جستجوی مقالات مرتبط با کلیدواژه « انتقال حرارت جابه جایی » در نشریات گروه « مکانیک »

The development of microchannel manufacturing technology has led to a growing interest in using them as heat exchangers. Microchannels are used to control the temperature of equipment and components that generate a high amount of heat flux. Heat transfer can be further increased by dispersing particles (nano-sized particles) with a low volume fraction into the base fluid (nanofluid). A nanofluid can change the thermophysical properties of a base fluid and improve its thermal performance. The purpose of this study was to examine the heat transfer characteristics of oil-based nanofluid within wavy microchannels in series and parallel arrangements of microchannels. In order to examine the performance of each microchannel separately and to make it easier to draw their diagrams, they are named 1 and 2. Experiments were performed on TiO2 and SiO2 oil-based nanofluids in volume fractions of 0.05 and 0.1, flow rates of 0.5, 1.0, and 1.5 lit/min, and inlet temperatures of 40°C, 45°C, 50°C, 55°C. The results show an increase in the Nusselt number of the base fluid up to 41.8% in the series arrangement and also a decrease in the surface temperature in the series arrangement compared to the parallel arrangement. Also, TiO2 and SiO2 nanofluids with a volume fraction of 0.1 caused the highest increase in heat transfer, up to 56% and 52.7% in parallel arrangement and up to 45.8% and 42% in series arrangement compared to the base fluid, respectively. The pressure drop of the test section in series arrangement was up to 82.1% higher than parallel.

The heat transfer and pressure drop of the air flow inside the square channel with perforated ribs are investigated experimentally. The effect of the perforation on the reduction of the low pressure region behind the ribs, and thus reduction in overall pressure drop, is evaluated. The Reynolds number is between 15000 and 50,000. Two values of 0.1 and 0.14 are selected for the ratio of the ribs height to the channel hydraulic diameter and three values of 20, 25 and 30 are considered for the ratio of the ribs pitch to the ribs height. The ratio of the hole diameter to the rib height are 0.3 and 0.5 respectively. It is found that the perforated ribs result in less pressure drop as well as reduced Nusselt number. Moreover, the effect of perforation is found to be unfeasible in the smaller pitch ratios. On the other hand, in long ribs, the perforation improves the performance coefficient up to 17%, the improvement is more evident in low Reynolds. In particular, for applications such as gas turbine or heaters, where the prime concern is to lower the wall temperature or increase the heat transfer and the blower energy consumption is less important, the current results show that using holes in the ribs can effectively enhance the flow thermal performance.

Cooling of blades is a challenge in advancement of gas turbines. Channels are used inside the blades to improve the heat transfer. Repeated ribs have significant impact on the heat transfer and pressure drop characteristics of a channel. In this study the effect of ribs with specified geometry is numerically investigated. In a relatively large range of Reynolds from 10000 to 80000, the fluid flow and heat transfer were simulated. The finite volume method was implemented. To model the turbulence, the standard k– ε model was used. The thermal boundary condition was the constant heat flux at the channel surfaces. The magnitude of the heat flux was assigned in a way to result in temperature difference from 10 K to 15 K. The results of current research were compared to the available experimental data under similar flow conditions. The coefficient of pressure drop was comparable with experimental data both in two dimensional and three dimensional cases. In the two dimensional simulation, the heat transfer coefficient was similar to the experimental data in the range of this study. In the three dimensional simulation, accuracy of the heat transfer coefficient in the case of low Reynolds was not satisfactory. However, it improved progressively as the Reynolds number increased. It was observed that, the maximum improvement in the heat transfer and pressure drop are about more than 200% and 300% respectively. Heat transfer augmentation could be explained by the presence of the secondary flows and disruption of the boundary layer growth due to the ribs.

In this paper, effect of the nanoparticles diameter on the forced convection heat transfer of turbulent flow of Al2O3-water nanofluids in a circular tube under constant heat flux on the wall using of two phase mixture model numerically investigated and was statistical analysis. The Volume fraction of nanoparticles is %3, the Reynolds number is 5×104 and the variation of diameter of nanoparticles is assumed in the range of 20- 110nm. In the statistical analysis, from the continuous probability distribution functions such as Gamma, Normal, Lognormal, Gumbel, Weibull and Frechet were used. After reviewing the results, it was found that by increasing the diameter of nanoparticles, the Nusselt number is reduced and this parameter due to changes in the nanoparticles of diameter has followed of the Frechet probability distribution function.

