This study aims to analyze the cooling performance of parallel microchannel heat sinks (PMCHS) under uneven heat flux distributions, taking into account different flow configurations including I, U, and Z. The objective is to demonstrate the potential of utilizing the flow maldistribution inherent in each configuration to effectively manage and mitigate the effects of uneven heat flux distributions. Four different heating arrangements have been considered, namely uniform, non-streamline, streamline, and across-streamline to generate the uneven heat flux distributions. A three-dimensional numerical simulation has been performed to analyze the combined effect of uneven heat flux distributions and flow maldistribution characteristics on the thermal performance of PMCHS. To assess the thermal performances; thermal resistance (Rth), Nusselt number (Nu), temperature nonuniformity (Ψ), and fin efficiency (ηfin) have been employed. The results show that all three flow configurations exhibit similar thermal performances for uniform heat load conditions (0.1 K/W for Rth, 5.5 kW for Nu, 0.3 for Ψ, and 0.98 for ηfin). However, in the case of uneven heat flux distributions, the thermal performance of each configuration is observed to be varying with respect to hotspot positions. This study reveals that each configuration has a huge discrepancy in terms of thermal performance with respect to uneven heat flux distributions. Also, the study concludes that a single flow configuration alone is insufficient to address the cooling challenges that arise due to uneven heat flux distributions. The cooling capability of any configuration to handle uneven heat distributions mainly depends upon the flow maldistribution characteristics of the respective configurations.

Effective thermal management of high thermal loads, especially asymmetric and localized hotspots, is a significant challenge for the safety and reliability of electronic devices. Software advancements and miniaturization today have devices which rely on multicore microprocessor architectures. Multiple active cores with high thermal loads risk inducing various hotspots and require sophisticated cooling of asymmetric thermal signatures, which can be realized via microfluidic interventions. This experimental research investigates the thermal performance of a parallel microchannel heat sink (PMCH) device and a proposed cylindrical pin fin heat sink (FHS) device to mitigate such practical heat load on microprocessors utilizing microfluidics devices. The thermo-fluidic performance of the heat sinks was studied for various uniform and non-uniform thermal loads, with a combined background heat load, to mimic several real-life asymmetric thermal signatures from current microprocessors. Non-uniform, asymmetric heat loads are studied here in the form of a single hotspot and as three-hot-spot thermal load to mimic the conditions of single and multicore operations. Three different micro heat-sink flow configurations (U, I, and Z types) are used to distribute the fluid effectively as per requirement, and their thermo-fluidic performance is comprehensively studied. Throughout the experiments, flow rates from 0.25 to 0.75 LPM, with an increment value of 0.25 LPM, were precisely maintained in all thermal heat load cases, and the thermal performance of two types of heat sinks was comprehensively analyzed. The work highlights that FHS outperforms PMCH at low flow rates due to enhanced mixing and effective hotspot cooling, while PMCH excels at higher flow rates (with up to ∼50% higher Nusselt numbers) and better temperature uniformity. Z-flow configuration consistently delivered the best thermal performance across both designs. These findings underscore the importance of flow rate and configuration optimization in microchannel cooling for advanced thermal management. The analysis provides effective cooling solutions for multicore microprocessors operating with different thermal design powers and shows that the FHS is a superior microfluidic thermal management device for asymmetric hotspots.

The electronic industry’s shift towards multicore processor technology which leads to an increase the power densities of the chip. In multicore processors the hotpot location arises depending on the computational load which leads to the generating the non-uniform heat flux. The uneven cooling of a multicore processor will affect the reliability and life span of the chip. In this study, employed parallel microchannel cooling systems (PMCHS) with different flow configurations by numerical simulations. The objective of the present work is to investigate the thermos-hydrodynamic characteristics of a PMCHS under a non-uniform heat load, the heat load is considered from an actively running 8-core processor. Here, considered that three types different flow configurations (U, I and Z) to determine the flow maldistribution, in additions the thermal performance of each flow configuration was analysed at non-uniform heat conditions. The size and shape of the PMCHS is equal to the octa-core processor which has been mimicked, and real-time heat load data of the processor has been retrieved. The present study exhibits that non-uniform thermal load creates additional non-uniform temperature distribution along with flow maldistribution in the PMCHS. Each flow configuration has a different flow maldistribution pattern, whereas the sometimes intended flow maldistribution helps to give better uniform cooling on the chip.

