Integral Microcryogenic Stirling Systems As A Part Of Cryostatting Photoreceiving Modules Based On Long Ir Region Matrix
SETTING THE PROBLEM
Domestic manufacturers of various types of cooled photoreceiving devices require large-scale production of microcryogenic systems (MCS) with higher efficiency, lower weight and size indicators and a life of up to 20 thousand hours and more. Today, no high-quality microcryogenic systems (MCS) are produced in Russia that may satisfy the manufacturers of PRM, while abroad there is an active systematic increase in the life of existing cryo-coolers, development and mastering of the production of fundamentally new systems for cryostatting is underway.
When designing the MCS, the specialists of JSC "ODB "ASTRON" applied the methods and process solutions to improve efficiency, improve the weight and size, energy and operational life indicators of the integrated MCSs with a cooling capacity of 500 and 750 mW (under normal climatic conditions and Те = 80 К) operating in a closed reverse thermogasdynamic regeneration Stirling cycle with internal heat recovery, using ultrapure helium gas as the working medium.
FEATURES OF THE MCS DESIGN
The developed MCS (Fig. 1) are the micromodules consisting of piston machines and heat exchangers in the form of a single unit. They have no valves, reduced size and weight, improved energy performance.
The specificity of the cycle of such MCSs is based on the processes of compression and expansion of the cryoagent, accompanied by heat and mass transfer between cavities with different temperatures, non-stationarity of processes in heat exchangers, temperature fluctuations, and loss of cooling capacity of various nature. These circumstances greatly complicate the mathematical modeling of such devices. The most complete accounting for the loss of cooling capacity of the MCS is the key to increasing their efficiency.
Total cooling capacity of the ideal Stirling cycle:
Qе = М R Tе ln (V1 / V2),
where M is the mass flow rate of the cryoagent gas; R is the gas constant of the working gas; Te is the temperature of cryostatting; V1, V2 is the volume of the cavities of compression and expansion, respectively.
Useful cooling capacity of the Stirling cooler:
Qп = Qe – ΔQ,
where ΔQ = Qнед. + Qтп + Qпер. + Qвт + Qгидр. + Qпроч. is the loss of cooling capacity due to inefficiency of heat exchange in the regenerator, heat leakage along the structural elements, overflows of the cryoagent, heat transfer by the propellant, hydraulic losses, other losses, respectively [1–3].
The MCS of this class is also a high-precision mechanism, the details of which are manufactured on precision machines with an accuracy of 0.1 µm and a repeatability of 1 µm. JSC "ODB "ASTRON" has a fleet of such machines. Fig. 2 shows some precision details of the MCS.
The integral-design MCSs have a sufficiently high efficiency, since the separation of blocks during the implementation of the split-Stirling cycle increases the "dead" volume of the device cavities, hydraulic losses in the channels, and most importantly, it is difficult to achieve the optimum phase shift angle between the maximum volume of the compression cavity and the minimum expansion cavity pressure .
The MCS is equipped with an electric motor with an adjustable rotational speed, which makes it possible to maintain the temperature of cryostatting at low power consumption (less than 3–4 W). Low power consumption is reflected in the increase in the life of the MCS and the equipment as a whole, the decrease in the level of vibration of the photoreceiving module (PRM), which is very important for obtaining the required photoelectric parameters. As the MCS drive, a highly efficient torque motor is used in a lightweight version jointly developed by LLC "Modem-Tekhno" and JSC "ODB "ASTRON". The motor consists of a stator with nine windings and a rotor made of a rare-earth magnetic material in a ring design. The Hall position sensor is installed on the shaft of the electric motor. The electronic control unit regulates the operation of the electric motor, supplying power to the stator windings. The magnitude and duration of the supplied current is formed according to the indications of the Hall sensors and a thermal sensor in the zone of the PRM cryostatting. The electronic control unit is built into the motor. An important element in the motor design is an insulating screen separating the cavity of the electric motor and the internal volume of the MCS. This eliminates the ingress of foreign particles from the stator side into the gas cavity of the machine.
During the start-up period, the system operates with the maximum number of revolutions, providing the required readiness time. When the working temperature of cryostatting is reached, the system switches to a mode with a speed that provides only suppression of heat leakages, which, when integrated, do not exceed 0.2–0.3 W for the MCS and PRM. Thus, the MCS operates in the energy-saving mode in the start-up period, consuming up to 30 W, and in the stationary mode – no more than 3–4 W. This significantly reduces the MCS’s vibratory activity, which is particularly important for the main mode of operation of the PEM, and the reduced number of revolutions also opens the way for improving the design and technology in the selection of anti-friction materials to bring the PRM working life up to 10,000 hours or more.
Since the system operates following the reverse Stirling cycle using a constant amount of a cryogenic agent, the impact of leaks and overflows, outgassing, dead volume, hydraulic losses, etc. significantly increases. Therefore, the processes used must meet the increased requirements for the production of the MCS.
One of the main features of this MCS is the availability of built-in heat exchangers in the working volume of the system. This explains the complex nature of the processes in the cycle, makes it difficult to assess the impact of changes in each parameter and design factor on the cooling capacity and efficiency of the system, since these changes affect not only the processes in a particular element of the system, but also the entire working cycle.
