Progress in knowledge of flamelet structure and extinction

In the past 25 years there has been a considerable amount of progress in studying flamelets, their structures and their responses to various perturbations. The term “flamelet” as used here really would mean “laminar flame” to most readers and is employed only because a major motivation is for ultimate use in connection with more complex flows, mainly turbulent. There is, however, no consideration here of how the knowledge reviewed may be employed in flamelet modeling of turbulent combustion. Not even time-dependent flamelets are addressed, although a few related references are provided. The focus is narrower, namely on quasisteady flamelets, and therefore the considerations of flamelet extinction that are presented concern quasisteady extinction, that is, not the dynamics of extinction. Even in this narrow context, it will be seen that a great deal has been accomplished. When such a long-term view is taken, it is found remarkable how much progress has been made. The progress is addressed separately for premixed, nonpremixed and partially premixed systems. Suggested directions of future research also are indicated. Despite the limited scope of the topic and the extensive advancement that has occurred, much more research remains to be done.     

Managing your process hazards as a means of conforming to ohsa requirements

There is considerable evidence that improper changes have directly caused or through the “domino sequence” led to many of the major accidents that have occurred in the chemical process industry and related industries that use hazardous chemicals and technology. Central Government has recognised the importance of careful management of change(MOC) for ensuring the safety of chemical operations and the quality of manufactured goods by the proposed adding of the Major Hazardous Installation Regulation to the OHSA (Act 85 of 1993).

The concept and the need to properly manage change are not new; many companies have adopted MOC procedures. It is common practice nowadays to perform detailed risk assessments on any new or intended project to ensure safety and reliability of the project. Yet, incidents and near misses attributable to inadequate management of changes on existing systems and processes continue to occur. To improve the performance of MOC systems throughout industry, managers need advice on how to better institutionalise MOC systems within their companies and plants.

The purpose of this paper is to define the important features of MOC systems and to assist in the implementation and management of process hazards. MOC systems help ensure that changes to design and operation of company facilities will not adversely affect employees, the public, or the environment. This document outlines a process that can be used for designing, developing, installing, operating, and maintaining MOC systems at individual company sites.

This paper is intended for an audience ranging from plant and corporate managers of process safety to workers who have differing levels of knowledge about the principles of safely managing change. It is primarily designed to equip people responsible for MOC systems with new ideas for implementing and improving MOC systems. It was indeed a privilege to have met the personnel of JBF Associates, Inc. and Process Safety Institute in Knoxville, Tennessee, USA, without whose lectures and study material (including the book “Managing process changes”) this paper would never have seen the light. They have succeeded in putting me firmly on the “process safety road”.     

Optimization and management of a solar refrigeration plant using latent storage

In a previous article, we presented the model, the responses and the parameters of a solar refrigeration plant using a battery storage BSS. The measurements collected on the BSS system in laboratory and rural sites (stand alone PV plant) at different climatic conditions have shown some problems coming from the battery and its regulator, such as maintenance, replacement, break-down and the limited battery life period. In order to surmount these problems, we thought of substituting the electric storage by a thermal or a latent storage LSS (accumulation of ice in a separate section). To have a good prediction, optimal control and dimensions of the LSS system, we have developed a simulation program “SIREF” by using an acquisition/treatment environment. The validation of “SIREF” required the design and the development of a real prototype LSS plant. By installing a data acquisition chain, we have acceded to have the measured PV-LSS responses which are used to identify the system parameters by the recursive least square algorithm (RLS). The measured and the simulated results have shown the importance of the thermal losses and the choice of the insulation thickness. Hence, a study of the energetic optimization was elaborated in order to reduce the total life cost TLC for the LSS plant.     

Environmental assessment and extended exergy analysis of a “zero CO2 emission”, high-efficiency steam power plant

Aim of this paper is to analyze the performance of an innovative high-efficiency steam power plant by means of two “life cycle approach” methodologies, the life cycle assessment (LCA) and the “extended exergy analysis” (EEA).

