基于热流片的冰的导热系数测量

设计了基于稳态法导热系数测量原理并结合热流片测量冰的导热系数的实验装置,对冰的导热系数进行测量。该实验装置采用的冰块尺寸(长×宽×高)为13 cm×8 cm×1.5 cm,冰块初始温度为0℃,以4℃的冰水混合物作为稳定的供热端,通过半导体制冷器获得-7℃的稳定冷端,采用热流片测量冰块中传导的热量,采用温差电偶测量温度差。测量的冰块导热系数基本稳定在2.00 W·m^(-1)·K^(-1)附近,与已有关于冰的导热系数的实验结果较为接近,可以为冰导热系数的测量提供新的思路。

非稳态条件下温度梯度型热流计片误差分析

温度梯度型热流计由于其价格低廉、应用方便等特点在热工测试中被广泛的运用,然而在实际工程运用当中其精度往往得不到保证。建立了典型的热流计片测试的数学模型,在非稳态条件下对其误差进行分析研究。最后得出结论,热流计片本身的热工属性以及不同的边界条件对于热流计片的现场测试精度都有很重大的影响。

基于微型空间相机的红外加热笼仿真与设计

在航天器热控技术领域,常采用红外加热笼模拟各表面的到达外热流,但随着模拟表面特征尺寸的逐渐变小,需要重新评估该模拟方法的合理性和准确性。本文基于某微型空间相机,以对外热流最为敏感的散热面为研究对象,开展红外加热笼仿真分析与优化设计的研究。采用有限元法建立红外加热笼-黑片热流计的系统仿真模型,分析了传统红外加热笼控制方法对模拟表面总到达能量和热流密度均匀性的影响。基于上述结果,通过适当扩大加热笼尺寸和调整热流计粘贴位置,提高模拟表面的热流密度均匀性,保证总到达能量满足保守设计原则。对比分析得出,优化设计前后散热面热流密度的统计方差由102.0下降至27.0、均匀性提升效果显著。本文研究内容也可为其他空间微小表面外热流的准确模拟提供参考、借鉴。

Numerical simulation of heat transfer enhancement by strip-coil-baffles in tube-bundle for a tube-shell heat exchanger

A novel strip-coil-baffle structure used to enhance heat transfer and support the tube bundle for a tube-shell heat exchanger is proposed. The new structure can sleeve the tubes in bundle alternatively to create a vortex flow in a heat exchanger. The numerical simulation on the flow and heat transfer characteristics for this new structure heat exchanger is conducted. The computational domain consists of two strip-coil sleeved tubes and two bare tubes oppositely placed at each comer of a square. The velocity and temperature fields in such strip-coil-baffled channel are simulated using FLUENT software. The effects of the strip-coil-baffles on heat transfer enhancement and flow resistance in relation to the Reynolds number are analyzed. The results show that this new structure bundle can enhance the heat transfer coefficient up to a range of 40% to 55% in comparison with a bare tube bundle; meanwhile, higher flow resistance is also accompanied. It is believe that the strip-coil- baffled heat exchanger should have promising applications in many industry fields.

Effective thermal and electrical conductivity of graphite nanoplatelet composites

The relationship between the thermal/electrical conductivity enhancement in graphite nanoplatelets （GNPs） composites and the properties of filling graphite nanoplatelets is studied. The effective thermal and electrical conductivity enhancements of GNP-oil nanofluids and GNP-polyimide composites are measured. By taking into account the particle shape, the volume fraction, the thermal conductivity of filling particles and the base fluids, the thermal and electrical conductivity enhancements of GNP nanofluids are theoretically predicted by the generalized effective medium theory. Both the nonlinear dependence of effective thermal conductivity on the GNP volume fraction in nanofhiids and the very low percolation threshold for GNP-polyimide composites are well predicted. The theoretical predications are found to be in reasonably good agreement with the experimental data. The generalized effective medium theory can be used for predicting the thermal and electrical properties of GNP composites and it is still available for most of the thermal/electrical modifications in two-phase composites.

Convective Heat Transfer Characteristics of the Elliptic Cylinder with Axis Ratio 4∶1 in Crossflow

The heat transfer features around the elliptic cylinder of axis ratio 4∶1 in crossflow were investigated experimentally within a wide range of Reynolds number. By means of heat-mass transfer analogy and the naphthalene sublimation technique, the local heat transfer distribution and the mean heat transfer coefficient are clarified. The result shows that the mean heat transfer coefficient is higher than that of a circular cylinder in most Reynolds number range regarded, and this superiority turns to be more significant with the increase of flow speed. Moreover, the effect of axis ratio on mean heat transfer coefficient was investigated tentatively. The oil-lampblack technique was employed to enable visualization of the flow pattern around the cylinder and on the cylinder wall.

Experimental Research on Flow Maldistribution in Plate-Fin Heat Exchangers

The flow maldistribution and the effect of different inlet configuration on the flow distribution in platefin heat exchangers were studied experimentally. It is found that the flow maldistribution is serious because of the defects of inlet configurations, while the inlet configuration and Reynolds number are the main factors affecting the flow distribution. The improved inlet configurations, which are the header with a two-stage distributing configuration and the guide vane with a fluid complementary cavity were proposed and tested in this paper. The experimental results show that the improved inlet configurations can effectively improve the performance of flow distribution in heat exchangers.

