Thermo-hydraulic characteristics of a tubular heat exchanger with air bubble injection and inserts: From bubble flow patterns to performance metrics

Heat transfer enhancement using inserts and air bubble injection is well established individually. However, despite improved performance under combined application, two-phase flow patterns with different inserts remain scarcely investigated. Therefore, the present experimental study investigates the thermo-hydraulic performance and flow patterns in a tubular heat exchanger with air bubble injection (ABI) and inserts. The inserts included twisted tape (TT), wire matrix (WM), wavy tape (WT), and wavy tape-wire coil (WT-c). Tests spanned superficial liquid Reynolds numbers (Re_SL) 1800–9000 and superficial gas Reynolds numbers (Re_SG) 340–1025 under constant heat flux. Initially, the energy parameters were evaluated and subsequently correlated with the observed flow patterns, which were validated against previously published studies. In plain tube, maximum 11.1% Nusselt number enhancement occurred at Re_SL:9000/Re_SG:340, while higher ABI caused 7.3% deterioration. The TT yielded highest PEC-I of 2.42, while WM witnessed 5.25. The WT showcased 7.5 followed 6.92 in case of WT-c. With respect to PEC-II, the observed trend was WM > WT-c > WT > TT, while PEC-III values ranged from 0.98 to 1.39 (TT), 1.31–3.30 (WT), 1.10–2.58 (WT-c), and 0.81–1.39 (WM). Further, cost-benefit ratio (CBR < 1) denoted economic viability for TT at high Re and WM at low Re, while WT showed reverse economics (CBR < 0). However, in-depth CFD analysis to capture local flow characteristics, identification of bubble size distribution, and scaling-up of models from laboratory-scale to industrial applications have been identified as potential future research directions.