Kadry A. Fathalah

Performance of multiple effect diffusion stills

Abstract Experimental testing of a three-effect diffusion still is carried out. The experiment is designed to test the effect of the heating fluid temperature input to the still T h and the feed rate of desalinated water to the still F on the performance of the still. The results produced several empirical relations to predict the following in terms of the T h and F : the average temperature of each effect, the rate of distillate produced by each effect, and the performance ratio of the still. In addition, a mathematical simulation is given for the n -effect diffusion still. The mathematical model is verified by comparing its results with the experimental results of the three-effect diffusion still. The model is further used to predict the performance of a ten-effect diffusion still at the following parametric conditions: various values of the dimensionless parameters N 3 and N 4 , various heating medium temperatures input to the still, and various feed rates to the still.

Study of a waste heat driven modified packed desiccant bed dehumidifier

A modified version of the cross cooled desiccant bed is presented. The parallel flow channels are subdivided longitudinally and packed with silica gel particles instead of being glued to the side walls. The bed is regenerated utilizing the rejected heat from a LiBrH2O absorption cooling machine and both are integrated in a solar powered air conditioning system. Two beds are alternately working in the humidification/dehumidification cycle. Treating the matrix and the desiccant separately, a five governing equation model, describing the heat and mass transfer between the passing air flow, the desiccant material and the matrix metallic structure, is introduced and given. This is numerically solved for both the humidification mode and the dehumidification mode with each occupying half of the bed cyclic duration. For a case study in Jeddah, with a cyclic duration of 15 minutes, the results showed an overall coefficient of performance of 1.2 which is 53% higher than that for the absorption machine alone. Moreover, adding the desiccant bed to the air conditioning system raised the evaporator temperature to 19.3°C compared with the 11.5°C required for the absorption machine alone.

Estimation and measurement of electric energy consumption due to air conditioning cooling load

Abstract Electric energy consumption due to air conditioning (A/C) in buildings is tremendous, especially in places having hot humid weather, such as Jeddah, Saudi Arabia. Building simulation programs, such as DOE2, are very valuable tools to study building energy consumption under different conditions. These programs can be used to investigate the effect of different building designs on their energy consumption. This paper presents the experimental and theoretical variations of electric power consumed by an A/C system. For the experimental part, a building has been chosen, and different sensors were installed to measure and record electric kW consumption at different indoor and outdoor conditions. In the theoretical part, the Visual DOE simulation program, which is based on the well known DOE2E program, has been used. Actual building data was fed to the program to generate the instantaneous electric consumption due to A/C cooling load. Comparison between measured and predicted electric consumptions is presented.

Theoretical study of a solar powered absorption/MED combined system

Abstract In this article, a theoretical study is presented for a solar powered combined system comprising a LiBrH 2 O absorption cooling machine and a multiple-effect distillator (MED). The MED has 8 VTE of the falling film type, and it replaces the condenser in conventional absorption machines. Steam released at the generator pressure is supplied to the effect which matches its conditions, and the condensate follows its usual route towards the evaporator of an A/C unit. Thus, the MED is powered by the waste heat of the absorption machine which improves the overall gain and the thermodynamic characteristics significantly. Governing equations for the combined system are given and are numerically solved. Medium parabolic concentrators are used to power the system, and a transient simulation for the combined arrangement is presented. Results are given for a typical design summer day in Jeddah, Saudi Arabia, for a range of firing temperatures 150–190°C with a storage temperature amplitude of 10–20°C over a daily working period of 12 h. For a given cooling load of 100 ton refrigeration, the system can produce up to 40m 3 of fresh water at a specific collector area of 12.41. H 2 O plus 0.03 TR/m 2 . The overall COP o reaches 1.44, which is more than twice that of a conventional absorption machine at the same temperature levels.

