Performance Of Pulsating Heat Pipe Research Articles

A novel method for introducing asymmetry in pulsating heat pipes to enhance performance

The primary objective of this study is to improve the performance of a pulsating heat pipe (PHP) by introducing a novel method of incorporating asymmetry into the PHP design. This asymmetry is achieved by inserting a copper wire into one column of a single-loop PHP. Five different configurations of single-loop PHPs were created for this study: one with a uniform internal diameter (ID) of 2.5 mm (UPHP), one with dual diameters (2.5 mm and 1.8 mm ID) (DPHP), and three with a uniform ID of 2.5 mm but with varying wire diameters (0.7 mm, 1.2 mm, and 1.7 mm) inserted into one column (WAPHP). The PHPs were constructed using borosilicate glass. The performance of these PHPs was evaluated through visualization techniques and by calculating equivalent thermal resistance. A comprehensive study was conducted by varying filling ratios, inclination angles, and heat loads to understand their influence on PHP performance. The results indicate that at a 60% filling ratio, the wired asymmetric pulsating heat pipe (WAPHP) with a 1.2 mm diameter wire exhibited superior performance compared to all other tested configurations achieving a thermal resistance of 1.27 K/W. The WAPHP performed effectively up to an inclination angle of 15°. Additionally, the WAPHP with a 1.7 mm wire diameter demonstrated the earliest startup, taking just 35 s, and showed the maximum heat load capacity at a 50% filling ratio in a vertical orientation. Furthermore, dry-out did not occur in the 1.2 mm and 1.7 mm diameter WAPHPs at 50% and 60% filling ratios up to the tested heat load.

Single-Loop Triple-Diameter Pulsating Heat Pipes at Reduced Heat Input: A CFD Study on Inner Diameter Optimization

The geometric configuration, particularly the inner tube diameter, plays a significant role in the thermal performance of pulsating heat pipes (PHPs). Previous experimental research has demonstrated that single-loop triple-diameter PHPs (TD-PHPs) outperform single-loop single-diameter PHPs (SD-PHPs) and dual-diameter PHPs (DD-PHPs) in terms of thermal performance under moderate heating input powers ranging from 25 W to 75 W. However, a reduction in heat input from 75 W to 25 W leads to a diminished impact of TD-PHPs on the thermal performance. Therefore, to improve the overall performance of TD-PHPs, this study used two-dimensional transient computational fluid dynamics simulations to identify the optimal inner tube diameters for TD-PHPs at a low heat input by evaluating the thermal resistance of five TD-PHPs with various inner diameters. The findings reveal that the TD-PHP configuration exhibits minimum thermal resistance, with inner diameters of 4.5 mm for the upper arch (the condenser section), 4.0 mm for the wide branch, and 2.5 mm for the narrow branch, primarily due to its full circulation flow pattern. Furthermore, the overall heat transfer performance of the optimal TD-PHP was compared with that of an SD-PHP at low heat inputs (10 and 18 W), indicating that although the optimal TD-PHP shows lower thermal resistance, it does not significantly affect the start-up time.

Experimental study on heat transfer characteristics of pulsating heat pipe under vibration environment

Understanding the heat transfer characteristics of pulsating heat pipe (PHP) under vibration condition is important for the specific applications of PHP in moving vehicles, working machines or handheld equipment. In this paper, an experimental system for testing the thermal performance of PHP under different vibration conditions was built, the effects of vibration frequency and vibration amplitude on the temperature oscillation characteristics, overall heat transfer performance (thermal resistance and equivalent heat conductivity), start-up performance (start-up temperature and start-up time) of PHP have been analyzed. The results show that more obvious influences of vibration frequency and amplitude on the heat transfer performance of PHP can be observed under lower heat input condition. Increasing vibration frequency will reduce the overall thermal resistance, start-up time and start-up temperature, and increase equivalent heat conductivity. However, increasing vibration amplitude will enhance the overall thermal resistance and reduce the equivalent thermal conductivity, and increase both start-up time and start-up temperature. Therefore, better heat transfer performance of PHP can be obtained under high-frequency vibration environment or small amplitude vibration environment, as well as the start-up performance. It is acknowledged that compared with non-vibration condition, the heat transfer performance of PHP is limited under low vibration frequency (fv ≤ 25Hz) or large vibration amplitude (a ≥0.8 mm).

