Bubble condensation with non-condensable gas is more common than pure steam bubble. In this study, a systematic experimental investigation on the air-steam bubble condensation with comparatively ...large ranges of the liquid subcooling from 10 K to 70 K and different initial mole fractions of non-condensable gas <0.3 are carried out.
Results show that the air-steam bubble trends to condense as a whole when the initial mole fraction of non-condensable gas x0 is >15%. When x0 is <15%, the pattern transition from bubble shrink to split can be observed and the values of x0 corresponding to this transition trend to increase with the increasing liquid subcooling. A similar phenomenon can also be found in the pattern transition from bubble split to collapse. Meanwhile, three typical bubble breakage processes are observed and identified, which are named as flatten-, stretch- and shrink-breakage. Moreover, a new Nusselt number correlation is proposed which is related to bubble Reynolds number, Prandtl number, Jacob number and x0. After comparison, our correlation can predict well not only the results in this study but also the data-sets of air-steam bubble condensation given in the literature, which shows good performance in robustness.
•A systematic experiment on air-steam bubble direct contact condensation is proposed.•Three typical bubble breakage processes are firstly observed and identified.•A regime map on the bubble deformation in the condensation process is presented.•A new correlation of Nusselt number is given with a larger range of subcooling.•Our correlation can predict well the data of this study and those in the literature.
In order to obtain a high-accuracy and adaptable condensation phase change model, this paper selects the Nusselt number correlation formula that Kim proposed based on the experimental data and ...adjusts the Nusselt number in the bubble condensation process by calculating the phase change coefficient of the Lee model in the UDF. Through the simulation and fine-tuning of the 12 groups of operating conditions, the formula for the change of the phase change coefficient of the Lee model during the bubble condensation process is obtained. The accuracy and wide applicability of the variation formula are verified by comparison with various types of experimental data. The Lee model provides a certain reference for the numerical simulation of the bubble condensation process. The numerical simulation of the condensation process of vapor bubbles is carried out by using the formula of the phase change coefficient. The error between the simulation result of the bubble volume change and the experimental result is lower than ±15%, which basically verified the reliability of the numerical model adopted in this study. The bubble condensation process has been analyzed under various operating conditions. The simulation results show that when the bubble rises, disturbance occurs with the fluid and several tiny eddies are generated on the side of the bubble. Micro-circulation of the vapor inside the bubble accelerates the heat and mass transfer rate at the gas–liquid interface. When condensation occurs, the mass transfer rate at the interface is different and the pressure inside the bubble is higher than that around it.
•Bubble formation and condensation process injected from a nozzle is studied numerically.•Euler-Euler two-fluid free surface model and species model from are coupled together.•Influence of ...non-condensable gas is considered.•Bubble shape variation histories are shown in comparison with experiments.
Bubble formation and condensation of steam-air mixture vertically injected in a subcooled water pool was simulated, combining thermal phase change model into the two continuous phase free surface model of ANSYS CFX 17.1. Continuous surface force model was used to calculate surface tension force and the influence of non-condensable gas was accounted for by component transportation equation and assumption of interface temperature equal to saturation temperature at local partial steam pressure. The thermal phase change model includes an experimental correlation for liquid side sensible heat transfer. Based on available experiment data from literatures, singular pure steam bubble and steam-mixture bubble in a pool were first simulated to see the predictability of the proposed method and then, the same method was applied to the bubble formation, detachment and condensation process of injected steam air mixture from a nozzle. Bubble dimeter, water subcooling and non-condensable gas concentration studied range from 4.9mm to 50mm, 12K to 40K, and 0 to 31.5% respectively. The results of the computations indicate that the present method can predict very well the bubble formation and condensation both for pure steam case and with non-condensable gas.
