相变面积对平行环路热管性能影响的研究

重力型环路热管作为一种高效的传热装置,其性能与运行时换热器的充注量及换热器倾斜角度密切相关。为了更深入研究在运行时换热器换热量、传热系数与充注量变化所引起的蒸发器、冷凝器相变换热面积之间的关系,设计了一种蒸发器与冷凝器平行放置的新型热管系统。分析实验结果表明:蒸发器在固定角度下换热量与相变有效面积的函数关系呈具有单波峰的曲线分布,其波峰随着倾斜角的增加而向浸湿面积变小的方向移动;倾斜角度越大达到最佳换热效果所需的换热面积越小,但最大换热强度会减小;在热管进口温差固定时,倾斜角度大于45°才能较为有效的通过提升进口温度来提升换热量;在蒸发器未被完全浸润时,冷凝器换热量处于下降阶段,可以增加冷凝器管长来提升换热;在蒸发器被完全浸润时,增加冷凝器管长无法提升换热;蒸发器与冷凝器的传热系数与相变有效换热面积成线性下降关系,下降速率与角度近乎无关。     

重力式热管串联运行的性能研究

对一种用于太阳能—土壤源热泵地板采暖系统中的碳钢/水重力式热管进行了实验研究。该热管全长0.8m,内外径分别为12mm和15mm。蒸发段采用水加热,冷凝段采用风冷。在不同的串联方式、倾角(-2°~90°)、热水温度(40~60℃)、热水流量(0.1~0.3m3/h)及蒸发段长度(30~180mm)下进行了串联热管的性能实验。实验结果表明:串联热管的传热功率和壁面温度均随供水温度和水流量的提高而持续增长,随倾角和蒸发段长度的增加而先增长后下降;串联热管在倾角为30°~40°、热水温度为60℃、热水流量为0.3m3/h、蒸发段长度为120mm、D型串联的条件下运行最佳。根据实验现象和数据分析了不同条件下串联热管的传热机理,给出了串联热管的工作特性。     

重力热管单线内螺纹分布强化换热实验研究

采用实验方法,研究了不同的内螺纹分布和油浴温度等因素对热管换热特性的影响。实验选用的热管材料为紫铜,外径16 mm,壁厚3 mm,长度为200 mm,传热工质为水,充液率为20%。实验结果表明:在同一油浴温度下,内螺纹重力管的启动特性要优于光滑重力热管。对比不同油浴温度下,布置内螺纹能够有效地降低热管的工作温度。实验选型的内螺纹仅布置在蒸发段不会提高热管的换热系数,而在绝热段和冷凝段布置内螺纹则能够使换热系数显著提升,且随油浴温度的增加,换热系数线性增加。     

水平布管重力型分离式热管实验研究

为了研究水平布管重力型分离式热管在空调余热回收领域的应用,本文设计了分离式热管的实验装置,进行了充液率的实验研究,并借助红外热力成像仪,首次应用图像可视化分析了不同充液率条件下热管蒸发器的工作状态。研究结果表明:热管的换热量随充液率的不同而发生变化,最佳充液率为45%~52%。通过能效比分析,最佳充液率时其能效比能达到8.4,表明水平布管重力型分离式热管节能效果显著,具有推广价值。     

超临界CO2倾斜螺旋槽管内冷却换热特性研究

为了提高气体冷却器内换热效率,对不同倾斜角下(-90°,-45°,0°,45°,90°)螺旋槽管内超临界CO2冷却对流换热特性进行了数值模拟,分析了各槽管内的湍动能和速度分布随倾斜角的变化趋势,并研究了不同螺旋角下倾斜角对换热特性的影响。结果表明:浮升力沿流动方向分量和垂直于流动方向分量对流动特性的影响并不相同;在类气区,流体速度对流动特性起主要作用,且换热系数随倾斜角的减小而增大;在类液区,流动特性的主要影响因素是速度梯度,此时换热系数随倾斜角的变化与类气区相反;螺旋角越大即螺旋程度越小,当流体倾斜向上流动时浮升力效应越为显著;当螺旋角为 0.70 rad时,最优倾斜角度为-45°,当螺旋角为0.94 rad时,最优倾斜角为45°。     

