مجله مهندسی مکانیک امیرکبیر
سال چهل و هشتم شماره 1 (بهار 1395)

The purpose of this study is to simulate numerically 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

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 work, application of hot wire, velocity distribution and turbulence intensity at the nozzle exit have been investigated. When the ratio of inlet to outlet area of the nozzle reduce from 12 to 6.25, the ratio of inlet to outlet turbulence intensity increase from 0.2 to 0.4. 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 percent 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 analysed 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 percent, 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 return 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 an appropriate and efficient ventilation system to reduce damages from fire hazards is necessary. Longitudinal ventilation system is widely used in tunnel ventilation. The critical ventilation velocity in longitudinal system is the amount of airflow necessary to prevent backlayering 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 moves to the downstream of the tunnel providing 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. The effect 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 used. The influence of temperature in surface tension is studied. Some validation is done as comparison of continues curves with theoretical maxwell ones and another two simple tests that their results are according to previous ones. After validation of code with previous works, drop penetration is investigated in layered porous medium which is made of four sections with the same porosity to produce more homogenous porosity. The effect of different factors like porosity, hydrophobicity/ hyrophilicity property of surfaces on the penetration rate and pattern is studied. The results illustrate that by decreasing the porosity, penetration rate would decrease too and in general hydrophilic surface in low porosity increase the penetration rate; Also, the difference of penetration pattern in two situations is illustrated, in hydrophilic situation penetration pattern is cloy and piston – type and in hydrophobic one penetration is like a finger or finger-type. Then, for more investigation of penetration in porous medium and showing the ability of written code, it is improved to simulate two component- two phase flows. After validation, the penetration pattern in different capillary numbers and viscosity ratios, viscous fingering and capillary fingering regimes are observed. At the end, change of penetration pattern by consideration the surface hydrophilic is 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 & 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 analyses of combined cooling, heating and power (CCHP) systems were performed here to select the nominal capacities of equipment system with gas engine as prime mover for a water sport complex. The analysis was performed for both different scenarios (selling (Ss) and no-selling (SNs) electricity )from (to) grid to specify design parameters such as the number and nominal power of prime movers, heating capacities of both backup boiler and the cooling capacities of electrical and absorption chillers. By defining an objective function multi criteria called the Relative Annual Benefit (RAB), Genetic Algorithm optimization method was used for finding the optimal values of design parameters. Then, how to choose nominal capacity of gas engine has been investigated by considering the economical (RAB, PB) and fuel energy saving ratio (FESR) and environmental (CO2). The optimization results indicated that two gas engines (with nominal powers of 130 and 150 kW) in selling scenario(Ss) and one gas engine (with nominal power of 120 kW) in no-selling scenario(SNs), provided the maximum value of the objective function. Furthermore the results of the how selection gas engine show, in both two scenarios sell and No-sell electricity , if two similar capacity instead optimized capacities are selected, the payback period increases and annual benefit decreases, but the ratio of fuel energy saving and reducing of emission CO2 ratio, decrease in sell scenarios and increase in No-sell scenarios.

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 improve 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 virtual mass and particle-particle interaction forces are negligible in such a way that they can be ignored.