Single-phase Rectangular Natural Circulation Research Articles

Stabilization of single phase rectangular natural circulation loop of larger diameter using orifice plate

Experimental investigations on the dependence of the stability threshold on different operating procedures have been carried out in a uniform diameter rectangular loop with orifice plates. The investigations revealed that there is no unique stability threshold for the different operating procedures with orifice confirming the existence of a hysteresis region just as in the loop without orifice. The hysteresis region can be further categorized into unconditionally stable, conditionally stable and unconditionally unstable regions even with the orifice plates. The stability region is found to increase with the introduction of the orifices. The predicted dimensionless steady state flow rate with orifice is found to be in good agreement with the data. One dimensional linear stability analysis accounting for the effective length of the orifices and elbows have been used to predict the stability map. The 1-D analysis can qualitatively predict the stabilizing effect of the orifice plates. However, the stable experimental data is not well characterized by the stability map predicted by the 1-D linear stability analysis which is attributed to the 3-D nature of the flow, the heat losses and the wall thermal inertia which are neglected in the 1-D analysis.

Non-linear dynamics of single phase rectangular natural circulation loop

The future nuclear reactors are being engineered to function completely on the phenomena of thermosiphoning. These reactors are equipped with various natural circulation-based systems which offer several benefits. The linear stability analysis examines the stability for such systems for small perturbations. If perturbations are slightly large, linear stability fails to fathom the stability of the system. In such cases, it becomes imperative to perform non-linear stability analysis. In the present work, a rectangular natural circulation loop has been simulated for small as well as large perturbations to underscore the importance of non-linear stability analysis. The key objectives of the present work include identification of the subcritical Hopf bifurcation and unravelling the repercussions of larger perturbation on the system in each region of the parametric space. The unfolding of dynamics from unstable limit cycles to chaos and then back to stable fixed points have been investigated through numerical simulations.

Investigations on the dependence of the stability threshold on different operating procedures in a single-phase rectangular natural circulation loop

During the past years, several studies were focused on the instability of single-phase rectangular natural circulation systems. However, experimental identification of instability threshold and categorization of instability regimes is of prime interest in many industrial applications which have been addressed by very few researchers that too limited to lower diameter loops only. Hence, in the present study, a larger diameter rectangular loop with horizontal heater horizontal cooler (HHHC) configuration has been designed to generate single phase natural circulation data on the steady state and stability behavior. Effect of different operating procedures (i.e., start-up from rest, power raising from stable steady state, and power reduction from an unstable state) on stability behavior are studied. Most of the oscillatory regimes are identified and noticed that the operating procedure can influence the instability threshold. The instability threshold was found to increase with the lowering of the coolant temperature and increasing its flow rate. The operating procedures i.e., power raising from stable steady state and start-up from rest gave the maximum and minimum values of the stability threshold respectively. As opposed to this, the minimum instability threshold in smaller diameter loops was reported for the operating procedure with power reduction from an unstable state. The experimental results confirm the occurrence of the hysteresis phenomenon. Due to the larger diameter and lower Lt/D ratio of the loop, all the oscillatory behaviors were aperiodic in nature in contrast to the observations in smaller diameter loops.

Computational and experimental investigation of steady state and transient characteristics of molten salt natural circulation loop

The aim of this study is to understand the steady state and transient characteristics of single-phase rectangular natural circulation loop working with molten salt. The same is achieved through experimental investigation as well as through CFD analysis. The in-house experimental facility and a CFD model which simulates, as closely as possible, the experimental facility, are developed for this purpose. CFD results are ensured to be mesh and time step independent. Using CFD studies, maximum and minimum values of heater power, which will keep the salt in desired range of temperature is established. Steady state characteristics are obtained over a range of heater power. The effect of power on induced mass flow rate, minimum and maximum temperatures in the loop, etc., is estimated through these studies. Analysis was performed for various transients like flow initiation transient, power step-up transient, power step back transient, power trip transient and loss of heat sink transient. The transient simulation results are also compared with experimental data. The key factor to achieve consistency between numerical results and experimental data is to be able to model the heat loss from various parts of the loop. The paper discusses the modelling issues and various results.

Dynamic response of a single-phase rectangular natural circulation loop to different excitations of input power

Dynamic analyses of system responses during sudden change in input power for single-phase natural circulation loops are only a very recent concept. The present study theoretically investigates the transient behaviour of rectangular loops, subjected to direct heat addition and convective cooling. Non-dimensional forms of governing equations have been used alongside suitable closure relations. Three fundamental signals, having step, ramp and exponential profiles, and a newly-defined modified exponential signal have been applied on input power during power increase and decrease. Step and exponential changes have been found to lead the system towards instability during power up-surge, due to the impulsive nature of transition. The system also takes long time to attain steady-state for power decrease to a stable state following a step change. Step signal can be broken into equal increments for better performance. Increasing the number of increments and time-lag between successive steps, higher power operation can be sustained for a longer duration. Modified exponential signal has been found to be superior compared to others, both during power increase and decrease. It allows a moderately increasing gradient during power transients and hence the rate of oscillation growth is much smaller.

