Thermal performance of embedded oscillating heat pipe for water, ethanol and methanol at various filling ratios

Oscillating heat pipes (OHPs) have attracted considerable attention as passive thermal management devices due to their high heat transport capability, simple fabrication, and design flexibility. Despite decades of research, the effect of filling ratio (FR) on OHP thermal performance—particularly through direct experimental measurement of condenser heat output—remains insufficiently explored. In this study, an embedded OHP consisting of fourteen channels machined into a copper plate and capable of experimentally measuring condenser heat output was fabricated and tested. The device was evaluated in a vertical bottom-heating configuration over fifty experimental runs. Three working fluids: deionized (DI) water, methanol, and ethanol were investigated at constant FR of 0.62, with DI water exhibiting the best overall thermal performance. For DI water, the FR was varied from 0 to 1 and the heat input from 60 W to 250 W, while startup behavior was examined across all filling ratios. The optimal FR was identified as 0.48 resulting in a minimum thermal resistance of 0.097 K/W, a 56% reduction relative to the copper plate baseline (0.22 K/W). Flow regimes of DI water within the channels were analyzed using MATLAB-based image processing, revealing that smaller liquid slugs and vapor plugs strongly correlated with higher heat output and improved thermal performance. These results suggest that intentionally inducing smaller slug/plug distributions can potentially reduce thermal resistance of OHPs.