Stabilization of a single-phase natural circulation loop by pressure drops

doi:10.1016/S0894-1777(01)00075-9

Experimental Thermal and Fluid Science

Volume 25, Issue 5, November 2001, Pages 277-282

https://doi.org/10.1016/S0894-1777(01)00075-9 Get rights and content

Abstract

Single-phase natural circulation is of interest in various energy systems, including solar heaters, nuclear reactors, geothermal power production, engine and computer cooling. The present paper deals with an experimental study on the influence of pressure drops on the behavior of a single-phase natural circulation loop. At DITEC two rectangular loops have been constructed and operated, differing in the inner diameter of the tubes and other geometric dimensions, but maintaining the same length/diameter ratio. In this paper the experimental results obtained using the larger loop are reported. Several series of tests have been already carried out in this loop, utilizing smooth pipes and varying the input power from 500 to 3400 W. The loop exhibited unstable behavior; hence, to stabilize it, two localized pressure drops (orifices of different diameters) were located in the vertical legs. The experimental data are analyzed and compared with results for smooth pipes, demonstrating the stabilizing effect of the pressure drops on the overall behavior of the loop.

Introduction

A natural circulation loop (thermosyphon) is a system in which the fluid motion is driven by thermally generated density gradients and body forces. These loops are extensively used in energy conversion systems, like solar heaters and cooling system of nuclear reactors, as well as in many other industrial fields, such as geothermal power production, turbine blade cooling, engine and computer cooling.

Several experimental and theoretical works are available in the literature dealing with the physics of the flow and how it influences the heat transfer in themosyphons. In particular, thermosyphonic flows in the most common geometry and their applications are reviewed by Zvirin [1] and Greif [2]. In the case of closed and open rectangular loops, particular attention has been devoted to transient and steady state behavior, as well as to stability analysis of the system under various heating and cooling modes.

Vijayan et al. [3] analyzed, experimentally and theoretically, the effect of loop diameter on the stability of single-phase natural circulation in a rectangular loop. Three loops characterized by different internal diameter (D=6, 11, and 22 mm) were investigated. Using linear stability analysis a map able to predict the behavior of the loop was proposed.

At least three thermal hydraulic behaviors could appear in a single-phase natural circulation loop: stable (invariant temperature difference across the heat sinks), neutral (oscillations of the temperature differences across the heat sinks without amplification), and unstable (amplification of the oscillations of the temperature differences across the heat sinks and flow reversals). In particular, in the present paper, this last behavior was experimentally investigated. As well known, the instability in the loop depends on the interaction between the buoyancy force generated by density gradients and the friction along the loop.

The authors of the present paper focused their attention on the thermohydraulic behavior of rectangular loops. Various test runs were conducted on the loop called MTT-1, utilizing water and FC-43 thus involving two different power thresholds, above which the loop behavior becomes unstable. Moreover, the Fast Fourier Transform was applied to analyze the frequency of the oscillations, [4], [5], [6]. The Relap5/Mod3.2 and Cathare2 V1.3u codes were used, having both purposes of code assessment and interpretation of experimental data [7].

In a previous paper, Misale and Frogheri [8], investigated the influence of two localized pressure drops (orifices of different diameters), evidencing that inserting the orifices (orifice diameters 6, 10, 14 mm) in the vertical legs of a rectangular loop, a stable behavior occurs for all values of power level. Furthermore, the authors observed that at the same power level, the temperatures across the heat sinks rise to a higher value in the case of tubes with orifices, but this effect is much more evident in case of high pressure drops. The amplitude of oscillations in the initial transient, as well as the time needed for their damping, decreases with the increase of pressure drops.

Preliminary experiments were conducted on a new loop (hereafter labeled as LOOP #1), characterized by larger dimensions but a similar length-diameter ratio, utilizing smooth pipes [9].

With the aim to study the stabilizing effect of pressure drops on this larger circuit, a new series of experiments was performed and is documented in the present paper. The loop was equipped with two sharp-edged orifices in the middle of both vertical legs. Different orifices diameters were used: 10, 22, 26, 30, and 36 mm, whereas the range of power was between 500 and 3400 W.

