Jeffrey R. Feller

A piezoelectric microvalve for cryogenic applications

This paper reports on a normally open piezoelectrically actuated microvalve for high flow modulation at cryogenic temperatures. One application envisioned is to control the flow of a cryogen for distributed cooling with a high degree of temperature stability and a small thermal gradient. The valve consists of a micromachined die fabricated from a silicon-on-insulator wafer, a glass wafer, a commercially available piezoelectric stack actuator and Macor TM ceramic encapsulation that has overall dimensions of 1 × 1 × 1c m 3 .A perimeter augmentation scheme for the valve seat has been implemented to provide high flow modulation. In tests performed at room temperature the flow was modulated from 980 mL min −1 with the valve fully open (0 V), to 0 mL min −1 with a 60 V actuation voltage, at an inlet gauge pressure of 55 kPa. This range is orders of magnitude higher flow than the modulation capability of similarly sized piezoelectric microvalves. At the cryogenic temperature of 80 K, the valve successfully modulated gas flow from 350 mL min −1 down to 20 mL min −1 with an inlet pressure of 104 kPa higher than the atmosphere. The operation of this valve has been validated at elevated temperatures as well, up to 380 K. The valve has a response time of less than 1 ms and has operational bandwidth up to 820 kHz. (Some figures in this article are in colour only in the electronic version)

A piezoelectric microvalve with integrated sensors for cryogenic applications

This paper describes a normally open, self-encapsulated, gas valve that has embedded sensors for pressure and temperature monitoring. The valve has been validated at operating temperatures over 80-380 K, and at pressures up to 130 kPa. A perimeter augmentation scheme for the valve seat has been implemented to provide higher flow modulation. Two kinds of suspensions are described for the valve seat. In tests performed at room temperature, the flow was modulated from 980 mL/min. with the valve fully open (0 V), to 0 mL/min. with 60 V actuation, at an inlet pressure of 55 kPa. Cryogenic flow rate tests show similar modulation with flow from 166 mL/min. with the valve fully open, to 5.3 mL/min. with 120 V actuation voltage, at an inlet pressure of 70 kPa. The platinum RTD temperature sensor is independently tested from 40-450 K with sensitivity of 0.23 %/K in its operational range of 150- 450 K. The pressure sensor has sensitivity of 250 ppm/kPa at room temperature.

A Low Power, Microvalve-Regulated Drug Delivery System using a SI Micro-Spring Pressurized Balloon Reservoir

This paper reports on a drug delivery system that provides modulated delivery of liquid-phase chemicals. The device uses silicon torsion springs on a 2times3 cm2 chip to pressurize a soft polymeric reservoir and regulate flow with a piezoelectricaly actuated silicon microvalve that is 1.5times1.5times1 cm3. Using the finished device, regulated diffusion of a fluorescent dye into agar gel was demonstrated. Fluid flow out of the 500 muL reservoir could be regulated from 10-500 muL/min with up to 80 kPa of delivery pressure. Typical regulation consumes 0.136 muW of power. Analysis of the valve, reservoir springs, and a model based on pressure-enhanced diffusion are presented and are validated by experimental data.

Cryogenic Boil-Off Reduction System Testing

Cryogenic propellants such as liquid hydrogen (LH2) and liquid oxygen (LO2) are a part of NASAs future space exploration due to the high specific impulse that can be achieved using engines suitable for moving 10s to 100s of metric tons of payload mass to destinations outside of low earth orbit. However, the low storage temperatures of LH2 and LO2 cause substantial boil-off losses for missions with durations greater than several days. The losses can be greatly reduced by incorporating high performance cryocooler technology to intercept heat load to the propellant tanks and by the integration of self-supporting multi-layer insulation. The active thermal control technology under development is the integration of the reverse turbo- Brayton cycle cryocooler to the propellant tank through a distributed cooling network of tubes coupled to a shield in the tank insulation and to the tank wall itself. Also, the self-supporting insulation technology was utilized under the shield to obtain needed tank applied LH2 performance. These elements were recently tested at NASA Glenn Research Center in a series of three tests, two that reduced LH2 boil-off and one to eliminate LO2 boil-off. This test series was conducted in a vacuum chamber that replicated the vacuum of space and the temperatures of low Earth orbit. The test results show that LH2 boil-off was reduced 60% by the cryocooler system operating at 90K and that robust LO2 zero boil-off storage, including full tank pressure control was achieved.

CHARACTERIZATION OF AN ACTIVELY COOLED METAL FOIL THERMAL RADIATION SHIELD

Zero boil‐off (ZBO) or reduced boil‐off (RBO) systems that involve active cooling of large cryogenic propellant tanks will most likely be required for future space exploration missions. For liquid oxygen or methane, such systems could be implemented using existing high technology readiness level (TRL) cryocoolers. However, for liquid hydrogen temperatures (∼20 K) no such coolers exist. In order to partially circumvent this technology gap, the concept of broad area cooling (BAC) has been developed, whereby a low mass thermal radiation shield could be maintained at temperatures around 100 K by steady circulation of cold pressurized gas through a network of narrow tubes. By this method it is possible to dramatically reduce the radiative heat leak to the 20 K tank. A series of experiments, designed to investigate the heat transfer capabilities of BAC systems, have been conducted at NASA Ames Research Center (ARC). Results of the final experiment in this series, investigating heat transfer from a metal foil fi...

A LOW TEMPERATURE REGENERATOR TEST FACILITY

Testing regenerators presents an interesting challenge. When incorporated into a cryocooler, a regenerator is intimately coupled to the other components: expander, heat exchangers, and compressor. It is difficult to isolate the performance of any single component. We have developed a low temperature test facility that will allow us to separate the performance of the regenerator from the rest of the cryocooler. The purpose of the facility is the characterization of test regenerators using novel materials and/or geometries in temperature ranges down to 15 K. It consists of the following elements: The test column has two regenerators stacked in series. The coldest stage regenerator is the device under test. The warmer stage regenerator contains a stack of stainless steel screen, a well-characterized material. A commercial cryocooler is used to fix the temperatures at both ends of the test regenerator, cooling both heat exchangers flanging the regenerator stack. Heaters allow varying the temperatures and allo...