日照作用对热源建筑自然通风影响的研究

参照变电站主变压器室，建立了双侧开口热源建筑数值模型，通过改变大气透过率（sunshine fraction）的大小改变日照强度，利用FL U EN T进行求解，考察日照作用对热源建筑自然通风流动的影响。计算结果表明：日照作用影响热源建筑室内外热量传递的大小和方向，当大气透过率为0．15时，室内外两侧传递热量达到平衡。较大的日照强度导致通风量增大，室内气流速度分布更均匀，日照作用下不同高度的气流温度受到的主要影响因素不同。     

Analysis of three-dimensional heat and fluid flow induced by piezoelectric fan

In this study, the characteristics of the three-dimensional heat and fluid flow fields generated by the vibrating fan are examined by performing numerical simulations and experimental measurements. This paper considers two different arrangements of the heat source, namely vertical and horizontal. In performing the simulations, the fluid domain is meshed using a dynamic meshing scheme in order to take account of the time-varying shape and position of the vibrating blade. The results show that the interaction between the normal force exerted on the air surrounding the moving blade and the impingement jet flow produced at the blade tip prompts the formation of two counter-rotating screw-type flow circulations on either edge of the blade. An infrared thermal camera is used to measure the temperature distribution on the heated surface to examine the numerical results. It is indicated that the piezoelectric fan improves the heat transfer coefficient by 1.6–3.4 times when the heat source is vertically arranged and 1.8–3.6 times when the heat source is horizontally arranged.     

会议室空调气流组织形式的数值模拟研究

办公区里的会议室为重要工作场所,其环境质量的优劣直接影响工作的效率。以计算流体力学和传热学为基础,利用CFD软件对室内混合通风与置换通风两种气流组织进行数值模拟。结果表明:在温度场、速度场、温度效率及人体热舒适性等方面,置换通风均优于混合通风方式。     

Fluid, heat and contaminant transport structures of laminar double-diffusive mixed convection in a two-dimensional ventilated enclosure

The objective of the present work is to investigate the characteristics of the airflow and heat/contaminant transport structures in the indoor air environment by means of a convection transport visualization technique. Laminar double-diffusive mixed convection in a two-dimensional displacement ventilated enclosure with discrete heat and contaminant sources is numerically studied. Based on the governing equations, the fluid, heat, and contaminant transport processes are respectively described by the corresponding streamfunction, heatfunction, and massfunction. Attentions are given to analyze the effects of the main factors—the strength of heat source indicated by the Grashof number (Gr), the strength of contaminant source by the buoyancy ratio (Br), the strength of ventilation by the Reynolds number (Re), and the ventilation mode—on the indoor air environment. Numerical results, presented by the contour function lines, namely, streamlines, heatlines, and masslines, illustrated that the indoor air, heat and contaminant transport structures are mainly determined by the interaction between the internal buoyancy natural convection induced by the discrete heat/contaminant sources and the external forced convection driven by the mechanical ventilation. It is found that the convection transport method could explicitly disclose the complicated philosophy of indoor air environment, and thus provides a simple but practical approach to see the indoor airflow and heat and contaminant transport structures.     

基于ANSYS模拟巷道围岩调热圈分布研究

应用ANSYS软件对在通风量不同情况下围岩调热圈温度分布及井巷在自然通风状态下与人工通风情况下各自围岩调热圈温度分布情况进行数值模拟。结果表明在自然通风情况下调热圈半径为3.09 m,人工通风作用下调热圈向岩层内部扩散较为显著,且其值为4.3 m。对高温深井进行通风量控制以及热害防治有指导意义。     

Influence of exit opening location on mixed convection in a channel with volumetric heat sources

Mixed convection heat transfer in a channel with volumetric heat source and different opening ratio at the exit has been analyzed numerically. Steady, laminar, two-dimensional model based on the finite-volume method is used for solving the mass, momentum and energy transfer governing equations. The flow is injected to the duct with a uniform velocity distribution. The problem is studied in three aspect ratios (A = 1, 3 and 5) and three opening cases as single opening near the ceiling, single opening near the bottom and double openings. The study is also tested for different Richardson numbers between 0.01 and 10. Streamlines, isotherms, Nusselt numbers and temperature profiles were obtained for indicated cases. It is found that both the Richardson number and the locations of exit openings have strong effects on flow and temperature distribution in the presence of volumetric heat sources. The highest heat transfer was formed when the outlet port is located onto top of vertical wall.     

