This draft explores an experimental investigation into improving pool boiling heat transfer through the use of eco-friendly hybrid nanofluids. The proposed eco-friendly hybrid nanofluids have been prepared using Rhamnolipid surfactant particles, Magnesium Oxide (MgO), and water. Four samples of hybrid nanofluids have been prepared by keeping Rhamnolipid surfactant at the critical micelles concentration and by varying the MgO concentrations as 0.5g/l, 0.05g/l, 0.005g/l, and 0.0005 g/l in deionized water. The detailed characterization and stability analysis of the hybrid nanofluids were performed. Roughness and wettability of fresh and post-boiling surfaces have been analyzed to check their influence on the heat transfer process. The hybrid nanofluid has a thermal conductivity of 4-9 % greater than DI water, where its surface tensions are 43 % - 53 % lower than those of pure DI water. The investigations were carried out on a smooth copper surface and at atmospheric conditions. The developed hybrid nanofluid enhances both the heat transfer rate and the critical heat flux (CHF) compared to pure water. The heat transfer coefficient (HTC) is observed to increase by 1.16 to 1.79 times, and the CHF is increased by 1.41 to 1.61 times. The result for improved heat transfer in eco-hybrid nanofluids is due to the decrease in surface tension, which has initiated more nucleation sites than pure water, increase in porosity and surface roughness due to the deposition of nanoparticles and surfactant monomers, improved surface wettability, and better rewetting phenomena.

The confinement of expanding vapor slugs during flow boiling in a microchannel heat sink (MHS) is one of the primary causes that triggers flow instability, resulting in substantial fluctuations in pressure and wall temperatures. Although several geometric modifications of microchannels have been explored, the pivotal role of the top cover plate in mitigating the flow boiling instabilities remains largely unexplored. The present study investigates the impact of flexible, porous, and non-porous membrane-based cover plates on mitigating flow boiling instabilities in open microchannel configurations. Flow boiling experiments are conducted on a flow boiling loop using deionized (DI) water as the working fluid, with heat fluxes ranging from 20 to 240 W/cm2 and coolant mass flows from 256 to 536 kg/m2s. The test module comprises an array of 11 rectangular microchannels with a hydraulic diameter of 0.5 mm (500 microns). The open microchannel features a 300 micron gap between the microchannel face and the cover plate. The influence of a flexible-porous PTFE (Polytetrafluoroethylene) membrane-based cover plate (OVV) and a flexible-non-porous Nitrile membrane-based cover plate (ONV-1& ONV-2) on flow boiling is analyzed in comparison to an open microchannel with a rigid cover plate (OMC). The porous PTFE membrane-based cover plate has considerably reduced the pressure drop and has effectively alleviated flow boiling instabilities. Continuous vapor venting through the pores enables periodic extraction of excess vapor slugs from the MHS, thereby reducing pressure and temperature fluctuations in the channels.

The thermomechanical behavior, including oxidation, of Zircaloy-4 cladding (internally pressurized at 10 bar) is studied by performing two-step isothermal heating in the steam environment. The first step of isothermal heating at 300 degrees C represents the normal reactor operation, whereas the second step at higher temperatures above 600 degrees C represents the loss-of-coolant accident (LOCA) severity. One preoxidized and three as-received samples have been used in this study. The preoxidized sample mimics the case of high burnup. Ballooning is insignificant at low temperatures, even if the cladding is heated for a longer duration, whereas it is large if the cladding is at high temperatures, even for shorter durations. Oxide layer thickness is proportional to temperature and exposure time. FESEM images measurements confirm the presence of circumferential and radial cracks. Circumferential cracks are found in long-duration heated samples. Ballooning causes the tearing of tube surfaces to form cavities, exposing the base metal for further oxidation. Moreover, the radial cracks are found on the newly grown oxide layer in these cavities of the preoxidized sample. XRD and Raman spectra confirm the presence of monoclinic and tetragonal zirconia phases within the oxide layers of both as-received and preoxidized samples. Nanoindenter is used to measure the mechanical properties, and it was found that the local hardness and elastic modulus of the zirconia are much higher than those of the base metal. The oxygen-rich alpha-Zr(O) beneath the oxide layer has properties lying between those of the zirconia and base metal. The FEA modelling of the nanonindentation process of the oxygen-rich Zircaloy-4 (having a higher elastic modulus) shows smaller plastic zones concentrated around the indentation mark.

