Solid-phase sintering process and forced convective heat transfer performance of porous-structured micro-channels

A solid-phase sintering process for the low-cost fabrication of composite micro-channels was developed. Three kinds of composite micro-channels with metallic porous structures were designed. The sintering process was studied and optimized to obtain porous-structured micro-channels with high porosity. The flow resistance and heat transfer performance in the composite micro-channels were investigated. The composite micro-channels show acceptable flow resistance, significant enhancement of heat transfer and dramatic improvement of flow boiling stability, which indicates a promising prospect for the application in forced convective heat transfer.

Numerical Investigation of Heat and Mass Transfer in Nanofluid-Filled Porous Medium

In this work, we numerically study the laminar mixed convection of fluid flow in a vertical channel filled with porous media during the drying process. The porous medium, modeled as a vertical wall, consists of solid and nanofluid phase (Water-Al2O3 or Water-Cu), as well as a gas phase. The established model is developed based on Whitaker’s theory and resolved by our numerical code using Fortran. Results principally show the influence of various physical parameters, such as nanoparticle volume fraction, ambient temperature, and saturation on heat and mass transfer on the drying process. This study brings the effect of the presence of nanofluids in porous media. It contributes not only to our fundamental understanding of drying processes but also provides practical insights that can guide the development of more efficient and sustainable drying technologies. .

Heat transfer study on solid and porous convective fins with temperature-dependent heat generation using efficient analytical method

A simple and highly accurate semi-analytical method, called the differential transformation method(DTM), was used for solving the nonlinear temperature distribution equation in solid and porous longitudinal fin with temperature dependent internal heat generation. The problem was solved for two main cases. In the first case, heat generation was assumed variable by fin temperature for a solid fin and in second heat generation varied with temperature for a porous fin. Results are presented for the temperature distribution for a range of values of parameters appearing in the mathematical formulation(e.g. N, εG, and G). Results reveal that DTM is very effective and convenient. Also, it is found that this method can achieve more suitable results in comparison to numerical methods.

Forced Convective Heat Transfer in a Porous Plate Channel

Forced convective heat transfer in a plate channel filled with metallic spherical particles was investigated experimentally and numerically. The test section, 58 mm×80mm×5mm in size, was heated by a 0.4 mm thick plate electrical heater. The coolant water now rate ranged from 0.015 to 0.833 kg/s.The local wall temperature distribution was measured along with the inlet and outlet fluid temperatures and pressures. The results illustrate the heat transfer augmentation and increased pressure drop caused by the porous medium. The heat transfer coefficient was increased 5-12 times by the porous media although the hydraulic resistance was increased even more. The Nusselt number and the heat transfer coefficient increased with decreasing particIe diameter, while the pressure drop decreased as the particle diameter increased. It was found that, for the conditions studied (metallic packed bed),the effect of thermal dispersion did not need to be considered in the physical model, as opposed to a non-metallic packed bed, where thermal dispersion is important.

Heat transfer in a porous saturated wavy channel with asymmetric convective boundary conditions

The viscous flow in a wavy channel with convective boundary conditions is investigated. The channel is filled with a porous viscous fluid. Two cases of equal and different external convection coefficients on the walls are taken into account. Effect of viscous dissipation is also considered. The governing equations are derived employing long wavelength and low Reynolds number approximations. Exact closed form solutions are obtained for the simplified equations. Important physical features for peristaltic flow caused by the wavy wave are pumping, trapping and heat transfer rate at the channel walls. These are discussed one by one in depth and detail through graphical illustrations. Special attention has been given to the effects of convective boundary conditions. The results show that for Bi1≠Bi2, there exists a critical value of Brinkman number Brc at which the temperatures of both the walls become equal. And, for Bi1>Bi2 and Br>Brc, the temperature of the cold wall exceeds the temperature of hot wall.

