Search

This article proposed an original comprehensive thermal treatment coupled with gasification and combustion (CGC) of oil sludge (OS), which was designed to produce hydrogen-rich syngas. Based on the experimental results of OS gasification with steam, the combustion characteristics of char from OS gasification were analyzed by thermogravimetric experiments under different heating rates of 10, 20 and 30 °C/min. The combustion process of OS gasification char can be divided into three stages, including water evaporation, volatile combustion and heavy component combustion. The average values of activation energy (

Oil sludge is a typical hazardous waste in the petrochemical and electric power industry. It has complex components and special properties, and has serious hazards to humans, plants, water, and soil. Therefore, how to realize the effective disposal of oil sludge has become an urgent issue to be solved worldwide. Among the existing oil sludge treatment approaches, combustion has been considered to be a promising technology to realize the large-scale industrial application. In the present work, the characteristics of oil sludge were described in detail. The application and development of oil sludge combustion technology were critically summarized and discussed, including factors affecting combustion, drying process, combustion characteristics, synergistic treatment technology, and formation and control of secondary pollution. Besides, the development of combustion equipment, and integrated thermal treatment technology for oil sludge were prospected. This work can be used for guiding the industrial disposal of oil sludge.

A double T-type microchannel consisting of two T-junctions is used as the base unit of tree-like microchannels. Studying the breakup process and behavior of bubbles in T-type microchannels can help enhance the capability of microfluidic systems and microchannel heat exchangers. In this study, the bubble breakup process in a double T-type microchannel was simulated using a volume of fluid model via numerical simulation. The simulation results show a total of five regimes of bubble breakup with capillary numbers between 0.001 and 0.008 and dimensionless bubble lengths between 1 and 9, which are the non-breakup, “tunnel” breakup, obstructed breakup, merging symmetric breakup, and merging non-breakup. These five breakup regimes were studied in detail. At a high velocity of the gas phase and with a small size of the generated bubble, the bubble does not break up. Symmetric breakup regimes can be divided into two regimes: tunnel breakup and obstructed breakup. Shear force plays a significant role in the tunnel breakup regime. The obstructed breakup regime is mainly caused by the increase in pressure at the T-junction, which elongates and makes the bubble break up. In the merging symmetrical breakup regime, the bubble has a tunnel breakup process at the beginning. The shear force is small and cannot break up the bubble. The merged bubble breaks up under the action of the obstructed breakup regime. Bubbles are in the merging non-breakup regime mainly because they are too long to break up.

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.

An ejector containing phase changing gas-liquid flow process acts as a popular and decisive device in multiple industrial applications, including the hydrogen production, electricity production, fuel cells, refrigeration, petroleum industry and desalination systems. However, non-condensable gas is inevitable for the usual operation of phase-changing gas-liquid ejector in the trigeneration or electrolyzer system for hydrogen production, and rarely research is concerned with this issue. In the present study, the effect of non-condensable gas contained in the condensable gas on the characteristics of gas-centered water ejector is presented, with steam, water and air acting as the gas, liquid and non-condensable gas, respectively. Experimentally, the flow rate of steam is controlled to be 1.45 g/s with an absolute pressure of 120 kPa, the air flow rate varies from 0 to 0.14 g/s, resulting in a non-condensable gas concentration ranging from 0 to 9%, and the resulted water flow rate at 100 kPa and 282.15 K changes from 34.7 to 37.3 g/s. Combined with the numerical methods, the performance of ejector expressed in ejected water flow rate was found to increase firstly with a small amount of non-condensable gas, and decrease when the non-condensable gas reaches a certain amount. In addition, the distributions of multiple local flow parameters including pressure, condensation rate and gas volume fraction, velocity and temperature inside the ejector were shown for different non-condensable concentration, by which the mechanism for the change of ejector performance under varying non-condensable concentration was demonstrated. These findings are initiative and insightful for the ejector design optimization in the trigeneration system for hydrogen production and the proposed numerical models can be utilized in analysis and design of steam ejector with non-condensable gas involved.

