مجله مهندسی مکانیک شریف
سال سی و چهارم شماره 2 (پاییز و زمستان 1397)

Direct contact condensation is used in industrial applications, such as steam jet pumps, direct-contact heat exchangers and nuclear reactor cooling systems, due to its highly ecient heat and mass transfer. When steam is injected into subcooled water, a steam plume is generated at the exit of the steam nozzle, surrounded by an interface around the steam plume. The direct and quick transfer of heat, mass and momentum across steamwater interface makes the physics of direct contact condensation very complex. To eciently design the above mentioned equipment of direct contact condensation, a proper understanding of heat and mass transfer in this phenomenon is required. Several experimental and theoretical works have been performed on steam jet condensation in water. However, there have been a few numerical investigations of this phenomenon. Dierent heat and mass transfer models have been used by researchers to develop a suitable numerical tool to simulate steam condensation process. Constant-rate mass transfer model is one of the models used in numerical simulation of direct contact condensation in steam injection into water. This model needs a specic empirical coecientfor eachsimulation. In this study, considering available experimental results of previous investigations, a numerical simulation was performed in ANSYS Fluent software by using constant-rate mass transfer model to study the eects of dierent steam and water ow parameters, including water subcooling degree, Reynolds number of water ow, and steam mass ux, on dimensionless steam plume length. Moreover, constant-rate mass transfer model was revised by using the results obtained from the simulation and a new correlation for calculation of the constant coecient of the mass transfer model was proposed as a function of ow parameters that in uence the phenomenon to increase the accuracy of future simulations. The proposed correlation agrees well with numerical data and most of the data lie in the range of20% of the correlation.

Recently, active ow control by dielectric barrier discharge (DBD) plasma actuators has been increased. These actuators are known as more ecient, low-cost, without the need of moving parts, low power consumption, small size, low weight, easy installation and without delay in control. All these features have attracted researcherstousethisactuatorinavarietyofcases, such as turbulence ow control, laminar to turbulent transition suppression, separation control, drag reduction and mitigationofnoisepollution. However,mostofthestudiessuerfromlackofanaccuratenumericalmodelwhich cansimulatethisphenomenonindetails. Computational analysisofthisphenomenonisverycomplexanddicult due to a combination of ionization phenomena and the interaction of the uid ow with the actuator eects. For the exact solution of this phenomenon in certain conditions, Maxwell and Navier-Stokes equations must be combined, while this non-linear solution combination willbeverydicult. Oneofthemodelsformodelingthe interactionbetweentheactuatorand uid owisanelectrostatic model which adds the actuator eect as source termsinthemomentumequationsbysolvingtheelectrical potential equation and charge density equation. In this study, an improvement is proposed to enhance the simulation accuracy of a model used for plasma actuator eect under interaction with uid ow. In the modied model suggested, a boundary condition for charge distribution on the charged surface is presented based on a relationship between the independent electrical potential and charge density equations. Further, semi-empirical relations are utilized to calculate the produced plasma extend. The eect of the actuator on induced jet shows a good agreement with experimental results and does not need experimental tests for parametric calibration.

In the present work, cross section size investigation of the plenum chamber of a marine gas turbine air supply system was done. Flow in ducts make noise and in the case of turbine inlet due to amount of ow this noise is very high and should be considered. Using numerical simulations, a method to nd a suitable design considering both the acoustic and aerodynamic performance presented. In this method, at First air supply channel system of marine gas turbine according to the requirements of the aerodynamic and acoustic were categorized into three sections; input, plenum chamber and output channels with circular cross sections. The geometric dimensions of input and output channels determined using the theory of plane waves within the channel, without considering the eects of ow also considering the limits of space inside the oat at dominant frequency. Then suitable size of the intermediate cross section of the channel in terms of both aerodynamic and acoustic requirements with regard to the eects of ow using numerical methods were studied and determined. Various 3 Dimensional turbulent ows inside chamber are considered for this work where large eddy simulation turbulence model is used. Ffowcs, Williams and Hawkings model is used for the sound propagation process based on Lighthill integral equation. Validity of the simulation is checked by comparing results (sound pressure level) against experimental data in a chamber and error is acceptable over range of studied frequencies. The results showed that channel system aerodynamic performance decreasesanditsacousticperformanceinitiallyincreases and then decreases with the increase in cross section of the plenum chamber than the cross section of the inlet / outlet channel. In addition, deviation from plane waves in considering the eects of ow is observed in channel. That is due to the eects of the current owing through Channel system and the eect of quadrupole sources in the production of sound in the channel system, which causes higher modes.

