Publication date: 14 July 2026
Source: Defect and Diffusion Forum Vol. 453
Author(s): Shendy Akbar Maryadi, Deendarlianto Deendarlianto, Sunandi Kharisma, Mulya Juarsa
Increasing demand for reliable and passive thermal management in modern energy systems, particularly in nuclear reactors, has elevated interest in natural circulation loops. Among the influencing factors, loop geometry and heating power are critical in natural circulation systems. This study investigates the effect of heating power on heat transfer in a rectangular VHVC natural circulation loop with an enlarged upper elbow radius of 350 mm. The analysis was conducted using CFD under steady-state conditions, employing a pressure-based solver with the realizable k-epsilon turbulence model and energy equation to simulate buoyancy-driven flow. Three power inputs 750 W, 1100 W, and 1540 W were applied to evaluate their effect on temperature distribution and energy absorption. Results show that increasing the heating power enhances buoyancy forces, leading to higher mass flow rate and stronger natural circulation within the loop. The fluid temperature difference between the heating and cooling sections rises with power input, which directly increases the convective heat transfer coefficient. Consequently, the obtained Nusselt number increased from 25.69 at 750 W to 31.23 at 1540 W. This finding confirms that higher heating power significantly improves the loop heat transfer performance, providing insight into the optimization of passive cooling systems in nuclear safety applications.
[1] D. Yuliaji, R. Waluyo, G.E. Pramono, M.G. Putra, N.R. Budiyanto, S.A. Mariadi, S. Kharisma, R. Oktaviandi, Deendarlianto, Indarto, M. Juarsa, Thermal-hydraulics performance and stability two-phase flow using Al2O3 nanofluids in an open natural circulation loop, Ann. Nucl. Energy 218 (2025) 111424.
[2] M. Juarsa, Giarno, D. Haryanto, A. Rosidi, K.G.B. Heru, A.E. Pamungkas, A.A. Budiman, Experimental on Transient Heating and Cooling of Natural Circulation Flow using A FASSIP-02 Large Scale Experimental Facillity, Evergr. Jt. J. Nov. Carbon Resour. Sci. Green Asia Strateg. 11 (2024) 1442–1449.
DOI: 10.5109/7183469
[3] S. Kharisma, D. Yuliaji, A.E. Pamungkas, A.A. Budiman, M. Rafel, S.A. Maryadi, R. Oktaviandi, N.R. Budiyanto, P.H. Setiyawan, W.N. Septiadi, M. Juarsa, Impact of Cooler Temperature Variations on Flow Boiling Phenomena in a Natural Circulation Rectangular Loop, J. Phys. Conf. Ser. 2972 (2025).
[4] P.H. Setiawan, A.S. Pamitran, D. Yuliaji, A.E. Pamungkas, A.A. Budiman, S.A. Maryadi, R. Irwansyah, M. Juarsa, Effect of water temperature variation on natural circulation flow regimes and reynolds number in passive cooling system, J. Polimesin 23 (2025). https://doi.org/.
[5] A. Wahyuni, D. Yuliaji, P.H. Setiawan, N.R. Budiyanto, S.A. Maryadi, A.E. Pamungkas, A.A. Budiman, S. Kharisma, R. Waluyo, M. Juarsa, Analisis Aliran Sirkulasi Alami di Bagian Pendingin Berdasarkan Variasi Temperatur Air Pemanas pada Untai Rektangular TP FASSIP-04 Ver.2, J. Rekayasa Mesin 19 (2024) 23–36.
[6] N.R. Budiyanto, Deendarlianto, R. Oktaviandi, D. Yuliaji, R. Waluyo, E.P.A. Raharjo, P.H. Setiawan, S.A. Maryadi, M. Rafel, M. Juarsa, Thermal Effect on Natural Circulation Flow using Water and Al2O3 Nanofluids Inside Rectangular Loop in Vertical Position, J. Phys. Conf. Ser. 2972 (2025).
[7] International Atomic Energy Agency, Passive Safety System in Advanced Water Cooled Reactors (AWCRs), IAEA, Vienna, 2013.
[8] R. Waluyo, Deendarlianto, Indarto, D. Yuliaji, V.I. Sri Wardhani, A. Afandi, M.G. Putra, R. Oktaviandi, S. Kharisma, S.A. Maryadi, A.H. Astyanto, M. Juarsa, Design of rectangular single-phase natural circulation loop based on working fluid and geometry parameters to enhancement stability and mass flow rates: A research review, Prog. Nucl. Energy 191 (2025) 106037.
