The growing industry of high-temperature superconducting magnets needs to be accompanied by the development of new cryogenic cooling techniques characterized by low fluid consumption, lightness, and adaptability. For this purpose, cryogenic pulsating heat pipes have been studied over the last decade as possible thermal links generally associated with cryocoolers. This paper presents the thermal performance of a one-meter long horizontal pulsating heat pipe using argon as working fluid. Operating at temperatures between 88 and 110 K, the system is able to transfer up to 25 W of heat load and attains a maximum equivalent thermal conductivity of 85 kW/m.K at 20 W. The experimental results have been compared with previous experiments using nitrogen and neon as working fluids. Analyzing in detail the pressure evolution and temperature distribution of the central capillary tube of the adiabatic part, differences in the thermo-hydraulic behavior of the fluid are observed and can be linked to the thermophysical properties of these three fluids. When operating with neon, the circulation of the fluid in the PHP is more dynamic, ensured by a larger amounts of vapor, while the heat is mainly carried by sensible heat through the liquid parts. With nitrogen, the heat is more transferred by phase change processes than with the two other fluids, which requires a higher filling ratio for the boiling and condensation phenomena leading to the highest filling ratio. The PHP with argon has heat transfer characteristics and filling ratios that lie between the other two fluids due to the combinations of its thermophysical properties, i.e. leading to an intermediate filling ratio but having the slowest fluid motion due to its thermo-physical properties associated with the fluid movement. (c) 2022 Elsevier Ltd. All rights reserved.

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.

Cooling dry superconducting magnets using cryocoolers as a cold source is becoming a standard. Due to their simple operation, compact configuration, lightness, and thermal performance, Pulsating Heat Pipes (PHP) are good candidates, as two-phase thermal links, for these kind of systems. Initially developed to cool small electronic devices at ambient temperature, PHP have been also studied, in the cryogenic field, using N-2, He, Ne and H-2 as working fluids. In this paper, we present the thermal performances of a 1m long horizontal PHP made of 36 stainless steel parallel tubes. The tube internal diameter is 1.5mm, close to the critical diameter (similar to 1.7 mm) to maintain capillary forces necessary to the PHP operation. Both, the evaporator and condenser sections are copper made and are separated by an adiabatic section. This PHP has been studied using N-2 as working fluid with the condenser temperature maintained at 75 K. Several pressure and temperature sensors placed in the three different sections allow monitoring the thermodynamic behavior of the PHP. Tests have been performed increasing and varying the input power as well as keeping it constant during long periods of time. The maximum equivalent thermal conductivity measured is 85 kW/m.K. This system can transfer a maximum heat power of 25 W before reaching its operation limit.

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.