抽吸式自然对流换热及扰流柱对其影响

用稳态法进行了竖直平行平板间加扰流柱的抽吸式自然对流换热实验研究,全面考虑了包括辐射损失在内的各项修正,得到了精度不低于10％的经验公式。实验发现,对于不加扰流柱的平行平板通道,当温差大于50℃时,换热系数随间距的变化存在极值;小间距加扰流柱时,扰流柱对流动的阻滞起主要作用,换热系数低于无扰流柱时的值。     

MAGNETIC DAMPING OF g-JITTER INDUCED DOUBLE-DIFFUSIVE CONVECTION

This article describes a numerical study of g-jitter driven double-diffusive convective flows and thermal and concentration distributions in binary alloy melt systems subject to an external magnetic field. The study is based on the finite element solution of transient magnetohydrodynamic equations governing the momentum, thermal, and solutal transport in the melt pool. Numerical simulations are conducted using synthesized single- and multi-frequency g-jitter as well as real g-jitter data taken during space flights with or without an applied magnetic field. It is found that for the conditions studied, the main melt flow follows approximately a linear superposition of velocity components induced by individual g-jitter components, regardless of whether a magnetic field exists or not. The flow field is characterized by a recirculating double-diffusive convection loop oscillating in time with a defined frequency equal to that of the driving g-jitter force. An applied magnetic field has little effect on the oscillating recirculating pattern, except around the moment when the flow reverses its direction. The field has no effect on the oscillation period, but it changes the phase angle. It is very effective in suppressing the flow intensity and produces a notable reduction of solute striations and time fluctuations in the melt. For a given magnetic field strength, the magnetic damping effect is more pronounced on the velocity associated with the largest g-jitter component present and/or the g-jitter spiking peaks. A stronger magnetic field is more effective in suppressing the melt convection and also is more helpful in bringing the convection in phase with the g-jitter driving force. The applied field is particularly useful in suppressing the effect of real g-jitter spikes on both flow and solutal distributions. With appropriately selected magnetic fields, the convective flows caused by g-jitter can be reduced sufficiently, and it is possible that diffusion dominates the solutal transport in the melt.     

Modeling of Thermal Properties of Thermal Insulation Layered with Transparent,Opaque and Reflective Film

The popular and effective food preservation technology based on refrigeration is not sufficient for high-quality products while undergoing logistic operations (transport and retail). One of the basic factors that affects the quality of chilled and frozen food products during storage and transport is packaging. A protective function of packaging strongly depends on the material used and its composition. There are different kinds of thermal insulation used for food packaging. One of them, proposed by the authors is a multilayer structure of insulation made of rectangular air cells. The insulation can be prepared by means of plastic film featuring various properties. The paper presents how to improve an effective material designed for food freezing and transport aiming to enhance its thermal resistance through the application of different transparency, reflectivity and emissivity of the film. Mathematical model based on heat exchange equations, including conduction, convection and radiation throughout a number of parallel internal sheets of film of multilayer structures was proposed. Thermal properties depending on different transparency, reflectivity and emissivity of the film were analyzed. The model was verified experimentally showing its compatibility and obtaining a significant influence of thermal resistance according to the type of film used to make air structures, the number and thickness of its layers as well as the gaps between internal folds. For multi-layer insulation designed for the insulation of packed frozen food in the shape of a rectangle, it was recommended to apply film transmittance as small as possible for the internal parts of the structure.     

The effect of inclination on the heat loss from flat-plate solar collectors

One of the many design variables that affects the heat losses from flat-plate solar collectors is the angle of inclination of the collectors to the horizontal. This is due to the variation in natural convection conductances in spaces between flat plates, with their angle to the horizontal. The top loss heat transfer coefficient is calculated for a series of plate temperatures, ambient temperatures, external convective heat transfer coefficients and plate emittances for angles of inclination from 0 to 90° using the natural convection correlation developed by Hollands et al.[4]. A sky temperature 12°C below ambient temperature is used as the radiant sink temperature and an effective sink temperature for the top losses is defined. Curves are presented showing the variation of the top loss coefficient with temperature and wind speed for two plate emittances at an angle of inclination of 45°. It is shown that the value of the top loss coefficient is insensitive to the effective sink temperature (as found by Duffie and Beckman [5]) and that the effective temperature is determined solely by the wind speed, for a given collector inclination.The top loss coefficient at any angle of inclination is expressed as a ratio of the top loss coefficient at 45°. The results indicate that there is a continual reduction in the top loss coefficient up to an inclination of 90°. The effect this has on the overall collector loss coefficient is illustrated and the change in collector instantaneous efficiency is estimated.     