Steam–water injectors have been widely applied in various industrial fields because of their compact and passive features. Despite its straightforward mechanical design, the internal two-phase ...condensing flow phenomena are extremely complicated. In present study, a numerical model has been developed to simulate steam–water interfacial characteristics in the injectors based on Eulerian–Eulerian multiphase model in ANSYS CFX software. A particle model is available for the interphase transfer between steam and water, in which a thermal phase change model was inserted into the model as a CFX Expression Language (CEL) to calculate interphase heat and mass transfer. The developed model is validated against a test case under a typical operating condition. The numerical results are consistent with experimental data both in terms of axial pressure and temperature profiles, which preliminarily demonstrates the feasibility and accuracy of particle model on simulation of gas–liquid interfacial characteristics in the mixing chamber of injector. Based on the dynamic equilibrium of steam supply and its condensation, interfacial characteristics including the variation of steam plume penetration length and steam–water interface have been discussed under different operating conditions. The numerical results show that steam plume expands with steam inlet mass flow rate and water inlet temperature increasing, while it contracts with the increase of water inlet mass flow rate and backpressure. Besides this, the condensation shock position moves upstream with the backpressure increasing.
To improve the transportation efficiency and reduce the supply cost, the liquefaction becomes an important technology to store and transport the natural gas. During the liquefaction, the various ...components (e.g. propane, ethane, methane etc.) undergo fractional condensation phenomenon due to their different boiling points. This means that when one component condenses, others play a role of non-condensable gas (NCG). In order to reveal the influence mechanism of NCG on this condensation process, a numerical method was employed in this paper to study the condensation characteristics of three non-azeotropic binary hydrocarbon vapor mixtures, namely the propane/methane (80%–95%), ethane/methane (65%–85%) and methane/nitrogen (2%–13%) mixtures, on a vertical plate. The model was proposed based on the diffusion layer model, and the finite volume method was used to solve the governing equations. A user defined function was developed by cell iterative method to obtain the source terms in the condensation process. The numerical results show that the gas phase boundary layer formed by the NCG becomes the main resistance to the reduction of heat transfer coefficient. And for the above three mixtures, there is a negative correlation between the NCG concentration and the heat transfer coefficient. Meanwhile, the results show a good agreement with the experimental data, meaning that the proposed model is reliable. Three mixtures within same non-condensable mole fraction of 20% were also investigated, indicating that the mixtures with a higher binary hydrocarbon molecular ratio have a lower heat transfer coefficient. As a result, the presence of the lighter NCG contributes to a thicker boundary layer.
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Frost layer on the outdoor air heat exchanger surface in an air-source heat pump (ASHP) can decrease the system coefficient of performance (COP). Although the common defrosting and anti-frosting ...methods can improve the COP, the periodic defrosting not only reduces the system energy efficiency but also deteriorates the indoor environment. To solve these problems, it is necessary to clearly understand the frosting phenomenon and to achieve the system frost-free operation. This paper focused firstly on the analyses of frosting pathways and frosting maps. Followed by summarizing the characteristics of frost-free technologies. And then the performances of two types of frost-free ASHP (FFASHP) systems were reviewed, and the exergy and economic analysis of a FFASHP heating system were carried out. Finally, the existing problems related to the FFASHP technologies were proposed. Results show that the existing frosting maps need to be further improved. The FFASHP systems can not only achieve continuous frost-free operation but reduce operating cost. And the total COP of the FFASHP heating system is approximately 30–64% higher than that of the conventional ASHP system under the same frosting conditions. However, the investment cost of the FFASHP system increases, and its reliability also needs further field test in a wider frosting environment. In the future, combined with a new frosting map, the control strategy for the FFASHP system should be optimized.
To improve the charging performance of the latent heat storage units, this paper performs a numerical simulation study of the melting of phase change material under various gradient straight fins ...employing the enthalpy-porous medium model. Based on the evolution of phase interface and temperature distribution, along with the variation of liquid phase fraction and Nusselt number, we analyze the sole effects of gradient-length and gradient-angle on the melting of the phase change material. Further, we gain the optimized-gradient fins by cooperatively adjusting the geometrical parameters of both the gradient-length and gradient-angle using response surface methodology. The introduction of natural convection of the formed liquid divides the melting process into three stages. Optimizing the geometrical parameters of straight fins could facilitate the fast second stage containing natural convection, leaving no (or less) room for the slow third stage without natural convection. Compared with the uniform fin structure, gradient-length and gradient-angle could reduce the total melting time at most by 48.9% and 8%, respectively. The largest reduction (56.6%) exists in the optimized straight fin structure with the gradient-length of 5.56 mm and the gradient-angle of 5.36°, involving only the first two melting stages without the third stage.
