Visualizing condensation

Niki Ritchey, Engineer, Research

Heptane and octane condense on plain tubes. The video is playing 20 times slower than real time.

A picture is worth a thousand words. At HTRI, the Low Pressure Condensation Unit (LPCU) is supplying us with previously untold stories of vacuum condensation. So what are these images saying? Let’s take a look!

We obtained high-speed videos of shellside vacuum condensation in the LPCU. A binary mixture of heptane and octane condenses around plain ½-inch tubes. At the top of the bundle, we see a fairly calm film of liquid hanging on the tube. Droplets gradually form and swing back and forth in a slow rhythm before raining down on the tubes below.

As we travel down the bundle, the liquid film becomes more hectic and unpredictable. Droplets from above form ripples on the film below. An occasional rogue drop hits the visualization window, and the resultant film slowly slides down. Some droplets disconnect in discrete, round packets while others form a liquid column temporarily connecting two tubes.

The bottom of the bundle is perhaps the most erratic. A constant stream of droplets from above feed into liquid draining below, culminating in a mass exodus of condensate leaving the test section and completing the condensation process. What began as hot vapor has transformed into cool liquid in a stunning display of heat transfer.

The chaotic dance of condensate drainage looks very different from Nusselt’s 1916 model of continuous, laminar sheets of liquid. With this newfound knowledge of reality, we can approach the development of future models with a fresh outlook. At HTRI, we have been able to capture so much activity all condensed in the span of a five-second video.

 

A fun extra, a haiku on condensation:

Hot vapor enters
Droplets fling around wildly
Cool liquid flows out