透水混凝土强度及透水性能影响因素研究

为研究透水混凝土强度及透水性能的影响因素,该文探索水胶比、砂率、粉煤灰和硅灰掺量对透水混凝土的抗压强度和透水性能的影响,并制备出高强、高透水性能的透水混凝土;为探究废弃橡胶应用于透水混凝土材料的可行性,该文探索不同橡胶粉掺量对透水混凝土抗压强度和透水性能的影响。试验结果表明:当水胶比为0.22、砂率为6%、矿物掺合料掺量为20%时,可制备出高强高渗透性的透水混凝土;当橡胶粉替代掺量为25%时,可提高透水混凝土抗压强度,且对透水性能无较大影响。

S型城市隧道污染物传播特性数值模拟

随着城市的发展,许多新型城市隧道采取上方局部开口形式,以缓解其内部环境污染以及出口污染物集中排放的问题,同时隧道内部设置射流风机也是改善隧道内空气质量的重要措施之一。以武汉水果湖隧道为研究对象,构建了一个顶部开口和隧道内部设置射流风机的S型城市隧道三维立体模型,并运用计算流体动力学(CFD)方法,模拟分析了5种不同工况下顶部开口和射流风机对S型隧道系统流场以及污染物传播特性的影响。结果表明:单开启射流风机工况下,S型隧道内污染物CO浓度的降低幅度约为10%;而单开启顶部开口对缓解S型隧道内空气污染无积极作用;当南北双顶部开口与射流风机同时开启时,S型隧道内污染物CO浓度的降低幅度约为35%。

滨水小区内气流和颗粒物扩散的特性研究

水体水面蒸发会改变滨水小区内气流和颗粒物扩散的特性。根据某滨水小区的几何参数建立三维模型,采用FLUENT软件对滨水小区内的气流场和颗粒物浓度场进行数值模拟。结果表明:相对湿度越大,小区流场内水蒸气的质量分数越大,但其增幅有所减小;不考虑颗粒物凝并的情况下,颗粒物质量浓度与相对湿度呈正比关系;随着水体水面蒸发的不断进行,水蒸气扩散范围越来越大,建筑物周围及横向主通道内水蒸气的含量越来越高,下风向处水蒸气的质量分数较上风向处低;水蒸气在水平方向上的扩散速率大于竖直方向上的扩散速率;水体可以有效降低滨水小区内的颗粒物质量浓度,促进颗粒物扩散。

隧道及其上方城市环境污染物传播规律数值模拟研究

构建了一个含地上建筑环境、地下S形城市隧道、地上丁字形机动车道等几何结构的三维城市模型,运用计算流体力学模拟方式,首先数值分析了整体计算域流场及污染物分布特性,进而探究了环境风向及风速等影响因素。结果表明:开口贯通处对缓解下方S形隧道内污染作用明显,经南开口贯通处后污染降低约60%,但上方建筑环境则因此污染加重,西南区受影响较为突出,且在北偏东NbE=60°时将受到最大程度的渗透及污染影响,约为最低值NbE=30°时的3.5倍;随风速增加建筑环境整体污染呈下降趋势,但风速增至2.5 m/s及以上时,污染降幅有所减小,风速敏感性减弱,且西南区建筑环境南半段较北半段受风速影响的敏感性更高。

Effect of the Filling Liquid Ratio on the Thermal Performance of a Novel Thermal Diode with Wick

The application of thermal diodes,which allow heat to flow more readily in one direction than the other,is an important way to reduce energy consumption in buildings and enhance the battery heat dissipation of electric vehicles.Depending on various factors including the specific design,materials used,and operating conditions,the convective thermal diode can exhibit the best thermal rectification effect in intended applications compared to the other thermal diodes.In this study,a novel convective thermal diode with a wick was proposed based on the phase change heat transfer mechanism.This design takes advantage of both capillary forces provided by the wick and gravity to achieve enhanced unidirectional heat transfer performance for the designed convective thermal diode.The effect of the filling liquid ratio on the thermal performance of the thermal diode was experimentally investigated,which was in good agreement with the theoretical analysis.The research findings showed that with an optimal liquid filling ratio of 140%,the thermal diode with a wick can achieve a better thermal rectification ratio when subjected to a lower heating power,and the maximum thermal rectification ratio of 21.76 was experimentally achieved when the heating power of the thermal diode was 40 W.

Optimization of Dimples in Microchannel Heat Sink with Impinging Jets Part A： Mathematical Model and the Influence of Dimple Radius

With increasing heat fluxes caused by electronic components, dimples have attracted wide attention by researchers and have been applied to microchannel heat sink in modern advanced cooling technologies. In this work, the combination of dimples, impinging jets and microchannel heat sink was proposed to improve the heat transfer performance on a cooling surface with a constant heat flux 500 W/cm^2. A mathematical model was advanced for numerically analyzing the fluid flow and heat transfer characteristics of a microchannel heat sink with impinging jets and dimples(MHSIJD), and the velocity distribution, pressure drop, and thermal performance of MHSIJD were analyzed by varying the radii of dimples. The results showed that the combination of dimples and MHSIJ can achieve excellent heat transfer performance; for the MHSIJD model in this work, the maximum and average temperatures can be as low as 320 K and 305 K, respectively when mass flow rate is 30 g/s; when dimple radius is larger than 0.195 mm, both the heat transfer coefficient and the overall performance h/ΔP of MHSIJD are higher than those of MHSIJ.

