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In many dispersed multiphase flows droplets, bubbles and particles move and disappear due to a phase change. Practical examples include fuel droplets evaporating in a hot gas, vapour bubbles condensing in subcooled liquids and ice crystals melting in water. After these `bodies' have disappeared, they leave behind a remnant `ghost' vortex as an expression of momentum conservation.
A general framework is developed to analyse how a ghost vortex is generated. A study of these processes is incomplete without a detailed discussion of the concept of momentum for unbounded flows. We show how momentum can be defined unambiguously for unbounded flows and show its connection with other expressions, particularly that of Lighthill (1986). We apply our analysis to interpret new observations of condensing vapour bubble and discuss droplet evaporation. We show that the use of integral invariants, widely applied in turbulence, introduces a new perspective to dispersed multiphase flows