Cooling methods for multiple hotspots with high heat flux pose a reliability threat to electronic devices. This study investigates the microchannel-based heat sink performance under various non-uniform heat load conditions for different geometry with three different flow configurations (I, U and Z). An in-house designed Heater Array Unit (HAU) facilitates the generation of both uniform and non-uniform heat loads using a heater power supply. Two different microchannel geometries were employed, namely, microchannel-1 (MC-1) with channel and fin widths of 0.6 mm and 0.33 mm, respectively, and MC-2 with dimensions of 0.64 mm and 0.572 mm. Each microchannel incorporates three manifold configurations (I, U, and Z). Each flow configuration is regulated by flow control valves. Various non-uniform heat load patterns were considered, including streamline, non-streamline, and across-streamline conditions. To assess the thermal performance of the heat sinks the parameters used are thermal resistance (Rth), Nusselt number (Nu), and temperature non-uniformity (Ѱ). Experimental findings indicate that the MC-2 design with an I flow configuration is more suitable for uniform heat load conditions. On the contrary, for some non-uniform heat load cases MC-1 also showed up as a suitable design over MC-2.

The parallel microchannel heat sink stands as a pivotal solution in managing high heat flux electronics due to its efficient heat transfer characteristics and ease of manufacturing. While numerous studies have explored the thermal performance and flow characteristics of microchannel heat sinks, most have focused on uniform heat loads or relied heavily on numerical methods. This study presents an experimental system tailored to generate data for analyzing the thermal performance of microchannel heat sinks under various conditions. Leveraging this dataset, four distinct machine learning models Artificial Neural Network (ANN), XGBoost, LightGBM, and K-nearest neighbor (KNN) were trained using 22 input features, totalling 560 data points categorised into geometry parameters, heating patterns, and boundary conditions details. The models were tasked with predicting six response variables: the average base temperature of the heat sink, temperature change (ΔT), hotspot temperature, heat transfer coefficient (h), Nusselt number (Nu), and thermal resistance (Rth). Among the four machine learning models, XGBoost exhibited a good predictive accuracy of an average R2 value of 0.98 and MAE values of 2.1 across all responses. Furthermore, the study delved into the impact of varying input features on prediction accuracy, revealing a consistent enhancement in accuracy with the inclusion of more features across all models.

Aerial inspection of solar PV plants using Unmanned Aerial Vehicles (UAVs) is gaining traction due to benefits such as no downtime and cost-effectiveness. This technology is proven to be the low-cost alternative to conventional approaches involving visual inspection and I-V curve tracing to identify physical damages and underperforming strings, respectively. Though the use of UAVs for thermographic solar PV inspection is a popular alternative in developed countries, its use in developing economies experience various challenges. Studies emphasizing these challenges especially in the context of rapid evolution of drones are limited. To overcome this limitation, literature scoping, a one-on-one survey, focus group discussion, and a flight campaign using a UAV with a thermal payload is conducted in India to identify the limitations. These are further categorized into Technical, Behavioural, Implementation, Pre-deployment, Deployment, and Post-deployment categories. The relevance and significance of each challenge are analysed using a hybrid multi-criteria framework developed in this study. Findings of this study highlight the importance of drone regulations, technology readiness, and workshops for drone pilots, industry professionals, and solar developers in India. This study aid developing economies in devising strategies that can promote the use of UAVs for solar PV plant commissioning activities.

