BITS Faculty Publications

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Author: search.filters.author.Yadav, Shyam SunderSubject: search.filters.subject.Mechanical Engineering

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Now showing 1 - 9 of 9
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    Oscillations of a Sessile Droplet in Contact and Non-contact Modes Under an AC Electric Field
    (Springer, 2016-09) Yadav, Shyam Sunder
    In the present work we numerically investigate the oscillations of a sessile conducting droplet in the contact and non-contact modes under an alternating electric field. We show that the oscillations in the non-contact mode, where the needle electrode remains away from the drop, are caused by the electric forces due to charge accumulation at the apex of the drop. In the contact mode case, where the needle remains dipped inside the drop, the electric charge accumulates at the drop surface just above the dielectric coating with a maximum value near the three phase contact line. These charges push the three phase contact line outwards with an oscillatory force which leads to drop oscillations. We also observe that higher needle voltage (~1 kV) is required for the non-contact mode while considerably less potential (~10 V) is enough for the contact mode to cause drop oscillations.
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    Numerical simulations of bubble formation from a submerged orifice and a needle: The effects of an alternating electric field
    (Elsevier, 2016) Yadav, Shyam Sunder
    In many applications, such as bubble column reactors, electric field is employed to provide a greater control on the sizes of bubbles forming at orifices and needles. In this study, we investigate the effects of an alternating electric field on the bubble dynamics. We perform numerical simulations of an alternating electric field coupled with two-phase flow using a Coupled Level-Set and Volume-of-Fluid method. We show that bubbles forming at orifices and needles decrease in size (up to ) only for a range of applied frequency and for other frequencies, the size of bubbles can be much bigger compared to the bubbles forming in the corresponding DC electric field case. The oscillating electric forces excite capillary waves on the bubble interface resulting in applied frequency dependent bubble oscillations. The numerically observed resonance for the needle case corresponds to , where is the frequency of the oscillation of the electric field force at the interface and is the capillary time scale, indicating that the resonance behavior is indeed governed by the interactions between the capillary and electric field force. A decomposition of bubble profile shapes into Legendre modes shows that for orifice as well as the needle case, second mode is most dominant followed by the fourth mode.
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    Numerical simulations of bubble formation from submerged needles under non-uniform direct current electric field
    (AIP, 2013-10) Yadav, Shyam Sunder
    In several chemical and space industries, small bubbles are desired for efficient interaction between the liquid and gas phases. In the present study, we show that non-uniform electric field with appropriate electrode configurations can reduce the volume of the bubbles forming at submerged needles by up to three orders of magnitude. We show that localized high electric stresses at the base of the bubbles result in slipping of the contact line on the inner surface of the needle and subsequent bubble formation occurs with contact line inside the needle. We also show that for bubble formation in the presence of highly non-uniform electric field, due to high detachment frequency, the bubbles go through multiple coalescences and thus increase the apparent volume of the detached bubbles.
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    Numerical investigation of a conducting drop’s interaction with a conducting liquid pool under an external electric field
    (Elsevier, 2020-06) Yadav, Shyam Sunder
    A charged conducting drop suspended in an insulating medium shows non-coalescence with an interface under high strength of an externally applied electric field. We perform numerical simulations of the non-coalescence phenomenon to understand the underlying physical mechanisms and the effect of electric field strength and fluid conductivity on the coalescence behavior of a conducting drop with a conducting liquid pool under highly viscous conditions. We show that two factors primarily govern the coalescence or non-coalescence of the drop with the interface. First, the magnitude of the charge transfer time scale (which governs the rate of charge transfer during contact between the drop and the pool) relative to the time scale of the capillary waves. Second, the strength of the electric forces compared to the viscous forces. We further show that for the case of macro drops ( mm), charge transfer by fluid convection dominates charge conduction at lower electric conductivities ( S/m) only. Finally, we explain the non-dependence of secondary droplet’s size and charge on the fluid’s electric conductivity as observed in the experiments.
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    Application of Fuzzy AHP Approach for Evaluation of Sustainable Energy Sources in India
    (CRC Press, 2021) Digalwar, Abhijeet K.; Yadav, Shyam Sunder
    Energy is an essential factor for the socio-economic development of societies and the nation. Energy consumption in India has risen very fast in the last few decades due to industrialization and urbanization. It will further increase in the future. To fulfill the increasing future energy demand, there is a necessity to find out the most sustainable energy source covering environmental, technical, economic, social, political and flexibility criteria. In this analysis both renewable (solar, wind, biomass, hydro) and nonrenewable (thermal, gas power, nuclear) energy sources are considered. Experts’ weight is collected in linguistic terminology and we employed a fuzzy analytical hierarchy process (AHP) approach to analyze the collected weights. By the analysis, solar energy is found as the most sustainable energy source in India. The sustainability order is followed by the wind, hydro, biomass, gas, nuclear and thermal energy respectively. Thermal energy is obtained as a least sustainable energy source in India because of scarcity in the availability of the fuels, a huge increase in fuel price and greenhouse gas emission.
