Eli Korin

Thermal analysis of a helical heat exchanger for ground thermal energy storage in arid zones

Abstract A mathematical model for thermal analysis of a helical heat exchanger for long-term thermal energy storage in soil for use in arid zones was developed. The helical heat exchanger was modeled as a series of horizontal rings with a constant pitch distance between them. The model was solved by a finite difference method, using a microcomputer, and validated with experimental data obtained from field experiments. Based on the model, theoretical results of the following parametric studies are presented: thermal properties of the soil, cycle period, and height and pitch distance of the helical heat exchanger.

An efficient numerical solution for the multidimensional solidification (or melting) problem using a microcomputer

Abstract The aim of this paper is to present a simple and efficient numerical technique for solving transient multidimensional heat transfer problems with melting/solidification processes. The proposed technique comprises an enthalpy-based method for solving the problems by a finite difference scheme, lump system behavior being assumed for each node. The computation technique is able to consider all kinds of boundary conditions, i.e. conduction, convection and radiation alone or in combination. The numerical method neglects convection effects in the liquid phase. The importance of this method lies in the fact that solutions are obtained with a personal microcomputer, thus providing a convenient and reliable tool for wide use in solving many problems of practical interest. The proposed method was verified against the two exact solutions available from the literature for a one-dimensional semi-infinite domain, one with constant temperature boundary condition and the second with constant heat flux. The technique was demonstrated by solving four different cases of two-dimensional problems. A comparison of the results obtained with a microcomputer using the technique presented in this paper with numerical results from the literature obtained using conventional methods, i.e. finite differences and finite elements methods, which generally involve the use of large computers, shows good agreement.

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.

Hydrophilic hollow fiber membranes for water desalination by the pervaporation method

Abstract Hydrophilic ion-exchange membranes based on sulfonated polyethylene hollow fibers were manufactured, and their suitability for a water pervaporation process was studied for possible application in water desalination systems. The effects of the following parameters on the average water flux were determined: membrane properties (diameter (0.4–1.8 mm) and wall thickness (0.05–0.18 mm)); charge density (0.6–1.2 meq g −1 ); and operating conditions (brine inlet temperature (30–68°C), air sweep velocity (0–6 m s −1 ), and salt concentration in the feed brine (0–3 M)). A water flux of 0.8–3.3 kg m −2 h −1 was obtained using this type of hollow fiber with an inlet brine temperature of 25–65°C. It was found that, for our application, the optimal specifications for the ion-exchange hollow fibers were an outside diameter of 1.2 mm, a wall thickness of 0.1 mm, and an ion-charge density of about 1.0 meq g −1 . This information is required as basic data for the design of a prototype water desalination system based on a pervaporation system that uses this type of ion-exchange hollow fiber membrane.

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.

Mediation at High Potentials for the Reduction of Oxygen to Water by Cobalt Porphyrin–Quinone Systems in Porous Aerogel Carbon Electrodes

Reduced p-, m-, and o-benzoquinones: hydroquinone (HQ), resorcinol (Res), and catechol (Cat), undergo irreversible monolayer adsorption in aerogel carbon (AEC) electrodes with rates of 1.7 × 10 ―4 , 7.1 × 10 ―5 , and 1.4 × 10 ―4 s ―1 for HQ, Res, and Cat, respectively. The adsorbed species showed electrochemical quasi-reversible behavior in 1 M H 2 SO 4 with half-wave potentials (E 1/2 ) of +0.45, +0.31, and +0.58 V vs Ag/AgCl/KCl (saturated) for AEC/HQ, AEC/Res, and AEC/Cat, respectively. Upon adsorption of Co(III) tetra(p-sulfonatophenyl)porphyrin in these electrodes, a single reduction wave was observed and its E 1/2 (∼+0.45 V) was independent of the nature of the adsorbed reduced quinone. This was attributed to a metalloporphyrin/ quinone complex, which formed and stabilized at the electrode surface. This species, after being reduced, reacted with oxygen with a rate of 1.8 × 10 5 M ―1 s ―1 . Mediation of oxygen reduction by these systems occurred at a relatively high potential (∼+0.5 V) almost completely via a four-electron-transfer process.

Electroprecipitation of Ag(II)/Ag(III) tetraphenylsulfonate porphyrin and electrocatalytic behavior of the films

The electrochemical precipitation on glassy carbon and gold electrodes of Ag(II) tetraphenylsulfonate porphyrin (Ag(II)TPPS) from aqueous HClO4 solutions, is reported. Electrochemical quartz crystal microbalance (EQCM) results indicate the possible formation of an Ag(II)–Ag(III) porphyrin dimer species. This species is oxidized and reduced in two consecutive steps: oxidation at +0.31 and +0.36 V (vs. SCE) and reduction at +0.11 and +0.07 V. The films show catalytic behavior toward O2 reduction in 10−2 M HClO4 at relatively low potentials (E<−0.1 V) but catalyze NO reduction at relatively high-reduction potentials (E<0.4 V). The electrochemical results seem to indicate that the catalytic cycle in the case of NO involves formation of Ag(II)TPPS–Ag(II)TPPS(NO)+ and its electroreduction to regenerate Ag(II)TPPS–Ag(III)TPPS and NO-reduction products.

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.