•A new radial vaneless diffuser design was proposed, numerically investigated and optimized.•Vaneless radial diffuser performance was enhanced by controlling its diffusion rate.•The optimized design ...showed up to 10% reduction of the diffuser loss coefficient.•The pressure coefficient of the optimized radial diffuser geometry was improved by up to 6.6%.
Centrifugal compressors are used in many heat pumps and refrigeration systems. Radial vaneless diffuser is a principal component in these compressors. Therefore, the present research aims at improving the centrifugal compressor performance by optimizing the design of the radial vaneless diffuser. Two radial vaneless diffuser geometries were proposed, investigated and numerically optimized. The optimization aimed at minimizing the diffuser loss coefficient and maximizing the pressure coefficient. Simulations were performed by solving the Reynolds averaged Navier–Stokes equation under 2D axisymmetric condition. A genetic optimization algorithm was implemented in order to conclude the optimum diffuser geometry. 2D axisymmetric simulations with air and R134a as working fluids showed that the optimized geometry reduced the diffuser loss coefficient by up to 10% and increased the pressure coefficient by up to 3.8%. Additional 3D simulations with an impeller located before the diffuser were performed. These 3D simulations showed that the optimized diffuser geometry reduced the diffuser loss coefficient by up to 4.7% and increased the pressure coefficient by up to 6.6% under jet-wake and swirl flow conditions.
Nano technology is developed in this decade and changes the way of life. Moreover, developing nano technology has effect on the performance of the materials and consequently improves the efficiency ...and robustness of them. So, nano scale thermal cycles will be probably engaged in the near future. In this paper, a nano scale irreversible Braysson cycle is studied thermodynamically for optimizing the performance of the Braysson cycle. In the aforementioned cycle an ideal Maxwell–Boltzmann gas is used as a working fluid. Furthermore, three different plans are used for optimizing with multi-objectives; though, the outputs of the abovementioned plans are assessed autonomously. Throughout the first plan, with the purpose of maximizing the ecological coefficient of performance and energy efficiency of the system, multi-objective optimization algorithms are used. Furthermore, in the second plan, two objective functions containing the ecological coefficient of performance and the dimensionless Maximum available work are maximized synchronously by utilizing multi-objective optimization approach. Finally, throughout the third plan, three objective functions involving the dimensionless Maximum available work, the ecological coefficient of performance and energy efficiency of the system are maximized synchronously by utilizing multi-objective optimization approach. The multi-objective evolutionary approach based on the non-dominated sorting genetic algorithm approach is used in this research. Making a decision is performed by three different decision makers comprising linear programming approaches for multidimensional analysis of preference and an approach for order of preference by comparison with ideal answer and Bellman–Zadeh. Lastly, analysis of error is employed to determine deviation of the outcomes gained from each plan.
In classical thermodynamic, maximum power obtained from system (or minimum power supplied to system) defined as availability (exergy), but availability term is only used for reversible systems. In ...reality, there is no reversible system, all systems are irreversible, because reversible cycles doesn’t include constrains like time or size and they operates in quasi-equilibrium state. Purpose of this study is to define limits of the all basic thermodynamic cycles and to provide finite-time exergy models for irreversible cycles and to obtain the maximum (or minimum) available power for irreversible (finite-time exergy) cycles. In this study, available power optimization and performance limits were defined all basic irreversible thermodynamic cycles, by using first and second law of thermodynamic. Finally, these results were evaluated in terms of cycles’ first and second law efficiency, COP, power output (or input) and exergy destruction.
In classical thermodynamics,
the maximum power obtained from a system is
defined as exergy (availability). However, the term exergy is used for
reversible cycles only; in reality, reversible cycles ...do not exist, and all
systems are irreversible. Reversible cycles do not have such restrictions as
time and dimension, and are assumed to work in an equilibrium state. The
objective of this study is to obtain maximum available work for SI, CI and
Brayton cycles while considering the aforementioned restrictions and
assumptions. We assume that the specific heat of the
working fluid varies with temperature, we define optimum compression ratios and pressure ratio in order to obtain maximum available
work, and we discuss the results obtained. The design parameter most
appropriate for the results obtained is presented.
