Equation of Rohsenow

This type of equation comes from dimensional analysis. The coefficient and exponents are found by experiment. If forced convection is used, the Reynolds number has the conventional meaning of DVp/p. If free convection is used, the Reynolds number is replaced by the Grashof number, which can be shown to have a meaning of a Reynolds number, owing to free convection (B8). 

The Rohsenow approach is to use Eq. (1) modified for boiling. The correct Reynolds number is taken to be the Reynolds number of a bubble just after it breaks loose from the hot solid. Previous workers (J2) found experimentally that the velocity of a released bubble is constant for a short time. In addition it was shown that the diameters of the released bubbles of water or carbon tetrachloride are inversely proportional to the frequency of emission of bubbles that size, or 

This relationship was verified at small heat fluxes only. Also a correlation for the average diameter of a bubble at the instant of break-off is available (F8) from experimental evidence for bubbles of steam and of hydrogen. 

If the bubbles are assumed to be spheres and if they leave the solid from n points per unit area, the estimated bubble velocity V becomes 

The bubble diameter can be eliminated because the product of the mass of vapor formed per unit time and the latent heat of vaporization must equal the heat transfer rate q = w.  

---

In addition to the equations of Rohsenow and of Forster and Zuber, which are the most theoretical equations now available, there exist many equations which are simply empirical correlations. Little pretense is made that these additional equations are defensible on theoretical grounds beyond simple dimensional analysis. A few of these are given below. 

Data from a single laboratory for tubes of different diameters are needed. Tubes are thicker than the bubbles produced, but the reverse is true for wires. The diameter effects may not be the same in the two cases. The equations of Rohsenow and of Forster and Zuber predict that the geometric arrangement is of no consequence. The prediction is not proved at present. 

---