The main purpose of this study is to experimentally investigate transient convective heat transfer from a fluid stored inside a closed reservoir. Different cooling methods using helical and straight tubes are considered for heat transfer from the fluid reservoir. This paper attempts to determine the thermal advantages of helically coiled versus a straight tube. Fluid stored in the reservoir is cooled by water flow in the tube section at 27oC inlet temperature. The pressure drop and heat transfer parameters are measured over a wide range of Reynolds numbers (covers 500 to 5500). In these experiments, the effects of a couple of different parameters such as Reynolds number, time and geometrical parameters have been studied. Both heat transfer and pressure drop fluid flow in two test sections have been also reported and discussed. The experimental data indicate that using of helical coil instead of straight tube leads to increase heat transfer sharply. The coiled tube results show 42% reduction in of reservoir temperature compared to straight tube.

In this paper, numerical simulation of flow and heat transfer of Al2O3/water nanofluid has been carried out through three different geometries involving a straight pipe, a 90o curved pipe and a 180o curved pipe under constant heat flux condition. Employing singe-phase model for the nanofluid, the Navier-Stokes and energy equations for an incompressible and laminar flow have been solved in a body fitted coordinate system using a homemade code based on control-volume approach, while all thermophysical properties of the nanofluid are dependent on considered temperature. The effects of different nanoparticle concentration and centrifugal forces on the temperature and pressure field have been examined in detail. The accordance of numerical results with experimental data expresses the accuracy of the employed numerical method for simulating flow and heat transfer in the curved pipes, as well as the accuracy of the single-phase model of the nanofluid. The Presented results indicated that both the nanoparticle and curvature effects improve the heat transfer characteristics dramatically, but at the expense of considerable increase in pressure drop. Furthermore, the results showed that in order to obtain the optimum operating conditions of nanofluids, different parameters such as heat transfer enhancement and pressure drop must be considered simultaneously. Finally, a method has been proposed to indicate the proper nanofluid and flow geometry for special practical applications.

Common fluids such as water, ethylene glycol and oil have low thermal conductivity compared to metals and metal oxides. Hence, in recent decades, use of nanofluids has received significant interest. These fluids are colloidal of a base fluid (such as water or oil) with metal, ceramic or polymer nanoparticles. In this study, multi walled carbon nanotubes/water nanofluid in a forced convection heat transfer was investigated numerically in turbulent flow through a curved tube (180°) with constant wall temperature. Effects of Reynolds number and nanoparticles volume fraction on convection heat transfer were considered. Results showed that with increase in Reynolds number from 4000 to 6000, convection heat transfer coefficient was enhanced by 24%. Also, with increasing the volume friction of nanoparticles up to 0.01, convection heat transfer coefficient increased by 21% compared to the base flow.

This research study presents a numerical study on forced convection heat transfer of an aqueous ferrofluid 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 and under different frequencies of the applied magnetic field in detail. The convective heat transfer coefficient for distilled water and ferrofluid was measured and compared under various conditions. The results showed that applying an alternating magnetic field can enhance the convective heat transfer rate. The effects of magnetic field, volume concentration and Reynolds Number on the convective heat transfer coefficient were widely investigated, and the optimum conditions were obtained. Increasing the alternating magnetic field frequency and thevolume fraction led to better heat transfer enhancement. The effect of the magnetic field in low Reynolds numbers was higher. The results showed that the modeling data were in a very good agreement with experimental data. The maximum error was around 10%.

Natural convection heat transfer from a vertical heated plate in a saturated porous medium is analyzed under thermal non-equilibrium assumptions. The plate is maintained at a non uniform temperature and the fluid is considered to be with internal heat generation. The coupled momentum and energy equations for fluid and solid phases are transformed to a similarity format، and solved numerically. The resulting velocity and temperature distributions are shown for different values of the problem parameters. Also the values of the local Nusselt numbers for both solid and fluid phases are shown. The effect of suction/injection on the free convection boundary layer is also studied. The obtained results are compared with those presented in the literature in the case of thermal equilibrium model.

In many industrial processes، heat exchangers play the important role of the cooling function. The effective heat transfer rate in a specific process strongly depends on the heat exchanger design. The most popular type of heat exchangers commonly used in the automotive industry، is called «radiator». In engines، radiators play the important role of the cooling process. The present study is devoted to develop a mathematical model for the simulation of the vehicle radiator by utilizing the genetic algorithm method. Beside available experimental/analytical studies for studying the behavior of heat exchangers، an analytical model to predict and simulate their performance is of great worth. In the present study، we are going to use results of experimental data to obtain a general correlation، which can simulate the performance of heat exchangers. In this regard، a program is written to develop a mathematical model applying experimental data as inputs. The model is based on thermodynamic and heat transfer calculations and calculates cooling rate in each time step with deviation of 1. 68%.