| Two-phase closed thermosyphon (TPCT) has many advantages, such as high efficiency, simple structure, low fabrication cost and so on. Although it has been widely used in many industrial devices, the heat transfer phenomena in a TPCT are complex, and many factors have effects on the heat transfer performance and are related to each other. Besides, there are different types of heat transfer limit. As a result, the available models cannot explain well the heat transfer process in the TPCT. With the rapid development of superconducting technology, the TPCT will play a more important role in high reliability requirement and cooling transfer in long distance and narrow space. However, the existing studies with cryogenic fluid as working fluid are not sufficient, which limits the applications in cryogenic field. The present work focuses on further developing the mathematical model for the TPCT and exploring its heat transfer performance at liquid nitrogen temperature. The following contents are included:1. Development of a new model for analyzing quantitatively the effects of filling ratio on the flow pattern inside a TPCT and the cooling temperatureFilling ratio is one of the most important factors for the heat transfer performance of a TPCT. It has significant effect on the flow pattern inside and the cooling temperature. For different filling ratios, there are diversiform distributions of liquid film and liquid pool. However, the available models only consider the flow patterns partly, and fail to explain well the different heat transfer mechanisms of liquid film and liquid pool. Consequently, their analyses on the effect of filling ratio are not sufficient. In the present work, based on the different heat transfer mechanisms, the individual models are developed for condenser region, liquid film region and liquid pool region in evaporator, respectively. Three types of flow pattern and three types of critical transition pattern, which could exist in a TPCT in the steady operation, are well considered. The flow pattern and the cooling temperature at different filling ratios can be determined by solving the total mass and energy conservation in the TPCT iteratively. The calculated results explain the effects of two types of critical filling ratio, which are related to the two types of critical transition pattern. One makes the TPCT in the operation with the best cooling performance. The other is the transition point for the different dependence of cooling performance on filling ratio. Finally, the range of filling ratio, which can keep the TPCT in the stable and effective operation, is obtained.2. Development of a novel model for a TPCT to predict dryout, flooding and boiling limit together. Establishment of the range of a TPCT in the steady operation by combining with the dependence of critical filling ratios on heat transfer rateThere are three types of heat transfer limit, which could happen in the TPCT:dryout limit, flooding limit and boiling limit. At present, the theoretical models for predicting heat transfer limit consider dryout and flooding limit together, and their validities on dryout limit have not been verified. In this work, based on the mechanisms of heat transfer limit, the concept of dryout-ratio is proposed for predicting dryout limit by combining with the general criterion-completely dryout liquid pool utilized in former models. The empirical correlation for the maximum gas Reynolds number is deduced for predicting flooding limit. The empirical void fraction, at the onset of annular flow in vertical liquid-vapor two-phase flow, is introduced into the model as the criterion for flow pattern transition in liquid pool, and predicting boiling limit. Consequently, a novel model, which can predict the three limits together, is developed in this work. By combining with the dependence of critical filling ratios on heat transfer rate, the range of a TPCT in the steady operation is established, and the effects of operating pressure and geometries of a TPCT are also discussed. 3. Experimental investigation on the heat transfer performance of a TPCT with nitrogen as working fluidAt present, the general refrigerants and liquids in the medium temperature range are mostly used as working fluid in the available experimental studies. In fact, some different factors, such as the properties of working fluid, boiling phenomenon and so on, result in the different heat transfer performance of the TPCTs working at different temperatures. However, the available studies with cryogenic fluids are not sufficient to explain the characteristic. In this work, the experimental setup of the cryogenic TPCT is designed and manufactured, and the investigation with nitrogen as working fluid is performed. In the cool-down process, it is observed that the operating pressure keeps relatively steady at the filling ratio (defined as the volume ratio of charged liquid to the evaporator) of 18.8%. When filling ratio is up to 49.6%, the pressure firstly increases to peak value, and then decreases gradually. It finally keeps steady when the system reaches the steady state at the heat transfer rates performed in the experiments. At the filling ratio of 62.0% and the heat transfer rate of 10 W, the pressure reaches the peak and then shows oscillation during decreasing to the steady state. When the heat tranfesr rate is up to 15 W, the amplitude of oscillation is augmented and cannot reach steady state. By calculating the transient heat transfer rate, it is found that the performance of operating pressure is closely related to that. At low heat transfer rate, they almost reach their peak vaules at the same time. At high heat transfer rate, the pressure peak is slightly behind. The comparisons between the model predictions and the experimental results are made and analyzed. The developed models are validated and some empirical values for the experimental condition in this work are determined, which can provide the reference for the relative studies.
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