Reentrant-type cavities are stable in generating vapor bubbles even at low wall superheat by combining the effects of their geometry and the thermodynamic aspects of the fluid .
where [DELTA][T.sub.sat] is the wall superheat (=[T.sub.s] - [T.sub.sat]), [sigma] is the surface tension of bulk liquid at saturation, [T.sub.l] is the bulk liquid temperature, [v.sub.lv] is the specific volume difference between vapor and liquid (=[v.sub.v] - [v.sub.l]), [L.sub.lv] is the latent heat of vaporization, and [R.sub.c] is the radius of liquid/vapor interface curvature in a cavity.
Consequently, the boiling curve can be drawn as a function of local values of the heat transfer coefficient (or heat flux) and wall superheat
from equation (i).
Furthermore, an understanding of the effects that varying wall superheat values have on the ratio of sensible to evaporative heat transfer is critical to optimizing the falling film heat exchanger.
Also, this investigation defines tube wall superheat as the difference between the inlet heating fluid temperature and saturation temperature of the system.
The data also show a larger increase in heat duty between 10 and 15[degrees]C wall superheat
than there is between 5 and 10[degrees]C.
However, assuming the heat flux was conservative in short length cylinder, it was much easier to extrapolate the wall temperature and calculate the wall superheat. If any test sample plate was attached to the heating surface, the thermal conductivity in (3) should be modified as composite material.
The aluminum surface's heat flux against wall superheat plots were shown in Figure 7.
Because the liquid is saturated at toe vapor-liquid interface, a low effective thermal conductivity requires a large amount of wall superheat
which, in turn, causes the liquid to boil.
Upward facing surface of this wafer has been immersed in deionised water at atmospheric pressure and uniformly heated maintaining constant wall superheat by circulating fluid on its lower side (Fig.
Onset of nucleate boiling process occurs as wall superheat reaches the value of 19 [degrees]C.
The wall superheat
([DELTA]T) is defined as the difference between the wall temperature ([T.sub.w]) and the system temperature ([T.sub.s]).