Pulsating Heat Pipe Performance Modeling with Liquid Metal Coolants Under Hypersonic Aerothermal Heating

Hypersonic heating loads concentrate at leading-edge compression areas to create excessively high local temperatures and thermally driven stresses. The fast and reliable thermal dispersion of heat pipes with significantly high thermal conductance can alleviate these localized thermal stiffness problems. The pulsating heat pipe (PHP) holds numerous advantages when compared to capillary, constant-conductance heat pipes for hypersonic thermal management applications, primarily because they lack a wicking structure. This paper numerically investigates thermal performances of a four-turn C103 niobium alloy PHP operating with lithium, potassium, sodium, and a eutectic sodium–potassium alloy (NaK-78) when exposed to heating conditions relevant to the hypersonic environment, investigating flight Mach numbers ranging from 6 to 8 and dynamic pressures ranging from 40 to 44 kPa. The robust thermofluidic properties of liquid metals, along with the powerful fluid pulsation induced in the PHP, can provide significant thermal transport from hot stagnant regions of the leading edge to the cooler trailing surfaces. Potassium showed superior thermal performance when compared to other liquid metal coolants under the presently tested conditions, with overall PHP thermal conductance as high as 34 W/K predicted for Mach 8 flight conditions. In contrast, sodium was associated with startup difficulties in the PHP; this paper attributes this to its significantly larger thermal conductivity, which can limit the vapor pressure difference over the liquid slug lengths. These predictions indicate an overall latent heat transfer dominance of around 70–95% in liquid metal PHPs.

Study on the effect of condenser configuration of pulsating heat pipes for space applications

In this study, the effect of condenser configuration on the thermal performance of pulsating heat pipes (PHPs) for space applications is experimentally investigated. The thermal performances of a single-condenser PHP and double-condenser PHP are compared under the constraints of the same evaporator and condenser areas. The PHPs are composed of a meandering channel with 30 turns and are made of a stainless-steel tube with the inner and outer diameters of 1.0 and 1.6 mm, respectively. R-134a is used as the working fluid, and the filling ratio is varied from 30 % to 60 %. Under various experimental conditions, the thermal performances of both single- and double-condenser PHPs are examined from two perspectives. First, the thermal resistances of the two PHPs are compared across a range of condenser temperatures from 10 to 30 °C and heat inputs from 0 to 700 W. According to the experimental observations, the double-condenser PHP exhibits a considerable reduction in thermal resistance, up to 55 % compared with the single-condenser PHP, across all the experimental conditions. Second, the operating ranges of the two PHPs are compared. For the single-condenser PHP, either an increase in the filling ratio or a decrease in the condenser temperature results in a narrower range of stable operation, with a maximum allowable heat input of approximately 200 W. On the other hand, the double-condenser PHP is mostly in the stable operating region regardless of the filling ratio or condenser temperature. Consequently, it exhibits an operating range approximately four times wider than that of the single-condenser PHP, indicating a significantly expanded range of stable operation. These improvements in thermal performance can be attributed to having the effects of doubling the number of evaporator-condenser pairs and halving the length between the evaporator and the condenser.

A spring-mass-damper model based on separated phase flow mode for pulsating heat pipe with adjustive-structured channels

Pulsating heat pipe (PHP) is a kind of efficient passive phase-change cooling device. The pulsating behaviors of the two-phase flow inside PHP significantly affect the heat transfer performance for PHP, the investigation of which will greatly contribute to the optimal design of PHP for electronic heat dissipation in small space. In the present work, the heat transfer performance of PHP is optimized via structure analysis and modeling calculation. A new “spring-mass-damper” model in terms of separated phase flow mode is established, where the frictional pressure loss of the real two-phase flow pattern − slug flow in PHP is considered. Besides, a prototype of PHP with adjustive-structured channel (ASCPHP) is proposed. The heat transfer performance of ASCPHP is evaluated with the newly established model. With theoretical computation method, the frequency of ASCPHP the superiority of ASPHP is also confirmed by comparison with other types of PHPs.

