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Abstract This paper reports a theoretical and experimental study on a thermo-fluid dynamics in an evaporator of a loop heat pipe (LHP). A steady-state model that includes liquid–vapor interface behaviors in the evaporator was developed. In this model, the changes in the heat transfer coefficient of the evaporator were considered. A flat-type evaporator was designed on the basis of the LHP model. The main characteristics of the evaporator are as follows: the number and width of the vapor grooves that were processed in a porous wick were 85 and 0.3 mm, respectively. As results of the experiments, the LHP transported heat up to 260 W (18.2 W/cm2) for the heat transport length of 550 mm. The minimum thermal resistance of the LHP was 0.13 K/W. The operating temperature of the experimental result agrees well with the calculation result. The liquid–vapor interface behaviors in the evaporator during the LHP operation were discussed. On the basis of the model, the relation between the liquid–vapor interface behaviors and the heat transfer performance of the LHP was quantitatively explained. In addition, it was found that the contribution of the nucleate boiling heat transfer increased with the applied heat. This phenomenon enhanced the heat transfer performance at high heat flux. Highlights • Liquid-vapor interface behavior in an evaporator for high heat flux LHP is studied. • LHP model that includes thermo-fluid dynamics in the evaporator is developed. • A porous wick that has 85 grooves of 0.3 mm width is designed based on the model. • LHP transported heat up to 260 W (18.2 W/cm2) for heat transport length of 550 mm. • Thermo-fluid dynamics in the evaporator during the LHP operation is explained. [ABSTRACT FROM AUTHOR]

• The effects of changing vapor groove width of a porous wick and improving wettability of a LHP evaporator on heat transfer performances were evaluated experimentally and theoretically. • The micro-grooves processed on the heating plate improved heat transfer performances regardless of the vapor grooves width. • The micro-grooves increased heat transfer performances more greatly as the vapor grooves width increased. • Numerical model explained that the length of triple phase contact line has a great influence on the evaporative heat transfer coefficient. In this study, the effects of an increased evaporation area known as the triple-phase contact line (TPCL), of the porous wick and improved evaporator wettability on the loop heat pipe (LHP) heat transfer performance were evaluated experimentally and theoretically. An infrared camera and a microscope were used to observe the thermo-fluid behavior of porous wicks with different vapor grooves widths and evaporator heating plates with different inner wall structures in the experiment. The porous wick's vapor groove widths were 1.0 mm, 0.5 mm, and 0.2 mm. A normal heating plate and a micro-grooved heating plate were used as the evaporator case. As a result, it was clarified that processing the micro-grooves on the heating plate improved the evaporative heat transfer coefficient and maximum heat flux, regardless of the vapor grooves width of the porous wick. Furthermore, it was proposed that both of them increased significantly as the vapor grooves width increased. The thermal-hydraulic numerical model that predicted the heat transfer coefficients for each case was developed. The calculation results showed a significant increase in the heat transfer coefficient as the TPCL appearance rate, which expresses how much the TPCL appears in the micro-grooves, increased, and this increasing tendency was in good agreement with the experimental results. Furthermore, it was proposed that the TPCL appearance rate at a high heat flux was reduced as the vapor grooves width decreased. The results indicate a new promising approach for improving the heat transfer performance of LHP evaporators. [Display omitted] [ABSTRACT FROM AUTHOR]

l Thermo-fluid dynamics in a micro-textured evaporator for LHPs was analyzed. l Thermo-fluid behavior was visualized by microscale infrared/visible observations. l Three types of stainless steel plates with various surface morphologies were used. l Micro-textured plates showed higher heat transfer performance than normal one. l Mechanisms for high heat transfer performance were explained based on modeling. This paper presents a thermo-fluid dynamic analysis in a micro-textured evaporator for loop heat pipes (LHPs) that has a super wetting surface with ethanol on the basis of an experiment and modeling. In the experiment, three kinds of stainless steel plates with various morphologies of heat transfer surfaces were tested: a normal flat plate, a sandblasted plate, and a micro-groove plate. The measured contact angles between the surfaces and ethanol were 8.2° ± 1.3°, 0°, and 0°, respectively. Evaluation of the heat transfer performance and liquid-vapor interface behavior was conducted using microscale infrared and visible observations. As a result, it was revealed that the sandblasted plate and the micro-groove plate showed 7 times and 20 times higher heat transfer coefficient under high heat flux conditions respectively, compared with the normal flat plate. Different liquid-vapor interface behaviors were found between the plate types. In the modeling, the heat transfer coefficient was predicted for each case. It was found that nucleate boiling heat transfer induced by rough surface increased the heat transfer performance in the case of the sandblasted plate. In addition, it was revealed that the thin film evaporation at the long triple-phase contact line significantly enhanced heat transfer performance in the case of the micro-groove plate. The findings indicate a new promising approach to enhance the heat transfer coefficient of LHP evaporators. [ABSTRACT FROM AUTHOR]