Some empirical equations for heat transfer during nucleate boiling in free flow

7 Some empirical equations for heat transfer during nucleate boiling in free flow 

In the following the equations introduced previously for heat transfer in nucleate boiling in free flow are summarised and supplemented by further equations. 

For pressures close to atmospheric, according to (4.96), it holds that 

The contact angle (30, in radians, for water lies between 0.22tt (=40°) and 0.257T (=45°), and for the halogenated hydrocarbons used as refrigerants it is around 0.1947T (=35°). For other pressures, a0 for the reference heat flux q0 = 20000W/m2 at a reference pressure p0 close to the ambient pressure, is first calculated from the equation above. The desired heat transfer coefficient a at pressure p and heat flux q is obtained from (4.98) 

In which, a0 is the heat transfer coefficient at pressure p0 and reference heat flux q0 = 20 000 W/m2 calculated from the equation given in 2 for a flat tube. F(p+/y/ip) is obtained from the function F(p+) in 2, when in place of p+ = p/pCT the quantity p+/y/ p is used with the area ratio ip = Ai/A. Af is the surface area of the finned tube, A that of a flat tube of the same core diameter as the finned tube. It is 

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