Study On Visualization And Heat Transfer Performance Of Improved Closed Loop Pulsating Heat Pipe

Pulsating heat pipe (PHP) is a kind of new efficient heat transfer element, whichis paid more attention for cooling technology requirement improving. It possessessimple structure. However, working fluid state in PHP is gas-liquid two-phase flow,which causes complex operation mechanism of PHP. Experiment and theory studyneed constant improvement, so practical engineering can be provided technicalsupport.Visualization and heat transfer performances were investigated by experiment.Research targets were as follows:(1) Observation of flow pattern and flow mode ofimproved closed loop PHP.(2) Analysis of start and operation characteristic curve.(3)Analysis of heat transfer performances.(4) Analysis of nonlinear chaoticperformance.Specific research contents were as follows.1. There were three visual specimens. Distilled water, ethanol and acetone wereused as the working fluids. It was found that flow pattern of working fluid includedbubbly flow, plug flow and mixed flow (plug flow and annular flow), but there wasno annular flow distributed in the whole channel. Flow modes of working fluid wereinvestigated with different stages (filling liquid stage, start, operation stage),inclination and filling ratios.2. Start and operation characteristic curve of improved closed loop flat plate PHPwas studied with specimen NO.2. Start of PHP is of temperature progressive start, thatis to say, temperature rows slowly to stable operation temperature and then steps intostable operation stage. Moreover, input power, inclination, filling ratio, thermalproperties of working fluid and cooling water flow on start were investigated.3. Heat transfer performances of improved closed loop flat plate PHP werestudied by specimen NO.2and specimen NO.3. It was found that there was theoptimal cooling water flow during operation. Heat transfer limit can be improved tosome extant with cooling water flow increasing, which is more obvious withinclination. Heat transfer performances were not obvious with inclination range from60°to90°. However, PHP cannot operate with small inclinations of45°and30°. Heat transfer performances of specimen NO.2were better than specimen NO.3at the sameconditions.4. Chaotic performance of PHP was analyzed with phase space reconstruction oftemperature time sequence. Operation state of PHP can be reflected by distribution ofthree-dimensional space attractor. Attractors firstly disperse in the whole phase space,then appear mass status, finally transit to banding and distribute in the whole spacewith input power increasing. The phenomenon of attractor showed three operationstages of PHP, which are unstable pulsation stage, stable pulsation stage anddry-burning stage. Improved closed loop PHP possesses chaotic dynamiccharacteristics by attractor distribution.