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。     

不同倾角条件下非均匀热流对超临界二氧化碳对流换热的影响

为了探究超临界二氧化碳（SCO2）对流换热的影响因素,在考虑了管道倾角和钢管壁厚引起热流密度不均的情况下,针对SCO2在上、下半周不同热流密度条件、管道放置倾角、质量流量以及压力值时的换热情况进行了研究。模拟计算中金属管外径16 mm、内径12 mm、长度1 500 mm;外壁面热流密度为425.6 kW/m2;管内出口压力工况参数分别为7.6,8.5和9.5 MPa;质量流量分别为101.788,76.341和50.894 kg/s;管道倾角分别为0°（水平）,30°,45°,60°和90°（垂直）。结果表明：在均匀加热条件下,由于钢管导热的影响使钢管内壁上半周的热流密度低于下半周;随着倾角的增大,二次流动能降低,上半周的热流密度逐渐接近下半周,同时,下半周的换热系数减小,上半周的换热系数增大。在非均匀加热条件下,沿着流动方向,初始阶段上半周的换热系数高于下半周的换热系数,随着流体温度增加,这种现象会发生逆转。因为当质量流量和压力增加时,上、下半周的换热系数均会增加,并且可以降低管道内壁面峰值温度。而不同加热方式下,上、下半周的温差与二次流动能有关。     

环路型脉动热管的工质流动和传热特性实验研究

建立了部分可视化的环路型铜-乙醇脉动热管试验台,研究了充液率、倾斜角度、环路数目等因素对脉动热管传热性能的影响。结果表明:不能形成脉动效应时工质的流型是间歇振动,形成脉动效应时工质的流型是弹状流或环状流;最佳倾角为70°~90,°最佳充液率在50%左右;热阻随着环路数目的增加而减小。     

海水淡化用波纹管降膜蒸发强化传热实验研究

通过利用水平管降膜蒸发换热试验台分别对Φ19×0.75 mm的波纹管和光滑管进行实验研究。实验在变喷淋密度(0.007~0.130 kg/m·s)、变热通量(52~143 k W/m~2)、变传热温差(1.5~10.0℃)、变蒸发压力(0.020~0.065MPa)条件下进行。通过实验数据得到波纹管和光滑管传热系数与各影响因素(喷淋密度、热通量、传热温差、蒸发温度)之间的变化规律。实验结果表明:在一定范围内,降膜蒸发器的传热系数K随喷淋密度γ、热通量Q的增大、蒸发温度T的升高而增大,随传热温差Δt的增大而降低。当喷淋密度大于0.178 kg/(m·s)时,总传热系数趋于稳定,当热通量大于130 k W/m~2时,总传热系数的增速明显变缓。此外,不凝气含量对传热系数K的影响显著,在同等实验条件下波纹管的传热系数比光滑管提高近30%。     

弯折和倾角对微槽道扁平热管传热特性的影响

研究了弯折和顺重力倾角对具有梯形微槽道的铝—丙酮扁平热管传热特性的影响,搭建了可变重力倾角热管传热性能实验台,在冷凝段采用第三类边界条件下进行测试。结果表明,顺重力倾角可显著提高热管传热极限,30°及以上的倾角可使传热极限从40 W左右提高至90 W以上,同时热阻在高热流时达到0.1 K/W的低位。弯折可有效降低热阻,30°弯折可使热阻整体降低23.6%以上,从而提高等效导热系数,在高热流和小倾角下效果更佳,最大等效导热系数可达1.47×10~4 W/(m·K)。在热管设计与应用时,合适的弯折和倾角可帮助提高热管传热性能。     

Correlation to predict heat transfer characteristics of a radially rotating heat pipe at vertical position

The heat transfer characteristics of a radially rotating heat pipe (RRHP) depend on a number of parameters. This paper is a study of the effects of these parameters. They are the inner diameter of the tube, aspect ratio, rotational acceleration, working fluid and the dimensionless parameters of heat transfer. RRHPs, made of copper tubes with inner diameters of 11, 26, and 50.4 mm, were used in the experiments. The aspect ratios were 5, 10, 20 and 40 respectively. The selected working fluids were water, ethanol and R123 (CHCl₂CF₃) with a filling ratio of 60% of evaporator volume. The experiments were conducted at inclination angles of 0–90° from horizontal axis and the rotational accelerations were lower, higher and equal to gravitational acceleration. The working temperature was 90 °C. The evaporator section was heated by electric power while heat in the condenser section was removed naturally by air. The evaporator and adiabatic section of the RRHP were well insulated with ceramic fibers. The experimental results showed that the heat flux decreases with an increasing inner diameter, and decreases with an increasing aspect ratio. The heat flux increases with an increasing rotational acceleration and decreases with an increasing liquid density of the working fluid. A correlation to predict the heat transfer rate at vertical position can be established.Further research will investigate a visual study of internal flow pattern and the formulation of a mathematical model.     

