CRITICAL AFFAIRS OF BOILING HEAT TRANSFER RELATED TO ENHANCEMENT TECHNIQUES AND INTERFACIAL INTERACTIONS

Abstract

To understand the fundamentals of the boiling process and enhance its performance, a thorough study on gas-liquid phase change heat transfer from macroscale level experiments and simulations to nanoscale level investigation is targeted in the present work. At first, a proposal for enhancement of flow boiling is made by developing tubes with discrete internal structures. Heat flux from 5 to 15 kW/m2 and mass flux from 40 to 300 kg/m2s has been considered as experimental range using R407C and R134a in an in-house experimental setup. The heat transfer characteristics of the newly developed tubes with internal micro-dimpled tube (IMD1 & IMD2 tubes) and the tube with internal bi-directional tunnel structure (BTS tube) are compared with the commercially available micro-fin tube (MF). Overall performance index of BTS tube is found to be higher than all other enhanced tubes considered in the present study for the experimental range considered. Comparison of the flow pattern of refrigerant inside newly developed tubes have been made to confirm the fluidic facts behind enhancement. By increasing nucleation site density, it has been shown that a higher heat transfer coefficient can be achieved with moderate pressure drop. With a target of understanding the interfacial dynamics in flow boiling, VOF based numerical simulations have been performed with R134a refrigerant flowing inside horizontal plain and structured tubes. The various stages of bubble dynamics like nucleation, growth, merging and departure have been observed for a range of applied heat and mass flux. An attempt has been made to estimate the number of nucleation sites and the bubble departure diameter for flow boiling of refrigerant inside the smooth tube. Numerical simulations have also been made in case of flow boiling inside the enhanced tube by providing artificial nucleation sites in the form of indentations or subsurface tunnel structures at the inner wall. An increase in nucleation sites has been observed for the structured tubes as compared to the plain tube and the corresponding change in bubble departure phenomenon have been reported. The heat transfer coefficients at different heat flux, mass flux, saturation temperature and tube configurations have been evaluated to link observed interfacial phenomenon with enhancement factor. Further, an effort has been made to numerically study the interactions between the vapour films generating due to boiling around rod bundle arrangement placed in a saturated water pool. Lagrangian Smoothed Particle Hydrodynamics (SPH) method has been used for domain discretization and interface reconstruction. It has been shown that the presence of cylinder in vertical neighbourhood results in suppression of vapour film generating from the bottom cylinder, whereas, the vapour films generating from two cylinders placed in horizontal neighbourhood experience a horizontal shift. Studies have also been made to observe multidirectional interactions of vapour films with heated cylinders placed in an inline array, vertically staggered and horizontally staggered arrangements. It has been found that the highest deviation from unconstrained growth occurs in the case of the centre cylinder in a horizontally staggered arrangement as compared to reported other arrangements. Mutual interaction coefficient and indices are proposed to judge the best possible arrangement in the case of stacked cylinders. Finally, an attempt has been made to understand the different phase change phenomena like evaporation, nucleate boiling and the rapid film boiling phenomena under the influence of the electric field from a molecular viewpoint. At first, a proposal is made to demonstrate features of thermodynamic evaporation at the nanoscale using only an external electric field. Further, the behaviour of water molecules on the solid copper surface during the rapid film boiling process in the presence of an electric field of different intensities has been studied. Molecular reasonings behind the suppression of the Leidenfrost phenomenon upon application of uniform electric field along the heating substrate is established. The effect of surface characteristics with different wettability on film boiling in the presence of an electric field has also been studied. The electric field produces a finger-like water column besides thinning of water film over a non-wetting surface. A similar phenomenon is also evident over hydrophilic surface only after reaching a threshold value of electric charge intensity. Molecular simulations have explained the phenomenon of nucleate boiling of water on hydrophilic and non-wetting surfaces. Finally, the ability to control the bubble formation and suppression at a required location using an electric field has also been demonstrated. The water molecules near the surface experience dispersion at a lower electric field and attraction force at a higher electric field, mimicking bubble nucleation and suppression, respectively.