Natural convection in a cavity with undulated walls filled with water-based non-Newtonian power-law CuO–water nanofluid under the influence of the external magnetic field

Natural convection heat transfer in a square cavity (with wavy or plane wall) filled with non-Newtonian power-law nanofluid has been elucidated for several input parameters like Ra spanning from 10⁵ to 10⁶, power-law index (n) from 0.6 to 1.4, and volume fraction of CuO nanoparticles (?) from to 0 to 0.12. Effect of external magnetic field on heat transfer has been illustrated by varying the Ha from 0 to 90. In the present study, our main objective is to explore the effect of nanoparticles on heat transfer enhancement in non-Newtonian power-law fluid. It is found that the addition of nanoparticles (?) to shear thinning fluid enhances the heat transfer approximately 15% when ? increases from 0 to 0.12 for Ha less than 60 at all Ra. For a shear thickening fluid, the same thing happens for all Ha at any Ra. The average surface Nusselt number for a cavity with wavy wall is less than that of a plane wall for all cases which is not true for the case of local Nusselt number.     

Experimental studies on the thermal performance of a parabolic dish solar receiver with the heat transfer fluids SiC + water nano fluid and water

An experimental investigation has been carried out with aa point focusing dish reflector of 12 square meters aperture area,exposed to the average direct normal irradiations of 810 W/m2.This work focuses on enhancinge the energy and exergy efficiencies of the cavity receiver by minimizing the temperature difference between the wall and heat transfer fluids.Two heat transfer fluids Water and SiC + water nano fluid have been prepared from 50 nm particle size and 1％ of volume fraction,and experimented separately for the flow rates of 0.2 lpm to 0.6 lpm with an interval of 0.1 lpm.The enhanced thermal conductivity of nano fluid is 0.800115 W/mK with the keff/kb ratio of 1.1759 determined by using the Koo and Kleinstreuer correlation.The maximum attained energy and exergy efficiencies are 29.14％ and 24.82％ for water,and 32.91％ and 39.83％ for SiC+water nano fluid.The nano fluid exhibits enhanced energy and exergy efficiency of 12.94％ and 60.48％ than that of water at the flow rate of 0.5 lpm.The result shows that the system with SiC+Water produces higher exergy efficiency as compared to energy efficiency;in the case ofwater alone,the energy efficiency is higher than exergy efficiency.     

Forced convective heat transfer between assemblages of spherical particles and power‐law non‐Newtonian liquids with velocity slip at the interface

Heat transfer from spheres can be influenced by a varying degree of slip at the fluid‐particle interface along with the rheology of the surrounding continuous liquid and adjacent spheres. Thus in this study, the effects of dimensionless velocity slip parameter (λ) along with power‐law fluid rheology and other pertinent kinematic flow and heat transfer parameters on isotherm contours, local and average Nusselt numbers of assemblages of spherical slip particles are presented. This is done by adopting a segregated approach where dimensionless momentum and energy equations are solved by SMAC algorithm formulated in spherical coordinates within the finite difference formulation. Before obtaining new results, grid independence studies for either extreme values of power‐law consistency index of non‐Newtonian fluids are carried out. Finally, the major contribution of this study is the development of a correlative equation for the average Nusselt number of assemblages of spherical slip particles in power‐law fluids based on the present results (5880 data points) as a function of pertinent dimensionless parameters.     

Performance of Microchannel Heat Sinks with Newtonian and Non-Newtonian Fluids

In this paper, the flow behavior and heat transfer performance of a microchannel heat sink is examined. Microchannel heat sink is a heat exchanger that is used to control the temperature of electronic devices with high heat flux capacity. A comprehensive thermal model for a microchannel should include a three-dimensional conduction analysis in the solid parts, followed by an extensive three-dimensional developing flow in the fluid region. The heat transfer analysis in the transition region of the fluid section is a crucial matter. Hydrodynamic and thermal entrance lengths are two important parameters, among others, which are studied in the solution. To examine the potential of using a non-Newtonian fluid, the power law model was used for both Newtonian and non-Newtonian fluids. The numerical solution of the problem was based on a finite difference approach using a control volume with staggered grid system. The SIMPLE algorithm was applied to the problem, and convection terms were estimated using QUICK method. A comparison of the Newtonian and non-Newtonian results showed that for shear thinning fluids, the pressure drop could reduce up to 45%, while for shear thickening fluids, it can increase up to 48%. The same comparison for the Nusselt number showed about a 160% increase with shear thinning fluids and a 43% decrease with shear thickening fluids. The thermal resistance at a Reynolds number of 50 will reduce approximately 25% with shear thinning fluids and will increase approximately 5% with shear thickening fluids. At higher values of the Reynolds number, the changes in the value of the thermal resistance are more pronounced.     

