Combined heat transfer of radiation and forced convection flow of participating gases in a three-dimensional recess

This research work presents a numerical investigation of three-dimensional combined convection-radiation heat transfer over a recess including two inclined steps in a horizontal duct. To simulate the inclined surface boundaries, the blocked off method is employed for both fluid mechanic and radiation problems. The fluid is treated as a gray, absorbing, emitting and scattering medium. In numerical solution of the governing equations including conservation of mass, momentum and energy, the three-dimensional Cartesian coordinate system is used. These equations are solved numerically using the CFD techniques to obtain the temperature and velocity fields. Discretized forms of the governing equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation terms are presented in the energy equation. For computation of radiative term in energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinates method (DOM) to find the divergence of radiative heat flux distribution inside the radiating medium. The effects of radiation-conduction parameter, optical thickness and albedo coefficient on heat transfer behavior of the system are presented. Comparison of numerical results with the available data published in open literature shows a good agreement.     

柴油机缸内辐射换热的多维数值模拟研究

对柴油机来说,辐射换热极为重要,在缸内总传热量中占有非常大的比重,直接关系到发动机热效率及因传热引起的各种热负荷、热强度问题,同时,辐射换热对燃烧系统的研究也十分重要,辐射热流量会深刻影响内燃机的燃烧性能,对发动机的各种燃烧产物的形成产生至关重要的影响。为此,利用离散传递法实现柴油机缸内辐射换热的多维数值模拟,通过多维模拟计算同时考察燃烧室部件表面发射率及喷雾提前角对柴油机缸内辐射换热的影响。结果表明：活塞的辐射热流量峰值高于缸盖的辐射热流;缸盖的辐射热流量的最大值并不在中心位置处,而是随时间变化;随着壁面辐射率的增加,缸内向燃烧室部件辐射换热量逐渐增大;喷雾提前角直接影响所有燃烧室部件表面的辐射热流密度。     

Effect of thermal radiation on convection heat transfer in cooling channel of photovoltaic thermal system

The effect of thermal radiation on convection heat transfer in flat-box type cooling channel of photovoltaic thermal system with tilt angle of 30 degree was studied by 3D numerical simulation under constant heat flux boundary condition. The temperature contours and velocity fields of fluid near the outlet were obtained. The variations of wall temperature and convection Nusselt number along flow direction for all the separate walls composing the cooling channel were compared and analyzed. The results show that due to thermal radiation, the deflection of the maximum velocity region to heated top wall, together with the asymmetry of temperature field is weakened. For natural convection, radiation promotes the formation of multi-vortices. For mixed convection, heat transfer on all the cooling channel walls is enhanced under the condition of lower heat flux while heat transfer on heated top wall is deteriorated when the heat flux is relative high. Also, pressure re-rising is promoted by thermal radiation.     

气膜冷却壁面温度的计算方法和计算曲线

用作图法求解了气膜冷却壁面的热平衡方程。计算方法中引用了本文作者最近做出的绝热壁面恢复温度的有效温比实验公式。算出了气膜冷却壁面的辐射热流量、气膜冷却热流量,外壁对流冷却和外壁辐射散热等。并用作图法求出了气膜冷却壁面的平衡温度和气膜冷却效率。有效温比、换热系数、气体黑度系数等都可用本文提供的图表直接查找。     

烧结开孔金属泡沫壁的辐射和自然对流耦合换热试验研究

开展烧结开孔金属泡沫壁在大空间条件下的辐射和自然对流耦合换热试验研究,研究壁面倾角、瑞利数Ra*、泡沫厚度对换热性能的影响。试验所得竖直光铜板的结果与文献结果符合一致,验证了试验方案的有效性。试验结果表明:在一定的加热功率下,随着倾角的增加,纯自然对流努塞尔数和复合努塞尔数先增加后减小,在倾角为60o～80o达到极大值,且辐射换热量占总换热量的33%～44%;竖直放置时,烧结泡沫壁的辐射换热量占总换热量的比值小于光壁,且该值随泡沫厚度增加而增加。与光壁相比,烧结泡沫竖壁复合换热的平均努塞尔数和纯自然对流换热的平均努塞尔数分别提高了1.52～1.98倍和1.16～1.66倍。通过对红外相机拍摄的泡沫表面温度分布进行分析验证了基于泡沫表面平均温度的辐射换热量计算模型的有效性。     

Application of spectral collocation method to conduction and laminar forced heat convection in eccentric annuli

