Constrained Optimization of Heat Transfer from an Extended Surface

Two different zero‐order optimization techniques are used to maximize the rates of heat transfer from a fin assembly of a specified cost and in the shape of several annular fins that are mounted on a central stem. The problem is formulated to account for two‐dimensional steady‐state heat transfer that is limited by several inequality constraints. The dimensionless governing equations are used to identify the relevant decision variables. The number of fins making up the assembly is treated as an input parameter. A digital computer is used to determine the required temperature distributions and to implement the optimization search algorithms. Three different fin materials are assessed—aluminum, copper and carbon steel. Design optimizations of the extended surface assembly were made over a range of operating conditions, encompassing several different convection heat transfer coefficients that are representative of free and forced convection in air, and several different overall temperature differences between the substrate surface and air. A few recommendations based on trends in the predicted results are given. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(6): 504–521, 2014; Published online 3 October 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21093     

Exergetic evaluation of 5 biowastes-to-biofuels routes via gasification

This paper presents the exergy analysis results for the production of several biofuels, i.e., SNG (synthetic natural gas), methanol, Fischer–Tropsch fuels, hydrogen, as well as heat and electricity, from several biowastes generated in the Dutch province of Friesland, selected as one of the typical European regions. Biowastes have been classified in 5 virtual streams according to their ultimate and proximate analysis. All production chains have been modeled in Aspen Plus in order to analyze their technical performance. The common steps for all the production chains are: pre-treatment, gasification, gas cleaning, water–gas-shift reactions, catalytic reactors, final gas separation and upgrading. Optionally a gas turbine and steam turbines are used to produce heat and electricity from unconverted gas and heat removal, respectively. The results show that, in terms of mass conversion, methanol production seems to be the most efficient process for all the biowastes. SNG synthesis is preferred when exergetic efficiency is the objective parameter, but hydrogen process is more efficient when the performance is analyzed by means of the 1st Law of Thermodynamics. The main exergy losses account for the gasification section, except in the electricity and heat production chain, where the combined cycle is less efficient.     

Round robin test of a wood stove: The influence of standards, test procedures and calculation procedures on the emission level

As a part of the IEA Bioenergy, Task X-Conversion, Combustion activity, an international round robin test of a wood stove supplied with a catalytic afterburner (JØTUL 3TDCI-2) has been performed to investigate and compare the emission level of CO, particles/tar, hydrocarbons and NO_x. The participating countries were Austria, Canada, Denmark, Finland, the Netherlands, Norway, Sweden. U.K. and U.S.A. The wood stove was tested according to national standards and test procedures. In addition, a comparison of the calculation procedures used to convert measured transient volumetric emission levels into average emission levels in g/kg dry fuel was performed, based on both arithmetic and weighted averaging. The results uncovered significant differences in ways of doing environmental evaluation. Particle emission measurements were found to be the best method to evaluate the environmental acceptability of the tested stove, since the particle emission level was least dependent of the national standards, test procedures and calculation procedures used. Finally, transient particle emission measurements are presented, which reveal a close relationship between particle and hydrocarbon emissions.     

A review on microcombustion: Fundamentals,devices and applications

Microcombustion research has flourished over the past decade. Yet, most of the commercial potential of microcombustion is still to come. Aside from portable electronics, emerging drivers stem from the energy problem of declining fossil fuel reserves and their large environmental footprint upon combustion. The need to capitalize on underutilized energy sources and renewables further stimulate energy research in microsystems. In this review paper, technological drivers, applications, devices, and fabrication protocols of microburners are presented. Then, a review of homogeneous, catalytic, homogeneous-heterogeneous and heat recirculating microburners is given. Results are presented that interpret literature findings. An outlook of microcombustion research is finally outlined.     

Minerals and iron-making reactions in blast furnaces

Coke is central to blast furnace operation, but because it is the most expensive raw material used, there is continuing pressure to minimize its use. Consequently, it has become increasingly pertinent to measure and predict the factors affecting coke performance more accurately. Coke performance is affected both by its properties and blast furnace operation. Recently, the importance of the minerals in coke in determining its performance in the blast furnace has been recognized. Minerals in coke influence its reaction with gas, metal and slag phases. This paper reviews coke behavior in an operating blast furnace with the main emphasis being on the role of its inherent mineral matter. Various techniques including advanced approaches such as scanning electron microscopy (SEM) and quantitative X-ray diffraction (XRD) have been used to identify and quantify coke minerals. Fundamental studies based on bench-scale reactors have highlighted the role of various mineral phases on the kinetics of gasification, hot-metal carburization and slag reactions. Because coke reaction rates are influenced by the constituent mineral phases differently, conventional ash analysis is not sufficient to determine the true impact of coke minerals on coke reactivity. The dominant catalytic phases of coke minerals can be identified and related to coke gasification with CO₂ at low temperatures. The kinetics of hot-metal carburization by coke and its temperature dependence is influenced by the melting behavior of minerals. Coke–slag reaction rates are largely influenced by total mineral matter content as well as composition. Coke changes its properties during descent through an experimental blast furnace (EBF) and some of these changes are presented. The increase in the ordering of the carbon in the coke as it descends the EBF can be related to increases in coke ordering in a bench-scale reactor, indicating that order in a particular coke may serve as a thermometer of its maximum exposure temperature. Moreover, coke fines emissions are influenced by the extent of graphitization in industrial blast furnaces. In contrast, coke reactivity in an operating blast furnace is influenced by recirculating alkalis as well as inherent mineral matter. Mineral phases of industrial cokes were found to be changed after CO₂ gasification with increasing reaction temperatures. Coke quality needs in current and emerging blast furnace process innovations are discussed to highlight that existing tests are not sufficient. A comprehensive coke quality index is required, particularly one that incorporates the heterogeneity of coke minerals, in order to make a reliable assessment of the impact of cokes on iron-making reactions.     