The plant object of the analysis is a hydrogen-fed steam power plant in which the H₂ is produced by a “zero CO₂ emission” coal gasification process (the ZECOTECH^© cycle). The CO₂ capture system is a standard humid-CaO absorbing process and produces CaCO₃ as a by-product, which is then regenerated to CaO releasing the CO₂ for a downstream mineral sequestration process.

The steam power plant is based on an innovative combined-cycle process: the hydrogen is used as a fuel to produce high-temperature, medium-pressure steam that powers the steam turbine in the topping section, whose exhaust is used in a heat recovery boiler to feed a traditional steam power plant.

The environmental performance of the ZECOTECH^© cycle is assessed by comparison with four different processes: power plant fed by H₂ from natural gas steam reforming, two conventional coal- and natural gas power plants and a wind power plant.     

Prospect of RBMK-NPPS strength monitoring system on-line module development

System for strength monitoring and residual lifetime (SMS) assessment of NPP equipment makes it possible to improve safety and effectiveness of operation.

According to SMS concept it includes two main modules: for on-line data acquisition and diagnostic and for off-line date processing and lifetime assessment.

Algorithm for RBMK SMS on-line module is proposed to be based on the basis of “DASHA” expert system shell.

Universal expert shell “DASHA” was created using the original matrix method of image identification theory and can be used for on-line diagnostic system development for any complex technological object of arbitrary physical nature.     

The derivations of several “Plate-efficiency factors” useful in the design of flat-plate solar heat collectors

All variables influencing the efficiency of a flat-plate solar heat collector as a heat exchanger can be combined into a single “efficiency factor.” These efficiency factors are more or less design constants of the particular collector design, and are only slightly influenced by operating conditions. Consequently they are extremely convenient for use in accurate design and performance calculations. The full mathematical derivations are presented for several of these efficiency factors for various types of collectors, together with graphical data and examples of their use.     

Economic optimisation of a wind power plant for isolated locations

This paper presents a model of a wind power plant for isolated locations composed of a vertical axis wind turbine connected to a self-excited induction generator operating at constant voltage and frequency; a back-up diesel generator and a battery system are moreover included in the system. Constant voltage and frequency are obtained only by controlling the generator appropriately. The control system is supposed to be optimised so that the system operates at the highest efficiency. In order to improve the total efficiency even further, a gear-box to vary the gear transmission ratio between the turinbe and the generator has been considered. A “Monte Carlo” type simulation has been used to analyse the operation of that system over a one year period. The model is based on a probability density function of the wind speed derived by statistical data concerning a given location and on the probabilistic curve of the load required by an isolated location. The cost per kWh for different dimensions of the main components has been evaluated and the optimum configuration has been identified.     

The calculation of wall and fluid temperatures for the incompressible turbulent boundary layer, with arbitrary distribution of wall heat flux

Numerical solution of the partial differential equation for heat transfer in the incompressible turbulent boundary layer has been obtained for uniform (q_w/ρC_pu_i)/√(c_f/2) and for Prandtl numbers 0–7, 1 and 7. The Spalding boundary-layer velocity law was assumed, and the Schmidt method of integration used. Boundary-layer temperature distributions up to x⁺ = 10⁶ are presented, together with the “Spalding function” St/√(c_f/2). A method is given for the application of the solutions to the case of arbitrary distribution of heat flux at the wall.     

A practical perspective on plant preservation and plant life extension in thermal power stations. Considering aspects of reliability, maintainability, cost and safety

Currently I am employed as the Assistant Power Station Manager, with responsibility for all maintenance aspects, in a thermal power station which has been on standby since 1986! Previously I have been employed as a maintenance engineer and engineering manager in base loading thermal power stations where plant life extension programmes have not only been conceived and planned, but, on most occasions, even completed.

This paper reflects on aspects of plant preservation and plant life extension programmes with which I have been, and am currently, associated; weighing the obvious desirability for such programmes against the need to achieve reliability and maintainability, at a reasonable cost. (Who knows, it may even answer the question continually asked by my wife, “If you are not generating electricity, what do you do all day?”).

