Electrokinetically forced turbulence in microfluidic flow.

While laminar flow heat transfer and mixing in microfluidic geometries has been investigated experimentally, as has the effect of geometry-induced turbulence in microfluidic flow (it is well documented that turbulence increases convective heat transfer in macrofluidic flow), little literature exists investigating the effect of electrokinetically-induced turbulence on heat transfer at the micro scale. Successful research in this area could be invaluable in creating more efficient heat exchangers for emerging microscale electronics as well as to fields requiring greater control of mixing in microfluidic devices

Challenges and progress on the modelling of entropy generation in porous media: a review

Depending upon the ultimate design, the use of porous media in thermal and chemical systems can provide significant operational advantages, including helping to maintain a uniform temperature distribution, increasing the heat transfer rate, controlling reaction rates, and improving heat flux absorption. For this reason, numerous experimental and numerical investigations have been performed on thermal and chemical systems that utilize various types of porous materials. Recently, previous thermal analyses of porous materials embedded in channels or cavities have been re-evaluated using a local thermal non-equilibrium (LTNE) modelling technique. Consequently, the second law analyses of these systems using the LTNE method have been a point of focus in a number of more recent investigations. This has resulted in a series of investigations in various porous systems, and comparisons of the results obtained from traditional local thermal equilibrium (LTE) and the more recent LTNE modelling approach. Moreover, the rapid development and deployment of micro-manufacturing techniques have resulted in an increase in manufacturing flexibility that has made the use of these materials much easier for many micro-thermal and chemical system applications, including emerging energy-related fields such as micro-reactors, micro-combustors, solar thermal collectors and many others. The result is a renewed interest in the thermal performance and the exergetic analysis of these porous thermochemical systems. This current investigation reviews the recent developments of the second law investigations and analyses in thermal and chemical problems in porous media. The effects of various parameters on the entropy generation in these systems are discussed, with particular attention given to the influence of local thermodynamic equilibrium and non-equilibrium upon the second law performance of these systems. This discussion is then followed by a review of the mathematical methods that have been used for simulations. Finally, conclusions and recommendations regarding the unexplored systems and the areas in the greatest need of further investigations are summarized

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

Determination of an analytical relationship between entropy generation and mixing efficiency for micromixer applications.

This thesis presents a detailed computational analysis for a simple tee micro-mixing geometry. Micromixers have broad applications in heat exchangers and lab-on-chip (LOC) devices. Simply, a micromixer seeks to efficiently and quickly exchange one or more physical quantities, such as temperature or molecular concentration. The measure of how completely these quantities are exchanged is known as the mixing efficiency. For LOC devices an effective design will be simple and cost effective to manufacture, and provide the greatest mixing efficiency for the smallest device as rapidly as possible. The work here has two main objectives. First, an analytical relationship is sought that functionally relates the entropy generation to the mixing index for a simple tee shaped micromixer. Second, the work will serve as a guide to improve an existing micromixer through its developed methods. A thorough computational fluid dynamics (CFD) analysis is performed for a wide range of Reynolds numbers typical to micromixers with varying flow parameters. The result are several analytical relationships that relate the relevant quantities of entropy generation rate and mixing efficiency to the known flow and fluid parameters. Additionally, a simple relationship is derived that relates the mixing efficiency directly to the entropy generation rate effectively proving a direct relationship between the two quantities. Finally, the relevant results are used to propose a design for a micromixer that provides high mixing efficiencies for a wide range of operating conditions

Transferência de calor por convecção em escoamentos de nanofluidos não newtonianos em micro permutadores de calor

