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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]

Falling film flow and heat transfer are extensively encountered in various industries of renewable and sustainable energy due to the outstanding heat transfer performance. The predictions in falling film hydrodynamics and heat transfer are crucial for the optimal design of falling film heat exchangers, and there have been considerable advancements made in this area over the past few decades. Considering the gaps in current literature predictions, this paper aims to present a comprehensive review on the correlations of flow pattern transition, film thickness, heat transfer, and liquid film rupture within falling film flowing over horizontal tubes and tube bundles. Through comparisons and summarizations, it can be found that the documented correlations appear to be significantly out of alignment with one another, which greatly restricts the applicability of these predictions. The correlations of falling film thickness that are obtained using regression approach based on dimensional analysis may take into account more factors than the ones modified from Nusselt theory. The predictions of flow pattern transition of falling liquid film with horizontal column are often described as the functions of the film Reynolds number, derived from the Kapitza number or the Galileo number. The functions of Nusselt number, film Reynolds number, Prandtl number, and Archimedes number are typically used to represent sensible and evaporative heat transfer correlations, whereas boiling heat transfer correlations also take into account the influence of heat flux, such as Boiling number. The full process of actual heat transfer is taken into account from both the standpoints of sensible convection and nucleate boiling using the combination functions of various heat transfer forms. Within the ranges of their own parameters, the well-established heat transfer models often offer good accuracy. While the predictions of phase change heat transfer often behave worse, the predictions of sensible heat transfer are more general and accurate. Generally speaking, the minimum film flow rate or the maximum heat flux were the functions that correlated with the inceptions of liquid film rupture. • Provides a comprehensive review of predictions for falling film hydrodynamics and heat transfer. • Describes in fully the most recent advancements in the relevant correlations. • Reviews respectively heat transfer prediction of sensible convection, evaporation, and boiling. • Provides an entire review of falling film heat transfer from subcooled convection to film dryout. • Proposed some recommendations and improvements for future research. [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]

The peripheral heat transfer correlation of the subcooled falling liquid film outside the horizontal tube was established using the present numerical data under different liquid flow rates, heat fluxes, tube sizes, distributor heights, and liquid temperatures. The results indicated that the present correlation can predicate 93% of 2654 data within ±25% in θ = 2°–15°, 92% of 2254 data within ±15% in θ = 15°–165°, and 80% of 1442 data within ±30% in θ = 165°–178°. The peripheral heat transfer correlation can predict 74% of 903 data and 80% of 700 data, respectively, within 0°–180° and 15°–165° with the deviations of ±30%. The sensitivity analysis indicated that the parameter angle and the distributor height have the most and least significant impact on the peripheral heat transfer correlation, respectively. [ABSTRACT FROM AUTHOR]

In the present work, a numerical method is utilized to simulate the nucleate pool boiling of R134a outside the horizontal plain tube. Eulerian multiphase model integrated with Rensselaer Polytechnic Institute (RPI) nucleate boiling model is adopted. The nucleate pool boiling is numerically investigated at the heat flux of 20–80 kW/m2 and saturation temperatures of 279 and 299.7 K. By analyzing the experimental results, a modified correlation of active nucleation site density is proposed. The numerical results based on the modified correlation agree well with the experimental data of the pool boiling heat transfer outside the plain tube. According to the simulation of azimuthal variations of temperature around the tube, it is found that the region between 90° and 130° has the best heat transfer performance for its high velocity and turbulence. The maximum temperature appears at the upper of the tube because the velocity and turbulence intensity at this region are lower. The investigation helps to explain the phenomena and observed facts in the experiment. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m2 to 300kW/m2. Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m2, and then the heat transfer degraded at higher heat flux. [ABSTRACT FROM AUTHOR]

The influence of downward vapor stream on the falling film evaporation with nucleate boiling outside a horizontal enhanced tube bundle arrayed in a vertical column is experimentally investigated. The variations in local and bundle averaged heat transfer coefficients with vapor velocity are presented. To simulate the practical condition in a falling film evaporator, the flow passages of the liquid/vapor are constructed using two quartz glass sheets and two columns of dummy half tubes. The test ranges are saturation temperature of 6 °C, film flow rate of 0.035–0.1 kgm –1 s –1 , heat flux of 20, 30, 40 kWm –2 . It is found that (1) the downward vapor effects are either positive or negative depending on the tube position, film flow rate, heat flux and vapor velocity; (2) the bundle effect becomes stronger under the effect of downward vapor stream; (3) the downward vapor has a significant effect on the heat transfer of the lower tubes than the upper tubes; and (4) the vapor stream effect on the bundle averaged heat transfer coefficient varies with the levels of film flow rate and heat flux. The experimental results provide significant guidance in the design of the vapor escape channel in a falling film evaporator. [ABSTRACT FROM AUTHOR]

