Heat-Generating Effects Involving Multiple Nanofluids in a Hybrid Convective Boundary Layer Flow on the Sloping Plate in a Porous Medium

The hybrid convective boundary layer circulation involving multiple nanofluids via a medium with pores is approaching a sloping plate. An investigation regarding the heat-generating effects upon the examined nanofluid flows has been carried out through computational analysis. A mathematical framework employing governing differential equations that are partial has been implemented to produce an ensemble of ordinary differential equations, which happen to be nonlinear that incorporate nanofluid flows by utilizing acceptable transformations. Through the combination of the Nachtsheim-Swigert shooting method and the Runge-Kutta method, the group of resulting non-dimensionalized equations is solved computationally. In a few special, confined cases, the corresponding numeric output is thereafter satisfactorily matched with the existing available research. The consequences of heat generation regarding local skin friction coefficient and rate of heat in conjunction with mass transfer have been investigated, evaluated, and reported on the basis of multiple nanofluid flows.

Slip Condition Effects on Unsteady MHD Fluid Flow with Radiative Heatflux over a Porous Medium

The objective of this paper is to study unsteady magneto hydrodynamic (MHD) free flow of viscoelastic fluid (Walter’s B) past an infinite vertical plate through porous medium. The temperature is assumed to be oscillating with time. The solution obtained shows different profiles of effects of slip conditions on primary and secondary velocity. Also, the effects of various parameters on temperature, concentration, primary and secondary velocity profiles were presented graphically. The result indicated the secondary velocity is enhanced with increase in slip parameter. Primary velocity demonstrated opposite trend.

Melting heat transfer effects on stagnation point flow of micropolar fluid saturated in porous medium with internal heat generation(absorption)

The effect of melting heat transfer on the two dimensional boundary layer flow of a micropolar fluid near a stagnation point embedded in a porous medium in the presence of internal heat generation/absorption is investigated. The governing non-linear partial differential equations describing the problem are reduced to a system of non-linear ordinary differential equations using similarity transformations solved numerically using the Chebyshev spectral method. Numerical results for velocity, angular velocity and temperature profiles are shown graphically and discussed for different values of the inverse Darcy number, the heat generation/absorption parameter, and the melting parameter. The effects of the pertinent parameters on the local skin-friction coefficient, the wall couple stress, and the local Nusselt number are tabulated and discussed. The results show that the inverse Darcy number has the effect of enhancing both velocity and temperature and suppressing angular velocity. It is also found that the local skin-friction coefficient decreases, while the local Nusselt number increases as the melting parameter increases.

Natural convection of nanofluid over vertical plate embedded in porous medium: prescribed surface heat flux

The aim of the present paper is to analyze the natural convection heat and mass transfer of nanofluids over a vertical plate embedded in a saturated Darcy porous medium subjected to surface heat and nanoparticle fluxes. To carry out the numerical solution, two steps are performed. The governing partial differential equations are firstly simplified into a set of highly coupled nonlinear ordinary differential equations by appropriate similarity variables, and then numerically solved by the finite difference method. The obtained similarity solution depends on four non-dimensional parameters, i.e., the Brownian motion parameter （Nb）, the buoyancy ratio （Nr）, the thermophoresis parameter （Nt）, and the Lewis number （Le）. The variations of the reduced Nusselt number and the reduced Sherwood number with Nb and Nt for various values of Le and Nr are discussed in detail. Simulation results depict that the increase in Nb, Nt, or Nr decreases the reduced Nusselt number. An increase in the Lewis number increases both of the reduced Nusselt number and the Sherwood number. The results also reveal that the nanoparticle concentration boundary layer thickness is much thinner than those of the thermal and hydrodynamic boundary layers.

Effects of Solid Matrix and Porosity of Porous Medium on Heat Transfer of Marangoni Boundary Layer Flow Saturated with Power-Law Nanofluids

The effect of the solid matrix and porosity of the porous medium are first introduced to the study of power-law nanofluids, and the Marangoni boundary layer flow with heat generation is investigated. Two cases of solid matrix of porous medium including glass balls and aluminum foam are considered. The governing partial differential equations are simplified by dimensionless variables and similarity transformations, and are solved numerically by using a shooting method with the fourth-fifth-order Runge-Kutta integration technique. It is indicated that the increase of the porosity leads to the enhancement of heat transfer in the surface of the Marangoni boundary layer flow.

