Cryogenic Pulsating Heat Pipe Research Articles

Visualized investigation of a nitrogen pulsating heat pipe

This paper presents the results of a visualization study of a cryogenic pulsating heat pipe (PHP). The impact of radiation heat leakage was reduced by minimizing optical windows. Visual images of the adiabatic section of a 2-turn PHP with nitrogen as working fluid were obtained at a filling ratio of 50 % and an inclination angle of 90°. The lengths of the evaporator, adiabatic section, and condenser were 50 mm, 100 mm, and 50 mm, respectively. The adiabatic section was made of quartz, whereas the evaporator and condenser were made of stainless-steel capillaries soldered onto copper plates. The flow channel of PHP was 1 mm. The fluid characteristics of the PHP under quasi-steady state conditions at different heat inputs were captured using a high-speed camera. Based on the experimental results, the evolution of flow patterns and fluid motion characteristics, and the coupling relationship between flow and heat transfer were illustrated and discussed.

High performance and working stability of an 18 W class neon pulsating heat pipe in vertical/horizontal orientation

This paper presents an experimental thermal and hydraulic characterization of a pulsating heat pipe (PHP) at neon temperature. In the search for efficient alternative cooling techniques, with a cryocooler as the cold source, this particular heat pipe is considered a potential candidate for passive thermal link to cool high-temperature superconducting devices. This wickless heat pipe is 0.4 m long with 0.1 m enclosed within the evaporator and condenser. It consists of 20 turns of 1.0 mm inner diameter stainless tube. A systematic study of the thermal performance and pressure of this PHP is presented as a function of thermal load and orientation covering the widest range of filling ratios (10 to 90 %). The cryocooler capacity limit is 18 W for which the PHP functions successfully. This cryogenic PHP has the best thermal performance ever measured with neon showcasing a thermal resistance as low as ∼0.17 K/W in vertical and ∼0.27 K/W in horizontal orientation. The limits of the operating conditions (start-up and dry-out) are also studied as well as the non-homogeneity in evaporator heat load. Finally, a 100 h stability test at constant operating conditions is also demonstrated.

Experimental investigation of a serial-parallel configuration helium pulsating heat pipe

Cryogenic pulsating heat pipe with the features of exceptional heat transfer performance and easy manufacturing is a good candidate to effectively transfer the cooling power energy in the conduction-cooled superconducting magnet system. In this work, a new type of serial-parallel 24-turn configuration pulsating heat pipe with helium as working fluid has been investigated experimentally under various working conditions by changing heat loads, inclination angles, and filling ratios. Experimental results demonstrate that for this pulsating heat pipe, working in vertical orientation is more appropriate. Additionally, the optimum filling ratio changes with the heat load input. The maximum effective thermal conductivity is 12357 W/(m∙K), obtained at the inclination angle of 90° and the filling ratio of 66.1%. The new configuration demonstrates high thermal performance well above the critical point of helium. At the filling ratio of 94.2% and the vertical position, this pulsating heat pipe can transport up to 25 W heat load without reaching dry-out occurrence. The influences of the number of turns and tube structure are discussed by comparing the serial-parallel 24-turn helium pulsating heat pipe with the 4-turn helium pulsating heat pipe and the serial 24-turn helium pulsating heat pipe with the same dimensions we tested previously. The result shows that the serial-parallel 24-turn helium pulsating heat pipe is more efficient concerning heat transfer capacity and thermal resistance. Increasing the number of turns can reduce the temperature gradient of the helium pulsating heat pipe, and changing the tube structure from series to series–parallel can greatly improve the heat transfer limit of the helium pulsating heat pipe. Moreover, another novel parallel 24-turn helium pulsating heat pipe is proposed. According to the theoretical analysis, The parallel 24-turn helium pulsating heat pipe could reach a higher maximum effective thermal conductivity, whereas the serial-parallel 24-turn helium pulsating heat pipe is outstanding in heat transfer capacity.

