不同加热功率下充液率对无芯环路热管性能的影响

设计了以铝为管材、丙酮为传热工质的无芯环路热管。其蒸发段采用加热带加热,冷凝段用风冷降温。热管依靠蒸发压头使工质循环,并依靠重力作用,使冷凝液回流到蒸发段。搭建试验台并研究了不同加热功率下充液率对无芯环路热管的传热温差、传热量、热效率、热阻和当量导热系数的影响。结果表明:加热功率为150.00 W、充液率为30%时,无芯环路热管的均温性最好;传热温差和热阻均最小,分别为6.75℃、0.045 K/W。传热量132.00 W、热效率0.88、当量导热系数168 125 W/(m·K),均达到最大值。所以,该无芯环路热管在本实验研究范围内的最佳工作条件为加热功率150.00 W、充液率30%。     

多壁碳纳米管水基纳米流体重力热管传热特性数值模拟

为揭示多壁碳纳米管水基纳米流体应用在重力热管中的传热特性,基于多相流模型(VOF)建立其重力热管数值模型,并将数值结果与实验数据进行对比验证。以热阻作为性能评价指标,改变加热功率和充液率,讨论二者对热管换热性能的具体影响。通过添加传热传质源项来编写用户自定义函数(UDF)完成内部流体蒸发冷凝过程中的相变模拟。模拟结果表明:该数值模型能够较好模拟多壁碳纳米管水基纳米流体应用重力热管内部复杂的流动与传热过程;在选定的加热功率及充液率参数范围内,该重力热管的整体热阻随蒸发段加热功率的增大而减小,随充液率的增大而增大。     

铜-R22分离式热管传热特性的试验研究

采用铜管模拟分离式热管蒸发段,以R22为工质,在热流密度1～5 kW/m2、蒸发温度30～60℃和蒸发段倾角10°～90°范围内,研究了这种分离式热管的传热特性.分析了热管的启动特性以及热流密度、蒸发段倾角和充液率对其传热特性的影响.结果发现:以R22为工质的分离式热管具有良好的启动性,蒸发段合理的充液率为80%～100%,在试验参数范围内,热流密度、蒸发段倾角和充液率对热管的传热性能没有明显影响.     

井筒重力热管传热特性的研究

井筒重力热管是利用热管将油藏自身能量即井底热量传递到井筒上部,在无需外加动力条件下实现对井筒近井口流体加热,改善井口流体温度分布,防止近井口结蜡和絮凝,从而降低采油成本。本文利用N-S方程,根据液膜内微元体的质量守恒、动量守恒和热平衡原理,模拟分析重力热管冷凝段冷却温度、加热段加热功率、冷凝段、绝热段长度以及热管内径等参数对热管运行的影响。研究变参数下热管内液膜厚度变化以及冷凝和蒸发换热系数的变化,进而分析得出变参数时重力热管传热特性,为优化重力热管参数和提高热管的换热性能提供了理论依据,从而使重力热管在最佳传热状态下运行,提高其换热效率。     

多片平板微热管阵列运行特性

将多片平板微热管阵列进行堆叠可以提高传热量。实验研究了不同叠加片数下平板微热管阵列在不同加热功率下的热阻、传热极限、温度分布及变化规律、不同层热管传热等运行特性。实验表明，随热管片数增加，热管整体的传热量增加，最大传热量由单片的60W提升至5片的180W；传热热阻降低，最小传热热阻由单片的1.09℃/W下降为5片的0.24℃/W。而且，随着片数增加，热管间的传热热阻开始影响多片平板微热管阵列的整体运行：外侧热管的传热量高于内侧热管，热阻低于内侧热管，且当片数为4片及以上时，热管整体性能提升会越来越不明显。在相同的加热功率下，多片平板微热管阵列的外侧蒸发段、外侧冷凝段和内侧蒸发段温度均随片数增加而降低，但内侧冷凝段温度先升高后降低。最理想的热管叠加片数为3-4片。     

热管换热器在工业炉上的应用

一、概述上海金桥节能设备厂，地处浦东金桥出口加工区，是研制开发热管技术的专业工厂。热管技术是六十年代开始发展起来的高新技术，按热管工质回流方法不同，大致分为标准热管和重力热管。我厂开发的属重力热管，该热管是一种内部形成真空并充适量介质的密封管，它是相变传热，即热管的一端（蒸发段）受热时，管内介质沸腾汽化弥漫至另一端（冷凝段），在另一端由于受到散热冷却，使汽雾凝结成液态回流至蒸发段，如此往复循环，热量由一端传至另一端，两端具有良好的等温性，传热速度快、效率高。由于我厂热管介质的特异性，热管工作温度…     