The falling film evaporation of R134a with nucleate boiling outside a triangular-pitch (2-3-2-3) tube bundle is experimentally investigated, and the effects of saturation temperature, film flow rate and heat flux on heat transfer performance are studied. To study the effect of cross vapor stream on the falling film evaporation, a novel test section is designed, including the tube bundle, liquid and extra vapor distributors. The measurements without extra vapor are conducted at the saturation temperature of 6, 10 and 16°C, film Reynolds number of 220 to 2650, and heat flux of 20 to 60 kWm−2. Cross vapor stream effect experiments are operated at three heat fluxes 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 smallest clearance in the tube bundle varies from 0 to 2.4 ms−1. The results indicate that: film flow rate, heat flux and saturation temperature significantly influence the heat transfer; the cross vapor stream either promote or inhibit the falling film evaporation, depending on the tube position, film flow rate, heat flux and vapor velocity. [ABSTRACT FROM AUTHOR]

The effects of film flow rate, heat flux, inlet liquid temperature, tube diameter and liquid distributor height on subcooled falling film heat transfer outside a horizontal smooth tube are numerically studied, and a heat transfer correlation based on the current data is developed. Comparisons between the predicted results and the published experimental data in the literature are also conducted. The calculation ranges are: film flow rate from 0.025 to 0.284 kg m −1  s −1 , heat flux from 1.0 to 100 kW m −2 , inlet liquid temperature from 2 to 104 °C, tube diameter from 6.35 to 50.8 mm and liquid distributor height from 3.0 to 50.8 mm. The results indicate that: (1) the numerical results of the local heat transfer coefficient are in good agreement with the experimental data in the literature; (2) the surface tension plays an important role in the calculations of heat transfer in two stagnation zones, (3) the heat transfer coefficient shows four distinct zones along with peripheral angle: stagnation zone, impingement zone, thermal layer development zone and departure zone; (4) the heat transfer coefficient increases with increase in film flow rate, tube diameter or liquid distributor height, while keeps constant with increasing heat flux; (5) the correlation predicts 92% of the total 141 calculated data with deviations within ±10%, and predicts 78% of 284 data available in literature with deviations within ±30%. [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]

The effect of vapor flow on the falling film evaporation of refrigerant R134a outside a horizontal tube bundle is investigated with an experimental approach. The test space is a cube with a rectangular cross section of 0.575 m (length) × 38.8 mm (width). The tube bundle was 3 × 6 (columns × rows) of staggered horizontal finned tubes made of copper. The longitudinal tube pitch is 22.5 mm and the transverse is 19.9 mm. The external fin density of test tube is 45 fpi (fins per inch), and outside diameter is 19.05 mm. The vapor flow velocity can be adjusted in the range of 0–3.1 m/s. Liquid falling film flow rate ranges from 0.07 to 0.2 kg/m·s. Experiment is firstly conducted at saturation temperature of 6 °C without the effect of additional vapor flow at the heat flux of 20, 60, 100 and 180 kW/m 2 (for the first tube row). Vapor flow effect experiment was carried out at three heat fluxes 20, 40 and 60 kW/m 2 . It is found that falling film flow rate is an important factor to influence the evaporating heat transfer coefficient. With the effect of vapor flow, both positive and negative effects are observed as the increment of vapor velocity. Positive effects are predominant for the two tubes in the top positions and higher vapor velocity. [ABSTRACT FROM AUTHOR]

Falling film heat and mass transfer, representatives of advanced and low-carbon heat and mass transfer technologies, has been applied in a variety of thermal processes due to its great prospect for heat and mass transfer enhancement. However, the hydrodynamics, heat and mass transfer are significantly influenced by the gas (vapor or air) streams during the operation of falling film heat exchangers. To acquire the detailed profiles of flow, temperature and concentration and to explore the influence mechanism of gas stream shearing on the falling film hydrodynamics and heat and mass transfer, researchers have adopted experimental, analytical and numerical methods for different attempts. This paper presents a comprehensive summarization of the relevant work concerning the hydrodynamic, heat and mass transfer characteristics of falling film components, including the evaporator, absorber, evaporative cooler and dehumidifier. Considering the specific applications, the cocurrent, countercurrent, and cross gas streams shearing on the liquid film on a horizontal tube and bundle, a vertical tube and a vertical plate are reviewed. The objective of this review is to cover sufficient knowledge of the results, findings and insights of gas stream impacts on the hydrodynamics, heat and mass transfer in building utilized heat exchangers to provide guidance on how to maximize the positive effect or at least minimize the negative impact of gas shearing on the falling film heat and mass transfer. Schematics of gas stream models encountered in various applications. [Display omitted] • Provide the first review of the gas stream effect on falling liquid film flow. • Clarified mechanisms of gas effect on liquid film flow, heat and mass transfer. • Summarized studies of gas stream effect on falling film flow, heat and mass transfer. • Concerned film thickness, heat and mass transfer prediction under gas shearing. • Proposed some recommendations and improvements for future research. [ABSTRACT FROM AUTHOR]

