Experimental and CFD analyses of a thermal radiation shield dimple plate for cryogenic pump application

M Scannapiego,C Day

doi:10.1088/1757-899x/101/1/012068

M Scannapiego, C Day

Open Access

https://doi.org/10.1088/1757-899x/101/1/012068

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Abstract

Large customized cryogenic pumps are used in fusion reactors to evacuate the plasma exhaust from the torus. Cryopumps usually consist of an active pumping surface area cooled below 5 K and shielded from direct outer thermal radiation by plates cooled at 80K. In nuclear fusion applications, cryopumps are exposed to excessively high heat fluxes during pumping operation, and follow-up regeneration cycles with rapid warm-up and cool-down phases. Therefore, high cryogenic operational mass flows are required and thus pressure drop and heat transfer characteristics become key issues for the design of the pump cryogenic circuits. Actively cooled dimple plates are a preferred design solution for the thermal radiation shield. A test plate with a rhomb pattern of dimples has been manufactured and tested in terms of pressure drop with a dedicated test facility using water. In the present work, computational fluid dynamics (CFD) models of the test dimple plate have been performed, and computed pressure drops have been compared to experimental results. Despite the complexity of the geometry, a good agreement with the experimental results was found. Then, the validated CFD approach has been further extended to relevant operation conditions, using gaseous helium at cryogenic temperature as working fluid. The resulting pressure drop and heat transfer characteristics are finally presented.

Highlights

In order to maintain the required pressure levels in the torus plasma chamber of a thermonuclear fusion reactor and to provide a good energy confinement of the plasma, very high gas throughputs have to be processed with a powerful and high speed cryogenic pumping system
The low-Reynolds formulation is used for mesh points located in the viscous sublayer to solve the details of near wall region with fine near wall grids, which is recommended for accurate pressure drop and heat transfer prediction
The computational fluid dynamics (CFD) model could not be validated for heat transfer coefficients against experimental data, but we are confident that the accurate results obtained for the pressure drop calculations provide a good indicator of a well resolved boundary layer, and an accurate heat transfer prediction as well

Summary

Introduction

In order to maintain the required pressure levels in the torus plasma chamber of a thermonuclear fusion reactor and to provide a good energy confinement of the plasma, very high gas throughputs have to be processed with a powerful and high speed cryogenic pumping system. The objective of the work presented in this paper was to gain confidence in the ability of a CFD approach to simulate with a satisfactory accuracy flows in this kind of components, before eventually extending the method to other cryopump components. For this purpose, CFD simulations of the test dimple plate are presented and the computed pressure drops compared to the experimental results in order to validate the model. The tried and tested CFD method with water is further extended to relevant operation conditions, using helium at cryogenic temperature as working fluid

Pressure drop test facility and test dimple plate

CFD model of the dimple plate

Computational grid

Hydraulic resistance and convective heat transfer characterization

Findings

Conclusion and perspective