Fluid dynamics of direct steam condensation under pressure (DENISE)

Background

For the safety assessment of nuclear power plants, flow and heat transfer processes that occur during a loss-of-coolant scenario and the injection of cold water are investigated. The resulting steam–water multiphase flows require the development of physically sound and experimentally validated models.

A key phenomenon is direct contact condensation, which occurs when cold water comes into contact with a saturated steam atmosphere. This leads to highly turbulent phase-change and heat transfer processes. Experimental single-effect studies conducted under realistic pressure conditions allow fundamental mechanisms to be investigated in isolation. In this way, a comprehensive database is generated to support the further development of turbulence, heat transfer, and mass transfer models for numerical codes.

Figure 1 illustrates the particularly relevant application scenarios. These include condensation processes at the phase interface of stratified flows (A), cold water entering as a free jet (B), and the condensation of steam bubbles entrained by a submerged water jet (C).

Figure 1: Direct Contact Condensation Processes: (A) in stratified flow, (B) at free jets, and (C) during steam entrainment.

Investigations of direct contact condensation are not only relevant for reactor safety analyses. The phenomenon also affects the operation of condensation chambers in boiling water reactors, desalination and water treatment processes, as well as the design of steam pipelines, where condensation-induced water hammer may occur. In addition, the well-measurable heat transfer at the phase interface provides a valuable model for other multiphase and mixing processes in industry, reactor engineering, and even climate modeling—for example, in gas and aerosol exchange at the ocean surface.

Experimental Setup

The experimental studies were carried out using the DENISE (Direct Condensation and ENtrainment Installation for Steam Experiments) facility inside the pressure vessel of the TOPFLOW test facility. The basin is designed to be particularly shallow in the viewing direction of the cameras, allowing for optimal observation conditions. A water depth of 50 mm has proven suitable for minimizing three-dimensional flow effects and wall influences. Additional dimensions are adapted to the geometry of the pressure vessel. Figure 2 shows the experimental setup inside the pressure vessel.

Figure 2: Simplified Representation of the Experimental Basin in the Pressure Vessel of the TOPFLOW Facility

The facility periphery includes steam injection, condensation, and a closed water circulation loop. Steam supplied from the TOPFLOW facility flows through the upper section of the basin and is fully condensed in the condenser. The water in the loop can be extracted at two locations, adjusted to the target temperature using heaters or coolers, and re-injected at two points. Valve positions and pump speeds are used to control the respective mass flow rates.

In addition to conventional process instrumentation, the following advanced measurement techniques were employed:

• High-speed temperature measurement (500 Hz)
• Temperature and pressure probe lances for position-adjustable measurements within the pressurized environment
• Optical imaging using a high-speed camera with mirror-reflector arrangements
• Infrared camera (0 to 300 °C, 10 fps at 30 mK sensitivity, approx. 1 mm/pixel resolution) using a 1 mm thin stainless steel sheet as an optical window

This setup enabled the acquisition of unique experimental data, such as temperature distributions within the steam region and integral condensation rates. For multidimensional characterization of the flow, a mirror arrangement was installed around the experimental setup (see Figure 3).

Figure 3: Arrangement of Imaging Measurement Techniques (left) for Multidimensional Characterization of Flow Phenomena in the Basin (right) during Gas Entrainment Studies.

Co-Current Flow Experiments

Various co-current flow conditions were established in the basin, involving subcooled water flowing alongside saturated steam. Temperature measurements and high-speed imaging provided insights into temperature and turbulence distributions within the basin (see Figure 4).

Figure 4: Idealized streamlines for (a) co-current flow and (b) partial counter-current flow in the basin.

The observed condensation rates for this type of direct contact condensation reached only about 2.5% of the theoretical maximum potential.

Free Jet Experiments

The behavior of subcooled free jets in saturated steam and nitrogen atmospheres was investigated using optical measurement techniques. The evolution of geometric parameters — such as jet length, jet diameter, and wave velocity — was analyzed, as illustrated in Figure 5.

Figure 5: Mean jet diameter as a function of (a) jet mass flow rate and (b) ambient pressure.

For this type of direct contact condensation, the observed condensation rates fully utilized the theoretical potential. The behavior could be described using a modified model based on Celata.

Gas Entrainment Studies

While entrained steam condenses shortly after entering the water reservoir, non-condensable gases form a characteristic plume of bubbles. Rising gas bubbles slow down the incoming jet (see Figure 6).

Figure 6: Single frames from gas entrainment experiments with nitrogen and steam in water.

The penetration depth of entrained steam bubbles increases with the mass flow rate of the free jet. The condensation rate at the jet increases with greater subcooling, which simultaneously leads to reduced penetration depth and less gas entrainment. Based on these investigations, a minimum impact velocity was identified above which gas entrainment occurs.

For this type of direct contact condensation, the observed condensation rates reached approximately 25% of the theoretical maximum potential.

Acknowledgements

This project was funded by the Federal Ministry for Economic Affairs and Energy under grant reference number 150 1411. The authors are responsible for the content.

Publications

• Seidel, T.; Beyer, M.; Lucas, D.; Hampel, U.
  Experimental Studies on high-pressure high-temperature Contact-Condensation at falling jets in the TOPFLOW Pressure-Tank
  Nuclear Engineering and Design 336(2018), 54-63
• Hampel, U.; Seidel, T.; Beyer, M.; Szalinski, L.; Lucas, D.
  Pressure-tank technology for steam-water two-phase flow experiments at elevated pressure and temperature
  Specialist Workshop on Advanced Instrumentation and Measurement Techniques for Nuclear Reactor Thermal Hydraulics (SWINTH), 15.-17.06.2016, Livorno, Italy
• Seidel, T.; Beyer, M.; Lucas, D.
  Direct Condensation and Entrainment steam experiments at the TOPFLOW-DENISE facility
  The 16th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-16)
• Seidel, T.; Lucas, D.
  Dampf-Experimente zur Kontaktkondensation und zum Blasenmitriss in der TOPFLOW-Anlage
  Jahrestagung der Kerntechnischen Gesellschaft, 17.-19.05.2011, Berlin, Deutschland