Microlayer evaporation

Microlayer evaporative mechanism (Fig. 2.19c) Section 2.2.5.5 gives the model for this mechanism. A theoretical heat transfer rate is possible through evaporation of a fluid into a receiver ... 

Estimation of microlayer evaporation The model, incorporating the evaporation from a microlayer surf ace underneath a bubble attached to the heater surf ace, was used by Hendricks and Sharp (1964). With water as the fluid, at somewhat subcooled conditions, the heat transfer rates were as high as 500,000 Btu/hr ft2, or... 

Figure 2.24 Comparison of experimental results of bubble period with predictions of a model involving different mechanisms (a) nucleate boiling only (b) nucleate boiling and natural convection (c) nucleate boiling, natural convection, and microlayer evaporation. (From Judd, 1989. Copyright 1989 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

A boiling heat transfer model incorporating nucleate boiling, natural convection, and microlayer evaporation was formulated as... 

Judd (1989) interpreted experimental results of Ibrahim and Judd (1985), in which the bubble period first increased and then decreased as subcooling varied over the range 0 < (7 t - Tm) < 15°C (27°F), by means of a comprehensive model incorporating the contributions of nucleate boiling, natural convection, and microlayer evaporation components. The mechanism responsible for the nucleation of bubbles at exactly the frequency required at each level of subcooling is the subject of their continuing research. 

Heat transport by continuous evaporation at the root of the bubble and condensation at the top of the bubble, while the bubble is still attached to the wall, q"b2 (microlayer evaporation)... 

Dzakowic, G. S., 1967, An Analytical Study of Microlayer Evaporation and Related Bubble Growth Effects in Nucleate Boiling, Ph.D. thesis, University of Tennessee, Knoxville, TN. (2)... 

Fath, H. S.,andR. L. Judd, 1978, Influence of System Pressure on Microlayer Evaporation Heat Transfer, Trans. ASME, J. Heat Transfer 100 49 55. (2)... 

Ignoring the contribution of microlayer evaporation (see Fig. 15.20c), Mikic and Rohsenow [47] considered only heat-transfer-controlled bubble growth (implying spherical bubbles) and obtained the following expression for bubble radius as a function of time for t > tw ... 

For very low pressures (where there is a very large change of specific volume between the liquid and vapor states), bubble growth may be controlled by microlayer evaporation this situation has been investigated in detail by Cooper and Lloyd [48] and van Stralen et al. [49], van Stralen et al. [49] proposed the following expression for bubble radius as a function of time ... 

Latent heat transport. Here, the vapor being transported from the surface into the bulk fluid transports heat from the surface in the form of latent heat. This process is particularly important when heat fluxes are high. It may also play a very important role when the situation is such as to promote microlayer evaporation (e.g., at very low pressures). 

A model analogous to that of Mikic and Rohsenow [120] has been recently developed by Benjamin and Balakrishnan [121], The latter authors took account of microlayer evaporation in the model and also produced a useful relationship between the number of active sites N as follows ... 

Hot Spot Growth Under a Bubble. When bubbles grow and detach from a nucleation center on a solid surface, evaporation of the liquid layer commonly occurs, separating the bubble from the solid surface. This microlayer evaporation process is particularly important at low pressures When a small zone under the bubble becomes dry as a result of this process, its temperature increases, and this increase can, under certain conditions, be sufficient to prevent rewetting of the surface on bubble departure, leading to a permanent hot spot and onset of the critical phenomenon. 

R. L. Judd and K. S. Hwang, A Comprehensive Model for Nucleate Pool Boiling Heat Transfer Including Microlayer Evaporation, J. Heat Transfer (98) 623-629,1976. 

Correlations for Boiling Heat Transfer Nucleate boiling is a complex phenomenon. Several mechanisms have been proposed to explain the boiling process, such as latent heat transport, microconvection, vapor-liquid exchange, wake flow, enhanced convection, and microlayer evaporation, details of which can be found in Ginoux (1978). 

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