Numerical simulation and performance optimization of a high capacity pulse tube cryocooler

Recent developments of superconductive industries require cryocoolers with cooling power higher than 1 W in the 70–80 K temperature range. High capacity pulse tube cryocoolers assure the cooling power required for operation of superconducting devices. This paper presents numerical simulation of a high capacity pulse tube cryocooler, intended to provide more than 200 W cooling power at 80 K. In this respect the behavior of the cryocooler is explained by applying the mass and energy balance equations to different components of the cryocooler. Nodal analysis technique is employed to simulate the tube section behavior numerically. To perform the system optimization the influence of key operating parameters on cryocooler cooling capacity and coefficient of performance is studied. The proposed model reports the optimum cooling capacity of 244 W at 80 K cold end temperature at frequency of 50 Hz with 3.5 kW net power delivered to the gas.     

Numerical investigation on heat transfer rate from the outside environment into the electronic compartment of the measurement-while-drilling tools

With the increase of drilling depth, a large number of high-temperature wells with bottom hole temperatures of more than 150°C appear. However, conventional measurement-while-drilling (MWD) instruments are no longer suitable. Therefore, it is necessary to design a new MWD instrument suitable for high-temperature downhole for a long time. Compared with other active refrigeration methods, a split-Stirling cryocooler is the best choice. In this paper, a new active cooling system is designed, based on the split-Stirling cryocooler. The heat transfer rate entering the instrument cabin of MWD from the external environment is calculated, and the effects of the heat transfer device, insulation material, circuit board, split-Stirling cryocooler, and working condition on heat transfer rate are discussed. The results show that the thermal conductivity of insulation material has a great influence on heat transfer rate. The thermal conductivity of the insulation material filled with the gap in the cabin must be small enough. Meanwhile, the influence of the length of the circuit board is greater than the height and width. If conditions permit, the double-layer circuit can be used. And the thermal power of the circuit board has no effect on the heat transfer rate entering the instrument cabin from outside.     

Numerical study of conduction and convection heat losses from a half-insulated air-filled annulus of the receiver of a parabolic trough collector

Grid-quality parabolic trough collectors utilize expensive receivers that maintain vacuum in their annuli to reduce convection losses. On the other hand, receivers with air-filled annuli, currently used mainly for process heat applications, are significantly less expensive, but their thermal performance is inferior to evacuated receivers. A promising technique that can bridge the cost and performance gap between the two types of receivers is introduced in this work. A heat-resistant thermal insulation material is fitted into the portion of the receiver annulus that does not receive concentrated sunlight. The presence of this insulation material is expected to reduce not only convection heat losses, but also radiation losses. This study focuses on the calculation of conduction and convection heat losses from the proposed receiver using numerical modeling. The performance of the proposed concept is compared to that of a conventional receiver with an air-filled annulus. The results have shown that the combined conduction and convection heat loss from the proposed receiver can be smaller than that from a receiver with an air-filled annulus by as much as 25% when fiberglass insulation is used. However, the fact that the thermal conductivity of the insulating material increases with temperature reduces the benefit of the proposed concept at high temperatures. As a result, the proposed receiver is expected to be suitable as a replacement for receivers with air-filled annuli or as an economical alternative to evacuated receivers that are used at the lower temperature end of utility-scale solar power plants.     

内部通道肋片结构对Mark Ⅱ叶片冷却性能影响的降维模型研究

一维冷却通道气热耦合计算是分层涡轮叶片冷却结构设计的重要方法。发展了以管道网络算法为核心的内部冷却特性计算程序,并与三维传热计算进行了耦合。通过与MarkⅡ叶片特定实验工况下的结果进行对比,验证了方法的有效性。此外,进一步将带肋结构流道传热特性相关经验公式集总在一维气热耦合算法中,分析了带肋通道改型的MarkⅡ叶片冷却性能。结果显示,带肋结构相比光滑流道能显著提升换热性能,在中径截面处较原方案温度下降15～30 K。     

反照率影响建筑热环境的实验

针对城市建筑物外墙墙面反照率变化，建立两个建筑物理模型，在不同反照率墙面材料的条件下，对夏季建筑室内热环境进行了分析研究，验证了采用高反照率的墙面材料是一种有效的、主动的隔热节能方式；采用积分拟合方法对墙面材料的反照率进行了拟合，其结果为实际情况下城市建筑外墙墙面材料的选择及其隔热性能分析、反照率的计算及其对建筑热环境的影响评价提供了一条有效的途径。     

