Study Of Cold Storage Characteristics Of Solid Nitrogen

Crycooler is easy to be operated with low cost and has compact structure.In cryogenic field,the cryocooler has gradually replaced the traditional method of using cryogenic refrigerants to provide cold energy.Because the cold energy provided by crycooler is limited at low temperature,the temperature stability of cryogenic system is easily be affected by thermal disturbances,such as heat leak from surroundings and heat generated by devices.Meanwhile,the mechanical vibration and electromagnetic noise generated by the crycooler during operation will affect the accuracy of measurement of the precision instruments.So it is necessary to switch off crycooler during the process of measurement.To maintain the temperature stability of the system under the above conditions,one way is to attach the cold storage unit with large heat capacities to the cryogenic device using a thermal link.Compared to other cold storage materials,nitrogen is easily obtained with low price and is very safe to use.At low temperature,solid nitrogen has large specific heat capacity and large latent heat of phase transformations at around 35.6K and 63.15 K.So the study of cold storage characteristics of solid nitrogen is meaningful and has practical value.This paper has designed two sets of cold storage units and the experimental system is built based on a two stage G-M cryocooler.During cooling process nitrogen gas is gradually charged into the void space of cold storage unit and converted to solid nitrogen at low temperature.After the cryocooler is switched off at the lowest temperature,the temperature of cold storage unit will constantly increase under the influence of heat leak from outside surroundings.The cold storage characteristic of solid nitrogen is tested during the warm-up process and experimental results are as follows: 1)At the temperature range of 4K~64 K,warm-up time of cold storage unit with 9.3g,18.6g and 27.9g nitrogen are 3.3%,6.5% and 10.1% longer than empty cold storage unit respectively.At the temperature range of 4K~93K,warm-up time of cold storage unit with 9.3g,18.6g and 27.9g nitrogen are 1.3%,3.7% and 5.3% longer than empty cold storage unit respectively.2)After the copper foam is inserted in solid nitrogen,the effective thermal conductivities are significantly increased and the cold energy release process is accelerated.Obvious temperature pauses can be seen at phase transition temperatures of solid nitrogen,i.e.,35.6 K and 63.15 K.At the temperature range of 4K~64 K,warm-up time of cold storage unit with 9.3g,18.6g and 27.9g nitrogen impregnated in copper foam are 3.7%,7.6% and 12.6% longer than empty cold storage unit respectively.At the temperature range of 4K~93K,warm-up time of cold storage unit with 9.3g,18.6g and 27.9g nitrogen impregnated in copper foam are 2.6%,4.2% and 6.1% longer than empty cold storage unit respectively.The numerical models are established based on experiments.Then the process of thermal conduction and the phase transition inside of cold storage unit with and without copper foam are compared.The VOF model and enthalpy porous medium model are adopted in the numerical simulation of heat transfer process in pure nitrogen.Besides,the effect of gravity is considered.The flow resistance is further considered after the copper foam is inserted in solid nitrogen and the two-temperature model is established to obtain the temperature field of nitrogen and copper foam respectively.The results show that the internal temperature gradient in nitrogen is reduced and phase transition process of nitrogen is accelerated under the effect of copper foam.The maximum temperature difference in pure nitrogen during heat conduction process is 0.7K and time of solid-solid phase transition and solid-liquid phase transition processes are 476 s and 700 s respectively.After the copper foam is inserted in solid nitrogen,the maximum temperature difference in nitrogen during heat conduction process is reduced to 0.2K and time of solid-solid phase transition and solid-liquid phase transition processes are shortened to 280 s and 640 s respectively.It can be known from the experimental and numerical results that the effective thermal conductivities are significantly increased and the phase transition processes of nitrogen are accelerated after the copper foam is inserted in solid nitrogen.Such cold storage unit can be used to enhance the thermal stability of cryogenic system under thermal disturbances.According to the heat leak power,the mass of nitrogen charged into cold storage unit can be adjusted to achieve great cold storage effect.