J. Energy Resour. Technol. 1998;120(2):97-101. doi:10.1115/1.2795032.

This paper reports the application of novel, digital image analysis techniques in the study of slug flow characteristics, under dynamic conditions in two-phase gas-liquid mixtures. Water and an oil of viscosity 18 cP were used for the liquid phase and carbon dioxide was used for the gas phase. Flow in a 75-mm i.d., 10-m long acrylic pipeline system was studied. Images of slugs were recorded on video by S-VHS cameras, using an audio-visual mixer. Each image was then digitized frame-by-frame and analyzed on a SGI™ workstation. Detailed slug characteristics, including liquid film heights, slug translational velocity, mixing length, and, slug length, were obtained.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):102-105. doi:10.1115/1.2795018.

A definition is given for a Froude number in the liquid film ahead of the slug and it is seen that slug characteristics are strongly influenced by the Froude number. The mechanisms in the mixing zone of the slug are described in detail and are shown to be a function of the film Froude number. It is shown that the Hubbard and Dukler model for mixing length is inadequate. A new expression is proposed for the slug mixing length as a function of the film Froude number.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):106-110. doi:10.1115/1.2795019.

The design of wet-gas pipelines and slug catchers requires multiphase flow simulations, both steady-state and transient. However, steady-state simulation is often inadequately conducted and its potential not fully utilized. This paper shows how mechanistic steady-state simulation models can be used to obtain not only pressure drop, liquid holdup and flow regime, but also to extract important operational information such as pig transit time, pig exit speed, liquid buildup rate behind the pig, and the time for the pipeline to return to a steady-state after pigging. A well-designed set of steady-state simulations helps to determine pipeline size, slug catcher size, and pigging frequency. It also serves as a starting point for subsequent transient multiphase flow simulations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):111-117. doi:10.1115/1.2795020.

This paper presents the results of a theoretical study, supported with the finite element analysis, into potential loss of external integrity around a casing shoe resulting from leak-off testing (LOT) in upper marine sediments (UMS). Three types of possible failures from LOTs were considered: vertical fracture, horizontal fracture, and a channel outside cemented annulus. It is proved in the paper that vertical fracture is the most unlikely failure of the three. The other two types of failure can be distinguished by different values of propagation pressures. Although horizontal fractures are initiated at low pressure in the plastic zone around the wellbore, they cannot propagate beyond the plastic zone until wellbore pressures exceed overburden pressures. Annular channels, on the other hand, may propagate upwards at pressures lower than overburden pressure. The paper shows that these channels are initiated at pressures equal to the contact stress between cement and rock and their propagation pressures are on average 3.5-fold greater than contact stress. It is also explained how to identify the UMS with high risk of annular channeling during LOTs.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):118-123. doi:10.1115/1.2795021.

In the mathematical modeling of bit penetration rate for tri-cone roller bits in permeable formations, virtually all of the current techniques assume that the differential pressure between the bottom-hole wellbore pressure and the formation is a “static” value. This work shows that the appropriate differential pressure is a dynamic quantity, because for overbalanced drilling, fluid filtrate from the wellbore requires a finite time to flow into the formation, producing a changing pressure gradient ahead of the bit. Moreover, this dynamic gradient is directly dependent upon the rate of drill bit penetration, which is in turn dependent upon the dynamic gradient itself. Accordingly, coupled penetration rate and dynamic gradient equations must be solved, which frequently result in the prediction of higher drilling penetration rates than when the static gradient is used. The appropriate dynamic differential pressure equations are developed and applied to an example drilling situation. It is shown that with water-based drilling fluids, for rock with permeability greater than a few microdarcies at virtually all penetration rates, and for penetration rates less than 3 m/h (9.84 ft/h) at permeabilities greater than 1 μd (microdarcy), the dynamic differential pressure is significantly less than the static differential pressure. Accordingly, using the conventional static differential pressure results in the prediction of penetration rates that are much too low. Moreover, using measured penetration rates from the field, the conventional approach yields predicted in-situ rock strength that is much too high.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):124-130. doi:10.1115/1.2795022.

This paper presents a historical view and present status of retractable bit design and drilling results. Despite the significant improvements in cone-sealed bearing rock bits, the special features of retractable bits provides a wider range of field applications. Scientific drilling, both continental and offshore and stratigraphic offshore boreholes, is an area for profitable application of retractable bits with a downhole motor. Another promising application is drilling and simultaneous casing where some prototypes have been successfully tested. Other applications such as horizontal and geothermal drilling look promising for future developments.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):131-136. doi:10.1115/1.2795023.