A compound parabolic concentrator (CPC) with a flat absorber is widely used in low-concentrating photovoltaic thermal (CPVT) systems. CPC certainly develops non-uniform heat flux distribution over the absorber surface which is significantly reduced by the integration of homogenizer referred as Elongated CPC (ECPC). The objective of the present work is to analyse the effect of homogenizer ratios, truncation ratios and concentration ratios on the heat flux distribution characteristics of a CPC collector. In this paper, a ray tracing simulation is carried out to obtain the heat flux distribution profiles and the same is incorporated within CFD software to obtain the temperature distribution profiles. As a result, it is observed that the optimum truncation ratio would be 0.7 at which uniformity in flux distribution is improved by 3%, with just 2% reduction of average heat flux value. Furthermore, with optimized homogenizer ratio of −0.35 at concentration ratio of 3, 64% improvement in uniformity of flux distribution has been noticed. From the study, it has been concluded that for different concentration ratios of 1.5, 2, 3, 4, 5, 6, 7 and 8, the optimum homogenizer ratio is observed to be −0.9, −0.55, −0.35, −0.25, −0.2, −0.15, −0.15 and −0.05 respectively.

Designing an effective parallel microchannel heat sink (PMCHS) is necessary for addressing the cooling challenges of high heat-dissipating electronics. This paper presents a shape optimization of PMCHS to minimize the thermal resistance and pressure drop for each U, I, and Z-type inlet/outlet manifold configuration with vertical intake and coolant delivery. The performance of PMCHS influencing design parameters, such as channel width, fin width, and channel height, is designed using the response surface methodology (RSM). In the present communications adopting the Artificial Neural Network (ANN) coupled NSGA-II method, a three-dimensional numerical simulation is executed to minimize the pressure drop and thermal resistance. Numerical simulation is performed using the finite volume method; the computational domain is taken as the entire microchannel system including the inlet/outlet plenum area, ports and microchannels. The overall analysis demonstrated that the pareto optimal design point has better hydraulic and thermal performances than the predefined design. The optimized design showed benchmark thermal resistance of 0.0306 ˚C/W, 0.0315 ˚C/W, 0.0316 ˚C/W and pressure drop of 3.1 kPa, 3.2 kPa, 3.19 kPa for U, I, Z configurations respectively.

The Compound Parabolic Concentrator (CPC), when coupled with the photovoltaic system, namely the Concentrated Photovoltaic Thermal System (CPVT), makes utilizing solar energy efficient. The major challenge that hinders the electrical and thermal performance of the CPC–CPVT system is the non-uniform heat flux distribution on the absorber surface. In the present paper, detailed ray-tracing simulations have been carried out to understand the heat flux distribution characteristics of CPC with different geometrical conditions, and those are concentration ratio, truncation ratio, incident angle, and average heat flux on the absorber surface. To have a thorough understanding, the analysis has been carried out in multiple steps. First, it is performed by analyzing the effect of concentration ratio and incident angle on heat flux distribution characteristics at a fixed truncation ratio. Second, investigations have been carried out to understand the heat flux distribution characteristics at different truncation ratios and different incident angles by keeping the concentration ratio constant. Local concentration ratio and non-uniformity index have been employed to quantify the non-uniformity of heat flux distribution on the absorber surface. It has been observed that the 0-deg incidence angle is the most effective angle to achieve uniform heat flux distribution on the absorber surface. This paper sheds insight into the heat flux distribution characteristics on the absorber surface of a CPC–CPVT system which can be used by the research community for designing an effective CPVT system from the perspective of uniform heat flux distribution on the absorber surface.

In this article, the hydrodynamic performance of microchannel cooling systems has been predicted analytically. The microchannel have a high surface area to volume ratio, due to that it has high heat transfer coefficients. The microchannel cooling systems have received prompt attention from researchers to address the cooling challenges of electronic components. However, due to the diameter of the order of microns, as the pressure drop is inversely proportional to the channel diameter, it leads to more pressure drop in the microchannel. Such that the investigation of flow characteristics in the microchannel is tremendously on-demand to understand hydrodynamics. Unfortunately, the applicability of conventional theories (Darcy pressure drop equations) in microchannel flows is still under debate. Kandlikar has come up with an expression for predicting pressure drop in microchannels by considering the Poiseuille number and aspect ratio of microchannels. This paper concentrated on validating the predictions of the Kandlikar pressure drop equation and Darcy pressure drop equation with experimental work taken from literature. The results show that available analytical methods are under-predicting as those will not consider the surface roughness and uncertainty present while conducting experiments. Among the analytical models, the Kandlikar equation predictions are better than the other methods, and the results of the prediction are well in agreement with experimental results.

