No abstract

Multiphase flow computations involve coupled momentum, mass and energy transfer between moving and irregularly shaped boundaries, large property jumps between materials, and computational stiffness. In this study, we focus on the immersed boundary technique, which is a combined Eulerian-Lagrangian method, to investigate the performance improvement using the multigrid technique in the context of the projection method. The main emphasis is on the interplay between the multigrid computation and the effect of the density and viscosity ratios between phases. Two problems, namely, a rising bubble in a liquid medium and impact dynamics between a liquid drop and a solid surface are adopted. As the density ratio increases, the single grid computation becomes substantially more time-consuming; with the present problems, an increase of factor 10 in density ratio results in approximately a three-fold increase in CPU time. Overall, the multigrid technique speeds up the computation and furthermore, the impact of the density ratio on the CPU time required is substantially reduced. On the other hand, the impact of the viscosity ratio does not play a major role on the convergence rates.

Cryogenic fluids have found many practical applications in today’s world, from cooling superconducting magnets to fueling launch vehicles. In many of these applications the cryogenic fluid is initially introduced into piping systems that are in excess of 150 degrees Kelvin higher than the fluid. This leads to voracious evaporation of the fluid and significant pressure fluctuations, which is accompanied by thermal contraction of system components. This process is known as chilldown, and although it was first investigated more than 4 decades ago, very little data are available on the momentum and energy transport during this transient process. Consequently, the development of predictive models for the pressure drop and heat transfer coefficient has been hampered. In order to address this deficiency, an experimental facility has been constructed that enables the flow structure to be observed while temperatures and pressures at various locations are measured. This study focuses on the inverse numerical procedure used to extract the transient heat transfer coefficient information from the data collected; this information is then used to evaluate the performance of various correlations for heat transfer coefficient in the flow boiling regime. The method developed utilizes flow structure information and temperature measurements, in conjunction with numerical computations for the temperature field within the tube wall, to calculate the heat transfer coefficient. This approach allows the transition point between the film boiling regime and the nucleate boiling regime to be determined, and it also elucidates the variation of the heat transfer coefficient along the circumference of a horizontal tube, with the heat transfer on upper portion being significantly smaller than that at the bottom.

This work describes an experimental investigation of the transient two-phase flow behavior of liquid nitrogen flowing through a pipeline during the chilldown process. The evaporation process and flow regime transitions were observed during chilldown. Initially pure vapor is observed in the visual test section. Then a moving film boiling front is observed to move through the test section. The length of the film boiling front appears to be very small compared with the length of the facility pipeline. After the film boiling front passes through the test section, a very high velocity stratified two-phase flow appears with a small liquid film thickness. As the pipeline is chilled, the film thickness grows. It appears that the high velocity two-phase flow suppresses nucleate boiling, and the dominant heat transfer mechanism is two-phase bulk turbulent convection. Evaporation occurs at the liquid/vapor interface. A high-speed video camera is used to observe the flow regime transitions during the transient chilldown process. Flow regime comparisons have been made with the Steiner [1], Kattan-Thome-Favrat [2], and Baker [3] flow regime maps.