Closed-loop Oscillating Heat Pipe Research Articles

Impact of particles on thermal performance of closed-loop oscillating heat pipe

The need to save and recover thermal energy requires new technological solutions. Passive phase change probes like closed-loop oscillating heat pipes (CLOHP) can help to meet these requirements. A current approach to improving the thermal performance of such devices is to modify the working fluid with particles. However, the working principle of such suspensions as heat transfer mediums in phase change probes is still not well understood. The aim of the present study is to investigate whether particles accumulate on the evaporator surface and improve the heat transfer coefficient there, rather than affecting the bubble-slug interaction. In addition, we investigate whether statistical measures help distinguish between different working regimes of CLOHPs.Experiments are carried out with a glass CLOHP with four loops and cooling volume fluxes of 6 and 40 l/h. The heat imposed on the evaporator ranges from 10 to 55 W. The results show that an increased number of nucleation sites on the coated evaporator surface improves thermal performance. Statistical parameters, namely skewness and kurtosis, adequately characterise the operating regimes. The achievable increase in transferred heat is between 5 and 12 %, depending on the heat supplied at the evaporator and the condenser cooling. Furthermore, such a coating suppresses the usual start-up behaviour and lowers the value of heat input at which stable operating conditions occur. Skewness and kurtosis of the condenser temperature difference indicate that this lowering is from 40 W down to 30 W at 6 l/h and from 40 W down to 10 W at 40 l/h.The study paves the way for a cost-effective in-situ manufacturing step - evaporator coating during the first minutes/hours of operation - leading to an improvement in the thermal performance of CLOHPs. Further studies are needed to stabilise this improvement in the long term.

Effect of Heat Input and Filling Ratio on Raise in Temperature of the Oscillating Heat Pipe with Different Working Fluids Using ANN Model

In the present article, feed forward multilayer perceptron neural network (FFMLPNN) model has been used to predict the rise in temperature in closed loop oscillating heat pipe filled with three different fluid i.e., Acetone, methanol and ethanol respectively. Experimental test was carried out for the inner diameter of 1.7mm copper tube for all the combinations of filling ratio, heat input and time taken to evaluate the performance of the OHP. Totally 2000 data sets have been used for Acetone and Methanol, 1500 data sets is used for ethanol in the present NN model. ANN model with FFMLPNN using three input parameter (Filling ratio, heat input and time taken) and rise in temperature has output parameter respectively. Levenberg-Marquardt algorithm with a 4-10 neurons has been used for the determination of optimal model. The 3-8-1 combinations predict the rise in temperature for ethanol and acetone whereas for methanol 3-7-1 is the optimal combinations was achieved. For all the combinations RMSE values are 0.3414, 0.1285 and 0.1237 (Training-70%), 0.3526, 0.1375, 0.1234 (testing-15%) and 0.3010, 0.1515, 0.1425 (validation-15%). The values for coefficient of determinations are 0.9941, 0.9975 and 0.9971 for methanol, acetone and ethanol was achieved. The results clearly indicated that the proposed MLPANN model can successfully predict the rise in temperature.

Thermal performance analysis of axial-rotating oscillating heat pipe and its prediction model based on grey system theory

Oscillating heat pipes (OHP) have attracted much attention in thermal management due to its high heat transport capacity and simple structure. OHP also have the potential to transfer heat under rotational conditions, while research on OHP heat transfer performance under a system rotation is still insufficient. The thermal performance of a single closed loop OHP under a centrifugal acceleration due to an axial rotation is here experimentally investigated. This OHP is then treated as a grey system, since the complexity of the internal physical processes is very high and only a limited amount of experimental data is available. The grey system theory, which analyzes an uncertain system with small samples and poor information, is used to provide a novel path of modeling the thermal performance of two-phase systems. The results show that the heat transfer characteristics of an OHP filled with methanol improve with the increase of centrifugal acceleration and heat flux. The data are then modeled by a GM(1,1) grey model to simulate and predict the heat transfer coefficient. The errors range from 3.36% to 16.6%, and the prediction of the performance is satisfying. A case study of applying the prediction model in designing abrasive-milling tool with OHP inside to enhance heat transfer is conducted. It shows that the grey system prediction model provides a reliable guidance for the engineering application of the OHPs.

