Figure 6.4 shows an idealized form of the flow patterns and the variation of the surface and liquid temperatures in the regions designated by A, B, and C for the case of a uniform wall heat flux. Under steady state one-dimensional conditions the tube surface temperature in region A (convective heat transfer to single-phase liquid), is given by:
and where q;SPMquot; is the heat flux, Per is the heated perimeter, G is the mass velocity, A is the flow area and 
is the liquid specific heat. Also 
is the temperature difference between the wall surface and the mean bulk liquid temperature at a given length z from the tube inlet, h is the heat transfer coefficient to single-phase liquid under forced convection. The liquid in the channel may be in laminar or turbulent flow, in either case the laws governing the heat transfer are well established; for example, heat transfer in turbulent flow in a circular tube can be estimated by the well-known Dittus-Boelter equation.
This relation is valid for heating in fully developed vertical upflow in z/D > 50 and Re > 10,000.
where 
is the hydraulic diameter, 
is the liquid viscosity and 
is the liquid thermal conductivity.
For the case of a given constant wall temperature, the temperature difference will decrease, as well as the heat flux. From an energy balance this is represented by a logarithmic decrease in the temperature difference.