Mixing, transport and combustion in sprays

Recent advances concerning analysis of sprays and drop/turbulence interactions are reviewed. Consideration is given to dilute sprays and related dilute dispersed flows, which contain well-defined dispersed-phase elements (e.g. spherical drops) and have dispersed-phase volume fractions less than 1%; and to the near-injector, dense spray region, having irregularly-shaped liquid elements and relatively-high liquid fractions.

Early analysis of dilute sprays and other dispersed flows assumed either locally-homogeneous flow (LHF), implying infinitely-fast interphase transport rates, or deterministic separated flow (DSF) where finite interphase transport rates are considered, but interactions between dispersed-phase elements and turbulence are ignored. These limits are useful in some instances; however, recent evidence shows that both methods are deficient for quantitative estimates of the structure of most practical dispersed flows, including sprays. As a result, stochastic separated flow (SSF) methods have been developed, which treat both finite interphase transport rates and dispersed phase (drop)/turbulence interactions using random-walk computations for the dispersed phase. Evaluation of SSF methods for particle-laden jets; nonevaporating, evaporating and combusting sprays; and noncondensing and condensing bubbly jets has been encouraging, suggesting capabilities of current SSF methods to treat a variety of interphase processes. However, current methods are relatively ad hoc and many fundamental problems must still be resolved for dilute flows, e.g. effects of anisotropic turbulence, modification of continuous-phase turbulence properties by the dispersed phase (turbulence modulation), effects of turbulence on interphase transport rates, and drop shattering, among others.

Dense sprays have received less attention and are poorly understood due to substantial theoretical and experimental difficulties, e.g. the idealization of spherical drops is not realistic, effects of liquid breakup and collisions are difficult to describe, spatial resolution is limited and the flow is opaque to optical diagnostics which have been helpful for studies of dilute sprays. Limited progress thus far, however, suggests that LHF analysis may provide a useful first-approximation of the structure and mixing properties of dense sprays near pressure-atomizing injectors. Since dense-spray processes fix initial conditions needed to rationally analyze dilute sprays, more research is this area is clearly warranted.     

Issues in biomass technologies for synthetic fuels from coal and biomass resources in the Midwestern U.S.A.

Discussions on the role of biomass in the development of a midwest synfuels industry brought general agreement on:

1. (1) the continued development of new techniques for ethanol production;

2. (2) use of corn as a feedstock for ethanol;

3. (3) large-scale plants are more economical than farm-scale ethanol plants; and

4. (4) tax subsidies to promote use of ethanol.

     

Issues in coal technologies for synthetic fuels development in the Midwestern U.S.A.

A two day meeting involving researchers, industrialists, and state policymakers on the subject of coal technologies and synfuels brought general agreement on:

1. (1) the lack of a cohesive energy policy regarding coal in the United States;

2. (2) the need for long-term constancy in the regulatory environment;

3. (3) the need for coal research and development; and

4. (4) the use of conflict resolution techniques to solve synfuel issues.

     

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.     

On enthalpy management in small buildings

Enthalpy management requirements of residential and small commercial buildings are analyzed and integrated approaches to energy-efficient and cost-effective heating and cooling schemes are proposed. Improved design and operating strategies are suggested to make more efficient use of off-the-self (or other readily accessible) technology for space conditioning. The use of Comfort Range Thermal Storage (temperatures in the approximate range of 65–75 °F) and special operational strategies are central to these approaches. Fossil-fuel heaters, heat pumps, solar collectors, electric driven air coolers, all can be used more efficiently when they are effectively interfaced with selected thermal storage systems. A central heated (fossil fuel) and cooled (electric air-conditioner) residence located in Long Island, New York, is considered as an example. With Comfort Range Thermal Storage, it is found that the revised operating approach leads to 50% savings in space conditioning costs, with the basic functional features of the building unchanged from those of a typical well-insulated frame residence. Additional insulation results in further savings.

Devices, methods and strategies employed to achieve these results include the use of Comfort Range Thermal Storage as well as one or more of the following:

1. (1) Variable firing rate fossil fuel heater.

2. (2) Thermally purgeable fossil fuel heater.

3. (3) Outside air for all fossil fuel heater requirements.

4. (4) Separation of the combustion and heat transfer function from the thermal storage function of a conventional boiler.

5. (5) Use of off-peak electrical energy.

6. (6) User-oriented controls for space conditioning applications to allow conscious reprogramming of temperature to suit variations in life-style of the occupants.

7. (7) Control of energy flow at windows during the heating and cooling seasons.

8. (8) Comfort Range Thermal Storage in the temperature range of 65–75 °F.