Investigation of the Effects of Some Physical Parameters and Hall Current on MHD Fluid Flow with Heat Flux over a Porous Medium

In this paper, an investigation of the effects of some physical parameters and Hall current on magneto hydrodynamics (MHD) fluid flow with heat flux over a porous medium was carefully examined, taking into consideration Hall effects where the temperature and concentration are assumed to be oscillating with time. Furthermore, perturbation method is used in solving the governing equations. The profiles of velocity, temperature and concentration are presented graphically, going into the problem the primary and secondary velocity are presented and compute for some physical parameters such as mass Grashof number (Gc), Schmidt number Sc, Prandtl number (Pr) viscoelastic parameter (K1) and hall current parameter (m). Results indicated that primary velocity increases with increase in values of Gc on one hand and on the other hand it decreases with increase in the values of Pr, K1 and m. Secondary velocity demonstrated opposite trend.

Piezoelectric Power Harvesting Process via Phase Changes of Low-Boiling-Point Medium Together with Water for Recovering Low-Temperature Heats

Low-temperature thermal energy conversions down to exergy zero to electric power must contribute energy sustainability. That is to say, reinforcements of power harvesting technologies from extremely low temperatures less than 373 K might be at least one of minimum roles for the current generations. Then, piezoelectric power harvesting process for recovering low-temperature heats was invented by using a unique biphasic operating medium of an underlying water-insoluble/low-boiling-point medium (i.e. NOVEC manufactured by 3M Japan Ltd.) in small quantity and upper-layered water in large quantity. The higher piezoelectric power harvesting densities were naturally revealed with an increase in heating temperatures. Excessive cooling of the operating medium deteriorated the power harvesting efficiency. The denser operating medium was surpassingly helpful to the higher piezoelectric power harvesting density. Concretely, only about 5% density increase of main operating medium (i.e. water with dissolving alum at 0.10 mol/dm3) came to the champion piezoelectric power harvesting density of 92.6 pW/dm2 in this study, which was about 1.4 times compared to that with the original biphasic medium of pure water together with a small quantity of NOVEC.

Heat and Mass Transfer in MHD Visco-Elastic Fluid Flow through a Porous Medium over a Stretching Sheet with Chemical Reaction

This paper presents the study of convective heat and mass transfer characteristics of an incompressible MHD visco-elastic fluid flow immersed in a porous medium over a stretching sheet with chemical reaction and thermal stratification effects. The resultant governing boundary layer equations are highly non-linear and coupled form of partial differential equations, and they have been solved by using fourth order Runge-Kutta integration scheme with Newton Raphson shooting method. Numerical computations are carried out for the non-dimensional physical parameters. Here a numerical has been carried out to study the effect of different physical parameters such as visco-elasticity, permeability of the porous medium, magnetic field, Grashof number, Schmidt number, heat source parameter and chemical reaction parameter on the flow, heat and mass transfer characteristics.

Gas Flow Development Through Tandem Heat Exchangers Inside Exhaust Nozzle by Using Porous Medium Model

A computational study on the flow development through tandem double-U-shaped-tubes compact heat exchangers inside exhaust nozzle is presented.In order to simplify the computational process on modeling the flow field,the compact heat exchanger is modeled as a porous matrix by using an isotropic porous medium assumption,which makes two-dimensional numerical simulation realistic.With the use of an existed quadratic relation which connects the pressure drop with the inlet velocity in the external part of the heat exchanger,the permeability and drag coefficient in the porous medium model are determined and a corresponding computational method validation is also made.Two schemes of tandem double-U-shaped-tubes compact heat exchangers are numerically analyzed.In relative to the baseline scheme,the modified scheme is improved by smoothing the nozzle expansion,varying heat exchanger mounting angle and installing boat-tail ramp at the trailing edge of the last heat exchanger.The results show that the pressure losses due to the existence of local recirculation zones and inappropriate distribution of the flow field are reduced in the modified scheme.The pressure loss coefficient is decreased from 1.7% under the baseline scheme to 1.2% under the modified scheme.

Analytic Solution for Fluid Flow over an Exponentially Stretching Porous Sheet with Surface Heat Flux in Porous Medium by Means of Homotopy Analysis Method

In this paper, the analytical solution of a viscous and incompressible fluid towards an exponentially stretching porous sheet with surface heat flux in porous medium, for the boundary layer and heat transfer flow, is presented. The equations of continuity, momentum and the energy are transformed into non-linear ordinary differential by using similarity transformation. The solutions of these highly non-linear ordinary differential equations are found analytically by means of Homotopy Analysis Method (HAM). The result obtained by HAM is compared with numerical results presented in the literature. The accuracy of the HAM is indicated by close agreement of the two sets of results. By this method, an expression is obtained which is admissible for all values of effective parameters. This method has the ability to control the convergence of the solution.

