Experimental investigation on heat transfer performance diamond nanofluid gravity heat pipe
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摘要: 热管是通过工质在管内的气液相态变化实现热量高效传递的换热元件,其中重力热管具有结构简单、工作稳定、成本低廉等优势,被广泛应用于工业生产的各个换热场合,尤其在节能、新能源的开发和利用方面发挥了显著的作用。本文中基于金刚石纳米流体开展重力热管的换热特性研究,探索特定条件下重力热管的最优工作参数。研究不同的纳米颗粒质量分数(0.5%~2.0%)、充液率(8%~26%)、纳米颗粒粒径(20和50 nm)、电源加热功率(3~18 W)和有无吸液芯等对金刚石纳米流体重力热管换热性能的影响,结果表明:当纳米颗粒质量分数为2.0%时,重力热管换热性能最佳,总热阻相比最大值降低28.4%~64.7%;当充液率为14%时,换热性能最好,总热阻相比最大值降低6.1%~8.5%;当选用粒径为50 nm的金刚石纳米流体时,重力热管换热性能整体优于20 nm的;当电源加热功率提高时,换热性能随之提升;当选用吸液芯重力热管时,其换热性能整体优于无吸液芯重力热管的换热性能。Abstract: Objectives: With the development of modern processing technology, heat accumulation has become an urgent processing problem that needs to be solved. A heat pipe is a heat exchange element that efficiently transfers heat through the gas-liquid phase change of the working fluid inside the pipe. Gravity heat pipe have advantages such as simple structure, stable operation, and low cost, and are widely used in various heat exchange scenarios in industrial production. They have played a significant role in energy conservation, the development and utilization of new energy, and in strengthening heat exchange during processing. This article prensents experimental research on diamond nanofluids, exploring the influence of different parameters on the heat transfer performance of diamond nanofluid gravity heat pipes, laying a foundation for the research and application of heat pipe technology in heat dissipation during machining processes such as drilling, milling, and grinding. Methods: The evaporation section is heated using a DC power supply and thermal resistance wire. K-type thermocouples and temperature acquisition cards are used to record the temperature of the evaporation and condensation sections of the gravity heat pipe. The influence of heating power, filling rate, nanofluid concentration, and nanoparticle size on the heat transfer performance of the gravity heat pipe is analyzed using thermal resistance R as an indicator. Results: The heat transfer performance of gravity heat pipes is investigated under a power range of 3-18 W, while maintaining a filling rate of 20% and