Abstract
The ever-growing demand for energy production calls for increasingly efficient power generation
systems. Originally developed at the Los Alamos National Laboratories in the late 80's, thermoacoustic
Stirling heat engines represent the state of the art in energy conversion technology. These devices can
convert heat from any source into acoustic and then electrical power with efficiencies reported to be up to
49% of Carnot's limit. Applications include nuclear-powered deep-space travel, natural-gas-fueled
household electrical generation and industrial refrigeration.
The first part of the talk will explore the functionality of thermoacoustic engines. Results from prediction
tools ranging from low-order analytical models to high-fidelity three-dimensional numerical simulations
will be discussed in depth. The analysis will focus on the theoretical traveling-wave thermoacoustic
engine shown in the figure. The core energy conversion process occurs in the regenerator -- a porous
metallic block, placed between a hot and a cold heat-exchanger, sustaining a mean temperature gradient
in the axial direction. Acoustic waves propagating through the regenerator are amplified via a
thermodynamic process resembling a Stirling cycle. As the acoustic energy in the system increases, the
fundamental nature of the fluid mechanics changes from linear to fully non-linear where the flow is
dominated by transitional turbulence and acoustic streaming. The latter is the result of cumulative net
fluid particle displacements over several high-amplitude acoustic cycles, and leads to the rectified flow
patterns such as the ones shown in the figure. Such flow is present in many advanced aeronautical
systems, such as rocket engines in thermoacoustically unstable modes of operation, and reduces their
overall performance.
A combination of analytical modeling and high-fidelity simulations is at the core of my future work,
which will focus on advanced energy systems. I propose a building-block approach starting from simple
systems such as high-amplitude resonating pipes or thermoacoustic annular systems, for which high
quality experimental measurements are already available in the literature. Long-term research plans are
to extend the developed modeling framework to complex problems such as shock-resonance. This
peculiar phenomenon is the premise for pulse-detonation, or ‘exploding’ engines, which are the highpower
counterpart of thermoacoustic engines. They exploit the unstable interaction between heat-release
from combustion and energy transport due to detonation waves. Fundamental modeling issues such as
non-equilibrium thermodynamics coupled with detailed chemistry models will need to be addressed.

Biography:
Carlo Scalo obtained his B. Eng. (2006) and M.Sc. (2008) in Aerospace Engineering at the Università
Federico II (Italy) where he focused on computational methods for fluid mechanics. He completed his
Ph.D. (2012) at Queen's University (Canada), working under the supervision of Prof. Ugo Piomelli on
subgrid-scale modeling of high-Schmidt-number turbulent mass transfer in equilibrium and nonequilibrium
geophysical flows. He is currently a Postdoctoral Fellow at the Center for Turbulence
Research at Stanford University working in collaboration with Prof. Sanjiva K. Lele on theoretical and
numerical modeling of thermoacoustic devices and sound-turbulence interaction in transonic boundary
layers.