As a kind of direct contact condensation, steam jet condensation in subcooled water flow is very common in various industrial applications. Particle model has been employed to investigate the interfacial characteristics of the steam jet condensation in subcooled water flow in a restricted channel, in which a thermal phase change model was inserted into ANSYS CFX as CFX Expression Language (CEL) to simulate the interphase heat and mass transfer process. To validate the model correctness, the steam plume and temperature profiles were compared with the experimental data. The interfacial characteristics including the variation of steam-water interface, local heat transfer coefficient and local condensation rate have been discussed under different operating conditions. Based on the dynamic equilibrium of steam supply and its condensation by the ambient water flow, the steam plume variation was analyzed. The axial profiles of thermodynamic parameters along the nozzle axis were studied with the theory of expansion and compression waves to reveal the formulation mechanism of ellipsoidal steam plume. Then the supersonic flow was observed at some distances from the straight nozzle exit in the numerical results as the related experimental results indicated. The numerical results show that steam plume expands with steam mass flux and water temperature increasing, while it contracts with the increase of water Reynolds number. With the increase of water inlet temperature, the local condensation rate decreases. As the Reynolds number of water flow increases, the local heat transfer coefficient increases and the local condensation rate increases as well.

Liquefied natural gas (LNG) is one of the key technologies in the development and utilization of natural gas. To clarify the condensation characteristics at the last stage of natural gas liquification, saturated methane mixed with nitrogen was chosen as the study object. A numerical model was proposed to predict the filmwise condensation characteristics of ultralow temperature (−162 °C) vapor with non-condensable gas on an isothermal vertical plate. The gas-liquid interface was captured using the Volume of Fluid (VOF) method. A user defined function (UDF), which was written using a cell iteration method and based on the mass conservation on the gas-liquid interface, was applied to deal with the mass and energy source term of the condensing process. The subcooled temperature (temperature difference between wall and saturated gas) and the molar content of non-condensable gas were 2-20 K and 2–13%, respectively. The numerical results show that the heat and mass transfer occur mainly near the cool wall in millimeter level, with the change of temperature, viscosity, phase and mole fraction. A small amount of non-condensable gas can reduce heat transfer coefficients significantly. When the mole fraction of non-condensable gas is 2%, the heat transfer coefficient is reduced by about 59%. And the heat and mass transfer resistance are mainly in the vapor-gas boundary layer rather than in the liquid film.

Small channel heat transfer with condensation or boiling is utilized to handle the more and more intense heat dissipation load nowadays. To further enhance the annular condensation heat transfer inside these channels, diverging-shaped small channel is considered in this work analytically. It is testified that diverging channels do improve condensation heat transfer significantly compared to small channels with constant cross-sectional area. With refrigerant mass flux density increasing or channel size decreasing, diverging channels become more and more efficient in enhancing condensation heat transfer.

Successful numerical simulation on heat transfer and fluid flow performances of plate heat exchangers is vital. Their complex structures often make the numerical calculation quite difficult and tim...

Thermal status or temperature distribution in terms of mixing or stratification inside a suppression pool (SP) is decisive for containment pressure and hence reactor safety. Numerical models were developed in this study to predict the temperature distribution inside a scaled-down SP with steam injected through a horizontal pipe. Regarding the modeling of injected steam, two methods were compared. Effective source (ES) method, which considers the effective heat and momentum source instead of calculating condensing steam directly, can predict the stratified thermal status but failed to reproduce the mixing thermal status. The reason may be attributed to the lack of an appropriate momentum source model. Direct contact condensation (DCC) method, which calculates the condensation process directly, can give agreeable results with experiments and successfully distinguish between mixing or stratification thermal status. In addition, the DCC method can also predict the transition between mixing and stratification. However, moment of the transition was found to deviate from measurement. It was assumed the measurement and proper modeling of the gas-liquid interface structures is necessary for improvement of the simulation. Besides, reasons for the above thermal statuses were analyzed by fluid velocity fields, and it was shown that stratification can be avoided if the injection location is moved downside. The numerical methods proposed can be used for design and improvement of thermal features of SP.

