Page 412 - Design and Operation of Heat Exchangers and their Networks

Experimental methods for thermal performance of heat exchangers 395

transfer surfaces. The measurement of heat transfer coefficient of a heat
transfer surface is based on the Newton’s law of cooling:
q ¼ α t w tð Þ (8.14)
where q is the local heat flux transferred from the wall to the fluid, t is the
local bulk temperature of the fluid, and t w is the local wall temperature.
Eq. (8.14) is in fact the definition of the heat transfer coefficient that is
defined by heat flux, fluid temperature, and wall temperature. Therefore,
we always wish to measure these three parameters accurately to obtain
the unknown heat transfer coefficient. For example, by electric heating,
the heat flux can be evaluated with the measured electric power applied
to the heater. By heating the working fluid with saturated water steam,
due to high condensation heat transfer coefficient at the hot side, the wall
temperature would very close to the saturation temperature of water steam.
The local bulk temperature of the fluid can be obtained from the energy
equation
dt A A
_ mc p ¼ q ¼ α t w tð Þ (8.15)
dz L L
(8.16)
z ¼ 0 : t ¼ t in
The difficulty in measuring the heat transfer coefficient lies that usually
not the all three parameters could be easily and accurately measured. The
direct measurement of the wall temperature is possible by embedding tem-
perature sensors such as thermocouples into the wall material. It would be a
very difficult and complicated task or even impossible for compact extended
heat transfer surfaces.
A lot of measurement techniques have been developed for measuring the
local or mean heat transfer coefficients of various heat transfer surfaces
directly or indirectly. The measurement methods can be classified as
steady-state test techniques and transient test techniques and will be intro-
duced in the next sections.

8.2 Steady-state test techniques

The steady-state test techniques are traditional measurement techniques. In
the steady-state test, after the specified operation conditions have been set,
one should wait until the test rig reaches a steady state, and then, the flow
rates, temperatures, and pressures or pressure drops are measured. With a
steady-state test technique, the transient terms in the energy equations for