An advanced turbulator with blades and semi-conical section for heat transfer improvement in a helical double tube heat exchanger

In present work,a helical double tube heat exchanger is proposed in which an advanced turbulator with blades,semi-conical part,and two holes is inserted in inner section.Two geometrical parameters,including angle of turbulator’s blades(θ) and number of turbulator’s blades(N),are considered.Results indicated that firstly,the best thermal stratification is achieved at θ=180°.Furthermore,at the lowest studied mass flow rate(m = 8 × 10^(-3) kg/s),heat transfer coefficient of turbulator with blade angle of 180° is 130.77%,25%,and 36.36% higher than cases including without turbulator,with turbulator with blade angle of θ =240°,and θ =360°,respectively.Moreover,case with N=12 showed the highest overall performance.At the highest studied mass flow rate(m = 5.842 × 10^(-2) kg/s),heat transfer coefficient for case with N=12 is up to 54.76%,27.45%,and 6.56% higher than cases including without turbulator,with turbulator with N=6,and with turbulator with N=9,respectively.

A nanofluid MHD flow with heat and mass transfers over a sheet by nonlinear boundary conditions: Heat and mass transfers enhancement

In this paper,we have numerically examined the steady boundary layer of a viscous incompressible nanofluid and its heat and mass transfers above a horizontal flat sheet.The boundary conditions considered were a nonlinear magnetic field,a nonlinear velocity and convection.Such nonlinearity in hydrodynamic and heat transfer boundary conditions and also in the magnetic field has not been addressed with the great details in the literature.In this investigation,both the Brownian motion and thermophoretic diffusion have been considered.A similarity solution is achieved and the resulting ordinary differential equations (nonlinear) are worked numerically out.Upon validation,the following hydrodynamic and heat and mass transfers parameters were found:the reduced Sherwood and Nusselt numbers,the reduced skin friction coefficient,and the temperature and nanoparticle volume fraction profiles.All these parameters are found affected by the Lewis,Biot and Prandtl numbers,the stretching,thermophoretic diffusion,Brownian motion and magnetic parameters.The detailed trends observed in this paper are carefully analyzed to provide useful design suggestions.

Simulation and design of a self-heating continuous-flow PCR chip

A novel continuous-flow PCR chip adopting self-heating, passive-cooling mode to realize the DNA fragments amplification was presented. Using the ANSYS finite element analysis, the temperature distribution of the chip is simulated and analyzed.The optimal size of the chip is 30×22 mm2, the roundabout micro-channel is the 90 μm width, 40 μm depth. Two micro-heater with the nickel-chrome alloy material film are formed on the side of silicon belonging to denaturation and renaturation zones needed for PCR reaction, and two adiabatic structures with groove on side of silicon by anisotropy etching. By the mode of heating local zones at single side, three wider constant temperature zones could be formed, which are 60 ℃,72 ℃,95 ℃ and suitable for PCR,and the temperature-difference could be restricted in less than 5 ℃.

Optimization of Heat Spreader Design for Electronic Cooling

The continuing increase in IC （Integrated Circuit） power levels and microelectronics packaging densities has resulted in the need for detailed considerations of the heat sink design for integrated circuits. One of the major components in the heat sink is the heat spreader which must be designed to effectively conduct the heat dissipated from the chip to a system of fins or extended surfaces for convective heat transfer to a flow of coolant. The heat spreader design must provide the capability to dissipate the thermal energy generated by the chip. However, the design of the heat spreader is also dependent on the convection characteristics of the fins within the heat sink, as well the material and geometry of the heat spreader. This paper focuses on the optimization of heat spreaders in a heat sink for safe and efficient performance of electronic circuits. The results of the study show that, for air-cooled electronics, the convective effects may dominate the thermal transport performance of the heat spreader in the heat sink.

A Straight-bladed Vertical Axis Wind Turbine with a Directed Guide Vane Row-Effect of Guide Vane Geometry on the Performance-

The objective of this study is to show the effect of guide vane geometry on the performance. In order to overcome the disadvantages of vertical axis wind turbine, a straight-bladed vertical axis wind turbine （S-VAWT） with a directed guide vane row has been proposed and tested by the authors. According to previous studies, it was clarified that the performance of the turbine can be improved by means of the directed guide vane row. However, the guide vane geometry of S-VAWT has not been optimized so far. In order to clarify the effect of guide vanegeometry, the effects of setting angle and gap between rotor blade and guide vane on power coefticlent and start- ing characteristic were investigated in the experiments. The experimental study of the proposed wind turbine was carried out by a wind tunnel. The wind tunnel with a diameter of 1.8m is open jet type. The wind velocity is 8 m/s in the experiments. The rotor has three straight blades with a profile of NACA0018 and a chord length of 100 mm, a diameter of 0.6 m and a blade height of 0.7 m. The guide vane row consists of 3 arc plates.

