Ehab Abdel-Rahman

Design and development of high-frequency thermoacoustic engines for thermal management in microelectronics

Thermoacoustic heat engines provide a practical solution to the problem of heat management in microcircuits where they can be used to pump heat or produce spot cooling of specific circuit elements. There are basically two types of thermoacoustic engines, a prime mover where heat is converted to acoustic energy, and a heat pump or cooler where sound can pump heat up a temperature gradient. Such devices are relatively simple, they can be efficient, and they are readily adaptable to microcircuit interfacing. Since this type of engines is usually operated in a resonant mode, the operating frequency determines its size. The devices presented here are pumped at frequencies ranging from 4 to 24 kHz. They have been developed for interfacing with microcircuits as heat pumps or spot coolers. Results of their performance are presented and suggestions for further improvements are discussed.

STUDY OF PHYSICAL PARAMETERS ON THE PROPERTIES OF PHONONIC BAND GAPS

In this work, we have introduced a comprehensive study concerning with the effects of some physical parameters on the dispersive properties of 1D phononic crystal (PnC). We have treated the propagation of elastic (in-plane) waves incident normally on 1D PnC. Based on the transfer matrix method, the reflection coefficients are calculated and plotted for the plane waves. We have studied many physical parameters effects on the properties of PnCs such as type of surrounding material, type of composites materials, temperature and propagation of waves in defect structures. Also the phenomenon of local resonance was discussed in this survey. These results can be useful in many applications such as sound suppressions and temperature sensor materials.

Tunability and Sensing Properties of Plasmonic/1D Photonic Crystal

Gold/one-dimensional photonic crystal (Au/1D-PC) is fabricated and applied for sensitive sensing of glucose and different chemical molecules of various refractive indices. The Au layer thickness is optimized to produce surface plasmon resonance (SPR) at the right edge of the photonic band gap (PBG). As the Au deposition time increased to 60 sec, the PBG width is increased from 46 to 86 nm in correlation with the behavior of the SPR. The selectivity of the optimized Au/1D-PC sensor is tested upon the increase of the environmental refractive index of the detected molecules. The resonance wavelength and the PBG edges increased linearly and the transmitted intensity increased nonlinearly as the environment refractive index increased. The SPR splits to two modes during the detection of chloroform molecules based on the localized capacitive coupling of Au particles. Also, this structure shows high sensitivity at different glucose concentrations. The PBG and SPR are shifted to longer wavelengths, and PBG width is decreased linearly with a rate of 16.04 Å/(μg/mm3) as the glucose concentration increased. The proposed structure merits; operation at room temperature, compact size, and easy fabrication; suggest that the proposed structure can be efficiently used for the biomedical and chemical application.

Characteristic-based non-linear simulation of large-scale standing-wave thermoacoustic engine.

A few linear theories [Swift, J. Acoust. Soc. Am. 84(4), 1145-1180 (1988); Swift, J. Acoust. Soc. Am. 92(3), 1551-1563 (1992); Olson and Swift, J. Acoust. Soc. Am. 95(3), 1405-1412 (1994)] and numerical models, based on low-Mach number analysis [Worlikar and Knio, J. Comput. Phys. 127(2), 424-451 (1996); Worlikar et al., J. Comput. Phys. 144(2), 199-324 (1996); Hireche et al., Canadian Acoust. 36(3), 164-165 (2008)], describe the flow dynamics of standing-wave thermoacoustic engines, but almost no simulation results are available that enable the prediction of the behavior of practical engines experiencing significant temperature gradient between the stack ends and thus producing large-amplitude oscillations. Here, a one-dimensional non-linear numerical simulation based on the method of characteristics to solve the unsteady compressible Euler equations is reported. Formulation of the governing equations, implementation of the numerical method, and application of the appropriate boundary conditions are presented. The calculation uses explicit time integration along with deduced relationships, expressing the friction coefficient and the Stanton number for oscillating flow inside circular ducts. Helium, a mixture of Helium and Argon, and Neon are used for system operation at mean pressures of 13.8, 9.9, and 7.0 bars, respectively. The self-induced pressure oscillations are accurately captured in the time domain, and then transferred into the frequency domain, distinguishing the pressure signals into fundamental and harmonic responses. The results obtained are compared with reported experimental works [Swift, J. Acoust. Soc. Am. 92(3), 1551-1563 (1992); Olson and Swift, J. Acoust. Soc. Am. 95(3), 1405-1412 (1994)] and the linear theory, showing better agreement with the measured values, particularly in the non-linear regime of the dynamic pressure response.

