Sharif Digital Repository

In this work, we study the gas mixing behavior in a micro T-mixer using the direct simulation Monte Carlo (DSMC) method. The gas mixing process is monitored through a T-mixer, which is fed by two different CO and N2 gases; flowing into the T-mixer through the upper and lower inlets. We investigate the effects of axial and lateral wall temperature gradients on the mixing evolution at different rarefaction levels. The achieved results show that any temperature difference between the channel walls would result in an increase in mixing length for the chosen wall temperature gradient ranges and the studies pressure cases. Our observations show that a positive temperature gradient toward the... 

The present study attempts to analyze the extended Graetz problem in combined electroosmotic and pressure driven flows in rectangular microchannels, by employing a variational formulation. Both the Joule heating and axial conduction effects are taken into consideration. Since assuming a uniform inlet temperature profile is not consistent with the existence of these effects, a step change in wall temperature is considered to represent physically conceivable thermal entrance conditions. The method of analysis considered here is primarily analytical, in which series solutions are presented for the electrical potential, velocity, and temperature. For general treatment of the eigenvalue problem... 

We use the direct simulation Monte Carlo (DSMC) method and investigate the subsonic rarefied gas flow through micro/nanochannels, imposing a constant pressure ratio and a constant temperature difference between the inflow and wall temperature. We further study the heat transfer characteristics of subsonic nitrogen gas flow under this imposed temperature difference. We show that, specifying a higher temperature magnitude would lead to more rarefactions even imposing a fixed temperature difference. This consequently results in a higher wall heat flux rate for a fixed inflow-wall temperature difference. Our investigating shows that the number of simulated particles need to increase suitably if... 

Heat pipes are important cooling devices that are widely used to transfer high heat loads with low temperature differences. In this paper, thermal performance of a novel type of sintered-wick heat pipe, namely, partly sintered-wick heat pipe, was investigated. The heat pipe was filled with degassed water and acetone, as working uids, and effects of filling ratio, orientation, and heat inputs were tested. Moreover, conditions at which dryout occurred were presented. The results showed that the best filling ratio for both working uids is about 20%. The heat pipe filled with water has better thermal performance than that filled with acetone; thus, the thermal resistances of the 20% water-filled... 

In this article, a novel solar still which is equipped with thermoelectric modules to utilize the heat of vapor condensation is experimentally investigated. The use of thermoelectric modules as devices for generating electricity from the temperature difference between two environments (hot vapor steam and cold ambient air) as well as taking outstanding advantages of evacuated tubes as collectors with high performances in adverse climatic conditions, led to utilizing of the energy dissipated due to vapor condensation and a considerable increase in the performance of the system. The effect of using the generated electricity to operate a small propeller fan for inducing forced convection was... 

Developing ocean energy extraction, including ocean thermal energy conversion (OTEC), has been of interest to researchers for many decades. OTEC is a free fuel technology and could be used as a baseline power generation. These advantages plus the engineering challenge have resulted in researchers striving to design and construct prototype devices. Making use of worldwide experience, all sections of a conceptual design including site selection, technical specifications and cost estimation were carried out for an Iranian OTEC power plant. A 5 MW closed cycle floating plant with an annual average temperature difference of 22°C was chosen at a 33 km distance from Chabahar harbour. Deep seawater... 

Pulsating Heat Pipes (PHPs) are cooling devices that are compact in size and have an ability to transfer heat in low temperature differences. Working fluids strongly affect the thermal performance of PHPs. In this paper, effects of some thermophysical parameters relating to working fluids, such as boiling point, latent heat of vaporization, surface tension, thermal conductivity and dynamic viscosity, are presented based on experimental and numerical studies done in recent years. Addition of nanoparticles to fluids, or making nanofuild, is a new method of improving thermophysical properties of fluids. Recently, many studies are carried out on thermophysical properties of nano-fuild. Results... 

A fluctuation relation for the heat exchange of an open quantum system under a thermalizing Markovian dynamics is derived. We show that the probability that the system absorbs an amount of heat from its bath, at a given time interval, divided by the probability of the reverse process (releasing the same amount of heat to the bath) is given by an exponential factor which depends on the amount of heat and the difference between the temperatures of the system and the bath. Interestingly, this relation is akin to the standard form of the fluctuation relation (for forward-backward dynamics). We also argue that the probability of the violation of the second law of thermodynamics in the form of the... 

Development of new techniques for detection and analysis of xenon/krypton mixtures is significant for both industrial and environmental purposes. In this research, direct simulation Monte Carlo is applied to analysis Xenon/Krypton gas mixtures through different molecular forces inside a new micro gas sensor (MIKRA). In this device, a temperature difference inside a rectangular enclosure with heat and cold arms as the non-isothermal walls induces a molecular force known as Knudsen force at low pressure condition. This force is proportional to the main characteristics of the gas mixture. In order to simulate a rarefied gas inside the micro gas detector, Boltzmann equations are applied to... 

Hot water heating coils are key components in air conditioning systems undergo heat transfer. The most widely used exchangers (coils) take the form of fin and tube configuration in association with the application of air conditioning systems. In this research, thermodynamic modeling and mathematical optimization of hot water heating coils at steady conditions, approaching minimum energy consumption are achieved. The modeling procedure for heating coils is done based on the log mean temperature difference (LMTD) method. The objective function for optimization is pressure drop of air crossing coil per heating load of the system. This unction comprises all thermal and geometrical parameters of... 

