Performance Of Oscillating Heat Pipe Research Articles

Experimental Study of a Novel Helix-Shaped Oscillating Heat Pipe and Design Parameters for Flow Stabilization

Abstract Abstract A novel helix-shaped oscillating heat pipe (OHP) designed for enhanced heat transfer in thermal management and heat recovery was studied experimentally. Two orientations were explored: side-heated, the intended orientation in which improved fluid circulation is expected, and bottom-heated, a control resembling traditional bottom-heated OHPs. Results showed stronger circulation, reduced temperature differences, and lower start-up thresholds in most side-heated cases. This orientation achieved higher maximum heat loads at a fill ratio of 0.5, although the maximum heat load decreased at a fill ratio of 0.7. Notably, the experimental OHP attained an effective thermal conductivity over 9,000 W m-K-1 in multiple tests and a maximum heat transport of 676 W. Additional parameters were explored including heat load, fill ratio, condenser temperature, and the presence of noncondensable gases (NCGs). NCGs increased the temperature drop as expected, but also raised the maximum heat transport, indicating potential benefits in certain applications. Elevated condenser temperatures decreased temperature drops, but also reduced maximum heat transport. A previously developed OHP performance model was expanded to evaluate the novel helix-shaped OHP. The model reasonably predicted temperature drops during degassed experiments under moderate heat loads. However, most data points fell outside the model's scope, emphasizing the need to extend it to handle condenser bubble collapse. The expanded analytical models for side-heated helix OHPs highlighted restrictions on circulation that differed from traditional, bottom-heated OHPs, which explains the superior performance of the novel design.

Oscillating heat pipe performance in various gravity force implementing openFOAM code

Oscillating heat pipe performance in various gravity force implementing openFOAM code

Effects of vibration conditions on thermo-hydrodynamic performances of oscillating heat pipes with various diameters

This paper investigates the thermo-hydrodynamic performance of oscillating heat pipes (OHPs) with different inner diameters (1.5 mm ~ 3.0 mm) under vibration conditions. The effects of vibration frequency (25 Hz, 50 Hz, and 100 Hz) and amplitude (0.4 mm, 0.8 mm, and 1.2 mm) on the startup and quasi-steady heat transfer of OHPs were analyzed. Results indicate that increasing vibration frequency promote fluid flow in OHPs, while increasing vibration amplitude has the opposite effect. The impact of vibration frequency and amplitude on hydrodynamic characteristics is more pronounced in OHPs with larger diameters due to additional forces induced by vibrations. The startup performance is firstly weakened and then gradually improved with the increasing vibration frequency, indicating the flow resistance is increased under low-frequency vibration condition. The initial fluid movement in the OHPs starts more easily under low-amplitude vibration condition, and thus the startup performance is optimum when the vibration amplitude is 0.4 mm. In addition, the startup performance of OHP with the inner diameter of 1.5 mm is affected mostly due to the finite fluid volume. The heat transfer performance, for the OHP with 3 mm inner diameter, will not be improved until the vibration frequency is higher than 100 Hz, and it is reduced most under the vibration amplitude of 0.8 mm.

Heat transfer and temperature characteristics of single-loop oscillating heat pipe under axial-rotation conditions

Thermal management is important for rotating machinery, such as spindles, grinding wheels or drillers, etc. If the heat cannot be dissipated in time, high temperatures will be generated, and abnormal operations or even destruction of the machinery may occur. Oscillating heat pipes (OHP) have attracted attention in thermal management and also have a demonstrated potential to transfer heat under rotation conditions. Research on OHPs under rotation is still insufficient: For example, the heat transport capacity and the temperature characterization of OHP under axial-rotation are not investigated, and also the motion of the working fluid is not yet recorded and analyzed. The thermal performance of a single-loop OHP under axial-rotations is here experimentally investigated. The temperature characteristics of the evaporator and condenser are analyzed. The inside flow is also visualized to better understand the heat transfer process. The results show that the heat transfer capacity of OHPs filled with acetone and methanol improves with the increase of centrifugal acceleration; while the thermal performance of an OHP filled with water peaks at rotation centrifugal acceleration of 30 m/s2. The centrifugal acceleration promotes movements of the working fluid and improves the heat transport capacity. Besides, the OHP shows a chaotic state under the axial-rotation based on nonlinear dynamic analysis.

