Sharif Digital Repository

This paper deals with numerical modeling of two-phase liquid jet breakup using the smoothed particle hydrodynamics (SPH) method. Simulation of multiphase flows involving fluids with a high-density ratio causes large pressure gradients at the interface and subsequently divergence of numerical solutions. A modified procedure extended by Monaghan and Rafiee is employed to stabilize the sharp interface between the fluids. Various test cases such as Rayleigh-Taylor instability, two-phase still water and air bubble rising in water have been conducted, by which the capability of accurately capturing the physics of multiphase flows is verified. The results of these simulations are in a good... 

The increment of using fossil fuels has caused many perilous environmental problems such as acid precipitation, global climate change and air pollution. More than 50% of the energy that is used in the world is wasted as heat. Recovering the wasted heat could increase the system efficiency and lead to lower fuel consumption and CO2 production. Organic Rankine cycle (ORC) which is a reliable technology to efficiently convert low and medium temperature heat sources into electricity, has been known as a promising solution to recover the waste heat. There are numerous studies about ORC technology in a wide range of application and condition. The main objective of this paper is to presents a... 

Electrospray cooling is experimentally examined for high volumetric flow rates (80 mL/h) stabilized by a novel hemispherical nozzle. Stability of the Taylor cone-jet mode of this nozzle is increased by interaction of the discharging liquid with the outer wall of a hemispherical cap, installed at the tip of a simple nozzle. The liquid can then be discharged at high flow rates at the same mode. The liquid is ethanol and the test surface temperature as well as the cooling heat flux are measured. The results of the surface temperature, the heat flux and the heat transfer coefficient are compared between the three cases. The first and main case is the electrospray using the novel hemispherical... 

Spray cooling characteristics of a flow blurring injector are investigated experimentally. Variations of the flow rates and the distance from the nozzle to the test surface are examined in terms of the cooling heat flux. The liquid flow rate is varied from 200 to 1200 mL/h. The effect of the air flow rate from 6 to 18 L/min is also studied and heat transfer coefficients are obtained. Depending on different effective parameters, the maximum cooling heat flux is measured between 61 and 193 W/cm2. Owing to formation of fine droplets at higher air flow rates, the maximum cooling heat flux is improved by about 60%. The present data is generated for a flow blurring type of injector to have four... 

A general regression neural network technique was applied to design optimization of a liquid-liquid coaxial swirl injector. Phase Doppler Anemometry measurements were used to train the neural network. A general regression neural network was employed to predict droplet velocity and Sauter mean diameter at any axial or radial position for the operating range of a liquid-liquid coaxial swirl injector. The results predicted by neural network agreed satisfactorily with the experimental data. A general performance map of the liquid-liquid coaxial swirl (LLCS) injector was generated by converting the predicted result to actual fuel/oxidizer ratios. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA  

A general regression neural network technique is applied for design optimization of liquid-liquid coaxial swirl injectors. Phase Doppler Anemometry measurements for velocity and SMD distributions for various Reynolds numbers are used to train the neural network to generate the injector's performance map. Excellent agreement between the predicted values and the measurements is obtained. It is observed that by reducing the number of randomly selected training samples to about one third of the number of prediction points, one may reconstruct the velocity field in the extrapolation regime with an accuracy of 93%. Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc.... 

A general regression neural network technique is proposed for design optimization of pressure-swirl injectors. Phase doppler anemometry measurements for velocity distributions are used to train the neural network. An overall optimized value for the width of the probability is determined. The velocity field in the extrapolation regime is reconstructed with an accuracy of 93%. Excellent agreement between the predicted values and the measurements is obtained. The results indicate that the capability of performing designand optimization studies for pressure-swirl injectors with sufficient accuracy exists by applying modest amount of data in conjunction with an overall optimized value for the... 

Cylindrical pipes are installed to a line-focusing solar system with a linear receiver tube for transmitting thermal energy to the working fluid. In this study, the effects of a novel forward ring step inside circular pipes on the heat transfer performance of linear solar receiver tubes were investigated using computational fluid dynamics. The rings are perforated, and their cross section is triangular. Although the applied heat flux is consistent with a solar collector with a linear receiver tube, the analysis can be performed for any given heat flux distribution on circular pipes. The model was verified by comparing the predicted Nusselt numbers to those of the Gnielinski correlation, and... 

Liquid impingement cooling in the form of jet or spray is an appropriate method to remove heat from small surfaces progressively utilized in industry. In this study, numerical simulations are conducted to investigate the effect of surface structure on heat transfer characteristics. Five different three-dimensional structured surfaces are investigated using Euler-Lagrangian approach. The effect of surface structures is discussed on the liquid film thickness, the liquid film velocity, and the heat flux with its uniformity for a given spray and temperature characteristics. The results show that the convective heat transfer strongly depends on both liquid film thickness and velocity. While the... 

For intermittent spray-cooling purpose, it is essential to study the unsteady aspects of film evaporation and heat-transfer characteristics. In the present study, total evaporation time and surface temperature variations are investigated for four different liquid films (water, ethanol, n-octane, and n-hexane). The evaporation process is analyzed using a three-dimensional spray-wall impact with Lagrangian wall-film model. The evaporation process occurs in three stages; at the initial moments, most of the heat is used to raise the film temperature, and slight evaporation also exists. The film temperature rises until it reaches the liquid saturation point to evaporate at a constant rate. In the... 