Several SAPO-34 samples with different Si/Al ratio (0.05-0.5) of synthesis gel were prepared hydrothermally at crystallization temperature of 190-215 °C. The products were characterized by XRD, SEM/EDX, NH3-TPD, FT-IR and nitrogen adsorption techniques. EDX data showed a deviation of elemental composition of products with respect to the Al, Si, and P from that of the gel mixture, but the increasing trend of Si/Al ratio of the products was confirmed. According to XRD and SEM results, crystallinity of samples increased with Si/Al ratio up to the low to medium range of ca. 0.13. However, a reduction in the crystallinity was observed with further increase of the silicon content of the synthesis... 

The goal of this study is the experimental and numerical investigation of using pulsating heat pipes (PHPs) as substitutes for fins in a typical air-cooled heat exchanger. Due to the small temperature difference between the cooling air and internal airflow in the case study, it was necessary that appropriate PHPs with the capability to work in small temperature difference conditions be investigated and an appropriate working fluid be selected. After reviewing various studies, R134a was selected as the best working fluid from the heat transfer standpoint. Afterward, a testing device was designed and fabricated. Additionally, in order to predict the behaviors of the main tubes with pulsating... 

In this paper, by using first-principle density functional theory (DFT) combined with non-equilibrium Green's function (NEGF), thermally induced spin current in zigzag and armchair Antimonene Nanoribbon (SbNR) is investigated. Also, we obtain higher spin current in Armchair nanoribbon (ANR) than zigzag nanoribbon (ZNR), because the start energy of transmission for ANR is closer to the Fermi level than ZNR. The results show that the device has a perfect spin Seebeck effect under temperature difference without gate voltage or bias voltage. For the ANR configuration, the competition between spin up holes and spin down electrons leads to negative differential behavior of charge current, which is... 

Natural convection in cavity has become one of the classical heat transfer problems with a large volume of research performed both experimentally and numerically. There are several permutations of the cavity problem related to its shape, its boundary conditions, the properties of circulating fluid, etc. Among them, the most interesting one is that of a rectangular cavity, which is maintained at hot and cold temperatures on its opposing side walls while its horizontal walls are thermally insulated. Contrary to the past works, we study the cavity with small to high temperature differences. This produces a wide range of compressibility effects in the cavity, which need to be treated carefully... 

In this paper, the application of neural networks for simulation and optimization of the cogeneration systems has been presented. CGAM problem, a benchmark in cogeneration systems, is chosen as a case study. Thermodynamic model includes precise modeling of the whole plant. For simulation of the steady sate behavior, the static neural network is applied. Then using dynamic neural network, plant is optimized thermodynamically. Multi layer feed forward neural networks is chosen as static net and recurrent neural networks as dynamic net. The steady state behavior of CGAM problem is simulated by MFNN. Subsequently, it is optimized by dynamic net. Results of static net have excellence agreement... 

The solar collectors absorb solar irradiation and then produce thermal energy; however, they cannot generate electrical power. Electricity and thermal energy can be produced by the photovoltaic thermal module, simultaneously, while the outlet temperature of the photovoltaic thermal module is not usually high enough to provide thermal requirements of a building such as hot water or space heating. This work proposes a novel compound system created by the connection of a solar collector in series with a photovoltaic thermal module to resolve the issue of low outlet temperature in the photovoltaic thermal module and lack of electrical power in solar collectors. To examine the feasibility of this... 

In this study, modeling of an irreversible thermoelectric heat pump was conducted, and its performance was assessed in terms of exergy for 10, 20, 30 and 40 K difference in temperature (∆T) by changing the values of the design parameters. By employing this model, positive impact of increasing cross-section area, current and thermocouple’s length which in turn increases the exergy efficiency is realized. In addition, diminishing adverse impact of adding more thermocouples on the exergy efficiency of the system is illustrated. Afterward, exergoeconomic performance of the thermoelectric heat pump is evaluated. Then, exergoeconomic factor for each of the system’s components is diagnosed. The... 

In this study, four new indirect heat pump assisted mechanical vapor compression humidification-dehumidification (HDH) systems are proposed and their superiorities over the reference system are demonstrated from thermodynamics and thermoeconomics viewpoints. The proposed models are configured based on an HDH unit and a simple cascade heat pump, an HDH unit and a heat pump with an ejector, an HDH unit and a cascade heat pump with an ejector, and an HDH unit and a vapor injection heat pump. Although employing a heat pump with cascade and ejector configurations improved gain-output-ratio (GOR) and specific power consumption (SPC) values in comparison with the base system, the performance... 

Three novel geothermal-based organic Rankine cycle (ORC) systems are proposed to enhance the efficiency and for waste heat recovery purpose. The proposed systems are modeled based on a basic ORC system (concept 1), an ORC system with an internal heat exchanger (concept 2), and a regenerative ORC system (concept 3). Accordingly, two thermoelectric generators (TEG) are introduced into the systems to exploit the waste heat of the system. The condenser is replaced with a TEG unit while the other TEG unit is used to recover the waste heat of the reinjected geothermal fluid. A comprehensive numerical investigation is conducted to compare the proposed systems from the thermodynamic and... 

In this study, a numerical and empirical scheme for increasing cooling tower performance is developed by combining the particle swarm optimization (PSO) algorithm with a neural network and considering the packing’s compaction as an effective factor for higher accuracies. An experimental setup is used to analyze the effects of packing compaction on the performance. The neural network is optimized by the PSO algorithm in order to predict the precise temperature difference, efficiency, and outlet temperature, which are functions of air flow rate, water flow rate, inlet water temperature, inlet air temperature, inlet air relative humidity, and packing compaction. The effects of water flow rate,...