Thermal Performance Of Heat Pipe Research Articles

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.

Optimal design of heat pipes for city gate station heaters by applying genetic and Bayesian optimization algorithms to an artificial neural network model

This study contains a novel idea to improve the performance of city gate station heaters, using pulsating heat pipes with optimized structure. In this research, a hybrid method including Bayesian and genetic algorithms is implemented to optimize the architecture of an artificial neural network (ANN) model and find the optimal operating point for heat pipes. In the previous study, the authors have proposed a neural network-based model to predict the thermal performance of heat pipes, based on a wide range of their design parameters, including dimensions of heat pipe, number of turns, working fluid, inclination angle, filling ratio, and heat input. In the present study, the prediction model architecture is optimized using Bayesian optimization algorithm. The best performance is achieved when iterations of the employed Bayesian optimization is set to 300 iterations or samples, which 150 samples are produced to train the Bayesian model and other 150 iterations are utilized in order to find the optimal ANN structure and hyper-parameter tuning of the model. The optimal ANN model with structure of 9-97-21-68-100-1 has evaluation parameters of 0.980 and 0.014 for R2 and MSE of total dataset, respectively. The performance metrics indicate an improvement in accuracy of the ANN model with a 53.33% reduction in MSE value and a 3.15% increase in R2 score. Then, the genetic algorithm is applied to the optimal model to find the optimum operating parameters of the heat pipe. In this regard, the design parameters of the best heat pipe in order to use in the city gate station heaters, are obtained by setting the genetic algorithm population equal to 300. Finally, as a practical case study, a real heater with capacity of 10,000 SCMH which is located in Iran is selected. Considering constraints and limitations of this heater, the proposed model indicates that a vertical pulsating heat pipe with four turns which is filled 55% with water as the working fluid, has the optimal thermal resistance of 0.104 K/W. Utilizing the optimal heat pipe in the mentioned heater is a promising solution for efficiency improvement and fuel consumption and CO2 emission reduction, which decreases the operating costs of the heater as well.

Experimental Investigation to Elucidate the Temporal Effect of Nanofluids on the Thermal Performance of Heat Pipes

The perplexity associated with the relevant practical application of the nanofluid lies with the alteration of the surface morphology. In the context of a heat pipe, it is established that the nanofluids, when used for a longer duration, are prone to agglomerate, and using them as working fluid can influence the heat pipe’s performance. The current work aims to examine the heat pipe performance deviation charged with Al2O3/de-ionized water nanofluid for extended operation periods of 0, 3, 6 and 12 months. The fabricated heat pipes charged with 40% of evaporator volume with various concentrations of Al2O3 base nanofluid (0.1%, 0.5%, and 0.8% volume fraction) were tested for various heat inputs (10, 30, and 60 W). The temporal effect of nanofluid on heat pipe performance was analyzed after successive intervals. The results propose that the heat pipe charged with nanofluid exhibits superior thermal performance in lieu of de-ionized water. Also, the performance characteristics of the heat pipe were more consistent and reliable for higher heat input of 60 W compared to 10 W when tested after a longer duration. The nanoparticles’ sedimentation and accumulation on the mesh clog adversely affect the contact angle and thus help in increasing the boiling limit.

Numerical Simulation of Heat Pipe Thermal Performance for Aerospace Cooling System Applications

The design of integrated and highly efficient solutions for thermal management is a key capability for different aerospace products, ranging from civil aircraft using hydrogen on board to miniaturized satellites. In particular, this paper discloses a novel numerical tool for the design and thermal performance assessment of heat pipes. To achieve this goal, a numerical Ansys Parametric Design Language code is set up to verify the effective subtractive heat flux guaranteed by the selected heat pipe arrangement. The methodology and related tool show their ability to provide good thermal performance estimates for different heat pipe designs and operating conditions. Specifically, the paper reports two very different test cases: (1) solid metal heat pipes to cool down the crotch leading-edge area of the air intake of a Mach 8 civil passenger aircraft, and (2) a copper-water heat pipe to cool down a Printed Circuit Board of a generic small LEO satellite. The successful application of the methodology and numerical code confirms the achievement of the ambitious goal of developing in-house tools to support heat pipe thermal performance prediction for the entire aerospace domain.