The concentrated solar dish collector is a promising technology for generating both electricity and thermal energy together and it is termed as concentrated photovoltaic thermal. The important component of the parabolic dish collector is the absorber; where all concentrated lights are falling. The present paper has investigated the heat flux distribution characteristics of the flat plate absorber based solar dish collector by using the ray-tracing simulations. In concentrating dish collector, rim angle and dish diameter are significant factor of the flux distributions. The present study reported the average heat flux distribution, maximum flux intensity and non-uniformity of flux distribution for different geometrical conditions. The maximum heat flux rate attains the rim angle between 35 to 55° for any dish diameter. Where the peak flux intensity raises concerning raises of rim angle and peak flux occurred at rim angle 90°. The increase of heat flux intensity causes the non-uniformity of heat flux distribution over the absorber surface. The non-uniformity factor is mainly influenced by the rim angle, not a dish diameter. When rim angle 15 & 75°, the non-uniformity is 2.5 & 10 respectively for whichever dish diameters. A critical rim angle produces the non-uniformity factor. Results shows that optimization of rim angle is a significant contribution for decreasing the non-uniformity index of concentrator; it most valuable for coupled thermal and electricity generating applications.

Thriving technologies in the electronics industry demands effective cooling systems for proper thermal management of the devices. To propose an effective cooling system, real-time challenges need to be taken into examination. One of the challenges is the non-uniform heat load emitted by the device. In this paper, a multicore processor has mimicked for the heat load and examined with U configured parallel microchannel cooling system (PMCS) for its capacity to cool the processor effectively. A detailed numerical examination has been conceded out by mimicking the AMD Ryzen-7 octa-core processor for its size, shape and heat load. To make it convenient for applying non-uniform heat load, the shape of the PMCS is divided into 4*4 array of a total of 16 heaters. The present has witnessed that, there is an uneven distribution of fluid flow across the channels. The initial channels do get more quantity of fluid whereas the end channels do get a very less quantity of fluid. Why because of the flow maldistribution, it formed the hot spots (high-temperature zones), which will be further intensified due to the non-uniform heat load emitted by the processor. It perceived from the present numerical analysis, there is a formation of hotspots not only due to the random location of active cores but also due to the flow maldistribution across the channels. The flow arrangement of U configuration having worse flow distribution among other existing configuration. The present work conclusively demonstrates that high maldistribution leads towards dropping the uniformity of cooling. But the existing maldistribution can be effectively exploited to tackle the non-uniform heat load released by the processor when the active core location falls in the place of initial channels.

Modification and control of the vaporization kinetics of microfluidic droplets can find utilitarian implications in several scientific and technological applications. The article reports the control over the vaporization kinetics of pendant droplets under the influence of competing internal electrohydrodynamic and ferrohydrodynamic advection. Experimental and theoretical studies are performed and the morphing of vaporization kinetics of electrically conducting, paramagnetic fluid droplets using orthogonal electric and magnetic stimuli is explored. Analysis reveals that the electric field has a domineering influence compared to the magnetic field. While the magnetic field is observed to augment the vaporization rates, the electric field is observed to decelerate the same. Neither the vapour diffusion dominated model, nor the field induced modified surface tension characteristics can explain the observed behaviours. Velocimetry studies within the droplet show extensively modified internal ferro and electrohydrodynamic advection, which is noted to be the crux of the mechanism towards modified vaporization rates. A mathematical analysis is proposed, which takes into account the roles played by the concomitant governing Hartmann, Electrohydrodynamic, Interaction, thermal and solutal Marangoni, and the electro and magneto Prandtl and Schmidt numbers. It is observed that the morphing of the thermal and solutal Marangoni numbers by the electromagnetic Interaction number plays the dominant role towards morphing the advection dynamics. The model is able to predict the internal advection velocities accurately. The findings may hold importance towards smart control and tuning of vaporization kinetics in macro and microfluidic systems.

