Optimisation and evaluation of NTU and effectiveness of a helical coil tube heat exchanger with air injection

Abstract

Air bubble injection has recently been confirmed as a beneficial technique for enhancing the thermal performance of a heat exchanger. The normal vertical motion of the bubbles due to buoyancy force leads to the entrainment of fluid from the shell side, thereby raising its velocity and turbulence level whilst improving the mix within the shell. Consequently, the thermal boundary layer that formed around the outer surface of the coil is disrupted, thus reducing heat transfer resistance and enhancing the heat transfer rate. Despite relatively considerable attention that has been given to assess this technique, numerous investigations have concentrated on only a few operational conditions. This consideration will be insufficient to understand the influence of air injection entirely on the thermal performance of a heat exchanger. Thus, this study aims to fill this gap by performing experiments over numerous operational conditions for hot and cold fluids and the injected air in a vertical counter-current coiled tube heat exchanger. The optimal air flow and the shell-side flow rates of the vertical coiled tube heat exchanger are determined, thus leading to the economic operation of a heat exchanger. Therefore, the implemented shell-side flow, air flow and coil-side flow rates were changed from 2 LPM to 10 LPM, from 0 to 10 LPM and from 1 LPM to 2 LPM, respectively, with three temperature differences (i.e., 20°C,30°Cand36°C). The number of heat transfer units (NTU) and the thermal effectiveness of the heat exchanger were investigated intensively. Results showed that the NTU and the effectiveness are influenced significantly by the injected air flow, shell-side flow and coil-side flow rates without noticeable effect on the temperature difference. In addition, the positive role of the injected air flow rate was diminished, and no further enhancement of NTU and effectiveness was observed when Qa>6LPM. The most effective shell-side flow rate was also 6 LPM. The maximum augmentation in the NTU and the effectiveness were 1.93 and 0.83, correspondingly, whilst the minimum value of the NTU and the effectiveness were 0.66 and 0.63, respectively.