a nanoparticle concentration of 1%. The results show that as the heating power increases, the temperatures of the evaporation and the condensation sections gradually increase, while the rise time gradually shortenes. The temperature difference between the evaporation and condensation sections shows a decreasing trend. When the heating power increases for the same concentration and filling rate of nanoparticles, the total thermal resistance shows a decreasing trend, but the magnitude of the decrease continues to decrease. Keeping the concentration of nanoparticles at 2% and the heating power at 6 W, the heat transfer performance of gravity heat pipes is investigated under conditions of filling rates of 8%, 14%, 20%, and 26%. The results show that the overall temperature of the 20 nm diamond nanofluid is higher than those of other filling rates at a 20% filling rate, while the overall temperature at a 26% filling rate is lower than at other filling rates. The overall temperature at a 26% filling rate is higher than at other filling rates. With the same mass fraction and heating power, as the filling rate increases, the total thermal resistance shows a trend of first decreasing and then increasing, with the minimum value of the total thermal resistance appearing at a filling rate of 14%. By maintaining a filling rate of 26% and a heating power of 12 W, the heat transfer performance of gravity heat pipes under 0.5%, 1.0%, 1.5%, and 2.0% mass fraction conditions is investigated. The results show that the overall temperature of 20 nm diamond nanofluid heat pipes is the highest at a 1% mass fraction, while the overall temperature is lower at a 2.0% mass fraction. The hot-end temperature of 50 nm diamond nanofluid heat pipes is the highest at a 1.5% mass fraction, and the cold-end temperature is the lowest. At a mass fraction of 2.0%, there is a situation where the hot-end temperature is lower and the cold-end temperature is higher. With the same filling rate and heating power, as the mass fraction increases, the total thermal resistance first increases and then decreases. At a mass fraction of 2.0%, the minimum total thermal resistance will appears. In