Determining the optimum cyclic operation of adsorption chillers by a direct method for periodic optimal control

Adsorption refrigeration systems provide a sustainable possibility to reduce the environmental impact of refrigeration and air-conditioning as they allow for sources of otherwise unused excess waste heat to be reused for cooling purposes. Adsorptive cooling is a discontinuously operated cycling process, and it is well known that the determination of an optimal cycling time yielding maximum cooling power is a key to the design of an efficient mode of operation. The optimal cycle time however strongly depends on operating conditions such as ambient air temperature, available heat source temperature, desired target cooling temperature, and achievable volume flow rates of the secondary heat transfer circuits. In this contribution, we apply a direct method for periodic optimal control to optimize two-bed adsorption chillers. We present a first principles dynamic model of the underlying thermal process. We show that direct methods for periodic optimal control allow for quick and reliable computation of optimal cycle times for a given set of parameters. Contrary to pre-existing methods, fast computation times and guaranteed optimality of the solutions computed by our approach makes it viable to extensively study the simulated optimal cyclic operation of two-bed adsorption chillers under a wide range of varying conditions.     

Heat transfer characteristics of cooked meats using different cooling methodsCaractéristiques du transfert de chaleur de viandes cuites réfrigérées par des méthodes différentes

Cooling rate and heat transfer characteristics of cooked meats using four different cooling systems of vacuum cooling, air blast cooling, water immersion cooling and slow air cooling were investigated. The experimental results show that only the vacuum cooling can achieve the requirement of cooling the cooked meats from about 74 to 10°C within 2.5 h. The vacuum cooling shows different heat transfer characteristics during the cooling process, as compared with other cooling methods. Vacuum cooling rate is controlled by the evaporation rate of water from the cooked meats, while the cooling rates of the other three cooling methods are governed by the thermal conductivity of the cooked meats. Therefore, it is impossible for air blast, water immersion and slow air cooling to obtain high cooling rates since these three methods are different only in the convective heat transfer from the surface of the cooked meat to the cooling medium.     

Multidisciplinary approach to materials selection for bipropellant thrusters using ablative and radiative cooling

Reduction of costs is a main consideration in every space mission, and propulsion system is an important subsystem of those missions where orbital maneuvers are considered. Lighter propulsions with higher performance are necessary to reduce the mission costs. Bipropellant propulsions have been widely used in launch vehicles and upper-stages as well as deorbit modules because of better performances in comparison with other propulsion systems. Unfortunately heat transfer and thermal control limit bipropellant propulsion performance and maximum performance cannot be achieved. Well-known cooling methods such as regenerative and film cooling increase the cost using extra equipment and high temperature materials. In this paper, a new approach for cooling is presented based on combined ablative and radiative cooling. Governing equations are derived for two or three layers of thermal protection system (TPS) to optimize the TPS mass. The first layer is used as an ablative layer to control the temperature where the second and third layers are used as an insulator to control the heat fluxes. Proposed cooling method has been applied for two real bipropellant thrusters. According to the results, the presented algorithm can suitably predict the heat fluxes and satisfy the wall temperature constraint. Then, the algorithm has been used to minimize the wall temperatures as low as possible and replace high temperature materials (platinum alloy) with common materials (composite or steel). It is shown that selection of TPS materials affects the TPS mass and Isp simultaneously, but conversely. Best solution should be derived by trading off between structure temperature (cost), Isp (performance), and TPS thicknesses (geometry). Multidisciplinary approach to TPS and structure material selection of a bipropellant thruster is presented for a case study. It has been shown that mass and performance penalties of using TPS are acceptable, considering the advantages of using steel alloy instead of platinum alloy.     

