Forced Convection: The most common mode of heat transfer is forced convection. A number of correlations have been developed for situations where fluid is flowing through pipes and annuli. These correlations are functions of Reynolds number (Re), Prandtl number (Pr), and Graetz number (Gz) (see Tables 5.4 and 5.5). During computation of these numbers, the properties of the fluid are taken at bulk temperatures.

Flow through a tube:

Example 5.5: Water is flowing through a tube having internal diameter 1.334" at a rate of 10000 lb/hr. It is heated from 80 °F to 120 °F in a counter-current arrangement. Find the heat transfer coefficient (h_i) if the following data apply:

Bulk temperature, t_b, = 100 °F

Specific heat, c, = 0.998 Btu/(lb·°F); density, ρ = 62.0 lb/ft³;

Absolute viscosity, μ = 1.65 lb/(ft·hr); thermal conductivity, k = 0.363 Btu/(hr·ft·°F);

Solution: Water is flowing through a circular tube. Heat transfer coefficient depends upon the value of Reynolds number and Graetz number.

$ The velocity of the fluid, V, can be found once the cross-sectional area, a, is known.

$

$ Reynolds number, Re:

$

$ Prandtl number, Pr:

$ Nusselt number, Nu: Appropriate relationship when Re is greater than 10000, so the following correlation will apply:

$ Film transfer coefficient, h: