Re: Steam And Gas Turbine By R Yadav Pdf 133 HOT
Sibyl Piccuillo
The paper presents details of a unique experimental facility along with necessary accessories and instrumentation for testing steam turbine cascade blades in wet and nucleating steam. A steam turbine rotor tip cascade is chosen for flow investigations. Cascade inlet flow measurements show uniform conditions with dry air and steam and dry air mixture of different ratios. Exit flow surveys indicate that excellent flow periodicity is obtained. Blade surface static pressure and exit total pressure distributions are also presented with dry air and with steam and dry air mixture of different ratios as the working medium at an exit Mach number of 0.52.
In spite of large importance of steam turbine testing, there are very few facilities available in the world for steam turbine testing. There are a few turbine test stands available with the steam turbine manufacturers. However these testing facilities have limited measurement capabilities. Hence understanding of the flow processes in the steam turbines is very limited. Although with the development of advanced instrumentation such as fast response miniature probe (Bosdas et al. [1]), it is possible to get detailed flow measurements behind the rotors of steam turbines (Duan et al. [2]), it is not easy to measure flow in the passages of the rotor blade. Optical methods proved helpful in obtaining detailed flow measurements in the steam turbine rotor passages. But these investigations are costly and time confusing. Modelling of flow in steam turbines is attempted by many researchers (Št'astný and Šejna [3]) but with limited success and with limited results. CFD is used to predict flow in steam turbines (Dykas et al. [4]). Extensive results are obtained, but these results have to be experimentally validated. Hence cascade testing of steam turbine blades provides useful information for understanding, modelling and improvement of flow in steam turbines. The starting point to study these problems in turbine flows satisfactorily, has been extension of the treatments of nucleating flows to two-dimensional fields. It is easy to investigate many of the problems resulting from the formation and behaviour of the liquid phase in steam turbine blading in two-dimensional cascades.
Mashmoushy et al. [5] carried out a comprehensive review on the blown-down tunnel results on steam turbine cascade tests. They concluded that the aerodynamic losses experienced by the flow are very similar under superheated and nucleating conditions in the majority of the cases. However, they found that the thermodynamic components of the losses in the nucleating tests are higher than the sum of the aerodynamic losses. They also found that the flow core is free from the effects of viscous dissipation in the tests with a subsonic outlet. The drop in the total pressure allows the nucleation loss incurred by these tests to be deduced. When the cascade is tested with wet steam, the droplets present in the flow offers some surface for condensation and lowers the super cooling attained. The number and the size of the droplets and the rate of expansion determines the extent to which this is achieved. The present paper describes a subsonic cascade tunnel useful for steam turbine blading under different steam conditions. A subsonic cascade tunnel already available in Turbomachines Laboratory, Department of Mechanical Engineering of IIT Madras is modified for investigating steam turbine blades under different working conditions encountered in steam turbines (saturated, super-heated or wet steam). The modification essentially consists of adding a boiler, which supplies steam of desired condition to the tunnel. The steam is supplied through a set of nozzles and mixed with the other working fluid, air. The paper describes these details and provides preliminary measurements. The experimental facilities available in the literature are presented in Table 1. Some of the facilities are blow down tunnels with running times of a few seconds. Some of the facilities are continuously operating with steam supplied from steam power plants near the facilities. This may not be always possible. The use steam as test fluid may not be always possible. The disadvantages of using steam are high cost and complexity. While dry air can be used as test fluid [5], it is not possible to determine the effects of wetness and superheat on the steam turbine cascade performance.
A subsonic cascade tunnel with maximum exit Mach number of 0.52 was commissioned in Turbomachines laboratory, Department of Mechanical Engineering, IIT Madras. The details of the cascade tunnel are given in Table 2. The tunnel is described in detail in reference 15. The working fluid in this tunnel is atmospheric air. This tunnel is upgraded to operate with steam in different conditions for studies on steam turbine blading. The details of the facility modification for testing with air and steam are given in the following sections.
The schematic of steam turbine cascade facility is shown in Fig. 1. A boiler supplies steam at different conditions to the cascade tunnel. The steam is mixed with dry air at the desired percentage to produce the desired working fluid. The details of the boiler along with its accessories and its operation are presented below.
