Axial flow turbines

An alternative type of downhole mud motor is the mud turbine, (multistage axial flow turbine) which directly drives the bit. The tool consists of an upper section containing the turbine blades and lower section with bearings. As mud is pumped through the upper section the blades are turned. Turbines are designed to rotate at higher speed than the displacement motor. The higher rotation speed requires diamond or composite bits. 

There are three types of mixing flow patterns that are markedly different. The so-called axial-flow turbines (Fig. 18-3) actually give a flow coming off the impeller of approximately 45°, and therefore have a recirculation pattern coming back into the impeller at the hub region of the blades. This flow pattern exists to an approximate Reynolds number of 200 to 600 and then becomes radial as the Reynolds number decreases. Both the RlOO and A200 impellers normally require four baffles for an effective flow pattern. These baffles typically are V12 of the tank diameter and width. 

FIG. 18-15 Typical flow pattern in a baffled tank with a propeller or an axial-flow turbine positioned on center. 

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. 

Axial-flow turbines are often used in blendiug pseudoplastic materials, and they are often used at relatively large D/T ratios, from 0.5 to 0.7, to adequately provide shear rate in the majority of the batch particularly in pseudoplastic material. These impellers develop a flow pattern which may or may not encompass an entire tank, and these areas of motion are sometimes referred to as caverns. Several papers describe the size of these caverns relative to various types of mixing phenomena. An effec tive procedure for the blending of pseudoplastic fluids is given in Oldshue (op. cit.). 

Data are not currently available on the dispersion with the newer fluidfoil impellers, but they are often used in industrial mixer-settler systems to maintain dispersion when additional resonance time holdup is required, after an initial dispersion is made by a radial- or axial-flow turbine. 

Figure 1-20 shows multistage high-pressure axial flow turbine rotor. The turbine rotor depicted in this figure has a low-pressure compressor followed by a high-pressure compressor. There are also two turbine sections, and the... 

There are two types of turbines used in gas turbines. These consist of the axial-flow type and the radial-inflow type. The axial-flow turbine is used in more than 95% of all applications. 

The inward-flow radial turbine has many components similar to a centrifugal compressor. There are two types of inward-flow radial turbines the cantilever and the mixed-flow. The cantilever type in Figure 1-34 is similar to an axial-flow turbine, but it has radial blading. However, the cantilever turbine is not popular because of design and production difficulties. 

The axial-flow turbine, like its eounterpart the axial-flow eompressor, has flow, whieh enters and leaves in the axial direetion. There are two types of axial turbines (1) impulse type, and (2) reaetion type. The impulse turbine has its entire enthalpy drop in the nozzle therefore it has a very high veloeity entering the rotor. The reaetion turbine divides the enthalpy drop in the nozzle and the rotor. Figure 1-37 is a sehematie of an axial-flow turbine, also depleting the distribution of the pressure, temperature and the absolute veloeity. 

Most axial flow turbines eonsist of more than one stage, the front stages are usually impulse (zero reaetion) and the later stages have about 50% reaetion. The impulse stages produee about twiee the output of a eompar-able 50% reaetion stage, while the effieieney of an impulse stage is less than that of a 50% reaetion stage. 

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 radial-inflow turbine has another advantage its eost is mueh lower than that of a single or multistage axial-flow turbine. The radial-inflow turbine has a lower turbine effieieney than the axial-flow turbine however, lower initial eosts may be an ineentive to ehoosing a radial-inflow turbine. 

The radial-inflow turbine is espeeially attraetive when the Reynolds number (Re = pUD/yi) beeomes low enough (Re = 10 — 10 ) that the effieieney of the axial-flow turbine is below that of a radial-inflow turbine, as shown in Figure 8-1. The effeet of speeifie speed (N = and speeifie... 

The boundary layer along the blade surfaces must be well energized so that no separation of the flow occurs. Figure 8-16 shows a schematic of the flow in a radial-inflow impeller. Off-design work indicates that radial-inflow turbine efficiency is not affected by changes in flow and pressure ratio to the extent of an axial-flow turbine. 

Axial-flow turbines are the most widely employed turbines using a eompres-sible fluid. Axial-flow turbines power most gas turbine units—exeept the smaller horsepower turbines—and they are more effieient than radial-inflow turbines in most operational ranges. The axial-flow turbine is also used in steam turbine design however, there are some signifieant differenees between the axial-flow turbine design for a gas turbine and the design for a steam turbine. 

The axial-flow turbine, like its eounterpart the axial-flow eompressor, has flow, whieh enters and leaves in the axial direetion. There are two types of axial... 

The degree of reaction in an axial-flow turbine is the ratio of change in the static enthalpy to the change in total enthalpy... 

The primary cause of efficiency losses in an axial-flow turbine is the buildup of boundary layer on the blade and end walls. The losses associated with a boundary layer are viscous losses, mixing losses, and trailing edge losses. To calculate these losses, the growth of the boundary layer on a blade must be known so that the displacement thickness and momentum thickness can be computed. A typical distribution of the displacement and momentum thickness is shown in Figure 9-26. The profile loss from this type of bound-ary-layer build-up is due to a loss of stagnation pressure, which in turn is... 

Horlock, J.H., Axial Flow Turbines, Butterworth and Company Ltd., London, 1966. 

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