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 oil the absorber it-all at the inlet of the receiver tube during the boiling of water at low mass flow of 1.6 X 10(-5) m(3)/sec (1 liter/min), and low pressure (4 X 10(2) kPa). It should be noted that the solar concentrator was focused oil 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.
