Bundle effects on the tubes’ heat transfer coefficients during spray evaporation of refrigerants R134a and R1234ze(E)

The increasing demand for higher energy efficiency drives enhancements in heat transfer. In shell-and-tube heat exchangers, the distribution of refrigerant via pressurized liquid spray on top of a bundle is effective in maintaining high heat transfer performance while reducing the refrigerant charge. Even when enough refrigerant is supplied to the bundle, localized dry patches can occur on some tubes, hindering the overall heat transfer performance. This study aimed to isolate and quantify the bundle effects of spray tube evaporators. It present the heat transfer coefficients (HTCs) of the tubes placed within the bundle. A novel test setup was constructed for spray evaporation of refrigerants R134a and R1234ze(E). Enhanced-surface copper tubes oriented horizontally were arranged in three columns and five rows, forming the bundle with a triangular pattern and a square pattern, and with a tube pitch to diameter ratio of 1.3. The tubes had an OD of 20 mm, and two types of enhanced tube surfaces, one with a condensing-like surface and the other with a boiling-like surface, were investigated. The HTCs of the tubes, when operating by themselves and when placed within the bundles, were compared. If the refrigerant inlet subcooling was controlled between 0.2 and 0.5 °C, high HTCs were measured because all tubes actively participated in the heat exchange process. The top tubes’ HTCs were within ±20% of single-tube pool boiling test data. However, the bundle effects reduced the overall bundle HTC by 50% or more because the tubes farther away from the nozzles could experience refrigerant starvation and localized dry-out. This study highlighted that the bundle pattern, tube type, and refrigerant distribution must be considered to ensure high performance of the spray evaporators across various conditions.