Karwa, Nitin (2012)
Experimental Study of Water Jet Impingement Cooling of Hot Steel Plates.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Liquid jet impingement cooling is critical in many industrial applications. Principle applications include extracting large heat flux from metal parts, such as hot fuel bundle post-loss-of-coolant-accident in nuclear reactors, heat treatment of steel plates post-hot-processing, etc. The ability of liquid jets to extract high heat flux at controlled rates from metal parts, with temperatures as high as 800-1000 ºC, at moderate flow rates has made them indispensable in these applications. Due to the complexity of the process, the mechanism of flow boiling heat transfer during jet impingement cooling is not well understood. Resultantly, the presently used design approaches are based more on experience and rule of thumb than science. The principle challenge in the study of jet impingement cooling for these high temperature applications has been the lack of reliable instrumentation for measuring the cooling rates. To add to this, the conjugate nature of boiling heat transfer, especially on low conductivity metal like steel, makes this problem very complicated to understand. Thus, much of the state of art on this subject has been limited to experiments where either the conjugate problem has not been addressed or the tests have been performed at temperatures that are much lower than in the above mentioned applications. The basic objective of the present work is to contribute to the understanding of the thermo-hydrodynamic phenomenon occurring during the cooling of a hot steel plate with an impinging water jet. This work also complements a parallel study being conducted at the Institute of Fluid Mechanics and Aerodynamics (Technische Universität Darmstadt), in which the complex transport processes are being treated theoretically and validated against the experimental results of this work. To achieve the objective, transient cooling experiments have been performed on an instrumented stainless steel AISI-type 314 cylinder. To measure the temperature variation within the stainless steel cylinder during the transient cooling, fast-response thermocouples have been embedded within holes that are precisely drilled though its bottom flat face. The cylinder is induction heated to a homogeneous initial temperature of 900 ºC and is subsequently cooled by means of an axisymmetric subcooled free-surface water jet that impinges on its top flat face (impingement surface). During the cooling, each thermocouple output has been recorded at the rate of 100 samples per second. A two-dimensional axisymmetric inverse heat conduction analysis using these measured temperature data has been performed to estimate the temporospatial variation of temperature and heat flux on the impingement face. Both low and high speed images have been recorded to visualize the two-phase flow. These images and the estimated heat transfer distribution are used to understand the boiling mechanism. The effect of jet parameters, namely subcooling and impingement velocity, on the heat transfer process has been studied. Additionally, the effect of spent liquid accumulation over the impingement surface has been studied in few exploratory plunging jet experiments. This study presents a systematic methodology for the measurement and estimation of the temporospatial variation of heat transfer on the impingement surface of a hot steel plate. Three distinct regions, with difference in the extent of liquid-wall contact, have been identified on the impingement surface from the recorded images. i) A wetted region surrounds the jet stagnation region. Nucleate boiling is the principle heat transfer mode in this region. The outer periphery of this region is called the wetting front. No boiling activity has been observed in the high speed images, most likely because the bubbles were small and were unable to reach the liquid free-surface. The maximum heat flux position is determined to be within this region. As the wetted region grows in size with time, the maximum heat flux position also moves radially outwards. The wetting front and maximum heat flux position velocity reduce with increasing radial distance from the impingement point because the liquid velocity and subcooling reduce at the wetting front. Likewise, the wetting front velocity increases with jet velocity and subcooling. ii) The liquid gets deflected at the wetting front due to the efflux of large vapor bubbles beyond the maximum heat flux position. A term ``wetting front region' has been coined in this thesis to describe this region. The width of this region could not be determined from the high speed images. Transition boiling within a thin superheated liquid film that is continuously replenished by the bulk flow is proposed to be the probable reason for the high heat flux in this region. Further, the radial heat conduction to the wetted region is also significant here. iii) The impingement surface outside the wetting front region is dry. The dry surface slowly cools down due to film boiling and radial heat conduction to the wetting front region. The film boiling rate is very low in the impingement region. After deflecting away from the impingement surface in the wetting front region, the liquid film breaks into droplets over this region. Looking from the side, droplet deflection angle is observed to be small; still these droplets do not come into direct contact with the impingement surface, as has been confirmed by looking down from the top. The velocity of the splashed droplets has been determined by analyzing the high speed images. It has been found that the drop velocity is much lower than the liquid film velocity calculated at the wetting front position using single-phase flow relations suggested by Watson. It has been hypothesized that the liquid film in the wetted region is decelerated by the bubbles growing on the impingement surface. Further, measurements reveal that the drop velocity increases with decreasing subcooling, which means that the film and the droplet are accelerated in the radial outward direction by the vapor released in the wetting front region. It has been shown that the rewetting temperature (analogous to the Leidenfrost temperature for a sessile droplet), which refers here to the temperature below which the direct liquid-wall contact is re-established and the heat flux increases, in both the impingement and radial flow regions is significantly higher than that reported in the literature for pool boiling. Removal of bubbles by the flowing liquid in the early stages of their growth and then their rapid condensation within the subcooled liquid avoids the buildup of vapor near the hot wall, which is the likely reason for the enhancement of the rewetting temperature. This observation confirms that high heat fluxes can be removed at large wall superheats by impinging liquid jets, as practiced in the industry. The boiling curve shifts to higher heat flux and superheat with the increase in the jet velocity and subcooling. The maximum heat flux and surface temperature at maximum heat flux increase with both the jet velocity and subcooling. Area-weighted average boiling curves have been determined, which clearly show the enhancement in the heat transfer with jet velocity over the average surface superheat of 100 to 800 K. The enhancement in jet subcooling is, however, noticeable only in the wall superheat range of 300 to 700 K. The maximum heat flux and surface temperature at maximum heat flux decrease with radial distance from the stagnation point before reaching a constant value. The radial distribution of maximum heat flux condition has been classified into three regions based on the relative size of the hydrodynamic/thermal boundary layer and the liquid film. In the plunging jet impingement studies, it has been found that the wetting front growth slightly slows down due to accumulation of the spent liquid over the impingement surface. Area-weighted average boiling curves show that the heat transfer reduces due to accumulation.

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