Relationships between lost available work and entropy generation cannot be
obtained in the general case, i.e., for systems in contact with arbitrary
heat and/or radiation reservoirs. Reservoirs in ...local (full or partial)
thermodynamic equilibrium are needed. The Gouy–Stodola theorem is a
particular case of a more general result obtained for systems in contact
with heat reservoirs in local thermal equilibrium. Radiation reservoirs
cannot be assimilated to heat reservoirs. A general model of a radiation
reservoir is shown. Particular cases are reservoirs containing black-body
radiation, deformed black-body radiation, and nuclear radiation. Despite the
case of heat reservoirs, knowledge of their temperature is not sufficient
for a complete characterization of radiation reservoirs. There is a need to
know other intensive thermodynamic parameters, such as the chemical
potential of radiation as well as some macroscopic parameters (such as the
geometric characteristics of the radiation reservoir) and microscopic
parameters (such as radiation spectra or band-gaps).
types of radiation
reservoirs may behave like heat reservoirs, but only in the
.
Nano technology is developed in this decade and changes the way of life. Moreover, developing nano technology has effect on the performance of the materials and consequently improves the efficiency ...and robustness of them. So, nano scale thermal cycles will be probably engaged in the near future. In this paper, a nano scale irreversible Braysson cycle is studied thermodynamically for optimizing the performance of the Braysson cycle. In the aforementioned cycle an ideal Maxwell–Boltzmann gas is used as a working fluid. Furthermore, three different plans are used for optimizing with multi-objectives; though, the outputs of the abovementioned plans are assessed autonomously. Throughout the first plan, with the purpose of maximizing the ecological coefficient of performance and energy efficiency of the system, multi-objective optimization algorithms are used. Furthermore, in the second plan, two objective functions containing the ecological coefficient of performance and the dimensionless Maximum available work are maximized synchronously by utilizing multi-objective optimization approach. Finally, throughout the third plan, three objective functions involving the dimensionless Maximum available work, the ecological coefficient of performance and energy efficiency of the system are maximized synchronously by utilizing multi-objective optimization approach. The multi-objective evolutionary approach based on the non-dominated sorting genetic algorithm approach is used in this research. Making a decision is performed by three different decision makers comprising linear programming approaches for multidimensional analysis of preference and an approach for order of preference by comparison with ideal answer and Bellman–Zadeh. Lastly, analysis of error is employed to determine deviation of the outcomes gained from each plan.
The optimization of forced cooling-down processes is performed using the optimal control theory. Four objective functions are shortly presented and minimized. They include the consummated cooling ...fluid mass and some dissipation measures, as the entropy generation and variants of the lost available work. The minimum duration for the cooling process with minimum lost available work normally exceeds the minimum duration for the cooling processes with minimum mass of cooling fluid but the reverse is sometimes true. The minimum lost available work does not depend on the initial and final temperatures of the cooled body but just on the duration of the cooling process. The cooling strategies with linear (or slightly non-linear) time dependence of cooled body temperature are based on rather strong non-linear increase in the optimum cooling fluid mass flow rate at the end of the cooling process.
Investigation of the combustion phenomena in a constant volume microcombustor was carried out on a constant volume microcombustor that is subject to significant heat loss to the wall in order to ...estimate the effects of combustor dimension on flame propagation. A cylindrical combustor of a variable height that is comparable to quenching limit of the combustible mixture of H
2
and air was built. Flame visualization and measurement of transient pressure during the flame propagation were made. Measurements were made at the chamber height ranging from 0.67 mm to 3 mm. A simple theoretical model that takes into account the heat loss to the wall during flame propagation was used to derive available work of the burnt gas from the pressure measurements. For a chamber depth of less than 1 mm, the pressure increase by combustion was negligible and the sensible heat in the burnt gas was not adequate to be converted to mechanical power. For a chamber height greater than 2 mm, however, the amount of heat generated by combustion was sufficient to produce a few Joules of theoretical work even after experiencing increased heat loss due to the small scale of the chamber.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Analyzed 2 types of applicants (n = 299), who applied to a temporary help firm during 1968. Although both types of applicants were interviewed and were hired, 1 group refused while the other accepted ...employment. Of 18 personal characteristics, 5 were predictive of acceptance: (a) type of job, (b) number of skills, (c) days/wk available for work, (d) expressed interest in a permanent job as a temporary worker, and (e) length of past experience. The analysis delineates several disadvantages in the use of temporary workers, especially when the labor market tightens.
This paper presents an optimization model for the problem of optimal scheduling of tamping actions on ballasted tracks. The model extends an existing one in the literature by including the unused ...life costs for track sections in the objective function and introducing available workforce constraints in the model. The aim is to obtain an optimal schedule for tamping actions minimizing the total cost in a finite time horizon using global optimization. The results of numerical study show that consideration of track layout in the problem leads to a significant increase in the total number of tamping actions and the amount of unused life for track sections. It is also found that it is important to use a big value for available workforce in the problem in order to reduce the total cost. The proposed model presents a more realistic solution for the studied problem than the existing work in the literature.
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