In this work, the perturbation length of vertical U-bend effects on the air–water two-phase flow in their adjacent straight tubes was investigated experimentally at ambient pressure and temperature. ...The experimental database covered test sections with three inner diameters of U-tubes (9, 12, and 16 mm), two curvature ratios (8.33 and 12.5), and two orientations (downward and upward). The air and water superficial velocities varied from 0.18 to 25.11 m/s and from 0.11 to 1.98 m/s, respectively. The dissipation of U-bend effects in the regions upstream and downstream of U-bends is characterized by the variation regularities of segmental pressure gradients along the contiguous straight tubes. It was found that the significant value of the dimensionless perturbation length (
L
p
/
D
) is less than 60 for the experimental conditions in this study. The influences of the inlet volume void fraction, two-phase mixed Froude number, mixed Weber number, and mixed Dean number on dimensionless perturbation length (
L
p
/
D
) are similar. With the increase in these parameters,
L
p
/
D
presents a unimodal distribution which first increases and then decreases. The maximum value of
L
p
/
D
takes place at a volume void fraction of around 90%. New dimensionless correlations to predict the influence length of U-bends on the straight tubes upstream and downstream have been provided. The prediction errors of 80% of data are within ± 30% compared with the present experimental data.
•Flow-induced oscillating tape is used a novel heat transfer enhancement element.•Oscillating features are shown for low Re turbulent airflow.•Heat transfer enhancement ratio reaches 1.7 times of ...bare tube.•Future research expectations are raised for flow-induced oscillation of membrane tape.
Flow-induced oscillation is an effective way for heat transfer enhancement, which embraces the advantages of requiring no extra energy consumption and better performance for preventing fouling or soot formation. In the present study, a thin membrane tape with length, width and thickness of 90 mm, 3.8 mm and 13.3 um respectively was mounted inside a horizontal pipe with an internal diameter of 13 mm. By visual measurement, the tape was demonstrated to be able to oscillate within airflow and the oscillation frequency was found to increase with the flow rate. Constant heat flux was applied at the pipe wall through electrical heating and the wall temperature was obtained by thermocouples. In low Reynolds number turbulent region (Re between 1800 and 4200), the mean heat transfer coefficient and flow resistance were found to be 1.3-1.7 times and 1.6-2.1 times than that of a smooth tube, respectively. However, the heat transfer enhancement effect is most pronounced at lower Re and as the Re increases, this effect becomes weak. Nevertheless, the spontaneous oscillating tapes can be a very promising heat transfer intensification element. This study is a primary investigation attempt and flow-induced oscillations of membrane tapes of different dimensions and the use of multiple membrane tapes are awaited to be studied deeply.
To enhance the energy discharging performance of the triple-tube latent heat storage unit (TTLHSU), this paper introduces a novel two-stage V-shaped branch fin design to uniform the heat transfer ...from different radial regions, and its transient solidification model is established and numerically simulated. The solidification process under the novel V-shaped branch fins is compared with that under straight fins based on phase interface/temperature distribution diagrams, liquid fraction, and temperature gradient curves. The sole impacts of three structural parameters (branch/base width ratio, branch angle, and branch/base length ratio) of the V-shaped branch fins on solidification are investigated. The optimized fin structure is obtained using the response surface methodology (RSM). The results indicate that the V-shaped branch fins promote solidification by synchronizing the radial rates. Structural parameters primarily affect the evolution of phase interfaces, the most largely with the length ratio, followed by the branch angle and then the width ratio. Smaller branch/base length and/or width ratios lead to shorter solidification times and faster heat release rates. As the branch angle increases, the solidification time decreases initially and then increases, and correspondingly, the heat release rate varies reversely. The optimized V-shaped fin structure has a branch angle of 44° and a branch/base length/width ratio of 0.5, reducing the solidification time by 64.2 % and enhancing the heat release rate by 186.2 % from those of straight fins. The results of this study could guide for using fins to improve the energy-discharging performance of TTLHSU.
•A novel V-shaped branch fins is proposed to enhance energy discharging of triple-tube latent heat storage unit.•Adding four V-shaped branch fins could perfectly synchronize the heat transfer of the difficult-to-solidify region.•The extend of influence to the solidification performance depends on the structural parameters of the adjustment.•The optimized V-shaped branch fins could give the best energy discharging performance.
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