Natural convection heat transfer in a circular enclosure

Natural convection heat transfer in a circular enclosure, one half of which was heated and the other half of which was cooled, was investigated experimentally, focusing on the effect of the inclination angle. The experiments were carried out with water. Flow and temperature field were visualized by using the aluminum and liquid-crystal suspension method. The results show that with downward heating the heat transfer coefficient increased as the inclination angle of the boundary between the heating wall and the cooling wall approached the vertical. But with upward heating, the heat transfer coefficient showed minimal change, exhibiting a small peak value when the inclination angle was γ ˜ –45°. The heat transfer coefficient of a flat circular enclosure was estimated from the circular enclosure's heat transfer coefficient. These results can be explained by the obtained flow and temperature fields. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 152–163, 1999     

耦合法在柴油机传热研究中的应用

利用流固耦合的分析方法,将柴油机气缸盖、气缸垫、气缸体、气缸套等柴油机主要零部件以及缸内气体、冷却介质作为一个耦合体,进行燃烧室部件的传热数值模拟实验。其中,冷却水侧的对流换热系数和温度由CFD软件Star-CD对整个水路进行模拟计算获得;底板火力面侧燃气的对流换热系数和温度由GT-POWER软件对缸内工作过程进行模拟获得;缸套燃气侧温度由活塞组——气缸套耦合传热模拟获得。最终的计算结果与实验数据较吻合,可以为柴油机热负荷分析和柴油机设计提供理论依据。     

Hysteresis phenomena in loop heat pipes

Testing of loop heat pipes (LHPs) has shown that the heat-load dependence of the operating temperature is not always unambiguous. It may have a hysteresis nature. It has been found that temperature hysteresis is connected with changes in the liquid distribution between the compensation chamber (CC) and the condenser. Analysis makes it possible to distinguish three types of temperature hysteresis. In the first case this redistribution is caused by the change in the amount of the parasitic heat flow that penetrates into the CC, which in its turn is a result of heat-transfer hysteresis in the evaporation zone. In the second, temperature hysteresis is connected with the liquid metastable state, which leads to a delay of formation of the vapor phase in the compensation chamber. The reason for hysteresis of the third type is the change of the initial liquid distribution in an LHP during a start-up.     

A data method using the inner temperature difference to improve the stability of the inverse heat conduction problems

Abstract

This article proposes a method to construct two series systems for improving the stability of the inverse heat conduction problems (IHCP) in a finite slab. The transfer function between the surface heat flux or temperature and the inner temperature difference is respectively obtained by Laplace transform technique firstly. Then the series systems which can solve IHCP based on the inner temperature difference are constructed by replacing the unsuitable zero and pole points of the transfer function approximated by è approximation. Finally the effects of the series systems are evaluated by a typical example. The results of the evaluation show that this method can obtain the surface heat flux and temperature by the inner temperature difference, and enhance the response speed of the measurement system at the same time. In addition this method can also improve the signal to noise ratio (SNR) of the inverse solutions by selectively amplifying the high SNR parts of the inner temperature difference. The present work provides an effective method to improve the stability of IHCP.     

Direct‐contact heat exchange between fluidizing particles and a heat transfer surface in a fluidized bed: Temperature visualization of fluidizing particles

Heat conduction during contact between a heat transfer surface and fluidizing particles, a phenomenon which is one of the effective heat transfer mechanisms in a gas–solid fluidized bed, has been empirically investigated. The temperature profile of the fluidizing particles during the contact period is visualized with the aid of an infrared imager. The visualization reveals that the particles have been considerably heated in the thermal boundary layer on the heat transfer surface before contact. Based on the visualized temperature profile of the particles, the contact conductance between a fluidizing particle and the heat transfer surface is estimated by an in inverse analysis. Using the evaluated contact conductance, the contributions of the conductive heat transfer to the total heat transfer are also evaluated. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 165–181, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10027     

Experimental assessment of heat storage properties and heat transfer characteristics of a phase change material slurry for air conditioning applications

A new microencapsulated phase change material slurry based on microencapsulated Rubitherm RT6 at high concentration (45% w/w) was tested. Some heat storage properties and heat transfer characteristics have been experimentally investigated in order to assess its suitability for the integration into a low temperature heat storage system for solar air conditioning applications. DSC tests were conducted to evaluate the cold storage capacity and phase change temperature range. A phase change interval of approximately 3 °C and a hysteresis behaviour of the enthalpy were identified. An experimental set-up was built in order to quantify the natural convection heat transfer occurring from a vertical helically coiled tube immersed in the phase change material slurry. First, tests were carried out using water in order to obtain natural convection heat transfer correlations. Then a comparison was conducted with the results obtained for the phase change material slurry. It was found that the values of the heat transfer coefficient for the phase change material slurry were higher than for water, under identical temperature conditions inside the phase change interval.     