The present paper explores the concept of improving the AC dielectric breakdown strength of insulating mineral oils by the addition of graphene or carbon nanotubes (CNTs) to form stable dispersions. Experimental observations of graphene and CNT nano-oils show that not only improved average breakdown voltage, but also significantly improved reliability and survival probabilities of the oils under AC high voltage stressing is achieved. Improvement of the tune of ∼70-80% in the AC breakdown voltage of the oils has been obtained. The study examines the reliability of such nano-colloids using a two-parameter Weibull distribution and the oils show greatly augmented electric field bearing capacity. The fundamental mechanism responsible for such observed outcomes is reasoned to be delayed streamer development and reduced streamer growth rates due to effective electron scavenging. A mathematical model based on the principles of electron scavenging is proposed to quantify the amount of electrons scavenged by the nanostructures.

Electrophoresis has been shown as a novel methodology to enhance heat conduction capabilities of nanocolloidal dispersions. A thoroughly designed experimental system has been envisaged to solely probe heat conduction across nanofluids by specifically eliminating the buoyancy driven convective component. Electric field is applied across the test specimen in order to induce electrophoresis in conjunction with the existing thermal gradient. It is observed that the electrophoretic drift of the nanoparticles acts as an additional thermal transport drift mechanism over and above the already existent Brownian diffusion and thermophoresis dominated thermal conduction. A scaling analysis based on the thermophoretic and electrophoretic velocities from classical Huckel-Smoluchowski formalism is able to mathematically predict the thermal performance enhancement due to electrophoresis. It is also inferred that the dielectric characteristics of the particle material is the major determining component of the electrophoretic amplification of heat transfer. Influence of surfactants has also been probed into and it is observed that enhancing the stability via interfacial charge modulation can in fact enhance the electrophoretic drift, thereby enhancing heat transfer calibre. Also, surfactants ensure colloidal stability as well as chemical gradient induced recirculation, thus ensuring colloidal phase equilibrium and low hysteresis in spite of the directional drift in presence of electric field forcing. The findings may have potential implications in enhanced and tunable thermal management of micro-nanoscale devices and in thermo-bioanalysis within lab-on-a-chip devices.

Electrorheological (ER) fluids are known to exhibit enhanced viscous effects under an electric field stimulus. The present article reports the hitherto unreported phenomenon of greatly enhanced thermal conductivity in such electro-active colloidal dispersions in the presence of an externally applied electric field. Typical ER fluids are synthesized employing dielectric fluids and nanoparticles and experiments are performed employing an in-house designed setup. Greatly augmented thermal conductivity under a field's influence was observed. Enhanced thermal conduction along the fibril structures under the field effect is theorized as the crux of the mechanism. The formation of fibril structures has also been experimentally verified employing microscopy. Based on classical models for ER fluids, a mathematical formalism has been developed to predict the propensity of chain formation and statistically feasible chain dynamics at given Mason numbers. Further, a thermal resistance network model is employed to computationally predict the enhanced thermal conduction across the fibrillary colloid microstructure. Good agreement between the mathematical model and the experimental observations is achieved. The domineering role of thermal conductivity over relative permittivity has been shown by proposing a modified Hashin-Shtrikman (HS) formalism. The findings have implications towards better physical understanding and design of ER fluids from both 'smart' viscoelastic as well as thermally active materials points of view.

The present paper illustrates experimentally the impact of non-uniform heat generation in a microprocessor on the hot spot distribution and the method to cool the same, efficiently employing parallel microchannel cooling configurations. It is often assumed during design of microprocessor cooling systems that the heat load emitted by the device is uniform, however real time tracking of the device shows that such assumptions are far from the reality. An Intel® Core™ i7–4770 3.40 GHz quad core processor has been experimentally mimicked using heat load data retrieved from a real microprocessor with non-uniform core activity. Parallel microchannel based heat spreader configurations using U, I and Z type flow configurations have been employed to mitigate the thermal load from the mimicked device. The observations clearly show that the microchannel cooling system experiences two forms of hot spots, one due to the flow maldistribution within the system and the other due to the additional non-uniform heat generation by the device. The hot spots have been shown to exhibit drastically different shapes and core temperatures and this has been verified through simulations and infrared thermography. To efficiently cool hot spot core temperatures, nanofluids have been employed and ‘smart cooling’ has been observed and the same has been explained based on nanoparticle slip mechanisms. The present work shows that the notion that high flow maldistribution leads to high thermal maldistribution, is not always true and existing maldistribution can be effectively utilized to tackle specific hot spot location. The present work can be important to design cooling mechanisms for real microprocessors with high core activity leading to non-uniform hot spot formation probabilities.

