Esmail M. A. Mokheimer

Performance of annular fins with different profiles subject to variable heat transfer coefficient

Abstract Performance of annular fins of different profiles subject to locally variable heat transfer coefficient is investigated in this paper. The performance of the fin expressed in terms of fin efficiency as a function of the ambient and fin geometry parameters has been presented in the literature in the form of curves known as the fin-efficiency curves for different types of fins. These curves, that are essential in any heat transfer textbook, have been obtained based on constant convection heat transfer coefficient. However, for cases in which the heat transfer from the fin is dominated by natural convection, the analysis of fin performance based on locally variable heat transfer coefficient would be of primer importance. The local heat transfer coefficient as a function of the local temperature has been obtained using the available correlations of natural convection for plates. Results have been obtained and presented in a series of fin-efficiency curves for annular fins of rectangular, constant heat flow area, triangular, concave parabolic and convex parabolic profiles for a wide range of radius ratios and the dimensionless parameter m based on the locally variable heat transfer coefficient. The deviation between the fin efficiency calculated based on constant heat transfer coefficient, reported in the literature, and that presently calculated based on variable heat transfer coefficient, has been estimated and presented for all fin profiles with different radius ratios.

A Review of Hybrid Solar–Fossil Fuel Power Generation Systems and Performance Metrics

A literature review of hybrid solar―fossil fuel power generation is given with an emphasis on system integration and evaluation. Hybrid systems are defined as those which use solar energy and fuel simultaneously, thus excluding the viable alternative of solar thermal plants which use fossil fuels as backup. The review is divided into three main sections: performance metrics, the different concentrated solar receiver technologies and their operating conditions, and the different hybridization schemes. In addition, a new linear combination metric for analysis of hybrid systems, which considers trade-off of different metrics at the fleet level, is presented. This metric is also compared to alternative metrics from multi-objective optimization. Some previous work only evaluates the hybrid cycle at a certain point in time, which can be misleading as this evaluation would not take into account certain aspects of hybrid cycle, such as fluctuating solar supply. Furthermore, almost all previous work designs the hybrid solar―fossil fuel systems for a certain point in time and then evaluates the performance of the system for an entire year. By not taking into account fluctuating solar supply and selling price of electricity in the design of the system, the best possible annual performance of the hybrid cycle may not be reached.

Free convection in vertical eccentric annuli with a uniformly heated boundary

The paper utilizes a boundary‐layer model in bipolar coordinates to study the developing laminar free convection in vertical open‐ended eccentric annuli with one of the boundaries uniformly heated while the other boundary is cooled and kept isothermal at the ambient temperature. This model has been solved numerically using finite‐difference techniques. Results not available in the literature are presented for a fluid of Prandtl number 0.7 in an annulus of radius ratio 0.5 for three values of the dimensionless eccentricity, namely, 0.1, 0.5 and 0.7. These results include the developing velocity profiles and the pressure along the annulus, the channel heights required to naturally induce different flow rates and the variation of the total heat absorbed by the fluid with the channel height.

Solar thermal catalytic reforming of natural gas: a review on chemistry, catalysis and system design

Solar radiation is an abundant and environmentally benign energy source. However, its capture and effective utilization is one of the most difficult challenges faced by modern science. An effective way to capture solar energy is to convert it to chemical energy using concentrated solar power and thermochemical conversion routes, such as methane reforming. Methane, the main component of natural gas, is poised to become a leading feedstock in the near term, partly due to recent developments in shale gas extraction. Solar-to-chemical energy conversion can be achieved by reforming methane into synthesis gas, a mixture of carbon monoxide and hydrogen, in a single, highly endothermic catalytic process when reacted with steam or carbon dioxide. This review highlights different aspects of solar thermal reforming of methane, including thermodynamics, challenges related to catalyst activity and stability and reactor design. Equilibrium limitations are discussed in detail with respect to solar thermal reforming. Recent developments in methane reforming catalysis are critically reviewed in a broad scope, addressing catalyst deactivation drawbacks and focusing on alternative catalysts. The potential of the low-temperature solar methane steam reforming and the related technological challenges are discussed, including catalyst requirements. Future directions are also outlined.

Prediction of the Thermal Conductivity of the Constituents of Fiber-Reinforced Composite Laminates: Voids Effect

There are inherent difficulties in the direct measurement of the thermal conductivity of fibers. The need to determine the thermal conductivity of fibers for design purposes has been the motivation of the present work. Four empirical formulas are developed to predict the thermal conductivities of fiberreinforced composite laminates (FRCLs) and their constituents. Two of these models utilize the parallel and series thermal models of composite walls in predicting the thermal conductivity of the fibers. The models are tested at different fiber-to-resin volume ratios (30/70, 45/55, 50/50, 60/40, and 75/25) and various fiber-to-resin thermal conductivity ratios (0.5, 1, 2, 3, 4, and 5). These ranges indicate the physically possible fiber to resin volume thermal conductivity ratios including the extreme possible cases. The effect of the air void volume ratio on the thermal conductivity of the composite laminates is investigated. The range of the investigated void fractions also represent the practical ranges. The predicted thermal conductivity of the fiber can be accurately predicted throughout the spectrum via three models. The first model is a first-order formula (R 2 = 0.9148) based on the parallel series structure of the constituents within the composite laminate. The second model is a second-order formula (R 2 = 0.9308) which is also based on the parallel series structure of the constituents. A third model is developed to predict the fiber thermal conductivity as a direct function of the composite thermal conductivity and other composite constituents and volume ratios. This correlation has a coefficient of determination (R 2 = 0.9632). A fourth model is developed to predict the effective thermal conductivity of the laminate. The effective thermal conductivity of the FRCL is predicted with very high accuracy (R 2 = 0.9948). Another use of these models is to determine the fiber to resin volume ratio or air void volume fraction (if all thermal conductivities of fiber, resin, and laminate are known).