Effect of heat recovery on the performance of a shape-memory effect heat engine

The performance of a shape-memory effect heat engine employing heat recovery has been modelled taking into account heat transfer to the working element and non-ideal material behaviour. Calculations of the energy flux to and from the working element show that a substantial fraction of the sensible heating requirements can be recovered by heat transfer between the heating and cooling stages. The increase in efficiency due to heat recovery and the resulting power output are shown to be a function of stress and rate of temperature change of the heat-transfer fluid. Working element-fluid heat transfer and hysteresis behaviour of the shape-memory working element are found to limit the efficiency improvement associated with heat recovery.     

Experimental Investigations on Boiling Heat Transfer Inside Miniature Circular Tubes Immersed in FC-72

To investigate the size effect on the characteristics of boiling heat transfer, boiling behavior of FC-72 in heated vertical miniature circular tubes immersed in a liquid pool was experimentally studied. Two AISI 304 stainless steel tubes with inner diameters of 1.10 mm and 1.55 mm correspondingly, were heated by swirled Ni-Cr wire heaters and sealed in Lucite blocks by silicon adhesive. Both the top and the bottom ends of the circular test sections were open to the liquid pool. The boiling curves and heat transfer coefficients were obtained experimentally. The boiling behaviors at the outlets of the miniature tubes were also visualized with a digital video camera. Experimental results show that the tube geometry has a significant effect on the boiling characteristics. Vapor blocking at the outlet of the smaller circular tube with a diameter of 1.10 mm caused severe boiling hysteresis phenomena. The CHF decreased with reducing in tube size.     

A Study of Bubble Behavior and Boiling Heat Transfer Enhancement under Electric Field

The effect of a D.C. electric field on nucleate boiling heat transfer for refrigerants, R11, R113, and FC72, was investigated experimentally in a single-tube shell/tube heat exchanger by using the temperature control method of wall superheat. Also the behavior of bubble under nonuniform electric field produced by wire electrodes was studied by numerical calculation. For R11, the electrohydrodynamic (EHD) enhancement for boiling heat transfer was observed for all ranges of wall superheat tested. However, the enhancement in boiling heat transfer disappeared if the wall superheat exceeded 13°C for R113, and no electric field effect on the boiling heat transfer was observed for FC72. An application of approximately 5 kV was enough to eliminate the boiling hysteresis for R11 and R113. Numerical study of the electric field in a single medium has hinted that the bubbles are forced away from the heating surface and toward the electrostatic stagnation point by the dielectrophoretic force. Such modified bubble motion turns out to promote the boiling heat transfer if one uses proper electrode configuration.     

Steady mixed convection in a differentially heated square enclosure with an active rotating circular cylinder

Mixed convection in a square enclosure with a rotating cylinder centered within it is numerically studied. Depending on the rotation of the cylinder, the natural and forced convection effects can be combined or opposite. However, due to the complex flow structure assumed by the flow, in some conditions the combined effects do not lead to the maximum overall heat transfer crossing the enclosure. The rotating cylinder participates on both the conductive and convective heat transfer processes, and exchanges heat with the fluid naturally, without imposition of a thermal condition at its surface. It is explored the influence of the cylinder through its radius, rotating velocity, thermal conductivity and thermal capacity on the resulting mixed convection problem. Thermal field is visualized using the isotherms, the flow structure is visualized through the streamlines, and the heat transfer process is visualized through the heatlines. For the first time, these visualization tools are applied to a moving solid. The overall thermal performance of the enclosure is analyzed through the overall Nusselt number. For a better understanding of the participation of the cylinder on the heat transfer process, the local heat exchanged between the cylinder surface and the fluid is also analyzed. Results clearly show how the rotating cylinder affects the thermal performance of the enclosure, and how the thermophysical properties of the cylinder are important on the overall heat transfer process across the enclosure.     

