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Vapor shearing is a common issue encountered in the operations of falling film heat exchangers. The vapor stream effect depends on its orientation. This study investigates liquid film hydrodynamics and heat transfer performance under the influence of vapor streams from different orientations. The results indicate that both orientation and velocity of vapor determine the encountering time and position of the films on the tube's two sides. The liquid film thickness uniformity and the liquid column deflection vary significantly depending on the orientation and velocity of the vapor. Zones of accelerated liquid film, climbing liquid film, liquid stagnation, and transition of liquid film flow pattern are observed. The gradient of film thickness along the tube axis and the deflection in time-averaged peripheral film thickness increase as the vapor orientation varies from 0° to 90° and subsequently decrease as the vapor orientation varies from 90° to 180°. Vapor streams have more pronounced effects on time-averaged peripheral film thickness in regions close to the liquid inlet and outlet. Vapor streams result in changes in peripheral heat transfer coefficients toward the downstream side depending on the orientation and velocity of the vapor. The impact of vapor streams on the overall heat transfer coefficient does not directly correlate with the velocity of the vapor when maintaining the same orientation. [ABSTRACT FROM AUTHOR]

Two-phase flow patterns are critical in falling film heat devices. Hydrodynamic characteristics of turbulent falling films across a horizontal triangular tube bundle were studied under a range of film Reynolds numbers and contact angles. A flow pattern map based on the k-means clustering approach was proposed after the intercolumn two-phase flow patterns were grouped. The results demonstrated that that the intercolumn liquid-vapor flow patterns in a horizontal tube bundle could be effectively and reasonably grouped using the k-means clustering approach according to the results of the void fraction and interface area. There are four intercolumn liquid-vapor flow patterns that were identified: bubbles flow, annular-slug-column flow, half-annular flow, and multi-bridges flow. The annular-slug-column flow pattern and the multi-bridges flow pattern are next to one another in the flow pattern map and are capable of transitioning one into the other. It is only through the two transitional flow patterns that the half-annular-column flow pattern and bubbles flow pattern can turn to one another. In the bubbles' flow pattern and half-annular-column flow pattern, respectively, the highest and least averaged heat transfer coefficients are attained. Simply raising the film Reynolds number or lowering the contact angle will not produce the best heat transfer results. [ABSTRACT FROM AUTHOR]

• Numerical study on falling film flow and heat transfer on porous tubes. • Compared falling film flow and heat transfer on plain and porous tubes. • Studied porosity, layer thickness, and material effects on flow and heat transfer. • Porous layer leads lower flow, thinner film, fewer rupture, better heat transfer. • Smaller porosity, thicker layer, aluminum and copper facilitate heat transfer. Porous surfaces often exhibit exceptional performance in liquid spreading and heat transfer, making them promising candidates for enhancing falling film heat transfer on solid surfaces. This paper delves into the flow and heat transfer characteristics occurring within falling films on horizontal tubes coated with a porous layer. To achieve this, a three-dimensional numerical model that couples the Volume of Fluid (VOF) method with porous media considerations was employed. The investigation comprehensively covers film spreading behavior, flow patterns, film velocity, film thickness, temperature distribution, and heat transfer coefficients, for both plain and porous tubes. Furthermore, we delve into the effects of key parameters, such as porosity, layer thickness, and material properties, on the overall heat transfer process. The results demonstrate the accuracy of our model in reproducing film thickness and heat transfer coefficients within falling films on horizontal porous tubes. When comparing plain tubes to porous ones, some key differences emerge. Specifically, the liquid film on porous tubes spreads more slowly, leading to increased film thickness and fewer occurrences of dry patches. Additionally, porous tubes result in lower film velocity and a more uniform temperature field compared to plain tubes. Interestingly, the findings reveal that smaller porosities yield thicker films and higher heat transfer coefficients, particularly when the porosity is below 0.4. Moreover, thicker porous layers result in thinner films, higher wetting ratios, and increased heat transfer coefficients. Among the materials we tested, aluminum and copper porous layers exhibit the highest heat transfer performance, while steel demonstrates the lowest. [ABSTRACT FROM AUTHOR]

• Tested cross vapor effect on the falling film evaporation of low pressure refrigerant. • Conducted larger vapor velocity effect on falling film evaporation in tube bundle. • Compared the cross vapor effect on falling film evaporation of R123 and R134a. • Visualized the cross vapor effect on falling liquid film flow and heat transfer. The falling film evaporation and the effect of cross vapor stream in falling film evaporator with triangular tube bundle using R123 was experimentally investigated. The variations of local and bundle average heat transfer coefficients were described. The tube bundle consisted of 4 × 3 (column × row) of triangular horizontal copper tubes. The tests without vapor effect were conducted with nominal heat fluxes of 20 to 60 kWm–2, saturation temperatures of 6 to 16 °C and film flow rates of 0.016 to 0.18 kgm–1s–1. Cross vapor stream effect experiments were operated at three heat fluxes of 20, 30 and 40 kWm–2 and two film flow rates of 0.035 and 0.07 kgm−1s−1, and the vapor velocity at the narrowest interstice in the tube bundle varies from 0 to 5.0 ms−1. It is indicated that the heat transfer is seriously influenced by the bundle effect while less affected by changing of saturation temperature. With the increase in vapor velocity, the heat transfer performance is generally weakened; the cross vapor stream has a strong influence on falling film evaporation of R123. [ABSTRACT FROM AUTHOR]

• Establishment and verification of a rig for refrigerants' pool boiling. • Heat transfer coefficients were obtained for the R134a pool boiling on 6 surfaces with various roughness and wettability under 3 saturation temperatures. • A phenomenon of bubble behavior above the boiling surface was spotted and observed with a high-speed camera. In order to investigate pool boiling heat transfer characteristics of refrigerants on different surfaces, an experimental rig is established and verified in this study. The pool boiling heat transfer experiment is conducted with R134a at three saturation temperatures. Six surfaces with Ra roughness ranging from 0.133 to 5.742 μm and two types of wettability are tested in the study. The heat flux varies from 40 to 280 kW/m2. Experimental results indicate that higher roughness is favorable to boiling heat transfer. Shieldex coating on copper surfaces can deteriorate heat transfer by 20% approximately. Visualization analysis was carried out with a high-speed camera to study bubble behavior. A new-found mode of bubble departing was observed at lower heat flux. Besides, the data were correlated in the form of Rohsenow correlation, and comparison and supplement are concluded based on data obtained in this study. [ABSTRACT FROM AUTHOR]

Falling film evaporation is an efficient heat transfer mode in refrigeration and air conditioning industries. In this paper, the falling film evaporation performances of R134a and R123 outside four enhanced tubes and a smooth tube are tested. The results reveal that: with the decrease of film flow rate the falling film heat transfer coefficients of both R134a and R123 on the five tubes exhibit two general stages (a plateau stage and a sharp drop stage); for R134a the plateau is quite uniform while for R123 a mild decrease occurs with the decrease in film flow rate. The four enhanced tubes behave differently in heat transfer performances for R134a and R123. R134a provides around 2–3 times of heat transfer coefficients of R123 for all tubes. [ABSTRACT FROM AUTHOR]

• Studied numerically the falling film flow and heat transfer under turbulent regime. • Clarified the dominant factors tube bundle effect in the turbulent condition. • Compared fluid flow and heat transfer results between laminar and turbulent regimes. • Recommended the key parameters and factors for a higher heat transfer performance. Turbulent falling films are commonly encountered in the practical operations of most falling film applications, but the relevant numerical study is very scarce. The hydrodynamics and thermal performance of falling films in horizontal tube bundles under the turbulent regime were numerically investigated in this study. The calculation ranges are: film Reynolds number from 3505 to 8034, heat flux of 47.3 kWm−2, inlet liquid temperature of 46 °C, tube diameter of 19.0 mm and liquid distributor height of 6.4 mm. Based on the results of the present numerical model, the Standard k - ω model is more suitable in reproducing the falling film heat transfer profile, as the present numerical model accurately predicted the turbulent falling film heat transfer characteristics. There are various hydrodynamics depending on the bundle arrangement and flow regime, including splashing, bridging, gas entrainment, Coanda effect, and liquid film thickness. The triangular bundle provides the highest heat transfer coefficient, but the square bundle provides the poorest one, and the square and rotated-triangular tube bundles behave the largest and smallest bundle effect, respectively. The bundle effect is generally independent of film Reynolds number, liquid feeder height and tube spacing. [ABSTRACT FROM AUTHOR]

Abstract An experimental investigation on the condensation of R134a, R1234ze(E) and R290 outside plain and enhanced titanium tubes was conducted. The saturation temperature in the experiments was 35 °C to 40 °C, and heat flux was in the range of 8–80 kW/m2. Effects of heat flux, refrigerant and saturation temperature on heat transfer were investigated. The heat transfer enhancement ratio of overall heat transfer coefficient decreased with increasing heat flux. It was found that the condensing heat transfer coefficient of R134a, compared with R1234ze(E) and R290, was the largest for plain and enhanced tubes. Saturation temperature had minor effect on condensing heat transfer coefficient for R134a, R1234ze(E) and R290 for plain tube. The effect of saturate temperature on the condensing heat transfer of R134a and R1234ze(E) was even negligible for enhanced tube. While, the condensing heat transfer coefficient of R290 for enhanced tube was increasing with increment of saturation temperature. Experimental condensing heat transfer coefficient were also compared with five predicting models for low-fin tube. It showed that the model of Briggs–Rose gave a better prediction result for R134a and R1234ze(E), while the model underestimated the experimental result for R290. [ABSTRACT FROM AUTHOR]

Nucleate pool boiling heat transfer of R1234ze(E), R1233zd(E) and R134a were investigated outside two reentrant cavity tubes. The two tubes have different fin density while the size of subsurface tunnel width and fin height are almost the same. In the experiment, the pool boiling heat transfer is tested at the saturation temperature of 6 °C and heat flux of 10–80 kW/m2. A brief summary of pool boiling experimental data of Hydrofluoroolefins(HFO) refrigerants on different enhanced tubes from recent work is also provided. It is found that the combinations of tube and refrigerant: R134a and R13234ze(E) outside Tube-B1 shows almost equivalent boiling heat transfer performance. The two combinations also yield the highest averaged overall heat transfer coefficient. At the lower heat flux less than 60 kW/m2, for the tube with larger fin density and thinner fin thickness, the number of nucleation cavities is found to be more than that with larger fin thickness. At the heat flux more than 40 kW/m2, the boiling heat transfer coefficient of R134a and R1234ze(E) for the two enhanced tubes nearly merges into a single curve. At the higher heat flux, boiling heat transfer shows weak dependence on the surface structures. The boiling heat transfer coefficient of R1233zd(E) is more than 40 percentages lower than R134a for the two tubes at the nearly identical conditions. The research is helpful for the designer to summarize the boiling heat transfer performance of some typical HFO refrigerants. [ABSTRACT FROM AUTHOR]

Highlights • Falling film evaporation heat transfer of R134a, R290 and R600a is characterized. • Heat transfer correlations for five refrigerants are suggested in two regimes. • Proposed correlations predict the base data and data from references reasonably. Abstract Falling film evaporation heat transfer of R134a and its potential substitutes R290 and R600a outside a single horizontal plain tube is experimentally investigated, and the effects of the saturation temperature, film flow rate and heat flux on heat transfer coefficient are studied. Heat transfer performance of R290 is slightly superior than that of R134a, while R600a is inferior than that of R134a. The threshold film Reynolds number is determined to separate the variation trend of HTC with film Reynolds number into full wetting and partial dryout regimes. Increase of heat flux benefits the heat transfer in both full-wetting and partial dry-out regimes. New heat transfer correlations based on the present data and data for R32 and R1234ze(E) in the authors' group are suggested for two regimes. The correlation for full wetting regime fits 96.7% of the total 542 correlated data within ±30% while fits 73.4% of the total 289 data in references from −30% to +15%, the correlation for partial dryout regime fits 97.5% of the total 162 correlated data within ±30% while fits 76.8% of the total 95 data from references within ±30%. [ABSTRACT FROM AUTHOR]

