(St/St)/(f/f)1/3 [36]. When both Re and Pr are controlled at the same values for the comparative groups with various HTE elements, the theoretical TPF in terms of (St/St)/(f/f)1/3 is reduced to ðNu=Nu1 Þ=ðf =f1 Þ, which is adopted by present study to compare the relative efficiencies of heat transmissions among the collective HTE elements summarized in Table 1