Pulsating heat pipe performance enhancement through porous metallic surfaces produced via physical dealloying

Physical dealloying (PD) is explored in this work as a way of creating a porous layer on metallic surfaces to be used for the enhancement of Pulsating Heat Pipe (PHP) thermal performances. PD can be applied to metal alloys consisting of components with a high difference between their partial vapour pressure, such as copper and zinc. Commercially available brass (Cu/Zn alloy) capillary tubes with OD = 2 mm and ID = 1.3 mm were shaped into a four-turn PHP, with a total length of 949 mm. One standard PHP with the same tube diameter, number of turns and total length was tested as benchmark, while other two PHPs were subjected to PD for 0.5 and 2 h, respectively. All PHPs were tested in the range of heat load between 3 and 40 W at a fixed 50 % filling ratio with ethanol as working fluid. The performed tests show that PHPs after PD display up to 30 °C lower average temperature at the evaporator and up to 7 °C lower average temperature of the condenser compared to the benchmark. PD was capable to lower the PHP thermal resistance by up to 4.2 times, from 11.2 to 2.65 K/W, at low heat powers. Furthermore, in the case of PD-treated PHP, the start-up takes place at lower power and temperatures when compared to untreated PHP. This characteristic holds significant value as it expands the range of applications for PHPs, while simultaneously enhancing their reliability, safety, and overall lifespan when used for thermal management in equipment. It is worth noting that this straightforward approach can be tailored for a wide range of thermal management equipment, including conventional, plate, and micro heat exchangers, as well as HVAC systems. This method is particularly suitable for situations where heat transfer takes place through phase change processes.

Computational Evaluation of Pulsating Heat Pipe for Fluid Flow Behavior and its Thermal Performance

Computational fluid analysis of a single circuit heat pipe was undertaken to ascertain the thermal efficiency of water at various fill ratios, specifically 40%, 50%, and 60% under varying heating conditions. The analysis employed the ANSYS Fluent computational fluid dynamics software, utilizing a k-epsilon to the heat pipe as the solver. The analysis was carried out on a copper tube with an internal diameter measuring 3/16 inches. During the analysis, the thermal inflow at the adiabatic region was adjusted to zero, while the condenser region’s temperature remained constant at 25°C. Meanwhile, the heat input at the evaporator region was systematically adjusted to 105°C, 110°C, 115°C, and 120°C. Key Performance characteristics of heat pipe i.e. thermal resistance, and water volume fraction were evaluated during analysis. The temperature variations noted in both regions affirmed that the pulsating pipe’s performance was influenced by the phase transition between liquid and vapor. Furthermore, the concentration of volume within the evaporator section was monitored, confirming the presence of a dual-phase phenomenon within the heat pipe. Through the analysis, it was noted that the thermal resistance of the water is minimized at a 50% fill ratio for each level of heat input. Notably, the promising results were obtained at an input temperature of 120°C with a value of 1.025°C /W which was 7.3% and 9.8% lower than thermal resistance at 40% and 60% fill ratios, respectively. The reduced thermal resistance in this scenario is attributed to the flow dynamics within the capillary, propelled by rapid phase change mechanisms. This shows the importance of Pulsating Heat Pipes (PHPs) in thermal control, as well as the intricacies of their operating mechanisms. Experimental and theoretical investigations have investigated numerous elements influencing PHP performance, with numerical modeling providing insight into thermal resistance and water volume fraction dynamics under varying heating temperatures and fill ratios.