An experimental investigation on the influence of condenser bypass area for the transient and steady-state heat-transfer performance of heat pipes

Fundamentally, an effective method for improving the performance of transient and steady-state heat transfer is reducing the flow resistance at the phase-change interface based on the counter flow of vapor and liquid within a typical heat pipe. The flow resistance at such an interface is proportional to the volume flow rates of liquid and vapor. Therefore, the flow resistance at the interface can be reduced by limiting the mass flow rate of vapor or liquid. In this study, experiments were performed on a liquid bypass to improve the steady-state thermal performance of heat pipes. A separate bypass tube allowed some liquid to be subcooled in the condenser to bypass the evaporator without passing through the heat pipe. Three liquid-bypass ports were designed and fabricated in a condenser to experimentally investigate the influence of the bypass port area on the thermal performance of the heat pipe under steady-state operation. An on/off valve was attached to each bypass tube port in the condenser to control the bypass flow rate. The results showed that the thermal resistance, an evaluation index of the performance of steady-state heat transfer, generally decreased with an increase in the bypass port area of the condenser. The thermal resistance of a heat pipe with a horizontal tilt angle and one at an inclination angle of 50° decreased by up to 25.5% and 51.6%, respectively.

A novel neural network and sensitivity analysis method for predicting the thermal resistance of heat pipes with nanofluids

It is critical to accurately predict the heat transfer performance of heat pipes with nanofluids for the design of the thermal management system. The current prediction models for the thermal performance of heat pipe with nanofluids are mostly developed based solely on their experimental data, resulting in poor universality and low predictive performance. Moreover, there is a lack of quantitative analysis of the correlation degree of selected input parameters on the thermal performance of heat pipe. The innovation of this work lies in the development of a novel thermal resistance prediction model based on neural network and sensitivity analysis method, characterized by a broad range of inputs and high accuracy. Firstly, a neural network database was established based on our own experimental data and published literature. The effects of the learning rate and the number of nodes in the hidden layer on the prediction performance were investigated. Subsequently, the Garson algorithm was used for sensitivity analysis of the input parameters. The results showed that when the Adam adaptive optimization algorithm and 28 nodes in the hidden layer were used, the neural network model had the highest prediction accuracy, with a mean square error (MSE) of only 0.0020 and a mean absolute error (MAE) of 0.03 K/W, and a mean relative error (MRE) of 7.86 %, demonstrating that the developed model can be used for predicting the performance of heat pipes. The ranking of the influence of each input variable on the thermal resistance from high to low is as follows: heat load, confinement number, inclination angle, cold source temperature, filling ratio, thermal conductivity, and concentration. Moreover, the influence of concentration on the accuracy of the model cannot be ignored. This work provides a direct guidance for developing a performance prediction model for heat pipe with nanofluids with high universality and accuracy.

Design and Thermal Performance of Ultra-Thin Flat Plate Heat Pipes with Composite Wick for Electronic Cooling

The rapidly increasing heat load and high-density packaging of electronic devices demand heat dissipation devices that possess miniaturization and high thermal performance. A series of ultra-thin flat plate heat pipes (UPHPs) with thicknesses of 1.2 mm was prepared to satisfy this demand. Strengthening rib optimization and capillary optimization were processed to restrict the entrainment limit and capillary limit of the UPHPs. Furthermore, surface modification technology with composite nanostructure coated was introduced to improve the surface wettability of capillary wick. Finally, the heat transfer capacity of the UPHPs was experimentally tested. The influence of capillary structure and liquid filling ratio on the heat transfer characteristic was investigated in detail. This work provides insight into the design and application of UPHPs for the new generation of electronic devices.

Investigation of thermal conductivity and thermal performance of heat pipes by structurally designed copolymer stabilized ZnO nanofluid

The present study concentrated on estimating the thermal conductivity, stability, efficiency, and resistance of a heat pipe for heat exchangers, which were essential for many industrial applications. To achieve this, copolymer of amphiphilic poly (styrene-co-2-Acrylamido-2-methylpropane sulfonic acid) poly (STY-co-AMPS) was synthesized by free radical polymerisation technique. The dispersant were used for homogeneous solution and stabilization of ZnO nanofluids. The effect of dispersant on the thermal conductivity of nanofluids was analysed using a KD2 pro thermal property analyser. There is a significant increase in fluid conductivity had a nonlinear relationship with the volume fraction. The maximum enhancement was observed at an optimized concentration of dispersant at 1.5 vol%. Same time, the influence of dispersant agent on the thermal conductivity of nanofluids were compared with linear polyelectrolytes. Further, the experimental values were compared to the existing classical models based on the reasonable aggrement, the prepared nanofluids were employed as a working medium. The conventional screen mesh heat pipe and the temperature distribution to the thermal resistance of the heat pipe was investigated experimentally. The result shows, optimum concentration of dispersants on nanoparticles exhibits an enhanced heat efficiency as compared with the base fluids. Further, the thermal resistance and temperature distribution show decreased behaviour by increasing the particle volume fraction and dispersant concentration.

