Person:

Ghiaasiaan, S. Mostafa

Permanent Link

https://hdl.handle.net/1853/71288

Associated Organization(s)

Organizational Unit

George W. Woodruff School of Mechanical Engineering

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Now showing 1 - 8 of 8

Effect of Frequency on Hydrodynamic Parameters of Mesh Fillers in Oscillatory Flow

(Georgia Institute of Technology, 2008-05) Landrum, E. C. ; Conrad, T. J. ; Pathak, M. G. ; Ghiaasiaan, S. Mostafa ; Kirkconnell, Carl Scott ; Crittenden, Thomas M. ; Yorish, S.

An experimental investigation was carried out to determine the effect of frequency on the porous media hydrodynamic closure relations for steady periodic flow. Using room temperature helium as the working fluid, stacked discs of #635 stainless steel and #325 phosphor bronze wire meshes were subjected to an oscillatory flow field. Dynamic pressure transducers recorded waveforms upstream and downstream of the porous section at charge pressures of 2.86 and 3.55 MPa. Tests were performed in the axial direction at frequencies ranging from 50 to 200 Hz. Hydrodynamic parameters were determined using a CFD-assisted methodology. The experimental test section and its vicinity were simulated using the Fluent code and mesh fillers were modeled as a porous structure. Model porous media hydrodynamic parameters were iteratively adjusted to match the model predictions to the experimental results. Directional resistances related to the Darcy permeability and Forchheimer‚Äôs inertial coefficients were obtained at discrete frequencies and errors were quantified. The experimental results indicate that Forchheimer‚Äôs inertial coefficient may depend rather strongly on frequency. More detailed experiments are needed to ascertain the observed trends.
Simulation of Boundary Layer Effects in the Pulse Tube of a Miniature Cryocooler

(Georgia Institute of Technology, 2008-05) Conrad, T. J. ; Ghiaasiaan, S. Mostafa ; Kirkconnell, Carl Scott

As pulse tube cryocoolers are miniaturized, boundary layer effects in the pulse tube may become more important than they are for larger refrigerators. Nearly uniform flow in the pulse tube is necessary for efficient cooling, and this condition is compromised as the pulse tube diameter becomes smaller relative to the thermal and viscous boundary layer thicknesses. As a result, miniature pulse tube cryocoolers are likely to experience enhanced acoustic streaming losses compared to larger PTC’s. This acoustic streaming results from thermal and viscous interactions between the working fluid and the pulse tube walls. The thermal and viscous penetration depths and their magnitudes relative to the pulse tube diameter and wall thickness are therefore important parameters for this phenomenon. A parametric study of the effects of the pulse tube diameter, scaled to a non-dimensional value by the relevant boundary layer thicknesses, on acoustic streaming in the pulse tube was performed using CFD modeling. The effect of the operating frequency was also considered through the frequency dependence of the viscous and thermal penetration depths. Temperature dependant material properties were included in the CFD models because they play an important role in acoustic streaming. Results indicated that close attention must be paid to the sizes of the boundary layers relative to the pulse tube physical dimensions when designing miniature pulse tube cryocoolers.
Impact of Small Regenerator Structural Flaws on the Performance of Miniature Pulse Tube Cryocoolers

(Georgia Institute of Technology, 2008-05) Conrad, T. J. ; Ghiaasiaan, S. Mostafa ; Kirkconnell, Carl Scott ; Crittenden, Thomas M.

Miniature cryocoolers are suitable for space applications and installation in portable devices. They can also be useful as final stages for applications where small cooling loads must be carried at temperatures lower than that required by the primary load. Strong regeneration, near plug-flow regime in the pulse tube and good flow control are essential for these cryocoolers to function. Miniature cryocoolers that use wire mesh as regenerator filler generally have a much larger ratio of regenerator filler pore size to regenerator diameter than their larger counterparts. For this reason, the significance of gaps existing between the porous regenerator filler and the interior wall of the regenerator shell will likely be greater for miniature cryocoolers. These gaps provide a low resistance flow path which may decrease the effectiveness of the regenerator. In this investigation the effects of such gaps on the performance of miniature pulse tube cryocoolers are examined using 2-D CFD simulations. Miniature scale pulse tube cryocooler designs whose suitability for cooling under ideal conditions that have been theoretically demonstrated are used as the basis for this study. The results confirm that extra precision and robustness are needed for miniature cryocoolers.
Thermal Dispersion and Convection Heat Transfer during Laminar Transient Flow in Porous Media

(Georgia Institute of Technology, 2008-05) Pathak, M. G. ; Ghiaasiaan, S. Mostafa

Solid-fluid thermal interactions during unsteady flow in porous media play an important role in the regenerators of pulse tube cryocoolers. Pore-level thermal processes in porous media under unsteady flow conditions are poorly understood. The objective of this investigation was to study the pore-level thermal phenomena during pulsating and unidirectional sinusoidal flow through a generic, two-dimensional porous medium by numerical analysis. Furthermore, an examination of the effects of flow pulsations on the thermal dispersion and heat transfer coefficient that are encountered in the standard, volume-average energy equations for porous media were carried out. Pulsating and unidirectional sinusoidal inlet flow rates were chosen as an intermediate step towards the more difficult problem of periodic flow. The investigated porous media are periodic arrays of square cylinders. Detailed numerical data for the porosities of 0.75 and 0.84, with flow pulsation frequencies of 0 - 80 Hz, were obtained at Reynolds numbers of 560 and 980. Based on these numerical data, the instantaneous as well as cycle-average thermal dispersion and heat transfer coefficients, to be used in the standard unsteady volume-average energy conservation equations for flow in porous media, were derived.
Small scale cryogenic refrigeration technology

(Georgia Institute of Technology, 2008-02-12) Ghiaasiaan, S. Mostafa
Hydrodynamics of dispersed liquid droplets in agitated synthetic fibrous slurries

(Atlanta, Georgia : the Institute,, 1997-08) Bose, Feler ; Ghiaasiaan, S. Mostafa ; Heindel, Theodore J. (Ted)
Macroscopic flow structures in a bubbling paper pulp-water slurry

(Atlanta, Georgia : the Institute,, 1995-01) Lindsay, Jeffrey D. ; Ghiaasiaan, S. Mostafa ; Abdel-Khalik, Said I.
Macroscopic flow structures in a bubbling paper pulp-water slurry

(Atlanta, Georgia : the Institute,, 1994-09) Taylor, Kevin Emory ; Ghiaasiaan, S. Mostafa ; Abdel-Khalik, Said I. ; Lindsay, Jeffrey D. ; George, Johnathan