9. (9) Functionally composite building materials.

Not all the above options are suitable for inclusion in new structures. Not all can be retrofitted to existing structures. Nevertheless, substantial energy and cost savings are selectively possible in all cases of fossil-fuel heating systems. Retrofitting of existing masonry buildings promises particularly significant economies. Such structures are typically uninsulated. Retrofit strategies which include insulation, permit economies substantially greater than those which result solely from the insulation-prescribed reduced heat losses. Such retrofitting, as well as the retrofitting of substantial thermal storage capacity to existing insulated frame buildings can result in substantial cost reductions for space heating and cooling.     

Municipal solid waste conversion to energy

In recent years it has been recognised by an increasing number of nations that there is considerable energy potential within MSW. As a result many countries have established R,D& D programmes to examine methods of exploiting this potential. The IEA's MSW Conversion Activity was set up in 1986 to provide an infrastructure for sharing information and co-ordinating work in this area internationally. This Activity was extended in 1989 and currently a total of 9 nations participate in it.

To cope with the wide scope of the area (encompassing both biological and thermal processing of MSW) the Activity was divided into three subgroups or Expert Working Groups (EWGs). Each of these dealt with a distinct area of expertise:

1. •Downstream effects of source separation and screening of MSW

2. •Sampling and analytical protocols

3. •Landfill gas

In addition to these groups a central secretariat based at Harwell (UK) has provided guidance, established and administered databases of contacts and produced a series of national reports.

This paper describes the achievements of the Activity and discusses work proposed for the future.     

An empirical model to describe the non-isothermal ageing behaviour of feroplug materials

Feroplug is a new temperature indicator developed by the authers and co-workers. The device makes use of the characteristics of ferrite transformation in duplex stainless steels, and has the following advantages over conventional methods of temperature measurement:

1. (1) It is a piece of stainless steel, with no delicate parts, and can be used in very hostile environments;

2. (2) It does not require any cables such as those in the case of thermocouples;

3. (3) It is very much cheaper than thermocouples.

This invention has been patented in the USA, UK and Europe with financial and legal support provided by the British Technology Group, UK. Patent application in Japan is still pending.

The invention is particularly useful for life assessment purposes in power and petrochemical plants. The underlying physical principle of the Feroplug is based on the transformation of ferrite, which is ferromagnetic at room temperature, into other phases such as carbide and sigma which are nonferomagnetic at room temperature. The amount of transformation depends on the temperature and duration of exposure. If the amount of transformation and time of exposure is known, the temperature of exposure can be deduced. Thus, the Feroplug makes use of the change in the amount of ferrite, which can be easily detected by room temperature magnetic measurement, to indicate the temperature of exposure. A model has been developed which can describe the ferrite transformation behaviour of Feroplug materials under non-isothermal conditions. The agreement between experimental data and the model is good at temperatures above 550°C.     

A decision analysis of the U.S. breeder reactor program

Past modelling efforts have failed to reach a consensus on breeder R & D strategy. These failures have led some to question the usefulness of benefit-cost analysis for a problem as complex and as politicized as this one has become. Our paper examines some of the major weaknesses of past benefit-cost analyses and then reformulates the breeder investment decision in a decision tree framework. Decision analysis is used to evaluate alternative R & D strategies that range from accelerating the current effort to abandoning the entire program.

Breeder benefits are assessed through ETA-MACRO, a model of interactions between the energy sector and the rest of the U.S. economy. A dynamic non-linear optimization procedure is used to examine how breeder timing could depend upon alternative assumptions with respect to:

1. (a) uranium supply,

2. (b) energy demand growth,

3. (c) environmental constraints upon coal and shale oil production, and

4. (d) the availability of alternative energy sources.

Subjective probabilities have been estimated through a Delphi questionnaire on the critical uncertainties. Each individual's responses have been used independently to calculate an optimal strategy. A fairly simple rule-of-thumb has emerged from this experiment. If a respondent believes that the probability of positive benefits (P) exceeds 0.03, there are positive expected dollar benefits from any breeder development program. If P is less than 0.03, however, the expected economic benefits of such a program would be negative. The costs of the R & D program would then exceed their actuarial value.     

The U.S. DOE least-cost utility planning program

This paper deals with the most significant accomplishments of recent projects sponsored by the U.S. Department of Energy's Least-Cost Utility Planning Program. This overview, with its accompanying bibliography, is designed to make the resources of these projects, completed in 1989, widely accessible to utility planners and regulators. The most significant contributions of the projects relate to three areas:

1. (1) DSM technology assessments and database development,

2. (2) assessments of current planning methods and future policy directions, and

3. (3) consensus building and the development of organizational structures to support least-cost planning.

