J. Energy Resour. Technol. 2003;125(2):85-93. doi:10.1115/1.1576266.

This paper presents results for the entropy generated internally during the charging and discharging processes of a direct, sensible thermal energy store. The two processes correspond to the inflow of either a low or high temperature liquid stream into an enclosure initially filled with a uniformly high or low temperature liquid, respectively. The level of internal entropy generation due to thermal mixing between the inflow and the initial liquid volume corresponds to losses in the usable fraction of the stored volume and therefore decreased efficiency. Empirically, the observed behavior of direct sensible storage devices spans the range of nearly mixed to highly stratified. In the present work, analytical models for the fully-mixed and ideally-stratified limits are used to bound these behaviors and to analytically determine the corresponding entropy generation levels. The ratio of total entropy generation for the ideally-stratified limit relative to that of the fully-mixed limit is shown to vary as 8/(πPe). The limiting behaviors therefore define a continuum of entropy generation levels separated by up to several orders of magnitude for typical Peclet numbers. A published numerical model which accounts for aspects of the observed thermal mixing is then examined in relation to these limits. The model predicts entropy generation levels midway between the limiting behaviors which suggests significant potential for improvements in the efficiency of direct sensible storage devices.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):94-100. doi:10.1115/1.1538186.

A polymer electrolyte membrane (PEM) fuel cell is analyzed by applying the conservation principle to the electrode backing, catalyst layers and polymer electrolyte. The conservation equations used are the conservation of species, momentum and energy, with the Nernst-Planck equation used for the electrolyte. Oxygen reduction at the cathode is modeled using the Butler-Volmer equation while the adsorption, desorption and electro-oxidation of hydrogen and CO at the anode are modeled by the Tafel-Volmer and “reactant-pair” mechanism, respectively. Temperature variations within the cell are minimized by decreasing current density or increasing temperature. An increase in pressure increases the cell voltage at low current density, but decreases the cell voltage at high current density. The electrochemical kinetics model used for the adsorption, desorption and electro-oxidation of hydrogen and CO is validated with published, experimental data.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):101-112. doi:10.1115/1.1538631.

Experiments are conducted on a heat pump with refrigerants R22, R407C and R407A. The performance is compared in terms of coefficient of performance, heating/cooling capacities, and exergy efficiency of the overall system. The results are analyzed from an exergy point of view to bring out the conditions under which the performance is maximum.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):113-118. doi:10.1115/1.1524330.

Char attrition is the main mechanism for the fine char generation in PFBC. The char attrition rates were significantly different for each of the two coal chars tested. Specific char attrition rate increased with the surface porosity of burning char particles. A coal with higher ratio of telocollinite/inertinite contents formed chars in PFBC with larger pores or higher porosity during devolatilization, which resulted in a higher specific attrition rate.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):119-125. doi:10.1115/1.1575776.

Recent increase in application of horizontal wells and in particular underbalanced drilling, has triggered the necessity of a powerful pneumatic downhole motor. To enhance the technology and make the system effective, a mathematical model is required to identify the opportunities for the modification of power section design. It is well known that the performance of positive displacement motor operating on compressible fluid drops down drastically as compared to the operation under incompressible fluids. The frequent motor replacement during the operation incrementally increases the operating cost despite deriving potential benefits from underbalanced drilling.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):126-130. doi:10.1115/1.1576264.

Pressure drop experiments on natural gas flow in 150 mm pipes at 80 to 120 bar pressure and high Reynolds number were carried out for pipes smooth to rough surfaces. The roughness was measured with an accurate stylus instrument and analyzed using fractal methods. Using a similar approach to that of Nikuradse the measured friction factor was related to the measured roughness values. Taking the value of the relative roughness and dividing it by the slope of the power spectrum of the measured roughness, a greatly improved fit with the measured friction factor was obtained. Indeed, a new friction factor correlation was obtained, but now formulated in terms of direct measurement of roughness.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):131-136. doi:10.1115/1.1576265.

Studies of wavy stratified and stratified/atomization two-phase flow in horizontal pipes are outlined. Notable features of this flow regime include the appearance of disturbance waves, the atomization onset and the drastic change of the gas/liquid interface profile from flat to “concave.” Liquid-to-wall shear stress tends to decrease circumferentially. A computational procedure for predicting main flow characteristics, which takes into account the above results in its design relations, is first assessed with detailed experimental data and is then combined with a CFD code, aiming at enhancing the predictive capability.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):137-144. doi:10.1115/1.1521710.

The mechanistic model of Taitel and Dukler [1] has previously been reported to exhibit a region in which three solutions exist for the height of the gas-liquid interface in the stratified/wave flow regime. Most laboratory studies have not dealt with flow rates in the ranges where the multiple solutions occur. An experimental study has been carried out at the University of Calgary to obtain such data. The object of this work is to review the relevant theory, investigate situations where theory suggests that multiple solutions should exist, and attempt to show whether or not such solutions do physically exist.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):145-153. doi:10.1115/1.1521711.

Current design and performance of the GLCC© 1 separator is dependent on the prediction of the upstream inlet flow conditions based on available models. It is expected that early detection of terrain slugging (slug length, slug velocity and holdup) and controlling the liquid level in the GLCC using feed forward mechanism can improve the operational range of the GLCC, by decreasing the gas carry under and liquid carry over, and thereby decreasing the control valve dynamics. The conventional feedback control loops can seldom achieve perfect control considering the impact of huge slugs that are keeping the output of the process continuously away from desired set point value. The reason is simple: a feedback controller reacts only after it has detected a deviation in the value of the level from the set point. Unlike the feedback systems, a feed forward control configuration measures the disturbance directly and takes control action to negate the effect of the disturbance on liquid level in the GLCC. Therefore, a feed forward control system has the theoretical potential for perfect control if the slug detection and characterization are perfect. A strategy for GLCC predictive control has been proposed which integrates the feedback and feed forward loops to compensate for error due to modeling and slug characterization. A model has been developed for predictive control system design and simulated in MATLAB-Simulink® . Experimental results obtained demonstrate that the proposed strategy is a viable approach for GLCC predictive control.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2003;125(2):154-159. doi:10.1115/1.1525002.

This work explores the possibility of utilizing the interaction between a pressure pulse propagating in a pipe with the blockages therein, as a means of blockage detection and characterization. Whereas an earlier work focused on a single blockage, the present work attempts to extend the strategy to multiple blockages. A one-dimensional isothermal non-compositional single-phase Eulerian model was used to describe the propagation of a pressure pulse through a pipe with multiple blockages. Pressure variations at the inlet caused by reflections of the propagating transient are monitored and analyzed. This analysis is used to make deductions about the internal configuration of the pipe. The results demonstrate that the technique is feasible and that accurate characterization of multiple blockages is possible.

Commentary by Dr. Valentin Fuster

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