Thermal performance and stability experiments of a 1.75-meter-long helium pulsating heat pipe

Experimental studies of the performance and thermal stability of extended-length helium pulsating heat pipes (PHPs) were performed at the University of Wisconsin – Madison. Multiple distinct testing procedures were carried out on helium PHPs with an adiabatic length of 1.75 meters, such as progressively increasing evaporator heat load, randomized heat load, and extended period tests. The results of these tests show that despite the stochastic nature of the fluid flow and persisting non-equilibrium conditions, long-distance helium PHPs can maintain stable and steady operation with excellent thermal performance. The progressively increasing heat load tests serve as a baseline for the 1.75-meter PHP’s performance, where the maximum effective conductivity observed was 443.4 kW/m-K at 570 mW heat load and a fill ratio of 56.42%. Furthermore, the randomized heat load test show that, with an optimized fill ratio, PHP performance is not strongly dependent on previous operating conditions and that normal operation can be swiftly recovered from a dry-out condition. The extended period test shows a stable, pseudo-steady operation for over 50 hours with no performance degradation or temperature deviations observed. The impact of PHP build quality is also discussed.

Experimental investigation on the heat transfer performance of pulsating heat pipe with self-rewetting Fe3O4-nanofluid

This paper conducted an experimental study on the heat transfer performance of pulsating heat pipe (PHP) under three different working fluids: ultrapure water, n-butanol self-rewetting fluid (SRWF), and self-rewetting Fe3O4-nanofluid (SRNF). The effects of filling ratio (FR), heating power, and magnetic field are studied. The results mainly show that the PHP with SRNF under a magnetic field, the start-up time increases, the evaporation section operating temperature increases, and the thermal performance decreases as the filling ratio increases. Increasing the FR, applying SRWF and SRNF can effectively increase the dry-out limit of PHP. At FR = 50 %, the overall thermal performance of PHP is best. In 10–130W, the thermal resistance of the SRWF and the SRNF with/without magnetic field is reduced by 14.5 %–27.8 %, 27.6 %–47.9 %, and 4.2 %–24.8 %, respectively, compared with that of ultrapure water. Similarly, compared without a magnetic field, the thermal resistance of SRNF with a magnetic field is reduced by 14.0 %–35.1 %. At FR = 50 %, the heat transfer performance of SRNF-PHP is best with a magnetic field. When the input power is 130W, the effective heat transfer coefficient keff is 5967.6 W/m∙ K, approximately 13.92 times greater than copper.

Experimental study on heat transfer performance enhancement of pulsating heat pipes induced by surfactants

Pulsating heat pipe (PHP) is considered as a promising technique to address the thermal dissipation issues in micro-electronic devices, but its heat transfer performance needs to be further enhanced urgently. In this study, the PHP comprehensive performance experimental system was established. Then, the influence of surfactant sodium dodecyl sulfate (SDS) aqueous solution on the heat transfer performance of PHP was experimentally investigated and the visualization of PHP with surfactant was also accomplished. Additionally, based on Marangoni effect and surface tension, a comprehensive analysis was conducted on the impact mechanism of surfactants on PHP. The experimental results indicated that the oscillatory disturbance of the liquid slug inside the PHP is enhanced by the addition of SDS, and the resulting pressure imbalance and temperature difference enhance the directional flow of the working fluid. Meanwhile, under the operating conditions of 30 W working thermal power and the solution concentration of 0.001 wt%, the thermal resistance of PHP was reduced by 30.7 % compared with that of deionized water (DI water). This indicated that the working fluid with low concentration SDS could increase the area of liquid film and reduce the thermal resistance, which is induced by the surface tension and Marangoni effect due to the uneven distribution of surfactants. However, the thermal resistance of PHP with the SDS concentration of 0.2 wt% is higher than that of DI. This means that high concentration of SDS would deteriorate the startup characteristics and the heat transfer performance of PHP. Therefore, high concentrations of surfactants should be avoided from practical application of PHP.

