NFLUENCE OF HEAT FLOWS ON THE COOLING CAPACITY OF A CRYOGENIC MICRO-REFRIGERATOR WITH THE JOULE-THOMSON EFFECT

Abstract

This paper presents an investigation into the influence of heat inflows on the performance characteristics of a Hampson-type microrefrigerator (MCR). This device, which operates based on the thermodynamic Joule-Thomson effect, is a critical component of modern high-tech systems, as it provides efficient cooling for infrared sensors, laser emitters, and various microelectronics used in medical, space, and defense sectors. The technological process of cooling target objects in such systems is implemented by supplying nitrogen under ultra-high pressure, ranging from 10 to 40 MPa, through a thin capillary tube of the heat exchanger, followed by rapid expansion through a throttle device to a pressure of approximately 1.1 atm and cryogenic temperatures in the range of 60–80 K. Given that direct experimental study of heat flows in systems of such a small scale is an extremely difficult technical task, the authors applied a method of complex computer modeling. This method is based on the integration of a three-dimensional geometric model of the housing and the holder with a nodal model of the internal thermodynamic processes occurring directly within the microrefrigerator channels. During the modeling, the influence of heat inflows was analyzed in detail as the vacuum level changed from high vacuum (0,01 Pa) to full atmospheric pressure

(101325 Pa). The generated results convincingly demonstrate that complete depressurization and loss of vacuum lead to an increase in total heat inflows and, consequently, to an almost twofold increase in nitrogen consumption to maintain the operating temperature. At the same time, it was established that under such conditions, the cooling time for sensors and microcircuits is reduced by 1,8 times, which creates a risk of unstable operation or thermal damage to the equipment. A significant conclusion of the work is the distribution of heat inflows: under high vacuum conditions, 58% of the heat enters through the end part of the Dewar vessel where the sensor is located, while the cylindrical wall allows only 6% to pass through. However, during depressurization, the share of heat inflow through the side wall increases sharply to 42%, which completely changes the thermal profile of the product. The scientific novelty of this study lies in the theoretical and practical substantiation of implementing additional thermal insulation for the holder, made of fluoroplastic 1 mm thick with a thermal conductivity coefficient of 0,25 W/(m·K). It was established that the use of such an insert allows for a local reduction in heat inflow through the holder assembly by 3 times, ensuring significant savings in working gas (a 30% reduction in consumption from 0,033 g/s to 0,025 g/s). The overall rate of reduction in heat inflows due to the use of insulating components reaches 16%. This opens up new opportunities for increasing operational reliability, extending autonomous operation time, and enhancing the overall efficiency of cryogenic systems in complex real-world operating conditions, allowing for the provision of scientifically based recommendations for MCR design. The results obtained can also be used to develop real-time vacuum state monitoring systems based on indirect gas consumption indicators. The paper emphasizes that it is precisely the comprehensive approach to modeling that allows for accounting for the mutual influence of design parameters and external conditions, which is critically important for miniature systems where even minor heat losses can lead to a disruption of cryogenic regime stability.