Sharif Digital Repository

In this study an expression for soot absorption coefficient is introduced to extend the weighted-sum-of-gray gases data to the furnace medium containing gas-soot mixture in a utility boiler 150 MWe. Heat transfer and temperature distribution of walls and within the furnace space are predicted by zone method technique. Analyses have been done considering both cases of presence and absence of soot particles at 100% load. To validate the proposed soot absorption coefficient, the expression is coupled with the Taylor and Foster's data as well as Truelove's data for CO2-H2O mixture and the total emissivities are calculated and compared with the Truelove's parameters for 3-term and 4-term gray... 

During the year, due to weather conditions, the temperature fluctuations at surface level cause problems in underground pipes as a result of freezing water. One of the best prevention strategies is the use of polyurethane floor insulation for keeping the temperature of clay above zero degrees Celsius. In this study to calculate the minimum thickness of polyurethane insulation layer, the differential equation of energy is solved based on principle of separation of variables using imaginary eigenvalues for consistency with the temperature distribution in multi-layer consist of asphalt, gravel and polyurethane with finite thickness and clay as a semiinfinite medium with periodic thermal... 

It is well established that nanofluids increase or decrease heat transfer in boiling phenomenon. The study acquired Quenching curve and boiling curve in two different surface roughnesses in two fluids: deionized water and the nanofluid MWCNT-water with four different concentrations. The cylinder was made of silver and two surface roughnesses of 129 and 690 nm. It was heated up and soaked in the fluid mentioned above. Temperature was recorded by a drilled to install thermocouple. The experiment was replicated in five times. To calculate the heat transfer quotient, assuming the cylinder to be thermally homogeneous, the Lumped capacity method was applied. The obtained results during quenching... 

The goal of the present study is to predict the formation of solidification induced defects in castings by thermal criteria functions. In a criterion function method, the heat transfer equation is firstly solved, and then the susceptibility of defect formation at every point in the casting is predicted by computing a local function, criterion function, using results of the thermal analysis. In the first part of the paper, some famous criteria functions, in particular, the Pellini and Niyama criteria, are analyzed and their shortcomings are discussed in details. Then, a new criterion function is suggested to decrease the shape-dependency issue of the former criteria. The feasibility of the... 

A twodimensional model, based on conservation of mass, momentum and energy equations, is represented in this paper in which the coke combustion process, for iron ore sintering in a packed bed, is simulated numerically. The aforementioned packed bed consists of iron ore, coke, limestone and moisture. The main objective of iron ore sintering is producing resistant agglomerates which can be used in blast furnaces. For this purpose, the sinter mixture is partially melted in high temperature and finally molten is allowed to cool. The molten production and subsequently, the solidification process are totally dependent on composition and components of mixture. Changes in bed porosity, caused by... 

We report improved performance of Li-ion polymer batteries through advanced gel polymer electrolytes (GPEs). Compared to solid and liquid electrolytes, GPEs are advantageous as they can be fabricated in different shapes and geometries; also ionic properties are significantly superior to that of solid and liquid electrolytes. We have synthetized GPE in form of membranes by trapping ethylene carbonate and propylene carbonate in a composite of polyvinylidene fluoride and N-methylpyrrolidinore. By applying phase-transfer method, we synthetized membranes with micro-pores, which led to higher ionic conductivity. The proposed membrane is to be modified further to have higher capacity, stronger... 

Microbial fuel cells (MFCs) have been a potential green energy source for a long time but one of the problems is that either the technology must be used on a large scale or special equipment have been necessary to keep the fuel cells running such as syringe pumps. Paper-based microbial fuel cells do not need to have a syringe pump to run and can run entirely by themselves when placed in contact with the fluids that are necessary for it to run. Paper-based microbial fuel cells are also more compact than traditional MFCs since the device doesn't need any external equipment to run. The goal of this paper is to develop a microbial fuel cell that does not require a syringe pump to function. This... 

The present study examines Alternating Current (AC) electroosmotic flows in a parallel plate microchannel subject to constant wall temperature. Numerical method consists of a central finite difference scheme for spatial terms and a forward difference scheme for the temporal term. Asymmetric boundary conditions are assumed for Poison-Boltzmann equation for determining the electric double layer (EDL) potential distribution. The potential distribution is then used to evaluate the velocity distribution. The velocity distribution is obtained by applying slip boundary conditions on the walls which accounts for probable hydrophobicity of surfaces. After determining the velocity distribution... 

Using reverse nonequilibrium molecular dynamics simulations the influence of intermolecular bridges on the thermal conductivity (λ) in carbon nanotube (CNT) bundles has been investigated. The chosen cross linkers (CH 2, O, CO) strengthen the transversal energy transport relative to the one in CNT bundles without bridges. The results showed that λ does not increase linearly with the linker density. The efficiency of the heat transport is determined by the number of linkers in the direction of the heat flux, the type of the linker, and their spatial ordering. The influence of a forced axial stress on the transversal λ has been also studied. The observed λ reduction with increasing axial... 

This research study presents an experimental investigation on forced convection heat transfer of an aqueous ferrofluid flow passing through a circular copper tube in the presence of an alternating magnetic field. The flow passes through the tube under a uniform heat flux and laminar flow conditions. The primary objective was to intensify the particle migration and disturbance of the boundary layer by utilizing the magnetic field effect on the nanoparticles for more heat transfer enhancement. Complicated convection regimes caused by interactions between magnetic nanoparticles under various conditions were studied. The process of heat transfer was examined with different volume concentrations... 

