Sharif Digital Repository

To compensate for drag increment due to the addition of nanoparticles to heat-transfer fluids, it seems that one could add drag reducer polymeric agents to these fluids. So, in this work, experiments were carried out for solutions of two types of polyacrylamide (FLOPAAM 3330S and FLOPAAM 3630S) at three distinct concentrations (25, 40, and 55 ppm), and TiO2-water nanofluid at concentrations of 0.015, 0.02, 0.025, and 0.03 L/L. The steady state turbulent convective heat transfer and the friction factor of the suspension of TiO2 in a dilute solution of very high molecular weight, polyacrylamide (hybrid fluid), in a coiled tube were analyzed. Experimental measurements were carried out from a... 

Steady-state convective heat transfer around a circular cylinder embedded in porous media is studied in the range of low and moderate Peclet numbers less than 40. The cylinder is at constant temperature and the Darcy model is used for the analysis of fluid flow and heat transfer in porous media. The governing equations are discretised using finite volume approach based on staggered grids. The powerlaw scheme is used in the numerical solution and a SIMPLE-like algorithm is developed and used in the solution process. It is found that the numerical algorithm is sufficiently efficient in the range of Peclet numbers less than 40. Parametric studies are done for better understanding of the porous... 

In this paper, the hydrodynamic and thermal behavior of a Fe3O4 ferrofluid flow was experimentally investigated inside a cupric u-shaped tube on a horizontal surface, under the effect of a variable magnetic field. The inlet flow regime was laminar and considered under the thermal boundary conditions of the tube with a uniform heat flux, which was affected by the variable magnetic field in some regions. The purpose of the study was to investigate the effect of parameters such as volume fraction of nanoparticles, Reynolds number of the flow, the radius of the curvature, and magnetic field frequency at three levels and four factors in the flow behavior in order to increase the convection heat... 

In the present study, a new swirling flow generator is studied which aims to enhance the convective heat transfer rate in a heat exchanger tube. This device has a four-point star cross section. The study mainly investigates the effect of swirl generator on heat transfer rate and pressure drop along the test tube which is under a constant and uniform heat flux. The working fluid in the experiments is the water–ethylene glycol mixtures with Prandtl numbers ranging from 5 to 150 at different Reynolds numbers from 12,000 to 27,000. The results clarify the potential of the applied swirl generator to make a significant enhancement in the heat transfer rate with a satisfactory rise in the pressure... 

In this paper, with the aim of enhancing the thermal conductivity of the fluid, a nanofluid is prepared based on SiO2. A series of experimental tests were carried out for both laminar and forced convection regimes in a horizontal tube with two different geometric shapes (circular and square cross section) subjected to constant wall heat flux (4735 W m−2). A comparative study has been done to investigate the effect of the geometry on the convective heat transfer. Moreover, the effect of the volume concentration on the behavior of the nanofluid and the base fluid was evaluated by comparing various volume concentrations (0.05, 0.07 and 0.2%). The experiments were done under two different... 

In this paper, with the aim of enhancing the thermal conductivity of the fluid, a nanofluid is prepared based on SiO2. A series of experimental tests were carried out for both laminar and forced convection regimes in a horizontal tube with two different geometric shapes (circular and square cross section) subjected to constant wall heat flux (4735 W m−2). A comparative study has been done to investigate the effect of the geometry on the convective heat transfer. Moreover, the effect of the volume concentration on the behavior of the nanofluid and the base fluid was evaluated by comparing various volume concentrations (0.05, 0.07 and 0.2%). The experiments were done under two different... 