Vehicle heating requires a substantial amount of energy. Electric vehicles (EVs) generate little waste heat, and using battery energy for heating may consume a substantial fraction of the energy storage capacity, reducing the vehicle range, which is one of the most important parameters in determining EV acceptability. This paper analyzes the applicability of a desiccant dehumidifier for providing comfort heating in an EV with reduced energy consumption. A dehumidifier can reduce the heating load in a vehicle because it adsorbs the water vapor generated by the passengers, thereby avoiding water condensation on the windows without requiring a high external air ventilation rate or a high window temperature. The results indicate that the desiccant dehumidifier can reduce the steady-state heating load by 60 percent or more under typical conditions. The reduction in heating load is such that waste heat may be enough to provide the required heating under most ambient conditions. Desiccant system dimensions and weight appear reasonable for packaging inside an EV.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):137-142. doi:10.1115/1.2795024.

This paper studies the application of insulated pressure vessels for hydrogen-fueled light-duty vehicles. Insulated pressure vessels are cryogenic-capable pressure vessels that can be fueled with liquid hydrogen (LH2 ); low-temperature (46 K) compressed hydrogen (CH2 ); or ambient-temperature CH2 . In this analysis, hydrogen temperatures, pressures, and venting losses are calculated for insulated pressure vessels fueled with LH2 or with low-temperature CH2 , and the results are compared to those obtained in low-pressure LH2 tanks. Hydrogen losses are calculated as a function of daily driving distance during normal operation; as a function of time during long periods of vehicle inactivity; and as a function of initial vessel temperature during fueling. The results show that insulated pressure vessels have packaging characteristics comparable or better than those of conventional, low-pressure LH2 tanks, with greatly improved dormancy and much lower boil-off, and therefore appear to be a good alternative for vehicular hydrogen storage.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):143-148. doi:10.1115/1.2795025.

A steady-flow approach for finite-time thermodynamics is used to calculate the maximum thermal efficiency, its corresponding power output, adiabatic temperature ratio, and thermal-conductance ratio of heat transfer equipment of a closed Brayton heat engine. The physical model considers three types of irreversibilities: finite thermal conductance between the working fluid and the reservoirs, heat leaks between the reservoirs, and internal irreversibility inside the closed Brayton heat engine. The effects of heat leaks, hot-cold reservoir temperature ratios, turbine and compressor isentropic efficiencies, and total conductances of heat exchangers on the maximum thermal efficiency and its corresponding parameters are studied. The optimum conductance ratio could be found to effectively use the heat transfer equipment, and this ratio is increased as the component efficiencies and total conductances of heat exchangers are increased, and always less than or equal to 0.5.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):149-153. doi:10.1115/1.2795026.

In multicomponent gaseous flows with chemical reactions, diffusion is usually considered to be a single-source entropy generator. Introducing the concept of dispersion, where the atom balance is dominant rather than the molecular, shows in an ideal setting that, even when there is diffusion, the dispersion part of entropy generation can be reduced to zero by proper choice of species flow velocities. Fuel cells and methane reformers are employed as examples to illustrate these concepts.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):154-160. doi:10.1115/1.2795027.

Cannot analyzed an engine operating between two reservoirs. Through a peculiar mode of reasoning, he found the correct optimum shaft work done during a cyclic change of state of the engine. Clausius justified Carnot’s result by enunciating two laws of thermodynamics, and introducing the concept of entropy as a ratio of heat and temperature of a thermodynamic equilibrium state. In this paper, we accomplish five purposes: (i) We consider a Carnot engine. By appropriate algebraic manipulations we express Carnot’s optimum shaft work in terms of available energies or exergies of the end states of one reservoir with respect to the other, and Clausius’ entropy S in terms of the energies and available energies of the same and states. (ii) We consider the optimum shaft work done during a cyclic change of state of an engine operating between a reservoir, and a system with fixed amounts of constituents and fixed volume, but variable temperature. We express the optimum shaft work in terms of the available energies of the end states of the system, and Clausius’ entropy in terms of the energies and available energies of the same end states. Formally, the entropy expression is identical to that found for the Carnot engine, except that here the change of state of the system is not isothermal. (iii) We consider the optimum shaft work done during a cyclic change of state of a general engine operating between a reservoir R and system A which initially is in any state A1 , stable or thermodynamic equilibrium or not stable equilibrium. In state A1 , the values of the amounts of constituents are n 1 , and the value of the volume is V 1 whereas, in the final state A 0 , n 0 ≠ n 1 and V0 ≠ V1 Using the laws of thermodynamics presented by Gyftopoulos and Beretta, we prove that such an optimum exists, call it generalized available energy with respect to R, and use it together with the energy to define a new property Σ1 We note that the expression for Σ is formally identical to and satisfies the same criteria as Clausius’ entropy S . The only difference is that Σ applies to all states, whereas Clausius’ S applies only to stable equilibrium states. So we call Σ entropy and denote it by S (iv) We use the unified quantum theory of mechanics and thermodynamics developed by Hatsopoulos and Gyftopoulos, and find a quantum theoretic expression for S in terms of the density operator ρ that yields all the probabilities associated with measurement results. (v) We note that the quantumtheoritic expression for S can be interpreted as a measure of the shape of an atom, molecule, or other system because ρ can be though of as such a shape, and provide pictorial illustrations of this interpretation. For given values of energy E , amounts of constituents n , and volume V , the value of the measure is zero for all shapes that correspond to projectors (wave functions), positive for density operators that are not projectors, and the largest for the ρ that corresponds to the unique stable equilibrium state determined by the given E , n , and V . Accordingly, spontaneous entropy generation occurs as a system adapts its shape to conform to the internal and external forces. Beginning with an arbitrary initial ρ this adaptation continues only until no further spontaneous change of shape can occur, that is, only until a stable equilibrium state is reached.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):161-166. doi:10.1115/1.2795028.

