Bubble Dynamics and Enhancement of Pool Boiling in Presence of an Idealized Porous Medium - a Numerical Study using Lattice Boltzmann Method

This paper reports our numerical investigations on enhancement of pool boiling heat transfer inside an array of solid cylinders of square cross section using lattice Boltzmann Method. The entire saturated pool boiling curve for the flat surface comprising of different nucleate boiling regimes has been obtained numerically. The effect of solid cylinder array has been quantitatively evaluated and expressed in the form of its corresponding boiling curve. It is found that the boiling incipience in presence of the cylinder array occurs at a lower surface superheat compared to that of a plane surface. Further, the solid array effectively delays the onset of film boiling. The bubble dynamics in such solid structure array including bubble nucleation, coalescence, growth, entrapment, splitting and escape is found to be very different compared to a flat surface. Based on the heat flux values and trends, the entire boiling curve could be classified into 4 distinct zones. To the best of our knowledge, this is the first instance where LBM could predict the entire pool boiling curve for any porous medium. Finally, two different cylinder arrays of porosity 90% and 98% are studied to examine the effect of porosity. It is found that the sensitivity of the heat transfer rates to porosity is significant especially at higher values of surface superheat.

Numerical Investigation of Pool Boiling Under Ocean Condition with Lattice Boltzmann Simulation. Part Ⅱ: Rolling Condition

Rolling motion caused by ocean condition will induce more complicated inertial forces with their force directions changing all the time, which results more complex bubble behaviors and unique heat transfer characteristics. In this work, pool boiling under rolling condition is numerically simulated using multiple relaxation time phase change lattice Boltzmann method (LBM). Pool boiling patterns, boiling curve of time-averaged heat flux, transient heat flux and rolling effects on different pool boiling regions are investigated. The results show that pool boiling curve of time-averaged heat flux between rolling condition and static condition are not obvious until close to critical heat flux, and 9.3% higher CHF is achieved under rolling condition while worse heat transfer is discovered at film boiling. Moreover, distinct fluctuation of transient heat flux of pool boiling under rolling condition is found for all boiling regimes, and its variation pattern along with the rolling motion and bubble behavior is investigated. Furthermore, tangential inertial force caused by rolling motion has positive influence on heat transfer of pool boiling, while the centrifugal force has negative influence on heat transfer, since it is opposite to the gravity and hence decreases the buoyancy force. Besides, larger rolling amplitude and smaller rolling period will induce larger additional inertial forces, and thus make greater influences on the bubbles’ behavior and pool boiling heat transfer.

Existence and uniqueness of generalized solutions to hyperbolic systems with linear fluxes and stiff sources

Motivated by the modeling of boiling two-phase flows, we study systems of balance laws with a source term defined as a discontinuous function of the unknown. Due to this discontinuous source term, the classical theory of partial differential equations (PDEs) is not sufficient here. Restricting to a simpler system with linear fluxes, a notion of generalized solution is developed. An important point in the construction of a solution is that the curve along which the source jumps, which we call the boiling curve, must never be tangent to the characteristics. This leads to exhibit sufficient conditions which ensure the existence and uniqueness of a solution in two different situations: first when the initial data is smooth and such that the boiling curve is either overcharacteristic or subcharacteristic; then with discontinuous initial data in the case of Riemann problems. A numerical illustration is given in this last case.

A Review of the Boiling Curve with Reference to Steel Quenching

This review presents an analysis and discussion about heat transfer phenomena during quenching solid steel from high temperatures. It is shown a description of the boiling curve and the most used methods to characterize heat transfer when using liquid quenchants. The present work points out and criticizes important aspects that are frequently poorly attended in the technical literature about determination and use of the boiling curve and/or the respective heat transfer coefficient for modeling solid phase transformations in metals. Points to review include: effect of initial workpiece temperature on the boiling curve, fluid velocity specification to correlate with heat flux, and the importance of coupling between heat conduction in the workpiece and convection boiling to determine the wall heat flux. Finally, research opportunities in this field are suggested to improve current knowledge and extend quenching modeling accuracy to complex workpieces.

Boiling Curve and Droplet Evaporation Lifetime on Hot Hemispherical Copper Surface

The objective of this paper is to investigate the droplet evaporation lifetime and boiling curve on hot copper surface using ethanol liquid. We focus our study to find the Critical Heat Flux (CHF) and Leidenfrost temperature in the boiling curve. Copper material which has a high thermal conductivity, k was chosen as a test material. The copper material dimension was approximately 28.0 mm in height and 50.0 mm in diameter. The copper surface was modified into hemispherical surface in order to maximize the evaporation lifetime. The hemispherical surface was constructed using Electrical Discharge Machining (EDM). After completing the EDM process, the dimension of the hemispherical surface area was approximately 15.0 mm in depth and 30.0 mm in diameter. Meanwhile, ethanol liquid which has a low boiling point of 78 °C was chosen as a test fluid. The droplet diameter was approximately 3.628 mm. The impact height was set to be around 4.0 mm corresponding to drop impact velocity of 0.886 m/s. As a result, it was found that the critical heat flux (CHF) and Leidenfrost temperature range on hemispherical copper surface was approximately TCHF = 100.4-117.7 °C and TL = 170.0-175.8 °C, respectively.