Faculty Publications
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Item Effect of CO2 based natural circulation loop for low temperature applications: CFD analysis(Toronto Metropolitan University, 2019) Wahidi, T.; Nagrani, P.P.; Yadav, A.K.Natural circulation loop (NCL) is a simple and economical heat transfer device in which flow occurs due to the buoyancy effect caused by thermally generated density gradient. In the present study, computational fluid dynamics (CFD) analyses are carried out to emphasize on the fluid ow and heat transfer characteristics of carbon dioxide (CO2 ) based NCL at low temperature (-38°C to 12°C). Studies are conducted in a three-dimensional (3-D) CFD model of NCL at different heat inputs i.e., 100W, 250W, 350W and 500W by keeping the loop fluid at pressure of 50 bar. Methanol is used as coolant in the heat exchanger at a fixed mass flow rate. Effect of loop operating pressure 50 bar on system performance is also investigated. Result are presented in the form of heat transfer rate, pressure drop, Reynolds number (Re) and temperature. Obtained results are validated with available correlations in the form of non-dimensional numbers, and found in good agreement. © 2019, Toronto Metropolitan University. All rights reserved.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 Stability enhancement of supercritical CO2 based natural circulation loop using a modified Tesla valve(Elsevier B.V., 2020) Wahidi, T.; Chandavar, R.A.; Yadav, A.K.This article deals with the comparative investigation of instability phenomenon in supercritical CO2 based regular natural circulation loop and a new modified Tesla natural circulation loop. Two-dimensional computational fluid dynamics simulation is carried out for square loops. Fluid flow behaviour and performance of both the loops are determined over a range of pressures (80–100 bar) and heat inputs (500–2000 W). Results show that the use of a modified Tesla valve leads to better stabilization for all supercritical pressures and heat inputs. It is also found that loop with Tesla mitigates the temperature and velocity oscillations without reducing the heat transfer performance. A good agreement with existing correlations is also obtained in the present study. The unidirectional fluid flow circulation achieved in loop with Tesla valve, makes it an efficient technique to combat instability. © 2020 Elsevier B.V.Item Instability mitigation by integrating twin Tesla type valves in supercritical carbon dioxide based natural circulation loop(Elsevier Ltd, 2021) Wahidi, T.; Yadav, A.K.Flow instability in supercritical fluid based natural circulation loop (NCL) is still an investigation aspect of physical and mathematical problems to comprehend. Therefore, NCLs require precise design assessment that focuses on the interaction of all the transient responses of buoyancy and friction forces which can ensure a stable zone of operation. To promote the uni-directional circulatory movement of loop fluid and to decrease the magnitude of instability, this research emphasizes the development of NCL integrated with two modified Tesla type valves. In this article, numerical simulations have been carried out for a range of supercritical pressures (80–100 bar) and heat inputs (500–2000 W) to do the comparative investigation of instability phenomenon in supercritical carbon dioxide based regular natural circulation loop and a new modified twin Tesla NCL. Results show that the use of modified Tesla valves leads to better stabilization for all supercritical pressures and heat inputs considered in the study. It is also found that the proposed Tesla NCL mitigates the temperature and velocity oscillations with a marginal drop of ?3% in the heat transfer performance. Using asymmetrical flow resistance to stimulate directional circulation is an efficient technique to combat this instability issue. Obtained results are validated with the existing correlations, and a good agreement is obtained. © 2020 Elsevier Ltd