• A tube with microfins is fabricated and hydrophobic coating is deposited on the fins. • Condensing heat transfer can be enhanced by a factor of 28 with coating and microfins. • The heat transfer can be 150% higher than the same finned tube without polymer coating. Dropwise condensation is extremely difficult to accomplish through the nanostructured surfaces for very low surface tension substances, such as the Freon refrigerants. To further improve the laminar film condensation heat transfer of the substances with very low surface tension, rely solely on the super hydrophobic layer might not work. In this work, a three dimensional micro-finned tube was fabricated and a layer of polymer coating was deposited outside the fins. Condensation of two substances, R1234ze(E) and R134a, were investigated at saturation temperature 30 and 40℃. Under the coupling effect of coating and fins, the condensing heat transfer coefficient can be up to 28 times larger than the smooth tube and the value can reach 43 kW/m2K at heat flux 20 kW/m2. In the lower heat flux, it can be 150% higher than the same finned tube without polymer coating. As the condensing heat transfer enhancement with only fin structure modifications would soon reach its limit, it should have a good application prospect for the micro-fins with surface treatment. [ABSTRACT FROM AUTHOR]

In this study, thin liquid film boiling heat transfer characteristics of R134a on a plain and two finned tubular surfaces were experimentally investigated. The dependence of heat transfer coefficient (HTC) on the composite effects of microstructure and hydrophobic coating was characterized. It is found that HTC increases monotonously with increasing heat flux until reaching a threshold heat flux (THF), beyond which the HTC starts to descend. The THF is highest for the plain surface and lowest for the boiling-enhanced surface due to the effect of reentrant cavities. The boiling-enhanced surface shows larger HTC under low heat fluxes (less than 60 kW m−2) compared with the condensation-enhanced counterpart, whereas the latter one is superior when the heat flux exceeds 60 kW m−2. Hydrophobic coating can substantially intensify the liquid film boiling heat transfer on boiling-enhanced finned surface with reentrant cavities. The intensification of heat transfer is more prominent in cases of high heat flux. [ABSTRACT FROM AUTHOR]

• Three dimensional structure with thicker fins is beneficial to the condensing heat transfer. • Condensing HTC of R-134a is higher than R-1234ze(E) and R-1233zd(E) is the lowest. • In the experiment, condensing HTC is increasing as the increase of heat flux for R-1233zd(E) Film-wise condensation of R-134a, R-1234ze(E) and R-1233zd(E) outside two enhanced tubes was experimentally investigated. The two tubes have the same fin density and similar fin height while the fin thickness is different. In the experiment, the saturation temperature was 36 °C. Heat flux was in the range of 20–90 kW·m−2. It was found that R-134a was more efficient than other refrigerants and gave the highest heat transfer performance outside the two tubes. R-1233zd(E) was the lowest. Condensing heat transfer coefficient of R-134a was approximately 2 times higher than R-1233zd(E). As heat flux was higher than 20 kW·m−2, condensing heat transfer coefficient of R-134a and R-1234ze(E) all decreased as the increasing of heat flux. While for R-1233zd(E), up to the heat flux of 90 kW/m2, the condensing heat transfer coefficient is ever-increasing as the increasing of heat flux. The condensing heat transfer of R-134a was 10–20% higher than R-1234ze(E). The trend of variations for the heat transfer performance of R-134a and R-1234ze(E) was similar. The present work examined the heat transfer characteristics of finned tubes with different fin thickness. It is helpful for the designers to summarize the heat transfer performance of some new HFOs refrigerants having the potential to be used in the water cooled chillers or heat pumps. [ABSTRACT FROM AUTHOR]