Effect of Internal Diameter and Working Fluids on Thermal Performance of a Vertical Closed-Loop Oscillating Heat Pipe with Double Heat Sources

This research investigates the effects of internal diameter and working fluids on the thermal performance of a vertical closed-loop oscillating heat pipe with double heat sources (VCLOHP w/DHS). The closed-loop oscillating heat pipe (CLOHP) tested was made of a copper capillary tube with various inner diameters and working fluids. Two evaporator sections in the outer end of CLOHP were heated by a Ni-Cr alloy resistance wire heater. The heat was removed from the condenser section in the middle of CLOHP by forced convection heat transfer of ambient air blowing the section. The results showed that, for the inner diameters of 1.5 and 2.0 mm, the thermal resistance decreased when the inner diameter and the latent heat of evaporation increased.

Flat plate solar water heater with closed-loop oscillating heat pipes

The flat plate solar water heater, using the closed-loop oscillating heat pipes, was constructed and investigated. The flat plate collector consisted of 10 pipes of closed-loop oscillating heat pipes and the collector area was 1.5?1 m2. Each closed-loop oscillating heat pipes was made of a copper capillary tube with a 1.5 mm inner diameter, a 2.8 mm outer diameter, and had 20 turns. The distilled water was used as the working fluid with a filling ratio of 50% the tube?s total internal volume. The evaporator section of the closed-loop oscillating heat pipes was placed on the absorber plate of the collector, and its condenser section was wrapped around the copper tube, in which hot water flowed through. The solar water heater was tested under the solar simulator with halogen lamps generating the uniform artificial solar energy. The irradiation intensity and the water flow rate of the solar water heater were adjusted. It was found that the thermal performance of the solar water heater clearly improved with an increase in the irradiation intensity from 480-1086 W/m2. However, the water flow rate in the range of 1.5-3.0 Lpm, had a thermal performance that was slightly different. The thermal efficiency of 0.67 was archived at the high irradiation intensity of 947-1086 W/m2. Moreover, the mathematical model to predict the thermal efficiency of the flat plate solar water heater with the closed-loop oscillating heat pipes was obtained.

Heat transfer of a heat pipe using titanium dioxide as working fluid

This research aims to study the heat transfer of a heat pipe using nanofluid as a working fluid by using a closed loop oscillating heat pipe with check valve (CLOHP/CV). The nanoparticles used this study are deionized water and TiO2 as a working fluid. The experiment the operating temperature was adjusted in the range of 60, 70 and 80 degrees Celsius and the inclination angle of the heat pipe in the range of 0 and 45 degrees. It was found that when using TiO2 as a working fluid, at inclination angle of 45 degrees, an operating temperature of 80 degrees Celsius, the highest heat transfer was 297.76 W, the highest heat flux was 2887.09 W/m2 the highest heat transfer coefficient was 1547. 53 W/m2 °C and the highest thermal efficiency was 0. 17. When using TiO2 as a working fluid, at inclination angle of 45 degrees, at operating temperature of 80 degrees Celsius the lowest heat resistance was 0. 26 C/W. In addition, we found that CLOHP/CV using TiO2, resulted in higher heat transfer ( 82. 23% ) than when using deionized water and lower heat resistance than using deionized water 92.85%.

Numerical study on heat transfer enhancement of closed loop oscillating heat pipe through active incentive method

In this paper, a mathematic model of a closed loop OHP with two branches was established to reveal the motion law of internal working fluid and the mechanism of heat transfer enhancement excited by different heat source modes, that is, uneven pulsing heat source mode (UP-mode), the even pulsing heat source mode (EP-mode) and the alternate heat source mode (A-mode). The simulation result shows that the vapor plug pressure and the liquid slug displacement both present the characteristic of variable amplitude fluctuation under the three heating modes. The amplitude generally changes periodically which is different from that of the continuous external heat source mode, which make the parameters above show additional fluctuation characteristics on the original oscillation law. The specific variation law is affected by the factors such as the heating period and heating interval. The three heat source modes introduced can enhance the transfer heat capability of the OHP under appropriate condition. But if the configuration condition is not suitable, the three modes above will weak the transfer heat ability, even result in that work stability of OHP deteriorates and stops.

