where NuL ¼ hdh=kL; ReLS ¼ ULSdh=mL; ReGS ¼ UGSdh=mG, h is the heat
transfer coefficient, kL is the liquid thermal conductivity, ULS and
UGS are the liquid and gas superficial velocities, respectively, and
mL and mG are the liquid and gas kinematic viscosity, respectively.
Fig. 11 shows the comparison of experimental Nusselt numbers to
predictions by Eqs. (11) and (12).