     

A cellwise method for the optimization of large central receiver systems

The total number of heliostats in the collecter field determines the approach to the optical simulation problem. For large central receiver systems, it is desirable to introduce a cell model which establishes an array of representative heliostats (see Ref.[1] for central receiver systems). We now have an arsenal of computer programs which allows us to optimize the arrangement of heliostats in the collector field subject to the approximations of the cell model. Each cell contains an arbitrary regular two dimensional array of heliostats. For practical reasons we have limited our current study of the 100 MWe commercial model to four categories of heliostats arrangement; (1) radial cornfields, (2) radial staggers, (3) N.-S. cornfields, and (4) N.-S. staggers.

The most important results from the 100 MWe commercial model optimization study are:

1. (1) Staggers are better than cornfields.

2. (2) The increased cost of the tower and receiver subsystems has moved the solution to a larger cell size and a shorter tower.

3. (3) No panels should be deleted from the south side of the cykindrical receiver, and

4. (4) The collector field trims to a 360° configuration.

The center of the collector field is north of the tower and some compromise may be made to prevent excessive panel power asymmetry. Currently, this problem is solved by using preheat panels in the southern part of the receiver.     

Analysis of a solar water heating system with n-tanks in series

A straightforward analysis of a solar water heating system with n-tanks connected in series has been presented. The long-term performance of the system has also been studied. On the basis of numerical calculations made for four successive days, the following conclusions have been drawn:

1. (1) The fluctuation in temperature variation decreases with increase of the number of tanks connected in series.

2. (2) The variation becomes smooth after the second successive day, which is more desirable from the point of view of users.

     

A review of drop size measurement in fuel sprays

A detailed review of selected drop size measurement techniques capable of application to practical combustion systems is presented. Attention is restricted to those methods producing quantitative drop size frequency distributions and methods yielding mean spray characteristics are ignored. Comment on the relative merits and restrictions of each method, together with its applicability to particular situations, is made. Some of the techniques described can be applied to measurement of angle of flight and velocity of drops, in addition to their size, in both burning and isothermal sprays. Some indication of future technological advances in the field of drop size measurement is given.     

Area- and time-specific marginal capacity costs of electricity distribution

Marginal costs of electricity vary by time and location. In the past, researchers attributed the variations to factors related to electricity generation and transmission. These authors, however, have not analyzed possible variations in marginal distribution capacity costs (MDCC). The objectives of this paper are:

1. (i) to show that large MDCC variations are due to the dispersion in distribution capital expenditures by time and space,

2. (ii) to propose a method for quantifying the area- and time-specific MDCC in the presence of lumpy investments, and

3. (iii) to compare our MDCC estimates to those commonly used in the electric utility industry.

Our proposed method and its results were adopted by the California Public Utilities Commission (CPUC) in 1992 for Pacific Gas and Electric Company (PG&E), the largest privately owned electric utility in the U.S.     

How competition might affect electric-utility DSM programs

This paper discusses competition in the electricity industry and how it might affect utility DSM programs. The roles that state regulatory commissions could play to affect retail competition and DSM programs are examined. Commissions could set exit or reentry fees for customers who want to buy electricity from an entity other than the local utility. Or they could ‘tax’ the use of the local distribution system to discourage uneconomic wheeling and to pay for DSM programs. The effects of DSM programs on retail electricity prices and how utilities might redesign their DSM programs for a more competitive environment are considered. In the future, utility DSM programs may

1. (1) focus more on customer service and less on system-resource benefits,

2. (2) emphasize capacity reductions more and energy savings less as utilities seek to minimize the lost revenues associated with DSM,

3. (3) become more cost-effective as utilities identify better ways to deliver DSM services at lower cost, and

4. (4) involve fewer inter- and intraclass transfers as utilities increasingly have individual customers pay for their own DSM services. While DSM programs in the future may be different from what they are today, they will continue to be important to utilities as powerful marketing tools and to society because of their environmental and economic-productivity benefits.

     

Different strategies for operation of flat-plate solar collectors

Three strategies for solar collector operation are defined. These involve keeping one of the following constant during the day:

1. (i) the average working fluid temperature,

2. (ii) the outlet temperature and

3. (iii) the inlet temperature.

A graphical and analytical method previously developed by the authors was generalized to analyse and compare these strategies. For a constant flow rate, the best strategy is to maintain a constant inlet temperature. A constant outlet temperature is recommended when flow-rate control is possible.     