N-point correlation functions are used for calculating properties of heterogeneous systems. Power and main advantage of statistical continuum technique is the direct link to statistical information of microstructures. Two-point correlation functions are the lowest order of correlation functions that can describe the relationship between morphology and properties of microstructures. Statisticalpaircorrelationfunctionsareachievedbyscattering electron microscopy or small-angle X-ray scattering experimentally. Higher order correlation functions must be calculated or measured to increase the accuracy statistical continuum approach. Microstructuretwo-pointcorrelationfunctionsarewell-knownclass of statistical descriptors for characterization and reconstructionofheterogeneousmicrostructures. Inthisstudy, a comprehensive review on statistical correlation functions that are used in micro and nanostructures is done and then, focused on reconstruction of these structures by using numerical and experimental approaches. Description and characterization of heterogeneous systems had high importance for scientists for last decades and dierent approaches have been developed for determining 3D descriptors of heterogeneous systems. Statistical continuum mechanics provided alternative approach for reconstruction and characterization of heterogeneous systems. Reconstructionapproacheshavebeenimproved by developing of numerous simulation techniques in recent years. Anisotropic structures, orientation distribution, shape, and geometrical features can be extracted from statistical correlation functions. 3D reconstruction of microstructures and nanostructures is a new way to investigate the behavior of these structures precisely. In this method the distribution of nanoparticles is determined, and their coordinates are saved which is very helpful for determination of their treatment. Mani different researches have used reconstruction approach to enter their experimental samples to nite element software. Then, by applying the properties of each phase they investigate about dierent behavior of their experimental samples. Depended on samples dierent approaches are used for reconstruction and researches have tried dierent ways to reduce reconstruction error. In this study, dierent approaches are discussed, and the calculation of error is presented.

The Counter-intuitive behavior which means that the permanent de ection of the elastic-plastic beam come to rest in opposite direction of the impulsive loading, normally appears and disappears abruptly, in certain small ranges of the loading and structural parameters. One of the most important issues in the study of this phenomenon is the determination of the in uence of the different parameters on this behavior. This present paper studied the eects of hardening in the counter-intuitive dynamic behavior of the elastic-plastic pin-ended beams under impulsive loading, by developing the proposed Galerkin's numerical model and by presenting a novel algorithm. Galerkin's method as well as Finite element code ANSYS/LS-DYNA were applied to study this phenomenon and the results of these two methods have been compared. In order to regard the hardening eects in Galerkin's method, a new algorithm was proposed. To test the validity of our proposed algorithm, the response of the elastic-perfectly plastic beam was rstly studied, and the hardening eects on the counter-intuitive response was investigated afterwards. The displacementtime history curves of mid-span of the beam were studied in detail and the region of the occurrence of the counterintuitive behavior was determined. Furthermore, using the nite element code, energy diagrams of the beam were also investigated. It was been found out that the counter-intuitive response is a phenomenon which is very sensitive to loading so that it may appear with a little change in the amount of loading. The results also showed that when considering the hardening eects, both methods predicted two continuous and distinct regions of loading for the occurrence of this phenomenon where the rst had a narrower band and occurred in the tight region of the impulsive loading while the next one had a wider band and occurred in the vast region of the impulsive loading. In addition, our investigations on the energy diagrams showed that, this anomalous behavior would occur in the proper proportions of kinetic to internal energy.

Microelectromechanical systems (MEMS) are used in many elds of industry like automotive, aerospace and medical instruments. Among the various ways to operate the MEMS devices, the electrostatic actuator is the common mechanism, due to simplicity and fast response. Previous experiments have shown that the mechanical behavior of devices, which their sizes are in order of micron and submicron, are dependent to size dependency. They also have illustrated that by decreasing the dimension of structures, the size dependent eect is highlighted. In this case, the classical theories are not capable to predict the size dependent eects and mechanical behavior of the microstructures properly. Therefore, nonclassical theories such as modied couple stress and strain gradient theories have been introduced. It was shown that the modied couple stress theory can accurately predict the size dependent behavior of microstructures. There are some in uences observed in the MEMS, that they have notable eects on the mechanical behavior of microswitches, such as fringing elds and large de- ection. When the air gap is larger than the electrode's width of microswitches, the impacts of fringing elds and geometric nonlinearity signicantly aect the mechanical behavior of the system. Therefore, neglecting the abovementioned eects leads to errors in the instability prediction of microswitches. Most of microswitches consist of a microcantilever with a proof mass and a xed substrate which there is an air gap between them. By applying voltage to the system, the microcantilever starts to de ect into the xed substrate. In this paper, pull-in instability and de ection of MEMS switches are investigated based on the size dependent model. The nonlinear model is introduced by considering modied couple stress theory and fringing eld eects as well as geometric nonlinearity. Utilizing the minimum total potential energy principle, the static equation of motion is derived in framework of the nonclassical theory. The eects of various parameters on static pull-in instability are studied and errors of considering the linear model or classical theories is calculated. The results show that the presented model is capable to predict the displacement and pull-in instability of the microswitches.

