After a brief review of developments in the field of optical masers in general, certain properties of the ruby optical maser are discussed in more detail. An account is given of a number of original experiments leading to new observations on the temporal properties of ruby maser light. A photomixing experiment is described which resulted in an unambiguous measure of the coherence time of light beams from ruby when the output consisted, as is usual, of a random sequence of spikes. The experiment made use of light beams from two independent masers, and led to a measured upper limit to the coherence time of microsecond which corresponds to the width of an individual spike. Next, a new effect, namely the appearance of a quasi-continuous output under conditions of non-uniform energy distribution, is reported. The attendant mode behaviour is described in detail, and additional experiments are outlined which were designed to observe beats between modes associated with different regions in the crystal. For reasons given, beats were primarily observed between modes associated with the same region. The duration of the beats pointed to a much longer coherence time than in the case of the earlier experiment. This conclusion relies upon evidence from the earlier experiment, that the beat spectrum is not appreciably affected by phase coupling between modes. The results are summarized and suggestions are made for further experiments.

Experiments, to measure the heat transfer from dry saturated steam to rotating plain cylindrical surfaces on which the steam is condensing, were carried out with specially built apparatus. The range of pressure of the steam was from 20 to 80 psia., and three plain cylindrical nickel-plated mild-steel specimens were employed having diameters of 4, 8, and 10 inches, rotating at various speeds from 0 to 1500 r.p.m. The axial speed of the steam was small. The heat fluxes attained varied from 20,000 to 80,000 B.Th.U/ft.2hr. and the heat transfer coefficients varied between 1,000 wild 4,000 B.Th.U/ft.2hr.°F. For centrifugal accelerations up to 2 g the heat transfer coefficients were similar to the stationary-cylinder values, but above 2 g them; was an increase in these. Photographs of the rotating cylindrical surface taken during the experiments showed that the condensate forms a complete film on the surface and protrusions of condensate occur, towards which the condensed water flows. Eventually the protrusions become drops large enough to be thrown off the rotating surface by centrifugal force. These protrusions covered only a small amount of the surface, and it was calculated that the flow of the condensate between protrusions was laminar. Existing laminar-film condensation theory was adapted to rotational conditions. The theory used assumes laminar flow of condensate towards a protrusion under the influence of the components of centrifugal force acting along the condensate surface. The relations between the heat transferred and other variables are presented in the report both graphically and in the form of equations using; standard dimensionless factors. The results of the present experiments are compared with previous work in this field, and extensions of the results of the present work to a wider range of conditions are considered.

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This thesis presents the research of my PhD, which is the study of two-phase flows by using the phase-field methods. The key point for this work is the thermodynamic con- sistency. We begin by introducing an extension of the Model H to study the two-phase flows with thermocapilliary effects, where we assume that the coefficient of the surface tension is temperature dependent, and the classical energy equation is coupled with the Model H. We then investigate numerically an established phase-field model (Quasi- incompressible NSCH model) for the two-phase flows with varible density. We design a numerical method where the energy law of the model is preserved at the discrete level. Fianlly we develop a new model to study the two-phase flows with thermocapil- liary effects where the model allows the two fluids to have different physical properties meanwhile maintaining the thermodynamic consistency. The pillbox argument is em- ployed to show that our model can reduce to the sharp-interface model where the jump conditions can be recovered.

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