Current from currents

Transcription

1 Current from currents (in main text) The use of large, efficient wind turbines to generate electrical power is becoming more commonplace throughout the world. Wind farms containing numerous turbines located at sites that have proper wind conditions can produce a significant amount of electrical power. Recently, researchers in the United States, the United Kingdom, and Canada have been investigating the possibility of harvesting the power of ocean currents and tides by using current turbines that function much like wind turbines. Rather than being driven by wind, they derive energy from ocean currents that occur at many locations in the 70% of the Earth s surface that is water. Clearly, a 4-knot (2.5 m s) tidal current is not as fast as a 40-mph (70 km hr) wind driving a wind turbine. However, since turbine power output is proportional to the fluid density, and since seawater is more than 800 times as dense as air, significant power can be extracted from slow, but massive, ocean currents. One promising configuration involves blades twisted in a helical pattern. This technology may provide electrical power that is both ecologically and economically sound.

2 1948 Buick Dynaflow started it (in main text) Prior to 1948 almost all cars had manual transmissions, which required the use of a clutch pedal to shift gears. The 1948 Buick Dynaflow was the first automatic transmission to use the hydraulic torque converter and was the model for present-day automatic transmissions. Currently, in the United States over 84% of the cars have automatic transmissions. The torque converter replaces the clutch found on manual shift vehicles and allows the engine to continue running when the vehicle comes to a stop. In principle, but certainly not in detail or complexity, operation of a torque converter is similar to blowing air from a fan onto another fan that is unplugged. One can hold the blade of the unplugged fan and keep it from turning, but as soon as it is let go, it will begin to speed up until it comes close to the speed of the powered fan. The torque converter uses transmission fluid (not air) and consists of a pump (the powered fan) driven by the engine driveshaft, a turbine (the unplugged fan) connected to the input shaft of the transmission, and a stator (absent in the fan model) to efficiently direct the flow between the pump and turbine.

3 Space Shuttle fuel pumps (in main text) The fuel pump of your car engine is vital to its operation. Similarly, the fuels (liquid hydrogen and oxygen) of each Space Shuttle main engine (there are three per shuttle) rely on multistage turbopumps to get from storage tanks to main combustors. High pressures are utilized throughout the pumps to avoid cavitation. The pumps, some centrifugal and some axial, are driven by axial-flow, multistage turbines. Pump speeds are as high as 35,360 rpm. The liquid oxygen is pumped from 100 to 7420 psia, the liquid hydrogen from 30 to 6515 psia. Liquid hydrogen and oxygen flowrates of about 17,200 gpm and 6100 gpm, respectively, are achieved. These pumps could empty your home swimming pool in seconds. The hydrogen goes from 423 F in storage to 6000 F in the combustion chamber!

4 Mechanical heart assist devices (in main text) As with any pump, the human heart can suffer various malfunctions and problems during its useful life. Recent developments in artificial heart technology may be able to provide help to those whose pumps have broken down beyond repair. One of the more promising techniques is use of a left-ventricular assist device (LVAD), which supplements a diseased heart. Rather than replacing a diseased heart, an LVAD pump is implanted alongside the heart and works in parallel with the cardiovascular system to assist the pumping function of the heart s left ventricle. (The left ventricle supplies oxygenated blood to the entire body and performs about 80% of the heart s work.) Some LVADs are centrifugal or axial-flow pumps that provide a continuous flow of blood. The continuous-flow devices may take some adjustment on the part of patients who do not hear a pulse or a heartbeat. Despite advances in artificial heart technology, it is probably still several years before fully implantable, quiet, and reliable devices will be considered for widespread use.

5 Hi-tech ceiling fans (in main text) Energy savings of up to 25% can be realized if thermostats in air-conditioned homes are raised by a few degrees. This can be accomplished by using ceiling fans and taking advantage of the increased sensible cooling brought on by air moving over skin. If the energy used to run the fans can be reduced, additional energy savings can be realized. Most ceiling fans use flat, fixed pitch, nonaerodynamic blades with uniform chord length. Because the tip of a paddle moves through air faster than its root does, airflow over such fan blades is lowest near the hub and highest at the tip. By making the fan blade more propeller-like, it is possible to have a more uniform, efficient distribution. However, since ceiling fans are restricted by law to operate at less than 200 rpm, ordinary airplane propeller design is not appropriate. After considerable design effort, a highly efficient ceiling fan capable of delivering the same airflow as the conventional design with only half the power has been successfully developed and marketed. The fan blades are based on the slowly turning prop used in the Gossamer Albatross, the human-powered aircraft that flew across the English Channel in (See Problem )

6 Fish friendly hydraulic turbine (in main text) Based on data about what actually kills fish as they pass through hydraulic turbines, Concepts NREC produced a rotor design that allows a larger flow passage, a more uniform pressure distribution, lower levels of shear stress, and other acceptable trade-offs between efficiency and fish survivability. Tests and projections suggest that the fish friendly turbine design will achieve 90% efficiency, with fish survivability increased from 60 to 98%.

7 Cavitation damage in hydraulic turbines (in main text) The occurrence of cavitation in hydraulic pumps seems to be an obvious possibility since low-suction pressures are expected. Cavitation damage can also occur in hydraulic turbines, even though they do not seem obviously prone to this kind of problem. Local acceleration of liquid over blade surfaces can be sufficient to result in local pressures low enough to cause fluid vaporization or cavitation. Further along the flow path, the fluid can decelerate rapidly enough with accompanying increase in local pressure to make cavitation bubbles collapse with enough intensity to cause blade surface damage in the form of material erosion. Over time, this erosion can be severe enough to require blade repair or replacement which is very expensive. (See Problem 12.4LL.)