| Compared with Stirling cryocoolers,pulse-tube cryocoolers have longer lifetime and less vibrations at the price of a lower coefficient of performance.Pulse tube cryocooler has been gradually used in small cooling fields during the history of scaling down the cryocoolers to micro size.Micro pulse tube cryocoolers mostly has been used in many fields,such as infrared devices,γ-ray detectors and X-ray detectors working in low background noise and high sensitivity environments,superconducting cooling,and so on.In general,the miniature cryocoolers with a mass of a few hundreds of grams and a length of about 6cm to 9 cm have cold-end temperatures higher than typically 100 K and a cooling power of a few hundred milliwatts.Typically,the inertance tube phase shifter was adopted in the micro pulse tube cryocooler system,due to its mature technique and high stability.But,the efficiency is always quite low,the key reasons might be the following three,firstly the expansion power generated at the warm end of pulse tube cannot be recovered.And then the phase shifting limit of inertance tube since inertance tubes are severely limited in their phase shifting capability for the big impedance.Finally,the heat transfer law in the pulse tube still is not so clear enough that needed to be investigated more,anf the heat transfer loss in the pulse tube is stronger than relugar size pulse tube.As a new type phase shifter of pulse tube cryocooler,the warm displacer has the functions of phase shifting and work recovery both.The efficiency of micro pulse tube cryocooler can be improved by it.To the warm displacer,specifically the gas-driving warm displacer,moving by the gas force differences between the compression space and expansion space of the displacer system.And the movement of displacer is highly correlated with the operating conditions of the pulse tube cryocooler.The complex and coupled relationships between them make the basic theory of the displacer hard to catch comprehensively.Correspondingly,the well-designed displacer type pulse tube cryocooler in experiments is getting harder to get the theoretical good performance.The basic heat transfer theory and specific phenomenon of micro pulse tube cryocooler are sort of important topics.These effectives to the performance of micro cryocooler might be stronger than the regular size cryocooler.And the more accurate simulation by CFD method is needed.Based on all above,the major parts of this work including:1.Investigation about the motion in a general view and phase shifting mechanisms of warm displacer.A more comprehensive theory about different operation modes of warm displacer of gas driving displacer(passive displacer),linear motor driving displacer(active displacer)and moving plug with orifice was carried out.Starting from the basic momentum equations,the mechanism of the piston motion was studied theoretically,and the function of moving plug with orifice was mentioned in passing.The phase relation between the gas force and displacer velocity was analyzed in several circumstances.In theory,both gas driving displacer and linear motor driving displacer can reach the optimal phase angle of the cryocooler by using suitable spring stiffness.A hybrid model with isothermal and adiabatic for displacer pulse tube refrigerator is introduced,it reveals the basic matching mechanisms between compressor,displacer and cold head.2.The heat transfer mechanism of pulse tube in micro size.The viscous resistance of the helium gas oscillating at near wall and heat transfer loss in the pulse tube become stronger in a micro pulse tube,which makes the temperature distribution worse,and decreases the expansion efficiency a lot.Consequently,the investigation of the heat transfer of micro pulse tube is very important.This work investigated the heat transfer phenomenon in the micro pulse tube by CFD.And it reveals that the heat transfer relation in pulse tube is quite complicated,which is hard to describe this phenomenon by any experimental equations.The temperature of the helium close to the centre of the pulse tube is influenced by its own pressure and motion,while the temperature of the helium gas at near wall postion is strongly related with the wall temperature.So,the heat transfer in the micro pulse tube needs 2D simulation at least to catch its characteristic accurately.3.CFD study of the 2D gas piston in pulse tube cryocoolers.The part of the gas that always moves in the pulse tube performs the same function as a solid piston;this part of the gas is called the gas piston.The shape and the position of the gas piston changes with time and the operating conditions rather than remaining fixed.In this study a CFD method is used to obtain the details of the 2D gas piston.The velocity field in the pulse tube is obtained by the commercial code ANSYS Fluent,and a La Grange tracing method is introduced to process the velocity data in order to obtain the boundary of the 2D gas piston.Additionally,this work investigates the influence of various parameters including pressure ratio,pulse tube aspect ratio and frequency on the shape of the gas piston.4.The regenerative effect in cold heat exchanger is studied with the CFD method.The regenerative effect of the cold heat exchanger of PTRs has been presented.The gas osciallting in the cold heat exchanger absorbs and releases heat aternaltively in a cycle,which is similar to the heat transfer characteristic in the regenerator.This phenomenon could be called regenerative effect of the cold heat exchanger.The centre position of cold heat exchanger facing the pulse tube cold end is cooled down by the low temperature gas at the cold end of the pulse tube,and a small temperature gradient is formed along the axial direction of cold heat exchanger gradually.Consequently,the gas at the cold end of the pulse tube will expand in a lower temperature,which will decrease the expansion work of the PTRs and lower the efficiency of the system,especially under bigger pressure ratio conditions.In addition,the basic heat transfer mechanism has been explained,including the local heat transfer coefficient,heat flux,the temperature distribution of the narrow tube,and the relation between the Nusselt number and the Reynolds number.5.A comprehensive investigation regarding the micro-pulse tube cryocooler with a displacer phase shifter is carried out,and the experimental study about a micro pulse tube cryocooler.Including the design of the major components of the cryocooler,and 2D simulation work addressing the performance of the cryocooler systems.In the optimum phase angle conditions of the 3 mm pulse tube size cryocooler,the mass flow rate leads the pressure wave by 28.8° at the regenerator warm end,and lags the pressure wave by 27.7°at the regenerator cold end.A cooling power of 1.5 W at 80 K is obtained with an input power of 7.4 W.For the optimum phase angle condition for the optimal COP of the 2mm-PT cryocooler,the mass flow rate leads the pressure wave by 24.9°at the regenerator warm end and lags the pressure wave by26.5°at the regenerator cold end.A cooling power of 0.65 W at 80 K is obtained with a frequency of 200 hz,an operating pressure of 3.5MPa,and an input power of 2.63 W.A micro pulse tube cryocooler experiment set-up was built and prepared for several phase shifters,and it has been tested by displacer and inertance tube phase shifters respectively.The lowest temperature of 124.8K was obtained with operating pressure of 3 MPa and frequency of 118 Hz under inertance tube phase shifter.Additionally,the micro pulse tube performance with inertance tube phase shifter was simulated by Sage.Eventually,the simulation results show that the lowest temperature of 86.5K was obtained with operating pressure of 3 MPa and frequency of 118 Hz.In the finality,the problems requiring further studies are discussed. |
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