Numerical approach based on the spectral collocation method has been utilized for analyzing heat convection and conduction in eccentric annuli. An eccentric instead of concentric annular duct is sometimes used as a fluid-flow and heat-transfer device especially in nuclear power plants. The hydrodynamically and thermally fully developed laminar flow with uniform heat flux through the inner and outer walls has been analyzed. Also, the conductive heat transfer problem, with uniform rate of internal heat generation in long hollow cylinder, has been solved. The governing equation for the present analysis is Poisson’s equation with constant nonhomogeneous term. Considering temperature and velocity distributions in eccentric annuli, Nusselt numbers and wall shear stresses are presented for various range of eccentricities. The spectral collocation method used in this study is verified by comparing the numerical solutions from the existing analytical solution and it is clear that this method is appropriate for assessing a more complicated heat transfer problem.     

A direct integration approach for the estimation of time-dependent boundary heat flux

In a one-dimensional heat conduction domain with heated and insulated walls, an integral approach is proposed to estimate time-dependent boundary heat flux without internal measurements. It is assumed that the expression of the heat flux is not known a priori. Hence, the present inverse heat conduction problem is classified as a function estimation problem. The spatial temperature distribution is approximated as a third-order polynomial of position, whose four coefficients are determined from the heat fluxes and the temperatures at both ends at each measurement. After integrating the heat conduction equation over spatial and time domain, respectively, a simple and non-iterative recursive equation to estimate the time-dependent boundary heat flux is derived. Several examples are introduced to show the effectiveness of the present approach.     

基于Workbench的高压圆盘气体轴承共轭传热研究

高压圆盘气体轴承流道间隙内高速气流的对流换热与轴承圆盘内部热传导紧密耦合在一起,是一个典型的共轭传热问题。基于ANSYS Workbench工作平台的Fluid Flow(Fluent)模块对高压圆盘气体轴承进行共轭传热数值模拟,获得轴承流道间隙内的速度和压力分布、流体域与固体域的温度分布以及共轭传热时流固耦合壁面的热流密度分布,并将其与非共轭传热恒温壁面条件下的计算结果进行对比,得到高压圆盘气体轴承共轭传热的一些基本特性。结果表明：2种情况下的计算结果存在较大差异,非共轭传热恒温壁面条件下,间隙内的气体只吸热,流体域耦合壁面上的热流密度均为正值;而共轭传热条件下流体域耦合壁面热流密度存在正负值,间隙内气体的吸热和放热同时存在,显示出轴承圆盘的热传导与间隙内气体的对流换热具有复杂的共轭作用机制;相比之下,采用共轭传热模型可以得到更为符合实际的结果。研究结果为该类轴承的设计和制造提供了有益的指导。     

基于瑞利分布的平面磨削温度场的仿真研究

考虑磨粒在砂轮表面随机分布对接触区热流密度分布的影响,基于未变形磨屑厚度的瑞利分布理论,假定流入工件的热流密度呈瑞利分布,建立了瑞利分布热源模型。采用基于有限元法的瑞利分布热源模型对典型磨削工况进行仿真计算,将计算结果与矩形分布及三角形分布仿真结果进行对比,系统地分析了热源模型、磨削液对温度场的影响规律。结合磨削实验测量值,发现基于瑞利分布的热源模型仿真结果与实验测量值吻合较好。     

Heat losses from the receivers of a multifaceted parabolic solar energy collecting system

Heat losses from the receivers of a dish-type solar energy collecting system at the Korea Institute of Energy Research (K.IER) are numerically investigated. It is assumed that a number of flat square mirrors are arranged on the parabolic dish structure to serve as a reflector. Two different types of receivers, which have conical and dome shapes, are considered for the system, and several modes of heat losses from the receivers are thoroughly studied. Using the Stine and McDonald model convective heat loss from a receiver is estimated. The Net Radiation Method is used to calculate the radiation heat transfer rate by emission from the inside surface of the cavity receiver to the environment. The Monte-Carlo Method is used to predict the radiation heat transfer rate from the reflector to the receiver. Tracing the photons generated, the reflection loss from the receivers can be estimated. The radiative heat flux distribution produced by a multifaceted parabolic concentrator on the focal plane is estimated using the cone optics method. Also, the solar radiation spillage around the aperture is calculated. Based on the results of the analysis, the performances of two different receivers with multifaceted parabolic solar energy collectors are evaluated.     

