| In the fields like waste heat recovery and utilization,heat utilization of new energy,and battery thermal management,the supply and demand of thermal energy both have the problems of mismatch between time and space,intermittency and volatility.In order to solve these problems,it is meaningful to develop phase change thermal energy storage(TES)enhancement technology.Combining heat transfer device with high efficiency reasonably to enhance the heat storage process passively,which does not require extra energy consumption,has become a hot spot of research.Oscillating heat pipes(OHPs),as efficient heat transfer device,have advantages of abundant external shape,flexible heating mode,high heat transfer limit and good resistance to gravity compared with traditional fins,capillary force driven heat pipes(HPs)and gravity driven HPs.It has great application potential used in TES system to enhance heat transfer.This paper is focus on the heat transfer law,internal pulse characteristics and strengthen heat transfer mechanism of OHP for TES process intensification application.The main work and conclusions are as follows:1.Parameterized experimental study on the characteristics of heat transfer and internal pulsation of large scale OHPs has been carried out.Also,theoretical analysis has been carried out on the instability phenomenon of gas-liquid distribution in the OHP with upper critical diameter.The results showed that: The long distance heat transport OHPs filled with self-rewetting fluid(SRWF)show much more excellent performance.The OHP with 3mm inner diameter(DI)under the filling ration(FR)of 50% to 60% might has the most considerable thermal performance in the stable working state and at large heat load.The thermal resistance is as low as 0.0024?/W.When the working medium,DI and FR are the same,the start-up temperature is inversely related to the installation angle.The force analysis model of the liquid slug in the OHP with upper critical diameter was established and the failure judgment equation of liquid slugs/vapor plugs distribution was derived.The longer the plug is,the more likely it is to fail at small angles.On the contrary,the higher the pressure difference between the lower plug and the upper plug is,the more likely it is to fail at large angles.Compared with the former two parameters,the influence of the inner diameter is relatively complex.The failure installation angle is mainly affected by the combination characteristics of interface and working medium,the internal diameter of OHP,the length of liquid slug and the external pressure of liquid slug.2.A OHP with external expansion structure was invented and fabricated,and its working characteristics and the strengthening law of different modes of external heat source field(EHSF)were studied experimentally.The results showed that: The OHP with novel external expansion structure exhibits well start-up performance under wide range of FRs and heating power when filled with water as working fluid.The minimum start-up temperature increases first and then keeps almost constant with the rise of FR.Meanwhile,it also shows strong adaptability to heat load under proper FR.The whole OHP has not obvious difference for each branch in the heat transfer performance under wide working conditions.However,higher uniformity for branches of the OHP does not mean better heat transfer performance.The pulsing EHSF mode and alternate EHSF mode are both helpful to increase the fluctuation frequency and amplitude of evaporator temperature.The thermal resistance of the OHP in different heating modes satisfies the following relationship in addition to a few conditions: the pulsating mode is the minimum,the alternate mode the second,and the continuous mode of is the largest.The thermal resistance at 100 W can be reduced by more than 10% with the heating period from 2s to 10 s.The thermal resistance at 200 W can be even reduced by over 30% for the pulsing heating mode with short heating period of 0.4s.3.A pulsating heat transfer model of "two liquid slugs-two vapor plugs" was established.The reliability of the model was verified by visual experiment,and the heat transfer mechanism of OHP was simulated and analyzed.The results showed that: The relative length of the liquid slug has a significant impact on the pulsating characteristics and even the working stability.Lower latent heat of gasification can improve internal driving force and increase pulsating strength of working medium.The sensible heat absorption is greater than the sensible heat release,and with the reduction of latent heat,the internal pulsation intensifies,the sensible heat absorption and sensible heat release both increase.When the FR is relative low or high,the OHP cannot work normally.There exists an appropriate range of FR for normal operation of the OHP,in which the internal pulsation intensity increases with the increasing of FR.It is conducive to improving the sensible heat transfer performance.As the DI is within a suitable range,the OHP can work stably.The average sensible heat released increases linearly with DI.4.A mathematical model of two branched OHP was established.The internal pulsation characteristics and heat transfer enhancement mechanism under variable EHSFs were studied numerically.The results showed that: Under the effect of the three variable EHSFs,the temperature and pressure of vapor plug,the displacement and velocity of liquid slug all show variable amplitude pulsation,which change periodically on the whole and show additional fluctuation characteristics upon the original basis.For the non-uniform pulsation mode with close heating time and interval time,the additional pulsation frequency and amplitude of the liquid slug movement generated by periodic excitation are relatively large.The sensible heat transfer enhancement ratio can reach 6% under appropriate heat source period when the heating/interval time ratio is 4:4,5:3 or 6:2.For the uniform pulsation mode,when the heat source period is short,the pulsations can be maintained.Longer heat source period would results in insufficient driving force or the driving force direction not matching the motion direction of working medium.With the increase of heat source period,the enhancement first grows exponentially to 12.3% and then decreases linearly approximately.The sensible heat transfer attenuation rate is 6.6% at 0.5s.For the uniform alternating mode,the additional fluctuation of liquid slug is more obvious.However,the influence trend of the heat source period is not clear.The heat source period contributing to enhance heat transfer is narrow,which may be related to the configuration of OHP,but the sensible heat enhancement rate can be as high as 25%.5.A phase change heat storage module based on OHP was designed.The heat transfer characteristics of the module under different working conditions,angle and heating power were studied.The results showed that: The highest temperature of the PCM locates in the middle region and The increasing speed has a positive correlation with the heating power during the charge process.When the phase transition of PCM completes,the rising speed of the evaporator temperature of the OHP increases,while the temperature difference between the evaporator and the PCM will decrease slightly and then become stable.The fluctuation occurs only when the evaporator temperature exceeds about 60oC during the discharge process.Compared to the coupling module without working medium,the temperature uniformity have been greatly improved for the case with water,SRWF and ETH,respectively. |
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