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9.3. Experimental Investigations

Stationary Pure Vapor

A number of earlier experimental results (before 1950) show some difference with the predictions of the Nusselt theory (McAdams, 1954). The differences can be attributed to one or more of the following reasons: 1) significant forced-convection effects; 2) presence of noncondensable gas; 3) waviness and turbulence within the condensate film; 4) presence of dropwise condensation.

More recently, Mills and Seban (1967) condensed steam on a copper vertical flat plate and Slegers and Seban (1969) conducted some experiments with n-butyl alcohol. These tests support the Nusselt theory for pure stationary vapor condensation.

Moving Pure Vapor

Mayhew and Aggarwal (1973) experimented with pure steam condensing on a flat surface. To avoid air in-leakage, the experiments were carried out at pressures slightly above atmospheric. Good agreement is obtained between the experimental results and the calculated values by their own theory. It is very interesting to note that the results for the counter-current flow cases are always appreciably higher than those predicted by the author's own model and indeed always higher than the corresponding co-current velocity vapor values. The reason was investigated and explained as follows in the original paper;

An obvious explanation was provided by dye-injection tests which showed that, with counterflow, no laminar film flow could be achieved. The film was torn off the plate (i.e. flooding occurred at quite moderate values of vapor velocity. Similar observations with parallel flow confirmed that the film was always both laminar and smooth. From work with noncondensing films it was expected that rippled flow would be encountered over part of the surface at the higher velocities used. In fact remarkable smooth films were observed suggesting that mass transfer, and possibly also surface tension effects on the non-isothermal film, must have had a stabilizing effect.

More recently Asano et al. (1978) reported their data for the condensation of pure saturated vapors on a vertical flat copper plate and showed good agreement with the authors' own model.

Stationary Vapor with a Noncondensable Gas

Perhaps the earliest definitive experiment of the effect noncondensable gas was done by Othmer (1929), who introduced air mole fractions of up to 11 The experimental heat transfer coefficient data of Hampson (1951) and Akers et al. (1960) were 20 Al-Diwany and Rose (1973) reported heat transfer measurements for steam condensing in the presence at air, argon, neon and helium. The vapor-gas mixture was passed into the steam chamber via flow straighteners which provided uniform flow of the mixture towards the condensing surface so as to preclude forced convection effects. The experimental data for steam-air, steam-argon and steam-neon showed satisfactory agreement with the predicted theoretical values of Sparrow but for steam-helium tex2html_wrap_inline793 showed a lower value than the theoretical values.

Recently, DeVuono and Christensen (1984) reported their experiment of natural convection of a steam-air mixture at pressures above atmospheric to 0.7 MPa to investigate the effect of pressure. The experiments were performed on a horizontal copper tube with 7.94 cm O.D. by 1.22 m of active condensation length. The tube was mounted in a cylindrical pressure vessel 1.52 m O.D. by 3.35 m long. Saturated steam was supplied by an external source and allowed to diffuse to the tube resulting in steady-state, natural convection conditions. An expression, which is a function of tex2html_wrap_inline729 , percent noncondensable gas by volume (Y

equation360

where tex2html_wrap_inline797 MPa

0.0 < Y < 14.0

tex2html_wrap_inline801 C.

Even though this experiment was done over a large range of pressure for a containment analysis and showed a significant effect of pressure, the pipe geometry and length scale make it questionable to apply this correlation to a large scale system. Unfortunately, the experiment results were not compared with any other theoretical model.

Moving Vapor with a Noncondensable Gas

Rauscher, Mills and Denny (1974) performed experiments of filmwise condensation from steam-air mixtures undergoing forced flow over 0.74 in. O.D. horizontal tube. The heat transfer coefficient at the stagnation point was reported for bulk air mass fractions 0 - 7


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