The dynamics of an impacting droplet can be controlled by microstructuring the surface. This work presents a numerical investigation of droplet impact characteristics on a hydrophobic surface microstructured with sinusoidal wavy patterns. The effects of amplitude and wavelength of the wavy surfaces on droplet deformation are analyzed for different impact velocities. A dynamic contact angle model is incorporated into the numerical method to track the three-phase contact line accurately. The influence of the Weber number on the wettability transition is analyzed, and the flow characteristics inside the droplet at different flow regimes are explained. A regime map is prepared to show the transition between different regimes for different surface attributes and Weber numbers. The initial contact of the impacting droplet with the surface is influenced by the amplitude and wavelength of the roughness element, leading to different wettability states. The Wenzel (wetting), Cassie (non-wetting), and mixed wetting states affect the droplet's spreading, recoiling, and rebound characteristics. The Cassie state of a plane surface is transformed into the Wenzel state due to microstructuring the surface with small amplitude and wavelength. A further increase in amplitude and wavelength leads to a transition from the Wenzel state to a mixed state, and finally from the mixed state to the Cassie state. The amplitude-controlled mixed state results in partial rebound of the droplet, whereas the wavelength-controlled state results in partial rebound and rebound with droplet breakup. The study may aid in designing droplet retention surfaces required for practical applications.

Cooling of modern gas turbine blades and vanes is crucial, as they are exposed to extremely high temperatures. The present investigation makes a three-dimensional conjugate heat transfer analysis of internal convection of a ribbed gas turbine vane-blade system under rotational conditions. Simulations have been performed for a single turbine stage, including stationary vanes and rotating blades. The performance of rib-roughened coolant channels is compared to that of smooth channels for different conditions. Flow turbulence is resolved using the Shear Stress Transport (SST k-omega) model with automatic wall function. The compressibility effects in the external hot-gas flow and the rotational effect-induced complex flow patterns inside the rib-roughened coolant channels have been analyzed. The Coriolis force, caused by the rotation of blades, affects the internal flow structures within the smooth and ribbed coolant channels. Ribbed channels significantly enhance local heat transfer by disrupting Coriolis-induced vortex structures and promoting secondary flow. The flow and heat transfer characteristics within coolant channels vary at different blade positions. A transition from supersonic flow at the leading edge to the transonic flow at the trailing edge of the suction side of rotor blades is observed. Compared to the smooth channels, the ribbed channels achieve a blade temperature reduction of up to 191.8 K and a 49.9% increase in the Nusselt number, with a moderate pressure penalty. The study highlights the critical role of rib geometry and rotational effects in optimizing the internal cooling performance of turbine blades for high-temperature applications.

Surface wettability significantly affects the dynamic behavior of impacting droplets. Recent studies have utilized the electrowetting (EW) effect to control the spreading and recoiling motions of droplets impacting hydrophobic substrates by modulating surface wettability. Electrowetting enhances droplet spreading while substantially reducing rebound tendencies. This study uses numerical methods to analyze the droplet’s behavior on a hydrophobic substrate under partial electrowetting conditions. When a droplet reaches its maximal spreading width, the electrowetting process is turned off. This method increases the maximal spreading width while decreasing the recoiling time compared to scenarios without electrowetting. Therefore, droplet rebounds from the surfaces quickly. Furthermore, the recoiling time decreases with higher frequencies and Weber numbers. Understanding the droplet dynamics has broad applications in areas such as self-cleaning surfaces, spray cooling, and microfluidic devices.

