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Journal Article•DOI•

A compromise between the temperature difference and performance in a standing wave thermoacoustic refrigerator

Mahmoud A. Alamir1, Ahmed A. Elamer2, Ahmed A. Elamer3•
Flinders University1, University of Bradford2, Mansoura University3
09 Nov 2020-International journal of ambient energy (Taylor & Francis)-Vol. 41, Iss: 13, pp 1441-1453
TL;DR: The operating conditions and geometric parameters are important for the thermoacoustic refrigeration as mentioned in this paper, which is an evolving cooling technology in which the acoustic power is used to pump heat.
Abstract: Thermoacoustic refrigeration is an evolving cooling technology in which the acoustic power is used to pump heat. The operating conditions and geometric parameters are important for the thermoacoust...

Summary (3 min read)

Jump to: [1. Introduction] – [2. Thermoacoustic Refrigerator Design] – [3. DeltaEC Model] – [4. Results and Discussion] – [4.1 Mean Pressure] – [4.2 Amplitude Pressure] – [4.3 Stack Position] – [4.4 Stack Length] – [4.5 Stack Spacing] and [5. Conclusion]

1. Introduction

  • Thermoacoustic refrigeration is a developing cooling technology.
  • Following this, the wave through the resonator produces hot and cold temperature regions due to the high and low-pressure areas distribution across the resonator.
  • Therefore, the operating conditions and the geometric parameters should be compromised to give a desired temperature difference across the stack with a high performance.

2. Thermoacoustic Refrigerator Design

  • The thermoacoustic effect occurs inside the stack walls, so the thermal contact and viscous losses are presented at the stack surface.
  • The stack geometric parameters influence the gained cooling load and the needed input power as demonstrated in Eqn. 4 and Eqn. 5 for the normalised cooling power ̇ and the normalised acoustic power ̇ respectively.
  • The parallel plate stack from Mylar material is selected with porosity 0.75.
  • The obtained results from the design steps are used in the DeltaEC model.

3. DeltaEC Model

  • The effect of the operating conditions and geometric parameters change on the coefficient of performance of the thermoacoustic refrigerator and the temperature difference across the stack at different cooling loads will be presented numerically with the help of the free simulation software DeltaEC version 6.3b11 [8].
  • DeltaEC solves the pressure and flow rate equations, which are concluded from the momentum equation and the continuity equations of fluid mechanics, respectively.
  • A number of trials are then performed to form solutions for p1(x) and U1(x) that make guesses meet targets.
  • The first boundary condition is to enforce the occurrence of resonance frequency [8,24] through the resonator, while the second boundary condition keeps pressure amplitude with known value independent of each trial of DeltaEC.
  • Temperature of the hot heat exchanger is kept constant to show how the cold temperature is lower than this specific temperature.

4. Results and Discussion

  • The effect of different operating conditions and geometric parameters on both the temperature difference across the stack and the performance of a thermoacoustic refrigerator is presented.
  • After that, a compromise is held for maximizing both the temperature difference across the stack and the performance according to the criteria of acceptable range shown in Table 2.
  • These criteria are considered reasonable for the required design parameters demonstrated in Section 2, which were primarily for small-size refrigerators.
  • After that, the compromised values for the operating conditions and geometric parameters were chosen to achieve two factors.
  • Second, the operating conditions and geometric parameters have values in between high performance and high temperature difference across the stack.

4.1 Mean Pressure

  • Increasing the mean pressure decreases the temperature difference as shown in Fig. 4a, as the pressure amplitude would be insignificant relative to the mean pressure.
  • The small thermal penetration depth decreases the temperature difference of the heat transfer between the gas parcels and the stack plates, as more gas parcels will be oscillating without interacting with the stack walls.
  • Also, increasing cooling load will lead to a decrease of the temperature difference due to the cold side temperature rise.
  • The acoustic energy is directly proportional to drive ratio, D, as the least wave amplitude from the input driver will lead to a good fluctuation.
  • Then, a compromise is made to select the suitable mean pressure for their design considering the acceptable range in Table 2.