Measurements of solar flux density distribution on a plane receiver due to a flat heliostat

Abstract An experimental facility is designed and manufactured to measure the solar flux density distribution on a central flat receiver due to a single flat heliostat. The tracking mechanism of the heliostat is controlled by two stepping motors, one for tilt angle control and the other for azimuth angle control. A x-y traversing mechanism is also designed and mounted on a vertical central receiver plane, where the solar flux density is to be measured. A miniature solar sensor is mounted on the platform of the traversing mechanism, where it is used to measure the solar flux density distribution on the receiver surface. The sensor is connected to a data acquisition card in a host computer. The two stepping motors of the heliostat tracking mechanism and the two stepping motors of the traversing mechanism are all connected to a controller card in the same host computer. A software “TOWER” is prepared to let the heliostat track the sun, move the platform of the traversing mechanism to the points of a preselected grid, and to measure the solar flux density distribution on the receiver plane. Measurements are carried out using rectangular flat mirrors of different dimensions at several distances from the central receiver. Two types of images were identified on the receiver plane—namely, apparent (or visible) and mirror-reflected radiation images. Comparison between measurements and a mathematical model validates the mathematical model.

Estimation of percentage useful area of a heliostat when considering shadowing and blocking

Abstract Analyses are presented to determine the percentage of the area of a heliostat that is suffering from shadowing and/or blocking of another heliostat in a solar central receiver system. The analyses are found to depend on the solar time, the day in the year, the location of the site, the height of the central receiver, the relative position of the heliostats, with respect to the tower and with respect to one another, and the dimensions of each heliostat. Examples are given to demonstrate the calculations at various parametric conditions.

Natural convection over a vertical flat plate due to absorption of thermal radiation

Heat transfer by simultaneous free convection and radiation in a participating fluid has received some attention during the past few years. However most of the previous work has been focussed on gases. The present work investigates the problem of combined radiation and natural convection in liquids. Analysis are given for an optically thick cold fluid layer adjacent to a non-emitting and non-reflecting radiation-transmitting plate. The external surface of the plate is subjected to heat loss to surroundings. The governing differential equations are transformed to a dimensionless form where the solution becomes dependent on the following parameters: the plate absorpitivity,αp; the dimensionless distance along the plate,ζ; the fluid Prandtl number,Pr; and dimensionless heat loss coefficient to surrounding,Nc. A local non-similar technique is adopted to obtain solutions atPr=6.5 and at a wide range ofαp,ζ, andNc. The results showed that both velocity and temperature are non-similar and they are greatly affected by the value ofαp whenζ is small. At large values of f the effect ofαp diminishes and for a plate without heat loss the velocity becomes similar, i.e. independent of C The heat loss from the external surface of the plate causes the maximum temperature of the fluid to depart far from the plate. The results also showed that for plates without heat loss the local heat transfer coefficient from the plate depends on the local Grashof number to the power 0.185.ZusammenfassungWärmeübertragung bei gleichzeitiger freier Konvektion und Strahlung in ein Fluid wurde in den letzten Jahren stärker beachtet. Die meisten der bisherigen Arbeiten beschäftigen sich jedoch mit Gasen, während hier Flüssigkeiten behandelt werden. Es werden Berechnungen durchgeführt für eine absorbierende kalte Fluidschicht an einer nicht emittierenden, nicht reflektierenden strahlungsdurchlässige Platte. An der äußeren Oberfläche erfolgen Wärmeverluste an die Umgebung. Die beschreibenden Differentialgleichungen werden dimensionslos dargestellt, die Lösung ist dann von folgenden Parametern abhängig: Dem Absorptionsgrad der Platteαp; der dimensionslosen Längekoordinate der Platteζ; der Prandtl-Zahl der FlüssigkeitPr und dem dimensionslosen Wärmeverlustkoeffizienten an die UmgebungNc. Eine örtliche Nichtähnlichkeitslösung wird angewandt, um Lösungen fürPr=6,5 und einen großen Bereich von αp,C und Nc zu erzielen. Die Ergebnisse zeigten, daß Geschwindigkeits- und Temperaturprofil nicht ähnlich sind und bei kleinen C stark von αp beeinflußt werden. Für große Werte von ζ nimmt der Einfluß von αp ab und für eine Platte ohne Wärmeverluste werden die Geschwindigkeitsprofile ähnlich, d. h. unabhängig von ζ. Der Wärmeverlust von der äußeren Oberfläche der Platte verursacht eine Verschiebung des Temperaturmaximums im Fluid von der Platte weg. Die Ergebnisse zeigen auch, daß der örtliche Wärmeübergangskoeffizient bei Platten ohne Wärmeverluste von der örtlichen Grashof-Zahl mit der Potenz 0.185 abhängt.