The steam is finally sent through the nozzle where it expands to the rig pressure. In the present setup a total 15 number of nozzles are installed, connected to three pipes. These nozzles are fixed to each pipe as shown in Fig. 3. The female threaded part is welded to 16 mm outer diameter pipe by TIG (tungsten inert gas) welding. The exit of the nozzles is adjusted in such a way that the steam sprayed will be in the flow direction. Hence uniform mixing of steam with air can be ensured.
The mixing duct connected in front of the test section is 800 mm in length and flow area is 250 mm height and 140 mm width. Autocad drawings of the mixing duct are shown in Fig. 4. It is fabricated with 10 mm thick stain steel sheets. The main function of the duct is to hold the spraying nozzles rigidly at one end. The length of the duct is calculated and it is found that it is sufficient to allow the steam to mix with dry air from the forced draft fan. Other end of the duct has a provision to measure static pressure. There are 17 static pressure taps in side plate and 8 static pressure taps in the bottom plate. Provision is made for mounting traverse mechanisms on the side plate and on the top plate of the duct to measure the total pressure. Figure 5 shows the arrangement of steam injection into mixing duct.
Pressure probes are extensively used to measure one, two and three dimensional aerodynamic flows. Bakhtar et al. [18] used total pressure tubes, yaw meters and static probes to measure flow in droplet, and mist and wet steam. For successful measurement of these flows, the characteristics of droplet are to be satisfactorily matched during the calibration of the instruments. Reference 19 presents the characteristics of three hole probe and static tube in superheated and wet steam.
In addition measurements of the cascade exit flow are carried out at 20 mm above and below the mid span section. The flow at the three spanwise stations is found to be in good agreement confirming that the flow at the mid span is two dimensional. The static pressures on the blade surfaces at an exit Mach number of 0.52 for four flow working mediums (dry air and with steam of 0.86, 1.30 and 1.73% of dry air) are presented in Fig. 12. The differences in the static pressures for the four working mediums seem to be very small. No direct comparison of blade surface static pressures with those of Bakhtar et al. [17] are not possible as working conditions are different. However the trend of blade surface static pressure distribution is very similar to that of Bakhtar et al. [17].
Flow traverses at the cascade exit at an exit Mach number of 0.52 are carried out using the precalibrated five hole probe and the pitch wise distribution of total pressure coefficient for the above four flow working mediums is presented in Fig. 13. With the introduction of steam, the total pressure loss increases. The cascade tunnel operating with steam air mixture did not experience any condensation in the tunnel. Wake traverses for the same cascade were available in Bakhtar et al. [21]. However no direct comparison is attempted as the operating conditions are different. However the trend of wake traverses for the three sets of experiments is similar.
From the total pressure distributions, total pressure loss coefficient is calculated and the pitch wise distribution of total pressure loss coefficient for the four flow working mediums at the cascade exit at an exit Mach number of 0.52 is presented in Fig. 14. The peak total pressure loss coefficient for the dry air as working flow medium occurs at slightly different pitch wise location compared to that for the steam and dry air working medium. Also its value is slightly lower than that for the steam and dry air working medium. From the pitch wise distribution of total pressure loss coefficients, its averaged value is calculated and presented in Fig. 15. From the figure, it is evident the averaged total pressure coefficient varies nearly linear with percentage of steam in dry air. However the increase in the total pressure loss coefficient with the percentage of steam is moderate, 0.075 at zero steam flow to 0.0975 (that is just about 25% or 17% per 1% steam flow) at 1.73% steam flow.
The existing subsonic tunnel is modified to accommodate the steam generation facility to test the steam turbine cascade blading with steam at different steam conditions as working medium. The subsonic tunnel is open loop continuous operation tunnel. The back pressure of the tunnel varies accordingly with the atmospheric conditions. The steam conditions can be changed by varying the pressure and the inlet temperature. This facility is of unique nature as the working fluid is air and steam mixture instead steam only. Hence the facility cost and complexity are low. To the best of our knowledge, this type of facility for testing in steam is unique.
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