Numerical study on waste heat recovery characteristics of non-Newtonian thermally hydrolyzed sewage sludge using dimpled tubes

Xiong, Songning and Wu, Z. and Lu, W. and Du, Y. and Zhang, G. and Xiong, Z. and Wu, Y. (2026) Numerical study on waste heat recovery characteristics of non-Newtonian thermally hydrolyzed sewage sludge using dimpled tubes. Chemical Engineering Science, 320 (B): 122552. ISSN 0009-2509

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Abstract

Following high-temperature thermal hydrolysis treatment, the sewage sludge contains substantial quantities of recoverable waste heat. Enhancing the waste heat recovery efficiency from this thermally hydrolyzed sludge can significantly reduce the operational costs of wastewater treatment plants. This study investigates the performance and entropy generation characteristics of waste heat recovery from thermally hydrolyzed sewage sludge with non-Newtonian fluid properties using dimpled tubes through numerical simulations. The experimental results demonstrate that dimpled tubes exhibit superior waste heat recovery efficiency compared to smooth tubes. Overall, the ellipsoidal dimpled tubes exhibit superior Performance Evaluation Criteria (PEC) values compared to spherical configurations, whereas spherical dimpled tubes demonstrate better entropy production performance. Specifically, under the same geometric parameters, the Nusselt number (Nu) and friction factor (f) of ellipsoidal dimpled tubes are 1.63–1.92 times and 2.65–3.32 times of smooth tubes, respectively, with a maximum PEC value of 1.39. Under the same geometric parameters compared to smooth tubes, the entropy generation is reduced by up to 50 % for spherical dimpled tubes. Furthermore, the results indicate that the relative influence of dimple geometric parameters on various performances exhibits a hierarchy of depth > number > pitch. These results provide valuable insights into the complex interplay between geometric parameters and non-Newtonian fluid flow characteristics.