The paper also looks at the retention of safety standards, and the need to improve safety standards on ageing plant, in line with ever more stringent legislation.     

The use of waste heat in a solar still

There are instances in remote areas where heat is being wasted, e.g., in internal combustion, engines, etc. Some of this heat can be recovered to produce distilled water in solar stills.

The solar still replaces the cooling tower, ponds, or radiators normally used to control the engine temperature. The diesel cooling water in such a system remains separate from the saline water in the solar still.

The advantages of using such a system compared with a conventional solar still are:

1. (a) water costs are very much reduced

2. (b) the area occupied is much less, i.e., about 1/5th

3. (c) production has much less seasonal variation

4. (d) the efficiency of the solar still is improved due to the higher operating temperatures.

From experiments conducted at Highett using a Mk VI solar still fitted with a simple heat exchanger and a separate electrically-heated source of hot water to simulate the waste heat, design data are not available for application to working systems. The information required to match a solar still to a diesel's cooling requirement is:

1. (a) engine efficiency

2. (b) hourly fuel consumption

3. (c) hourly solar radiation

4. (d) hourly ambient temperatures.

A by-product of this work has been the production of a “solar water heater” which costs less than that of the cheapest conventional system. This “solar” hot water system uses a heat exchanger similar to what is used to transfer the waste heat to the saline water. It is envisaged to have hot water productions approximately the same as the distilled water productions. The influence of hot water production on the output of the waste heat solar still is discussed.     

Thermal evaluation of a “Winter House”

In this paper, the transient thermal model of a “Winter House” has been presented for a cold climatic region of Srinagar in India. In order to reduce the fluctuations in living room temperature, the effect of an isothermal mass has been incorporated in the thermal model. An analytical expression for room temperature has been derived to evaluate the performance of the “Winter House” by incorporating the effect of movable insulation during the night. Numerical calculations were done in terms of system as well as climatical parameters. Some isothermal mass, like water, has been considered to stabilize the room temperature throughout the day and right. For further heating, the effect of the roof as an air collector has also been taken into account.     

Enhancing the elevated temperature performance of Li/LiMn2O4 cells by reducing LiMn2O4 surface area

Lithium-rich spinels were obtained with the same structure but different surface area by two different synthesis routes, namely the “once-annealed” and the “twice-annealed” methods. The elevated temperature performance of Li/Li_1+xMn₂O₄ cell is significantly improved using a spinel cathode with a small surface area: the cell at 50°C lost 5% of the initial capacity over the first 100 cycles based on a spinel cathode with the small surface area of 1.2 m²/g compared to 8% based on a large one of 6.2 m²/g. Also the mechanism responsible for the reaction of LiMn₂O₄ with LiOH to form lithium-rich spinel has been investigated.     

Facility risk review as a means to addressing existing risks during the life cycle of a process unit, operation or facility

In today's process industry environment, it is becoming more and more important for companies to manage the risks associated with their plants. Amongst others, some reasons for this are that 1) Process Safety is featuring high on the agenda of Trade Unions; 2) that Management is coming under increased pressure to provide a safe workplace; 3) that Companies are trying to survive in the current competitive environment by adopting the “zero accidents” ideal; 4) because the effects of accidents that do occur are becoming more devastating due to increased inventories and the exotic nature of products and 5) the fact that overseas companies are looking at safety and conservation track records before choosing business partners or considering trade agreements.

Regulatory directives such as OHSA'S (Occupational Health and Safety Act, Act 85 of 1993) Major Hazard Installation Regulation in conjunction with SABS 0228 (hazard group 1) is another newcomer entering the playing fields of South African Companies and is in the future going to have a serious impact on Company philosophies and policies. In this Regulations “Risk Assessment” is defined as “a planned programme to reduce, to remove, and to control a major incident”. This can be interpreted as having the meaning/objective to involve some type of Risk Management programme which will co-ordinate inter alia the risk assessment function. Also defined is “Major Incident”, which reads: “It is an occurrence (including in particular, a major emission, fire or explosion) resulting from uncontrolled developments in the course of an industrial activity, leading to a serious danger to persons, whether immediate or delayed, inside or outside the installation, or to the environment, and involving one or more dangerous substance”

