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IN THIS ISSUE

### RESEARCH PAPERS

J. Energy Resour. Technol. 2007;129(3):169-176. doi:10.1115/1.2748810.

Impellers with splitter blades have been used in turbomachinery design for both pumps and compressors. Increasing the number of blades increases the head of the pump, however, it causes a decrease in efficiency due to the blockage effect of the blade thickness and friction. The impellers with splitter blades between two long blades can be used to alleviate the serious clogging at the inlet of the impeller caused by more blades. In this study, impellers having a different number of blades ($z=3$, 4, 5, 6, and 7) with and without splitter blades (25, 35, 50, 60, and 80% of the main blade length) were tested in a deep well pump. The effects of the main blade number and lengths of splitter blades on the pump performance have been investigated. While the number of main blades and the lengths of the splitter blades of a principal impeller were changed, the other parameters such as pump casing, blade inlet and outlet angles, blade thickness, impeller inlet and outlet diameters, were kept the same.

Topics: Impellers , Blades , Pumps
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(3):177-189. doi:10.1115/1.2748811.

The development of the new API RP2A (22nd edition) parametric static strength prediction equations for planar circular hollow section tubular joints is described. Prediction equations are presented for brace axial, brace in-plane bending, and brace out-of-plane bending loads. The prediction equations are based on screened test databases, augmented, and extended by an extensive new series of validated nonlinear finite element simulations for nonoverlapping K joints, double tee (DT/X) joints, and T joints. The increased reliability (reduced scatter) provided by the new static strength formulation was used to justify a reduction of the load factor of safety to 1.6 from the previous value of 1.7.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(3):190-199. doi:10.1115/1.2748812.

The power generation efficiency and power cost of an independent microgrid that distributes the power from a small diesel engine power generator was investigated using numerical analysis. The fuel consumption of a small diesel engine and the relation between power generation and heat power were obtained in experiments using a prototype. The independent microgrid built using one to six sets of 20 average houses in Sapporo and the distributed engine generators were examined using these test results. However, the operation of a diesel engine power generator controls the number of operations according to the magnitude of the power load of the microgrid. When a diesel engine power generator is distributed, since the power generation capacity per set decreases compared with the central system, the load factor of each engine generator rises. As a result, the operation of an engine at partial load with low efficiency can be reduced. When the number of distributions of the engine generator increases as a result of numerical analysis, the cost of the fuel decreases. However, when the rise in facility cost is taken into consideration, the number of engine generators for distribution is in fact 3 or 4.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(3):200-213. doi:10.1115/1.2748813.

In this paper, a different control strategy of a small wind generator, including a permanent magnet synchronous machine (PMSM), has been studied. The objective is to analyze the produced power quality of small wind turbines connected to weak AC grids. The extraction of the electrical energy from the wind turbine is based on a maximum power point tracking (MPPT) algorithm to control a pulse width modulation (PWM) rectifier. The grid connection is realized by means of a PWM voltage source inverter via a filter. This inverter is controlled by three different methods based on current or active and reactive power control. These methods are applicable for different power ranges. The obtained results demonstrate the efficiency of the system and the energetic contribution, mainly for rural weak AC grids.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(3):214-222. doi:10.1115/1.2748814.

In direct sensible thermal storage systems, both the energy discharging and charging processes are inherently time-dependent as well as rate-dependent. Simplified models which depict the characteristics of this transient process are therefore crucial to the sizing and rating of the storage devices. In this paper, existing models which represent three distinct classes of models for thermal storage behavior are recast into a common formulation and used to predict the variations of discharge volume fraction, thermal mixing factor, and entropy generation. For each of the models considered, the parametric dependence of key performance measures is shown to be expressible in terms of a Peclet number and a Froude number or temperature difference ratio. The thermal mixing factor for each of the models is reasonably well described by a power law fit with $Fr2Pe$ for the convection-dominated portion of the operating range. For the uniform and nonuniform diffusivity models examined, there is shown to be a Peclet number which maximizes the discharge volume fraction. In addition, the cumulative entropy generation from the simplified models is compared with the ideally-stratified and the fully-mixed limits. Of the models considered, only the nonuniform diffusivity model exhibits an optimal Peclet number at which the cumulative entropy generation is minimized. For each of the other models examined, the cumulative entropy generation varies monotonically with Peclet number.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(3):223-231. doi:10.1115/1.2751504.

