Flow and Heat Transfer Characteristics of Liquid Nitrogen in Mini-/Microchannels

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Abstract
Flow and heat transfer of liquid nitrogen in mini-/microchannels have many particular characteristics and are very important for many cooling applications. In this study, the investigation of flow and heat transfer characteristics of liquid nitrogen in mini-/microchannels is presented by summarizing the experimental studies carried out in the author's group. In addition, some recent results about flow and heat transfer of liquid nitrogen in microchannel heat sink are also presented. It is found that small viscosity of liquid nitrogen enables the single-phase liquid flow in mini-/microchannels to be turbulent state, which proves that the classical theory for pressure drop is still valid if the surface roughness of the passage is properly taken into consideration. Experiments of flow boiling of liquid nitrogen are conducted under both adiabatic and diabatic conditions. It is shown that confinement number can be applicable in classifying the heat transfer characteristics of liquid nitrogen in macro- and microchannels. Flow visualization in microchannels at low temperatures poses big challenges on experimental aspects, which have been subtly overcome and clear images have been obtained. The flow patterns and flow regimes of two-phase flow of liquid nitrogen exhibit different features from the room-temperature fluids. Furthermore, three-dimensional (3D) flow visualization by only one high-speed camera is conducted to obtain more detailed information of flow patterns. Finally, the experiments of flow boiling of liquid nitrogen in microchannel heat sink are also presented and discussed.

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