NUMERICAL STUDY ON NATURAL CONVECTION HEAT TRANSFER INSIDE RECTANGULAR ENCLOSURE FILLED WITH ANISOTROPIC POROUS MEDIA

Natural convection heat transfer inside horizontal rectangular enclosure filled with the anisotropic porous media, with isothermally heated bottom and cooled top while the vertical walls are adiabatic, is analyzed numerically by applying the Brinkman model-a modified form of Darcy model giving consideration to the viscous effect. The results show that: (1)the permeability ratio (K*=Ky/Kx) is an important factor affecting natural convection heat transfer in the porous media. As K' decreases, the circulation intensity of the natural convectioncells increase significantly, resulting in an enhancement of heat transfer coefficient; (2)the increase of Darcy number (aa=Ky/H2) implies that the viscous effect is more significant. As Da≥10-, there exists a certain difference between the Darcy model and the Brinkman model. It is more significant at a lower permeability ratio. In particalar, with K*≤0. 25, the Nusselt number for Da=10-3 would differ form that of Darcy model up to an amount of 30K. The Darcy flow as depicted by Darcy model is no longer existing and an analysis neglecting the viscous effect will inevitably be of considerable error.

Effect of cavitating flow on forced convective heat transfer: a modeling study

With water as working medium,a numerical study on liquid convection heat transfer accompanied by cavitating flow in a circular tube was conducted by combining mixture model and Schnerr–Sauer cavitation mode in the commercial code,Ansys Fluent.Cavitation is instigated by setting a restriction orifice in the circular tube.The simulation results show that cavitation occurs around the wall of the restriction orifice and disappears at the downstream regions.The comparison of local heat transfer coefficients under the same mass flow rate indicates that,heat transfer is significantly augmented at the downstream regions of the restriction orifice because of the occurrence of cavitation.The analysis on the characteristics of cavitating and noncavitating flows confirms that the occurrence of cavitation can increase the turbulence intensity under the same mass flow rate,which is the origin for heat transfer to be enhanced.Based on it,the effects of such factors as inlet pressure and ratio of orifice to pipe diameter on cavitation enhanced heat transfer were further investigated for the purpose of application.

Optimizing Heat Sink Performance by Replacing Fins from Solid to Porous inside Various Enclosures Filled with a Hybrid Nanofluid

The current study generally aims to improve heat transfer in heat sinks by presenting a numerical analysis of natural convection of an enclosure with hot right and cool left walls,and thermally insulated top and bottom walls.The cold wall included configurations(half circle/half square)in various sizes(S=0.1,0.2,and 0.3),numbers(N=1,2,3,and 4),and locations(C=0.35,and 0.65).A heat sink is constructed of Aluminum attached to the hot wall,and composed of five fins with protrusions.Fins of the heat sink will be examined in a solid and porous structure.The enclosure is filled with a hybrid nanofluid of Nanoparticles(MWCNT and Fe3O4)and water.The current study utilized COMSOL Multiphysics software due to its efficacy in addressing scientific and technical challenges involving partial differential equations.The solving of the governing equations is achieved using the finite element method with various parameters:Rayleigh number(Ra=10^(3)–10^(6)),Darcy number(Da=10^(-2),10^(-3)),solid volume fraction(ϕ=0–0.06)to determine stream function,isotherms lines,and average Nusselt number(Nu).The results of numerical simulations show that heat sink with solid fins have a 97%higher stream function when Ra is raised from 10^(3) to 105.Whilst with porous fin heat sink,a stream function 96%for Da=10^(-3) and 94%for Da=10^(-2).Changing solid fins to porous increases stream functions by 9%at Da=10^(-3) and 20%at Da=10^(-2).It has been found that Ra increases Nu by 44%for solid fins and 50%for porous fins.Making solid fins porous increases Nu by 54%at Ra=10^(6).The high increase in the percentage of(Nu)indicates the importance of the improvement in heat transfer,and this distinguishes the results of the current study from previous studies.Nu values were found highest for(half square)compared to(half circle),with 2%increases for numbers,11.6%for sizes,and 11%for location.Solid volume fractions for all Ra at a solid-finned heat sink increased Nu by 23%.

Self-Similar Solution of Heat and Mass Transfer of Unsteady Mixed Convection Flow on a Rotating Cone Embedded in a Porous Medium Saturated with a Rotating Fluid

A self-similar solution of unsteady mixed convection flow on a rotating cone embedded in a porous medium saturated with a rotating fluid in the presence of the first and second orders resistances has been obtained. It has been shown that a self-similar solution is possible when the free stream angular velocity and the angular velocity of the cone vary inversely as a linear function of time. The system of ordinary differential equations governing the flow has been solved numerically using an implicit finite difference scheme in combination with the quasi-linearization technique. Both prescribe wall temperature and prescribed heat flux conditions are considered. Numerical results are reported for the skin friction coefficients, Nusselt number and Sherwood number. The effect of various parameters on the velocity, temperature and concentration profiles are also presented here.

