Experimental Investigation of a Large Scale-oscillating Heat Pipe at Different Inclinations

The oscillating heat pipe (OHP) is an up-and-coming passive thermal transfer device that transports heat through the thermally excited oscillating motions of a working fluid. OHP has become one of the popular research topics since its discovery by Akachi in 1990 (Akachi, 1990). Although more than two decades have passed since its invention, lots of information regarding the OHP, such as information on the hydrodynamic and thermodynamic coupling leading to complex combinations of two-phase instabilities and a metastable fluid state, remains unknown (Khandekar & Groll, 2004; Lips et al., 2010).

The OHP is built from a small-diameter tube formed into a meandering snake shape tube and joined end to end. First, the tube is evacuated and then filled partially with a working fluid, which distributes itself naturally in the form of liquid-vapor plugs and slugs inside the capillary tube. The device essentially works with two main different pressures between the evaporator and the condenser. These two main different pressures generate the driving force for the oscillatory motion of the working fluid.

The OHP structure typically consists of the following three parts: the evaporator, adiabatic, and condenser sections.  Heat transfer  occurs in  the evaporator  and  condenser  sections,  while the adiabatic section connects both of them. The evaporator absorbs heat from an object, heat which is then transferred to the condenser by the oscillating motion of the working fluid. If the evaporator is in contact with the thermal load, then the working fluid inside will evaporate and increase the local vapor pressure. Due to bubble formation, local high pressure will expel the liquid and form vapor towards the condenser section. In the condenser section, the vapor plugs will collapse and condense because of the low saturation pressure. The growth and collapse of bubbles in the two different sections leads to the oscillating or pulsating motions inside the tube. As a result, a pressure imbalance is generated inside the OHP tube. As a result, working fluid oscillations appear and deliver heat from the evaporator to the condenser. Latent heat transfer processes are involved during OHP operation, mainly as a driving force of the working fluid motions. These working fluid motions will continue as long as the temperature difference exists. Although the heat transfer phase change does not dominate the total heat transfer rate, as in a conventional heat pipe, the oscillating heat pipe has the potential for better heat transfer capability, especially at high heat flux (Mameli et al., 2012).

A PHP is essentially a nonequilibrium heat transfer device whose performance success depends primarily on the continuous maintenance of these nonequilibrium conditions within the system. Because the length of each liquid slug and vapor plug are different, it is not surprising that this working fluid flow both undergoes complex oscillatory displacements and displays circulatory characteristics (Tong et al., 2001). Ma stated that the OHP motion is a mechanical vibration system with a vapor plug acting as a constant spring (Ma, 2015). Contrary to conventional heat pipes, OHP has a wickless structure inside. Avionics and extraterrestrial applications need more lightweight cooling devices, thus OHP is more preferable than conventional heat pipes with wick structures.

Cui et al. studied OHP with distilled water, methanol, acetone, and ethanol as working fluids (Cui et al., 2014). They found that dry-out appeared locally on some individual pipes in the evaporator. Elevating the power input would cause the dry-out to spread to several other locations. Naik et al. examined acetone, methanol, and ethanol as working fluids at various filling ratios (Naik et al., 2013). They found that acetone with a 60% filling ratio had the lowest thermal resistance and the highest heat transfer coefficient. Verma et al. demonstrated that methanol worked efficiently in a variety of orientations compared with distilled water (Verma et al., 2013). Tong et al. conducted a study of OHP visualization with methanol as the working fluid (Tong et al., 2001). The visual study showed a high amplitude of oscillation during the start-up stage. They also discovered that the bubble displacement of methanol oscillation versus time is in the form of quasi-sine waves. The water OHP had periodic “stationary–fast movement” oscillation motion behavior. Xu et al. observed a difference in the advancing and receding angles of water when traveling inside the channel due to high surface tension (Xu et al., 2005). An experimental study by Saha et al. showed that methanol and water should be the first consideration when choosing a working fluid for an open loop OHP with vertical and horizontal orientations (Saha et al., 2012). Senjaya and Inoue conducted an OHP simulation considering the dry-out phenomenon (Senjaya & Inoue, 2014). These research studies stated that dry-out occurs because there is not a sufficient supply of liquid to the evaporator. The performance of the heat pipe seriously deteriorates if dry-out occurs. Xian et al. tested an OHP with an evaporator length of 200 mm with water and ethanol as the working fluids (Xian et al., 2010). They found that the maximum thermal conductivity for the water OHP and the ethanol OHP peaks at 295 kW/m?K and 111 kW/m?K, respectively. Based on their results, there is a potential high thermal transfer capability over long distances using an OHP design with the longest effective length (l_eff). Lin et al. conducted an experiment with different heat transfer lengths and inner OHP diameters (Lin et al., 2011). In their study, all the OHPs used water as the working fluid. They showed that the inner diameter of the OHP should be greater than 0.8 mm in vertical bottom heating mode. At high heating power, the performance of MOHP is at almost the same level when compared with a sintered heat pipe in the horizontal orientation. Yang et al. conducted an experimental work with an OHP length of approximately 600 mm for a solar collector application (Yang et al., 2009). They found that the OHP could be applied properly as a solar collector. The relative importance of testing the OHP under high heat flux to prove that OHP could withstand a higher heat flux, as stated by Akachi et al. that could operate as passive cooling up to 30 W/cm² (Mameli et al., 2012).

Varying the effects of gravity on the OHP orientation has become one of the popular topics in the recent investigations (Mameli et al., 2014; Mameli et al., 2015; Ayel et al., 2015; Mangini et al. 2017). The results of such studies show that both gravity and heat input level influence the device operation. One of the recent popular topics of experimental OHP research is varying the effect of gravity. The change of performance of OHP due to gravitation is still growing as a hot topic in many publications. Even though, there are still rare data about these topics.

At the beginning, the OHP was designed with an effective length (l_eff) of no more than 20 mm. There is a lack of data on the thermal characteristics of OHPs with effective lengths more than 200 mm. This scarcity makes sense because the OHP was originally developed to provide thermal management solutions for small electronic devices, especially electronic devices that have strict requirements for space limitations and high heat flux rejection. However, OHPs are starting to be investigated in heat exchange or heat recovery applications using an l_eff exceeding 350 mm (Supirattanakul et al., 2011; Arab et al., 2012; Mahajan et al., 2017; Winarta et al., 2017).

The objective of this research is to experimentally study the heat transfer characteristics of an OHP using methanol as the working fluid for different orientations and higher heat flux. The OHP was manufactured with an l_eff of approximately 500 mm, which is not typically used in previous OHP data experimental tests. Most of the heat transfer performance for OHPs was designed for electronic thermal management. However, recent trends include the implementation of OHPs in heat recovery and heat exchanger design areas. The results of this experimental data will also provide more experimental data for improving the characteristic behavior of the thermal process in an OHP.