Page 425 - Design and Operation of Heat Exchangers and their Networks

408 Design and operation of heat exchangers and their networks

of the hot fluid at the tube outlet, t i,out,cal ¼t i,x¼L , will be compared with the
measured outlet fluid temperature. The unknown C i , m, C o , and n are deter-
mined by minimizing the deviation between the measured data and pre-
dicted results:

N
X
2
min t i,out,cal, j t i,out, exp, j (8.67)
C i,m,C o,n
j¼1
The experiment procedure of Jin (2003) was used for the measurement,
as has been aforementioned. It is very important to get two sets of experi-
mental data. The one contains the test runs with large Re range of the hot
fluid under high heat flux (the heat flux does not need to be kept constant),
and the other contains the test runs with large heat flux range under high Re
of the hot fluid (Re does not need to be kept constant).

8.3 Transient test techniques
The single-blow transient testing technique is a rapid and accurate method
to determine the mean heat transfer coefficients of compact heat transfer
surfaces. The experiment uses one fluid stream (usually the air flow) flow-
ing through the heat transfer surface to be measured (test core). The exper-
imental system and procedure are relatively simple: a fluid flows steadily
along the heat transfer surface. Before the test, the test core and the fluid
have the same uniform temperature. Then, a sudden change in the fluid
temperature at the inlet of the test core is applied, and the temperature var-
iations at the inlet and outlet of the test core are measured continuously.
The measuring time is usually about 20s. These data are then compared
with the theoretical results to obtain the corresponding heat transfer coef-
ficient between the test core and the fluid. If the single-blow technique is
applied to a real heat exchanger, the flow passage for the other fluid is con-
sidered to be adiabatic and should be evacuated. Usually, it is filled with
stationary air without violating the adiabatic boundary condition. The
single-blow problem deals with the simplest dynamic process of heat
exchangers, in which only the disturbance in inlet fluid temperature needs
to be considered.
The theory of single-blow test techniques is based on earliest studies of
Anzelius (1926), Nusselt (1927), Hausen (1929), and Schumann (1929),in
which the transient heat transfer between a porous medium and a fluid