Performance assessment of some ice TES systems

In this paper, a performance assessment of four main types of ice storage techniques for space cooling purposes, namely ice slurry systems, ice-on-coil systems (both internal and external melt), and encapsulated ice systems is conducted. A detailed analysis, coupled with a case study based on the literature data, follows. The ice making techniques are compared on the basis of energy and exergy performance criteria including charging, discharging and storage efficiencies, which make up the ice storage and retrieval process. Losses due to heat leakage and irreversibilities from entropy generation are included. A vapor-compression refrigeration cycle with R134a as the working fluid provides the cooling load, while the analysis is performed in both a full storage and partial storage process, with comparisons between these two. In the case of full storage, the energy efficiencies associated with the charging and discharging processes are well over 98% in all cases, while the exergy efficiencies ranged from 46% to 76% for the charging cycle and 18% to 24% for the discharging cycle. For the partial storage systems, all energy and exergy efficiencies were slightly less than that for full storage, due to the increasing effect wall heat leakage has on the decreased storage volume and load. The results show that energy analyses alone do not provide much useful insight into system behavior, since the vast majority of losses in all processes are a result of entropy generation which results from system irreversibilities.     

Mineralogical characterization of the intraseam layers of Lofoi lignite deposits in Florina basin (western Makedonia,northwest Greece)

The mineralogical composition of intraseam layers from Lofoi lignite deposits (northwest Greece) is the subject of the present study. The samples were examined by means of X-ray diffraction (XRD), thermo-gravimetric (TG/DTG) and differential thermal analysis (DTA), and Fourier transform infrared (FT-IR) spectrometry. The clay minerals prevail in most samples, with illite-muscovite being the dominant phase, and kaolinite and chlorite being the other major clay components. No smectite was found. Quartz and feldspars, dominate in two cases. The studied materials are characterized as clays to clayey sands, showing significant similarities with the intraseam layers of the adjacent Achlada lignite deposits.     

Model development and numerical procedure for detailed ejector analysis and design

The main purpose of this work was to develop a mathematical model and computer programs for ejector studies in refrigeration cycles. Version A of the program was written for optimal ejector design while Version B, with more built-in flexibility, was intended for simulation. The study is a one-dimensional analysis of compressible refrigerant flow, based on a forward marching technique of solution for the conservation equations. Refrigerant properties were evaluated using NIST [NIST Standard Reference Database 23, NIST Thermodynamics and Transport Properties of Refrigerants and Refrigerant Mixtures, 1998, REFPROP, Version 6.01] subroutines for equations of state solutions. The approach assesses the flow locally and provides the flexibility of returning upstream for correcting adjustments. Model validation against the R141b data of Huang et al. [Int. J. Refrig. 22 (1999) 354] has shown very good agreement under all conditions. Analysis with refrigerant R142b was performed for typical refrigeration conditions. The entrainment ratio ω, the compression ratio P₆/P₂ and geometric parameters such as diameters and axial dimensions were used to assess performance. Local distributions of pressure, temperature and Mach number were obtained for typical conditions and the mixing chamber was found to greatly impact operation and performance, by controlling the shock wave occurrence and intensity.     

Gasification of biomass: comparison of fixed bed and fluidized bed gasifier

Gasification as a thermochemical process is defined and limited to combustion and pyrolysis. A systematic overview of reactor designs categorizes fixed bed and fluidized bed reactors. Criteria for a comparison of these reactors are worked out, i.e. technology, use of material, use of energy, environment and economy. A utility analysis for thermochemical processes is suggested. It shows that the advantages of one of the reactor types are marginal. An advantage mainly depends on the physical consistency of the input. As a result there is no significant advantage for the fixed bed or the fluidized bed reactor.     

Design of an implantable power supply for an intraocular sensor,using POWER (power optimization for wireless energy requirements)

The reduction in size and power usage of MEMS (microelectromechanical systems) devices has enabled development of fully implantable medical devices [K.D. Wise, IEEE Eng. Med. Biol. Magaz. 24(5) (2005) 22–29], though major obstacles remain in developing devices of very small scale (<1 mm) [T. Simunic, L. Benini, G. De Micheli, IEEE Trans. Very Large Scale Integr. (VLSI) Syst. 9 (2001) 15–28]. One of the most challenging applications; an intraocular sensor (IOS) developed by the Wireless Integrated Micro-Systems-Engineering Research Center (WIMS-ERC) at The University of Michigan; is the subject of the present study. Our specific objectives are fourfold: (1) to model the power usage of an intraocular sensor (IOS); (2) to develop a methodology for optimization of Hybrid Implantable Power Systems (HIPS); (3) to apply the selection tool to identify candidate power systems; and (4) to establish a methodology to fabricate and test the performance of an optimized power supply. In the present study we fabricated and tested three different cells. For one of these, 10 complete discharge and recharge cycles were successfully obtained. The experimental capacity was 7.70 mAh (15% of theoretical) for a discharge rate of C/5. As part of future work, a microbattery will be built for the WIMS-ERC IOS and tested in a fully integrated testbed.