Multiphase numerical modeling and investigation of heat transfer during quenching of metallic specimen by means of water jets

The necessity of advanced efficient materials for engineering applications such as structural, aerospace, automotive etc., is expanding. To improve the usage characteristics for instance hardness, strength, heat treatment utilizing the water jet quenching process has been conventionally adopted by metal industries. To achieve this, metal plates are heated and rapidly cooled with water jets or array of jets. During this process, the spatial and temporal thermal history in the material based on the process control parameters determines the resulting material properties of the metal plate. Since the metal plate is quenched from a very high surface temperature, this creates immense temperature gradients within the plate, which may lead to distortion and even cracking. In addition, for applications such as direct chill casting and hot rolling, the metal plate traverses under the water jets, escalating the process complexity. Due to the Leidenfrost effect, the challenge is to establish a controlled heat transfer where the water contacts with the hot surface (wetting), which is particularly hindered by the strong vapor generation at the hot surface. The heat transfer is dominated by boiling heat transfer (film, transient, nucleate boiling). Therefore, a thorough investigation of the spatial and temporal heat transfer of this conjugate heat transfer process is necessary to identify the relevant process control parameters. In this thesis, the quenching of a hot metal plate with circular water jets (full jet) is investigated for stationary as well as moving plates. The impact of the nozzle arrangement in groups or fields with the configurations such as inline and staggered on boiling heat transfer is analyzed for stationary plate quenching. The plate materials investigated are aluminum alloy (AA6082) and stainless steel (1.4828), where the quenching fluid is normal tap water at room temperature. To address this conjugate heat transfer problem, a two-phase numerical model based on the Euler-Euler approach has been developed to simulate the full quenching of both the stationary and the moving metal plate, where water acts as a continuous phase and the water vapor as a dispersed phase. To model the phase transfer during quenching (water/water vapor), the source and sink terms of the corresponding conservation equations for mass, momentum and energy are modified accordingly. The model for the translational motion of the metal plate has been realized by adopting the concept of a sliding mesh. Experiments are performed to validate the developed numerical models in a two-way approach by means of high-speed and infrared imaging considering the hydrodynamic and thermodynamic perspectives. This validated numerical model can provide the relevant hydrodynamic and thermodynamic engineering parameters directly from the impinging side of the plate such as wetting front propagation, heat flux, HTC (heat transfer coefficient), cooling curve etc., which cannot be easily obtained from conventional experiments. The influence of process control parameters on the boiling heat transfer during quenching is analyzed numerically by varying the jet Reynolds number, plate velocity, plate material, plate thickness, plate initial temperature and subcooling, providing a detailed insight into the hydrodynamic and thermodynamic behavior during the jet quenching of a metal plate in the heat treatment process.