Blade thickness

A (3 is caused by boundary-layer effects A 3 is caused by the blade thickness... 

Shock in rotor losses. This loss is due to shock occurring at the rotor inlet. The inlet of the rotor blades should be wedgelike to sustain a weak oblique shock, and then gradually expanded to the blade thickness to avoid another shock. If the blades are blunt, a bow shock will result, causing the flow to detach from the blade wall and the loss to be higher. 

The incidence angle o is for a 10% blade thickness. For blades of other than 10% thickness, a correction factor K is used, which is obtained from Figure 7-25. 

Figure 7-25. Correction factor for blade thickness and incidence angle calculation.

Impeller size relative to the size of the tank is critical as well. If the ratio of impeller diameter D to tank diameter T is too large (Z)/r is > 0.7), mixing efficiency will decrease as the space between the impeller and the tank wall will be too small to allow a strong axial flow due to obstruction of the recirculation path (21). More intense mixing at this point would require an increase in impeller speed, but this may be compromised by limitations imposed by impeller blade thickness and angle. If P/Pis too small, the impeller will not be able to generate an adequate flow rate in the tank. 

The middle of the specimens is notched at both sides by inserting a razor blade (American safety single edged blades, thickness 1.0 mm). The razor blade is inserted by means of a mechanical testing machine at low speed (0.20 mm/min) to minimize the introduction of internal stresses in the specimen. For every notch a new razor blade is used to make sure that every notch has the same sharpness. This notching procedure is carried out according to the notch method used for polyethylene specimens, ASTM F1473 [15]. The notches (2.5 mm) reduce the cross-section of the specimen by 50% (from 40 mm to 20 mm ). 

Since for strength reasons b cannot be reduced without any limit, it is suggested that the paddle edges be slanted. From a range of different paddle profiles, that shown in Fig. 5.19 has proved to be the most favorable. For a stirrer diameter d = 192 mm the blade height is 48 mm, the blade thickness 4 mm, the length of the slant 15 mm and the edge width 0.5 mm. Measurements showed, that in this way the stirrer speed could be reduced by 5% and the stirrer power reduced by 17%. 

For measurements well off the resonance, i.e. with wavelengths larger than 720 nm, p-TS6 crystals were prepared by thermal polymerisation of monomer crystals, grown out of a saturated solution and manually cut using a shaver blade. Thickness of these crystals varies between 40-100 pm. 

For now, the characteristic length scale Lc is assumed to scale with the impeller diameter, not the tank diameter. If geometric similarity is observed and all impeller dimensions are scaled with the impeller diameter (including details such as blade thickness), the characteristic length scale (CpD) will scale any of the impeller dimensions equally well only Cl will change. The constants and Cl are a function of the impeller and tank geometry selected. For now, however, we retain them as constants. 

One must design the mechanical components, such as shaft diameter, impeller blade thickness, baffles and supports, bearings, seals, etc. (see Chapter 21). 

To design an effective stirred tank, an efficient impeller should be chosen for the process duty. More than one impeller may be needed for tanks with high aspect ratio (Z/T > 1.5). Sizing of the impeller is done in conjunction with mixer speed to achieve the desired process result. The appropriate size and type of wall baffles must be selected to create an effective flow pattern. The mixer power is then estimated from available data on impeller characteristics, and the drive size is determined. The mixer design is finalized with mechanical design of the shaft, impeller blade thickness, baffle thickness and supports, inlet/outlet nozzles, bearings, seals, gearbox, and support structures. 

The commonly known or calculable force acting on the impeller blade is the force related to torque, which is horsepower, P [hp] W, divided by rotational speed, N [rpm] rps, divided by the number of blades, nb, or the first term inside the parentheses in eq. (21-26). Because the pressure force acts normal to the blade and the torsional force must be horizontal for a vertical rotating shaft, a factor of the reciprocal of the sine of the blade angle enters the expression for blade thickness. The equivalent pressure force must act at some moment arm from the center of rotation, which would be the impeller radius, D/2 [in.] m, if the force acted at the blade tip. However, because pressure forces are lost around the tip of the blade, causing a vortex flow pattern, the effective force must act at a... 

The coefficient in this blade thickness, t [in.] m, calculation returns the appropriate value with the units shown previously. 

Calculations for the stub blades or welded attachment points of impeller blades can be done like calculations for the extension blade thickness. The details of welding, casting, or other methods of attachment become critical in the design. Conventional calculations for structural strength may be adequate, but for com-phcated geometry, finite element models can provide better design information. 

Liewhiran, C. and Phanichphant, S., 2008. Doctor-bladed thick films of flame-made Pd/ZnO nanoparticles for ethanol sensing. Current Applied Physics, 8(3/4), pp. 336-339. 

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