sustainability Article Prediction of the Oscillatory Heat Transfer Coefficient in Thermoacoustic Refrigerators Mosa Machesa, Lagouge Tartibu * and Modestus Okwu   Citation: Machesa, M.; Tartibu, L.; Okwu, M. Prediction of the Oscillatory Heat Transfer Coefficient in Thermoacoustic Refrigerators. Sustainability 2021, 13, 9509. https:// doi.org/10.3390/su13179509 Academic Editor: Amir Mosavi Received: 29 May 2021 Accepted: 16 August 2021 Published: 24 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Mechanical & Industrial Engineering Technology, Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg 2028, South Africa; mosamachesa@gmail.com (M.M.); mechanicalmodestus@yahoo.com (M.O.) * Correspondence: ltartibu@uj.ac.za Abstract: Thermoacoustic refrigerators are emerging devices that make use of meaningful high- pressure sound waves to induce cooling. Despite the accelerated progress in the field of thermoacous- tics, knowledge of the heat transfer process in the heat exchange of the devices is still developing. This work applies different soft computing techniques, namely, an artificial neural network trained by particle swarm optimisation (ANN-PSO), adaptive neuro-fuzzy inference system (ANFIS), and artificial neural networks (ANNs) to predict the oscillatory heat transfer coefficient in the heat ex- changers of a thermoacoustic device. This study provides the details of the parametric analysis of an artificial neural network model trained by particle swarm optimisation. The solution model considers the number of neurons, the swarm population, and the acceleration factors to develop and analyse the architecture of several models. The regression model (R 2 ) and mean squared error (MSE) were used to evaluate the accuracy of the models. The result showed that the proposed soft computing techniques can potentially be used for the modelling and the analysis of the oscillatory heat transfer coefficient with a higher level of accuracy. The result reported in this study implies that the prediction of the OHTC can be considered for the enhancement of thermoacoustic refrigerators performances. Keywords: thermoacoustics; soft computing techniques 1. Introduction Traditional ways of refrigeration have played an important role in modern life. The traditional refrigeration process is stimulated by the vapour compression cycle that utilises certain refrigerants. Since its introduction at the dawn of the nineteenth century, the struc- ture of the refrigeration system has undergone several developments [1]. Even though there have been numerous developments, the traditional refrigeration system presents a great threat to the earth through its generation of greenhouse gases that deplete the ozone layer. Gases such as fluorine, carbon, and chlorine are blended to obtain the correct refriger- ation temperature, which has proven its potential to contribute to global warming [1]. The construction of the traditional system also involves significant energy costs. An analysis of the disadvantages of traditional refrigerators has led to efforts to move away from their use. Thermoacoustic refrigeration is an excellent substitute, as it has shown the potential to provide cooling without any negative impact on the environment at a relatively low cost. Thermoacoustics is the science of the interaction between thermodynamics and the oscillatory stream of sound waves when introduced to a temperature gradient. The move- ment of the working gas causes variations in the pressure and initiates the heat transfer between the walls of the regenerator to begin the thermoacoustic cycle [2]. Two distinct thermoacoustic systems exist. Thermoacoustic refrigerators use high-amplitude sound waves to pump heat from one point to another and thermoacoustic engines use heat energy to generate sound waves that are typically used to power the thermoacoustic refrigerators. Sustainability 2021, 13, 9509. https://doi.org/10.3390/su13179509 https://www.mdpi.com/journal/sustainability