Molecular dynamics study of heat transfer characteristics of bubble induced nanochannel flow

Abstract

The study of phase change heat transfer in liquids has become increasingly significant due to the growing demand for efficient thermal management in compact systems, particularly in applications requiring high heat flux in small-scale devices. However, addressing the challenges posed by small dimensions and high heat densities requires advanced approaches. Boiling heat transfer in bubble induced nanochannel flows has thus emerged as a critical area of research. This study utilizes molecular dynamics simulations to examine the effects of surface interaction potential (W), channel width (D), heater intensity (T) and length (H) on phase change heat transfer in nanochannels. The findings reveal that higher heat source temperatures under hydrophilic conditions promote earlier bubble nucleation, while bubble propagation transitions from gradual at lower temperatures to erratic at higher ones. Multiple nucleation sites are activated for bubble nucleation for a higher heater length due to the lack of accumulation of heat within a single region, as occurs for the case of lower heater length. During heating, a reverse liquid flow towards the liquid pool is observed, driven by a sudden temperature surge at the heater. Additionally, higher surface interaction potentials lead to increased liquid atom accumulation at the heater surface, enhancing energy transfer and bubble propagation, thereby improving heat transfer efficiency. In contrast, under hydrophobic conditions, bubble nucleation is delayed, and the vapor shielding effect which is the formation of a vapor layer between the heat source and liquid, becomes more prominent. This insulating barrier significantly reduces heat transfer efficiency. For identical conditions, hydrophilic surfaces exhibit up to 90% enhancement in heat transfer compared to hydrophobic surfaces. Location of bubble nucleation shifts from middle of the channel to the closest of the heater wall as the wetting condition changes from hydrophilic to hydrophobic. After forming stable bubble nucleus, it propagates gradually in both directions, with upward propagation dominating over time, due to the cooling effect of liquid pool. Lower channel width facilitates bubble nucleation earlier due to the distribution of available heat within a small number of liquid atoms compared to the higher width. By reducing the channel width, nucleation time can be significantly advanced without changing the heating condition and wetting condition. These findings provide valuable insights into phase change heat transfer in bubble induced nanochannel flows, underscoring the pivotal roles of heat source temperature and surface wetting conditions regarding thermal performance of the system.