Dynamic Crushing Strength Analysis of Auxetic Honeycombs

The in-plane dynamic crushing behavior of re-entrant honeycomb is analyzed and compared with the conventional hexagon topology.Detailed deformation modes along two orthogonal directions are examined,where a parametric study of the effect of impact velocity and cell wall aspect ratio is performed.An analytical formula of the dynamic crushing strength is then deduced based on the periodic collapse mechanism of cell structures.Comparisons with the finite element results validate the effectiveness of the proposed analytical method.Numerical results also reveal higher plateau stress of re-entrant honeycomb over conventional hexagon topology,implying better energy absorption properties.The underlying physical understanding of the results is emphasized,where the auxetic effect(negative Poisson's ratio) induced in the re-entrant topology is believed to be responsible for this superior impact resistance.     

Effects of thermal dispersion on heat transfer in cross-flow tubular heat exchangers

Effects of thermal dispersion on heat transfer and temperature field within cross-flow tubular heat exchangers are investigated both analytically and numerically, exploiting the volume averaging theory in porous media. Thermal dispersion caused by fluid mixing due to the presence of the obstacles plays an important role in enhancing heat transfer. Therefore, it must be taken into account for accurate estimations of the exit temperature and total heat transfer rate. It is shown that the thermal dispersion coefficient is inversely proportional to the interstitial heat transfer coefficient. The present analysis reveals that conventional estimations without consideration of the thermal dispersion result in errors in the fluid temperature development and underestimation of the total heat transfer rate.     

Experimental investigation of TiO2/water nanofluid laminar forced convective heat transfer through helical coiled tube

Coiled tubes and nanofludics are two significant techniques to enhance the heat transfer ability of thermal equipments. The forced convective heat transfer and the pressure drop of nanofluid inside straight tube and helical coiled one with a constant wall heat flux were studied experimentally. Distilled water was used as a host fluid and Nanofluids of aqueous TiO₂ nanoparticles (50 nm) suspensions were prepared in various volume concentrations of 0.25–2 %. The heat transfer coefficient of nanofluids is obtained for different nanoparticle concentrations as well as various Reynolds numbers. The experiments covered a range of Reynolds number of 500–4,500. The results show the considerable enhancement of heat transfer rate, which is due to the nanoparticles present in the fluid. Heat transfer coefficient increases by increasing the volume concentration of nanoparticles as well as Reynolds number. Moreover, due to the curvature of the tube when fluid flows inside helical coiled tube instead of straight one, both convective heat transfer coefficient and the pressure drop of fluid grow considerably. Also, the thermal performance factors for tested nanofluids are greater than unity and the maximum thermal performance factor of 3.72 is found with the use of 2.0 % volume concentration of nanofluid at Reynolds number of 1,750.     

A separated flow model for predicting two-phase pressure drop and evaporative heat transfer for vertical annular flow

A separated flow model has been developed that is applicable to vertical annular two-phase flow in the purely convective heat transfer regime. Conservation of mass, momentum, and energy are used to solve for the liquid film thickness, pressure drop, and heat transfer coefficient. Closure relationships are specified for the interfacial friction factor, liquid film eddy-viscosity, turbulent Prandtl number, and entrainment rate. Although separated flow models have been reported previously, their use has been limited, because they were tested over a limited range of flow and thermal conditions. The unique feature of this model is that it has been tested and calibrated against a vast array of two-phase pressure drop and heat transfer data, which include upflow, downflow, and microgravity flow conditions. The agreements between the measured and predicted pressure drops and heat transfer coefficients are, on average, better or comparable to the most reliable empirical correlations. This separated flow model is demonstrated to be a reliable and practical predictive tool for computing two-phase pressure drop and heat transfer rates. All of the datasets have been obtained from the open literature.     

