Other affiliations: Brunel University London
Bio: Sateesh Gedupudi is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topic(s): Heat transfer coefficient & Heat transfer. The author has an hindex of 10, co-authored 50 publication(s) receiving 412 citation(s). Previous affiliations of Sateesh Gedupudi include Brunel University London.
05 May 2017-European Physical Journal E
TL;DR: The present paper conclusively explains several physical phenomena observed, yet hitherto unexplained, in the case of the surface tension of such complex fluids by segregating the individual contributions of each component of the colloidal system.
Abstract: A systematically designed study has been conducted to understand and demarcate the degree of contribution by the constituting elements to the surface tension of nanocolloids. The effects of elements such as surfactants, particles and the combined effects of these on the surface tension of these complex fluids are studied employing the pendant drop shape analysis method by fitting the Young-Laplace equation. Only the particle has shown an increase in the surface tension with particle concentration in a polar medium like DI water, whereas only a marginal effect of particles on surface tension in weakly polar mediums like glycerol and ethylene glycol has been demonstrated. Such behaviour has been attributed to the enhanced desorption of particles to the interface and a theory has been presented to quantify this. The combined particle and surfactant effect on the surface tension of a complex nanofluid system showed a decreasing behaviour with respect to the particle and surfactant concentration with a considerably feeble effect of particle concentration. This combined colloidal system recorded a surface tension value below the surface tension of an aqueous surfactant system at the same concentration, which is a counterintuitive observation as only the particle results in an increase in the surface tension and only the surfactant results in a decrease in the surface tension. The possible physical mechanism behind such an anomaly happening at the complex fluid air interface has been explained. Detailed analyses based on thermodynamic, mechanical and chemical equilibrium of the constituents and their adsorption-desorption characteristics as extracted from the Gibbs adsorption analysis have been provided. The present paper conclusively explains several physical phenomena observed, yet hitherto unexplained, in the case of the surface tension of such complex fluids by segregating the individual contributions of each component of the colloidal system.
TL;DR: In this article, a 3D numerical simulation of bubble growth from nucleation to full confinement was performed using the volume of fluid (VOF) method in commercial CFD code FLUENT.
Abstract: This Part II of the paper reports the three-dimensional (3-D) numerical modelling on bubbly flow in confined mini-/micro-channels using the volume of fluid (VOF) method in commercial CFD code FLUENT. The numerical simulation aims to provide detailed information of the fields of velocity, temperature and pressure so as to further understand the effect of bubble growth on the flow field and heat transfer from the channel wall. In Part I, the experiment of flow boiling in a mini-/micro-channel of rectangular cross-section was carried out and a simple one-dimensional (1-D) model for the interaction of the pressure fluctuations during the growth of a confined bubble with various kinds of upstream compressibility was developed as an aid to the rational specification of inlet resistance. In Part II, the experimental observers and the theoretical model developed in Part I are tested by performing the 3-D numerical simulation of bubble growth from nucleation to full confinement. The simulation involves some approximations based on a concept of pseudo-boiling to avoid the well-known difficulties of modelling bubble generation and growth. During the simulation, the volumetric growth rate of the bubble is defined to match the experimental observations. At small times prior to bubble detachment, a vapour flow was injected through a small hole in the wall to simulate nucleation. Following partial confinement, vapour injection was stopped and growth was driven by the generation of vapour at a defined rate at the contact area between the bubble and the superheated wall. The 3-D simulation reproduces the experimental observations of the distorted profile of the bubble and its trajectory during partially confined growth and provides information about flow and heat transfer in the bulk liquid outside the thin film region. The 3-D and 1-D predictions of the development of axial pressure distributions during partially and fully confined growth are in satisfactory agreement.
TL;DR: In this article, a simple 1-D model for bubble growth in a single channel with a compressible volume in its upstream plenum is developed as a tool for the rational design of measures known to reduce flow instabilities, namely inlet resistance and enhanced nucleation in every channel.