The droplet’s impact dynamics on the superhydrophobic substrate can be manipulated by varying the viscosity of the droplet. A phase-field numerical technique with a dynamic contact angle (DCA) approach is used to investigate the droplet impact on a superhydrophobic surface. With the increase in the Weber number (We), the spreading factor ( β) of the impacting droplet increases, and the non-dimensionalized spreading height decreases for all Ohnesorge numbers. The minimum value of non-dimensionalized spreading height is attained in the recoiling phase rather than when the droplets attain maximum spreading (D_max ) . It means the droplet velocity decreases at the center when the droplet is at its maximum spreading factor. The spreading factor ( β) decreases, and the minimum value of non-dimensionalized spreading height increases at the droplet's center with the increase in the Oh for all We because of the more losses of kinetic energy (K.E) into the viscous dissipation energy. The time required to rebound the droplet is the same up to Ohnesorge number = 0.0916, and after an increment in Ohnesorge number, bouncing time increases. With the further increase in Ohnesorge number, the droplet completely suppresses the rebound behavior of the droplets. The droplet's height decreases as the Ohnesorge number increases for all Weber numbers when the droplet starts to rebound. The percentage change in β_max is 61.75

Flow boiling in microchannels involving phase-change heat transfer is a promising technique for high heat dissipation from electronic devices. However, two-phase heat transfer in microchannels is adversely affected by flow boiling instabilities. The present work explores a passive vapor venting technique featuring a superhydrophobic stainless steel mesh screen and a fluidic dampener to suppress flow boiling instabilities in microchannels. Experimental investigations have been made to study the flow and heat transfer characteristics in conventional microchannels, i.e., non-venting (NV) and microchannels with passive vapor venting (VV) configuration. Microchannels have been fabricated on a copper block, chosen for its high thermal conductivity, and enclosed within a Teflon housing to enhance thermal insulation. A superhydrophobic coating of Cytonix Fluropel is applied on the stainless steel mesh to make it superhydrophobic. Flow boiling experiments have been performed using deionized water, with mass fluxes ranging from 255-535 kg/m2s and heat fluxes from 10 to 220 W/cm2. An early evacuation of large vapor slugs in the VV configuration eliminates the vapor clogging and backflow, thus promoting frequent rewetting of the hotspots in the microchannels. The high amplitude and low frequency flow fluctuations in NV configurations are converted into low amplitude and high frequency fluctuations in the VV configuration. The temperature and pressure fluctuations are drastically reduced in the VV configuration, resulting in a 95 % higher heat transfer coefficient (HTC) than in the NV configuration. It also produces a 47 % reduction in pressure drop compared to the NV configuration.

Droplet impact on a hydrophobic surface is affected by various parameters. In this draft, the effects of droplet shape, size, and velocity during its impact on a hydrophobic surface have been reported. Numerical simulations have been performed to investigate the impact characteristics of ellipsoid droplets with different aspect ratios. After impact, the spreading and recoiling rates of droplets with a low aspect ratio (oblate droplets) are higher than those with a higher aspect ratio (prolate droplets). However, the maximum spreading factor increases with the aspect ratio, indicating that droplets achieve a greater spread than oblate droplets. Additionally, oblate droplets demonstrate shorter spreading, recoiling, and bouncing durations than prolate droplets. The initial contact surface area and falling orientation of an ellipsoid droplet significantly influence these behaviors. The role of initial kinetic energy, represented by the Weber number, is more pronounced in prolate droplets than in oblate ones, with the aspect ratio's effects becoming more evident at higher kinetic energy levels.

