CONTENTS Page X-RAY OBSERVATIONS OF FLOW PATTERNS IN A MERCURY BOILER by Alex Vary Lewis Research Center SUMMARY Flow patterns in an electrically heated, forced-flow mercury boiler were studied with a high-resolution X-ray image system. The boiler consisted of a thin-wall metallic tube with a swaged helical groove along its length. This groove was a swirling device that helped separate the liquid phase into an essentially helical streamlet flowing in contact with the boiler wall. There were long periods of stable operation during which X-ray observations showed good separation of the liquid phase and no detectable liquid carryover into the superheater. An attempt was made to explain the observed flow patterns in terms of patterns predicted by previous adiabatic studies made with other fluids. There were close similarities between the observed and predicted patterns, but the observed patterns depended strongly on the intimate liquid-to-boiler-wall contact produced by the helical motion of the mercury. Because of the high surface tension of mercury, the stratified flow with good phase separation that was observed cannot be readily reproduced in straight-tube boilers. Prior X-ray observations of straight-tube prototype boilers indicated that, with mercury, phase separation (swirling) devices are needed. X-ray observations were also made during metastable intervals when there were flow and pressure oscillations and excursions. In these instances, the X-ray aided in identifying phenomena that appeared to trigger and sustain metastable conditions in the boiler. Using flow pattern and other observations, it was concluded that metastable oscillations were promoted by nucleation effects in the boiler inlet region. INTRODUCTION The advent of advanced nuclear space power systems has generated investigative effort for improvement and development of power conversion devices based on two-phase flow of mercury and alkali metals. Two topics of interest have therefore been flow stability and flow pattern control in liquid-metal boilers. In particular, flow pattern con trol efforts have been concerned with the prevention of undesirable liquid entrainment in vapor flows. An X-ray method for observing flow patterns was developed as a part of a forcedflow mercury-loop design. The loop was primarily an instrument for investigating mercury corrosion under two-phase flow conditions. Prior to the construction of the loop described herein, several prototypes were tested. Each of these also incorporated the X-ray image system for flow observation. The first prototypes used straight cylindrical boiler tubes. The problems of producing stable boiling and preventing liquid entrainment (carryover) with mercury became apparent at once. Because of surface tension and dewetting effects, there is a tendency for liquid droplets and slugs of mercury to form and separate from the mainstream. These can be carried downstream by high-velocity vapor. Under this condition, boiler and superheater performances degenerate, and stable operation with high-quality vapor cannot be attained. Therefore, phase separation devices are useful in forced-flow mercury boilers to promote sustained intimate wall contact for inducing wetting and, hence, improving stability and performance. Recently developed mercury boilers contain swirling devices that promote phase separation, increase boiling stability, and reduce the tendency for slug formation (refs. 1 to 4). Because of the wide use of swirling devices for flow pattern control, there is interest in observing flow conditions produced with swirlers. A number of investigators have studied actual and simulated liquid-metal flow under both adiabatic and diabatic conditions (refs. 3, 5, and 6). An opportunity for direct observation of actual high-temperature flow patterns in a mercury boiler was afforded by the corrosion loop. By use of an X-ray photographic system coupled with an electronic image amplifier, the mercury loop provided clearer flow pattern observations than heretofore available, although X-ray methods have been used before (refs. 7 to 9). A motion-picture film supplement that shows the flow patterns observed in this investigation has been prepared and is available on loan. A request card and a description of this film are included at the back of the report. The purpose of this report is to describe the observed flow patterns obtained under both stable and metastable conditions. Included herein are conclusions concerning methods for phase separation in mercury boilers and causes of metastable flow based on X-ray observations. The post-test corrosion results that were relevant to flow pattern phenomena are also described. A full account of the corrosion results is given in reference 10. APPARATUS The mercury loop was a tubular flow circuit having the following major components: preheater, boiler, superheater, vapor throttle, condenser, condensate column, electrodynamic mercury pump, and flowmeter. The arrangement of the loop components is shown in figure 1. The loop material was a cobalt-base alloy, HS-25. With the exception of the pump cell and flowmeter, all major parts of the loop were arranged in a vertical plane (figs. 2 to 4). The purpose of this layout was to permit con |