Theoretical study of a solar powered absoprtion/MED combined system

Abstract In this article a theoretical study is presented for a solar powered combined system comprising a LiBr-H 2 O absorption cooling machine and a multi effect distillator (MED). The MED has 8 VTE of the falling film type and it replaces the condenser in conventional absorption machines. Steam released at the generator pressure is supplied to the effect which matches its conditions and the condensate follows Its usual root towards the evaporator of an A/C unit. Thus the MEP is powered by the waste heat of the absorption machine which improves the overall gain and the thermodynamic characteristics significantly. Governing equations for the combined system are given and are numerically solved. Medium parabolic concentrators are used to power of the system and a transient simulation for the combined arrangement is presented. Results are given for a typical design summer day in Jeddah Saudi Arabia for a range of firing temperature 150–190°C with a storage temperature amplitude of 10–20 °C over a daily working period of 12 hours. For a given cooling load of 100 ton refrigeration, the system can produce upto 40 m 3 of fresh water at a specific collector area of 12.4 liters of H 2 O plus 0.03 TR per square meter. The overall COP O reaches 1.44 which is more than twice that of a conventional absorption machine at the same temperature levels.

Numerical study of evaporation-condensation in a vertical diffusion gap

The present work deals with mass transfer between a vertical falling film over a heated plate and a condensing film over a parallel cooled plate in a diffusion gap. This is typically the case of the distillation process in a diffusion still. The governing equations for mass, momentum, and energy are considered for the evaporating film, the diffusion gap, and the condensing film, together with the boundary and interfacial conditions. The local similar technique is used to solve the problem numerically and to get the velocity and temperature distributions in the gap. A comprehensive analysis of the effect of evaporating temperature, condensing temperature, and diffusion gap width over the diffusion mass flow rate and evaporation heat transfer coefficient are carried out. Performance charts of air and helium diffusion gaps are given. Additionally, the analytical results are experimentally validated.ZusammenfassungDie vorliegende Arbeit befaßt sich mit dem Stofftransport zwischen einem vertikalen Rieselfilm über eine beheizte Platte und einem kondensierenden Film über eine parallele gekühlte Platte, wobei im Spalt dazwischen Diffusion auftritt. Dies ist ein typischer Fall für einen Destillationsprozeß in einer Diffusionsanlage. Es werden die Bilanzgleichungen für Masse, Impuls und Energie für den verdampfenden Film, den Diffusionsspalt und den kondensierenden Film zusammen mit den Bedingungen für die Grenzschicht und für die Phasengrenzflächen betrachtet. Für die numerische Lösung des Problems und für die Berechnung der Verteilung von Geschwindigkeit und Temperatur im Spalt wird eine örtliche Simulationstechnik verwendet. Es wird eine umfassende Analyse über die Einflüsse von Verdampfungstemperatur, Kondensationstemperatur und Abmessungen des Diffusionsspaltes zur Berechnung des diffusiven Mengenstromes und des Wärmeübergangskoeffizienten infolge Verdampfung vorgenommen. Weiterhin wird das Verhalten von Luft- und Heliumdiffusion im Spalt behandelt. Die analytischen Ergebnisse werden experimentell verifiziert.

RATED MW FROM A HELIOSTAT FIELD ON CYLINDRICAL EXTERNAL RECEIVER

Some of the reflected beam radiation from a heliostat field bypasses the receiver surface. The spillage factor which is a measure of how much of reflected beam radiation actually intercepted by the receiver surface, is calculated and plotted for easy access. The variation of ihe spillage factor with tower height, external cylindrical receiver size, dimensionless radial distance from the tower is computed and plotted. The value of the rated MW energy absorbed by an external cylindrical receiver, is investigated, and its relations to the tower height, the site location and the field radius are given. The effect of changing the radial spacing on the rated MW and the total number of heliostats in the field is also computed and depicted. The developed set of charts for the spillage factor are believed to be very useful for solar central receiver system design.