Effect of Filling Ratio on Thermal Characteristics and Performance of a Pulsating Heat Pipe

Pulsating heat pipes are complex devices for heat transfer and their optimal thermal performance depends mainly on different parameters. This work is about the thermal efficiency of a closed-loop oscillating heat pipe with a diameter of 2.0 mm and 3.0 copper tube inner and outer. For all experiments, the filling ratio (FR) was used 40%, 50 %,70%,80% and heat inputs of 20W, 40W, 60W, and 80W was provided to PHP. The position of the PHP was vertical bottom heat type. The length of evaporator, adiabatic and condenser section was maintained 52 mm,170mm,60mm. Water and benzene were selected as working fluids. From the available literature it is observed that working fluid and filling ratio are key factors in PHP's performance. The results show that the thermal resistance decreases rapidly with the increase in the heat input to 20 to 40 W., while it decreases gradually over 40 to 80W.Simulation is done in CFD and experimental data were equated to the results.

Experimental investigation of oscillating heat pipe efficiency for a novel condenser by using Fe3O4 nanofluid

This paper presents a novel study on the performance of closed-loop oscillating heat pipe (CLOHP) using iron oxide (Fe3O4) as the working fluid for three types of condensers. The tested CLOHP consists of six turns made of copper tubes, 4.5 mm outer diameter and 3 mm inner diameter with heating power input in a range of 0–200 W. The experimental results showed that the thermal performance of the CLOHPs has been improved when the corrugated horizontal condenser was used compare to straight and corrugated vertical condensers. Based on 800 sets of available experimental data, the results show that the CLOHPs with corrugated horizontal condenser had better thermal performance when charged with Fe3O4/water at 2% mass concentration.

Thermal Resistance Modeling of Oscillating Heat Pipes for Nanofluids by Artificial Intelligence Approach

In this study, thermal resistance of a closed-loop oscillating heat pipe (OHP) is investigated using experimental tests and artificial intelligence methods. For this target, γFe2O3 and Fe3O4 nanoparticles are mixed with the base fluid. Also, intelligent models are developed to predict the thermal resistance of the OHP. These models are developed based on the heat input into evaporator section, the thermal conductivity of working fluids, and the ratio of the inner diameter to length of OHP. The intelligent methods are multilayer feed-forward neural network (MLFFNN), adaptive neuro-fuzzy inference system (ANFIS) and group method of data handling (GMDH) type neural network. Thermal resistance of the heat pipe (as a measure of thermal performance) is considered as the target. The results showed that using the nanofluids as working fluid in the OHP decreased the thermal resistance, where this decrease for Fe3O4/water nanofluid was more than that of γFe2O3/water. The intelligent models also predicted successfully the thermal resistance of OHP with a correlation coefficient close to 1. The root-mean-square error (RMSE) for MLFFNN, ANFIS, and GMDH models was obtained as 0.0508, 0.0556, and 0.0569 (°C/W) (for the test data), respectively.

Experimental Study on Closed Loop Oscillating Heat Pipe for Different Filling Ratios

An experimental study was performed on Oscillating Heat Pipe (OHP) with closed loop structure. The capillary tube made up of copper of this OHP has inner diameter of 2.15 mm and thickness of wall is 1 mm. Different set of experiments were performed with filling ratios ranging from 30% to 100%, heating power ranging from 115 to 430 W at evaporator section and free convection condition at condenser section. The water has been considered as a working fluid. The thermal resistance was calculated for measuring the thermal performance of OHP. The changes in thermal resistance with heat input for various filling ratios has been carried out. The thermal resistance achieved lowest for 70 % filling ratio.