Optimization of Dimples in Microchannel Heat Sink with Impinging Jets-Part B： the Influences of Dimple Height and Arrangement

The combination of a microchannel heat sink with impinging jets and dimples(MHSIJD) can effectively improve the flow and heat transfer performance on the cooling surface of electronic devices with very high heat fluxes. Based on the previous work by analysing the effect of dimple radius on the overall performance of MHSIJD, the effects of dimple height and arrangement were numerically analysed. The velocity distribution, pressure drop, and thermal performance of MHSIJD under various dimple heights and arrangements were presented. The results showed that: MHSIJD with higher dimples had better overall performance with dimple radius being fixed; creating a mismatch between the impinging hole and dimple can solve the issue caused by the drift phenomenon; the mismatch between the impinging hole and dimple did not exhibit better overall performance than a well-matched design.

Influence of Dust Accumulation on the Solar Reflectivity of a Linear Fresnel Reflector

Solar reflecting and heat collecting systems are generally operated outdoors year-round,and the optical performance of the reflector is reduced by dust accumulation on the surface.In this study,the dust accumulation on a linear Fresnel reflector was investigated.The relative reflectivity of the mirror before and after dust accumulation and the physical and chemical characteristics of the dust were measured using an ultraviolet spectrophotometer,scanning electron microscope,and X-ray diffractometer.The results showed that the dust density on the mirror increased,and the relative reflectivity decreased with an increase in the dust accumulation time.During 48 days of dust accumulation,the average relative reflectivity decreased 9.4%for a 1 g/m^(2)increase in the dust density.Additionally,the dust density on the mirror increased while the relative reflectivity decreased with a decrease in the mirror tilt angle.The rate of decrease of the relative reflectivity was higher for the aluminum mirror than that of the silver mirror after dust accumulation.The main component of the dust particles in the test area was SiO_(2),and the particle size range of the dust was 0.9 um to 87µm.According to the physical and chemical properties of the dust and the shielding effect of the dust on the mirror,a model to predict the influence of natural dust accumulation on the relative reflectivity of the linear Fresnel reflector was proposed.The predicted results deviated about 1%from the test results.

Analysis of the Light Concentration Loss of a Fresnel CPV/T System after Dust Accumulation

Dust accumulation is one of the reasons for the performance degradation of concentrating photovoltaic and thermal(CPV/T) systems due to the deposition of dust particles with different compositions, shapes, sizes, and masses. In this work, an optical model was developed to investigate the influence of the particle size, diameter, shape, and deposition density on the light concentration efficiency, using the Monte Carlo raytracing(MCRT) method in the Tracepro software. The triangular particles had a larger influence on the light ray deflection and energy flux degradation than the circular and square particles. An average increase in the dust density of 1 g/m^(2) decreased the light concentration efficiency of particles with sizes smaller than 50 μm and 60 μm by 3.31% and 3.26%, respectively. Furthermore, the effect of the incidence angle on the light concentration efficiency was considered at an angle less than 2°.

A Model to Evaluate the Device-Level Performance of Thermoelectric Cooler with Thomson Effect Considered

In this paper,a one-dimensional thermodynamic model was developed to evaluate the device-level performance of thermoelectric cooler(TEC)with the Thomson effect,contact resistance,gap heat leakage,heat sink,and heat load taken into account.The model was generalized and simplified by introducing dimensionless parameters.Experimental measurements showed good agreement with analytical results.The parametric analysis indicated that the influence of the Thomson effect on cooling capacity continued to expand with increasing current,while the effect on COP hardly changed with current.Low thermal contact resistance was beneficial to obtain lower hot-junction temperature,which can even reduce 2 K compared with the electrical contact resistance in the case study.The gap heat leakage was a negative factor affecting the cooling performance.When the thermal resistance of the heat sink was small,the negative effect of heat leakage on performance would be further enlarged.The enhancement of heat load temperature would increase the cooling power of the TEC.For example,an increase of 5 K in heat load can increase the cooling capacity by about 4%.However,once the current exceeded the optimum value,the raising effect on the cooling power would be weakened.The research can provide an analytical approach for the designer to perform trade studies to optimize the TEC system.

A Strategy to Reduce the Peak Temperature of the Chip Working under Dynamic Power Using the Transient Cooling Effect of the Thin-Film Thermoelectric Cooler

The thin-film thermoelectric cooler(TEC)is a promising solid-state heat pump that can remove the high local heat flux of chips utilizing the Peltier effect.When an electric current pulse is applied to the thin-film TEC,the TEC can achieve an instantaneous lower temperature compared to that created by a steady current.In this paper,we developed a novel strategy to reduce the peak temperature of the chip working under dynamic power,thus making the semiconductor chip operate reliably and efficiently.A three-dimensional numerical model was built to study the transient cooling performance of the thin-film TEC on chips.The effects of parameters,such as the current pulse,the heat flux,the thermoelement length,the number of thermoelements,and the contact resistance on the performance of the thin-film TEC,were investigated.The results showed that when a current pulse of 0.6 A was applied to the thin-film TEC before the peak power of the chip,the peak temperature of the chip was reduced by more than 10℃,making the thin-film thermoelectric cooler a promising technology for the temperature control of modern chips with high peak powers.