We study numerically the heat transfer of steady laminar flow in a square cavity filled with electrically conducting fluids, in the presence of an external uniform magnetic field. Imposing a large temperature gradient between two opposite vertical walls, there are substantial temperature and density variations in the domain. The fluid is treated as an ideal gas. Indeed, high temperature gradient thermo-buoyant cavity flows result in natural convection flow domains with high Rayleigh number. To implement the temperature variation effect, the fluid properties, including the conductivity and viscosity coefficients, are considered to vary with temperature in accordance to the Sutherland's law.... 

Oil leakage of transportation pipelines causes a change in its environmental temperature. This effect is employed to detect the leakage and its positions. The Raman Optical Time Domain Reflectometer (ROTDR) and Brillouin Optical Time Domain Amplifier (BOTDA) sensors are two of the most precise oil pipeline leak detector systems operate based on this effect. The position of leakage is determined by the time difference between the sending and backscattered laser pulses. In this paper, the transient response of BOTDA and ROTDR sensors are obtained through solution of the mass, energy and heat transfer in soil and fiber cable. It is shown that the mechanical rise time is of the order of a few... 

Pulsating heat pipes (PHPs) are interesting heat transfer devices. Their simple, high maintaining, and cheap arrangement has made PHPs very efficient compared to conventional heat pipes. Rotating closed loop PHP (RCLPHP) is a novel kind of them, in which the thermodynamic principles of PHP are combined with rotation. In this paper, effect of rotational speed on thermal performance of a RCLPHP is investigated experimentally. The research was carried out by changing input power (from 25. W to 100. W, with 15. W steps) and filling ratio (25%, 50%, and 75%) for different rotational speeds (from 50. rpm to 800. rpm with an increment of 125. rpm). The results presented that at a fixed filling... 

Both experimental and numerical studies are unanimous for enhancing Nusselt number for forced convection of nanofluids with slight difference, but there is inconsistency for natural convection heat transfer of nanofluids. In this paper attempt is made to study the effects of nanoparticles migration on the natural convection behavior of nanofluids. For analysis, a mixture model is used by including important phenomena such as Brownian motion and thermophoresis effects. These two mechanisms are taken into account to compute the slip velocities between the base fluid and nanoparticles. The governing equations are solved numerically and good agreements are observed in comparison with... 

Purpose - The purpose of this paper is to study a two-dimensional steady convective flow of a micropolar fluid over a stretching sheet in the presence of radiation with constant temperature. Design/methodology/approach - The corresponding momentum, microrotation and energy equations are analytically solved using homotopy analysis method (HAM). Findings - To validate the method, investigate the accuracy and convergence of the results, a comparison with existing numerical and experimental results is done for several cases. Finally, by using the obtained analytical solution, for the skin-friction coefficient and the local Nusselt number as well as the temperature, velocity and angular velocity,... 

Pulsating heat pipes (PHPs) are simple, cheap, and efficient heat transfer devices. They have applications in electronic cooling. In the present research, an experimental investigation is conducted on startup and steady thermal performances of open loop pulsating heat pipes (OLPHPs). Effects of working fluid, heat input, non-condensable gases (NCGs), ferrofluid concentration, magnets location, and inclination angle on the thermal performance of OLPHPs have been considered. Obtained results show that using ferrofluid can improve the thermal performance in steady state condition. Furthermore, applying a magnetic field enhances the heat transfer characteristics of ferrofluidic OLPHPs in both... 

In order to reach a more efficient and compact heat exchanger, it is essential to optimize the design, having in mind the impact of different geometrical parameters. Many of the previously cited studies in the area of heat transfer enhancement using vortex generators were confined only to defined points in the possible design space. Thus, a multiobjective optimization study is particularly suitable in order to cover this space entirely. A CFD simulation along with Pareto method were used to simulate the air flow and heat transfer and optimize the design parameters. The angle of attack of a pair of delta-winglets mounted behind each tube is varied between β = -90° and ß = +90°. Three... 

In the present research an experimental investigation is performed to explore the effects of working fluid, heat input, ferrofluid concentration, magnets location, and inclination angle on the thermal performance of an Open Loop Pulsating Heat Pipe (OLPHP). Obtained results show that using ferrofluid can improve the thermal performance and applying a magnetic field on the water based ferrofluid decreases the thermal resistance. It shows that at an inclination angle of the OLPHP to be zero, the thermal performance of the present OLPHP reduces. Best heat transfer capability was achieved at 67.5 degree relative to horizontal axis for all of working fluids. Variation of the magnets location... 

The present work investigates the thermal performance of a five turn Open Loop Pulsating Heat Pipe (OLPHP). The effects of working fluid namely water and ferrofluid, heat input, ferrofluid concentration, charging ratio, and orientation will be considered. Experimental results show that using ferrofluids can enhance the thermal performance in comparison with the case of distilled water. In addition, applying a magnetic field on the OLPHP charged with ferrofluid reduces its thermal resistance. Variation of the ferrofluid concentration results in different thermal performance of the OLPHP. Best charging ratio for the distilled water and ferrofluid without magnetic field is 60 % in most of the... 

Shell and tube heat exchanger with single twisted tube bundle in five different twist angles, are studied using computational fluid dynamics (CFD) and compared to the conventional shell and tube heat exchanger with single segmental baffles. Effect of shell-side nozzles configurations on heat exchanger performance is studied as well. Heat transfer rate and pressure drop are the main issues investigated in the paper. The results show that, for the same shell-side flow rate, the heat transfer coefficient of heat exchanger with twisted tube bundle is lower than that of the heat exchanger with segmental baffles while shell-side pressure drop of the former is even much lower than that of the...