Cooling of Superheated Refrigerants Flowing inside Tubes Filled with Porous Media： Study of Heat Transfer and Pressure Drop, Carbon Dioxide Case Study

In this work an experimental study combined with an analytical investigation for cooling superheated Carbon Dioxide （CO2） gas were carried out. This work is intended to be part of the super critical Gustav Lorentzen refrigeration cycle of CO2. Experimental and analytical works concentrated on heat transfer and pressure drop for single phase flow during gas cooling inside tubes filled with porous media. Analytical empirical correlations were formulated for the coefficient of convectional heat transfer and for the pressure drop. A comparison between experimental results and that obtained by developed correlations were carried out, and a comparison between these results and literature published ones were carried out too. The results of this research showed that for cooling process the proposed correlations were proved to be acceptably accurate for pressure drop with difference from experimental results of 2%, while for convective heat transfer the difference from experimental results reached about 3%. More than 90% agreement with literature results was obtained. This work can enhance the calculations of heat flux and pressure drop of gases flow inside porous media filled tubes, and can help in the design procedure of heat exchangers and cooling processes.

LAMINAR DISSIPATIVE FLOW IN A POROUS CHANNELBOUNDED BY ISOTHERMAL PARALLEL PLATES

The effects of viscous dissipation on thermal entrance heat transfer in a parallel plate channel filled with a saturated porous medium,is investigated analytically on the basis of a Darcy model.The case of isothermal boundary is treated.The local and the bulk temperature distribution along with the Nusselt number in the thermal entrance region were found. The fully developed Nusselt number, independent of the Brinkman number, is found to be 6. It is observed that neglecting the effects of viscous dissipation would lead to the well-known case of internal flows,with Nusselt number equal to 4 93.A finite difference numerical solution is also utilized. It is seen that the results of these two methods, analytical and numerical, are in good agreement.

NATURAL CONVECTIVE HEAT AND MASS TRANSFER BY THE COUPLING OF TEMPERATURE CRADIENT AND CONCENTRATION GRADIENT IN A POROUS MEDIUM

The coupling of temperature gradient and concentration gradient and it's effects on heat and mass transfer are considered for natural convection in a porous enclosure. By use of the penalty finite element method, the distributions of flow, temperature and concentration fields are numerically simulated in four kinds of boundary condition. The profiles of heat and mass transfer coefficients along with the vertical wall are shown in the peper.

Forced Convection Heat Transfer in Porous Structure: Effect of Morphology on Pressure Drop and Heat Transfer Coefficient

A light-weight structure with sufficient mechanical strength and heat transfer performance is increasingly required for some thermal management issues.The porous structure with the skeleton supporting the ambient stress and the pores holding the flowing fluid is considered very promising,attracting significant scientific and industrial interest over the past few decades.However,due to complicated morphology of the porous matrices and thereby various performance of the pressure drop and heat transfer coefficients (HTC),the comprehensive comparison and evaluation between different structures are largely unclear.In this work,recent researches on the efforts of forced convection heat transfer in light-weight porous structure are reviewed;special interest is placed in the open-cell foam,lattice-frame,structured packed bed,and wire-woven structures.Their experimental apparatus,morphological of the porous structures,effect of morphology on pressure drop and HTC,and further applications are discussed.The new method which measure morphology accurately should be paid more attention to develop more accuracy correlation.Also,the most research focused on low Reynolds number and existing structure,while very few researchers investigated the property of forced convection heat transfer in high velocity region and developed new porous structure.

Influence of Brownian Motion, Thermophoresis and Magnetic Effects on a Fluid Containing Nanoparticles Flowing over a Stretchable Cylinder

The influence of Brownian motion and thermophoresis on a fluid containing nanoparticles flowing over a stretchable cylinder is examined.The classical Navier-Stokes equations are considered in a porous frame.In addition,the Lorentz force is taken into account.The controlling coupled nonlinear partial differential equations are transformed into a system of first order ordinary differential equations by means of a similarity transformation.The resulting system of equations is solved by employing a shooting approach properly implemented in MATLAB.The evolution of the boundary layer and the growing velocity is shown graphically together with the related profiles of concentration and temperature.The magnetic field has a different influence(in terms of trends)on velocity and concentration.