Study on high efficient heat recovery cycle for solid-state cooling

Traditional vapor compression cycles (VCCs) use mainly halogenated refrigerants such as hydrochloroflurocarbons and hydrofluorocarbons that are considered as greenhouse gases. Therefore, their regulations are imposed on a global scale. As an alternative cooling technology other than VCCs, solid-state cooling technologies, such as magnetic cooling, thermoelastic cooling and electrocaloric cooling, demonstrate its advantage of not using greenhouse gases as working fluids. However, one of the most challenging issues of these solid-state cooling technologies is the relative high parasitic internal latent heat loss, which could significantly deteriorate the system performance. In order to improve the system performance of solid-state cooling, the authors propose a novel and high-efficient heat recovery (HR) cycle for solid-state materials with high thermal conductivity. The novel heat recovery process was first compared as an analog of spatial scale counter-flow heat transfer process. A simplified ideal model was developed to quantitatively investigate the heat recovery process performance limit and the physics behind the analogy of the spatial scale counter-flow heat transfer process. Experiment was conducted and 60% HR efficiency was achieved. In addition, a detailed dynamic model validated by the experiment results was developed and used to further investigate the limiting factors together with the theoretical analysis.     

一种新型复合相变材料导热性能的实验研究

针对相变材料在实际应用过程中交替存在升温液化和降温固化的复杂传热过程,采用JW-Ⅲ建筑材料热流计式导热仪,分别对升温和降温过程中处于固态、混合态、液态的新型复合相变材料导热性能进行了测试和分析。研究结果表明,复合相变材料在加热和冷却过程中的导热系数随温度的变化存在明显的规律性差异,导热系数在混合态时差值达到20%;升温过程中,复合相变材料在混合态和液态时的导热系数值相差不大,但与固态时相比有明显减小;降温过程中,在液-固相变的过程中导热系数随温度减小而增大,有利于加速相变材料的固化。     

轨道车辆空调相变蓄冷换热器放冷过程的数值仿真研究

通过建立传热过程的数学模型,对轨道车辆空调相变蓄冷换热器放冷过程进行数值仿真计算,得到圆管型和扁管型蓄冷换热器在不同条件(环境温度、风速和蓄冷材料导热系数)和不同结构尺寸(圆管的管径、扁管的厚度)下的总放冷量和放冷时间,根据计算结果分析对比了蓄冷换热器在不同条件下放冷效果的优劣,结果表明影响放冷效果的重要因素为风速和环境温度,导热系数对放冷效果的影响非常小。     

Analytic study of radiative heat transfer in ultra‐fine powder insulations with dependent scattering and absorption

Abstract

In this work, the radiative heat transfer of the ultra‐fine powder insulation Aerosil 380, with dependent scattering and absorption, is investigated theoretically. The radiative transport process is modeled by the two‐flux model and the diffusion approximate method to solve the government equations of transfer. The radiative properties of Aerosil 380 have been determined by the Rayleigh scattering theory because of the small values of particle size parameter. The results show that the dependent effect of scattering will reduce the scattering efficiency; however, the absorption efficiency will be increased due to the dependent absorption. The overall thermal radiation resistance is increased by the dependent effect. A comparison of radiative thermal conductivity has been calculated by the two models. The comparison reveals that the difference is small at a mean temperature of 300°K, but that the difference goes up to about 30 percent at a mean temperature of 400°K.     

Thermal uniformity and rapid cooling inside refrigerators

The authors developed a new air-supply system for improving the thermal uniformity and the cooling rate inside a fresh food cabinet of a household refrigerator. For these purposes, we added a blower and jet slots to a conventional cooled air supply system. The jet slots circulate the air inside the cabinet at a higher velocity to optimize airflow velocity and its distribution. The jet stirs the air inside the cabinet and improves thermal uniformity, which resulted in half the temperature deviation in the cabinet as that of the conventional systems. The jet also improves the heat transfer on the surface of foods. We achieved a four times higher cooling rate with the new system than that with the conventional ones. In order to cut down the development period, we applied computational fluid dynamics to study air distribution inside the cabinet with the new system. We also derived the model of the cooling process by the jet using theoretical and empirical equations and applied it to decide the jet velocity for the rapid cooling.     

