The influence of the bulk liquid thermal boundary layer on saturated nucleate boiling (original) (raw)
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Vapor bubble growth in heterogeneous boiling—I. Formulation
International Journal of Heat and Mass Transfer, 1995
Abstraet--A numerical analysis is carried out to study bubble growth in saturated heterogeneous boiling. The bubble growth is determined by considering the simultaneous energy transfer among the vapor bubble, liquid microlayer, and heater. Finite difference solutions for the temperature fields in the microlayer and heater are obtained on expanding coordinates as the bubble grows. The parameters characterizing the bubble shape and microlayer wedge angle are determined by matching the existing experimental data. The predicted bubble growth rate compares very well with the reported experimental data over a wide range of conditions.
Dynamics of vapor bubbles and associated heat transfer in various regimes of boiling
2018
The dynamics of bubble formation during boiling is highly significant considering its influence on the heat transfer rate associated with various applications. Depending on the heat flux, the mode of boiling transforms from the nucleate boiling regime to the film boiling regime. The present thesis is focused on the study of the varying characteristics of boiling regimes through direct numerical simulations. The liquidvapor interface-capturing is performed using the CLSVOF (Coupled Level-Set and Volume of Fluid) approach. In the film boiling regime, the phenomenon of bubble formation is governed by the instabilities at the liquid-vapor interface instigated by the combined influence of surface tension, buoyancy, heat flux, vapor thrust or any other applied external field (electric field in the present study). The dynamical disturbances destabilize the interface which results in bubble formation with the passage of time. The bubble release during film boiling is found to be more of a d...
Numerical Study of Bubble Coalescence Heat Transfer During Nucleate Pool Boiling
Heat Transfer Engineering, 2018
Bubble growth during nucleate boiling in a large pool of liquid was modeled by numerically solving the unsteady Navier-Stokes laminar flow equations with the energy equation to predict the vapor and liquid flow fields. The analysis assumed two-phase, transient, three-dimensional, laminar flow with the Boussinesq approximation for the buoyancy. The volume of fluid method was used with the level set method to predict the bubble interface motion. The numerical investigations studied the dynamics and heat transfer rates associated with the coalescence of bubbles generated on two microheaters. The results for various wall superheats and liquid subcoolings illustrate the bubble growth and interaction dynamics throughout the coalescence process and the wall heat fluxes associated with the bubble nucleation and coalescence. In some cases, the bubble coalescence traps an evaporating liquid layer between the bubbles that then quickly evaporates resulting in high heat fluxes. In other cases, the bubbles very quickly coalescence while the bubbles are still in the fast inertial controlled growth regime and the liquid layer between the bubbles is pushed out without evaporating, resulting in low heat fluxes as the surfaces are covered with vapor. These results show how similar conditions can lead to very different heat fluxes during coalescence as has been seen experimentally.
International Journal of Heat and Mass Transfer, 1975
Recently a new mechanistic model for pool and nucleate flow boiling was developed in our group. This model is based on the balance of forces acting on a bubble and considers the evaporation of the microlayer underneath the bubble, thermal diffusion around the cap of bubble due to the super-heated liquid and condensation due to the sub-cooled liquid. Compared to other models we particularly consider the temporal evolution of the microlayer underneath the bubble during the bubble growth by consideration of the dynamic contact angle and the dynamic bubble base expansion. This enhances, in our opinion, the model accuracy and generality. In this paper we further evaluate this model with experiments and direct numerical simulation (DNS) in order to prove the importance of dynamic contact angle and bubble base expansion.
Results from numerical simulation and experimental validation of the growth and departure of single and multiple merging bubbles and the associated heat transfer on a horizontal heated surface during pool nucleate boiling under low and earth normal gravity conditions have been reviewed here. A finite difference scheme was used to solve the equations governing mass, momentum and energy in the vapor and liquid phases. The vapor-liquid interface is captured by a level set function while including the influence of phase change at the liquid-vapor interface. Water and PF5060 were used as test liquids.
A bubble dynamics-based model for wall heat flux partitioning during nucleate flow boiling
International Journal of Heat and Mass Transfer, 2017
Many physical mechanisms are responsible for wall heat transfer during nucleate flow boiling, such as evaporation of microlayers, gradual rewetting, transient conduction, and forced convection. The nature of these mechanisms tightly connects with the complex dynamics of nucleating bubbles (e.g., growth, sliding, and merger), leading to considerable challenges of modeling the partitioning of wall heat flux into these mechanisms. In this study, we proposed a mechanistic model for wall heat flux partitioning relying on the coupling of heat transfer mechanisms with relevant bubble dynamics. The heat transfer via evaporation of superheated liquid (including microlayers) and gradual quenching over dry spots during the bubble growth period was determined as the latent heat transported to growing bubbles using bubble energy balance and growth equations. The heat transfer over the areas swept by bubbles while sliding and merger whose thermal effect is counted from after the bubble departure to the instant it changes to forced convection or nucleation was quantified by the conventional transient conduction combining with the bubble growth equation and wall functions. The residual wall heat transfer corresponds to forced convection over the region unoccupied by bubbles and the region it replaces transient conduction during the remaining period of bubbling cycle. These three primary mechanisms mechanistically constitute the present wall heat flux partitioning model that is physically concrete and confirmed to has good predictability against experimental data for nucleate boiling at a variety of flow conditions.
