A new model is developed for gas-liquid segmented flows in ducts and channels. This model is an improvement of an earlier analysis presented and published by the author. In the present work, it is shown that for constant wall temperature, the dimensionless mean wall flux has two characteristic behaviours: thermal boundary layer flow and fully developed flow. These can also be viewed as short and long residence times as the plug train moves through the tube or channel. The boundary layer limit is dominant for short residence times while fully developed flow occurs for longer residence times. An additional parameter, the plug length to duct length ratio, (Ls/L), is shown to have significant impact on the rate of heat transfer. This parameter has the limits 0 < Ls/L < 1. The new model is compared with data from several published studies in which the variables were well controlled. It is also shown that careful experiments must be undertaken to demonstrate the characteristics of this type of flow under constant wall temperature conditions.

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