Cooling performance and efficiency of night sky radiators

A theoretical equation was derived to predict the surface temperature of night sky radiators as a function of power, Q, from radiator properties and sky conditions. The power of an ideal radiator, R_I, which is perfectly black in the 8–14 μm atmospheric window, perfectly reflective outside the window and has a transparent cover was used to define radiator efficiency as . Plots of against , where T_a and T_s are the air and radiator surface temperatures, were primarily dependent on radiator properties and only slightly on environmental conditions. These curves provide a means to compare different radiators and to aid in the design and prediction of performance of night sky radiators.Performance measurements were obtained with three night sky radiators constructed with surfaces of aluminum, white TiO₂ paint, and black paint covered with polyethylene. Similar measurements were also obtained with a fourth radiator that had an uncovered black paint surface. Depressions below air temperature for Q = 0 of 6 and 2.5°C were observed with the aluminum and the black-uncovered radiators at vapor pressures of 0.5 and 2 kPa, respectively. Depressions of the white and black paint covered radiators were about 11 and 6°C at vapor pressures of 0.5 and 2 kPa. Fair agreement with theory was achieved. Calculations of cooling losses from various radiators for the Phoenix, AZ, climate were made. Generally the losses were too small for practical use in July and August, but had potential for other months. The procedure presented can be used to predict the feasibility of radiator use for other application temperatures, climates and radiator properties.     

Cooling by evaporation of flowing water over a hollow roof

This paper discusses the thermal modelling of the technique for cooling buildings by means of open evaporation of water over the roof. A comparative study of cooling by means of (i) a roof pond, (ii) a water spray and (iii) moving water over the roof is presented. The influences of parameters such as wind speed, relative humidity and water flow velocity on the performance of the system are numerically examined.     

Performance of a heat pipe assisted night sky radiative cooler

A passive radiative cooling system was designed, constructed and tested under clear skies. This refrigerator operates by losing heat to the night sky through infrared (i.r.) radiation emission. It consists of a radiator, an array of heat pipe elements and a cold chamber. The heat pipe elements are so arranged that they act as thermal diodes, transferring heat from the cold chamber to the radiator. Performance tests show that the system has a cooling capacity of 628 kJ/m² per night with a sky coefficient of performance of 0.26. The lowest temperature attained in the cold chamber was 12.8°C for an ambient temperature of 20°C. The overall results indicate that the system has a great potential for providing a cold storage facility in developing countries and in remote areas.     

New formulae for the equivalent night sky emissivity

The knowledge of the equivalent sky emissivity is important in computations of radiation cooling systems, as well as in the evaluation of the thermal losses in solar collectors. A new empirical formula has been developed which permits the determination of the equivalent nocturnal sky emissivity as a function of the site's altitude, the ambient temperature, the relative humidity and the degree of cloudiness, of mist or of haze present at the time. The formula has been found to agree well with the equivalent nocturnal sky temperature values measured at various Venezuelan sites and other places. Comparison is made with formulae from other investigators. A modification of Brunt's. Elsasser's and Angström's formulae for clear nocturnal sky is also suggested.     

Cooling load reduction of building by seasonal nocturnal cooling water from thermosyphon heat pipe radiator

In this study, a concept of using thermosyphon heat pipe to extract heat from water in a storage tank to generate cooling water was proposed. Heat pipe condenser was attached with an aluminum plate and acted as a thermal radiator while its evaporator was dipped in the water storage tank. Cooling water in the tank could be produced during the nighttime and used to serve the cooling load in a room during the daytime. A heat transfer model to calculate the water temperature and the room temperature during both the nighttime and daytime was developed. The input data were ambient temperature, dew point temperature, area of the radiator, volume of cooling water and room cooling load. The experiment was setup to verify the heat transfer model. A 9.0 m² tested room with six cooling coils, each of 0.87 m² was installed at the ceiling, was constructed along with the 1.0 m³ water storage tank. A 500–2000 W adjustable heater was taken as an artificial load inside the room. A 6.36 m² radiator is installed on a 45° tilting roof of the tested room. The simulated results agreed very well with those of the experimental data. With the developed model, a simulation to find the sizing of the radiator area and the volume of cooling water for cooling water production during winter of Chiang Mai, Thailand was carried out. The cooling water was used for cooling during summer in an air‐conditioned room with different cooling loads. The parameters in terms of room temperature, radiator area, volume of cooling water, cooling load and UA of cooling coil were considered to carry out the percent of cooling load reduction. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass transfer from hot dry rock to water flowing through a circular fracture

Numerical simulation of heat and mass transfer from hot dry rock to flowing water in a circular fracture was conducted to estimate the concentration of the dissolved silica at the production well. The local mass transfer coefficients between the rock and the fluid, obtained by using the electrochemical method during the laboratory experiments, were used in the calculation. The results of the simulation indicate that the concentration of the silica at the production well increases with increasing distance between the injection and the production wells, increasing fracture diameter and increasing pressure. It decreases with increases in flow rate and porosity in the fracture. The concentration of silica at the production well first increases and then decreases with increasing initial rock temperature due to the decrease in the solubility of silica at high temperatures. Finally, the mass transfer by forced convection of flowing water plays an important role in the variation in the concentration of silica in water, and the assumption that the concentration of dissolved silica is equal to that at the fracture surface is not valid for estimation of the concentration at the production well.     

