Heat transfer model of spray evaporation on tube bundles for refrigerant and lubricant mixtures

Spray-type evaporators are often used in large chillers for building air conditioning and industrial refrigeration. Spray-type evaporators reduce the refrigerant charge while maintaining high energy efficiency. Studies in the literature focused on heat transfer models for spray evaporation. However, the insights gleaned from single-tube heat transfer experiments cannot be directly applied to tube bundles. In addition, models in the literature overlook the inevitable presence of lubricant in the system.

This paper introduces an innovative semi-empirical heat transfer model that predicts the heat transfer coefficients (HTCs) of the tubes within the bundle while factoring in the lubricant. The model, which was also experimentally validated for R1234ze(E)/oil and R134a/POE mixtures, accounted for the type of bundle configuration (square vs. triangular) and type of tube surface enhancements (boiling vs condensing).

Utilizing a novel segmented tube bundle approach, the model considered the spatial distribution of lubricating oil to predict the decrease in the tube HTC due to their vertical placement in the bundle. The local film flow rates decreased for the lower tubes of the bundle, especially at high heat fluxes. When considering the HTC from single tube experiments, approximately 50% of the HTC degradation was attributed to the bundle effect, and the remaining 50% was due to the lubricant. The lubricant affected boiling-like tubes negatively, while sometimes increasing the HTC of condensing-like tubes used in the bundle. Foaming and film breakdown when oil was present affected the bottom tubes of the bundles the most, especially if the feed ratio was less than 2. At a concentration of 1%, the lubricant decreased the HTC by 30% or more if the heat flux was above 15 kW/m². The new model enables engineers to forecast the performance of spray-based tube bundle evaporators based on tube geometry, bundle configuration, refrigerant conditions, and the specific refrigerant-lubricant pair in use.