Performance assessment of planar and non-planar cryogenic pulsating heat pipe with novel condenser

Pulsating heat pipe (PHP) is gaining popularity as a viable solution for thermal conductive links associated with a cryogenic cooling system. A competitive commercial environment requires PHP with a more compact configuration, high heat load removal capacity, and free of orientation dependency. Making orientation-free PHP, a planar PHP (2D layout), requires a large number of turns, increasing the overall size of the PHP device. A PHP with a non-planar (3D layout) layout is capable of accommodating a large number of turns in a short area to tackle both compactness and orientation-dependent issues. The present paper focuses on a comparative thermal performance assessment of planar and non-planar configurations of cryogenic PHP by developing a novel cylindrical shell-type condenser (CSTC). Liquid nitrogen is used as a working fluid. Two separate test setups are developed with 2D Four-Turn PHP (2D FT-PHP) and 3D Four-Turn PHP (3D FT-PHP) with a novel CSTC. The condenser, adiabatic, and evaporator lengths of PHP are kept as 60 mm, 120 mm, and 50 mm, respectively. The thermal performance is assessed by varying Filling Ratio (FR) (38−95 %) and heat load (1 W–130 W) with an evaporator at the bottom position. It is observed that, for a particular heat load, a 3D FT-PHP performed better than a 2D FT-PHP, irrespective of FR. The anti-dry-out ability of 3D FT-PHP is observed to be superior to 2D FT-PHP at a low FR. The frequency analysis results with Wavelet transform show that the working fluid oscillates with a more dominant frequency in 3D FT-PHP than in 2D FT-PHP configuration.

Novel condenser based cryogenic pulsating heat Pipe: Investigations on influence of geometrical parameters

Pulsating heat pipes (PHPs) are becoming popular among passive cooling devices as a possible replacement for the thermal conductive links associated with cryocooler applications. To cool high-temperature superconductors, the need for a large cooling capacity cryocooler is rapidly increasing. The available literature studies show that the heat transfer capability of cryogenic PHP depends on the cryocooler's cooling power. Therefore, findings of a higher heat transport limit and optimal geometrical parameters for a cryogenic PHP remain unsolved. The present paper focuses on developing a novel cylindrical shell-type condenser (CSTC) to provide a high cooling load in a cryogenic PHP. Liquid nitrogen is used as a cryogen. The experiments are performed to study the influence of a number of turns (Single-Turn (ST), Two-Turn (TT)) and adiabatic length (60 mm, 120 mm) designated as ST-60, ST-120, TT-60 and TT-120 PHPs. The inner and outer diameters of PHPs are 1.3 mm and 3.2 mm, respectively. The filling ratio (FR) of cryogen is kept between 38 % and 95 %. The heat load varies from 1 W to 60 W with a bottom heating position. For a case of varying adiabatic length, the lowest thermal resistance increases by 24.78 % for ST-120 PHP compared to ST-60 PHP and 3.23 % for TT-120 PHP compared to TT-60 PHP. For a case of a number of turns, the lowest thermal resistance decreases by 36.57 % for TT-60 PHP compared to ST-60 PHP and 47.5 % for TT-120 PHP compared to ST-120 PHP. The maximum effective thermal conductivity (Keff) of 40,590 W/m-K is observed for an ST-120 PHP. A hybrid cooling technique is proposed with a combination of a novel CSTC and cryocooler to provide a high cooling load with a lower need for cryogen replenishment in the condenser section of a PHP device for an on-ground application.

CFD Modelling of a Helium Cryogenic Pulsating Heat Pipe

This paper explores the development of a 3D model using ANSYS Fluent to characterize the two-phase (gas/liquid) flow of helium in a cryogenic pulsating heat pipe. The model includes condensation and evaporation phenomena and uses the VOF model to capture aspects of the two phase flow. Temperature dependent properties were applied for helium in the range of 2.2 K and 20 K. Bubble formation and movement were observed, as well as flow direction at various times in each of the individual pipes. Grid independence tests were performed by plotting the average temperature of different sections. The influence of the number of turns on the performance of PHP is also investigated.

Design of a cryogenic two-phase flow visualization system for cryogenic pulsating heat pipe

A cryogenic pulsating heat pipe (PHP), as a novel two-phase heat transfer device, has been studied for cooling superconducting magnets because of its high transfer coefficient, simple configuration, and flexible arrangement. From the results of the room temperature PHP experiments, it was noticed that the flow characteristics and the flow pattern of the PHP are complex, which affect the heat transfer performance of the PHP. In this work, a cryogenic two-phase flow visualization system for cryogenic PHP by using a GM cryocooler is developed to get a better understanding of the oscillating behavior, the distribution of vapor slug and liquid plug, and the flow pattern of the cryogenic working fluid. The details of the design of this system are presented. The performance of the GM cryocooler is verified and the temperature distribution of the cooling system is discussed. The liquid slugs and vapor bubbles were observed during the cool-down period. The visualization experiment will be carried out with the cryogenic PHP prototype installed and operated in a vertical position using nitrogen as a working fluid.

