FYFD

Tag: heat transfer

  • Bubbles Sliding

    Bubbles Sliding

    Two-phase flows involve more than one state of matter – in this case, both gas and liquid phases. Flows like this are common, especially in applications involving heat transfer. In some heat exchangers, bubbles will rise and then slide along an inclined surface, as shown above. The motion of these bubbles is a complicated interplay between the surface, the bubble, and the surrounding fluid. The bubble’s wake, visualized here using schlieren imaging, is unsteady and turbulent. Although the bubble oscillates in its path, the wake spreads even wider, and its turbulence stirs up the liquid nearby, increasing the heat transfer. (Image and research credit: R. O’Reilly Meehan et al., source)

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    Staying Cool in the Outback

    Daytime temperatures in the Australian outback can soar, creating a harsh environment for life. Red kangaroos use several methods to regulate their body temperature during the hottest part of the day. They shelter under trees to escape the sun, they dig away the solar-heated topsoil and flop down in cooler soil, and they lick their forearms. Like our wrists, kangaroo forearms have a network of blood vessels near the surface. As their saliva evaporates, it cools the skin and the blood vessels beneath it. Humans are cooled the same way when our sweat evaporates, but a more kangaroo-like trick for cooling off is running cold water over your wrists. (Video credit: BBC/Planet Earth)

  • Pouring Molten Aluminum on Dry Ice

    Pouring Molten Aluminum on Dry Ice

    What happens when you pour molten aluminum on dry ice? As the Backyard Scientist shows, you get what looks like slippery, sliding, boiling metal. In fact, what you see may remind you of the Leidenfrost effect, where a liquid can slide around over an extremely hot surface on a thin film of its own vapor. Despite the opposite temperature extremes–this is a very cold surface rather than a very hot one–a very similar thing is happening here. The molten aluminum is so much hotter than the dry ice that it causes the dry ice to sublimate, releasing gaseous carbon dioxide that the aluminum slides around on. For the same reason, the aluminum appears to boil in the bottom animation. What we’re really seeing is carbon dioxide gas rising and escaping the aluminum so violently that it carries some of the metal with it. Be sure to check out the full video for more awesome physics!  (Image credit: The Backyard Scientist, source; via Gizmodo)

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  • Boiling Water to Snow

    Boiling Water to Snow

    When it’s really cold outside–to the tune of -40 degrees (Fahrenheit or Celsius)–physics can get a little crazy. In this photo, boiling-hot water from a thermos turns into an instant snowstorm when tossed. How is this possible? It turns out there are a combination of factors that affect this. Firstly, the rate of heat transfer between two objects depends on the magnitude of the temperature difference between them. The bigger the difference in temperature, the faster the hot object cools. Of course, as the hot object cools down, the temperature difference between it and its surroundings is smaller and the rate of heat transfer decreases.

    The second important factor here is that the water is being tossed. When you throw water, it breaks into droplets, and droplets have a large surface area compared to their volume. As it turns out, the rate of heat transfer also depends on surface area. By breaking the hot water into smaller droplets, you increase the surface area exposed to the cold air, allowing the hot water to freeze faster. (Image credit: M. Davies et al.; via Gizmodo)

    Also: Since there are a few events scheduled around the country over the next couple months, I’ve added an events page where you can find details for those appearances. And as always, if you’re interested in scheduling a talk or event, feel free to contact me directly.

  • Penguins Can Be Colder Than Their Surroundings

    Penguins Can Be Colder Than Their Surroundings

    Thermal imaging of emperor penguins in Antarctica shows that, in still conditions, large portions of their bodies remain colder than ambient temperatures. In the image above, the heads, beaks, eyes, and flippers of this pair of penguin are the warmest while much of their feathered surface remains several degrees colder than the temperature around them. Not only does this indicate that the penguins’ skin and feathers are extremely effective insulators–the core temperature of each penguin is roughly the same as a human’s–but it means that the penguins are losing heat via radiative cooling toward the sky, the same way your car does when frost forms. The measurements in the study are for penguins at least one body length away from any other penguins; of course penguins typically huddle together to generate additional warmth. The mathematics of this behavior are under active research. (Photo credit: D. McCafferty et al.; via Wired)

  • Reader Question: Frosty Cars

    Reader Question: Frosty Cars

    Reader Mike L asks:

    Why do I never see frost on my car when I park in a detached garage or under a carport?