Experimental investigations on control of flow instability in single-phase natural circulation loop

Abstract Natural circulation systems offer simplicity, enhanced safety and reliability and thus are advantageous over their forced circulation counterpart. However, natural circulation is susceptible to flow instabilities. These instabilities are undesirable for various reasons. Literature suggests the use of orifices at the inlet to suppress instability. However, orificing introduces pressure drop which limits the flow rate and hence the heat carrying capacity of the fluid. In the present study, investigations have been carried out with different mechanical gadgets to control the natural circulation flow instabilities. Extensive experiments have been carried out in a single-phase rectangular natural circulation loop to study the effect of these mechanical gadgets on instability. The paper brings out the results of experimental investigations carried out on the role of mechanical gadgets in controlling instability.

Influence of geometry and operating parameters on the stability response of single-phase natural circulation loop

An investigation on the influence of different geometric and operating parameters on the stability of a single-phase rectangular natural circulation loop having horizontal heating–cooling orientation is presented. The major objective is to identify the most plausible option of expanding stable zone of operation, which also improves steady-state flow. Decreasing loop diameter stabilizes the flow, but also reduces steady-state flow rate drastically. Increasing vertical length has been identified as a much better option, as that yields the dual benefit of enhanced stability margin and a slightly enhanced steady-state flow rate. A correlation for predicting the threshold of instability has also been proposed in terms of four geometric variables, namely, inner diameter, height, horizontal length and length of heating section. Introduction of minor losses in the form of valves or orifices has been found to be a weak option, both due to rapid deterioration of steady-state flow and a strong dependence of minor loss requirement on power supply. Tilting the loop from vertical, while maintaining the rectangular shape, has been observed to be a much better choice. A small tilt angle in the range of 10°–15° stabilizes the loop even for very high power input, with a fairly small reduction in steady-state flow.

Steady-state analysis of CO 2 based natural circulation loops with end heat exchangers

In the present work, results obtained from the steady-state analysis of single-phase rectangular natural circulation loop with parallel flow, tube-in-tube type heat exchangers are presented. Due to certain favorable properties, CO 2 is chosen as the secondary fluid in the loop. Water is used as the external fluid in both hot and cold heat exchangers. Using a simple analytical expression, it is first shown that use of CO 2 in place of water can result in a compact loop. The results obtained are found to match with the results obtained using the analytical expressions, which are generally used as first approximations for the steady-state behaviour of natural circulation loops [Y. Zvirin, A review of natural circulation loops in pressurized water reactors and other systems, Nucl. Eng. Des. 67 (1981) 203–225]. However, unlike the classic expressions, the method suggested here is general as it considers the heat exchanger configuration, non-symmetric geometry and variable properties, so that it can be used for optimal system design. The results obtained on a rectangular loop show that for a given input data there exist optimum values for heat exchanger length and tube diameters. It is also seen that the external fluid flow rate does not influence the loop performance significantly beyond a certain value. However, the temperatures of external fluids affect the loop performance strongly.

Conjugate heat transfer simulation of a rectangular natural circulation loop

In this article, conjugate heat transfer characteristics of a single-phase rectangular natural circulation loop were investigated numerically. Effects of the wall thermal conductivity and the wall thickness on the heat transfer behavior in a circulation loop with fixed geometrical configuration were investigated in detail. The presence of axial conduction through thick and highly conductive loop wall tends to strengthen markedly the buoyancy-induced circulating flow in the loop at lower Ra*.

Effect of heat loss to ambient on steady-state behaviour of a single-phase natural circulation loop

This study aims at modelling of a single-phase rectangular natural circulation loop as a conjugate problem to investigate the influence of ambient temperature on the steady-state temperature profile for the loop fluid and wall temperature. Finite volume technique has been employed for the solution of non-dimensional governing equations. Numerically predicted values have been compared with available test data from literature. Natural convection heat loss to the ambient is found to cause significant changes in the loop fluid and wall material temperature along the bare sections. Warmer ambient conditions increase the fraction of input power transferred to the cooling stream and hence cause a rise in coolant temperature across the cooler. Increase in ambient temperature results in higher coolant outlet temperature and thus higher heat exchange effectiveness for the system. The loop temperature profiles gradually approach the ideal condition of perfect insulation with increase in ambient temperature. The combined effect of change in power input and ambient temperature predicts a particular ambient condition, when the system produces a constant effectiveness regardless of the power input.

EFFECT OF END HEAT EXCHANGER PARAMETERS ON THE PERFORMANCE OF A NATURAL CIRCULATION LOOP

The effect of end heat exchanger parameters on the performance of a single phase rectangular natural circulation loop with a constant circular cross section is reported in the present paper. Based on one-dimensional governing equations, the mass flow rate of the coupling fluid has been determined as a function of physical properties and geometrical parameters of the loop. The relevant non-dimensional numbers for the loop performance have also been identified. The model developed has been used for parametric study to determine the circulation rate as a function of (Ntu*, Ntu max , CR h -CR c combinations and θ o ).