A non-intrusive measurement technique based on spontaneous Raman imaging was proposed for investigating microscale flow structures. It has the advantage that it does not require tracer particles or fluorescent dye to measure fluid velocity and scalar quantities. Raman scattering from ions in solution is substance specific and is observed as Raman spectra that contain peaks due to molecular species. The spontaneous Raman intensity from an electrolyte solution strongly depends on the electrolyte concentration. Thus, a bandpass filter attached to an EM-CCD camera detects only the strong Raman scattering at the selected Raman shift. The spontaneous Raman image obtained was converted to an electrolyte concentration distribution by using a calibration curve that expressed the relationship between the Raman intensity and the concentration. The flow velocity in a millichannel was calculated from the peak-value displacement of the time-series concentration distribution.
Evaporation of liquids is of major interest for many topics in process engineering. One of these is chemical process engineering, where evaporation of liquids and generation of superheated steam is mandatory for numerous processes. Generally, this is performed by use of classical pool boiling and evaporation process equipment. Another possibility is creating mixtures of gases and liquids, combined with a heating of this haze. Both methods provide relatively limited performance. Due to the advantages of microstructure devices especially in chemical process engineering  the interest in microstructure evaporators and steam generators have been increased through the last decade. In this publication several microstructure devices used for evaporation and generation of steam as well as superheating will be described. Here, normally electrically powered devices containing micro channels as well as non-channel microstructures are used due to better controllability of the temperature level. Micro channel heat exchangers have been designed, manufactured and tested at the Institute for Micro Process Engineering of the Karlsruhe Institute of Technology for more than 15 years. Starting with the famous Karlsruhe Cube, a cross-flow micro channel heat exchanger of various dimensions, not only conventional heat transfer between liquids or gases have been theoretically and experimentally examined but also phase transition from liquids to gases (evaporation) and condensation of liquids. However, the results obtained with sealed microstructure devices have often been unsatisfying. Thus, to learn more onto the evaporation process itself, an electrically powered device for optical inspection of the microstructures and the processes inside has been designed and manufactured . This was further optimized and improved for better controllability and reliable experiments . Exchangeable metallic micro channel array foils as well as an optical inspection of the evaporation process by high-speed videography have been integrated into the experimental setup. Fundamental research onto the influences of the geometry and dimensions of the integrated micro channels, the inlet flow distribution system geometry as well as the surface quality and surface coatings of the micro channels have been performed. While evaporation of liquids in crossflow and counterflow or co-current flow micro channel devices is possible, it is, in many cases, not possible to obtain superheated steam due to certain boundary conditions . In most cases, the residence time is not sufficiently long, or the evaporation process itself cannot be stabilized and controlled precisely enough. Thus, a new design was proposed to obtain complete evaporation and steam superheating. This microstructure evaporator consists of a concentric arrangement of semi-circular walls or semi-elliptic walls providing at least two nozzles to release the generated steam. The complete arrangement forms a row of circular blanks. An example of such geometry is shown in Figure 7. A maximum power density of 1400 kW·m-2 has been transferred using similar systems, while liquid could be completely evaporated and the generated steam superheated. This is, compared to liquid heat exchanges, a small value, but it has to be taken in account that the specific heat capacity of vapour is considerably smaller than that of liquids. It could also be shown that the arrangement in circular blanks with semi-elliptic side walls acts as a kind of micro mixer for the remaining liquid and generated steam and, therefore, enhances the evaporation.
Droplet evaporation has attracted much interest recently, being relevant to a wide range of biological and technological applications. The underlying mechanisms for this phenomenon are still poorly understood. We report on experimental results, from micro-Particle Image Velocimetry (µPIV), of the spatial and temporal velocity field within pure water and ethanol-water mixture droplets evaporating on a glass substrate. The drop profile, evaporation rate and surface temperature were also measured. For pure water droplets, the redial velocity is found to exhibit a maximum spatially towards the three-phase contact line and to increase dramatically towards the end of the drop lifetime. For ethanol-water droplets, three flow phases of (I) vortical flow, (II) transient flow and (III) radial flow were observed. Phase I has vortices, driven, we believe, by concentration differences arising during the preferential evaporation of ethanol. Phase II sees an exponential decay in vorticity with remaining vortices migrating towards the contact line, accompanied by the formation of one large toroidal vortex, possibly due to ethanol depletion at the apex of the drop leading to a surface tension instability. Phase III is characterized by radial flow towards the contact line, matching the evaporative flux and identical to the flow measured for pure water.
