Evaporator Section Research Articles

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.

A Molecular Dynamics Perspective on the Impacts of Random Rough Surface, Film Thickness, and Substrate Temperature on the Adsorbed Film’s Liquid–Vapor Phase Transition Regime

While recent studies have proven an unexpected liquid–vapor phase transition of adsorbed liquid films, a comprehensive description of the mechanisms of different types of phase change regimes over realistic representations of random rough surfaces is absent in the literature. The current comprehensive study investigates the effects of a gold random rough surface, liquid film thickness, and substrate temperature on the liquid–vapor phase change regime of an adsorbed sodium liquid film, considering the evaporator section of a wicked heat pipe (WHP) using a molecular dynamics (MD) simulation. At first, to generate a realistic random rough surface, a new and promising method is proposed that is entirely based on MD simulations. Then, to simulate the evaporator section of a WHP, a unique configuration for eliminating the vapor domain is developed. The simulation results reveal that three distinct regimes, namely, normal evaporation, cluster boiling, and film boiling, could be identified, which are presented on two-dimensional diagrams with the substrate temperature and liquid film thickness as coordinates for the ideally smooth and random rough surfaces. The results also manifest that even though using the random rough surface could lead to different phase transition regimes, the type of regime depends mainly on the substrate temperature and liquid film thickness. Furthermore, this study displays two different modes for normal evaporation. Also, it is shown that the impacts of the liquid film thickness and substrate temperature on the mode of normal evaporation are much more significant than the surface roughness.

Evaluation of thermo-fluidic performance of micro pulsating heat pipe with and without evaporator side surface structures

Interfacial behaviour and thermal performance of a 4-loop micro pulsating heat pipe are examined through experiments performed at different filling ratio (30–60 %), working fluid (acetone, ethanol, and methanol), and heat input (0–40 W). During the heat pipe operation, nucleation, growth and merging of bubbles, elongation, and movement of vapor plug, and dry out conditions are noticed upon ramp heating (0–40 W) inside smooth surface evaporator pulsating heat pipe. Experiments show that preparation and boiling inception lead to steady operation of heat pipe. Pulsating heat pipe showed minimum thermal resistance when operated with 40 % filling ratio and acetone as working fluid, which has low viscosity, surface tension and boiling point. Additionally, effect of surface structures (rectangular pillar protrusion and hemispherical dimple cavity) on the evaporator section is tested against base smooth surface micro pulsating heat pipe to understand augmentation of heat transfer. Hemispherical dimple cavities show better nucleation, bubble growth, and vapor plug elongation and thereby reduction in thermal resistance. On the other hand, rectangular pillar protrusions showed clogged bubble, least bubble detachment and vapor plug length in the adiabatic zone which results in higher thermal resistance than even smooth surface.

Features of fluoroethers-filled flat plate pulsating heat pipe: Influence of cooling temperature and semi-empirical modeling

We studied the effects of heat input and cooling temperature on the operational characteristics and heat transfer characteristics of flat plate pulsating heat pipes (FPPHP) using HFE-7100, HFE-7200 and HFE-7300 as working fluids. Empirical correlation equations were fitted to predict the thermal performance of FPPHP. The results showed that the start-up heat input, ultimate heat input, thermal resistance, and evaporation section temperature were positively correlated with boiling point under most operating conditions. At a cooling temperature of 20℃, the FPPHP start-up heat inputs for HFE-7100, HFE-7200, and HFE-7300 are 50 W, 70 W, and 110 W, respectively. After the start-up of the FPPHP, with the increase of cooling temperature, the flow patterns of slug flow, churn flow, annual flow, and vapor region appear successively. FPPHP has lower thermal resistance and ultimate heat input at higher cooling temperatures. At a cooling temperature of 80℃, the ultimate heat inputs of the three fluoroethers in FPPHP are 50 W, 50 W, and 90 W, respectively. In addition, it fills the gap in the empirical equations related to FPPHP and fluoroether working solutions and expands the Ku value range in the current experimental equations to 0.084.

