Sateesh Gedupudi

In the present study, a one-dimensional model is proposed to estimate the pressure drop and heat transfer coefficient for flow boiling in a rectangular microchannel. The present work takes into account the pressure fluctuations caused due to the confined bubble growth and the effect of pressure fluctuations on the heat transfer characteristics. The heat transfer model considers five zones, namely, liquid slug, partially confined bubble, fully confined (elongated) bubble, partial dryout and full dryout. The model incorporates the thinning of liquid film due to shear stress at liquid-vapour interface in addition to evaporation. The transient fluctuations in pressure and heat transfer coefficient, along with the time-averaged ones, are verified with the experimental data available in the literature. Heat transfer characteristics with flow reversal caused by inlet compressibility are also presented.

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 fluids in the case of particle alone and combined particle and surfactant case. We also discuss the role of imbibitions across the particle wedge in the precursor film in tuning spreading dynamics. We propose an analytical model to predict the nature of dependency of contact radius on time for the complex colloids. A detailed theoretical examination of the governing factors, the interacting forces at the three phase contact line, and the effects of interplay of surfactants and the nanoparticles at the precursor film in modulating the spreading dynamics has been presented for such complex colloids.

The present work reports the findings from an experimental study on the effect of bypass inlet on the performance of flow boiling of water in a copper mini/micro-channel of dimension 2.5 mm wide x 0.6 mm deep x 25 mm long. The mass fluxes considered were 430 kg/m2 s, 640 kg/m2 s and 850 kg/m2 s and the heat flux was in the range 87–548 kW/m2. The results show a significant increase in heat transfer coefficient with the increase in bypass ratio for the subcooled boiling conditions and the enhancement decreases with the increase in the exit quality. An increase in pressure drop was also noticed due to the bypass inlet. The study indicates potential for further investigation and optimization.

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. Graphical abstract: [Figure not available: see fulltext.].

Heat Transfer Engineering: Fundamentals and Techniques reviews the core mechanisms of heat transfer and provides modern methods to solve practical problems encountered by working practitioners, with a particular focus on developing engagement and motivation. The book reviews fundamental concepts in conduction, forced convection, free convection, boiling, condensation, heat exchangers and mass transfer succinctly and without unnecessary exposition. Throughout, copious examples drawn from current industrial practice are examined with an emphasis on problem-solving for interest and insight rather than the procedural approaches often adopted in courses. The book contains numerous important solved and unsolved problems, utilizing modern tools and computational sources wherever relevant. A subsection on common issues and recent advances is presented in each chapter, encouraging the reader to explore a greater diversity of problems.

Incorporating insulation material in the building envelope is one of the simple yet effective passive techniques aimed at mitigating the energy consumption in buildings. The development and utilization of sustainable materials with heat insulating capacity as an alternative to synthetic insulations is a promising path towards reduced carbon footprint of buildings. The insulation potential of straw, an agricultural waste, in the context of the wide-ranging climate of India is the focus of the current work. Cooling and/or heating load analysis over 24 h of a representative summer and winter day is performed through transient numerical analysis for five climatic zones of India. Three building envelope configurations possible with retrofitting straw insulation (placed on the outside or inside or equally on either side) over existing walls/roof are compared. Four different thicknesses (10, 20, 30 and 40 cm) of the straw insulation are considered for analysis. Recommendations of envelope configuration with insulation are proposed based on energy and cost savings for the different climatic zones. Overall, the case of straw insulation split on either side performs the best. Energy savings in the range of 67–96% is achievable with the addition of just 10 cm thick straw across different climatic zones. The work also presents a comparison of the straw envelope performance against different types of envelopes based on sustainable materials found in literature.

The heat insulating potential of straw, an agricultural residue, used as part of the envelope of unconditioned buildings is the focus of the current work. Using a CFD software with experimentally validated boundary conditions, transient thermal analysis is performed on a 3-D model room of dimensions 3 m × 3 m × 2.5 m with vents and heat source representing a naturally ventilated indoor space. The thermal performances of a reference uninsulated concrete envelope and a straw insulation-based envelope are compared in terms of quantitative parameters such as indoor air temperature, decrement factor, and time lag as well as a qualitative discussion on the need for occupant adaptation, namely closing or opening of inlet vent and requirement of additional heating. The assessment and subsequent recommendation of straw insulation utilization is done for representative summer and winter days of the five different climatic zones of India. A maximum reduction of up to 3.5 °C in the peak indoor air temperature is achievable with inlet temporarily closed when straw insulation is included in the envelope in summer conditions. In winter conditions of cold climatic zone, with inlet always closed, use of straw insulation leads to the lessening of additional active heat source requirement by as much as 600 W for the considered model room. Further, the effects of increased straw insulation thickness, removal of heat source, varying inlet air velocity, and changing the position of the straw insulation to the outside are also analyzed for the extreme warm and cold conditions.

