Unsteady-state heat transfer effective coefficients

Effective Coefficient for Unsteady-State Heat Transfer/ /V... 

In some cases it is convenient to treat unsteady-state heat transfer using an effective coefficient rather than the exact equations or plots such as Fig. 10.6. For example, the rate of heat transfer to a spherical particle can be approximated using an internal coefficient equal to 5k/r , and the external area 47tr. The unsteady-state heat balance for a sphere then becomes... 

With drops and bubbles the resistance to mass transfer in both phases may be significant. Diffusion inside a stagnant noncirculating) drop is an unsteady-state process, but for convenience in combining coefficients an effective internal coefficient can be used, as was done for heat transfer in spheres [see Eqs. (11.38) and (11,40)] ... 

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