[9] V.I. Sri Wardhani, I.M. Joni, C. Panatarani, E.S. Hanam, S. Kharisma, S.A. Maryadi, A. Rosidi, A.A. Budiman, S. Ismarwanti, P.H. Setiawan, I. Roswandi, E.P. Ariesta, R. Akbar, H.A. Gunawan, A.E. Pamungkas, M. Juarsa, Enhancing the natural circulation flow using ultrafine bubbles: Simulation and experimental, Int. J. Therm. Sci. 218 (2025) 110181.
[10] S. Gangwar, P.K. Vijayan, G. Dutta, Insights on the steady-state performance of single-phase natural circulation loops, Nucl. Eng. Des. 440 (2025) 114128.
[11] D.N. Elton, U.C. Arunachala, P.K. Vijayan, Investigations on the dependence of the stability threshold on different operating procedures in a single-phase rectangular natural circulation loop, Int. J. Heat Mass Transf. 161 (2020). https://doi.org/10.1016/j.ijheatmasstransfer. 2020.120264.
[12] A.K. Nayak, M.R. Gartia, P.K. Vijayan, Thermal-hydraulic characteristics of a single-phase natural circulation loop with water and Al2O3 nanofluids, Nucl. Eng. Des. 239 (2009) 526–540.
[13] M. Hashemi-Tilehnoee, M. Misale, S.M. Seyyedi, E. Palomo del Barrio, A. Marchitto, S.S. Rahim Hosseini, M. Sharifpur, Overview of fundamental aspects of natural circulation loops, Appl. Therm. Eng. 276 (2025) 126936. https://doi.org/10.1016/j.applthermaleng. 2025.126936.
[14] X. Wang, C. Shen, L. Liu, M. Liu, H. Gu, Development of heat transfer correlation of turbulent forced convection in subchannels with rough surfaces based on CFD, Ann. Nucl. Energy 181 (2023).
[15] S.A. Maryadi, Deendarlianto, S. Kharisma, M. Juarsa, Simulation on Power Effect in Natural Circulation Flow using U-top Rectangular Loop, Int. Conf. Digit. Soc. Sci. 02 (2025) 1613–1625.
DOI: 10.62201/22wwx959
[16] J.Y. Wang, T.J. Chuang, Y.M. Ferng, CFD investigating flow and heat transfer characteristics in a natural circulation loop, Ann. Nucl. Energy 58 (2013) 65–71.
[17] M. Zhou, O. Costa Garrido, S. Ma, N. Zhang, Numerical investigation of turbulent thermal stratification at a horizontally oriented 90° pipe-elbow with varied elbow radiuses, Int. J. Therm. Sci. 185 (2023).
[18] B.E. Launder, B.I. Sharma, Application of the energy-dissipation model of turbulence to the calculation of flow near a spinning disc, Lett. Heat Mass Transf. 1 (1974) 131–137.
[19] L.C. Pinheiro, A.C.M. Alvim, CFD investigation of fluid and heat transfer in a single-phase natural circulation toroidal loop, Nucl. Eng. Des. 415 (2023).
[20] T. Wahidi, R.A. Chandavar, A.K. Yadav, Supercritical CO2 flow instability in natural circulation loop: CFD analysis, Ann. Nucl. Energy 160 (2021) 108374.
[21] B.T. Swapnalee, P.K. Vijayan, A generalized flow equation for single phase natural circulation loops obeying multiple friction laws, Int. J. Heat Mass Transf. 54 (2011) 2618–2629.
[22] P.K. Vijayan, Experimental observations on the general trends of the steady state and stability behaviour of single-phase natural circulation loops, Nucl. Eng. Des. 215 (2002) 139–152.
[23] A. Crabtree, M. Siman-Tov, Thermophysical Properties of Saturated Light and Heavy Water for Advanced Neutron Source Applications, Oak Ridge Natl. Lab. (1993).
DOI: 10.2172/6306919
[24] V. Gnielinski, New equation for heat and mass transfer in turbulent pipe and channel flow, Int. Chem. Eng. 16 (1976) 359–368.
[25] R.H.S. Winterton, Where did the Dittus and Boelter equation come from?, Int. J. Heat Mass Transf. 41 (1998) 809–810.