A 1-D numerical modelling of the dynamics of inclined closed loop buoyancy driven heat exchangers with inclusion of the wall conduction effect at the heat exchanger is presented in the current study. A Coupled Natural Circulation Loop (CNCL) is an ideal system for studying the closed loop buoyancy driven heat exchanger. The modelling utilises a Fourier series based approach to develop a 1-D model of the Conjugate CNCL system, which is then verified with the 3-D CFD studies of the respective cases. A good agreement is observed with the 3-D CFD data, which demonstrates the suitability of the 1-D model for transient behaviour prediction. The non-dimensional numbers and thermal coupling sensitivity coefficients which govern the dynamics of the Conjugate CNCL are identified and an appropriate parametric study is conducted. Results show that the wall conduction and inclination have a significant effect on the transient behaviour of the Conjugate CNCL system. A jump in the heat transfer coefficient with variation in the inclination of the Conjugate CNCL system is observed. The 1-D model is also able to capture the flow direction reversal with change in the inclination of the Conjugate CNCL system for zero flow field initial conditions.

To elucidate the pure physics of evaporation which is free from surface effects, the pendant mode of evaporation is employed in the present study. The present study brings out the evaporation kinetics of a combined surfactant and nanoparticle colloidal system. We also segregate the contributing effects of surfactants alone, particle alone, and the combined effect of surfactant and particles in modulating the evaporation kinetics. It is observed that the rate of evaporation is a strong function of the particle concentration for nanocolloidal suspensions of particle alone and concentration of surfactant molecules up to the micellar concentration and thereafter insensitive to concentration for an aqueous surfactant solution. The combined colloidal system of nanoparticles and surfactant exhibited the maximum evaporation rate, and the rate is a strong function of the concentration of both the particle and surfactant. The theoretical classical diffusion-driven evaporation falls short of the experimentally observed evaporation rate in aqueous surfactant and colloidal solutions. Evidence of convective currents was observed in flow visualization studies in aqueous surfactant solutions, nanocolloidal solution of particle alone, and an oscillatory convective circulation in a combined surfactant-impregnated nanocolloidal solution. Thermal Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found not potent enough to induce strong circulation currents in such systems from a stability map. Scaling analysis of solutal Marangoni is observed to be capable of inducing circulation from a stability map in all the systems and the enhanced thermophoretic drift and Brownian dynamics, and enhancement in the diffusion coefficient of the nanoparticles is also contributing to the enhanced evaporation rate for only nanocolloidal solutions. The oscillatory convective current arising out of two opposing driving potential enhances the evaporation rate of surfactant-impregnated nanocolloids. The present findings could reveal the effect of surfactants in tuning the evaporation rate of colloidal solutions.

Two-phase microchannel heat sinks generate higher pressure drop along with higher heat transfer coefficient. Literature review indicates that the addition of a vapour venting membrane to a microchannel reduces the flow boiling pressure drop, but the literature does not report the effect of the channel aspect ratio with a vapour venting membrane. The present study experimentally investigates the influence of vapour venting on the flow boiling pressure drop and heat transfer of water in copper mini/micro-channels of three different aspect ratios – 0.31, 0.92 and 3.7, with a hydraulic diameter of 0.60 mm and a length of 40 mm. Experiments were conducted for two different cases – one with a Lexan cover on top and the other with a PTFE hydrophobic membrane on top of the channel. Two membranes of different pore sizes 0.22 μm and 0.45 μm were chosen. The mass fluxes maintained were 270, 450 and 650 kg/m2s and the channel heat flux varied from 200 to 620 kW/m2. For the minimum mass flux case, the highest aspect ratio channel with vapour venting membrane showed a maximum reduction of 60% in the two-phase pressure drop along with a 10% reduction in the heat transfer coefficient, compared to that without membrane. The lowest aspect ratio channel with vapour venting membrane showed a maximum increase of 30% in the two-phase heat transfer coefficient accompanied by a 10% reduction in the two-phase pressure drop, compared to that without membrane.