一种硅基集成PCR微芯片的传热数值分析

设计和制作了一种基于MEMS技术的硅基集成PCR（聚合酶链式反应）微芯片,采用有限容积法,边界条件考虑自然对流换热和辐射换热,对PCR微芯片的传热过程进行了数值模拟。主要分析了样品的升、降温速度和样品内部的温度均一性。分析结果表明芯片具有较高的升、降温速度,而且样品内部的温度均一性也满足PCR反应的要求。芯片在温度控制系统下进行了热循环反应,实现了GUS基因的扩增,获得了良好的实验结果。     

Cooling of molten core material within a pressurised water reactor vessel lower head: interaction of surface radiation and wall conduction with free convection

The phenomenon of cooling of molten core material inside the lower head of a pressurised water reactor pressure vessel has been analysed numerically. A detailed analysis of interaction of surface radiation and conjugate wall conduction with free convection inside the semi-circular cavity has been presented. The effect of parameters which govern the radiative heat transfer from the pool surface has been investigated. The results show that for a certain set of parameters radiative heat transfer from the pool surface is more efficient than convection towards vessel circular wall. For all the cases considered in the present analysis well-stratified corium pool has been obtained.     

Inverse thermal mold design for injection molds

The thermal mold design and the identification of a proper cooling channel design for injection molds becomes more and more complex. To find a suitable cooling channel system with objective rules based on the local cooling demand of the part a new methodology for the thermal mold design based on an inverse heat transfer problem was introduced. Based on a quality function regarding production efficiency as well as part quality, additional aspects to model the injection molding process are discussed. Aim of those extensions is the improvement of the inverse optimization of the problem.     

Equivalent Thermal Conductivity of Open-Cell Ceramic Foams at High Temperatures

At high temperature, heat transfer in open-cell foams occurs by thermal radiation through the whole medium as well as by conduction through the solid matrix and air filling the pores. This paper applies the body-centered cubic cell model to predict radiative properties and the thermal conductivity of the open-cell foams. The model is validated by comparing the results with previous published works. Effects of structural characteristic parameters (cell diameter and porosity) and optical properties of the solid matrix (reflectivity and specularity parameter) on extinction coefficients and the radiative conductivity are discussed. The influence of temperature on the thermal conductivities including the effective, radiative, and the equivalent conductivity of open-cell ceramic foams are analyzed.     

Modeling of thermal barrier coating temperature due to transmissive radiative heating

Thermal barrier coatings are generally designed to possess very low thermal conductivity to reduce the conduction heat transfer from the coating surface to the metal turbine blade beneath the coating. In high-temperature power generation systems, however, a considerable amount of radiative heat is produced during the combustion of fuels. This radiative heat can propagate through the coating and heat up the metal blade, and thereby reduce the effectiveness of the coating in lowering the thermal load on the blade. Therefore, radiative properties are essential parameters to design radiative barrier coatings. This article presents a combined radiation and conduction heat transfer model for the steady-state temperature distribution in semitransparent yttria-stabilized zirconia (YSZ) coatings. The results of the model show a temperature reduction up to 45 K for YSZ of high reflectance (80%) compared to the YSZ of low reflectance (20%). The reflectivities of YSZ and metal blade affect the temperature distribution significantly. Additionally, the absorption and scattering coefficients of YSZ, the thickness of the coating, and the thermal conductivities of YSZ and metal blade affect the temperature distribution.     

地源热泵能效比试验与研究

采用对比试验,对不同型号井埋管的散热能力、传热能力进行研究,从而得出不同井埋管对地源热泵的不同影响。对地源热泵能效比研究前景作简单介绍,在试验部分,通过对传感器进行校正与误差分析,保证试验中数据的精确性,使散热试验和取热试验一样。应用传热模型,根据散热和取热中管内水流的流量和温度,就可以计算出水平管在各种工况下的平均散热和取热能力,鉴于在地源热泵系统应用中,地埋管换热器的设计是最关键的部分,而地源热泵的能效比又与其有密切联系,可以由此知道不同型号井埋管对地源热泵能效比的影响。     

Characterization of Thermal Radiative Properties of Nanofluids for Selective Absorption of Solar Radiation

Nanofluids are used in device cooling, heat pipes, and other applications. Researchers in thermal engineering have extensively investigated the thermal conductivity and enhanced heat transfer of nanofluids. This study investigates the thermal radiative properties of nanofluids and discusses the characteristics of the selective absorption of solar radiation of nanofluids. Several kinds of nanoparticles are prepared using the two-step method, and the effects of dispersants, mass fractions, and nanoparticle materials on the radiative properties of nanofluids within the wavelength range of 300 nm to 2500 nm are analyzed. Dispersants can reduce the transmittance of water by <5 % within the visible spectrum. ZnO- and AlN-water nanofluids selectively absorb solar radiation, whereas ZrC- and TiN-water nanofluids absorb most of the solar radiation applied to them. The findings of this study are beneficial to research on the application of nanofluids in solar energy utilization.     