The main reason for the decrease in the efficiency of the MCS is the regenerator losses. Depending on the level of cryostatting, the system’s size and type, the losses can be from 30 to 90% of the available cooling capacity. This is explained by the specifics of the operation of the embedded regenerators, since the gas cavities of the regenerator are included in the MCS’s working volume. They constitute about 70% of the total "dead" volume of the system, i. e. they are one of the main reasons for reducing the specific cooling capacity, and, therefore, the efficiency of the MCS. As a consequence, a strict limitation on the size of the regenerator occurs .
The design peculiarity of the MCS regenerators is mainly determined by the large specific surfaces of the regenerator nozzles (from 104 to 5 · 104 m2 / m3) as well as the small transverse dimensions of the heat transfer elements and the small hydraulic diameter of the regenerator channels (from 0.1 mm or less), so that it is highly effective filtering material. Built-in regenerators are sensitive to the degree of purity of the working fluid. Hardening impurities, gassing and wear products, while precipitating in the "cold" sections of the regenerator, dramatically increase the hydraulic resistance and, therefore, reduce the cooling capacity of the MCS. In the flow section of the regenerator, the laminar gas flow is characterized by a pressure drop caused by friction forces and, according to the Darcy-Weisbach equation, is equal to:
∆р = (ξ · ρ · ν2 · l) / 2 · dэкв,
where ξ is the coefficient of resistance to friction forces; ρ is the specific gravity of the gas; ν is the average gas velocity in the gap; l is the channel length; dэкв – equivalent channel diameter.
In the case of helium laminar flow in the regenerator section (the mode when the equivalent diameter is much less than the channel length) ξ = 64 ν / (ν · dэкв), ν is the kinematic viscosity of the gas . Then ∆р = 32 · ν · ρ · ν2 · l / dэкв2.
Thus, the pressure drop due to friction forces is directly proportional to the average velocity of helium in the regenerator section and inversely proportional to the squared equivalent diameter ∆р ~ ν / dэкв2. And since the volume flow rate of the passing gas: G = ν · S, where S is the cross-sectional area of the channel (S ~ dэкв2), then the resulting pressure drop along the regenerator varies quite noticeably with the change equivalent diameter (proportional to the power of 4).
The wire-meshed nozzle of the regenerator is the most common one, it allows you to greatly simplify the manufacturing technology of the regenerator and increase its efficiency due to the uniform distribution of metal throughout the apparatus. The mesh is made of soft annealed wire with a diameter of 30 microns.
The optimal layout of the regenerator of the MCS of this type is to be accommodated inside the displacer. This constructive solution is used in the developed MCS, it allows to reduce the lateral dimensions of the low-temperature part. Since in order to increase the operational life of the system, the number of cycles is reduced, the travels of the piston and displacer are reduced, all this makes it possible to slightly increase their diameter and obtain efficient regenerator sizes .
ENSURING SUSTAINABLE OPERATION OF THE MCS
The features of the operation of the MCS define stringent requirements for vibratory activity. Permissible vibrations of the cold cylinder of the system in different planes should not exceed 10–20 µm. This requirement is met by the choice of the size of the mechanism and the balancing of the inertia forces that arise and the moments of the rotating masses. The correct choice of the kinematic relations of the movement mechanism contributes to the improvement of the overall and resource characteristics of the systems. In this system, the balancing issues play an important role, since it constructively implements the Stirling integral cycle, which does not have flexible junctions .
The MCS includes a two-row crank mechanism that drives the compressor piston and the displacer, and the pressure above and below the displacer is almost the same and differs only in the influence of the hydraulic resistance of the regenerator. The angular arrangement of the pistons ("gamma" scheme) ensures maximum cycle efficiency, since in this design, the optimum phase shift of the piston movement is implemented .
A very important issue, necessary for the normal operation of the MCS, is the purity of the gas cavities, which ensures the system’s operation during the entire operational life without refueling and replacing the cryoagent.
All parts of the system contain gases dissolved in their depth or adsorbed on the surface. These gases should be removed in advance, since they will be released into the working volume of the system, contaminate its gas cavity, precipitate in the regenerator, thereby reducing the cooling capacity of the MCS, as well as the durability and reliability of the system .
Ensuring the required reliability and durability of the MCS is especially demanding towards the condition of the cryoagent. The works carried out showed that the highest purity helium gas with a volume fraction of 99.9999% for helium most fully meets the required standards.
FEATURES OF THE MCS INTEGRATION
Fully assembled and filled with helium, the MCS enters the integration with MPRD. Considering the specifics of the MCS interface unit with PRM, the integration process is rather complicated and is a responsible set of technological operations, since the MCS tightness is violated due to its implementation, the gas cavities come in contact with the surrounding air, new static sealing elements are installed at the MCS docking site with PRM, tolerances of the shape and size of the working pair of a sleeve-displacer are checked. In this regard, the process of integrating the MCS with PRM requires more careful monitoring, increased demands on the room cleanliness and conducting operations on evacuating the internal cavity of the MCS and filling it with helium, followed by checking the tightness of the system.