In this research, it is aimed to enhance the heat transfer properties of the carbon nanotubes through nitrogen doping. To this end, nitrogen-doped multiwall carbon nanotubes (N-CNTs) were synthesized via chemical vapor deposition method. For supplying carbon and nitrogen during the synthesis of N-CNTs, camphor and urea were used, respectively, at 1000 °C over Co–Mo/MgO nanocatalyst in a hydrogen atmosphere. N-CNTs with three different nitrogen loadings of 0.56, 0.98, and 1.38 mass% were synthesized, after which, water/N-CNT nanofluids of these three samples with concentrations of 0.1, 0.2, and 0.5 mass% were prepared. To obtain a stable nanofluid, N-CNTs were functionalized by nitric acid... 

In this study, a mixture of nano-encapsulated phase change materials in water has been used as the working fluid in a pulsating heat pipe in order to investigate its thermal performance. The results of the experiments showed that using the nano-encapsulated paraffin dispersed in water as working fluid improves heat transfer and decreases the thermal resistance of the pulsating heat pipe. On the other hand, among the tested concentrations for nano-encapsulated paraffin, there is an optimal value in which increasing the concentration causes an increase in the thermal resistance of the pulsating heat pipe due to the increased dynamic viscosity of the fluid. Enhancement in the thermal... 

In this paper, a study is conducted to determine the influences of perforated anchors on heat transfer intensification of turbulence nanofluid flow in a pipe. Six different turbulence models are used, and the results obtained by these models are benchmarked with the existing theoretical data to select the best turbulence model. The outputs showed that the k–ε–RNG–scalable wall function model has higher accuracy and so it is selected to simulate this problem. The influences of various parameters including the addition of perforation on the anchors, the perforation diameter (in the range of 1–5 mm), the Re number (in the range of 5000–25,000), and the volumetric concentration of nanoparticles... 

In the present study, a new swirling flow generator is studied which aims to enhance the convective heat transfer rate in a heat exchanger tube. This device has a four-point star cross section. The study mainly investigates the effect of swirl generator on heat transfer rate and pressure drop along the test tube which is under a constant and uniform heat flux. The working fluid in the experiments is the water–ethylene glycol mixtures with Prandtl numbers ranging from 5 to 150 at different Reynolds numbers from 12,000 to 27,000. The results clarify the potential of the applied swirl generator to make a significant enhancement in the heat transfer rate with a satisfactory rise in the pressure... 

This study numerically investigates uid ow and heat transfer enhancement of a two-dimensional developing laminar ow in an axisymmetric pipe with partially filled porous material attached to the wall. The effects of porous layer in the range of 0 ≤ δ=R < 1 and Darcy number in the range of 10-6 ≤ Da ≤ 10-4 are investigated. It is found that thermal entrance length increases with porous layer thickness and is longer than a plain pipe. In addition, the dependence of Nusselt number on the thickness of the porous layer is not monotonic. A critical value of the porous layer thickness exists at which the Nusselt number value reaches the minimum. As the porous layer is thickening, more uid is pushed... 

The heat transfer simulations of turbine blades with internal cooling are faced with so many uncertainties, of which some originate from the secondary air system, including the inlet hot gas temperature and pressure and the cooling side boundary conditions, and the blade material. The main objective of this work is to carry out a suitable sensitivity analysis on a specific novel turbine vane to improve the thermal performance of its internal cooling system and to quantify how the uncertainties on the designed/calculated values can desirably/undesirably affect the maximum blade surface temperature, which can consequently affect the gas turbine engine efficiency. Furthermore, the sensitivity... 

In this paper, a fully coupled thermo-hydro-mechanical model is presented for two-phase fluid flow and heat transfer in fractured/fracturing porous media using the extended finite element method. In the fractured porous medium, the traction, heat, and mass transfer between the fracture space and the surrounding media are coupled. The wetting and nonwetting fluid phases are water and gas, which are assumed to be immiscible, and no phase-change is considered. The system of coupled equations consists of the linear momentum balance of solid phase, wetting and nonwetting fluid continuities, and thermal energy conservation. The main variables used to solve the system of equations are solid phase... 

