Experimental study of direct contact condensation of steam in turbulent duct flow

This thesis deals with an experimental study on direct injection of steam into a crossflow of water. The main goals are to identify the main parameters that influence direct steam injection in liquids and to quantitatively access their impact on the interaction of the condensation and the liquid cross-flow both in the proximity of the injection nozzle and in the far-field single-phase flow. The experiments are carried out in a pressurized flow loop consisting of a measurement section with a square inner geometry of dimensions 30 x 30 mm2 . The measurement section is optical accessible near the steam injection point and also includes a hydraulic development section with a length of 1200 mm (40Dh). To verify whether the flow is fully developed after passing the hydraulic development section, velocity measurements, using PIV 1 are performed in the centre plane of the duct parallel to the main direction of the flow. For two Reynolds lltunhers, profiles of the mean axial and lateral velocity components as well as distributions of the higher order statistics are measured and compared with DNS and LES results at corresponding Reynolds numbers. The agreement between the measured and numerical profiles is satisfactory, apart from some deviations of the experimental profiles in the near-wall wall regions due to measurement inaccuracies. The experimentally observed dependency of the Reynolds stress gradient on the Reynolds number is in close agreement with the trend estimated from Prandtl's law of friction for fully developed pipe flow. This indicates that the duct flow has reached its fully developed state at a distance of 43Dh from the inlet of the hydraulic development section. If any streamwise variations in the flow properties are present, they are expected to be small and would remain undetected due to the limited height of the observed domain. The condensation of steam in the liquid cross-flow has been investigated in the proximity of the steam injection point by means of high speed photography at various steam mass fluxes, liquid cross-flow rates and approaching liquid temperatures. The high-speed recordings clearly reveal an intermittent character of steam pocket growth and collapse/condensation. Pocket length typically grows linearly until it reaches a maximum penetration depth. Subsequent disappearance of the pocket occurs either via detachment and collapse or via instantaneous break-up. The initial steam pocket shape predominantly resembles that of a truncated sphere while in later stages of growth either a spherical or ellipsoidal shape is observed, depending on the steam mass flux and temperature of approaching liquid. The main effect of the liquid crossflow is a significant reduction of the steam pockets' maximum length and growth time. Heat transfer coefficients between water and steam have been determined, based on the smoothed steam pocket interfacial area. The measured trends show an increase of the heat transfer coefficient with increasing liquid cross-flow rate. A new correlation to predict the Nusselt number of intermittent steam condensation is presented, using a characteristic steam plume length, as the physically relevant length scale. The effect of cross-flow is incorporated in the Nusselt correlation via an extra Reynolds number based on the liquid cross-flow rate. A model for topology history prediction has been developed to facilitate interpretation of measurement results and to increase our predictive capacity of intermittent steam condensation. With a correction factor on the input value of the heat transfer coefficient, both the steam pockets' growth time and its maximum penetration depth, are predicted reasonably well. The growth of a steam pocket in intermittent condensation regimes is found to be controlled by fluid inertia and injected momentum of steam, while drag is negligible. Velocity measurements in the region upstream the steam injection point have been carried out to investigate the far-field single-phase jet that is induced by the condensation of steam and deflected by the liquid cross-flow. The injected steam mass flux, liquid cross-flow rate and the liquid approach temperature are varied to study their influence on the jet centreline, velocity distributions and turbulence properties. The measured velocity fields show that the ratio of injected steam momentum and cross-flow momentum is largely governing the flow field, while effects of the liquid approach temperature are found to be minor. Jet center lines trajectories, based on the loci of maximum velocity magnitude in the jet appear to collapse onto a single curve if scaled with the product of the nozzle diameter and the effective velocity ratio. This collapse of centerlines is similar to that observed in non-condensing jets in cross-flow. A power-law correlation is proposed to describe the position of the collapsed centreline trajectories for the condensing jet in cross-flow. A similarity analysis has been applied to the lateral distributions of the mean streamwise velocity component by evaluating the velocity components, measured in cartesian coordinates, in a rotated frame of axes. Lateral distributions of the jets' mean velocity excess, scaled with the maximum excess, at successive streamwise coordinates collapse onto a single curve when the spanwise coordinate is scaled with the jets' half-width. In addition, the centerline velocity excess appears to be inversely proportional to the streamwise coordinate whereas the jets' half-width is found to increase linearly in that direction. This demonstrates that the jet flow displays self-similarity properties which resemble those of a free turbulent jet. turbulence intensity profiles have been investigated in the rotated frame, for two momentum flux ratios. The distributions along the spanwise axis show that the streamwise and lateral turbulent fluctuations exhibit maximum values at the centreline of the jet and that they rapidly decrease in spanwise direction to the normal turbulence level of the cross-flow. turbulence intensities appear to increase with increasing momentum flux ratio. Finally, scaling laws are proposed for the centreline decay of the streamwise and lateral RMS-fluctuations.