Enhancing combi-boiler effectiveness through geometric optimization: a computational and experimental study on recuperator with elliptical tubes
Journal of Thermal Analysis and Calorimetry, 2026 (SCI-Expanded, Scopus)
- Yayın Türü: Makale / Tam Makale
- Basım Tarihi: 2026
- Doi Numarası: 10.1007/s10973-026-16470-8
- Dergi Adı: Journal of Thermal Analysis and Calorimetry
- Derginin Tarandığı İndeksler: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Aerospace Database, Chemical Abstracts Core, Chimica, Compendex, Index Islamicus, INSPEC, Academic Search Ultimate (EBSCO), Engineering Source (EBSCO), Materials Science & Engineering Collection (ProQuest), Technology Collection (ProQuest)
- Anahtar Kelimeler: Condensing combi-boiler, Elliptical tube, Recuperator, Tube inclination angle
- Eskişehir Osmangazi Üniversitesi Adresli: Evet
Özet
This study investigates the modification of the tube geometry in an aluminum recuperative heat exchanger used to recover waste heat from flue gases in a hermetic combi-boiler. The existing recuperator featuring four circular finned tubes was redesigned into six elliptical tubes, maintaining a constant external fin surface area. Numerical simulations based on the realizable k-ε turbulence model were conducted under non-condensing conditions to evaluate different elliptical tube inclination angles (0°, 35°, 40°, and 45°). Evaluating the velocity vectors, pressure fields, and temperature distributions revealed that inclined configurations induced severe flow maldistribution, thermal channeling, and aerodynamic dead zones. Consequently, the 0° inclined elliptical tube was identified as the optimal design, as it ensured uniform flow distribution, maximized thermal surface contact, and yielded the lowest gas-side pressure drop. To validate these numerical findings under real operating and critical condensing conditions, a physical prototype of the 0° elliptical tube recuperator was manufactured and tested against the conventional circular model. Experimental results demonstrated that, despite a negligible 3.5% difference in the total heat transfer area, the actual useful heat transferred to the water significantly increased from 1563 to 1841 W. This significant enhancement in heat transfer performance was primarily driven by superior latent heat recovery, evidenced by the condensate mass flow rate increasing from 1.15 kg·h−1 to 1.65 kg·h−1. Ultimately, the overall effectiveness of the newly developed recuperator improved substantially from 63.2% to 74.2%. This was accompanied by an optimized aerodynamic profile that reduced the gas-side pressure drop from 33.2 Pa to 30.9 Pa, thereby lowering the required fan power. Driven by this enhanced pressure drop profile and improved aerodynamic behavior, CO emissions decreased from 42 to 24 ppm, while NOx emissions dropped from 180 mg kWh−1 to 168 mg kWh−1. These findings demonstrate that the proposed 0° elliptical design yields an efficient recuperator with improved aerodynamic and environmental performance for modern combi-boilers.