Lee, Hao-En (2016-05). Effects of Confined Structures on Pool Boiling Heat Transfer. Master's Thesis.
Pool boiling heat transfer is one of the most effective heat transfer processes used in a host of applications. The enhancement of pool boiling has been studied for decades by considering a variety of surface modifications and configurations. For that reason, this study investigates the effects of the confined structures on the pool boiling heat transfer. The confined structures consist of flanges, a plate with a central orifice and a mesh with a central orifice. The diameters of the plate orifice are 2 and 4 mm; the diameter of the mesh orifice size are 2.5, 3.5, 4.5 mm. By comparing the boiling curves of different confined structures, the effects of confinement on heat transfer performance can be evaluated. An infrared camera and a high speed camera were used to capture bubble images and for measuring surface temperature. Furthermore, a pump assisted system was used to determine the effect of confinement on vapor quality and system pressure. The test results show that pool boiling heat transfer can be generally enhanced by using confined structures. The level of enhancement depends on the orifice size of the plate and the mesh. Smaller orifice size leads to higher heat transfer enhancement. The results of the pump assisted test indicate that the boiling heat transfer enhancement can be attributed to the bubble coalescence process and the induced shear flow caused by the coalesced bubble departure. Results also indicate than an increase of vapor generation (quality) and the induced shear flow rate (shear stress) can be found when increasing the level of confinement on the pool boiling process. Furthermore, using a mesh in confined structure can provide a higher heat transfer coefficient when compared to the no-mesh cases when a pump is used to facilitate bubble departure. In summary, the results show that using meshes leads to better heat transfer performance, which cannot be replicated using a solid surface as confinement structure.