To address this problem of managing the risks in a facility/plant/operation is no mean feat. Cost factors, time constraints, manpower availability, legislatory pressures and uncertainty on the most applicable risk assessment method to use, are just some of the problems facing management. Adding to this dilemma is the fact that the reviewing specialist has to choose from a variety of risk assessment methods that have varying degrees of complexity, cost and applicability. Past experience has however shown that Hazard and Operability Analysis (Hazop), Failure Mode and Effects Analysis (FMEA), Fault Tree Analysis and Event Tree Analysis are the most widely used and well accepted.

Choosing the right assessment method which will provide the information management needs to address the facility's risks is often the most difficult part in the whole assessment process. With this paper we are providing an overview of such a risk assessment method, namely Facility Risk Review (FRR) which uses a combination of techniques of a qualitative and quantitative nature.

These methods are similar to those currently being used with great success in the risk assessment process at the facilities of the Sasol Group of Companies. These methods are in use from the initial conceptual design stage of any project or change up to the stage where the facility has been in operation for many years.     

Non-ideal equations of state for combusting and and detonating explosives

This paper deals with the formulation and evaluation of the non-ideal equation of state for products developed when burning or detonating energetic solid propellants and/or solid explosives. Included is the review of non-ideal gas equations of state (EOS) based on (a) the virial EOS; (b)the Haar-Shenker EOS; (c) the Becker-Kistiakowsky-Wilson EOS; and (d) the Jacobs-Cowperthwaite-Zwisler EOS. A new methodology is presented for evaluating the parameters in the BKW formulation, based on explosion data for different loading densities of the same explosive. An alternative approach to determine the single species equivalent molecular well-depth and σ for JCZ EOS has been significantly improved by adjusting the values of γ, the equivalent specific heat ratio. Sufficient details for each EOS are included for one explosive constituent, namely HMX (Octagen).

A general non-ideal EOS with a non-constant co-volume function has been obtained by utilizing a “multi-equation combination” procedure, which can be used for product densities ranging from ideal gas values to values of gas density equivalent to the condensed phase density. Comparison of the sensitivity of the general non-ideal EOS with various other forms was made by utilizing a DDT (Deflagration-to-Detonation Transition) analyses (work of Krier and Butler⁴). The results show that the accuracy of predictions of the DDT modeling using this general EOS has been improved.     

Some environmental and safety aspects of using hydrogen as a fuel

At present, less than 7% of the hydrogen produced in the U.S. is delivered to users as “merchant” hydrogen. The balance is produced and used as “captive” hydrogen as a chemical intermediate. In the future it is probably that this pattern will change and that the predominant use for hydrogen will be as fuel. In this paper the discussion of environmental and safety considerations related to the use of hydrogen is focused primarily on its use as a fuel for autos, trucks and buses, and for aircraft. The objective is to bring some of the interesting generalities of the subject to the attention to those who are just acquiring an interest in the potential of hydrogen, as well as for those specialists in hydrogen technology who may not have given much thought to these considerations.     

Natural convection from isothermal flat surfaces

Local heat-transfer coefficients along a flat plate in natural convection in air were measured using Boelter-Schmidt type heat flux meters. Experiments were carried out for different temperature differences in heating and cooling, and with inclinations varying from the horizontal “facing upwards” position, through the vertical position, to the horizontal “facing downwards” position.

The results are presented in terms of local Nusselt number as a function of the local Grashof number “tangential component”. All runs were in the range accepted as that of laminar boundary layer flow. However, under certain conditions when the normal velocity component of the air is directed away from the surface, separated flow is indicated along the trailing part of the surface, well before turbulence sets in in the boundary layer. Separation starts at a certain point along the surface. This point is nearer to the leading edge the higher the temperature difference, and the larger the inclination of the surface to the vertical.

In a separation region, the flux density is uniform. In all other regions the results agreed closely with established theories of laminar boundary layer flow.

