Lift and Drag Drag-based wind turbine In drag-based wind turbines, the force of the wind pushes against a surface, like an open sail. In fact, the earliest wind turbines, dating back to ancient Persia, used this approach. The Savonius rotor is a simple drag-based windmill that you can make at home (Figure 1). It works because the drag of the open, or concave, face of the cylinder is greater than the drag on the closed or convex section. Lift-based Wind Turbines More energy can be extracted from wind using lift rather than drag, but this requires specially shaped airfoil surfaces, like those used on airplane wings (Figure 2). The airfoil shape is designed to create a differential pressure between the upper and lower surfaces, leading to a net force in the direction perpendicular to the wind direction. Rotors of this type must be carefully oriented (the orientation is referred to as the rotor pitch), to maintain their ability to harness the power of the wind as wind speed changes. Airflow over any surface creates two types of aerodynamic forces— drag forces, in the direction of the airflow, and lift forces, perpendicular to the airflow. Either or both of these can be used to generate the forces needed to rotate the blades of a wind turbine.
Heat pipe Best PPT
Presented by:HARSHA.M.N(1ks09me036 )
A heat pipe heat exchanger is a simple device which ismade use of to transfer heat from one location to another,using an evaporation-condensation cycle.Heat pipes are referred to as the "superconductors" of heatdue to their fast transfer capability with low heat loss.What is a Heat Pipe?
Working Principle• The heat input region of the heat pipe is called evaporator, thecooling region is called condenser.• In between the evaporator and condenser regions, there may bean adiabatic region
• Container• Working fluid• Wick or Capillary structure
1.ContainerThe function of the container is to isolate the working fluidfrom the outside environment.Selection of the container material depends on manyfactors. These are as follows:Compatibility (both with working fluid and externalenvironment)Strength to weight ratioThermal conductivityEase of fabrication, including welding, machineability andductilityPorosityWettability
Container materialsOf the many materials available for the container, three are byfar the most common in use—name copper, aluminum, andstainless steel.Copper is eminently satisfactory for heat pipesoperating between 0–200◦C in applications such as electronicscooling.While commercially pure copper tube is suitable, the oxygen-free high conductivity type is preferable.Like aluminum and stainless steel, the material is readilyavailable and can be obtained in a wide variety of diameters andwall thicknesses in its tubular form.
The prime requirements are:1.compatibility with wick and wall material2.Good thermal stability3.wettability of wick and wall materials4.vapor pressure not too high or low over the operatingtemperature range5.high latent heat6.high thermal conductivity7.low liquid and vapor viscosities8.high surface tension9.acceptable freezing or pour point
Examples of Working FluidMediumMelting Point (°C)Boiling Point at Atm. Pressure(°C)Useful Range(°C)Helium -271 -261 -271 to -269Nitrogen -210 -196 -203 to -160Ammonia -78 -33 -60 to 100Acetone -95 57 0 to 120Methanol -98 64 10 to 130Flutec PP2 -50 76 10 to 160Ethanol -112 78 0 to 130Water 0 100 30 to 200Toluene -95 110 50 to 200Mercury -39 361 250 to 650Sodium 98 892 600 to 1200Lithium 179 1340 1000 to 1800Silver 960 2212 1800 to 2300
1. It is a porous structure made of materials likesteel,alumunium, nickel or copper in various ranges of poresizes.2. The prime purpose of the wick is to generate capillarypressure to transport the working fluid from the condenser tothe evaporator.3. It must also be able to distribute the liquid around theevaporator section to any area where heat is likely to bereceived by the heat pipe.
4. Wicks are fabricated using metal foams, and more particularlyfelts, the latter being more frequently used. By varying thepressure on the felt during assembly, various pore sizes can beproduce.5. The maximum capillary head generated by a wick increases withdecrease in pore size.6. The wick permeability increases with increasing pore size.7. Another feature of the wick, which must be optimized, is itsthickness. The heat transport capability of the heat pipe is raisedby increasing the wick thickness.8. Other necessary properties of the wick are compatibility with theworking fluid and wettability.
Wick DesignTwo main types of wicks: homogeneous and composite.1.Homogeneous- made from one type of material ormachining technique. Tend to have either high capillarypressure and low permeability or the other way around.Simple to design, manufacture, and install .2.Composite- made of a combination of several types orporosities of materials and/or configurations. Capillarypumping and axial fluid transport are handledindependently . Tend to have a higher capillary limit thanhomogeneous wicks but cost more.
Three properties effect wick design1. High pumping pressure- a small capillary pore radius(channels through which the liquid travels in thewick) results in a large pumping (capillary) pressure.2. Permeability - large pore radius results in low liquidpressure drops and low flow resistance. Designchoice should be made that balances large capillarypressure with low liquid pressure drop. Compositewicks tend to find a compromise between the two.3. Thermal conductivity - a large value will result in asmall temperature difference for high heat fluxes.
Fig: The actual test results of heat pipe with different wick structure athorizontal and vertical (gravity assist) orientations.
Types of Heat PipesThermosyphonLeading edge-Rotating and revolving-Cryogenic pumped loop heat pipeFlat Plate-Micro heat pipes-Variable conductance-Capillary pumped loop heat pipe-
Advantages Of Heat PipesMay reduce or eliminate the need fir reheat,Allow cost effective manner to accommodate newventilation standards,Requires no mechanical or electrical input,Are virtually maintenance free,Provide lower operating costs,Last a very long time,Readily adaptable to new installations and retrofitingexisting A/C units andAre environmentally safe.
Heat transport limitationHeat transportlimitationdescription Cause Potential solutionViscous Viscous forces prevent vapor flowin the heat pipeHeat pipe operating belowrecommended operatingtemperatureIncrease heat pipeoperating temperature orfind alternative workingfluidSonic Vapor flow reaches sonic velocitywhen exiting heat pipe evaporatorresulting in a constant heattransport power and largetemperature gradientsPower/temperaturecombination, too much powerat low Operating temperatureThis is typically only aproblem at start up. Theheat pipe will carry a setpower and the largetemperature will selfcorrect as heat pipe warmsupEntrainment(Flooding)High Velocity vapor flow preventscondensate from returning toevaporatorHeat pipe operating abovedesigned power input or at toolow an operating temperatureIncrease vapor spacediameter or operatingtemperatureCapillary Sum of gravitational, liquid andvapor flow pressure drops exceedthe capillary pumping head of theheat pipe wick structureHeat pipe input power exceedsthe design heat transportcapacity of the heat pipeModify heat pipe wickstructure design or reducepower inputBoiling Film boiling in heat pipeevaporator typically initiates at 5-10 W/cm2 for screen wicks and 20-30 w/cm2 for powder metal wicksHigh radial heat flux causesfilm boiling resulting in heatpipe dry out and large thermalresistancesUse a wick with a higherheat flux capacity orspread out the heat load
Heat Pipe ApplicationsElectronics cooling- small high performance componentscause high heat fluxes and high heat dissipation demands.Used to cool transistors and high density semiconductors.Aerospace- cool satellite solar array, as well as shuttleleading edge during reentry.Heat exchangers- power industries use heat pipe heatexchangers as air heaters on boilers.Other applications- production tools, medicine andhuman body temperature control, engines and automotiveindustry.