addition, for diamond nanofluids with different particle sizes, there is a trend of heat transfer capacity decreasing first and then improving with increasing mass fraction. Maintaining a filling rate of 14% and a mass fraction of 2.0%, the heat transfer performance of gravity heat pipes with particle sizes of 20 nm and 50 nm was investigated. The total thermal resistance of 50 nm diamond nanofluid gravity heat pipes was always lower than that of 20 nm diamond nanofluid gravity heat pipes. However, as the heating power increases, the advantage of 50 nm diamond nanofluid gravity heat pipes tends to weaken. Maintaining a liquid filling rate of 14% and a mass fraction of 2.0%, the heat transfer performance of gravity heat pipes with and without a liquid absorbing core was investigated. The total thermal resistance of gravity heat pipes with suction cores is lower than that of heat pipes without suction cores, but as the heating power increases, the advantage tends to weaken. Conclusions: When the mass fraction is 2.0%, gravity heat pipes have the best heat transfer performance, with a total thermal resistance increase of approximately 28.4%-64.7% compared to the maximum value. When the filling rate is 14%, the heat transfer performance is the best, and the total thermal resistance decreases by about 6.1%-8.5% compared to the maximum value. When using diamond nanofluids with a particle size of 50 nm, the overall heat transfer performance of gravity heat pipes is better than that of 20 nm. When the heating power of the power supply increases, the heat exchange performance also improves. When using a gravity heat pipe with a liquid absorbing core, its overall heat transfer performance is better than that of a gravity heat pipe without a liquid absorbing core.
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Key words:
- gravity heat pipe /
- heat transfer coefficient /
- thermal resistance /
- diamond nanofluid
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热管凭借导热性能好而成为备受关注、最为有效的传热元件之一。除此之外,热管还具有恒温性能好、冷热两端位置可以灵活调整、无动力设备、热流密度可变等优点,使得热管比常规的换热元件应用范围广、安全可靠性高、节能效果显著。重力热管又称两相闭式热虹吸管,管内腔无吸液芯,重力是工作液体循环的唯一动力,蒸发段须置于冷凝段的下方。重力热管与内腔有吸液芯的热管相比较,内部结构更简单、制造成本更低廉、安全可靠性更高。重力热管可以与成熟的工业设备结合使用,实现高效的热量传递、热量再利用等目的。