EFFECTS OF TRANSVERSE MAGNETIC FIELD,PRANDTL NUMBER AND REYNOLDS NUMBER ON NON-DARCY MIXED CONVECTIVE FLOW OF AN INCOMPRESSIBLE VISCOUS FLUID PAST A POROUS VERTICAL FLAT PLATE IN A SATURATED POROUS MEDIUM

The effect of transverse magnetic field parameter (Hartmann number, Ha), Reynolds number (Re) and Prandtl number (Pr) on the mixed convection flow past a semi-infinite vertical porous plate in a non-Darcian porous medium with variable viscosity and porosity, viscous dissipation and fluid–solid thermal conductivity ratio in the presence of plate transpiration (lateral mass flux) is investigated theoretically and numerically using Keller's implicit finite difference scheme. It is shown that the Hartmann number acts as a retarding force and increases the momentum boundary layer thickness, analogous to the flow against a positive pressure gradient, simultaneously decreasing local skin friction (shear stress). The heat transfer rate is however enhanced by the magnetic field (for positive values of the Eckert number) since the fluid is heated and temperature gradients become reduced between the fluid and the plate, with important potential applications in MHD power generators, materials processing and geothermal systems containing electrically-conducting fluids. The effects of high velocity flow (larger Re) and different Prandtl numbers corresponding to different industrial and geophysical fluids on heat transfer are also discussed. © 1997 by John Wiley & Sons, Ltd.     

磁场作用下纳米磁流体热管传热速率的研究

提出了一种利用外加磁场来强化纳米磁流体真空热管传热的实验研究方法,设计和建立了纳米磁流体真空热管传热测试实验台,并在不同种类、不同强度的外加磁场作用下分别对纳米磁流体真空热管的传热速率进行了实验研究.结果表明：纳米磁流体热管在各种外界条件下都优于磁流体热管和水工质热管;各种磁场都能强化纳米磁流体热管的传热,其中静态直流磁场作用下纳米磁流体真空热管的传热速率提高最大.     

Heat source and radiation effects on magneto-convection flow of a viscoelastic fluid past a stretching sheet: Analysis with Kummer''s functions

An analysis is carried out to study heat source and radiation effects on two-dimensional steady flow of an electrically conducting, incompressible, viscoelastic fluid (Walter's liquid-B′) past a stretching sheet in the presence of transverse uniform magnetic field. Two cases are studied namely (i) the sheet with prescribed power law surface temperature (PST case) and (ii) the sheet with prescribed power law surface heat flux (PHF case). Kummer's functions are used to obtain temperature field and wall temperature gradient. The variations in the velocity and temperature field with change in parameters encountered into the equations are obtained and depicted graphically. The numerical values of the variations in wall temperature gradient due to change in physical parameters are presented in the tables. The results obtained have been discussed.     

A numerical investigation on the heat and fluid flow of various nanofluids on a stretching sheet

Following the necessity of investigating fluid flow and heat transfer in the stretching sheet problem and effect of nanofluids on them, performance of various nanofluids were investigated in the present study. Three base fluids (deionized water, ethylene glycol, and engine oil) in combination with 18 nanoparticles (metals and their oxides) were investigated. While experimental methods are preferable, a mathematical model was developed and solved by applying differential quadrature method due to lack of such experimental data. With the results obtained in the real dimensions, the error caused by the cancellation of the viscosity effect due to the dimensionless variables was omitted. Effects of magnetic field and volume fraction of nanoparticle on the fluid flow and heat transfer characteristics were investigated. Highest heat transfer rate as well as small amounts of shear stress was obtained for deionized water–Al and deionized water–Mg nanofluids. Increasing volume fraction of nanoparticle was observed to increase both heat transfer and shear stress rates, while presence of a magnetic field caused an increase in shear stress and decrease in heat transfer rate.     

Suitable Heat Transfer Model for Self-Similar Laminar Boundary Layer in Power Law Fluids

The classical power law non-Newtonian fluids energy boundary layer equation is proved improper to describe the self-similar heat transfer. A theoretical analysis for momentum and energy boundary layer transfer behavior is made and the full similarity heat boundary layer equation is developed, which may be characterized by a power law relationship between shear stress and velocity gradient with the Falkner-Skan equation as a special case. Both analytical and numerical solutions are presented for momentum and energy boundary layer equations by using the similarity transformation and shooting technique and the associated transfer characteristics are discussed.     

