جستجوی مقالات مرتبط با کلیدواژه "انتروپی تولیدی" در نشریات گروه "مکانیک"

In recent years, with the continuous growth of natural gas consumption in Iran, the number of pressure drop stations has increased significantly. In throttling valves of these stations, the temperature drop due to the Joule-Thomson effect causes the gas to hydrate, freeze the valves, and block the transmission path. Hence, about 14,000 indirect-fired water-bath heaters have a duty for preheating high-pressure gas before entering them. Unfortunately, the 30% average efficiency of indirectly fired water-bath heaters wastes nearly one billion cubic meters of processed natural gas every year, equivalent to a 400 MW power plant capacity. In this article, intending to optimize, indirect-fired water-bath heaters were modeled thermodynamically and thermo-economically, and three objective functions including thermal efficiency, entropy generation number, and wasted cost number are defined and the mathematical model was proposed in two scenarios. Then the model was solved based on the multi-objective genetic algorithm, using the entropy generation minimization method, and the Pareto optimal fronts of the scenarios were determined. The model implementation results with a deviation of less than ±10% compared to the results of a real sample indicate its acceptable performance. Based on the techno-economic justified results, it is possible to improve the efficiency of indirectly fired water-bath heaters between 48 and 55% depending on the gas volume flow rate. The relations, curves, and dimensionless groups obtained, can be used as a reference for the optimal design of indirect-fired water-bath heaters.

The rolling process is one of the most common processes of making the machinery parts and the workpiece cooling rate is one of the most important parameter in the improved of product properties. In this paper, the flow of nanofluids over a nonlinear stretching sheet with a magnetic field is studied as a simplified model of a rolling process. The stretching velocity of the sheet is assumed to vary as a power function of the distance from the origin. The governing equations are including continuity, momentum and energy balances that by using of similarity solution method are transformed to nonlinear ordinary differential equations and are solved. The similarity solution results of momentum and energy equations are compared with published data, and have achieved appropriate compliance. The results show that increasing the solid volume fraction of the nanoparticles φ, the thermal conductivity of the fluid has increases and Increment of the nonlinear stretching parameter n, decreases heat transfer. Also increment of magnetic parameters M, has reduced the cooling rate. In this paper, Bijan number is introduced as a normative for the qualitative cooling rate in the rolling process. Based on the results increase the solid volume fraction of the nanoparticles φ, the nonlinear stretching parameter n and magnetic parameters M, increased, decreased and decreased the number Bijan respectively. The results of this research can be extended for the increasing of the cooling rate in the rolling process of metals.

Prior to entering to the throttling valve of the City Gate Stations (CGS), high-pressure natural gas flow in pipelines is transmitted through Water Bath Indirect Heaters (WBIH), which is increasing its temperature to compensate for the temperature drop caused by the Joule-Thomson effect and preventing the occurrence of the hydration phenomenon, gas freezing, and subsequent blockage of the gas flow path. Because of feeding of processed gas of the network on a large scale, optimizing the WBIHs has a lot of significance. In the present study, each WBIH is simulated by a type of thermodynamic machine, consisting of two distinct thermal systems. According to the problem geometry and governing equations, the thermodynamic analysis of these two systems results in the formulation of a relationship between their thermal efficiencies together and the definition of a parameter was defined as the Thermodynamic Similarity Coefficient (TSC). Then, the results showed that always, a constant logarithmic relationship exists between of the Number of Heat Transfer Units (NTU) values difference of the fire tube and heat coil of the WBIHs with their TSC as well as a constant power relationship between their NTU values ratio with this coefficient too. Finally, by solving the equation system obtained from these two relations, it was possible to determine the optimal values of NTU for the fire tube and heat coil as functions of TSC of the WBIH and to achieve the relationship between their optimum geometric dimensions together in the most ideal heat transfer state with a maximum relative error of about 13%.

It is convenient method to use a drag-reducing polymer to reduce entropy generation in annular two-phase flow. The drag-reducing polymer is a high molecular weight poly-alpha-olefin. In present paper for the first time, the explicit equation has been presented for the prediction the entropy generation in annular flow with drag-reducing polymer. The equation is function of the pipe diameter and superficial velocities. Entropy generation versus drag-reduction and holdup in different pipe's diameter (0.0127, 0.0254 and 0.0953 ) and in different superficial velocities are presented. The results are shown that entropy generation decreases with increasing , and pipe diameter but entropy generation increases with increasing gas and liquid superficial velocities. Therefore by adding drag-reducing polymer and correct determination of pipe diameter and superficial velocities using the presented explicit equation in this research, could be reduced the losses and increased in the production in related industries.

Welding has been employed at an increasing rate for its advantage in high joint efficiency, water and air tightness, weight saving, reduction in fabrication cost and design flexibility. However, welded structures are not free from problems. Residual stress and distortion are major difficulties with welded structures. The buckling distortion of welded structures caused by welding stress is seen in relatively thin welded structures. Buckling distortion causes loss of dimensional control, structural integrity and increased fabrication costs. Thus, it is necessary to achieve a predictive analysis technique to determine the susceptibility of a particular design to buckling distortion. This paper presents a predictive buckling distortion method to determine if welded structures buckle during welding or not. In this method, a 3D thermal transient process is coupled to a 3D eigenvalue analysis to determine the total entropy generation due to welding and critical total entropy generation, respectively. In this work, because of the small size of the melted region (weld pool), the heat source model developed by the moving isotherm pool is used. Welding time in this work is calculated by dividing the welder speed obtained from the practical welding characteristics data by the length of the welded region. The temperature dependent thermal properties of material were also incorporated into the model. In this research, temperature distribution producing the minimum value of longitudinal stress to cause the buckling of welded shells is predicted by 3D eigenvalue analysis. This temperature distribution creates negative longitudinal strains in the welded region, causes tensile longitudinal stress in regions close to the weld line and compressive stress in regions away from the weld line. Total entropy generation, due to this temperature distribution, which is named critical entropy (Scri), is calculated. The comparison between total entropy generation due to completely welding shells (SW) and critical entropy is a good criterion to predict the possibility of buckling welded shells. In this method, the computational time is much less than in the previous methods because of omitting the mechanical analysis of welding. The correctness of this predictive method is proved by analytical results.

In this paper, energy and exergy analysis of fuel cell systems has been considered in simple and CHP modes. System model consists of following components: fuel cell stacks, burner, reformer, heat exchanger, battery and water heater. Thermodynamic analysis of system has been investigated with consideration of variations of energy efficiency, exergy efficiency and entropy generation versus internal air temperature to the fuel cell for different air stoichiometries. Results show that by increasing of internal air temperature to the fuel cell, entropy generation and exergy efficiency increase in both simple and CHP modes. Also, in simple mode, not in CHP mode, increasing of intenal air temperature leads to decreasing of energy efficiency.