尽管重力热管已经获得了日益广泛的应用,但人们对于换热能力更强、运行更稳定的重力热管的需求也在逐渐增加。因此,学者们对重力热管展开了各方面的研究,陈家绪[1]对重力热管的换热特性开展了实验与数值模拟研究,发现热管的总热阻随加热功率的增大而减小,热管的响应时间随输入功率的增大而增大。于涛[2]通过实验测定了铜-丙酮、铜-甲醇和铜-水重力热管在相同的启动温度和启动时间下的不同换热性能,发现热管稳态工作时的温度主要由热管工质的物性参数和冷凝条件来决定,在相同的加热功率下,铜-水重力热管的等温性能优于其余2种热管。郭浩等[3]对以水、乙醇和 FC-72 为工质的重力热管的换热性能、温度均匀性以及温度波动进行了分析,当以水和乙醇为工质时,蒸发段壁面温度均匀性呈先升高后降低的趋势,并且转折点所对应的韦伯数We=1。韩振兴等[4]应用电容层析成像技术,对玻璃-乙醇重力热管冷凝段进行了可视化实验研究,发现当蒸发段温度较低时,乙醇蒸气在冷凝段壁面凝结形成条索状流动;随着温度升高,冷凝液流动过渡至环状流,出现液膜增厚甚至闭合脱落的周期性现象,并且频率随温度升高而升高。金志浩等[5]对氧化石墨烯-水重力热管开展了换热性能研究,在加热时间和质量分数相同的条件下,60%充液率时传热系数最大,相比于基液提升了60%。杨文斌等[6]对SiO2-乙醇纳米流体重力热管进行了换热性能试验研究,发现一定体积分数下,SiO2纳米颗粒的粒径越小,有效导热系数越大;相同粒径和浓度下,充液率为32%时热管传热系数最大。GOU等[7]通过对比试验研究,发现平面微槽重力热管的等效导热系数是不带微槽的平面重力热管的2.55倍,并且微槽更窄更深的平板重力热管凹槽表现出更好的热性能。SUDHAN等[8]对沟槽热管在重力和反重力条件下进行试验研究,结果表明最佳倾角为45°时,热阻降低51.1%,传热系数提高36.6%。刘时安[9]对水平热管研究发现,存在吸液芯结构的水平热管比无吸液芯热管的导热性和均温性都更好,且吸液芯节距为1.5 mm的热管的等温性和导热性要优于节距为0.75 mm的热管的,前者的传热温差和热阻更低,换热量和当量导热系数更大。郭威[10]针对未修饰丝网芯和超亲水修饰丝网芯的毛细芯分别进行了超薄平板热管换热性能试验,研究发现超亲水修饰后的芯体显著提升了热管的传热极限和反重力换热性能,水平放置时的极限传热功率提升了20%,反重力条件下运行时,传热极限可提升50%。仲宁波[11]设计了一种具有仿叶脉复合吸液芯的柔性超薄平板热管,研究发现超亲水处理使得热管极限功率提升17.6%,热阻降低51.2%,达到20 W和2.0 K/W。WU等[12]通过实验研究了吸液芯脉动热管的启动和准稳态热流体动力学特性,并与无吸液芯脉动热管进行了比较,结果表明吸液芯可以改善蒸发器内的核态沸腾以及增强热驱动力,并提供额外的毛细作用以促进液体回流至蒸发器。ZHAO等[13]通过实验研究了毛细芯毛细压力和平板微热管的传热特性,结果表明,与烧结单尺寸铜粉吸液芯相比,多尺寸铜粉吸液芯的液体传输时间缩短了16.59%,动态响应时间常数降低了0.9 s,整体热阻降低了14.22%。ZHAO等[14]通过实验观察了复合吸液芯结构对同心环形高温热管抗重力换热性能的影响,结果发现其有效热导率为
1303 W/(m·K),与重力工作的同心环形高温热管相似,这表明复合吸液芯有助于引导液体在反重力模式下的流动。虽然已有许多学者对重力热管和吸液芯热管进行研究,但对于纳米流体重力热管和吸液芯重力热管的研究较少。本文中对金刚石纳米流体重力热管进行了换热性能试验,研究了金刚石纳米颗粒质量分数、充液率、纳米颗粒粒径和加热功率对重力热管换热性能的影响。
1. 试验装置与试验方案
1.1 试验平台
图1所示是金刚石纳米流体重力热管换热性能测试平台,热管内部填充金刚石纳米颗粒(如图2所示)。试验中通过直流电源及热阻丝对蒸发段进行加热,热阻丝外圈通过绝热棉进行绝热处理。通过K型热电偶及温度采集卡记录重力热管蒸发段(T1、T2)和冷凝段(T3、T4)的温度。为确保热管换热过程达到稳态,每组试验持续10 min。
1.2 试验方案
加热功率、充液率、纳米颗粒质量分数和纳米颗粒粒径是影响重力热管换热性能的主要因素,本研究将通过上述换热试验平台开展试验,以探究这4种因素对重力热管换热性能的影响。表1列出了4种试验参数的取值范围及水平并给出了不变参数的取值。重力热管的换热性能通过式(1)计算得到的热阻进行评价。
表 1 换热试验参数Table 1. Heat transfer test parameters参数 取值 加热功率 Q / W 3,6,9,12,15,18 充液率 φ / % 8,14,20,26 纳米颗粒质量分数 ωnp / % 0.5,1.0,1.5,2.0 纳米颗粒粒径 rnp / nm 20,50 环境温度 θ' / ℃ 20 热管直径 d / mm 8 $$ R=\left(T\mathrm{_e}-T_{\mathrm{c}}\right)/P_{\text{额定 }} $$ (1) 式中:R为热阻,Te为蒸发段温度,Tc为冷凝段热阻,P额定为热阻丝加热的额定功率。
