Study On Acoustic Power Match In A Stirling Pulse Tube Cryocooler

Stirling pulse tube cryocoolers(SPTCs)have potential to meet the growing requirements of High-Temperature Superconductor technology,because of the advantages of high efficiency,high reliability and long life.Based on studies over the past ten years,it seems that the development of pulse tube cryocoolers(PTCs)has come to a bottleneck.Two common problems exist in studies of SPTCs.One is the difficulty in quantifying the energy flows and the losses in the PTC.The other is that the methods for matching the linear compressor and the cold end are imperfect.This paper proposes a new way to match the linear compressor and the cold head from the perspective of acoustic power,and seeks to guide cold head design and compressor selection Combined with the measurement of acoustic power,a new loss equation is proposed which is able to quantify the main losses of cooling power in the SPTC and consequently assist in the optimization of SPTC performance.The theoretical and experimental studies are listed as follows:1.Exploring the intrinsic relationship mechanism between acoustic power and the losses of cooling power in the cold head.It shows that the mass flow rate in the regenerator is the key to losses in the regenerator,and the losses could be optimized by modifying the the phase angle and pressure amplitude for a constant input acoustic power.The energy flow distributions and loss mechanisms are the central problems of PTCs.In this paper,the unsteady model has been transformed into a steady model,based on the time-averaged analyses.Then,the relationship between the acoustic power and the expansion power is studied based on control volume and control mass methods.The irreversible losses generated by the flow are in essence the losses in cooling capacity,since the acoustic power is delivered by the fluid.In this study,three different kinds of irreversible flow losses have been analyzed,according to the characteristics of each component of the cold head.A new loss equation has been deduced to analyze the effect of the losses on the cooling capacity.The equation reveals that the major loss occurs in the regenerator,and that the main contribution to loss is the high mass flow rate in the regenerator.The pressure amplitude and phase angle are two aspects that could reduce the mass flow rate in the regenerator,without changing the acoustic power input.Therefore,the irreversible losses analyses shows that the match between the compressor and the cold end should consider the intensity of the acoustic power,the phase angle and the pressure amplitude.2.Proposing a new method of generating high acoustic power and high pressure amplitude while maintaining high efficiency with a linear compressor.This provides the theoritical suppport for matching the cold end and the linear compressor.A good compressor should provide high acoustic power at high efficiency.Since the pressure amplitude is very important to the cold end,the comprosser should also generate a high pressure amplitude.In this study,the efficiency and the output acoustic power of a linear compressor are optimized to the operating parameters,instead of the classic impedance match method.Then,by using the phasor analysis method,the pressure amplitude could be represented by the hypotenuse of a triangle model.The new method shows that resonance is the necessary condition to make the most of the compressor according to the acoustic power equation and the efficiency equation.At the resonant state,the acoustic power equation and the efficiency equation are turned into single-parameter equations by introducing the ratio of velocity and current.Thus the curve of maximum acoustic power could be taken as the reference curve for balancing acoustic power and efficiency,since the linear compressor is limited by the rated current and the rated displacement.Combining with the phasor triangle model of pressure amplitude,the curve of maximum pressure amplitude,the curve of pressure amplitude for the maximum acoustic power and the curve of pressure amplitude for a given acoustic power can be derived geometrically.The optimal balance between the acoustic power,the efficiency and the pressure amplitude of the compressor can be realized by reference to the three curves,in the selection and utilization of linear compressors.Therefore,the method meets the demand of the cold head to the acoustic power and the pressure amplitude.3.Proposing simple methods to measure the acoustic power in the PTC as well as providing technical support on matching the cold head and the linear compressor.The measurement technique of acoustic power can not only assist in matching acoustic power between the cold head and compressor,but also helps to quantify the system losses.The difficulty in measurement of acoustic power derives from the measurements of velocity and phasor angle.This paper suggests that using the energy conservation law to gauge the acoustic power in the compressor could avoid the measurement of the phase angle.As to the acoustic power in the cold end,it can be deduced from the local measurement of the temperature and the pressure,because the projections of mass flow rates on the pressure phasor are almost equal.This new method avoids the measurement of the phase angle and velocity,and it has been verified by simulations of REGEN on a large scale.4.Experimental verification of the acoustic power match method has been done on a large capacity SPTC.The performance of the SPTC is significantly increasead by using the acoustic power match method.Experiments include three parts:verifications on models of the linear compressor,optimizations of the key components in the cold head and the optimization of the SPTC basedon the first two optimizations.It shows that calculated pressure amplitudes of the phasor triangle model are consistent with the experimental values;and that the phasor triangle model can be used to explain the propotional phenomena between pressure amplitudes and displacements.The structures of the transition tube and the cold end heat exchanger markedly affect the cooling performance which was improved in the cold end experiments.Since parallel transfer tubes show the same performance with serial ones,parallel transfer tubes have been used to reach the optimal match volume.Ultimately,the cooling capacity of 187.1 W @ 80K is achieved with a 8023 W electrical power input.According to the loss equation,the maximum loss of the system is due to excessive mass flow rate in the regenerator.