Experimental investigations on turbulent mixing of hot upward flow and cold downward flow inside a chimney model of a nuclear reactor

• Simulated mixing of hot upward and cold downward flows in a chimney of a reactor.
• Experiments in chimney model (2:9 scale) at Reynolds number (Re)—1.5 to 4.5 × 105.
• Hot upward flow comes out of the chimney when bypass flow ratio (R) is zero.
• Increase in ratio (R) reduces jet height, vortex spread height and temperature front height.
• Effects of Re, chimney height and temperature differential are not significant.

Experiments were conducted to study the turbulent mixing of hot upward flow and cold downward flow inside a scaled down model of chimney structure of a pool type nuclear research reactor. Open pool type nuclear reactors often use this type of chimney structures to prevent mixing of radioactive core outlet water directly into the reactor pool so that radiation field at the reactor pool top can be kept to a lower limit. The chimney structure is designed to facilitate guiding of the radioactive water towards the two outlet nozzles of the chimney and simultaneously allows drawing water from the reactor pool through the chimney top opening. The present work aims at studying flow mixing behaviour of hot and cold water inside a 2/9th scaled down model of the chimney structure experimentally. The ratio between the cold downward flow and the hot upward flow is varied between 0 and 0.15 to predict the extent of suppression of the hot upward flow within the chimney region for various bypass flow ratios. The Reynolds number of the hot upward flow considered in the experiment is about 1.5 × 105 which corresponds to a flow rate of about 500 l min−1. The upward jet height and the temperature distribution were predicted from the experiment. It was observed that increase in bypass flow ratio reduces the upward jet height of hot water. Experiments were also carried out by increasing the flow rate to 1000 and 1500 l min−1 corresponding to Reynolds numbers of 3 × 105 and 4.5 × 105, respectively. It was observed that the jet height did not vary significantly with changing upward flow rate provided the bypass flow ratio was held constant. Experiments were also done by increasing the chimney height from 500 mm to 600 mm and no significant change in mixing behaviour of the hot and cold water was observed.