Precise control over microdroplets is vital in various microfluidic applications like drug delivery and cancer cell separation, among others. At micro/nanoscale dimensions, the conventional no-slip wall boundary condition becomes unreliable, giving rise to slip velocity at channel walls. This study investigates the influence of wall slip on droplet splitting dynamics in a microfluidic T-junction using a conservative two-phase level set method implemented in COMSOL Multiphysics (R) (version 5.3). The slip length ( beta) has been varied from 0.1 to 3 mu m, with capillary numbers ( Ca) ranging from 0.0071 to 0.0338, while fluid properties were kept constant to isolate the effect of wall slip on droplet dynamics. The splitting process has been categorized into three stages: entering, squeezing, and post-splitting. Increasing the slip length leads to a flatter velocity profile, reduced shear-induced resistance, and altered pressure gradients, all of which influence droplet deformation and splitting. At smaller slip lengths, enhanced fluid-solid interactions lead the droplet tip to advance farther into the daughter channel before undergoing splitting. Results also reveal that higher slip lengths and capillary numbers accelerate neck thinning and shorten splitting time. As the slip length increases and the capillary number decreases, the neck center of the droplet interface shifts closer to the junction inlet. Finally, flow map diagrams with power-law correlations identify distinct flow regimes, including no splitting, splitting with tunnel, and splitting with obstruction, with regime boundaries shifting toward higher droplet lengths as slip length increases at a fixed capillary number. This work provides key insights into the interplay between slip length and droplet dynamics, offering a pathway for optimizing microfluidic platforms in biomedical and lab-on-a-chip technologies.

An attempt has been made to explore the combined effect of structured surfaces (segmented finned surface and uniform cross-section surface) and Rhamnolipid biosurfactant to improve heat transfer rate in pool boiling. Experimental investigations were performed at normal atmospheric condition and till reaching critical heat flux. The results shows that the segmented finned surface has shown better heat transfer than plain surfaces, with a 200% increase in heat transfer coefficient. In contrast, a uniform cross-section surface showed an improvement in heat transfer coefficient of 107% over a plain surface. The reasons for better thermal performance in segmented finned configuration can be associated to the reduced surface tension of the working fluid increasing more nucleation sites. Additionally, better rewetting phenomena and bubble evolution from the segmented finned surface have also assisted in improving the heat transfer rate. Moreover, the pool boiling results of all the heating surfaces with surfactant solution is compared with pure water. It has been observed that all heating surfaces have performed better with surfactant solution compared to pure water. However, the CHF obtained on all heating surfaces with surfactant solution were lesser than pure water. Low CHF is due to the foamability of the surfactant solution.

Flow boiling in microchannels can effectively address the challenges of high power density heat dissipation in electronic devices. However, the intricate bubble dynamics during the two-phase flow in microchannel necessitates understanding the characteristics of complex bubble hydrodynamics. In this study, we perform 2D numerical simulations of flow boiling using the Cahn-Hilliard phase-field method for a 200-mu m width microchannel with single and multiple cavities in COMSOL Multiphysics (V5.3). The numerical model successfully captures bubble dynamics, encompassing vapor embryo generation, bubble growth, departure, coalescence, sliding, and stable vapor plug formation. The heat transfer mechanism inside the microchannel is dominated by bubble nucleation and thin-film evaporation. Elevated wall superheats in a single nucleation cavity, and increased mass flux facilitates higher bubble departure frequency and heat transfer performance. Temporal pressure fluctuations are observed inside microchannels in multiple cavities due to bubble coalescence, departure, and subsequent nucleation. Increasing the nucleating cavities from 2 to 5 within the microchannel while maintaining consistent cavity spacing of 100 mu m has resulted in nearly 32% enhancement in heat transfer performance. This study offers valuable findings that can help improve the thermal management of electronic devices.

Lithium titanate oxide is becoming a prominent alternative to graphite as an anode in lithium-ion batteries due to its long cycle life, fast charging/discharging, and ability to function at low ambient temperatures. However, lithium-ion batteries are susceptible to catastrophic thermal runaway under extreme and abusive conditions. The present study proposes a novel channeled dielectric fluid immersion cooling system for the 23Ah lithium titanate oxide batteries modeled using an equivalent circuit model within a multi-scale, multi-domain framework using the commercial solver ANSYS. The novel cooling system reduced maximum temperature and improved temperature uniformity with less immersion fluid requirement than the generic designs. Hydrofluoroether HFE-6120 turned out to be the most effective coolant, and 0.5LPM is the optimum flow rate. When subjected to dynamic loading, the proposed cooling system restricted the maximum temperature to 299.2 K in the aged battery pack with 3972 cycles. Further, the reduced order model is utilized in lieu of the equivalent circuit model to reduce computational time for analyzing the battery pack. Compared to the full-order equivalent circuit model, the reduced order model efficiently captures the battery dynamics with minimal deviations of 1 % and 1.5 % in voltage and temperature, respectively. The computational time is reduced by 26 % with the reduced order model. The proposed cooling system limits the maximum temperature and non-uniformity in the battery pack to 302 K and 1 K under aggressive scaled-down US06 loading conditions with continuous 9 degrees gradeability. This research will be helpful for the future development and understanding of immersion cooling systems for high-energy LTObased batteries under the impact of aging and gradeability.

