Jürgen Brandner

Thermal improvements for high power UV LED clusters

We present a high power density UV LED module for a wavelength of 395 nm with an optical power density of 27.3 W/cm2. The module consists of 98 densely packed LED chips soldered onto an Al2O3 ceramic board. Thermal and optical measurements were conducted. The module was cooled by a forced air heat sink for the characterization experiments. A room temperature liquid gallium alloy was used as thermal interface material between substrate and heat sink with a TiN barrier layer to prevent corrosion. Further a technology to replace Al2O3 ceramics by aluminum as substrate is presented. An initial characterization shows that aluminum with a dielectric layer for electrical insulation is a competitive substrate material for efficient heat removal.

High power UV-LED-clusters on ceramic substrates

We present a high power density UV-LED module for a wavelength of 395 nm with an optical power density of 13.1 W/cm2. The module consists of 98 densely packed LED chips adhesively bonded to an Al2O3-ceramic board. Thermal simulations and measurements as well as optical measurements were conducted. The module was cooled by a forced air heat sink for the characterization experiments. A surface micro cooler with water as a coolant is proposed to improve thermal performance of the module. To drive the LED module, we developed an efficient current source powered directly from AC mains supply with integrated power factor correction using a single switching component.

Development of a continuous emulsification process for a highly viscous dispersed phase using microstructured devices

Abstract A highly viscous oil was used to create oil-in-water emulsions. Rheological properties of the dispersed phase, the emulsifier, and their combination were measured to study the flow behavior. The temperature dependence of the viscosity and the influence of different mixing orders on emulsion quality were examined. Furthermore, the stability of the produced emulsions was studied. Finally, the initial emulsification process was analyzed and optimizations were suggested.

Index matched fluidic packaging of high power UV LED clusters on aluminum substrates for improved optical output power

We present an improved cooling for a high power density UV LED module for a wavelength of 395 nm. The module consists of 98 LED chips soldered on a thick film printed alumina substrate on an area of 2.11 cm2. We investigated cooling by a commercial water cooler as well as by a surface micro cooler developed by our own. Further we describe a technology to replace alumina by aluminum as substrate material. A module consisting of 25 UV LEDs was optically characterized without and with liquid encapsulation. Finally we conducted numerical studies to develop an easily producible, sufficiently powerful, and robust water cooler. Based on the results we present a water cooler design with cooling channels embedded in the aluminum substrate of an LED module, removing the interface between LED substrate and heat sink.

Quantitative measurement of gas pressure drop along T-shaped micro channels by interferometry

The study of gas flows in microchannels has received considerably more attention in the literature from a simulation perspective than an experimental. The majority of the experimental work has emphasis on the global measurements at the inlet or exit of the microchannel instead locally along it. In this paper some efforts were made to measure the pressure drop along T-shaped micro channel by using interferometry. The two side channels were served as gas entrances and they were both open to air and the channel outlet was being vacuumed during experiments. A Mach-Zehnder interference microscopy was built for the measurement of gas pressure drop along the mixing channel. Some points along the mixing channel were selected for interferometric measurements. Simulations were first developed in unsteady condition by using Ansys Fluent to verify the nonexistence of transient phenomena of gas flow in the defined condition and then run again in steady condition to get the theoretical pressure drop that was would be used for comparison with experimental results.

Micro-structured heat exchanger for cryogenic mixed refrigerant cycles

Mixed refrigerant cycles (MRCs) offer a cost- and energy-efficient cooling method for the temperature range between 80 and 200 K. The performance of MRCs is strongly influenced by entropy production in the main heat exchanger. High efficiencies thus require small temperature gradients among the fluid streams, as well as limited pressure drop and axial conduction. As temperature gradients scale with heat flux, large heat transfer areas are necessary. This is best achieved with micro-structured heat exchangers, where high volumetric heat transfer areas can be realized. The reliable design of MRC heat exchangers is challenging, since two-phase heat transfer and pressure drop in both fluid streams have to be considered simultaneously. Furthermore, only few data on the convective boiling and condensation kinetics of zeotropic mixtures is available in literature. This paper presents a micro-structured heat exchanger designed with a newly developed numerical model, followed by experimental results on the single-phase pressure drop and their implications on the hydraulic diameter.