非线性热环境下高温合金蜂窝板隔热性能研究

金属蜂窝板结构在高温热环境下的隔热特性是高速飞行器热防护设计的重要参数. 使用自行研制的高速飞行器瞬态气动热试验模拟系统, 对高温合金蜂窝平板结构在高达800℃的非线性热环境下的隔热性能进行实验研究, 获得了蜂窝板结构的瞬态和稳态传热特性以及在多种不同温度下金蜂窝平板结构隔热效果的实验数据. 在考虑结构内部蜂窝芯壁面间辐射、金属结构的传热以及蜂窝腔内空气传热的多重热交换条件下, 采用三维有限元计算方法对蜂窝板的隔热特性进行了数值模拟, 计算结果和试验结果的吻合性良好, 验证了数值模拟方法的可信性和有效性, 并为数值模拟方法能够在一定程度上较好地替代价格昂贵的气动热模拟试验打下了基础. 讨论了在复杂非线性高温环境下金属蜂窝板隔热效率的变化, 加热面温度的升降速度与隔热效率的关联性以及金属蜂窝板表面发射率的选取等问题, 对高速飞行器金属蜂窝结构的热防护研究具有重要的参考价值.      

Simultaneous transfer of heat and mass occurring in casting process

Studied in this paper is simultaneous transfer of heat and water vapor which takes place in a green sand mold in a very short period of time after pouring a molten metal in its cavity. Governing equations describing heat and mass transfer in a mold are solved by finite difference method and the results are compared with the actually measured values to examine the validity of the calculated results. The effect of thermal properties and permeability of the mold, the amount of water contained, the heating temperature (i.e., temperature of casting metal) and other factors on the heat transfer rate at the interface between the molten metal and the mold, the pressure rise in the mold and the development of dried zone around the casting are investigated to propose some empirical relations available for predicting those transfer phenomena by using dimensionless parameters presented.     

Local heat transfer coefficients on the circumference of a tube in a gas fluidized bed

A heated horizontal heat transfer tube was installed 14.8 cm above the distributor plate in a square fluid bed measuring 30.5 × 30.5 cm. Four different Geldart B sized particle beds were used (sand of two different distributions, an abrasive and glass beads) and the bed was fluidized with cold air. The tube was instrumented with surface thermocouples around half of the tube circumference and with differential pressure ports that can be used to infer bubble presence. Numerical execution of the transient conduction equation for the tube allowed the local time-varying heat transfer coefficient to be extracted. Data confirm the presence of the stagnant zone on top of the tube associated with low superficial velocities. Auto-correlation of thermocouple data revealed bubble frequencies and the cross-correlation of thermal and pressure events confirmed the relationship between the bubbles and the heat transfer events. In keeping with the notion of a “Packet renewal” heat transfer model, the average heat transfer coefficient was found to vary in sympathy with the root-mean square amplitude of the transient heat transfer coefficient.     

蜂窝密封内流动传热及转子动力特性的研究进展

蜂窝密封由于其具有优良的密封性能和转子动力特性而广泛应用于叶轮机械中.本文从实验研究、数值模拟和理论分析3个方面对蜂窝密封的研究现状及其进展进行了综述.主要内容包括:蜂窝密封泄漏特性及其密封机理的研究进展;蜂窝密封流动总温升特性的研究进展;蜂窝密封流热耦合作用下的流动传热特性研究进展;蜂窝密封非定常气流激振和转子动力特性的流固耦合机理研究进展.最后在蜂窝密封研究现状综述的基础上, 展望了蜂窝密封的发展方向.      

Heat transfer in open cell polyurethane foam insulation

This paper study systematic investigates the combined conductive and non-gray radiative heat transfer of open cell polyurethane (PU) foam in the pressure range between 760 and 0.02 Torr. Direct transmission measurements are also taken using Fourier transform infrared (FTIR) spectrometer. In doing so, experimental results are obtained for the spectral extinction coefficient from 2.5 to 25 μm. In addition, the P-3 approximation method along with the box model is employed to calculate the non-gray radiative heat flux. The diffusion approximation method is also applied to calculated the radiative conductivity. Also tested herein are three samples with different cell sizes ranging from 330 to 147 μm. According to those results, the spectral extinction coefficient increases with a decrease of cell size, leading to a decrease of thermal conductivity. Moreover, evacuating the gases in the foam cells can reduce the thermal conductivity of the PU foam by as much as 75%. Furthermore, radiative heat transfer accounts for about 4% of total heat transfer at 760 Torr and increases to 20% at 0.02 Torr. Received on 20 April 1998     

Bi-functional optimization of actively cooled, pressurized hollow sandwich cylinders with prismatic cores

