Flow boiling over modified surfaces in mini channels

Contact Person: M. Vlachou [marvlach@gmail.com]
 

The present project is dealing with flow boiling in a single rectangular channel. The main objectives of the current work are (a) to understand the phenomena that govern flow boiling in a small channel with a hydraulic diameter of a few millimeters and (b) to determine the experimental conditions that will allow currying away high heat fluxes. For this reason the effect of the following parameters on flow boiling heat transfer characteristics are investigated:

  • channel’s geometrical characteristics (height)
  • channel’s orientation (0-360º)
  • heat flux
  • mass flow rate
  • topography of the heated surface (plain& treated)

Experiments are conducted with subcooled water (Twater, inlet <Tsaturation) and take place in both terrestrial (AUTH) and hypergravity conditions (Large Diameter Centrifuge at ESA/ESTEC). The experimental setup used to for the experiments at AUTh is shown in figure 1, while the experimental setup placed inside the gondola of the LDC is shown in figure 2.

Figure 1: Test section at AUTH

Figure 1: Test section at AUTH

Figure 2: Large diameter centrifuge and experimental device at the LDC.

Figure 2: Large diameter centrifuge and experimental device at the LDC.

The parameters that are monitored during the experiments are:

  • Working fluid flow rate
  • Working fluid temperature (inlet: one position and outlet: two positions)
  • Heated surface temperature (six positions)
  • Pressure in the test section (inlet and outlet)
  • High speed video recordings (side and top of the channel at 8000 & 1000 fps respectively)
  • Infrared images (test section)
  • Void fraction of the two phase exit flow by a novel technique, IVED (In-Vivo Embolic Detector, ESA Project).

Surface modification techniques (i.e. laser engraving) are applied in order to increase flow boiling heat transfer coefficient.

Summary
Working conditions such as channel size, channel orientation, mass flow rate, hypergravity and surface topography affect heat transfer performance, as expected. Assessing bubble dynamics will provide the necessary evidence to explain the behavior of the heat transfer mechanisms during those changes.

Figure 3: Effect of (a) inclination and (b) channel size on boiling curves at terrestrial gravity (same mass flow rate).

Figure 3: Effect of (a) inclination and (b) channel size on boiling curves at terrestrial gravity (same mass flow rate).

Figure 4: Effect of increased gravitational acceleration on boiling curves at (a) horizontal and (b) vertical orientation (same mass flow rate).

Figure 4: Effect of increased gravitational acceleration on boiling curves at (a) horizontal and (b) vertical orientation (same mass flow rate).