Retraction

Thermal Science 2022 Volume 26, Issue 2 Part A, Pages: 921-929 https://doi.org/10.2298/TSCI201110210M by Vukman Bakic, Editor-in-Chief of the journal Thermal Science request that it is necessary to retract paper PERFORMANCE AND IMPACT OF CRYOGENIC PULSATING HEAT PIPE USING A DIFFERENT NUMBER OF TURNS AND HELIUM GAS by Arunkumar MUNIMATHANa*, Mohanavel VINAYAGAMb, Prabhu RAJALINGAMc, Ganesamoorthy RAJUd, and Ashraff Ali KAVERIPAKKAM SUBANe aDepartment of Agriculture Engineering, Sri Shakthi Institute of Engineering and Technology, Coimbatore, Tamil Nadu, India bCentre for Materials Engineering and Regenerative Medicine, Bharath Institute of Higher Education and Research, Chennai, Tamil Nadu, India cDepartment of Mechanical Engineering, Dhanalakshmi Srinivasan College of Engineering, Coimbatore, Tamil Nadu, India dDepartment of Mechanical Engineering Chennai Institute of Technology, Chennai, Tamil Nadu, India eDepartment of Mechanical Engineering, C. Abdul Hakeem College of Engineering and Technology, Vellore, Tamil Nadu, India published in the journal THERMAL SCIENCE 2022 Volume 26, Issue 2 Part A, Pages: 921-929, since this paper is completely plagiarized the article Effect of Number of Turns and Configurations on the Heat Transfer Performance of Helium Cryogenic Pulsating Heat Pipe (DOI: 10.1016/j.cryogenics.2018.09.005) published in CRYOGENICS, Vol. 96, 2018, pp.159-165, by authors Monan Li, Laifeng Li, Dong Xu: Effect of number of turns and configurations on the heat transfer performance of helium cryogenic pulsating heat pipe Monan Li, Laifeng Li, Dong Xu Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China Corresponding authors. E-mail addresses: laifengli@mail.ipc.ac.cn (L. Li), xudong@mail.ipc.ac.cn (D. Xu). https://doi.org/10.1016/j.cryogenics.2018.09.005 Received 11 April 2018; Received in revised form 22 August 2018; Accepted 12 September 2018 Available online 14 September 2018 0011-2275/ ? 2018 Elsevier Ltd. All rights reserved. Following presented data, paper https://doi.org/10.2298/TSCI201110210M must be treated as a serious academic misconduct - plagiarism. <br><br><font color="red"><b> Link to the retracted article <u><a href="http://dx.doi.org/10.2298/TSCI201110210M">10.2298/TSCI201110210M</a></b></u>

Performance and impact of cryogenic pulsating heat pipe using a different number of turns and helium gas

The present work involves the development of helium based pulsating heat pipe (PHP), which containing 48 parallel tubing parts. The PHP is considered as one of the best alternatives for conducting metals and it is used for long distance heat transfer process. Their heat transfer capability and efficient thermal conductivity are the prominent properties which considered for applications. The region of the condenser was thermally sealed to the giffored mcmohanon cryo-cooler using a cooling cap of 1.49 W at 4.2 K while 1.1 W of heat are allowed to the evaporator section at a filling rate of 70%, through comparing the 48-turn PHP and 8-turn PHP, a most intense efficient thermal conductivity of 12329 W/ mK was achieved in the 48 turn PHP. The influence of no turns of warm movement execution was observed with the same operating parameters and topographical parameters. Observations revealed that the temperature variations of PHP 48-turn was significantly less than that of PHP 8-turn. It exhibited efficient thermal conductivity, high capacity heat transfer and a good dry-out temperature response. Thus PHP 48-turn of series and parallel configurations are defined as excellent system de-signs and are accessible to the PHP cryogenics framework architecture. <br><br><font color="red"><b> This article has been retracted. Link to the retraction <u><a href="http://dx.doi.org/10.2298/TSCI220630084E">10.2298/TSCI220630084E</a><u></b></font>

The influence of aspect ratio on the thermal performance of a cryogenic pulsating heat pipe