    Great question! Frost forms on surfaces when their temperature drops below the freezing point of water and the dew point of the surrounding air. The water vapor in the air gets deposited as a solid directly; this is called deposition. This means that the surface–in this case your car–has to be colder than the nearby air. Neither conduction nor convection of heat between your car and the surrounding air can cause this drop; heat transfer between your car and the surrounding air would tend to make them the same temperature, not make the car colder than the air. The third–and typically least effective–type of heat transfer, radiation, is the answer because it allows heat transfer between two objects that are not in direct contact like the air and car are.

    Frost typically forms on still, clear nights with little clouds or wind. A car sitting beneath a clear night sky will radiate heat out into space. Since space is much, much colder than the air, this radiation cooling to space allows the car’s surface temperature to drop below that of the surrounding air, which is not a good radiator by comparison. On a night with little wind (and thus little convection), this radiation cooling can be quite effective. Frost will tend not to form on one’s car under a carport because the car is sheltered from the night sky, blocking such radiative cooling. Having a tree or house blocking the car from the night sky is also effective at preventing frost formation. (Photo credit: N. Sharp; with thanks to Keri B and Jerry N for the meteorological assistance)

  • Reader Question: Snow from Boiling Water?

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    Reader kylewpppd asks:

    Have you seen the post of a man in Siberia throwing boiling water off of his balcony? Can you provide a better explanation of what’s going on?

    As you can see in the video (and in many similar examples on YouTube), tossing near boiling water into extremely cold air results in an instant snowstorm. Several effects are going on here. The first thing to understand is how heat is transferred between objects or fluids of differing temperatures. The rate at which heat is transferred depends on the temperature difference between the air and the water; the larger that temperature difference is the faster heat is transferred. However, as that temperature difference decreases, so does the rate of heat transfer. So even though hot water will initially lose heat very quickly to its surroundings, water that is initially cold will still reach equilibrium with the cold air faster. Therefore, all things being equal, hot water does not freeze faster than cold water, as one might suspect from the video.

    The key to the hot water’s fast-freeze here is not just the large temperature difference, though. It’s the fact that the water is being tossed. When the water leaves the pot, it tends to break up into droplets, which quickly increases the surface area exposed to the cold air, and the rate of heat transfer depends on surface area as well! A smaller droplet will also freeze much more quickly than a larger droplet.

    What would happen if room temperature water were used instead of boiling water? In all likelihood, a big cold bunch of water would hit the ground. Why? It turns out that both the viscosity and the surface tension of water decrease with increasing temperature. This means that a pot of hot water will tend to break into smaller droplets when tossed than the cold water would. Smaller droplets means less mass to freeze per droplet and a larger surface area (adding up all the surface area of all the droplets) exposed. Hence, faster freezing!

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    Peering Inside the Kettle

    Here natural convection is explored experimentally in a quasi-2D environment. The researchers demonstrate how this phenomenon, which is much like that seen in a boiling pot, can be investigated by measuring the refractive distortions caused by the thin heated fluid layer. They also demonstrate types of boiling that can occur.  Typically, bubbles nucleate at the heated surface and then rise to pull hot fluid with them.  At high enough temperatures above the liquid’s boiling point, however, an unstable layer of vapor can form over the heated surface.  This “boiling crisis” or critical heat flux produces a marked reduction in heat transfer due to the insulation provided by the vapor layer. (Video credit: S. Wildeman et al.)

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    Creating Lava

    In Syracuse, NY, artists and scientists work together to study volcanic flows by melting crushed basalt in a special furnace before releasing the lava into the parking lot.  This particular flow is very prone to boiling behavior, likely because of the cold air and ground temperatures (less than 0 C).  The outer layers of rock cool quickly, leaving bubble-shaped chambers which hotter lava can fill before melting out. (via It’s Okay To Be Smart; submitted by @jpshoer)

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    New CPU Fan

    This video discusses a new quieter and more efficient CPU fan developed by engineers at Sandia National Labs. As the impeller spins, it draws ambient air down the center of the impeller while the shape of the fins forces air past the fins and out radially. As the air flows over the fins, it draws heat from the CPU away. In a sense, the design combines a heat sink with a fan. (Video credit: Sandia National Labs; submitted by Adam L)