In this study, the eect of tube-insert as a passive method to increase heat transfer, instant gaslight water heater performance was evaluated. The simulation results revealed that for rotary tube-insert with dierent cross section shapes. The article present a numerical study of turbulent heat transfer and  ow characteristics inelliptical tube equipped with two-bladed rotary tubeinsert. Seven dierent tube-insert are tested in the current work; they included the two-bladed tube-insert with four dierent length of pitch 75, 100, 150 and 200 mm and three blade angle 40, 55 and 70 degree. In this study, the eect of tube-insert as a passive method to increase heat transfer, instant gaslight water heater performance was evaluated. To analyze the eciency and eectiveness tube-insert geometric parameters and physical parameters of the  ow, steady and incompressible  ow gaslight water heater, using the commercial software ANSYS CFX 14 was modeled. Turbulent  ow regime of the  uid is used. The numerical simulation is performed using SST turbulence model and wall function near the walls in ANSYS CFX to study the phenomena of  ow eld in a elliptical tube with helical blade rotors. The SIMPLE algorithm pressure-velocity coupling method, the standard pressure, the high resolution scheme discretization scheme for momentum, energy are selected in the simulation. The results of the simple tube compared with the theoretical Gnielinski and Petukhov relationship for validate of results. Using modeling results, performance parameters such as the coecient of friction, heat transfer coecient, and thermal performance has been calculated and analyzed. Eect of Tube-insert geometric parameters such as the length and angle of blades were evaluated on performance gaslight boilers. Reynolds numbers range from 3580 to 14320 is investigated. The average coecient of thermal performance for gaslight water heater equipped with two-bladed tubeinsert with length of pitch 75, 100, 150 and 200 mm, respectively, 1/017, 1/027, 1/033 and 1/042have been. By observing the results and compare them two-blade tubeinsert with blade angle 55 and 70 degrees with an average coecient of thermal performance 1/215 and 1/193 best performance in gaslight water heater have been studied. Therefore, the use of rotary tube-insert as a passive method can signicantly increase heat transfer in the heat exchangers. The increase in  ow velocity of the heat exchanger tube equipped with a rotary tube-insert reduce friction and prevent the accumulation of fouling. This method can also be used in heat exchangers.

The Counter-intuitive behavior which means that the permanent de ection of the elastic-plastic beam come to rest in opposite direction of the impulsive loading, normally appears and disappears abruptly, in certain small ranges of the loading and structural parameters. One of the most important issues in the study of this phenomenon is the determination of the in uence of the different parameters on this behavior. This present paper studied the eects of hardening in the counter-intuitive dynamic behavior of the elastic-plastic pin-ended beams under impulsive loading, by developing the proposed Galerkin's numerical model and by presenting a novel algorithm. Galerkin's method as well as Finite element code ANSYS/LS-DYNA were applied to study this phenomenon and the results of these two methods have been compared. In order to regard the hardening eects in Galerkin's method, a new algorithm was proposed. To test the validity of our proposed algorithm, the response of the elastic-perfectly plastic beam was rstly studied, and the hardening eects on the counter-intuitive response was investigated afterwards. The displacementtime history curves of mid-span of the beam were studied in detail and the region of the occurrence of the counterintuitive behavior was determined. Furthermore, using the nite element code, energy diagrams of the beam were also investigated. It was been found out that the counter-intuitive response is a phenomenon which is very sensitive to loading so that it may appear with a little change in the amount of loading. The results also showed that when considering the hardening eects, both methods predicted two continuous and distinct regions of loading for the occurrence of this phenomenon where the rst had a narrower band and occurred in the tight region of the impulsive loading while the next one had a wider band and occurred in the vast region of the impulsive loading. In addition, our investigations on the energy diagrams showed that, this anomalous behavior would occur in the proper proportions of kinetic to internal energy.