Nonlinear coupling between thermal mass and natural ventilation in buildings

An ideal naturally ventilated building model that allows a theoretical study of the effect of thermal mass associating with the non-linear coupling between the airflow rate and the indoor air temperature is proposed. When the ventilation rate is constant, both the phase shift and fluctuation of the indoor temperature are determined by the time constant of the system and the dimensionless convective heat transfer number. When the ventilation rate is a function of indoor and outdoor air temperature difference, the thermal mass number and the convective heat transfer air change parameter are suggested. The new thermal mass number measures the capacity of heat storage, rather than the amount of thermal mass. The analyses and numerical results show that the non-linearity of the system does neither change the periodic behaviour of the system, nor the behaviour of phase shift of the indoor air temperature when a periodic outdoor air temperature profile is considered. The maximum indoor air temperature phase shift induced by the direct outdoor air supply without control is 6 h.     

Acknowledgment

An experimental and numerical study has been carried out in order to investigate mixed and natural convection heat transfer in a two-dimensional enclosure. A discrete isothermal heat source is located at one of the vertical walls. Also, two ventilation ports are at the bottom and on top of the opposite wall. A forced flow condition was imposed by providing an inlet of air at the bottom port. A Mach–Zehnder interferometer was used to visualize the temperature field within the enclosure and to determine the local and average heat transfer characteristics of the heat source. Five heater positions on the vertical wall and different Rayleigh numbers (4.5 × 10⁵ to 1.15 × 10⁶) and Reynolds numbers (120 to 1600) were considered in the experiments. A finite volume code has been developed based on the SIMPLE algorithm and hybrid discretization scheme for the numerical study. It is observed that the interaction of natural convection with the forced flow leads to various flow fields depending on the Richardson number, Reynolds number and the heater position. Also, results show different trends for variation of the average Nusselt number with the heater position at low and high Reynolds numbers. An optimum position for the heat source, at which the maximum heat transfer is achieved, exists for high Reynolds numbers and has been found to be at the middle of the vertical wall.     

Distribution ratio of radiant heat and its effect on cooling load

Distribution ratio of radiation heat on each inside surface of building enclosure of a room has an important effect on the accurate calculation of air conditioning loads. Cooling loads are calculated and analyzed under different distribution ratios on the building enclosure of a room with underfloor air conditioning system and with different constructions. It is found that the previous approximate methods of distribution ratio will result in serious errors when calculating the cooling loads accurately formed by radiant heat gains from indoor heat sources. The accurate method should be calculating the loads based on the actual distribution ratio to each of the inside surfaces of the room. Factors affecting the ratio of radiant heat sent out by indoor heat sources are analyzed. And some regular results are concluded with the ratio calculated under different conditions. The results can supply reference to the people concerned.     

低温热水地板辐射供暖间歇运行研究

建立了低温热水地板辐射供暖系统间歇运行时室内热环境数学模型，利用模型计算了不同运行方式、内围护结构和内热源作用下室内热环境变化规律。发现此类供暖系统采用适当间歇运行即可满足室内热环境要求，运行时间主要受室外温度和内热源变化规律影响。当系统按冬季室外供暖计算温度设计时，在冬季室外平均条件下，系统夜间运行半天左右即可基本保证全天室内热环境要求。     

Optimal configuration of discrete heat sources in a vertical duct under conjugate mixed convection using artificial neural networks

This paper reports the results of a numerical investigation of the problem of finding the optimum configuration for five discrete heat sources, mounted on a wall of a three-dimensional vertical duct under mixed convection heat transfer, using artificial neural networks (ANN). The objective is to locate the positions for the five heat sources in such a way that the maximum temperature of any of the heat sources in a given configuration is a minimum. The three-dimensional governing equations of mass, momentum and energy equations for the fluid flow and the energy equation for the solid regime have been solved by using FLUENT 6.3 and a database of temperature versus configuration was generated. The temperature database developed from CFD simulations is used to train the neural network. The trained neural network predicts the temperature of the heat sources very accurately and much faster than the CFD software. With the use of this network, an exhaustive search for all possible configurations was done that resulted in a global optimum for the problem.     

