Axial-flow combine

Mechanical Impulse The principle of mechanical impulse when applied to fluids is usually combined with one of the other means of imparting motion. As mentioned earher, this is the case in axial-flow compressors and pumps. The turbine or regenerative-type pump is another device which functions partially oy mechanical impulse. 

The fluidfoil impellers in large tanks require only two baffles, but three are usually used to provide better flow pattern asymmetiy. These fluidfoil impellers provide a true axial flow pattern, almost as though there was a draft tube around the impeller. Two or three or more impellers are used if tanks with high D/T ratios are involved. The fluidfoil impellers do not vortex vigorously even at relatively low coverage so that if gases or solids are to Be incorporated at the surface, the axial-flow turbine is often required and can be used in combination with the fluidfoil impellers also on the same shaft. 

Medium-sized gas turbines between 5-50 MW are a combination of aero-derivative and frame type turbines. These gas turbines have axial flow compressors and axial flow turbines. 

The accurate calculation and proper evaluation of the losses within the axial-flow compressor are as important as the calculation of the bladeloading parameter, since unless the proper parameters are controlled, the efficiency drops. The evaluation of the various losses is a combination of experimental results and theory. The losses are divided into two groups (1) losses encountered in the rotor, and (2) losses encountered in the stator. The losses are usually expressed as a loss of heat and enthalpy. 

TTiere are two main types of expansion turbines axial flow and radial flow. Axial flow expansion turbines are like conventional steam turbines. They may be single-stage or multistage with impulse or reaction blading, or some combination of the two. Turbines of this type are used as power recovery turbines. They are used where flow rates, inlet temperatures, or total energy drops are very high. 

The spiral assembly can be fitted with covers to provide three flow patterns (1) both fluids in spiral flow, (2) one fluid in spiral flow and the other in axial flow across the spiral, and (3) one fluid in spiral flow and the other in a combination of axial and spiral flow. 

A vaneaxial fan consists of an axial flow wheel within a cylinder combined with a set of air guide vanes located either before or after the wheel. The vaneaxial fan is designed to move air or gases over a wide range of volumes and pressures. It is generally constructed of sheet metal, although cast metal fan wheels are sometimes furnished. 

In the more favored multi-stage axial flow design the bladeing is of the impulse type (known as a Rateau or Curtis type stage) or of the reaction type. It is popular among manufacturers to design multi-stage machines with a combination of these two types, and the most common combinations are ... 

Double-impeller combinations Bouaifi et al. (2001) derived the following correlations for stirred gas-liquid reactors with various combinations of double impellers. The impellers used were the lightning axial flow impeller (A-310), the four 45° pitched blade turbine pumping down (PBTD) and the Rushton disk turbine (RDT). Furthermore, the tank was a dish-bottom cylindrical tank equipped with four baffles, while the gas was introduced by a ring sprager. The gas-flow rate ranged from 0.54 to 2.62 L/s, whereas the rotational speed was from 1.66 to 11.67 s. The gas holdup is... 

Figure 7.10. Some types of impellers for centrifugal pumps, (a) Open impeller, (b) Semiopen impeller, (c) Shrouded impeller, (d) Axial flow (propeller) type, (e) Combined axial and radial flow, open type, (f) Shrouded mixed-flow impeller, (g) Shrouded impeller (P) in a case with diffuser vanes (V). (h) Turbine impeller.

If we combine the flow generated by the down channel and cross channel flows, a net flow is generated in axial or machine direction (w ) of the extruder, schematically depicted in Fig. 6.5. As can be seen, at open discharge, the maximum axial flow is generated, whereas at closed discharge, the axial flow is zero. From the velocity profiles presented in Fig. 6.5 we can easily deduce, which path a particle flowing with the polymer melt will take. 

As mentioned previously, axial flow impellers are typically used for solids suspension. It is also typical to use radial flow impellers for gas-liquid mass transfer. In combination gas-liquid-solid systems, it is more common to use radial flow impellers because the desired power level for mass transfer normally accomplishes solids suspension as well. The less effective flow pattern of the axial flow impeller is not often used in high-uptake-rate systems for industrial mass transfer problems. There is one exception, and that is in the aeration of waste. The uptake rate in biological oxidation systems is on the order of 30 ppm/hr, which is about to the rate that may be required in industrial processes. In waste treatment, surface aerators typically use axial flow impellers, and there are many types of draft tube aerators that use axial flow impellers in a draft tube. The gas rates are such that the axial flow characteristic of the impeller can drive the gas to whatever depth is required and provide a very effective type of mass transfer unit. 

Oosthuizen, P.H. and Rangarajan, N., "Experimental Study of Combined Convective Heat Transfer from Horizontal Cylinders in an Axial Flow . Trans. CSME. Vol. 6. No. 2. pp. 103-105, 1980. 

---