Heat--Transmission

Boiling heat transfer associated with bubble growth is perhaps one of the most efficient cooling methodologies due to its sizeable latent heat during phase change. Despite significant advancement, numerous questions remain regarding the fundamentals of bubble growth mechanisms, a primary source of enhanced heat dissipation. This thesis provides a comprehensive examination of the mechanisms involved in the growth of bubbles during nucleate boiling. By conducting a combination of experiments and numerical analyses, the goal is to enhance our understanding of bubble growth phenomena and their impact on heat transfer. Initially, the experimental work focuses on comparing the heat transfer performance and parameters related to bubble dynamics between regular and modified fin structures. The findings demonstrate that the modified fin structure, which featured artificial nucleation sites, exhibits superior heat transfer characteristics. This improvement is attributed to changes in the bubble departure diameter, bubble departure frequency, and growth time. Subsequently, an artificial neural network is developed to accurately predict the bubble departure diameter based on the wall superheat and subcooling level. This predictive model provides valuable insights into bubble behavior originating from artificial nucleation sites.

A Computer Automated Radioactive Particle Tracking (CARPT) facility was designed and implemented for the investigation of hydrodynamics in two phase flows. This facility was complemented by a versatile fluidized bed facility capable of handling high air flow rates. Solids mean dynamic behavior and heat transfer to internals in a 29.21 cm diameter fluidized bed were investigated for different operating conditions. Different flow parameters like the solids ensemble-averaged velocity, stagnancy and the phase density in the presence of horizontal tubes were determined using the CARPT facility. Local circumferential variations of heat transfer coefficients at the surface of horizontal tubes were measured at different locations in a large particle fluidized bed using a miniature heat transfer probe assembly. The influence of solids hydrodynamics on the heat transfer coefficient in gas-fluidized beds was investigated. The data obtained in the present study was compared to current heat transfer models for large particle gas-fluidized beds.