Calculated heat transfer development in bundles

In many cases heat transfer in rod bundles can be considered as a superposition of several simple heat transports. A number of practical problems can thus be solved if solutions for these elementary transport are available. Two elementary heat transports in rod bundle geometry are investigated, namely the transport from a fuel rod surface into the adjacent subchannel and the transport from one subchannel into the next one. The first transport is characterized in terms of the Nusselt or Stanton numbers, while the latter in terms of the Stanton gap number or mixing factor. Hydraulically developed flow is assumed, with no feedback of heat transport on the flow condition. A three-dimensional numerical calculation by means of the finite difference method is applied to determine the eigenvalue solution of the response on a step change in heating for both cases. The method is tested on an internally heated concentrical annulus. The result is compared with available experimental and theoretical predictions. It is found that the heat transfer between subchannels is developed considerably slower in comparison with the development of the heat transport from fuel rod to subchannel.