Introduction: Injecting bubbles into a liquid is widely regarded as a promising approach to enhancing heat transfer in free convection for industrial processes. Although previous studies have demonstrated that bubble injection can improve heat transfer, the underlying mechanisms remain insufficiently understood. This study experimentally investigates the heat transfer characteristics of laminar free convection in a liquid infused with millimetre-sized bubbles. The effects of bubble diameter, the distance between the bubbles and the heated wall, and the gas injection rate on heat transfer between the heated wall and the liquid (hereafter, HTBHWL) were analyzed in detail. Method: This study established an experimental system to assess the effect of bubble injection on HTBHWL. Heat Transfer Enhancement (HTE) attributable to bubble injection was quantified by measuring the time taken for the liquid temperature to increase from 30°C to 33°C. The effects of bubble size (by varying needle specification), bubble frequency (by adjusting the syringe pump flow rate), and bubble position were systematically examined. All experiments were repeated to ensure data reliability, thereby facilitating a comprehensive analysis of the HTE mechanism. Results: At a fixed bubble generation frequency, the closer the initial bubble position is to the heated wall, the larger the bubble size, and the more significant the THE is induced by bubble injection. When both bubble position and needle specification remain constant, a higher gas injection rate produces more bubbles, thereby yielding a more pronounced THE between the HTBHWL. Discussion: Bubble ascent enhances heat transfer by promoting the mixing of cold and hot fluids and thinning the thermal boundary layer. The characteristic temperature dip followed by recovery observed for single bubbles validates this "pumping action": it transports cold fluid toward the high-temperature region while displacing heated fluid away from the wall. This is consistent with established findings that bubble motion thins the upstream thermal boundary layer, thereby enhancing heat transfer. The distinct oscillation behaviors of bubbles of varying sizes, together with the regional effects of gas flow rate, collectively demonstrate that bubble dynamics play a decisive role in Heat Transfer Enhancement (HTE). Conclusion: The injection of isolated bubbles significantly enhances the heat transfer by free convection between the heated wall and the liquid. The heat enhancement is significant due to the injection of large bubbles, the bubble release point closer to the wall, and the large gas injection rate. A maximum heat transfer enhancement (i.e., the reduction of 24% in heating time) is achieved under optimal conditions. This investigation provides quantitative guidance for designing bubble-assisted heat transfer systems and contributes to understanding gas-liquid multiphase heat transfer.
Zhu et al. (Fri,) studied this question.