Stefan Maikowske

A Microstructure Device for Single Phase Surface Cooling

A microstructure device for cooling of hot surfaces at liquid single phase laminar flow is presented. The initial design as well as the theoretical background is described in detail. It consists of numerous short micro channels acting as overflow structures and providing a relatively large hydraulic diameter, used in parallel between large inlet and outlet channels. The design was chosen to be scalable as well as appropriate for mass production in different materials. The fluid distribution was optimized as well as the dimensions of the overflow structures in terms of heat transfer, both by CFD simulations. Several devices were tested. They provide very high heat flux at reasonably low pressure drop. The temperature difference to achieve, heat flux and pressure drop can be adjusted easily by control of the applied mass flow. The design was tested as liquid-liquid heat exchanger in a simple lab-scale test facility. Moreover, using a copper electrically powered surface heat focus, some devices were tested as surface coolers.Copyright

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.

High power density LED modules with silver sintering die attach on aluminum nitride substrates

Current research studies deal with the investigation of the thermal and optical properties of four LED modules on different substrate materials. The LED modules consist of arrays of 98 blue emitting LEDs with an emission wavelength of 457 nm within an area of 2.11 cm2. The modules are based on aluminum oxide or aluminum nitride substrates and the LED chips are attached by using a soldering or a pressureless silver sintering process. The modules are mounted on a high performance microstructured heat exchanger. By using the water driven cooler a maximum optical power density of 106.2 W/cm2 at a forward current of 2100 mA and 837.5 W electrical input power is achieved. A saturation of the optical power density over the input current due to thermal degradation is not observed.

Very high power density LED modules on aluminum substrates with embedded water cooling

We present optical measurements of an LED module consisting of 98 UV LEDs with an emission wavelength of 395 nm soldered onto a ceramic substrate within an area of 211 mm2. The module is mounted to a high performance mi-crostructured water cooler. This cooler enables a maximum optical power density of 45.9 W/cm2 at a forward current of 1350 mA and 447.9 W electrical input power. Further we describe the development of an LED module based on an aluminum substrate with thick film printed insulator and conductor layers and embedded, meander shaped water cooling channels. Numerical and experimental studies with different channel cross-sections are shown. Finally experimental results for this kind of UV LED module with 98 LED chips are presented and compared to the ceramic based module.

Pre-Calculation of Evaporation of Water in Parallel Microchannels Using Measurable Fluid Inlet Conditions

Evaporation of water in parallel microchannels is examined in the current research study. A simple model approach, based on thermodynamic considerations, enables the prediction in terms of a pre-calculation of a possible full evaporation of water in arrays of parallel microchannels. The model contains easily measureable values of the liquid fluid at the channel inlets such as temperature, pressure and mass flow. The model is verified using water with different microchannel designs and different microstructured inlet distribution geometries. A pressure range of up to ∼106 Pa with liquid fluid flow inlet velocities up to ∼0.8 m · s−1 is examined. Additionally, the model is verified using different heat fluxes and different phase transition lengths, the length of two-phase flow from bubble generation to total evaporation, in the microchannels. An empirical correction function is introduced to fit the idealized model with good agreement of about 5% to the real condition of the measurements and the process conditions. The adjusted model enables the pre-calculation of a possible total evaporation of water in the microchannel arrays by calculating the maximum liquid flow velocity at the channel inlets to achieve saturated steam inside the microchannels.Copyright

Efficient Heat Transfer by Phase Transition in Microstructured Devices

Devices with microchannels or similar structures with dimensions in the range of a few 100 micrometers, so-called microstructured devices, have become a powerful tool in modern process engineering for transferring huge amounts of thermal energy. The high internal surface of these devices, caused by small characteristic channel dimensions, lead to very high specific heat transfer rates. Additional increase of these high heat transfer capabilities is enabled by taking advantage of the latent heat of evaporation. During fundamental research activities phase transition and accompanying phenomena in arrays out of straight microchannels as well as novel microstructures were investigated to obtain new and additional information about these processes. A novel microstructure which is based on a new innovative design away from straight channels is able to enhance evaporation. This design, based on semicircular and semi-elliptical microstructures, leads to mixing effects as well as flow acceleration by pressure release effects including increased heat transfer properties. This novel microstructure reaches highly enhanced evaporation performance compared to linear microchannels.Copyright

Optical Studies of Evaporation in Microchannel Arrays

Phase transition in microchannels has become an interesting filed of research within the last few years. Using arrays out of a multitude of parallel microchannels, it is possible to transfer a huge amount of thermal energy by taking advantage of the latent heat of evaporation. Another point of interest concerning this research field is the stable generation of steady vapor with homogeneous parameters such as vapor quality, mass flow, pressure or temperature. Phase transition and accompanying phenomena during evaporation of water in microchannel arrays as well as influences of microstructure geometry were observed during these research studies. Optical investigations have been done using a digital high-speed camera for visualization of transient processes, e.g. explosively and confined bubble growing behavior or phase transition fluctuation. Beside this, a novel test device for optical investigation is presented in this report. The test device enables to vary several variables like temperature, microstructure or pressure drop, to name but a few. Furthermore, results and influences of different microstructure geometries on phase transition as well as different shapes of phase transition fronts in microchannel arrays are presented. Additionally, the visualization of complete and stable phase transition in microchannel arrays with steam superheating is shown.Copyright