Experimental investigations on single-absorber solar air heaters

Flat, corrugated and vee-corrugated solar air heaters having a single absorber are designed, fabricated and tested. The rear plate in these collectors is replaced by rigid synthetic foam. It is found that the vee-corrugated absorbers give the best performance. The efficiency of the commercially corrugated absorber is about 5% lower than that of the vee-corrugated absorber. Correlations for predicting forced convection heat transfer coefficients for these solar air heaters have been obtained. The analytical model for these air heaters has also been developed.     

Experimental investigations on showerhead cooling on a blunt body

In modern gas turbines, the turbine airfoil leading edge is currently protected from the hot gases by specific film cooling schemes, so-called showerhead cooling. The present paper shows an experimental study of different showerhead cooling geometries on a blunt body. For these tests, TLC (thermochromatic liquid crystals) have been used for measuring the film cooling performance and the heat transfer. Detailed experimental results for the aerodynamics, the film cooling effectiveness and the heat transfer enhancement are presented for different film cooling geometries.     

An experimental investigation of the heat transfer in a ribbed triangular cooling channel

One of the most challenging aspects of gas turbine cooling is the cooling of the first stages of turbine blades. Here the highest external heat load is seen at the leading edge of the blade. The present study investigates the internal cooling in a triangular channel with a rounded edge as a model of a leading edge cooling channel for a gas turbine blade. A transient liquid crystal method is used to measure the heat transfer. Experimental results are reported for a number of new 3D rib configurations for Reynolds numbers between 50 000 and 200 000. From the experimental results it has been found that 60 deg. ribs provide in general higher heat transfer enhancements than 45 deg. ribs. However, this results in extremely high friction factors for the 60 deg. ribs. Taking the local and mean distributions of the heat transfer coefficients (as well as the increase in friction factors) into consideration, it was found that the most promising rib arrangement for leading edge cooling is a 3D rib with 45 deg. angle and double-sided fully overlapped ribs in the arc area. These ribs provide uniform heat transfer in the arc area as well as a high level of the heat transfer coefficients in the channel. The resulting friction factors are in an acceptable range for these ribs.     

高压涡轮工作叶片综合冷却效果试验研究

为探究不同气动参数对涡轮工作叶片冷却性能的影响,通过试验研究了中温中压工况下高压涡轮工作叶片综合冷却效果分布,主要讨论燃气与冷气的温比、冷气与燃气的质量流量比和栅后雷诺数对综合冷却效果的影响。试验温比变化范围为1.3～2.1,质量流量比变化范围为0.002～0.02,栅后雷诺数变化范围为2.9×10⁵～4.8×10⁵。结果表明：不同气动参数下,综合冷却效果较低位置出现在叶片前缘和尾缘,较高冷却效果出现在叶背1/4和1/2相对弧长位置;当质量流量比从0.008增加到0.014、温比分别为1.40,1.67和1.90时的综合冷却效果分别提高了44.4%,44.5%和34.4%;当温比从1.8增加到2.1、质量流量比为0.008,0.011和0.015时的综合冷却效果分别降低了3.6%,1.6%和4.2%;当栅后雷诺数从2.9×10⁵增加到4.8×10⁵、温比为0.011、质量流量比为1.67时,综合冷却效果降低了20.7%。     

冷却通道参数对发汗冷却性能影响的数值研究

为了解决传统多孔材料孔隙结构不可控的问题,制备具有可控微冷通道的冷却结构对提高涡轮叶片冷却效率有着重要意义。为了研究不同冷却通道参数对叶片发汗冷却效率的影响,通过数值模拟方法研究了不同注入比下,仿生树形通道和传统直孔通道发汗冷却多孔板的换热特性及流动机理。同时,研究了6种不同模型参数多孔板在不同注入比下的冷却性能及流场的变化情况。研究结果表明：在内表面比和冷却剂出口面积基本一致的条件下,仿生树形多孔板具有更高的冷却效率;当注入比为2%时,仿生树形多孔板的平均冷却效率提高了5%,且存在一个最佳的注入比使得整体的冷却效率最高;冷却剂的出口面积是影响发汗冷却效率的关键性独立参数,与冷却剂的注入比大小有关;孔隙率对整体的冷却效率影响较小,内表面积比越大,发汗冷却的整体冷却效率越高。     

