Jake Fisher is a legendary member of the HYPER lab because…
he accomplished the improbable.
To paraphrase one of his customers during his thesis defense, “Jake, when you started this project we didn’t think it was possible. Not only have you succeeded, you’ve given us valuable measurements to help us out.”
Jake was the first graduate student in the HYPER lab back in 2010. He inherited an incredible challenge: build the world’s first twin-screw solid hydrogen extruder that can measure inter-screw extrudate temperature. Suffice it to say, cryogenic systems of this magnitude are incredibly complex. This was a lesson Jake learned during two of his summer internships with the Pellet Fueling of Fusion Plasmas group at ORNL of Larry Baylor, Steve Combs, and Steve Meitner.
The build started from scratch by designing and commissioning our first cryogenic vacuum chamber (a.k.a. cryostat).
After several years worth of meticulous instrumentation and construction, we had a working Diagnostic Twin Screw Extruder (DTSE).
Jake’s resulting dissertation provided the first measurements showing that interscrew extrudate temperatures did not elevate significantly above the bulk material temperatures and identified temperature thresholds below which stable extrusion occurs.
Thanks for the quality time and work Jake. Good luck at Aerojet-Rocketdyne!
My Story:
In the first semester of graduate school at WSU I attended a seminar and part of it was about twin screw extrusion of solid hydrogen. I am a supporter of alternative energy so to hear about fusion energy and how twin screw extrusion can help really resonated with me. After the seminar I contacted the presenter Dr. Jacob Leachman and told him I wanted to research twin screw extrusion for fueling fusion reactors. He became my adviser and we began designing and building the diagnostic twin screw extruder experiment. He was a new faculty member with an empty lab and I had experience in running a lab from my time at the Institute for Shock Physics so I helped set up Dr. Leachman’s HYPER lab. I designed the extruder, worked with the College of Engineering and Architecture machine shop to build many of the components, and machined several of the components myself. I assembled and tested the extruder with nitrogen. Then I operated it with neon, hydrogen, and deuterium. The data has been extremely insightful and has led to discoveries of operating conditions that haven’t been seen before. Now I am working on a mathematical model of the extruder that fusion scientists can use to design twin screw extruders to meet the fueling needs of fusion reactors.
My Research: Diagnostic Twin Screw Extruder for Fueling Fusion Tokamaks
A prototype hydrogenic twin screw extruder has demonstrated the feasibility of producing fuel for fusion tokamaks however for it to be used on the ITER tokamak it will have to produce 0.33 g/s throughput at 99.9% reliability. Throughput instability and unknown scaling factors of the prototype extruder has prompted the design and construction of the Diagnostic Twin Screw Extruder (DTSE) to measure fundamental parameters and develop a predictive throughput model to insure the next generation extruders can meet ITER requirements. The ITER tokamak is an experimental reactor being built to exhibit fusion technology needed for commercial power plants. Fusion power plants have the potential to meet the future world energy demand without producing green house gases or long lived radioactive waste.
Hydrogenic twin screw extruder operation has been modeled previously by Dr. Jacob Leachman using numerical heat transfer methods to predict the temperature distribution. This model assumed a throughput efficiency of 100% meaning backflow was not taken into account. Backflow or leakage flow has been identified as a major proponent of throughput instabilities and reduces extruder throughput to zero in some cases. Backflow in twin screw extruders has been modeled in polymer literature with work done by L.P.B.M. Janssen who indicated four clearance gaps as areas of leakage flow. However the equations developed for flow through these gaps do not directly translate to hydrogenic material because polymer melts have different flow properties. Rheology studies on hydrogenic material have been done by J.W. Leachman which produced the temperature and shear rate dependence of shear stress; a thermophysical property needed for heat transfer and fluid flow modeling. The literature on hydrogenic twin screw extrusion is short: only two twin screw extruders have been built at Oak Ridge National Lab to experiment with producing solid hydrogenic material. Neither device was instrumented to record screw temperature distributions need for model validation.
A finite volume numerical model combining heat and mass transfer has been developed to predict the throughput of hydrogenic twin screw extruders. The model expands Leachman’s temperature distribution model to include a mass balance therefore incorporating temperature and leakage flow effects on throughput. The leakage flow equations from Janssen are used in the mass balance and have been modified to better emulate hydrogenic fluid properties based on experimental results from the DTSE and other references. The model can be used to produce plots that compare extruder throughput to screw speed and required cooling power for different extruder geometry. These results can be used by fusion scientists to size and operate twin screw extruders to fuel fusion tokamaks like ITER.
The throughput model will be validated by data taken from experiments on the specifically designed and built Diagnostic Twin Screw Extruder. The DTSE is cooled by a cryogenic refrigerator using a special helium refrigeration cycle to cool the DTSE to -448 °F (6 K). The DTSE is suspended in a custom modular vacuum chamber to eliminate convection heat transfer. Thermal radiation is also eliminated using a special made multi layer insulation blanket wrapped around a copper radiation shield that is actively cooled by the cryogenic refrigerator. Temperature sensors are embedded in the screw threads of both screws to provide the most accurate extrudate temperature measurements of any hydrogenic twin screw extruder. The DTSE also has sensors to monitor torque, speed, and mass flow rate during testing. The DTSE operates by condensing gas to a solid from an external reservoir. A DC motor powers the screws which convey the solid through a barrel and out a nozzle where it melts and the liquid follows a circulation loop back to the top of the extruder. The benefit of a closed loop system is that when a test is complete the solid changes phase back to a gas and returns to the reservoir reducing cost and required cooling over a once-through system.
Thus far the DTSE has extruded solid nitrogen and neon producing torque, speed, and temperature measurements. Preliminary analysis suggests higher temperatures in the intermeshing zone than in the screw channels. If the temperature in the intermeshing zone exceeds the extrudate melting temperature then high leakage flow through the calendaring gap would occur. This helps guide throughput model development by adjusting fluid properties in specific backflow equations. Initial measurements also showed change in extrudate temperature along the barrel being only 2 K suggesting higher heat transfer than predicted. Further tests are being conducted on neon, deuterium, and hydrogen with the goal of collecting data over a wide extruder operating range. The data will improve the accuracy of the throughput model and allow it to predict deuterium-tritium extrudate, the fuel to be used in the ITER tokamak.