Low-T, Low-Q Cryocoolers for Science Instruments

The first objective of the proposed research was to design a pulse tube cryocooler capable of handling cooling loads in the 10-15W range at 35 K, in addition to achieving this goal with an efficiency goal of 100 W of input power to every 1 W of cooling. In order to attain the proposed goal, a linear pulse tube cryocooler with a precooler was the approach deemed the highest rate of success. The precooler is designed to precool the gas before it reaches the 1st stage regenerator. The temperature range on the precooler was set to be 300-100K, while the regenerator was set to remain in the 100- 35K temperature range. For the design of the 35 K cooler, the first step was to determine an appropriate material to handle the 100-35K temperature gradient. Using material properties available through the resources from NIST, it was determined that there were two possible materials that would give the best performance. The criterion used to determine which material would give the best performance was the volumetric heat capacity over the temperature range desired. Gd-Sb and Er60Pr40 were the materials that met the desired criteria. Due to the complex physics that take place within a regenerator at this point, it was decided that the following analysis would be performed for both materials. The second step was to determine the optimal operating conditions. This was accomplished through the use of the computer code REGEN 3.3. REGEN 3.3 has the capability to completely model the thermal and fluid interactions that occur within a regenerator in a cryocooler system. An operating parameter sweep was performed to determine the optimal operating frequency and operating pressure. For this sweep, the frequency ranged from 20-70 Hz in 2.5 Hz increments. The pressure ranged from 1.5- 4.5 MPa, in 0.25 MPa increments. The important parameter that determines the effectiveness of a regenerator is the coefficient of performance, or the COP for short. It is the net refrigeration power normalized by the input PV work to the regenerator. With the optimal operating conditions determined, the next step was to determine the optimal dimensions of the regenerator. Another sweep was performed using REGEN 3.3 where the length ranged from 4.5-6.75 cm in 0.15 cm increments, while the diameter was varied from 4-6cm in 0.25 cm increments. Figure 2 shows an example plot of the dimension sweep for the Gd-Sb regenerator The results of this analysis showed that using Er60Pr40 gives the best performance over the dimension range. To further optimize the regenerator, the sphere size and mass flow were varied separately to see parametrically their effect on the system. With the regenerator fully optimized, the precooler was promptly designed. To ensure mass, momentum, and energy conservation in the system, the outputs of the regenerator were used as the boundary conditions to the precooler. A dimension sweep was performed to determine the optimal precooler. Unlike a regenerator, a precooler is not designed to provide any cooling. Therefore the COP is not an entirely accurate criterion to judge the performance of the precooler. Instead, the goal is to minimize the heat leak across the precooler to the adjacent regenerator. This is represented by the combination of two of REGEN 3.3 outputs, ENTFLX and TUBECD. The ENTFLX represents the enthalpy flux at the cold end of the precooler, while the TUBECD represents the thermal conduction through the tube containing the precooler mesh. In this scenario, stainless steel mesh was chosen to fill the precooler. Stainless steel was chosen due to having a favorable volumetric heat capacity in the 300-100K range. This completes the regenerator and precooler design. The pulse tube, inertance tube, and surge volumes were designed next to achieve specific phasing within the system. A well-designed heat exchanger minimizes pressure loss while maximizing heat transfer. This was accomplished using a custom Matlab program written to interlink the previously designed components with empirical rules for pulse tube design and a transmission-line-model for the inertance tube and surge volume sizing. In addition to a custom Matlab program, the computer code ISOHX was used in the heat exchanger design. With the analytical design finished, the results were used in the software package SAGE to further optimize the system. SAGE is a computer code that is capable of modeling the thermal-fluid processes in an entire cryocooler system. SAGE optimizations allow changes to be made to the every parameter, such as charge pressure, frequency, and dimensions, to see how it affects the system as a whole. Through several rounds of optimizations the final design was able to attain the proposed goals. The designed cooler surpasses the plan 100 W per W by a significant amount. The implications of this are directly applicable for NASA goals of collecting data into the depths of space. A more efficient cryocooler allows it to use less power, thus being able to be used for longer space missions to explore and learn more about uncharted areas of space. In addition the computational work for the fellowship, experimental work was promised to complement it. The results of the preliminary CFD simulations suggest that due to the large-size system components, multidimensional effects are likely to be important. To accomplish the desired experimental goals, a new experimental setup needed to be designed and implemented. This setup will be used to conduct experimental tests that will include separate effects tests to examine experimentally the performance of the key components of the cryocooler, in particular the regenerator, and compare them with model predictions. In addition, the apparatus allows us to study the sensitivity of pulse tube cryocoolers to the tilt angle with respect to gravitational acceleration, and in the near future to sudden jolts, to name a few. The experimental setup consists of a fully rotational 27 in. diameter stainless steel high vacuum dewar with 27 in. depth with an aluminum cover housing a 4K G-M cryocooler, a 500 W pressure wave generator, and a turbomolecular vacuum pumping system. In addition to the overall housing items, it contains all the equipment necessary to obtain all pertinent measurements at cryogenics temperatures. This includes a calibrated thermal mounting system to measure heat flows to and from experimental sections, a calibrated oscillating mass flow meter, precision temperature measurement and control loops, high sensitivity cryogenic pressure sensors, and a subminiature thermal anemometry system. One of the most important aspects of the experimental setup is its ability to accurately measure temperatures and heat flows. This is accomplished through the use of multiple thermocouples, silicon diodes, and RTD’s to allow for multiple temperature measurements.