Characterization of structures and properties of amorphous nanostructured SiC thin films deposited on AISI 304 stainless steel using pulsed laser deposition

Amorphous nanostructure silicon carbide (a-SiC) recently received great attention for its use as protective coating for metallic substrates due to its good mechanical properties and corrosion resistance. In this article, a-SiC thin films were deposited on AISI 304 stainless steel substrates at room temperature for deposition times of 4 and 6 h using pulsed laser deposition technique. The deposition process was stopped every 2 h then resumed for an hour. The effect of interval time during deposition process and substrate type on the properties of the produced films was extensively investigated. The morphological features of the deposited SiC films were investigated using field-emission scanning electron microscope and atomic force microscope. The film structures were determined by transmission electron microscopy, X-ray photoelectron spectroscopy and energy-dispersive X-ray. The mechanical and tribological properties, such as Young’s modulus, hardness and scratch resistance, were determined using nanoindenter. The results showed the formation of uniform monocrystalline nanostructured Si interface between two a-SiC layers after the 2 h of no-deposition time intervals. The formation of crystalline Si interface attributed to the effect of high kinetic energy of the incoming ablated particles deposited on the grown a-SiC layers. The a-SiC films were amorphous having nanostructure grains with dimensions ≤ 100 nm. All films showed smooth surfaces with fine cracks due to the presence of intrinsic stresses. The deposited films showed low mechanical properties due to their amorphous structures.

Numerical simulation of key linear alternator performance indicators under thermoacoustic-power-conversion conditions

Thermoacoustic power converters consist of thermoacoustic heat engine and linear alternator. The linear alternator converts the acoustic power generated by the thermoacoustic engine to electric output. Efficient and stable operation of a thermoacoustic power converter requires acoustic matching between the engine and the alternator. It also requires matching between the linear alternator and the connected load. An experimental setup was built to measure and analyze the linear alternator performance under different thermoacoustic power converter operating conditions. The effects of the different design and operation factors on the key linear alternator performance parameters such as mechanical stroke, the generated electric power, the acoustic-to-electric conversion efficiency, the mechanical motion loss, the electric loss, and the fluid-seal loss were investigated experimentally and numerically. The experimental results were simulated using DeltaEC and reasonable agreement was obtained.

A method to suppress harmonics in standing-wave thermoacoustic engines

Harmonic generation in thermoacoustic engines is regarded as a non-linear loss mechanism that extracts acoustic power from the fundamental wave into harmonics. It is shown that the use of specifically designed insert that limits the gas flow area over a very-limited part of the resonator can suppress this non-linear loss mechanism causing a significant increase in the generated acoustic power in the fundamental mode. In this experimental work, a standing-wave thermoacoustic engine is built and operated without inserts and with inserts of different shapes, porosities, and thicknesses. The self-generated dynamic pressure waves are captured under different operating conditions and then are decomposed into a fundamental component and harmonics. Results for different inserts are presented and discussed. All inserts caused lower harmonic content with respect to the no-insert case but inserts of low gas flow area cause severe flow blockage and thus a severe reduction in the produced acoustic powers. Results anal...

Jet pump oscillating-flow behavior at different Womersley numbers

Few numerical works simulate the laminar-flow behavior within the diverging passages of typical thermoacoustic jet pumps. The associated boundary-layer separation is mostly delayed by maintaining a slowly increasing flow passage and thereby the corresponding pressure drop and consumed acoustic energy are noticeably reduced. Of equal importance is the effect of the transition to turbulence on the boundary-layer separation and the resulting flow-pattern. To our knowledge, no simulation has been developed that numerically predict and capture the conditionally turbulent flow regime within typical jet pumps. Here, a new finite-volume model is reported that employs the non-linear CFD solver of ANSYS FLUENT. The k-kl-ω transition model is considered and its parameters are particularly adjusted for present application. A sinusoidal pressure oscillation is enforced at one end, while the far-field approximation is considered at the other end to model the non-reflecting boundary condition. An axisymmetric model alon...

Sensitivity analysis of the performance of a linear alternator at mechanical resonance under thermoacoustic power conversion conditions

A linear alternator lie at the interface between the acoustic power generated by a thermoacoustic engine and an electric load, and thus, its performance is significantly controlled by its matching with the thermoacoustic engine and with the electric load. Under thermoacoustic power conversion conditions, small unavoidable changes in the operating conditions may incur significant effects on the alternator performance. In this work, the sensitivities of several alternator performance indices, namely, the acoustic-to-electric conversion efficiency, the mechanical stroke, and some of the main alternator losses (mechanical damping loss, seal loss, and electric copper loss), are examined to small changes in four operating conditions. Using the methodologies of the design of experiments and sensitivity analysis, a scheme of experiments is designed and carried-out to analyze the sensitivity of these indices to ± 10% changes in operating conditions at mechanical resonance. The operating conditions considered are t...

Synthesis of nanoporous chromium film by DC electrodeposition on aluminum substrate

Porous Anodic Alumina (PAA) is widely employed as a template for fabrication of nanostructures due to its ability to generate self-organized, well-ordered pore structures. A new technique has been developed to deposit porous Chromium (Cr) film on Al substrate by DC electrodeposition. The chemical composition and morphological characteristics of fabricated PAA membrane and Cr film were investigated by scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopy (EDX). In addition, the growth mechanism of the fabricated nanoporous Cr film was discussed. Keywords: Nanoporous Cr film; PAA membrane; DC electrochemical deposition; Growth mechanism