A bayonet tube heat exchanger with fins being employed outside and inside outer tubes of bayonet-elements is recommended to be used in the ultra high temperature environment by our previous research, such as Externally Fired Combined Cycle and syngas production processes. The present study is concerned with the heat transfer and pressure drop characteristics in internally finned bayonet tubes. Two kinds of thermal boundary conditions applied to the model, i.e., constant wall temperature boundary and convective heat transfer boundary, are calculated and compared. Effects of different fluid flow patterns, inlet Reynolds number and tube length on the heat transfer and pressure drop characteristics are performed. Thermal stress is produced due to the large temperature difference between the inner and outer tube under ultra high temperature environment. Considering the safety of the heat exchanger, the alternative designs are suggested that there should be a small spacing between the inner fin and inner tube surface. The effects of various spacing on the heat transfer and pressure drop characteristics are discussed as well. Based on the results, the suitable spacing is proposed.

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