While known to be superior coolants in stand–alone conditions, some scepticism exists with respect to the hydrodynamic and thermodynamic performance of nanofluids in real life applications. The present work employs theoretical investigations (supported by simulation results) on the entropy generation characteristics in parallel microchannel cooling systems (PMCS) employing water and nanofluid as working fluids. Alumina-water nanofluid of different concentrations and PMCS of three different configurations, viz. U, I and Z have been employed for the present study. In order to shed more clarity onto the real thermodynamic performance of nanofluids, an Eulerian–Lagrangian discrete phase approach (DPM) has also been used to model nanofluids alongside the conventional effective property approach (EPM). The thermodynamic performance of twin component nanofluid model in PMCS over the base fluid and single component counterpart has been investigated in view of flow friction generated entropy and heat transfer generated entropy. To quantify thermodynamic irreversibility of nanofluids in PMCS due to heat transfer, the Bejan number has been employed. The entropy generation due to particle migration effects reveal that the effective property model overestimates the entropy generation in case of nanofluids and essentially nanofluids generate lesser degree of entropy than estimated by use of simplistic models. The Bejan number analysis reveals that although hydrodynamically inferior to water, nanofluids are thermodynamically superior fluids when employed as coolants in complex microscale flow systems. The article sheds insight onto the entropy generation behaviour of such dispersed system flows with respect to flow and heat transfer characteristics such as particle concentration, flow Reynolds number, and heat load for proper design of such systems.

While parallel microchannel based cooling systems (PMCS) have been around for quite a period of time, employing the same and incorporating them for near–active cooling of microelectronic devices is yet to be implemented and the implications of the same on thermal mitigation to be understood. The present article focusses on a specific design of the PMCS such that it can be implemented at ease on the heat spreader of a modern microprocessor to obtain near-active cooling. Extensive experimental and numerical studies have been carried out to comprehend the same and three different flow configurations (U, I and Z) of PMCS have been adopted for the present investigations. Additional to focussing on the thermofluidics due to flow configuration, nanofluids (as superior heat transfer fluids) have also been employed to achieve the desired essentials of mitigation of overshoot temperatures and improving uniformity of cooling. Two modelling methods, Discrete Phase Modelling (DPM) and Effective Property Modelling (EPM) have been employed for numerical study to model nanofluids as working fluid in micro flow paths and the DPM predictions have been observed to match accurately with experiments. To quantify the thermal performance of PMCS, an appropriate Figure of Merit (FoM) has been proposed. From the FoM It has been perceived that the Z configuration employing nanofluid is the best suitable solutions for uniform thermal loads to achieve uniform cooling as well as reducing maximum temperature produced with in the device. The present results are very promising and viable approach for futuristic thermal mitigation of microprocessor systems.

Design of effective microcooling systems to address the challenges of ever increasing heat flux from microdevices requires deep examination of real-time problems and has been tackled in depth. The most common (and apparently misleading) assumption while designing microcooling systems is that the heat flux generated by the device is uniform, but the reality is far from this. Detailed simulations have been performed by considering nonuniform heat load employing the configurations U, I, and Z for parallel microchannel systems with water and nanofluids as the coolants. An Intel® Core™ i7-4770 3.40 GHz quad core processor has been mimicked using heat load data retrieved from a real microprocessor with nonuniform core activity. This study clearly demonstrates that there is a nonuniform thermal load induced temperature maldistribution along with the already existent flow maldistribution induced temperature maldistribution. The suitable configuration(s) for maximum possible overall heat removal for a hot zone while maximizing the uniformity of cooling have been tabulated. An Eulerian-Lagrangian model of the nanofluids shows that such "smart" coolants not only reduce the hot spot core temperature but also the hot spot core region and thermal slip mechanisms of Brownian diffusion and thermophoresis are at the crux of this. The present work conclusively shows that high flow maldistribution leads to high thermal maldistribution, as the common prevalent notion is no longer valid and existing maldistribution can be effectively utilized to tackle specific hot spot location, making the present study important to the field.