Constant-flux and operation thermal behavior of ground heat exchangers are meaningful for system design and control strategy of ground-coupled heat pump systems. Based on a ground-coupled heat pump project, four thermal performance tests and an operation test were conducted to compare the constant-flux and operation thermal behavior of spiral-tube energy piles with that of double U-tube borehole heat exchangers. The thermal performance of energy piles is more efficient than that of borehole heat exchangers. A large radial scale of heat source, a long heat exchange tube per unit depth of holes and a backfilled material with large thermal conductivity is helpful to improve the heat transfer efficiency of ground heat exchangers. The short-term thermal response of energy piles is more rapid than that of borehole heat exchangers. The short-term thermal resistance of energy piles is much less than that of borehole heat exchangers. In operation test heat transfer rate per unit length of pile and per unit length of pipe for energy piles is greater than those for borehole heat exchangers. The regular fluctuations in fluid temperature and heat transfer rate in operation should be determined by heating capacity of heat pumps and thermal loads of buildings.

To improve the heat transfer characteristics of lubricant, graphene-based lubricants were prepared by adding graphene particles, due to its advantages of excellent thermal conductivity and two-dimensional sheet structure. In the present study, its physical properties were measured. A flow heat transfer experiment platform was built to study the flow and heat transfer characteristics of the graphene lubricating oil in a horizontal circular tube. The results show that the graphene lubricant prepared using a two-step approach had good stability, and the dispersibility was good without the agglomeration phenomenon, according to measurements undertaken using an electron microscope and centrifuge. The thermal conductivity and viscosity of graphene lubricant increased with the increase of the graphene concentration, and the thermal conductivity of graphene lubricant with the same concentration decreased with the increase of temperature. When the concentration was equal, the convective heat transfer Nusselt number (Nu) of graphene lubricant increased with the increase of Reynolds number (Re). When Re was equal, the convective heat transfer Nu increased with the increase of graphene particle concentration, and the maximum Nu increased by 40%.

Gas-liquid two-phase flow in curved channels is widely found in various heat exchangers and transport pipes. The fluid flow and heat transfer characteristics are closely related to flow patterns. This work experimentally investigated flow patterns for air-water flows in a U-bend and its contiguous straight tubes at a pressure of 0.1 MPa and a temperature of 25 °C, and proposed a new flow pattern identification method. A flow visualization technique along with frequency spectral (power spectral density, PSD) and chaos analysis (multi-scale entropy, MSE) of pressure drop time series were used. The bend had an inner diameter of 16 mm with curvature radius of 100 mm, and it was installed with either a downward or an upward arrangement. The superficial air and water velocities were set to the range of 0.17–16.58 m/s and 0.10–2.03 m/s, respectively. Five flow patterns were identified on the basis of captured flow images, pressure drop time series and PSD distributions. Flow pattern maps for both flow orientations were proposed. Two unique flow patterns (stratified-plug flow in downward flow and unstable flow in upward flow) were obtained under low superficial gas and liquid velocities. Combining the skewness of PSD and the rate of MSE enabled us to objectively identify flow patterns in the U-bend unit, which facilitates identification of flow patterns in other non-transparent curved channels.

Nanofluids performance counts on nanoparticle configuration. The evolutionary topology optimization algorithm is applied in this work for direct nanoparticle configuration optimization. The initial random distribution of nanoparticles inside base fluid is found to well simulate practical nanofluids. The optimized nanofluids configuration proves to be continuous strips assembled by nanoparticles with width comparable to size of nanoparticles and length comparable to size of heat-transferring system. System overall thermal resistance is significantly reduced by the optimized nanoparticle configuration. Future nanofluids should be fabricated with strip-shaped nanoparticles in order to perform better.