Liquid cooling is an efficient way of thermal management for high CPV cells, increasing the system performance by reducing cell's temperature. Thermal management complexity rises upon using an array of cells while the system becomes sizeable. The present numerical simulations follow a single cell study to investigate spray cooling for high CPV arrays with minimum number of injecting nozzles. The idea is to cool the cells by the liquid film formed on the thermal paste surface. Cooling the whole surface by the help of the liquid film decreases the number of nozzles to less than the number of cells. Both mass flow rate and pumping power are reduced, improving the system efficiency and lowering... 

An experimental investigation of the flow field of pressure swirl atomizers for different pressure drops is performed. Two experimental methods are employed. For low injection pressures, a visual study is conducted while for higher pressures the velocity components and size of the droplets are measured by using Phase Doppler Anemometer. The spray formation, spray cone angle, and breakup length are visually investigated. Velocity and size of the droplets are measured on a plane of measurement along the axis of the spray so that the properties are carried out in a radial direction as a function of both pressure drop and axial distance from the nozzle. Self-similar mean axial velocity profiles... 

Radial Basis Function, a neural network based interpolation method, is used to simulate swirling spray phenomenon. From this model, the profile of mean velocity components' distribution along the main axis of the injector at various pressure drops and spray measurements' plane are obtained and analyzed. Experimental data obtained through Phase Doppler Anemometer is used to train this model. The results are compared with experimental data as well as to those obtained through General Regression Neural Network method as another neural network based interpolation method investigated previously by the authors. © 2003 by M.R. Soltani  

Experiments are performed to investigate the atomization characteristics of mixed-interaction regions of sprays of two swirl injectors installed side by side. Both droplet size and velocity distributions on a plane perpendicular to the axes of the injectors are measured using a PDA system. As a result of the interaction phenomenon, a region of secondary atomization is identified that differs significantly from the hollow region spray of a single swirl injector. A neural network algorithm is used to reconstruct the entire spray field for both droplet size and velocity distribution in extrapolation regimes for injector spacing as well as three dimen sional spatial coordinates. Excellent... 

An experimental investigation is performed to explore the characteristics of sprays produced by a liquid-liquid coaxial swirl injector in a non-combusting environment. Phase Doppler anemometry is used for the measurement of velocity and Sauter Mean Diameter of droplets across and along the injection axis for various inner- and outer mass flow rates. Results of the combined spray are compared with those of inner- and/or outer spray alone. The results indicate that the inner injector has a larger influence on the flow field of the combined spray compared to the outer one. Further, the maximum velocity of the combined spray is close to the center of the spray thickness and the velocity... 

A stable capillary liquid jet formed by an electric field is an important physical phenomenon for formation of controllable small droplets, power generation and chemical reactions, printing and patterning, and chemical-biological investigations. In electrohydrodynamics, the well-known Taylor cone-jet has a stability margin within a certain range of the liquid flow rate (Q) and the applied voltage (V). Here, we introduce a simple mechanism to greatly extend the Taylor cone-jet stability margin and produce a very high throughput. For an ethanol cone-jet emitting from a simple nozzle, the stability margin is obtained within 1 kV for low flow rates, decaying with flow rate up to 2 ml/h. By... 

A novel MED-based desalination system for low-grade heat sources with a temperature between 65 °C and 90 °C has been proposed. Low-grade heat-driven desalination systems are of the most important methods for increasing energy efficiency and assisting the environment along with alleviating the water scarcity problem. The new proposed system is known as cascade multi-effect distillation (CMED). It is designed based on multi-step heat extraction from the heat source fluid (HSF). Multi-step heat absorption helps to form two arrays of MED effects and to utilize two boosters to achieve better performance. The advantage of this system is less complexity (for example, lower vapor injection), and... 

Chiral recognition has long been a challenging issue to deal with in biological systems, drug design and food authentication. Implementing nanoparticle-based probes with intrinsic or induced chirality in this field has addressed several issues concerning sensitivity, reliability, rapidness and the cost of chiral sensing platforms. Yet, research into chiral nanoprobes that can be used for visual monitoring of chiral substances is still in its infancy. As part of this study, a visual chiral recognition platform has been developed in which a combination of blue-emitting carbon dots (BCDs) and mercaptopropionic acid-capped CdTe quantum dots (MPA-QDs) with inherent chiroptical activity were... 

Phase Doppler Anemometer (PDA) was used to measure velocity distribution in the mixed spray field of two identical pressure-swirl atomizers placed beside each other. A memory-based network that provides estimates of continuous variables was developed to simulate the results. This general regression neural network (GRNN) is a one-pass learning algorithm with a highly parallel structure and can be used for any multidimensional regression problem, where an assumption of linearity is not justified. The approach is not limited to any particular form and requires no pre-knowledge of the appropriate form that any usual correlation technique requires. The predicted results are in excellent agreement... 

The proof of the first Carnot theorem in classical thermodynamics is revisited in this study. The underlying conditions of a general proof of this principle presented by Senft (1978 Phys. Educ. 13 35-37) are explored and discussed. These conditions are analysed in more detail using a physical description of heat and work to present a simpler proof of the first principle prior to using the violation of the second law of thermodynamics. Finally, a new simple proof is also presented based on Gibbs relation. This discussion will benefit the teaching of classical thermodynamics and promote better understanding of the proof of the first Carnot theorem in general form