Page 441 - Design and Operation of Heat Exchangers and their Networks

424 Design and operation of heat exchangers and their networks

This method can be extended to include the effect of the axial heat con-
duction in the primary wall and the axial heat dispersion in the fluid. How-
ever, if we do not neglect the axial heat conduction in fins, the fin
temperature cannot be solved alone. Even in the Laplace domain, the fin
temperature is coupled with the temperatures of the fluid and the wall.
Therefore, the present method cannot be used. Zhu et al. (2004) investi-
gated the effect of the axial heat conduction in fins on the outlet fluid tem-
perature dynamics and found that for the commonly used fins, this effect is
very small and no precise consideration is needed. If we simply add the axial
heat conduction area of fins to that of the primary wall, the effect can be
taken into account with sufficient accuracy. According to their conclusion,
we can apply the present analytical model to the single-blow test technique
for plate-fin heat exchangers.
Another model for single-blow technique is called temperature oscilla-
tion method, in which a steady oscillation of inlet fluid temperature is used.
Roetzel et al. (1993) developed a temperature oscillation method that can be
used to determine both heat transfer coefficient and axial dispersion coeffi-
cient simultaneously. More details can be found in (Luo, 1998).

8.3.4 Test rig and test procedure
The wind tunnel for single-blow test is shown schematically in Fig. 8.12.
Such a wind tunnel at the Institute of Thermodynamics, Helmut Schmidt
University/University of the Federal Armed Forces Hamburg, Germany,
2
has a free-flow area of 0.3 0.3m . The air is drawn in by a centrifugal
blower with variable rotating speed. The heater in the wind tunnel consists
of a nine-layer stainless steel wire mesh with 0.1-mm wire diameter. A photo
of the heater is shown in Fig. 8.13. The electrical power for the heater is
applied by a computer-controlled 30-kVA DC power supply. The temper-
atures at the inlet and outlet of the test section are measured by inlet and
outlet thermocouple screens, each of them consists of 25 NiCr-Ni thermo-
couples connected in series, as is shown in Fig. 8.14. The diameter of the
thermocouple wire is 0.1mm, so the thermal capacity of the thermocouples
is negligible.
In the experiment, first, the air velocity is set to a specified value. After
the air velocity has been set, the heater is turned on, and the voltage of the
heater is regulated until the temperature difference between the outlet and
inlet of the test section is about 10°C. Then, the value of the voltage is
stored, and the heater is switched off. The heater and the test core are cooled