Energy pile is a cost-effective solution for ground-coupled heat pump systems to take the place of borehole ground heat exchanger partly. This paper focuses on the influence of spiral pitch on the transient thermal behaviors of an energy pile with spiral-tube heat exchanger. A transient 3D numerical model was established and solved by using the finite element method. The thermal resistances and the heat transfer rates were obtained and analyzed for the energy piles with four kinds of spiral pitch. The proposed g-functions of circulated fluid temperature were used to study fluid temperature and coefficient of performance based on the superposition method. As spiral pitch decreases, the change in both fluid temperature and internal thermal resistance diminishes but the change is narrowed down, although the total rate of the heat transfer on pile surfaces is almost same for all piles. For both heating and cooling, the difference between the extreme fluid temperatures diminishes and the mean coefficient of performance increases as spiral pitch decreases. The energy pile with a small spiral pitch is energy-efficient and has a better applicability. The increase of spiral pitch (SP = 0.25–2.0) leads to a reduction of the mean coefficients of performance (0.77–16.49%) according to the influential factors.

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.

Unprecedented heat dissipation requirements drive investigation and further enhancement of condensation heat transfer inside small channels significant. Converging-shaped small-sized channel is coined，and its annular condensation performance is studied analytically in this work. The influence of channel size and gravity are considered to better fulfill heat dissipation goals under varying-gravity conditions. The analytical results accord with predictions of the Comprehensive Shah Correlation satisfactorily. The nondimensional correlation for condensation heat transfer performance is derived. Channel with smaller radius ensures better condensation performance when channel cross-sectional area remains constant, which is more pronounced for small-gravity conditions. Converging channel enhances the local condensation performance after channel central cross section and the overall condensation performance even with very small converging angles. As channel size and gravity decrease, converging channel performs even better.

To investigate the hydraulic characteristics of double-source and ring-shaped heating networks, this paper proposes a new hydraulic modeling method that, to a large extent, solves the modeling problem of hydraulic intersection users (HIUs) and common pipes (CPs). Additionally, an experimental network and its hydraulic model including double heat sources, double rings, and 12 users were established, and simulations and experiments under three sets of typical hydraulic conditions were carried out. The results indicate that the maximum relative error between the simulated and experimental data of the pressure drops in the users were −2.83%, −2.40%, and −3.17%, and the maximum relative errors of the flow rates in the user were −2.07%, −3.60%, and 2.18%. The HIU of the simulation model was in good agreement with that of the experimental network regarding both number and position. Thus, the proposed modeling method and simulation model are validated.

Recycling utilization of straw ash as a fertilizer in farmland is expected to play an important role in the sustainable development of both agriculture and biomass energy. However, the ashing temperature and the aqueous solution characteristics may affect the recycling properties of the nutrients contained in the ash. The solubilities of both the ash and its elements can represent the above recycling properties. This paper presents a systematic experimental investigation on the acid solubilities of both rice straw ash and its major elements produced from combustion at 400&ndash, 800 &deg, C, and these findings are compared with the corresponding water solubilities obtained from the authors&rsquo, previous work. Meanwhile, the correlations of two solubilities with the ashing temperature were given based on the experimental data. Results show that the acid solubility of rice straw ash decreases linearly by approximately 76% as the ashing temperature increases from 400 to 800 &deg, C, while it is significantly higher than the corresponding water solubility at different temperatures. The acid solubilities of K, P, Ca, Mg, and Na are higher than their water solubilities, whereas two solubilities of S and Cl have almost no dependence on the temperature and the acidity of solution. This study also reveals a strong negative linear relationship between the solubility of K and the temperature. The solubilities of other elements (P, S, Na, Ca, Mg, and Cl) with the temperature have quadratic curve or cubic curve relationships. Furthermore, it is recommended that the ashing temperature should be lower than 600 &deg, C to avoid the loss of some nutrients in the straw ash.

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.