Influence of Inclination Angle on the Start-up Performance of a Sodium-Potassium Alloy Heat Pipe

ABSTRACT

Alkali metal heat pipes play significant role in various high-temperature engineering applications because of their excellent heat transfer capacity. Inclination angle is one of major factors which significantly affect start-up and heat transfer characteristics especially for thermosiphons. A sodium-potassium alloy (Na-K) gravity-driven heat pipe (GHP), in which the content of potassium in Na-K is wt. 55%, was fabricated to study the effect of inclination angle on start-up and heat transfer capacities of high-temperature GHPs. The Na-K GHPs was fixed by the adjusting bracket in 9 inclination angles (0°, 10°, 20°, 30°, 40°, 50°, 60°, 70° and 80°). Outside wall temperature was measured by eleven thermocouples which calibrated by the China Institute of Metrology prior to using them in the experiments. Results show that inclination angle has a significant impact on start-up and heat transfer performances of the Na-K GHP because of the impact of gravity on the two-phase flow inside the heat pipe and effective heating area in the evaporator. Start-up and heat transfer characteristics are dramatically improved and temperature difference significantly decreases as the inclination angle increases from 0° to 50°, but slightly decreases when the inclination angle exceeds 60°.     

Flow and Heat Transfer Characteristics of Two‐Phase Fluid in Inclined Pipes on a Rotation Platform

A rotating platform was used to create dynamic load, and the mixture air–water two‐phase flow and boiling steam–water two‐phase flow were obtained in an inclined test pipe. By changing the parameters, such as inclination of the test pipe, rotational speed, inlet temperature, flow rate, and so on, the experiments for two‐phase flow in the pipe at inclination of 0°, 45°, and 66° were conducted, respectively. The effects of acceleration and inclination on their flow and heat transfer characteristics were investigated. The two‐phase flow patterns in inclined pipes under rotation conditions were caught with a video camera. The images show that the impact mixed flow and churn flow were found in this research. The results show that the acceleration and pipe inclination significantly influence the flow characteristic and heat transfer of the two‐phase pipe flow. As the directions of the dynamic load and the gravity are opposite to the flow direction, the greater the dynamic load and inclination, the higher the pressure drop and the heat emission, and the lower the flow rate, the void fraction, and the fluid temperature. Therefore, the dynamic load and gravity will improve the flow resistance, enhance heat emission and reduce the heat gained by the fluid.     

Effects of the inclination angle on pool boiling in an annulus

To identify effects of the inclination angle on pool boiling heat transfer an experimental study has been executed. For the test a single tube of 30 mm diameter and an annulus of 12.7 mm gap size submerged in the saturated water at atmospheric pressure have been considered. The inclination angle changes heat transfer much. The change of the inclination angle from 0° to 45° results in 29.8% and 11.2% decrease in the heat transfer coefficient at 40 kW/m² for the single tube and the annulus, respectively. For the single tube, no specific changes in heat transfer are observed as the inclination angle increases up to 15° whereas the angle for the annulus is 30°. The major heat transfer mechanisms are considered as the intensity of liquid agitation and bubble coalescence due to the enclosure by the outer tube.     

Thermal performance of inclined screen mesh heat pipes using silver nanofluids

This study presents the effect of silver nanofluid on thermal performance of inclined screen mesh heat pipe in cooling applications. Four cylindrical copper heat pipes containing two layers of screen mesh were fabricated and tested with distilled water and water based silver nanofluids with mass concentrations of 0.25%, 0.5% and 0.75% as working fluids. The experiments were performed at four inclination angles of 0°, 30°, 6° and 90°. The main focus of this study is to investigate inclined heat pipe performance with nanofluid. Experimental results indicate that the thermal performance of heat pipes was improved with nanofluids compared to water and thermal resistance of the heat pipes decreased with the increase of nanoparticle concentration. Moreover, the thermal performance of the heat pipes at inclination angle of 60° is found to be higher than other tested inclination angles, which shows the effect of gravity on heat pipe performance.     