基于有限元分析的强迫对流条件下开关柜温升特性研究

12kV大电流开关柜广泛用于配电网中,一般采取强迫对流的方式进行散热。为研究开关柜在强迫对流条件下的散热过程,基于有限元仿真技术,分别分析了强迫鼓风和抽风两种条件下开关柜的散热能力。通过对开关柜进行三维建模、热源计算和边界条件设立,仿真分析了开关柜在强迫鼓风和抽风条件下的温升分布及气流场分布情况,并与试验结果进行对比。结果表明,开关柜在两种强迫对流条件下散热效果均较好,温升符合国标温升要求,但不同的散热方式对开关柜温升和气流场分布影响较大。     

Study of species, temperature distributions and the solid oxide fuel cells performance in a 2-D model

A two-dimensional mathematical model for a planar SOFC (solid oxide fuel cell) is constructed. The distribution of the chemical species, the temperature and the performance (power) and the current density were calculated using a single-unit model with double channels of co-flow pattern. The finite volume method was employed for the calculation. The method was based on the fundamental conservation laws of continuity, momentum, energy and mass. The equations are implemented in FORTRAN language. The effects of several heat sources and flow rates on the calculated results were also investigated. The reference SOFC polarization curve has been calculated by imposing a uniform temperature of 800 K, a pressure equal to 1 atm; H₂ and O₂ molar fractions equal to 0.97 and 0.21 respectively. Results of temperature, chemical species distributions, performance and efficiency under several heat sources are shown and discussed. At a current density of about 23500 A/m², the power densities under all sources and chemical sources reached their maximums of 12965 W/m² and 16209 W/m² (i.e. 25% lower) respectively. However the temperature increment in the anode is analyzed toward all sources and chemical reaction. The temperature maximum values for each heat source type reached 1005.81 K and 984.69 K respectively.     

A new model and solutions for a spiral heat exchanger and its experimental validation

A spiral heat exchanger was applied in a ground source heat pump (GSHP) system that is primarily used for residential indoor heating. Studies that have been performed on the heat transfer of spiral heat exchanger have focused on field measurements and numerical analysis; however, theoretical research on the subject is absent in the literature. In this study, a methodology is proposed to analyze the heat performance of a spiral heat exchanger. A ring source model was established and solved analytically to describe the temperature variation of the ground caused by a spiral heat exchanger. The validity of the model was examined by an experiment on the soil temperature variation with a spiral heater. The virtual ring tube surface temperature response of unit ring circle was calculated by a superposition of the contributions of the ring source itself and adjacent ring sources. Furthermore, a fast algorithm was created to compute the average tube surface temperature resulting from the dimensionless temperature rise at a point far from the ring source that is constant when the non-dimensional distance is less than 0.13. The author confirmed that the calculation time of this proposed algorithm decreased by a factor of 100 compared with the traditional integration method. A system designer will find this algorithm helpful when determining the size of a heat exchanger under a required heating load, particularly for different arrangement of spiral heat exchangers.     

The BEM for point heat source estimation: application to multiple static sources and moving sources

This paper deals with an inverse problem, which consists of the identification of point heat sources in a homogeneous solid in transient heat conduction. The location and strength of the line heat sources are both unknown. For a single source we examine the case of a source which moves in the system during the experiment. The two-dimensional and three-dimensional linear heat conduction problems are considered here. The identification procedure is based on a boundary integral formulation using transient fundamental solutions. The discretized problem is non-linear if the location of the line heat sources is unknown. In order to solve the problem we use an iterative procedure to minimize a quadratic norm. The proposed numerical approach is applied to experimental 2D examples using measurements provided by an infrared scanner for surface temperatures and heat fluxes. A numerical example is presented for the 3D application.     