The closed-type heat-source tower (CTHST) heat pump system is a more promising alternative to the conventional air-source heat pump (ASHP) unit compared with the open-type heat-source tower (OTHST) heat pump system. However, the heat transfer efficiency of the CTHST is significantly lower than that of the OTHST, resulting from its indirect heat transfer way, liquid film break-up and high gas phase pressure drop. To provide a theoretical basis for the heat-mass transfer enhancement and structural optimization design of the high-performance CTHST, a three-dimensional (3D) model taking gas-liquid interfacial friction force into consideration was established in the present paper to study counter-current and co-current gas-liquid falling film flow characteristics on the airside of the finned-tube heat exchanger (FTHE) with a typical large fin pitch used in the CTHST. The predicting results of liquid film thickness based on the established model show good agreement with those of experiments in the reference, with a maximum deviation less than 5%, meaning that this model is reliable. Then the effects of contact angle, air flow direction, air Reg and liquid film Rel on liquid film characteristics were studied. The prediction correlation of average liquid film thickness was presented. The pressure drop characteristics of gas phase were analyzed in the co-current gas-liquid flow under three different liquid film Rel. Meanwhile, the change law of interfacial friction force with air Reg was revealed. The numerical results show that the critical mass flow rate of complete film flow on the finned-tube surface with contact angle of 30° is 0.06 kg/(m⋅s). The average liquid film thickness on the surface of the FTHE is obviously higher than that on the vertical plate. There is a close and positive quadratic curve relation between average interfacial friction force and air Reg. To obtain a complete film flow and small air pressure drop, the air Reg should be less than 2190.7 and 3286.0 in the counter-current and co-current gas-liquid falling film flow, respectively.

Two-phase Taylor flow in microchannels as a critical way to enhance heat dissipation has attracted extensive attention due to the high integration and miniaturization development of electronic devices. To further elucidate the pressure drop and development process of heat transfer characteristics in Taylor flow, a 2-D planar T-junction microchannel with a width of 1 mm was investigated numerically by using the fixed frame computational domain method. The local Nusselt number distribution is divided into four parts for analysis, and the effects of mixture velocity and void fraction on the heat transfer coefficient are also discussed. The results indicate that the injection of gas-phase leads to an increase in pressure gradient, and the flow-pattern related model has a better predictive result. Strong evidence of internal circulation was found when the shear stress in Y direction is not equal to zero in Taylor flow. The investigation of Nusselt number has shown that the mixture velocity and the void fraction affect the temperature field distribution and heat absorption capacity in the liquid slug, respectively. Together these simulation data, nearly 1.8 times higher of the Nusselt number was observed compared to pure water flow.

This paper experimentally investigates the influence of non-condensable gas on water vapor condensation heat transfer on an isothermal vertical aluminum plate in an air mass fraction range of 0.05–0.5. A high-speed digital camera was employed to capture real-time images of water vapor condensation in non-condensable gas under different wall temperatures and air concentrations. The experimental results showed that the condensation modes varied with surface subcooling and air concentration. Four condensation modes were observed in the experiments: drop, drop-streak, film, and streamlet. Under identical air concentrations, the condensation mode varied from the fine drop and film mode to the larger drop-streak mode; the condensation drop diameters departing the plate became obviously bigger, the drops fell off the plate more quickly, and the heat transfer coefficients decreased gradually as surface cooling increased. Under identical surface cooling conditions, the condensation drops grew and fell off more quickly when the air mass fraction was low, the condensation drops irregularly varied from drop, drop-streak, to ribbon stream, and the condensation had a tendency to spread out. The heat transfer coefficients decreased with the increase of the air mass fraction. The heat transfer coefficient declined greatly under a lower air mass fraction (5–10%), while it declined slightly under a higher air mass fraction (40–50%).

By coupling the heat transfer equation with semi-global chemical reaction kinetic equations, a onedimensional, unsteady mathematical model is developed to describe the pyrolysis of single biomass pellet in the pyrolysis zone of downdraft gasifier. The simulation results in inert atmosphere and pyrolysis zone agree well with the published experimental results. The pyrolysis of biomass pellets in pyrolysis zone is investigated, and the results show that the estimated convective heat transfer coefficient and emissivity coefficient are suitable. The mean pyrolysis time is 15.22%, shorter than that in inert atmosphere, and the pellet pyrolysis process in pyrolysis zone belongs to fast pyrolysis. Among the pyrolysis products, tar yield is the most, gas the second, and char the least. During pyrolysis, the temperature change near the center is contrary to that near the surface. Pyrolysis gradually moves inwards layer by layer. With the increase of pyrolysis temperature and pellet diameter, the total pyrolysis time, tar yield, char yield and gas yield change in different ways. The height of pyrolysis zone is calculated to be 1.51–3.51 times of the characteristic pellet diameter.