• Cryogenic pulsating heat pipes perform best with large vertical to horizontal ratio. • Aspect ratio dependence of thermal resistance is characterized for a helium PHP. • The major thermal resistance for a helium PHP occurs at the wall – fluid interface. • Method provided to reduce heat load and orientation sensitivity for a hydrogen PHP. Pulsating heat pipes (PHPs) are expected to serve as a significant design component for the thermal management of superconducting magnets and various cryogenic space applications, including cryogen storage tanks and low temperature detectors on telescopes. A growing body of data is accumulating regarding the performance of cryogenic PHPs, including operation with helium, hydrogen, neon, and nitrogen, and the notable dependence on their orientation with respect to gravity. However, in view of the expectation that the thermal path between a low temperature heat source and the corresponding cryocooler will involve a convoluted route including vertical, horizontal, and sloping segments, we have begun a systematic study regarding the influence of the vertical-to-horizontal aspect ratio of a PHP on its thermal performance. Although various studies available in the literature characterize the performance of completely vertical or completely horizontal PHPs, this study quantitatively displays the relative impact of the vertical and horizontal segments on the performance of a mixed orientation PHP. This systematic investigation is the first of its type to characterize the influence of the vertical-to-horizontal aspect ratio of a PHP on its thermal performance. Data gathered with both helium and hydrogen filled PHPs including five different aspect ratios confirm the expectation that vertically dominated geometries perform better than horizontally dominated geometries. Minimum thermal resistance values ranging from 1.5 K/W to 20 K/W for the 1-meter-long PHPs, with aspect ratios varying from 4:1 to 1:4, are reported as a function of heat load, fill ratio, and fluid type. Here the aspect ratio is defined by the length of the vertical adiabatic section divided by the length of the horizontal adiabatic section. The results with the hydrogen PHP demonstrate that a fill ratio of 70% significantly reduces the influence of orientation and heat load on the PHP’s thermal resistance. The combined results provide valuable guidance for their application as heat transfer components in superconducting magnets and various cryogenic space applications.

Experimental investigations on two-turn cryogenic pulsating heat pipe with cylindrical shell-type condenser

• Cryogenic Pulsating Heat Pipe is experimentally investigated. • A cylindrical shell-type condenser is proposed for Liquid nitrogen-based Cryogenic PHP. • The influence of heat input and Filling Ratio (FR) on the performance of Cryogenic PHP is studied. • Fast Fourier Transform (FFT) analysis is carried out to comprehend the Cryogenic PHP system. Pulsating heat pipe (PHP) is a two-phase passive cooling device and has become an efficient heat transport system to replace bulky conductive straps in cryogenics applications. A two-turn cryogenic PHP is experimentally investigated with liquid nitrogen as working fluid in the present paper. A cylindrical shell-type condenser is proposed that provides a high cooling load with easy refilling of liquid nitrogen. The inner diameter of the capillary tube is 1.3 mm, and the outer diameter is 3.2 mm. The filling ratios are kept as 38%, 57%, 76%, and 95%. Heat input is varied in the range of 1 W-80 W. The maximum heat transport capacity of a two-turn cryogenic PHP with cylindrical shell-type condenser is observed 70 W before dry out. The maximum effective thermal conductivity ( K eff ) of 16350 W/m-K is observed at an optimum filling ratio of 76%. Fast Fourier Transform (FFT) analysis showed cryogenic PHP's deterministic behaviour at the optimal stage with a frequency range of 0.62–1.56 Hz.

Effects of filling ratio of a long cryogenic Pulsating Heat Pipe

In the development of cryogenic pulsating heat pipes for cooling superconducting devices, numerous progressive heat load tests have been performed at different filling ratios with neon, argon and nitrogen as working fluids. The tests have been carried on with a pulsating heat pipe system connected to an external buffer volume during the entire test (open configuration) and with the pulsating heat pipe system isolated from any external volume (closed configuration). From this two group of tests, it has been possible to determine the minimum filling ratio required to operate in stable conditions and also to determine the optimum filling ratio giving the highest thermal performance of the system for each working fluid. This filling ratios are always below 50%, revealing the key role of the fluid flow circulation in pulsating heat pipes, mainly ensured by the expansions and contractions of the vapor parts, in the global heat transfer. In addition, the analysis of the temperatures and pressure evolution of a progressive heat load test with a high filling ratio reveals that an excess of liquid impedes the flow circulation and therefore reduces the heat transfer from the evaporator to the condenser of the pulsating heat pipe. • A cryogenic Pulsating Heat Pipe has been tested with nitrogen, neon and argon. • Filling ratios ranging from 20 to 90% have been tested. • Progressive heat load tests have been performed in open and closed configuration. • Optimum filling ratios giving the highest thermal performance have been identified. • Minimum filling ratios giving stable operating conditions have been identified.