辐射通道一维稳态温度场数值模拟

为得到辐射对流通道中的温度分布,依据能量守恒原理,建立了辐射、对流非线性边界条件下圆形管壁与管内空气的传热数学模型,提出了管壁温度、管内冷却空气温度一维稳态换热有限差分求解方法,其中辐射换热计算采用基于辐射传递系数的蒙特卡罗法。分析了相关参数对辐射通道温度分布的影响,所研究的参数包括辐射器表面温度、管道长度与半径比、管内冷却空气流速等。计算结果表明：辐射器表面温度是影响辐射通道最高温度的主要因素。此方法可为辐射通道精细的热工特性计算提供温度场数据。     

Thermodynamic analysis and parametric study of a closed Brayton cycle thermal management system for scramjet

A closed Brayton cycle thermal management system is proposed for a regeneratively cooled scramjet to reduce the hydrogen fuel flow for cooling, through converting part of the heat from fuel to other forms of energy to decrease the heat that must be taken away by hydrogen fuel. Fuel heat sink (cooling capacity) is thus indirectly increased. Instead of carrying excess fuel for cooling or seeking for any new coolant, the fuel flow for cooling is reduced, and fuel onboard is adequate to satisfy the cooling requirement for the whole hypersonic vehicle. A parametric study of an irreversible closed Brayton cycle thermal management system for scramjet has been performed with external as well as internal irreversibilities. It is known through performance analyses that closed Brayton cycle thermal management system has excellent potential performance over conventional regenerative cooling, due to the reduction in fuel flow for cooling and additional power output.     

Experimental and Numerical Investigation of a Microjet-Based Cooling System for High Power LEDs

The optical extraction efficiency and reliability of light emitting diodes (LEDs) relies heavily on successful thermal management due to their inherit dependence on the low junction temperature of LED chips. In this paper, a microjet-based cooling system is proposed for the thermal management of high power LEDs. Experimental and numerical investigations on such an active cooling system were conducted. Thermocouples were packaged with LED chips to conduct an online measurement of the temperature and evaluate the cooling performance of the proposed system. The experimental results demonstrate that the microjet-based cooling system has good cooling performance. For a 2 × 2 LED chip array, when the input power is 5.6 W and the environmental temperature is 28°C, the temperature of the 2 × 2 LED chip array reaches 72°C within 2 minutes and continues to increase sharply if no active cooling technique is applied. By using the proposed cooling system to cool the LEDs, however, the maximum LED temperature measured by thermocouples will remain stable at about 36.7°C, when the flow rate of the micropump is 9.7 mL/s. With consideration of the experimental difficulty, a numerical investigation was conducted on flow and temperature distribution in the microjet device. The feasibility of the numerical model was proven by comparison with experimental results. The numerical results showed that at a flow rate of 3.2 mL/s, the heat transfer coefficient of the impinging jets in the proposed system was about 5523 W/m²·K, and the pressure drop in the microjet device was about 1368 Pa.     

燃气隧道窑热工测试与节能分析

我国工业炉窑是能耗大户,总体水平较低,具有相当大的节能潜力。以某高铝砖隧道窑为测试对象进行了详细的热工测试,测量了炉气温度、烟气参数、干燥段的气流参数以及各主要壁面的温度,通过计算窑车及物料带出的热量、干燥段排气热损失、排烟热损失和壁面散热损失等参数,对隧道窑进行了热平衡分析。结果表明,所测试隧道窑的热效率为33.4%,造成热损失的原因包括,砖坯码放方式、干燥段气流组织不合理、助燃空气量过大、围护结构保温性能差等,通过改善急冷段和干燥段气流组织,减小预混空气量,增强窑顶和烧成段保温等措施可实现该类隧道窑的节能。     

Investigation on the optimal performance and design parameters of an irreversible solar-driven Braysson heat engine

An irreversible solar-driven Braysson thermal engine has been investigated, in which finite rate heat transfer with the radiation–convection mode from the high-temperature reservoir to the heat engine and the convection mode from the heat engine to the heat sink, and irreversible adiabatic processes are taken into account. Based on the thermodynamic analysis method, the analytic expressions of the power output and efficiency of the Braysson heat engine are derived. By using numerical value calculation, the effects of the isobaric temperature ratio, internal irreversibility parameter, temperature ratio of the thermal reservoirs as well as the allocation parameters involving the heat-transfer coefficients, and areas on the performance characteristics of the Braysson heat engine are analysed and discussed in detail. The results obtained in this paper are more general than the related conclusions published in the literature and may provide some parameter design reference for solar-driven heat engines.     

