**Hüseyin Yapici, Nesrin Kayatas, Nafiz Kahraman and Gamze Bastürk**

Erciyes Üniversitesi Mühendislik Fakültesi, 38039 Kayseri,Turkey

Tel: 00-90-352-437 49 01/32125

Fax: 00-90-352-437 57 84
E-mail: yapici@erciyes.edu.tr

*Received: 8 December 2004 / Accepted: 8 February 2005 / Published: 11 February 2005*

**Abstract:**
This study presents the investigation of
the local entropy generation in compressible flow
through a suddenly expanding pipe. Air is used as fluid.
The air enters into the pipe with a turbulent profile
using 1/7 th power law. The simulations are extended to
include different expansion ratios reduced gradually
from 5 to 1. To determine the effects of the mass flux, φ"
the ambient heat transfer coefficient, h_{amb}, and the
inlet temperature, T_{in}, on the entropy generation rate,
the compressible flow is examined for various cases of
these parameters. The flow and temperature fields are
computed numerically with the help of the Fluent
computational fluid dynamics (CFD) code. In addition to
this CFD code, a computer program has been developed to
calculate numerically the entropy generation and other
thermodynamic parameters by using the results of the
calculations performed for the flow and temperature
fields. The values of thermodynamic parameters in the
sudden expansion.

(SE) case are normalized by dividing by their base
quantities obtained from the calculations in the uniform
cross-section (UC) case. The contraction of the radius
of the throat (from 0.05 to 0.01 m) increases
significantly the maximum value of the volumetric
entropy generation rate, (about 60%) and raises
exponentially 11 times the total entropy generation rate
with respect to the its base value. The normalized merit
number decreases 73% and 40% with the contraction of the
cross-section and with the increase of the ambient heat
transfer coefficient (from 20 to 100 W/m^{2}-K),
respectively, whereas it rises 226% and 43% with the
decrease of the maximum mass flux (from 5 to 1 kg/m^{2}-s)
and with the increase of the inlet temperature (from 400
to 1000 K), respectively. Consequently, the useful
energy transfer rate to irreversibility rate improves as
the mass flux decreases and as the inlet temperature increases.

**Keywords:**
high-speed flow; sudden pipe expansion; local entropy generation;
exergy; computational fluid dynamics.

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