J. Energy Resour. Technol. 1996;118(3):169. doi:10.1115/1.2793858.
Commentary by Dr. Valentin Fuster


J. Energy Resour. Technol. 1996;118(3):170-179. doi:10.1115/1.2793859.

Jet fuel requirements have evolved over the years as a balance of the demands placed by advanced aircraft performance (technological need), fuel cost (economic factors), and fuel availability (strategic factors). In a modern aircraft, the jet fuel not only provides the propulsive energy for flight, but also is the primary coolant for aircraft and engine subsystems. To meet the evolving challenge of improving the cooling potential of jet fuel while maintaining the current availability at a minimal price increase, the U.S. Air Force, industry, and academia have teamed to develop an additive package for JP-8 fuels. This paper describes the development of an additive package for JP-8, to produce “JP-8+100.” This new fuel offers a 55°C (100°F) increase in the bulk maximum temperature (from 325°F to 425°F) and improves the heat sink capability by 50 percent. Major advances made during the development of JP-8+100 fuel include the development of several new quantitative fuel analysis tests, a free radical theory of autooxidation, adaptation of new chemistry models to computational fluid dynamics programs, and a nonparametric statistical analysis to evaluate thermal stability. Hundreds of additives were tested for effectiveness, and a package of additives was then formulated for JP-8 fuel. This package has been tested for fuel system materials compatibility and general fuel applicability. To date, the flight testing has shown an improvement in thermal stability of JP-8 fuel. This improvement has resulted in a significant reduction in fuel-related maintenance costs and a threefold increase in mean time between fuel-related failures. In this manner, a novel high-thermal-stability jet fuel for the 21st century has been successfully developed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):180-186. doi:10.1115/1.2793860.

New fuels with high-energy-density are desirable for many combustion applications. Two types are reviewed in this paper, namely, mixtures resulting from addition of certain metallic or nonmetallic elements to conventional hydrocarbon fuels, and newly synthesized hydrocarbon fuels with strained molecular conformations or more densely packed molecular structures. Despite the favorable effects of high-energy content, these materials often exhibit low reactivity and their ability to improve the performance of practical combustion systems relies strongly on their interaction with the dynamics of the surrounding fluid flow. The intensity of the combustion processes of these materials is dictated, in general, by the melting, evaporation, pyrolysis, mixing, and exothermic reactions processes. Unlike other conventional hydrocarbon fuels, all these processes time scales are often comparable with each other, causing difficulties to devise simpler theoretical models to predict the combustion characteristics. Both the advances made in recent years and the needs for future research and development in the field of energetic fuels are discussed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):187-192. doi:10.1115/1.2793861.

The United States generates the largest amount of solid waste per person in the world. The old practice of direct landfilling and storage is receiving greater public resistance and is attributing to the search for alternative disposal methods. The evergrowing problem of solid wastes requires environmentally benign and good public acceptance for the safe and ultimate disposal of the various kinds of solid wastes. Incineration and various kinds of mass burn-type systems have been used to reduce the volume and mass of the wastes, which can be characterized by their operational temperature. In all types of incineration systems, different kinds of gas clean-up devices are used to meet the local, state, and federal regulations for the gases before being released into the environment. A major concern over these systems have been in the by-products produced from these systems during their normal design and off-design point of operation. Indeed, the by-products generated from some incineration systems, under certain operational conditions, can be a health hazard and the solid residue may be leachable. Recent trends in advanced thermal destruction systems are described which can destroy the solid waste to the molecular level. Advanced systems can be designed to meet almost any emission standards. The use of oxygen-enriched air in place of air for the combustion of gases released from the solid waste reduces the amount of effluent gas, and, hence, the reduced size and cost of the gas clean-up system. The use of an excess enthalpy system offers attractive benefits in which the energy released from the waste is recycled back into the system under controlled conditions with the final desired objectives of reduced emissions, higher efficiency, and lower costs. Thermal destruction of solid wastes using advanced techniques makes good technical, environmental, economical, and human health and safety. The issues concerning recyclability, life cycle integration, and health effects from incineration are only expected to grow in the future.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):193-200. doi:10.1115/1.2793862.

Since recent reviews cover the issues in NOx formation under gas-turbine canditions, and since regulations essentially dictate use of the premixed mode of combustion for minimum NOx , this review concentrates on phenomena that can arise in premixed combustion. Specifically, 1) the initial unmixedness in a fuel-air premixer has been shown to make overall lean mixtures autoignite sooner than might be expected based on the overall fuel-air ratio, because the richer portions of the mixture lead the process;2) combustion pressure oscillations caused by the interplay between acoustic waves and unsteady heat release in a one-dimensional system can be calculated in good accordance with measured data, and set the stage for multi-dimensional CFD;3) carbon deposition arising from the flow of liquid fuel over metal surfaces such as found in fuel injectors and swirl cups has been described as a function of temperature and of surface composition; and 4) quenching and subsequent emissions of carbon monoxide can be minimized by preservation of a boundary-layer rather than an impingement type of flow over combustor liners.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):201-208. doi:10.1115/1.2793863.

Computer modeling of low-emissions gas-turbine combustors requires inclusion of finite-rate chemistry and its intractions with turbulence. The purpose of this review is to outline some recent developments in and applications of the physical models of combusting flows. The models reviewed included the sophisticated and computationally intensive velocity-composition pdf transport method, with applications shown for both a laboratory flame and for a practical gas-turbine combustor, as well as a new and computationally fast PSR-microstructure-based method, with applications shown for both premixed and nonpremixed flames. Calculations are compared with laserbased spectroscopic data where available. The review concentrates on natural-gas-fueled machines, and liquid-fueled machines operating at high power, such that spray vaporization effects can be neglected. Radiation and heat transfer is also outside the scope of this review.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):209-213. doi:10.1115/1.2793864.

