EE- Enerji Bilim ve Teknoloji Lisansüstü Programı - Doktora

Bu koleksiyon için kalıcı URI

http://hdl.handle.net/11527/199

Gözat

Sustainable Development Goal "none" ile EE- Enerji Bilim ve Teknoloji Lisansüstü Programı - Doktora'a göz atma
  • Öge
    Experimental and numerical investigation of single and multiple droplet interactions with high-temperature surfaces
    (Lisansüstü Eğitim Enstitüsü, 2021) Gültekin, Ahmet ; Çolak, Üner ; 698448 ; Enerji Bilim ve Teknoloji
    The phenomenon of droplet impingement onto solid surface can be seen in several industrial applications such as ink-jet printing, fuel injection process in internal-combustion engines, spray coating, and spray cooling systems. For several decades, the importance of this phenomena has inspired several researchers' investigations in pursuit of a thorough understanding of the interactions between mass, momentum and heat transfer. However, despite many experimental and numerical studies, the effect of droplets on a solid surface is not fully understood. This problem becomes more complicated if the surface is heated during the impact and interacts with other droplets. In the literature, most of the studies have focused on single droplet impact, while those relating multiple droplet impacts are quite more limited. After the single droplet impact onto the solid surface, droplet starts to spread and a circular lamella takes shape. However, after multiple droplet impingement onto a solid surface, the interaction phenomenon will occur if the droplets are too close to each other based on their impact parameters. The hydrodynamic outputs and heat transfer activities of the droplets are very distinct from single droplet cases due to this interaction. This interaction phenomenon leads to uprising sheets which causes lesser spreading area per droplet on solid surface. The challenge of understanding of physical mechanism and modeling the phenomenon of spray cooling comes from the droplets' randomness and untraceable behavior. For that purpose, it is important to examine a simplified method with a known number of droplets. In addition, most of the studies available in the literature have provided limited quantitative data, such as spreading diameter, and lamella height. Recent developments in visualization and imaging technology provide significant advantages in measurement techniques. Spreading phase after droplet impingement is characterized by rapid energy conversions and dissipations within a small area. The major limitation lies in the difficulty of optically reaching the target region for non-invasive physical quantity measurements. The success of particle image velocity (PIV) measurement is especially noteworthy in such circumstances. This technique makes it possible to evaluate complex actions over time in more detail. Therefore, time-dependent radial velocity distribution of the droplets in the spreading phase was measured using the PIV technique. This thesis consists of two main sections: experimental and numerical studies. Experimental studies were carried out in the Visualization Laboratory at Tokyo University, Nuclear Engineering and Management Department. An experimental setup was designed and built to carry out experimental studies. To get simultaneous multiple droplet, a droplet generation and control system has been developed. In the experimental investigations, shadowgraph and PIV methods are used. In the simulation part of this thesis, hydrodynamics behavior and cooling performance of single and multiple droplets were investigated by using Computational Fluid Dynamics (CFD). StarCCM + (version 2019) has been used to perform numerical investigations on a workstation (4 cores 16 GB RAM). New numerical models have been developed for the liquid and gas phases including mass, momentum, and heat transfer equations based on the "Volume of Fluid" (VOF) method. These numerical models were validated with experimental data. 2-D axisymmetric VOF model has been used only for single droplet investigations whereas, 3-D VOF model has been used for multiple droplet interactions. The results obtained in this thesis are summarized as follows: Variation of uprising sheet height with dimensionless time is compared with an existing theoretical model. The theoretical model agrees with experimental results for initial phases. However, the experimental findings do not fully match with the theoretical findings in later phases owing to uprising sheet losing its balance. The contact time of a single droplet in the film boiling has been compared with available correlations in the literature and it has been observed that the experimental results are largely in agreement with available correlations. Also, a new empirical correlation is proposed. It is seen that MAPE value is 3.12% and correlation successfully represents the contact time. It has been observed that the rebound phenomenon for simultaneous and non- simultaneous double droplet cases take place faster comparing with a single droplet. This is more probably due to the less spreading area per droplet owing to droplet interaction. Also, using PIV method, the variation of radial velocity was examined inside the droplets for different temperatures. It was observed that radial velocity is linear over a comparatively wide range of spreading radius, but due to capillary and viscous forces with time, the radial velocity profiles took a non-linear shape in the exterior radial positions. At the high surface temperatures, particularly in later stages, increases the uncertainties in the radial velocity distribution in exterior radial locations because of intense disturbances are formed at the interface by the bubble nucleation. Afterwards, PIV data within the lamella were compared with a theoretical model at ambient temperature. For high We case, the analytical model highly agrees with linear sections of the radial velocity profiles. For moderate We case, during the early spreading phase, the model highly agrees with the linear pieces of radial velocity profiles in the inner radial positions. Partial linearity is still identified in the latter stages, but the theory differs from radial velocity profiles. Furthermore, the spreading velocities within droplet pair are examined at ambient temperature using PIV. Creation of uprising sheet leads to an upward flow which causes this extra stagnation point in the interaction region. Computational models for single droplet were validated by comparing qualitative shadowgraph images, spreading factor as well as radial velocity distributions within the droplet. For multiple droplet case, to validate computational model variation of the dimensionless uprising sheet with time was also compared. Total number of mesh elements are 125 000 and 7.5 × 106 for 2-D Axisymmetric and 3-D model, respectively. When vertical distance is close to each other, uprising sheet created by spreading liquid lamella collision still can be detected. However, when vertical distance is wider the uprising sheet could not be observed. Also, it is observed that decreasing vertical spacing leads to reaching maximum values of total spreading area and heat flow earlier. As the horizontal distance between the droplets gets shorter, the magnitude of interaction increases and the spreading area covered on the surface and heat transfer decreases. A mathematical expression is proposed to predict the dimensionless spreading area per droplet for multiple droplets impacting simultaneously on the solid surface taking into account the variation of spreading factor for a single droplet. Cooling efficiency and performance loss are defined to see the effect of droplet number taking into account the variation of heat flow for a single droplet. It was found that the interaction strongly effects the cooling performance especially in the first stages after the multiple droplet interaction.