Hamdy M. Shafey

Heat, Water, and Solute Transfer in Unsaturated Porous Mdeia: II -- Compacted Soil Beneath Plastic Cover

Soil surface dynamics involve coupled transferof heat, water, and solute. An experimental andtheoretical study of heat, water, and solute transferin closed compacted soil columns under surfacetemperature wave amplitudes is presented. Thetemperature wave amplitudes ranged from 17.9 to 21.0°C. Potassium chloride solution was used tomoisten Clarinda clay and Fayette silty clay loamsoils. Initial water contents of 0.403 and 0.279 andinitial solute concentrations of 0.062 and 0.052 mol kg-1were used in Clarinda and Fayette soils,respectively. The moistened soils were packed andcompacted in PVC columns (0.075 m diameter and 0.30 mhigh). Bulk densities of the compacted Clarinda andFayette soils were 1403 and 1585 kg m-3,respectively. The columns were buried in soil suchthat column surfaces were exposed to natural as wellas artificial radiation and thermal conditions. Thecoupled nonsteady-state balance equations of mass andenergy were solved numerically to predict soiltemperature, water content, and solute concentrationdistributions. The theoretical model described soiltemperature, water content, and solute concentrationwell as compared with the measured values. TheFickian diffusive solute flux was one or two orders ofmagnitude greater than salt-sieving and thermal-diffusion solute fluxes.

Experimental study on a bench-scale, batch-type fluidized-bed combustor for energy production from waste-derived fuels

We report experimental results on a bench-scale, atmospheric fluidized-bed combustor with batch feeding of solid fuels. The combustor has an inside diameter of 10 cm and is 1.5 m high; it is provided with a packed-bed, sandwich-type air distributor. Cooling water is circulated in tubes that are partially immersed in the fluidized bed to control the bed temperature during the combustion experiments. Measurements of the pressure drop and temperature were made at different locations in the combustor. The fuels used were naturally available coal with 3.5% sulfur, petroleum coke, and corncobs. Silica sand was used as the inert-bed material. Test runs lasting 40 hours were carried out to study the hydrodynamics and combustion processes in the fluidized-bed combustor. The cold-flow tests included investigations of the effects of the material and size of the bed particles on the fluidization characteristics. Tests were carried out to study the combustion of coals and corncobs at temperatures as low as 650 °C. Temperature uniformity was verified and the heat-transfer coefficient was estimated between the bed and tubes. Values of 600 W/m2 -K are typical of fluidized-bed combustion and were obtained for average bed temperatures from 400 to 800 °C.

Thermodynamics of the Conversion of Solar Radiation

An important parameter in the analysis provided by the second law of thermodynamics for the evaluation of solar energy conversion systems is the radiation temperature. The solar radiation is diluted due to atmospheric scattering and absorption, and reflection at the absorber surface. This dilution reduces the temperature of the sun (as a blackbody source) to an effective radiation temperature. In the presented study, spectral dilution functions were derived for three components of the absorbed global terrestrial solar radiation (direct beam component + forward Mie component + diffuse component). The energy and entropy fluxes and effective temperature for each component were expressed in terms of these dilution functions. They were calculated for different atmospheric conditions using numerical integration over the wavelength. The effects of the air mass and the atmospheric parameters on these thermodynamic quantities and the maximum conversion efficiency of solar energy were investigated. An apparent temperature assigned for the sun as a high temperature reservoir for Carnot engine was calculated and found to vary between about 3600 K for clear sky and about 2000 K for highly turbid sky under the given atmospheric conditions.

Performance Analysis of a Plastic Shell-and-Tube Heat Exchanger

This paper presents an experimental investigation of the performance of an all-plastic heat exchanger. Plastic conical turbulators were inserted in the exchanger tubes to enhance the overall heat transfer process 1.5 to 3.5 folds. The results of performance tests indicated a limiting value for the overall heat transfer coefficient corresponding to the thickness and the conductive thermal resistance of the plastic tubes. In addition, the results did not show significant increase in the total tube-side pressure drop due to the use of turbulators. The heat transfer enhancement was evaluated on a thermoeconomic basis using a dimensionless exergy destruction function. Physical and chemical characteristic tests conducted on the plastic material used, prevealed its excellent solubility resistance and relatively high softening temperature, which makes it attractive in some heat recovery applications.

