MODELING TRANSITIONAL CONDITIONS TO CHARACTERISTIC MODES OF THE COURSE OF TWO-PHASE CRYOGENIC MEDIA IN PIPES AND RING CHANNELS

Abstract

Adiabatic and diabatic two-phase flows are widespread in cryogenic technology. The organization of the forced circulation of boiling liquid in channels of various orientations is one of the effective ways of cooling numerous heat exchangers. However, to create a rational design of heatexchange channels, it is necessary to know the conditions corresponding to the transition to undesirable flow regimes, in the presence of which the heat transfer intensity along the perimeter of the horizontal channel is significantly impaired. A comprehensive study of hydrodynamics and heat transfer during forced movement of a two-phase flow of helium (as well as nitrogen) in pipes and annular channels with visual observation of flow regimes was conducted at a special experimental bench at USPTU. Conventionally, it is possible to distinguish several flow patterns of a two-phase flow in the horizontal channel that are characteristic of hydraulic resistance and heat transfer. This is a stratified flow, including stratified and wave flows, slug and wave flows with jumpers, as well as flow regimes that are little dependent on the channel orientation - bubble and ring, including wave-ring and dispersed-ring flows. In the annular channel, the flow regimes are in qualitative agreement with the indicated flow regimes in the pipe. However, one of the determining parameters for identifying the conditions for stratification of a two-phase flow in an annular channel is its outer diameter. In the projectile and wave modes with jumpers, liquid jumpers support the flow of a liquid at the upper forming channel wall. In this case, the reduced resistance (as a function of the vapor content) is on average close to the calculated one for the homogeneous model. The heat transfer intensity near the upper generatrix of the wall (for small thickness and heat conductivity of the wall) depends on the ratio between the rate of evaporation of the near-wall liquid film and the hydrodynamic characteristics of the projectile and wave flow with jumpers. So, at small values of the heat flux density, the heat transfer intensity on the upper generatrix of the wall is close to the heat transfer intensity on the lower generatrix. When the values of the heat flux density correspond to the region of developed bubble boiling, as a rule, the upper generatrix of the wall undergoes a transition to film boiling (more precisely, convective heat transfer in the gas phase), and heat transfer compared to heat transfer to lower generatrix decreases by about an order of magnitude. The resulting liquid jumpers cause a significant temperature pulsation on the upper forming wall. When the conditions corresponding to the transition to the annular flow are reached, the heat exchange intensifies on the upper forming wall.