Minichannel versus MicroChannel

A channel may behave as a microchannel for certain fluids and operating conditions and as a macrochannel for certain other conditions. Channel size can also be the distinguishing factor between macro- and microchannel flows. The boundary between macro- and microchannel flows is related to the ratio of bubble size to channel diameter for two-phase flows. The case with larger ratio between bubble size and channel size behaves more like a microchannel flow. 

Kew and Cornwell (1997) used this rationale to define a confinement number for determining the boundary between microchannel and macrochannel flows as 

a is surface tension, p is density, Df is hydraulic diameter of microchannel, subscript f corresponds to saturated liquid, and subscript g corresponds to saturated vapor. Confinement number is based on the ratio of surface tension force to buoyancy force. It is a good representation of pool boiling, that is, in a confined channel. The macrochannel assumption falls apart when the channel becomes too confining at co 0.5. 

Combining equations (9.88) and (9.89) and substituting G = PfV, we have the criterion for transition from macro- to microchannel flow as 

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