Nano-oils comprising stable and dilute dispersions of synthesized Graphene (Gr) nanoflakes and carbon nanotubes (CNT) have been experimentally observed for the first time to exhibit augmented dielectric breakdown strengths compared to the base transformer oils. Variant nano-oils comprising different Gr and CNT samples suspended in two different grades of transformer oils have yielded consistent and high degrees of enhancement in the breakdown strength. The apparent counter-intuitive phenomenon of enhancing insulating caliber of fluids utilizing nanostructures of high electronic conductance has been shown to be physically consistent thorough theoretical analysis. The crux mechanism has been pin pointed as efficient charge scavenging leading to hampered streamer growth and development, thereby delaying probability of complete ionization. The mathematical analysis presented provides a comprehensive picture of the mechanisms and physics of the electrohydrodynamics involved in the phenomena of enhanced breakdown strengths. Furthermore, the analysis is able to physically explain the various breakdown characteristics observed as functions of system parameters, viz. nanostructure type, size distribution, relative permittivity, base fluid dielectric properties, nanomaterial concentration and nano-oil temperature. The mathematical analyses have been extended to propose a physically and dimensionally consistent analytical model to predict the enhanced breakdown strengths of such nano-oils from involved constituent material properties and characteristics. The model has been observed to accurately predict the augmented insulating property, thereby rendering it as an extremely useful tool for efficient design and prediction of breakdown characteristics of nanostructure infused insulating fluids. The present study, involving experimental investigations backed by theoretical analyses and models for an important dielectric phenomenon such as electrical breakdown can find utility in design of safer and more efficient high operating voltage electrical drives, transformers and machines.

A deep understanding of fluidic maldistribution in microscale multichannel devices is necessary to achieve optimized flow and heat transfer characteristics. A detailed computational study has been performed using an Eulerian–Lagrangian twin-phase model to determine the concentration and thermohydraulic maldistributions of nanofluids in parallel microchannel systems. The study reveals that nanofluids cannot be treated as homogeneous single-phase fluids in such complex flow situations, and effective property models drastically fail to predict the performance parameters. To comprehend the distribution of the particulate phase, a novel concentration maldistribution factor has been proposed. It has been observed that the distribution of particles does not entirely follow the fluid flow pattern, leading to thermal performance that deviates from those predicted by homogeneous models. Particle maldistribution has been conclusively shown to be due to various migration and diffusive phenomena such as Stokesian drag, Brownian motion and thermophoretic drift. The implications of particle distribution on the cooling performance have been illustrated, and smart fluid effects (reduced magnitude of maximum temperature in critical zones) have been observed for nanofluids. A comprehensive mathematical model to predict the enhanced cooling performance in such flow geometries has been proposed. The article clearly highlights the effectiveness of discrete phase approach in modeling nanofluid thermohydraulics and sheds insight on the specialized behavior of nanofluids in complex flow domains.

