All the foregoing Nusselt number expressions are restricted to situations for which the surface temperature Ts is uniform. A common exception involves existence of an unheated starting length (Ts = T∞) upstream of a heated section ( ) T T s ≠ ∞ . As shown in Figure 7.5, velocity boundary layer growth begins at x = 0, while thermal boundary layer development begins at x = ξ. Hence there is no heat transfer for 0 ≤ x ≤ ξ. Through use of an integral boundary layer solution [5], it is known that, for laminar flow