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ethonodon

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- 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...