Juergen J. Brandner

Heat Transfer in Microchannels—2012 Status and Research Needs

Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.

Microstructure Heat Exchanger Applications in Laboratory and Industry

In this article, heat transfer in microstructure devices and its application in laboratory and industry will be described. Basic principles of microstructure heat exchangers made of metal, ceramics, and polymers will be presented. A variety of laboratory prototype applications will be shown, as well as some examples for industrial use of not only microstructure heat exchangers, but also microstructure devices as chemical reactors. A brief outlook will describe possible future application fields.

Low-Frequency Instabilities in the Operation of Metallic Multi-Microchannel Evaporators

Visualization by high-speed videography and infrared surface thermography was used to compare the spatial and temporal maldistribution of flow, manifesting itself in pulsation and hot spot formation, respectively, in water evaporators consisting of either a single metallic foil with an array of mechanically micromachined microchannels or of several such foils assembled into an electrically powered micro heat exchanger. In the single layer devices examined by high-speed videography, pulsation in the frequency range below 20 Hz was found to be dominated by the generation of large bubbles in the inlet plenum. A redesign of the inlet with microchannels instead of a large plenum eliminated the pulsation at sub-audio frequencies, at the expense of a significantly increased pressure drop across the device. Infrared thermography of an electrically powered micro heat exchanger operated as an evaporator showed the formation of metastable hot spots as the result of maldistribution among different microchannel array layers. The formation of these hot spots could be eliminated by operating the device under heater cartridge temperature control conditions instead of constant power conditions.

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.

Hydraulic and thermal design of a gas microchannel heat exchanger

In this paper investigations on the design of a gas flow microchannel heat exchanger are described in terms of hydrodynamic and thermal aspects. The optimal choice for thermal conductivity of the solid material is discussed by analysis of its influences on the thermal performance of a micro heat exchanger. Two numerical models are built by means of a commercial CFD code (Fluent). The simulation results provide the distribution of mass flow rate, inlet pressure and pressure loss, outlet pressure and pressure loss, subjected to various feeding pressure values. Based on the thermal and hydrodynamic analysis, a micro heat exchanger made of polymer (PEEK) is designed and manufactured for flow and heat transfer measurements in air flows. Sensors are integrated into the micro heat exchanger in order to measure the local pressure and temperature in an accurate way. Finally, combined with numerical simulation, an operating range is suggested for the present micro heat exchanger in order to guarantee uniform flow distribution and best thermal and hydraulic performances.

Velocity field measurements in gas phase internal flows by molecular tagging velocimetry

The goal of the present work is to implement Molecular Tagging Velocimetry (MTV) for the analysis of internal gas flows in mini-channels. A MTV experimental setup has been designed. Tagging and detecting steps are respectively insured by a UV laser and a CCD camera coupled to intensified relay optics. A specific channel with 1 × 5 mm2 rectangular cross section has been designed and equipped with integrated temperature sensors along its 20 cm length. It has been manufactured in PEEK (PolyEtherEther-Ketone) and Suprasil® optical windows have been integrated for the tagging access. Image processing allows extraction of velocity profiles for a pressure driven steady flow of argon through this channel. These profiles are compared to the theoretical profiles of laminar flows and the accuracy of the method is discussed. The MTV potential for the analysis of internal gaseous flows is commented on, with a discussion on perspectives for velocity measurement in rarefied flows and direct access to slip velocity at the walls.

Fabrication and Testing of Microstructure Heat Exchangers for Thermal Applications

For a couple of years, microstructure heat exchangers have been numerically simulated, designed, manufactured and tested at the Institute for Micro Process Engineering of the Forschungszentrum Karlsruhe. The microstructure heat exchangers consist of single metal foils which are connected by a diffusion bonding process to form a nearly monolithic body. The number of integrated microchannels is in the order of several hundreds to several thousands. This internal numbering-up leads to a sufficiently low pressure drop. The devices have an extremely high heat transfer to volume ratio of about 30,000 m2 · m−3 which makes it possible to transfer thermal power in the range of several kilowatt within a volume of some centimeters cubed only.Copyright

Design and Experimental Investigation of a Gas-to-Gas Counter-Flow Micro Heat Exchanger

In this work, a double-layered microchannel heat exchanger is designed for investigation on gas-to-gas heat transfer. The micro-device contains 133 parallel microchannels machined into a polished polyether ether ketone plate for both the hot side and cold side. The microchannels are 200 μm high, 200 μm wide, and 39.8 mm long. The design of the micro-device allows tests with partition foils in different materials and of flexible thickness. A test rig is developed with the integration of customized pressure and temperature sensors for in situ measurements. Experimental tests on the counter-flow micro heat exchanger have been carried out for five different partition foils and various mass flow rates. The experimental results, in terms of pressure drop, heat transfer coefficients, and heat exchanger effectiveness are discussed and compared with the predictions of the classic theory for conventionally sized heat exchangers.

Comparison of Crossflow Micro Heat Exchangers With Different Microstructure Designs

Microstructure heat exchanger are well known for their superior heat transfer capabilities and the good temperature management for applications. Most microstructure heat exchangers presented so far consisted of a number of microchannels in a defined arrangement to provide the transfer of thermal power from one fluid to another, mostly in laminar flow regime. In this publication, several crossflow microstructure heat exchangers are presented. The devices design ranges from simple linear microchannels of different dimensions and shape, complex micro column arrangements up to three dimensional flow structures like crossed sinusoidally shaped microchannels, a kind of split-and-recombine structure. By comparing experimental results of the most interesting devices, microstructure devices with good performance can be chosen.Copyright

Microstructure devices for efficient heat transfer

Microstructure devices provide unique properties with regard to heat and mass transfer. Due to the tremendously high surface-to-volume ratio they are very well suited for many thermal and chemical processes in which large amount of heat has to be transferred. Metal microstructure devices also provide very high stability against high pressure, combined with an adjustable mass flow range of up to several thousand kg of liquid per hour and per passage, depending on the size and number of the integrated microstructures. Aside of fluid driven metallic microstructure devices like the famous Karlsruhe Cube, electrically powered devices have been developed and applied for temperature ranges where thermoliquids reach their limits or the use of gases may be disadvantageous due to their high viscosity and the arising pressure drop. In this publication several microstructure devices for heating and evaporation of fluids as well as for chemical reactions are presented in overview style. Details on manufacturing and device properties are given. Some process examples and experimental data for different types of microstructure devices are shown. Fouling problems are discussed briefly by an example.