entropy generation

The purpose of this analysis using lattice Boltzmann method (LBM) is to investigate the matter with which strategies can be used to control the amount of heat transfer and entropy formation. For this goal, the mixed convection heat transfer of non-Newtonian nanofluid under the impact of the magnetic field inside the square chamber containing the conductor wall has been analyzed. The results depicted that the flow characteristics and heat transfer are strongly affected by changing the position in these places. If the magnetic field is applied at the same location as the speed application position, the effect of increasing the Hartmann number in reducing the average Nusselt number becomes more evident. In order to have the highest value of Nusselt number, CASE3 should be considered, however, the greatest impact of magnetic field on fluid flow was observed for CASE2. By increasing the distance between the conductive wall and the moving wall, the average Nusselt number increases up to 1.5 times. By decreasing the thermal conductivity ratio from 10 to 0.5, the average Nusselt number decreases to about 55%. The entropy value has a direct/inverse relationship with the Richardson number/power-law index.

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.

In this study, the cylinder drag coefficient is reduced by using passive flow control. Installing a flat plate in two heights and different longitudinal distances in upstream flow increases the upstream flow momentum of the cylinder, leading to the higher boundary layer flow resistance against adverse pressure gradient which delays the flow separation. The flow separation delay enhances the pressure on the cylinder downstream. Then, the net pressure on the cylinder in the flow direction and, consequently, the cylinder drag coefficient are decreased. In case that the higher flat plate is utilized, the pressure on the upstream side is reduced more, leading to lower drag coefficient. However, for both heights of the flat plate at specific longitudinal distances from the cylinder due to the cavity flow formation between the cylinder and the flat plat, the vortex shedding is suppressed and the cylinder upstream is changed from the pressure side to suction side, leading to lower net pressure on the cylinder in the flow direction and as a result, less drag coefficient. At the optimal flat plate configuration at and , the minimum cylinder drag coefficient reached 90% reduction in comparison to the single cylinder case in the same flow condition. Results show that the drag coefficient reduction behavior is similar for different sub-critical Reynolds numbers due to the constant flow pattern and no considerable variation of the separation point. The entropy generation for the single cylinder and the case where the flat plate is located in its optimal configuration were investigated. The single cylinder has the highest entropy value, while the entropy of the optimal flat plate configuration with the cylinder reaches the lowest value, the same as the drag coefficient. Then, the drag coefficient is reduced by decreasing entropy generation, indicating the direct relation between drag coefficient and entropy generation.

The purpose of this paper is to investigate the heat transfer and entropy generated due to mixed convection of hybrid nanofluid inside the lid-driven semi-oval chamber. The magnetic field is applied uniformly and non-uniformly. The simulation is performed by MRT-LBM and by writing computer code in Fortran language. Effect of Richardson number (0.1, 1 and 10), nanoparticles volume fraction (0-0.06), Hartmann number (0-60), inclination angle of the chamber (-90, 0 and +90 degrees) and type of magnetic field applied (uniform and non-uniform) on the flow formed in the chamber is evaluated. The results show that increasing the volume fraction of nanoparticles increases the flow strength, average Nusselt number, total volumetric entropy and Bejan number and most of the effect is observed for Richardson number 10 and inclination angle +90°. In all cases, increasing the Hartmann number decreases maximum values of streamlines and heat transfer, and this effect decreases with increasing Richardson number. By applying a magnetic field non-uniformly, it can change the flow strength by more than 80% and increase the heat transfer to 35%. Increasing the Hartmann number makes the effect of the non-uniform magnetic field more obvious, and the heat transfer has the largest share in the total volumetric entropy.

A compact fin-tube heat exchanger is used to transfer current fluid heat inside the tubes into the air outside. In this study, entropy production and optimized Reynolds number for finned-tube heat exchangers based on the minimum entropy production have been investigated. As a result, the total entropy of compact heat exchangers, which is the summation of the production rate of fluid entropy inside the tube, and the entropy production rate of fluid in the air, is minimum due to the changes in the Reynolds number of the fluid inside the tubes. Based on thermodynamic analysis and study of parameters affecting the entropy production, one expression is proposed for the optimal Reynolds number. The expression is a function of the thermodynamic properties of the fluid in the pipe, fluid thermodynamic properties of air, the effects of ambient temperature, input temperature of the cooling fluid, and geometrical dimensions of the heat exchanger. According to this optimization, effective information for the design of a compact finned-tube heat exchanger would be obtained. Therefore, in practical conditions, using optimal Reynolds number, the system has the lowest irreversibility, and as a result, the best exergy will be accessible. Finally, an empirical correlation is proposed for the optimum Re number that can predict Reopt with less than 0.6 % error.

The proper design of the stator turbine supercharger is of great help in increasing its performance and overall engine efficiency. In this research, with the help of FLUENT software, the local entropy generation and rate of exergy destruction of the turbine stator vane has been investigated. In the analysis, local entropy generation is divided into two sections: viscous entropy generation and thermal entropy generation, and the governing equations are defined with the help of UDF within the FLUENT software. After calculating the local entropy generation and determining the value of irreversibility quantities, According to the Gouy-Stodola equation, the amount of exergy destruction is calculated by numerical method. The results show that the two models of the Spalart Allmaras and k-ω(SST) have performed the best prediction wake of the vane. The amount of viscous entropy generation and thermal entropy generation is 69% and 31%, respectively. The values of the local entropy production calculated with the results of a stator turbine vane of authentic paper are validated with acceptable adaptation in it. The calculated exergy destruction is 281 kW, located in the range of modern gas turbines for a high-pressure turbine stage.