Measurement of the lamella thickness during droplet impact onto differently wettable smooth surfaces using an extension of the LASER Pattern Shift Method with naturally occurring patterns

This study shows that the LASER Pattern Shift Method (LPSM) is a powerful measurement technique for film thickness measurements. In this paper, the approach of the LPSM is extended, which is now able to measure the lamella thickness during droplet impacts on smooth surfaces using the naturally occurring air bubble shadows. With the help of this rather new measurement technique, the influence of different experimental parameters on the lamella thickness could be systematically assessed, e.g., the influence of impact velocity, liquid properties, and surface wettability. Upon comparing the obtained results to an analytical correlation in the literature, good agreement could be found for its validity range.

Velocity distributions in trapped and mobilized non-wetting phase ganglia in porous media

Understanding the mobilisation of trapped globules of non-wetting phase during two-phase flow has been the aim of numerous studies. However, the driving forces for the mobilisation of the trapped phases are still not well understood. Also, there is little information about what happens within a globule before, at the onset and during mobilization. In this work, we used micro-particle tracking velocimetry in a micro-fluidic model in order to visualise the velocity distributions inside the trapped phase globules prior and during mobilisation. Therefore, time-averaged and instantaneous velocity vectors have been determined using fluorescent microscopy. As a porous medium, we used a polydimethylsiloxane (PDMS) micro-model with a well-defined pore structure, where drainage and imbibition experiments were conducted. Three different geometries of trapped non-wetting globules, namely droplets, blobs and ganglia were investigated. We observed internal circulations inside the trapped phase globules, leading to the formation of vortices. The direction of circulating flow within a globule is dictated by the drag force exerted on it by the flowing wetting phase. This is illustrated by calculating and analyzing the drag force (per unit area) along fluid-fluid interfaces. In the case of droplets and blobs, only one vortex is formed. The flow field within a ganglion is much more complex and more vortices can be formed. The circulation velocities are largest at the fluid-fluid interfaces, along which the wetting phase flows and decreases towards the middle of the globule. The circulation velocities increased proportionally with the increase of wetting phase average velocity (or capillary number). The vortices remain stable as long as the globules are trapped, start to change at the onset of mobilization and disappear during the movement of globules. They reappear when the globules get stranded. Droplets are less prone to mobilization; blobs get mobilised in whole; while ganglia may get ruptured and get mobilised only partially.

Single Acetone Droplets at Supercritical Pressure: Droplet Generation and Characterization of PLIFP

An experimental investigation of isolated acetone droplets under supercritical pressure is presented. An optically accessible pressure chamber was set up for pressures ranging up to 60 bar and temperatures of up to 530 K. A novel method for the generation of acetone droplets on demand in the vicinity of the critical point is presented. High speed shadowgraphy imaging revealed remarkable reproducibility of the droplet generation regarding their size and shape and the timing of the detachment.

The photo-physical properties of acetone in nitrogen as well as nitrogen-oxygen mixtures were investigated using planar laser induced fluorescence and phosphorescence (PLIFP). The measurements revealed a strong sensitivity and selectivity of the phosphorescence-fluorescence-ratio relative to the oxygen concentration. This demonstrates the potential of PLIFP to distinguish between the liquid and gaseous phase of acetone injected into mixtures of nitrogen and oxygen.

Composition measurements of binary mixture droplets by rainbow refractometry

So far, refractive index measurements by rainbow refractometry have been used to determine the temperature of single droplets and ensembles of droplets. Rainbow refractometry is, for the first time, to the best of our knowledge, applied to measure composition histories of evaporating, binary mixture droplets. An evaluation method is presented that makes use of Airy theory and the simultaneous size measurement by Mie scattering imaging. The method further includes an empirical correction function for a certain diameter and refractive index range. The measurement uncertainty was investigated by numerical simulations with Lorenz-Mie theory. For the experiments, an optical levitation setup was used allowing for long measurement periods. Temperature measurements of single-component droplets at different temperature levels are shown to demonstrate the accuracy of rainbow refractometry. Measurements of size and composition histories of binary mixture droplets are presented for two different mixtures. Experimental results show good agreement with numerical results using a rapid-mixing model.

Heat transfer technology for internal passages of air-cooled blades for heavy-duty gas turbines

The present review paper, although far from being complete, aims to give an overview about the present state of the art in the field of heat transfer technology for internal cooling of gas turbine blades. After showing some typical modern cooled blades, the different methods to enhance heat transfer in the internal passages of air-cooled blades are discussed. The complicated flows occurring in bends are described in detail, because of their increasing importance for modern cooling designs. A short review about testing of cooling design elements is given, showing the interaction of the different cooling features as well. The special focus of the present review has been put on the cooling of blades for heavy-duty gas turbines, which show several differences compared to aero-engine blades.

Facial-selective allylation of methyl ketones for the asymmetric synthesis of alpha-substituted tertiary homoallylic ethers

The asymmetric synthesis of enantiomerically pure a-substituted tertiary homoallylic ethers 4a, 11 and 12a-c by the allylation of ethyl methyl ketone (la) with gamma-substituted allylsilanes 9a-h is described. The allylsilanes were obtained by a nickel-catalysed Grignard cross-coupling reaction of (E)- and (Z)-(3-iodoallyl)trimethylsilane with various Grignard reagents. The reaction of the allylsilanes with la in the presence of the trimethylsilyl ether of N-trifluoroacetylnorpseudoephedrine (3), and catalytic amounts of a mixture of trimethylsilyl triflate and trifluoromethanesulfonic acid led to the homoallylic ethers 4a, 11 and 12a-c with two new stereogenic centres, with a selectivity of 1:9 to >20:1 for the homoallylic and of 1:99 to >60:1 for the allylic centre. The facial selectivity does not depend on the configuration of the allylsilane, and in all reactions the anti product is preferentially formed. Interestingly, a pronounced switch of facial selectivity takes place with increasing length of the alkyl group of the allylsilane.

Synthesis of enantiopure homoallylic ethers by reagent controlled facial selective allylation of chiral ketones

The stereoselective allylation of chiral methyl ketones to give tertiary homoallylic ethers, which can easily be transformed into homoallylic alcohols, is described. Reaction of the enantiopure ketones 8a-d and the racemic ketones 26a-d with the norpseudoephedrine derivative 2 or ent-2 and allylsilane in the presence of a catalytic amount of trifluoromethanesulfonic acid, led to a series of homoallylic ethers with good to excellent diastereoselectivity (85:15 to > 97:3). The allylation is reagent controlled and nearly independent from the stereogenic centers in the substrates. A partial kinetic resolution was observed using the racemic ketones 26a-d. In the reaction of the chiral ketones 8a-d with the achiral reagents ethoxytrimethylsilane and allylsilane only a low diastereoselectivity was observed.