INVESTIGATION OF INTRINSIC DYNAMIC CHARACTERISTICS IN AN OSCILLATING HEAT PIPE

Conjugate heat transfer devices based on oscillating phenomenon allow thermal instability to play a crucial role in thermal management. This study proposed and tested a hollow cylindrical oscillating heat pipe (OHP) for managing waste heat sources. Deionized water was filled as a working medium at ratios of 10%-90%. Experiments were conducted with heat inputs 10-120 W; the performance was dependent on the features of self-sustained oscillations. Dry-out was found only under low filling conditions, and this operating fault can be avoided by increasing the working medium. A medium level of approximately 50% is optimal. The intrinsic thermal instability related to the performance of an oscillating thermal design has attracted considerable academic interest. Simplifications and assumptions of traditional modeling approaches render them unsuitable for probing the dynamic characteristics of such thermal designs. A nonlinear autoregressive exogenous (NARX) modeling based on machine learning techniques was introduced to identify the causality of the heat transfer mechanism of the OHP. The discrete models estimated using the NARX modeling accurately simulated OHP dynamics, which were further revealed through spectrum analysis. Energy excitation, shown as frequency resonances, was distinctly related to OHP performance.

Experimental study of transparent oscillating heat pipes filled with solar absorptive nanofluids

Nanoparticle-based volumetric solar absorption has been shown to be a potential technique to realize efficient solar harvesting. However, most of studied systems are stationary and cannot achieve long distance solar energy transport, which limits their applications. A novel idea of using directive absorptive nanofluids in oscillating heat pipes (OHP) is investigated in this work, which would achieve efficient solar energy capture and transportation simultaneously without the use of any additional pumping power. The influence of a variety of parameters such as nanoparticle type, nanoparticle concentration, nanofluids filling ratio and solar radiation intensity on the performance of OHPs are investigated. It is found that there exists an optimal filling ratio of the nanofluid for the OHP (i.e., 83%), under which single direction circulation of the working fluid is achieved and the thermal resistance of the OHP reaches the minimum. An extremely high thermal conductivity, i.e., 6000 W/m·K, is obtained when the OHP is filled with 3.0 wt% MWCNT nanofluid. The maximum energy conversion efficiency reaches to 92% under the current experimental settings. Strong absorption of solar energy, efficient vapor generation inside the OHP and proper configuration of the OHP should be responsible for the efficient operation of such a system.

Heat transfer characteristics of plug-in oscillating heat pipe with binary-fluid mixtures for electric vehicle battery thermal management

The development of highly-efficient, cost-effective, and reliable cooling schemes of electric vehicle (EV) battery is still a big challenge for massive commercialization of EVs. In this work, a proof-of-concept plug-in oscillating heat pipe (OHP) with flat-plate evaporator and tube condenser has been developed and experimentally tested for the potential applications in EV battery thermal management. Pure water, ethanol, and binary-fluid mixtures of them at different mixing ratios (MRs) ranging from 1:1 to 4:1 were used at volumetric filling ratios (FRs) of 30%, 40% and 50%. Owing to the zeotropic properties in phase transition and the complex molecular interactions between the ethanol component and water component, the OHP charged with ethanol-water mixtures exhibited better start-up and heat transfer performance. The average battery pack temperature could be controlled below 46.5 °C under the power input of 56 W. Besides, well-satisfied temperature uniformity was achieved on the battery module and the maximum temperature differences were mostly in the range of 1–2 °C. The complementary features for ethanol-water mixture thermo-physical properties and temperature/concentration gradients induced mass transport account for the OHP performance improvement. The novel plug-in OHP associated with desirable heat transfer performance provides a promising candidate for battery thermal management system.

Variation in oscillating heat pipe performance

Oscillating heat pipes have been shown to be an effective means for thermal management. However, the constant, phase-change-powered oscillation can also produce variations in performance. The goal of this research is to quantify performance variation of oscillating heat pipes through repeated tests with closed-loop flat plate oscillating heat pipes. The heat pipes were filled with two different working fluids, butane and R134a, and tested in both double-sided and single-sided cooling configurations, two or three different orientations, and four different power levels, and repeated to quantify the variation in performance, including average thermal resistance, intratest variation (repeatability), intertest variation (temperature fluctuations), and reliability (likelihood of temperature overshoot). Orientation, configuration, working fluid, and power level all influenced the level of variation in performance. In a double-sided configuration, OHPs had lower thermal resistance when oriented vertically. In a single-sided configuration, OHPs had lowest thermal resistance when the evaporator was below the condenser. OHPs were significantly more reliable and experienced smaller temperature fluctuations when in the double-sided than single-sided configuration.

Experimental investigation of the heat transfer performance of an oscillating heat pipe with graphene nanofluids

The heat transfer performance of oscillating heat pipes (OHPs) with graphene nanoplatelet (GNP) nanofluids was investigated experimentally. In these experiments, the GNP nanofluid concentrations were 1.2, 2.0, 5.7, 9.1, 13.8, and 16.7 vol%, the heating power ranged from 10 to 100 W, and the filling ratios were 45%, 55%, 62%, 70%, and 90%. The results indicate that the heat transfer performance of OHPs is improved by using GNP nanofluids as the working fluid compared to an OHP with deionized water (DW). At appropriate filling ratios (55%, 62%, and 70%), the optimum range of GNP nanofluid concentrations was 2.0–13.8 vol%. The maximum reduction in thermal resistance was 83.6% for an OHP filled with a 2.0 vol% GNP nanofluid compared to the DW OHP at the same filling ratio (62%) and heating power (80 W). At a low filling ratio (45%), adding GNP nanofluids to DW could alleviate dry-out and improve the heat transfer performance of the OHP.

Effects of filling ratio and condenser temperature on the thermal performance of a neon cryogenic oscillating heat pipe

A cryogenic oscillating heat pipe (OHP) made of a bended copper capillary tube is manufactured. The lengths of the condenser section, adiabatic section and evaporator section are 100, 280 and 100 mm, respectively. Neon is used as the working fluid. Effects of liquid filling ratio and condenser temperature on the thermal performance of the OHP are studied. A correlation based on the available experimental data sets is proposed to predict the thermal performance of the neon cryogenic OHP with different filling ratios and condenser temperature. Compared with the experimental data, the average standard deviation of the correlation is about 15.0%, and approximately 92.4% of deviations are within ±30%.

Experimental investigation of oscillating heat pipes under direct current and pulse width modulation heating input conditions

An experiment was designed to quantify any change in thermal performance of an oscillating heat pipe (OHP) subjected to Pulse Width Modulation (PWM) power input as opposed to the more traditional Direct Current (DC) power input/heating. Two identical ThermAvant Technologies aluminum 152.4×50.8×2.0mm3, 34 turn closed-loop OHPs with 0.76mm diameter channels were built. One OHP was filled with acetone (54% by volume at room temperature) and another filled with R-134a (55% by volume at room temperature). The OHPs were tested horizontally with DC and PWM (at 2.88Hz) heat inputs of 0–40W. Three trials of each OHP and power type were performed. It was found that nearly all trials of each OHP were within one standard deviation of one another. Based on these results and prior findings in OHP performance repeatability it is reasonable to conclude that the use of PWM power input does not influence the thermal performance of a flat-plate OHP of this design.

Experiment investigation on thermal performance of a large-scale oscillating heat pipe with self-rewetting fluid used for thermal energy storage

The maximization of oscillating heat pipe (OHP) has important significance on OHP based large-scale thermal energy storage (TES) system. In order to understand the thermal performance of OHP with long heat-transport distance for thermal energy storage (TES) application, a large-OHP testing platform was constructed. The influence of working medium type, heating load, cooling condition on the thermal efficiency, thermal resistance and effective thermal conductivity of OHP were investigated and analyzed. The results mainly showed that the OHP with self-rewetting fluid (SRWF) has greater heat transfer limit and can work normally under larger heating load compared to that with water or ethanol as working medium. The OHP with filling ratio (FR) of 40–80% can endure higher heating load compared to the case of FR=30%. It can be concluded that the best FR is about 40% under large heat load (the effective thermal conductivity reaches 5676Wm−1°C−1 at 700W). The SRWF based OHP with long heat-transport distance in this paper has lower thermal resistance and larger heat density compared to those of some other research groups.

Experimental investigation of thermo-hydrodynamic behavior in a closed loop oscillating heat pipe

A simultaneous visualization and temperature measurement has been performed to investigate the interaction of bubble size distribution, bubble movement, temperature variation in a glass oscillating heat pipe (OHP) with an internal diameter of 2.5mm. Ethanol was used as the working fluid with volumetric filling ratios of 50%, 60% and 70%. Fluid temperatures were recorded under four different heating power inputs of 38, 50, 75, and 110W with thermocouples inserting into tube inside. The experimental results show that a higher heating power input or filling ratio usually leads to larger oscillation frequency and amplitude of bubble movement, contributing to better OHP performance. The quasi-sine oscillation wave of bubble movement was occasionally observed and its appearance also depends on the filling ratio and power input. Visual observation indicates that the bubble size distribution is associated with bubble movement and temperature variation, which is available to indirectly evaluate the OHP performance. This study provides a new approach to understand the thermo-hydrodynamic behavior of OHPs.