The effect of using non-Newtonian fluids as a heat transfer medium in microchannel heat exchangers (MCHE) is numerically investigated. The study of Non-Newtonian fluids for heat transfer purposes is a research area with growing interest as a result of the development of nanofluids with enhanced heat transfer characteristics, as these usually show a complex rheologic behaviour. In the present work, water-based carbon multi-walled carbon nanotubes (MWCNT) fluids, showing a shear-thinning rheological behaviour were considered. The heat transfer performance of MWCNTs nanofluids in microchannels is assessed for a wide range of operating conditions and comprising the parameters known to directly influence the thermophysical and transport properties of this sort of heat transfer fluid, namely base fluid, nanoparticle geometry and concentration. The overall problem is computationally solved using CFD tools, considering a single-phase, 2D, laminar, steady state flow numerical model for a specific micro heat exchanger geometry. The thermophysical properties of the considered MWCNTs nanofluids, experimentally obtained, were made available and conveniently modelled. As the physical properties of the considered fluids are directly related to the fluids morphology, the study provides a means to establish the relative influence of MWCNTs nanofluids properties on the overall heat transfer and fluid flow effectiveness of such systems, providing a mean to indirectly support the tailoring of the heat transfer fluid to specific heat exchanger applications, e.g.: micro-electromechanical systems, solar energy, aerospace applications. This thesis addresses several goals of the 2030 Agenda for Sustainable Development, adopted by all United Nations Member States in 2015, namely those directly and indirectly related with energy use and availability (7, 9, 11 and 13, see: https://sustainabledevelopment.un.org/sdgs), since heat transfer effectiveness dictates the overall energy efficiency of most thermal systems, broadening the scope towards more rational use of energy sources and hence mitigate the environmental impact of the system’s life cycle.Neste trabalho é estudado numericamente o impacto do uso de fluidos não-newtonianos como meio de transferência de calor em micro permutadores de calor (MCHE). O estudo da utilização de fluidos não newtonianos para fins de transferência de calor é uma área de pesquisa com interesse crescente como resultado do desenvolvimento de nanofluidos com características de transferência de calor melhoradas, já que estes geralmente apresentam um comportamento reológico complexo, i.e., não newtoniano. No presente trabalho, foram considerados nanofluidos de base aquosa produzidos a partir de nanotubos de carbono (MWCNT), caracterizados por um comportamento reológico pseudoplástico. O desempenho da transferência de calor destes nanofluidos em microcanais é avaliado para uma extensiva gama de condições de operação e compreendendo variações nos parâmetros constitutivos e morfológicos do fluido de trabalho, identificados como determinantes das propriedades termofísicas e de transporte deste tipo de fluido, nomeadamente, fluido de base, geometria da nanopartícula e concentração. O problema em análise é estudado computacionalmente usando ferramentas de CFD, considerando escoamento monofásico bidimensional, em condições estacionárias e regime laminar, para uma geometria específica de micro permutador de calor. As propriedades termofísicas dos nano fluídos considerados, obtidas experimentalmente, foram disponibilizadas e convenientemente modeladas, de modo a serem integradas neste estudo. Como as propriedades físicas dos fluidos considerados estão diretamente relacionadas à composição e morfologia dos fluidos, os resultados deste estudo podem ser usados para estabelecer a influência relativa da propriedades deste tipo de nanofluido sobre a transferência de calor e eficácia do processo de transferência neste tipo de sistema (micro canais), proporcionando uma forma de indireta de estabelecer as características do fluido de trabalho em função da aplicação específica pretendida, por exemplo: micro sistemas eletromecânicos, energia solar, aplicações aeroespaciais, sensores, atuadores etc. Esta tese aborda vários objetivos da Agenda 2030 para o Desenvolvimento Sustentável, adotada por todos os estados-membro das nações Unidas em 2015, nomeadamente as relacionadas direta e indiretamente com a utilização e disponibilidade de energia (7, 9, 11 e 13, ver: https://sustainabledevelopment.un.org/SDGs), uma vez que a eficácia da transferência de calor dita a eficiência energética da maioria dos sistemas térmicos, potenciando uma utilização mais racional das fontes de energia disponíveis, e consequentemente, um menor impacto ambiental quando se considera o ciclo de vida do sistema.Programa Doutoral em Sistemas Energéticos e Alterações Climática

Mathematical Modeling for Nanofluids Simulation: A Review of the Latest Works

Exploiting nanofluids in thermal systems is growing day by day. Nanofluids having ultrafine solid particles promise new working fluids for application in energy devices. Many studies have been conducted on thermophysical properties as well as heat and fluid flow characteristics of nanofluids in various systems to discover their advantages compared to conventional working fluids. The main aim of this study is to present the latest developments and progress in the mathematical modeling of nanofluids flow. For this purpose, a comprehensive review of different nanofluid computational fluid dynamics (CFD) approaches is carried out. This study provides detailed information about the commonly used formulations as well as techniques for mathematical modeling of nanofluids. In addition, advantages and disadvantages of each method are rendered to find the most appropriate approach, which can give valid results

