04. Boiling and Multi-Phase Flow

Recently, it is required in the refrigeration and air conditioning field to use low GWP (Global Warming Potential) refrigerants and reduce the amounts of refrigerant used from the viewpoint of reducing environmental load. A falling film evaporator is considered to be one of the effective ways to reduce the amount of refrigerant used as an alternative to a flooded evaporator. The important issues in the falling film evaporator are to keep liquid film even at low flow rate and to obtain nucleate boiling in a thick liquid film at a sufficient flow rate. A thermal spray coating that can enhance nucleate boiling was applied to the heat transfer surface. On a thermal spray coating, a large number of cavities are formed that are expected become nucleation sites. The increase in nucleation site density is expected to lead to smaller vapor bubbles and thus a more stable liquid film. In addition, evaporative heat transfer is enhanced by the nucleate boiling. On the other hand, the growth of vapor bubbles in the liquid film may cause a risk of dry-patch formation with vapor bubble breakup. In this study, the relationship between superficial thickness of liquid film with or without vapor bubbles and heat transfer performance are investigated for falling film evaporation on an inclined plate with a thermal spray coating. HFO-1233zd(E) was used as the working fluid. The effects of liquid film flow rate, heat flux and inclination angle on liquid film thickness and evaporative heat transfer coefficient were evaluated. The inclination angles are 15 and 30 degrees. Obtained results will be reported with the liquid film flow behavior.