一种新型微热管传热性能的实验研究

对一种新型的平板式微热管一零切角曲面微热管进行了实验研究。以热阻为基础,研究不同倾角、工质、充液比下微热管的热性能。为便于分析,将热管总热阻分解为4个部分：加热热阻、蒸发段热阻、冷凝段热阻和热沉热阻。通过实验得出如下结论：微热管总热阻的主要变化因素是冷凝段热阻和蒸发段热阻;与相应的无工质平板式换热器相比,实验件主要热阻变为热沉热阻．蒸发段和冷凝段热阻所占比例较低。根据不同的充液比和倾角。微热管传热极限分别由局部干烧和核态沸腾向膜态沸腾转化引起。实验表明。这种新型的微热管具有良好的应用前景,但是对于其机理还需要更深入的研究。     

脉动热管水平及小倾角条件下的传热性能

为了研究脉动热管放置方式对其传热性能的影响,以超纯水作为工质,对水平及倾角为30°放置的脉动热管的传热性能进行研究,用壁面温度振荡性能和传热热阻来描述其传热能力。在不同的放置条件下,着重分析不同加热功率和充液率(35%,50%,70%)对其传热性能的影响。研究表明：水平放置时,充液率为35%和50%时脉动热管不能启动,充液率70%时可以启动运行;脉动热管在运行时存在临界热量输入值,倾角为30°时,临界值为60 W,但水平放置条件下临界值为90 W;水平放置下的脉动热管传热热阻在不同加热功率下,显著高于倾角为30°的情况;倾角为30°,充液率为35%时的脉动热管适合在低加热功率范围运行,此时传热热阻要低于充液率为50%的情况,但传热范围很窄,传热极限低;30°倾角时,与充液率35%和50%相比,高充液率70%的脉动热管整体传热性能最优。     

不同充液率和加热功率下乙烷脉动热管传热性能研究

通过实验研究了在充液率为30%～70%,加热功率为10～60 W的工况下乙烷脉动热管的传热性能。结果表明:随着加热功率的增加,冷凝段和蒸发段的温度波动依次经历了低幅低频、低幅高频、高幅高频和高幅低频的振荡模式;在中低加热功率下,蒸发段和冷凝段的温度振荡波形相位角相差180°,而当高加热功率时,蒸发段和冷凝段的温度变化是同步的;在不同的加热功率下,脉动热管均在50%充液率时达到最佳传热性能;脉动热管的传热性能随加热功率的增大先增强后减弱,其存在最佳加热功率使得脉动热管的换热效率最高。     

单回路紫铜_水脉动热管传热性能的实验研究

实验研究了单回路紫铜—水脉动热管在水冷方式和定传热功率时,冷却水流量、倾角、管径和充液率4种因素对热管传热性能,包括管壁测点温度、冷热段均温、传热温差、传热热阻和温度振幅的影响规律,得到提高传热性能的一些措施。结果显示:水平放置的单回路脉动热管无法启动;30°以上倾角管内可产生振荡,增加倾角可降低传热热阻;定加热功率下,冷却水流量存在最佳值,过大和过小都会增加传热热阻;在脉动热管允许管径范围内,增加管径可大大降低传热热阻;相同传热功率时,30%充液率热管的传热热阻明显低于70%充液率管;小而均匀的壁温振荡比大幅锯齿状振荡时的传热性能好。     

Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors: Part 1: Indoor experiment

An especial open thermosyphon device used in high-temperature evacuated tubular solar collectors was designed. The indoor experimental research was carried out to investigate the thermal performance of the open thermosyphon using respectively the deionized water and water-based CuO nanofluids as the working liquid. Effects of filling rate, kind of the base fluid, nanoparticle mass concentration and the operating temperature on the evaporating heat transfer characteristics in the open thermosyphon were investigated and discussed. Experiment results show the optimal filling ratio to the evaporator is 60% and the thermal performance of the open thermosyphon increase generally with the increase of the operating temperature. Substituting water-based CuO nanofluids for water as the working fluid can significantly enhance the thermal performance of the evaporator and evaporating heat transfer coefficients may increase by about 30% compared with those of deionized water. The CuO nanoparticles mass concentration has remarkable influence on the heat transfer coefficient in the evaporation section and the mass concentration of 1.2% corresponds to the optimal heat transfer enhancement.     