An experimental investigation on the validation of Cooper correlation at the heat flux of 10,000 to 370,000 W/m 2 is conducted with refrigerant R134a. The slope of best-fitting straight line for heat transfer coefficient versus heat flux is mostly of 0.67 at heat flux less than 250 kW/m 2 . A sustainable decrease of slope is observed when heat flux is larger than 250 kW/m 2 . At the higher heat flux greater than 250 kW/m 2 , a significant deviation of experimental data and the prediction result of Cooper’s correlation is observed. The valid range of heat flux for Cooper’s correlation to predict the pool boiling heat transfer of R134a should be less than 250 kW/m 2 . [ABSTRACT FROM AUTHOR]

• A method is proposed to assess the efficiency of heat transfer enhancement techniques. • Reynolds number and ln(Nu e / Nu r)/ln(f e / f r) were taken as the abscissa and ordinate. • The data in higher levels in the coordinate will be more effective in energy saving. In this paper, a revised performance evaluation method was proposed to estimate the thermal hydraulic performance of various heat transfer enhancement techniques. The energy saving effectiveness for the enhanced structures can be identified via a plot. In the plot, Reynolds number and the efficiency index ln(Nu e / Nu r)/ln(f e / f r) are taken as the X and Y coordinates, respectively. The efficiency can be ranked into four different levels. For experimental data or simulation results in Level 1, enhanced heat transfer is accompanied with more power consumption. The data in Level 2 is that the heat transfer can be intensified at the same pumping power. In Level 3, it is characterized by the augmented heat transfer at the same pressure drop. The efficiency in Level 4 is the highest. The ratio of heat transfer augmentation is higher than the increment ratio of friction factor at the same Reynolds number. Four levels correspond to four horizontal regions with Level 1 at the bottom and Level 4 at the top. The data at higher levels in the ordinate will be more competitive for energy saving. The examples on how to use the method for evaluating the performance of enhanced surfaces and typical heat exchangers were also presented. [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]

• Factors affecting methane hydrate decomposition under microwave heating are studied. • Hydrate decomposition time decreased with increase in microwave power. • Heating cycle invovled 5 min of microwave stimulation and 1 min of no stimulation. • This intermittent microwave heating improved the efficiency ratio by 59.32%. Microwave stimulation is a new method for natural gas hydrate exploitation. In this study, methane hydrate was synthesized in quartz sand with a particle size of 106–150 μm, and the decomposition characteristics were investigated using continuous and intermittent microwave heating. During the initial stage of continuous microwave heating, methane hydrate decomposed immediately due to microwave stimulation. Subsequently, due to a decrease in microwave penetration depth, microwaves did not affect the areas further away from the microwave source and macroscopic heat transfer became the main heat source for hydrate decomposition. The microwave power used during the study ranged from 200 W to 500 W. The results indicated that hydrate decomposition time decreased as power increased, and the average gas production rate increased at the same time. However, the energy efficiency ratio did not vary with the microwave power. The maximum energy efficiency ratio was 1.320 at a microwave power of 400 W under continuous microwave heating. Compared with continuous microwave heating, intermittent microwave heating improved the efficiency ratio by 59.32 %, with a heating cycle involving microwave stimulation for 5 min (Microwave ON time) and no stimulation for 1 min (Microwave OFF period). As microwaves provide heat for hydrate decomposition, a high-speed gas production can be obtained. Heat reservoirs were utilized to maintain hydrate decomposition during the OFF period, which also resulted in energy saving. Although intermittent microwave heating reduced the average gas production rate slightly, it improved the energy efficiency significantly. Therefore, the number of heating cycles and microwave heating periods should be carefully considered to obtain high hydrate exploitation efficiency. Intermittent microwave heating is recommended for hydrate exploitation to improve extraction efficiency and save energy. [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]

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]

• An experimental system is set up for studying the Peltier effect. • The optimal cooling water flow rate is identified. • The impact of Peltier effect on the output performance of TEG is investigated. • Peltier effect makes the maximum power point deviate from the theoretical point. Thermoelectric generator technology can convert heat into electricity based on Seebeck effect, but the Peltier effect cannot be neglected in the thermoelectric conversion process. In order to explore the influence of Peltier effect on the output performance of thermoelectric generator under the fixed heat transfer boundary conditions, an experimental system is set up. In the experiment, the hot side of the thermoelectric generator is heated by a thermostatic heater which can keep the heating temperature constant, and the cold side is cooled through the convective heat transfer with water. The effect of cooling water flow rate on the open-circuit voltage is first analyzed and the optimum cooling water flow rate is confirmed. Furthermore, the influence of Peltier effect on the cold side temperature and output performance of thermoelectric generator is investigated. The result shows that Peltier effect results in the less actual output power than the theoretical value and makes the maximum power point deviate from the point where the load resistance equals the internal resistance. Besides, as hot side temperature advances from 67 °C to 185 °C, the ratio of load resistance to internal resistance increases from 1.2 to 1.307 in the case of maximum output power. [ABSTRACT FROM AUTHOR]