Experimental Investigation on Surface Particle Interactions During Pool Boiling of Nanofluids

The pool boiling characteristics of nanofluids is affected by the interaction between the nanoparticles and the heater surface which forms a sorption layer and this layer increases the surface wettability and thereby enhances the CHF. While deteriorated nucleate boiling has been attributed to the decreased activation of cavities due to the increased wettability, it fails to explain the enhanced performance observed by several researchers, which can be explained only by an increase in surface roughness and hence a direct increase in the number of cavities, thereby compensating for the increase in wettability. Attempts to characterize the roughness of heater surfaces have been restricted to magnified visualizations and intrusive probing. No non-intrusive tests have been reported on flat heaters, which are ideal to conduct surface analyses. The present work is aimed at conducting a non-intrusive experimental study to analyse the surface roughness modification due to the sorption layer on flat plate heaters. Experiments have been carried out using electro-stabilized aluminium oxide water based nanofluids of different concentrations with heaters having varying values of surface roughness. The burn-out heat flux was measured and the effect of sedimentation time was studied. The surface-particle interaction parameter (Ra/dp) was varied to capture the phenomena of plugging as well as splitting of nucleation sites. An experiment having a high value of the interaction parameter shows enhanced boiling performance and that with a value close to 1 shows deteriorated performance. Further it was seen that this behaviour is dependent on the particle concentration. Detailed surface characterization has been done using an optical measurements setup and atomic force microscopy. Boiling on nano-coated heaters has been investigated and presented as an effective solution to counter the disadvantageous transient boiling behavior of nanofluids.

Numerical Investigation of the Combined Effects of Biomolecular Adsorption and Microdroplet Evaporation on the Performance of the Electrocapillary-Based Digital Microfluidic Systems

In this article, microdroplet motion in the electrocapillary-based digital microfluidic systems is modeled accurately, and the combined effects of the biomolecular adsorption and micro-droplet evaporation on the performance of the device are investigated. An electrohydrodynamic approach is used to model the driving and resisting forces, and Fick’s law and Gibbs equation are used to calculate the microdroplet evaporation and adsorption rate. Effects of the adsorption and evaporation rates are then implemented into the microdroplet dynamics by adding new terms into the force balance equation. It is shown that mass loss due to the evaporation tends to increase the protein concentration, and on the other hand, the increased concentration due to the mass loss increases the biomolecular adsorption rate which has a reverse effect on the concentration. The modeling results indicate that evaporation and adsorption play crucial roles in the microdroplet dynamics.

Two-Phase Heat Sinks With Microporous Coating

To minimize flow boiling instabilities in two-phase heat sinks, two different types of microporous coatings were developed and applied on mini- and small-channel heat sinks and tested using degassed R245fa refrigerant. The first coating was epoxy-based and was sprayed on heat sink channels while the second coating was formed by sintering copper particles on heat sink channels. Mini-channel heat sinks had overall dimensions 25.4 mm × 25.4 mm × 6.4 mm and twelve rectangular channels with a hydraulic diameter 1.7 mm and a channel aspect ratio of 2.7. Small-channel heat sinks had the same overall dimensions, but only three rectangular channels with hydraulic diameter 4.1 mm and channel aspect ratio 0.6. The microporous coatings were found to minimize parallel channel instabilities for mini-channel heat sinks and to reduce the amplitude of heat sink base temperature oscillations from 6 °C to slightly more than 1 °C. No increase in pressure drop or pumping power due to the microporous coating was measured. The mini-channel heat sinks with porous coating had in average 1.5-times higher heat transfer coefficient than uncoated heat sinks. Also, the small-channel heat sinks with the “best” porous coating had in average 2.5-times higher heat transfer coefficient and the critical heat flux was 1.5 to 2-times higher compared with the uncoated heat sinks.

Optimizing Energy Output of Piezoelectric Microcantilevers

This work presents some new methods in optimizing electrical energy, harvested using a micro piezoelectric cantilever. Both mechanical and electrical aspects have been considered. Mechanically, two items have been considered to maximize the generated voltage: geometry of the cantilever and placement of the electrodes. It has been shown that for given sizes of length and width of the harvester and for a given natural frequency, the output voltage can be increased by adjusting the thickness of the beam and the proof mass and consequently increasing the amplitude of vibration. As well, the placement of the electrodes plays a very important role in optimizing output voltage. It has also been shown that piezoelectric cantilevers with shorter top electrodes induce higher voltage than cantilevers with longer top electrodes. Overall results agree with the analytical equations reported in literature so far. Moreover, distribution of top electrodes along the width of the cantilever has been taken into consideration. It has been shown how output voltage can be approximately doubled by using two narrower top electrodes along the width of the cantilever. All analysis in this work was carried out in ANSYS. In this research, to improve the electrical efficiency, diodes have been considered in the circuit to reduce electrical losses in comparison to rectifiers which have been used in conventional harvesters. Applying these methods to particular test cases, a 71% increase in output voltage was observed for the case of geometry optimization, a 116% increase was observed for the case of shortening the top electrode and losses in the electrical circuit were reduced by approximately 50% by using diodes comparing to using rectifiers. While these results focused on cantilever based harvesters, the ideas contained are equally applicable to other structures.