An analysis of the combined conductive-radiative heat transfer between a surface and a gas-fluidized bed at high temperature

Abstract A theoretical investigation of the combined conductive radiative wall-to-fluidized bed heat transfer is presented. The packet of emulsion is assumed to be an absorbing, emitting, and scattering nongray medium. Equations of energy and radiative transfer in the packet are solved simultaneously using an iterative numerical method. The bubble is modelled as a hemisphere of gas enclosed by the emulsion and the nongray heal transfer surface. Heat transfer coefficients were calculated for different bed systems in the temperature range 300–1000 °C. The predictions by the present analysis were in good agreement with available experimental data. The radiative contribution of as much as 35% varies directly with bed temperature, particle size and emissivity, surface temperature and emissivity. Both isotropic scattering and gray medium approximations were found to be acceptable for practical applications.

Experimental study on spectral reflective properties of a painted layer

A new diffuse reflectometer using a paraboloidal mirror has been developed and employed to measure various reflective properties of painted layers for the case of normal incidence. The effects of the pigment volume concentration, the scattering properties of the pigment (TiO2, Fe2O3, carbon, and ZnO), the thickness of the painted layer, the reflection characteristics of the substrate (specular or diffuse) on the spectral normalhemispherical reflectance, and the angular distribution of the diffuse reflection are studied. The wavelength dependence of the results is examined over the range of 0.45 ~ 10 jim. The comparison with the analysis indicates that the experimental results, with the exception of the interference effect at large pigment volume concentrations, are in reasonable agreement with the analytical results.

PREDICTION OF ATMOSPHERIC PARAMETERS FOR SPECTRAL AND TOTAL SOLAR IRRADIANCES

ABSTRACT A study is carried out on the inverse problem of deriving various clearness indices and atmospheric parameters required for estimating total and spectral solar irradiances using available clear sky measurements of total irradiance. Optimum values of these parameters obtained for Assiut city were about 0.43 cm of O3, 5.33 cm of H2O and 0.0093 for Rayleigh scattering constant. The annual variations of daily solar radiation index and turbidity parameters are studied. The average daily global solar irradiance is expressed as a Fourier series for transient case studies.

RADIATIVE PROPERTIES OF A PAINTED LAYER CONTAINING NONSPHERICAL PIGMENT

The radiative properties of a painted layer containing nonspherical pigment particles are studied theoretically. The scattering properties are calculated using a new method based on the volume integral form of Maxwells equations. The radta* tive transfer is treated by Chandrasekhars theory. The effects of the optical properties of the pigment and the optical thickness are examined. It is found that the assumption for nonspherical pigments having the scattering properties calculated by the Mie theory for equivalent spherical particles leads to a considerable error in predicting the reflectances of an optically thin layer, and a small error in the case of a thick layer.

Design and performance analysis of a solar pond power plant

Abstract This paper reports on a study for the design and performance analysis of a solar pond power plant for a site of known geographical and atmospheric conditions. Modelling and thermal analysis of the pond are carried out, including treatment of solar radiation transfer. The factors affecting selection of the optimum ORC power loop are also discussed, and overall system performance analysis is carried out under varying load conditions.

Theoretical Study on Radiative Properties of a Painted Layer Containing Spherical Pigment : Cases of Oblique and Hemispherical Incidence

吸収-非等方散乱媒体からなる塗膜に光が斜入射,軸対称入射および半球入射する場合の反射性質について理論解析を行った.斜入射の場合の方位角依存性については拡散光をフーリエ級数展開により表して輸送方程式に導入し解を得た.光学的厚さ,入射角,顔料の光学的性質,基板反射性質が指向,半球反射率に及ぼす影響を検討した.等強度半球入射の場合の反射性質は天頂角60°の軸対称入射でもって近似できることもわかった.