Accessible Unlicensed Requires Authentication Published by De Gruyter October 23, 2021

Heat transfer characterization under radial jet and falling film induced rewetting

Charakterisierung des Wärmeübergangs bei radialer Strahl- und Filmwiederbenetzung
M. Kumar and D. Mukhopadhyay
From the journal Kerntechnik

Abstract

Empirical correlations are developed for rewetting velocity and maximum heat transfer coefficient during rewetting phase of single hot vertical Fuel Pin Simulator (FPS) by using radial jet impingement and falling film. Emergency Core Cooling System (ECCS) has been designed for Advance Heavy water Reactor (AHWR) to rewet the hot fuel pin under the loss of coolant accident. Coolant injection takes place from a water rod which is located at the center of the fuel bundle in form of jets to rewet hot surface of fuel pin under loss of coolant accident. This kind of design to reflood the fuel bundle is different than bottom and top spray reflooding practiced in PWR and BWR type of nuclear reactors. There are two different kinds of rewetting found during radial jet induced cooling. The first one is due to radial jet impingement and the second one is due to falling film which is below the jet impingement point. Rewetting velocity has been predicted along the length of fuel pin due to radial jet impingement cooling. Temperature of FPS has been varied from 400°C to 700°C with help of different powers supply, simulating decay heat of reactor. A variation of coolant radial jet mass flow rate is from 0.5 lpm to 1.8 lpm. It is considered during ECCS injection. It has been observed from the experiments that rewetting velocity decreases with increasing the clad surface temperature and increases with increasing the coolant mass flow rate. The rewetting velocity in falling film is found to be nearly 1.8 times higher than rewetting velocity predicted in circumferential direction. Further, it is found that maximum heat transfer coefficient increases with increasing the radial jet coolant mass flow rate. The maximum heat transfer coefficient in case of radial jet impingement is found to be nearly 1.5 times the falling film rewetting. Developed correlation predicts the maximum heat transfer coefficient with experimental data well within the error band of ±10%.

Abstract

Es werden empirische Korrelationen für die Wiederbenetzungsgeschwindigkeit und den maximalen Wärmeübergangskoeffizienten während der Wiederbenetzungsphase eines einzelnen heißen vertikalen Brennstoffstabsimulators (FPS) unter Verwendung von Radialstrahlen und fallenden Filmes entwickelt. Das Emergency Core Cooling System (ECCS) wurde für den fortgeschrittenen Schwerwasserreaktor (AHWR) entwickelt, um den heißen Brennstoffstift bei einem Kühlmittelverlust wieder zu benetzen. Die Kühlmitteleinspritzung erfolgt durch einen Wasserstab, der sich in der Mitte des Brennstoffbündels befindet, in Form von Düsen, um die heiße Oberfläche des Brennstoffstabs bei einem Kühlmittelverlust wieder zu benetzen. Diese Art der Wiederbenetzung des Brennelementes unterscheidet sich von der Wiederbenetzung durch Sprühen von unten und von oben, wie sie in Kernreaktoren vom Typ DWR und SWR praktiziert wird. Bei der radialstrahlinduzierten Kühlung gibt es zwei verschiedene Arten der Wiederbenetzung. Die erste ist auf das Auftreffen des Radialstrahls zurückzuführen, die zweite auf den herabfallenden Film, der sich unterhalb des Strahlauftreffpunkts befindet. Die Wiederbenetzungsgeschwindigkeit wurde entlang der Länge des Brennstoffstabs aufgrund der radialen Strahlabkühlung vorhergesagt. Die Temperatur des Brennelementes wurde von 400°C bis 700°C mit Hilfe unterschiedlicher Energiezufuhr variiert, um die Nachzerfallswärme des Reaktors zu simulieren. Der Massendurchsatz des Kühlmittels im Radialstrahl variiert von 0,5 lpm bis 1,8 lpm. Sie wird bei der ECCS-Einspritzung berücksichtigt. Aus den Experimenten geht hervor, dass die Wiederbenetzungsgeschwindigkeit mit steigender Oberflächentemperatur der Umhüllung abnimmt und mit steigendem Kühlmittelmassenstrom zunimmt. Die Wiederbenetzungsgeschwindigkeit im fallenden Film ist fast 1,8-mal höher als die Wiederbenetzungsgeschwindigkeit in Umfangsrichtung. Ferner wird festgestellt, dass der maximale Wärmeübergangskoeffizient mit zunehmendem Radialstrahl-Kühlmittelmassenstrom steigt. Der maximale Wärmeübergangskoeffizient bei radialer Strahlbeaufschlagung beträgt fast das 1,5-fache der Fallfilmrückbenetzung. Die entwickelte Korrelation sagt den maximalen Wärmeübergangskoeffizienten mit den experimentellen Daten innerhalb eines Fehlerbereichs von ±10% voraus.

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Received: 2021-02-16
Published Online: 2021-10-23

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