Everyday thermals: boiling pot of water

Calibrating our thermal vision

With a thermal imaging camera, we'll break down each step of boiling water. A thermal imaging camera outputs a color image representing different temperatures. In this thermal contour scale, we have dark blues as the coolest temperatures, to greens, yellows, orange, reds and up to white as the hottest temperatures. Here is our empty aluminum pot that we’re going to fill with water. It’s sitting on a glass top electric stove in the kitchen. This is what we will consider our baseline.

Notice we have a few hot spots before we add water or turn on the heat. Since thermal cameras typically adjust their color range to whichever temperatures it sees, there isn’t much temperature difference between these objects. Unfortunately, the images from our thermal camera don’t save with the whole scale, so each image shows the whole color scale relative to the temperatures in that specific instant.

So what’s going on with these hot spots when we haven’t even turned on the stove?

Radiation

Next we have the two bright spots on the pot that has no heat on it yet. The bright yellow spot is actually a reflection of light and heat coming in from the kitchen window. And the yellow green spot? That’s our heat reflection. Since thermal energy can radiate, it acts just like light. If there are shiny surfaces, like a shiny pot, it will reflect the heat radiation.

Time to add some water and turn on the stove. Here we've added enough water to fill the pot halfway. Again in this image we see the LED of the stove and the electrical outlet on the top of the image. Notice how they’re much more subdued in this image than the first one. The relationship between the temperature of the stove and those things has dramatically changed and the colors of the thermal image updated to display that. We can also tell, based off the gradual change from mid green to blue and darker blue, there’s conduction bringing heat up along the sides of the pot.

We see the heat generated by the electric stove and how some of it’s conducting up and away from the bottom of the pot. We can also see some of the thermal radiation being reflected out from the inside of the pot in the green area. As the stove top gets hotter, black splotches start showing up on the hottest areas. This is an artifact of the thermal camera since the temperature in those areas are too hot for the camera to measure. For our camera that limit is 302.4°F or 150°C.

Convection

We’ve seen radiation and conduction, so let’s find some convection. If we watch the pot closely, we can see some vapor forming before it starts to boil. As a side note, those blue dots in the pot are where the handles are riveted into the pot. Those are a little cooler than the rest of pot since those handles essentially act like heat sinks, pulling heat away from the side walls of the pot.

A rolling boil

Once the pot starts boiling, more vapor stops rolling inside the pot and starts rising up and out of the pot. There’s generally a central plume of hot air rising, but it’s behavior is fairly unpredictable. Natural convection is heavily dependent on surrounding conditions. Small variations in the ambient air can have a drastic effect on how the central plume of vapor moves.

Learn more about the Fundamentals of heat transfer or contact our team to help you with your heat transfer challenges.