Liquid Protection Schemes for Inertial Fusion Energy

 

In inertial fusion energy (IFE), a specially designed target (usually a few mm in diameter) containing deuterium and tritium fuel is heated and compressed in a reactor chamber (typically with a radius of 4-6 m) by more than 100 laser or heavy-ion beams to the temperatures and pressures required to create fusion. The lead American IFE research facility, the National Ignition Facility in Livermore, CA, should demonstrate laser IFE by 2015. The energy released by the fusion explosion consists of neutrons, photons, and charged particles that dissipate their energy in the walls surrounding the reactor cavity, eroding the reactor chamber first wall. Various types of molten-salt or liquid-metal films or curtains have been proposed to absorb this radiation and reduce material degradation of the chamber first walls. By increasing chamber lifetime and reducing chamber size, such liquid protection schemes will help make IFE commercially feasible, reducing the capital and operations costs for an IFE power plant.

Our work focuses on the basic “building block flows” for two types of liquid protection schemes: 1) Thick liquid protection, where high-speed liquid curtains, or turbulent liquid sheets, are used to absorb virtually all the radiation from the fusion event; and 2) Thin liquid protection , where liquid films attached to the reactor walls (created by low-speed injection of liquid through a porous wall and high-speed injection of liquid tangential to the reactor walls), are used to absorb photons and charged particles. The important design issues for all liquid protection schemes are:

  • Provide robust coverage and hence effective protection of the entire chamber first wall
  • Avoid formation of droplets or vapor that can interfere with target injection and target ignition
  • Minimize geometric, or free-surface, fluctuations in these flows, since surface ripple can also interfere with target injection and ignition

Thick Liquid Protection

The HYLIFE-II conceptual IFE power plant design proposes using a combination of oscillating and stationary turbulent sheets of a molten salt, Flibe (Li2BeF4) at a Reynolds number Re = 240,000 to protect the reactor chamber first walls from damaging radiation and charged particles. We are experimentally modeling these flows using sheets of water issuing from a rectangular exit into atmospheric pressure air at Re up to 150,000 (Figure 1). A variety of flow diagnostics are employed to characterize this flow including. laser-Doppler velocimetry (LDV), planar laser-induced fluorescence (PLIF), flow visualization, and droplet collection.

 

Figure 1 Photo of turbulent sheet of water at Re = 34000 issuing into air from a nozzle with exit dimensions of 10 cm x 1 cm.  Flow is in x-direction.


Figure 2 Direct imaging of the free surface of the turbulent liquid sheet using the PLIF technique.  Resulting images show cross-sectional shape of the jet.

 

Figure 3 Determination of initial conditions of the turbulent liquid sheet using Laser Doppler Velocimetry (LDV).  The laser lines are simulated in this figure.

The objectives of this research involving fluid mechanics, fusion engineering and optical diagnostics are:

  • To experimentally characterize turbulent liquid sheets at high Re : How are the cross-sectional shape and free-surface fluctuations in these flows (Figure 2) affected by initial conditions (Figure 3) such as nozzle geometry (Figure 4) and flow conditioner design and flow parameters such as Re and Weber number We? How is droplet formation (Figure 5) and subsequent breakup of the liquid sheet affected by initial conditions and flow parameters?
  • Use this knowledge to create smooth, stable turbulent liquid sheets

 

Three Nozzles

 

Figure 4 Photo of three different nozzles (all with exit dimensions 10 cm x 1 cm) fabricated using stereolithography rapid prototyping: from left to right, a matched circular-arc contraction, a fifth-order polynomial contraction, and a fifth-order polynomial contraction with rounded corners.

Figure 5 Edge view of turbulent liquid sheet.  Droplets appear to emanate from the free surface.  Exposure time for this image is 5 ms.  The streaks are due to the droplet ejection speeds and trajectories. 