Energy recovery is gaining importance in various transportation and industrial process applications because of rising energy costs and geopolitical uncertainties impacting basic energy supplies. Various advanced thermoelectric (TE) materials have properties that are inherently advantageous for particular TE energy recovery applications. Skutterudites, zero- and one-dimensional quantum-well materials, and thin-film superlattice materials are providing enhanced opportunities for advanced TE energy recovery in transportation and industrial processes. This work demonstrates (1) the potential for advanced thermoelectric systems in vehicle energy recovery and (2) the inherently complex interaction between thermal system performance and thermoelectric device optimization in energy recovery. Potential power generation at specific exhaust temperature levels and for various heat exchanger performance levels is presented showing the current design sensitivities using different TE material sets. Mathematical relationships inherently linking optimum TE design variables and the thermal systems design (i.e., heat exchangers and required mass flow rates) are also investigated and characterized.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(3):232-242. doi:10.1115/1.2751505.

Computational simulations of flow and heat transfer in heat recovery steam generators (HRSGs) of vertical- and horizontal-tube designs are reported. The main objective of the work was to obtain simple modifications of their internal configuration that render the flow of combustion gas more spatially uniform. The computational method was validated by comparing some of the simulation results for a scaled-down laboratory model with experimental measurements in the same. Simulations were then carried out for two plant HRSGs—without and with the proposed modifications. The results show significantly more uniform combustion gas flow in the modified configurations. Heat transfer calculations were performed for one superheater section of the vertical-tube HRSG to determine the effect of the configuration modification on heat transfer from the combustion gas to the steam flowing in the superheater tubes.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(3):243-247. doi:10.1115/1.2748815.

The possibility of exploiting low-temperature heat sources has been of great significance with ever increasing energy demand. Optimum and cost-effective design of the power cycles provide a means of utilization of low-temperature heat sources which might otherwise be discarded. In this analysis, the performance of the Kalina cycle system 11 (KCS11) is examined for low-temperature geothermal heat sources and is compared with an organic Rankine cycle. The effect of the ammonia fraction and turbine inlet pressure on the cycle performance is investigated in detail. Results show that for a given turbine inlet pressure, an optimum ammonia fraction can be found that yields the maximum cycle efficiency. Further, the maximum cycle efficiency does not necessarily yield the optimum operating conditions for the system. In addition, it is important to consider the utilization of the various circulating media (i.e., working fluid, cooling water, and heat resource) and heat exchanger area per unit power produced. For given conditions, an optimum range of operating pressure and ammonia fraction can be identified that result in optimum cycle performance. In general, the KCS11 has better overall performance at moderate pressures than that of the organic Rankine cycle.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(3):248-253. doi:10.1115/1.2748816.

Sugar cane bagasse, a biomass material that is readily available, has been used as a fuel for well over 4 decades. However, combustion of bagasse has its own special set of problems which appear to be due largely to the high moisture content of the fuel. In this present research work, in order to gain insight into the effect of moisture on the flame front, an experimental program is carried out on an operating, industrial size bagasse-fired furnace. The furnace is modeled by the three-dimensional CFD package FLUENT . The results of modeling show a considerable delay to ignition due to the drying of fuel. The effect of fuel moisture on drying and heating up of the fuel is the key feature for the investigation done in this work.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(3):254-265. doi:10.1115/1.2751506.

