Over the past decade, the development of theoretical models used to simulate capillary tube performance has steadily progressed. Currently, the major obstacle to improving performance model accuracy is the inability to accurately predict the onset of vaporization (flash point) and thereby the metastable liquid region length. It has been demonstrated that neglecting this region's effect on the overall flow resistance can lead to an underprediction by 5 to 25% of the actual mass flow rate through a capillary tube. Before an accurate metastable liquid region model can be developed, however, a more thorough understanding of the region behavior is necessary. In response, an experimental study has been completed with R-134a using a single adiabatic capillary tube. Testing was undertaken to determine if the metastable liquid region effect on steady-state mass flow rate measurements was repeatable and dependent on controllable test parameters, as had been suggested in a recent study. The experimental parameters were: steady-state inlet subcool level (three levels), the direction of approach to the steady-state inlet temperature (increasing or decreasing temperature), and the forced inlet temperature response rate used in attaining the steady-state flow condition. The resulting general data trend demonstrated increasing scatter in repeated flow rate measurements as the inlet subcool level decreased. Further, there appeared to be some dependence of the flow rate data on the experimental procedure used in setting the inlet condition. However, even under controlled test conditions the dependence was not always repeatable. Using a theoretical flow model, it was shown that most of the data scatter among repeated measurements was caused by variation in the flash point location and thereby the metastable liquid region length. These results precipitated a second experimental phase designed to determine if the flash point location could be controlled. If so, the scatter in measured mass flow rate data for a common inlet condition would be reduced. Two modifications were made to the capillary tube. First, a thin wire was run through its entire length and the original test matrix repeated. The overall measured flow rate levels were lower due to the decrease in flow area caused by the wire. But in comparison to the measurements without the wire, the proportional range of data scatter was cut in half. The second modification was made by drilling five 0.020 in. (0.51 mm) diameter holes through the tube wall, which were spaced at 5 in. (127 mm) toward the end of the tube. Each hole was sealed along the outer tube surface, thus creating a series of wall cavities acting as potential nucleation sites. The overall measured flow rate levels were reduced by roughly 13% in comparison to the original unmodified tube. But more interestingly, the range of data scatter was reduced by nearly an order of magnitude. In conclusion, it was proven that the data scatter in repeated measurements of mass flow rate was caused by variation in the flash point location. It was then demonstrated that the variation in the flash point location could be controlled. Once controlled, an accurate prediction of the metastable liquid region length is potentially possible, which will in turn improve the accuracy of theoretical flow models used to simulate capillary tube performance.

Reprinted from the International Journal of Heating, Ventilating, Air-Conditioning and Refrigerating Research, Vol. 7, No. 2, April 2001, pp. 107-123.

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