 

 

 

Thin Liquid Protection

The Prometheus conceptual IFE power plant design proposes using a film of liquid lead to protect the reactor chamber first walls from x-rays and charged particles. At the top of the reactor chamber, which consists of a cylinder with hemispherical endcaps, liquid is injected through slots at high speed tangential to the first wall surface, while liquid is injected at low speed through a porous wall normal to the first wall surface over the rest of the reactor chamber. We are experimentally studying turbulent liquid films on downward facing surfaces, or the underside of planar surfaces at 0 to 45 degrees below the horizontal. We are also using the finite-volume level contour reconstruction method to numerically simulate the flow through a downward-facing porous planar surface 0 to 45 degrees below the horizontal , and carrying out experimental studies with water validate these simulations.

The objectives of this research involving fluid mechanics, fusion engineering and heat transfer are:

  • To experimentally characterize turbulent liquid films using flow visualization and laser-Doppler velocimetry (LDV) at Re up to 20,000 on downward-facing surfaces: How are film detachment and droplet formation from the downward-facing free surface (Figure 4) affected by initial conditions including slot aspect ratio and flow parameters such as Re , We and Froude number Fr? What is the film thickness as a function of initial conditions and flow parameters? How does this film flow around cylindrical obstructions typical of (ignition) beam entry ports?
  • To numerically and experimentally study the “wetted wall” concept, or the flow of liquid slowly injected through a porous wall normal to the surface: How is droplet formation affected by initial conditions including wall porosity and initial film thickness and flow parameters such as Re , We and Fr (Figure 5)? What is the volume of detached fluid? What is the film thickness as a function of initial conditions and flow parameters?

 

Liquid film

 

Figure 4 Side view of water film (flowing from left to right) at Re = 9000 on the underside of a horizontal glass plate. The scale at the top is in inches measured from the slot exit.



Drop formation


Figure 5 Time sequence from a finite-volume numerical simulation showing drop formation for liquid lead at 700 deg C with an initial film thickness of 0.5 mm injected with a speed of 1 mm/s down through the underside of a horizontal porous plate. The ratio of the liquid to surrounding gas density is essentially infinite. Time increases from left to right, with the first drop forming 0.31 s after the start of injection. [simulation courtesy S. Shin]


This project, a collaboration with S. Abdel-Khalik and D. Sadowski in Mechanical and Nuclear Engineering, is supported by the Department of Energy through the Office of Fusion Energy Sciences and the ARIES-IFE study.  Financial support for S. Durbin is provided by the Fusion Energy Sciences Fellowship Program administered by Oak Ridge Institute for Science and Education under a contract between the U.S. Department of Energy and the Oak Ridge Associated Universities.


Publications (contact M. Yoda for reprints)

  1. S.G. Durbin, T.P. Koehler, J.R.R. Reperant, M. Yoda, S.I. Abdel-Khalik, and D.L. Sadowski “Surface Fluctuation Analysis for Turbulent Liquid Sheets,” to appear in Fusion Science and Technology (2004)
  2. S.G. Durbin, M. Yoda, S.I. Abdel-Khalik and D. L. Sadowski “Turbulent liquid sheets for protecting IFE reactor chamber first walls.” Fusion Science and Technology 44 (2,II), 307-311 (2003)
  3. J. K. Anderson, S. G. Durbin, D. L. Sadowski, M. Yoda and S. I. Abdel-Khalik “Experimental studies of high-speed liquid films on downward facing surfaces.” Fusion Science and Technology 43 (3,III), 401-407 (2002)
  4. S. Shin, F. F. Abdelall, S. I. Abdel-Khalik, M. Yoda and D. L. Sadowski “Fluid dynamic aspects of the porous wetted wall protection scheme for IFE reactors”. Fusion Science and Technology 43 (3,III), 366-377 (2002)
  5. J. J. R. Reperant, S. G. Durbin, M. Yoda, S. I. Abdel-Khalik and D. L. Sadowski “Studies of turbulent liquid sheets for protecting IFE reactor chamber first walls”. Fusion Engineering and Design 63-64, 627-633 (2002)
  6. J. A. Collins, M. Yoda and S. I. Abdel-Khalik “Direct measurements of free-surface smoothness in turbulent liquid sheets”. Fusion Technology 39 (2,II), 721–725 (2001)
  7. L. C. Elwell, D. L. Sadowski, M. Yoda and S. I. Abdel-Khalik “Dynamics of oscillating turbulent liquid sheets”. Fusion Technology 39 (2,II), 716–720 (2001)