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Design, Measurements and Performance Predictions for a Micromolded Refrigerant Expansion Device.


pdf icon Design, Measurements and Performance Predictions for a Micromolded Refrigerant Expansion Device. (4629 K)
Yashar, D. A.

NISTIR 7327; 137 p. April 2006.

Keywords:

refrigerants; heating; ventilation; air conditioning; microfabrication; refrigerant expansion

Abstract:

The design, fabrication and testing of a thermopneumatic microfabricated valve for controlling refrigerant mass flow rate during expansion is presented in this report. This device was fabricated through nickel electroplating within very thick SU8 molds, thereby realizing expansion devices useful for small vapor compression systems through an inexpensive modified Ultraviolet-LIGA (German acronym for lithography, electrodeposition and molding) process. This work begins with process development to create meso-scale nickel pieces through electroplating into SU8 molds. Two test benches were constructed and used to benchmark the performance of a prototype with compressed air and R134a. Three dimensional numerical simulations were performed on the flowfield within the device to predict its ability to control compressed air. A code was also developed to predict the device's temporal response and relationship between actuation level and power input. The prototype was able to reduce the mass flow rate of the compressed air by 22% at the conditions used in the analysis. The performance was then demonstrated in a 1.5 kW to 2 kW R134a vapor compression system to characterize both steady state and transient response. Steady state data showed that the mass flow rate of refrigerant could be controlled using the valve. The level of refrigerant subcooling defined the magnitude of the response. Steady state data taken at 750 kPa inlet pressure shows the mass flow rate was reduced by 4.2% at 1 oC subcooling and up to 10.8% at 5 oC subcooling for a given level of actuation. Transient system response was characterized using cyclic actuation of the device. The change in capacity was approximately 5%, at the conditions used during these tests.