Abstract:
Heat exchangers are used in industrial processes to recover heat between two
processes fluids and are widely used. Although the equations for heat transfer and
pressure drop characteristics in a double pipe heat exchangers are available, there is
still need to completely understand how these characteristics interact which
geometrical factors like annular diameter ratio or some thermal boundaries conditions
which have not yet drawn more attention from the research community.
The purpose of this study was to experimentally measure the heat transfer and
pressure drop characteristics of a concentric annular duct of ratio 0.593 for different
heat fluxes simultaneously on the inner and outer tube in the turbulent flow regime
and to describe or discuss the impact or interaction of heat flux ratios on the flow and
heat transfer behaviour.
An experimental set]up was designed to achieve this goal. It consisted of an overall
facility and a removable test section. The test section allowed for the measurement of
the temperature along the length of the test section, the pressure drop, the heat flux
inputs and the flow rate. These quantities were used to determine the heat transfer
coefficients and friction factors of the!system.
The concentric duct was an annulus formed of a single (15.88]mm]outer diameter and
14.46]mm]inner diameter) copper tube inserted inside a 0.91mm] thick] copper tube of
26.76 mm of inner diameter. The overall length of the annular duct was 4.84 m. To
transfer heat, a heating element made of constantan wire was wrapped around each
heat transfer area.
Heat transfer and pressure drop data were obtained on heating the inner and the outer
wall separately with four different heat flux densities and eight heat flux ratio were
used for the case of simultaneously heating both walls. Reynolds numbers for
unilateral heating range from 5800 to 12 000 while bilateral heating were focus around
two Reynolds numbers, 6 500 and 9 500.
Satisfactory results were found between the measurements of this experiment and
currently available literature for the case of unilateral heating. An estimate of the
accuracy of the experimental setup showed the maximum relative error was about 5 %
in the determination of the Nusselt number and 1.8 % for the friction factor.
Diabatic friction factors have been presented using adiabatic friction factors with a
correction term which considered the effect of temperature difference between the
fluid and walls. Heat flux density ratio showed to have an impact on the heat transfer
characteristics. The Nusselt number on the inner wall could be enhanced by 19% with
increasing the heat flux ratio up to 2.3 times.