纳米流体强化导热系数机理初步分析

从添加纳米粒子改变了液体结构和纳米粒子微运动两个方面,分析了纳米流体强化导热系数的机理,研究表明,相对于在液体中添加毫米或微米级固体粒子以增加导热系数而言,纳米流体强化导热系数的原因主要来自于纳米粒子的微运动,通过测量不同温度下纳米流体的导热系数,验证了纳米粒子微运动是纳米流体强化导热系数的主要因素。     

Transient rotating flow over a moving surface with a magnetic field

The transient flow and heat transfer on a moving surface in a rotating fluid in the presence of a magnetic field have been investigated. The unsteadiness in the flow field has been introduced by the sudden change in the surface velocity or the fluid angular velocity. The parabolic partial differential equations governing the unsteady flow and heat transfer have been solved by using an implicit finite-difference scheme in combination with the quasilinearization technique. The computations have been carried out from the initial steady state to the final steady state. The effects of the sudden change in the surface velocity on the flow and heat transfer are found to be more significant than those of the impulsive change in the angular velocity of the fluid. When the surface velocity is suddenly reduced, the surface shear stress is found to vanish in a small time interval after the start of the impulsive motion, but it does not imply flow separation. The surface shear stress for the primary flow increases with the magnetic field and the fluid angular velocity, but the surface heat transfer decreases. The surface shear stress for the secondary flow increases with the angular velocity of the fluid, but decreases with increasing magnetic field.     

Experimental study of pool boiling heat transfer of water‐based magnetic fluid on a horizontal heater

The effect of mass concentration of magnetic particles and an applied magnetic field on pool boiling heat transfer of water‐based magnetic fluid on a horizontal heater was investigated. The experimental results show that high‐concentration magnetic fluid deteriorates boiling heat transfer, while middle‐ and low‐concentration magnetic fluid enhances the boiling heat transfer. There was an optimum concentration in which the enhancement of boiling heat transfer was the best. Conclusions were the same with an applied magnetic field that enhances the boiling heat transfer of magnetic fluid further. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(3): 180–187, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20054     

Momentum and Heat Transfer Phenomena for Power–Law Liquids in Assemblages of Solid Spheres of Moderate to Large Void Fractions

This work extends our previously reported results for the flow of and heat transfer from expanded beds of solid spheres to power–law fluids by using a modified and more accurate numerical solution procedure. Extensive results have been obtained to elucidate the effects of the Reynolds number (Re), the Prandtl number (Pr), the power–law index (n), and the bed voidage (ε) on the flow and heat transfer behavior of assemblages of solid spheres in the range of parameters: 1 ≤ Re ≤ 200, 1 ≤ Pr ≤ 1000, 0.6 ≤n ≤ 1.6, and 0.7 ≤ε ≤ 0.999999. The large values of bed voidage are included here to examine the behavior in the limit of an isolated sphere. As compared to Newtonian fluids, for fixed values of the Reynolds number and the voidage, the total drag coefficient decreases and the average Nusselt number increases for shear thinning fluids (n < 1); whereas, for shear thickening fluids (n > 1), the opposite behavior is observed. The drag results corresponding to bed voidage, ε = 0.99999, are very close to that of a single sphere; whereas, the heat transfer results approach this limit at ε = 0.999. Based on the present numerical results, simple correlations for drag coefficient and average Nusselt number are proposed which can be used to calculate the pressure drop for the flow of a power–law fluid through a bed of particles, or rate of sedimentation in hindered settling and the rate of heat transfer in assemblages of solid spheres in a new application. Broadly speaking, all else being equal, shear-thinning behavior promotes heat transfer, whereas shear-thickening behavior impedes it.     