EFFECT OF A MAGNETIC FIELD AT INCLINATION ANGLES ON THE PERFORMANCE OF COPPER OSCILLATING HEAT PIPES

This paper presents the result of experimental investigation concerning the use of Fe2O3 nanoparticles added to kerosene as a working fluid under a magnetic field for a copper oscillating heat pipe with inclination angle of 0° (horizontal), 15°, 30°, 45°, 60°, 75°, and 90° (vertical). The temperature distribution and heat transfer rate on an oscillating heat pipe (OHP) were examined, with magnetic field under different angles. The results showed that the addition of Fe2O3 nanoparticles under a magnetic field improved the thermal performance of OHP, especially at 75°.

Effects of Superhydrophobic and Superhydrophilic Surfaces on Heat Transfer and Oscillating Motion of an Oscillating Heat Pipe

The effects of superhydrophobic surface and superhydrophobic and superhydrophilic hybrid surface on the fluid flow and heat transfer of oscillating heat pipes (OHPs) were investigated in the paper. The inner surfaces of the OHPs were hydrophilic surface (copper), hybrid surface (superhydrophilic evaporation and superhydrophobic condensation section), and uniform superhydrophobic surface, respectively. Deionized water was used as the working fluid. Experimental results showed that superhydrophobic surface influenced the slug motion and thermal performance of OHPs. Visualization results showed that the liquid-vapor interface was concave in the OHP with copper surface. A thin liquid film existed between the vapor plug and the wall of the OHP. On the contrary, the liquid-vapor interface took a convex profile in the OHP with superhydrophobic surface and the liquid-vapor interface contact line length in the hybrid surface OHP was longer than that in the uniform superhydrophobic surface OHP. The liquid slug movements became stronger in the hybrid surface OHPs as opposed to the copper OHP, while the global heat transfer performance of the hybrid surface OHPs increased by 5–20%. Comparing with the copper OHPs, the maximum amplitude and velocity of the liquid slug movements in the hybrid surface OHPs increased by 0–127% and 0–185%, respectively. However, the maximum amplitude and velocity of the liquid slug movements in the uniform superhydrophobic OHPs was reduced by 0–100% and 0–100%, respectively. The partial dryout phenomenon took place in OHPs with uniform superhydrophobic surface. The liquid slug movements became weaker and the thermal resistance was increased by 10–35% in the superhydrophobic surface OHPs.

Effects of non-condensable gas on the performance of oscillating heat pipe, part II: Experimental study

In this paper, we investigate the effects of non-condensable gas (NCG) on the flow characteristics of liquid slugs–vapor plugs and heat transfer performance (λeff) of oscillating heat pipe (OHP), experimentally, in order to validate our simulation results of part I. The experimental results show that NCG decreases the heat transfer performance of OHP, which are the same with our simulation results. From experimental results, NCG increases the time-average temperature of heating section. This means that the liquid slugs enter the heating section less frequent (bubbles generation is also less frequent), then the total amount of evaporation and total amount of heat transfer through working fluid decrease. In the adiabatic section, NCG decreases the fluctuation of wall temperature, which means that the motion of liquid slugs and vapor plugs becomes less active. And in the cooling section, NCG decreases the fluctuation and also time-average of wall temperature, which means that NCG obstacles the condensation. The experimental results also show that the effects of NCG on the heat transfer performance of OHP are smaller than that of the loop heat pipe and conventional heat pipe, which agree and validate our simulation results.

Effects of non-condensable gas on the performance of oscillating heat pipe, part I: Theoretical study

In this paper, we investigate the effects of non-condensable gas (NCG) on the phase change phenomena, flow characteristics of liquid slugs-vapor plugs, pressure difference of heating-cooling section and heat transfer performance of oscillating heat pipe (OHP), theoretically through simulation. In the simulation, the NCG is assumed to be un-dissolved in the liquid slugs. The NCG is also assumed to be perfectly mixed with vapor, as form of plugs. The simulation results show that NCG decreases the pressure difference between heating and cooling section, which means the decreasing of driving force. The decreasing of driving force due to the decreasing the total amount of condensation, which means that NCG disturbed the condensation phenomena. Finally, the heat transfer performance of OHP also decreases due to the decreasing of driving force. Compared with the other types of heat pipe from previous studies, the effects of NCG in the OHP is smaller than that in the loop heat pipe and conventional heat pipe.

