Heat Transfer Enhancement of Falling Film Evaporation

Purpose

Since most of refrigerant used in refrigeration and air conditioning systems has high global warming potential (GWP), the charging amount of refrigerant is required to be reduced. Falling film evaporator is expected as a substitute of typical flooded evaporator because it can operate with less refrigerant. Liquid flows down on the outer surface of heat transfer tubes as liquid film, and heat is transferred by convection, evaporation from the liquid film surface or nucleate boiling on the tube surface. The important issues to improve heat transfer performance are to avoid dryout for thin liquid film thickness under high heat flux or low mass flow rate conditions, and to reduce the wall superheat for the onset of nucleate boiling for thick liquid film thickness under high mass flow rate condition. To suppress the liquid entrainment in nucleate boiling is also important issue.
The purpose of this study is to develop a heat transfer enhancement tube for falling film evaporator, especially surface structure is investigated. The heat transfer characteristics were experimentally evaluated for various refrigerants under the varied heat flux and liquid flow rate conditions.

　

Research content

　

In this study, a porous structure processed by thermal spraying is applied to the heat transfer surface to satisfy the requirements on the increase in bubble site density for nucleate boiling and easy liquid spreading. Thermal spray coating is one of the surface reforming methods where melted metal is sprayed onto the target surface. As seen in the cross-section, melted copper is stacked on a copper surface with cavities. It was shown from our experimental results that the thermal spray coated surface produced higher heat transfer coefficients because fine porous structure on the surface enhanced the liquid spreading and the activation of nucleation site in nucleate boiling. Evaporation behaviors for the smooth and thermal spray coated surface for the same film flow rate and heat flux are shown in the left and right figures, respectively. Liquid is supplied to the test tube through the dummy tube for uniform liquid distribution. The test tube was heated by an inserted cartridge heater. It can be seen that partial dryout was observed on the smooth tube, while nucleate boiling was observed on the whole area of the thermal spray coated tube.

　

Publications

T. Ubara, K. Sugimoto, H. Asano
Film thickness and heat transfer characteristics of R1233zd(E) falling film with nucleate boiling on an inclined plate, International Journal of Heat and Mass Transfer 198, 123423 (2022). https://doi.org/10.1016/j.ijheatmasstransfer.2022.123423　　

T. Ubara, H. Asano, K. Sugimoto
Falling Film Evaporation and Pool Boiling Heat Transfer of R1233zd(E) on Thermal Spray Coated Tube, Applied Thermal Engineering, 117329 (2021). https://doi.org/10.1016/j.applthermaleng.2021.117329

T. Ubara, H. Asano, K. Sugimoto
Heat transfer enhancement of falling film evaporation on a horizontal tube by thermal spray coating, Applied Sciences, 10(5), 1632 (2020). https://doi.org/10.3390/app10051632

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