Inverse radiation-conduction estimation of temperature-dependent emissivity using a combined method of genetic algorithmand conjugate gradient

The solution of an inverse, conduction-radiation problem in a two-dimensional rectangle is analyzed to determine the temperature-dependent emissivity at the boundary. The medium is gray, absorbing, emitting and isotropically scattering. The bounding surfaces are assumed to be opaque and diffuse. The inverse problem is solved by minimizing the performance function, which is expressed by the sum of square residuals between estimated and exact heat fluxes, using a combined method of genetic algorithm and conjugate gradient. The emissivity is assumed to be represented as a function of boundary temperature with unknown variables. Therefore, the inverse problem is treated by the estimation of these variables. Finally, four examples are presented to show the accuracy of the algorithm. The effect of the measurement errors on the accuracy of the inverse analysis is also investigated. Results show the algorithm can estimate the unknown emissivity when the measurement errors are neglected. Also it is found that increasing the measurement error decreases the accuracy of estimation of temperature-dependent emissivity.     

A numerical study on the conjugate natural convection in a circular pipe containing water

In this paper, the effect of material property of pipe on the conjugate natural convection in a circular pipe containing water was investigated by solving the unsteady incompressible Navier-Stokes equations coupled with energy equations of the water and pipe. Natural convection and conduction of water inside the pipe was coupled with the conduction of the pipe whose bottom was subject to uniform heat source. From the present grid resolution and time-step independent solutions, it has been confirmed that the water temperature inside a PVC pipe was higher than that inside a steel pipe due to the smaller heat capacity of PVC and that the streamline patterns of the two cases were found to be opposite because the thermal diffusivity of steel (PVC) is larger (smaller) than that of water such that steel (PVC) pipe is heated faster (slower) than water. Furthermore, a quantitative comparison of heat flux to water was performed by examining the distributions of the heat flux along the inside walls of steel/PVC. The average temperature of water inside steel was found to be higher than that inside PVC at the initial stage of heating. On the other hand, PVC provided a larger heat flux to water when it reached a steady value.     

Near- and far-field second-harmonic imaging of quasi-phase-matching crystals

Second-harmonic scanning near- and far-field optical microscopy of an electric-field poled KTiOPO₄ quasi-phase-matching crystal has been accomplished. This has been done in order to reveal the walls that form the intersections between inverted and non-inverted crystal domains. The domain walls are seen clearly only in images recorded by means of second-harmonic generation because of a large nonlinear contrast, and they appear as bright stripes when studied in a reflection geometry but they are dark when studied in transmission. The images show that the duty cycle of the quasi-phase-matching crystal differs from the ideal and that the walls are not completely smooth. These effects, in combination with the observed scattering from the domain walls, are expected to lower the output of the crystal when used for frequency doubling. We conclude that the wall thickness is no more than approximately 100 nm, which makes it a suitable test object for the resolution capabilities of scanning near-field optical microscopes that are used for nonlinear imaging.     

新型微小型平板CPL蒸发器流动与传热分析

提出一种新型微小型平板毛细抽吸两相流体回路(Capillary pumped loop,CPL)的蒸发器结构,使其能够适应高热流密度散热的要求。分析蒸发器由于微型化后侧壁导热对系统传热能力的影响。建立新型蒸发器毛细多孔芯内的传热传质数学模型和液体补偿腔的流动与传热模型以及蒸汽槽道和金属外壁区域的导热模型,并用SIMPLE算法对蒸发器进行整场耦合求解。数值结果表明,工质蒸发发生在多孔芯上表面以及侧壁附近,采用热导率较大的铝外壁时,蒸发器加热表面的温度水平较低且温度均匀性较好,但侧壁导热的影响导致CPL的传热能力不高。外壁采用热导率较小的不锈钢可以明显提高CPL的传热极限能力,但同时却较大地增加了加热表面的温度水平以及不均匀性。采用组合结构的蒸发器一方面可以提高系统的传热能力,同时降低了加热表面的温度水平和温度梯度。     

Extended Graetz Problem Including Axial Conduction and Viscous Dissipation in Microtube

Extended Graetz problem in microtube is analyzed by using eigenfunction expansion to solve the energy equation. For the eigenvalue problem we applied the shooting method and Galerkin method. The hydrodynamically isothermal developed flow is assumed to enter the microtube with uniform temperature or uniform heat flux boundary condition. The effects of velocity and temperature jump boundary condition on the microtube wall, axial conduction and viscous dissipation are included. From the temperature field obtained, the local Nusselt number distributions on the tube wall are obtained as the dimensionless parameters (Peclet number, Knudsen number, Brinkman number) vary. The fully developed Nusselt number for each boundary condition is obtained also in terms of these parameters.     