A leading adiabatic section, used in some of the experiments, did not affect the results. An appendix gives relations recommended for engineering calculations.     

Scenarios of energy demand and efficiency potential for Bulgaria

The paper presents aggregated results on macroeconomic and final energy demand scenarios developed within the Bulgarian Country Study on Greenhouse Gas Emissions Mitigation, supported by US Country Studies Program. The studies in this area cover five main stages: (1) outline of “Basic” and “Energy Efficiency” socio-economic and energy policy scenarios (2) modeling of macroeconomic and sectoral development till 2020; (3) expert assessments on the technological options for energy efficiency increase and GHG mitigation in the production, transport and households and services sectors; (4) bottom-up modeling of final energy demand; (5) estimation of sectoral and overall energy efficiency potential and policy. Within the Bulgarian Country Study, the presented results have served as a basis for the final integration stage “Assessment of the Mitigation Policy and Measures in the Energy System of Bulgaria.”     

Strain gradient-induced diffusion of hydrogen in palladium and nickel membranes

The “uphill” diffusion of hydrogen during permeation through flat sheets of palladium and nickel has been studied by an electrochemical permeation method at 303 K. For both annealed and “as cold rolled” Pd samples, uphill diffusion effects on hydrogen absorption and desorption have been observed over a range of initial hydrogen contents from about H/Pd = 0.01, i.e. near or slightly less than the _max composition, up to H/Pd = 0.25–0.3. The occurrence of a non-Fickian component of permeation flux has been associated with temporary formation of lattice volume differences across the ( + β)/β and ( + β)/ interfaces during absorptions and desorptions, respectively. Influences of the magnitudes of galvanostatic hydrogen fluxes and of the membrane thickness on the uphill effects were examined. Analogous uphill effects were observed in similar studies with nickel membranes also in both annealed and “as cold rolled” states, which were much larger than those observed for palladium.     

Thermal performance of air-heating solar collectors with thick, poorly conducting absorber plates

Expressions are derived for the efficiency and loss factors of a coverless, air-heating solar collector in which it is possible to assign finite values for the thickness and thermal conductivity of the absorber plate. In the geometry treated, air flow is beneath a flat absorber, and heat transfer is both steady state and one dimensional. The expressions are validated by outdoor measurements from a fullscale tile roof used as a collector and by indoor measurements from tile and metal roof sections tested in a heating simulator. For the tile roofs, the expressions presented give better agreement with experiment than do expressions based on the usual “thin plate” models since the absorber is thick and poorly conducting. Efficiency characteristics of the full-scale tile roof are also presented. Trapezoidally profiled metal panels are commonly used as a cladding for industrial buildings; it is shown that their performance as collectors may be described by a “fin and tube” model. Information is presented in a format that is useful for design and may be used in computer models of the thermal performance of solar-assisted buildings comprising these, or similar, collection devices.     

Optimization of a wind-power fuel-cell hybrid system in an autonomous electrical network environment

Stability considerations associated with intermittency prevent high wind energy penetration in small electrical networks. The case of the islands of the Aegean is presented. [Ntziachristos L, Kouridis C, Samaras Z, Pattas K. A wind-power fuel-cell hybrid system study on the non-interconnected Aegean islands grid. Renewable Energy 2005;30(10):1471–1487] proposed a wind turbine(WT)–fuel-cell hybrid as a means to store wind energy and increase penetration in these islands. The effect of network restraints was, however, not included in that study. Simulation results including network restrictions prove that when network restrictions are relatively “strict”, hybridizing a WT using the scheme presented in the same paper will indeed increase the WT's energy output. However, in the case of “lenient” network restrictions, that hybridization scheme will in fact decrease the WT's energy output. Moreover, the system configurations presented that paper could not achieve financial viability at current electricity prices due to high capital costs. Two alternative operating principles for the hybrid system are presented in this study. These operating principles significantly improve the hybrid system's energy performance even under “lenient” network restrictions. In some cases, these operating principles manage to yield hybrid systems that are financially viable assuming current electricity prices.