Numerical Investigation of Heat Transfer by CuO–Water Nanofluid in Rectangular Enclosures

This paper analyzes heat transfer and fluid flow of natural convection in inclined cavity filled with CuO–water nanofluid and differentially heated. Conservation of mass, momentum, and energy equations are solved numerically by a control volume finite-element method using the SIMPLER algorithm for pressure–velocity coupling. The Prandtl number is fixed at 7.02, corresponding to water. Aspect ratio and solid volume fraction are varied from 0.5 to 4 and from 0% to 4%, respectively. The inclination angle is varied from 0° to 90° and used as a control parameter to investigate flow mode-transition and the accompanying hysteresis phenomenon (multi-steady solutions). It is found that the efficiency of heat transfer is improved by the addition of nanoparticles into base fluid; however, there is an optimum solid volume fraction that maximizes the heat transfer rate. Numerical results show also that the diameter of solid particle is an important parameter that affects the heat transfer efficiency; its impact is more important than the concentration itself. Effects of inclination angle on streamlines and on thermal boundary layer are presented. Combined effects of aspect ratio and inclination angle on heat transfer and hysteresis region are analyzed.     

Heat transfer of air-water dispersed flow in a vertical pipe

Heat transfer of air-water dispersed flow in a vertical heating pipe and its enhancement have been studied. The axial and circumferential wall temperature distributions were measured using various mist ratios and wall heat fluxes. The measured wall temperature increased sharply at a particular streamwise location, with a notable variation in the circumferential profile. This sharp increase was conceivably caused by a breakdown of the water film rather than by its dryout. A separate unheated experiment was carried out to estimate the droplet deposition velocity and the water-film flow rate. A numerical analysis, taking into account heat and mass transfer from the water film to the bulk flow, was performed in order to estimate the mean wall temperature. Good agreement was obtained with the experimental results in the area where the entire inner surface of the pipe was covered with the water film. In this area, the rate of heat transfer was approximately seven times larger than that for single phase air flow. This enhancement was shown to be due mainly to evaporation of the water film. The mechanism of heat transfer enhancement is discussed in detail using the numerical analysis results. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 255–270, 1998     

车用内燃机整机热平衡动态传热模型

提出了一个基于集总参数法的发动机动态热平衡模型，考虑了发动机燃烧室的传热、发动机主要部件的传热、冷却系统和润滑系统的工作。建立了燃烧气体与发动机部件、各部件之间、部件与冷却液、部件与润滑油、部件与周围空气之间的热耦合计算公式，对一台单缸柴油机的热平衡及主要部件的温度进行了计算．结果证实该模型可用于发动机热平衡的研究。     

高温相变蓄热容器传热的隐式求解

空间太阳能热动力发电系统是非常有前景的未来空间能源供应系统，关键部件之一是吸热/蓄热器，高温蓄热容器是其中的基本部位，它的传热性能直接影响玻整个热动力系统的性能。采用一种预测-修正的隐式迭代法对相变蓄热容器进行了三维热分析。并且与显式求解方法在计算时间和计算结果上进行了比较分析。证明这种方法可以明显提高计算速度，并可以获得较好的结果。     

通道中串列倒转柔性体的运动与传热

为研究柔性结构运动对通道换热性能的影响,采用任意拉格朗日-欧拉(Arbitrary Lagrangian-Eulerian, ALE)法,对通道中串列布置的倒转柔性体进行数值模拟。研究不同长宽比的柔性体在抗弯刚度系数相同时的拍动状态、流场特性以及对通道换热性能的影响。结果表明：随抗弯刚度系数变化柔性体存在稳定模式、大幅度拍动模式和偏转拍动模式3种运动模式;随抗弯刚度系数减小,小长宽比柔性体的拍动状态变化具有滞后性;大长宽比柔性体在净换热量提升上更有优势,在合理的机械能损失范围内拥有更高的热效率系数,相较小长宽比通道,平均净换热量提高了3.87倍,最高热效率系数可达1.3;在实际工程应用中,应采用大长宽比并处于大幅度拍动模式下才能有效提升通道内传热性能。     

Recent Advances in Closed Loop Spray Cooling and its Application in Airborne Systems

