Thermodynamic models and energy distribution of single-phase heated surface in a boiler under unsteady conditions

Abstract

A coal-fired power unit frequently operates under unsteady conditions; thus, in order to acquire scientific energy analysis of the unit, thermodynamic analysis of a single-phase heated surface in a boiler under such conditions requires investigation. Processes are analyzed, and distributions of energy and exergy are qualitatively revealed. Models for energy analysis, entropy analysis, and exergy analysis of control volumes and irreversible heat transfer processes are established. Taking the low-temperature superheater of a 610 t/h-boiler as an example, the distribution of energy, entropy production, and exergy is depicted quantitatively, and the results are analyzed.

Key words： thermodynamic model; energy distribution; boiler; unsteady conditions

Acknowledgements

This study was supported by the National Basic Research Program of China (No. 2009CB219801), the National Natural Science Foundation of China (Grant Nos. 50776028 and 50606010) and the Fundamental Research Funds for the Central Universities.

Notation

F	heat transfer area/m2
V	volume/m3
m	mass/kg
\dot{m}	flow rate of mass/(kg·s-1)
p	pressure/Mpa
T	absolute temperature/K
v	specific volume/(m3·kg-1)
e	specific internal energy/(kJ·kg-1)
h	specific enthalpy/(kJ·kg-1)
\dot{Q}	time rate of heat transfer/kW
\dot{W}	time rate of work/kW
s	specific entropy/(kJ·kg-1·K-1)
\mathrm{\Delta }\dot{S}	time rate of enthalpy change/(kJ·s-1·K-1)
ex	specific exergy/(kJ·kg-1)
{\dot{E}}_{\mathrm{d}}	time rate of exergy destruction/kW
{\dot{E}}_Q	time rate of exergy transfer accompanying heat transfer/kW
Greek symbols
\tau	time/s
\rho	density/(kg·m-3)
\alpha	heat transfer coefficient/(kJ·m-2·K-1)
\dot{\sigma }	time rate of entropy production/(kJ·s-1·K-1)
Subscripts
CV	control volume
j	locations on the boundary of a system
i	inlets
e	exits
f	fluid
b	solid surface
0	environment

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