Numerical investigation of a nitrogen based cryogenic pulsating heat pipe

In cryogenic conditions, a Pulsating heat pipe (PHP) can be used to transport a high heat flux across a relatively small temperature difference. Using empirical techniques to design such a PHP is unreliable due to the shortage of experimental dataset available for cryogenic operation. A numerical model developed for cryogenic temperatures and compared with experiments conducted at those temperatures is missing from the literature. In this study, a model available for ambient temperature conditions is modified to simulate the working of a nitrogen-based PHP. The effect of various geometric and operational parameters on the internal heat transfer mechanism of the device is also presented. The results indicate the presence of optimum inner tube diameter, fill ratio, and number of turns at which peak performance can be extracted.

Numerical study of liquid nitrogen based pulsating heat pipe for cooling superconductors

Numerical simulation of a 2-D Cryogenic Pulsating Heat Pipe (CPHP) is carried out. Liquid nitrogen is considered as working fluid. The Volume of Fluid (VOF) model is used to simulate two-phase liquid–vapor flow in a CPHP. Filling Ratio (FR) is kept in the range of 25–70%. The evaporator temperature is set in the range of 85–115 K. The condenser temperature of 76 K is set constant. The volume fraction results of liquid nitrogen are studied for different operational conditions. A detailed qualitative and quantitative analysis is performed for working fluid temperature, adiabatic wall temperature and flow circulation velocity. Power spectrum density (PSD) and the Autocorrelation function (ACF) tools are used to analyze adiabatic wall temperature-time series data. Thermal oscillations are observed with dominant frequency in the range of 0.25–1.85 Hz.

CFD Analysis of Cryogenic Pulsating Heat Pipe with Near Critical Diameter under Varying Gravity Conditions

Numerical simulation of 2D Cryogenic Pulsating Heat Pipe (CPHP) is carried out with near critical diameter. Liquid nitrogen is used as working fluid for targeting cryogenic applications in the range from 77 to 123 K. The volume of fluid (VOF) model is considered for two phase simulations. The evaporator and condenser section are kept at constant wall temperature condition while adiabatic section is considered insulated. The evaporator temperature varied from 85 to 110 K and condenser section kept at 75 K. The filling ratio (FR) of working fluid is varied as 50, 60 and 70%. CPHP model is first tested with on ground gravity condition of 9.81 m/s2. It is then tested with low gravity conditions (0.981 m/s2) and milli-gravity condition (0.01 m/s2). In low gravity condition, surface tension force is observed more dominant than body force which significantly altered the performance of a CPHP. In comparisons with ground level condition, more stable flow patterns are observed which led to the improved heat transfer performance of a CPHP in case of low gravity.

Thermal Study of a one-meter long Neon Cryogenic Pulsating Heat Pipe

Cryogenic Pulsating Heat Pipes (PHP) are excellent candidates for cooling superconducting magnets due to their lightness, simple configuration and thermal performance. Long cryogenic PHP using Neon as working fluid have been developed and are under consideration to be used as thermal links between a superconducting magnet (the hot source) and a cryocooler (the cold source) maintaining the latter out of the influence of the magnetic field. Tests in a horizontal 1 m long PHP composed of 36 parallel tubes have been performed. Using Neon as working fluid, the device shows high performance, even though the inner diameter of the tubes (1.5 mm) is larger than the critical diameter determined by the most commonly accepted criterion in the literature. During the tests, the condenser temperature was maintained at 27 K while the heat load was increased gradually at the evaporator part. The PHP was able to transfer a heat load of 50 W from the evaporator to the condenser with an equivalent thermal conductivity of 70 kW/mK.

Effect of number of turns and configurations on the heat transfer performance of helium cryogenic pulsating heat pipe

The pulsating heat pipe (PHP) is a potential alternative to highly conductive metals such as copper for long distance heat transfer. Effective thermal conductivity and heat transfer capacity of a PHP are two of the most critical factors for practical applications. In this paper, a helium based PHP, which consists of 48 parallel tubing sections, was developed. The lengths of the evaporator, adiabatic and condenser sections are 50 mm, 100 mm and 50 mm respectively. The condenser section was thermally anchored to a Gifford-McMahon cryocooler (GM cryocooler) with a cooling capacity of 1.5 W at 4.2 K. A maximum effective thermal conductivity of 12330 W/m∙K was obtained when 1.1 W heat was applied to the evaporator section at a fill ratio of 70.5%. With the same geometric parameters and operational parameters, the effect of the number of turns on the heat transfer performance was figured out by comparing the 48-turn PHP with an 8-turn PHP. The results show that the temperature difference between the evaporator and condenser sections of the 48-turn PHP is much smaller than that of the 8-turn PHP. The dry-out temperature response, effective thermal conductivity and heat transfer capacity of them are obtained and analyzed. Furthermore, two configurations of the 48-turn PHP, a parallel configuration and a series configuration, are defined. An optimum configuration is proposed and makes a reference to the design of a cryogenic PHP for applications.

