Improving the Convective Heat Transfer Coefficient of a New Design Helical Coiled Tube Heat Exchanger via Air Injection Technique

This study experimentally investigates the effect of air injection on enhancing the overall heat transfer coefficient in a newly designed vertical helical coiled tube heat exchanger. Unlike conventional or uniform helical configurations, the new design increases the shell area coverage by coiled tube, significantly boosting the probability of bubble-coil interactions and enhancing disruption of the thermal boundary layer around the tube. In addition, the new coil geometry ensures that the mixing of the shell fluid due to bubble injection occurs effectively near the coil boundary, unforming the temperature in this zone, thereby reducing temperature polarisation and maximising the temperature gradient between the coil surface and surrounding fluid. To do so, initially, the heat transfer performance of the novel coil configuration was theoretically validated by comparing its heat transfer coefficient, expressed through the Nusselt number (Nu), with that of a conventional helical coil using an appropriate heat transfer correlation. The study further explored the influence of air injection, introduced as microbubbles on the shell side of the heat exchanger, across a broad spectrum of operating conditions. The microbubbles were generated using a porous sparger with an average pore size of 100 mu m. During the experiments, the temperature difference was maintained constant Delta T=20 degrees C\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\Delta T = 20<^>\circ {\text{C}}} \right)$$\end{document}, while variations in shell-side Reynolds number Res=4825,7238and9650\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left({\text{Re}}_{s}=4825, 7238 \text{and} 9650\right)$$\end{document}, coil-side Reynolds number Rec=2600,5200,7800,10400and13000\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left({\text{Re}}_{c}=2600, 5200, 7800, 10400 \text{and} 13000\right)$$\end{document}, and injected air Reynolds number Rea=0,2600,5200,7800,10400and13000\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left({\text{Re}}_{a}=0, 2600, 5200, 7800, 10400 \text{and} 13000\right)$$\end{document} were systematically tested. The baseline tests, conducted without air injection, revealed that the new heat exchanger design outperformed the traditional coiled tube heat exchanger by approximately (average) 26%. Moreover, air injection substantially improved the overall heat transfer coefficient, achieving a maximum enhancement of 119% under the conditions of Rec=9650,Res=825andRea=10400\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Re}}_{c}=9650, {\text{Re}}_{s}=825 \text{and} {\text{Re}}_{a}=10400$$\end{document}. Conversely, the minimum enhancement of 41% was observed at Rec=9650,Res=275andRea=2600\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Re}}_{c}=9650, {\text{Re}}_{s}=275 \text{and} {\text{Re}}_{a}=2600$$\end{document}. The ratio of U/UNE\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$U/{U}_{NE}$$\end{document}, representing the overall heat transfer coefficient with air injection relative to that without air injection, reached its optimal value under these optimal operating conditions.