The bending of a receiver tube in two-phase flow under stratified conditions when water is first introduced to the hot steel receiver of a 14.5-m long parabolic trough concentrator is presented in this paper. Thermal gradients were observed on the absorber wall at the inlet of the receiver tube during the boiling of water, at low mass flow of $1.6×10−5m3/sec$ (1 liter/min), and low pressure $4×102kPa.$ It should be noted that the solar concentrator was focused on the receiver tube, which contained static air before the water was introduced. The introduction of the water produced a change in the temperature difference between the upper and lower sides of the receiver, from 40-60 K to much lower temperatures, in about 45 seconds. The bending of the steel receiver tube occurred when the two-phase flow began. Maximum deflection was observed when the thermal gradient reached a minimum value. We conclude that, when the flow of steam, water, and air exist in a stratified pattern, the combination of these three elements produces the bending phenomenon. The theoretical model, developed to evaluate the experimental data, confirms that the change in temperature gradient produces the bending of the steel receiver tube during this transient stage.

1.
Pederson, R. J., and May, E. K., 1982, “Flow Instability during Direct Steam Generation in a Line-Focus Solar Collector System.” Technical Report SERI/ TR-632-1354, National Renewable Energy Laboratory (NREL), Golden, CO.
2.
Murphy, L. M., and May, E. K., 1982, “Steam Generation in Line-Focus Solar Collectors: A Comparative Assessment of Thermal Performance, Operating Stability, and Cost Issues.” Technical Report, SERI/TR-632-1311, Solar Energy Research Institute (NREL), Golden, CO.
3.
Griffth P., 1973, “Two Phase-Flow,” Handbook of Heat Transfer, W. M. Ronsenow and J. P. Hartnett (Eds.), Chap. 14, pp. 14-1–14-21, McGraw-Hill.
4.
Zarza, E., Ajona, J. I., and Hennecke, K., 1997, “Development of a New Generation of Solar Thermal Power Stations,” Solar Thermal Concentrating Technologies, M. Becker and M. Bo¨hmer, (eds.), Proc. of 8th Int. Symp., Vol. 2, pp. 397–415, C.F. Mu¨ller Verlag, Heidelberg, Germany.
5.
Goebel, O., and Hennecke, K., 1997, “Investigation of Thermohydraulic in Parabolic Trough Absorber Tube with Direct Steam Generation (DSG).” Solar Thermal Concentrating Technologies, M. Becker and M. Bo¨hmer (Eds.), Proc. of 8th Int. Symp., Vol. 2, pp. 787–813, C.F. Mu¨ller Verlag, Heidelberg, Germany.
6.
Goebel, O., 1997, “Modelling of Two Phase Stratified and Annular Flow in Heated Horizontal Tubes.” Proc. of Int. Engineering Foundation 3rd Conf., Irsee, Germany, pp. 303–310, Taylor and Francis.
7.
Hahne, E., Herrmann, U., and Rheinla¨nder, J., 1997, “The Effect of Tilt on Flow Patterns of Water/Steam Flow Through Heated Tubes,” Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics, Edizioni ETS, pp. 925–934.
8.
Almanza
,
R.
,
Lentz
,
A.
, and
Jime´nez
,
G.
,
1997
, “
Receiver Behavior in Direct Steam Generation with Parabolic Troughs.
Sol. Energy
,
61
(
4
), pp.
275
278
.
9.
Almanza
,
R.
, and
Lentz
,
A.
,
1998
, “
Electricity Production at Low Powers by Direct Steam Generation with Parabolic Troughs
,”
Sol. Energy
,
64
(
1-3
), pp.
115
120
.
10.
ASHRAE Handbook, 1993, pp. 34.3, Equipment, USA.
11.
Valde´s, A., Almanza, R., Soria, A., and Mazari, M., 1998, “Mathematical Model for Direct Steam Generation in Parabolic Trough Collectors with Compound-Wall Receiver,” Proc. of 1998 Annual Conf., American Solar Energy Society, pp. 271–275.
12.
Almanza, R, Valde´s, A., and Lo´pez, S., 1982, “Solar Concentrators,” NTIS, PB-82-157553.
13.
Valde´s, A., 1988, “Stochastic Model of the Solar Beam Irradiance Focused in a Parabolic Trough,” Proc. of XII Mexican Solar Energy Society (ANES), pp. 63–66 (in Spanish).
14.
Martinez
,
I.
,
Almanza
,
R.
,
Mazari
,
M.
, and
Correa
,
G.
,
2000
, “
Parabolic Trough Reflector Manufactured with Aluminum First Surface Mirrors Thermally Sagged
,”
Sol. Energy Mater. Sol. Cells
,
64
(
1
), pp.
85
96
.
15.
Almanza
,
R.
,
Lentz
,
A.
,
Santiago
,
L.
, and
Valde´s
,
A.
,
1999
, “
Some Experiences on Electricity Production at Low Powers with DSG Using Parabolic Troughs
,”
J. Phys. IV
,
9
, pp.
229
232
(9th SolarPACES Int. Symp. on Solar Thermal Concentrating Technologies, France).
16.
Flores, V., 2001, Bimetallic Receiver to Produce Direct Steam Generation in Parabolic Troughs, Ph.D. dissertation in process at National University of Mexico (private communication).