Several experiments on single-phase natural circulation loops are reported in the literature. The loops studied can be categorised as Uniform Diameter Loops (UDLs) and Nonuniform Diameter Loops (NDLs).

5.1.2.1. Uniform diameter loops (UDLs)

The UDLs experimentally studied include both closed-loop and open-loop thermosyphons. Considering the shape of the loop, the closed loops can be further classified into rectangular, toroidal and figure-of-eight loops. Holman-Boggs (1960), Huang-Zelaya (1988), Misale et al. (1991), Bernier-Baliga (1992), Vijayan et al. (1992), Ho et al. (1997) and Nishihara (1997) obtained experimental natural circulation data in UDLs of rectangular shape. Uniform diameter open-loops were investigated by Bau-Torrance (1981, 1981a) and Haware et al. (1983).

Fig. 8 shows a comparison of the data with the theoretical correlations for laminar and turbulent flow for uniform diameter loops. The experimental data reported by Misale et al. (1991), Bernier-Baliga (1992) and Ho et al. (1997) are well predicted by the theoretical correlation. Most of these data points are from laminar flow region where the total local loss coefficient is negligible compared to Lt/D due to the large value of the friction factor. For the
range 2 x 107<GrmD/Lt<3 x 108 significant deviation is observed between the data and the

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correlation. Beyond GrmD/Lt of 3 x 10 , the agreement is found to be better.

For all the steady state data reported in Fig. 8, it is assumed that Sleff=1. But for all practical configurations of loops, local losses are present so that Sleff>1. Hence, the observed deviations with the theoretical correlation may be attributed partly to the unaccounted local pressure losses in these loops. To study its effect, the local pressure loss coefficients due to elbows, bends, Tees, orifices, etc. given in Streeter and Wylie (1983) were used. The results are shown in Fig. 9. The turbulent flow data is now closer to the theoretical correlation indicating the significance of the local pressure losses at high Reynolds numbers where the friction factor is very low. The laminar region data is practically unaffected. The data for the transition region 2 x 10 <Grm/NG<3 x 10 is now closer to the theoretical correlation, but the deviation is still significant. This is the region where the correlation is not applicable as the loop is neither fully laminar nor fully turbulent. Instead the loop is partly in the laminar region and partly in either transition or turbulent regions.

FIG. 8. Steady state natural circulation flow in FIG. 9. Steady state natural circulation flow UDLs without considering local losses. in UDLs with local losses.