Page 426 - Design and Operation of Heat Exchangers and their Networks

Experimental methods for thermal performance of heat exchangers 409

steadily flowing through the porous medium was investigated and the outlet
fluid temperature response to a sudden change in the inlet fluid temperature
was obtained. Furnas (1932) would be the first one to use the Schumann’s
model for the measurement of heat transfer coefficients of compact heat
transfer surfaces and developed a transient experimental method—the
single-blow testing technique. The dynamic properties of the outlet temper-
ature response can also be the maximum inclination and the first moment of
the temperature profile at the outlet (Kohlmayr, 1966, 1968a,b). The tran-
sient response to arbitrary inlet temperature variation and arbitrary initial
temperature distribution was obtained by Kohlmayr (1968c) with double
Laplace transform. Since the inlet temperature changes of a single-blow test
rig can usually be represented by an exponential function, the analytical
solution to the exponential inlet boundary condition of Liang and Yang
(1975) has special significance. Cai et al. (1984) considered the effect of axial
thermal conduction in the wall using a finite difference method. The effect
of the lateral thermal conduction resistance on the outlet temperature
response was investigated by Zhou and Cai (1988). The outlet temperature
response to arbitrary inlet temperature variation, taking into account the
axial heat conduction in the wall, was solved by Mullisen and Loehrke
(1986) using a finite difference method. Based on their numerical results,
the effects of the time constant of the inlet temperature curve, the axial heat
conduction in the wall, the variable local heat transfer coefficient, and the
inaccuracy of the heat capacity of the heat exchanger wall were discussed
(Loehrke, 1990). With a perturbation method, the mathematical model
considering the axial heat conduction in the wall was solved by Zhou
et al. (1991). A more complicated model considering the fin dynamics of
plate-fin heat transfer surfaces was proposed by Luo and Roetzel (2001)
in which the lateral heat conduction resistance along the fin height is taken
into account. Detailed illustrations and review of previous research work can
be found in the literature (Heggs and Burns, 1988).
The theory of single-blow test techniques is in principle based on the
plug-flow assumption. To take the effect of the nonuniform velocity distri-
bution in the test core into account, Luo et al. (2001) introduced the axial
(longitudinal) heat dispersion term into the single-blow model. New eval-
uation methods for transient experiments have recently been developed by
Roetzel and Na Ranong (2014, 2015, 2018), which are based on the new
unity Mach number dispersion model (Roetzel et al., 2011; Na Ranong and
Roetzel, 2012).