Abstract: Heat sinks using evaporation in arrays of parallel microchannels have potential for the removal of high heat fluxes from small areas. They suffer from flow instabilities and uneven distribution between channels that may cause local dryout and overheating. The current state of the art is reviewed critically. A simple 1-D model for bubble growth in a single channel with a compressible volume in its upstream plenum is developed as a tool for the rational design of measures known to reduce flow instabilities, namely inlet resistance and enhanced nucleation in every channel. The model considers two stages of partially and fully confined bubble growth in a single channel of rectangular cross-section, suggested by experimental observations, followed by venting of vapour to the downstream plenum. The experiments also show the influence of apparently minor changes in rig design and operation on upstream compressibility and flow reversal. The model considers upstream compressibility due to subcooled boiling in a preheater or trapped non-condensable gas and the reduction of flow reversal by inlet resistance. The feasibility of measuring transient axial variations in pressure within small channels using inexpensive transducers is demonstrated.
TL;DR: In this article, the combined effect of surfactants and nanoparticles on the wettability of complex fluids such as nanocolloids on different substrates, ranging from hydrophilic with a predominantly polar surface energy component (silicon wafer and glass) to near hydrophobic range with a predominant dispersive component of surface energy (aluminum and copper substrates).
Abstract: Even though there are quite large studies on wettability of aqueous surfactants and a few studies on effects of nanoparticles on wettability of colloids, to the best of authors’ knowledge, there is no study reported on the combined effect of surfactant and nanoparticles in altering the wettability. The present study, for the first time, reports an extensive experimental and theoretical study on the combined effect of surfactants and nanoparticles on the wettability of complex fluids such as nanocolloids on different substrates, ranging from hydrophilic with a predominantly polar surface energy component (silicon wafer and glass) to near hydrophobic range with a predominantly dispersive component of surface energy (aluminum and copper substrates). Systematically planned experiments are carried out to segregate the contributing effects of surfactants, particles, and combined particle and surfactants in modulating the wettability. The mechanisms and the governing parameters behind the interactions of nanocol...
TL;DR: The present study experimentally proves that the classical Tanner's law is disobeyed by the complex fluids in the case of particle alone and combined particle and surfactant case and proposes an analytical model to predict the nature of dependency of contact radius on time for the complex colloids.
Abstract: While a body of literature on the spreading dynamics of surfactants and a few studies on the spreading dynamics of nanocolloids exist, to the best of the authors’ knowledge, there are no reports on the effect of presence of surfactants on the spreading dynamics of nanocolloidal suspensions. For the first time the present study reports an extensive experimental and theoretical study on the effect of surfactant impregnated nanocolloidal complex fluids in modulating the spreading dynamics. A segregation analysis of the effect of surfactants alone, nanoparticle alone, and the combined effect of nanoparticle and surfactants in altering the spreading dynamics have been studied in detail. The spreading dynamics of nanocolloidal solutions alone and of the surfactant impregnated nanocolloidal solutions are found to be grossly different, and particle morphology is found to play a predominant role. For the first time the present study experimentally proves that the classical Tanner’s law is disobeyed by the complex ...
25 Mar 2017-Applied Thermal Engineering
TL;DR: In this paper, a discussion of the possible applications of flow boiling in microchannels in order to highlight the challenges in the thermal management for each application is presented. But, several fundamental issues are still not understood and this hinders the transition from laboratory research to commercial applications.