Heat transfer in gas–solid fluidized bed with various heater inclinations

The study explored the heat transfer properties in an air-fluidized bed of sand, heated with an immersed heat transfer tube positioned at several angles of inclination. Operating with fluidizing velocity up to 0.5 m/s; and particles of 150–350 μm diameter, the effect of air velocity and particle size on the average and maximum achieved heat transfer coefficient was examined for the heat transfer tube at angles of inclination in the range 0–90°. Experimental results showed that the angle of inclination altered the bubble size and behavior close to the heat transfer tube hence the expected heat transfer coefficient, with the influence of tube inclination being less pronounced for smaller particles. The optimum angle of inclination was in the range of 10–15° relative to the direction of the flow, while the heat transfer coefficient had its lowest values at the angle of 45°, and thereafter improved upon transition to 90°. Upon comparison with existing correlations, a correction factor is proposed to account for the impact of the angle of inclination on the heat transfer coefficient calculated by the Molerus–Wirth semi-empirical correlation.     

Experimental investigation of micro heat pipes of different cross-sections having same hydraulic diameter

Effects of micro heat pipe （MHP） cross-sections and orientations on its thermal performance are experimentally investigated in this study. Tests are conducted using five different cross-sections （circular, semicircular, elliptical, semi-elliptical and rectangular） of micro heat pipes having same hydraulic diameter of 3 rnm placed at three different inclination angles （0°, 45°, 90°）, where water is used as the working fluid. Evaporator section of the MHP is heated by an electric heater and the condenser section is cooled by circulation of water in an annular space between condenser section and the water jacket. Temperatures at different locations of the MHP are measured using five calibrated K type thermocouples. Heat supply is varied using a voltage regulator which is measured by a precision ammeter and a voltmeter. It is found that thermal performance tends to deteriorate as the MHP is flattened. Thus among all cross-sections of MHP, circular one exhibits the best thermal performance in terms of heat flux dissipation followed by semi-elliptical, semi-circular, elliptical and rectangular cross-sections. Moreover, its heat transfer capability also decreases with decreasing of its inclination angle. Finally, a correlation is developed which covers all the experimental data within ＋7%.     

Investigation on the heat transfer characteristics of propylene glycol–water mixture in the shell side of a spiral‐wound heat exchanger

The heat transfer characteristics of propylene glycol–water (PG–W) mixture (10%, 20%, and 30% propylene glycol) on the shell side of a spiral‐wound heat exchanger (SWHE) were investigated experimentally. Among the SWHE selected, there are 18 twined tubes with a diameter of 8 mm. PG–W mixture is on the shell side and water is on the tube side. The results show that the heat transfer coefficient of PG–W mixture flowing downwards is higher than upwards under countercurrent conditions. The heat transfer coefficient decreases with the increasing of concentration of PG–W mixture. When the inclination angle of the SWHE is 90°, the heat transfer coefficient of PG–W mixture is the largest; and when the inclination angle is less than 90°, the heat transfer coefficient decreases with the decrease of inclination angle. The inclination angle has a great effect on the heat transfer coefficient at a high concentration. The fitting correlation equations between Nu, Re, Pr, and inclination angles of SWHE are established.     

Experimental study of closed‐loop thermosyphon with a different evaporator geometry

In this study, the effect of evaporator geometry on the loop thermosyphon's heat transfer coefficient is experimentally verified by using water as a working fluid with three filling ratios (50%, 70%, 90%), constant heat input (185 W), and condenser cooling water flow rate remaining constant at 2 Lpm. Three evaporator pipes are used (I: straight; II: helical coil evaporator with a diameter of 100‐mm coil and two turns; III: helical coil evaporator with a diameter of 50‐mm coil and four turns). From the experimental results, it can be observed that the performance of evaporator III is higher than the two other forms. A greater heat transfer coefficient value is found in case of type III evaporator and is equivalent to 2456 W/m²·°C. The maximum thermal resistance reduction occurs in the type III evaporator (37.32%), and the highest effective thermal conductivity for the same type is 6.123e + 05 W/m·°C. The experimental results demonstrate good agreement with the empirical equations.