A numerical study on heat transfer enhancement and design of a heat exchanger with porous media in continuous hydrothermal flow synthesis system

The aim of this study is to use a new configuration of porous media in a heat exchanger in continuous hydrothermal flow synthesis(CHFS)system to enhance the heat transfer and minimize the required length of the heat exchanger.For this purpose,numerous numerical simulations are performed to investigate performance of the system with porous media.First,the numerical simulation for the heat exchanger in CHFS system is validated by experimental data.Then,porous media is added to the system and six different thicknesses for the porous media are examined to obtain the optimum thickness,based on the minimum required length of the heat exchanger.Finally,by changing the flow rate and inlet temperature of the product as well as the cooling water flow rate,the minimum required length of the heat exchanger with porous media for various inlet conditions is assessed.The investigations indicate that using porous media with the proper thickness in the heat exchanger increases the cooling rate of the product by almost 40% and reduces the required length of the heat exchanger by approximately 35%.The results also illustrate that the most proper thickness of the porous media is approximately equal to 90% of the product tube's thickness.Results of this study lead to design a porous heat exchanger in CHFS system for various inlet conditions.

Impact of Heat Transfer Media on Performance of Solar-Hydrogen Power Generation

Solar-hydrogen system has great potential for contributing to sustainable and clean energy supply. The aim of this study is to clarify the impact of heat transfer media in solar collector such as methane, ammonium, hydrogen, air and water on the performance of solar-hydrogen system. After estimating the highest temperature attainable by each heat transfer media, the amount of thermal energy that could be saved in the production of hydrogen or preheat for power generation by fuel cell was calculated. The power generation performance of fuel cell using each heat transfer media was also investigated. As a result, it has been revealed that the temperature changes of methane, ammonium and air follow the horizontal solar radiation intensity irrespective of seasons, and their highest temperatures are almost the same among them. The temperature response of hydrogen is slower than methane, ammonium and air. This study defines the ratio of saving thermal energy which indicates the effect of solar thermal utilization for production of hydrogen or preheat for power generation by fuel cell without using utility gas. It has been found that the biggest thermal energy saving is obtained when hydrogen and air are used as the heat transfer media. The power generated by PEFC system per effective area of evacuated tube collector in the case of using methane or ammonium is 3.309×10-2 kWh/m2 and 2.076×10-2 kWh/m2, respectively, while it is 2.466×10-2 kWh in the case of using hydrogen and air.

The Quasi-Static Approximation of Heat Waves in Anisotropic Thermo-Elastic Media

The equilibrium equations of anisotropic media, coupled to the heat conduction equations, are studied here based on the standard spaces of the physical presentation, in which an new thermo-elastic model based on the second law of thermodynamics is induced. The uncoupled heat wave equation for anisotropic media is deduced. The results show that the equation of heat wave is of the properties of dissipative waves. In final part of this paper, we discuss the propagation behaviour of heat waves for transversely isotropic media.

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. .

Microscale Infrared Observation of Liquid-Vapor Phase Change Process on the Surface of Porous Media for Loop Heat Pipe

Loop Heat Pipe (LHP) performance strongly depends on the performance of a wick that is porous media inserted in an evaporator. In this paper, the visualization results of thermo-fluid behavior on the surface of the wick with microscopic infrared thermography were reported. In this study, 2 different samples that simulated a part of wick in the evaporator were used. The wicks were made by different two materials: polytetrafluoroethylene (PTFE) and stainless steel (SUS). The pore radii of PTFE wick and SUS wick are 1.2 μm and 22.5 μm. The difference of thermo-fluid behavior that was caused by the difference of material was investigated. These two materials include 4 different properties: pore radius, thermal conductivity, permeability and porosity. In order to investigate the effect of the thermal conductivity on wick’s operating mode, the phase diagram on the q-keff plane was made. Based on the temperature line profiles, two operating modes: mode of heat conduction and mode of convection were observed. The effective thermal conductivity of the porous media has strong effect on the operating modes. In addition, the difference of heat leak through the wick that was caused by the difference of the material was discussed.

甲烷/乙烷混合工质纳米液膜蒸发传质特性的研究

采用分子动力学模拟方法研究了五种不同配比甲烷/乙烷混合工质纳米液膜的汽化过程,从蒸发现象、能量变化、涨落耗散规律及相对分子汽化量四个方面分析了混合工质的蒸发传质特性.结果表明:混合工质纳米液膜中甲烷配比越大,汽化起始时间越早,汽化过程越剧烈,汽化的持续时间越短,固定吸附层越薄,模拟体系动能和势能的增量越小,乙烷的存在会抑制甲烷的汽化;由涨落耗散分析可得,甲烷配比越大,对乙烷汽化的促进作用越强,汽化弛豫时间越短,整体汽化速率越快,传质效果越好.