4.2 Amplitude Pressure

  • The temperature difference starts at a low value in the first part of Fig. 6a due to the weakness of the pressure amplitude to make the change.
  • After that, the increase of amplitude pressure increases the temperature difference, until it reaches a maximum value obtained near a drive ratio of 3 %.
  • The input acoustic energy for a fixed mean pressure increases with the acoustic pressure increase, which means a lower performance as shown in Fig. 6b.
  • The maximum temperature difference occurs at drive ratio equals 3%, but the performance is another key parameter and the authors considered factors shown in table 2.
  • Therefore, a drive ratio equals 2 % is chosen to improve the performance and to account for the driver abilities to provide that drive ratio.

4.3 Stack Position

  • The input acoustic signal changes with a sine wave, so the temperature distribution is also changed.
  • The values of temperature differences at normalised stack positions from 0.25 to 0.3 do not change with a sensible change, so a normalised stack position equals 0.3 is chosen.

4.4 Stack Length

  • Increasing the stack length means that larger number of the working fluid molecules will interact with the stack plates leading to the increase of temperature difference as shown in Fig. 8a.
  • The stack is the place where the thermoacoustic effect and pumping heat takes place, so increasing the stack length will lead to more gas particles interact with the stack plates, and thus the acoustic power consumption will increase, and the performance will decrease as shown in Fig. 8b.
  • Further increasing the stack length will result in a decrease in the temperature difference across the stack.
  • This observed decrease in temperature difference across the stack could be attributed to the variability of the acoustic field inside the resonator.
  • For different stack positions and cooling load of 5 W, the normalised stack length effect on the temperature difference and the coefficient of performance will be as shown in Fig.

4.5 Stack Spacing

  • The thermal and viscous penetration depths parameters make a deep understanding of plate spacing change effect.
  • Thus, increasing the spacing between the plates reduces the viscous penetration depth effect and the viscosity losses.
  • After that, a weak thermal interaction with the plates is observed with the increase in the stack spacing, and the temperature difference is then considerably decreased.
  • Increasing the plate spacing will increase the coefficient of performance in the studied range of normalized stack spacing, as the viscous losses are declined leading to a significant decrease in the consumed acoustic power and higher heat transfer rates between the gas particles and the cold heat exchanger according to the results obtained by DeltaEC.
  • Until it reaches a region, where the change is less sensitive and the performance will remain constant as shown in Fig. 10b.

5. Conclusion

  • Theoretical study using DeltaEC is presented to show the effect of changing the geometric parameters and operating conditions of a thermoacoustic refrigerator on both the coefficient of performance and the temperature difference across the stack.
  • In addition, the physical phenomenon of the effect of the operating conditions and the geometric parameters is introduced.
  • Moreover, depending on the designed thermoacoustic refrigerator, compromised values for the operating conditions and geometric parameters are collected as following: A drive ratio of 2 % will compromise the temperature difference and the coefficient of performance.
  • A normalised stack position of 0.3 will compromise both the temperature difference across the stack and the performance.
  • Nomenclature Latin Letters Resonator area, Sound velocity, Porosity, c Specific heat, ( ) Drive ratio, Frequency, Thermal conductivity, ( ) Length,.

