Page 17 - Kỷ yếu hội thảo quốc tế: Ứng dụng công nghệ mới trong công trình xanh - lần thứ 9 (ATiGB 2024)

8 TRƯỜNG ĐẠI HỌC SƯ PHẠM KỸ THUẬT - ĐẠI HỌC ĐÀ NẴNG

inserts P = 69 mm, the Nusselt number was enhanced
by 1.77 times as compared with the plain heat-
exchanger. Arjmandi et al. [14] conducted a
numerical investigation on the effect of employing the
new combined vortex generators, the twisted tape
turbulator and Al2O3-H2O nanofluid as the involved
base fluid. Their results show that the pitch ratio has a
predominant effect on the Nusselt number and the
friction factor, which causes an efficiency increase up
to five times compared to the original one. In
addition, by decreasing the angle of the vortex
generators in the new combined turbulator, both
Nusselt number and the friction factor are increased.
Hereby shows the research is done to improve
efficiency of double pipe heat exchanger has been
investigated numerically and experimentally. Fig.1. 3D overview and cross sections of the studied
However, there has been few works reported on the double-pipe heat exchanger
effects of geometrical parameters of fins on the 3. THEORY BACKGROUND
efficiency of a double pipe longitudinal fined heat A. Governing Equations
exchanger. Thus, this study is oriented to further
clarify this issue. Continuity, momentum and energy equations for
an incompressible flow in a steady regime are
in this study, through CFD simulation combined expressed as follows,
with analysis to consider the influence of longitudinal
rectangular fin geometric parameters: fin heigh,  .V = 0 (1)
number of fins,… on temperature distribution, heat   ) P ( ( .  +  )
( . VV = − +
) V
transfer rate, effectiveness, which are investigated t (2)
(
under a range of reynolds number. + − V V ' )
'
.
2. PROBLEM STATEMENT
 C V. T =  ( . k  T ) (3)
Heat transfer process inside a double pipe p eff
longitudinal fined heat exchanger was investigated at Here, V is fluid velocity (m/s), ρ is fluid density
different longitudinal rectangular fins geometrical (kg/m ), P is pressure (Pa), μ is fluid dynamic
3
parameters of the inner pipe. To this end, a 500 mm viscosity (kg/m.s), μt is fluid turbulent viscosity
long double-pipe heat exchanger was made of steel, (kg/m.s), V’ is velocity fluctuation (m/s), T is
d 54 temperature (K), Cp is heat capacity (J/kg.K), and keff
having a 1 = mm for outer pipe and a is effective heat conductivity (W/m.K).
d 2 60
d 3 = 16 mm for inner pipe was considered. In B. Definition of performance indicators
d 4 22 Changes on physical properties of the fluids with
addition, longitudinal rectangular fins of inner pipe temperature were ignored on both sides of the heat
exchanger in this study to eliminate extra
were made of steel, 1 mm thick, designed by complexities.
changing the ratio of the fin height to the thickness of
the space between the two pipes (annulus), is defined Overall the heat transfer rates of the hot fluid flow
h and the cold fluid flow are
as x = (fin height as h) but must ensure
0,5. (d − d 4 ) Q = h m .C p,h ( . T − in T out ) , W (4)
h
h
1
that the heat transfer area of the inner tube remained Q = m .C ( . T − T ) , W (5)
constant. Accordingly, three case x = 75%, 50%, c c p,c out n i c
37.5% was considered for the study (Fig. 1). Flow Here, mh is inlet mass flow rate for the hot fluid
direction of hot fluid in the inner pipe and that of cold (kg/s), Cp,h is specific heat capacity of the hot fluid
fluid in the annulus are opposite. (J/kg.K), mc is inlet mass flow rate for the cold fluid
(kg/s), Cp,c is specific heat capacity of the cold fluid
(J/kg.K), Tin and Tout show temperatures at inlet and
outlet (K).
Mean heat transfer rate is defined as [15]:

ISBN: 978-604-80-9779-0