Optimal roof structure with multilayer cooling function materials for building energy saving

Both cool roof and phase change thermal storage are promising technologies in decreasing building energy consumption. Combining these two technologies is likely to further enhance the thermal comfort of the building as well as reduce air condition loads. In this paper, the cooling performance and energy-saving effects of four types of roof (normal roof, phase change material [PCM] roof, cool roof, and cool PCM roof [cool roof coupled with PCM]) were investigated under a simulated sunlight. Experimental results indicate that compared with normal roof, the other three roofs are able to narrow the indoor temperature fluctuation and decrease the heat flow entering into the room. Among them, cool PCM roof gave the best energy-saving effect that can lower the indoor temperature and heat entering into rooms by 6.6°C and 52.9%, respectively. Besides, the PCM location, PCM thickness, and insulation thickness exerted great impacts on the cooling performance of the roof. Placing the PCM on the internal layer beneath the extruded polystyrene (XPS) insulation board can make the indoor temperature 1.2°C lower than that on the middle layer. Although thicker PCM panels or insulation boards can provide a better thermal insulation, 5 mm in PCM thickness and 20 mm in insulation thickness are enough to guarantee the indoor temperature of cool PCM roof system at a comfortable range (22°C-28°C) for a whole day. These findings will give guidance in designing buildings with a light and compact roof structure to decrease energy consumption and improve comfort level.     

辐射屏法输热管网隔热保温的参数确定方法及计算结果

在实验和工程应用的基础上，介绍了利用辐射屏法对热网管道进行隔热保温中的确定辐射屏的层数及间隔的方法，并给出了40种不同管径或温度的计算结果，在工程应用中可予以采用。     

平流层电子设备温度数值模拟

分析了平流层电子设备内外部热环境,考虑平流层大气对流、设备内部自然对流、太阳直射辐射、大气辐射、地面反射太阳辐射、地球红外辐射以及设备自身辐射等因素的基础上,建立了计算电子设备温度分布特征的对流、辐射耦合模型,模拟了其在不同功率、不同对流换热、不同环境条件下的温度分布。结果表明:对于平流层电子设备散热,对流换热和辐射换热都会影响电子设备的温度分布,尽管由于平流层大气压力低、对流换热弱,但对流换热量占到散热总量的60%以上,是散热的主要方式。因此,在平流层电子设备热设计时,可以优先考虑采取开孔等强化对流散热方法来控制设备的温度。最后,开展了平流层模拟环境的实验验证,典型工况实验值与计算值吻合较好,验证了计算模型的正确性。对平流层电子设备热设计有重要的指导意义。     

复杂热源下不可逆诺维科夫热机最大功率输出

考虑实际热机工作下的旁通热漏和内部耗散等不可逆因素,建立了包括连续均匀分布、三角形分布、二次分布和帕累托分布等四种不同的统计概率分布高温热源温度下的广义不可逆诺维科夫热机模型,导出了热机最大输出功率及相应的热效率和熵产率随高温热源温度、内部不可逆性等因素变化的关系式。结果表明：热漏和内部耗散分别对热机性能有着不同的影响,热漏使统计热源温度分布下最大功率输出对应的热效率减小,同时也增大了熵产率,但对热机的最大功率输出无影响;内部耗散不可逆性使热机的最大输出功率及相应热效率均明显减小,但使熵产率先增大后减小;熵产率随高温热源温度的标准差增大而减小。研究结果对太阳能发电厂性能提升具有一定理论指导意义。     

电熔炉用于冶金渣制备岩棉热工计算及可行性分析

冶金渣用于制备岩棉等建筑保温材料是冶金渣有效利用的新途径,电熔炉熔炼冶金渣及其保温过程是该工艺的技术关键。研究了基于某钢厂电熔炉设计方案,对其进行了热平衡计算,并提出改进方法。计算结果表明,对冶金渣容量为10 t的电熔炉,冶金渣入炉温度为1 550℃,出炉温度为1 450℃时,功率500 k W电极在加盖工况下理论上可达到热量平衡,开盖工况下还需增加大于413k W功率的电极;对空气的热辐射是炉内冶金渣热损失的关键因素,因此减小电熔炉开口面积是设备节能的方法之一;优化保温层(耐火层)厚度可降低热传导损耗。     

Active cooling of a mobile phone handset

Power dissipation levels in mobile phones continue to increase due to gaming, higher power applications, and increased functionality associated with the internet. The current cooling methodologies of natural convection and radiation limit the power dissipation within a mobile phone to between 1-2 W depending on size. As power dissipation levels increase, products such as mobile phones will require active cooling to ensure that the devices operate within an acceptable temperature envelop from both user comfort and reliability perspectives. In this paper, we focus on the applied thermal engineering problem of an active cooling solution within a typical mobile phone architecture by implementing a custom centrifugal fan within the mobile phone. Its performance is compared in terms of flow rates and pressure drops, allowable phone heat dissipation and maximum phone surface temperature as this is the user constraint for a variety of simulated PCB architectures in the mobile phone. Perforated plates with varying porosity through different size orifices are used to simulate these architectures. The results show that the power level dissipated by a phone for a constant surface temperature may be increased by ～50 - 75% depending on pressure drop induced by the internal phone architecture. Hence for successful implementation and efficient utilization of active cooling will require chip layout to be considered at the design stage.     

