Convective heat transfer to non-newtonian fluids

In this thesis, the perturbation method was implemented to analytically solve the governing equations relevant to both hydrodynamically and thermally fully developed power-law fluid and plug flows through parallel-plates and circular microchannels under constant isoflux thermal and slip boundary condition. The temperature-dependent properties, being viscosity and thermal conductivity, were considered along with nonlinear slip condition in the analysis in addition to viscous dissipation. The velocity, temperature and constant property Nusselt number closed form expressions were derived and then the Nusselt number corresponding to temperature-dependent thermophysical properties was numerically obtained due to their complexity nature. Numerical simulations were also performed for verifying the analytical results. The results indicated that the property variations and slip condition significantly affected thermo-fluid characteristics. The second law analysis was further performed for both constant and variable properties. Furthermore, an experimental study was performed on nucleate pool boiling of polymeric solutions (aqueous Xanthan gum solutions) by the dissolution of Xanthan gum powder in different amounts into deionized water. Their advantage over new generation fluids such as nanofluids is that they have no side effects such as agglomeration and sedimentation of particles, which is common for nanofluids. The results revealed that heat transfer coefficients of prepared polymeric solutions were lower than those of pure water, while concentration played a significant role in the performance of the heat transfer. In visualization studies, different pool boiling patterns were recorded particularly for high concentrations, which bolsters the heat transfer results

Droplet Production and Handling in Microchannels Using Electric Fields

Droplet-based protocols in microfluidic devices have found numerous applications in such different areas as bioanalytics, chemical synthesis, drug delivery etc. Droplets can either be produced in a continuous stream or on-demand. Employing an active technique via applying external sources such as temperature, acoustic, magnetic or electric fields, potentially in combination with a passive technique, could enhance the utility and controllability of droplet generation. Among these approaches, probably the most versatile and flexible one is based on the application of electric fields, because electric actuation tends to be faster and requires less complex components than mechanical actuation. This thesis addresses electrically manipulation of droplets inside microchannels generated both on-stream and on-demand along with some particular applications such as using the droplets as biological reaction compartments or as carriers to transfer tiny amounts of dissolved species. For the on-stream case, the effect of DC electric fields on the generation of droplets of water and xanthan gum solutions in sunflower oil at a microfluidic T-junction is experimentally studied. The electric field leads to a significant reduction of the droplet diameter, by about a factor of 2 in the case of water droplets. The droplet size is tuned by varying the electric field strength, an effect that can be employed to produce a stream of droplets with a tailor-made size sequence. Compared to the case of purely hydrodynamic droplet production without electric fields, the electric control has about the same effect on the droplet size if the electric stress at the liquid/liquid interface is the same as the hydrodynamic stress. The focus of the thesis, however, is the manipulation of droplets generated on-demand via electric fields. In the first scenario for droplets being utilized in the context of artificial genetic circuits in biological systems as outlined by the LOEWE CompuGene project (managed by TU Darmstadt), a method is presented allowing to produce monodisperse droplets with volumes in the femtoliter range in a microchannel on demand. The method utilizes pulsed electric fields deforming the interface between an aqueous and an oil phase and pinching off droplets. Water and xanthan gum solutions are considered as disperse-phase liquids, and it is shown that the method can be applied even to solutions with a zero-shear rate viscosity more than 104-times higher than that of water. The droplet formation regimes are explored by systematically varying the pulse amplitude and duration as well as the salt concentration. The dependence of the process on the pulse amplitude can be utilized to tune the droplet size. To demonstrate the applicability of the electric-field-driven droplet generator, it is shown that the droplets can be used as versatile biological reaction compartments. It is proven that droplets containing a cell-free transcription–translation system execute gene transcription and protein biosynthesis in a timely and programmable fashion. Moreover, it is verified that biomolecules inside the aqueous droplets such as small RNAs can be diffusionally activated from the outside to induce a ligand-driven biochemical switch. In another scenario of using droplets as carrier, adding and subtracting the smallest amounts of liquid in a well-controlled manner is a key step. A principle is demonstrated allowing the transfer of tiny amounts of dissolved species to an aqueous femtoliter droplet reciprocating between two aqueous reservoirs (or interfaces) under the influence of a DC electric field. Mass transfer is shown to be size selective and adaptive, for example, via tuning the viscosity of the surrounding oil phase or the electric-field strength. A map of the dynamic regimes is provided, indicating under which conditions the reciprocating droplet motion occurs. A model based on diffusive mass transfer is formulated that describes the amount of species taken up and transferred by the droplet. Interestingly, in some cases, the droplets reciprocating between two aqueous interfaces show simultaneously volume losses (at most contacts with the reservoirs) under certain conditions, a phenomenon called ‘partial coalescence’. Accordingly, a scaling model is provided allowing the prediction of daughter droplet size during partial coalescence. Overall, the results significantly help to facilitate the handling, production and manipulation of femtoliter droplets

