Heat transfer and flow characteristics of condensing refrigerants in small-channel cross-flow heat exchangers
Zietlow, David Charles
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Permalink
https://hdl.handle.net/2142/21977
Description
Title
Heat transfer and flow characteristics of condensing refrigerants in small-channel cross-flow heat exchangers
Author(s)
Zietlow, David Charles
Issue Date
1995
Doctoral Committee Chair(s)
Pedersen, Curtis O.
Department of Study
Mechanical Science and Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Language
eng
Abstract
This study is the first to model and experimentally validate refrigerant inventory of R134a in small-channel cross-flow condensers$\sp\dagger.$ 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 $\pm$10% of the experiment and the pressure drop was predicted within $\pm$30% for the majority of the data.
More importantly, the model predicts refrigerant inventory within $\pm$10% of the experiments for ninety five percent of the data. In the inlet header, the slip ratio was correlated to the Reynolds and Froude numbers, and the homogeneous liquid volume fraction. In the small channels, the Reynolds and Weber numbers, and the homogeneous liquid volume fraction were used to correlate the slip ratio. For the outlet header, the dispersed liquid in the core was modeled using an unsteady gravity model and the annulus was modeled using the liquid-film Reynolds number and inverse viscosity.
"Finally, the flow regimes were documented for the pipes, headers and small channel condenser tubes. Intermittent flow was the predominate flow regime in the small channels which is consistent with the Damianides flow map and Kelvin-Helmholtz stability criteria. Through the inlet header the flow transitioned from a dispersed liquid to a bubble flow regime. The flow regime in the outlet header was always a dispersed ""gravity driven"" liquid in the core with a thin liquid annulus on the wall. Visual data collected for the headers were in qualitative agreement with the refrigerant inventory model. ftn$\dagger$ Small-channel heat exchanger technology is the subject of United States and foreign patents applied for and issued to Modine Manufacturing Company, Racine, Wisconsin, USA. Current United States patents include 4,615,385, 4,688,311 and 4,998,580."
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