Characteristic Of Microscale Boiling Over Microheaters And Thermodynamic Analysis Of Nucleation Based On GIBBS Free Energy

Rapid advances in microelectronic and micromachining technology have provided a great opportunity for the development of micro therofluidic devices. Many applications, such as microfluidic chip, microactuator, micromixer, micro-chemical reactor, micro fuel cell etc.have been fabricated. The involved microscale heat and mass transfer processes play an important role in the performance of these micro theromofluidic devices. As a result, the investigation on the characteristics and mechanism of microscale heat and mass transfer has aroused general interest in recent years. For example, microbubble generation in microfluidics, microbubble manipulation by electric field, the application of microbubble in micropump and the EHD enhancement on heat transfer have been the hot research topics in the field of engineering thermophysics. However, due to the limit of traditional theory and experimental mothods, the microscale boiling and nucleation processes are not well understood. A lot of phenomenon has not been effectively explored and analyzed, and in-depth understandings of the mechanisms involved are needed. This thesis focuses on the study of various effects on microscale boiling under steady and pulsed heat flux. The corresponding thermodynamic models are developed to understanding the mechanism of microscale boiling nucleation and experiments were carried out to verify the results of the analyses. During the course of experimental studies, Pt microheaters and micro electrodes were fabricated in the silicon microchannels using MEMS technology. With the excellent temperature-resistance characteristics of Pt, the temperature response during boiling was recorded. Using high speed CCD camera and microscope, the microscale boiling phenomenon was observed and recorded. During the course of theoretical analyses, based on the chemical potential, thermodynamic analyses on heterogeneous nucleation in superheated liquid layer are carried out considering various effects and conditions.For microscale boiling nucleation under steady heat flux, the microscale boiling and microbubble dynamics were first investigated experimentally. By varying heat flux and mass flux, three different kinds of micro/nano bubble emission were found. Through the analysis on periodic boiling with periodic temperature response, it is found that the micro/nano bubble emission can reduce surface temperature and slow temperature rise. A flow map for micro/nano bubble emission is obtained. Hereafter, thermodynamic analyses on heterogeneous nucleation in superheated liquid layer under steady heat flux are carried out, based on the Gibbs free energy and availability. In these analyses, the effects of wall temperature gradient and wettability are considered. For heterogeneous nucleation with temperature gradient under steady heat flux, it is found that the critical radius decreases with increasing contact angle (more hydrophobic surface), making the nucleation easier. For same heat flux, nucleation temperature decreases with increasing contact angle. While the nucleation temperature increases with wall temperature gradient. The predicted heat flux by present model matches well with exiting experimental data and critical model.For microscale boiling nucleation under pulsed heat flux, based on the experimental investigations on microscale boiling and microbubble dynamics under pulsed heat flux, the flow map of microscale boiling regime in terms of heat and mass flux for fixed pulse width is obtained. Through the investigation on the change in microbubble diameter under varying heat fluxes, it is found that boiling is advanced and bubble diameter increases with increasing heat flux. After the investigation on effects of pulse with and mass flux on boiling inception time, four regimes are identified as single phase, nucleate boiling, film boiling and dry out with increasing heat flux at fixed pulse width and mass flux. The increase of pulse width brings the transition of boiling regimes forward. Either the pulse width or mass flux has little effects on boiling inception time versus heat flux, which matches well with the classic nucleation formula by Hsu. For theoretical analysis on nucleation under pulsed heat flux, the previous steady model for steady heat flux is modified based on transient heat conduction theory. Through the analysis on heterogeneous nucleation of water, alcohol and R113 under pulsed heat flux, it is found that the critical radius increases with pulse width, which is largest for water and smallest for alcohol. The transitional pulse widths from steady model to transient model for the three fluids are identified according to the experimental data of nucleation heat flux and temperature. For pulse width shorter than this transitional pulse width, the nucleation depends on pulse width and the critical radius and nucleation heat flux are applicable to transient model, which should be applicable to steady model when the pulse width is longer than the transitional pulse width. It is also found that both the critical radius and total energy required for nucleation increase with pulse width. While on the other hand, the power required for nucleation decreases with pulse width, meaning that increasing pulse width makes nucleation easier.The effects of surfactant and electric field on microscale boiling under pulse heating are investigated experimentally. After the studies on the effects of non-ionic surfactant Triton X-100 on boiling regimes, boiling inception and heat transfer, it is found that heat flux required for nucleation increases with increasing concentration. The nucleation temperature decreases with increasing concentration when the concentration is lower than critical micelle concentration (cmc), while the nucleation temperature increases even higher than pure water when the concentration is beyond cmc. With the addtition of surfactant, boiling becomes more violent and the heat transfer coefficent is greatly enhanced until cmc, which decreases when the concentration is beyond cmc. Thereafter, the effects of electric field on boiling inception heat flux, nucleation temperature, nucleate sites and heat transfer coefficent are studied. It is shown from experimental results that ONB is delayed by electric field, which increases the heat flux required for boiling and decreases the nucleation time at fixed heat flux. For the same heat flux, wall temperature gradient decreases with increasing electric field strength, leading to decreasing nucleation temperature and smaller bubble. At high heat flux, there appear multiple nucleate sites when the electric field strength is increased high enough. A map for number of nucleate sites in terms of heat flux and electric field strength is obtained. For theoretical analysis on nucleation under electric field, a thermodynamic model based on Gibbs free energy on onset of heterogeneous nucleation field under an externally imposed electric field is made. For the heterogeneous nucleation under external electric field, the change in permittivity with temperature is considered. It is found that critical radius and nucleation barrier increase with electric field strength, making nucleation more difficult. For same nucleation wall temperature, wall temperature gradient decreases with increasing electric field strength.