Cogeneration can improve energy utilization efficiency significantly. In this paper, a new ammonia-water system is proposed for the cogeneration of refrigeration and power. The plant operates in a parallel combined cycle mode with an ammonia-water Rankine cycle and an ammonia refrigeration cycle, interconnected by absorption, separation, and heat transfer processes. The performance was evaluated by both energy and exergy efficiencies, with the latter providing good guidance for system improvement. The influences of the key parameters, which include the basic working solution concentration, the cooling water temperature, and the Rankine cycle turbine inlet parameters on the cycle performance, have been investigated. It is found that the cycle has a good thermal performance, with energy and exergy efficiencies of 27.7% and 55.7%, respectively, for the base-case studied (having a maximum cycle temperature of $450°C$). Comparison with the conventional separate generation of power and refrigeration having the same outputs shows that the energy consumption of the cogeneration cycle is markedly lower. A brief review of desirable properties of fluid pairs for such cogeneration cycles was made, and detailed studies for finding new fluid pairs and the impact of their properties on cogeneration system performance are absent and are very recommended.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2006;129(3):266-277. doi:10.1115/1.2751507.

An optimization for the geometrical parameters of continuous fins on an array of tubes of a refrigeration evaporator is developed in this paper using the exergy method. The method is based on exergy, economic analysis, and optimization theory. As there are humid air and refrigerant single- and two-phase streams involved in the heat transfer process, then there are irreversibilities or exergy destruction, due to pressure losses $İΔP$, due to temperature difference $İΔT$ and due to specific humidity gradient $İΔω$. These principal components of total irreversibility are not independent, and their relative contribution to total irreversibility of a cross-flow refrigeration evaporator is investigated. A change in geometry was obtained by varying the evaporator tube diameter for a selected evaporator capacity, and hence the evaporator tube length and total heat transfer area are calculated for a fixed evaporator face length. In this way, the effect of changes in the geometry on the total number of exergy destruction units of the heat exchange process is investigated. The optimum balance between the three components of irreversibility ($İΔP,İΔT$, and $İΔω$) is also determined, thereby giving the optimum solution for the heat exchanger area. The total cost function, which provides a measure of the contribution of the evaporator to the total cost of the refrigeration system, is expressed on the basis of annual capital and electrical energy costs. The total cost function is minimized with respect to the total heat transfer area and the total number of exergy destruction units $(NI)$. The relationship between the operational variables, heat transfer area, refrigerant and air irreversibilities, and the total annual cost for this type of evaporator are developed, presented, and discussed. The pressure, temperature, and specific humidity irreversibilities are found to be 30.34%, 33.78%, and 35.88%, respectively, of the total irreversibility, which is 8.5% of the evaporator capacity.

Commentary by Dr. Valentin Fuster

### DISCUSSION

J. Energy Resour. Technol. 2007;129(3):278. doi:10.1115/1.2751508.
FREE TO VIEW

I was interested in the above paper since the engine involved incorporates the same spherical rocker arm seals as were used on the valve mechanism of a solar thermal water pump which I built and tested in the 1980s (Solar Energy, 31 (5), pp. 523–525). This form of transverse rocker beam drive and seal design was suggested to me by Richard Kinnersly, the designer of the Stirling engine, which is the subject of Dr. Mahkamov’s paper.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(3):279. doi:10.1115/1.2751509.
FREE TO VIEW

As the designer of the engine featured in Dr. Mahkamov’s paper, I welcome his pioneering application of CFD modeling to the Stirling cycle. The technique would have been helpful in 1996 when this engine was on the drawing board and then during testing in 2003, when absence of dynamic pressure measuring equipment hindered diagnosis of the problem he subsequently identified by modeling—flow restriction caused by a dimensional error in the crown of the power piston.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(3):280. doi:10.1115/1.2751510.
FREE TO VIEW

Some very impressive modeling work lies behind this paper, but leaves the reader with many questions which Dr. Mahkamov may care to answer.

Commentary by Dr. Valentin Fuster