Turbines radial-inflow

Radial-Inflow Turbine The radial-inflow turbine, or inward-flow radial turbine, has been in use for many years. Basically a centrifugal compressor with reversed-flow and opposite rotation, the inward-flow radial turbine is used for smaller loads and over a smaller operational range than the axial turbine. Radial-inflow turbines are only now beginning to be used because little was know about them heretofore. Axial turbines have enjoyed tremendous interest due to their low frontal area, making them suited to the aircraft industiy. However, the axial machine is much longer than the radial machine, making it unsuited for certain vehicular and helicopter applications. Radial turbines are used in turbochargers and in some types of expanders. 

The efficiency of expansion turbines (partial admission axial, full admission axial, and radial inflow turbines) is a function of the following four basic parameters. 

For the preliminary estimate of the expected efficiency of expansion turbines, in most cases it is sufficient to neglect Reynolds number effects (Rg > 10 ) and use the efficiency and specific speed correlations shown in Figure 2-12 for partial admission axial impulse, reaction radial inflow and full admission impulse and reaction axial turbines. Due to the economic advantage of the radial turbine, die radial inflow turbine is die best selection when operating in die specific speed range 20 < Nj < 140, whereby die optimum efficiency will be achieved at N, = 80. 

Because high-pressure ratio requirements coincide in most cases with low specific speed designs, only partial admission axial or radial inflow turbines are seriously considered. 

To reeap, a turboexpander is a radial inflow turbine. The expansion proeess is aeeomplished in two steps primary and seeondary expansion. Primary expansion oeeurs in the inlet guide vanes and seeondary expansion oeeurs in the radial wheel. The proeess is isentropie and thermally effieient with reeoverable eold energy. Turboexpanders used in dew point eontrol require the following ... 

Power is generated by the pressurized gas expanding through an 11,000 rpm single-stage, radial-inflow turbine expander, which drives a synchronous generator. Exhaust gas from the expander is liquified by air-cooled condensers and is pumped back to the heat exchangers to repeat the cycle. 

With a lower temperature, the turbine is best used by allowing the baek-pressure to fall and thus obtain more power. In radial inflow turbines, the relative veloeity at the turbine inlet is small. Any ehanges are, therefore, far less signifieant than with high relative veloeity impulse wheels. Commonly, a turboexpander tolerates as mueh as a 30% ehange from its designed enthalpy. The effeet on effieieney was shown earlier in Figure 3-12. 

Dakin, R., Radial Inflow Turbines for Geothermal Application, Geothermal Resources Council, February 1986. 

Small Gas Turbines. These gas turbines are in the range from about 0.5 MW-2.5 MW. They often have eentrifugal eompressors and radial inflow turbines. Effieieneies in the simple eyele applieations vary from 15-25%. 

The two types of turbines—axial-flow and radial-inflow turbines—can be divided further into impulse or reaction type units. Impulse turbines take their entire enthalpy drop through the nozzles, while the reaction turbine takes a partial drop through both the nozzles and the impeller blades. 

The radial-inflow turbine, or inward-flow radial turbine, has been in use for many years. Basically a centrifugal compressor with reversed flow and opposite rotation, the inward-flow radial turbine is used for smaller loads and over a smaller operational range than the axial turbine. 

Small standby power turbines less than 2-MW. The smaller size of these turbines in many cases have centrifugal compressors driven by radial inflow turbines, the larger units in this range are usually axial... 

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 radial-inflow turbine has many eomponents similar to those of a eentrifugal eompressor. Flowever, the names and funetions differ. There are two types of radial-inflow turbines the eantilever radial-inflow turbine and the mixed-flow radial-inflow turbine. Cantilever blades are often two-dimensional and use nonradial inlet angles. There is no aeeeleration of the... 

Figure 8-4. Mixed-flow-type radial-inflow turbine.

The rotor or impeller of the radial-inflow turbine eonsists of a hub, blades, and in some eases, a shroud. The hub is the solid axisymmetrieal portion of the rotor. It defines the inner boundary of the flow passage and is sometimes ealled the dise. The blades are integral to the hub and exert a normal foree on the flow stream. The exit seetion of the blading is ealled an exdueer and it is eonstrueted separately like an indueer in a eentrifugal eompressor. The exdueer is eurved to remove some of the tangential veloeity foree at the outlet. 

The general prineiples of energy transfer in a radial-inflow turbine are similar to those already outlined in the eompressor seetion. Figure 8-6 shows the veloeity veetors in turbine rotor flow. 

The overall effieieney of a radial-inflow turbine is a funetion of effieieneies from various eomponents sueh as the nozzle and rotor. A typieal turbine expansion enthalpy/entropy diagram is shown in Figure 8-7. The total enthalpy remains eonstant through the nozzle, sinee neither work nor heat is transferred to or from the fluid. Within the rotor, the total enthalpy ehanges. Downstream of the rotor the total enthalpy remains eonstant. 

Radial-Inflow Turbines 329 Turbine Design Considerations... 

To design a radial-inflow turbine of the highest effleieney, the exit veloeity leaving the turbine must be axial. If the exit veloeity is axial, the Euler turbine equation reduees to... 

The flow entering the rotor of a radial-inflow turbine must have a eertain ineidenee angle eorresponding to the slip flow in a eentrifugal impeller and not to zero ineidenee. By relating this eoneept to the radial-inflow turbine, the following relationship ean be obtained for the ratio of whirl veloeity to blade tip speed ... 

With the aid of the previous relationships, a veloeity diagram for the flow entering a radial-inflow turbine ean be drawn as shown in Figure 8-10. 

Losses in a radial-inflow turbine are similar to those in a eentrifugal impeller. The losses ean be divided into two eategories internal losses and external losses. Internal losses ean be divided into the following eategories ... 

Figure 8-11. An example of a radial-inflow turbine characteristic. (Courtesy Institution of Mechanical Engineers.)...

Exit loss. The fluid leaving a radial-inflow turbine constitutes a loss of about one-quarter of the total exit head. This loss varies from about 2-5%. 

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. 

---