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Concerns over global warming and ozone depletion will limit or phase out several refrigerants currently used in commercial and residential cooling and heating equipment. The environmental criteria for the future refrigerants include zero ozone depletion potential (ODP), low global warming potential (GWP), and high efficiency. This project will benchmark the heat transfer properties of the leading replacement candidates and will measure and model their overall thermal performance in a vapor compression system. The new refrigerants will likely also be mildly flammable, and this project will measure and model the flammability of possible blends of agents.
Objective - To develop heat transfer, system performance information, and burning velocity behavior of low-GWP refrigerants, which will assist in the selection and implementation of the best replacements for high-GWP hydrofluorocarbon (HFC) refrigerants.
What is the new technical idea? Most of the currently used HFC refrigerants will be phased out or phased down. Consequently, candidate replacement fluids must be researched and evaluated. The scope of the evaluation includes two-phase heat transfer and cycle measurements that will support the development of a prediction model for a comprehensive and fair comparison of low-GWP refrigerants. For complete characterization of new fluids, fundamental heat transfer and pressure drop measurements will be taken in a convective-boiling heat transfer apparatus, and cycle performance will be measured in a laboratory heat pump apparatus. The prediction model will be a new cycle simulation-based methodology that will determine refrigerant performance parameters, volumetric capacity, and coefficient of performance while accounting for the refrigerant's thermodynamic and transfer properties. The new methodology will provide more realistic predictions of low-GWP refrigerant performance than conventional theoretical models based on refrigerant thermodynamic properties alone.
The new low-GWP refrigerants are likely to be mildly flammable. To optimize the thermodynamic and fluid mechanic properties of the refrigerants, while minimizing the flammability, industry will use blends of the new agents. Hence, the capability to predict the flammability of pure agents as well as arbitrary blends would be very useful. The laminar burning velocity is the preferred flammability metric upon which standards are being developed. The laminar burning velocity of select blends of agents will be measured and predicted numerically. Modeling of the burning velocity will provide an understanding of the physical properties of the agents that influence their behavior in both test methods and in full-scale fires, facilitating their safe use.
What is the research plan? The research plan encompasses four tasks. Within Task 1, the local convective-boiling heat transfer coefficient of candidate replacements for high-GWP refrigerants inside a micro-fin tube will be determined. In FY12 through FY15, the apparatus was used to test R134a and several low-GWP test refrigerants. The test fluids were selected by theoretically evaluating the evaporative and condensing heat transfer performance of 40 potential low-GWP refrigerants as identified by the Air-Conditioning, Heating, and Refrigeration Institute's (AHRI) Low-GWP Alternative Refrigerants Evaluation Program (AREP). Three more low-GWP refrigerants will be tested during the FY16 effort. For each refrigerant, roughly eighty discrete operating points will be recorded to characterize the heat flux and mass flow versus thermodynamic quality. The heat transfer measurements for low-GWP refrigerants will broaden the fluid database in the literature, which will be used to extend the validity of an existing NIST heat transfer correlation to low-GWP fluids.
NIST will be working closely with chemical manufacturers that have supplied the test refrigerants. Some of these test refrigerants may be mildly flammable having an ASHRAE 2L designation . Consequently, modifications have been made to the test rigs to safely test the 2L refrigerants.
Task 2 consists of the laboratory characterization of low-GWP refrigerants in a mini-breadboard heat pump (MB-HP). In FY13, this laboratory apparatus was completely rebuilt and equipped with specialized, variable-area heat exchangers, a variable-speed compressor, and two water chillers to control the temperatures of the heat source and the heat sink. In FY14 and FY15, the MB-HP was instrumented and calibrated. Shakedown-baseline tests with R134a were done followed by tests with the first low-GWP fluid (to be selected). In FY16, the rig will be used to evaluate the cycle performance of four new low-GWP refrigerants and mixtures.
The modeling element of this project (Task 3) supports two NIST Standard Reference Databases (SRDs): CYCLE_D and REFLEAK. FY11 produced updates of both programs with new versions released in FY12. Also in FY12, the simulation methodology and design specifications were developed for a new generation cycle model, Cycle-ΔT/UA, which accounts for both thermodynamic and transport properties of refrigerants. Inclusion of transport phenomena within the heat exchangers enables the analysis and optimization of the fundamental trade-off between the pressure drop penalty and refrigerant heat transfer enhancement with increasing refrigerant mass flux. The FY13 effort produced the first version of this model, which is limited to the basic vapor compression cycle. During FY 2015, this model was expanded to include three advanced cycle options offering improved efficiency by limiting throttling irreversibilities. In FY16, The CYCLE_D-HX program will be updated and subjected to Beta testing and review before it is submitted to the Office of Standard Reference Data to become part of a new NIST Standard Reference Database in FY17.
The goal of the flammability work is to develop predictive tools for the burning velocity, so that it does not have to be measured for each blend of interest (which would be extremely time consuming and prohibitively expensive). Task 4 involves the measurement and prediction of burning velocity for the agents and blends. A 30 L combustion chamber with shadow-graph imaging and high-speed video will allow direct measurement of the burned-gas flame propagation rate. With suitable data reduction and analysis, the 1-D unstretched, laminar burning velocity can be determined. Using available numerical techniques, together with a chemical kinetic model and datasets for the thermodynamics and transport properties, the 1-D unstretched, laminar burning velocity will be calculated directly, and then compared with the experimental measurements. As a first step in the development of the predictive tool, the measured and predicted burning velocities of blends of R32 and R125 (components of the widely used refrigerant R-410a) will determined, and the predictive ability assessed.
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Start Date:October 1, 2011
Lead Organizational Unit:el
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