Incipience of nucleation boiling

There are a number of experiments directed at studying the incipience of nucleate boiling in heated channels of hydraulic diameter db = 0.04-20 mm. The overall... 

Davis EJ, Anderson GH (1966) The incipience of nucleate boiling in forced convection flow. AlChE 12 774-780... 

Natural convection. This process is obviously dominant up to the incipience of nucleate boiling. As more and more of the surface becomes covered by bubbles, there is less area over which the natural convection can operate. However, the growth and motion of the bubbles may more than compensate for this by inducing additional convection motions. 

For qualitative analysis of the conditions at which the boiling incipience was studied experimentally the parameter D = ATsub.oNB / s may be used. Depending on the value of D, the channels can be subdivided into two groups D < 1 and D < .. When D < D = 0.125—0.25) the onset of nucleate boiling occurred at a bulk temperature significantly less than saturation. When D nucleate boiling occurred at values of the bulk temperature close to saturation. 

Because the bubble population increases with heat flux, a point of peak flux may be reached in nucleate boiling where the outgoing bubbles jam the path of the incoming liquid. This phenomenon can be analyzed by the criterion of a Hemholtz instability (Zuber, 1958) and thus serves to predict the incipience of the boiling crisis (to be discussed in Sec. 2.4.4). Another hydrodynamic aspect of the boiling crisis, the incipience of stable film boiling, may be analyzed from the criterion for a Taylor instability (Zuber, 1961). 

The relationship between the wall temperature and the coolant temperature can be seen in Figure 4.5. The wall temperature starts to bend at the incipience of subcooled boiling, where the coolant temperature is defined as Tm. The wall temperature follows a curve of partial boiling and then reaches an approximately constant value at a fully developed nucleate boiling where the coolant temperature... 

Partial nucleate flow boiling of ordinary liquids Bergles and Rohsenow (1964), using data obtained from several commercially finished surfaces, have developed a criterion for the incipience of subcooled nucleate boiling by solving graphically the... 

Effect of surfactants on the ebullient behavior, nucleate boiling enhancement, incipi-... 

Heat transfer rates in modern boilers are relatively high, and when the first stage of boiling (incipient boiling point) is quickly reached, small bubbles of steam begin to form on the heated, waterside metal surface (steam bubble nucleation) but initially collapse when cooled by contact with the bulk water. 

Sato and Matsumura (1964), and Bergles and Rohsenow (1964) have proposed the equation for the incipient boiling condition in the case that surface cavities of all sizes are available for nucleation. Hino and Ueda (1985) studied incipient boiling of fluorocarbon R-113 in a stainless steel tube of d = 1 mm at mass velocity G = 158—1,600 kg/m s, and inlet subcooling ATsub,m = 7s — Tin ranged from 10-... 

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. 

Visual observation in the studies by Hetsroni et al. (2002a,b, 2003), Qu and Mudawar (2002) proved bubble behavior at incipient boiling in micro-channels (d i < 1 mm) and concluded that it was quite different from that in larger channels. After nucleation, bubbles first grew to detachment size before departing into the liquid flow. The detachment size was comparable to that of the micro-channel... 

The effects of microchannel size, mass flow rate, and heat flux on boiling incipience or bubble cavitation in a microchannel were investigated by Li and Cheng [56], The effects were also estimated of contact angle, dissolved gas, and the existence of microcavities and corners in the microchannel on bubble nucleation and cavitation temperature. 

Nonequilibrium models taking into account superheating normally reflect experimental results. Since incipient boiling of liquid metals under convective flow conditions is an area of current research, some uncertainty is involved in the calculation model formulation relating nucleation to superheat. Variables that appear to affect superheat behavior include the heat flux, nature of the heating surface, fluid velocity, and physical properties (75). As a result, it is often useful to consider the superheat as a parameter of the study. 

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