Fig. 5b showed heat transfer enhancement
ratios versus Reynolds numbers. Hereby the heat transfer enhancement
ratio φ was defined as the Nusselt numbers for the MHTT
divided by those for the BT under the same operating conditions. With
Re variation from 2109 to 20,175, the enhancement ratios were always
larger than unity, indicating the heat transfer enhancement. It was
noted that the transition flow regime happened in the Re range of
2300–6000.