Effect of Thermal Conductivity on Cooling of Square Heat Source Array under Natural Convection in a Vertical Channel

Abstract

This article presents experimental and numerical investigation on natural convection air-cooling of discrete square heat source array in a vertical channel. Conjugate heat transfer for three-dimensional laminar developing flows over an array of square heat sources representing integrated circuit components for electronic cooling has been studied. Experiments are conducted using three-substrate board materials viz. FR4, Bakelite, and copper clad board having thermal conductivities of 0.3, 1.4, and 8.8?W/m K to study the effects of substrate thermal conductivity on fluid flow and heat transfer. A finite element-based software is used to solve the coupling between heat transfer in solids and fluid region. Incompressible flow over discrete square heat sources is modeled using Navier–Stokes equations under Boussinesq approximation. Air-cooling of circuit boards populated with heat sources is modeled and simulated to present heat transport in combination with the fluid flow resulting from the natural air circulation at constant heat fluxes of 1,000, 2,000, and 3,000?W/m². Multilayer copper clad board of thermal conductivity of 40.5?W/m K have been studied numerically. The results show that single sided copper clad board is the preferred candidate. Experiments indicate a deviation of under 5% with simulations.     

Generalized Magnetothermoelasticity for Isotropic Media

The theory of generalized magnetothermoelasticity is applied to a two-dimensional problem of a homogeneous isotropic perfectly conducting thick plate with heat source subjected to a time-dependent compression under constant magnetic and electric intensities. Double integral transforms (Laplace transform for time variable and Fourier transform for space variable) are used, and the resulting equations are written in the form of a vector-matrix differential equation, which was solved by the eigenvalue approach. The inversion of the Laplace transform is carried out numerically by the Zakian method. Finally, numerical computations of the displacement and stress components, temperature distribution, induced magnetic and electric fields have been done and are represented graphically.     

A heuristic approach to optimal arrangement of multiple heat sources under conjugate natural convection

Steady state numerical computations and experiments were performed to study three-dimensional, conjugate laminar natural convection heat transfer from multiple identical heat generating modules (heat sources) in a vertical duct. The heat sources were mounted on a wall at different positions in a defined grid of 5 × 5 positions. Air is used as the cooling medium. The governing flow and energy equations were solved using FLUENT 6.3. The optimum geometric configuration of the five heat sources that maximizes the heat transfer was determined by the introduction of a dimensionless distance parameter and an exhaustive search. The heuristic procedure based on the geometric parameter was tested for varying number of heat sources and for different heat source strengths. Experiments were performed to study the effect of modified Grashof number and the duct spacing on maximum temperatures of different configurations in order to support the numerical findings. Additionally, the temperatures of the heat sources arranged in the optimum configuration obtained by the heuristic approach have been experimentally validated.     

室温传感器偏差故障对武汉地区暖通空调系统的影响研究

暖通空调系统的高效节能运行高度依赖于传感器测量的准确性。在传感器全寿命运行周期中,不可避免发生各种故障,影响其准确性。为探究传感器故障对不同暖通空调系统的影响,文章以室温传感器偏差故障为例,针对武汉地区某办公建筑,同时开展地源热泵和"冷水机组+锅炉"两种暖通空调系统形式的能耗建模,对比分析-5℃~+5℃偏差故障对两种系统运行能耗、工作性能及室内热舒适性的影响差异。结果表明:室温传感器故障的偏差幅值方向对两种系统运行能耗、工作性能及室内热舒适性的影响规律不同。其中,地源热泵系统能耗受室温传感器偏差故障影响相对更小。     

闭式外腔循环太阳能墙集热效果实验研究

文章设计了闭式外腔循环太阳能墙系统,从理论和实验两方面研究太阳能墙传热传质规律。以大连市冬季为研究背景,基于对实验测温点不同时刻的持续监测,从逐时曲线中分析了不同环境下闭式外腔循环系统内部温度以及室内获得热量后室温的变化情况。结果表明,太阳能墙集热效率与太阳辐射强度密切相关,在晴朗天气下,白天部分时段单一太阳能墙就能满足采暖要求,不满足采暖要求时可以选择空气源热泵辅助采暖。