In this research, a Kalina system integrated with the parabolic trough solar collector is proposed to produce power-cooling load and power-heating load for summer and winter, respectively. The proposed system is modeled using the conventional and advanced exergy, exergoeconomic and exergoenvironmental analyses. Based on exergy modeling, in summer collector and in winter auxiliary heater are the worst components respectively with 19629 kW and 11561 kW exergy destruction rate while according the advanced exergy analysis the major portion of their exergy destructions are related to the endogenous part while the avoidable endogenous parts are zero. On the other hand, the avoidable exogenous subdivision of the collector in summer mood and auxiliary boiler for winter are signicant with values of 4492.84 kW and 2676.31 kW, respectively. From the viewpoint of economic, thermal storage tank with minimum investment cost rate (13.24 $/h) has the maximum exergy destruction cost rate within 1724.13 $/h and 1783.37 $/h respectively for summer and winter while the collector. Advanced exergoeconomic analysis indicates that the substantial portion of its avoidable part is exogenous indicating that the technological improvement of remaining components lead to the decrement of the exergry destruction cost rates of the auxiliary boiler. Moreover, the maximum value of the investment cost rate is related to the collector containing the maximum endogenous avoidable part with values of 646.465 $/h for summer and 455.015 $/h for winter. Similarly, in exergoenvironmental analysis, auxiliary heater with the lowest investment environmental impact has the maximum environmental impact associated with the exergy destruction with values of 242.11 Pts/h and 306.95 Pts/h respectively for summer and winter due to the utilization of the natural gas. Advanced exergoenvironmental analysis, the auxiliary heater has the maximum exogenous avoidable environmental impact associated with exergy destruction indicating the improvement of other components has a positive eect on decreasing of its environmental impact.

Carbon nano tube (CNT) and coiled carbon nano tube (CCNT) have exceptional properties and can improve mechanical, electrical and thermal properties of nano composites. Among carbon nanostructure, a great deal of interests has been recently focused on CCNTs due to their specic properties which stem from the specic helical structure, which this eld is one of the most interesting elds in engineering. In this paper, a multi-scale model is developed to study the mechanical properties in polyethylene reinforced by CNTs and CCNTs. In uence of volume fraction (VF) and geometric of the nano particle that is in two types of the CNT and CCNT, on overall stress - strain response have been investigated. Elastic modulus for each sample is calculated based on the linear part of stress-strain curve, and compared to each other. Results show with adding 1 percent of the CNT and CCNT to the polymer elastic modulus increases 17.5% and 9.5%, respectively, by using 1.5 percent of the CNT elastic modulus increase 21.4%. As a result, when the volume fraction increases the strength of nano composites enhances. In this paper also eect of orientation of CNT in matrix has been studied. Samples with 1 percent volume fraction in two isotropic and anisotropic states are generated and mechanical properties of these samples are studied. Results show a huge reduction in the stress-strain response of the anisotropic composite compared to the isotropic composite. Also eect of coil shape of CCNT were studied and compared with straight CNT. According to the results straight CNT is a better reinforcement compared to CCNT and as the coil angle increases, the overall stress-strain response improves as well.

Due to important rule of edge stresses in the delamination and local failure of laminated structures, the objective of this work is to analyze the interlaminar stresses in laminated hybrid composite cylindrical shell which is subjected to centrifugal body force using layerwise theory of Reddy (LWT). The model is under centrifugal forces caused by rotation of the cylinder about its axis. The principle of minimum total potential energy is used to obtain the governing equations of the rotating cylinder. The layerwise theory (LWT) of Reddy is employed for discretization of the governing equations to a system of ordinary dierential equations in terms of the displacement of numerical surfaces. To solve the equations of motion of the cylinder in the LWT, the equation is written in the decoupled form by denition of modal space variables. An analytical solution is presented for governing equation. The solution is completed by imposing the boundary conditions in the edges of the cylinder. The interlaminar and in-plane stresses are obtained using the displacement eld of layerwise theory for the free and clamped boundary conditions. The interlaminar stresses in the Hybrid cylindrical shell are obtained using two dierent methods, the constitutive law of the numerical layers, and integrating the equilibrium equations of motion. To validate the results, the results of LWT for cross-ply cylindrical shell are compared to those of Finite element solution in the commercial nite element code Ansys. The results showed an excellent accuracy of the layerwise theory in calculating the interlaminar and in-plane stresses in hybrid cylinder. Various gures from the distribution of the stresses along the interfaces of layers and through the thickness of rotating cylinder are presented in the numerical results. The eects of the geometry, the types of layering, the speed of rotation and hybridation of laminated cylindrical shell on the interlaminar stresses are analyzed. Consequently, the delamination of the layered can be predicted and prevented.

