Gas turbine failures associated with sea-salt ingestion and sulfur-containing fuel impurities have directed attention to alkali sulfate deposition and the associated hot corrosion of gas turbine (GT) blades under some GT operating conditions. These salt deposits form thin, molten films which undermine the protective metal oxide coating normally found on GT blades. The prediction of molten salt deposition, flow and oxide dissolution, and their effects on the lifetime of turbine blades are examined. Goals include rationalizing and helping to predict corrosion patterns on operational GT rotor blades and stators, and ultimately providing some of the tools required to design laboratory simulators and future corrosion-resistant high-performance engines. Necessary background developments are reviewed first, and then recent results and tentative conclusions are presented along with a brief account of the present research plans

Rosner, D. E.

English

NASA Technical Reports Server

N87-I1191
_Dm_[1_ AI]l_rIL BB_81TION ]_
Daniel E. Rosner
Yale University
Department of Chemical Engineering
High Temperature Chemical Reaction Engineering Laboratory
1. _ONo
Gas turbine failures associated with sea-salt ingestion and sulfur-containing fuel impur-
ities have directed attention to alkali sulfate deposition and the associated "hot corro-
sion" of gas turbine (GT) blades under some GT operating conditions. These salts deposit and
form thin, molten films which undermine the protective metal oxide coating normally found on
GT blades. This research project deals with the prediction of molten salt deposition, flow
and oxide dissolution and their effects on the lifetime of turbine blades. Our goals include
rationalizing and helping to predict corrosion patterns on operational GT rotor blades and
stators, and ultimately providing some of the tools required to design laboratory simulators
and future corrosion-resistant high-performance engines. In the program summary below, we
fxrst revxew necessary background developments (Sections 2, 3 and 4) and then introduce our
recent results and tentative conclusions (Sections 5, 6), along with a brief account of our
present research plans.
2. VAPOR llmq_l"rloN (VD--) _Te
The first step in a complex sequence of events leading to hot corrosion failures is the
deposition of alkali salts from the combustion prod!acts onto turbine stator vanes and rotor
blades. Our NASA-supported laboratory experiments,l-Sand those carried out in collaboration
with NASA-Lewis Research Laboratories, 4 have enabled advances in salt deposition rate theox_
as briefly outlined here. A comprehensive but tractable method for predicting the rates of
chemical vapor deposition (CVD) of, say, alkali sulfates from multicomvonent salt-laden com-
bustion products was first developed, illustrated and tested in part. 4-'5'8 This theory pre-
dicts important effects of multicomponent diffusion and thermal (Soret) diffusion 6 on dew
points, CVD-rates and deposit compositions. 7 For present purposes an important feature of the
deposition rate is its dependence on surface temperature, which ordinarily reveals the exis-
tence of a dew point temperature (above which a macroscopic surface condensate is not stable)
and a temperature-insensitive plateau deposition rate at temperatures sufficiently far below
4,5,7,8the dew point -- i.e., features included in the above-mentioned CVD-rate theory. While
in its most general form, allowance is made for the transport of each chemical species (i =
1_ .... ) containing the elements of interest (Na, S ...) across the diffusion boundary layer
(BL) (Fig. 1), to study the dynamics of the resulting molten salt condensate layer (Section 3
below), we use the single-component limiting form of the CVD theory to economically provide
the spatial distribution of condensate "arrival." This approach also applies without funda-
mental modifications to the deposition rate of pre-existing fine particles, 9'10 i.e., mist
droplets small enough to behave like '_eavy molecules" in the prevailing carrier gas flow
field. The inclusion of chemical reactions and/or phase change (e.g., '*mist formation')
within the BL requires further research, some of which is in progress (Section 6).
"and update bibliographic information on our recently published papers.
NAG 3-201
lOl
https://ntrs.nasa.gov/search.jsp?R=19870001758 2020-03-20T14:04:20+00:00Z
3. _M_BqSA3Z l_wm ECS
An important process in the molten salt attack of GT blades is evidently the dissolution
("fluxing") of the normally protective metal oxide, and this in turn, implies dynamic proces-
ses which allow dissolution/reaction to occur without saturating the molten salt solvent.