2. 重力热管换热性能评价
2.1 加热功率的影响
蒸发段的加热功率对重力热管的换热性能有显著影响。保持充液率为20%,纳米颗粒质量分数为1.0%不变,分别探究3、6、9、12、15、18 W条件下重力热管的换热性能。随加热功率变化,热管的温度信号如图3和图4所示。随着加热过程的持续,蒸发段和冷凝段温度逐渐上升,最终保持在一恒定值,说明重力热管达到稳态。由此可见,在不同加热功率下,换热过程均已达到稳态。另外随着加热功率的增大,蒸发段和冷凝段的温度均逐渐提升,而上升时间逐渐缩短,意味着重力热管在高功率下能够更快速地启动。此外,随着功率的增大,蒸发段和冷凝段温差有减小趋势,意味着重力热管在高输入功率下具备更好的均温性能。
不同颗粒粒径条件下,热管总热阻随加热功率增大的变化规律如图5所示。相同纳米颗粒质量分数和充液率下,加热功率增大时,总热阻呈现出降低的趋势,因为随着加热功率增大,蒸发段温度升高,一方面会加剧核态沸腾,另一方面沸腾形成的对流和高温的共同作用导致纳米颗粒运动加剧,纳米颗粒相互碰撞并对基液形成扰动,使得工质的对流换热效果增强[15]。此外,50 nm金刚石纳米流体重力热管的总热阻明显低于20 nm的,这可能是由于小粒径的纳米颗粒更容易发生团聚,影响了其对基液的强化换热效果。随着加热功率的增大,液体内部核态沸腾加剧,纳米颗粒受到的扰动变得剧烈,团聚现象有所缓解,使得不同粒径下热阻的差距逐渐减小,换热效果在18 W时基本持平。
因此,重力热管的总热阻随加热功率的增大而降低,并且变化趋势逐渐平缓,总热阻在加热功率为18 W时达到最小值1.18 ℃/W,与3 W时的热阻相比降低了80%。
2.2 充液率的影响
保持纳米颗粒质量分数为2.0%,加热功率为6 W不变,分别探究充液率为8%、14%、20%和26%条件下重力热管的换热性能。随充液率的变化,20和50 nm金刚石纳米流体重力热管的温度信号变化情况如图6和图7所示,可以看出20 nm金刚石纳米流体重力热管在20%充液率时的总体温度高于其他充液率时的,而26%充液率时的总体温度低于其他充液率时的,这是因为随着充液率的升高,20 nm金刚石颗粒逐渐变得更易于团聚,但由于充液率上升而确保了换热效果的提升;50 nm金刚石纳米流体重力热管在8%充液率时的总体温度低于其他充液率时的,而26%充液率时的总体温度高于其他充液率时的,这是因为50 nm金刚石纳米颗粒相较于20 nm更易发生团聚,当充液率为26%时,充液率升高对于换热性能的提升已经无法弥补团聚作用对换热性能的减弱。
不同充液率条件下,热管蒸发段与冷凝段的总热阻随充液率增加的变化规律如图8所示。由图8可知:相同质量分数和加热功率下,随着充液率的升高,总热阻呈现先减小后增大的趋势,在充液率为14%时出现总热阻的最小值,这是由于充液率较低时,热管中液相含量太少,对流作用较弱,且容易发生干涸;充液率升高时,液相流动速度加快,传热作用也就增强;而充液率过高时,蒸发段热阻过大,传热作用又减弱[16]。此外,无论在何种充液率下,50 nm金刚石纳米流体重力热管的总热阻均小于20 nm的。
因此,重力热管的总热阻随充液率的升高呈现先减小后增大的趋势,不同纳米颗粒粒径的重力热管均在最佳充液率14%时热管总热阻达到最小。20 nm金刚石纳米流体重力热管的总热阻在充液率为14%时达到3.23℃/W,相较于8%充液率时约降低了8.5%,相较于26%充液率时约降低了6.1%。
2.3 纳米颗粒质量分数的影响
保持充液率为26%、加热功率为12 W不变,探究0.5%、1.0%、1.5%和2.0%纳米颗粒质量分数条件下重力热管的换热性能。随质量分数的变化,20和50 nm金刚石纳米流体热管的温度信号变化情况如图9和图10所示,可以看出20 nm金刚石纳米流体重力热管在1.0%质量分数时的整体温度最高,而在2.0%质量分数时的整体温度较低,这是因为随质量分数的升高,纳米颗粒也更易于发生团聚,但由于纳米颗粒的增多,整体换热能力还是得到了提升;50 nm金刚石纳米流体重力热管在1.5%质量分数时的热端温度最高,冷端温度最低,这是因为颗粒发生团聚后沉积在热端阻碍了纳米流体吸热,液相吸收热量的时间相对延长,冷端的气相也因此减少了放热,而在2.0%质量分数时又出现热端温度较低、冷端温度较高的情况,这是因为颗粒虽然发生了团聚,但由于固相增多液相减少,固相与液相间的换热效率提高了,液相变为气相所需的总热量减少,单位时间内就有更多气相到达冷端进行放热,于是换热速率得到了提高[17]。
不同纳米颗粒质量分数条件下,热管蒸发段与冷凝段间的总热阻随加热功率增加的变化规律如图11所示。不同质量分数下的试验结果表明:相同充液率和加热功率下,质量分数升高时,总热阻将会先增大后减小,在质量分数为2.0%时出现总热阻的最小值,这是因为随着质量分数升高,单位体积工质中的金刚石纳米颗粒增多,固相与固相、固相与液相间的换热效率也相应提高。另外对于不同粒径的金刚石纳米流体,随着质量分数的升高,均出现了换热能力先降低再升高的趋势。这可能是由于质量分数较低时,纳米颗粒不容易发生团聚,此时纳米颗粒的加入对基液造成的扰动及纳米颗粒的碰撞能够提高工质的换热能力[18];当纳米颗粒质量分数提高时,纳米颗粒之间团聚现象逐渐加剧,导致热管换热能力下降;随着纳米颗粒质量分数的进一步升高,由于纳米颗粒的高热导率,其浓度增大对工质导热能力的增强有可能抵消了因团聚造成的换热能力下降,表现出总体换热性能的强化。