基于CFD DEM微细带肋内冷通道中颗粒沉积特性比较研究

航空发动机全天候全域长航时运行时,颗粒随着二次流空气系统进入到涡轮叶片内部,沉积堵塞在涡轮叶片内冷通道中,严重影响了涡轮叶片的冷却性能。本文采用计算流体力学和离散单元法(CFD-DEM)相结合的方法研究了涡轮叶片带肋细小矩形内冷通道中微尘颗粒的流动和沉积特性。所研究的内冷通道肋片周期性布置在通道的一侧,肋片阻塞比和肋间距比分别为0.024和10,考虑了平行直肋、45°斜肋和45°V肋3种肋结构,详细分析了雷诺数、颗粒斯托克斯数、入口颗粒体积分数和肋片的类型对颗粒流动和沉积特性的影响规律。结果表明：颗粒沉积主要发生在第1根肋片的前缘处;颗粒的沉积质量均随着雷诺数、斯托克斯数和颗粒体积分数增加而减小;在所有的肋片类型中,直肋布置时颗粒沉积现象最明显,其次是V肋,斜肋拥有最小的颗粒沉积质量。     

冷却空气量对涡轮冷却性能影响综述

涡轮冷却技术被广泛应用于航空发动机及燃气轮机涡轮研发中,冷却空气的引气量成为影响整机效率的重要因素之一。本文基于现代燃气轮机及航空发动机涡轮叶片采用外部冷却与内部冷却结合的复合冷却的技术发展背景,综述了国内外在冷却空气量对涡轮叶片冷却性能影响方面的研究进展,分析并总结了冷却空气量对气膜冷却、交错肋冷却以及对综合冷却效率的影响规律,并对未来的研究方向给出了一定的建议。分析表明：对气膜孔形状的探索是未来气膜冷却技术研究的重点;交错肋研究主要处于定性研究阶段,对定量研究方法的探索是目前的发展趋势;对综合冷却效率的研究还处于起步阶段,未来可以从外部冷却和内部冷却之间的相互作用关系方面对综合冷却效率开展进一步的研究。     

Experimental investigations of a small-scale solar-assisted absorption cooling system

The present study deals with a small-scale solar-assisted absorption cooling system having a cooling capacity of 3.52 kW and was investigated experimentally under the climatic conditions of Taxila, Pakistan. Initially, a mathematical model was developed for LiBr/H₂O vapor absorption system alongside flat-plate solar thermal collectors to achieve the required operating temperature range of 75°C. Following this, a parametric analysis of the whole system was performed, including various design and climate parameters, such as the working temperatures of the generator, evaporator, condenser, absorber, mass flow rate, and coefficient of performance (COP) of the system. An experimental setup was coupled with solar collectors and instruments to get hot water using solar energy and measurements of main parameters for real-time performance assessment. From the results obtained, it was revealed that the maximum average COP of the system achieved was 0.70, and the maximum outlet temperature from solar thermal collectors was 75°C. A sensitivity analysis was performed to validate the potential of the absorption machine in the seasonal cooling demand. An economic valuation was accomplished based on the current cost of conventional cooling systems. It was established that the solar cooling system is economical only when shared with domestic water heating.     

