JOURNAL OF MECHANICAL ENGINEERING AMIRKABIR (AMIRKABIR)
Year:2016 | Volume:48 | Issue:1|Papers Count:11

The purpose of this study is to numerically simulate water-vapor two-phase flow in ducts with variable cross-section. The homogeneous equilibrium model is used to describe the two-phase in a converging-diverging nozzle with the inlet vapor quality in the rage of 0.05<x<0.8 and back pressure in the range of Pb>0.1 atm. The flow passage is assumed to be adiabatic and frictional. Fluid properties are calculated basis on the thermodynamic tables. The governing equations are solved by flux-vector splitting method explicitly. The numerical results indicate that the vapor quality decreases along the nozzle before the location of the shock wave and it increases after that. In other words, the behavior of the nozzle depends on the inlet vapor quality. Therefore, the condensing behavior of the nozzle changes to the evaporating behavior when the flow passes over the shock wave with the inlet vapor quality greater than about 0.5. In addition, the pressure distribution along the nozzle with the inlet vapor quality of 0.73 is in a good agreement with the experimental data.

In order to decrease construction cost of vertical wind tunnel, it is necessary to reduce the wind tunnel nozzle length. In this regard, it is adequate to reduce the ratio of inlet to outlet diameters of the nozzle and ratio of nozzle length to its inlet diameter. In addition, shifting of the inflection point of the nozzle curves to the flow upstream and reduction of the exit section of the nozzle can result in reduction in nozzle length. These modifications may cause change in the flow quality at the nozzle exit, which has to be studied. In this experimental study, by using hot wire, velocity distribution and turbulence intensity at the nozzle exit have been investigated. The ratio of inlet to outlet turbulence intensity increases from 0.2 to 0.4, when the ratio of inlet to outlet area of the nozzle reduces from 12 to 6.25. Using the results, the nozzle length can be reduced by about 62% so that air quality in the short nozzle output is acceptable.

In this study, the stability of deionized water based copper oxide nanofluid with weight concentration of 0.1% is investigated experimentally. The experiments are designed to investigate the influence of rotational speed and dispersion time of nanoparticles in the base fluid, ultrasonic waving time, type and concentration of surfactants and pH on the nanofluid stability and achieve to an optimal stability condition. The results are statistically analyzed using Taguchi method by implementing Qualitek-4 software. Furthermore, nanofluid stability is evaluated by investigation of sedimentation photographs also, zeta potential method. The results showed that using sodium dodecyl sulphate with weight concentration of 0.1%, ultrasonic waving by ultrasonic probe device for an hour and changing the pH to 10.72, provide the best conditions for dispersing copper oxide nanoparticles in deionized water. In this condition, prepared nanofluid is maintained it̕s stability with no trace of sedimentation of nanoparticles for forty days at least.

The reaction of thermal NO is highly sensitive to temperature and if a technique can reduce the flame temperature, it would be effective to reduce NOx formation. The dilution of the fuel and also producing swirling flows can reduce the flame temperature and as a result, decrease the rate of NOx formation. In the present study, the effect of dilution and the swirling flow on NOx emission in the premixed propane-air mixture is investigated experimentally. The experiments were carried out in an axially symmetric cylindrical furnace for an equivalence ratio of 0.7 to 1.3 and (0.0-1.0) dilution ratios. The swirling is achieved by a swirler with 45-degree angle corresponding to the swirl number of 0.7. The results show that by increasing the dilution ratio, the flame temperature and as the result, the NOx emissions are decreased. The results also reveal that the swirler causes better mixing of the fuel, air and the diluents and parts of combustion products are returned to the reaction zone and since the present species have high heat capacities, they absorb the heat of combustion, which in turn decreases the temperature of the furnace and consequently decreases the NOx emissions. The experimental results are in a good agreement with the results reported by other researchers.

