Faculty Publications
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Item Microstructure evolution in AI-7Si-0.3Mg alloy during partial melting and solidification from melt: A comparison(2006) Nyamannavar, S.; Ravi, M.; Prabhu, K.N.In the present work, a comparative study of microstructure evolution in Sr modified AI-7Si-0.3Mg alloy isothermally held at semi-solid state under conditions of (i) cooling from liquid state and (ii) partial melting from solid state to the semi-solid temperature was carried out. The effect of cooling rate (0.01 to 100 K/s) on the microstructure during solidification of semisolid alloy is studied. Partial melting of alloy results in the fine and more spherical solid phase compared to cooling of the same alloy from liquid state. Chemical modification of the eutectic silicon by Sr addition was found to remain same for both cooling the melt from liquid as well as partial melting from solid state, contrary to the reported results. The morphology of eutectic silicon corresponding to the liquid entrapped in solid phase is finer compared to that in interconnected liquid channel.Item Constitutional undercooling and growth of globuletic particle(2008) Nyamannavar, S.; Ravi, M.; Prabhu, K.N.Isothermal holding of an alloy at semi-solid temperature and quenching/cooling, results in formation of instabilities on the globuletic α-particles. The aspect of instability formation is necessary input for accurate simulation and modeling of microstructure evolution for semi-solid metal forming (SSM) process. In the present work instability formation is studied for Al-7Si-0.3Mg alloy. Small cylindrical samples (10 mm, height 10 mm) were subjected to isothermal holding at semisolid temperature followed by quenching/cooling to room temperature. Instabilities were found to form at cooling rates 30 and 100 K/s. Instability formation is explained by interface stabilization theory based on the constitutional undercooling of liquid ahead of the solid/liquid interface.Item Stability analysis of Tesla valve based natural circulation loop for decay heat removal in nuclear power plants(Institute of Electrical and Electronics Engineers Inc., 2019) Chandavar, R.A.In Nuclear power plants, the process of Decay heat removal is very crucial for safe operation of the plant. Passive methods such as using Natural Circulation Loops make the process reliable and cheaper. However, such systems are subjected to instability when both heater and condenser are horizontally positioned. One of the methods to overcome such difficulties is to use a Tesla valve. A Tesla valve is a one way valve with no moving parts which allows fluid to flow easily in one direction but provides a very high impedance in opposite direction. The present study deals with numerical analysis of single phase natural circulation loop with and without Tesla valve having water as the fluid. The loop with Tesla valve stabilizes about 100 seconds earlier than loop without Tesla and incorporation of Tesla results in unidirectional circulation every time. Hence Tesla valve can be used in the loop for stability without appreciable flow resistance. © 2019 IEEE.Item Numerical Instability Assessment of Natural Circulation Loop Subjected to Different Heating Conditions(Springer Science and Business Media Deutschland GmbH, 2022) Thimmaiah, S.; Wahidi, T.; Yadav, A.K.; Arun, M.Natural circulation loop (NCL) is a passive system in which the driving action of the buoyancy force establishes fluid circulation by overcoming the frictional force without the help of any external power source. NCLs are prone to several kinds of instabilities due to the nonlinearity of the natural convection process. In fact, it is an inability of NCLs to sustain themselves against small perturbations to which any physical system is subjected. This instability in fluid flow creates flow oscillation, chaotic non-linear dynamic behaviour and flow reversal. In this article, three-dimensional computational fluid dynamics (CFD) numerical simulations have been carried out for a range of supercritical pressures (80 bar to100 bar) and heat inputs (250 W to 2500 W) to do the comparative investigation of instability phenomenon in supercritical CO2-based regular natural circulation loop configured with two different types of heat sources, i.e. heater and isothermal wall at the source with a cold heat exchanger (CHX) at sink. Results show higher instabilities for heater-exchanger loop (Heater-CHX) than an isothermal heater with heat-exchanger loop (ISO-CHX). With an increase in heat input, loops attain stability at a faster rate for a given operating pressure. At a lower heat input, both the loops show bidirectional fluctuation, whereas it is unidirectional at high heat input. Nusselt number shows that the Heater-CHX loop’s heat transfer capability is more compared to ISO-CHX loops. Obtained