表面活性剂对水工质超长重力热管传热性能的影响

Effect of Surfactant on Heat Transfer Performance of Ultra-Long Gravity Heat Pipe with Water as Working Fluid

超长重力热管是近年来被提出的用于干热岩地热能开采的一种新技术。该技术方案通过工质的沸腾−冷凝相变来进行热量传输从而在地面获得地下数千米深的热量,突破了常规热管的热力输运距离。表面活性剂能降低液体的表面张力,从而改变液体工质的沸腾特性,能在一定程度上提升常规热管的热力性能,但在超长重力热管中的作用仍有待研究。本文在自行搭建的超长重力热管实验系统(L=40 m,D=7 mm)中,以不同浓度的十二烷基硫酸钠(SDS)水溶液为工质,研究了表面活性剂的加入对超长重力热管采热性能的影响。实验发现SDS的加入降低了热管的最佳注液量。纯水工质的最佳注液率为30%(注液高度为6 m),随SDS浓度升高,最佳注液率降低至10%(注液高度为2 m)。实验还发现,SDS的影响在不同注液量条件下有很大区别:注液量较低(2 m)时,加入SDS后热管性能改善,随着SDS溶液浓度的升高,热管的采热性能提高;注液量较高(6 m)时,加入SDS后热管性能下降,随着SDS溶液浓度的增大,采热性能下降。分析热管测温点温度波动发现,加入SDS对不同注液高度工况的影响机制并不相同。注液量较低时,SDS的加入使得热管整体壁温下降,这可能和常规热管中一样,是由于沸腾相变更加剧烈,壁面润湿性提升,从而提高了热管的采热性能。但在注液量较高时,加入SDS后,温度波动幅度减小且沿高度迅速衰减,这可能是由于工质表面张力的降低,沸腾时气泡聚并减弱,热管工作时产生的间歇沸腾减弱或消失,导致超长重力热管的采热性能下降。因为间歇沸腾在一定程度上有利于降低热管注液段与外界环境的温差,减少散热。

The ultra-long gravity heat pipe has been proposed to use as a new technology for the extraction of hot dry rock geothermal energy recently.This technology utilizes the phase change of the working fluid to transfer heat from underground with thousands-meters in depth,far exceeding the transportation distance of conventional heat pipes.Adding surfactants can reduce the surface tension of the working liquid,thereby changing the boiling characteristics as well as the thermal performance of conventional heat pipes.However,the role of surfactant in ultra-long gravity heat pipes has not yet been researched.In this work,using different concentrations of sodium dodecyl sulfate(SDS)aqueous solutions as working fluids,the effects of surfactant on the thermal performance of ultra-long gravity heat pipe were experimentally studied.The experimental heat pipe was 40 m in length and 7 mm in inner diameter(L=40 m,D=7 mm).It was found that the optimal filling ratio of the heat pipe reduced from 30%(pure water)to 10%(2000 mg/L)by adding SDS.The effect of SDS was dependent on the filling ratio:the thermal performance of the heat pipe with low filling ratio was improved by adding SDS,while decreased with high filling ratio.The surface temperature profile analyses revealed that the predominant influence mechanism of SDS on the heat pipe thermal performance varied with filling heights.At low filling height(2 m),the overall wall temperature of the heat pipe decreased by adding SDS,similar with conventional heat pipes.This should be mainly due to the improved wall wettability and the enhanced boiling phase transition.However,at high filling height(6 m),after adding SDS,the temperature fluctuation was weakened and diminished rapidly along the pipe height.This implied that due to the reduction of the surface tension of the working fluid,the Geysering boiling,which was beneficial to alleviate the suppression of boiling imposed by the deep liquid pool,was restrained.Therefore,the heat pipe thermal performance decreased at high liquid filling height.