Abstract: The rapid advances in performance and miniaturization of electronics and high power devices resulted in huge heat flux values that need to be dissipated effectively. The average heat flux in computer chips is expected to reach 2–4.5 MW/m2 with local hot spots 12–45 MW/m2 while in IGBT modules, the heat flux at the chip level can reach 6.5–50 MW/m2. Flow boiling in microchannels is one of the most promising cooling methods for these and similar devices due to the capability of achieving very high heat transfer rates with small variations in the surface temperature. However, several fundamental issues are still not understood and this hinders the transition from laboratory research to commercial applications. The present paper starts with a discussion of the possible applications of flow boiling in microchannels in order to highlight the challenges in the thermal management for each application. In this part, the different integrated systems using microchannels were also compared. The comparison demonstrated that miniature cooling systems with a liquid pump were found to be more efficient than miniature vapour compression refrigeration systems. The paper then presents experimental research on flow boiling in single tubes and rectangular multichannels to discuss the following fundamental issues: (1) the definition of microchannel, (2) flow patterns and heat transfer mechanisms, (3) flow instability and reversal and their effect on heat transfer rates, (4) effect of channel surface characteristics and (5) prediction of critical heat flux. Areas where more research is needed were clearly mentioned. In addition, correlations for the prediction of the flow pattern transition boundaries and heat transfer coefficients in small to mini/micro diameter tubes were developed recently by the authors and presented in this paper.
TL;DR: In this article, a large pool of published papers on computational simulation of boiling and condensation is reviewed and compared, as well as identification of future research needs to improve predictive computational capabilities.
Abstract: Developments in many modern applications are encountering rapid escalation in heat dissipation, coupled with a need to decrease the size of thermal management hardware. These developments have spurred unprecedented interest in replacing single-phase hardware with boiling and condensation counterparts. While computational methods have shown tremendous success in modeling single-phase systems, their effectiveness with phase change systems is limited mostly to simple configurations. But, given the complexity of phase change phenomena important to many modern applications, there is an urgent need to greatly enhance the capability of computational tools to tackle such phenomena. This article will review the large pool of published papers on computational simulation of boiling and condensation. In the first part of the article, popular two-phase computational schemes will be discussed and contrasted, which will be followed by discussion of the different methods adopted for implementation of interfacial mass, momentum and energy transfer across the liquid-vapor interface. This article will then review papers addressing computational modeling of bubble nucleation, growth and departure, film boiling, flow boiling, and flow condensation, as well as discuss validation of predictions against experimental data. This review will be concluded with identification of future research needs to improve predictive computational capabilities, as well as crucial phase change phenomena found in modern thermal devices and systems that demand extensive computational modeling.
TL;DR: In this article, numerical simulations of single elongated bubbles in flow boiling conditions within circular microchannels were performed, where thin-film evaporation was proved to be the dominant heat transfer mechanism in the liquid film region between the wall and the elongated bubble.
Abstract: Flow boiling within microchannels has been explored intensively in the last decade due to their capability to remove high heat fluxes from microelectronic devices. However, the contribution of experiments to the understanding of the local features of the flow is still severely limited by the small scales involved. Instead, multiphase CFD simulations with appropriate modeling of interfacial effects overcome the current limitations in experimental techniques. Presently, numerical simulations of single elongated bubbles in flow boiling conditions within circular microchannels were performed. The numerical framework is the commercial CFD code ANSYS Fluent 12 with a Volume Of Fluid interface capturing method, which was improved here by implementing, as external functions, a Height Function method to better estimate the local capillary effects and an evaporation model to compute the local rates of mass and energy exchange at the interface. A detailed insight on bubble dynamics and local patterns enhancing the wall heat transfer is achievable utilizing this improved solver. The numerical results show that, under operating conditions typical for flow boiling experiments in microchannels, the bubble accelerates downstream following an exponential time-law, in good agreement with theoretical models. Thin-film evaporation is proved to be the dominant heat transfer mechanism in the liquid film region between the wall and the elongated bubble, while transient heat convection is found to strongly enhance the heat transfer performance in the bubble wake in the liquid slug between two bubbles. A transient-heat-conduction-based boiling heat transfer model for the liquid film region, which is an extension of a widely quoted mechanistic model, is proposed here. It provides estimations of the local heat transfer coefficient that are in excellent agreement with simulations and it might be included in next-generation predictive methods.