Heat transfer to water flowing turbulently through a rectangular duct with asymmetric heating

Experimental, fully established friction factors and Nusselt numbers are reported for the turbulent flow of water in a 2:1 rectangular duct. The measured friction factors are in good agreement with the Blasius prediction if the Reynolds number proposed by Jones is used. The measured Nusselt values are in good agreement with the results of Novotny et al. for the turbulent flow of air if the results are corrected for the effects of the Prandtl number.The influence of asymmetric heating on the average Nusselt number was found to be much less for water as a working fluid than for air. This difference is explained by the fact that water has a higher Prandtl number than air.     

A thermosyphon water cooler with a parallel, flat-plate radiator

The performance of a thermosyphon water cooler with a parallel, flat-plate radiator has been studied, both theoretically and experimentally. Some theoretical predictions of water temperatures in the storage tank and of the cooling intensity have been obtained from energy and momentum balances. Theoretical values are in agreement with experimental data. This research will be useful in assessing the combination of solar heating and radiative cooling in a single device.     

夜空辐射换热器冷量采集性能的试验研究

以内蒙古工业大学土木馆屋顶上安装的夜空辐射换热器为研究对象,建立了夜空辐射换热器冷量采集的理论模型,对表面未涂油漆和涂有白色油漆的换热器的表面温度及进出口温度进行了试验测试。结果表明,涂有白色油漆的辐射换热器较未涂油漆的辐射换热器的空气降温效果显著;换热器壁面温度随换热器长度的增加而降低,温度变化率由大变小;天气对换热器冷量采集性能有很大影响。此外,计算了换热器的冷量采集功率和效率。     

Optimization of a central-heating radiator

An approximate analytical model has been used to evaluate the optimum dimensions of a central-heating radiator. The radiator problem is divided into three one-dimensional fin problems and then the temperature distributions within the fins and heat-transfer rate from the radiator are obtained analytically. The optimum geometry maximizing the heat-transfer rate for a given radiator volume and the geometrical constraints associated with production techniques, and thermal constraints have been found. The effects of geometrical and thermal parameters on the radiator’s performance are presented.     

新型复合式冷却器的研制

主要从结构设计、传热原理和试验研究等方面,对电力机车复合式冷却器的研制情况进行了比较全面的介绍。     

Forced—Flow Convection for Liquid Methanol Flowing through Microchannels

Experiments were conducted to investigate the single phase forced-flow convection of methanol flowing through microchannels with rectangular cross-section. The fully-developed turbulent convection regime was found to be initiated at aboutRe=1000 ∼ 1500. The fully developed turbulent heat transfer can be predicted by the well-known Dittus-Boelter correlation with mere modification of the original empirical constant coefficient 0.023 to 0.00805. The transition and laminar heat transfer behaviors in microchannels are highly peculiar and complicated, and heavily affected by liquid temperature, velocity and microchannel size.     

工业用水的冷却原理

当今工业冷却水的供水系统一般可分为直流式、循环式和混合式3种。为了重复利用吸热后的水以节约水资源,常采用循环冷却水系统。由于水冷却的重要性以及目前水冷却存在的一些问题,探求新型的水冷却原理。     

Analytical solutions of triple-diffusive magnetohydrodynamic fluid flowing through a stretching/shrinking sheet

The aim of this article is to investigate the dual nature solutions of the triple diffusive magnetohydrodynamic flow due to stretching/shrinking surfaces. The system of nonlinear partial differential equations is transformed into nonlinear ordinary differential equations with the help of compatible transforms. Analytical dual solutions are obtained for every unknown velocity, temperature, and concentration profile in terms of known physical parameters. Heat and mass transfer analyses have been carried out in the presence of convective boundary conditions. The graphic interpretation of the possible dual solutions of dimensionless velocity, temperature, concentration, skin-friction coefficient, and Nusselt and Sherwood numbers is analyzed under the influence of different known physical parameters. The obtained results are validated against previously published results for a special case of the problem.     

Condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes

This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000 mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7 mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2 mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46 mm. Similarly, the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The test conditions are performed at saturation temperatures of 40–50 °C, heat fluxes of 5–10 kW/m², mass fluxes of 200–700 kg/m² s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes.     