Transport in nanofluidic systems: a review of theory and applications

In this paper transport through nanochannels is assessed, both of liquids and of dissolved molecules or ions. First, we review principles of transport at the nanoscale, which will involve the identification of important length scales where transitions in behavior occur. We also present several important consequences that a high surface-to-volume ratio has for transport. We review liquid slip, chemical equilibria between solution and wall molecules, molecular adsorption to the channel walls and wall surface roughness. We also identify recent developments and trends in the field of nanofluidics, mention key differences with microfluidic transport and review applications. Novel opportunities are emphasized, made possible by the unique behavior of liquids at the nanoscale

Analysis, Design and Fabrication of Micromixers, Volume II

Micromixers are an important component in micrototal analysis systems and lab-on-a-chip platforms which are widely used for sample preparation and analysis, drug delivery, and biological and chemical synthesis. The Special Issue "Analysis, Design and Fabrication of Micromixers II" published in Micromachines covers new mechanisms, numerical and/or experimental mixing analysis, design, and fabrication of various micromixers. This reprint includes an editorial, two review papers, and eleven research papers reporting on five active and six passive micromixers. Three of the active micromixers have electrokinetic driving force, but the other two are activated by mechanical mechanism and acoustic streaming. Three studies employs non-Newtonian working fluids, one of which deals with nano-non-Newtonian fluids. Most of the cases investigated micromixer design

Recent Advancements in Thermal Performance Enhancement in Microchannel Heatsinks for Electronic Cooling Application

Thermal management of electronic equipment is the primary concern in the electronic industry. Miniaturization and high power density of modern electronic components in the energy systems and electronic devices with high power density demanded compact heat exchangers with large heat dissipating capacity. Microchannel heat sinks (MCHS) are the most suitable heat exchanging devices for electronic cooling applications with high compactness. The heat transfer enhancement of the microchannel heat sinks (MCHS) is the most focused research area. Huge research has been done on the thermal and hydraulic performance enhancement of the microchannel heat sinks. This chapter’s focus is on advanced heat transfer enhancement methods used in the recent studies for the MCHS. The present chapter gives information about the performance enhancement MCHS with geometry modifications, Jet impingement, Phase changing materials (PCM), Nanofluids as a working fluid, Flow boiling, slug flow, and magneto-hydrodynamics (MHD)

A thermodynamic analysis of forced convection through porous media using pore scale modeling

The flow thorough porous media is analyzed from a thermodynamic perspective, with a particular focus on the entropy generation inside the porous media, using a pore scale modeling approach. A single representative elementary volume was utilized to reduce the CPU time. Periodic boundary conditions were employed for the vertical boundaries, by re-injecting the velocity and temperature profiles from the outlet to the inlet and iterating. The entropy generation was determined for both circular and square cross-sectional configurations, and the effects of different Reynolds numbers, assuming Darcy and Forchheimer regimes, were also taken into account. Three porosities were evaluated and discussed for each cross-sectional configuration, and streamlines, isothermal lines and the local entropy generation rate contours were determined and compared. The local entropy generation rate contours indicated that the highest entropy generation regions were close to the inlet for low Reynolds flows and near the central cylinder for high Reynolds flows. Increasing Reynolds number from 100 to 200 reveals disturbances in the dimensionless volume averaged entropy generation rate trend that may be due to a change in the fluid flow regime. According to Bejan number evaluation for both cross-section configurations, it is demonstrated that is mainly provoked by the heat transfer irreversibility. A performance evaluation criterion parameter was calculated for different case-studies. By this parameter, conditions for obtaining the least entropy generation and the highest Nusselt number could be achieved simultaneously. Indeed, this parameter utilizes both the first and the second laws of thermodynamics to present the best case-study. According to the performance evaluation criterion, it is indicated that the square cross-section configuration with o=0.64 exhibits better thermal performance for low Reynolds number flows. A comparison between the equal porosity cases for two different cross-sectional configurations indicated that the square cross-section demonstrated a higher performance evaluation criterion than the circular cross-section, for a variety of different Reynolds numbers