Faculty Publications
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Item Transient analysis of subcritical/supercritical carbon dioxide based natural circulation loops with end heat exchangers: Numerical studies(Elsevier Ltd, 2014) Yadav, A.K.; Ram Gopal, M.; Bhattacharyya, S.Transient analysis of carbon dioxide based natural circulation loop (NCL) with end heat exchangers has been carried out. Subcritical and supercritical phases of CO2 are considered with operating pressures in the range of 50-100 bar for an operating temperature range of 323 K to 363 K. Studies are carried out for various loop tilt angles, different initial conditions, and different water mass flow rates. Results: are obtained for various inlet temperatures of water in the hot heat exchanger while keeping the inlet temperature of cooling water in the cold heat exchanger fixed. Effect of tilting the loop in XY and YZ planes on transient as well as steady state behaviour of loop are also studied. Validation of simulation results against experimental and numerical results reported in the literature in terms of modified Grashof number (Grm) and Reynolds number (Re) show good agreement. © 2014 Elsevier Ltd. All rights reserved.Item Effect of tilt angle on subcritical/supercritical carbon dioxide-based natural circulation loop with isothermal source and sink\(American Society of Mechanical Engineers (ASME) infocentral@asme.org, 2016) Yadav, A.K.; Ram Gopal, M.R.; Bhattacharyya, S.In recent years, a growing popularity of carbon dioxide (CO2) as a secondary fluid has been witnessed in both forced as well as in natural circulation loops (NCLs). This may be attributed to the favorable thermophysical properties of CO2 in addition to the environmental benignity of the fluid. However, an extensive literature review shows that studies on CO2-based NCLs are very limited. Also, most of the studies on NCLs do not consider the three-dimensional variation of the field variables. In the present work, threedimensional computational fluid dynamics (CFD) models of a NCL with isothermal source and sink have been developed to study the effect of tilt angle in different planes. Studies have been carried out employing subcritical (liquid and vapor) as well as supercritical phase of CO2 as loop fluid at different operating pressures and temperatures. Results are obtained for a range of tilt angles of the loop, and a significant effect is observed on heat transfer, mass flow rate, and stability of the loop. It was also found that changing the orientation of the loop could be an elegant and effective solution to the flow instability problem of NCLs.Item Optimum Operating Conditions for Subcritical/Supercritical Fluid-Based Natural Circulation Loops(American Society of Mechanical Engineers (ASME) infocentral@asme.org, 2016) Yadav, A.K.; Bhattacharyya, S.; Ram Gopal, M.R.Natural circulation loop (NCL) is simple and reliable due to the absence of moving components and is preferred in applications where safety is of foremost concern, such as nuclear power plants and high-pressure thermal power plants. In the present study, optimum operating conditions based on the maximum heat transfer rate in NCLs have been obtained for subcritical as well as supercritical fluids. In recent years, there is a growing interest in the use of carbon dioxide (CO2) as loop fluid in NCLs for a variety of heat transfer applications due to its excellent thermophysical environmentally benign properties. In the present study, three-dimensional (3D) computational fluid dynamics (CFD) analysis of a CO2-based NCL with isothermal source and sink has been carried out. Results show that the heat transfer rate is much higher in the case of supercritical phase (if operated near pseudocritical region) than the subcritical phase. In the subcritical option, higher heat transfer rate is obtained in the case of liquid operated near saturation condition. Correlations for optimum operating condition are obtained for a supercritical CO2-based NCL in terms of reduced temperature and reduced pressure so that they can be employed for a wide variety of fluids operating in supercritical region. Correlations are also validated with different loop fluids. These results are expected to help design superior optimal NCLs for critical applications. © 2016 by ASME.Item Transient analysis of subcritical/supercritical carbon dioxide based natural circulation loop with end heat exchangers: experimental study(Springer Verlag service@springer.de, 2017) Yadav, A.K.; Ramgopal, M.; Bhattacharyya, S.Carbon dioxide (CO2) based natural circulation loops (NCLs) has gained attention due to its compactness with higher heat transfer rate. In the present study, experimental investigations have been carried out to capture the transient behaviour of a CO2 based NCL operating under subcritical as well as supercritical conditions. Water is used as the external fluid in cold and hot heat exchangers. Results are obtained for various inlet temperatures (323–353 K) of water in the hot heat exchanger and a fixed inlet temperature (305 K) of cooling water in the cold heat exchanger. Effect of loop operating pressure (50–90 bar) on system performance is also investigated. Effect of loop tilt in two different planes (XY and YZ) is also studied in terms of transient as well as steady state behaviour of the loop. Results show that the time required to attain steady state decreases as operating pressure of the loop increases. It is also observed that the change in temperature of loop fluid (CO2) across hot or cold heat exchanger decreases as operating pressure increases. © 2017, Springer-Verlag Berlin Heidelberg.