Due to its precise control over droplet sizes and generation rates, droplet-based microfluidics has become widely utilized across various fields, such as food processing, medical diagnostics, and drug delivery. The integration of nanoparticles has notably advanced droplet synthesis by preventing undesirable coalescence. However, despite these advancements, a thorough comprehension of how nanoparticles influence microdroplet generation within microfluidic channels remains elusive. In this experimental study, we have explored the impact of SiO2 nanofluid as a dispersed phase at various concentrations within a T-junction microchannel, with silicone oil serving as the continuous phase. This investigation elucidates the non-Newtonian characteristics of SiO2 nanofluid, demonstrating shear thickening behavior that correlates with elevated viscosity, interfacial tension, and contact angle as nanoparticle concentration rises. The distinct regimes and stages of droplet generation for different concentrations of nanoparticles have been reported. The increase in nanoparticle concentration typically leads to extended production times and larger droplet sizes, primarily driven by simultaneous viscosity and surface tension increments. Increased primary phase flow rates lead to a decrease in droplet length alongside an increase in droplet formation frequency. Elevating the flow rate of the secondary phase fluid results in an augmentation of both droplet length and droplet generation frequency. This study highlights the interplay between rheology and the microfluidic generation of nanofluid droplets, marking a significant milestone in colloid science.

In recent years, the automobile industry has witnessed a revamp of its fossil fuel-driven conventional vehicles by electric vehicles (EVs) and hybrid electric vehicles (HEVs). The recent EV fires are the predominant hindrances to the market rise of EVs. This study addresses this problem with the easily retrofitted phase change material (PCM) embedded battery thermal management system. A multi-scale multi-dimensional (MSMD) framework's equivalent circuit model (ECM) is employed to model the battery. The solidification and melting model is used to analyse the n-octadecane PCM. The results concluded that the optimal thickness of the PCM enclosure is 3 mm as the highest reduction of 2.8 K in the maximum temperature of the battery pack (Tmax) is witnessed. The PCM embedded design has lowered the Tmax by 2.82 K, 2.82 K, and 2.63 K when the 4S2P battery pack is discharged at constant C-rates of 10C, 8C, and 6C, respectively. The batteries in immediate contact with PCM (side-BATT) have shown significantly lower Tmax values than central batteries (centre-BATT). A reduction of 43.38 K, 1.24 K in Tmax is observed in side-BATT, whereas 2.48 K, 0.6 K in centre-BATT when the battery pack is discharged at constant 10 C-rate and dynamic loading, respectively.

For effective heat dissipation from turbine blades, consistent efforts have been made to explore several passive heat transfer enhancement techniques in the internal coolant channels of modern gas turbine blades. Various researchers have studied different types of turbulators and vortex generators, like ribs, dimples, and protrusions inside the channel. The present work investigates the thermal performances of a rectangular-shaped cooling channel of a 4-aspect ratio with multiple arrays of compound rib dimples arranged on top and bottom walls for realistic conditions. Numerical simulations have been performed for two types of ribs, i.e., 45 degrees V-rib and broken V-rib with the spherical dimple structures for Reynolds numbers ranging from 20,000 to 80,000. The effects of rib height and dimple depth on flow and heat transfer characteristics are presented for different combinations of rib-dimple structures. The compound rib structures result in the formation of longitudinal vortices, recirculating vortices, and counter-rotating vortices in different scales associated with flow recirculation, flow separation, and flow reattachment. Consequently, significant turbulence and heat transfer improvements are observed in specific regions of the coolant channels. The maximum heat transfer enhancement factor of 2.46, i.e., the ratio of heat transfer rate with rib structure and heat transfer rate in smooth channels, is observed for the configuration of 45 degrees V rib, 1.5 mm height with spherical dimple of 4.0 mm depth. Similarly, a 45 degrees V rib with a height of 1.0 mm and a spherical dimple of 2.0 mm depth produces the maximum thermal performance factor of 1.205.