Effects of hydrophilic surface on heat transfer performance and oscillating motion for an oscillating heat pipe

In this paper, a series of experiments were performed to investigate the effects of superhydrophilic and hydrophilic surfaces on the heat transfer performance and slug motion for oscillating heat pipes (OHPs). Deionized water was used as the working fluid. The surfaces of the OHPs were copper, superhydrophilic, hydrophilic and hydrophobic surfaces with contact angles of 73.4°, 0°, 12.9° and 141.5°, respectively. The heat transfer performance in six-turn OHPs was higher than in four-turn OHPs. Experimental results showed that the surface wettability remarkably influenced the slug motion and thermal performance of OHPs. The liquid slug movements became stronger both in superhydrophilic and hydrophilic OHPs as opposed to the copper OHP, while the global heat transfer performance of the superhydrophilic and hydrophilic OHPs increased in the six-turn OHP. For six-turn OHPs, the maximum displacement of the liquid slug in the hydrophilic OHPs and superhydrophilic OHPs increased by 5–60% and 25–60%, respectively, in comparison with that of copper OHPs. The heat transfer performance of superhydrophilic OHPs and hydrophilic OHPs increased by 5–15% and 15–25%, respectively, in comparison with that of copper OHPs. The hydrophilic surface improved the heat transport capability of OHPs, and the maximum displacement and velocity of the liquid slug increased. The startup temperature of the four-turn and six-turn OHPs varied from 45 to 55°C and 35 to 45°C, respectively. The startup temperature of the four-turn and six-turn superhydrophilic OHPs varied from 40 to 55°C and from 35 to 50°C, respectively. The startup temperature of the four-turn and six-turn hydrophilic OHPs varied from 40 to 50°C and from 30 to 40°C, respectively. The hydrophilic OHP also showed a better startup performance than the copper OHP. At heat input of 150W, the thermal resistance for the four-turn hydrophobic OHPs was about two times greater than that of copper OHPs. The global heat transfer performance of the superhydrophilic OHP for the four-turn OHP was lower than that of the four-turn OHPs with hydrophilic and pure copper OHPs.

Research of the Heat-transfer Performance on Methanol/Acetone Oscillating Heat Pipe

As a high efficient heat transfer element,the oscillating heat pipe(OHP) has unique advantages in solving the problem of electronic device heat dissipation with tiny space and high heat flux and its heat transfer performance is greatly affected by its working medium.The methanol/acetone binary mixture,as well as the pure methanol,acetone,have been selected as working mediums of OHP,then the heat transfer characteristics of methanol/acetone OHP has been researched by analysing the physical properties of the working mediums and its interaction characteristics on heat transfer performance of OHP under different ratios(7︰1,4︰1,1︰4,1︰7),filling ratios(45%,62%,70%)and heat inputs(10-100 W).The results show that all evaporator sections of OHP are dried out obviously with small filling ratios.Compared with pure working mediums,the thermal resistance of mixture working medium is smaller when dried out,namely when the heat input is 50 W,the pure methanol,acetone are 1.509 oC/W and 1.48 oC/W,the methanol/acetone 1︰7 thermal resistance is 0.88 oC/W,between them are the thermal resistances of the rest ratios.In addition,the effect is more obvious when little methanol is added into acetone as working medium(such as methanol/acetone 1︰7);With large filling ratios,the thermal resistances of mixture working mediums increased with little gradient and get close to each other as heat inputs increased gradually,the heat transfer performances are generally great.

Nonlinear thermomechanical finite-element modeling, analysis and characterization of multi-turn oscillating heat pipes

Oscillating heat pipes (OHPs) are promising heat dissipation devices for modern electronic systems due to their high heat transfer rate, simple construction and low manufacturing cost. Despite the unique features of OHPs, how to initiate and sustain the slug flow within the capillary tube and how different parameters affect the performance of an OHP are not well understood. This paper presents an advanced, fully nonlinear thermomechanical finite-element model that can simulate the parametrically excited oscillation of the liquid slug, the temperature distribution along the two-phase flow and the heat transfer performance of OHPs. The model can account for the influences of nonlinear spring effect of vapor slugs, interphase mass-transferring effect, fluid filling ratio, operating temperatures of the evaporator and condenser, different heating modes (top- or bottom-heating), gravity, bending pressure loss, properties of the working fluid, and different random distributions of initial velocities and lengths of fluid slugs. An Euler predictor–corrector method with convergence check is used to solve for the oscillation of and the temperature distribution within each fluid plug. The dynamics of OHPs is characterized using a newly developed time–frequency analysis technique. Numerical results show that an OHP is a parametrically excited nonlinear thermomechanical system. Latent heat transfer provides the driving force for the oscillation, and sensible heat transfer induced by forced convection contributes more than 80% of the total heat transfer rate. Generally speaking, working fluids with high thermal conductivity, low latent heat, and low viscosity are favorable for efficient heat transfer. These results provide better understanding of heat transfer mechanisms of OHPs and can be used for design optimization of OHPs.