Liquid-metal/water direct contact heat exchange

One concept being considered for steam generation for advanced liquid-metal reactor applications, involves water coming into direct contact with the circulating liquid-metal. The vigorous agitation of these two fluids, their direct liquid-liquid contact and the consequent large interfacial area can give rise to higher heat transfer coefficients and rapid steam generation. For an optimum design of such direct contact heat exchange and vaporization systems, detailed knowledge is necessary of the various flow regimes, their interfacial transport phenomena, heat transfer and operational stability.; In order to investigate the characteristics of such a liquid-metal/water direct contact heat exchanger, a series of experiments were performed in a two dimensional experimental facility. The facility primarily consists of a liquid-metal melt chamber, a heated test section, water pumping/injection system, and steam condenser. The effects of water injection from a 2.0 mm nozzle at flow rates from 1.5 g/s to 8 g/s into a high density liquid-metal (Pb or Pb/Bi alloy) at temperatures of 450--500°C and system pressures ranging from 1--10 bar were investigated. These conditions were chosen to span the major flow regimes that may be present in reactor based direct contact systems.; A real time high energy X-ray imaging system along with extensive temperature and flow measurements were developed and utilized to measure the multiphase flow and obtain an empirical database of local as well as overall system heat transfer performance. Results include volumetric void fraction between 0.025--0.16, overall volumetric heat transfer coefficient ranging from 0.4--2.0 kW/m 3-K, evaporation zone length between 5--60 cm depending on the water injection rate and system pressure and local heat transfer coefficients varying between 200--5000 W/m2-K depending on the inlet water injection conditions and system pressure. Time dependent void fraction and water/vapor bubble characteristics (i.e. bubble formation time, bubble rise velocity, and bubble surface area) were determined using an X-ray image analysis technique. These measurements aided in the determination of the volumetric heat transfer coefficient as well as the first detailed information on local interfacial phenomenon. This information in turn resulted in the first experimental measurements of the local heat transfer coefficient at different locations within the liquid metal pool.; The effect of the system operating pressure and the water injection rate on the performance and the stability of the liquid-metal/water direct contact heat exchange are reported in detail. Detailed information regarding the thermal performance and the operational stability of a liquid-metal/water direct contact heat exchange were obtained for the mentioned pressure and water flow rate conditions. The resulting extensive experimental database can be used to aid in the design of reactor scale components dealing with high temperature direct contact heat transfer.