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2.6 A Study of Countercurrent Flow and Flooding in Parallel Channels

DOI: 10.1615/ICHMT.1982.AdvCourHeatTransfNucReactSaf.180
pages 361-377

B. D. G. Piggott
Central Electricity Generating Board, Performance and Safety Division, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire, GL13 9PB, UK

M. C. Ackerman
Central Electricity Generating Board, Performance and Safety Division, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire, GL13 9PB, UK


The phenomena of countercurrent flow and flooding are of importance in the emergency cooling of light water reactors following a postulated loss of coolant accident. For instance, downward penetration of water into the core from the upper plenum during reflood determines top quenching behaviour. Most previous studies have been concerned with single channels although physical situations often consist of large pin arrays or parallel channels. The existing literature on pin bundle and multiple channel behaviour is sparse and has uncovered instability and hysteresis phenomena for which no explanations were offered. A detailed investigation was therefore undertaken to improve the understanding of interactions between parallel channels in countercurrent flow and flooding and a simple but accurate method was developed by which multiple channel behaviour can be predicted from a knowledge of single tube characteristics.
Behaviour of a single channel in air/water counterflow was investigated in detail. Pressure drop characteristics were obtained as a function of air and water flowrates for both constant volume and constant pressure drop modes of air delivery. In the constant pressure drop mode of operation any flooding column height could be obtained, whereas for constant volume operation the flooding column always filled the tube. Void fraction in the flooded region was found to be independent of column height for a given air/water flowrate. Particular attention was paid to the detailed behaviour, in both modes of air supply, close to the flooding transition. Hysteresis effects were noted, a knowledge of which proved very important for a proper understanding of multiple channel behaviour.
Pressure drop versus airflow characteristics were obtained for two to six channels. These characteristics showed a number of peaks equal to the number of channels in operation, the peaks corresponding to channels going through a flooding transition one by one. The order in which channels flooded was, apparently, random. Instability phenomena were noted which could be explained by reference to the detailed single channel measurements. A method has been developed with which it is possible to predict very precisely multiple channel behaviour from single channel measurements. The measured multiple channel characteristics were shown to be in very close agreement with predictions.

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