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A compromise between the temperature
difference and performance in a standing
wave thermoacoustic refrigerator
Item Type Article
Authors Alamir, M.A.; Elamer, Ahmed A.
Citation Alamir MA and Elamer AA (2018) A compromise between the
temperature difference and performance in a standing wave
thermoacoustic refrigerator. International Journal of Ambient
Energy. Accepted for publication.
Rights © 2018 Taylor & Francis. The Version of Record of
this manuscript has been published and is available
in International Journal of Ambient Energy https://
doi.org/10.1080/01430750.2018.1517673.
Download date 09/08/2022 16:58:05
Link to Item http://hdl.handle.net/10454/16622
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=taen20
International Journal of Ambient Energy
ISSN: 0143-0750 (Print) 2162-8246 (Online) Journal homepage: http://www.tandfonline.com/loi/taen20
A compromise between the Temperature
Difference and Performance in a Standing Wave
Thermoacoustic Refrigerator
Mahmoud Alamir & Ahmed A. Elamer
To cite this article: Mahmoud Alamir & Ahmed A. Elamer (2018): A compromise between the
Temperature Difference and Performance in a Standing Wave Thermoacoustic Refrigerator,
International Journal of Ambient Energy
To link to this article: https://doi.org/10.1080/01430750.2018.1517673
Accepted author version posted online: 29
Aug 2018.
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1
Publisher: Taylor & Francis & Informa UK Limited, trading as Taylor & Francis Group
Journal: International Journal of Ambient Energy
DOI: 10.1080/01430750.2018.1517673
A compromise between the Temperature Difference and
Performance in a Standing Wave Thermoacoustic Refrigerator
Keywords: DeltaEC; Performance; Refrigeration; Thermoacoustics; Temperature difference.
Highlights
A theoretical DeltaEC model of a standing wave thermoacoustic refrigerator is built.
Compromised values for the geometric parameters and operating conditions are
collected.
The physical description of the performance and the temperature difference change
behavior is presented.
2
Abstract
Thermoacoustic refrigeration is an evolving cooling technology where the acoustic power is
used to pump heat. The operating conditions and geometric parameters are important for the
thermoacoustic refrigerator performance, as they affect both its performance and the
temperature difference across the stack. This paper investigates the effect of the stack
geometric parameters and operating conditions on the performance of a standing wave
thermoacoustic refrigerator and the temperature difference across the stack. DeltaEC software
is used to make the thermoacoustic refrigerator model. From the obtained results, normalised
values for the operating conditions and geometric parameters are collected to compromise
both the performance and the temperature difference across the stack.
1. Introduction
Thermoacoustic refrigeration is a developing cooling technology. It has many positives over
other alternative refrigeration technologies, as it uses environment friendly working gases, the
cooling capacity is continuously controlled, the design is simple, and it can operate quietly [1–
3]. This cooling technology is now in the research and development process, and it is expected
for noticeable spread commercially [4].
3
Thermoacoustic refrigeration uses the vibrational sound pressure waves. The heat is pumped
from low temperature source to high temperature sink by the sound waves. Fig. 1 shows a
typical standing wave thermoacoustic refrigerator. The function generator and the amplifier
feed the signal to the acoustic driver, and transmit the required frequency and power into the
resonator. Following this, the wave through the resonator produces hot and cold temperature
regions due to the high and low-pressure areas distribution across the resonator. The stack
which has low thermal conductivity separates the hot and cold areas inside the resonator, and
two heat exchangers are bounded the stack for heat transfer.
Insert Fig. 1 about here.
The temperature difference is a key parameter in refrigeration area, as a large temperature
difference may be required in some applications that need low temperatures. This can be on
the expense of the performance or even the obtained cooling loads. Further, the operating
conditions and geometric parameters of thermoacoustic refrigerators can have an influence on
both the temperature difference across the stack and the consumed acoustic power. Therefore,
the operating conditions and the geometric parameters should be compromised to give a
desired temperature difference across the stack with a high performance.
Recently, researchers have shown an increased interest in optimizing thermoacoustic
refrigerators. A number of researchers have reported design and optimization algorithms for
the thermoacoustic devices. Wetzel and Herman [5] developed an algorithm for
thermoacoustic refrigerators. The total acoustic power was introduced as follows,
󰇗

󰇗
󰇗

󰇗

(1)
Citations
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Journal Article•DOI•
An artificial neural network model for predicting the performance of thermoacoustic refrigerators

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Mahmoud A. Alamir1•
Flinders University1
01 Jan 2021-International Journal of Heat and Mass Transfer
TL;DR: The results showed that the proposed model can predict the TAR cooling temperature and performance with high accuracy and the use of ANNs could enhance the predictability of TAR performance using different operating conditions and geometric parameters.
Abstract: This paper proposes an artificial neural network (ANN) model for predicting the cooling temperature and performance of a thermoacoustic refrigerator (TAR). Two ANN models were presented with two different numbers of neurons in the hidden layers. The results showed that the proposed model can predict the TAR cooling temperature and performance with high accuracy. An R squared value reaching 0.97 can be obtained when using ten neurons in the hidden layer as compared to 0.77 when using five neurons. The use of two hidden layers was sufficient for the highest model accuracy when compared to one or three hidden layers. A feature analysis using the “RReliefF” algorithm was used to investigate the relative importance of operating conditions and geometric parameters to the performance. The operating frequency had the highest importance weights to the performance as compared to the stack geometric parameters. This confirms the results of previous research which showed that the frequency can affect the performance of thermoacoustic refrigerators significantly. The use of ANNs could, therefore, enhance the predictability of TAR performance using different operating conditions and geometric parameters. The feature analysis also shows the important parameters that need further optimisation to be targeted in future TAR designs. The use of ANN models for predicting TAR cooling temperature and performance can also minimise the need for conducting costly and time-consuming experiments.

