Abstract: The production of sound-wave in thermo-acoustic device is necessary to induce cooling or generate electricity. The magnitude of the sound-wave is normally proportional to the amount of heat provided to the device. The possibility to use waste heat in any locations can be enough to justify the use of thermo-acoustic technology for sustainable electricity generation or refrigeration. In this work, an adjustable thermoacoustically-driven engine has been developed using the Design Environment for Low-amplitude ThermoAcoustic Energy Conversion (DELTAEC). Many studies have highlighted the relationship between the geometry of the stack and the performance of the device. Unlike previous studies, the resonator of this thermoacoustically-driven device, made of two portions, was adjusted. The performance of the device has been analysed in order to evaluate the influence of the alteration of the resonator on the heat-to-sound conversion. Performance indicators like the acoustic power, the temperature difference across the stack and the frequency of the sound-wave have been studied. This work point out the possibility to regulate the performance of thermo-acoustic engine by adjusting the geometry of the resonator

Alcock, A.C.

Balonji, S.

Jen, T.C

Tartibu, L.K.

English

University of Johannesburg Institutional Repository

 Available online at www.sciencedirect.com ScienceDirect Procedia Manufacturing 00 (2019) 000–000    www.elsevier.com/locate/procedia  2351-9789 © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of SMPM 2019.  International Conference on Sustainable Materials Processing and Manufacturing, SMPM 2019, Sun City Resort, South Africa, 08 – 10 March 2019 Performance alteration of standing-wave thermoacoustically-driven engine through resonator length adjustment S. Balonjia, A.C Alcocka, L.K Tartibua,* and T.C Jenb aDepartment of Mechanical Engineering Technology, University of Johannesburg, Doornfontein Campus, Johannesburg 2028 bDepartment of Mechanical Engineering Science, University of Johannesburg, Auckland Park Campus, Johannesburg 2006, South Africa. Abstract The production of sound-wave in thermo-acoustic device is necessary to induce cooling or generate electricity. The magnitude of the sound-wave is normally proportional to the amount of heat provided to the device. The possibility to use waste heat in any locations can be enough to justify the use of thermo-acoustic technology for sustainable electricity generation or refrigeration. In this work, an adjustable thermoacoustically-driven engine has been developed using the Design Environment for Low-amplitude ThermoAcoustic Energy Conversion (DELTAEC). Many studies have highlighted the relationship between the geometry of the stack and the performance of the device. Unlike previous studies, the resonator of this thermoacoustically-driven device, made of two portions, was adjusted. The performance of the device has been analysed in order to evaluate the influence of the alteration of the resonator on the heat-to-sound conversion. Performance indicators like the acoustic power, the temperature difference across the stack and the frequency of the sound-wave have been studied. This work point out the possibility to regulate the performance of thermo-acoustic engine by adjusting the geometry of the resonator.  © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of SMPM 2019. Keywords: Thermo-acoustic; sound-wave; DELTAEC; engine.    * Corresponding author.       E-mail address: ltartibu@uj.ac.za   2 Balonji S./ Procedia Manufacturing 00 (2016) 000–000 1. Introduction Thermo-acoustic engines (TAEs) are potentially good candidates in the search for suitable heat recovery technology that can be used in industrial processes for instance. This could result in the decrease of the dependency on fossil fuels, mitigate the environmental concerns and make industrial process more sustainable. TAE consist of a high temperature heat exchanger, a stack, an ambient heat exchanger and a resonator tube (Fig. 1). The energy conversion from heat energy into acoustic energy takes place within the stack, considered as the heart of the device. A compressible gas is generally used to fill up the engine. The acoustic waves generated are the result of the heat transfer taking place between the compressible fluid and the stack when the latter is subjected to a temperature gradient. This transfer creates local changes of velocity and pressure within the fluid. With a suitable phasing between the pressure and the velocity, an acoustic oscillations is generated resulting ultimately in a sound-wave [1]. This technology can be used to generate clean energy and drive refrigeration systems.   