Inverse Problem on Heat Conduction in Heterogeneous Medium

Under consideration is a nonclassical stationary problem on heat conduction in a body with the pre-set surface temperature and heat flow. The body contains inclusions at unknown locations and with unknown boundaries. The body and inclusions have different constant thermal conductivities. The author explores the possibility of locating inclusions. The article presents an integral criterion based on which a few statements on identification of inclusions in a body are proved.

基于压缩CO_(2)的新型储能技术研究进展

储能技术是新型电力系统的重要装备基础,不仅能够实现能源时间上的转移,平滑可再生能源的功率输出,还可以实现供给侧和用户侧的能源管理。本文首先综述了压缩CO_(2)储能的技术原理及应用现状;随后,基于CO_(2)的储存状态,将压缩CO_(2)储能技术路线进行分类,并总结了不同技术路线的性能特点和研究进展;最后,对压缩CO_(2)储能系统未来发展方向做出预测。(1)CO_(2)的临界点为7.38 MPa/31.3℃,比空气更容易被液化而具有储能密度大的优点,在相同功率等级下压缩CO_(2)储能系统相比于压缩空气储能系统更加紧凑。目前,国内外均已有CO_(2)储能验证项目建成投运。(2)根据低/高压侧储存装置中CO_(2)状态的不同,可将压缩CO_(2)储能技术的储存方案分为七种:气态-超临界状态CO_(2)储存、气态-液态CO_(2)储存、超临界态-超临界态CO_(2)储存、液态-超临界状态CO_(2)储存、液态-液态CO_(2)储存、吸附态-液态CO_(2)储存和吸附态-超临界态CO_(2)储存。(3)在未来压缩CO_(2)储能的技术开发上,主要着重于气态-液态CO_(2)储能技术、液态-液态CO_(2)储能技术,前者可以充分发挥其高循环效率、系统结构简单优势,后者可以发挥其高储能密度优势。(4)在面向压缩CO_(2)储能技术多元化发展方面,一是推动“可再生能源+压缩CO_(2)储能”模式,实现可再生能源的持续平稳供电,辅助电网运行,减少弃光弃电现象;二是深入研究基于压缩CO_(2)储能技术的集成系统,通过耦合喷射器制冷循环、有机朗肯循环等实现CO_(2)的冷凝以及余热再利用,提高系统性能和能量利用率。目前,已完成的压缩CO_(2)储能验证项目还很少,应加快CO_(2)储能技术发展,实现由理论到实践、由概念研发到项目示范、再到大量推广。

MHD Heat and Mass Transfer of an Oscillatory Flow over a Vertical Permeable Plate in a Porous Medium with Chemical Reaction

The problem of magneto-hydro-dynamic (MHD) mass and heat transfer of an oscillatory fluid in two-dimensional viscous, electrically conducting over an infinite vertical permeable moving plate in a saturated porous medium with the presence of a transverse magnetic field and chemical reaction is analytically presented. The governing equations, momentum, energy, and concentration are solved. Various flow parameters effects on velocity, temperature and concentration fields are discussed. It is found that, the fluid velocity increases with increasing both the permeability and chemical reaction parameters. While, it increases with decreasing the magnetic field parameter. Furthermore, the concentration increases with increasing chemical reaction parameters.

Consideration of transient heat conduction in a semi-infinite medium using homotopy analysis method

In the current work, transient heat conduction in a semi-infinite medium is considered for its many applications in various heat fields. Here, the homotopy analysis method （HAM） is applied to solve this problem and analytical results are compared with those of the exact and integral methods results. The results show that the HAM can give much better approximations than the other approximate methods： Changes in heat fluxes and profiles of temperature are obtained at different times and positions for copper, iron and aluminum.

An Investigation into the Effect of Porous Medium on Performance of Heat Exchanger

In this paper, a detailed numerical investigation of two-dimensional laminar forced convection in a porous channel with inlet and outlet slot is presented. A uniform heat flux is applied on one wall of channel and another wall is isolated. The flow in the porous medium is modeled using the Brinkman-Forchheimer-extended Darcy model in which the inertia and boundary effects are taken into consideration and thermal dispersion effects are not included in the energy equation. Parametric studies are conducted to evaluate the effects of particle diameter, Reynolds number on the heat transfer and friction factor. Nusselt number and friction factor are developed for efficient design of a porous heat exchanger based on the present configuration. In order to solve this problem Fluent software was used.