TL;DR: In this paper, the authors reviewed the effects of NPs in low salinity water (LSW) and surfactant on the IFT of sandstone and carbonate reservoirs.
Abstract: Nanoparticles (NPs) are currently gaining wide acceptance in the field of petroleum engineering. They are applied in different areas of petroleum exploration and production such as drilling, well logging, reservoir management, and enhanced oil recovery (EOR). Due to the size of NPs, they have special physical and chemical properties. Therefore, NPs can influence the properties of the fluid system, including viscosity, magnetism, and interfacial tension (IFT). The injection of NPs into the reservoirs for EOR is more effective than water injection but not as effective as chemical flooding. Consequently, NPs are injected along with low salinity water (LSW) or chemicals such as surfactant in order to improve the recovery of oil. NPs are used to prevent the fines migration during LSW injection, control the mobility of formation water, and reduce the surfactant adsorption on the pore walls of the reservoir. The improvement in oil recovery, when NPs are injected in combination with LSW or chemicals in the reservoir, can be attributed to the variations in the properties of the fluid system and the rock-fluid interactions. This study comprehensively reviews the mechanisms behind these variations. LSW injection improves the oil recovery by altering the rock wettability from oil-wet to water-wet. However, this study reveals that the dispersion of NPs in LSW does not necessarily change the rock wettability towards water-wet. The wettability of the system may shift towards oil-wet instead of water-wet depending on the concentration of NPs. Improvement in oil recovery depends on the effective surface charge and the volume fraction of dispersed NPs in the solution. Aggregation of NPs in solution should be avoided because it lowers the recovery by plugging the pore throats. The stability of NPs dispersed in solution with increasing concentrations of salt and surfactant is reviewed and the resulting effect on the IFT of the solution is analyzed. NPs dispersed in different types of surfactant show different behaviors of IFT. The behavior of the IFT depends on the concentration of surfactant, the amount of dispersed NPs (concentration), the type of surfactant (anionic, cationic, and non-ionic), and the effective charge of NPs (positive, negative, and neutral). The combination of LSW with surfactant for oil recovery has two opposing impacts. The IFT reduces while the contact angle increases with an increase in salinity. The mechanisms responsible for the variations in the properties of the system when the combination of LSW, surfactant, and NPs are used for oil recovery are reviewed in this study. Understanding the mechanisms behind the interactions at the fluid-fluid and the fluid-solid interfaces will aid in designing an effective fluid system that combines LSW, surfactant, and NPs for successful implementation of EOR in sandstone and carbonate reservoirs.
29 Apr 2019-Physics Reports
TL;DR: In this article, the mass transfer process, the formation and evolution of phase fronts and the identification of mechanisms of pattern formation are discussed, and several potential directions for future research in this area are discussed.
Abstract: Evaporation of a drop, though a simple everyday observation, provides a fascinating subject for study. Various issues interact here, such as dynamics of the contact line, evaporation-induced phase transitions, and formation of patterns. The explanation of the rich variety of patterns formed is not only an academic challenge, but also a problem of practical importance, as applications are growing in medical diagnosis and improvement of coating/printing technology. The multi-scale aspect of the problem is emphasized in this review. The specific fundamental problem to be solved, related to the system is the investigation of the mass transfer processes, the formation and evolution of phase fronts and the identification of mechanisms of pattern formation. To understand these problems, we introduce the important forces and interactions involved in these processes, and highlight the evaporation-driven phase transitions and flows in the drop. We focus on how the deposited patterns are related to and tuned by important factors, for instance substrate properties and contents of the drop. In addition, the formation of crust and crack patterns are discussed. The simulation and modeling methods, which are often utilized in this topic, are also reviewed. Finally, we summarize the applications of drop evaporation and suggest several potential directions for future research in this area. Exploiting the full potential of this topic in basic science research and applications needs involvement and interaction between scientists and engineers from disciplines of physics, chemistry, biology, medicine and other related fields.