A three dimensional model was established based on Euler–Euler two-fluid model to simulate the steam jet direct contact condensation with air involved. Condensation of steam in the gas mixture was realized using thermal phase change model. Species transport equation for the gas phase was employed to account for composition variation of the steam air mixture. The investigated gas mass flux at nozzle exit was within 300 kg/m2/s, with non-condensable air of less than 10%. Numerical results show the steam in gas plume condenses rapidly after leaving the nozzle, until only air and a little uncondensed steam remain in the gas plume. The addition of air in the jet deteriorates the condensation heat transfer, leading temperature and void fraction around the nozzle to decrease more slowly in both axial and radial directions. Five cases of experimental results were demonstrated with three of them validating the temperature distribution and two of them validating the void fraction. The comparison between numerical and experimental results demonstrates the CFD model can predict the steam–air jet condensation process quite reasonably.Classification: K. Thermal hydraulics

Diverging-shaped small channel is considered analytically aiming to further enhance condensation heat transfer inside channels. To fulfill heat dissipation requirements under varying-gravity conditions, effectiveness of the diverging small channel is also verified under different gravities. The refrigerant mass flow rate, total heat transfer area and thermal boundary conditions are fixed to investigate effect of channel shape. Smaller channel possesses better overall condensation performance, especially for small-gravity conditions. Diverging channel enhances local condensation heat transfer even with very small diverging angles. Overall condensation heat transfer becomes worse as diverging angle is relatively larger when channel size and gravity decrease. Converging-shaped small channel is therefore proposed and verified to effectively enhance the overall condensation heat transfer performance.

Steam jet in subcooled water is a typical mode of direct contact condensation (DCC) where condensation occurs on the interface between steam and water. A visualization study of sonic steam jet condensation in subcooled water flow in a restricted channel has been performed for a convergent nozzle over a wide range of operating conditions. The DCC characteristics, including steam plume shapes, penetration length and average heat transfer coefficient, were discussed in detail. Method of digital image processing using MATLAB software was employed to evaluate steam plume shapes, penetration length, interface area and contours of gas volume fraction. Using a high-speed camera, five different steam plume shapes were typically observed, i.e. hemispherical, conical, contraction–expansion-contraction, ellipsoidal and divergent shapes, which are described in a three-dimensional map of plume shapes depending on steam mass flux, subcooled water temperature and Reynolds number of water flow. The measured data of dimensionless penetration length and average heat transfer coefficient are found to be in the range of 0.8–4.5 and 1.6–5.5 MW/(m2·K), respectively. By means of fitting 140 groups of measured data, the empirical correlations for dimensionless penetration length and average heat transfer coefficient as a function of dimensionless steam mass flux, condensation driving potential and Reynolds number of water flow were proposed. Both of the measured data and predictions show a good agreement with each other, which are suggested to predict the condensation characteristics of sonic steam jet in water flow in a restricted channel.

In this study, condensation of air–steam mixture bubble with mass fraction of steam above 0.5 was investigated first by a visual experiment at atmosphere. Then a 3-D numerical model based on volume of fluid (VOF) model and species model was developed to simulate the bubble condensation. In order to model steam condensation, mass and energy transfer between phases were modeled as source terms of conservation equations, using a correlation obtained from the experiment to predict the condensation heat transfer coefficient (HTC). After validation of the numerical model with the experimental results, influences of steam fraction and diameter of bubble on condensation characteristics are studied numerically. Moreover, steam concentration distribution in bubble was also analyzed along the time series of bubble condensation process. It was found that condensation HTC decreases with the increase of bubble diameter. With increase of steam fraction in bubble, the bubble volume shrinks more quickly due to the increased condensation rate, and the bubble accelerates more quickly reaching a higher terminal velocity. Aggregation of non-condensable air inside the bubble near the gas and water interface deteriorates the condensation heat and mass transfer.