Experimental study of Large-scale cryogenic Pulsating Heat Pipe

Pulsating Heat Pipes (PHP) are passive two-phase heat transfer devices consisting of a long capillary tube bent into many U-turns connecting the condenser part to the evaporator part. They are thermally driven by an oscillatory flow of liquid slugs and vapor plugs coming from phase changes and pressure differences along the tube. The coupling of hydrodynamic and thermodynamic effects allows high heat transfer performances. Three closed-loop pulsating heat pipes have been developed by the DACM (Department of Accelerators, Cryogenics and Magnetism) of CEA Paris-Saclay, France. Each PHP measures 3.7 meters long (0.35 m for the condenser and the evaporator and 3 m for the adiabatic part), being almost 20 times longer than the longest cryogenic PHP tested. These PHPs have 36, 22 and 12 parallel channels. Numerous tests have been performed in horizontal position (the closest configuration to non-gravity) using nitrogen as working fluid, operating between 75 and 90 K. The inner and outer diameters of the stainless steel capillary tubes are 1.5 and 2 mm respectively. The PHPs were operated at different filling ratios (20 to 90 %), heat input powers (3 to 20 W) and evaporator and condenser temperatures (75 to 90 K). As a result, the PHP with 36 parallel channels achieves a certain level of stability during more than thirty minutes with an effective thermal conductivity up to 200 kW/m.K at 10 W heat load and during forty minutes with an effective thermal conductivity close to 300 kW/m.K at 5 W heat load.

Experimental heat transfer analysis of a cryogenic nitrogen pulsating heat Pipe at various liquid fill ratios

The experimental results of extensive testing of a cryogenic Pulsating Heat Pipe (PHP) using nitrogen as the working fluid operating between 77 K and 80 K is presented. Fifty-two different test runs were analyzed over a range of different fill ratios, heater input powers, and orientations in order to map thermal performance as a function of the system operating parameters. The temperature difference between the evaporator and condenser section were obtained using platinum resistance thermometers (PRTs), and a pressure transducer was used to record the fluid’s pressure oscillations. As opposed to other N2 pulsating heat pipes which use less than 16 parallel tubes, this experimental PHP consists of 40 parallel tubes. The PHP was operated at different liquid fill ratios ranging between 10% and 40% and heat loads between 1 W to 4.5 W. Results show that the PHP’s temperature difference between section are small and resulted in high effective thermal conductivities up to 70,000 W/m/K at fill ratios around 20%. Also, it was shown the PHP stopped oscillating at heat loads below 1.5 W but operated successfully above 2 W. Results from experiments conducted in both horizontal and vertical orientations show that gravity has a significant effect on the thermal performance of the PHP even with a high number of turns.

Experimental investigation on the thermal performance of helium based cryogenic pulsating heat pipe

An experimental investigation was conducted to study the thermal performance of a cryogenic pulsating heat pipe (PHP) filled with helium in terms of thermal resistance, effective thermal conductivity and the maximum transferred power. The stainless steel helium PHP in this paper with an inner diameter of 0.5mm was bent to 8 parallel channels, and the lengths of the evaporator, condenser, and adiabatic section were 50mm, 50mm, and 100mm, respectively. A mechanical–thermal switch worked as a novel pre-cooling system for the helium PHP was developed, which was on during the pre-cooling process and off during the test process. Tests were performed with different heat power, tilt angle and liquid filling ratio. For the helium PHP at +30° with a liquid filling ratio of 70.8%, the effective thermal conductivity was 4800–13,000W/mK, which was higher than that of copper. With the increment of heat power, the thermal resistance decreased at first and then increased, while the effective thermal conductivities were the opposite. The effective thermal conductivity and the maximum transferred power increased with the increment of the tilt angle. In addition, the helium PHP possessed the optimal liquid filling ratio which could make the maximum thermal conducting effectivity, and the optimal liquid filling ratio changed with the heat power. The optimal liquid ratio, by its very nature, meant reasonable proportion of sensible and latent heat.