槽式太阳能集热器中萘热虹吸管启动特性的研究

对槽式太阳能集热器中萘热虹吸管的启动特性进行了理论分析,并对萘热虹吸管在不同受热方式、不同倾角和不同充液率下的启动特性进行了试验研究,同时对4°倾角、半周向受热方式下的启动特性进行了试验与理论计算的对比.结果表明:萘热虹吸管在较小工作倾角、半周向受热方式下能正常启动和稳定运行;受热方式对萘热虹吸管启动特性的影响不大;充液率对冷凝段开始启动的时间有一定的影响,充液率越小,开始启动所需的时间越短;冷凝段转折温度的试验值稍低于理论计算值,启动过程与理论分析过程基本一致.     

Effect of Inclination Angle and Filling Ratio on Thermal Performance of a Two-Phase Closed Thermosyphon under Normal Operating Conditions

In this paper, the effect of the inclination angle on the thermal performance of a two-phase closed thermosyphon with different filling ratios has been investigated experimentally under normal operating conditions. A series of experiments were carried out for inclination angle range of 5°–90° and filling ratios of 15%, 22%, and 30%. A copper thermosyphon with an outside diameter of 16 mm, an inside diameter of 14.5 mm, and a length of 1000 mm was employed. Distilled water was used as the working fluid. The results show that the two-phase closed thermosyphon has the highest thermal performance in the inclination angle range of 15°–60°. A good agreement was observed between the experimental results of this study and those available in the open literature. The interesting phenomenon of geyser boiling occurred in our experiments for filling ratios equal or greater than 30%. The geyser boiling puts no limitation on thermal performance of thermosyphon, but it should be avoided because it damages the condenser end cap due to the slug striking.     

Experimental investigation of small diameter two-phase closed thermosyphons charged with water,FC-84, FC-77 and FC-3283

An experimental investigation of the performance of thermosyphons charged with water as well as the dielectric heat transfer liquids FC-84, FC-77 and FC-3283 has been carried out. The copper thermosyphon was 200 mm long with an inner diameter of 6 mm, which can be considered quite small compared with the vast majority of thermosyphons reported in the open literature. The evaporator length was 40 mm and the condenser length was 60 mm which corresponds with what might be expected in compact heat exchangers. With water as the working fluid two fluid loadings were investigated, that being 0.6 ml and 1.8 ml, corresponding to approximately half filled and overfilled evaporator section in order to ensure combined pool boiling and thin film evaporation/boiling and pool boiling only conditions, respectively. For the Fluorinert? liquids, only the higher fill volume was tested as the aim was to investigate pool boiling opposed to thin film evaporation. Generally, the water-charged thermosyphon evaporator and condenser heat transfer characteristics compared well with available predictive correlations and theories. The thermal performance of the water-charged thermosyphon also outperformed the other three working fluids in both the effective thermal resistance as well as maximum heat transport capabilities. Even so, FC-84, the lowest saturation temperature fluid tested, shows marginal improvement in the heat transfer at low operating temperatures. All of the tested Fluorinert? liquids offer the advantage of being dielectric fluids, which may be better suited for sensitive electronics cooling applications and were all found to provide adequate thermal performance up to approximately 30–50 W after which liquid entrainment compromised their performance.     

Experimental investigation of the loop thermosyphon with different adiabatic lengths charged with different working fluids

This paper reports an experimental investigation of a closed-loop thermosyphon system charged with water and other low saturation fluids, such as ethanol, acetone, and methanol, for different adiabatic lengths, filling ratios, and heat loads. The closed-loop thermosyphon with two inline vertical heaters in the evaporator section and forced air-cooled plate-type heat exchanger in the condenser section, connected by a changeable adiabatic length, is investigated at different working conditions. Out of five filling ratios used in the analysis, at 0.6 filling ratio, the loop thermosyphon is seen to be operated at its best. The acetone-charged loop thermosyphon shows the lowest values (up to 72% reduction) of overall thermal resistance than that of other fluids and significantly higher effectiveness, due to the plate-type forced air-cooled condenser. For the acetone-filled thermosyphon, an almost 15% increase in the effectiveness is observed by changing the adiabatic length from 800 to 200 mm. This study suggests that the limitation of the loop thermosyphon with a water-cooled condenser to cool electronic components, computational clusters, and data centers is well fulfilled by the loop thermosyphon with plate-type forced air-cooled condenser. The nucleate pool boiling correlation is developed and validated for the loop thermosyphon system to determine the evaporator heat transfer coefficient.     