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TL;DR: An enhanced CNN classification model using the late fusion between CNNs and ensemble classifiers to predict different classes of acoustic scenes has robust applicability for future ASC problems.
Abstract: Recent evidence suggests that convolutional neural networks (CNNs) can model acoustic scene classification (ASC) with high accuracy. Ensemble classifiers have also shown high accuracy in different machine learning areas. However, little is known about fusion models between CNNs and different ensemble classifiers for ASC. This study presents an enhanced CNN classification model using the late fusion between CNNs and ensemble classifiers to predict different classes of acoustic scenes. A CNN model was first built to classify fifteen acoustic scene environments. Different ensemble classifier models were then used for this classification problem. Late fusion of CNN and ensemble classifier models was then applied. The results showed that late fusion models have higher classification accuracy, as compared to individual CNN or ensemble classifier models. The best model was obtained by fusion of the CNN and discriminant random subspace classifier with an increase in the average accuracy of 10% as compared to the average accuracy of the CNN model. When compared with previous research on ASC, the late fusion model between CNN and ensemble classifiers showed higher accuracy. Therefore, this method has robust applicability for future ASC problems.

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Experimental study of the stack geometric parameters effect on the resonance frequency of a standing wave thermoacoustic refrigerator

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Mahmoud A. Alamir1•
Flinders University1
15 Apr 2019-International Journal of Green Energy
TL;DR: The resonance frequency is important for the performance of thermo-acoustic refrigerators, as it... as mentioned in this paper, as it is the resonance frequency that is selected by the system.
Abstract: Thermoacoustic refrigeration is an emerging technology that makes use of acoustic power to pump heat. The resonance frequency is important for the performance of thermoacoustic refrigerators, as it...

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Cites background from "A compromise between the temperatur..."

  • ...…the role of the stack geometric parameters on the thermoacoustic refrigerator performance (N Atiqah Alamir 2017, 2019; Alamir and Elamer 2018; Alcock, Tartibu, and Jen 2017; Elnegiry, Eltahan, and Alamir 2016; Ibrahim, Omar, and Abdel-Rahman 2011; Napolitano, Romano, and Dragonetti 2017;…...

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Journal Article•DOI•
Experimental study of the temperature variations in a standing wave loudspeaker driven thermoacoustic refrigerator

[...]

Mahmoud A. Alamir1•
Flinders University1
01 Jun 2020-Thermal science and engineering
TL;DR: In this paper, the effect of the stack geometric parameters on the temperature difference across the stack in a standing wave loudspeaker driven thermoacoustic refrigerator was investigated, and the authors provided guidelines for increasing the performance of thermo-acoustic refrigerators, which still lack competitiveness because of their relatively low performance.
Abstract: The temperature difference across the stack is essential to the performance of thermoacoustic refrigerators. This paper experimentally investigates the effect of the stack geometric parameters on the temperature difference across the stack in a standing wave loudspeaker driven thermoacoustic refrigerator. In addition, it investigates the temperature variations through the resonator and its cross-sections. The effect of this variability on the performance of the refrigerator is also presented. Celcor Ceramic stacks are used at five normalised stack positions of 0.286, 0.764, 1.05, 1.43 and 1.72, four normalised stack lengths of 0.076, 0.114, 0.153 and 0.191, and two stack porosities of 0.8 and 0.85. Results show that the maximum and minimum temperatures through the resonator are across the stack, particularly at the centre point of each side of the stack. Moreover, at certain stack positions, hot and cold sides across the stack are altered. The coefficient of performance also increases at high-temperature difference positions. As a result, this study provides guidelines for increasing the performance of thermoacoustic refrigerators, which still lack competitiveness because of their relatively low performance. It also helps design some parts such as heat exchangers to consider the maximum and minimum temperature positions of thermoacoustic refrigerators.