By just applying a temperature difference to a micro-system filled with rarefied gas, it is possible to engender a displacement or a compression of the gas in the temperature gradient direction. This is the Thermal Transpiration phenomenon. In the present work, thermal transpiration has been studied both through an experimental approach, which exploits an original measuring system, and through a numerical approach, which is modeled on the basis of the Shakhov model kinetic equation. In both studies, a circular cross section glass micro-tube is submitted to a temperature gradient. The obtained results for Helium, such as the thermal molecular pressure difference, the thermal molecular pressure ratio and the thermal molecular pressure exponent at the final zero-flow stage, are analyzed in the case of a tube submitted to a temperature difference of 51 K. Finally, the obtained thermal molecular pressure ratio results are also compared to the semi-empirical formulas of Liang (1951) and Takaishi and Sensui (1963). These semi-empirical formulas are still in use nowadays to introduce correction factors for pressure measurements done when the pressure gauge functions at different temperatures in respect to the temperature of the operating gas. For the here working pressure conditions and the used tube dimensions the gas rarefaction conditions go from near free molecular to slip regime.
In recent film boiling heat transfer studies with nanofluids, it was reported that deposition of nanoparticles on a surface significantly increases the nominal minimum heat flux (MHF) or Leidenfrost Point (LFP) temperature, considerably accelerating the transient cooling of overheated objects. It was suggested that the thin nanoparticle deposition layer and the resulting changes in the physico-chemical characteristics of the hot surface, such as surface roughness height, wettability and porosity, could greatly affect quenching phenomena. In this study, a set of water-droplet LFP tests are conducted using custom-fabricated surfaces which systemically separate the effects of surface roughness height (0-15 um), wettability (0-83°) and nanoporosity (∼23 nm). In addition, high-speed imaging of the evaporating droplets is used to explore the influence of these surface characteristics on the intermittent solid-liquid contacts in film boiling. The obtained results reveal that nanoporosity (not solely high surface wettability) is the crucial feature in efficiently increasing the LFP temperature by initiating heterogeneous nucleation of bubbles during short-lived solid-liquid contacts, which results in disruption of the vapor film, and that micro-posts on the surface intensify such effects by promoting intermittent liquid-surface contacts.
In this study, high-speed digital interferometry was used to measure heat transfer from the liquid phase to an isolated boiling bubble on a MEMS boiling sensor. The interferometric measurement results indicated variations in the macroscopic thermal field around the isolated boiling bubble, such as development of a superheated liquid layer on the heating wall, swelling of the superheated liquid layer in the bubble growth process, hot wake accompanied by a rising bubble, and thermal boundary layer around the bubble indicating condensation in subcooled boiling. However, the interferometry could not detect the positive temperature gradient driving the evaporation near the liquid-vapor interface during the bubble growth process, because the spatial resolution of about thirty microns was insufficient. The thickness of the boundary layer driving the evaporation was estimated to be a few dozen microns by a two-dimensional heat transfer simulation with the experimental results as calculation conditions. Finally, an improvement plan of the high-speed interferometer based on the result of the heat transfer analysis was presented.
This paper reports a particle accumulation driven by alternating-current electroosmosis (ACEO) in a microfluidic device with co-planar electrode. Accumulation processes of particles in single- and double-gap electrode device were investigated. The flow field of ACEO and flow-induced particle accumulation process were measured by the micron-resolution particle tracking velocimetry and fluorescent intensity analysis, respectively. Particles in a solution are concentrated gradually from an electrode edge close to gap at the entrance and converged into a certain location downstream. Contribution of ACEO to particle transportation and eventual accumulation was discussed, and dependences of experimental parameters on the accumulating position were evaluated as well. The particle concentration behavior can be classified into two types; one has similar accumulating characteristics in both gap patterns, the other is the case in which particles are concentrated at the center span of the channel. Consequently, it is indicated from the results in this study that an estimation of the particle concentration is possible in a device with more complicated electrode geometry based on that in the single-gap device. The particle focusing method by ACEO can contribute to an improvement of detection sensitivity in the microfluidic system.
We conduct an experimental study focusing on the spreading droplet in the vicinity of the boundary line of solid-liquid-gas interface, which is called macroscopic contact line (M-CL). When the droplet spreads on the solid surface completely, a very thin film whose thickness is of a few nm is formed ahead of the M-CL. This thin film is so-called precursor film. We pay our attention to the spatio-temporal growth of the precursor film in terms of its thickness and length at an early stage of the droplet spreading. The target system is a tiny droplet of 2-cSt and 5-cSt silicone oil spreading on the glass substrate. We apply confocal laser displacement sensor to measure the temporal variation of the precursor film thickness at a designated point, and the Brewster angle microscope to detect the precursor film ahead of the M-CL and to evaluate its existing length. We show the effects of the liquid viscosity on the development of precursor film through the results of both its length and thickness.
Edited and published by : The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Produced and listed by : Showa Joho Process Co., Ltd.(Vol.8 No.3-) Sanbi Printing Co., Ltd.(Vol.1 No.1-Vol.8 No.2)