Three-Dimensional Oscillating Heat Pipe: Flow Pattern Transition and Heat Transfer Performance

Abstract The three-dimensional oscillating heat pipe (OHP) could utilize asymmetric heating and cooling to facilitate stable unidirectional flow. At low power levels, the working fluid mainly exhibits a reciprocating oscillation pattern and unidirectional flow is unstable due to insufficient driving force. As power increases, the working fluid can establish stable unidirectional flow within the OHP. With higher power, flow velocity increases, accompanied by localized small-scale oscillations in this process. During low-power oscillatory flow, temperature on both sides of the adiabatic section of the same bend fluctuate significantly. In contrast, during unidirectional flow there is a clear temperature stratification. Additionally, based on flow direction, temperature at the connected pipeline with the higher-temperature adiabatic side. Under anti-gravity conditions, the OHP consistently operates in an oscillatory manner, with temperatures on both sides of the adiabatic section of the same bend being chaotic and overlapping. The flow and flow patterns of the working fluid inside the OHP significantly impact heat transfer performance. Compared to gravity environment, heat transfer resistance and temperature difference increase significantly under anti-gravity conditions, resulting in deteriorated heat transfer performance and lower efficiency.