In recent years,the problem of heat dissipation in airborne directed energy weapons has attracted considerable research interest.Spray cooling can be applied to cool airborne directed energy weapons,owing to its several advantages such as a large heat transfer coefficient,absence of boiling hysteresis and uniform surface temperature.To examine the potential of an airborne spray cooling system,the typical high heat flux dissipation methods were compared,and the state of the art research on spray cooling was reviewed.This review was focused on studies related to the spray cooling hydrodynamic mechanism,experimental studies of closed loop spray cooling,numerical simulation studies about spray cooling and the identification of the factors influencing spray cooling systems,and investigations related to the multiple nozzle spray cooling technology and heat transfer correlation predictions.Overall,there is a need for further research to investigate the failure phenomenon after the critical state,matching operation of the total system and microscopic characteristics of airborne specific parameters.     

燃煤锅炉三维CFD数值模型水冷壁热边界条件设定方法

目前,燃煤锅炉三维CFD数值模拟中对炉膛水冷壁传热分布的预测大都基于给定的壁面温度边界条件。然而,此方法无法体现锅炉运行状态对壁面传热与壁温分布的影响。提出了一种基于锅炉烟气侧放热与汽水侧吸热间热平衡关系的壁面传热计算方法,并重点讨论了壁面传热系数的物理意义及取值方法。研究发现,壁面传热系数基本由壁面结渣状态决定,因此可根据壁面渣层的传热系数确定。本文方法将影响壁面传热的关键因素合理地体现在计算过程中,同时在模型复杂性与工程适用性之间保持了合理的平衡。采用此方法对一台320 MW锅炉的燃烧与传热分布进行了数值模拟,水冷壁吸热量的预测结果与锅炉运行数据吻合良好。     

流固耦合仿真技术在发动机稳态传热计算中的应用

为解决发动机传热研究中冷却水与缸套、机体之间的流动与传热问题，将有限元软件中提供的流固耦合仿真技术应用到发动机稳态传热计算中。建立了发动机活塞组-缸套-冷却水-机体流固耦合传热模型，该模型既包括了固体与固体之间的接触传热，也包括了流体与固体之间的耦合传热。同时，零件之间的传热边界条件也变得既简单又合理。以某型号柴油机为例进行了有限元仿真计算，并与活塞和缸套的温度场测量数据进行了对比分析。结果表明：仿真结果与试验测量数据误差较小，应用流固耦合仿真方法可以较好的模拟发动机稳态传热状态。     

边界点法在传热问题数值分析中的应用

将一种新的数值分析方法-边界点法应用于传热问题的研究，对无内热源稳态热传导问题，通过传统边界元法将边界积分方程离散化，发现可以不直接求解影响系数矩阵，而是通过对偶关系，由域外虚源构造方程组的特解场形成边界已知和未知温度，热流密度的系数矩阵，而且域内温度和热流密度的求解将不依赖于边界未知参数的求解，对于有内热源的问题，可以将非齐次方程的解转换为齐次方程的解和某一确定解的叠加，对于非线性问题，可以通过基尔霍夫变换，将非线性问题转化为线性问题求解，这种边界点方法不但具有边界元法降维的优势，而且不须求解奇异积分，大大节约了计算时间，计算精度极高，以有内热源非线性稳态热传导问题的实例印证了这种方法的高效性。     

中深层地热单井循环系统传热强化方法研究

针对中深层地热单井循环系统井内热贯通导致的换热功率低的问题,提出一种内管末端变径的井下传热强化方法,并建立数值模型,利用FLUENT进行为期30 d的模拟计算。结果表明,采用内管末端变径的方式能有效增强地下水“互动”,充分利用含水层的高温来提高单井换热功率。将井下换热分为导热区和采灌区两部分,随着封堵比例的增加,抽水中的含水层补给占比增加,且采灌区换热功率在系统换热功率中的占比逐渐增加。当封堵比例增大到100%时,采灌区换热功率达到导热区的1.76倍,井口出水温度可基本稳定在58℃,系统换热功率稳定在约995.46 kW,相较于内管等径系统,换热功率可提高84.71%。同时,单井循环系统仅导热区的延米换热量就可达到154.23～216.89 W/m,超过了闭式同轴套管换热系统稳定运行的最高延米换热功率,而系统换热功率可达到闭式系统的3.57～6.60倍,在单井换热系统中具有显著优势。