The experimental data are analyzed and compared with previous results obtained on MTT-1 loop (in Table 1 its geometric characteristics are reported in comparison with LOOP #1), both with smooth pipes and with localized pressure drops. As observed in [8], the presence of orifices stabilizes the behavior of MTT-1 loop, whereas for LOOP #1 it was found that the orifice diameter able to stabilize the loop is between 22 and 26 mm. For the former diameter, stable behavior was always observed, whereas for the latter, regular oscillations characterized by constant amplitude were detected.

Section snippets

Experimental apparatus

During the experiments a rectangular loop was used. The circuit consisted mainly of two copper horizontal tubes (heat transfer sections) and two vertical plexiglas tubes, connected with four 90° bends made of stainless steel.

The lower heating section consisted of nichrome electrical heating wire of on the outside of the copper tube; the upper heat extraction system was a coaxial cylindrical heat exchanger with tap water flowing in the annulus. In this way, the loop had an imposed heat flux in

Results

During these experiments different values of the orifice diameters were investigated at different power levels. In this paper the authors focused their attention on the influence of the presence of the localized pressure drops on the themohydraulic behavior of a single-phase natural calculation loop. In particular, the experiment results reported in this paper concern the data obtained using loop labeled LOOP #1.

In Fig. 2 the temperature difference across the heater as a function of time and

Conclusions

The behavior of a rectangular loop with different localized pressure drops realized by two sharp-edged orifices located in the middle of the vertical legs has been investigated experimentally.

During the experiments, the internal diameter of the orifices was increased from 10 mm up to 36 mm and the input power was varied from 500 up to 3400 W.

The data were compared with those measured in previous experimental activities.

The main conclusions can be summarized as follows:

•
The pressure drops could

Acknowledgements

This paper was supported by grants “Cofinanziamento MURST 1999” and Italian Space Agency.

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Analysis of single-phase natural circulation experiment by system codes
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M. Misale et al.
Influence of pressure drops on the behavior of single-phase natural circulation loop: preliminary results
Int. Commun. Heat Mass Transfer
(1999)
Y. Zvirin
A review of N.C. loops in PWR and other systems
Nucl. Eng. Des.
(1981)
R. Greif
Natural circulation loops
J. Heat Transfer
(1988)

There are more references available in the full text version of this article.

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Citation Excerpt :
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Series coupled natural circulation system with the last loop being an open-air loop finds application as an infinite mission time passive decay heat removal system in nuclear power plants and solar thermal systems. Hence the stability behavior of series coupled rectangular loops with horizontal heater and horizontal cooler orientation was experimentally studied. The loop was subjected to various operating procedures viz., start up from rest, power raising from a stable steady state and power step down from an unstable state to witness that the stability threshold was not a unique value, thus confirming the existence of a hysteresis phenomenon. The stability threshold enhancement of the system with orifice plates was maximum for the lowest orifice diameter. The coupled system was chosen with an unstable first loop and the instability was found to transmit from loop#1 to the subsequent loops. However, the oscillation amplitude was highly attenuated and seemed to be nearly stable in the 2nd and 3rd loop without any flow reversals even when the first loop was unstable with chaotic bidirectional pulsing. In case of bi-directional pulsing in loop#1, the heat exchanger between the first two loops operated alternatively in co-current and counter-current mode with no change in the flow direction of loop#2. A delay in the flow initiation was found for start-up from rest, which increased from the first loop to the last loop and decreased with the increase in power input. Further, the numerical simulation with the one-dimensional model reproduced the oscillatory behavior in loop#1, with the predicted stability threshold being considerably lower compared to the experiments. The attenuation of the oscillations in the loop#2 as observed in the experiments could be reproduced. Predicted transients showed practically no oscillations in loop#3 corroborating the experimental observations. The attenuation of the oscillations in loop#2 and the absence of oscillations in loop#3 were attributed to the damping caused by the wall and also due to the higher Lt/D ratio in loop#2 and a vertical air flow channel in loop#3 respectively. The phase plots indicated a near periodic oscillation compared to the highly chaotic pattern in the experiments, presumably due to the higher power at which instability was observed in the experiments. The deviations were also due to the secondary flows being generated in the large diameter loop as the simulation was based on 1D along with the heat loss being neglected.

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Stabilization of a single-phase natural circulation loop by pressure drops

Abstract

Introduction

Section snippets

Experimental apparatus

Results

Conclusions

Acknowledgements

Int. J. Thermal Sci.

Int. Commun. Heat Mass Transfer

A review of N.C. loops in PWR and other systems

Nucl. Eng. Des.

Natural circulation loops

J. Heat Transfer