肋片间距对表冷器性能影响的实验研究

表冷器肋片间距是影响表冷器传热性能的主要因素。在实验研究和理论分析的基础上，以单位体积换热量和单位阻力换热量为衡量依赖，找出了表冷器性能相对优化的肋片间距值。结果表明，在常用的间距范围内，3.3mm左右较好。     

内螺纹管空预器的设计和优化

在大量设计计算的基础上，讨论了用内螺纹管代替光管改造锅炉设备中的空气预热器时，烟气流速对于设计结果的影响。最后，从工程的角度出发对内螺纹管的优化设计给出了一些建议。     

Experimental Investigations of Micro Air Injection to Control Rotating Stall

Steady discrete micro air injection at the tip region in front of the first compressor rotor has been proved to be aneffective method to delay the inception of rotating stall in a low speed axial compressor.Considering the practicalapplication a new type of micro injector was designed and described in this paper,which was imbedded in thecasing and could be moved along the chord.In order to verify its feasibility to other cases,such as high subsonicaxial compressor or centrifugal compressor,some other cases have been studied.Experimental results of the samelow speed axial compressor showed that the new injector could possess many other advantages besides success-fully stabilizing the compressor.Experiments performed on a high subsonic axial compressor confirmed the ef-fectiveness of micro air injection when the relative velocity at the blade tip is high subsonic.Meanwhile in orderto explore its feasibility in centrifugal compressor,a similar micro injector was designed and tested on a lowspeed centrifugal compressor with vaned diffuser.The injected mass flow was a bit larger than that used in axialcompressors and the results showed micro injection could also delay the onset of rotating stall in the centrifugalcompressor.     

Experimental flow field investigations of a film cooling hole featuring an orifice

This paper presents the flow field downstream of a film cooling hole geometry featuring orifice, referred to as nozzle hole, on a flat plate using PIV. The experiments were performed with blowing ratios from 0.5 to 2.0, density ratio of 1.0 and mainstream Reynolds number of 115,000. Velocity fields and vorticity fields of nozzle hole jet are compared with that of cylindrical hole jet. The results indicate that nozzle hole jet features double-decker vortices structure, resulting in vortices canceling out and significant reduction in CRVP strength. The streamwise vorticity of nozzle hole jet averages a drop of 55% at low blowing ratio 0.5 in comparison to cylindrical case. At high blowing ratio from 1.0, 1.5 and 2.0, the average drop is 30%–40%. A round jet bulk is observed to merge from the two legs of a typical kidney-shaped jet and the merged jet brings better coverage over the surface. In addition, it is found that CRVP strength might not have strong impact on jet lift-off but influences jet-mainstream mix characteristics.     

CO2应用于燃机进口空气冷却的实验研究

运用CO2作冷却介质建立高低压闭式循环系统是对燃气轮机预冷的一种尝试。针对某自备电厂联合循环机组的实际情况,设计搭建了一套实验系统。本文主要介绍了实验装置的结构尺寸、数据采集控制系统以及实验数据处理方法。通过实验,验证了该套冷却设备对于燃机进口空气的冷却效果,并达到实际燃气轮机压缩机进口空气冷却的工作要求。     

Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling

This paper reports the results of an experimental investigation of the performance of finned heat sinks filled with phase change materials for thermal management of portable electronic devices. The phase change material (PCM) used in this study is n-eicosane and is placed inside a heat sink made of aluminium. Aluminium acts as thermal conductivity enhancer (TCE), as the thermal conductivity of the PCM is very low. The heat sink acts as an energy storage and a heat-spreading module. Studies are conducted for heat sinks on which a uniform heat load is applied for the unfinned and finned cases. The test section considered in all cases in the present work is a 80 × 62 mm² base with TCE height of 25 mm. A 60 × 42 mm² plate heater with 2 mm thickness is used to mimic the heat generation in electronic chips. Heat sinks with pin fin and plate fin geometries having the same volume fraction of the TCE are used. The effect of different types of fins for different power level (ranging from 2 to 7 W) in enhancing the operating time for different set point temperatures and on the duration of latent heating phase were explored in this study. The results indicate that the operational performance of portable electronic device can be significantly improved by the use of fins in heat sinks filled with PCM.     

CFD investigation on the effect of air temperature on air blowing cooling system for preventing tube rupture

In this study a solution for preventing tube rupture in the superheater region of the 320 MW Bisotoun power plant boiler, Iran, was suggested. Based on our previous study, the air blowing system which locally cools the tube in place of tube rupture was proposed. Due to huge differences in air temperature at different seasons in the region the air with different inlet temperatures was blown over the modeled tube. The CFD results showed that the inlet air temperature has no significant effect on cooling performance due to high rate of heat transfer to the air carrier tube.     

