Wettability and nanostructure effect on oscillating heat pipe

As electronics technologies rapidly develop with a demand for more power and miniaturization, effective thermal management of these systems becomes much more important. The oscillating heat pipe (OHP) is a promising highly efficient heat transfer device that is great for high heat flux applications common in the electronics industry. In the current investigation, the wettability effect on the heat transfer performance of OHPs has been conducted: 1) Extensive experimental investigation of the inner surface wettability on the heat transfer performance of an OHP has been conducted. The inner surface of OHP evaporator was first treated to determine whether the hydrophilic surface treatment can enhance the heat transfer performance. Then, an experimental investigation of hybrid half hydrophilic/half hydrophobic OHP was conducted to determine their effects on the oscillating amplitude, frequency and heat transfer performance. The overall performance of configuration of hydrophilic evaporator/ hydrophobic condenser and hydrophobic evaporator/ hydrophilic condenser was worse than the nontreated OHP, however; the oscillations were much damper when comparing the amplitudes. 2) In order to visualize the wettability effect on the oscillating motion and frequency, neutron image technology has been utilized to study the hydrophilic surface effect on the oscillating motion. An OHP with dimensions of 88 mm x 40 mm was fabricated and tested. Results show that high oscillating motion occurs in the OHP with the hydrophilic surface while low oscillating motion occurs in the untreated OHP. 3) In order to find the mechanism how the wettability affects the oscillating motion and amplitude, a mathematical model was developed. The model considers the effects of frictional force, driving force, and receding and advancing contact angles. Results show that as contact angle increases the oscillating motion decreases. 4) In addition, based on the Taylor bubble flow, a theoretical model predicting the operating limitation is developed. The model considers the effects of radius of the channel, charging ratio, length of the OHP, and multiple working fluids Results show that radius and charging ratio has a large effect on the maximum heat transfer limit while length has a relatively small contribution. When comparing working fluids such as water, acetone, and R123, water has a significantly higher upper heat transfer limit than acetone and R123.