| Heat transfer augmentation has always been a concern among industrial and academic investigators.Environmental protecting requirements and increasing energy efficiency standards poses greater challenges to the efficiency and reliability of enhanced heat transfer technology.Phase change heat transfer is a widely used thermal management approach in the fields of refrigeration and electronic heat dissipation.However,there is still a gap in understanding the thermal-hydraulic performance of micro/multi-scale enhanced heat transfer and its mechanisms,which is of great importance for the prediction and a further optimization.In this dissertation,convective heat transfer and pressure drop characteristics during flow boiling and condensation in conventional channels are experimentally explored,and the inner surface of these channels are enhanced by various single or compound enhanced methods.In addition,influences of surface characterization and working fluids on heat transfer and pressure drop of micro-fin tubes are investigated numerically and theoretically,and new predictive methods are proposed.Experimental results indicated that in terms of convective condensation,the herringbone grooved/dimpled enhanced tube has the highest heat transfer coefficients at the test conditions,which can reach 1.4–1.74 times higher than those of the equivalent smooth tube.The herringbone grooves can effectively reduce the equivalent liquid film thickness at the convergence,while the dimples would promote fluid turbulence and reduce the thermal resistance of liquid film at the tips.The detailed tests under varied vapor quality showed that heat transfer coefficients(HTCs)of the smooth tube mainly depend on the film condensation heat transfer at the top of the tube for a low mass velocity.Therefore,HTCs flatten out initially and then start to grow quickly while the convective heat transfer part becomes more prominent with increasing mass velocities and vapor quality.However,there is a different trend on the effects of mass velocities for two multi-scale enhanced tubes,especially for the herringbone grooved/dimpled enhanced tube,and an obviously positive relationship is found for them between mass velocity and HTCs at the entire test conditions.The difference may be attributed to the augmentation of convective parts by the enhanced surfaces.By introducing the area enlargement ratio to account for the geometrical effects,a new heat transfer model is proposed,which is capable of providing an excellent heat transfer prediction for two enhanced tubes.As for the frictional pressure drop,the herringbone groove/dimpled tube provides a friction pressure drop penalty ratio of 1.30–1.63.Then the enhancement structures are treated as roughness using the ratio of height to diameters and incorporated in the friction factor to generalize specific correlations.As a result,all data points can be predicted within±20%deviation range.In terms of flow boiling,HTCs increase with increasing mass velocities and heat fluxes for a fixed vapor quality range.The herringbone grooved/dimpled tube provides the highest HTCs when mass velocities(G)are less than 120 kg/(m~2s),while the herringbone grooved/rough tube seems to perform best at higher mass velocities.The uneven axial heat transfer caused by the incomplete wetting of the inner surface leads to a strong correlation between wall temperature differences and phase distribution along the tube.Besides,the relationship is validated using the predicted flow patterns in the smooth tube.HTCs of the smooth and the herringbone groove/rough tubes tend to decrease firstly and then be flat with increasing vapor quality,and the HTC at the top of the tube is significantly lower than that at the bottom at these conditions.However,the HTCs of the herringbone grooved/dimpled tube show a deep“V”-shaped transition at G=140 kg/(m~2s),at which HTCs start to increase rapidly with increasing vapor quality,and the temperature differences between the upper and lower walls approach zero.The enhancement units on the inner surfaces can promote surface wetting,and then contribute to the early transition of stratified-wavy/annular flow.Given the effects of surface structures on the wetted angle,new heat transfer correlations are proposed for the smooth and the herringbone grooved/dimpled tubes.A numerical investigation is also conducted to investigate heat transfer characteristics during annular condensation,and results indicate that heat transfer coefficients increase with increasing vapor quality(x)and mass velocities,but drops with increasing saturation temperature.A smaller apex angle means larger inter-fin space,and the equivalent film thickness would be smaller at the same vapor quality.Therefore,the micro-fin tube with a smaller apex angle provides a better thermal performance especially at high vapor quality.A higher fin can result in a larger area enlargement ratio,reduce the film thickness near the fin tip,and promote the turbulence viscosity in the vapor and liquid phases.In the annular flow,circumferential liquid film distributes evenly due to the centrifugal force by helical fins.Mass transfer only occurs near the gas-liquid interface during condensation heat transfer,and the interface near fin tips provides the highest mass transfer coefficient.Correspondingly,the local HTCs at fin tip regions are much higher than those at fin root regions.As for the thermal-hydraulic performance during flow boiling in micro-fin tubes,existing correlations are evaluated using collected databases,and most of them are unsuitable for a wider range of tube diameters and refrigerants.Given different parametric effects of mass velocities and heat fluxes in conventional and small channels,a critical fin-root diameter and an equivalent heat flux are introduced to the new heat transfer correlations.As a result,the new model provides the best prediction against the heat transfer database with a mean absolute error of18.2%,and it shows a robust predictive ability against different data sources.Besides,an existing frictional pressure drop correlation is modified to accurately convey the effects of vapor quality,and the updated one outperforms other correlations.At last,the two proposed models are validated by experimental investigations of several micro-fin tubes of different geometries,and results indicate that all the data points could be predicted within±30%deviation range. |
PDF Full Text Request