For these reasons, we have carried out studies of the dynamics of thin condensate liquid
layers 11, allowing for the interplay of salt arrival rate (Section 2) and runoff induced by
aerodynamic shear stress _w and blade rotation rate, fl (Fig. 2). These studies should ulti-
mately prove useful in the interpretation of observed hot corrosion maps on blades removed
from operational engines. They have already guided the development and interpretation of
laboratory sinmlator experiments 3"4'8 in which molten salt deposition and runoff necessarily
occur at seed levels such that Tdp > Trap.
In ref. 11 the necessary liquid layer theory was developed and used to predict steady-
state laminar single component condensate layers on smooth non-rotating, isothermal targets,
with emphasis on the circular cylinder in high Reynolds number cross-flow (a common test
configuration). In our recent studies, this approach has been extended to include the treat-
ment of single component condensate layer flow along smooth non-isothermal rotor blades (Fig.
2). 111ustrative calculations have thus far been made for a test turbine (NASA TP 1018
(1977)), but most of our methods will carry over to operational engine vanes and blades
(except in the innediate vicinity of cooling slots/holes). From the inviscid stream velocity
data, we compute, using efficient integral methods, the corresponding distributions of gas-
side momentum; heat- and mass-transfer coefficients, and the distribution of blade "recovery"
temperature (used to estimate this root-cooled blade temperature distribution). The corre-
sponding condensate arrival rate distribution and liquid viscosity distributions are then
inserted into the partial differential equation (PDE) governing the liquid layer thickness
8_-_x,z-_ (Fig. 2). This nonlinear first-order PDE is numerically solved (by the method-of-
characteristics) to provide the condensate layer streamline pattern (Fig. 3), as well as the
corresponding liquid layer thickness 8^_x,z_. Two important by-products of these calculations
are (a) the solvent inventory on the blade, and (b) relative tip and trailing edge salt
runoff rates. It is also interesting to note that surface flow can lead to the presence of
condensate on portions of the blade which are hotter than the prevailing dew point. One such
region is shown in Fig. 4, in which the span-wise flow associated with blade rotation causes
the steady-state presence of molten salt at Tw ) %, and hence, the possibility of apprecia-
ble local hot corrosion rates above %.
4. _ G_IDK "DLqSfl[,U_GI_' _ DISIRIHJ_(lqS
As mentioned at the outset of Section 3, non-stoichiometric molten sodium sulfate is
known to be a solvent for A1203(s) or Cr203(s) _ the oxides ordinarily relied upon for
corrosion protection of the underlying blade alloys. Accordingly, we have utilized the above-
mentioned fornmlation to predict dissolution rate distributions associated with solvent flow
patterns of the type shown (Fig. 3). Imleed, it will be instructive to compare such predic-
tions with hot corrosion rate patterns observed on blades removed from operational GT engines.
Toward this end, estimates have been made of the oxide diffusion coefficient in the solvent,
equilibrium solubility, and maximum (kinetic) rate of dissolution This information was then
used to predict the solute diffusion-limited dissolution rate along each streamline (cf. Fig.
$) using a generalization of Leveque-Levich BL-theory. The corresponding arrival rate dis-
tribution is shown in Fig. 6 for comparison. Preliminary dissolution rate maps are being
checked, generalized and discussed in terms of their parametric dependencies, and to the
limited extent possible, agreement with operational experience. This research will be sumnar-
ized in our final report on Grant NAG 3-201, and in a paper 12 currently in preparation.
Pablication seems to be timely, since even though high-quality operational data are not
generally available, we believe that the formalism we have developed will make possible in the
U.S.-GT industry instructive and economical parametric studies for both stator and rotor
102
blades over a wide variety of enviromnental conditions (salt level, turbine inlet temperature,
stagnation pressure level, blade contour and cooling, tip speed, etc.). Moreover, the stage
has been set to generalize many aspects of the theory, including commonly encountered mixed
alkali deposits, condensate arrival by both vapor diffusion and thermophoretic capture of BL-
nucleated droplets, 13 and transient condensate flows. It should also be useful to examine in
greater detail the effects on localized dissolution rates of secondary flows, produced in part
by surface tension gradients associated with the oxide dissolution process itself.
All properties of thin liquid condensate films, including their aggressiveness as corro-
dants, are expected to be composition-dependent. Moreover, appreciable amounts of adulter-
ants (e.g. Ca- and Mg-compounds) are often found in GT-blade deposits upon engine shut-down
and blade removal. _his information, combined with the remarkable effect the addition of a
second salt can have on the deposition rate of the primary salt (see ref. 14, based on our
experiments carried out under NASA Grant NSG 3169) suggests that attention should be directed
toward the deposition, dynamics and dissolution rate properties of multicomponent molten salt
layers, not just the behavior of pure Na2S04(_). Toward this end, we are proposing intensive
studies of the expected behavior of binary sulfate salt layers.
Among the effects anticipated for bina_ molten salt cases, perhaps the most significant
are the alteration of:
(a) deposition rates and associated dew point temperature,
(b) liquid properties, including the important oxide solubility and associated Fick
diffusivity,
(c) freezing point of the solution.
While the property effects (b) remain to be studied, it is clear that effects (a) and (c) can
considerably broaden the important temperature interval: Tdp - Trap within which hot corro-
sion is known to take its toll on GT blade alloys.
To illustrate the consequences of the altered freezing point, we have performed a prelim-
inary steady-state analysis of the binary _S04 + N_S04 ) condensate layer, expected along the
leading edge of a non-isothermal (root-cooled) rotor blade, with a specifxed root temperature
(of. T (Na2SOiow) = I157K and T__(]_SOA) = 1343K) and _ = 6786 rad/s. Figure 7 reveals that ifTw(0,0_Pis as as lll0K the_Vth_ c_ndensate on the leading edge would be entirely solid in
the absence of potassium addition to the mainstream (with Td_(Na2SO 4) = 1242K). However, with
xncreasing potassium levels, an increasing fractxon of the condensate would become molten
(becoming fully molten at ([K]/[lqa])®-ratios above about 0A3. More comprehensive prototyp-
ical examples, including effects (a), (b) and (c) above, are planned.
6. _ _01, II_ICATI_I_, mrnlm
While much of the required input information remains to be sharpened up (based on inde-
pendent experiments), we believe that the present formalism has reached the point where
instructive parametric studies can now be economically performed for both GT stator vanes and
rotor blades over a wide variety of environmental conditions (salt level, turbine inlet
temperature, stagnation pressure level, blade contour and cooling, tip speed, etc.).
For the present, the following conclusions have emerged from our recent investigations:
(a) On rotor blades, qualitatively different condensate behavior (thickness, composition)
is observed depending upon whether the liquid streamlines emerge from the blade lead-
ing edge or root (see e.g. Fig. 3).
103
(b) For surfaces everywhere below Tdv predicted oxide dissolution rate distributions
(Fig. 5) are strongly coupled to the predicted local condensate arrival rate dis-
tribution (Fig. 6).
This observation, combined with formal ccmputatlons which presume highly undersaturated liquid
layers, lead us to conclude that:
(c) The tendency toward local saturation of the condensate layer strongly influences the
distribution of metal oxide dissolution rate.
(d) Owing to liquid layer convection across blade isotherms, liquid layers (hence corro-
sion) can occur on some areas of a GT blade which are above the prevailing dew point
temperature.
(e) The simultaneous presence of chemical elements capable of entering into the conden-
sate solution considerably broadens the temperature interval TdD-Tmv (within which
hot corrosion is 1/kely to take its toll on GT blade _l_oys-) bu-t improves the
chances of reduced deposition rates via BL mist formation. I_'14
Finally, we realize that to fully exploit and test the presently developed liquid layer
theory:
(f) Additional fundamental data will be needed (e.g. temperature and stoichiometry depen-
dence of the solubility of A1203 and Cr203 in Na2SO4(_)).
(g) Well-documented test data on hot-corrosion rate distributions from simulated and/or
real turbine blades run under nearly constant environmental conditions will be
needed.