因此,重力热管的总热阻随纳米颗粒质量分数的升高呈现先增大后减小的趋势,20 nm金刚石纳米流体重力热管的总热阻在质量分数为1.0%时出现最大值3.17 ℃/W,50 nm金刚石纳米流体重力热管的总热阻在质量分数为1.5%时出现最大值2.41 ℃/W,它们均在质量分数为2.0%时取得最小值,其相较于最大值分别降低了28.4%和64.7%。
2.4 纳米颗粒粒径的影响
保持充液率为14%、纳米颗粒质量分数为2.0%不变,探究纳米颗粒粒径为20 nm和50 nm的重力热管换热性能。根据粒径的变化,金刚石纳米流体重力热管的温度信号变化情况如图12所示,50 nm金刚石纳米流体重力热管的总体温度低于20 nm的,且均温性方面也是50 nm的优于20 nm的,但随着加热功率的增大,50 nm金刚石纳米流体的优势逐渐减弱。不同纳米颗粒粒径条件下,热管蒸发段与冷凝段间的总热阻随加热功率增大的变化规律如图13所示。由图13可知:当充液率和质量分数不变时,50 nm金刚石纳米流体重力热管的总热阻总是低于20 nm的,并且随着加热功率的增大,50 nm金刚石纳米流体重力热管的优势趋于减弱,这是因为同样质量分数的情况下,20 nm金刚石纳米流体中纳米粒子更多,在较高功率下,分子运动更为剧烈,更多的碰撞形成了更好的热交换[19-20],使得其换热能力在18 W时超过了50 nm金刚石纳米流体的。
因此,在本研究的试验条件下,以颗粒粒径为50 nm的纳米流体作为工质的重力热管,其总热阻总是低于颗粒粒径为20 nm的,但这种优势在加热功率达到一定值后趋于减弱。
2.5 吸液芯的影响
保持充液率为14%、质量分数为2.0%不变,探究有无吸液芯情况下重力热管的换热性能。吸液芯热管与无吸液芯重力热管的温度变化情况如图14和图15所示,有吸液芯热管在低功率时的整体温度低于无吸液芯重力热管,这是因为热管内壁的铜网吸液芯加速了冷凝段液滴回流,加快了换热,从而降低了热管的整体温度[21]。有吸液芯和无吸液芯条件下,热管蒸发段与冷凝段间的总热阻随加热功率增大的变化规律如图16所示。由图16可知:有吸液芯重力热管的总热阻比无吸液芯重力热管的总热阻要低,但随着加热功率的增大,优势趋于减弱。有吸液芯重力热管内壁嵌有铜网吸液芯,纳米流体蒸发上升后在冷凝段形成液滴,液滴在吸液芯作用下加速向蒸发段回流,因此有吸液芯热管的换热性能整体优于普通热管[22]。并且增加吸液芯会提高其换热面积,对其换热能力的提高也有一定帮助。
因此,在本研究的试验条件下,吸液芯重力热管的总热阻往往低于无吸液芯重力热管,但这种优势也随着加热功率的升高而减弱。
3. 结论
通过开展重力热管换热性能试验,探究了充液率、纳米颗粒质量分数、加热功率、纳米颗粒粒径和有无吸液芯等参数对重力热管换热性能的影响,主要包括:
(1)重力热管的总热阻随加热功率的增大而降低,并且变化趋势逐渐平缓,总热阻在加热功率为18 W时达到最小值1.18 ℃/W,与3 W时的热阻相比降低了80%。
(2)重力热管的总热阻随充液率的升高呈现先减小后增大的趋势,不同纳米颗粒粒径的重力热管均在最佳充液率14%时热管总热阻达到最小。20 nm金刚石纳米流体重力热管的总热阻在充液率为14%时达到3.23 ℃/W,相较于8%充液率时约降低了8.5%,相较于26%充液率时约降低了6.1%。
(3)重力热管的总热阻随纳米颗粒质量分数的升高呈现先增大后减小的趋势,20 nm金刚石纳米流体重力热管的总热阻在质量分数为1.0%时出现最大值3.17 ℃/W,50 nm金刚石纳米流体重力热管的总热阻在质量分数为1.5%时出现最大值2.41 ℃/W,它们均在质量分数为2.0%时取得最小值,其相较于最大值分别降低了28.4%和64.7%。
(4)在本研究的试验条件下,以颗粒粒径为50 nm的金刚石纳米流体作为工质的重力热管,其总热阻总是低于颗粒粒径为20 nm的,但这种优势在加热功率达到一定值后趋于减弱。
(5)在本研究的试验条件下,吸液芯重力热管的总热阻往往低于无吸液芯重力热管,但这种优势也随着加热功率的升高而减弱。
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表 1 换热试验参数
Table 1. Heat transfer test parameters
参数 取值 加热功率 Q / W 3,6,9,12,15,18 充液率 φ / % 8,14,20,26 纳米颗粒质量分数 ωnp / % 0.5,1.0,1.5,2.0 纳米颗粒粒径 rnp / nm 20,50 环境温度 θ' / ℃ 20 热管直径 d / mm 8 -
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