Limiting Values for Free-Convection Induced Flow Rates in Vertical Eccentric Annuli with an Isothermal Boundary

Fully developed laminar natural convection in vertical eccentric annuli has been investigated under fundamental thermal boundary conditions of the first and third kinds. For conditions of the first kind, numerical solutions are presented showing the effect of eccentricity on the velocity profiles and the local Nusselt number in an annulus of radius ratio 0.5. Limiting values for the induced flow rate and the average Nusselt numbers are presented for a fluid of Pr = 0.7 over wide ranges of eccentricity and annulus radius ratio (0.1-0.9). Closed-form analytical solutions are obtained for fundamental conditions of the third kind and the corresponding variations in the limiting induced flow rates with eccentricity are presented.Fully developed laminar natural convection in vertical eccentric annuli has been investigated under fundamental thermal boundary conditions of the first and third kinds. For conditions of the first kind, numerical solutions are presented showing the effect of eccentricity on the velocity profiles and the local Nusselt number in an annulus of radius ratio 0.5. Limiting values for the induced flow rate and the average Nusselt numbers are presented for a fluid of Pr = 0.7 over wide ranges of eccentricity and annulus radius ratio (0.1-0.9). Closed-form analytical solutions are obtained for fundamental conditions of the third kind and the corresponding variations in the limiting induced flow rates with eccentricity are presented.

A New Study for Hybrid PV/Wind off-Grid Power Generation Systems with the Comparison of Results from Homer

This article presents the development of a computational model for the sizing optimization of an off-grid hybrid solar wind electric power generation system. The model includes a PV model, wind power model, and a model for the required battery. The developed mathematical model also comprises a model for a diesel generator as an alternative for the storage battery. A simulation code has been developed using MATLAB to solve the mathematical model and simulate the performance of the hybrid system with different configuration for performance analysis and optimal sizing purpose. The mathematical model and the computer code have been developed using a general approach. This approach can be used to simulate, analyze and optimize any hybrid solar/wind/battery/diesel off-grid electric power generation systems. Also, it can be utilized for any remote area in the world depending on the weather data used as input to the developed simulation code. The results obtained via the presently developed model and code have been validated against previously published work and also against that obtained via the widely used software (HOMER). The model is used for the sizing optimization and assessment of a hybrid solar wind system based on the weather conditions for Dhahran city in the eastern province of the Kingdom of Saudi Arabia.

On the Use of Spreadsheets in Heat Conduction Analysis

Detailed methodology and different techniques for simply utilizing the widely available and user friendly spreadsheet programs in heat conduction analysis are presented. Evaluation of analytical and numerical solution of heat conduction problems via spreadsheets is investigated. Detailed techniques of obtaining spreadsheet numerical solutions for one- and two-dimensional steady and transient heat conduction problems are introduced. A new technique of marching the transient numerical solution with time, in a single layer spreadsheet, for one- and two-dimensional heat conduction is explained. Creating macros that automate the spreadsheet processes, particularly calculations, is detailed. Utilization of the powerful graphical facility that is built in the spreadsheets to graphically represent the obtained solutions is outlined.

Maximum possible induced flow rates in open‐ended vertical eccentric annuli with uniform heat flux

Purpose – Obtaining the maximum possible flow rates that can be induced by free convection in open‐ended vertical eccentric annuli under fundamental thermal boundary conditions of the fourth kind (heating or cooling one of the annulus walls with a uniform heat flux while keeping the other wall at ambient temperature). Obtaining the maximum possible flow rates that can be induced by free convection in open‐ended vertical eccentric annuli under fundamental thermal boundary conditions of the fourth kind (heating or cooling one of the annulus walls with a uniform heat flux while keeping the other wall at ambient temperature).Design/methodology/approach – The fully‐developed laminar free convection momentum equation has been solved numerically using an analytical solution of the governing energy equation.Findings – Results are presented to show the effect of the annulus radius ratio and the dimensionless eccentricity on the induced flow rate, the total heat absorbed by the fluid, and the fully developed Nussel...

Spreadsheet numerical simulation for developing laminar free convection between vertical parallel plates

Abstract A detailed spreadsheet simulation for laminar free convection with simultaneously thermally and hydrodynamically developing boundary layer in the entrance region between two vertical parallel plates is presented. The boundary layer governing equations are expressed in an implicit finite difference form. This results in sets of algebraic equations. These sets of algebraic equations are solved simultaneously using a non-iterative scheme. Spreadsheet setup for the developing fluid and heat flow problems, the powerful abilities of the spreadsheet and macro creation for automated solutions are emphasized. Plots of the results are presented using the spreadsheet-built-in powerful graphical facilities. The spreadsheet simulation results are compared with presently obtained results for the same problem via a FORTRAN code. Moreover, the numerically obtained developing axial velocity and temperature profiles at large distance downstream the entrance, for high enough channels, show excellent agreement with the available fully-developed analytical profiles. Flow and heat transfer parameters such as the channel height required to naturally induce a specific flow rate, the local and averaged Nusselt number and the amount of heat absorbed by the fluid through the channel have been calculated for a fluid of Pr=0.7, air, under two types of thermal boundary conditions. These boundary conditions are namely boundary conditions of the first and third kind.