Applied Thermal Engineering 182 (2021) 116087 Available online 1 October 2020 1359-4311/© 2020 Elsevier Ltd. All rights reserved. Instability mitigation by integrating twin Tesla type valves in supercritical carbon dioxide based natural circulation loop Tabish Wahidi, Ajay Kumar Yadav * Advanced Heat Transfer Laboratory, Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India A R T I C L E INFO Keywords: Tesla valve Supercritical CO 2 Single-phase Instability Rectangular natural circulation Loop Flow transition ABSTRACT Flow instability in supercritical fuid 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 fuid and to decrease the magnitude of instability, this research emphasizes the development of NCL integrated with two modifed 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 modifed twin Tesla NCL. Results show that the use of modifed 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 ve- locity oscillations with a marginal drop of ⩽3% in the heat transfer performance. Using asymmetrical fow resistance to stimulate directional circulation is an effcient technique to combat this instability issue. Obtained results are validated with the existing correlations, and a good agreement is obtained. 1. Introduction Natural convection loop (NCL) is a passive heat transfer device in which fuid fow in the system is driven by favourable density gradient caused by differential thermal expansion of working fuid. Due to simple geometry and passive operation compared to the loops with aided cir- culation (i.e. forced circulation), NCL enhances the level of intrinsic safety and provides an inherent beneft in terms of price, loop reliability, maintenance and can be inherent in many applications like solar heater [1], geothermal process [2], turbine blades cooling [3], electronic components cooling [4,5], and nuclear reactor cooling systems [6,7]. Environmental pollution due to energy demand is an appealing aspect for the researcher to expand their investigation not only to enhance the energy effciency of the system but also to fnd eco-friendly energy resource. Environmental benign nature of CO 2 (Global warming potential = 1 and Ozone depletion potential = 0) made it one of the most popular secondary fuid in recent years, and its use in various heating/ cooling systems reduces the amount of greenhouse gases in the atmo- sphere. The admirable thermophysical properties of CO 2 as working fuid offers a compelling alternative, so that it can be used in the process of deposition and preparation of materials [8,9] nuclear reactor appli- cations [10], chemical extraction [11], refrigeration [12,13], and for the heat pump systems [14]. When CO 2 operates above a temperature of 304.25 K and pressure of 7.37 MPa, it attains a supercritical fuid state. For water or other fuids, where at high-temperature, a phase change occurs, and the fow becomes abrupt, in that situation where phase change is to be avoided supercritical fuid can serve as a feasible alter- native. Research indicates that supercritical CO 2 based NCLs (scCO 2 - NCLs) have higher effciency in a compact design than other working fuid [15,16]. scCO 2 -NCL creates a signifcantly high driving head and high Reynolds number even for small differences in temperature be- tween source and sink, which in turn induces a high circulation rate in the system than conventional fuid-like subcritical water [16–18]. Zhang et al. [19] explored the fuid behaviour of supercritical CO 2 (scCO 2 ) by performing a computational simulation and observed an enhanced heat transfer phenomenon for scCO 2 -NCLs. Numerical and experimental research by Yadav et al. [20,21] on the transient behaviour of carbon dioxide-based NCLs operating in subcritical/supercritical conditions shows that supercritical CO 2 provides a much higher heat transfer rate for a specifed loop geometry compared to water. Thippeswamy et al. [22] conducted an experimental study to investigate the subcritical and * Corresponding author. E-mail address: ajaykyadav@nitk.edu.in (A.K. Yadav). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng https://doi.org/10.1016/j.applthermaleng.2020.116087 Received 9 May 2020; Received in revised form 25 August 2020; Accepted 15 September 2020