R290微细通道流动沸腾摩擦压降特性研究

研究R290在管径为3、2和1 mm的水平不锈钢微细通道内,质量流率为73～505 kg/(m~2·s)、热流密度为12. 74～66. 05 k W/m~2、饱和温度为-10～25℃、干度为0～1的范围内的摩擦压降特性,分析R290流动沸腾过程中的摩擦压降变化。结果表明:换热管径尺度微型化使相同条件下的管内压降剧烈上升,质量流率对压降的影响最显著;热流密度的增加对压降的影响很小,几乎为零; R290不同饱和温度物性的改变是造成其不同温度时压降特性差异的主要原因,随着饱和温度的升高,摩擦压降变小;压降随着干度的变化在某个干度存在极值。     

微细圆管内CO2两相沸腾换热实验研究

本文对CO_2在水平微细管内流动沸腾特性进行实验研究。实验结果表明:热流密度增加对强化核态沸腾换热和高干度区域流型转变具有显著影响,随着热流密度的增加换热系数增加,对摩擦压降影响很小;质量流率对于换热系数的影响较小,但随着质量流率的增加摩擦压降大幅增加,质量流率的大小直接决定了换热过程所经历流态;饱和温度升高换热系数相应升高,摩擦压降减小,且对流态转变特性有重要影响。在同样工况下摩擦压降最大值先于换热系数最大值出现,理论分析采用的流态形式与实际CO_2管内流动流动沸腾换热流态基本一致。     

5mm小管径内R290凝结换热研究

对R290在5 mm小管径内的凝结换热特性进行了实验。实验工况:热流密度5～10 kW/m~2、质量流率180～250 kg/(m~2·s)、饱和温度40～55℃、管径5 mm。研究了质量流速、饱和温度、热流密度及管型对管内换热系数的影响。研究表明:换热系数随质量流率的增大而增大,随饱和温度的上升而下降,且在干度较大区域,影响更加明显;换热系数随热流密度的增大而增大,且存在最佳热流密度使其达到最大值;相同工况下,内肋管换热系数大于光管,在质量流速低、干度小的区域内肋管的强化效果更优。     

水平光滑细管内R32的冷凝换热特性

对流体R32在内径2 mm的水平光滑圆管内的冷凝换热特性进行实验研究。实验设定的流体饱和温度为35、40和45℃,质量流量为100~500 kg/(m2·s),热流密度7~28 k W/m2。实验获得R32在不同工况下的冷凝换热系数和摩擦压降梯度。发现其换热系数随质量流量增加而增大,随饱和温度提高而减小。入口干度和热流密度对其影响不大。摩擦压降梯度随质量流量增加而增加,相同质量流量下,随饱和温度升高而降低。并将该次实验值与其他经典换热模型和压降模型进行对比分析,发现Baird模型对该次实验的换热系数预测较好,Müller-Heck模型和Chisholm模型对R32的摩擦压降预测较好。     

水平微细管内CO_2流动沸腾换热与干涸特性研究

为分析饱和温度、热流密度、质量流率和管径对CO_2流动沸腾换热特性和干涸特性的影响,对水平微细管内CO_2流动沸腾换热进行了实验研究。实验工况:饱和温度-40～25℃,热流密度5～40 kW/m~2,质量流率180～1 400 kg/(m~2·s),管径0. 50、1. 0和1. 5 mm。实验结果表明:热流密度的增加显著影响核态沸腾换热,加快干涸发展进程的同时,降低干涸起始干度;换热系数受质量流量的影响较小,但质量流率的增加会降低干涸起始干度,干涸后的换热系数有所增加;不同饱和温度下换热特性差异的主要原因是CO_2的热物性受饱和温度的影响较大,饱和温度升高后干涸起始干度具有降低的趋势,且干涸后换热系数下降更为剧烈;在符合微尺度效应的前提下,管径的减小会极大地提高换热系数,同时降低干涸起始干度,干涸后换热系数下降剧烈。     

水平内螺纹铜管内R404A的冷凝压降特性

对R404A在内螺纹铜管内冷凝压降进行了实验研究。为了给R404A在小管径换热器中应用的可行性提供依据,研究缩小管径带来压降上升的特性,设置了不同的影响因素来研究R404A在内螺纹铜管中的冷凝压降特性。实验工况为:饱和温度35～45℃,质流密度200～900 kg/(m~2·s),热流密度10 kW/m~2,入口干度0.1～0.9,内螺纹铜管管径分别为5、7和9.52 mm。实验结果表明:饱和温度对冷凝压降的影响主要集中在较高的质流密度区间,压降随饱和温度的升高而降低;压降随质流密度的增大而上升,随管径的减小而增大;随着干度值的降低,冷凝压降从一个峰值开始逐渐下降,其中5 mm管的压降下降速率最大;通过对比相对传热系数得出3种内螺纹管中,5 mm管的综合性能更好。     

