Schlieren Studies of the Surface Sub-layers

A cryogenic Schlieren system shown in Fig. 4.5 was used in conjunction with a TV camera and video recorder to observe the convective motion within the surface sub-layer directly. 

A parallel beam of white light from a point source is directed onto a stainless steel mirror submerged 10-30 nun deep in the cryogenic liquid, i.e. sufficiently deep to be clear of both the surface evaporation sub-layer and the radial inflow across the surface of superheated liquid from the wall heated boundary flow. The reflected beam of white light passes out and is focussed on to a viewing screen or camera, after passing a knife-edge to remove the undeviated beam. 

Bearing in mind that the vertical-axis Schheren optics detect horizontal gradients in refractive index, which relate directly to changes in density produced by horizontal gradients in temperature, local convection patterns are revealed in extraordinary detail. 

Combining these findings with the intermittent cold and hot temperature spikes discovered with the fixed micro-thermometers, it is concluded that flie intermittent convection is co-operative in nature. Indeed, the Schlieren pictures show patterns of convection lines which are similar to those observed during extensive studies of the surface evaporation of water by Rayleigh-Benard convection [14,15]. 

In conventional Benard convection [14], flie liquid surface is stationary in the laboratory frame of reference, and hence the cells are stationary, and are observed as a regular array of hexagonal convection or Benard cells. In each cell, hot fluid rises in the centre and cold fluid falls along its boundary (see Fig. 4.7). 

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