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.