Specific enthalpy pressure, definition

The temperature profile of a planetary atmosphere depends both on the composition and some simple thermodynamics. The temperature decreases with altitude at a rate called the lapse rate. As a parcel of air rises, the pressure falls as we have seen, which means that the volume will increase as a result of an adiabatic expansion. The change in enthalpy H coupled with the definition of the specific heat capacity... 

Fortunately, we never need to know the absolute values of H or // at specified states we only need to know AH and AH for specified changes of state, and we can determine these quantities experimentally. We may therefore arbitrarily choose a reference state tor a species and determine AH = 0 - Href for the transition from the reference state to a series of other states. If we set Href equal to zero, then H(= AH) for a specified state is the specific internal energy at that state relative to the reference state. The specific enthalpies at each state can then be calculated from the definition, H = U + PV, provided that the specific volume (V) of the species at the given temperature and pressure is known. 

The specific enthalpy change associated with the transition of a substance from one phase to another at constant temperature and pressure is known as the latent heat of the phase change (as distinguished from sensible heat, which is associated with temperature changes for a singlephase system). For example, the specific enthalpy change AH for the transition of liquid water to steam at I00°C and 1 atm, which equals 40.6 kJ/mol, is by definition the latent heat of vaporization (or simply the heat of vaporization) of water at this temperature and pressure. 

For the case when the pressure is kept constant during the heating process, we begin by noting the definition of specific enthalpy, namely ... 

Standard thermodynamic formalism for the total differential of specific enthalpy in terms of its natural variables (i.e., via Legendre transformation, see equations 29-20 and 29-24b) allows one to calculate the pressure coefQdent of specific enthalpy via a Maxwell relation and the definition of the coefficient of thermal expansion, a. 

Heat capacities. In the demonstration of the ideal gas equation, the variations of the capacitive (internal) energy 11 and the enthalpy J-f have been merged into a single equation describing the variation of the PVproduct. If these differential equations had been kept separate and the same procedure was followed for defining the gas constant as the constant common value of two second derivatives, one would have found the definitions of the molar heat capacities (also called specific heats). Depending on the energy form, there are two heat capacities, at constant volume and at constant pressure ... 

Equation (2.86) comes from H=U + PV the definition of specific enthalpy in terms of specific internal energy, U, pressure and volume, P and V, respectively. 

Chemically reacting flows in CFD and multiphysics applications are usually written in terms of the specific heat at the constant pressure Cp and temperature T. By recalling the definition of enthalpy as... 

---