Surface wettability influences the droplet impact characteristics, especially for a droplet impacting with low inertia. The present work reports an experimental investigation of droplet impact on homogeneous and heterogeneous wettability surfaces for different Weber numbers. Droplet impact characteristics on surfaces with three homogeneous surface wettabilities, i.e., hydrophilic, hydrophobic, and superhydrophobic, and two heterogeneous surface wettabilities, i.e., hydrophilic–hydrophobic and hydrophilic–superhydrophobic, have been analyzed. The symmetric deposition, spreading, and recoiling on homogeneous surfaces are affected by the surface wettability gradient across the droplet on heterogeneous surfaces resulting in asymmetric behavior. Furthermore, hybrid wettability surfaces suppress the partial rebound, complete rebound, and complete rebound with droplet breakup observed in the homogeneous hydrophobic and superhydrophobic surfaces. The initial inertia force of the droplet significantly affects the asymmetric and droplet migration behavior. The average recoiling velocity of the droplet increases with the inertia of the droplet. The rate of increase in droplet migration is maximum for a Weber number of 12 for both surfaces with hybrid wettability. The analysis of asymmetric spreading and migration of impacting droplets on heterogeneous surfaces is important in enormous applications, such as microfluidic devices, self-transport of liquid, and water harvesting.

An extensive experimental investigation was performed to study the oxygen embrittlement of the Indian Pressurized Heavy Water Reactor (PHWR) fuel pin under simulated Loss-of-Coolant Accident (LOCA) conditions. Zircaloy fuel cladding experiences creep and corrosion simultaneously during service and LOCA conditions. Zircaloy-4 fuel pins were pre-oxidized to attain different oxide layer thicknesses, achieving in-service conditions. These pre-oxidized tubes were then subjected to burst tests in the steam environment to mimic the LOCA scenario. The present study aims to improve the understanding of the effect of oxidation on the cladding microstructure and the mechanical response of the fuel pin in a LOCA scenario by accounting for the cross-influence, during transient heating, of oxidation and deformation on the behavior of the clad in the LOCA domain. The oxide layer morphology in pre- and post-burst samples was studied using FESEM, XRD, and Raman spectroscopy. In some cases, the inner oxide layer grew faster than the outer oxide layer when the fuel pin was heated in steam during the burst test. The evolution during transient heating of radial and circumferential crack growth in the oxide layer and the occurrence of delamination facilitated faster oxygen and hydrogen uptake. The hydrogen uptake in pre and post-burst samples was related to the oxygen uptake. The hydrogen concentration increases with the oxygen concentration in the pre-oxidized samples. Small oxygen and hydrogen concentrations were found in the post-burst as-received samples due to the formation of a protective oxide layer. The hightemperature oxide layer was formed at extremely high heating rates.

Droplet impact on surfaces integrated with the electrowetting effect has been recently explored to control droplet spreading and recoiling behavior on hydrophobic surfaces. With the integration of electrowetting, the spreading of the impacting droplet on hydrophobic surfaces increases, whereas the rebound tendency is suppressed. The present work numerically investigates the droplet impact on hydrophobic surfaces under partial electrowetting (EW) effects. In partial EW, the electrowetting effect is immediately cut off once the droplet attains the maximum spreading diameter. In addition, based on the energy conservation principle, a mathematical model is developed to predict the maximum spreading diameter of the droplet and its kinetic energy during bouncing for different parameters. The partial EW technique enhances the maximum spreading diameter and reduces the recoiling time compared to full EW and no EW effect. The recoiling time decreases with the increase in voltage amplitude, frequency, Weber number, and surface wettability.