Numerical study on the heat transfer performance of a hybrid oscillating heat pipe and its application in the recovery of low-grade waste heat systems

The heat transfer performance of oscillating heat pipe (OHP) depends on various parameters, and one such vital parameter is the wettability of its surface. In a conventional OHP (COHP), the wettability on all sections is uniform. In the present numerical study, a hybrid OHP having different wettability at different sections, that is an OHP with a hydrophilic evaporator and a superhydrophobic condenser, is proposed to recover larger quantities of low-grade waste heat (WH). The performance of the hybrid OHP is numerically investigated for different filling ratios (FRs) varying the range of 30%–90% and varying low-grade WH temperatures (WHT) between 320 K and 350 K, with water as the working fluid. Incorporation of a hydrophilic evaporator reduced the start-up time by 20%–80%, which is attributed to early onset of bubble nucleation. At the same time, the superhydrophobic condenser, owing to lower surface tension forces, increased the rate of condensation by promoting dropwise condensation, resulting in increased sensible and latent heat transfer in the hybrid OHP. Also, it was observed that the slug motion in hybrid OHP increased with an increase in low-grade WHT. The heat transfer performance of hybrid OHP increased with an increase of FR up to 80%, and thereafter the heat transfer performance is decreased. It was envisaged from the results that the heat transfer performance of the hybrid OHP is increased with increase in low-grade WHT. The highest mean heat transfer coefficient of 1270 W/m2-K is obtained in hybrid OHP at an FR of 80% and WHT of 350 K, which is 70.52% higher than COHP, indicating its suitability in low-grade WH recovery systems.