All metallic, hollow sandwich cylinders having ultralight two-dimensional (2D) prismatic cores are optimally designed for maximum thermo-mechanical performance at minimum mass. The heated cylinder is subjected to uniform internal pressure and actively cooled by forced air convection. The use of two different core topologies is exploited: square- and triangular-celled cores. The minimum mass design model is so defined that three failure modes are prevented: facesheet yielding, core member yielding, and core member buckling. The intersection-of-asymptotes method, in conjunction with the fin analogy model, is employed to build the optimization model for maximum heat transfer rate. A non-dimensional parameter is introduced to couple the two objectives—structural and thermal—in a single cost function. It is found that the geometry corresponding to maximum heat transfer rate is not unique, and square-celled core sandwich cylinders outperform those having triangular cells. The eight-layered sandwich cylinders with square cells have the best overall performance in comparison with other core topologies. Whilst a sandwich cylinder with shorter length is preferred for enhanced thermo-mechanical performance, the influence of the outer radius of the cylinder is rather weak.     

Effects of the heat transfer fluid velocity on the storage characteristics of a cylindrical latent heat energy storage system: a numerical study

A numerical study of the effects of the thermal fluid velocity on the storage characteristics of a cylindrical latent heat energy storage system (LHESS) was conducted. Due to the low thermal conductivity of phase change materials (PCMs) used in LHESS, fins were added to the system to increase the rate of heat transfer and charging. Finite elements were used to implement the developed numerical method needed to study and solve for the phase change heat transfer (melting of PCM) encountered in a LHESS during charging. The effective heat capacity method was applied in order to account for the large amount of latent energy stored during melting of the PCM and the moving interface between the solid and liquid phases. The effects of the heat transfer fluid (HTF) velocity on the melting rate of the PCM were studied for configurations having between 0 and 18 fins. Results show that the overall heat transfer rate to the PCM increases with an increase in the HTF velocity. However, the effect of the HTF velocity was observed to be small in configurations having very few fins, owing to the large residual thermal resistance offered by the PCM. However, the effect of the HTF velocity becomes more pronounced with addition of fins; since the thermal resistance on the PCM side of the LHESS is significantly reduce by the large number of fins in the system.     

Flow and heat transfer performance of a mini-channel radiator with cylinder disturbed flow

Mini-channel heat sinks have relatively low Nusselt number due to small Reynolds number. For heat transfer enhancement purpose, a mini-channel radiator with cylinder disturbed flow was proposed. The disturbed flow was created by a circular cylinder placed horizontally in front of channels entrance. The performance of heat transfer and pressure drop with/without disturbed flow was studied experimentally. It was found that the friction factor of mini-channel flow was larger than that of the macro-channel flow due to larger surface roughness, and the pressure drop caused by cylinder disturbed flow was less than 5%. It also concluded that the average Nusselt number increases with augment of Reynolds and Prandtl number. The Nusselt number correlations as the function of the Reynolds and Prandtl number were given for evaluation the heat removal performance of similar heat radiators. There is an inflexion point in the empirical formulas when the channel length equals to the thermal entrance length. For the mini-channels heat radiators with disturbed flow, the inflexion Reynolds number is larger than that of without disturbed flow. Due to the flow pulsing caused by circular cylinder placed in front of channels entrance, the thermal entrance length increases. On the other hand, for both mini-channels with or without disturbed flow, the thermal resistance increases with the decrease of pressure drop.     

Two-phase annular flow and evaporative heat transfer in a microchannel

A physical and mathematical model has been developed to predict the two-phase flow and heat transfer in a microchannel with evaporative heat transfer. Sample solutions to the model were obtained for both constant wall temperature and constant wall heat flux conditions. Results are provided for evaporation rate, liquid film thickness, liquid and vapor phase pressure and temperature distributions. In addition to the sample calculations that were used to illustrate the transport characteristics, computations based on the current model were performed to generate results for comparisons with the experimental results of Qu and Mudawar (2004) where two different mass flow rates of the working fluid were used in the experiment. The comparisons of total pressure drops with the experimental data of Qu and Mudawar (2004) cover the wall heat flux range of 142.71-240 W/cm² with a total channel mass flux of 400.1 kg/m² s and also the wall heat flu range of 99.54-204.39 W/cm² with total channel mass flux of 401.9 kg/m² s. The calculated results from the current model match closely with those of Qu and Mudawar (2004).     