Response of saturated porous media subjected to local thermal loading on the surface of semi-infinite space

Heat source function method is adopted in the present paper to derive elementary solutions of coupled thermo-hydro-mechanical consolidation for saturated porous media under conjunct actions of instantaneous point heat source, instantaneous point fluid source and constant volume force. By using the so-called fictitious heat source method and images method, the solutions of a semi-infinite saturated porous medium subjected to a local heat source with time-varied intensity on its free surface are developed from elementary solutions. The numerical integral methods for calculating the unsteady temperature, pore pressure and displacement fields are given. The thermomechanical response are analyzed for the case of a circular planar heat source. Besides, the thermal consolidation characteristics of a saturated porous medium subjected to a harmonic thermal loading are also given, and the fluctuation processes of the field variables located below the center of heat source are analyzed.

Soret and Dufour effects in strongly endothermic chemical reaction system of porous media

Taking account of the thermal-diffusion (Soret) and the diffusion-themo (Dufour) effects, the properties of the heat and mass transfers in a strongly endothermic chemical reaction system for a porous medium are numerically studied. Through the theory of the thermodynamics of irreversible processes, a coupled mathematical model describing the heat and mass transfers in aporous system for the calcination of limestone is formulated. The governing partial differential equations are numerically solved by the implicitly finite volume method through decomposing the equations to a set of coupled differential equations. The results indicate that when the convectional velocity is lower or when the initial temperature of the feeding gas is higher, Soret and Dufour effects can’t be ignored. The distribution figures for the temperature field of the gas in the system, the concentration field of the product gas and the solid conversion ratio are provided.

Heat Transfer and Energy Utilization of Waste Heat Recovery Device with Different Internal Component

Steel industry is high energy-consuming industry, and its waste?heat recovery is critically?important for energy utilization. In this study, pipeline bundle is used to enhance heat transfer in?waste?heat recovery device,?and?associated gas-solid heat transfer and energy utilization performance with different pipeline arrangement, pipe diameter and shape of internal component are further analyzed. The temperatures of gas and particle in device with pipeline bundle periodically fluctuate in horizontal direction, and those in staggered system distribute more uniformly than those in paralleled system. Compared with paralleled device, exergy and waste heat utilization efficiency of staggered device have been improved, and they are both higher than?those without pipeline. As pipe diameter increases, exergy and waste heat utilization efficiency first increases and then decreases, and they reach the maxima with optimal pipe diameter.?As the width of internal component keeps constant, influence of its shape on heat transfer is very little.

Numerical Study of Heat Transfer and Contaminant Transport in an Unsaturated Porous Soil

Penetration of chemicals in the soil ground through irrigation water or rainfall induces important risks for the environment. These risks are badly known and may lead to direct contamination of the environment (atmosphere or ground water) or harmful effects on organisms living at ground level, indirectly affecting men. It is thus necessary to estimate these potential chemical risks on the environment. For that reason, the gradual change of these products (fertilizers, solutions, pollutants, ...) in the ground has been the subject of a lot of recent research works, based in particular on the study of non-saturated porous media in a theoretical, numerical or experimental way. Most of these works are incomplete and, in order to simplify the problem, they don’t take into accounts some process, which may be of prime importance under particular natural conditions. Complexity of such studies results from their multidisciplinary nature. In this communication, we study simultaneous transport of pollutant, the water that provides transport and the heat transfer in a 200 cm long cylindrical column full of sand taken as a non-saturated porous medium. We consider two kinds of conditions on the temperature at the column surface: the case of constant temperature and the case of sinusoidal temperature. We evaluate the influence of this temperature on the transfers. This study is purely numerical. We use the control volume method to determine hydrous, thermal and pollutant concentration profiles.

Combining the Radiative, Conductive and Convective Heat Flows in and around a Skylight

Normal skylights bring light into the spaces located below them. By the use of IR （infrared radiation） transmissive polymer films and IR-emitting and absorbing gases, an advanced version of the skylight may supply passive cooling and thermal insulation to the room located below it. This novel radiative skylight can, in its cooling mode, lead heat from the room below, to the cool skies located above the skylight. When cooling is no longer needed or attainable, the skylight will in its cooling mode provide the room with an optimal amount of thermal resistance. This article is a progress reporting on the modeling of the skylight. The main work is done to combine the different heat transfer methods into one single model by the use of the commercial program Comsol 4.1. The results show that a cooling effect of 100 W/ma is achievable when the skylight is compared with a similar skylight containing only air.

