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INTRODUCTION

The tendency of a fluid to stay in its liquid state even if the fluid pressure falls below its saturation pressure is termed as metastability. This is a non-equilibrium state of the fluid in which the liquid exists in a superheated state. Thus, the phenomenon of metastability always exists whenever a fluid undergoes a transition from liquid phase to vapor phase. The phenomenon of superheating of liquid above the saturation temperature had been mainly investigated for the cases of critical efflux in the flow boiling and of the superheating of a water bubble in a higher-boiling liquid. However, the present discussion has been limited to the metastability inside a capillary tube only, an expansion device used in the low capacity vapor compression refrigeration systems. The performance of a refrigeration system greatly depends on the appropriate selection of the capillary tube size (bore and length) for a given set of input conditions. To achieve the desired pressure drop from a capillary tube, the length of the capillary tube for given capillary tube diameter is to be determined. The refrigerant while passing though the capillary tube undergoes a phase change from liquid state to vapor state. Thus, the entire capillary tube can be divided in two three distinct regions- single phase subcooled liquid region, the metastable region and the two-phase region. To determine the length of capillary tube, the length of each of the three regions has to be evaluated. The accurate prediction of metastable length is a must for effective designing of capillary tube for a given application. Hence, the study of the phenomenon of metastability becomes important as it helps to determine the initial conditions of the flashing process.

The objective of the present work is to not only highlight the significance of the metastable flow in a capillary tube but also to discuss the available correlations for the prediction of underpressure of vaporization in a capillary tube. Furthermore, this paper presents a critical review of the metastable flow in a capillary tube.