Abstract:
An experimental study was performed in which an ammonia-water solution
was desorbed within a branching fractal-like microchannel array. The solution entered
in the center of a disk, and flowed out radially until discharging in to a gravity-driven
separation chamber. Heat was added to the ammonia-water through a thin wall, above
which flowed heat transfer oil in a separate branching fractal-like microchannel array.
Such arrays have been shown to utilize the increased heat transfer coefficients seen in
parallel channel arrays; however, they do so with a lower pressure drop.
An experimental flow loop consisting of ammonia-water and heat transfer oil
sub-loops was instrumented along with a test manifold for global measurements to be
taken. Temperature, pressure, density and mass flow rate measurements permitted
calculation of desorption and heat transfer characteristics. Parameters included oil
mass flow rate, oil inlet temperature, and strong solution flow rate, while strong
solution concentration, temperature, and weak solution pressure were kept constant.
The desorber was assumed to achieve equilibrium conditions between the
vapor and weak solution in the separation chamber. The exit plenum was large and
acted as a flash chamber, making the assumption reasonable. The vapor mass fraction
was determined from knowledge of the weak solution saturation temperature.
Heat exchanger analyses (LMTD and ε-NTU) were done to determine the heat
transfer characteristics of the desorber. Calculated values of UA are shown to be as
high as 5.0 W/K, and desorber heat duties were measured as high as 334 W. Strong
solution, at 0.30 mass fraction, was desorbed into weak solution and vapor with
concentrations ranging from 0.734 to 0.964. Circulation ratios, defined as strong
solution mass flow rate per unit desorbed vapor mass flow rate, varied in this study
from 3.4 to 20.
A method for specifying desorber operating conditions is described, in which a
minimum desorber heat input per unit vapor flow rate is determined at an optimum
circulation ratio. A description of how the circulation ratio behaves as a function of
strong solution mass flow rate, oil flow rate, and the maximum temperature difference
between oil and ammonia-water solution is shown.
