Transient thermal behavior of a neon Pulsating Heat Pipe (PHP)

INTRODUCTION R ECENTLY, severals experiments have been performed to characterize pulsating heat pipes connected to a cryocooler to cool down superconducting magnets. These kind of devices work with different orientations, without gravity, require small amount of fluid and the distance between the hot and the cold part can be relatively long. This last characteristic is interesting because the performances of cryocoolers tend to decrease in presence of high magnetic fields [1]–[3]. The PHP devices, invented by Akachi in 1990 [4], are composed of one single capillary tube, bent in many turns and fixed to the hot and cold section of the device. As the fluid is close to the saturation state in the PHP, it vaporizes in the evaporator section and liquefies in the condenser. In normal operation, a train of alternating liquid slug and vapor bubbles surrounded by liquid film (cf. Fig. 1) is created in the PHP. The expansion and compression of the vapor bubbles are responsible of the flow oscillating movement whereas, the liquid parts carry the heat from the hot to the cold part. Many experiments, focused on the cooling of superconducting magnets [5]–[10], have been performed at cryogenic temperature using mostly nitrogen, neon or helium as working fluids. The operating temperatures with neon are between 26-32 K. Mito and al. [5] has tested a closed loop vertical PHP, 160 mm long, composed of 10 parallel tubes with an inner diameter of 0.78 mm and the performances achieved during the tests are about 8000 W/m.K with filling ratio between 31% and 70%. The same PHP achieved 18000 W/m.K with nitrogen as working fluid. Liang and al. [10] has developed a 700 mm long vertical neon PHP (12 parallel tubes and 1 mm inner diameter) with a maximum equivalent thermal conductivity of 30000 W/m.K. Our laboratory has made, for the European SR2S project, a test bench to study pulsating heat pipes to cool down future massive superconducting magnet (about 10 m diameter and 20 m long), designed to protect astronauts from cosmic rays in deep space [11]. A long PHP configuration composed of 36 parallel tubes filled with nitrogen has been studied horizontally to perform the experiments with the closest conditions to space (zero gravity). The 3.6 m long PHP shows interesting stable thermal performances (equivalent thermal conductivity about 300 kW/(m.K)) during 40 minutes after the insertion of the working fluid, then the evaporator temperature increases [12]. On the contrary, the 1 m long PHP transfers up to 25 W with the temperature of the evaporator oscillating around a stable value [13]. Above this heat load, the oscillating flow stops inside the PHP and the heat is no longer transferred. When these systems reach their limit (dry out phenomena), some vapor bubbles are no longer surrounded by the wall liquid film and the performance of the PHP decreases [14], [15]. The same PHP, with neon as working fluid [16], can transfer 50 W of heat load from the hot to the cold part with an equivalent thermal conductivity about 65000 W/(m.K). During the quench of a superconducting magnet, an extra amount of energy needs to be extracted by the cryogenic system in a short period of time. Despite the important number of experiments on the thermal characteristics of PHP, only one study [17] has presented transient thermal behavior of these devices when high heat loads (above stable conditions) have been delivered on the evaporator section of a horizontal PHP. Using nitrogen as working fluid, the PHP does not retrieve its former thermal performances (the equivalent thermal conductivity is twice as small). Due to the lack of transient heat transfer data, this paper reports transient experiments, performed with neon in a 1 m long horizontal PHP, with a sudden high increase of the heat load on the evaporator part. It presents the thermal behavior of the PHP after applying those extreme conditions and the dryout evolutions.