Inception conditions of density-wave oscillations were determined in two sodium heated once-through steam generator tubes, i.e., in a 44.43 m long helical coil of 0.018 i.d. and in a tube of 0.0131 m i.d., comprised of a 9.40 m long vertical- and an 11.05 m long, V-shaped horizontal tube. The operating conditions on the water/steam side for the experiments were: pressure: 6–19.1 MN/m2; mass velocity 187–1020 kg/m2 s; inlet subcooling: 3.2–168 K; outlet steam quality: 1.15–2.38. The 306 data obtained and the 74 data found in the literature from sodium and electrically heated once-through steam generator tubes were correlated within 7.5 percent accuracy for 98 percent of the time. The R.M.S. error for all the 380 data is 3.33 percent. The observed density-wave oscillations are time-delay oscillations and the length of the superheated steam region and the transit time in this region practically govern the mechanism of these oscillations. An empirical relation has also been established for this mechanism.

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