Most practical applications of natural circulation employ non-uniform diameter loops. Common examples are the nuclear reactor loop, solar water heater, etc. Most test facilities simulating nuclear reactor loops also use non-uniform diameter loops. Depending on the operating pressure, the non-uniform diameter loops can be categorised as High pressure loops and Low pressure loops. Most studies are conducted in the high-pressure test facilities simulating nuclear reactor loops. Typical examples are the SEMISCALE, LOBI, PKL, BETHSY, ROSA, RD-14, FISBE, etc. Some studies, however, are carried out in low-pressure facilities. Examples are the experiments carried out by Zvirin et al. (1981), Jeuck et al. (1981), Hallinan-Viskanta (1986), Vijayan (1988) and John et al. (1991). Most of the available experimental data in a usable form (i. e. full geometrical details are known) are from the low — pressure test facilities. High-pressure test data in a usable form was available only from FISBE. The data from NDLs are plotted in Fig. 10, which shows reasonable agreement with theoretical correlation in the laminar and the turbulent flow regions. Significant deviation is observed for the intermediate values of Grm/NG where the flow is neither fully laminar nor fully turbulent. Similar trend was observed in the case of UDLs. Interestingly, the parallel channel data of John et al. (1991) and parallel loop data of Jeuck et al. (1981) are also found to be in reasonable agreement with the theoretical correlation.