Boiling, flow nucleate

The part a B emanating from the bubble formation is based on the heat transfer coefficient aB in nucleate boiling in free flow. However because the temperature rise in the boundary layer of a forced flow is steeper than in free flow nucleate boiling, more heat will be released from the wall by conduction and the bubble formation will be partially suppressed in comparison to that in free flow. Chen accounted for this effect with a suppression factor S < 1, which the heat transfer coefficient aB in nucleate boiling in free flow is multiplied by, a B = SaB. 

The heat transfer mechanisms that are active in boiling in micro-channels can be summarized as follows (i) in bubbly flow, nucleate boiling and liquid convection would appear to be dominant, (ii) in slug flow, the thin film evaporation of the liquid film trapped between the bubble and the wall and convection to the liquid and vapor slugs between two successive bubbles are the most important heat transfer mechanisms, also in terms of their relative residence times, (iii) in annular flow, laminar or turbulent convective evaporation across the liquid film should be dominant, and (iv) in mist flow, vapor phase heat transfer with droplet impingement will be the primary mode of heat transfer. For those interested, a large number of two-phase videos for micro-channel flows from numerous laboratories can be seen in the e-book of Thome [22]. 

Figure 9.16 Schematic of boiling flow regimes (a) nucleate boiling dominant heat transfer and (b) convective boiling dominant heat transfer...

The code determines the onset of nucleate boiling and critical heat flux corresponding to flow instability in each individual channel and burnout of fuel plate. Heat flux for the onset of nucleate boiling and critical heat flux are calculated using actual power distribution, coolant velocity, local pressure and saturation temperature at each individtial code. Margins to nucleate boiling, flow instability and burnout are also calculated. 

Satisfactory performance is obtained with tubes having helical ribs on the inside surface, which generate a swirling flow. The resulting centrifugal action forces the water droplets toward the inner tube surface and prevents the formation of a steam film. The internally rifled tube maintains nucleate boiling at much higher steam temperature and pressure and with much lower mass velocities than those needed in smooth tubes. In modern practice, the most important criterion in drum boilers is the prevention of conditions that lead to DNB. 

In a boiler, with the continued application of heat, steam under pressure is produced via a combination of steam bubble formation (nucleate boiling) and direct evaporation at the steam-water interface (convective boiling), as shown in the sketch of different generated steam flow forms in Figure 1.1. 

Figure LI Steam generation from a heated surface, showing nucleate boiling, leading to bubbly, intermediate, annular and mist flow forms of convective boiling. Steam bubbles in water (a) leading to water droplets in steam (b).

Typically, FT boilers tend to have lower rates of overall heat-flux and lower steam/water quality, and nucleate boiling predominates. Water tube (WT) boilers tend to have higher heat fluxes and higher steam/water quality under these conditions, annular flow convective boiling tends to dominate. 

Essentially, except for once-through boilers, steam generation primarily involves two-phase nucleate boiling and convective boiling mechanisms (see Section 1.1). Any deposition at the heat transfer surfaces may disturb the thermal gradient resulting from the initial conduction of heat from the metal surface to the adjacent layer of slower and more laminar flow, inner-wall water and on to the higher velocity and more turbulent flow bulk water. 

The conclusion to be drawn from the above examples and many others is that softness in a boiling system, preceding the boiling channel inlet, may cause flow oscillations of low frequency. It is probably the pressure perturbations arising from the explosive nature of nucleate boiling that initiates the oscillation, and the reduced burn-out flux which follows probably corresponds to the trough of the flow oscillation, as a reduction in flow rate always drops the burn-out flux in forced-convection boiling. 

H7. Hines, W. S., Forced convection and peak nucleate boiling heat transfer characteristics for hydrazine flowing turbulently in a round tube at pressures to 1000 psia, Rept. No. 2059, Rocketdyne, Canoga Park, California (1959). 

Shah RK, London AL (1978) Laminar flow forced convection in ducts. Academic, New York Sher I, Hetsroni G (2002) An analytical model for nucleate pool boiling with surfactant additives. Int J Multiphase Elow 28 699-706... 

During the subcooled nucleate flow boiling of a liquid in a channel the bulk temperature of the liquid at ONB, 7b, is less than the saturation temperature, and at a given value of heat flux the difference ATsub.oNB = 7s - 7b depends on L/d. The experimental parameters are presented in Table 6.2. 

The same conclusion is evident from results obtained by Hino and Ueda (1975) and presented above in Fig. 6.4. The conclusion that A7s is almost unaffected by inlet flow velocity as at 7) -C 1 as at Z) < 1 was established from experiments carried out in the channels of diameters about d = 1—10 mm. What has been commonly observed at incipient boiling for subcooled flow in channels of this size is that small bubbles nucleate, grow and collapse while still attached to the wall, as a thin bubble layer formed along the channel wall. 

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