Influence of external magnetic manipulation on thermal transport characteristics of the bubble-slug flow of ferro-nanocolloids

Abstract

Bubble-slug flows are an important two-phase flow pattern known for their superior transport characteristics, which predominantly depend on the phase distribution (bubble and slug lengths). In the present work, it is shown that the thermal transport characteristics of such flows can be substantially enhanced and tuned as per requirement by utilizing ferro-nanocolloids or ferrofluids as the primary liquid phase. The magnetic properties of ferrofluids provide an additional means to manipulate their flow characteristics externally. The flow morphology of the Taylor bubble flow of ferrofluid is altered through an externally applied magnetic field (MF). Smaller bubble-slug units are generated in the resulting flow through external magnetic manipulation while all other parameters are kept identical. The ensuing heat transfer of different flow morphologies generated through such manipulation is examined using micro-thermometry. It is shown that the smaller bubble-slug units can considerably augment thermal transport in such flows. The movement of smaller units creates a frequent disruption to the boundary and interfacial regions of the flow, and additional recirculation regions are formed, resulting in the augmentation of both local and overall convective transport. Augmentation in heat transfer of up to 80% is seen in the present case, which mainly depends on the homogeneous gas fraction of the flow. The present study demonstrates that for a given homogeneous gas fraction (> 0.50), smaller bubbles and unit-cells are superior for achieving a higher heat transfer coefficient. In addition, the utilization of ferrofluids provides an excellent alternative for heat transfer augmentation via external manipulation in non-boiling two-phase flows. With all other flow parameters kept identical, the results clearly show the role of smaller bubble-slug units in augmenting heat transfer.