微细通道内R290流动沸腾换热特性实验研究

对R290制冷剂在微细通道内的流动沸腾换热特性进行了实验研究。研究管径分别为1和2 mm,热流密度为20~65 k W/m~2,质量流率为100~200 kg/m~2·s,饱和温度为15和25℃,干度范围为0.1~0.9。通过实验数据分析管径、热流密度、质量流率、饱和温度对流动沸腾换热的影响。结果表明:随着管径的下降,换热系数呈现出大幅上升的趋势,其平均增幅为31%;随着热流密度的上升,换热系数呈现出大幅上升的趋势,其平均增幅达到了131%;随着质量流率的上升,换热系数呈现出小幅上升的趋势,其平均增幅为14%;随着饱和温度的上升,大部分换热系数呈现出小幅上升的趋势,其平均增幅为12.6%。     

3mm微细通道内R134A沸腾换热压降特性研究

环保型制冷剂R134A作为R22的替代品已广泛应用于各种制冷技术,在不断探索更完美的制冷剂替代品过程中,微细通道换热技术也逐渐成为近些年的研究热点。为了研究R134A在3 mm紫铜管内沸腾换热过程中的压降特性,在饱和温度为0～20℃、热流密度为5～10 kW/m~2、干度变化在0～1、质量流率在300～500 kg/(m~2·s)的实验工况下进行实验,通过对压力、温度和干度等重要物理参数的控制和试验数据的分析,得出以下结论:压降在相同干度区随质量流率的增大而增大,但在高干度区和低干度区的增幅不同;干度对压降的影响很大程度上与沸腾过程中的流动型态发生变化相关;饱和温度与压降的关系主要呈现负相关;而热流密度在压降影响中的占比则是很小的一部分。     

5 mm水平强化管内R410A/润滑油流动冷凝压降实验研究

在质量流率200～400 kg/(m~2·s)、干度0.1～0.9、冷凝温度40℃及润滑油浓度0～5%的工况范围内,对5 mm水平强化管内R410A/润滑油混合物流动冷凝压降特性进行了实验研究,分析不同质量流率、干度和润滑油质量分数对冷凝压降特性的影响。实验结果表明:摩擦压降随着质量流率和干度的增大而增大;润滑油的存在对压降特性具有重要影响,在中低干度区域摩擦压降和油影响因子随着润滑油浓度的增大而减小,在高干度区域摩擦压降和油影响因子随着润滑油浓度的增大而增大。     

水平微细管道内R717沸腾换热特性研究

搭建了氨(R717)沸腾换热测试台,对内径3 mm水平光管内R717的沸腾换热特性进行了测试,分析热流密度、干度、饱和温度及质量流率对沸腾换热及换热方式的影响。实验热流密度15～40 kW/m~2,质量流率40～160 kg/(m~2·s),饱和温度-5、0和5℃,干度0.1～0.9。结果表明:在氨制冷剂管内沸腾换热的过程中,质量流率过低和热流密度过高会导致干涸传热恶化,换热形式由核态沸腾换热向气态氨制冷剂强制对流换热转变,同时也影响干涸的起始干度;在干涸发生前,沸腾换热系数随着干度的增加而增大,逐渐达到峰值;在干涸发生后,传热恶化导致换热系数急剧降低;饱和温度升高会加快核态沸腾气泡生成速率,强化沸腾换热,但干涸的起始干度随着饱和温度升高而降低。     

Ash-forming elements in four Scandinavian wood species. Part 1: Summer harvest

Woody biomass in Finland and Sweden comprises mainly four wood species: spruce, pine, birch and aspen. To study the ash, which may cause problems for the combustion device, one tree of each species were cut down and prepared for comparisons with fuel samples. Well-defined samples of wood, bark and foliage were analyzed on 11 ash-forming elements: Si, Al, Fe, Ca, Mg, Mn, Na, K, P, S and Cl. The ash content in the wood tissues (0.2–0.7%) was low compared to the ash content in the bark tissues (1.9–6.4%) and the foliage (2.4–7.7%). The woods’ content of ash-forming elements was consequently low; the highest contents were of Ca (410–1340 ppm) and K (200–1310), followed by Mg (70–290), Mn (15–240) and P (0–350). Present in the wood was also Si (50–190), S (50–200) and Cl (30–110). The bark tissues showed much higher element contents; Ca (4800–19,100 ppm) and K (1600–6400) were the dominating elements, followed by Mg (210–2400), P (210–1200), Mn (110–1100) and S (310–750), but the Cl contents (40–330) were only moderately higher in the bark than in the wood. The young foliage (shoots and deciduous leaves) had the highest K (7100–25,000 ppm), P (1600–5300) and S (1100–2600) contents of all tissues, while the shoots of spruce had the highest Cl contents (820–1360) and its needles the highest Si content (5000–11,300). This paper presented a new approach in fuel characterization: the method excludes the presence of impurities, and focus on different categories of plant tissues. This made it possible to discuss the contents of ash element in a wide spectrum of fuel-types, which are of large importance for the energy production in Finland and Sweden.     

