Leonid Bronfenbrener

Kinetic model for crystallization in porous media

Abstract A kinetic model for analyzing phase front propagation during freezing of a fine porous medium under conditions of moisture diffusion is presented. Crystallization is assumed to take place in a kinetic zone according to an experimental function characterizing the crystallization rate. The method was demonstrated for the crystallization of a 1-D fine-grained soil medium subject to constant boundary conditions. The numerical results were validated against experimental data from the literature. The following conclusions were inferred from the theoretical results: (1) in a closed system the rate of phase front propagation can oscillate even under constant boundary conditions; (2) as the phase front reaches a stationary state, the diffusional moisture flux from the non-frozen zone to the kinetic zone vanishes.

Experimental studies of water crystallization in porous media

Abstract An experimental system was designed and built for determination of thermodynamic and kinetic parameters of water crystallization processes in porous media. Experimental measurements were carried out for two different types of soil, sand and sandy loam (fined–grained soil). The results showed that the kinetic data fit a simple first-order expression in which the overall crystallization rate is directly proportional to the driving force, with rate constant 1/ τ , where τ is the characteristic time of the system. The driving force in this expression is defined as the difference between the prevailing and the equilibrium unfrozen water contents in the soil.

Two-phase zone formation conditions under freezing of porous media

Experimental studies on the freezing of porous media show that in case of a fine-grained skeleton matrix a thin transition zone (often called frozen fringe zone) exists between the propagating frozen phase interface and the thawing zone. In this paper a simple analytical criterion for the formation of the freezing zone is presented. The criterion is derived from a quasi-steady model solution, which takes into account moisture diffusion of unfrozen water in both the freezing and thawing zones and neglects convection effects. The model assumes that in the existing temperature range of the freezing zone the thermodynamic equilibrium of unfrozen water can be expressed as a linear function of temperature and that the thermal and mass diffusion coefficients are constant in each zone. The analytical criterion was found to be consistent with experimental results on the freezing zone formation of sandy and silty clay type soils reported in literature.

Thawing and refreezing around a buried pipe

Abstract An approximate two-dimensional theoretical model based on a quasi-steady approach is presented for thermal analysis of phase-change processes around an insulated pipeline buried horizontally in semi-infinite frozen soil. The model is verified by comparison with numerical and other approximate solutions from the literature. The theoretical results of this study show that, under constant boundary conditions, the propagation of the thawing/freezing interface is limited. The reverse process occurring as a result of interruption of the fluid flow is also examined. Based on the solution for prediction of the boundary location of the thawing region, an analytical equation for determination of the fluid temperature as a function of time is developed.

Experimental Study of Heat Transfer Intensification under Vibration Condition

Experimental and theoretical models for enhancement of heat transfer from a tube with rings rotating on the external surface were investigated. The rings were rotated on acting vibration forces (hula-hoop phenomenon). The working fluid flowing into the tube was water. The Reynolds number ranged from 800 to 2000. The amplitude range of the parameters of vibration was 0.1 mm to 1 mm, and the frequency range was 10 to 120 Hz. On the basis of a dimensionless analysis, a mathematical model for the heat-transfer process was developed. It was shown that the mean heat transfer coefficient became higher as the velocity of vibration increased. The experimental results were in good agreement with the theoretical model.