Experimental study of heat pipes for battery cooling technology in EVs

Abstract The modern world is moving towards electric vehicles (EV) due to the increment in greenhouse gas (GHG) emissions, global warming, and the lack of fossil fuels. EVs can overcome these issues by using batteries instead of fuel. But increasing and maintaining the batteries is a major challenge in EVs because of the large heat emissions from the batteries. In order to overcome these issues and increase the performance of the batteries, a heat pipe (HP) is attached to the passive cooling system. This study aims to improve the performance of batteries and the thermal conductivity of HP with a combination of refrigerant and nanofluid (nanorefrigerant) as working fluids. Copper HP with R-134a or SWCNT is selected for this study. The thermal resistance and thermal conductivity of HP with R-134a and SWCNT were observed for several heat conditions. From the study, it was well observed that changing the working fluid inside the HP affects the thermal performance and the cooling capacity of batteries. Fixing an HP to a battery would decrease the battery’s temperature effectively. Furthermore, increasing the heat power in an evaporator section decreases the thermal resistance and enhances thermal conductivity with the shortest time limit because of Brownian motion.

Thermal performance against gravity of an AlSi10 AM heat pipe with a diamond lattice structure

Metal Additive Manufacturing has gained momentum as a viable production technique for specialized heat transfer devices, in particular for industrial sectors characterized by small production numbers associated with high-performance requirements. Within the space industry, ESA is leading and funding several applied research programs to explore the possibility to employ AM for building electronic boxes with embedded heat pipes, in order to reduce manufacturing post-processing steps and contact thermal resistances. In the context of tender 1-10238, the project “Heat Pipe Solutions for High Power Systems” (HPS2) has developed and tested lattice-based heat pipes, that are intended to be integrated in electronic modules. In this paper, the heat transfer performances as function of input power and tilt angle against gravity of a 150 mm long heat pipe with a 20 mm evaporator section and a 40 mm condenser section are presented and compared with the results of the models of performance limits, based on the measured properties of the lattice.

Results of a nitrogen-based pulsating heat pipe with varied configurations

In this paper, the results of a nitrogen-based pulsating heat pipe (PHP) with the ability to change the number of turns are presented. The experiment assembly consists of two identical PHP subsections with their own fill lines. The length of the condenser, the adiabatic section, and the evaporator section of each PHP are 90mm, 1000mm, and 60mm respectively. Because of the unique design of the condenser section of the PHP, it can change the configuration into 1-turn, 3-turn, 5-turn, and 7-turn PHP. The PHP was tested at the condenser temperatures 77.4K, and 84.5K, which are at, and above the boiling point of the liquid nitrogen at ambient pressure. At each condenser temperature, the PHP was tested at three different initial fill ratios (50%, 63%, and 75%). The experimental results and analysis for the 1-turn setup of this nitrogen-based PHP assembly are presented in this paper.

Experimental investigation and optimization of heat pipe heat exchanger performance under two distinct heat transport mediums through TOPSIS approach

This paper presents a comprehensive thermal performance analysis of the heat pipe heat exchanger by varying the input process parameters and optimizing the best condition for waste heat recovery through the TOPSIS technique. In this work, the heat pipe is constructed with a copper tube and segmented into evaporator and condenser sections carrying hot and cold fluids with a half-fill ratio. The heat pipe is oriented in 0° horizontally with the wick material made from stainless steel and works based on the capillary principle. The efficiency of the HPHE is evaluated by varying the input operational parameters, namely heat input, mass flow rate of hot fluid, inlet temperature of hot fluid, and mass flow rate of cold fluid, on response variables, specifically thermal resistance, energy balance ratio, universal heat transfer coefficient, and effectiveness. The experiment was designed for four factors at three levels as per Taguchi’s L27 orthogonal array, and the best optimising condition was determined by employing the multi-objective technique for order preference by similarity to ideal solution (TOPSIS) approach. The best input condition found was A3B1C1D3, in which the heat input was at 90 W, the mass flow rate of hot fluid was at 0.2 kg min−1, the mass flow rate of cold fluid was at 0.1 kg min−1, and the inlet temperature was at 60 °C. The effect of process parameters and their significance on output responses was analysed by executing an analysis of variance (ANOVA) statistical method. The mathematical regression model was generated, and validation was made to investigate the prediction error percentage.

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.

Study of the characteristics of the separated gravity heat pipe of a self-activated PCM wall system

Self-activated phase change material (PCM) wall integrated with radiative sky cooling (RSC) is a novel wall system that uses natural energy directly for low-energy buildings to support carbon peaking and neutrality goals. A separated gravity heat pipe (SGHP) is an effective heat transfer component for heat transport from the wall body to the radiative cooler without using mechanical energy. Its heat transfer characteristics affect the thermal performance of the wall system. In this study, a numerical Volume of Fluid (VOF) model of the SGHP is established. The thermal and flow characteristics under the small temperature difference boundary of building scenarios are simulated and analyzed. Results show that the average temperature of the working fluid inside the SGHP in the “steady stage” is about 26.3 °C when the boundary temperature of the evaporation and condensation sections are respectively 28 °C and 20 °C. The heat exchange can reach 356 W/m2 and the flow velocity of the working fluid is about 0.1 m/s. Influences of different evaporation/condensation boundary temperatures on the heat transfer effect are further studied. Compared to increasing the evaporation section temperature, decreasing the condensation section temperature is a better strategy for improving the heat exchange capacity of the SGHP.