To investigate the dynamic characteristics of the thermal conditions of hot-water district-heating networks, a dynamic modeling method is proposed with consideration of the heat dissipations in pipes and the characteristic line method is adopted to solve it. Besides, the influences of different errors, space steps and initial values on the convergence of the dynamic model results are analyzed for a model network. Finally, a part of a certain city district-heating system is simulated and the results are compared with the actual operation data in half an hour from 6 secondary heat stations. The results indicate that the relative errors for the supply pressure and temperature in 5 stations are all within 2%, except in one station, where the relative error approaches 4%. So the proposed model and algorithm are validated.

Steam&ndash, 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&ndash, water interfacial characteristics in the injectors based on Eulerian&ndash, 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&ndash, 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&ndash, 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.

The characteristics of the longitudinal vortex induced by trapezoid-winglets in a circular tube are investigated by the Particle Image Velocimetry (PIV) Technique with flow Reynolds number in the range of 500–13 000. In the experimental test section, four trapezoid-winglets are fixed symmetrically on the tube wall in two different ways: up-flow and down-flow. The results show that a counter-rotating vortex pair is formed behind each winglet and they distribute as a symmetrical vortex array in the transverse section. Between the two vortexes in a vortex pair the fluid flows towards the wall in the up-flow winglet case and away from the wall in the down-flow winglet case, corresponding also to the regions of peak values of the velocity components normal to the mainstream. Both of the flow patterns enhance the velocity in the near wall region, leading to the intensification of the transverse mixing and the mass transfer in the tube. With Reynolds number increasing, the flow maintains the vortex pattern in the case of the up-flow winglets, indicating better persistence of the longitudinal vortex, while the vortexes in the case of the down-flow winglets are more scattered and tend to breaking into small eddies. The trapezoid winglet shows the preferable turbulent disturbance characteristics in the tube and the experimental results provide benchmark data for further CFD studies.

Experimental and theoretical investigations of water-side fouling have been performed inside four corrugated plate heat exchangers. They have different geometric parameters, such as plate height, plate spacing, and plate angle. Fouling experiments including particulate fouling tests and composite particulate and precipitation fouling tests have been performed. The tests are primarily focused on the effects of concentration and average velocity. Scanning electron microscope (SEM) was used to investigate the microscopic structures of composite fouling. Heat transfer coefficients and friction factors have been obtained in clean tests. The plate heat exchanger with the largest d e and height to pitch ratio shows the best anti-fouling performance. The Von-Karman analogy was used to obtain mass transfer coefficient to develop appropriate semi-theoretical fouling correlations; The wall shear stress is obtained from the pressure drop. The model predictions agree well with experimental data.

A three-dimensional numerical study was made to investigate effects of fin angle, fin surface emissivity, and tube wall temperature on heat transfer enhancement for a longitudinal externally-finned tube placed vertically in a small chamber. The numerical model was first validated through comparison with experimental measurements and the appropriateness of general boundary conditions was examined. The numerical results show that the mean Nusselt number increases with Rayleigh number for all the fin angles investigated. The maximum heat transfer rate per mass occurs when the fin angle is about 60° for fin surface emissivity between 0.7 and 0.8 and 55° when the surface emissivity increases to 0.9. With increasing tube wall temperature, both the natural convection and radiation heat transfer are enhanced, but the fraction of radiation heat transfer decreases in the temperature range studied. Radiation fraction increases with increasing fin surface emissivity. Both convection and radiation heat transfer modes are important.

A three-dimensional numerical model is presented for studying the convection-condensation of mixture with vapor in a tube with edgefold-twisted-tape inserts under transition flow. According to the diffusion layer theory and laminar species transport, a condensation model with user defined function is proposed and compared with heat and mass transfer analogy and experimental test. With the condensation model, the influences of gap width and operating parameters on thermal-hydrodynamics performance are simulated. As the gap width increases, convection and condensation heat transfer increase initially and then decrease, while convection heat transfer increases sharply and then decreases slightly. Increasing vapor fraction has a significant effect on condensation heat transfer but it has little effect on convective heat transfer. With the increase of inner wall temperature both convection and condensation heat transfer all decrease and the ratio of condensation to total heat decrease dramatically. Increases inlet temperature mainly affects convection heat transfer.