Critical heat flux in vertical annulus under zero flow conditions with radial heat losses

At the boiling condition in a bottom end closed vertical heated channel the countercurrent flow limitation (CCFL) will lead to liquid deficiency at the heated zone and burnout of the channel wall. That heating limitation is called flooding CHF and it is used in nuclear reactors accidents analysis such as fuel channel blockage or Loss of Flow Accident (LOFA). It is well known that the flooding CHF is influenced by channel diameter, heated length and fluid properties, however no references were found in the literature regarding the influence of the radial heat losses from the channel, e.g. in case of immersed fuel channel. In the present work, flooding CHF was studied using an annular flow at atmospheric pressure. Some experiments were conducted with transparent annular test section consists of an 830 mm heated length 44.85 mm – ID and 48.6 mm OD . In these experiments, the radial heat loss through the quartz were negligible as compared with the input electrical power. Another version of the test section consists of a cooling jacket instead of the quartz tube. That version was used to examine the influence of the radial heat loses. In the adiabatic version (quartz tube) the influence of the upper reservoir temperature was examined within the range of 30 to 80 °C. It was found that the flooding CHF values varied by about 10% within this range. Existing correlations exhibit a similar variation of the flooding CHF due to the influence of the temperature on the water surface tension. A significant effect of the radial heat losses (up to a 100%) was observed on the measured flooding CHF values with the cooling jacket system version.