A new monitoring system for piping systems in fossil fired power plants

A CEC-funded project has been performed to tackle the problem of producing an advanced Life Monitoring System (LMS) which would calculate the creep and fatigue damage experienced by high temperature pipework components. Four areas were identified where existing Life Monitoring System technology could be improved:

1. 1. the inclusion of creep relaxation

2. 2. the inclusion of external loads on components

3. 3. a more accurate method of calculating thermal stresses due to temperature transients

4. 4. the inclusion of high cycle fatigue terms.

The creep relaxation problem was solved using stress reduction factors in an analytical in-elastic stress calculation. The stress reduction factors were produced for a number of common geometries and materials by means of non-linear finite element analysis. External loads were catered for by producing influence coefficients from in-elastic analysis of the particular piping system and using them to calculate bending moments at critical positions on the pipework from load and displacement measurements made at the convenient points at the pipework. The thermal stress problem was solved by producing a completely new solution based on Green's Function and Fast Fourier transforms. This allowed the thermal stress in a complex component to be calculated from simple non-intrusive thermocouple measurements made on the outside of the component. The high-cycle fatigue problem was dealt with precalculating the fatigue damage associated with standard transients and adding this damage to cumulative total when a transient occurred.

The site testing provided good practical experience and showed up problems which would not otherwise have been detected.     

Studies on membrane viscosity stabilized solar pond

In this nonsalt type of solar pond, the nonconvecting layer is composed of a viscous polymer solution partitioned by a number of transparent films. An advantage of partitioning is that a thinner polymer solution can be used and that the light transmittance increases. Results of experimental and theoretical investigations on the performance of this solar pond are summarized as follows:

1. 1. Ionized polyacrylamide solution was chosen as the thickener based on tests about solubility, viscosity, light transmittance and stability.

2. 2. The critical temperature difference for the onset of convection in the polymer layer (ΔT/L)_cr [°C/m] was given by the following formula based on the measurements in various thicknesses of the polymer layers (L) [m] and various concentrations (ζ) [%],

(ΔT/l)_cr=(55−185lnL)exp(4.66L^0.505lnζ

3. 3. An outdoor model pond, 200 × 150 cm surface and 100 cm depth, was constructed in Osaka. Four types of model ponds were tested, and the availability of membrane type with partition films was confirmed.

4. 4. The theoretical temperature rise of the pond using a one-dimensional model was calculated by solving the equations of the heat balance in the pond. As a result, the optimum values of thickness of polymer layer and number of films was determined

     

An optical study of interfacial turbulence in a liquid-liquid system

In order to make clear the relation between the mass-transfer rate and the mechanism of interfacial turbulence in a liquid-liquid system a fundamental, qualitative study of interfacial turbulence was carried out. The mass transfer of propionic acid in the di-n-butylphthalate-water system was observed in a well devised test cell, and the concentration profile of propionic acid near the interface measured by microinterferometric and Mach-Zender interferometric methods. When these concentration profiles had a typical pattern, we defined them as the interfacial turbulence (turbulence) and observed them qualitatively.

The following results were obtained from these experiments:

1. (1) The existence of stable and oscillatory turbulence in the interfacial turbulence.

2. (2) Information about the process of how some small concentration discrepancies, after making the interface, develop to the interfacial turbulence.

3. (3) Information about the process how interfacial turbulence changes with time.

Fluid flow around the interfacial turbulence was investigated by following the movement of small dispersed particles in the system and the relation between the interfacial turbulence and the fluid flow is discussed.     

Fluidization characteristics of gas-fluidized beds: Air and glass beads system

Good gas-solids contacting and solids mixing are essential features, both for efficient coal combustion and desulfurization of coal in a fluidized-bed combustor. It will therefore be very useful if suitable procedures could be developed to characterize the quality of fluidization in gas-solid systems. Three different procedures are discussed in this article with reference to air and glass beads of four different average sizes:

1. (i) in one scheme, the bed voidage is measured at minimum fluidization and the variation of this quantity with Reynolds number at minimum fluidization is correlated with the powder classification scheme of Saxena and Ganzha and hence with the corresponding fluidization characteristics. A major effort is directed at substantiation and confirmation of this characterization scheme, which relates the fluidization behavior to the Reynolds number at incipient fluidization.

2. (ii) The local heat-transfer coefficient of a probe element immersed in the bed is also measured as a function of the superficial fluidizing velocity and the nature of variation is taken as representing the quality of bed fluidization.

3. (iii) A technique is developed to measure the time-history of the temperature fluctuation of a probe element and to relate this parameter to the bed-fluidization characteristics.

Additional experimental data with other solids will be generated in the future to develop these proposed three schemes further for the characterization of bed-fluidization behavior.