Forced Convective Heat Transfer in a Plate Channel Filled with Solid Particles

A numerical study of fluid flow and convective heat transfer in a plate channel filled with solid (metallic)particles is presented in this paper.The study uses the thermal equilibrium model and a newly developed numerical model which does not assume idealized local thermal equilibrium between the solid particles and the fluid.The numerical simulation results are compared with the experimental data in reference[2].The paper investigates the effects of the assumption of local thermal equilibrium versus non-thermal equilibrium,the thermal conductivity of the solid particles and the particle diameter on convective heat transfer.For the conditions studied,the convective heat transfer and the temperature field assuming local thermal equilibrium are much different from that for the non-thermal equilibrium assumption when the difference between the solid and fluid thermal conductivities is large. The relative values of the thermal conductivities of the solid particles and the fluid also have a profound effect on the temperature distribution in the channel.The pressure drop decreases as the particle diameter increases and the convective heat transfer coefficient may decrease or increase as the particle diameter increases depending on the values of ε,λs,λf,λd,αv, ρu.

Investigation of Ti6Al4V and AA7075 alloy embedded nanofluid flow over longitudinal porous fin in the presence of internal heat generation and convective condition

The thermal attributes of porous fin due to radiation and natural convection have been carried out in the presence of nanofluid flow.The geometry of the fin taken for the analysis is rectangular profiled longitudinal fin.The temperature-dependent internal heat generation condition is also considered along with Darcy’s model.The two types of nanofluid containing titanium alloy(Ti6Al4V)and aluminium alloy(AA7075)immersed in water is considered for the investigation.The modelled nonlinear ordinary differential equation is numerically solved by the Runge–Kutta–Fehlberg technique.The impact of geometric parameter on the heat transfer analysis of the fin due to the flow of both nanofluids is plotted and consequences are physically interpreted.It is observed that the presence of the water-based titanium alloy better enhances the fin heat transfer rate.

A Coupled Discrete Unified Gas-Kinetic Scheme for Convection Heat Transfer in Porous Media

In this paper,the discrete unified gas-kinetic scheme(DUGKS)is extended to the convection heat transfer in porous media at representative elementary volume(REV)scale,where the changes of velocity and temperature fields are described by two kinetic equations.The effects from the porous medium are incorporated into the method by including the porosity into the equilibrium distribution function,and adding a resistance force in the kinetic equation for the velocity field.The proposed method is systematically validated by several canonical cases,including the mixed convection in porous channel,the natural convection in porous cavity,and the natural convection in a cavity partially filled with porous media.The numerical results are in good agreement with the benchmark solutions and the available experimental data.It is also shown that the coupled DUGKS yields a second-order accuracy in both temporal and spatial spaces.

管内插物强化换热性能分析及应用

管内插物的种类很多,扭带、螺旋线圈以及绕花丝是三种较常用的管内插物强化换热技术,对它们的强化换热性能以及应用进行分析比较,是非常有必要的。从综合强化性能来看,螺旋线圈内插物比扭带内插物效果好;绕花丝内插物是一种新型综合强化换热技术,它优于前两种内插物的特点是:可使流体在流动方向上做复杂的三维混合流动,并且所产生的阻力降非常小,所以综合强化换热性能最好。

三维内肋管内插入螺旋扭带的强化传热实验

本文分别以水和乙二醇为工质，在Ｒｅ数范围为：６００～４００００，Ｐｒ数范围为：５．５～１１０之间，对四根分别插入三种不同扭率螺旋扭带的三维内助管内的换热和流阻特性进行了实验研究。结果表明：三维内肋管内加装扭带的强化传热技术适用于低Ｒｅ数下高Ｐｒ数工质的管内对流换热强化。根据实验值得到了流阻和换热关联式。

波形刃铣刀片温度场边界条件的研究

通过传热学、流体力学、切削理论的综合运用对波形刃铣刀片温度场边界条件进行了研究。采用相似原理对面铣加工过程中波形刃铣刀片表面的对流换热系数进行了分析 ,得出了对应的计算公式 ;结合铣削温度试验 ,推导出了前刀面刀—屑接触面输入的热流密度的函数表达式。给出了波形刃铣刀片温度场有限元分析的边界条件 ,为进一步研究铣刀片的粘结破损和槽型优选提供了理论依据。