paper brings further an illustrative explanation of why the wall heat flux in the wedge region may reverse its direction.

Survival conditions of a vapour bubble in saturated liquid flowing inside a micro-channel

Abstract The conditions dictated by a vapour bubble to exist in saturated liquid flowing inside a micro-channel are the main subject of the paper. The liquid flow is laminar and fully established at a constant wall heat flux. Under these conditions, an equilibrium bubble is shown to require a heat flux, which sensitively increases with decreasing channel radius. Given a mass flow density, the hydrodynamic forces acting on the bubble generated in a macro-channel may cause its premature detachment thereby shifting formation of “visible” bubbles towards higher heat fluxes in comparison to macro-channels.

Kondensation von Dampfgemischen nicht mischbarer Flüssigkeiten

Im Gegensatz zur Kondensation reiner Dampfe und von Dampfgemischen mischbarer Flussigkeiten ist die in dieser Arbeit betrachtete Kondensation von Dampfgemischen nicht mischbarer oder nur teilweise mischbarer Flussigkeiten immer noch unzureichend erforscht. Die Modelle zur Beschreibung des Warmebergangs bei der letzteren Kondensationsart weichen nicht nur quantitativ, sondern auch qualitativ stark voneinander ab. Ursachlich hierfur sind vor allem die vielfaltigen Stromungsformen des zweiphasigen Kondensats, die sich von reiner Tropfenkondensation bis zur Filmkondensation erstrecken. In der vorliegenden Arbeit werden die Ergebnisse einer Literaturauswertung hinsichtlich der Stromungsformen und des Warmebergangs bei der Kondensation im Bereich der Mischungslucke des Kondensats vorgestellt. Wie sich zeigt, ist das qualitative Wissen auf den erfassten Gebieten zufriedenstellend. Zur Beschreibung des Warmebergangs liegen mehrere Korrelationen vor. Ihre Abweichungen voneinander fuhren aber zu erheblichen Unsicherheiten bei der Wahl der „richtigen“ Beziehung. Eigene Untersuchungen mit Benzol/Wasser-Gemischen an einem senkrechten Rohr haben ergeben, dass unter den Bedingungen unserer Experimente eine von Hoon und Burnside vorgeschlagene Rechenvorschrift zufriedenstellende Ergebnisse liefert. Condensation of Vapour Mixtures of Immiscible Liquids In contrast to the condensation of pure vapours and vapour mixtures of miscible liquids, the condensation of vapour mixtures forming immiscible or only partly miscible condensates is still poorly understood. Models describing heat transfer in the latter kind of condensation deviate so much from one another, not only quantitatively but also qualitatively, that reliable design of condensers is impossible. The reason lies primarily in the variety of flow patterns of two-phase condensates, which range from pure droplet to pure film condensation. This paper surveys literature reports of the flow patterns and heat transfer occurring during condensation of vapour mixtures in the miscibility gap of their liquids. Qualitative knowledge in this condensation area is shown to be satisfactory. Several models have been proposed for heat transfer. However, the heat transfer coefficients obtained from these models differ too much from one another to allow one to choose the “best” correlation. Our own experiments with a eutectic benzene–water mixture on a vertical tube show that the calculation scheme proposed by Hoon and Burnside leads to acceptable results.

Boiling Heat Transfer Enhancement by Controllable Tailoring of the TPL

Novel surface treatment technology was developed in the University of Paderborn in partnership with MiCryon Technik GmbH, Quedlinburg, Germany. The technology allows creation of micro-pin structures, which enhance the boiling heat transfer up to 18 times in comparison with a smooth surface, and provide an independency of the surface superheat on the applied heat flux [1–6]. The micro pins as basic structure elements can be created with the diameters of 0.1 μm to 25 μm at pins density ranging up to 109 pins/cm2 . Such pin structures are created by electro-deposition of metallic ions on the basic surface. The microstructure provides for the very first time in thermal engineering a possibility to adjust the available length of the three phase line (TPL) on demand, correspondingly tailoring the shape of a boiling curve. The TPL, formed by the micro pins piercing the vapor-liquid interface, acts as an extremely efficient heat sink, providing high heat transfer coefficients and the constancy of the wall superheat at heat fluxes up to 125 kW/m2 . Present article delivers the summary on boiling experiments performed with the novel microstructure, and reveals the quantitative dependencies of the heat transfer enhancement rate on the TPL length, having the pressure and the liquid type as further parameters. A newly discovered phenomenon of the vapor bubble chains formation on microstructured surfaces is discussed as well. Experiments were conducted with the refrigerants R134a, R141b and the fluorocarbon liquid FC-3284 at pressures, ranging between 0.5 bar and 9 bar.Copyright

On the Mass and Energy Balances of Two-Phase Systems

Mass and energy balances of a two-phase system undergoing a phase transition are given for different flow arrangements of the phases, taking the system boundary as diabatical. The expressions obtained are illuminated from different points of view stressing the consequences of the corresponding energy balances occasionally adopted in literature.