Study of enhanced boiling heat transfer from porous graphite foam structures

• Main Author: Pranoto, Indro
• Other Authors: Leong Kai Choong
• Format: Theses and Dissertations
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Mechanical engineering::Energy conservation; DRNTU::Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/65449
• Institution: Nanyang Technological University

Rapid developments of new-generation electronic devices in the last few decades have provided the impetus to develop compact and effective cooling systems that are able to dissipate high heat fluxes and maintain the devices at desirably low temperatures. To address this need, much effort had been put in by engineers and researchers to develop densely-packed electronic cooling systems. Two-phase cooling is widely recognised as a promising technique to remove high heat fluxes. In addition, porous materials such as porous graphite foams of low density, high effective thermal conductivity and large total exposed surface area have been recognised as potential heat sinks or evaporators. However, the boiling process is a complex phenomenon. Combining it with the complexity of the internal structure of the graphite foams has rendered the boiling mechanism from such structures to be not well understood. This study represents a comprehensive experimental and analytical study of boiling heat transfer from porous graphite foams. Pool and flow boiling experimental facilities were developed to study the boiling heat transfer performance, bubble characteristics, and boiling parameters with graphite foam evaporator and dielectric liquid coolant. Pool boiling experiments with different graphite foam types, foam geometry and working fluid types were conducted in the study. “Pocofoam” of 61% porosity, “Pocofoam” of 75% porosity, “Kfoam” of 78% porosity, and “Kfoam” of 72% porosity were tested as evaporator inserts. Three dielectric liquids viz. FC-72, HFE-7000, and HFE-7100 were selected as the working fluids. Important boiling parameters such as bubble departure diameter, bubble departure frequency and nucleation site density were determined. Bubble growth and boiling phenomena from the porous graphite foam were analysed by a visualisation study. Boiling heat transfer enhancements of up to 3 times were obtained with porous graphite foam evaporators. It was also found that the critical heat flux (CHF) was enhanced by about 2 times to 225 W/cm2. The role of internal pore structure of the graphite foam was investigated in this study by testing block and fins structures of different fin-to-block-surface-area ratios. The study shows that the block structure attained higher boiling heat transfer performance of up to 1.6 times as compared to fin structures which indicates the important contribution of the internal pore structure of the porous graphite foams to the enhancement of boiling performance. The boiling phenomena from the block and fin structures were captured and analysed through a visualisation study. It was concluded that the entire pore structure of the foam provides possible nucleation sites for generation of bubbles which contribute to the boiling heat transfer coefficient. From the experimental data, pool boiling heat transfer correlations from the porous graphite foams were proposed. The Rohsenow correlations were modified by accounting for the equivalent boiling surface area of the foam. Beside pool boiling experiments, flow boiling heat transfer on the porous graphite form structure was also investigated. The effects of foam properties, coolant mass flux and evaporator gaps on the cooling performance were studied. Bubble flow characteristics from the porous graphite foam structure were captured and analysed. The experimental results show that the foam properties, coolant mass flux, and evaporator gaps have affected significantly the cooling performance. The results suggested that higher coolant mass fluxes are able to sweep away the generated and flowing bubbles more effectively and increase the liquid replacement rate to the pore structures. It was also found that a smaller evaporator gap produces larger coalesced bubbles and causes bubble confinement and the formation of vapour layers on the top foam surface resulting in significant increase of the wall temperatures. Working fluid replenishment rate was reduced accordingly by reducing the evaporator gap and resulted in the decrease of the bubble departure frequency. Semi-analytical models of bubble departure diameter and active nucleation site density for porous graphite foams were developed in the study. It was found that the proposed bubble departure diameter model is a function of pore radius, coolant properties, contact angle and bubble growth time. The bubble departure diameter from the graphite foams were measured from the captured boiling images and compared with the predicted values. Two models which predict the active nucleation site density from the foam structures were developed. The first model was constructed based on a heat and mass balance analysis, while the second model was developed based on the probability density function of cavity radius in porous graphite foams. The deviations from both models are about 27%. It is the author’s hope that the results of this present study can be used as guidelines in the design and use of porous structures in current and future high heat flux electronic device cooling systems.