A Micro-Sized Generic Demonstrator for Studying Synthetic Biological Processes

In the field of synthetic biology, enzymatic pathways are used to enhance the efficiency of chemical production processes. These pathways consist of micro reactors that are filled with porous media and enzymes that catalyze the partial reactions. In the present study we summarize the requirements of the micro reactors used for these applications and give an overview of the different problems that have to be solved. Furthermore we present the idea of a generic demonstrator as a research tool. In the interdisciplinary field of synthetic biology it can enhance the interaction between different groups. It will serve as a common base for numerically as well as for experimentally working groups. We propose a definition of a generic demonstrator for a micro fluidic reactor as it is typically used in this field. Besides, present work of our group is presented, that analyzes the flow field in the micro channels of the reaction zone that is filled with porous media. We show how computational costs can be saved by studying the entry lengths of these channels.

Homogeneous Oxidation of Hydrogen in Catalytic Mini/Microchannels

Gas phase reaction effects in the catalytic oxidation of hydrogen on platinum-coated minichannels and microchannels are investigated numerically in planar geometry. The main objective of this work is to identify the relative importance of the gas phase and surface reactions under different operating conditions. A collocated finite-volume method is used to solve the governing equations. Detailed gas phase and surface reaction mechanisms along with a multi-component diffusion model are used. As the channel size is reduced, heat and radical losses to the walls can significantly alter the combustion behavior. While catalytic walls help in sustaining the gas phase reactions at very small length scales by reducing the heat losses to the walls owing to heat release associated with the surface reactions, they may inhibit homogeneous reactions by extracting radicals due to typically high absorption rates of such species at the walls. Thus, the radical chain mechanisms can be significantly altered by the presence of wall reactions, and the build-up of radical pools in the gas phase, which lead to homogeneous ignition, can be suppressed as a consequence. In the present study, the effects of two key parameters, i.e. channel height and the inlet mass flux on the interaction of gas phase and surface reactions will be explored. In each case, the limiting values beyond which the gas-phase reactions become relatively negligible compared to surface reactions will be identified for hydrogen/air mixtures.

Decision of the Best Blend Proportional Factor Value of Two Staggered Grids Used in LBM for PEFC Simulation

The appropriate blend proportional factor value which combines two kinds of staggered grids used in Lattice Boltzmann Method (LBM) for simulating the multiphase flow phenomena with large density ratio in the Polymer electrolyte fuel cell (PEFC) is fixed on. The shape deformation of the water droplet is found when using the two kinds of staggered grids to prevent the pressure oscillation when solving the Poisson equation of this LBM model and the shape of the water droplet varies with the changes of the blend proportional factor values. Two methods are adopted to find out the two staggered grids’ appropriate blend proportional factor value that can diminish or minimize the deformation of the droplet. The first one is to compare the simulation results of different blend proportional factors with the theoretical value and find the one mostly approaches the theoretical value; the second one is to compare the current velocity divergences of the two staggered grids using the results calculated by different blend proportional factor values. A water droplet resting in a tunnel is simulated with different blend proportional factor values and the appropriate value is decided.

Microflow Heat Transfer Effectiveness: Questioning the “Area/Volume-Argument”

The often used argument that heat transfer in micro-sized devices is superior due to the fact that the transfer area scales like L2 but the volume like L3 with L as a characteristic length is critically analyzed for various heat transfer situations. It turns out that for steady state heat transfer cases the thermal boundary layer behavior is more important. In general, dimensional analysis should be applied to understand how the heat transfer performance changes when scales are reduced from macro- to micro-size.

Magnetogasdynamic Flow and Heat Transfer in a Microchannel

The presence of current flow in an electric and magnetic field results in electromagnetic force and joule heating. It is desirable to understand the roles of electromagnetic force and joule heating on gas microflow and heat transfer. In this study, a mathematical model is developed of the pressure-driven gas flow through a long isothermally heated horizontal planar microchannel in the presence of an external electric and magnetic field. The solutions for flow and thermal field and characteristics are derived analytically and presented in terms of dimensionless parameters. It is found that an electromagnetic driving force can be produced by a combined non-zero electric field and a negative magnetic field and results in an additional velocity slip and an additional flow drag. Also, a joule heating can be enhanced by an applied positive magnetic field and therefore results in an additional temperature jump and an additional heat transfer.