Abstract
Hot spots appearing in an operating high temperature gas-cooled reactor (HTGR) core have been considered as the most possible reason leading to a severe accident like fission production releasing to the environment, therefore, investigation on their positions and thus seeking ways to reduce the possibility of their appearance have attracted scientists’ attention. In our previous studies, heat transfercharacteristics of a face–centered–cubic (FCC) structured pebble–bed have been discussed,and a correlation on heat transfer coefficient with Reynolds number was presented. In this study, a method, placing a small sphere in thegap area, which is able to enhance the convective heat transfer wasproposed and the effect verifiedas well. The influence of the sphere diameter on heat transfer performances wasinvestigated in details. It is concluded through results analysis that (1) inserted sphere lowered thelocal surface temperature of adjacent pebbles by varying surrounding flow field;(2) maximum velocity of the fluid and average heat transfer coefficientincreased with sphere diameter, particularly, comparing with no small sphere case, 12.95% enhancement was achieved. Such findings may provide dataand information for reactor designers, andhelp to develop a safer HTGR pebble–bedcore.
Highlights
Pebble-bed type high temperature gas-cooled reactor is considered as a candidate reactor for the very high temperature reactor (VHTR) which is one of the Generation-IV reactor types and could be used for hydrogen production because of high outlet temperature of the coolant
The results indicated that placing an additional small sphere in the gap formed by adjacent pebbles improved heat transfer performance, in addition, average heat transfer coefficient of the pebble bed increased as the additional sphere got larger
It is really difficult to tell how overall temperature profile is affected by the small sphere only based on these 3 figures at this moment, pebble surface temperatures at positions shown in Fig. 4 were obtained and their variation trends were analyzed in details
Summary
Pebble-bed type high temperature gas-cooled reactor is considered as a candidate reactor for the very high temperature reactor (VHTR) which is one of the Generation-IV reactor types and could be used for hydrogen production because of high outlet temperature of the coolant. The possible appearance of hot spots on pebble surfaces may affect the integrity of the pebbles and cause a serious accident like fission products releasing[1], which has driven researchers to investigate flow patterns of the coolant [2,3] and to observe the temperature profiles of structured pebble-beds by simulations [4,5]. If the convective heat transfer is enhanced, temperatures of the hot spots will be lowered down, which helps to sustain the integrity of pebbles and safety of the reactor as well In this present study, a method of enhancing heat transfer in an FCC structured pebble-bed was proposed for the first time and its effect was examined through simulations. That model has been validated by experimental results, and the only difference exists in geometry part, that is a small sphere without heat generation with changeable diameter is placed in the gap formed by pebbles, it is believed even with such a small change the model is still applicable to this study
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