The Woodruff School

SUMMARY

The divertor, an important plasma-facing component in future long-pulse magnetic fusion energy (MFE) reactors, is vital in sustaining fusion reactions by removing fusion products, impurities, and debris from the core plasma. This thesis focuses on the helium-cooled modular divertor with multiple jets (HEMJ) design. An HEMJ module employs an array of impinging jets to cool the inner surface of an endcap brazed to the plasma-facing surface, a tungsten tile. Originally proposed for the European demonstration DEMO fusion reactor, this concept has been experimentally shown to remove steady-state incident heat fluxes of at least 10 MW/m^2. Individual HEMJ “fingers” are initially assembled into bundles, or units, of nine fingers with a common inlet and outlet to form the divertor with a plasma-facing area of O(100 m^2). To date, most of the studies and models of the HEMJ are based on a single finger, and there are few studies of even a single HEMJ unit. The main objective of this thesis was therefore to evaluate the thermofluids characteristics of a representative HEMJ bundle to verify that the results obtained from a single HEMJ finger can be used for a multi-finger bundle. The outer shell of the HEMJ bundle has a significantly different geometry beyond the endcap region and therefore may have different thermofluids behavior. The experimental studies presented here use a reversed heat flux (RHF) approach, whereby heat is removed (rather than added) at the plasma-facing surface, thereby reducing the operating temperatures for the test section. This approach was first successfully validated in experimental studies of a single HEMJ finger by comparing RHF and normal heat flux tests for dimensionless heat transfer coefficient, or Nusselt number Nu, and dimensionless pressure drop, or pressure loss coefficient K_L. Experimental studies were then conducted on a seven-finger HEMJ bundle where a central finger was surrounded by six outer fingers using the RHF method at the prototypical He pressure of 10 MPa, He temperatures as great as 300 °C and incident heat flux magnitudes as great as 5.9 MW/m^2. For this purpose, a larger helium loop with a mass flow rate as great as 100 g/s was designed and built. The results indicate that the Nu correlation developed from single-finger HEMJ studies is applicable to the seven-finger HEMJ unit at the prototypical mass flow rate of 6.8 g/s. In all cases, the KL results are higher than those for the single-finger HEMJ due in part to the addition of common inlet and outlet chambers. Computational fluid dynamics (CFD) studies of a seven-finger HEMJ unit were carried out with a commercial software package ANSYS and used to design the seven-finger test section and clarify some of the experimental results, including the impact of partial blockage of the jet exit holes on Nu and K_L.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Shekaib Musa

TIME:	Thursday, February 15, 2024, 5:00 p.m.

PLACE:	Love Building, 295

TITLE:	Investigation of a Helium-Cooled Modular Divertor with Multiple Jets Using a Reversed Heat Flux Approach

COMMITTEE:	Dr. Minami Yoda, Co-Chair (ME) Dr. Said I. Abdel-Khalik, Co-Chair (NRE) Dr. Yogendra K. Joshi (ME) Dr. S. Mostafa Ghiaasiaan (ME) Dr. Chris Chungkei Lai (CEE)

SUMMARY The divertor, an important plasma-facing component in future long-pulse magnetic fusion energy (MFE) reactors, is vital in sustaining fusion reactions by removing fusion products, impurities, and debris from the core plasma. This thesis focuses on the helium-cooled modular divertor with multiple jets (HEMJ) design. An HEMJ module employs an array of impinging jets to cool the inner surface of an endcap brazed to the plasma-facing surface, a tungsten tile. Originally proposed for the European demonstration DEMO fusion reactor, this concept has been experimentally shown to remove steady-state incident heat fluxes of at least 10 MW/m^2. Individual HEMJ “fingers” are initially assembled into bundles, or units, of nine fingers with a common inlet and outlet to form the divertor with a plasma-facing area of O(100 m^2). To date, most of the studies and models of the HEMJ are based on a single finger, and there are few studies of even a single HEMJ unit. The main objective of this thesis was therefore to evaluate the thermofluids characteristics of a representative HEMJ bundle to verify that the results obtained from a single HEMJ finger can be used for a multi-finger bundle. The outer shell of the HEMJ bundle has a significantly different geometry beyond the endcap region and therefore may have different thermofluids behavior. The experimental studies presented here use a reversed heat flux (RHF) approach, whereby heat is removed (rather than added) at the plasma-facing surface, thereby reducing the operating temperatures for the test section. This approach was first successfully validated in experimental studies of a single HEMJ finger by comparing RHF and normal heat flux tests for dimensionless heat transfer coefficient, or Nusselt number Nu, and dimensionless pressure drop, or pressure loss coefficient K_L. Experimental studies were then conducted on a seven-finger HEMJ bundle where a central finger was surrounded by six outer fingers using the RHF method at the prototypical He pressure of 10 MPa, He temperatures as great as 300 °C and incident heat flux magnitudes as great as 5.9 MW/m^2. For this purpose, a larger helium loop with a mass flow rate as great as 100 g/s was designed and built. The results indicate that the Nu correlation developed from single-finger HEMJ studies is applicable to the seven-finger HEMJ unit at the prototypical mass flow rate of 6.8 g/s. In all cases, the KL results are higher than those for the single-finger HEMJ due in part to the addition of common inlet and outlet chambers. Computational fluid dynamics (CFD) studies of a seven-finger HEMJ unit were carried out with a commercial software package ANSYS and used to design the seven-finger test section and clarify some of the experimental results, including the impact of partial blockage of the jet exit holes on Nu and K_L.