Hamid Saffari

Numerical investigation of porous rib arrangement on heat transfer and entropy generation of nanofluid flow in an annulus using a two-phase mixture model

ABSTRACT The effect of porous rib arrays on the heat transfer and entropy generation of laminar nanofluid flow inside annuli is studied numerically, using a two-phase mixture model for nanofluid flow simulation. Porous media, nanoparticles, and vortex formation are simultaneously affecting the characteristics of the system. Results showed that the permeability and height of porous ribs have significant effects on the thermal performance of system. Vortex zones also affect the trend of variation of entropy and performance numbers, and local optimums exist for these two parameters. The role of nanofluid in heat transfer enhancement in recirculating zones is more significant for higher volume fractions.

A New Alpha-function for the Peng-Robinson Equation of State: Application to Natural Gas

Abstract Cubic equations of state (EOSs) are simple and easy at calculation. One way of improving the accuracy of a cubic EOS is through the modification of temperature-dependent energy parameter by using alpha-function. The industrial applications of natural gas are very wide and as a result, prediction of thermodynamic properties and phase behavior of natural gas is an important part of design for such processes. In this work we develop a new α-function for the Peng-Robinson (PR) EOS with the parameters optimized especially for natural gas components. The parameters are generalized as a linear function of acentric factor. The results are compared to the predictions from original PR EOS and other α-functions in literature. It is shown that the new α-function presents a good accuracy with the average deviation of 1.42% for natural gas components.

Modeling and Exergy and Exergoeconomic Optimization of a Gas Turbine Power Plant Using a Genetic Algorithm

In this paper, the thermodynamic modelling of a gas turbine power plant in Iran is performed. Also, a computer code has been developed based on Matlab software. Moreover, both exergy and exergoeconomic analysis of this power plant have been conducted. To have a good insight into this study, the effects of key parameters such as compressor pressure ratio, gas turbine inlet temperature (TIT), compressor and turbine isentropic efficiency on the total exergy destruction, total exergy efficiency as well as total cost of exergy destruction have been performed. The modelling results have been compared with an actual running power plant located in Yazd city, Iran. The results of developed code have shown reasonable agreement between the simulation code results and experimental data obtained from power plant. The exergy analysis revealed that the combustion chamber is the must exergy destructor in comparison with other components. Also, its exergy efficiency is less than other components. This is due to the high temperature difference between working fluid and burner temperature. In addition, it was found that by the increase of TIT, the exergy destruction of this component can be reduced. On the other hand, the cost of exergy destruction is high for the combustion chamber. The effects of design parameters on exergy efficiency have shown that increase in the air compressor ratio and TIT, increases the total exergy efficiency of the cycle. Furthermore, the results have revealed that by the increase of TIT by 350°C, the cost of exergy destruction is decreased about 22%. Therefore, TIT is the best option to improve the cycle losses. In addition, an optimization using a genetic algorithm has been conducted to find the optimal solution of the plant.Copyright

Numerical investigation of flow boiling heat transfer in helically coiled tube under constant heat flux

Numerical study of subcooled and saturated flow boiling in the curved and helically coiled tubes in presence of phase change is one of the challenging area of CFD studies. In this paper, the CFD modeling of the nucleate and convective flow boiling in the small helically coiled tube at low vapor quality (up to the 18.93 percent) region is studied. A proper Eulerian-based mathematical model is used for interphase exchange forces and heat transfer between two phases in CFD modeling using Bulk boiling model. The results show that, the inner and the bottom wall of the helically coiled tube have the lowest and the highest heat transfer coefficient, respectively. The effect of change in coil diameter, helical pitch and tube diameter is investigated on the counters of vapor volume fraction. It is seen that at low vapor quality flows, the heat transfer coefficient is enhanced by decreasing in coil diameter, tube diameter and increasing in coil pitch of helically coiled tube.

Effect of Nanostructured Microporous Surfaces on Pool Boiling Augmentation

ABSTRACT Nanostructured microporous surfaces were electrodeposited at various electrolyte temperatures on copper substrate to investigate the saturated pool boiling enhancement of distilled water at atmospheric pressure. Surface structure topography and wickability were analyzed to investigate their relation to critical heat flux. Scanning electron microscope showed that the micro-clusters have nanostructures from cubic at 5°C to dendritic at 60°C electrolyte temperature. Rate-of-rise experiments demonstrated that dendritic copper structure has the best capillary performance. The experimental results of pool boiling heat transfer indicate that the critical heat flux increased with surface wickability. Electrodeposited porous surface in hot electrolyte showed the highest critical heat flux and heat transfer coefficient of the 124 W/cm2 and 17 W/cm2K, respectively, which is 50% and 270% higher than that of plain surface. However, the two-step electrodeposition and annealing were used in fabrication of surfaces, but the mechanical strength of layer needs more improvement by changing the electrochemical process parameters.

Influence of variation of pipe diameter on pressure drop predictions of the new modified three-fluid model inside condensing vertical pipes

When a loss of coolant accident occurs in the primary system of a water-cooled nuclear reactor, a large amount of steam is released into the reactor containment. Therefore, there is the possibility of over-pressurization of the reactor containment. In such a condition, the released steam is often condensed using a passive containment cooling system. This system consists of condensing vertical pipes with diameters of 30–50 mm. Thus, condensation inside vertical pipes with annular flow happens, which is usually analyzed using the three-fluid models. In the present work, the effect of variation of condensing vertical pipe diameter on pressure drop predictions in downward condensing annular flow of steam is studied using the new modified three-fluid model. In a new pipe diameter, D = 0.03 m, the pressure drops are calculated using the new modified three-fluid model and the correlation of Stevanovic et al. for steam–liquid film interfacial friction coefficient and compared.

Length of the Entrance, Fully developed and hydraulic characteristics of bubbly flows in helical coils

In the present study an experimental and computational fluid dynamic carried out for entrance length flow in helical coils for single and two-phase flow. Development region length and the detailed characteristics of fluid flow has been investigated by varying coil parameters such as tube diameter, coil diameter and void fraction. The considered Reynolds number is in the range of 10000 < Re < 80000 and the void fraction is in the range of 0.01 < α < 0.09. For CFD simulation the two-phase fluid flow in this article, the Eulerian-Eulerian model was employed. To calculate the turbulent fluctuations, the k-e realizable turbulence model has been used. The experimental and numerical simulation of the local parameters demonstrates that developing length (L/D) increases with increase Reynolds in the single and two-phase flows. The obtained results showed that the development entrance length increases with the increase of pipe diameter and decreases with the increase of coil diameter. Also, entrance length decrease with increases void fraction. According to the secondary motion in a curved tube, a major difficulty in describing the problem has been recognition of the importance of flow development at the entrance of a helical tube. Therefore, correlation has been suggested to predict the entrance length as functions of various variables of the helical coil.