BUBBLE DYNAMICS FACTORS OF STRUCTURED SURFACES WITH RECTANGULAR FIN BASE FOR NEW, LOW-GWP REFRIGERANTS
Structured surfaces consisting of sub-surface tunnels and surface pores or gaps are used to enhance boiling heat transfer resulting into compact heat exchangers. One of the applications of enhanced surface tubes is in flooded evaporators in water chillers. The fundamental mechanisms in nucleate boiling on structured surfaces are not still well understood, especially for new, low-GWP refrigerants. Mechanistically based analytical models to predict boiling performance of structured, enhanced surfaces are available in published literature. There are 3 possible nucleate boiling mechanisms viz. "flooded mode", "suction evaporation mode" and the "dried-up mode". Out of these the mode for saturated boiling is observed to be "suction evaporation mode" as confirmed by published experimental results. The base of this work is Chien and Webb model for nucleate boiling on enhanced structured surfaces. An inhouse code in Matlab is developed to predict the bubble dynamics factors for inputs of surface geometry and refrigerant properties. Chien and Webb model was developed based on experimental data with circular fin base i.e. with 2 menisci at the fin tip. For rectangular fin base i.e. with menisci at all the four corners, the surface would exhibit better heat transfer performance for the same degree of wall superheat. The aim of this study is to predict and compare the bubble dynamics factors for circular and rectangular fin base for the new, low GWP refrigerants viz. R1234ze (E), R513A and R450A which are the targeted replacements for the currently used, high GWP refrigerant R134a. The results show bubble frequency, site density and heat flux are higher for rectangular fin base compared to circular fin base due to higher volume of liquid which gets evaporated from 4 menisci instead of 2 as in circular fin base. The range of increase in these factors is observed to be from 10 % to 30 %.