04-10-2012, 01:37 PM
Cool sound: the future of refrigeration? Thermodynamic and heat transfer issues in thermoacoustic refrigeration
Cool sound.pdf (Size: 424.22 KB / Downloads: 49)
Abstract
During the past two decades the thermoacoustic
refrigeration and prime mover cycles gained
importance in a variety of refrigeration applications.
Acoustic work, sound, can be used to generate temperature
differences that allow the transport of heat from a
low temperature reservoir to an ambient at higher temperature,
thus forming a thermoacoustic refrigeration
system. The thermoacoustic energy pumping cycle can
also be reversed: temperature difference imposed along
the stack plates can lead to sound generation. In this
situation the thermoacoustic system operates as a prime
mover. Sound generated by means of this thermoacoustic
energy conversion process can be utilized to drive
different types of refrigeration devices that require
oscillatory flow for their operation, such as thermoacoustic
refrigerators, pulse tubes and Stirling engines. In
order for a thermoacoustic refrigeration or prime mover
system as well as a thermoacoustic prime mover driving
a non-thermoacoustic refrigeration system to be competitive
on the current market, it has to be optimized in
order to improve its overall performance.
Introduction
In recent years environmental issues have become a key
concern in the design and development of refrigeration
systems. Current research and engineering efforts focus
on the development of alternative refrigerants as well as
alternative technologies, such as Stirling engines, pulsetube
refrigerators, thermoelectric refrigerators, etc.,
which can reduce the need for hazardous refrigerants [1].
One promising approach in the class of alternative
technologies is thermoacoustic refrigeration [1–5].
Thermoacoustic refrigeration was developed during the
past two decades as a new, environmentally safe refrigeration
technology [6–8]. The operation of thermoacoustic
refrigerators employs acoustic power to pump
heat.
Parameter spaces
The first step in the optimization algorithm introduced
byWetzel and Herman [9] is the design and optimization
of the thermoacoustic core. The design begins by
establishing the design requirements for a specific device,
depending on the application. The basic design
requirements for a thermoacoustic refrigerator are
twofold: (1) the refrigerator has to supply the desired
cooling load (Qload in Fig. 2) and at the same time (2) it
has to achieve the prescribed cooling temperature (Tr in
Fig. 2). The result of the short stack boundary layer
approximation yields two equations describing the enthalpy
flux H and work flux W. These equations were
originally derived by Rott [14] and developed further by
Swift [7].
Conclusions
In this paper a systematic design approach is discussed
that provides fast engineering estimates for initial design
calculations of thermoacoustic refrigerators. Their performance
was evaluated using two criteria. The methodology is based on a first law analysis that suggests
a separate optimization of the four main modules
of a thermoacoustic refrigerator: (1) thermoacoustic
core, (2) resonance tube, (3) heat exchangers and (4)
acoustic driver. An upper limit on the thermoacoustic
refrigerator’s performance is set by the heat pumping
capacity of the thermoacoustic core. The short stack
boundary layer approximation was applied to evaluate
and optimize the performance of the thermoacoustic
core. This simplified linear model of thermoacoustic
theory was not directly suitable for design and optimization
purposes in its original form. Calculations of the
thermoacoustic core’s performance indicate that thermoacoustic
refrigeration can achieve COPRs competitive
to commercially available refrigerators. Of course,
when comparing data with other refrigeration systems,
we have to keep in mind that the presented calculations
do not include energy losses in the other three modules
(resonance tube, heat exchangers and acoustic drivers)
of a thermoacoustic refrigerator.