An experimental and analytical evaluation of the insulative properties of double glazing and thick curtains

Theoretical and experimental evaluations are made of thermal curtains applied to double-glazed greenhouse structures. The reflectance and emittance of the curtain is permitted to be different on the two surfaces: It may be partially transparent or opaque to thermal radiation, and it may have thick blanket insulative properties. The effect of cloud cover upon the night-sky radiation temperature is included in the model. Convection coefficients are prescribed for the curtain and glazed surfaces, and the effect of air leakage into the space between the curtain and inner layer of glazing is included as an empirically defined parameter. The thermal balance for the system is obtained in terms of the radiative, convective and air-leakage parameters relevant to each component. The resulting set of thermal balance equations are solved iteratively using existing matrix-solution programs at the University of Waterloo. An experimental rig was constructed to determine nighttime heat loss from a conventional double-walled polycarbonate glazing material which is partially transparent to thermal radiation. The rig was operated with and without an insulating curtain. An opaque multilayer curtain and a semitransparent bubble film curtain were tested. The agreement between the thermal model and the test rig was within ±5% for all cases. Cloud cover was found to be a significant parameter that alters heat loss upwards by 11% from fully cloudy to clear-sky conditions. Intermediate cloud cover conditions were investigated as well and were found to be well predicted by an existing correlation for sky temperature vs cloud cover. The analysis is extended to provide a simple resistance coefficient for heat loss from the glazed structure based upon inside and outside temperatures for a specified cloud cover. It is found that the semiopaque curtain and double-glazed combination has 3.74 times the thermal resistance of a single glass structure as compared to a value of 3.81 for the opaque curtain and double-glazed arrangement. In addition the semiopaque curtain can be used for solar shading during the summer time, thereby serving a dual purpose that significantly increases its economic advantage.     

平行平板间有圆台形蜂窝板的不同组合体时自然对流传热的实验研究

报道了平行平板间装有圆台形蜂窝及其不同组合体时自然对流传热的实验研究工作，介绍了实验装置及实验方法，比较了有无蜂窝及蜂窝的不同组合方案对自然对流传热的影响。实验结果表明，采用本工作研究的蜂窝结构，可有效地抑制空气夹层的自然对流传热。     

MOLECULAR SIMULATION OF SOME THERMOPHYSICAL PROPERTIES AND PHENOMENA

Molecular dynamics study has been performed on ultra-thin liquid film sheared between two solid surfaces, which has a direct relation to lubrication. Energy and momentum transfer in the liquid film and at the solid-liquid interface accompanied by viscous heating are analyzed. The system consists of liquid film where molecules are modeled by the Lennard-Jones (12-6) potential and two parallel solid walls having a spacing of several nanometers. The solid walls have a constant temperature and move at a velocity in the opposite directions to each other, which causes a shear in the liquid film. A layered structure has been formed in the liquid by the effect of interaction with solid molecules, in which highly nonequilibrium distribution of thermal energy among the degrees of freedom for molecular motion is observed.     

玻璃衬底和硅衬底沉积TZO透明导电薄膜的对比研究

利用直流磁控溅射法在室温水冷玻璃衬底和硅片衬底上制备出掺钛氧化锌(ZnO∶Ti)透明导电薄膜。SEM和XRD研究结果表明,两种衬底上的ZnO∶Ti薄膜均为为六角纤锌矿结构的多晶薄膜,且具有c轴择优取向。讨论了衬底对掺钛氧化锌透明导电薄膜光学、电学性能的影响。当玻璃衬底薄膜厚度为568nm时,薄膜电阻率达到最小值1.64×10-4Ω.cm,硅衬底上薄膜厚度为641nm时有最小电阻率2.69×10-4Ω.cm。两种衬底所制备薄膜都具有良好的附着性能,玻璃衬底薄膜样品在波长为500~800nm的可见光中平均透过率都超过了91%,硅衬底上薄膜样品的折射率约为2.05,ZnO∶Ti薄膜可以用作薄膜太阳电池的透明电极。     

Brine clarity maintenance in salinity-gradient solar ponds

Brine transparency is an important part of the maintenance of a salinity-gradient solar pond as it affects the amount of solar radiation reaching the storage zone and hence has an influence on the thermal performance. There is a wide range of factors that can hinder the transmission of light in a solar pond. Algal and microbial growths are the most common problems encountered in working solar ponds and control of their densities is essential to maintain transparency. Two different chemical treatment methods for algae growth prevention are described in this paper: chlorine and a novel chemical product – copper ethylamine complex. The latter method has never been implemented previously in a working pond. This paper discusses the theory of the algae control methods used and presents the experimental results of the chemical treatments. The results showed that Cupricide is more effective than chlorine and is therefore the recommended chemical for algae control in solar ponds; it improves the water transparency especially in the upper convective zone and lower convective zone with all measurement values less than 1 NTU. Chlorine was found to be more corrosive than Cupricide due to the acidic effect it has on the pH. The preliminary cost analysis showed that granular chlorine is the cheapest chemical. A more detailed financial analysis is nevertheless required to refine these costs.     

Effect of gravity on methanol diffusion flames burning within a forced convective environment

Characteristics of methanol diffusion flames burning under three different configurations with respect to the directions of forced and natural convective flow fields are presented for zero and normal gravity conditions. Combustion of a spherical methanol particle and of a methanol film within a mixed convective environment, have been numerically simulated. The effects of gravity on aiding, opposing and perpendicular flow are presented. The gravitational field considerably changes the flow field, flame shapes and fuel burning rates, especially for the opposing flow case.