Effect of Working Fluid, Orientation, and Cooling Mode on Thermal Performance of Miniature Flat Heat Pipe

Abstract Flat heat pipes (FHPs) are commonly used as a passive cooling system in portable electronic gadgets due to their compact profile. The present study investigates the effect of different working fluids on the thermal performance of a miniature FHP under different orientations and condenser cooling mechanisms and the start-up performance of FHP. Deionized water, acetone, ethanol, and methanol are chosen as working fluids in the FHP. Five different inclinations (0 deg (horizontal), 30 deg, 45 deg, 60 deg, and 90 deg (vertical)) and two different condenser cooling methods (natural convection and forced convection with fan cooling) are considered in this experimental study. The FHP thermal performance is quantified in terms of overall temperature difference, thermal resistance, and effective thermal conductivity. The results indicate that comparatively higher effective thermal conductivity values are obtained for methanol and acetone heat pipes at low heat loads and under natural convection. At higher heat loads, the ethanol heat pipe had higher effective thermal conductivity values for the same condenser cooling method. For the case of the forced convection cooling mode, the methanol heat pipe had enhanced thermal performance as compared to the other three fluids for all heat load ranges and different inclinations. Due to the higher boiling point of water, as a working fluid water is not suitable in most of the experimental trials except at high heat load under forced convection cooling and in a horizontal orientation. The maximum effective thermal conductivity of 7846 W/mK is obtained for FHP filled with methanol at 24 W heat load and 90 deg orientation under forced convection condenser cooling.

Investigative Studies on Thermal performance of Heatpipe with suspended Nanoparticles

This paper presents an experimental investigation regarding the use of solid nanoparticles added to water as a working fluid. Tests were made on a thermosyphon heat pipe. The experiment was performed in order to measure the temperature distribution and compare the heat transfer rate of the thermosyphon heat pipe with nanofluid and with DI-water. The iron oxide nanoparticles were obtained by the laser pyrolysis technique. The tested concentration level of nanoparticles is 0%, 2%, and 5.3%. Results show that the addition of 5.3% (by volume) of iron oxide nanoparticles in water presented improved thermal per-formance compared with the operation with DI-water.

Numerical analysis of transient heat and mass transfer for a water heat pipe under non-uniform heating conditions

Non-uniform loads are widely present in heat pipe applications and have significant effects on their performance. In this work, a transient 3D CFD model is developed to investigate the liquid–vapor flow and heat transfer for a water heat pipe under various non-uniform loads. The source terms are added to the governing equations using the User-Defined Function (UDF) to simulate the evaporation and condensation process. The model is verified using the experimental and simulated data under uniform and non-uniform loads in the literature. The effects of key parameters including adiabatic length, heating power, and heat distribution on the thermal performance of heat pipes are analyzed. The results show that the non-uniform conditions would shift the maximum axial velocity of vapor to the unheated side and induce a significant circumferential flow in the wick region, especially in the evaporator section. The short adiabatic length and high heating power could reduce the elapsed time of reaching the steady state. The heat distribution in the evaporator section exhibits little effect on the elapsed time reaching the steady-state and the average steady-state vapor temperature

Thermal performance of heat pipe at different internal groove ratios and working fluids: An experimental investigation

Thermal performance of heat pipe at different internal groove ratios and working fluids: An experimental investigation

Analysis of thermal performance a heat pipe for concentrated photovoltaic cooling

Heat pipes are used in this study for the concentrated photovoltaic (CPV) passive cooling. Investigations have been done in two steps: in the first step, the heat pipe's thermal performance in the horizontal position has been tested. Two sets cylindrical heat pipes with identical diameters (6 mm) but of different lengths (250 mm and 150 mm, respectively) were employed. Each set consists of three heat pipes. The results of the study showed that the heat pipe has a maximum operating point that is dependent on both the intensity of the incident light and the surrounding temperature. Moreover, this operating point could be determined. For high light intensity, a long heat pipe performs better than a short one, and vise versa. In the second step, a heat pipe passive cooled CPV system has been investigated for both long and short heat pipe systems. The performance of heat pipe cooled CPV system was compared to that of the traditional fins-cooled CPV system. Results showed that at light concentrations of 55 X, cooling a solar cell with a heat pipe enhances its maximum output power by 6% for a long heat pipe and by 7% for a short heat pipe, respectively. While at 210 X illumination concentration, long and short heat pipes, respectively, produced gains of 16 and 12% in the cell's maximum output power.

