Boiling incipience

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. 

The point of ONB located at the channel exit corresponds to minimum heat flux at which the boiling incipience is possible for the channel of the given length. Kennedy et al. (2000) presented the measurements that were carried out at different values of the inlet average velocity t/m, whereas the inlet temperature Tin, was constant. 

Under some conditions boiling incipience in surfactant solutions may be quite different from that in Newtonian fluids. Hetsroni et al. (2007) presented results for... 

Fig. 6.16 Boiling incipience in degraded solutions. Reprinted from Hetsroni et al. (2007) with permission...

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. 

When D < 1 (Tin C Ts) incipient boiling heat flux increases with increasing mass velocity. When D heat flux weakly depends on mass velocity. For micro-channels boiling incipient heat flux may weakly depend on inlet temperature. This case corresponds to flow boiling in parallel micro-channels, in which vapor penetrates the inlet manifold. 

For some kind of surfactant solutions boiling incipience was accompanied with hysteresis and the wall superheat up to 24 K was observed. 

Desorption of the dissolved gases formed bubbles of gas and a limited amount of bubbles containing gas-water vapor mixture. Under these conditions, during flow boiling of water boiling incipience occurred at channel wall temperatures below that of saturation temperature. Addition of surfactants led to an increase in wall temperature. In this case the boiling occurred in the range of heat flux of 5.2-21 W/cm at wall temperatures of 107-121 °C. 

Ghiaasiaan SM, Chedester RC (2002) Boiling incipience in microchannels. Int J Heat Mass Transfer 45 4599-4606... 

Desorption of the dissolved bubbles formed bubbles of gas and a limited amount of bubbles containing gas-water vapor mixture. As a result, boiling incipience oc-... 

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. 

---