The thermal state of a porous wall under conditions of transpiration cooling

Results of numerical experiment are used for analysis of fields of temperature in a laminar boundary layer, in a porous wall, and in a cooling gas delivery chamber, as well as for analysis of heat transfer and of distribution of the temperature difference between the cooling gas and the porous wall frame and cooling efficiency. It is demonstrated that heat transfer between a porous wall of finite thickness and a high-temperature gas flow differs significantly from heat transfer with preassignment of the same intensity of injection and of the homogeneous thermal boundary condition directly on the surface subjected to flow. One of the reasons for this is the formation of wall temperature variable along the boundary layer.     

Estimation of the refrigeration capacity required to cool horticultural produce

The provision of sufficient refrigeration capacity to satisfy the maximum initial cooling rates achieved by convective heat transfer during pressure-cooling is often unnecessary and uneconomic. Provision of 45% of the maximum capacity will increase the seven-eights cooling time by only 15–20%. An equation is derived for the refrigeration capacity in terms of the rate of heat loss from the cooling produce, so that cooling can be achieved within the time required for maintenance of quality without the need for an excessively large plant. Cooling rate is defined in terms of the seven-eights cooling time, S, and the lag factor, j, which is an empirical measure of the thermal properties of the produce.     

无接触热阻全铝换热器空调系统制冷性能研究

本文用新型无接触热阻全铝换热器对传统家用空调换热器进行了替代设计,利用空气焓值法对使用新型换热器和管片式换热器的家用窗式空调器进行对比试验,优化并测试了毛细管规格和制冷剂充灌量对新型换热器空调系统制冷性能的影响。研究结果表明：新型无接触热阻全铝换热器在换热面积减小37.53%时,制冷量反而提高3.59%,能效比EER提高7%。新型换热器有更强的换热能力,是目前家用空调换热器的理想替代产品。     

单晶铸件凝固过程工艺优化的数值模拟

采用ProCAST软件系统研究了LMC(Liquid Metal Cooling)以及HRS(High Rate Solidification)工艺下,不同工艺参数对单晶铸件凝固过程中纵向温度梯度、温度梯度角、凝固界面位置的影响。结果表明:HRS工艺受型壳厚度影响很小,型壳表面的辐射散热是HRS工艺的主要影响因素,型壳的导热或者型壳和合金之间的换热是LMC工艺的主要影响因素;提高保温炉温度有利于提高纵向温度梯度;拉速是影响定向凝固最重要的参数,随拉速的增加,单晶铸件的纵向温度梯度先增大后减小,因此,制备不同合金铸件时应当采用不同的拉速;不同浇注温度时,经过10min的静置时间后,单晶铸件的初始温度分布趋于一致,对后续凝固过程影响很小。提出了以纵向温度梯度G∥、温度梯度角θ以及凝固界面位置Rp考察定向凝固工艺参数优劣的标准,纵向温度梯度、温度梯度角、凝固界面位置是评价定向凝固参数优劣的有效手段。     

新型热管电子器件散热器的实验研究和数值模拟

随着电力电子技术的不断发展,高热流密度器件热控制问题以及出现的热流分布不均匀的现象,是电子电器设备亟待解决的关键技术。热管作为高效传热元件,已经广泛应用于电子器件的散热技术研究中。本文对新型平板式热管散热器进行了数值模拟和实验研究。根据电子器件的运行工况,建立了散热器性能测试系统,并对平板热管型电子器件散热器进行了在不同工况下的性能实验。结果表明采用平板热管散热器可以有效提高CPU芯片的散热性能,芯片发热量在160W后为其最佳工作状态。同时用数值模拟方法对平板热管散热器底面的导热效果进行了优化设计,通过改变导热系数以及扩大模拟芯片尺寸来达到改善CPU冷却散热器的散热效果的目的,并得到了一些有价值的结论,这对改进管散热器的散热效果有一定的指导意义。