Abraham Dayan

Heat and mass transfer within an intensely heated concrete slab

Abstract A model for the heat and mass transfer within a surface heated concrete slab has been developed. The concrete is treated as a porous material containing water, vapor and air. The solution of the problem for various temperature and heat flux boundary conditions has been obtained with an explicit numerical scheme. The results consist of temperature, pore pressure and moisture distributions as a function of time. The rate of steam release from the drying concrete is also calculated. The code predictions were successfully tested with preliminary experimental data. The results demonstrate the importance of the evaporation-recondensation mechanism in enhancing the rate of heat transfer.

A new thermography-based approach to early detection of cancer utilizing magnetic nanoparticles theory simulation and in vitro validation

This work describes the utilization of tumor-specific magnetic nanoparticles together with an alternating magnetic field as a means to thermally mark a tumor so as to detect it using a thermal imaging system. Experiments were conducted using an in vitro tissue model, an inductive heating system, and an infrared camera. The thermal images, recorded by the infrared camera during the experiments, were analyzed using an algorithm that was developed as part of this work. The results show that small tumor phantoms (diameter of 0.5 mm) that were embedded under the surface of the tissue phantom (up to 14 mm below the surface) can be detected and located, indicating that the proposed method could potentially offer considerable advantages over conventional thermography and other methods for cancer early detection. Nevertheless, several issues should be clarified in future studies before the method can be offered for clinical use.

Thermodynamic and hydrodynamic response of compressed air energy storage reservoirs: a review

Abstract Installation of large-scale compressed air energy storage (CAES) plants requires underground reservoirs capable of storing compressed air. In general, suitable reservoirs for CAES applications are either porous rock reservoirs or cavern reservoirs. Depending on the reservoir type, the cyclical action of air injection and subsequent withdrawal produces temperature and pressure fluctuations within the reservoir. An accurate prediction of these fluctuations is essential for the design of the reservoir and its associated turbomachinery. Being mutually dependent, the selection of the turbomachinery and reservoir characteristics must be conducted simultaneously to obtain an integrated cost-effective plant. The present review is intended to encompass the pertinent literature on the temperature and pressure variations within CAES reservoirs. The principal experimental and operational data sources are described, as well as important results of theoretical modeling efforts. Conclusions derived from those investigations and their relevance to CAES plant designs are discussed.

Self-similar temperature, pressure and moisture distributions within an intensely heated porous half space

Abstract A model for the transient heat and mass transfer within an intensely heated porous half space is presented. The model is based on three principal transport phenomena: heat conduction, vapor convection under pressure gradients and the evaporation-recondensation mechanism. Local liquid-vapor equilibrium is assumed. The governing equations consist of two sets: one for the porous region that has dried and one for the remaining moist region. A self-similar solution is obtained for the combination of uniform initial conditions with a step change in boundary conditions. Transient temperature, pressure and moisture distributions are described in closed form for the dry region and numerically for the moist region. The model was applied to study the rate of moisture loss from a drying concrete wall. The results correspond well with results of previously reported investigations.

Axial dispersion and enterainment of particles in wakes of bubbles

Abstract The entrainment and dispersion of solid particles in bubble columns was investigated experimentally and heoretically. A mechanistic model for the dispersion caused by entrainment in wakes of large solitary bubbles was developed. The dispersion coefficient was found to be dependent on the bubble size, bubble frequency, particle settling velocity and column surface area. Experimental tests were conducted in a rectangular bubble column. The system consisted of air, water and copper powder. Spherical cap bubbles were produced by a single nozzle. Significant entrainment of particles in wakes of rising bubbles observed in the lower region of the column, whereas, turbulence seemed to dominate the dispersion in the upper region of the tank. Calculated particle distributions were found to be in good agreement with experimental data.

Temperature distributions around buried pipe networks in soil with a temperature dependent thermal conductivity

Abstract The temperature distribution in soil around a network of subsurface horizontal warm water pipes was investigated. Computations of soil thermal conductivity revealed a linear dependence on temperature due to the presence of moisture and induced vapour diffusion within the pore space. Solutions of the temperature distributions around various buried pipe networks were obtained in closed forms and numerically. Experiments were conducted with a single buried pipe. The data showed that the present model yields better predictions than previous constant property models. Analyses were extended to account for effects of desiccation around the heated pipes. The results are applicable in horticulture for improved designs of soil warming networks.

Laminar free convection underneath a hot horizontal infinite flat strip

Abstract Analyses were conducted to study the problem of natural convection underneath a hot and isothermal horizontal infinite flat strip. It included a numerical and an analytical investigation of the problem. The work offers simple closed form solutions for the critical flow depth, boundary layer thickness and heat transfer coefficient. The governing equations were solved by the integral method. The justification for applying self-similar boundary layer profiles was demonstrated both numerically and from analyses of published experimental results. The solution was improved through the use of numerical analyses as well as from analytical inspection of limiting cases. The results were successfully tested against published experimental data. Furthermore, the work offers an explanation for the discrepancy that exists amongst the various heat transfer correlations found in the literature.

Thermodynamic Models for the Temperature and Pressure Variations Within Adiabatic Caverns of Compressed Air Energy Storage Plants

The temperature and pressure variation limits within the cavern of a compressed air energy storage (CAES) plant affect the compressor and turbine works, the required fuel consumption and therefore the overall plant performance. In the present work, the thermodynamic response of adiabatic cavern reservoirs to charge/discharge cycles of CAES plants are studied. Solutions for the air cavern temperature and pressure variations were derived from the mass and energy conservation equations, and applied to three different gas state equations, namely, ideal, real, and a self-developed simplified gas models. Sensitivity analyses were conducted to identify the dominant parameters that affect the storage temperature and pressure fluctuations. It is demonstrated that a simplified gas model can adequately represent the air thermodynamic properties. The stored air maximal to minimal temperature and pressure ratios were found to depend primarily on, both the ratio of the injected to the initial cavern air mass, and the reservoir mean pressure. The results also indicate that the storage volume is highly dependent on the air maximum to minimum pressure ratio. Its value should preferably be in between 1.2 and 1.8, where the exact selection should account for design and economic criteria.

Cogeneration With Concentrating Photovoltaic Systems

Simultaneous production of electrical and high-grade thermal energy is proposed with a Concentrating Photovoltaic/Thermal (CPVT) system operating at elevated temperature. The CPVT may operate at temperatures above 100°C and the thermal energy can drive processes such as refrigeration, desalination, and steam production. An example of CPVT with single-effect absorption cooling is investigated in detail. The results show that under a wide range of economic conditions, the combined solar cooling and power generation plant can be comparable and sometimes even significantly better than the conventional alternative.Copyright

Design procedures for subsurface soil-warming pipe systems

Abstract Soil warming has the potential to be an alternative to conventional heating methods in agriculture. It has been proven that crop yield is enhanced and growing periods are shortened with relatively little energy input. A subsurface soil-warming system consists of a network of parallel horizontal hot-water pipes buried at an appropriate depth in the soil to maintain a favourable temperature field throughout the root zone. Theoretical steady-state, constant property soil-warming models for four pipe configurations are presented in dimensionless form. Experimental verification of one of the configurations is presented. A procedure for establishing optimal pipe depth and spacing according to root-zone heating requirements is described and an example is illustrated. Effects of diurnal temperature fluctuations are also considered. Applications for these models include greenhouse crops and small unprotected crops of several hectares.