Cooling of Laser Slab by Forced Convection through a Heat Sink

The present study addresses a novel cooling scheme for the high-power solid-state laser slab. The scheme cools the laser slab by forced convection in a narrow channel through a heat sink. Numerical simulations were conducted to investigate the thermal effects of a Nd:YAG laser slab for heat sinks of different materials, including the undoped YAG, sapphire, and diamond. The results show that the convective heat transfer coefficient is non-uniform along the fluid flow direction due to the thermal entrance effect, causing a non-uniform temperature distribution in the slab. The heat sink lying between the coolant fluid and the pumped surface of the slab works to alleviate this non-uniformity and consequently improve the thermal stress distribution and reduce the maximum thermal stress of the slab. The diamond heat sink was found to be effective in reducing both the highest temperature and the maximum thermal stress; the sapphire heat sink was able to reduce the maximum thermal stress but not as effective in reducing the highest temperature; and the undoped YAG heat sink reduced the maximum thermal stress but tended to increase the highest temperature. Therefore, cooling with the diamond heat sink is most effective, and that with the sapphire heat sink follows; cooling with the undoped YAG heat sink may not apply if the highest temperature is a concern.     

Hydrogen production from coal gasification in supercritical water with a continuous flowing system

The technology of supercritical water gasification can convert coal to hydrogen-rich gaseous product efficiently and cleanly. A novel continuous-flow system for coal gasification in supercritical water was developed successfully in State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF). The experimental device was designed for the temperature up to 800 °C and the pressure up to 30 MPa. The gasification characteristics of coal were investigated within the experimental condition range of temperature at 650–800 °C, pressure at 23–27 MPa and flow rate from 3 kg h⁻¹ to 7 kg h⁻¹. K₂CO₃ and Raney-Ni were used as catalyst and H₂O₂ as oxidant. The effects of main operation parameters (temperature, pressure, flow rate, catalyst, oxidant, concentration of coal slurry) upon gasification were carried out. The slurry of 16 wt% coal + 1.5 wt% CMC was successfully transported into the reactor and continuously gasified in supercritical water in the system. The hydrogen fraction reached up to 72.85%. The experimental results demonstrate the bright future of efficient and clean conversion of coal.     

Preliminary study of polymer melt rheological behavior flowing through micro-channels

Determination of melt rheological behavior within micro-structured geometry is very important for the accurate simulation modeling of micro-molding. Yet investigation of micro-melt rheology is difficult due to the lack of commercial equipment. In this study, melt viscosity measurement system for polymer melt flowing through micro-channel was established using a micro-channel mold operated at a mold temperature as high as the melt temperature. Form measured pressures and volumetric flow rate both capillary flow model and slit flow model were used for the calculation of viscosity utilizing Rabinowitsch and Walters corrections. It was found that the measured viscosity values in the test ranges are significantly lower (about 30% to 80% lower) than those obtained from traditional capillary rheometer no matter what capillary flow model or slit model are used for analysis. As micro-channel size decreases, the percentage reduction in viscosity also increases. This indicates microscopic scale melt rheological behavior is different from that of macroscopic scale and that current simulation packages are not suitable for micro-molding simulation without considering this difference.     

太阳能低温热水地板辐射采暖

随着塑料工业的飞速发展 ,开发出了具有高柔韧性、抗老化、耐高温、耐高压的塑料软管。从 70年代开始 ,塑料管低温热水地板辐射采暖技术在北美、欧洲等发达国家及日本、韩国等得到了广泛应用。我国已引进该项技术 ,并开发出了国产的这种塑料管材。北京亚运村康乐宫、沈阳夏宫等国内一批大型公共建筑已成功使用了地板辐射采暖技术。节能和提高房间的热舒适度是地板辐射采暖的两大优点。地板辐射采暖的热媒是低温热水 ,从而使利用太阳集热器作为热源成为可能。在建设部制定的《中国“住宅阳光计划”纲要》(草稿 )中提出 :根据当前太阳能技术与…     

Modelling of a porous flowing electrolyte layer in a flowing electrolyte direct-methanol fuel cell

The flowing electrolyte-direct methanol fuel cell is a developing technology that may have practical uses in the future. Its main advantage over a direct methanol fuel cell is that it limits methanol crossover using a flowing electrolyte layer. The flowing electrolyte layer (or flowing electrolyte channel) involves an ion-conducting fluid that allows protons to be transported from the anode to the cathode, and flows through a porous material to wash away crossed-over methanol. In this study, the flowing electrolyte layer is modelled as a porous domain in ANSYS CFX. General flow behaviour and the effects of volume flux, channel thickness, and porous material properties are investigated. It is found that the flow has a flattened velocity profile with thin boundary layers that are virtually independent of volume flux and channel thickness. The pressure drop is mainly dependent on the volume flux and the permeability. It is recommended that cell performance could be improved by using a flowing electrolyte channel that is thinner, and selecting a sufficiently high volume flux and a sufficiently permeable porous material to achieve an optimal combination of pressure drop and methanol removal characteristics.