An experimental study conducted to determine the effects of lifting the flame base off the burner rim on the differences between the flame characteristics of diffusion flames from circular and elliptic burners is presented. The in-flame profiles of temperature, concentrations of fuel and combustion product species, and the mean and fluctuating components of axial velocity are presented. This study has shown that the effects of burner geometry in turbulent lifted flames are considerable only in the near-burner region. In the midflame and far-burner regions, the effects traceable to burner geometry are much weaker, contrary to those observed in the attached flame configuration. The observations are attributed to the turbulence and additional air entrainment into the jet prior to the flame base accompanying the lift-off process, which mitigate the effects of burner geometry.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):167-171. doi:10.1115/1.2795029.

The blowout limits of a co-flowing turbulent methane jet diffusion flame with addition of diluent in either jet fuel or surrounding air stream is studied both analytically and experimentally. Helium, nitrogen, and carbon dioxide were employed as the diluents. Experiments indicated that an addition of diluents to the jet fuel or surrounding air stream decreased the stability limit of the jet diffusion flames. The strongest effect was observed with carbon dioxide as the diluent followed by nitrogen and then by helium. A model of extinction based on recognized criterion of the mixing time scale to characteristic combustion time scale ratio using experimentally derived correlations is proposed. It is capable of predicting the large reduction of the jet blowout velocity due to a relatively small increase in the co-flow stream velocity along with an increase in the concentration of diluent in either the jet fuel or surrounding air stream. Experiments were carried out to validate the model. The predicted blowout velocities of turbulent jet diffusion flames obtained using this model are in good agreement with the corresponding experimental data.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):172-178. doi:10.1115/1.2795030.

This paper investigates the effect of heat exchanger allocation on overall system performance using both reverse Carnot and vapor compression refrigeration cycle models to calculate system performance and entropy generation rate. The algebraically simple constraints applied in previous studies are shown to be justifiable. The vapor compression model considers nonideal compressor performance, compressor volumetric efficiency, refrigerant properties, and throttling, in addition to mechanistic heat exchanger models. The results support the conclusions of previous studies in that maximum performance is observed when the condenser and evaporator thermal sizes are approximately equal. For air-to-air systems, this result indicates that the areas of the heat exchangers should be approximately equal. However, it is found that minimizing the entropy generation rate does not always result in the same design as maximizing the system performance unless the refrigeration capacity is fixed. Minimizing the entropy generation rate per unit capacity is found to always correspond to maximizing the coefficient of performance of refrigeration systems.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1998;120(2):179-184. doi:10.1115/1.2795031.

Geothermal energy has been used for power generation, space and process heating, and to a lesser extent, space cooling. However, it is rarely used for cogeneration. This paper shows how a district heating/cooling system can be incorporated into an existing geothermal power plant to make the best use of extracted hot brine. In the power plant analysis, exergy destruction throughout the plant is quantified and illustrated using an exergy cascade. The primary source of exergy destruction in the plant is determined to be the reinjection of used brine into the ground, which accounts for 48.1 percent of the total exergy destruction. The overall first and the second law efficiencies of the plant are calculated to be 5.6 and 28.3 percent, respectively, based on the exergy of the geothermal fluid at downwell, and 5.7 and 28.6 percent, respectively, based on the exergy of the geothermal fluid at wellhead. A binary system is considered for the heating/cooling district to avoid corrosion and scaling problems. The heating system, as designed, has the capability to meet the entire needs of the Reno Industrial Park under peak load conditions, and has 30 percent reserve for future expansion. An absorption system will be used for the cooling of the intended 40 percent floor space of the industrial park. An economic analysis shows that the incorporation of the district heating/cooling system with 2,785,000 m2 of floor space connected to the geothermal grid appears to be feasible, and financially very attractive. Further, using the returning freshwater from the district heating/cooling system for partial cooling of the binary fluid of the power plant can save up to 15 percent of the fan work.

Commentary by Dr. Valentin Fuster

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