results are validated with the existing correlations, and a good agreement is obtained. © 2022, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.Item Comparative Numerical Appraisal of Subcritical and Supercritical CO2-Based Natural Circulation Loop(Springer Science and Business Media Deutschland GmbH, 2022) Wahidi, T.; Yadav, A.K.A natural circulation loop (NCL) is a passive heat transfer system in which circulation occurs solely due to density differences caused by thermal imbalance and the elevational difference between the source and sink of the loop. Carbon dioxide (CO2)-based NCL is highly sensitive to operating conditions and vulnerable to unstable behaviour, mainly due to intense changes in the thermo-physical properties of CO2. Therefore, NCLs always require precise design assessment that focusses on the interaction of all the transient responses of buoyancy and friction forces, ensuring a stable zone of operation. In this article, a three-dimensional computational fluid dynamics study has been carried out for over a range of pressures (30 to 100 bar) and heat inputs (500 to 1500 W) to do the comparative investigation of fluid flow and heat transfer phenomenon of subcritical/supercritical CO2-based NCLs with water-based NCL. The simulations quantify the degree of instability and heat transfer rate for subcritical/supercritical CO2 and water. A possible mechanism for continuous flow oscillation and measurement of instability with different pressure in unstable loops is also proposed in this study. Obtained results are validated with the correlations available in the literature; it shows an amicable agreement. © 2022, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.Item Supercritical CO2 flow instability in natural circulation loop: CFD analysis(Elsevier Ltd, 2021) Wahidi, T.; Chandavar, R.A.; Yadav, A.K.Three-dimensional computational fluid dynamics (CFD) simulation on a supercritical CO2 based natural circulation loop (scCO2-NCL) is carried out to explore the effects of various parameters (i.e., pressure and heat inputs) on the loop's transient and stability behaviour. Results show that for supercritical CO2, there is a threshold point that decides the flow's nature. Lower than threshold heat inputs flow shows repetitive-reversal flow while at higher heat input the flow changes to stable or single-direction flow. With an increase in heat input, the system attains stability for a given operating pressure. In addition, a possible mechanism for continuous flow oscillation and measurement of instability with different pressure in unstable loops is also proposed in this study. It is found that the Nusselt number decreases with an increase in operating pressure for given heat input. Obtained simulation results are validated with the two existing correlations and found a good agreement. © 2021 Elsevier LtdItem Numerical assessment of stability behaviour in supercritical CO2 based NCLS configured with heater, heat exchanger and isothermal wall as heat sources(Yildiz Technical University, 2023) Thimmaiah, S.; Wahidi, T.; Yadav, A.K.; Mahalingam, A.Three-dimensional numerical analysis is presented in this study to assess the transient and stability behaviour of supercritical CO2 (sCO2) based NCLs configured with three different types of heat sources, i.e., heater, a hot heat exchanger (HHX) and isothermal wall (ISO) at the source, and a cold heat exchanger (CHX) at the sink in all three NCLs. Unsteady threedimensional conservation equations (mass, momentum and energy equations) are solved to assess the transient and stability behaviour of sCO2 mass flow rate, temperature and velocity as a function of time. Further, the effect of pressure on sCO2 mass flow rate is also assessed to compare the loops performance. Performance of the loop has been studied for various heat inputs at the source by keeping constant mass flow rate and temperature at the sink. It is observed that for any boundary condition at the source, the loop experiences some initial disturbances or instabilities before reaching the steady-state. However, the time needed to attain a steady-state varies with the nature of heat input employed at the source. Results show a higher magnitude of instabilities in the Heater-CHX loop than HHX-CHX and ISO-CHX loops, and these instabilities mitigate at a faster rate in the ISO-CHX loop at all levels of heat input and operating pressure of the loop. It is also observed that as loop fluid operating pressure increases, the instability of the system decreases and the loop fluid mass flow rate increases. Further, the Nusselt number in the Heater-CHX loop is more than other loops because of its high turbulent kinetic energy. The findings of this study are validated with the published experimental and numerical data and found a good agreement © Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/)