The DEPLOY! Project: Development of a Deployable Pulsating Heat Pipe experiment on a parabolic flight

DEPLOY! Project aims at analysing the behaviour of Deployable Pulsating Heat Pipe (PHP) shaped as a helicoidal torsional spring in the adiabatic section on board a Parabolic Flight platform. A PHP is a passive thermal control device where the heat is efficiently transported by means of the self-sustained oscillatory fluid motion driven by the phase change phenomena. The microgravity environment allows to eliminate the buoyancy force contribution in the liquid phase momentum. Consequently, it is possible to isolate the contribution of the pressure drop caused by the 3D arrangement and infer on their effect on the PHP performance. As a result, a proper design based on the previous considerations would increase the flexibility of the PHP for use in space applications without significant reductions in efficiency. The goal of DEPLOY! is to demonstrate the functionality of a Deployable Pulsating Heat Pipe in various unfolding configurations by analysing its thermal-hydraulic response throughout a Parabolic Flight. The presented Deployable PHP is composed of an aluminium tube (inner/outer diameters 1.6mm/2.6 mm) and filled with HFE-7000. It is heated at the evaporator using a flat heater and cooled at the condenser with a water-cooled cold plate. T-type thermocouples are used to measure the wall temperature in several locations, while two pressure transducers (one located in the evaporator section and another in the condenser section of the same pipe) measure the local fluid pressure. Additionally, an IR Camera will be used to observe a section of the pipe for further analysis of the flow frequency. The device operation will be tested on ground and 0-g at different heat loads (24W, 34W), in multiple static positions corresponding to different opening angles (0°, 45°, 90°, 135°, 180°) and during its dynamic opening from 0° to 180°, thanks to a remotely controllable motion system.

Numerical Study on Single Turn Pulsating Heat Pipe with Additional Branch Configuration

The simple wickless structure and strong heat transfer potential of pulsating heat pipes (PHP) have prompted widespread interest in transferring heat from electronic devices. This work proposes a novel single-turn PHP with convergent and divergent shaped additional branches (ABs) (C-PHP and D-PHP) in the evaporator section, which could increase latent heat transfer in the evaporator by minimizing the saturation temperature and improving the unidirectional flow and thermal performance of PHP. Numerical simulations were performed using computational fluid dynamics (CFD) techniques to understand the thermo-hydrodynamics and properties of the proposed novel PHPs. This study provides the outcomes of novel PHPs and a comparison with the performance of traditional PHP (T-PHP) without AB and straight AB PHP (S-PHP). The results demonstrated that the startup time of novel C-PHP and D-PHP were improved by 32.6% and 22.4%, respectively, compared to the T-PHP, “stopovers” and “local flow reversals” are minimized by the use of AB. The average flow velocity of novel C-PHP and D-PHP was improved by 16% when compared to T-PHP and 11.4% compared to S-PHP, the directional component in C-PHP is substantially stronger than S-PHP and D-PHP.

Innovative experimental approach for the dynamic Multi-Variable investigation of Pulsating heat Pipes

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices characterized by a simple structure and high heat transfer capabilities. Despite this, their large-scale application is still hindered by the actual unpredictability of their dynamic behavior during the startup and the thermal crisis. An innovative experimental apparatus is designed to systematically investigate the above-mentioned phenomena. It consists in a square loop made of four borosilicate transparent glass tubes joined at corners by means of brass connectors. The external tube surface is coated with several transparent Indium Tin Oxide heaters. The device is used to topologically reproduce 5, 7, and 11 evaporator/condenser sections PHPs with a 2 mm inner diameter tube, filled with pure ethanol and tested in horizontal position. First a steady-state characterization is performed. The condenser temperature is varied from 10 °C to 40 °C; the input power goes from 10 W to 40 W. Results show that both the increase of number of evaporator sections and condenser temperature enhance the PHP performance in terms of equivalent thermal resistance. Then, the effect of the condenser temperature on the initial liquid phase distribution is analyzed. It is observed that at low condenser temperatures (10 °C, 20 °C), the liquid phase tends to gather in the condenser sections rather than the evaporator sections and it is arranged in short liquid plugs; conversely, for high condenser temperatures (30 °C, 40 °C), the amount of liquid phase in evaporators is higher and it is arranged in long liquid plugs. Finally, the transient behavior is studied in terms of startup time. It is observed that the parameter influencing most the startup time is the initial fluid distribution; the startup time increases with the increase of evaporator sections volume occupied by liquid phase and with the length of liquid plugs; on the other hand, the startup time decreases with the increase of number of menisci in the evaporator sections.

