Experimental Investigation of Droplet Dynamics and Spray Atomization inside Thermostatic Expansion Valves

In this paper, experimental investigation on spray atomization and droplet dynamics inside a thermostatic expansion valve (TXV), a component commonly used in vehicle refrigeration system, was conducted. A needle and an orifice were copied from a commercial TXV and machined to be mounted inside a chamber with optical access so that the flow inside the TXV is simulated and visualized at the same time. The break-up and atomization of the refrigerant were documented near the downstream of the orifice under different feed conditions for two TXV with different geometry. A Phase Doppler Anemometry (PDA) system was used later to measure the size and velocity of atomized refrigerant droplets. The results showed that the droplet size variation along the radial direction is slightly decreased at near downstream and increased at farther downstream due to the coalescence. The increase of the pressure drop through throttling induces more droplets splashed from the needle plate and more violent secondary breakup at the downstream for TXV 1, which is featured by wide expansion space between the needle and orifice. Such trend is less apparent on TXV 2, of which the expansion space is much narrower. Further study of the impingement effect indicate that a more active coalescence of incoming droplets and ejected droplets is taking place in TXV2, which potentially offset the effect of the secondary breakup induced by the impingement. This makes TXV2 operating more stable and less likely to be subjected to the hunting.