Roy Tew

Review of Computational Stirling Analysis Methods

Nuclear thermal to electric power conversion carries the promise of longer duration missions and higher scientific data transmission rates back to Earth for both Mars rovers and deep space missions. A free-piston Stirling convertor is a candidate technology that is considered an efficient and reliable power conversion device for such purposes. While already very efficient, it is believed that better Stirling engines can be developed if the losses inherent its current designs could be better understood. However, they are difficult to instrument and so efforts are underway to simulate a complete Stirling engine numerically. This has only recently been attempted and a review of the methods leading up to and including such computational analysis is presented. And finally it is proposed that the quality and depth of Stirling loss understanding may be improved by utilizing the higher fidelity and efficiency of recently developed numerical methods. One such method, the Ultra HI-Fl technique is presented in detail.

Status of NASA’s Advanced Radioisotope Power Conversion Technology Research and Development

NASA’s Advanced Radioisotope Power Systems (RPS) development program is funding the advancement of next generation power conversion technologies that will enable future missions that have requirements that can not be met by either the ubiquitous photovoltaic systems or by current Radioisotope Power Systems (RPS). Requirements of advanced radioisotope power systems include high efficiency and high specific power (watts/kilogram) in order to meet mission requirements with less radioisotope fuel and lower mass. Other Advanced RPS development goals include long‐life, reliability, and scalability so that these systems can meet requirements for a variety of future space applications including continual operation surface missions, outer‐planetary missions, and solar probe. This paper provides an update on the Radioisotope Power Conversion Technology Project which awarded ten Phase I contracts for research and development of a variety of power conversion technologies consisting of Brayton, Stirling, thermoelectri...

Fast Whole-Engine Stirling Analysis

An experimentally validated approach is described for fast axisymmetric Stirling engine simulations. These simulations include the entire displacer interior and demonstrate it is possible to model a complete engine cycle in less than an hour. The focus of this eort was to demonstrate it is possible to produce useful Stirling engine performance results in a time-frame short enough to impact design decisions. The combination of utilizing the latest 64-bit Opteron computer processors, ber-optical Myrinet communications, dynamic meshing, and across zone partitioning has enabled solution times at least 240 times faster than previous attempts at simulating the axisymmetric Stirling engine. A comparison of the multidimensional results, calibrated one-dimensional results, and known experimental results is shown. This preliminary comparison demonstrates that axisymmetric simulations can be very accurate, but more work remains to improve the simulations through such means as modifying the thermal equilibrium regenerator models, adding uid-structure interactions, including radiation eects, and incorporating mechanodynamics.

Two-Dimensional Compressible Non-Acoustic Modeling of Stirling Machine-Type Components

Starting with an existing two-dimensional incompressible e ow computer code, a two-dimensional code was developed for modeling enclosed gas volumes with oscillating boundaries. The incompressible code was modie ed to use compressible nonacoustic Navier‐ Stokes equations. The devices modeled have low Mach numbers and are sufe cientlysmallthatthetimerequiredforacousticstopropagateacrosstheinteriorsissmallcomparedtothecycle period.Therefore,acousticswereexcludedtominimizecomputingtime.Thecompressiblenonacousticassumptions are discussed. The governing equations are presented in transport equation format. The numerical methods are briee y described.Codepredictionsarecompared with experimentaldata.Compressiblenonacousticpredictionsof gas spring losses agreed well with 10-rpm test data, and »50- and 500-rpm calculated and experimental pressure‐ volume diagrams agreed well. For a heat-exchanger/piston-cylinder test rig, calculations of heat exchanger heat e uxes at various axial locations over the cycle agreed well qualitatively with the data, but quantitative agreement was not good.

Comparison of GLIMPS and HFAST Stirling engine code predictions with experimental data

Predictions from GLIMPS and HFAST design codes are compared with experimental data for the RE-1000 and SPRE free-piston Stirling engines. Engine performance and available power loss predictions are compared. Differences exist between GLIMPS and HFAST loss predictions. Both codes require engine-specific calibration to bring predictions and experimental data into agreement.