Joule Heating and Thermal Radiation Effects on MHD Boundary Layer Flow of a Nanofluid over an Exponentially Stretching Sheet in a Porous Medium

A numerical study on boundary layer flow behaviour, heat and mass transfer characteristics of a nanofluid over an exponentially stretching sheet in a porous medium is presented in this paper. The sheet is assumed to be permeable. The governing partial differential equations are transformed into coupled nonlinear ordinary differential equations by using suitable similarity transformations. The transformed equations are then solved numerically using the well known explicit finite difference scheme known as the Keller Box method. A detailed parametric study is performed to access the influence of the physical parameters on longitudinal velocity, temperature and nanoparticle volume fraction profiles as well as the local skin-friction coefficient, local Nusselt number and the local Sherwood number and then, the results are presented in both graphical and tabular forms.

高海拔地区铁路隧道施工期围岩热流密度数值模拟

冷负荷预测是隧道施工的重要任务,目前传统隧道冷负荷预测方法无法满足高海拔铁路隧道施工环境。为高海拔铁路隧道的冷负荷预测提供准确方法,建立一个热湿耦合多孔介质模型,考虑围岩孔隙率和低压环境对隧道围岩热流密度的影响。建立高海拔铁路隧道围岩热流密度预测模型,分析渗流水和衬砌的存在对围岩热流密度的影响,对比不同环境参数对围岩热流密度的影响。结果表明,隧道的冷负荷取决于围岩的热流密度。预测模型可以准确计算隧道围岩热流密度,其均方根误差为0.573 W/m^(2)。围岩渗流水和衬砌对隧道热湿环境产生显著影响。压力对围岩热流密度的影响最小。围岩温度每升高10℃时,围岩初始热流密度增加了58.74 W/m^(2)。此外,每当隧道目标温度增加4℃或孔隙率增加0.1,分别导致围岩初始热流密度减少了23.5、0.14 W/m^(2)。研究结果可以为实际高海拔铁路隧道工程的冷负荷预测提供新的理论方法和参考依据。

中温铯热管启动与升温试验研究

为解决热管对中温热源的热传输难点,该研究研制了一种中温铯热管。该研究通过对金属铯作为中温热管工质优缺点进行了分析,确定了铯作为中温热管工质的可行性,研制了3根试验用中温铯热管。该研究利用热管启动与升温实验装置对3根不同规格中温铯热管开展了启动与升温试验。研究得到:铯热管能够在500 K附近进入正常工作状态,实现启动。铯热管蒸汽腔直径为17 mm时,热管启动温度为510 K;蒸汽腔直径为22 mm时,启动温度为495 K;蒸汽腔直径25 mm时,启动温度为485 K。对比试验验证了铯热管的启动温度随热管蒸汽腔直径的增大而降低。在500~700 K中温区间,中温铯热管轴向各点能够同步升温,具有良好的等温性。

海上平台烟气余热利用分析与工艺优化

依托南海某在建钻采平台,从工艺原理、工艺流程、有机工质的选择、安全性、能源节约及降低二氧化碳排放等方面进行分析,论证有机朗肯循环(Organic Rankine Cycle,ORC)技术在海上平台应用的可行性。根据能量梯级利用原则,设计一套优化的烟气-导热油-热用户系统,利用透平废热实现ORC发电、生产系统加热及溴化锂制冷的热电冷联产系统。

沼气多孔介质燃烧换热器的燃烧换热特性研究

为促进沼气的实际应用,解决燃烧器对沼气热值变化的适应性问题,设计了一台基于沼气的多孔介质燃烧换热器.通过试验研究了沼气热值变化时,不同燃烧强度、不同水量下多孔介质燃烧换热器的燃烧室壁面温度分布、热效率及污染物排放情况.结果表明:沼气中甲烷含量为50%~70%时均可在燃烧换热器中稳定燃烧,燃烧室壁面温度变化在75℃以内;热值瞬时变化时,燃烧换热器仍能维持稳定燃烧,出口水温相较于燃烧室壁面最高温度响应速度略滞后;燃烧换热器有良好的换热能力,热效率随燃烧强度的增加而减小,随燃气热值和水量的增加而增加;尾部换热器传热系数和燃烧室传热系数都随燃烧强度的增加而增加;NOx排放随燃烧室壁面温度降低而降低,最高为27 mg/m^(3);CO排放量最大为151 mg/m^(3)。