The Use of Models to Evaluate Corrosion Effects on Mild Steel Heat Exchanger in Water and Mono Ethanol Amine (MEA)

Heat exchanger is an important equipment used in process industries for cooling and heating purposes. Its design configuration which involves the flow of cold and hot fluids within the exchanger subjects it to corrosion attack. The article utilized the principle of mass and energy conservation in the development of weight and temperature models to study the effect of corrosion on mild steel coupon inside the exchanger containing water and Mono ethanol amine (MEA). The models developed were resolved analytically using Laplace Transform and simulated using Excel as simulation tool and data obtained from experiment in the laboratory to obtain profiles of weight loss and temperature as a function of time. The weight loss and performance of mild steel under various corrosive conditions were examined which indicates the effect of corrosion on the mild steel heat exchanger in water and MEA media. The result shows that water is more corrosive than MEA at higher temperatures and at lower temperatures of 35°C and 1 atm, MEA has inhibitive properties than water as indicated by the weight loss result with time. The comparative analysis between the results obtained from the model simulation and experimental results shows that the result obtained from the model is more reliable and demonstrated better performance characteristics as it clearly shows mild steel heat exchanger experiences more corrosive effect in water medium than MEA at higher temperatures. And at lower temperatures, MEA becomes more inhibitive and less corrosive than water. The model simulation results correlate with various literatures and hence, it is valid for future referencing.

Effect of Exponentially Temperature-Dependent Viscosity on the Onset of Penetrative Ferro-Thermal-Convection in a Saturated Porous Layer via Internal Heating

The effect of viscosity depending exponentially on temperature on the onset of penetrative ferro-thermal-convection (FTC) in a saturated horizontal porous layer in the presence of vertical magnetic field is investigated. The bounding surface of the ferrofluid layer is considered to be rigid-rigid and insulated to temperature perturbations. The resulting eigenvalue problem is solved numerically using the Galerkin technique and also analytically by a regular perturbation technique with wave number as a perturbation parameter. The analytical and numerical results are found to be concurrence. The characteristics of stability of the system are strongly dependent on the viscosity parameter B. The effect of B on the onset of ferroconvection in a porous layer is dual in nature depending on the choices of physical parameters and a sublayer starts to form at higher values of B. Whereas, increase in magnetic number M1 and the Darcy number Da is to advance the onset of ferroconvection in a porous layer. The nonlinearity of fluid magnetization M3?is found to have no influence on the onset of ferroconvection.

CONJUGATE MODEL FOR HEAT AND MASS TRANSFER OF POROUS WALL IN THE HIGH TEMPERATURE GAS FLOW

Heat and mass transfer of a porous permeable wall in a high temperature gas dynamical flow is considered. Numerical simulation is conducted on the ground of the conjugate mathematical model which includes filtration and heat transfer equations in a porous body and boundary layer equations on its surface. Such an approach enables one to take into account complex interaction between heat and mass transfer in the gasdynamical flow and in the structure subjected to this flow. The main attention is given to the impact of the intraporous heat transfer intensity on the transpiration cooling efficiency.

Non-Darcy Mixed Convection between Differentially Heated Vertical Walls Filled with a Porous Material: Application of New Modified Adomian Decomposition Method

This paper presents non-Darcy mixed convective flow of an incompressible and viscous fluid in a differentially heated vertical channel filled with a porous material in the presence of a temperature dependent source/sink. The analytical solution of fourth order non-linear ordinary differential equation for temperature field, which is formed by eliminating velocity field from system of governing equations in non-dimensional form, is obtained by using new modified Adomian decomposition method (NMADM) in terms of various parameters. In order to illustrate the interactive influences of governing parameters on the temperature and velocity fields, a numerical study of the analytical solution is performed with respect to three categories of transport processes i) when forced convection is dominated, ii) when forced and natural convection are equal and iii) when natural convection is dominated. Analysis of all categories has revealed that the temperature and velocity profiles are increasing function of modified Darcy number while decreasing function of Forchheimer number.

