Smaller heat exchangers

Transcription

1 Nanoscale flows (in main text) The term nanoscale generally refers to objects with characteristic lengths from atomic dimensions up to a few hundred nanometers (nm). (Recall that 1 nm 10 9 m.) Nanoscale fluid mechanics research has recently uncovered many surprising and useful phenomena. No doubt many more remain to be discovered. For example, in the future researchers envision using nanoscale tubes to push tiny amounts of water-soluble drugs to exactly where they are needed in the human body. Because of the tiny diameters involved, the Reynolds numbers for such flows are extremely small and the flow is definitely laminar. In addition, some standard properties of everyday flows (for example, the fact that a fluid sticks to a solid boundary) may not be valid for nanoscale flows. Also, ultratiny mechanical pumps and valves are difficult to manufacture and may become clogged by tiny particles such as biological molecules. As a possible solution to such problems, researchers have investigated the possibility of using a system that does not rely on mechanical parts. It involves using light-sensitive molecules attached to the surface of the tubes. By shining light onto the molecules, the light-responsive molecules attract water and cause motion of water through the tube. (See Problem 8.9.)

2 Smaller heat exchangers (in main text) Automobile radiators, air conditioners, and refrigerators contain heat exchangers that transfer energy from (to) the hot (cold) fluid within the heat exchanger tubes to (from) the colder (hotter) surrounding fluid. These units can be made smaller and more efficient by increasing the heat transfer rate across the tubes surfaces. If the flow through the tubes is laminar, the heat transfer rate is relatively small. Significantly larger heat transfer rates are obtained if the flow within the tubes is turbulent. Even greater heat transfer rates can be obtained by the use of turbulence promoters, sometimes termed turbulators, which provide additional turbulent mixing motion than would normally occur. Such enhancement mechanisms include internal fins, spiral wire or ribbon inserts, and ribs or grooves on the inner surface of the tube. While these mechanisms can increase the heat transfer rate by 1.5 to 3 times over that for a bare tube at the same flowrate, they also increase the pressure drop (and therefore the power) needed to produce the flow within the tube. Thus, a compromise involving increased heat transfer rate and increased power consumption is often needed.

3 Listen to the flowrate (in main text) Sonar systems are designed to listen to transmitted and reflected sound waves in order to locate submerged objects. They have been used successfully for many years to detect and track underwater objects such as submarines and aquatic animals. Recently, sonar techniques have been refined so that they can be used to determine the flowrate in pipes. These new flowmeters work for turbulent, not laminar, pipe flows because their operation depends strictly on the existence of turbulent eddies within the flow. The flowmeters contain a sonarbased array that listens to and interprets pressure fields generated by the turbulent motion in pipes. By listening to the pressure fields associated with the movement of the turbulent eddies, the device can determine the speed at which the eddies travel past an array of sensors. The flowrate is determined by using a calibration procedure that links the speed of the turbulent structures to the volumetric flowrate.

4 Safety in fluids (in main text) The following incident is taken from a NIOSH (National Institute for Occupational Safety and Health) fatality investigation report that emphasizes the dangers of oxygen-deficient environments in confined spaces. A 35-yearold male water system operator (victim) was asphyxiated after entering a valve vault at a municipal water system plant. The victim was assigned to turn on a waterline valve serving a nearby tree farm. The valve was located at the water treatment plant inside an underground valve vault that always had normal air. The victim entered the valve vault through a ground-level manhole without testing or ventilating the vault to atmosphere. A coworker, who had last seen the victim one hour earlier, checked the manhole and saw the victim lying on his back at the bottom. The victim did not respond to any calls. Other workers summoned from the plant building and local fire department personnel ventilated the valve vault and removed the victim. The vault atmosphere was subsequently found to be oxygen deficient. There were no witnesses to the incident, but evidence suggests that the victim lost consciousness and fell from the ladder railings to the bottom of the vault.

5 New hi-tech fountains (in main text) Ancient Egyptians used fountains in their palaces for decorative and cooling purposes. Current use of fountains continues but with a hi-tech flair. Although the basic fountain still consists of a typical pipe system (i.e., pump, pipe, regulating valve, nozzle, filter, and basin), recent use of computer-controlled devices has led to the design of innovative fountains with special effects. For example, by using several rows of multiple nozzles, it is possible to program and activate control valves to produce water jets that resemble symbols, letters, or the time of day. Other fountains use specially designed nozzles to produce coherent, laminar streams of water that look like glass rods flying through the air. By using fast-acting control valves in a synchronized manner it is possible to produce mesmerizing three-dimensional patterns of water droplets. The possibilities are nearly limitless. With the initial artistic design of the fountain established, the initial engineering design (i.e., the capacity and pressure requirements of the nozzles and the size of the pipes and pumps) can be carried out. It is often necessary to modify the artistic and/or engineering aspects of the design in order to obtain a functional, pleasing fountain. (See Problem 8.60.)

6 Deepwater pipeline (in main text) Pipelines used to transport oil and gas are commonplace. But south of New Orleans, in deep waters of the Gulf of Mexico, a not-so-common multiple pipe system is being built. The new so-called Mardi Gras system of pipes is being laid in water depths of 4300 to 7300 ft. It will transport oil and gas from five deepwater fields with the interesting names of Holstein, Mad Dog, Thunder Horse, Atlantis, and Na Kika. The deepwater pipelines will connect with lines at intermediate water depths to transport the oil and gas to shallowwater fixed platforms and shore. The steel pipe used is 28 in. in diameter with a wall thickness of in. The thick-walled pipe is needed to withstand the large external pressure which is about 3250 psi at a depth of 7300 ft. The pipe is installed in 240-ft sections from a vessel the size of a large football stadium. Upon completion, the deepwater pipeline system will have a total length of more than 450 miles and the capability of transporting more than 1 million barrels of oil per day and 1.5 billion cu ft of gas per day. (See Problem )