Magnetic field influence on double-diffusive natural convection in a square cavity – A numerical study

In this paper double diffusive natural convection in a square cavity in the presence of external magnetic field has been studied numerically by Galerkin’s weighted residual finite element method using velocity-vorticity formulation. Simulation results are reported for 0?相似文献   

超临界有机朗肯循环低温余热发电系统的分析

当采用朗肯循环方式回收低温余热（350℃以下）的动力时,不宜采用水作工质,而使用一些低沸点有机物的有机朗肯循环（ORCs）,则能获得较高的能量转换效率。有机朗肯循环可分为亚临界条件下的动力循环与超临界条件下的动力循环,超临界条件下的动力循环在热端换热器中（余热加热蒸汽发生器）能获得较好的温度匹配和较高的[火用]效率。     

First and Second Law Analysis for the MHD Flow of Two Immiscible Couple Stress Fluids between Two Parallel Plates

This paper aims to analyze the heat transfer by the first and second laws of thermodynamics for the flow of two immiscible couple stress fluids inside a horizontal channel under the action of an imposed transverse magnetic field. The plates of the channel are maintained at constant and different temperatures higher than that of the fluid. The flow region consists of two zones, the flow of the heavier fluid taking place in the lower zone. No slip condition is taken on the plates and continuity of velocity, vorticity, shear stress, couple stress, temperature, and heat flux are imposed at the interface. The velocity and temperature distributions are derived analytically and these are used to compute the dimensionless expressions for the entropy generation number and Bejan number. The results are presented graphically. It is observed that the imposed magnetic field reduces the entropy production rate near the plates.     

Thermal flow of fluid with magnetic particles in the presence of magnetic field

A magnetic fluid is a stable colloidal solution composed of magnetic particles, surfactant, and a carrier fluid. Magnetic fluids have numerous applications. In this study, we used the computational fluid dynamics model to simulate the behavior of a magnetic fluid in a 2-D square under different conditions such as different positions and intensities of the magnetic source. The preliminary model was established and used in conjunction with experimental data obtained from a present study, in order to determine the influence of particle size and mass fraction on fluid behavior. Our results show that particles with a smaller size have better ability to dissipate heat, and a larger mass fraction would provide a stronger driving force which leads to the velocity and temperature profile. We anticipate that our model would be useful to develop newer applications of magnetic fluids and magnetic flow.     

Effect of magnetic field on two-layered natural/thermocapillary convection

Heat transfer in a two-layered fluid system is of great importance in a variety of applications. Control and optimization of convective heat transfer of the immiscible fluids needs complete understanding of all phenomena, especially those induced by surface tension at the fluid interface. The present work is focused on rather complex convective flow and heat transfer phenomena in a cavity, which can be subject to both buoyancy and thermocapillary effects in addition to the influence of magnetic field applied for flow control. With the encapsulant liquid posing magnetic properties, a magnetic force can arise to either enhance or counterbalance the gravity effect when the cavity is placed in a non-uniform magnetic field. In our study, the velocity and temperature distribution of the system can be significantly altered to change the heat transfer by varying intensity and gradient of the applied magnetic field. Preliminary results of numerical computation presented here are for a two-layered liquid cavity MnCl₂·4H₂O and Fluorinert FC40 under various magnetic fields intensities.     

理想流体对流传热问题的理论解

研究理想流体受迫对流传热和自然对流传热问题的理论解。采用流体无垂直于壁面法线方向运动(即无穿透)的条件取代黏性流体在壁面无滑移条件，解决了流体在边界上有滑移时计算对流传热系数的困难，给出了理想流体与平壁受迫对流传热、理想流体与竖直壁面自然对流传热和理想流体在管内受迫对流传热的理论解。结果表明：理想流体的对流传热与黏性流体同样存在着热边界层。在外部流动的情况下，无论受迫对流传热还是自然对流传热，对流传热系数都与流体的导热系数、密度和比热三乘积的二分之一次方成正比。在管内受迫对流的情况下，当无因次长度大于0．05时，局部Nu和界面无因次温度分布都不再变化，对于恒热流边界条件，Nu等于8，截面无因次平均温度等于2；对于恒壁温边界条件，Nu等于5．782，截面无因次平均温度等于2．316。     

Design method of automatic energy transport devices based on the thermomagnetic effect of magnetic fluids

The present work is aimed at developing a design method for automatic energy transport devices based on the thermomagnetic effect of magnetic fluids. A model for describing thermomagnetic convection of a temperature-sensitive magnetic fluid in a loop is established, which includes the coupling of the three fundamental phenomena, i.e. magnetic, thermal, and fluid dynamic features. The thermomagnetic convection of the temperature-sensitive magnetic fluid in a loop-shape energy transport device is simulated in the presence of an external magnetic field. Comparison between experimental data and numerical results is carried out to validate the model. The effects of different factors such as input heat load, heat sink temperature and magnetic field distribution along the loop on the performance of the energy transport device are analyzed and discussed according to the numerical simulation results.