|Title:||Heat Transfer Flow Regimes of Refrigerants in a Horizontal-Tube Evaporator
|Contributor(s):||Chato, J.C.; Christoffersen, B.R.; Gaibel, J.A.; Ponchner, M.; Kenney, P.J.; Shimon, R.L.; Villaneuva, T.C.; Rhines, N.L.; Sweeney, K.A.; Allen, D.G.; Hershberger, T.T.
|Subject(s):||refrigerant-side evaporation condensation studies
|Abstract:||An experimental study of flow boiling heat transfer of refrigerants in a horizontal-tube
evaporator was conducted. A single-tube evaporation test facility was designed and developed to
measure the evaporation characteristics of alternative refrigerants. Measurements were made in
several instrumented, horizontal copper tubes with inside tube diameters ranging from 0.277 to
0.430 in and lengths ranging from 4 to 8 ft using R-12, R-22, R-134a, and a 60%/40% azeotropic
The two main flow regimes found during objective and visual evaluation of the flow
patterns during adiabatic and diabatic flow in smooth, horizontal tubes were wavy-stratified flow
and annular flow. High speed pressure and differential pressure measurements were taken for a
variety of mass flux and quality combinations, and were analyzed both spectrally and statistically.
The normalized power spectral density of these measurements had sharp peaks near zero
frequency, indicative of separated flows. Analysis of the standard deviation of pressure drop
divided by the mean pressure drop showed that wavy-stratified flow occurred for values above
0.20, while annular flow occurred for values below 0.10.
For annular flow at low heat fluxes, convective boiling was the dominant mode of heat
transfer. As heat flux increased, nucleate boiling enhanced the heat transfer coefficient, especially
for low qualities and high reduced pressures. For wavy-stratified flows, convective boiling was
diminished due to loss of available convective surface area, while nucleate boiling did not appear to
be suppressed at higher qualities or lower heat fluxes. The heat transfer coefficients were well
correlated using an asymptotic model, which combined the benefits of the "greater of the two" and
superposition models for flow boiling heat transfer. A Froude number dependent term accounted
for stratification effects on the heat transfer coefficient.
The heat transfer correlation and previously developed pressure drop correlations were
combined in a computer program which simulated the two-phase portion of evaporators. This
program was used to examine whether an optimum diameter existed for evaporators with fixed airside
resistances and refrigerant mass flow rates. The results revealed that over a wide range of
diameters, the required length of the evaporator was relatively insensitive to the tube diameter, but
the required surface area had a definite minimum. As the diameter became sufficiently small,
pressure drop decreased much of the driving temperature difference, increasing the length of the
|Publisher:||Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
|Series/Report:||Air Conditioning and Refrigeration Center TR-55
|Sponsor:||Air Conditioning and Refrigeration Center Project 37
|Date Available in IDEALS:||2009-03-13
|Identifier in Online Catalog:||3873038