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Numerical Analysis of Standing Wave Thermoacoustic Nonlinear Processes with Flow Field Investigation

Babaei Zarch, Abbas | 2024

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 57297 (45)
  4. University: Sharif University of Technology
  5. Department: Aerospace Engineering
  6. Advisor(s): Mazaheri, Karim
  7. Abstract:
  8. This dissertation investigates and analyzes the nonlinear effects within standing wave thermoacoustic systems. These effects, often overlooked, can play a crucial role in the behavior of such systems. Two-dimensional numerical simulations of the Navier-Stokes equations were conducted using a commercial software, leading to the identification and discovery of new nonlinear physics governing the interaction of heat transfer and hydrodynamic oscillations of fluid particles. The numerical solution method and mesh quality were validated against a similar problem with a well-established numerical solution. Furthermore, a parametric study was performed on a stationary-wave thermoacoustic refrigeration system to determine the optimal placement of stack plates for achieving the maximum temperature difference across them. The best location for these plates was found to be λ/10 away from the closed end of the system. In a separate numerical analysis, the thermodynamic and hydrodynamic behavior of particles in the region close to the stack of a stationary-wave thermoacoustic engine was investigated. This analysis revealed that the nonlinear thermoacoustic behavior led to a temperature increase of 49°C on the left side of the hot heat exchanger (higher than the hot heat exchanger) and a decrease of 23°C on the right side of the cold heat exchanger (lower than the cold heat exchanger). Due to nonlinear phenomena, non-sinusoidal temperature oscillations were observed near the engine's stack, seemingly contradicting the second law of thermodynamics. A closer examination of the oscillatory behavior of particles within the system revealed that this unexpected temperature difference and oscillatory behavior are attributed to the large amplitude of longitudinal particle oscillations coupled with heat transfer. No violation of thermodynamic laws occurred. This phenomenon was analyzed in detail by examining the thermodynamic cycles of oscillating particles. Based on this understanding of nonlinear particle behavior, a thermoacoustic superheater was designed and modeled. Acoustic power for this superheater was supplied by two stationary-wave thermoacoustic engines positioned at either end of the resonator tube. Superheated heat exchanger plates were placed in the middle of the resonator tube, coinciding with the location of maximum oscillatory velocity within the tube. In this superheater, the hot heat exchanger temperature was 500 K, while the superheated heat exchanger temperature reached 800 K. Observations showed that the maximum oscillatory velocity in the middle of the tube, interacting with the superheated heat exchanger plate surfaces, led to the generation of this high-temperature heat. A thorough analysis of these phenomena in the superheater system was conducted using the particle tracking method, and the thermodynamic cycles of the particles were accurately analyzed
  9. Keywords:
  10. Thermoacoustic Engine ; Standing Waves ; Particles Tracking ; Computational Fluid Dynamics (CFD) ; Numerical Analysis ; Waste Heat Recovery Power Generation (WHRPG)Technology ; Super Heat Exchanger ; Nonlinear Thermoacoustic ; Coaxial Motors

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