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Background |
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The use
of vapor compression cycles for high-temperature-lift space-conditioning
systems and concern about the depletion of the stratospheric ozone level has
created an interest in high pressure, zero-ozone-depletion-potential, near-azeotropic
refrigerant blends such as R404A and R410A as substitutes for refrigerants
such as R22. At the desired high heat rejection temperatures in such
space-conditioning and water heating systems, the saturation pressures of
these blends approach and even exceed the critical pressure.
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Motivation |
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Phase change at
near-critical pressures in these refrigerant blends is not well understood.
Much of the literature available for the prediction of condensation heat
transfer is at relatively low operating pressures, and the validity of
extrapolating these models to near-critical pressures is not well
established |
| Past
Work |
|
Most prior investigations focused on:
 | Annular Flow |
| Pure refrigerants |
| Low critical pressures |
| Pressure drop in adiabatic flow |
Commonly used correlations fail to predict the heat
transfer and pressure gradient at higher reduced pressures.
|
Liquid-Vapor Dome for 410A
|
|
Specific Objectives |
| Experimentally determine local h and ΔP
across V-L dome in small quality increments at Pr= 0.8
and 0.9×Pcritical |
| Mass flux range 200 < G < 800 kg/m2-s |
| Round, horizontal tubes, with diameters ranging from
0.5 < D < 9 mm |
| Develop flow mechanism
based h and ΔP models |
|
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Approach |
|
Experimental Facility |
Schematic of Test Facility
|
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Condensation:
| Pre-condenser energy balance -> Inlet quality
(enthalpy) |
| Post-condenser energy balance -> Outlet quality
(enthalpy) |
| Secondary coolant energy balance -> Test section
heat duty |
|
|
Thermal Amplification Technique
|
Thermal
Amplification Technique
| Decouple heat duty determination and resistance
ratio issues |
| Use high flow rate primary coolant (closed loop) to
establish high resistance ratio (Rrefg/Rcoolant) |
| Address heat duty determination in secondary coolant
(open loop) at low flow rates |
| Low secondary coolant flow rate ensures high DT,
low uncertainty in Q |
|
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Sample Findings |
|
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Experimental Data
| h increases at higher x and G |
| h slightly decreases at higher Pr |
Model Development
For larger diameters (6 and 9 mm I.D.), the data is
divided into the following flow regimes:
·
Wavy Flow
·
Annular Flow
·
Wavy-Annular Transition
FrSO (Soliman modified Froude
Number) is used to divide the different flow regimes.
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Applications |
|
This study will aid the
understanding of refrigerant blend behavior at elevated pressures and will
help the HVAC industry to develop safer and more efficient
high-temperature-lift space-conditioning and water heating systems with less
adverse impact on the environment. |
|
Ongoing Work/Future Directions |
|
The Air-Conditioning
and Refrigeration Technology Institute (ARTI) is the primary sponsor of this
project and has particular interest in small diameter tubes, which can
withstand higher operating pressures. In addition to the diameter, the
effects of mass flux, operating pressure, quality (during condensation) will
be experimentally investigated. |
|
Sponsors |
|
ARTI |
|
Papers |
- Mitra, B. and S. Garimella (2003), "Heat
Transfer and Pressure Drop for Condensation of Refrigerant R410a at
near-Critical Pressures," 2003 ASME International Mechanical
Engineering Congress, Nov 15-21 2003, Washington, DC., United
States, American Society of Mechanical Engineers, pp. 87-97.
- Jiang,
Y. and S. Garimella (2003), "Heat Transfer and Pressure Drop for
Condensation of Refrigerant R-404a at near-Critical Pressures,"
Technical and Symposium Papers Presented At the 2003 Winter Meeting of
The ASHRAE, Jan 26-29 2003, Chicago, IL, United States, Amer. Soc.
Heating, Ref. Air-Conditoning Eng. Inc., pp. 677-688.
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