Micro-grooved circular miniature heat pipe for thermal management: Experimental and analytical investigations

In this paper, thermal performance of a cylindrical micro-grooved heat pipe is experimentally and mathematically investigated. The experimental tests are conducted included the optimum working fluid filling ratio, heat pipe inclination, and input heat load. The new correlations of micro-grooved heat pipe condensation and evaporation heat transfer coefficients are proposed basing on the experimental data. A coupled thermal and hydraulic model is developed to predict the heat pipe local thermal resistances and operating temperature regarding the effect of the architectural size. The Experimental results show that the heat pipe thermal performance is not affected by the inclination angle. Whereas the heat pipe performance is better at horizontal position. It showed that the micro-grooved heat pipe could provide an efficient performance even if the condenser is longer that 40% of the total length and the heat pipe microgrooves aspect ratio higher than 37%.

Experiments on thermal performance of heat pipes using nanoparticle layer as the wick

To enhance the usability of heat pipes and vapor chambers as the cooling device for portable electronic systems, necessity of thin wick is rapidly increasing. In the present work, the layer of silica nanoparticles of about 10 μm in thickness was formed on the copper tube inner wall to explore experimentally if it works as the thin wick. It was demonstrated that in comparison with the ordinary heat pipe using a wire mesh as the wick, the thermal resistance was even improved (0.62 to 0.23 K/W) and the deterioration of the maximum heat transfer rate was not significant (120 to 78 W). This indicated that the present nanoparticle layer can be used as a very thin wick of the heat pipes and vapor chambers. If the mass of nanoparticles deposited on the heat pipe inner wall was sufficient, the thermal performance was not deteriorated even after the maximum heat transfer rate condition was experienced. It was also demonstrated that the maximum heat transfer rate can be enhanced up to 100 W when the nanoparticles were suspended in the working fluid enclosed in the container.

Thermal performance analysis of heat pipe using response surface methdologyUdayvir

Heat pipes are the specific class of heat exchangers. They are used in thermal management of electronic components. Research community is continuously working to obtain the optimum heat transfer performance. In present work, parametric study of heat pipe using nano- fluid has been carried out. The operating parameters of heat pipe like power supply, orientation (gravity assisted angle), filling- ratio, and nano-fluids concentration are being investigated to find the optimum thermal performance of heat pipe. Response surface method (RSM) is used to analyze the effect of operating parameters on thermal performance. The optimum value of thermal resistance and thermal efficiency are 0.3994 °C/Watt and 68.44% respectively. Most suitable power supply, inclination angle, filling ratio and nanofluid concentration are 185.85 W, 60.09°, 50.7% and 1.05 % respectively. The experimental results confirm and validate the RSM predicted results.

Heat transfer coefficient and thermal performance of heat pipe with R134a/mineral oil nanodiamond+Fe3O4 hybrid nanorefrigerant

Experimental research has been done on the heat transfer coefficient and thermal performance of heat pipe operating with R134a/mineral oil (5:1% weight percent) based nanodiamond+Fe3O4 hybrid nanorefrigerants. The in-situ growth approach was used to create the hybrid nanoparticles, which were subsequently characterized by X-ray diffraction. Using a vibrating sample magnetometer, the prepared sample magnetic measurement was determined. The studies were conducted with varying heat inputs of 100, 150, and 200 W and with particle loadings of 0.1%, 0.5%, and 1.0%, respectively. The results show that when using hybrid nanofluids instead of base fluid, the wall temperatures at the evaporator and condenser sections of heat pipe are lower. At a heat supply of 200 W, the average temperature in evaporator and condenser sections are reduced by 30.08% and 19.21%, respectively. At [Formula: see text] = 1.0% vol. loading and 200 W, the thermal resistances in the evaporator and condenser sections drop to 24.58% and 21.74%, respectively. In comparison to the base fluid, the heat transfer coefficients of the evaporator and condenser are greater to 37.08% and 30.24%, respectively, at a heat input of 200 W. The thermal performance of the heat pipe is enhanced by employing the hybrid nanoparticles based nanorefrigerant, and it is also increased with increasing particle loadings.