Bubble coalescence at constant wall temperatures during subcooled nucleate pool boiling
Experimental Thermal and Fluid Science, 2013
Bubble coalescence during subcooled nucleate pool boiling was investigated experimentally using constant wall temperature boundary conditions while the wall heat flux was measured at various locations to understand the effects of coalescence on the heat transfer. The effects of the subcooling on the coalesced bubble shape, size and departure time were also investigated. The observations showed that the coalesced bubble moved and oscillated on the heater surface with significant heat transfer variations prior to departure. The coalesced (or departure) bubble size and departure frequency decreased as the subcooling increased. The heat flux for boiling with coalescence fluctuated much more than for single bubble boiling with the heat flux fluctuations due to oscillations of the coalesced bubble on the heater surface resulting in frequent rewetting of the heated surfaces. In some cases, many additional small bubbles nucleated very near a larger bubble and then rapidly coalesced with the larger bubble which enhanced the overall heat transfer. The observations also showed coalescence with no increase in the heat transfer rate for very fast bubble coalescence such as would occur for large surface superheats as the boiling conditions approach CHF. A mechanistic model comparing the inertia and heat flux controlled bubble growth phases for inhomogeneous bubble growth shows that this coalescence without increased heat transfer corresponds to the inertial growth phase.
Bubble growth characterization during fast boiling in an enclosed geometry
International Journal of Heat and Mass Transfer, 2009
Microboiling is commonly used in thermal inkjet atomizers (TIJ) and microelectromechanical (MEM) devices. The TIJ and MEM devices performance is closely related to the dynamics of the bubble used to operate them; therefore, it is important to determine the conditions of input energy and power leading to specific bubble dynamics. The objective in this work is the characterization, in a confined space, of the bubble dynamics on a range of input conditions of energy and power and what is the effect of the input conditions on the bubble extractable mechanical efficiency. Mechanical efficiency is defined by the ratio of the integral of the mechanical work (work done by the bubble expansion due to the elevated internal pressure relative to atmospheric pressure minus the increase in bubble surface energy) to the total energy input to the microheater. Bubbles are generated with energies of 7-17 lJ under high heating rates and short pulses in deionized water. Resulting nucleation temperature measurements are consistent with homogeneous nucleation. The bubble lifecycle shows strong dependence on the input heater energy and input heating rate. This work presents new results in bubble growth where growth-shrink-growth derived from specific energy conditions. The bubble growth-shrink-growth may be due to subcooled fluid, local variation in the pressure field, and by the surface tension driven change in curvature of the bubble. Mechanical bubble efficiencies result in small values suggesting most of the energy applied to the heater is distributed in other processes which may include increasing the internal energy of the heater film and the fluid.
International Journal of Multiphase Flow, 2016
This second of two companion papers presents an analysis of sliding bubble and wall heat transfer parameters measured during subcooled boiling in a square, vertical, upward flow channel. Bubbles were generated only from a single nucleation site for better observation of both the sliding bubble characteristics and their impact on wall heat transfer through optical measurement techniques. Specific interests include: (i) bubbles departure and subsequent growth while sliding, (ii) bubbles release frequency, (iii) coalescence of sliding bubbles, (iv) sliding bubbles velocity, (v) bubbles size distribution and (vi) wall heat transfer influenced by sliding bubbles. The results showed that sliding bubbles involve two distinct growth behaviors: (i) at low mass fluxes, sliding bubbles grew fast near the nucleation site, subsequently shrank, and then grew again, (ii) as mass flux increased, however, sliding bubbles grew more steadily. The bubbles originating from the single nucleation site coalesced frequently while sliding, which showed close relation with bubbles release frequency. The sliding bubble velocity near the nucleation site consistently decreased by increasing mass flux, while the observation often became reversed as the bubbles slid downstream due to the effect of interfacial drag. The sliding bubbles moved faster than the local liquid (i.e., u r <0) at low mass flux conditions, but it became reversed as the mass flux increased. The size distribution of sliding bubbles followed Gaussian distribution well both near and far from the nucleation site. The standard deviation of bubble size varied insignificantly through sliding compared to the changes in mean bubble size.