The need for alternative fuels to replace liquid petroleum-based fuels has been accelerated in recent years by environmental concerns, concerns of shortage of imported liquid hydrocarbon, and congressional prompting. The fact is accepted that natural gas is the cheapest, most domestically abundant, and cleanest burning of fossil fuels. However, socio-economical and technical handicaps associated with the safety and efficiency of on-board fuel storage inhibit its practical use in vehicles as an alternative fuel. A concept is presented for safely storing fuel at low pressures in the form of hydrates in natural gas vehicles. Experimental results lead to gas storage capacities of 143 to 159 volumes/volume. Vehicle travel range could be up to 204 mi. Controlled decomposition rate of hydrates is possible for feeding an automotive vehicle. Upon sudden pressure decrease in the event of a vehicle accident, the rate of release of hydrocarbons from the hydrates at constant temperature is 2.63 to 12.50 percent per min, slow enough to prevent an explosion or a fireball. A model is given for predicting the rates of gas release from hydrates in a vehicle wreck. A storage tank design is proposed and a process is suggested for forming and decomposing hydrates on-board vehicles. A consistent fuel composition is obtained with hydrates.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):214-220. doi:10.1115/1.2793865.

The local instantaneous temperature, heat transfer coefficient, and pressure data, gathered around a horizontal tube in a fluidized bed, have been analyzed using the deterministic chaos theory. A stainless steel heat transfer tube, carrying a hot water flow, was placed in a cold bubbling fluidized bed. The tube was instrumented in the circumferential direction with five fast-responding surface thermocouples and a vertical pressure differential sensor. The local temperature and pressure data were measured simultaneously at a frequency of 120 Hz. Additionally, the local instantaneous heat transfer coefficient was evaluated by solving the transient two-dimensional heat conduction equation across the tube wall numerically. The mutual information function (MIF) has been applied to the signals to observe the relationship between points separated in time. MIF was also used to provide the most appropriate time delay constant τ to reconstruct an m-dimensional phase portrait of the one-dimensional time series. The distinct variation of MIF around the tube indicates the variations of solid-surface contact in the circumferential direction. The correlation coefficient was evaluated to calculate the correlation exponentv, which is closely related to the fractal dimension. The correlation exponent is a measure of the strange attractor. The minimum embedding dimension as well as the degrees of freedom of the system were evaluated via the correlation coefficient. Kolmogorov entropies of the signals were approximated by using the correlation coefficient. Kolmogorov entropy considers the inherent multi-dimensional nature of chaotic data. A positive estimation of Kolmogorov entropy is an indication of the chaotic nature of the signal. The Kolmogorov entropies of the temperature data around the tube were found to be between 10 bits/s and 24 bits/s. A comparison between the signals has shown that the local instantaneous heat transfer coefficient exhibits a higher degree of chaos than the local temperature and pressure signals.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):221-228. doi:10.1115/1.2793866.

The effects of moisture on the performance of thermoelectric air conditioning systems and heat pumps equipped with a heat exchanger were studied. Coefficients of performance and fluid temperature variations were calculated for heat capacity ratios from 1 to 10 and relative humidities ranging from 0 to 100 percent at the cold fluid inlet. Only the energy effects of the water condensation are considered as it is assumed that the heat transfer coefficients are those of a dry heat exchanger. It was found that different flow arrangements and the energy associated with condensation on the cold fluid side have no strong effects on the variation of the hot fluid temperature. The coefficient of performance decreases and the cold fluid exit temperature increases when condensation occurs. When the moisture content at the cold fluid inlet increases most of the cases studied show a decrease in the difference between the optimum and uniform current results. The difference among different flow arrangements also becomes smaller as more water vapor condenses in the cold flow.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):229-236. doi:10.1115/1.2793867.

Adsorption processes can be used for operating environment-friendly refrigeration cycles. When combined with the thermal regeneration process, these cycles can have quite high performance. The second law analysis of the adsorption cycles with thermal regeneration is fully developed. The different heat transports between heat transfer fluid and adsorbent, between adsorbate and condenser/evaporator heat sources, and between heat transfer fluid and heat sources are analyzed. The entropy balance is then completely established. Consistency between the first law and second law analysis is verified by the numerical values of the entropy productions. The optimal Operation of an adsorber is then described, and the study of those optimal conditions lead to some correlation between the different internal entropy productions.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):237-241. doi:10.1115/1.2793868.

An investigation of a ground thermal energy storage system, which includes storage units containing phase-change materials (PCM), is presented. This study is related to a large-diameter helical heat exchanger, which is placed vertically in the ground. The PCM storage units under consideration have a cylindrical shell shape and are located inside and/or outside the helix. A modified numerical scheme for the Solution of heat transfer in the ground, in the PCM units, and within the heat exchanger pipe, is presented. The theoretical results show that the thermal diffusivity of the PCM dominates the thermal performance of the system. Incorporation of PCM storage units containing paraffin wax results in a reduction of the thermal efficiency in comparison with a system not containing these units. However, incorporation of PCM having the same thermal diffusivity as of the soil results in a significant improvement of the thermal performance.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1996;118(3):242-248. doi:10.1115/1.2793869.

This paper presents an exergetic analysis of the charge process in energy storage using multiple phase-change materials (PCMs). Thermal storage using two, three, five, as well as an infinite number of PCMs is analyzed with a distributed model for the heat transfer fluid. Analytical results show the relative merits of using multiple PCMs compared with a single PCM for thermal energy storage. Sample results are presented and discussed.

Commentary by Dr. Valentin Fuster

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