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 LtdItem Effect of loop tilting on the heat transfer and pressure drop in two-phase CO2 based natural circulation loop: An experimental study(American Society of Mechanical Engineers (ASME), 2021) Thippeswamy, L.R.; Yadav, A.K.The natural circulation loop (NCL) is widely used where the safe and economic heat transfer device is desired. However, the instability associated with the regular change in fluid flow behavior due to the imbalance between friction and buoyant forces is a major disadvantage. One of the erudite solutions to overcome this is to tilt the entire loop by a certain angle, with an inherent penalty in heat transfer and pressure drop. In the present study, experimental studies have been carried out on two-phase carbon dioxide (CO2) based NCL, which has gained popularity because of its compactness and higher heat transfer rate. Pressure drop and heat transfer performance of the loop for various tilt angles (0 deg, 30 deg, and 45 deg) in different planes (XY and YZ planes) have been investigated. Methanol is used as the external fluid in cold and hot heat exchangers in order to maintain low operating temperature in the loop. Results show that the tilting of the loop causes a marginal drop in the heat transfer rate of two-phase CO2 based NCL. Hence, tilting of the loop could be a solution to instability problem without conceding the performance of the loop. © © 2020 by ASMEItem Investigation of preheated Dhupa seed oil biodiesel as an alternative fuel on the performance, emission and combustion in a CI engine(Elsevier Ltd, 2021) Kodate, S.V.; Satyanarayana Raju, P.; Yadav, A.K.; Kumar, G.N.The present study investigates the suitability of preheated Vateria indica methyl ester (VIME) as an alternative fuel for a diesel engine. VIME is a renewable, non-toxic and sustainable alternative biodiesel obtained from Dhupa fat by transesterification. This study aims to evaluate the combustion, performance, and emission characteristics of four different blends such as B0 (0% VIME and 100% mineral diesel), B30, B50 and B100 at elevated fuel inlet temperatures ranging from 35 °C to 95 °C. The tests are carried out in a single cylinder diesel engine at optimum loading condition and fixed speed. Results are obtained in terms of brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), in-cylinder pressure, heat release rate and exhaust emissions (CO, HC, NOX, CO2 and soot). It is observed that the preheating of blends decreases the viscosity which enhances fuel spray characteristics, leading to higher engine performance, lower CO and HC emissions with a slight increase in NOX and CO2 emissions. BTE and peak in-cylinder pressure for B100 at 95 °C and 75% load are increased by 7.44%, 2.97% respectively compared to unheated B100 biodiesel. BSFC, CO, HC emissions at 75% load for B100 at 95 °C are reduced by 26.73%, 28.08%, 42.7% respectively compared to unheated B100. © 2021 Elsevier LtdItem CFD and exergy analysis of subcritical/supercritical CO2 based naturally circulated solar thermal collector(Elsevier Ltd, 2022) Biradar, M.K.; Parmar, D.N.; Yadav, A.K.Solar water heating system is inefficient during winter due to the chances of water freezing and higher viscosity at low temperatures. Several investigations are being done to increase the efficiency of the solar water heater using various secondary fluids for different climatic conditions. This paper emphasises on the study of heat transfer and fluid flow behavior of CO2 based naturally circulated indirect solar water heating system. Subcritical (liquid and vapour) and supercritical CO2 are considered as loop fluid, and the results are compared with water based system. Three-dimensional computational fluid dynamics simulations are carried out for two different weather conditions i.e., winter (278 K) and summer (305 K). Results are obtained for 33° collector inclination angle from horizontal at various operating pressures 50–70 bar for subcritical and 80–100 bar for supercritical CO2. The CO2 based system yields very high Reynolds number (subcritical liquid: ∼160 times; subcritical vapour: ∼204 times; supercritical vapour: ∼260 times) and very high Nusselt number (subcritical liquid: ∼14 times; subcritical vapour: ∼19.5 times; supercritical vapour: ∼48 times) compared to water based system. Supercritical CO2 based system exhibits 12% higher energy efficiency compared to water. Whereas, subcritical vapour based system exhibits 140% higher exergy efficiency relative to water based system. © 2022 Elsevier Ltd