Thermodynamic analysis of polymer-electrolyte-membrane fuel-cell performance under varying cooling conditions

The cooling capacity and cooling load of a fuel-cell cooling loop govern the operating temperature of the fuel-cell module and its electrical output, efficiency and other thermodynamic aspects. The aim of this work was to analyze the performance of a polymer-electrolyte-membrane fuel-cell (PEMFC) under changing cooling conditions. A back-iteration algorithm was employed to determine the operating temperature of a PEMFC for which thermodynamic performance models were developed for the entropy generation, exergy-destruction and second-law efficiency using an entropy-analysis method. Electrochemical equations for the calculation of the voltage, power and first-law efficiency of the cell were also formulated. A parametric study was performed to evaluate the effects of varying cooling conditions on the energy and exergy efficiency of the PEMFC. The parameters considered include the electric-current density governing the cooling load, the mass flow rate of the coolant and the external thermal resistance of the cooler, which together determine the cooling ability of the fuel-cell cooling loop. Their influences on operating temperature, voltage, power, energy and exergy efficiencies were numerically investigated. The results indicate that although the power output and exhaust heat of PEMFC is mainly dominated by the electric-current density, the impacts of the coolant's mass flow rate and the cooler's external thermal resistance on the voltage, energy and exergy efficiencies of PEMFC module can't be neglected. In the investigated ranges, the gross energy and exergy efficiencies increase with the cooler's external thermal resistance by 3.2% and 2.45%, and decrease with the increase in coolant's mass flow rate by 1.2% and 0.92%, respectively.     

冷却流量对涂层叶片冷却性能影响的数值研究

采用气热耦合数值方法研究冷却流量对热障涂层气冷涡轮叶片冷却性能的影响。分析表明：叶片的冷却效率随冷却流量的增加而增大,但增幅则逐渐下降。吸力面上,附加热障涂层的效果更好。基准工况下,温度的降幅为72.6K,冷却效率的增幅为6.5%。尾部部分区域内部冷却不足,热障涂层阻碍热量从金属表面向流体传递,金属表面温度升高,综合冷却性能下降,因此,只有配合高效的内冷技术,才能达到理想的冷却效果。     

Simulation studies on gax based Kalina cycle for both power and cooling applications

A detailed parametric analysis is carried out on both simple and GAX based combined power and cooling cycle. The effect of various parameters such as heat source temperature, refrigeration temperature, sink temperature, split ratio (refrigerant flow ratio between power and cooling systems), split factor (solution flow ratio between absorber and GAX heat exchanger) on the performance of the cycle is studied. The results of the analysis show that using the GAX heat exchanger about 20% of internal heat is recovered within the cycle. The optimum split factor is 0.8–0.9 and the split ratio is 0.5:0.5. The maximum combined thermal efficiency of 35–45% and coefficient of performance of about 0.35 is attained at the optimum conditions.     

Thermal management of solid oxide fuel cells with liquid metal

Effective thermal management of SOFCs is necessary for their long life and highly efficient operation, while the conventional method through excess air cooling is limited due to the inherently low thermal conductivity and capacity of air. In this study, a novel temperature control strategy is proposed by using liquid metal as a new kind of coolant that can work in both the stable operation stage and start-stop stage of an SOFC stack. A three-dimensional model is developed considering chemical/electrochemical reactions, mass, momentum and heat transfer processes to assess the effect of liquid metal cooling. The simulation results show that liquid metal has an excellent ability to improve the temperature uniformity and electric performance of the cell unit. The temperature difference of the cell unit cooled by air cooling is 60 K, which can be decreased to 15 K with liquid tin cooling. Furthermore, inlet air has little effect on the performance of the cell unit when liquid metal is chosen as coolant. The pumping powers of the air and liquid metal are compared at different excess air ratios and inlet velocities of liquid metal. The total pumping power consumption could be dramatically decreased when liquid metal is utilized as the coolant. Furthermore, the variations in the conductivity, heat capacity and convective resistance at different liquid metal inlet velocities are discussed.