Contents in Volume 23,2014

<正>~~     

NEXT GENERATION ENGINEERED MATERIALS FOR ULTRA SUPERCRITICAL STEAM TURBINES

正1 ABSTRACT To reduce the effect of global warming on our climate,the levels of CO2emissions should be reduced.One way to do this is to increase the efficiency of electricity production from fossil fuels.This will in turn reduce the amount of CO2emissions for a given power output.Using US practice for efficiency calculations,then a move from a typical US plant running at 37%efficiency to a 760℃/38.5 MPa(1 400/5 580 psi)plant running at 48%efficiency would reduce CO2emissions by 170kg/MW.hr or 25%.     

Performance assessment of some ice TES systems

In this paper, a performance assessment of four main types of ice storage techniques for space cooling purposes, namely ice slurry systems, ice-on-coil systems (both internal and external melt), and encapsulated ice systems is conducted. A detailed analysis, coupled with a case study based on the literature data, follows. The ice making techniques are compared on the basis of energy and exergy performance criteria including charging, discharging and storage efficiencies, which make up the ice storage and retrieval process. Losses due to heat leakage and irreversibilities from entropy generation are included. A vapor-compression refrigeration cycle with R134a as the working fluid provides the cooling load, while the analysis is performed in both a full storage and partial storage process, with comparisons between these two. In the case of full storage, the energy efficiencies associated with the charging and discharging processes are well over 98% in all cases, while the exergy efficiencies ranged from 46% to 76% for the charging cycle and 18% to 24% for the discharging cycle. For the partial storage systems, all energy and exergy efficiencies were slightly less than that for full storage, due to the increasing effect wall heat leakage has on the decreased storage volume and load. The results show that energy analyses alone do not provide much useful insight into system behavior, since the vast majority of losses in all processes are a result of entropy generation which results from system irreversibilities.     

Aerodynamic performance effects of leading-edge geometry in gas-turbine blades

The purpose of this paper is to illustrate the advantages of the direct surface-curvature distribution blade-design method, originally proposed by Korakianitis, for the leading-edge design of turbine blades, and by extension for other types of airfoil shapes. The leading edge shape is critical in the blade design process, and it is quite difficult to completely control with inverse, semi-inverse or other direct-design methods. The blade-design method is briefly reviewed, and then the effort is concentrated on smoothly blending the leading edge shape (circle or ellipse, etc.) with the main part of the blade surface, in a manner that avoids leading-edge flow-disturbance and flow-separation regions. Specifically in the leading edge region we return to the second-order (parabolic) construction line coupled with a revised smoothing equation between the leading-edge shape and the main part of the blade. The Hodson–Dominy blade has been used as an example to show the ability of this blade-design method to remove leading-edge separation bubbles in gas turbine blades and other airfoil shapes that have very sharp changes in curvature near the leading edge. An additional gas turbine blade example has been used to illustrate the ability of this method to design leading edge shapes that avoid leading-edge separation bubbles at off-design conditions. This gas turbine blade example has inlet flow angle 0°, outlet flow angle −64.3°, and tangential lift coefficient 1.045, in a region of parameters where the leading edge shape is critical for the overall blade performance. Computed results at incidences of −10°,   −5°,   +5°,   +10° are used to illustrate the complete removal of leading edge flow-disturbance regions, thus minimizing the possibility of leading-edge separation bubbles, while concurrently minimizing the stagnation pressure drop from inlet to outlet. These results using two difficult example cases of leading edge geometries illustrate the superiority and utility of this blade-design method when compared with other direct or inverse blade-design methods.     

Effect of co-cultivation of Chlamydomonas reinhardtii with Azotobacter chroococcum on hydrogen production

Chlamydomonas reinhardtii cc124 and Azotobacter chroococcum bacteria were co-cultured with a series of volume ratios and under a variety of light densities to determine the optimal culture conditions and to investigate the mechanism by which co-cultivation improves H₂ yield. The results demonstrated that the optimal culture conditions for the highest H₂ production of the combined system were a 1:40 vol ratio of bacterial cultures to algal cultures under 200 μE m^?2 s^?1. Under these conditions, the maximal H₂ yield was 255 μmol mg^?1 Chl, which was approximately 15.9-fold of the control. The reasons for the improvement in H₂ yield included decreased O₂ content, enhanced algal growth, and increased H₂ase activity and starch content of the combined system.     

Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions

Natural gas is a fossil fuel that has been used and investigated extensively for use in spark-ignition (SI) and compression-ignition (CI) engines. Compared with conventional gasoline engines, SI engines using natural gas can run at higher compression ratios, thus producing higher thermal efficiencies but also increased nitrogen oxide (NO_x) emissions, while producing lower emissions of carbon dioxide (CO₂), unburned hydrocarbons (HC) and carbon monoxide (CO). These engines also produce relatively less power than gasoline-fueled engines because of the convergence of one or more of three factors: a reduction in volumetric efficiency due to natural-gas injection in the intake manifold; the lower stoichiometric fuel/air ratio of natural gas compared to gasoline; and the lower equivalence ratio at which these engines may be run in order to reduce NO_x emissions. High NO_x emissions, especially at high loads, reduce with exhaust gas recirculation (EGR). However, EGR rates above a maximum value result in misfire and erratic engine operation. Hydrogen gas addition increases this EGR threshold significantly. In addition, hydrogen increases the flame speed of the natural gas-hydrogen mixture. Power levels can be increased with supercharging or turbocharging and intercooling. Natural gas is used to power CI engines via the dual-fuel mode, where a high-cetane fuel is injected along with the natural gas in order to provide a source of ignition for the charge. Thermal efficiency levels compared with normal diesel-fueled CI-engine operation are generally maintained with dual-fuel operation, and smoke levels are reduced significantly. At the same time, lower NO_x and CO₂ emissions, as well as higher HC and CO emissions compared with normal CI-engine operation at low and intermediate loads are recorded. These trends are caused by the low charge temperature and increased ignition delay, resulting in low combustion temperatures. Another factor is insufficient penetration and distribution of the pilot fuel in the charge, resulting in a lack of ignition centers. EGR admission at low and intermediate loads increases combustion temperatures, lowering unburned HC and CO emissions. Larger pilot fuel quantities at these load levels and hydrogen gas addition can also help increase combustion efficiency. Power output is lower at certain conditions than diesel-fueled engines, for reasons similar to those affecting power output of SI engines. In both cases the power output can be maintained with direct injection. Overall, natural gas can be used in both engine types; however further refinement and optimization of engines and fuel-injection systems is needed.     

Controls on the Karaha–Telaga Bodas geothermal reservoir,Indonesia

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.     

Hydrogen production by steam-gasification of carbonaceous materials using concentrated solar energy – V. Reactor modeling,optimization, and scale-up

A chemical reactor for the steam-gasification of carbonaceous particles (e.g. coal, coke) is considered for using concentrated solar radiation as the energy source of high-temperature process heat. A two-phase reactor model that couples radiative, convective, and conductive heat transfer to the chemical kinetics is applied to optimize the reactor geometrical configuration and operational parameters (feedstock's initial particle size, feeding rates, and solar power input) for maximum reaction extent and solar-to-chemical energy conversion efficiency of a 5 kW prototype reactor and its scale-up to 300 kW. For the 300 kW reactor, complete reaction extent is predicted for an initial feedstock particle size up to 35 μm at residence times of less than 10 s and peak temperatures of 1818 K, yielding high-quality syngas with a calorific content that has been solar-upgraded by 19% over that of the petcoke gasified.     

Assessment methods of material ageing using Sdobyrev''s creep rupture model

The physical aspects of the activation energy, in higher and high temperatures, of the metal creep process were examined. The research results of creep-rupture in a uniaxial stress state and the criterion of creep-rupture in biaxial stress states, at two temperatures, are then presented. For these studies creep-rupture, taking case iron as an example the energy and pseudoenergy activation was determined. For complex stress states the criterion of creep-rupture was taken to be Sdobyrev's, i.e. σ_red = σ₁ β + (1 − β)σ_i, where: σ₁-maximal principal stress, σ_i-stress intensity, β-material constant (at variable temperature β = β(T)). The methods of assessment of the material ageing grade are given in percentages of ageing of new material in the following mechanical properties: 1) creep strength in uniaxial stress state, 2) activation energy in uniaxial stress state, 3) criterion creep strength in complex stress states, 4) activation pseudoenergy in complex stress states. The methods 1) and 3) are the relatively simplest because they result from experimental investigations only at nominal temperature of the structure work, however, for methods 2) and 4) it is necessary to perform the experimental investigations at least at two temperatures.