Performance of a flashing driven natural circulation loop with dual risers over a range of operating conditions

Numerical studies have been carried out to investigate supercritical flow instabilities in a CO2 natural circulation loop. For the steady state and dynamic analyses of the loop under supercritical conditions, a single-channel, one-dimensional model is developed. In this model, equations for the conservation of mass, momentum and energy are discretized using an implicit finite difference scheme. A computer code called FIASCO (Flow Instability Analysis under SuperCritical Operating conditions) is developed in FORTRAN90 to simulate the dynamics of natural circulation loops with supercritical fluid. Results obtained for the stability boundary substantially deviate from the results reported by previous investigators, and thus contradict some of the reported findings. The disagreement in results is most likely due to the undesirable dissipative and dispersive effects produced from the large time steps used in previous studies, thereby leading to a larger stable region than those found using smaller time step. Results presented here suggest that the stability boundary of a natural circulation loop with supercritical fluid, is not confined to the near-peak region of the (steady state) flow-power curve. Additional and more extensive experimental data are needed to resolve the differences between results obtained here and those reported earlier. However, results obtained for the range of parameter values used in this investigation always predict the stability threshold to be in the positive slope region of the (steady state) flow-power curve. Parametric studies for different operating conditions reveal the similarity of stability characteristics under supercritical conditions with those in two-phase flows.

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Steady-state and nonlinear stability analysis for the feasibility of different fluids in a supercritical natural circulation loop

Proceedings of the Institution of Mechanical Engineers Part E Journal of Process Mechanical Engineering ◽

10.1177/09544089211043970 ◽

2021 ◽

pp. 095440892110439

Author(s):

Neetesh S Raghuvanshi ◽

Goutam Dutta ◽

Manoj K Panda

Keyword(s):

Stability Analysis ◽

Nonlinear Stability ◽

Natural Circulation ◽

Stability Boundary ◽

Operating Conditions ◽

Circulation Loop ◽

Marginal Stability ◽

Nonlinear Stability Analysis ◽

Natural Circulation Loop ◽

Stable Operating

A numerical model for a supercritical natural circulation loop is developed to examine the flow instabilities by nonlinear stability analysis. The supercritical natural circulation loop is a loop geometry, which is driven by natural circulation with supercritical fluids as a coolant. A mathematical formulation is developed to study the steady-state and transient solution procedure for supercritical natural circulation loop. This mathematical model is then used to perform various parametric studies with different supercritical fluids (water, [Formula: see text], R134a, ammonia, R22, propane, and isobutane). The behavior of all the fluids is analyzed on identical geometrical and operating conditions. A comprehensive numerical study of the nonlinear stability analysis is presented with particular emphasis on the feasibility of various fluids in a natural circulation loop environment. The 50% increment in loop diameter and height increased the stable operating zones and shifted the marginal stability boundary upward respectively by approximately three times and 25–40% of the previous value. However, further increase in diameter and height reduces the increment of stable operating zones; hence the marginal stability boundary shifts upward marginally than the previous value. Furthermore, the marginal stability boundaries are generated to identify the stable and unstable zones for the available geometrical and operating conditions.

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

Journal of Heat Transfer ◽

10.1115/1.4038541 ◽

2018 ◽

Vol 140 (6) ◽

Cited By ~ 2

Author(s):

Sayan Sadhu ◽

Maddali Ramgopal ◽

Souvik Bhattacharyya

Keyword(s):

Steady State ◽

High Temperature ◽

Natural Circulation ◽

Boussinesq Approximation ◽

Operating Conditions ◽

Circulation Loop ◽

Design Parameters ◽

Axial Conduction ◽

Operating Variables ◽

Natural Circulation Loop

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.

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