Conjugate heat transfer in square enclosures

Building elements represented by square vertical enclosures encircled with finite walls or with centered solid body, could maintain the equivalent fluid volumes through the volume ratio scale. Present work aims to investigate the fluid flow and heat transfer in these two building elements. Complete two-dimensional numerical simulation of the conjugate heat conduction and natural convection occurring in both enclosures is carried out. An analytical expression for the minimum size of the inserted body at which the body begins to suppress the natural convection flow is proposed and validated by the numerical results. The fluid flow and heat transfer characteristics are analyzed through the streamlines, heatlines, and total heat transfer rates across both enclosures. Results reveal that heat transfer rates across both enclosures are complex functions of the volume ratio scale, Rayleigh number, and the relative thermal conductivity.     

Periodic heat transfer in the fins with variable thermal parameters

ＰＥＲＩＯＤＩＣＨＥＡＴＴＲＡＮＳＦＥＲＩＮＴＨＥＦＩＮＳＷＩＴＨＶＡＲＩＡＢＬＥＴＨＥＲＭＡＬＰＡＲＡＭＥＴＥＲＳ￥ＹａｎｇＸｉａｎｇｘｉａｎｇ（杨翔翔）;ＨｅＨｏｎｇｚｈｏｕ（何宏舟）（ＤｅｐａｒｔｍｅｎｔｏｆＭｅｃｈａｎｉｃａｌＥｎｇｉｎｅｅｒ...     

Computational analysis of heat transfer and pressure drop performance for internally finned tubes with three different longitudinal wavy fins

Turbulent pressure drop and heat transfer characteristics in tubes with three different kinds of internally longitudinal fin patterns (interrupted wavy, sinusoidal wavy and plain) are numerically investigated for Re = 904–4,520. The channel velocity, temperature, and turbulence fields are obtained to discern the mechanisms of heat transfer enhancement. Numerical results indicate that the steady and spatially periodic growth and disruption of cross-sectional vortices occur near the tube/fin walls along the streamwise locations. The thermal boundary layers near the tube/fin surfaces are thereby periodically interrupted, with heat transfer near the recirculation zones being enhanced. The overall heat transfer coefficients in wavy channels are higher than those in a plain fin channel, while with larger pressure drop penalties. At the same waviness, the interrupted wavy fin tube could enhance heat transfer by 72–90%, with more than 2–4 times of pressure drop penalty. Among the fins studied, the sinusoidal wavy fin has the best comprehensive performance.     

Aqueous Al2O3 nanofluids: the important factors impacting convective heat transfer

A high accuracy, counter flow double pipe heat exchanger system is designed for the measurement of convective heat transfer coefficients with different nanofluids. Both positive and negative enhancement of convective heat transfer of alumina nanofluids are found in the experiments. A modified equation was proposed to explain above phenomena through the physic properties of nanofluids such as thermal conductivity, special heat capacity and viscosity.     

Heat (mass) transfer and friction loss in two-pass ducts with various parallel rib arrangements

The present study investigated the effects of four parallel rib arrangements on heat/mass transfer and pressure drop characteristics in rotating two-pass ducts. The experiments of mass transfer and pressure measurement are performed to obtain heat transfer distributions and friction losses. The highest averaged heat (mass) transfer and friction loss in each region appeared in the turning region in the stationary cases, but appeared in the upstream region of the second pass in the rotating cases. Among the tested rib arrangements, the averaged heat (mass) transfer and the friction factor in the second pass in the stationary ducts were high in the cases with the NN- and PP-type ribs; however, in the rotating ducts, they were high in the cases with the NN- and PN-type ribs. The thermal performance in the four different rib arrangements was almost the same.     

Enhancement of natural convection heat transfer from a fin by triangular perforation of bases parallel and toward its tip

This study examines the heat transfer enhancement from a horizontal rectangular fin embedded with triangular perforations （their bases parallel and toward the fin tip） under natural convection. The fin＇s heat dissipation rate is compared to that of an equivalent solid one. The parameters considered are geometrical dimensions and thermal properties of the fin and the perforations. The gain in the heat transfer enhancement and the fin weight reduction due to the perforations are considered. The study shows that the heat dissipation from the perforated fin for a certain range of triangular perforation dimensions and spaces between perforations result in improvement in the heat transfer over the equivalent solid fin. The heat transfer enhancement of the perforated fin increases as the fin thermal conductivity and its thickness are increased.