Local temperature drop
Difference between the temperatures of the medium and the local wall (phase boundary) temperature, or the temperature difference between two media in a given combination within a heat exchange system.
Comments. 1. In external flow past bodies the temperature of the outer flow is usually taken as the temperature of the medium, whereas for flow in pipes, the bulk temperature at a given section is taken.
2. The term "pipe" refers here to pipes and tubes (or channels closed over the entire perimeter) of arbitrary cross section but constant with length, in which fluid flow takes place.