Vapor blanketing

Slow burn-out tends to be associated with high-quality burn-out conditions and to produce a not unduly excessive wall-temperature rise. In fact, there appears to be an extreme condition in which the temperature rise may hardly be noticeable, and it becomes difficult to say whether burn-out has occurred. These circumstances probably coincide with the jump discontinuity in Fig. 3 ceasing to exist for certain values of system parameters. The condition is effectively one in which, at the burn-out point, the heat-transfer coefficient is the same whether the surface is vapor-blanketed or liquid-wetted. 

Point C in Figure 15.14 is termed the critical heat flux or maximum boiling flux or peak boiling flux as bubbles coalesce on the surface creating a vapor blanket. Critical heat flux occurs because insufficient liquid is able to reach the heat transfer surface due to the rate at which vapor is leaving. Beyond Point D, the surface is dry and entirely blanketed by vapor and heat is transferred by conduction and radiation. 

If the vapor stream velocity exceeds this value, vapor cannot easily get away and thus a partial vapor blanketing (film boiling) may occur. This result is used to predict the maximum heat flux by relating the heat flux to the vapor velocity (see Sec. 2.4.4). 

B) The DNB in a medium- or low-subcooling bubbly flow is caused by near-wall bubble crowding and vapor blanketing. 

Near-wall bubble crowding and vapor blanketing... 

Hence, Ub is a function of UbL, Db, and fluid properties. The equivalent diameter of the vapor blanket, Db, can be obtained from the correlation for the bubble departure diameter (Cole and Rohsenow, 1969). To calculate the liquid velocity, UbL,... 

To determine the thickness of the micro layer, 8m, a force balance on the vapor blanket was used in the direction normal to the wall. The inertial force, F, due to vapor generation was used to balance the force due to vapor blanket circulations, Fr ... 

With this model, it is predicted that there should be a liquid drop size which, if too large, will result in long delay times and excessive numbers of new embryos to vapor blanket the surface. Thus, small drops (or thin layers) are more prone to escape termination by vapor blanketing. Also, if experimental variables are modified so as to reduce the growth rate of embryos from A to B, e.g., by increasing pressure, again one would expect a lower probability of escalation. These two predictive conclusions appear to be substantiated by experiment. 

The model also predicts that if the interface temperature exceeds the critical temperature of the colder liquid, vapor blanketing should always result and no rapid phase transitions could occur. 

Increases in system pressure lead to more rapid vapor blanketing of small drops so, to obtain rapid phase transitions at elevated pressures, more extensive prefragmentation is necessary. 

There are upper and lower limits of applicability of the equation above. The lower limit results because natural-convection heat transfer governs at low temperature differences between the surface and the fluid. The upper limit results because a transition to film boiling occurs at high temperature differences. In film boiling, a layer of vapor blankets the heat-transfer surface and no liquid reaches the surface. Heat transfer occurs as a result of conduction across the vapor film as well as by radiation. Film-boiling heat-transfer coefficients are much less than those for nucleate boiling. For further discussion of boiling heat transfer, see Refs. 5 and 6. 

A typical layout is shown in Figure 12.8. The tube arrangement, triangular or square pitch, will not have a significant effect on the heat transfer coefficient. A tube pitch of between 1.5 to 2.0 times the tube outside diameter should be used to avoid vapor blanketing. Long, thin bundles will be more efficient than short, fat bundles. 

Near-Wall Bubble Crowding and Vapor Blanketing. Here, a layer of vapor bubbles builds up near the wall and this prevents the ingress of liquid to the tube surface, leading to a decrease in efficiency of cooling and to the critical phenomenon. 

Vapor blanketing should be assumed to occur in a partial condenser if the vapor product route fails (60). For instance, if the re-... 

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