Gene proliferation is vital for infectious and genetic diseases diagnosis from a blood sample, even before birth. In addition, DNA sequencing, genetic finger-print analyzing, and genetic mutation detecting can be mentioned as other procedures requiring gene reproduction. Polymerase chain reaction, briefly known as PCR, is a convenient and effective way to accomplish this task; where the DNA containing sample faces three temperature phases alternatively. These phases are known as denaturation, annealing, and elongation/extension which in this study -regarding the type of the primers and the target DNA sequence- are set to occur at 95, 58, and 72 degrees of Celsius. In this study, a PCR device... 

This work focuses on numerical validation of natural convection heat transfer of TiO2-water nanofluids in a cylindrical container using COMSOL. The main aim of this study is to examine different available approaches to calculate effective thermal conductivity and compare them with experimental data available in the literature. Simulation results show that for considered mixture, average Nusselt number decreases by increasing Rayleigh number and particle volume fraction. It has been found that only one model was able to represent similar trends for given particle volume fractions, compared to experimental results  

Effects of inclination angle on natural convective heat transfer and fluid flow in an enclosure filled with Al2O3-water nanofluid are studied numerically. The left and right walls of enclosure are kept in hot and cold constant temperature while the other two walls are assumed to be adiabatic. Considering Brownian motion and thermophoresis effect (two important slip velocity mechanisms) the two-phase mixture model has been employed to investigate the flow and thermal behaviors of the nanofluid. The study was performed for various inclination angles of enclosure ranging from γ = 0° to γ = 60°, volume fraction from 0% to 3%, and Rayleigh numbers varying from 105 to 107. The governing equations... 

This research investigated the forced convection heat transfer by using the swirling impinging jets. This study focused on nozzles, which equipped with twisted tapes via a numerical approach. The computational domain created by utilizing the fully structured meshes, which had very high quality from the viewpoint of aspect ratio and skewness. The numerical simulations were performed at four different jet-to-plate distances (L/D) of 2, 4, 6 and 8, four Reynolds numbers of 4000, 8000, 12,000 and 16,000, and also four different twist ratios (y/w) of 3, 4, 5 and 6. The mesh-independent tests were conducted based upon the average Nusselt number. The obtained results revealed good agreement with... 

Lack of a direct method to control combustion timing is one of the main disadvantages of homogeneous charge compression ignition (HCCI) engines. Fuel blending, in which two fuels with different auto-ignition characteristics are blended, can be used to control combustion timing. Utilizing different additives is another method for HCCI combustion control. The aim of this research is investigation on the effect of reformer gas addition on the availability terms in HCCI engines fueled with primary reference fuels (PRFs). A multi zone model (MZM) coupled with a semi detailed chemical kinetics mechanism is used for calculation of different terms of exergy analysis. Heat and mass transfer between... 

The effect of a permanent magnetic field on the heat transfer characteristics of hybrid graphene-magnetite nanofluids (hybrid nanofluid) under forced laminar flow was experimentally investigated. For this purpose, a reduced graphene oxide-Fe3O4 was synthesized by using two-dimensional (2D) graphene oxide, iron salts and tannic acid as the reductant and stabilizer. Graphene sheets acted as the supporting materials to enhance the stability and thermal properties of magnetite nanoparticles. The thermo-physical and magnetic properties of this hybrid nanofluid have been widely characterized and it shows that the thermal conductivity increased up to 11%. The hybrid nanofluid behaves as a Newtonian... 

This work is dedicated to finding a suitable measure to judge thermal performance of nanofluids. The importance of this issue arises from misleading claim of excess heat transfer of nanofluids compared to the base fluid, neglecting the hydraulic effects such as increase in pressure drop. To clarify the issue, the experimental setup with capability to create constant Reynolds number and constant pumping power is constructed. Thermal behavior of nanofluids of silicon oxide/water and aluminum oxide/water and distilled water in developing region of laminar flow regime is investigated. In this regard, the convective heat transfer coefficient within the finned tube heat exchanger is evaluated....