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Thermoacoustic Energy Conversion Devices: Novel Insights

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Mahmoud A. Alamir1•
Flinders University1
14 Nov 2020
TL;DR: Important considerations of the design and state-of-the-art developments in thermoacoustic energy conversion devices are summarized and the use of neural networks and metadata as optimisation methods are presented.
Abstract: Thermoacoustic engines and refrigerators have many advantages. They use environment-friendly working gases, their design is simple, and they can operate quietly. However, they have many design characteristics from geometric parameters and operating conditions. Besides this, they still have low efficiencies and performance. This paper summarises important considerations of the design and presents the state-of-the-art developments in thermoacoustic energy conversion devices. This includes recent studies and designs of the thermoacoustic refrigeration devices towards more efficient thermoacoustic engines and refrigerators. New insights into the design of resonators, the different sources of the power sources, the different stack geometries and working mediums were considered. The challenges that face the development of thermoacoustic devices were also discussed. Far too little attention has been paid to looking at these devices comprehensively. In further research, the use of neural networks and metadata as optimisation methods could be a means of significantly increasing the performance of these devices. There is also abundant room for further progress in enhancing oscillatory heat transfer. Moreover, further recommendations and studies were proposed for a better understanding of the interrelationship between the geometric parameters and operating conditions.

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TL;DR: The Los Alamos thermoacoustics code, available at www.lanl.gov/thermoACoustics/, has undergone extensive revision this year, and a Python-based graphical user interface wrapped around that core provides improved usability as discussed by the authors.
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"A compromise between the temperatur..." refers background or methods in this paper

  • ...DeltaEC [8] was used as a verification tool for this algorithm....

    [...]

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    [...]

  • ...The four unknown guesses are the unknown hot heat exchanger power required for each DeltaEC trial, the resonance frequency (which varies with the operating conditions and geometric parameters in each trial [6,21]), volume flow rate and the mean temperature ( that are dependent on the power of the acoustic driver [8], which is variant in this study) at the beginning segment....

    [...]

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Abstract: In this paper the design of thermoacoustic refrigerators, using the linear thermoacoustic theory, is described. Due to the large number of parameters, a choice of some parameters along with dimensionless independent variables will be introduced. The design strategy described in this paper is a guide for the design and development of thermoacoustic coolers. The optimization of the different parts of the refrigerator will be discussed, and criteria will be given to obtain an optimal system.

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Johns Hopkins University1
01 Jan 1997-International Journal of Refrigeration-revue Internationale Du Froid
TL;DR: The simplified linear model of thermoacoustics - the short stack boundary layer approximation - is implemented into a systematic design and optimization algorithm and serves as an easy-to-follow guideline for the design of therMOacoustic refrigerators.
Abstract: Thermoacoustic refrigeration was developed during the past two decades as anew, environmentally safe refrigeration technology. The operation of thermoacoustic refrigerators employs acoustic power to pump heat. Nowadays, as commercial applications are sought, it is important to be able to obtain fast and simple engineering estimates for the design and optimization of prototypes. This paper provides such estimates by implementing the simplified linear model of thermoacoustics - the short stack boundary layer approximation - into a systematic design and optimization algorithm. The proposed algorithm serves as an easy-to-follow guideline for the design of thermoacoustic refrigerators. Performance calculations applying the algorithm developed in this paper, predict values of 40–50% of Carnot's efficiency for the thermoacoustic core, the heart of a thermoacoustic refrigerator. One reason that these efficiencies have not yet been achieved in devices built to date, is the poor performance of the heat exchangers in thermoacoustic refrigerators. This issue and other remaining challenges for future research are also addressed in the paper. Solving these problems in the near future, we believe, will bring an environmentally safe refrigeration technology a step closer to commercial use.

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Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "A compromise between the temperature difference and performance in a standing wave thermoacoustic refrigerator" ?

In this paper, the effect of different operating conditions on the thermo-acoustic performance of a standing wave thermoacoustic refrigerator was investigated.