The augmentation of the heat recovery by using evaporative cooling in HVAC applications: Experimental study

An experimental study was conducted to investigate the impact of evaporative cooling on heat recovery within HVAC systems. This study involved the development of a specialized test rig comprising two air passages connected by a wickless heat pipe heat exchanger (wickless HPHE). Additionally, an evaporative cooling pad was integrated into the cold air return duct. The evaporative cooling process effectively lowered the temperature of the cold air, which was subsequently directed through the wickless HPHE condenser. This occurred at a variable mass flow rate ratio, in contrast to the constant mass flow rate of warm air passing through the wickless HPHE evaporator section. The study encompassed a range of mass flowrate ratios, specifically 1, 1.5, and 2. The thermosiphon heat pipe was charged with acetone at a 60 % filling ratio. The results clearly demonstrated that evaporative cooling played a pivotal role in enhancing the heat exchanger's performance. Notably, the temperature differential observed in the wickless HPHE evaporator surpassed that in the HPHE condenser, primarily due to the influence of evaporative cooling. The highest temperature difference recorded in the HPHE evaporator was 17.8 °C, indicative of the most substantial heat recovery achieved. Importantly, the effect of evaporative cooling significantly improved the performance of the wickless HPHE by more than 48 %.

Experimental investigation of the oscillation threshold to limit state operation of a single branch pulsating heat pipe

The study investigates the characteristics of oscillations and associated thermo-hydrodynamic phenomena for startup, limit state, and intermediate transitional behavior of a pulsating heat pipe (PHP) with the help of its single branch. Experiments are performed on a transparent single branch PHP (SBPHP) at different orientations, evaporator and condenser section temperatures using n-pentane as the working fluid. The entire two-phase flow behavior is recorded with the help of shadowgraph based high speed imaging synchronized in real-time domain with pressure data acquisition at evaporator end. Instability threshold is determined in terms of evaporator temperature at different tube orientations and condenser temperatures. The oscillation startup mechanism along with the associated intermittency is studied in detail. The SBPHP is found to have characteristic variations in oscillation amplitude and frequency as it undergoes a gradual transition from its oscillation threshold to limit state. The mechanism by which the limit state creeps-in and intumesces further is an intrinsic function of liquid-vapor distribution in SBPHP, which in turn depends on the applied boundary conditions. The operational limit is governed by evaporator dryout exceeding permissible limits marked by the steep rise in the dry length of evaporator. As SBPHP approaches or operates at its limit state, evaporator is found to have different regimes across its length, viz., mild convective evaporation combined with subcooled nucleate boiling, saturated nucleate boiling, forced convective boiling, film breakdown and permanent dryout. The liquid film distribution, pressure fluctuations, and heat transfer components in evaporator at the limit state have also been studied.

Thermal performance of a large-diameter thin flattened heat pipe with novel composite wick structure

Flattening large-diameter cylindrical heat pipes to fabricate thin heat pipes can significantly improve their heat transfer performance in theory, but their appropriate wick structures and thermal performance are still unclear. In this work, a novel composite wick structure sintered from spiral woven mesh and orthogonal woven mesh was developed for large-diameter thin flattened heat pipes. Their diameter and thickness were 8 and 1.0 mm, respectively. Seven types of wick structures for the experimental heat pipes were designed to investigate the effects of the length and position of the orthogonal woven mesh on the thermal performance. The results indicated that based on the spiral woven mesh, sintering the orthogonal woven mesh in the evaporator and adiabatic sections of the heat pipe can significantly improve the thermal performance. Compared with the single spiral woven mesh wick, the maximum heat transport capacity of the experimental heat pipe with optimal composite wick increased by 41.67%, and the evaporation and total thermal resistances decreased by 68.41% and 61.49%, respectively. This study provided very useful guidance for improving the thermal performance of the large-diameter thin flattened heat pipes for cooling laptops.