Heat Conduction in PVD Nanocoatings

The model of heat conduction in nonisostructural multilayered coatings is presented in this paper. It is based on well known phonon and electron heat transfer mechanisms. The volume of heat conduction coefficient depends on the mean free path, which plays a significant role when the sublayer thickness becomes lower than the critical one. The value of the mean free path of the heat carriers was determined by means of Monte Carlo numerical experiment and on this base, the analytical model describing the coating modulation parameter influence on the heat conduction coefficient was elaborated. The same amorphisation degree of the structure was taken into account, what is connected with the electron role limitation in heat transfer process, on the one side and very strong mean free path decrement of the phonons on the other side, so the results are very low values of heat conduction coefficients of the interlayer boundaries. The modified equations describing effective heat and temperature conduction coefficient were introduced to connect the single sublayer properties and interlayer boundaries.Experimental verification was realized with use of the variable-temperature camera method. This method is based on the indirect temperature change measurement of the coating and the substrate during specified heat flux flow with use of the X-ray diffraction. The changes in thermal expansion of substrate in temperature function were used as a reference base.     

Influence of tube wall longitudinal heat conduction on temperature measurement of cryogenic gas with low mass flow rates

Longitudinal heat conduction is an important parameter in the cryogenic field, especially in cryogenic heat exchangers. In the present study, the parasitic effect of tube wall longitudinal heat conduction on temperature measurement within the tube has been studied for cryogenic gas with low mass flow rates by finite element method and experimental tests. The effects of various parameters such as tube outlet temperature, tube wall thermal conductivity, mass flow rate, and tube wall thickness have been investigated. Axial positioning errors of temperature sensor due to tube wall longitudinal heat conduction were higher for lower gas flow rates. The results showed that the tube wall thermal conductivity leads to axial heat conduction within the tube wall, but the higher tube wall thermal conductivity does not lead to bigger axial positioning error of temperature sensor at tube outlet. According to data obtained from simulations and experiments, sensor with distance of 5 mm from tube outlet had 14.92% and 8.51% temperature measurement error (with respect to gas flow temperature at tube outlet) for tube wall thermal conductivities of 16 and 400 W m⁻¹ K⁻¹, respectively.     

Temperature transients on engine combustion chamber walls—I. Finite-difference formulation, validation and grid optimization

A finite-difference (FD) code is developed to solve the 1-D transient heat conduction problem through combustion chamber walls of internal combustion (IC) engines. The FD equations are derived for Cartesian, cylindrical and spherical coordinate systems from basic principles, i.e. heat balance and rate equations, and are capable of handling temperature-dependent material properties. In order to minimize the computational time, the wall is divided into layers of increasing thickness in geometric progression. The code developed is a useful tool for studying surface temperature swings on combustion chamber surfaces having different geometries, especially of significance in ceramic engines. It can also handle several different material layers together, thus making it possible to study thin-wall deposit effects on engine heat loss or surface thermocouple readings.     

Film condensation on horizontal tube with wall suction effects

This study performs a theoretical investigation into the problem of two-dimensional steady filmwise condensation flow on a horizontal tube with suction effects at the tube surface. An effective suction function is introduced to model the effect of the wall suction on the thickness of the liquid condensate film. The local condensate film thickness and the local Nusselt number are then derived using a simple series numerical method. The results show that the Nusselt number varies as a function of the Jakob number Ja, the Rayleigh number Ra, and the suction parameter Sw. It is found that the wall suction effect has a significant influence on the heat transfer performance. An analytical solution is derived for the mean Nusselt number for the case in which the wall suction effect is ignored. Finally, a closed-form correlation is presented for the mean Nusselt number subject to a wall suction effect.     

Inverse estimation of bi-directional reflectivity in long-wave infrared region by using RPSO algorithm

The Infrared signature is used in the fields of remote sensing, military defense and auto navigation system by detecting objects against the background. Infrared signature of objects depends on many factors, including the shape of objects, surface temperature, emissivity, bidirectional reflectivity and radiation sources such as solar irradiation and sky shine. It is essential to procure radiative surface properties in predicting the infrared signature of objects. Especially the bidirectional reflectivity is difficult to obtain due to the limited data in the literature. In this study, we develop a software which estimates radiative properties of painted surfaces inversely from the measured temperature and radiance variations with time by using the Repulsive particle swarm optimization (RPSO) algorithm. In this study surface radiative reflection properties in the LWIR region are estimated for 3 different paints by using the pre-determined radiance in LWIR region and surface temperature. The results obtained from this study show fairly good agreement with the original BRDF data used to estimate the LWIR radiance and temperature as the input data.