Overview 2004 of NASA‐Stirling Convertor CFD Model Development and Regenerator R&D Efforts

This paper reports on accomplishments in 2004 in (1) development of Stirling‐convertor CFD models at NASA GRC and via a NASA grant, (2) a Stirling regenerator‐research effort being conducted via a NASA grant (a follow‐on effort to an earlier DOE contract), and (3) a regenerator‐microfabrication contract for development of a “next‐generation Stirling regenerator.” Cleveland State University is the lead organization for all three grant/contractual efforts, with the University of Minnesota and Gedeon Associates as subcontractors. Also, the Stirling Technology Co. and Sunpower, Inc. are both involved in all three efforts, either as funded or unfunded participants. International Mezzo Technologies of Baton Rouge, LA is the regenerator fabricator for the regenerator‐microfabrication contract. Results of the efforts in these three areas are summarized.

Overview of NASA Multi‐Dimensional Stirling Convertor Code Development and Validation Effort

A NASA grant has been awarded to Cleveland State University (CSU) to develop a multi‐dimensional (multi‐D) Stirling computer code with the goals of improving loss predictions and identifying component areas for improvements. The University of Minnesota (UMN) and Gedeon Associates are teamed with CSU. Development of test rigs at UMN and CSU and validation of the code against test data are part of the effort. The one‐dimensional (1‐D) Stirling codes used for design and performance prediction do not rigorously model regions of the working space where abrupt changes in flow area occur (such as manifolds and other transitions between components). Certain hardware experiences have demonstrated large performance gains by varying manifolds and heat exchanger designs to improve flow distributions in the heat exchangers. 1‐D codes were not able to predict these performance gains. An accurate multi‐D code should improve understanding of the effects of area changes along the main flow axis, sensitivity of performance...

Recent Stirling Engine Loss-understanding Results

For several years, the National Aeronautics and Space Administration and other U.S. Government agencies have been funding experimental and analytical efforts to improve the understanding of Stirling thermodynamic losses. NASAs objective is to improve Stirling engine design capability to support the development of new engines for space power. An overview of these efforts was last given at the 1988 IECEC. Recent results of this research are reviewed here.

Overview of NASA Supported Stirling Thermodynamic Loss Research

NASA is funding research to characterize Stirling machine thermodynamic losses. NASAs primary goal is to improve Stirling design codes to support engine development for space and terrestrial power. However, much of the fundamental data is applicable to Stirling cooling and heat pump applications. The research results are reviewed. Much was learned about oscillating flow hydrodynamics, including laminar/turbulent transition, and tabulated data was documented for further analysis. Now, with a better understanding of the oscillating flow field, it is time to begin measuring the effects of oscillating flow and oscillating pressure level on heat transfer in heat exchanger flow passages and in cylinders.

Initial Comparison of Single Cylinder Stirling Engine Computer Model Predictions with Test Results

A NASA developed digital computer code for a Stirling engine, modelling the performance of a single cylinder rhombic drive ground performance unit (GPU), is presented and its predictions are compared to test results. The GPU engine incorporates eight regenerator/cooler units and the engine working space is modelled by thirteen control volumes. The model calculates indicated power and efficiency for a given engine speed, mean pressure, heater and expansion space metal temperatures and cooler water inlet temperature and flow rate. Comparison of predicted and observed powers implies that the reference pressure drop calculations underestimate actual pressure drop, possibly due to oil contamination in the regenerator/cooler units, methane contamination in the working gas or the underestimation of mechanical loss. For a working gas of hydrogen, the predicted values of brake power are from 0 to 6% higher than experimental values, and brake efficiency is 6 to 16% higher, while for helium the predicted brake power and efficiency are 2 to 15% higher than the experimental.