Numerical simulation analysis of high-temperature bent sodium heat pipes

Abstract High-temperature bent heat pipes are safety-level heat transfer components used in heat pipe cooled reactor, and their applications are becoming increasingly widespread. In this study, a comprehensive three-dimensional numerical model was established to evaluate the effects of different bending angles and radii on the performance of high-temperature heat pipes. The model solved for the temperature, pressure, and velocity fields in the wall, wick, and evaporator, considers the compressibility effect of the vapor. The study investigated parameters such as vapor pressure, vapor velocity, radial secondary flow field, wall temperature, wick temperature, and equivalent thermal resistance of powder sintered core sodium heat pipes with bending angles of 45°, 90°, 135°, 180°, and a bending radius under 90°. The results showed that when sodium vapor enters the bent pipe, the vapor flow deviates towards the outer side, resulting in higher vapor pressure at the outer side than the vapor pressure at the inner side, and the generation of radial secondary vortex. This leads to higher temperature in the evaporator and adiabatic sections of the bent heat pipe, while lower temperature in the condenser. Fractal patterns are observed in the outer and inner wall temperature curve in the adiabatic section, with lower temperatures on the inner side and higher temperatures on the outer side. The bent heat pipe exhibits higher equivalent thermal resistance than that of a straight heat pipe, and the equivalent thermal resistance increases with increasing bending angle, and the equivalent thermal resistance decreases with increasing bending radius.

Performance assessment of a geometrical modification on thermosyphon heat pipe with the assistance of novel convergent-divergent truncated cones for energy recovery systems

Heat pipe technology has evolved significantly in addressing thermal management challenges. Still, existing heat pipe designs have limitations in efficiently optimizing heat transfer, particularly in scenarios with spatial constraints and variable heat loads. To overcome these challenges, this study deals with incorporating truncated cones in different forms, such as convergent and divergent, in the thermosyphon heat pipe. The parametric analysis and the performance of this research highlight the ability to handle varying heat loads and determine that a 1:1:1 ratio between the evaporator, adiabatic, and condenser sections of 300 mm in total length thermosyphon heat pipe may be expected to meet spatial constraints efficiently. Placement of the convergent truncated cone in the evaporator and the divergent truncated cone in the condenser helps improve the evaporation and condensation performance by 62.07% and 108%, respectively, when compared to the conventional geometry due to the increasing vapour and liquid velocities. Interestingly, the wall temperature decreases by 11.9%, and the thermal resistance is reduced, increasing the heat transfer coefficient by 85.68% compared to the uniform geometry thermosyphon heat pipe. Hence, these findings provide a path for promising applications in heat recovery and thermal management systems.

Operating mechanism of pulsating heat pipe with different wettability from the perspective of thermo-hydrodynamic characteristics of vapor–liquid interface region

Pulsating heat pipe (PHP) is considered to be one of the most promising efficient heat sinks in electronic thermal management for good cooling capacity and compact structure. Understanding the complex operating mechanism of PHP from various angles is crucial for advancing its large-scale application. Among various factors, the thermo-hydrodynamic characteristics of the vapor–liquid interface region have a critical impact on the operation of PHP. Based on this, the transient numerical simulations were conducted in this work on PHPs with 20° and 150° contact angles (hydrophilic and hydrophobic, respectively), and the influence of different thermo-hydrodynamic characteristics of the vapor–liquid interface region on the operating mechanism of PHPs was analyzed. The results found that the presence of a widely spreading liquid film at the interface of hydrophilic PHP significantly changed its thermo-hydrodynamic characteristics compared to hydrophobic PHP. As a consequence, the hydrophilic PHP and hydrophobic PHP presented better heat transfer performance in the evaporation section and condensation section, respectively. Under low heat inputs, with regular vapor–liquid distribution in the interface region, the hydrophobic PHP presented better startup and heat transfer performance due to better hydrodynamic behavior caused by more easily formed pressure gradients. Under high heat inputs, the hydrophilic PHP showed significantly better heat transfer performance due to the intense evaporation and continuous spreading of new liquid film in the interface region, which also played a key role in stronger anti-dry-out performance.

