Enthalpy potential

The most generally accepted theory of the coohng-tower heat-transfer process is that developed by Merkel (op. cit.). This analysis is based upon enthalpy potential difference as the driving force. 

Here, St (x, s) represents the local Stanton number on a plate at a uniform temperature for x > s preceded by an unheated portion of length s, and St (x, 0) is the Stanton number on a plate with a uniform temperature over its entire length. The first term in parentheses in the enthalpy potential arises from the difference between the leading-edge enthalpy of the plate and the recovery enthalpy. The integral term accounts for the portions where continuous surface enthalpy variations occur. The last term sums over a k number of discontinuous jumps in surface enthalpy that may occur downstream of the leading edge. The terms L(sf) and iw(sf) represent the surface enthalpy just upstream and downstream of st where the y th jump in enthalpy occurs. 

The subscript 0 in this equation signifies zero mass transfer at the surface (/(0) = 0). The subscript 2 indicates that the enthalpy potential contributing to the heat flux by conduction alone is dependent only on the specific heat of the air and not that of the coolant. 

The intuitive approach for including variable fluid properties is to evaluate the local St (x, 0) from the variable-property techniques of the previous section, employing the local surface temperature or enthalpy as a parameter. Equations 6.210 and 6.211 are used primarily to define the form of the enthalpy potential appropriate to an arbitrary surface temperature. (See Eq. 6.65). 

If the cell potential equals the enthalpy potential, there is no net heat loss, which is why the enthalpy potential is often termed the thermo-neutral potential. However, the enthalpy energy is not fully accessible as it is composed of both a reversible or entropic (Qrev = TAS) component, as well as irreversible components. 

For a cell that operates at a voltage, V, below the equilibrium and enthalpy potentials, the net heat lost from the cell is given by... 

The basic equations covering combined mass- and heat-transfer phenomena have been covered in the literature. The analysis combines the sensible and latent heat transfer into an overall process based on enthalpy potential as the driving force. 

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