Design and performance evaluation of pulsating heat pipe using metallic nanoparticles based hybrid nanofluids

Pulsating heat pipes (PHP) use two-phase flows to transport heat between evaporators and condensers. The performance of PHP largely depends on the thermal conductivity of the working fluid. The research objective of this study is to investigate PHP performance enhancement with HNF consisting of metallic and metallic oxide nanoparticles and compare the PHP performance with different working fluids. A three-turn PHP made from copper tubing was fabricated and charged with Al2O3-Cu HNF, Al2O3 mono nanofluid, Cu mono nanofluid, and pure water for performance evaluation. A total of ninety-eight (98) experiments were conducted with four working fluids over a range of operating conditions, including nanoparticles weight concentration (0.1 % and 0.2 %), filling ratio (50 % and 60 %) and heat input (30–90 W). Results showed that the Al2O3-Cu HNF achieved 30–54 % lower thermal resistance than water, and 17–43 % lower than the mono nanofluids at the same filling ratio, weight concentration and heat input, which indicates superior heat transfer enhancement. The optimal condition for the PHP filled with Al2O3-Cu HNF examined in this study was achieved at a 60 % filling ratio and a 0.2 % weight concentration of nanoparticles. However, maintaining the stability of Al2O3-Cu HNF poses challenges due to the hydrophobic nature of Cu nanoparticles. Future studies are planned to use molecular dynamics simulation to investigate factors influencing the stability of metallic nanoparticles-based mono and hybrid nanofluids.

An Experimental Investigation on the Heat Transfer Characteristics of Pulsating Heat Pipe with Adaptive Structured Channels

In recent years, the development of electronic chips has focused on achieving high integration and lightweight designs. As a result, pulsating heat pipes (PHPs) have gained widespread use as passive cooling devices due to their exceptional heat transfer capacity. Nevertheless, the erratic pulsations observed in slug flow across multiple channels constitute a significant challenge, hindering the advancement of start-up and heat dissipation capabilities in traditional PHP systems. In this paper, we introduce a flat plate pulsating heat pipe (PHP) featuring adaptive structured channels, denoted as ASCPHP. The aim is to enhance the thermal performance of PHP systems. These adaptive structured channels are specifically engineered to dynamically accommodate volume changes during phase transitions, resulting in the formation of a predictable and controllable two-phase flow. This innovation is pivotal in achieving a breakthrough in the thermal performance of PHPs. We experimentally verified the heat transfer performance of the ASCPHP across a range of heating loads from 10 to 75 W and various orientations spanning 0 to 90 degrees, while maintaining a constant filling ratio (FR) of 40%. In comparison to traditional PHP systems, the ASCPHP design, as proposed in this study, offers the advantage of achieving a lower evaporation temperature and a more uniform temperature distribution across the PHP surface. The thermal resistances are reduced by a maximum of 37.5% when FR is 40%. The experimental results for start-up characteristics, conducted at a heating power of 70 W, demonstrate that the ASCPHP exhibits the quickest start-up response and the lowest start-up temperature among the tested configurations. Furthermore, thanks to the guiding influence of adaptive structured channels on two-phase flow, liquid replenishment in the ASCPHP exhibits minimal dependence on gravity. This means that the ASCPHP can initiate the start-up process promptly, even when placed horizontally.

Experimental investigation of the flooding phenomenon in a pulsating heat pipe unit cell

This study presents an experimental investigation on characterization and quantification of flooding phenomenon in a pulsating heat pipe (PHP) unit cell at different orientations, evaporator and condenser temperatures. A transparent capillary tube having a diameter of 2.5mm is used as a PHP unit cell, and n-pentane is used as the working fluid. The meniscus oscillations along with flooding events are recorded using a high-speed camera. Three different flooding mechanisms are recognized, viz., wave transport with film drainage delay, wave transport with slug formation, and droplet entrainment. The flooding frequency is determined for each flooding mechanism at different combinations of the operating boundary conditions. The adverse impact of flooding on the performance of PHP is realized apparently by considering its influence on the amplitude and frequency of meniscus oscillations. The approximate liquid deficit in evaporator caused by flooding is also estimated for one of the mechanisms, viz. wave transport with slug formation. Thus, role of flooding in accession of the operational limit of a PHP is signified. Further, the hydrodynamics of flooding is studied in detail. The superficial velocity of vapor at the onset of flooding and film-vapor interfacial wavelength are determined for a few cases and compared to the respective predictions based on some of the most widely accepted correlations. It is found that none of the considered correlations could predict the flooding velocity of vapor for the present experiments. The study signifies the need to study flooding more intensively for PHPs in near future.

