I Enthalpy change in a flow process

[ethonodon]

TL;DR

In a flow process, why do we substitute dH = c*dT if pressure is dropping during the process

I am looking at simple steady-state flow processes, say for flow through a pipe. The general energy balance for pipe flow reduces to simply heat evolved = change in enthalpy between the two states (or heat rate = mass flow rate * change in specific enthalpy). However, for a flow process, why is it justified to replace change in enthalpy with heat capacity * change in temperature. I understand that for closed systems, one can only do so for processes taking place at constant pressure (or for cases in which the enthalpy is a function only of temperature.) In the case of pipe flow, pressure obviously drops - do we simply approximate the enthalpy of the fluid to only depend on temperature? Otherwise, the dH = c*dT substitution does not seem valid to me...

 

[gmax137]

TL;DR Summary: In a flow process, why do we substitute dH = c*dT if pressure is dropping during the process

I am looking at simple steady-state flow processes, say for flow through a pipe. The general energy balance for pipe flow reduces to simply heat evolved = change in enthalpy between the two states (or heat rate = mass flow rate * change in specific enthalpy). However, for a flow process, why is it justified to replace change in enthalpy with heat capacity * change in temperature. I understand that for closed systems, one can only do so for processes taking place at constant pressure (or for cases in which the enthalpy is a function only of temperature.) In the case of pipe flow, pressure obviously drops - do we simply approximate the enthalpy of the fluid to only depend on temperature? Otherwise, the dH = c*dT substitution does not seem valid to me...

In general, dH = c*dT is not valid. If the specific heat c is constant enough, it may be an acceptable approximation, depending on the nature of the calculation. Or it may not be.

 

[Chestermiller]

Are you talking about a liquid or a gas?