Fig. 1. Typical standing wave TAE  It is expected that a large acoustic power can be generated if the geometrical configuration of the device is optimum. An experimental investigation on the performance of simple TAE was conducted by Hariharan et al. [2]. This study found that the stack length and its position within the resonator affect significantly the thermal efficiency of TAE. This study considered the temperature gradient, the frequency and the pressure amplitude as performance indicators. Tartibu’ experimental results suggests that some of the geometrical parameters namely the stack length, its position and its porosity are interdependent [3]. Tao et al. [4] focus on the operating frequency and the onset temperature. This study point out that the resonator length is crucial in the operation of the TAE. In addition, the possibility to drop the onset temperature below 100oC is demonstrated. A numerical analysis performed with Computational Fluid Dynamics shows that the frequency and the pressure amplitude of oscillations are higher in tapered resonator as compared to straight resonator [5]. This findings is supported by Bao et al. [6] who suggest that tapered resonator reduces the velocity amplitude and minimize the total acoustic loss in the resonator tube. The resonant frequency is proportional to the sound speed (a) and inversely proportional to the resonator length (L). This is demonstrated by Equation 1 pointing out the relationship between these parameters:  (1) Intuitively, an extension of the resonator length results in a drop of the resonant frequency. This has been demonstrated experimentally by Cahyadi et al. [8]. This study emphasizes the importance of selecting the resonator length carefully in order for TAE to work optimally and produce higher acoustic power. Although several studies have demonstrated the impact of the geometry of TAE on its performance, to the authors’ knowledge, no study have shown how to take this aspect into account during the designing. We are proposing a new TAE model made of two portions that could be adjusted during operation in order to allow a single device to have different operating configurations (Fig. 2 and Fig. 3). The aim of this work is to perform a preliminary investigation on the influence of the length adjustment of the proposed TAE model and get an insight into the benefit of this designing approach. This analysis was performed numerically using the modelling code called DELTAEC (Design Environment for Low-amplitude Thermo-Acoustic Energy Conversion). The resonator length constitutes the focus of this analysis. This modelling code is useful in predicting the performance of thermo-acoustic system. It is freely available on the Los Alamos National Laboratory website. Details about the designing of this adjustable system are available in Ref. [7]. Key (1.) Heating source (2.) Resonator (3.) Heat exchanger (4.) Stack (5.) Sound wave (6.)  Front view of stack δQh Hot heat load δQc Cold heat load Q Heat input  Balonji S./ Procedia Manufacturing 00 (2016) 000–000 3  Fig. 2. TAE set-up  Fig. 3. Adjustable TAE model  2. Modelling with DeltaEC The computer program DelaEC (Design Environment for Low Amplitude) was used to study the behaviour of the adjustable TAE. This model was built in order to approximate the geometry of the TAE and get an insight into the trend of the frequency, the onset temperature, the acoustic power and the efficiency when the resonator is extended/retracted. The major segments used to build the devices are BEGIN, DUCT, HX, CONE, HARDEND, COMPLIANCE and RPN. A summary of the main parameters adopted in this study is provided in Table 1. The model incorporated five guesses (G) and five targets (T).   Description 1. Buffer volume 2. Adjustable resonator (0-500 mm extension) 3. Shell and tube heat exchanger 4. Honeycomb ceramic stack 5. 2 mm 12-gauge nichrome heating coil coupled to stack (1) (2) (3) (4) (5) 4 Balonji S./ Procedia Manufacturing 00 (2016) 000–000   Fig. 4. DeltaEC model  Table 1. Main  design parameters Segment  Parameters (0) BEGIN Pm: 1.2 E+6 Pa; f: 60.084 Hz (G); T: 323.89 K  (G); |p| : 1.79E+4 (G); |U|: 0.576 m3/s (G) (2) DUCT A: 8.3323E-3 m2; L: 0.132 m (3) HX A: 8.3323E-3 m2; L: 3.0E-2 m; yo: 5.0E-4 m; HeatIn 2000 W (4) STKSLAB (Celcor) Ls: 0.10 m; yo=2.243E-4 m; Lplate: 1.25 E-4 m (5) HX HeatIn -5171 W (G); T: 300 K (T) (8) CONE L: 6.0E-2 m (9) DUCT A: 2.463E-3 m2; L: 0.5-1.5 m (11) CONE L: 6.0E-2 m (12) DUCT L: 5.0E-2 m; (13) CONE A: 2.52E-2 m2; L: 0.10 m (14) COMPLIANCE Surface area: 1 m2; Volume: 2.5E-2 m3 3. Illustration of the simulation procedure Fig. 4 shows the model DeltaEC adopted for this study. The analysis focuses on the resonator length described by “DUCT 9”. This component has been adjusted in the range of 0.5 to 1.5 m corresponding to the resonator extension/retraction. After each run, the performance indicators namely the onset temperature, the frequency, the acoustic power and the efficiency have been recorded.  