Nano-oils synthesized by dispersing dielectric nanostructures counter common intuition as such nano-oils possess substantially higher positive dielectric breakdown voltage with reduced streamer velocities than the base oils. Nano-oils comprising stable and dilute homogeneous dispersions of two forms of titanium (IV) oxide (TiO2) nanoparticles (Anatase and Rutile) have been experimentally examined and observed to exhibit highly enhanced dielectric breakdown strength compared to the conventional transformer oils. The present study involves titania dispersed in two different grades of transformer oils, both with varied levels of thermal treatment, to obtain consistent and high degrees of enhancement in the breakdown strength, as well as high degrees of increment in the survival of the oils at elevated electrical stressing compared to the base oils, as obtained via detailed twin parameter Weibull distribution analysis of the experimental observations. The experimental results demonstrate higher augmented breakdown strength for Anatase compared to the Rutile phase of titania. In-depth survey of literature indicates that mostly Rutile based oils are used. However, the present study shows that they exhibit relatively less breakdown strength enhancement compared to the Anatase based oils. It is also observed that heat treatment of the nano-oils further enhances the dielectric breakdown performance. The differences in the performance of Anatase and Rutile has been explained based on the electronic structure of the two and the affinity towards electron scavenging and the theory has been found to validate the experimental observations.

Readily synthesizable nano-graphene and poly ethylene glycol based stable gels have been synthesized employing an easy refluxing method, and exhaustive rheological and viscoelastic characterizations have been performed to understand the nature of such complex gel systems. The gels exhibit shear thinning response with pronounced yield stress values which is indicative of a microstructure, where the graphene nanoflakes intercalate (possible due to the refluxing) with the polymer chains and form a pseudo spring damper network. Experimentations on the thixotropic behavior of the gels indicate that the presence of the G nanoflakes leads to immensely augmented structural stability capable of withstanding severe impact shears. Further information about the localized interactions of the G nanoflakes with the polymer chains is revealed from the amplitude and frequency sweep analyses in both linear and non-linear viscoelastic regimes. Massively enhanced cross over amplitude values are recorded and several smart effects such as enhanced elastic behavior at increasing forcing frequencies are registered. Structural resonance induced disruption of the elastic behavior is observed for the gels for a given range of frequency and the proposition of resonance has been justified mathematically. It is observed that, post this resonance bandwidth, the gels are able to self-heal and regain their original elastic behavior back without any external intervention. More detailed information on the viscoelastic nature of the gels has been obtained from creep and recovery compliance tests and justifications for the spring damper microstructure has been obtained. Smart features such as enhanced stress relaxation behavior with increasing strain have been observed and the same explained, based on the proposed microstructure. The viscoelastic response of the gels has been mathematically modeled and it has been revealed that such complex gels can be accommodated as modified Burger's viscoelastic systems with predominant elastic/plastic behavior. The present gels show promise in microscale actuators, vibration isolation, and damping in devices and prosthetics, as active fluids in automotive suspensions, controlled motion arrestors, and so on.

Mitigation of 'hot spots' in microelectromechanical systems (MEMS) employing in situ microchannel systems requires a comprehensive picture of the maldistribution of the working fluid and uniformity of cooling within the same. In this paper, detailed simulations employing parallel microchannel systems with specialized manifold-channel configurations, i.e., U, I, and Z, have been performed. Eulerian-Lagrangian discrete phase model (DPM) and effective property model with water and alumina-water nanofluid as working fluids have been employed. The distributions of the dispersed particulate phase and continuous phase have been observed to be, in general, different from the flow distribution, and this has been found to be strongly dependent on the flow configuration. Accordingly, detailed discussions on the mechanisms governing such particle distribution patterns have been proposed. Particle maldistribution has been conclusively shown to be influenced by various migration and diffusive phenomena, such as Stokesian drag, Brownian motion, thermophoretic drift, and so on. To understand the uniformity of cooling within the device, which is of importance in real-time scenario, an appropriate figure of merit has been proposed. It has been observed that uniformity of cooling improved using nanofluid as working fluid as well as enhanced relative cooling in hot zones, providing evidence of the 'smart' nature of such dispersions. To further quantify this smart effect, real-time mimicking hot-spot scenarios have been computationally probed with nanofluid as the coolant. A silicon-based microchip emitting nonuniform heat flux (gathered from real-time monitoring of an Intel Core i7-4770 3.40-GHz quad-core processor) under various processor load conditions has been studied, and the evidence of enhanced cooling of hot spots has been obtained from DPM analysis. This paper sheds insight on-the behavior of nonhomogeneous dispersions in complex flow domains and the caliber of nanofluids in cooling MEMS more uniformly and 'smarter' than base fluids.