Thermal performance of an ammonia loop heat pipe using a rectangular evaporator with liquid-guiding channels

In order to enhance the heat transfer capacity of a stainless steel-ammonia loop heat pipe using a rectangular evaporator, liquid-guiding channels were adopted to significantly reduce the liquid flow resistacne in the evaporator wick. An experimental system was established, and extensive experimental studies have been implemented to investigate its startup characteristics, heat transfer capability and thermal resistance. To be specific, the effects of heat load, evaporator attitude and antigravity on the startup performance were evaluated. According to the experimental results, the loop heat pipe showed excellent startup performance, which could realize the startup at small heat loads for different attitudes of the evaporator. Both adverse evaporator attitude and anti-gravity can cause an increase of the operating temperature of the loop heat pipe, due to poor cooling effect of return liquid and increased heat leak from the evaporator respectively. The loop heat pipe could achieve a heat transfer capability up to 300 W, corresponding to a heat flux of 20 W/cm2. The minimum system thermal resistance reached as low as 0.048 K/W at the head load of 140 W. The present study contributes greatly to the future loop heat pipe applications in high heat flux electronics cooling.

Predicting the thermal performance of heat pipes applying various machine learning methods and a proposed correlation

The key purpose of this study is to predict the thermal performance of heat pipes by means of artificial intelligence methods. In this regard, several different prediction models have been employed. These models are proposed with totally nine different parameters as inputs such as dimensions of heat pipes, number of turns, working fluids, inclination angle, filling ratio, and heat input. The output of these models is the thermal performance of the heat pipe. For training the prediction models, a dataset of totally 1196 experimental data was collected from previous studies which includes several different working fluids. The employed prediction methods contain ten machine learning regression methods, seven ANN models with different structures and a correlation model. At first, the machine learning regression methods have been employed and based on the results, the best performance belongs to random forest regressor method with the R2 score and MSE value of 0.90 and 0.003, respectively. Then inspiring physics of the problem, several feed forward back-propagation neural network models have been designed for simulating and estimating the thermal performance of heat pipes. Results verified that the proposed models could be used on a wide range of working fluids with an acceptable accuracy. The values of R2 and MSE of the optimal ANN model are 0.95 and 0.03, respectively which are pretty acceptable for a predicting model working on such a wide range of working fluids. Moreover, a new correlation is proposed to estimate the thermal performance of heat pipes on the basis of the same inputs. This correlation can estimate the thermal performance of the whole dataset with R2 score of 0.60 and MSE value of 0.27, while the minimum and maximum of R2 scores are 0.47 and 0.88 for water and ethanol, respectively. Finally, the results obtained from the mentioned machine learning methods, the ANN models and the proposed correlation were compared. It was found that due to complexity of the problem and wide variety of inputs, the results of using methods based on artificial intelligence are far more accurate than those of the proposed correlation. In fact, choosing more accurate and complicated methods (e.g., the optimal ANN model with structure of 9–32–40-50-64-1), the R2 score of prediction significantly increased from 0.60 to 0.95. Since there is a trade-off between the accuracy and simplicity of the methods, selecting the best method should be considered as a choice problem.

A Review of Working Fluids and Flow State Effects on Thermal Performance of Micro-Channel Oscillating Heat Pipe for Aerospace Heat Dissipation

A MCOHP (micro-channel oscillating heat pipe) can provide lightweight and efficient temperature control capabilities for aerospace spacecraft with a high power and small size. The research about the heat flow effects on the thermal performance of MCOHPs is both necessary and essential for aerospace heat dissipation. In this paper, the heat flow effects on the thermal performance of MCOHPs are summarized and studied. The flow thermal performance enhancement changes of MCOHPs are given, which are caused by the heat flow work fluids of nano-fluids, gases, single liquids, mixed liquids, surfactants, and self-humidifying fluids. The use of graphene nano-fluids as the heat flow work medium can reduce the thermal resistance by 83.6%, which can enhance the maximum thermal conductivity by 105%. The influences of gravity and flow characteristics are also discussed. The heat flow pattern changes with the work stage, which affects the flow mode and the heat and mass transfer efficiency of OHP. The effective thermal conductivity varies from 4.8 kW/(m·K) to 70 kW/(m·K) when different gases are selected as the working fluid in OHP. The study of heat flow effects on the thermal performance of MCOHPs is conducive to exploring in-depth aerospace applications.