Moving blades

Figure 9-6 shows a diagram of a single-stage impulse turbine. The statie pressure deereases in the nozzle with a eorresponding inerease in the absolute veloeity. The absolute veloeity is then redueed in the rotor, but the statie pressure and the relative veloeity remain eonstant. To get the maximum energy transfer, the blades must rotate at about one-half the veloeity of the gas jet veloeity. Two or more rows of moving blades are sometimes used in eonjunetion with one nozzle to obtain wheels with low blade tip speeds and stresses. In-between the moving rows of blades are guide vanes that redireet the gas from one row of moving blades to another as shown in Figure 9-7. This type of turbine is sometimes ealled a Curtis turbine.

Another impulse turbine is the pressure eompound or Ratteau turbine. In this turbine the work is broken down into various stages. Eaeh stage eonsists of a nozzle and blade row where the kinetie energy of the jet is absorbed into the turbine rotor as useful work. The air that leaves the moving blades enters the next set of nozzles where the enthalpy deereases further, and the veloeity is inereased and then absorbed in an assoeiated row of moving blades. 

Fan guards of wire grating or hardware cloth are mounted helow the fan to prevent accidental contact with the moving blades and to keep newspapers, leaves, and other light objects from being drawn into the fan. The use of a wire fence around the entire unit is good to keep unauthorized individuals away from all of the equipment how-... 

Moving Stationary Moving blades blades blades... 

Reaction turbine design also makes use of steam jets (which are produced by steam flowing across static vanes), although the turbine has rotor discs that incorporate movable blades rather than buckets. The design utilizes the reactive force produced by steam accelerating through a nozzle (created by the combination of a stationary vane and a moving blade) to rotate the shaft. 

That is, the maximum efficiency for energy transfer from the fluid to the blade occurs when the velocity of the impinging fluid is twice that of the moving blade. 

The n-A curves were measured with a trough equipped with a moving blade and a piezoelectric device (Figure 2). Both the trough (286 mm long and 70 mm wide) and blade were coated with Teflon. The subphase temperature was kept within 0.1 °C by use of a water jacket connected with a thermostated circulation system, and the environmental air temperature was kept at 18 °C. The surface tension was measured with a Wilhelmy plate made of filter paper (25 x 25 x 0.25 mm) using a piezoelectric device. The surface pressure(ji) is defined as ... 

In practice, it is sometimes possible to incorporate moving blades in the filter equipment so that the thickness of the cake is limited to the clearance between the filter medium and the blades. Filtrate then flows through the cake at an approximately constant rate and the solids are retained in suspension. Thus the solids concentration in the feed vessel increases until the particles are in permanent physical contact with one another. At this stage the boundary between the slurry and the cake becomes ill-defined, and a significant resistance to the flow of liquid develops within the slurry itself with a consequent reduction in the flowrate of filtrate. 

Suspend 10 g dry or 300 ml swollen beads in a total volume of 350 ml water, stir with a moving blade agitator, and heat to 60 °C. Now add dropwise a solution of 2 g dye in 60 ml water. Add 45 g of sodium chloride 30 min after finishing the addition of dye and continue stirring at 60 °C for an additional hour. Next raise the temperature to 80 °C and add 4 g of anhydrous sodium carbonate. Continue stirring at 80 °C for 2 h. 

A propeller pump is another type of positive displacement pump. It operates similar to a fan blade inside of a pipe or tube. As fluid moves past the blade, the energy of the moving blade delivers fluid through the outlet at a higher pressure than delivered. 

The liquid/solid (L/S) transition temperature as determined by the moving blade technique. 

We assume that a moving blade creates a channel of depth d as it moves across the ice. If fj =0.5 and there is no strain recovery, the dynamic and static values of d are equal. Part of the resistance encountered by the blade is the force,, required to plough through the ice at... 

Positive displacement pumps are often built directly into skimmers to recover more viscous oils. These pumps have a variety of operating principles, all of which have some common schemes. Oil enters a chamber in the pump where it is pushed by a moving blade, shoe, or piston to the exit of the pump. The oil and other material with it must move through the chamber because there is no alternative passage, thus the name positive displacement. ... 

Dashed arrows indicate velocity direction relative to the moving blade Figure 15.1 Plan view of turbine stage single nozzle-blade combination. 

Enthalpy drop in moving blades Enthalpy drop in the reaction stage... 

Loss of kinetic energy at the entry to a moving blade... 

Kinetic energy will be lost when the gas does not enter the moving blade smoothly, that is to say its angle of approach does not match the inlet angle of the blade. Figure 15.4 shows the very tip of the turbine blade receiving the inlet stream of gas for the two situations ... 

Now the increase in specific kinetic energy of the gas passing over the moving blades results from the decrease in specific enthalpy over the moving blades (see equation (14.3)). Hence... 

For shockless entry, the moving blade inlet angle needs to match the angle of gas entry ... 

An important consequence is that both the fixed and moving blades have the same cross-sectional shape (although the blade height will increase as the expansion proceeds along the turbine). Figure 15.8 shows the effect diagrammatically. 

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