A new drift flux relation is derived for low speed vertical bubble flows by balancing the hydrostatic pressure with the liquid eddy stresses; the effective eddy viscosity is related to the energy dissipated by the gas flow. Comparisons with steam-water data show that the observed variation of void fraction with gas flux, and the effects of pipe diameter, are well predicted by the model. However, to represent the pressure effects it is found necessary to postulate that the eddy viscosity increase systematically with pressure. Further experimental data are required to test this hypothesis.

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