4. Results and discussions Eleven runs have been performed in order to study the influence of the resonator extension/refraction on the performance of the TAE. The results obtained are reported in Table 2. In order to analyze the results trend, these results have been presented graphically in Fig. 5 and Fig. 6. Fig. 5 confirms the fact that extending the resonator length results in a drop of the resonant frequency as per Equation. 1. It appears that the onset temperature increases from 320oC to 330oC approximatively when the resonator is extended from 0.5 m to 1.5 m respectively. This might be due to the necessity to have a higher input power in order to overcome the viscous losses proportional to the resonator length. Fig. 6 suggests that an extension of the resonator length affects the acoustic power and results in a drop of the TAE efficiency. This could be explained by the relatively higher viscous losses proportional to the resonator length. It appears that an adjustment of the resonator length affect the frequency, thermal efficiency and  Balonji S./ Procedia Manufacturing 00 (2016) 000–000 5 sound pressure of the TAE. Longer resonators resulted in lower frequency outputs and lower sound pressures. Thermal efficiency decreases when the resonator was extended.  Table 2. DeltaEC results Duct extension Frequency Acoustic power  Onset temperature Efficiency 0.5 60.084 2960.1 323.89 0.413 0.6 56.181 2966.9 324.83 0.394 0.7 52.944 2977.0 325.71 0.377 0.8 50.196 2972.2 326.47 0.362 0.9 47.827 2969.6 327.19 0.348 1.0 45.758 2968.8 327.88 0.335 1.1 43.924 2954.7 328.46 0.323 1.2 42.282 2923.1 328.92 0.313 1.3 40.811 2917.6 329.49 0.304 1.4 39.482 2922.4 330.09 0.295 1.5 38.264 2907.9 330.26 0.286      Conclusion In this paper, a numerical model describing an adjustable TAE have been built within DeltaEC. The resonator extension has been investigated in order to get an insight into its impact on the performance of the TAE. The results trend obtained from the simulation performed, suggest that a change of the resonator length affect the onset temperature, the resonant frequency, the acoustic power and the thermal efficiency of the device. Although this fact was known from previous studies, this work point out the advantage of developing an adjustable TAE in order to be able to operate the device at different configuration. It appears that an adjustment of the resonator length affect the frequency, thermal efficiency and sound pressure of the TAE. Longer resonators resulted in lower frequency outputs and lower sound pressures. Thermal efficiency decreases when the resonator was extended.   Fig. 6. Acoustic power and efficiency as a function of the duct extension Fig. 5. Onset temperature and frequency as a function of the duct extension 6 Balonji S./ Procedia Manufacturing 00 (2016) 000–000 Acknowledgements The authors would like to acknowledge the financial support received from the Faculty of Engineering of the University of Johannesburg, Johannesburg, South Africa. On behalf of all authors, the corresponding author states that there is no conflict of interest. References [1] J.A. Mumith, C. Makatsoris and T.G. Karayiannis, (2014). Design of a thermoacoustic heat engine for low temperature waste heat recovery in food manufacturing: A thermoacoustic device for heat recovery. Applied Thermal Engineering, 65(1-2), pp.588-596. [2] N.M. Hariharan, P. Sivashanmugam and S. Kasthurirengan, (2012). Experimental and theoretical investigation of thermoacoustic prime mover. HVAC&R Research, 18(6), pp.1112-1121. [3] L.K. Tartibu, (2018). Impact of ceramic substrates geometry on the performance of simple thermo-acoustic engines. Experimental Techniques, 42, pp. 155–176. [4] T. Jin., B.S. Zhang, K. Tang, R. Bao and G.B. Chen, (2007). Experimental observation on a small-scale thermoacoustic prime mover. Journal of Zhejiang University-SCIENCE A, 8(2), pp.205-209. [5] H. Muralidharan, N.M. Hariharan, V.T. Perarasu, P. Sivashanmugam and S. Kasthurirengan, (2013). Computational fluid dynamics simulation of open ended thermoacoustic prime mover with straight and tapered resonator. HVAC&R Research, 19(3), pp.208-212. [6] R. Bao, G. Chen, K. Tang, Z. Jia and W. Cao, (2006). Influence of resonance tube geometry shape on performance of thermoacoustic engine. Ultrasonics, 44, pp.1519-1521. [7] A.C. Alcock, L.K. Tartibu and T.C. Jen, (2017), Design and construction of a thermoacoustically driven thermoacoustic refrigerator, In: Proceedings of the 2017 International Conference on the Industrial and Commercial Use of Energy (ICUE). DOI:10.23919/ICUE.2017.8103430. [8] D.D. Cahyadi, Y.N. Adhitama, I. Setiawan and A.B. Utomo, (2017). Experimental Study of Resonance Frequency at Prime Mover Thermoacoustic Standing Wave. Journal of Physics: Theories and Applications, 1(2), pp.157-166.   

Performance alteration of standing-wave thermoacoustically-driven engine through resonator length adjustment

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Performance alteration of standing-wave thermoacoustically-driven engine through resonator length adjustment

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