|Title:||Heat Transfer and Flow Characteristics of Condensing Refrigerants in Small-Channel Cross-Flow Heat Exchangers
|Author(s):||Zeitlow, D.C.; Pederson, C.O.
|Abstract:||This study is the first to model and experimentally validate refrigerant inventory of
R134a in small-channel cross-flow condenserst. This heat exchanger is used in automotive
applications and uses smaller internal volumes than conventional heat exchangers to perform
the same task. Since the cost of refrigerants continues to rise due to the phase out of
chlorofluorocarbons (CFCs), internal volume becomes a key design parameter.
The model is a one-dimensional, two-fluid model which divides the condenser into
several segments and modules. This model accurately predicts the rate of heat transfer and
refrigerant pressure drop: the heat transfer was predicted within ±10% of the experiment and
the pressure drop was predicted within ±30% for the majority of the data.
More importantly, the model predicts refrigerant inventory within ±10% of the
experiments for ninety five percent of the data. In the inlet header, the slip ratio was
correlated to the Reynolds and Froude numbers, and the homogeneous liquid volume
fraction. In the small channels, the Reynolds and Weber numbers, and the homogeneous
liquid volume fraction were used to correlate the slip ratio. For the outlet header, the
dispersed liquid in the core was modeled using an unsteady gravity model and the annulus
was modeled using the liquid-film Reynolds number and inverse viscosity.
Finally, the flow regimes were documented for the pipes, headers and small channel
condenser tubes. Intermittent flow was the predominate flow regime in the small channels
which is consistent with the Damianides flow map and Kelvin-Helmholtz stability criteria.
Through the inlet header the flow transitioned from a dispersed liquid to a bubble flow
regime. The flow regime in the outlet header was always a dispersed "gravity driven" liquid
in the core with a thin liquid aimulus on the wall. Visual data collected for the headers were
in qualitative agreement with the refrigerant inventory model.
|Publisher:||Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
|Series/Report:||Air Conditioning and Refrigeration Center TR-73
|Sponsor:||Air Conditioning and Refrigeration Center Project 03
|Date Available in IDEALS:||2009-04-17
|Identifier in Online Catalog:||3900927