Visual study on two-phase flow in a horizontal closed-loop oscillating heat pipe

Visual study on two-phase flow in a horizontal closed-loop oscillating heat pipe

Effect of position of heat mode to heat transfer of a closed loop oscillating heat pipe with check valves on fins

This research aimed to determine the effect of position of heat mode to closed loop oscillating heat pipe with check valves (CLOHP/CV) on fins. To check on heat transfer rate needed to focus on evaporator temperature, air velocity, and position of heat mode. All heat exchangers used ethanol as working fluid with the filling ratio of 50% by total volume of tube, evaporator temperature at 60, 70, and 80 degrees Celsius, and air velocity at 0.5, 1.0, and 1.5 meter per second, size of fin was 0.5 centimetre. All heat exchangers used different positions at evaporator section and condenser section of heat pipe. The CLOHP/CV was made from copper capillary tube of (inner diameter: 5 mm), 24 turns 2 check valves. The length of evaporator and condenser and adiabatic section is 20, 20, and 10 centimetre respectively. From the experiment it’s was found that the heat exchanger of bottom heat mode had a temperature of 80 degrees Celsius and air velocity of 1.5 m/s with heat flux and thermal effectiveness the highest.

Heat transfer performance of polytetrafluoroethylene oscillating heat pipe with water, ethanol, and acetone as working fluids

The startup mechanism, temperature oscillation, liquid slug oscillation, and heat transfer performance of polytetrafluoroethylene (PTFE)-based oscillating heat pipes (OHPs) are investigated experimentally. The effects of working fluid, heat input, filling ratio on the startup performance, surface temperature oscillation, and thermal resistance of OHPs are investigated and analyzed. A closed loop OHP with six turns and an inner diameter of 2.41 mm was tested in a vertical heating orientation with working fluids of deionized water, ethanol, and acetone, respectively. The results show that the acetone-filled OHP provides the best thermal performance among three OHPs. Average peak-to-peak amplitudes of temperature oscillations in the acetone- and ethanol-filled OHPs are greater than the ones in the water-filled OHP. Temperature oscillation frequencies are highest in the acetone-filled OHP and lowest in the water-filled OHP among three OHPs. Liquid slug oscillation characteristics behave the similar trend. The amplitude and velocity of liquid slug oscillation are highest in the acetone-filled OHP and lowest in the water-filled OHPs. For the PTFE-based OHP, the thermal performances of ethanol- and acetone-filled OHPs are higher than the thermal performance of the water-filled OHP at a heat input between 60 and 300 W, which are different from a copper OHP. This conclusion is different from results of literature research showed that water-filled OHPs gave a better heat performance than acetone- and ethanol-filled OHPs.

Thermal performance of a novel helically coiled oscillating heat pipe (HCOHP) for isothermal adsorption. An experimental study

Helically Coiled Oscillating Heat Pipes (HCOHPs) have been designed and tested under laboratory conditions to investigate their potential to achieve isothermal adsorption when integrated with a cylindrical solid desiccant packed bed system. The HCOHPs fabricated out of copper, are essentially single turn closed loop oscillating heat pipes with their evaporator and condenser sections helically coiled. They were charged with ethanol, methanol and deionized water respectively at approximately 60% volume fill ratio and tested by slotting through their helically coiled evaporators an empty cylindrical copper vessel which allowed hot air to be blown through at various heat loads to ascertain their thermal performances.The results showed there were critical heat fluxes which varied with heat input amount at the evaporator, beyond which dry-out commenced and thermal resistance increased. These heat fluxes were ≤70W for the ethanol HCOHP and ≤105W for both the methanol and deionized water HCOHPs. Performance instabilities owing to liquid phase of the working fluid transitioning in the drying-out stage was observed for the methanol HCOHP beyond 234W. The variation of the effective thermal conductivities at the evaporators were found to influence the thermal contact resistance experienced at the contact interface of integration and the maximum heat input amount at the evaporators. Optimum performance between the HCOHPs was observed with the deionized water type. Overall, the HCOHPs were capable of managing relatively large amounts of heat input due to their helically coiled sections creating comparatively larger evaporator sections holding relatively more working fluid than the conventional serpentine single turn closed loop OHP system of the same volume and fill ratio. Investigations involving the visualization of the internal flow dynamics is recommended for future studies.

