Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

20010024910 Johns Hopkins Univ., Dept. of Chemical Engineering, Baltimore, MD USA
Using Surfactants to Control Bubble Coalescence in Nucleate Pool Boiling
Stebe, Kathleen J., Johns Hopkins Univ., USA; Balasubramaniam, R., National Center for Microgravity Research on Fluids and
Combusiton, USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December 2000,
pp. 465-485; In English; See also 20010024890; No Copyright; Avail: CASI; A03, Hardcopy; A10, Microfiche
Nucleate boiling is the preferred mode of heat transfer in heat exchangers operating not only on the earth, but also in reduced
gravity conditions, where the size of the heat exchanger is constrained. In nucleate boiling, large amounts of heat are transported
with small changes in the system temperature as consequence of the latent heat of vaporization. For efficient operation in this
regime, the dynamics of the vapor bubbles must be understood and controlled. It has been established experimentally that vapor
bubble dynamics are strongly influenced by surfactant additives in that greater heat fluxes are realized at smaller superheat. This
effect, however, is non-monotonic in surfactant concentration and depends strongly on the degrees superheat, which determine
the rate of bubble growth and detachment. The aim of this work is to understand the mechanisms behind the non-monotonic heat
flux improvement caused by the surfactant additives, and to identify regimes which promote improved heat flux. Surfactants
adsorb at fluid interfaces, where they reduce the surface tension and give rise to Marangoni stresses when the rate of surfactant
mass transfer is slow compared to the prevailing surface convective flux at the vapor-liquid interface. Surfactant also adsorbs at
solid substrates, changing the balance of surface tensions and interfacial energies that determine the wetting conditions on the solid
substrate. As bubbles grow, they create liquid-vapor interface. If surfactant transport to these interfaces is far slower than the rate
of bubble growth and detachment, surfactant will be ineffective in changing the bubble dynamics. If the mass transfer rates are
comparable, reduced surface tensions and Marangoni stresses both favor the formation of smaller bubbles which detach more
easily, and hence higher heat flux. If surfactant mass transfer is far faster than the bubble growth rate, the surface tension will be
reduced, but no Marangoni stresses will occur. Therefore, it is imperative to understand the surfactant mass transfer kinetics and
the hydrodynamic behavior of the growing vapor bubble as a function of surfactant concentration in order to control this process.
We propose to study the dependence on surfactant concentration of the bubble formation, growth and detachment both numerically
and experimentally. Ground-based, drop-tower and flight experiments are proposed. In the laboratory, the wetting conditions on
the solid surface will be varied independently in a controlled manner using self-assembled monolayers (SAMs). The equilibrium
and dynamic surface tension for surfactant systems will be studied for temperatures of interest, where data are extremely scarce.
The dynamics of individual vapor bubbles will be studied both in the laboratory and in drop tower experiments. Bubble coalescence
will be studied as a function of surfactant properties by creating two neighboring nucleation sites of the substrates. In the
numerical modeling of this process, the surfactant effects on the stress conditions at a strongly deforming bubble interface will
be studied for a single bubble growing on a heated surface. This is a problem in which the surfactant mass transfer, the temperature
field and the momentum equation are coupled. The surfactant data obtained in the laboratory, including the surface equation of
state which relates the surface tension to the local surface concentration and the surfactant mass transfer kinetics, will provide the
material parameters required in the numerical model.
Author
Nucleate Boiling; Surfactants; Heat Transfer; Coalescing; Bubbles; Additives; Heat Flux; Microgravity; Space Processing
20010024911 California Univ., Center for Risk Studies and Safety, Santa Barbara, CA USA
The Physics of Boiling at Burnout
T. Theofanous, California Univ., USA; Tu, J. P., California Univ., USA; Dinh, T. N., California Univ., USA; Salmassi, T., California
Univ., USA; Dinh, A. T., California Univ., USA; Gasljevic, K., California Univ., USA; Proceedings of the Fifth Microgravity
Fluid Physics and Transport Phenomena Conference; December 2000, pp. 486-526; In English; See also 20010024890; No Copyright;
Avail: CASI; A03, Hardcopy; A10, Microfiche
The basic elements of a new experimental approach for the investigation of burnout in pool boiling are presented. The
approach consists of the combined use of ultrathin (nano-scale) heaters and high speed infrared imaging of the heater temperature
pattern as a whole, in conjunction with highly detailed control and characterization of heater morphology at the nano and micron
scales. It is shown that the burnout phenomenon can be resolved in both space and time. Ultrathin heaters capable of dissipating
power levels, at steady-state, of over 1 MW/sq m are demonstrated. A separation of scales is identified and it is used to transfer
the focus of attention from the complexity of the two-phase mixing layer in the vicinity of the heater to a micron-scaled microlayer
and nucleation and associated film-disruption processes within it.
Author
Nucleate Boiling; Infrared Imagery; Heat Transfer; Heat Flux; Burnout; Temperature Measurement; Mixing Layers (Fluids)
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