Influence of Internal Channel Geometry of Gas Turbine Blade on Flow Structure and Heat Transfer

This paper presents the study of the influence of channel geometry on the flow structure and heat transfer, and also their correlations on all the walls of a radial cooling passage model of a gas turbine blade. The investigations focus on the heat transfer and aerodynamic measurements in the channel, which is an accurate representation of the configuration used in aeroengines. Correlations foi： the heat transfer coefficient and the pressure drop used in the design of internal cooling passages are often developed from simplified models. It is important to note that real engine passages do not have perfect rectangular cross sections, but include a comer fillets, ribs with fillet radii and a special orientation. Therefore, this work provides detailed fluid flow and heat transfer data for a model of radial cooling geometry which has very realistic features.

Thermo-flow characteristics and air flow field leading of the air-cooled condenser cell in a power plant

Special A-frame geometry of the air-cooled condenser cell and the complicated flow field at the exit of the axial flow fan bring on the air mal-distribution on the surface of the finned tube bundles and the deteriorated thermo-flow performances of a condenser cell. It is of benefit to the design and operation optimization of the direct dry cooling system in a power plant to investigate the thermo-flow characteristics of the condenser cell and propose the flow leading measures of cooling air. On the basis of the representative configuration of the air-cooled condenser cell in a 600 MW direct dry cooling power plant, the computa- tional models of the air side fluid and heat flows are built, in which the actual fan blade geometric details are considered. Various flow field leading ways of cooling air are presented and the thermo-flow characteristics in the A-frame condenser cell and through the finned tube bundles are compared. Results show that the flow field leading measures can result in the increased volumetric flow rate and heat rejection, thus bringing on the improved performance of the condenser cell. The improvement of thermo-flow oerformances depends upon the geometric details of the flow guiding device.

Comparative study on constructal optimizations of T-shaped fin based on entransy dissipation rate minimization and maximum thermal resistance minimization

The constructal optimizations of T-shaped fin with two-dimensional heat transfer model are carried out by finite element method and taking the minimization of equivalent thermal resistance based on entransy dissipation and the minimization of maximum thermal resistance as optimization objectives, respectively. The effects of the global parameter a (integrating the coefficient of convective heat transfer, the overall area occupied by fin and its thermal conductivity) and the volume fraction ? of fin on the minimums of equivalent thermal resistance and maximum thermal resistance as well as their corresponding optimal configurations are analyzed. The comparison of the results based on the above two optimization objectives is conducted. The results show that the optimal structures based on the two optimization objectives are obviously different from each other. Compared with the optimization result by taking the minimization of maximum thermal resistance as the objective, the optimization result by taking the equivalent thermal resistance minimization as the objective can reduce the average temperature difference in the fin obviously. The increases of a and ? can all improve the working status of local hot spot and the global heat transfer performance of the system. But the improvement effects of the increases of a and ? on the minimization of equivalent thermal resistance are different from those on the minimization of maximum thermal resistance. For either objective, the effect of a is different from that of ?. The T-shaped fin with minimum equivalent thermal resistance is much taller than that with minimum maximum thermal resistance; for either optimization objective, the stem of fin is thicker than the branches of fin, and the stem thickness is relatively close to branch thickness when the minimization of equivalent thermal resistance is taken as the optimization objective. The T-shaped fin with flat stem and slender branches can benefit the reduction of the maximum thermal resistance.

Experiment of Flow Regime Map and Local Condensing Heat Transfer Coefficients Inside Three Dimensional Inner Microfin Tubes

This paper developed a new type of three dimensional inner microfin tube. The experimental results of the flow patterns for the horizontal condensation inside these tubes are reported in the paper. The flow patterns for the horizontal condensation inside the new made tubes are divided into annular flow, stratified flow and intermittent flow within the test conditions. The experiments of the local heat transfer coefficients for the different flow patterns have been systematically carried out. The experiments of the local heat transfer coefficients changing with the vapor dryness fraction have also been carried out. As compared with the heat transfer coefficients of the two dimensional inner microfin tubes, those of the three dimensional inner microfin tubes increase 47-127% for the annular flow region, 38-183% for the stratified flow and 15-75% for the intermittent flow, respectively. The enhancement factor of the local heat transfer coefficients is from 1.8-6.9 for the vapor dryness fraction from 0.05 to 1.

Airside fin efficiencies for finned-tube heat exchangers with forced convection

The heat transfer and mass transfer fin efficiencies were analyzed numerically to show that popular models for heat transfer fm efficiency for circular fins are not always reasonable. The numerical results show that the effective heat transfer area of a circular fin increases several times faster than that of a straight fin for the same tube radius. Then, a simple but accurate heat transfer fin efficiency model was developed and verified by numerical results for a wide range of fin designs. This model predicts the heat transfer fin efficiency with absolute errors of less than 1%. The heat transfer and mass transfer fin efficiencies were found to be quite different for typical air flow with low relative humidity. Thus, these two fin efficiencies should not be assumed to be equal and a mass transfer fin efficiency model was developed, based on the heat transfer fin efficiency model. These heat transfer and mass transfer fin efficiencies are very useful for more accurate prediction for a wide range of practical applications.