Due to the importance of safety in underground tunnels and the health of passengers in emergency modes, analysis and simulation of fires in tunnels and design of an appropriate and efficient ventilation system to reduce damages of fire hazards is necessary. Longitudinal ventilation system is widely used in tunnel ventilation. Critical ventilation velocity in longitudinal system is the amount of airflow necessary to prevent back layering of smoke and heat to upstream of fire region. The lower air velocity leads to influence of smoke and heat of fire to the fire upstream, and resulting in reduction of visibility and fresh air in the tunnel. In critical velocity, smoke and heat move to the downstream of the tunnel and provide fresh air and a safe passage for passengers to escape. The aim of this research is to investigate the critical ventilation velocity and effective parameters on it. CFD simulation were performed in this paper to study the critical ventilation velocity by using the code FDS. Effects of fire source shape, vehicle such as a train inside the tunnel, tunnel geometry and slope on the critical ventilation velocity were investigated.

In this study, drop penetration with high density ratio in layered porous medium is simulated with pseudo- potential lattice Boltzmann model. Due to inherent weakness of this model in simulation of flows with high density ratio, equations of state as Redlich-Kwong and Peng-Robinson are employed in the simulation. The influence of temperature in surface tension is also studied. Drop penetration is investigated in a layered porous medium which is made of four different sections. Effects of different factors like porosity, hydrophobicity/ hyrophilicity of surfaces on the penetration rate and pattern is studied. The results illustrate that by decreasing the porosity, the penetration rate is changed. In hydrophilic situation, the penetration pattern is piston-type and in hydrophobic one, the penetration is similar to the finger or finger-type. The penetration pattern in different capillary numbers, viscosity ratios and different regimes are analyzed. The change of penetration pattern by consideration of non-homogenous hydrophilicity is also studied.

The ongoing developmental studies on the application of subscale liquid rocket engines as small thruster and laboratory tester are briefly reviewed. Then a detailed design and manufacturing process of a laboratory liquid subscale engine with single swirl double base injector of 300 N thrust for this reaserch is presented. For the preparation of pressurized water, fuel and oxide, a test facility has been prepared. Results of water analogy tests are presented. Initial firings using the real fuel and oxide were not successful. Low fuel flow, low mixing area of the fuel and oxide, and contamination in the TR-1 were considered to be the reasons. Overcoming to these problems resulted in successful firing of the subscale engine. Obtained results were adapted to design expected results.

Energy, economic and environmental analysis have been employed in order to optimize optimal (nominal) capacity of a combined cooling, heating and power system in a water sports complex. Design parameters are nominal capacity and number of gas engines and their partial load, heat capacity of boiler and cooling capacity of electricity and absorption chillers. These parameters are optimized either in the scenarios of possibility of selling electricity (SS) or impossibility of selling scenarios (SNS). Design parameters are optimized using genetic algorithm and a multi criteria objective function that is called relative annual profit (RAB). Next, how to select optimal capacity of gas engine has been investigated economically, saving fuel and controlling the pollutants (CO, NOx, and CO2). Optimization results illustrate that two gas engine with capacity of 130 kW and 150 kW have the maximum value of objective function in the scenarios of possibility of selling electricity, while in the impossibility of selling scenarios the maximum value of goal function is for on 120 kW gas engine. In addition, the results reveal that payback periods on investment are increased (decreased) when two similar capacity are chosen in the cases of SS (SNS). Moreover, fuel consumption and CO2 emissions are decreased (increased) in the scenarios of SS (SNS).

This paper deals with water flow in a backward-facing step with blowing of different nanofluids. The objective is to evaluate the effect of nanofluid blowing on the heat transfer rate. For this purpose, the Eulerian-Eulerian two-phase model is employed. The accuracy of the current simulations is demonstrated by comparing the obtained results with those of open literature. The results show that increasing the nanofluid blowing as well as nanoparticles fraction therein improves heat exchange from different surfaces of the channel. Comparing the results of different nanofluids leads one to conclude that the bottom wall heat transfer attains its maximum value when the blowed nanofluid contains nanoparticles with the highest thermal conductivity. However, it is found that maximum heat transfer in the top wall is achieved during blowing of a nanofluid with the highest nanoparticle penetration into the channel flow. Moreover, it is observed that discrepancies appearing between the results of different nanofluids become more remarkable as one increases the nanofluid blowing or nanoparticles fraction therein. Finally, the Eulerian-Eulerian model demonstrates that among the interphase forces, the effects of the virtual mass force and the particle-particle interaction force are negligible in such a way that they can be ignored.