Mounir B. Ibrahim

Analysis of Thermal Energy Storage Material With Change-of-Phase Volumetric Effects

NASAs Space Station Freedom proposed hybrid power system includes photovoltaic arrays with nickel hydrogen batteries for energy storage and solar dynamic collectors driving Brayton heat engines with change-of-phase thermal energy storage (TES) devices. A TES device is comprised of multiple metallic, annular canisters which contain a eutectic composition LiF-CaF[sub 2] phase change material (PCM) that melts at 1,040 K. A moderately sophisticated LiF-CaF[sub 2] PCM computer model is being developed in two stages considering first one-dimensional and then two-dimensional canister geometries. One-dimensional model results indicate that the void has a marked effect on the phase change process due to PCM displacement and dynamic void heat transfer resistance. Equally influential are the effects of different boundary conditions and liquid PCM free convection. For the second stage, successful numerical techniques used in the one-dimensional phase change model are extended to a two-dimensional (r, z) PCM containment canister model. A prototypical PCM containment canister is analyzed and the results are discussed.

A Kalina cycle application for power generation

A multi-component (NB3/H2O) Kalina-type cycle that utilizes the exhaust from a gas turbine is investigaed in this paper. The turbine-inlet pressure of 5.96 × 106N/m2 and temperature of 755.372 K were kept constant, as well as the working fluid temperature at the condenser outlet (at 290K). The HN3 mass fraction at the turbine inlet was varied along with the seperator temperature and the effects on the cycle efficiency were studied. The relationships between turbine-inlet flow and the seperator-inlet. How are shown in addition to the upper and lower HN3 mass-fraction bounds. The multi-component working-fluid cycle investigated is 10–20% more efficient than a Rankine cycle with the same boundary conditions.

Laminar/turbulent oscillating flow in circular pipes

Abstract A two-dimensional oscillating flow analysis was conducted simulating the gas flow inside Stirling engine heat exchangers. Both laminar and turbulent oscillating pipe flow were investigated numerically for Remax = 1,920 (Va = 80), 10,800 (Va = 272), 19,300 (Va = 272), and 60,800 (Va = 126). The results are here compared with experimental results of previous investigators. Predictions of the flow regime on present oscillating flow conditions are also checked by comparing velocity amplitudes and phase difference with those from laminar theory and quasi-steady profile. A high Reynolds number k-ϵ turbulence model was used for turbulent oscillating pipe flow. Finally, the performance of the k-ϵ model was evaluated to explore the applicability of quasi-steady turbulent models to unsteady oscillating flow analysis.

Separation Control on a Very High Lift Low Pressure Turbine Airfoil Using Pulsed Vortex Generator Jets

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. Jet pulsing frequency, duty cycle, and blowing ratio were all varied. Computational results from a large eddy simulation of one case showed reattachment in agreement with the experiment. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was possible even at the lowest Reynolds number. Pulsed VGJs were more effective than steady jets. At sufficiently high pulsing frequencies, separation control was possible even with low jet velocities and low duty cycles. At lower frequencies, higher jet velocity was required, particularly at low Reynolds numbers. Effective separation control resulted in an increase in lift and a reduction in total pressure losses. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can reseparate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times.

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.

Experimental and Computational Investigations of Phase Change Thermal Energy Storage Canisters

Two sets of experimental data for cylindrical canisters with thermal energy storage applications were examined in this paper: 1) Ground Experiments and 2) Space Experiments. A 2-D computational model was developed for unsteady heat transfer (conduction and radiation) with phase-change. The radiation heat transfer employed a finite volume method. The following was found in this study: 1) Ground Experiments, the convection heat transfer is equally important to that of the radiation heat transfer; Radiation heat transfer in the liquid is found to be more significant than that in the void; Including the radiation heat transfer in the liquid resulted in lower temperatures (about 15 K) and increased the melting time (about 10 min.); Generally, most of the heat flow takes place in the radial direction. 2) Space Experiments, Radiation heat transfer in the void is found to be more significant than that in the liquid (exactly the opposite to the Ground Experiments); Accordingly, the location and size of the void affects the performance considerably; Including the radiation heat transfer in the void resulted in lower temperatures (about 40 K).

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...

Two-Dimensional Model of a Space Station Freedom Thermal Energy Storage Canister

The Solar Dynamic Power Module being developed for Space Station Freedom uses a eutectic mixture of LiF-CaF[sub 2] phase-change salt contained in toroidal canisters for thermal energy storage. This paper presents results from heat transfer analyses of the phase-change salt containment canister. A two-dimensional, axisymmetric finite difference computer program which models the canister walls, salt, void, and heat engine working fluid coolant was developed. Analyses included effects of conduction in canister walls and solid salt, conduction and free convection in liquid salt, conduction and radiation across salt vapor-filled void regions, and forced convection in the heat engine working fluid. Void shape and location were prescribed based on engineering judgment. The salt phase-change process was modeled using the enthalpy method. Discussion of results focuses on the role of free convection in the liquid salt on canister heat transfer performance. This role is shown to be important for interpreting the relationship between ground-based canister performance (in 1-g) and expected on-orbit performance (in micro-g). Attention is also focused on the influence of void heat transfer on canister wall temperature distributions. The large thermal resistance of void regions is shown to accentuate canister hot spots and temperature gradients.

UNSTEADY FLUID DYNAMICS SIMULATION OF A STIRLING ENGINE HEATER HEAD

This paper presents hot-wire anemometry data from an experiment that replicates important features of oscillatory flows in Stirling engines. These features include impinging and sink flows, large-scale separation zones, recirculation bubbles, unsteady shear layers and spatially varying transition and relaminarization zones. In addition to a characterization of oscillatory flow, this paper presents and compares results gathered in a unidirectional investigation to determine when, during the oscillation cycle, quasi-steady results can be applied to Stirling engine design. This paper is part of a program that focuses on characterization of fluid flow and heat transfer within Stirling engines. The unsteady velocity data presented herein serves to support the development of 3-D CFD models and verify the conditions under which 1-D systems models may be applied. The ultimate goal is that Stirling engine design codes can be used with a greater degree of confidence.

Measurements in oscillatory flows with separation in support of Stirling engine model development

This work is a contribution to a larger program that seeks to improve computation of unsteady flows in Stirling engines. It presents flow visualization results from an experiment that replicates important features of oscillatory flows; such as large-scale separation and adverse and favorable, spatial and temporal acceleration. The geometry is a tube perpendicular to a space between two circular discs, one of which the tube penetrates such that the end of the tube is flush with the disc inner surface. The flow, driven by a piston in the tube, is oscillatory and acts on the exhaust stroke as a jet impinging on the center of a circular disc and on the intake stroke as a sink flow drawn into a tube from the space between the two discs. Time-varying recirculation zones and periodic appearances of highly turbulent flow are observed. In addition to a characterization of oscillatory, separating flow, this program provides code validation support for Stirling engine design models. The project goals are to identify the areas where 1-D modeling is especially weak and to provide support for developing more sophisticated design models that can be used with greater confidence. A complete description of the experiment and results is given. It is shown that a change in geometry can affect the flow in major way; i.e. whether a recirculation eddy survives an entire cycle or is eliminated upon flow reversal. Future work is outlined.