Automatic line-break control valve can be used in zones with no access to national electricity network, hazardous or impassable area enclosure, for passive defence conditions or to protect dierent ecosystems such as rivers and ground water, forests and fertile land and etc. The pipeline break causes a growth in oil velocity magnitude and uid pressure reduction consequently but the pressure drop is not a good signal for line break detection because it is low, therefore reference tank is used. The reference tank is connected to the pipeline through a calibrated orice with check valve. The tank pressure is higher than pipeline pressure when pressure reduction occurs by line breaking. If the pressure dierence between tank and pipeline is higher than the sustainable pressure value of diaphragm valve, the valve will be closed by actuator. In this paper, the eects of orice diameter, pipeline initial pressure and pipeline break pressure drop rate on the dierential pressure of diaphragm valve in automatic line-break control valve have been studied with nitrogen by 27 experiments. The dierential pressure of diaphragm valve is increased by the growth of pipeline break pressure drop rate. The occurrence time of maximum dierential pressure of diaphragm valve just depends on orice diameter. The maximum of dierential pressure increases with decrease of orice diameter. The dierential pressure of diaphragm valve increases by decrease of pipeline initial pressure for constant orice diameters and pipeline break pressure drop rates. The curves of maximum dierential pressure generated by dierent pipeline break pressure drop rates are shown for dierent orice diameters and pipeline initial pressure. The values of curves in this diagram with 10 percent safety factor can be used in automatic line-break control valve setting which are installed on gas transportation pipelines because of the nitrogen gas using instead of the natural gas.

In order to create a laser beam, one common method is to use beam pumping. In this method, beam is rstly emitted to the active medium of laser crystal and the electrons in the last shell of atoms are stimulated. With some internal interactions, laser beam will be created and some pumping energy is converted to heat in the crystal. In solid-state lasers, large amounts of pumping power may lead to heat generation in the active laser medium and a non-uniform temperature distribution in the crystal is generated. The temperature gradient causes thermal stress in the crystal structure and can be eective on thermomechanical behavior of the crystal and the quality of the laser beam. With increasing the pumping power, the heat generation within the active media is increased and consequently the thermal eects including the thermal lensing, polarization of output beam, mechanical stress, reducing the eciency of output beam, are intensied. Even in this case, thermal stress causes failure. Hence to achieve a high quality beam laser, it is necessary to investigate the thermomechanical behavior of the laser crystal, through the designing of solid state lasers. In the past works, thermomechanical behavior of the crystal was investigated in framework of Fourier heat conduction theory (Heat wave diusion speed is considered to be innite), while this theory is not appropriate to study the heat conduction for medium under pulsed heat with appreciable relaxation time. Hence, in this research, by providing a nite element formulation based on the non-Fourier thermoelasticity (with considering relaxation time), distributions of temperature and stress in the Nd:YVO4 crystal are calculated. The results show that the maximum temperature and stress calculated by non-Fourier theory, are larger than those calculated by the Fourier theory and also the non-Fourier theory predicts lower failure power for crystal. In addition, if the relaxation time is lower than a certain value, the results of classical and nonclassical theories are closely equal to each other.

In this paper, the performance of the rigid polyurethane (PU) foams used in composite armors to mitigate the impact pressure and energy of a projectile has been studied numerically through nite element methods. The armors of this study comprise two layers, namely, a metal layer and a PU-foam layer. A sophisticated FE-Model was developed to simulate the dynamic performance of the composite armor under the impact of a projectile using the software package ANSYS-Autodyn. The FEModel was veried with the results of a previous experimental study in which a projectile impacted a certain two-layer armor comprising a steel front face that was attached on a layer of PU-foam. The main objectives of this research study are to study the eects of design parameters such as the thickness and density of the PU-foam and the impact velocity of the projectile (representing the strain rate) on the performance of a composite armor. To achieve the research objectives, PU-foams of dierent densities of 80, 160, 288 and 320 kg/m3, at six dierent thicknesses of 1.25, 2.5, 5, 7.5, 10 and 12.5 mm were modeled. Additionally, the armors were subjected to projectiles with dierent impact velocities of 400, 450, 509 and 550 m/s. Results of the FE-Analysis runs conducted in this paper indicate that the response mitigation of a composite armor is signi- cantly aected by both the density and thickness of its PU-foam layer. For a given thickness of foam layer, it was observed from the FE-analysis runs that the optimum value for the foam-density that results in the maximum response mitigation is approximately 150 kg/m3. The impact pressure that is dissipated by a composite armor is signicantly decreased as the foam-density diverts from its optimal value. For a given foam-density, it was found that the response mitigation of the armor was improved with an increase in the thickness of the foam layer. For the composite armors investigated in this study, the rate of improvement in the response mitigation with increased foam-layer thickness was found to be very slow for the thickness values beyond 5 mm. The impact velocity of the projectile, as an indicator of strain rate, was also investigated in this paper. The dynamic response of foam layers of relatively lower thickness and density was found to be more sensitive to the variations of strain rate.