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    Investigation of effect of suction nozzle pressure drop and degree of sub-cooling on co2 transcritical ejector system
    (Begell House, 2017) Dasgupta, Mani Sankar; Yadav, Shyam Sunder
    A two-phase ejector is often used to improve the performance of a CO2 transcritical refrigeration system, such systems are also, mostly equipped with an internal heat exchanger. Any effect on performance due to suction nozzle pressure drop (SNPD) is generally neglected for theoretical studies using constant pressure mixing model. A few authors, in the recent past, have highlighted the effect of SNPD. A detailed investigation of effect of SNPD as well as sub-cooling on the performance parameters like pressure recovery, ejector efficiency, entrainment ratio and overall COP of the system are presented here. Optimized ejector SNPD values are determined for maximizing COP across a wide range of operating conditions. COP increment upto 4.95% is observed at optimized SNPD condition. Further, the effect of degree of sub-cooling on cycle parameters is studied and optimum degree of sub-cooling for COP improvement for a range of operating conditions is also determined.
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    A Comparison of Three Different Flow Solvers For Simulating Steam Condensation Inside a Nozzle
    (Springer, 2023-04) Dasgupta, Mani Sankar; Yadav, Shyam Sunder
    In this work the condensation process of steam inside a converging diverging nozzle is simulated, the experiments on which were performed by Moses and Stein [1]. Three different flow solvers are used for this purpose, namely, Ansys Fluent, Ansys CFX and the open source flow solver OpenFOAM. The aim of the current work is to narrow down the choice of the flow solver which can finally be used for simulating non-equilibrium condensation of Carbon Dioxide inside ejectors. The pressure distribution predicted inside the nozzle by the three solvers follow closely the experimental one. However, the location and amount of pressure rise due to the condensation onset are differently predicted by the flow solvers. Only one plateau is observed in the pressure curve experimentally while at least two different plateaus are predicted numerically. For the vapor temperature and supercooling, the three solvers predict similar values before the condensation onset. After start of condensation, Ansys Fluent and OpenFOAM give similar results while Ansys CFX predicts at least 10 degree higher values. Ansys CFX predicts a wider nucleation zone compared to the two other solvers. The highest discrepancy being displayed by the solvers appears in the distribution of droplet diameter and droplet number density. However, the three solvers predict similar trend for the liquid mass fraction distribution inside the nozzle
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    Ejector expansion transcritical R744 refrigeration system analysed for various climate zones of India
    (AIP, 2023-05) Dasgupta, Mani Sankar; Yadav, Shyam Sunder
    Ejectors can provide advantage over conventional expansion device, in terms of energy as well as exergy, especially in high pressure transcritical R744 refrigeration systems. This paper presents a simulation-based comparative study of annual energy consumption and environmental impact of four different configurations of ejector expansion R744 systems for thirty different cities in India, spread across five different climatic zones. A control strategy is also adopted in order to ensure smooth transition of operation between subcritical, transcritical and transition modes for round the clock operation. A configuration having multi-compression ejector expansion system equipped with internal heat exchanger (MCEETRC+IHX) along with suggested control strategy exhibited lowest annual energy consumption. However, all the components are not equally effective for all the cities or weather conditions. Ambient temperature and evaporator temperature are found to play an important role. A simplified economic analysis is also presented which suggests overall cost-effectiveness for supermarket food storage application.
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    Classical nucleation theory based simulations of non-equilibrium condensation of carbon dioxide inside converging-diverging nozzles
    (Begell House, 2021) Dasgupta, Mani Sankar; Yadav, Shyam Sunder
    In the current work, we perform numerical simulations of the phase change process of Carbon Dioxide inside three different converging diverging nozzles, the experimental data on which is available in open literature. The simulations are performed with the classical nucleation theory based non-equilibrium phase change solver available in Ansys CFX with the thermophysical properties of CO2 obtained from NIST Refprop. We focus on the supercooling levels attained by the fluid and the distribution of the liquid mass fraction of CO2 during its high speed expansion inside the nozzles. The nozzle shape, expansion rate and fluid inlet conditions have a strong influence on the supercooling levels and the maximum liquid mass fractions obtained inside the nozzles. The results show much lower supercooling levels attained by CO2 (~ 2K) inside the Claudio Lettieri nozzle, the inlet state for which is near to the critical point. The supercooling attained by the vapor inside the Gyarmathy nozzle is around 22.5 K, the inlet state for which is far from the critical point. The case with the Nakagawa nozzle fails to converge properly.