In the double tube heat exchanger, a high-temperature fluid flows inside a small pipe and a low-temperature fluid flows through an annular space between the two tubes. The flow of the two fluids is turbulent and the surfaces of all tubes are smooth. When the temperature distribution function of the low temperature fluid in the radius r-direction of the tube near the small tube wall is T=Tw2(r-r0), the number of Nusselts for the low temperature fluid is? (However, r0 is the radius of a small tube, Deq is the equivalent diameter of the annular space, Tm is the average temperature of the low-temperature fluid, and Tw is the wall temperature of the small tube in contact with the low-temperature fluid.)