Pulsating heat pipe and embedded heat pipe heat spreaders for modular electronics cooling

Two-phase-based heat spreaders are highly sought electronics cooling solutions due to their superior thermal performance over conventional conduction plates. Along with good thermal performance, Pulsating Heat Pipes (PHPs) potentially have thickness, shape matching and cost benefits over Embedded Heat Pipe (EHP) heat spreader solutions. In this manuscript, the thermal performance of PHP charged with three different fluids-propylene, R245fa, and acetone was experimentally determined for different sink temperatures and compared to an EHP heat spreader. A mathematical model to predict the thermal performance of the PHP was developed and validated against experimental results. It was determined that at low sink temperatures, PHP charged with propylene performs better than or similar to an EHP heat spreader with up to 7.8 times higher effective thermal conductivity over a conduction plate. However, at moderate temperatures above >40 °C, propylene PHP dries out faster than the EHP plate. PHP performance with R245fa and acetone improves with increasing heater power. Effective thermal conductivity improved by 2.5 times with R245fa PHP and by 4 times with acetone PHP over the conventional plate but was lower than the EHP plate whose effective thermal conductivity was 17.3 times higher. However, weight comparison showed PHP to be lighter than the EHPs.

Improvements in performance and durability through surface modification of aluminum micro-pulsating heat pipe using water as working fluid

PHPs (pulsating heat pipes) using water are typically made of Cu and Si. Although Al is more sustainable than Cu and cheaper than Si (in addition to weight and durability advantages), water–Al PHPs have not been extensively investigated because noncondensable H2 gas is generated during Al–water reactions at high temperatures, stopping the flow in PHPs and degrading the thermal performance. In this study, a nontoxic micro-nano surface treatment method that is cheaper and simpler than the only reported method for water–Al PHPs was applied to the internal surface of an Al PHP. This prevented H2 gas production by forming Al(OH)3, thereby eliminating possible reaction sites on the surface. The internal two-phase flows, which significantly affected the thermal performance of PHPs, were compared using a flow visualization technique. The dry-out region gradually expanded in the original PHP with increasing operation time. In contrast, the modified surface PHP exhibited flow regeneration, preventing dry-out, as well as a thermal resistance 45.8% lower than that of the original surface PHP. Further, the critical heat flux of the PHP remained at 4.57 W/cm2 with the modified surface. Therefore, the proposed method improved the performance and durability of water–Al PHPs.

Effect of filling ratio, number of loops, and transverse distance on the performance of pulsating heat pipe in a microchannel heat sink

In this study, three-dimensional modeling of a pulsating heat pipe is carried out, and the effect of physical and geometrical parameters such as filling ratio (FR), transverse distance (Z), and the number of loops (N) on its performance is investigated. In this study, it is assumed that the heat pipe is embedded in the microchannel heat sink, and its boundary condition is the thermal condition in the microchannel heat sink. The volume of fluid (VOF) multiphase flow model is used in the current numerical analysis. For the PHP, four different filling ratios (FR = 40, 50, 60, 70%), four transverse distances (Z = 2, 5, 8, 18 mm), and four numbers of loops (N = 1, 2, 3, 4) are considered, and the results will be compared with each other. It is shown that PHP with FR = 60% has the best performance. Also, comparing different values of transverse distance, Z = 18 mm has the lowest thermal resistance. The results show that by embedding a 4-loop PHP, the thermal resistance of MCHS decrease from 0.0268 to 0.0221 K/W, which indicates a 21.27% increase in the thermal performance of the microchannel heat sink compared to the basic design (without PHP).