Soret-Dufour Effects on the MHD Flow and Heat Transfer of Microrotation Fluid over a Nonlinear Stretching Plate in the Presence of Suction

In this work, the Micropolar fluid flow and heat and mass transfer past a horizontal nonlinear stretching sheet through porous medium is studied including the Soret-Dufour effect in the presence of suction. A uniform magnetic field is applied transversely to the direction of the flow. The governing differential equations of the problem have been transformed into a system of non-dimensional differential equations which are solved numerically by Nachtsheim-Swigert iteration technique along with the sixth order Runge-Kutta integration scheme. The velocity, microrotation, temperature and concentration profiles are presented for different parameters. The present problem finds significant applications in hydromagnetic control of conducting polymeric sheets, magnetic materials processing, etc.

Sodium-Modified Fluorapatite: A Mild and Efficient Reusable Catalyst for the Synthesis of <i>α,α</i>’-Bis(Substituted Benzylidene) Cycloalkanones under Conventional Heating and Microwave Irradiation

A versatile and environmentally friendly method for α,α’-bis(substituted ben-zylidene) cycloalkanones has been developed using a heterogeneous catalysis technology. We have synthesized a series of the α,α’-bis(substituted benzylidene) cycloalkanones, a biologically important class of compounds, via the cross aldol condensation between arylaldehydes and cycloketones using sodium-modified fluorapatite (Na/FAP) as a highly efficient solid catalyst under conventional heating in aqueous media and solventless conditions under microwave. Catalyst reuse, ease of separation of the pure product, and high yields are some of the unique features of this process. Shorter reaction times (4 - 7 min) and higher yields (80% - 94%) were achieved under microwave irradiation conditions.

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.

多孔介质传热模型在多孔壁湍流中的适用性

为了考查不同的多孔介质传热模型在不同工况下的适用性,对带有高孔隙率多孔介质壁面槽道湍流及其传热进行了直接数值模拟研究。在多孔介质层外流体区域,通过有限差分方法求解不可压缩Navier-Stokes方程和温度对流扩散方程;在多孔介质层内,使用修正的Darcy-Brinkman-Forchheimer模型描述高孔隙率多孔介质阻力,以及分别采用局部热平衡(local thermal equilibrium,LTE)模型、局部非热平衡(local thermal non-equilibrium,LTNE)模型、理想金属(ideal metal foams,IMF)模型计算温度分布。通过对所得热场的统计特性的分析比较,探究了不同Biot数下水和空气两类流体介质的多孔介质传热模型的有效性。研究表明:LTE模型不足以准确预测金属泡沫多孔介质内传热问题,其等效导热系数因仅考虑孔隙率因素而低估了多孔介质层的传热能力;IMF模型在小比热容流体介质的算例中表现良好,可以代替LTNE描述多孔介质层内的传热,而在大比热容流体介质的算例中表现不佳,需要考虑比热容以及流固两相间的传热能力对预估的固体相温度分布进行修正。

纳米流体地热循环换热实验研究

提升换热介质的换热性能是高效开采地热资源的有效手段之一。添加纳米级金属氧化物可有效提升流体的换热能力,而纳米颗粒种类、质量分数、粒径、分散剂质量分数等物性参数以及流速对纳米流体换热性能具有重要影响。采用球形纳米CuO和Al_(2)O_(3)(粒径20~50 nm)作为换热介质,十二烷基苯磺酸钠(SDBS)作为分散剂配制纳米流体,利用自主搭建的纳米流体基础换热实验装置进行室内换热实验,优选纳米流体参数。此外,通过自主搭建循环流动换热实验装置,以湖北英山某水热型地热井中地热水作为热源,讨论了在现场实际热源边界条件下,流速对纳米流体和去离子水的换热性能影响规律。结果表明:(1)CuO纳米流体换热性能优于Al_(2)O_(3)纳米流体;(2)纳米流体的换热性能与纳米颗粒质量分数呈负相关关系,CuO质量分数为1%时纳米流体升温效率最高,在150 s内温度可由25℃上升到79.2℃,同时间内比去离子水高4.1℃,同时,随着纳米颗粒质量分数的增加,纳米流体与热源界面的润湿性减小;(3)纳米流体换热性能随着纳米颗粒粒径增加呈现先增加后减小的趋势,在纳米颗粒粒径为40 nm时纳米流体换热性能最佳;(4)纳米流体的换热性能与分散剂质量分数呈负相关关系,当分散剂质量分数为1%时换热性能最佳;(5)层流状态下纳米流体的换热性能与流速呈负相关关系;在湍流状态下纳米颗粒运动状态逐渐剧烈,有利于纳米流体传热。研究成果可为纳米流体应用于地热换热从而提升地热系统的换热效率提供依据,并为纳米流体参数以及流速参数的选择提供理论依据。