Heat transfer performance of high-temperature heat pipe startup in small heat pipe reactors

High-temperature heat pipe is an important heat transfer component in small heat pipe reactors (SHPR). Research on the transient heat transfer characteristics during heat pipe startup is crucial for safety analysis of SHPR. This manuscript investigates the startup performance of a lithium heat pipe from the frozen state with various influencing factors, including heating power, inclination angle, and convection heat transfer coefficient. The objective of this work is to speed up the heat pipe startup process in a safe and economic manner based on its heat transfer characteristics. In order to illustrate the startup characteristics and further to reduce the startup time, this manuscript constructs a CFD heat pipe model via the multi-physics simulation software, COMSOL. The phase change process of heat pipe startup is described by the Phase Change Material method, with the fluid velocity and pressure fields of wick established using the Brinkman equation. Results show that the heat pipe thermal resistance decreases with increasing heating power, positive inclination angle, or convection heat transfer coefficient. On the other hand, the heat pipe startup time reduces with increasing heating power, increasing positive inclination angle, or decreasing convection heat transfer coefficient. Moreover, numerical analysis indicates that heat pipe temperature rises faster in the evaporation section than in the condensation section at the beginning of startup. Temperature rise process in the condensation section occupies the majority of the entire heat pipe startup process. Therefore, to speed up the startup process, this work proposes a novel extra heating method applying to the condensation section at early startup. Compared with the conventional startup, the proposed method is able to shorten the startup time by 10.5 % and 15.8 % with an extra heating power of 10 % Full Power (FP) and 20 %FP respectively.

Numerical study of Two-Phase flow behavior during dryout in a Two-Phase closed thermosyphon under thermal boundary conditions

This paper presents a two-dimensional axisymmetric two-phase closed thermosyphon (TPCT) model filled with R-134a refrigerant using the Volume of Fluid (VOF) method, Lee model, and Continuum Surface Force (CSF) method within Computational Fluid Dynamics (CFD) and simulates the flow characteristics inside the TPCT during the occurrence of dry out. Specifically, it discusses the two-phase flow in the condensation and evaporation sections when dry out occurs. The results indicate that the condensation section exhibits varying forms of vapor–liquid interface waves at different times. Phenomena such as liquid carryover and entrainment become increasingly prominent over time, contributing significantly to refrigerant blockages and dryout in the evaporation section. The mechanism behind dryout in the evaporation section is revealed to be the result of the initially continuous falling film being disrupted by fluctuations in the liquid–vapor interface, leading to the premature drying of thin liquid films in the troughs of these waves.

Influence of vapor core design on thermal and hydraulic performance of mesh-type ultra-thin vapor chamber

The mesh-type ultra-thin vapor chamber(UTVC), with an internal mesh porous structure, is a cost-effective and efficient heat transfer element for compact spaces. The design of the vapor core plays an important part in enhancing the heat transfer performance of the mesh-type UTVC within the confined space requirements. The present study proposes to design cavities in the supporting coarse mesh inside the vapor core to form a multi-scale combination of vapor channels with the mesh channels. Utilizing a numerical model that considers microporous evaporation as well as the Marangoni effect, the effects of different flow channel designs and designed channel widths on the performance of the UTVC were analyzed. The study determined the flow and heat transfer characteristics of the working medium within the mesh-type UTVC, and found that the most significant performance enhancement was achieved when the designed flow channel passed through part of the UTVC evaporation and condensation sections, reducing the thermal resistance by 17.45%. Whereas the design flow channel width has less influence on the performance of mesh-type UTVC, it satisfies the changing trend of vapor flow pressure drop and thermal resistance decreasing firstly and subsequently increasing with the expansion of the designed channel width.

Numerical study on the characteristics of sodium heat pipes for space application at operation limits

Sodium heat pipes are important thermal conductive components in space nuclear power sources. They efficiently transfer heat within the range of 500–1000 °C by utilizing the latent heat of phase change. However, their maximum heat transfer capacity is limited by various factors. In this study, numerical simulations were conducted to investigate the operating conditions of sodium heat pipes when encountering different heat transfer limits, with a focus on analyzing the vapor–liquid distribution and axial temperature distribution in evaporator section. It is revealed that as reaching the heat transfer limits, sodium heat pipes exhibit wall temperature fluctuations in evaporator section. When encountering the viscous limit, liquid aggregation occurs within the wick, causing discontinuous flow. As reaching the sonic limit, a vapor blockage zone forms at the outlet of evaporator section, disrupting the flow circulation within heat pipe. For the entrainment limit, the liquid inside wick is carried away by vapor flow, reducing the amount of liquid involved in phase change cycle. As in the capillary limit, inadequate liquid reflux leads to thinner liquid layer, weakening the flow circulation. During the transition from the viscous limit to the sonic limit, no significant phenomena of phase distribution occur in the evaporator section. As the sonic limit and the entrainment limit act together, both vapor blockage and liquid entrainment occur, accompanied by liquid accumulation. The phenomena when the entrainment limit and the capillary limit act together differ from those occurring individually. The simulation results provide references for a deeper understanding of heat transfer limit mechanism and other operating characteristics.