Numerical investigation of slip flows through 2-d u-shaped microchannels

Micro-electro-mechanical systems (MEMS) offer vast applications and those involving fluid flow/gas flows typically operate in the slip flow regime, where the normal non-slip condition assumptions for boundary conditions are not valid. In this study, two-dimensional numerical simulations of continuum and slip flows for U-shaped microchannels were performed to evaluate flow characteristics by means of the ANSYS-FLUENT software. In order to model slip condition existing at the solid-liquid interface of microchannels, a user defined subroutine was linked to the software. The effect of Reynolds number and geometric parameters such as the bend shape and the distance between legs on fluid flow and heat transfer characteristics were examined in detail. The computational results showed that increasing either the slip length or Reynolds number decreased the average friction factor, while the distance between legs does not have any effect on it. Moreover, it was found that the averaged Nusselt number would increase with Reynolds number and the slip length, but increasing the distance between legs decreased the averaged Nu for square bend compared to its negligible increase for circular bend

Manipulation of Single Sub-Femtoliter Droplets via Partial Coalescence in a DC Electric Field

The data are the x- and y-values of the graphs shown in figures 3, 4 and S2. The type and relevance of these data is described in the article and in the corresponding supplementary material

Numerical investigation of fully developed fluid flow and heat transfer in double-trapezoidal microchannels

Fully developed fluid and heat transfer characteristics of double-trapezoidal microchannels with constant wall temperature are numerically investigated in the slip flow regime by applying slip/ jump conditions. The governing equations are solved together with the appropriate boundary conditions using a finite volume method. The influence of rarefaction on the Poiseuille number, Po, and the Nusselt number, Nu, is studied in the range of Knudsen number, Kn, between 0 and 0.1. The effects of base angle, B, and aspect ratio, A, on the fluid flow and the heat transfer characteristics are also examined. The results reveal that the rarefaction and the cross-section shapes have prominent effects on these characteristics of double-trapezoidal microchannels. According to the results, the Poiseuille number decreases with increasing Kn, while the values of the Nusselt number completely depend on the impacts of the rarefaction and the fluid-surface interaction. Po and Nu decrease with aspect ratio when A<1, while the effect of aspect ratio on the Poiseuille number and the Nusselt number becomes unclear for those of A>1. Further, an increase in the base angle increases Po and Nu, but the increasing trend is less observable for the case of both B > 60 o and A < 1.67

Convective heat transfer and entropy generation analysis on Newtonian and non-Newtonian fluid flows between parallel-plates under slip boundary conditions

In this study, convective heat transfer and entropy generation in Newtonian and non-Newtonian fluid flows between parallel-plates with velocity slip boundary condition were analytically investigated for both isoflux and isothermal thermal boundary conditions. Accordingly, the governing equations of hydrodynamically and thermally fully developed laminar flows were analytically solved using wall slip boundary conditions while also including viscous dissipation. As a result of this analysis, some closed form expressions for velocity, local and mean temperature distributions, Nusselt number, entropy generation and Bejan number in terms of different parameters such as slip coefficient, power-law index, and Brinkman number were obtained. According to the results, it was found that heat transfer characteristics of non-Newtonian micro flows are strongly influenced by these governing parameters. The derived expressions can be also generalized to Newtonian fluids and to macro scale by letting the power-law index equal to unity and the slip coefficient equal to zero, respectively. The results indicated that an increase in the slip coefficient leads to an increase in both Nusselt number and Bejan number, whereas it gives rise to a decrease in global entropy generation rate. Brinkman number and power-law index had opposite effects on Nusselt number, Bejan number, and entropy generation rate compared to slip coefficient

Experimental investigation on convective heat transfer of non-Newtonian flows of Xanthan gum solutions in microtubes

Convective heat transfer of thermally developing flows of non-Newtonian Xanthan gum solutions, a potential candidate for cooling and heating applications, was experimentally investigated in a microtube. Xanthan gum solutions of different concentrations (0.1, 0.5, 1 and 4 g/L), whose properties matched with the Carreau-Yasuda model, were tested at different fixed flow rates at constant uniform heat flux. The results revealed that the heat flux was effective at more downstream locations, and the enhancements were attained at low concentrations (c=0.1 g/L) and low flow rates. Therefore, at the same flow rate, Xanthan gum solutions are not very good candidates for enhancement of convective heat transfer unless they are used at both low concentrations and low flow rates

Pool boiling and flow boiling on micro- and nanostructured surfaces

This study reviews recent experimental investigations performed on pool and flow boiling over nano- and micro engineered structures for enhancements in boiling heat transfer, namely heat transfer coefficient (HTC) and critical heat flux (CHF). Modified surfaces having nano/micro porous features of mainly irregular shapes through anodic oxidation processes, coating of metallic and non-metallic layers, deposition of powder materials, and roughening for improving boiling heat transfer have been of research interests of many researchers. In addition, pool boiling and flow boiling studies on artificial structures, mainly fabricated on a plain surface, such as pins, pillar fins, grooves (in different shapes, i.e. rectangular, square, cylinder, etc.) for increasing the heated surface area, or cavities created on substrates for increasing bubble nucleation sites were also considered for both micro and nano scale. The results reported in recent investigations on pool boiling and flow boiling from micro/nanostructured surfaces were included, and a comprehensive overview was provided