## Review Papers/ Books

**Ho-Young Kwak and Yoon-PyoLee, “Shock and thermal waves emanating from asonoluminescing gas bubble in Shock Focussing Effect in Medical Science and Sonoluminescence,” Edited by R.C. Srivastava, D.Leutloff, K. Takayama and H. Gronig, Springer, 2003.**

**Abstract**

The generation and propagation of the shock pulse from a sonoluminescing gas bubble whose wall acceleration reaches 10^{12}m/s^{2} near the collapse is considered by using the bubble wall motion developed by Keller and Miksis in conjunction with the analytical solutions for the gas inside bubble and the Kirkwood-Bethe hypothesis for the outgoing wave. The propagation of the pressure wave inside the bubble, where there are in homogeneous of density, pressure and temperature induced by the rapid bubble collapse, is also treated. The propagation of a solition-like heat wave which is generated by “thermal spike” due to the rapid increase and subsequent decrease in the bubble wall acceleration is also discussed.

**Ho-Young Kwak, “Vapor bubble nucleation: a microscopic phenomena,” KSME International Journal, Vol. 18, pp. 1271-1287, 2004.**

**Abstract**

In this article, vapor bubble nucleation in liquid and the evaporation process of a liquid droplet at its superheat limit were discussed from the viewpoint of molecular clustering (molecular cluster model for bubble nucleation). For the vapor bubble formation, the energy barrier against bubble nucleation was estimated by the molecular interaction due to theLondon dispersion force. Bubble nucleation by quantum tunneling in liquid helium under negative pressure near the absolute zero temperature and bubble nucleation on cavity free micro heaters were also presented as the homogenous nucleation processes.

**Ho-Young Kwak, “Bubbles: Homogeneous Nucleation,” Encyclopedia of Surface and Colloid Science, 2**^{nd}Edition, pp. 1048-1071, CRC Press, 2006.

**Abstract**

In this article, gas bubble nucleation in gas-supersaturated solutions, vapor bubble nucleation in liquid and the evaporation process of a liquid droplet at its superheat limit were discussed from the viewpoint of molecular clustering (molecular cluster model for homogeneous bubble nucleation) in metastable liquid. A puzzling phenomenon of carbon monoxide gas bubble formation in Fe-C-O melts and gas bubble nucleation in polymer solutions were also discussed by using the molecular cluster model based on the interaction between solute gas and solvent molecules. For the vapor bubble formation, the energy barrier against bubble nucleation was estimated from the molecular interaction due to theLondon dispersion force. Bubble nucleation by quantum tunneling in liquid helium under negative pressure near the absolute zero temperature, bubble nucleation on cavity free micro heaters, and the bubble nucleation due to laser irradiation were also presented as the homogeneous vapor bubble nucleation processes.

**Key words**; bubble nucleation, CO bubble in melts, droplet explosion, evaporation, gas bubble, laser-inducedcavitation, microcellular foam, molecular cluster, quantum tunneling, superheat limit of liquid, tensile strength, vapor bubble

**Ho-Young Kwak and Si-Doek Oh, “Exergetic and thermoeconomic analyses optimal planning of cogeneration systems” in Energy Efficiency Research Advances, ed. By D.M. Bergmann, Nova Science Publishers, Inc., 2008.**

**Abstract**

Increasing demand in fossil fuels which will remain the dominant energy source to 2030 might cause a drastic change in global climate. To curb such fossil fuels demand and reduce CO_{2} emissions correspondingly, more efficient use of the fossil fuels are strongly required. In this article thermodynamic and economic evaluation methods and optimal planning of gas turbine cogeneration systems which can return fossil fuel energy savings up to 30% are treated. A general exergy balance equation that is applicable to any component of thermal system is presented. A cost-balance equation formulated by assigning a unit exergy cost to each disaggregated exergy stream in the exergy balance equation is also presented. Applications of the exergy and cost-balance equations to a gas turbine cogeneration are shown. A method to determine the optimal configuration and optimal operation mode of cogneration system is also presented based on the energy demand data for commercial buildings such as a hotel, a hospital and an office building.

**Ho-Yong Kwak, “Nonlinear bubble behavior due to heat transfer” in Heat Transfer—Theoretical Analysis, Experimental Investigations and Industrial Systems, INTECH, 2011**.

**Abstract**

Previous studies of the forced oscillation of a spherical bubble in solution have been investigated by using the Rayleigh equation to obtain the time dependent bubble radius and a polytropic relation to obtain the gas pressure inside the bubble depending bubble volume. In fact, the polytropic approximation with proper index values has been widely used for the gas undergoing quasi-equilibrium process in which there is heat transfer. However, the polytropic pressure-volume relationship fails to account the thermal damping due to heat transfer through the bubble wall because *P**b**dV* is a perfect differential and consequently its integral over a cycle vanishes where *P**b *is the gas pressure inside the bubble and *V* is the bubble volume. Furthermore, the polytropic approximation assumes the uniform temperature for the gas intrinsically, which is valid only for a particular case and it is hard to tell whether the gas inside the bubble oscillating under ultrasound behaves isothermally or adiabatically.

In this study, we have formulated a general bubble dynamics model, which is as follows. The density, velocity and pressure distributions for the gas inside a spherical bubble were obtained by solving the continuity and momentum equations analytically. With the set of analytical solutions for the conservation equations, the temperature distribution for the gas inside the bubble was also obtained by solving the energy equation for the gas. The heat transfer through the bubble wall was considered to obtain the instantaneous thermal boundary layer thickness from the mass and energy conservations for the liquid layer adjacent to the bubble wall by the integral method. The mass and momentum equations for the liquid outside the bubble wall provided the well known equation of motion for the bubble wall, the Rayleigh-Plesset equation in incompressible medium or the Keller-Miksis equation in compressible medium. The bubble dynamics model was applied to an evolving bubble formed form the fully evaporated droplet at the superheat limit and phenomena of sonoluminescence which is light emission associated with the catastrophic collapse of a micro-bubble oscillation under ultrasound.

With uniform density, temperature and pressure approximations which are valid for the characteristic time scale of ms, the calculated values of the far field pressure signal from the evolving the bubble formed form the fully evaporated droplet at its superheat limit are in good agreement with the experimental results. With uniform pressure approximation which is valid for the characteristic time scale of *μ**s*, the calculated values of the minimum velocity of the bubble wall, the peak temperature and pressure are excellent agreement with the observed ones for the sonoluminescing xenon bubble in sulfuric acid solutions. Furthermore, the calculated bubble radius-time curve displays alternating pattern of bubble motion which is apparently due to the heat transfer for the sonoluminescing xenon bubble, as observed in experiment. The bubble dynamics model presented in this study has also revealed that the sonoluminescence for an air bubble in water solution occurs due to the increase and subsequent decrease in the bubble wall acceleration which induces pressure non-uniformity for the gas inside the bubble during ns range near the collapse point. The calculated sonoluminescence pulse width from the instantaneous gas temperature for air bubble is in good agreement with the observed value of 150 ps. Due to enormous heat transfer the gas temperature inside the sonoluminescing air bubble at the collapse point is about 20000~40000 K instead of 10^{7} K which estimated to be in the adiabatic case. Molecular dynamics (MD) simulation results for the sonoluminescing xenon bubble were compared to the theoretical predictions and observed results.