The dependence of heat transfer performance on fin spacing has been investigated for condensation of steam on horizontal integral-fin tubes. Thirteen tubes have been used with rectangular section fins having the same width and height (0.5 mm and 1.6 mm) and with fin pitch varying from 1.0 mm to 20.5 mm. For comparison, tests were made using a plain tube having the same inside diameter and an outside diameter equal to that at the root of the fins for the finned tubes. All tests were made at near-atmospheric pressure with vapor flowing vertically downward with velocities between 0.5 m/s and 1.1 m/s. The observed heat transfer enhancement for the finned tubes significantly exceeded that to be expected on grounds of increased area. Plots of enhancement against fin density were repeatable and showed local maxima and minima. The dependence of enhancement on fin density did not depend appreciably on vapor velocity or condensation rate for the ranges used. The maximum vapor-side enhancement (i.e., vapor-side heat transfer coefficient of finned tube/vapor-side coefficient for plain tube) was found to be around 3.6 for the tube with a fin spacing of 1.5 mm.
A theoretical approach for calculating the rate of deposition of fog droplets on steam turbine blades by turbulent diffusion is described. The theory is similar to that which has proved successful for predicting deposition of small particles in pipe flow and includes a recent correlation for the inertia-moderated regime. A reliable estimate of the blade surface shear stress distribution is required and is obtained by a quasi-three-dimensional inviscid flow calculation to give the blade surface velocity distribution, followed by a two-dimensional boundary layer calculation. The theory has been applied to two representative case studies. The first involves deposition on the final stage blading of the low-pressure cylinder of an operating 500 MW turbine, and the second concerns deposition in a high-pressure, wet steam turbine. Results are presented showing the effect of fog droplet size, surface roughness, and other flow parameters on the deposition rate. A comparison is made between the rates of deposition by diffusional and purely inertial mechanisms. In low-pressure turbines these are of comparable magnitude, but in high-pressure machines diffusional deposition may dominate.
The paper describes a theoretical and experimental study of fog droplet deposition and coarse water formation in the LP cylinders of two 500 MW steam turbines. Measurements of coarse water flow rates entering and leaving the final stage of each turbine were performed using a new design of water absorbent probe. From these measurements it was possible to deduce the rate of deposition of fog droplets onto the last stage blading of each machine. Aerodynamic and optical traverses provided experimental data on the fog droplet mean diameter and wetness faction, and the application of an inversion procedure generated an approximation to the droplet size spectrum itself. Using these data and theoretical methods for predicting inertial and diffusional deposition rates, a second estimate was obtained for the stage deposition rates. The two different approaches show excellent agreement, in contrast with previously published work, which was unable to reconcile (to within one order of magnitude) deposition theory with measured fog droplet sizes and coarse water quantities.
A theoretical approach for calculating the rate of deposition of fog droplets on steam turbine blades by inertial impaction is described. Deposition rates are computed by tracking a number of droplet path lines through a specified blade-to-blade vapor flowfield and identifying the limiting trajectories that just intersect the blade surface. A new technique for performing the calculations efficiently has been developed whereby the mathematical stiffness of the governing equations is removed, thus allowing the numerical integration to proceed stably with comparatively large time increments. For high accuracy, the vapor flowfield is specified by a quasi-three-dimensional flow calculation involving both meridional and blade-to-blade plane calculations. Results are presented for two representative “test cases,” namely the final stage blading of the low-pressure cylinder of a 500 MW turbine and a typical stage in a high-pressure wet steam turbine. The effect on the deposition rate of fog droplet size and blade profile geometry is investigated for both on- and off-design flowfields. Comparisons are made with the predictions of a simplified theory for inertial deposition and the effect of blade rotation in flows with high pitch angles is discussed.
The paper reports a continuation of an experimental investigation of the effect of fin pitch on the heat transfer performance of horizontal, integral-fin tubes for condensation of steam at near-atmospheric pressure. The effects of “drainage strips” located along the lower edge of finned and plain tubes have been studied. These gave significant increases in the heat transfer coefficient for finned tubes but had only marginal effect for the plain tube. Condensate retention angles have also been measured for simulated condensation using water, ethylene glycol, and refrigerant-113 for finned tubes with and without drainage strips. In the latter case the data agreed satisfactorily with theory. Drainage strips were found to reduce the extent of holdup significantly.
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