Heat transfer characteristics of an external-fin-assisted two-phase closed thermosyphon: An experimental study

In the current paper, the performance of an external-fin-assisted thermosyphon is investigated experimentally. The thermosyphon is produced with a copper tube and includes three parts—the evaporator, the adiabatic, and the condenser. The condenser part is enhanced with external longitudinal fins. In this study, different number of fins, filling ratios (FRs), coolant flow rates, a wide range of heat inputs, and initial absolute pressures are considered. The experiments are carried out by measurement of temperature distribution of the thermosyphon's wall and the temperature difference of the coolant. The results depict that increasing the heat input and FR reduces the thermal resistance, while raising the coolant flow rate augments the thermal resistance. Adding external fins to the condenser causes further condensation, which enhances the thermosyphon thermal performance by a reduction of 26.32% in thermal resistance and an increment of 28.55% in the thermosyphon efficiency.     

Feasibility of using multiport minichannel as thermosyphon for cooling of miniaturized electronic devices

The feasibility of using multiport minichannel (MPMC) as thermosyphon for cooling miniaturized electronic products is experimentally investigated with acetone as the working fluid. A detailed analysis on thermal performance and entropy generation due to heat transfer and pressure drop with the effects of heat load (10-50 W), filling ratio (FR; 40%, 50%, and 60%), and inclination angle (45°, 60°, and 90°) has been carried out. The results showed a reduction of 22.2% and 9.31% in thermal resistance and evaporator wall temperature at optimum filling ratio (OFR) of 50%. Reduction in entropy generation due to heat transfer and pressure drop of 16.6% and 12.3%, respectively, was observed at OFR. Internal fins in MPMC increase the surface area and evaporation rate by enhancing heat transfer leading to a decrease in the rate of entropy generation. Multiport increases surface tension of condensate at right angles to the flow direction along with the effects of gravity and enhancing rate of condensation. A new correlation is developed to predict evaporator wall temperature as a function of heat load and FR. The proposed correlation agrees well with a deviation of ±20% with present experimental results and also with the published literature. Thus, the obtained results will be useful in cooling miniaturized electronic devices.     

Effects of Magnetic Field and Inclination on Natural Convection in a Cavity Filled with Nanofluids by a Double Multiple-Relaxation-Time Thermal Lattice Boltzmann Method

Abstract

The present study numerically investigates the magneto-hydrodynamic flow and heat transfer of copper (Cu)-water nanofluids in an inclined cavity with one heat and one cold source. Simulations have been done via double multiple-relaxation-time thermal lattice Boltzmann method. Impacts of Hartmann number, Rayleigh number, inclination angle and the volume fraction of nanoparticles on the fluid flow and heat transfer performance are illustrated in terms of streamlines, isotherms, local, and average Nusselt numbers. Outputs demonstrate that the average Nusselt number decreases remarkably first as the inclination angle increases and then the average Nusselt number increases continuously and approaches a maximum value at a certain inclination angle for high Rayleigh numbers. In addition, the position where the average Nusselt number is maximized moves toward the lower inclination angle with increasing the Hartmann number for high Rayleigh numbers.     

Flower-type pulsating heat pipe for a solar collector

Inspired by the sunflower, we report a new structure of a solar collector that integrates a pulsating heat pipe (PHP) into a flat-plate collector. The proposed flower-type PHP solar collector is designed after a sunflower with petals that absorb sunlight and transfer nutrients to the stem after photosynthesis. The evaporator section adopts the shape of a flower to absorb sunlight fully, and the condenser section is rolled into a cylinder and placed in the lower part of the structure. A systematic experimental study is conducted upon start-up, and the performance characteristics, with acetone as the working fluid, are evaluated. We also did a heat loss analysis, which has a deviation of 8%. The effects of the mass flow rate of cooling water, filling ratio, length of the condenser section, and solar intensity are assessed. As the temperature of the heat absorber plate increases, the thermal resistance of the PHP can decrease to a minimum of 0.14°C/W. Under sunny weather conditions, the instantaneous thermal efficiency of the system with a filling ratio of 50% reaches 50%. Besides, we discussed the unstable operation conditions and possible dryout phenomenon that happened inside the PHP.