Thermal Performance of Oscillating Heat Pipe with Ethanol/Methanol for Heat Recovery Application Design

As one of the two-phase heat transfer technology, Oscillating Heat Pipe (OHP) gains popularity in heat pipe technology recently. OHP has an excellent performance, simple structure, low production cost and high flexibility for a wide range of applications. OHP is also suitable for applications in waste heat recovery system. This paper experimentally examines a closed loop OHP design for heat recovery application. Therefore, an OHP was manufactured and tested to find out the thermal characteristics for heat recovery application. OHP was made from capillary copper tube with 1.6 mm inner diameter based on the critical diameter for ethanol and methanol working fluids. The length of OHP evaporator, adiabatic and condenser were 260, 240 and 260 mm respectively. The total length of the OHP was 13.6 m and had 18 U-turns. The result showed that evaporator temperature for methanol was lower than ethanol at high heating input. Quasi-sine wave oscillation occurs at methanol OHP (Q=64 Watt). Methanol also had the lowest thermal resistance which was 0.258 °C/W at 79.5 Watt heat input.

Thermal performance comparison of oscillating heat pipes with and without helical micro-grooves

This paper presents an experimental investigation to compare the thermal performance of three closed loop oscillating heat pipes (OHPs) with and without internal helical microgrooves at vertical and horizontal orientations. All of these OHPs were made from copper tubes and have three turns with lengths of 70, 230 and 110 mm at the evaporator, adiabatic and condenser sections, respectively. Deionized water was used as the working fluid at a volumetric filling ratio of 50%. The internal diameters (IDs) of two smooth-tube OHPs are 4.0 and 4.8 mm, respectively, and the internal diameter of micro-grooved OHP without groove structures is about 4.5 mm. Experimental results demonstrated that the addition of groove structures make the OHP remarkably outperform smooth-tube OHPs in both effective thermal conductivity and thermal resistance. The thermal resistance of vertically-oriented micro-grooved OHP could be lowered to 0.057 °C/W associated with an effective thermal conductivity of 6.1 × 104 W/ (m·K) at the input heat flux of 3.8 × 104 W/m2. Compared to smooth-tube OHPs, preliminary mechanism analysis reveals that local heat transfer coefficients both at the heating and cooling sections of micro-grooved OHP could be significantly improved. Moreover, enhanced liquid backflow to the evaporator due to microgroove-induced capillarity is also responsible for the OHP performance enhancement.

Closed loop oscillating heat pipe as heating device for copper plate

In manufacturing parts by molding method, temperature uniformity of the mold holds a very crucial aspect for the quality of the parts. Studies have been carried out in searching for effective method in controlling the mold temperature. Using of heat pipe is one of the many effective ways to control the temperature of the molding area to the right uniform level. Recently, there has been the development of oscillating heat pipe and its application is very promising. The semi-empirical correlation for closed-loop oscillating heat pipe (CLOHP) with the STD of ±30% was used in design of CLOHP in this study. By placing CLOHP in the copper plate at some distance from the plate surface and allow CLOHP to heat the plate up to the set surface temperature, the temperature of the plate was recorded. It is found that CLOHP can be effectively used as a heat source to transfer heat to copper plate with excellent temperature distribution. The STDs of heat rate of all experiments are well in the range of ±30% of the correlation used.

Experimental investigations of effect of sound waves on oscillation and startup characteristics of oscillating heat pipe at different orientations

This research deals with the effects of working fluid and use of sound waves on the startup and heat transfer characteristics in terms of thermal resistance of a closed loop oscillating heat pipe. The performance of the oscillating heat pipe is checked for different orientations as 90? (vertical position), 60?, and 30?. Initially water is used as working fluid and performance of the oscillating heat pipe is checked with and without sound waves. Then 0.1 wt.% Al2O3-water nanofluid is utilized as working fluid in oscillating heat pipe and its performance is analyzed with and without sound waves. In this work, sound waves of 1 kHz frequency are passed through the evaporator section of closed-loop oscillation heat pipe. Application of sound waves improved the oscillation characteristics of the CLOHP with reduced startup time and enhanced thermal performance at all orientations. In comparison between working fluids, 0.1 wt.% Al2O3-water nanofluid showed better oscillation characteristics at all orientations of CLOHP except at 90? where use of sound waves leads to dry-out condition.

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.