Measurements of the second-sound velocity in dilute 3He4He mixtures

A Helmholtz resonator was used to measure the velocity of second sound in dilute solutions of ³He in ⁴He. The results are compared with values reported by Brubaker et al. and de Voogt et al.     

A mutual inductance compensator with varying primary current

Summary An ac mutual-inductance compensator is described in which compensation of the secondary voltage is achieved by calibrated variation of the primary current in a standard mutual inductor of a fixed value. A resistive current-dividing network is used containing a commercial precision potentiometer. The ac frequency range employed can easily be extended to 10 kHz without seriously affecting the accuracy of the measurements.     

The response of the level of a liquid fluidized bed to a sudden change in the fluidizing velocity

Summary When the fluidizing velocity in a liquid fluidized bed of solid particles is suddenly changed, a discontinuity in the porosity is introduced at the bottom of the bed. This discontinuity is propagated upwards through the bed. The boundary between the old and the new porosity broadens or remains sharp depending on whether the porosity is increased or decreased. This behaviour is reflected in the way in which the bed level changes as a function of time. For a few different systems such response curves have been measured by means of a specially designed follow-up system. On the basis of the above mechanism a quantitative theory was developed for the response of the bed level to a step-wise change in the fluidizing velocity. This theory proved to give a satisfactory agreement with the observed facts.List of symbols m.k.s. units have been used for the purpose of calculation - d _p diameter of particle - h instantaneous height of the fluidized bed - n constant in eq. (2) - t time - u average velocity of the fluidized particles with respect to the wall, positive in the direction of the liquid flow - U _s settling velocity of single particle in tube; constant in eq. (2) - v average liquid velocity with respect to the wall - w() velocity of propagation of a disturbance d at a porosity - x coordinate in the direction of flow - porosity, void fraction - _p particle density - liquid velocity in region above particles, volumetric flow per unit area of empty tube - index 0 refers to the steady situation for t 0 - index 1 refers to the steady state situation state reached after t = t ₁