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System Coefficient (system + coefficient)
Selected AbstractsThe performance analysis of a two-stage transcritical CO2 cooling cycle with various gas cooler pressuresINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 14 2008Arif Emre Özgür Abstract A theoretical analysis of a two-stage transcritical CO2 cooling cycle is presented. The effect of a two-stage cycle with intercooling process on the system coefficient of cooling performance is presented for various gas cooler pressures. However, the performance comparison between one-stage and two-stage cycles is presented for same operating conditions. Gas cooler pressure, compressor isentropic efficiency, gas cooler efficiency, intercooling quantity and refrigerant outlet temperature from the gas cooler are used as variable parameters in the analysis. It is concluded that the performance of the two-stage transcritical CO2 cycle is approximately 30% higher than that of the one-stage transcritical CO2 cycle. Hence, the two-stage compression and intercooling processes can be assumed as valuable applications to improve the transcritical CO2 cycle performance. Copyright © 2008 John Wiley & Sons, Ltd. [source] Thermoeconomic optimization for a finned-tube evaporator configuration of a roof-top bus air-conditioning systemINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 4 2008M. Khamis Mansour Abstract This paper presents a methodology of a design optimization technique that can be useful in assessing the best configuration of a finned-tube evaporator, using a thermoeconomic approach. The assessment has been carried out on a direct expansion finned-tube evaporator of a vapor compression cycle for a roof-top bus air-conditioning (AC) system at a specified cooling capacity. The methodology has been conducted by studying the effect of some operational and geometrical design parameters for the evaporator on the entire cycle exergy destruction or irreversibility, AC system coefficient of performance (COP), and total annual cost. The heat exchangers for the bus AC system are featured by a very compact frontal area due to the stringent space limitations and structure standard for the system installation. Therefore, the current study also takes in its account the effect of the variation of the design parameters on the evaporator frontal area. The irreversibility due to heat transfer across the stream-to-stream temperature difference and due to frictional pressure drops is calculated as a function of the design parameters. A cost function is introduced, defined as the sum of two contributions, the investment expense of the evaporator material and the system compressor, and the operational expense of AC system that is usually driven by an auxiliary engine or coupled with the main bus engine. The optimal trade-off between investment and operating cost is, therefore, investigated. A numerical example is discussed, in which a comparison between the commercial evaporator design and optimal design configuration has been presented in terms of the system COP and evaporator material cost. The results show that a significant improvement can be obtained for the optimal evaporator design compared with that of the commercial finned-tube evaporator that is designed based on the conventional values of the design parameters. Copyright © 2007 John Wiley & Sons, Ltd. [source] Investigation of solid,gas reaction heat transformer system with the consideration of multistep reactionsAICHE JOURNAL, Issue 9 2008C. Wang Abstract The performance of solid,gas reaction heat transformer system using MnCl2 and CaCl2 was investigated with the consideration of multistep reactions between CaCl2 and NH3. The reactions between CaCl2 and NH3 could be CaCl2·2/4NH3 and CaCl2·4/8NH3. The simulated results were verified by the experimental data. From the analysis results, it was concluded that the two reactions between CaCl2 and NH3 existed simultaneously. The favored conditions for the simultaneous occurrence of multistep reactions were discussed. The main reaction in the system with the initial state of CaCl2·2NH3 and CaCl2·4NH3 were designated as the reaction of CaCl2·2/4NH3 and CaCl2·4/8NH3, respectively. It was concluded that high driving temperature TM, large relative gas volume, and large specific heat transfer area were the favored conditions for the simultaneous occurrence of multistep reactions in the systems both with the initial state of CaCl2·2NH3 and CaCl2·4NH3. High initial charging pressure P0 was favored for the system with the initial state of CaCl2·2NH3, while low P0 was favored for the system with the initial state of CaCl2·4NH3. The impacts of the simultaneous occurrence of multistep reactions on the system performance indicators [temperature lift, specific power, and system coefficient of performance (COP)] were also investigated in this article. It was concluded that the occurrence of the reaction of CaCl2·4/8NH3 for the initial state of CaCl2·2NH3 led to better system performance, i.e., larger temperature lift, larger specific power, and larger system COP. However, the occurrence of the reaction of CaCl2·2/4NH3 for the initial state of CaCl2·4NH3 would lead to lower specific power. The temperature lift and system COP were larger for the initial state of CaCl2·4NH3; while the cycle period was shorter for the initial state of CaCl2·2NH3. © 2008 American Institute of Chemical Engineers AIChE J, 2008 [source] Monomeric and polymeric anionic gemini surfactants and mixed surfactant systems in micellar electrokinetic chromatography.ELECTROPHORESIS, Issue 2 2005Part II: Characterization of chemical selectivity using two linear solvation energy relationship models Abstract Sodium di(undecenyl) tartarate monomer (SDUT), a vesicle-forming amphiphilic compound possessing two hydrophilic carboxylate headgroups and two hydrophobic undecenyl chains, was prepared and polymerized to form a polymeric vesicle (i.e., poly-SDUT). The anionic surfactants of SDUT and poly-SDUT (carboxylate head group) and sodium dodecyl sulfate, SDS (sulfate head groups) as well as mixed surfactant systems (SDS/SDUT, SDS/poly-SDUT, and SDUT/poly-SDUT) were applied as pseudostationary phases in micellar electrokinetic chromatography (MEKC). Two linear solvation energy relationship (LSER) models, i.e., solvatochromic and solvation parameter models, were successfully applied to investigate the effect of the type and composition of pseudostationary phases on the retention mechanism and selectivity in MEKC. The solvatochromic and solvation parameter models were used to help understand the fundamental nature of the solute-pseudostationary phase interactions and to characterize the properties of the pseudostationary phases (e.g., solute size and hydrogen bond-accepting ability for all pseudostationary phases). The solute types were found to have a significant effect on the LSER system coefficients and on the predicted retention factors. Although both LSER models provide the same information, the solvation parameter model is found to provide much better results both statistically and chemically than the solvatochromic model. [source] A diffusion approach to the electrolyte leakage from plant tissuesPHYSIOLOGIA PLANTARUM, Issue 1 2005Konstantina V. Kocheva The exchange of ions between plant tissues and the external solution in vitro exhibits prominent biphasic kinetics. This is generally ascribed to the different contribution of the two compartments , apoplast and symplast , involved in the process. In this regard, an electro-diffusion model of the leakage is proposed in the paper. On the basis of the balance of fluxes through the plasmalemma and the cell wall, a system of differential equations describing the ion concentration in the outer solution is found. For a wide range of the system's coefficients, its behaviour is well approximated by a previously obtained analytical function. The values of the function's parameters, derived from the fit with experimental data, correlate adequately with the water deficit conditions of the samples. Hence, these parameters may be used to characterize the physiological status of the investigated plants. [source] | ||||||||||