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The pressure drop characteristics and flow rates encountered in measurements of flashing water flow in parallel-sided pipes cannot be simply related to the thermodynamic equilibrium theory of the velocity of sound in homogeneous two-phase one-component mixtures. It is shown in the paper that the velocity of propagation through such a mixture is different in magnitude, both to the thermodynamic equilibrium theory and to the theory postulating no phase change. Measurements of the velocity of propagation of compression and rarefaction waves in atmospheric pressure stagnant boiling water are described and interpreted. Some aspects of bubble motion in a pressure gradient are discussed. Measurements in stagnant water are not precisely relevant to the case of flowing liquid in which the vapour component is slipping positively with respect to the liquid. The effect of slip is discussed.I t seems to the authors that some relationship between the sonic velocity obtained from their tests and the critical velocity correctly defined in terms of local pressure slip and void fraction should exist. The data which should be extracted from a critical flow experiment are suggested. * Project Manager, Dounreay Experimental Reactor Establishment, t Principal ScientiJfc Oficer, Dounreay Experirnentul Reactor $ Scientific Oficer, Dounreay Experimental Reactor Establishment, S References are given in Appendix 23.1. Proc Instn Mech Engrs 1969-70 1969 and accepted for publication on 23rdJuly 1969. 3

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