Abstract
This study is mainly focused on applying the Particle Image Velocimetry (PIV) technique to the unique fluid system of He II. Challenges associated with the application are identified and discussed in the context of the exceptional physical properties and experimental environment of He II. The particle dynamics in He II is studied, and two important parameters, slip velocity and relaxation time, are derived. Based on this information, the tracking characteristics of a variety of candidate tracer particles, including commercially available solid particles and particles generated by freezing liquids and gases, are discussed, as well as their potential applications to liquid helium. It is indicated that polymer particles with a mean diameter of 1.7 ƒÝm and specific gravity of 1.1 are the most suitable for tracking the thermal counterflow in our experiments. To introduce these very fine particles into liquid helium, a simple seeding method based on the two-phase fluidized bed technology was developed. The seeding results show an adequate concentration and also a quite uniform spatial distribution of seeded particles.
Using the PIV technique, velocity fields of He II thermal counterflow in steady state have been measured in a range of bath temperatures from 1.61 K to 2.0 K and applied heat fluxes from about 1.1 kW/m2 up to 13.7 kW/m2. A significant discrepancy between the measured particle velocity vp,a and theoretical normal fluid velocity vn,t is present at all the temperatures, and the ratio of vp,a/vn,t is most likely a temperature-independent constant around 0.5. Careful analysis suggests that this velocity discrepancy may be caused by an additional force from the superfluid component. A semi-empirical correlation for this force is developed. By adding the force to the particle dynamics equation, the analytical results are shown to be consistent with the experimental results.
Also, the propagation of second sound shock and heat diffusion has been studied by measuring the instantaneous velocity fields of induced transient thermal counterflow. The arrival of shock front, effect of expansion fan, passage of shock tail, and the onset of heat diffusion are clearly observed from the particle velocity profiles versus time. The generated particle velocity profiles are compared and discussed in respect of the critical energy flux for the onset of quantum turbulence, and the additional force from the superfluid component is further addressed regarding its application to the transient state.
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