Numerical Investigations Of Surface Wettability Effects In Pool Boiling Heat Transfer By Lattice Boltzmann Method

Mechanisms and enhancements of microscale heat transfer have become one of hot spots among academic circles. Surface wettability near the three-phase contact line is a key factor and controlling the wettability is an effective way to enhance microscale heat transfer. However, it is very difficult to get rid of roughness effects when studying pure wettability effects on boiling heat transfer by experiments. On the other hand, existing macroscopic models are unable to compute the nucleation process, which is a key of this study. In this thesis, a lattice Boltzmann liquid-vapor phase-change model is proposed to study the wettability effects in pool boiling heat transfer. The detailed research contents are described as follows:1. Development of a pseudo-potential lattice Boltzmann model for multiphase flows. An improved lattice Boltzmann model for multiphase flows is proposed by combining a new interparticle interaction force scheme and the exact difference method to incorporate the force term. The proposed model can give more accurate and stable numerical results in a wider temperature range and avoid the unphysical phenomenon of relaxation time dependence. To test the proposed model, droplet movement, coalescence and splitting processes on surfaces with wettability gradients are numerically investigated and the mechanisms of droplet motions under wettability gradients are illustrated in details.2. Development of a lattice Boltzmann model for liquid-vapor phase-change heat transfer. A lattice Boltzmann model for liquid-vapor phase-change heat transfer is proposed by combining our proposed model for multiphase flows and an energy equation model. A new form of the source term in the energy equation is derived and our improved pseudo-potential lattice Boltzmann multiphase flow model is used to improve the numerical stability. There is no need to track the interface explicitly in this model and the phase change is determined by the thermodynamic relations given by the equation of state for real gases. As a direct simulation method for liquid-vapor phase-change heat transfer, this lattice Boltzmann phase-change model is able to simulate the entire ebullition cycle including the bubble nucleation process, no need to place a small vapor bubble at the beginning of computation. Pool boiling from a micro-heater(i.e., point heat source) is studied by this proposed lattice Boltzmann model for liquid-vapor phase change heat transfer. It is shown that pool boiling characteristics on a micro-heater may differ a lot from those on a macro-heater. Important information, such as nucleation time and nucleation temperature under constant heat flux conditions, which was unable to obtain by other numerical simulation methods, is obtained and analyzed for the first time. Effects of wettability on bubble nucleation time and nucleation temperature during pool boiling from a micro-heater are illustrated.3. Investigation of wettability effects on pool boiling heat transfer from uniform hydrophilic/hydrophobic surfaces. Effects of wettability on pool boiling heat transfer from smooth uniform hydrophilic/hydrophobic surfaces with a finite thickness are studied by our proposed phase-change lattice Boltzmann method. The heat conduction process is considered in the heated substrate. It is shown that bubble departure diameter increases with the increasing contact angle and superheat. Hydrophilic surfaces and hydrophobic surfaces exhibit quite different bubble departure modes: the whole bubble departs from the hydrophilic surface with a waiting period in the ebullition cycle while a residual bubble will be left on the hydrophobic surface when bubble departs and hence no waiting period exists. The bubble departure mode on the hydrophobic surface contributes to the high boiling heat transfer on a hydrophobic heating surface. It is also demonstrated that the pool boiling heat transfer mechanism on a hydrophilic surface differs from that on a hydrophobic surface. For pool boiling from a hydrophilic surface, a microlayer of liquid exists between the bubble and the heated surface and microlayer evaporation is the important heat transfer mechanism; For pool boiling from a hydrophobic surface, no microlayer exists on the heated surface and the three-phase contact line region exhibits the highest local heat flux and lowest local temperature, hence three-phase contact line heat transfer is the important heat transfer mechanism. Boiling curves from natural convection regime to nucleate boiling regime, to transition boiling regime to film boiling regime for hydrophilic and hydrophobic heating surfaces are obtained by numerical simulation for the first time, and onset of nucleate boiling(ONB), critical heat flux(CHF) and Leidenfrost point are captured numerically.4. Investigation of pool boiling heat transfer enhancement on mixed wettability surfaces. Pool boiling heat transfer from smooth surfaces with mixed wettability is studied based on our proposed phase-change lattice Boltzmann method. Using numerical simulations makes it possible to study pure wettability effects by excluding roughness effects on the surface completely. It is demonstrated that addition of hydrophobic spots on smooth hydrophilic surfaces promotes bubble nucleation and enhances boiling heat transfer. The mixed wettability surface is also expected to enhance critical heat flux(CHF) by regulating vapor spreading behaviors over the heating surface. Local heat flux distributions on the hydrophobic and the hydrophilic regions are analyzed, and the hydrophobic regions where phase change process takes place exhibit higher local heat flux than hydrophilic regions where no bubble nucleation occurs. Effects of the pitch distance between the hydrophobic spots on boiling heat transfer are illustrated and it is shown that there exists an optimal pitch distance for the maximum boiling heat transfer. Furthering decreasing the pitch distance causes inhibition of nucleation sites while further increasing the pitch distance decreases the bubble release frequency, both of which decrease the boiling heat transfer rate.Research results in the present work provide new ideas for the numerical simulation of liquid-vapor phase-change heat transfer and deep insights for the understanding of wettability effects on pool boiling heat transfer, with important theoretical basis for the boiling heat transfer enhancement by surface wettability control.