I. Rosner, D. E. and Atkins, I_ M., in Foul_n_ _nd Slaggin_ Resulting from Impurities in
Combustion Gases (R. Bryers, ed.) Engineering Foundation (New York) 1983, pp. 469-492.
2. Rosner, D. E. and Seshadri, K., in Eighteenth Int, Symposium on Combustion, The Combus-
tion Institute (Pittsburgh, PA), 1981, pp. 1385-1394.
3. Seshadri, K. and Rosner, D. E., AIChE J. 3_O0(2), 187 (1984).
4. Kohl, F. J., Santoro, G. J., Stearns, C. A., Fryburg, G. C. and Rosner, D. E., Y.
Electrochem. Soc. _ 1054 (1979).
5. Rosner, D. E., Chen, B.-K., Fryburg, G. C. and Kohl, F. J., Comb. Sci. and Tech. 2__O, 87
(19791.
6. Rosner, D. E., 7. Physicochem. Hydrodynamics 1, 159 (1980).
7. Rosner, D. F_ and Nagarajan, R., "Transport-lnduced Shifts in Condensate Dew Point and
Composition in Multicomponent Systems with Chemical Reaction," Chemical __. Sci__. (in
press, 1984).
i04
8 Santoro, G. 3, gohl, IZ. _., Stearns, C. A., Rosner, D. E. and Gokoglu, S. A., Experimen-
tal and Theoretical Deposition Rates from Salt-seeded Combustion Gases of a Math0,3 Bur-
ner Rie. NASA TP 225 (1984).
9. Gokoglu, S. A. and Rosner, D. E., Int. J. Heat and Mass Trans. _ 639 (1984).
I0. Rosner, D. E., Gokoslu, S. A. and Israel, R., in Foulin_ of Heat ExchanKe Surfaces (R.
Bryers, ed.) Engineering Foundation (New York) 1983, pp. 235-256.
11. Rosner, D. E., Ounes, D. and Anous, N., Che_ Engrg. Commm. _ 275 (1983).
12. Rosner, D. E. and Nagarajan, I_, "Theoretical Model to Predict/Understand the Corrosion
Rate Consequences of Molten Alkali Sulfate Condensation on Combustion Turbine Blades"
(in preparation, 1984).
13. Castillo, 3. L. and Rosner, D.E., "Mass Transfer Consequences of Droplet Thermophoresis
and Local Vapor/Liquid Equilibrium in Dilute Binary Condensible Vapor Laminar Boundary
Layers" (in preparation, 1984).
14. Rosner, D. E. and Liang, B., '_)evelopsent and Demonstration of Flash Evaporation Tech-
nique for Binary Alkali Sulfate Salt-Seeded Flames" (in preparation, 1984).
105
Fig. 1: Vapor phase boundary layer near turbine blade surface source of moment_ and mass
for thin condensate layer.
Fig. 2: Factors influencing the evolution of liquid condensate layer dynamics on a turbine
rotor blade.
106
O_
_.4
0
0
Fig. 3:
,. ..,.
ORIGINAL PAGE IS
I_ POORQUALIFY
J II
Predicted streamline pattern in molten salt condensate layer along suction surface of
test turbine rotor blade (o = 6786 tad/s).
Fig. 4:
,_y,_o)=n._ -
_ _ '_;_o;_ _i/di,_ _'
.O,(o l I I J I
Predicted molten salt layer thickness distribution along leading edge of root-cooled
test turbine blade. (Note flow of liquid layer onto region of blade above prevailing
dew point.)
107
Fig. 5: Predicted distribution of metal oxide dissolution rates caused by molten salt conden-
sate layer flow on root-cooled turbine rotor blade.
i
O
.t
Fig. 6: Normalized Na2SO 4 arrival rate distribution (from vapor phase boundary layer) on
tu__ine rotor blade suction surface.
108
Fig. 7: Predicted distribution of condensate phase and composition along leading edge of a
root-cooled turbine rotor blade (Tw_O,O) = 1110K) vs. mainstream potassium: sodium
rat io.
109


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