Influence of pressure on Leidenfrost effect

• Main Author: Buchmüller, Ilja
• Format: Others
• Language: German; en
• Published: tuprints 2014
• Online Access: https://tuprints.ulb.tu-darmstadt.de/4072/13/Druckeinfluss_x3c.pdf; Buchmüller, Ilja <http://tuprints.ulb.tu-darmstadt.de/view/person/Buchm=FCller=3AIlja=3A=3A.html> (2014): Influence of pressure on Leidenfrost effect.Darmstadt, tuprints, Technische Universität, [Ph.D. Thesis]

The Leidenfrost effect influences substantially the contact of a liquid droplet with a hot surface. Contact between the liquid and solid is crucial for cooling applications such as fire-fighting, hot-mill steel rolling, thermal power plants and microprocessor cooling. In automotive or aerospace internal combustion engines, the combustion chamber is pressurized prior to ignition. Despite various effects of elevated pressure, the combustion process needs to be controlled. The reaction time in a combustion chamber is limited, but mixture preparation involves prior evaporation. Fuel in contact with a combustion chamber wall evaporates in a uncontrolled manner. Furthermore, the fuel reacts with lubricants and decomposes into coke residue. The fundamental physics of the Leidenfrost effect are yet to be fully understood. This applies especially to the influence of pressure on the Leidenfrost effect. For the question, if injected water droplets would stay in contact with the heated combustion chamber wall, current models would need to be extrapolated from ambient pressure, although there is no validation experiment available. This experimental study addresses the influence of elevated pressure on the Leidenfrost effect, providing observations and measurements suitable to validate theories, hence extending the knowledge about the Leidenfrost effect. The experiment is implemented inside a pressure chamber and results for single water droplets impinging onto a hot aluminium substrate are presented. The droplet impingement Weber number was 5. The experiments were conducted at chamber pressures from 1 to 25 bar (0.1 to 2.5 MPa) and wall temperatures from 100 to 460 °C (373 to 733 K). Based on video observations, phenomenological boiling states are identified and mapped on a pressure-temperature diagram. The various states of impact behaviour shift to higher temperatures with increasing pressure. Nucleate boiling and critical temperature models of the bulk liquid serve as the lower and the upper bounds for the transition for all observed states of droplets, respectively. A new nucleation model, accounting for the fluid flow inside the impacting droplet, agrees reasonably well with experimental results for the nucleate boiling in the experiment. All previous theories and correlations predict transition temperatures which are constant or deviate from experimental values at elevated pressures. Therefore, the theoretical part of this study tests refined hypotheses for transition from nucleate boiling to film boiling. A Landau instability model and a bubble percolation model propose explanations for transitions in the boiling phenomena, but these models deviate from experimental results. Refinement of these approaches is still needed with respect to the characteristic length of instability and the active nucleation site count. The experimentally observed onset of the transition state exhibits a linearity between the reduced pressure value and the reduced contact overheat. The boiling states are further quantified with the measurement of the residence time upon the target. In the wetting state at ambient pressure, the residence time is equal to the evaporation time of the droplets. Residence time is lower for the transition and the Leidenfrost rebound states. The droplet detaches from the surface prior to complete evaporation. Residence time thresholds mark the observed transition state. Asymptotic rebound time marks the rebound state. The time thresholds follow the state borders in the pressure-temperature map. Secondary droplets are detected with the shadowgraph technique. Characterization of the secondary droplets has been achieved using a new image processing algorithm. It is based on the irradiance model of a semi infinite screen. The Sauter mean diameter of the secondary droplets in transition boiling state increases with increasing pressure. An increasing trend of the Sauter mean diameter of secondary droplets to the bubble departure diameter was observed.