Edgar Hansjosten

Numerical investigation of interfacial mass transfer in two phase flows using the VOF method

ABSTRACT A mass transfer model is developed using the volume-of-fluid (VOF) method with a piecewise linear interface calculation (PLIC) scheme in ANSYS FLUENT for a free-rising bubble. The mass flow rate is defined via the interface by Ficks law and added into the species equation as a source term in the liquid phase using the user-defined functions (UDFs) in ANSYS FLUENT. The interfacial concentration field for the mass flow rate is discretized by two numerical methods. One of them is based on the calculation of the discretization length between the centroid of the liquid volume and the interface using the liquid void fraction and interface normal vectors at the interface cells, while in the second method the discretization length is approximated using only the liquid void fraction at the interface cells. The influence of mesh size, schemes, and different Schmidt numbers on the mass transfer mechanism is numerically investigated for a free-rising bubble. Comparison of the developed mass transfer model with the theoretical results shows reasonable and consistent results with a smaller time-step size and with cell size.

Microstructure devices generation by selective laser melting

Selective Laser Melting (SLM) is a generative manufacturing procedure mainly known for the application with metal powders. From these, metallic structures are produced in a layer-by-layer way. This layer-related procedure is comparable to the stereolithographic manufacturing of polymer devices. On a base plate, a thin layer of metal powder is spread. The powder is locally completely melted by the application of a focused laser beam. The base plate is then lowered by a value defined by the thickness of the metal layer, metal powder is spread again, and the local melting process is re-initiated. The complete procedure is continued as described, until the device is manufactured in the defined way. Commercially available metal powder can be used as base material. In principle, the SLM process should be suitable for the generation of metallic microstructures. The main precondition for the generation of microstructures by SLM is that the spatial resolution of the laser focus is small and precise enough to generate microstructure walls of around 100μm thickness in a reproducible way by melting metal powder. The walls should be gas- and leak-tight. In this publication, experimental results of the generation of metallic microstructure devices by SLM will be given. The process will be described in details. Process parameters for the generation of stainless steel devices having wall thicknesses in the range of about 100μm will be given. Examples for microstructure devices made by SLM will be shown. The devices can be manufactured in a reproducible way. Moreover, very first preliminary results on the use of ceramic powder as base material will be presented.

Micro Heat Changers and Surface-Micro-Coolers for High Heat Flux

This publication describes the development of a new microstructure to transfer high heat fluxes. With a simple mathematical model based on heat conduction theory for the heat transfer in a micro channel at laminar flow conditions it was deduced that for the transmission of high heat fluxes only the initial part at the beginning of the micro channels is of importance, i.e. the micro channels should be short. Based on this principle a micro structure was designed with a large number of short micro channels taken in parallel. With this newly developed microstructure a prototype of a micro heat exchanger and a surface micro cooler was manufactured and tested. Using the prototype of the micro heat exchanger, manufactured of plastic, heat fluxes up to 500 W/cm2 were achieved at a pressure loss of 0.16 MPa and a mass flow of the water of 200 kg/h per passage. Due to the use of materials with a higher temperature resistance and higher stability like aluminum or ceramic, higher water throughputs and higher flow velocities could be realized in the micro channels. Thus it was possible to increase the heat flux up to approx. 800 W/cm2 at a pressure loss of approx. 0.35 MPa and a mass flow of 350 kg/h per passage. The important focus of investigation of the surface micro cooler was set on the examination of the surface temperatures for different heat fluxes and different velocities of the water in the micro channels. The experimental results of these surface micro coolers are summarized to characteristic maps. With this characteristic maps it is possible to determine whether a micro surface cooler can be used for a specific application.Copyright

Development on Manufacturing Process for Integrating Glass Plates With Microchannel Walls Made by Micro Stereolithography

Micro stereolithography is frequently used to manufacture functional models of microstructured devices and components. However, the products polymerized directly from the micro stereolithography process can significantly distort the imaging wavefront and introduce substantial error into qualitative and quantitative interpretations. The present work focuses on the improvement of optical properties of the product. An open micro channel device was specially designed and the correspondent manufacturing process was also developed. Efforts were mainly made regarding the integration of glass material into the micro structures manufactured by micro stereolithography. Different from the normal micro stereolithography process, a junction between the glass material and polymer micro structures was introduced. The manufactured sample channel was cut and the cut section was imaged by scanning electron microscope (SEM). The deviation on channel dimension and the channel quality in the junction area were studied in this work.Copyright

Fabrication of Microchannels by Stereolithography for Optical Use

In this paper the feasibility of obtaining optical access for micro devices made by stereolithography is investigated, to meet demands set by microfluidic research efforts. A commercially available resin SL5530 was selected for the UV polymerisation process. By optimising the design of the CAD models and the fabrication process, the suitability of the final product was improved for optical metrology. Transparent T-shaped microchannels were manufactured for future application in gas mixing study on interferometric system. Fringes patterns were first obtained from interferometric experiments and then the signals were extracted from the fringes pattern to analyse its suitability.© 2011 ASME