Publications

The capability for statically pre-compressing fluid targets for Hugoniot measurements utilizing gas gun driven flyer plates has been developed. Pre-compression expands the capability for initial condition control, allowing access to thermodynamic states off the principal Hugoniot. Absolute Hugoniot measurements with an uncertainty less than 3% on density and pressure were obtained on statically pre-compressed fluid helium utilizing a two stage light gas gun. Helium is highly compressible; the locus of shock states resulting from dynamic loading of an initially compressed sample at room temperature is significantly denser than the cryogenic fluid Hugoniot even for relatively modest (0.27-0.38 GPa) initial pressures. The dynamic response of pre-compressed helium in the initial density range of 0.21-0.25 g/cm3 at ambient temperature may be described by a linear shock velocity (us) and particle velocity (up) relationship: us = C0 + sup, with C0 = 1.44 ± 0.14 km/s and s = 1.344 ± 0.025.

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This final report on SNL/NM LDRD Project 141536 summarizes progress made toward the development of a cryogenic capability to generate liquid helium (LHe) samples for high accuracy equation-of-state (EOS) measurements on the Z current drive. Accurate data on He properties at Mbar pressures are critical to understanding giant planetary interiors and for validating first principles density functional simulations, but it is difficult to condense LHe samples at very low temperatures (<3.5 K) for experimental studies on gas guns, magnetic and explosive compression devices, and lasers. We have developed a conceptual design for a cryogenic LHe sample system to generate quiescent superfluid LHe samples at 1.5-1.8 K. This cryogenic system adapts the basic elements of a continuously operating, self-regulating {sup 4}He evaporation refrigerator to the constraints of shock compression experiments on Z. To minimize heat load, the sample holder is surrounded by a double layer of thermal radiation shields cooled with LHe to 5 K. Delivery of LHe to the pumped-He evaporator bath is controlled by a flow impedance. The LHe sample holder assembly features modular components and simplified fabrication techniques to reduce cost and complexity to levels required of an expendable device. Prototypes have been fabricated, assembled, and instrumented for initial testing.

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A first order model developed for the design analysis and optimization of Inertance Tube Pulse Tube Refrigerators (ITPTRs) is integrated with the code NIST REGEN 3.2 capable of modeling the regenerative heat exchangers used in ITPTRs. The model is based on the solution of simultaneous non-linear differential equations representing the inertance tube, an irreversibility parameter model for the pulse tube, and REGEN 3.2 to simulate the regenerator. The integration of REGEN 3.2 is accomplished by assuming a sinusoidal pressure wave at the cold side of the regenerator. In this manner, the computational power of REGEN 3.2 is conveniently used to reduce computational time required for parametric analysis and optimization of ITPTRs. The exergy flow and exergy destruction (irreversibility) of each component of ITPTRs is calculated and the effect of important system parameters on the second law efficiency of the refrigerators is presented. © 2008 American Institute of Physics.

In most Stirling and pulse tube refrigerators the regenerative heat exchanger is the major contributor to the irreversibility of the refrigerators. Exergy analysis is a convenient method to quantify the losses in regenerators. NIST Code REGEN 3.2 provides a powerful tool to quantify exergy flow in the regenerators to evaluate the exergy destruction (irreversibility) and losses due to heat transfer and fluid friction. The effect of important parameters, with emphasis on the phase shift between the pressure and the mass flow rate at the cold side of regenerator, on the exergetic efficiency and the important components of exergy destruction in the regenerator is presented. The efficiency of the regenerator based on exergy analysis is compared to other methods of evaluating regenerator performance. The ability of the code to quantify the exergy flow and destruction at low temperatures where the ideal gas assumption is not applicable is discussed. © 2008 American Institute of Physics.