“…This paradox can be solved using the twoscale thermodynamics. 20,21 Actually the inner surface is not smooth enough (see Figure 1), so a continuum model with smooth boundary on a large scale leads to a wrong result; however, if we observe the problem on a smaller scale so that the unsmooth inner surface can be measured, the paradox can be completely solved by the fractal calculus, 22,23 which is to study various phenomena in discontinuous space, and has widely applied in electrochemistry, 24 biomechanics, 25,26 Tsunami model, 27 wool fiber, 28 thermal insulation, 29 fractal solitary wave, 30 and fractal convection-diffusion model. 31 As shown in Figure 1, the inner surface of the hollow fiber is not smooth, and the inner boundary of the cross section is a coastline-like curve with fractal dimensions larger than 1, so the fractal calculus has to be adopted in our study.…”