Steady-State Analysis of a High-Temperature Natural Circulation Loop Based on Water-Cooled Supercritical CO2

Abstract

A high-temperature natural circulation loop (NCL) using supercritical carbon dioxide as loop fluid is modeled to study the effects of operating variables and relevant design parameters on loop performance. The steady-state system model duly considers the axial conduction through loop fluid as well as loop wall and heat transfer with surroundings. The heat source is considered to be a heater with controlled heat flux and the heat sink is modeled as an end heat exchanger with water as the external cold fluid. The governing conservation equations for mass, momentum, and energy are nondimensionalized and are solved numerically discretizing in finite volume method. The numerical results are validated against experimental results reported in the literature in terms of modified Grashof number (Grm) and Reynolds number (Re). Results show that heat loss to the ambient affects the loop performance significantly for the high-temperature loop. It is also observed that the heat input at which the circulation becomes maximum can be increased by increasing either the diameter and/or the loop height. However, better performance is obtained with larger diameter tubes instead of longer loop heights. Axial conduction is seen to have a negligible effect on the overall loop performance. Boussinesq approximation appears to be reasonable as the operating conditions of the supercritical loop are away from the critical point.

The steady behavior of the supercritical carbon dioxide natural circulation loop

“…The steady state of supercritical natural circulation loops has previously been investigated using both numerical and experimental approaches. In the numerical literature, a onedimensional transient model is most commonly used to predict both the steady and unsteady behavior of the considered loops [1]- [4], although three-dimensional approaches have also been undertaken [5]. The mass flow rate of a natural convection loop has been predicted to attain a maximum with varying heating rates [1], [2].…”

“…In the numerical literature, a onedimensional transient model is most commonly used to predict both the steady and unsteady behavior of the considered loops [1]- [4], although three-dimensional approaches have also been undertaken [5]. The mass flow rate of a natural convection loop has been predicted to attain a maximum with varying heating rates [1], [2]. Furthermore, a rise in mass flow rate is expected with increasing loop heights, and increasing channel diameters [1], [3], [5].…”

“…The mass flow rate of a natural convection loop has been predicted to attain a maximum with varying heating rates [1], [2]. Furthermore, a rise in mass flow rate is expected with increasing loop heights, and increasing channel diameters [1], [3], [5]. On the contrary, an increase of the loop length is expected to have a limiting effect on the flow rate of the loop [5] The influence of thermodynamic state on the steady behavior of supercritical NCLs is briefly touched upon in [4].…”