Natural convection in a triangular cavity filled with air under the effect of external air stream cooling

Natural convection in a triangular cavity filled with air is investigated numerically. In this paper, the cavity is exposed to air stream cooling exerted on its sides and it is heated by a fixed heat flux from the base. The air inside the cavity is assumed to be laminar and obeying Boussinesq approximation. The governing equations are solved numerically using the finite volume technique with SIMPLE algorithm. The results are achieved with a range of Rayleigh number (10⁴ < Ra < 10⁷), free stream Reynolds number (10³ < Re_∞ < 1.5 × 10⁴), four aspect ratios (AR = 0.25, 0.5, 0.866, and 1) and five inclination angles (? = 0°, 30°, 45°, 60°, 90°). The influence of these parameters is displayed on the stream function, isotherms lines, local and average Nusselt numbers. The results reveal that the heat transfer rate increases as Rayleigh number, free stream Reynolds number and AR increase. The highest heat transfer rate is obtained at ? = 0° while the lowest one is obtained at ? = 90°. Furthermore, as the AR augments, the local and average Nusselt numbers are enhanced and the stream function is formed of two symmetric counter‐rotating vortices.     

Mixed convection air cooling of protruding heat sources mounted in a horizontal channel

A numerical study of laminar mixed-convection heat transfer to air from two identical protruding heat sources, which simulate electronic components, located in a two-dimensional horizontal channel, is presented in this paper. The finite volume method and the SIMPLER algorithm are used to solve the conservation equations of mass, momentum, and energy for mixed convection. Results show that the heat transfer increases remarkably for Pr = 0.71 and 5 ≤ Re ≤ 30. It was also found that the increase of separation distance, the height and the width of the components has a considerable enhancement of the heat removal rate from the components, and therefore, on the improvement of the heat transfer inside the channel.     

A model and heat transfer correlation for rooftop integrated photovoltaics with a passive air cooling channel

Photovoltaic (PV) panels can experience undesirably high temperatures due to the heat input by that part of the absorbed solar radiation which is not converted into electricity. Regulation of the temperature rise is necessary to maintain maximum solar to electric conversion. One approach for temperature regulation, suitable for rooftop integrated PV, involves fitting an open channel beneath the PV module. The panels are cooled by radiation and free convection as ambient air rises through the channel. A scale analysis and numerical study of PV modules with a back mounted air channel provides heat transfer rates over a practical range of operating conditions and channel geometries. A generalized correlation for the average channel Nusselt number for the combined convective-radiative cooling is developed for modified channel Rayleigh numbers from 10² to 10⁸, channel aspect ratios between 15 and 50 and inclination angles between 30° and 90°. The usefulness of a passive cooling channel to improve PV efficiency is illustrated by system analyses of typical PV modules.     

Effect of droplet characteristics on heat transfer of mist/air cooling in a pin-finned channel

Effects of droplet characteristics of mist/air cooling on heat transfer for three pin-fin structures are investigated. The round-tip pin-fin structure is newly proposed with partial detachment from one endwall with a round-shaped tip structure. A flat-tip pin-fin with partial detachment and a traditional pin-fin with full attachment serve as references. Reynolds-averaged Navier-Stokes equations and the shear-stress-transport turbulence model are applied. Influences of initial mist temperature, initial mist diameter and initial mist velocity are analyzed in the Reynolds number range 15,000 to 50,000. The round-tip pin-finned channel has highest heat transfer coefficient and lowest pressure loss among the structures. Heat transfer enhancement increases first gradually and then decreases sharply with increasing initial mist diameter but an optimal diameter exists for the highest Nusselt numbers. Nusselt number decreases monotonically with increasing initial mist temperature. Droplet movement and heat transfer are nearly independent of initial mist velocity.     

Forced Convective Air Cooling from Electronic Component Arrays in a Parallel

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