双螺杆挤出机发展

TWIN-SCREW EXTRUDER DEVELOPMENT FOR THE ITER PELLET INJECTION SYSTEM
S. J. Meitner, L. R. Baylor, S. K. Combs, D. T. Fehling, J. M. McGill, D. A. Rasmussen
The ITER pellet injection system is comprised of devices to form and accelerate pellets, and is connected to inner wall guide tubes for fueling, and outer wall guide tubes for ELM suppression [1,2,3]. The ITER plasma fuel is injected as solid hydrogen isotope pellets. The ITER pellet injection system will need to provide a solid extrusion rate of ~1500 mm3/s
J. W. Leachman
Mechanical Engineering Department University of Wisconsin - Madison Madison, WI USA
(1800 mbar-L/s) at durations of up to 3000 s [4]. The pellets used to trigger ELMs are to be injected at high frequencies >15 Hz. A method of producing fuel pellets approaching the ITER requirements with a single screw extruder has proven feasible [5,6]. The extruder provides a stream of hydrogen isotopes to a secondary section of the pellet injection system, where cylindrical pellets are punched/formed and accelerated in a gas gun with room temperature propellant gas into the plasma. A novel method of using a twin screw extruder, cooled by supercritical helium, has been proposed [2] to utilize the advantages of the twin-screw extruder. Twin-screw extruders used in the food industry, when compared to single-screw extruders, offer the proven advantages of positive displacement pumping without relying on wall friction to move the extrusion through the screws and reduced pulsation at the die [7]. A one-fifth ITER scale prototype has been built at the Oak Ridge National Laboratory [8] which has demonstrated the production of a continuous solid deuterium (D2) extrusion. Design improvements have been made to the pre-cooler and liquefier heat exchangers, and to limit the loss of extrusion through gaps in the screws. II.
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Fusion Energy Division Oak Ridge National Laboratory Oak Ridge, TN USA
Abstract— The ITER pellet injection system is comprised of devices to form and accelerate pellets, and will be connected to inner wall guide tubes for fueling, and outer wall guide tubes for ELM pacing. An extruder will provide a stream of solid hydrogen isotopes to a secondary section, where pellets are cut and accelerated with a gas gun into the plasma. The ITER pellet injection system is required to provide a plasma fueling rate of 120 Pa-m3/s (900 mbar-L/s) and durations of up to 3000 s. The fueling pellets will be injected at a rate up to 10 Hz and pellets used to trigger ELMs will be injected at higher rates up to 20 Hz. A twin-screw extruder for the ITER pellet injection system is under development at the Oak Ridge National Laboratory. A one-fifth ITER scale prototype has been built and has demonstrated the production of a continuous solid deuterium extrusion. The 27 mm diameter, intermeshed, counter-rotating extruder screws are rotated at a rate up to ≈5 rpm. Deuterium gas is pre-cooled and liquefied and solidified in separate extruder barrels. The precooler consists of a deuterium gas filled copper coil suspended in a separate stainless steel vessel containing liquid nitrogen. The liquefier is comprised of a copper barrel connected to a Cryomech AL330 cryocooler, which has a machined helical groove surrounded by a copper jacket, through which the pre-cooled deuterium condenses. The lower extruder barrel is connected to a Cryomech GB-37 cryocooler to solidify the deuterium (at ≈15 K) before it is forced through the extruder die. The die forms the extrusion to a 3 mm x 4 mm rectangular cross section. Design improvements have been made to improve the pre-cooler and liquefier heat exchangers, and to limit the loss of extrusion through gaps in the screws. This paper will describe the design improvements for the next iteration of the extruder prototype. Keywords-ITER; twin-screw extruder; pellet injection
HEAT EXCHANGER IMPROVEMENTS
I.
INTRODUCTION
A. Improved Pre-Cooler Design The pre-cooler is used to reduce the temperature of the incoming fuel gas (295 K) to 80 K before it is passed on to the liquefier. If the exhaust temperature of the D2 from the precooler does not reach 80 K, additional cooling load is placed on the liquefier and solidifying barrel, exceeding their cooling capacity. Design changes to the pre-cooler were implemented to improve the heat exchanger performance. The previous precooler design consisted of a copper reservoir containing LN2, surrounded by a D2 trace copper coil [8]. The redesigned pre-cooler now contains the D2 copper coil inside the reservoir in a LN2 bath. The copper coil diameter was reduced from 12.7 mm to 6.35 mm for ease of coiling. The required tube length was determined by analyzing a differential control volume on the gas travelling in the xdirection, shown in Fig. 1, as outlined by Nellis and Klein [9].