Featured Research

from universities, journals, and other organizations

How to tame hammering water droplets

Date:
January 23, 2011
Source:
Massachusetts Institute of Technology
Summary:
A water hammer can occur when a valve is suddenly opened or closed in a pipe carrying water or steam, causing a pressure wave to travel down the pipe with enough force that it can sometimes cause the pipes to burst. Now, new research shows that a similar effect takes places on a tiny scale whenever a droplet of water strikes a surface.

This color-enhanced image shows a droplet of water being deposited on a superhydrophobic surface, just before it separates from the dropper used to deposit it.
Credit: Image courtesy of Kripa Varanasi

A water hammer can occur when a valve is suddenly opened or closed in a pipe carrying water or steam, causing a pressure wave to travel down the pipe with enough force that it can sometimes cause the pipes to burst. Now, new research shows that a similar effect takes places on a tiny scale whenever a droplet of water strikes a surface.

MIT's Kripa Varanasi, co-author of a report on the new finding published in the journal Physical Review Letters, says the phenomenon could help engineers design more durable condensing surfaces, which are used in desalination plants and steam-based power plants. Other co-authors include MIT mechanical-engineering graduate students Hyuk-Min Kwon and Adam Paxson, and associate professor Neelesh Patankar of Northwestern University.

Varanasi, the d'Arbeloff Assistant Professor of Mechanical Engineering, says the effect explains why blades used in power-plant turbines tend to degrade so rapidly and need to be replaced frequently, and could lead to the design of more durable turbines. Since about half of all electricity generated in the world comes from steam turbines -- whether heated by coal, nuclear fuel, natural gas or petroleum -- improving their longevity and efficiency could reduce the down time and increase the overall output for these plants, and thus help curb the world emissions of greenhouse gases.

There has been widespread interest in the development of superhydrophobic (water-repelling) surfaces, Varanasi says, which in some cases mimic textured surfaces found in nature, such as lotus leaves and the skin of geckos. But most research conducted so far on how such surfaces behave have been static tests: To see the way droplets of different sizes spread out on such surfaces (called wetting) or how they bead up to form larger droplets, the typical method is to add or subtract water slowly in a stationary droplet. But this is not a realistic simulation of how droplets react on surfaces, Varanasi says.

"In any real application, things are dynamic," he says. And Varanasi's research shows the dynamics of moving droplets hitting a surface are quite different from droplets formed in place.

Specifically, such droplets undergo a rapid internal deceleration that produces strong pressures -- a small-scale version of the water-hammer effect. It is this tiny but intense burst of pressure that accounts for the pitting and erosion found on power-plant turbine blades, he says, which limits their useful lifetime.

"This is one of the biggest unsolved problems" in power-plant design, he says. In addition to damaging the blades, the formation and growth of water droplets mixed with the flow of steam saps much of the power, accounting for up to 30 percent of the system losses in such plants. Since some steam-based power plants, such as natural-gas combined-cycle plants, can already have efficiencies of up to 85 percent in converting the fuel's energy to electricity, if these droplet losses could be eliminated it could provide almost a 5-percent boost in power.

Small-scale texturing of surfaces can prevent the droplets from wetting the surfaces of turbine blades or other devices, but the spacing and sizes of the surface patterns need to be studied dynamically, using techniques such as those developed by Varanasi and his co-authors, he says. Regularly spaced bumps or pillars on the surface can produce a water-shedding effect, but only if the size and spacing of these features is just right. This research showed that there seems to be a critical scale of texturing that is effective, while sizes either larger or smaller than that fail to produce the water-repelling effect. The analysis developed by this team should make it possible to determine the most effective sizes and shapes of patterning for producing superhydrophic surfaces on turbine blades and other devices.

The work is related to Varanasi's research on how to prevent ice formation on airplane wings, also using nano-texturing of surfaces, but the potential applications of this latest research are much broader. In addition to power-plant turbines, this could also affect the design of condensers in desalination plants, and even the design of inkjet printers, whose operation is based on depositing droplets of ink on a surface.

Funding: MIT Energy Initiative, the National Science Foundation, the Dupont-MIT Alliance, and the Initiative for Sustainability and Energy at Northwestern. MIT's Edgerton Center also provided high-speed video equipment.


Story Source:

The above story is based on materials provided by Massachusetts Institute of Technology. Note: Materials may be edited for content and length.


Journal Reference:

  1. Hyuk-Min Kwon, Adam Paxson, Kripa Varanasi, Neelesh Patankar. Rapid Deceleration-Driven Wetting Transition during Pendant Drop Deposition on Superhydrophobic Surfaces. Physical Review Letters, 2011; 106 (3) DOI: 10.1103/PhysRevLett.106.036102

Cite This Page:

Massachusetts Institute of Technology. "How to tame hammering water droplets." ScienceDaily. ScienceDaily, 23 January 2011. <www.sciencedaily.com/releases/2011/01/110121150954.htm>.
Massachusetts Institute of Technology. (2011, January 23). How to tame hammering water droplets. ScienceDaily. Retrieved September 17, 2014 from www.sciencedaily.com/releases/2011/01/110121150954.htm
Massachusetts Institute of Technology. "How to tame hammering water droplets." ScienceDaily. www.sciencedaily.com/releases/2011/01/110121150954.htm (accessed September 17, 2014).

Share This



More Matter & Energy News

Wednesday, September 17, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Space Race Pits Bezos Vs Musk

Space Race Pits Bezos Vs Musk

Reuters - Business Video Online (Sep. 16, 2014) Amazon CEO Jeff Bezos' startup will team up with Boeing and Lockheed to develop rocket engines as Elon Musk races to have his rockets certified. Fred Katayama reports. Video provided by Reuters
Powered by NewsLook.com
MIT's Robot Cheetah Unleashed — Can Now Run, Jump Freely

MIT's Robot Cheetah Unleashed — Can Now Run, Jump Freely

Newsy (Sep. 16, 2014) MIT developed a robot modeled after a cheetah. It can run up to speeds of 10 mph, though researchers estimate it will eventually reach 30 mph. Video provided by Newsy
Powered by NewsLook.com
Manufacturer Prints 3-D Car In Record Time

Manufacturer Prints 3-D Car In Record Time

Newsy (Sep. 15, 2014) Automobile manufacturer Local Motors created a drivable electric car using a 3-D printer. Printing the body only took 44 hours. Video provided by Newsy
Powered by NewsLook.com
Refurbished New York Subway Tunnel Unveiled After Sandy Damage

Refurbished New York Subway Tunnel Unveiled After Sandy Damage

Reuters - US Online Video (Sep. 15, 2014) New York officials unveil subway tunnels that were refurbished after Superstorm Sandy. Nathan Frandino reports. Video provided by Reuters
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

    Technology News



    Save/Print:
    Share:

    Free Subscriptions


    Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

    Get Social & Mobile


    Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

    Have Feedback?


    Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
    Mobile: iPhone Android Web
    Follow: Facebook Twitter Google+
    Subscribe: RSS Feeds Email Newsletters
    Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins