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Highlights • The flow distribution in parallel manifolds has been numerically investigated. • Three types of flow distribution have been analyzed. • The effect of the header match on the flow distribution have been investigated. • The best choice of DCR for different types of flow distribution are given. Abstract The present study numerically investigates the effect of the header match on the flow distribution in a central-type heat exchanger. Past studies have shown that the appropriate choice in two different header diameters (header match) will significantly help improve the flow distribution in the Z-type and U-type parallel heat exchangers. Under this circumstance, investigations are carried out on the central-type heat exchangers. According to our previous work on the effect of the geometric parameters on the flow distribution in a central-type exchanger (Zhou et al., 2017), three different types flow distribution have been pointed out. Considering the differences in characteristics among three types of the flow distribution. The investigations have been separately made on each type of flow distribution. The results indicate that for the different types of the flow distribution, the best choice of the cross-sectional area ratio of dividing header to combining header (DCR) is different. Besides, for all of three types of the flow distribution, when the DCR values less than 1, a great deal of flow maldistirbution will be brought. The present study is intended to figure out the effect of the header match on the flow distribution and provide constructive suggestions for the design of a central-type heat exchanger. [ABSTRACT FROM AUTHOR]

In order to design thermal protection system, it is important to predict heat transfer rate precisely on the hypersonic vehicles. In present paper, considering chemical reaction, interal energy excitation and thermo-chemical non-equilibrium effects of high temperature air, the thermo-chemical non-equilibrium flow-fields are simulated using the numerical code AEROPH_Flow. An aero physical numerical code system for Hypersonic vehicles, the distributions of heat transfer rate is obtained, and the analysis is done for the influence of the chemical reaction model, thermodynamic non-equilibrium model, surface temperature, surface material catalytic action on the computational results. The results show that thermo-chemical non-equilibrium models have a more important influence on the computational results, it is important for the selection of the correct air reaction model and thermal non equilibrium model according to the flight environment and thermo-chemical non-equilibrium mechanism of the flow field around the hypersonic vehicles; the heat transfer rate isn't always the highest for lower surface temperature, and it become very complicated for the variation law of heat transfer rate along with the surface temperature under the condition of high Mach number or thermo-chemical non-equilibrium flow, so the surface temperature may as well be fixed choosed according to the distribution of the real flight condition or different constant for computing exactly heat transfer rate on the hypersonic vehicles. [ABSTRACT FROM AUTHOR]

• The effects of different geometrical parameters on flow and heat transfer in the extended domain are studied. • The influence of the extended domain is explained from the angle of relative pressure and pressure gradient. • The criterion for judging the accuracy of numerical simulation results of the extended domain is put forward. • The optimal geometric parameters of the extended domain are selected. The natural convection flow in the vertical rectangular channels is frequently encountered in many engineering applications. Currently, most relevant simulation studies neglected the influence of the extended domain on the calculation results, it could cause the heat transfer coefficient to deviate by more than 10% from the actual result. This paper aims to study the influence of the extended domain on the natural convection heat transfer in a vertical rectangular channel. It is found that the mass flow rate in the channel can rise by approximately 8% and the heat transfer coefficient by up to 13.5% with an increase in the extended domain. This influence is explained from the angle of relative pressure and pressure gradient. The influence of different geometric parameters on the extended domain and the selection of the optimal geometric parameters of the extended domain are discussed. Furthermore, it is expanded to different heated boundary conditions and geometric structures of the channel. This study provided a reference for the selection of extended domains in the corresponding natural convection. [ABSTRACT FROM AUTHOR]

Under severe accident conditions, in addition to steam and air, there may also be hydrogen produced by the reaction of zirconium alloys and steam, CO 2 produced by the molten corium concrete interaction (MCCI) and other gases in the containment. These gases, whose properties are quite different from air, can significantly affect the steam condensation process outside the tube bundle. This paper aims to investigate the effects of non-condensable gas mixtures with varying component concentrations on the steam condensation process outside a 3 × 5 tube bundle using computational fluid dynamics (CFD) methods. The thermal conditions studied are set to a pressure of 0.4 MPa, a subcooling of 40 K, and a molar concentration of steam of 0.66. The results show that CO 2 has an acceleration effect on the flow field outside the tube bundle, which promotes the diffusion of steam to the inside of the bundle, thus increasing the condensation heat transfer coefficient (CHTC) of the bundle and decreasing the difference in CHTC between different heat transfer tubes inside the bundle. Hydrogen, on the other hand, inhibits the downward flow of the gas mixture, causing a decrease in CHTC. However, at high hydrogen concentration (X H2 >0.1), the gas mixture will form a buoyant flow upward, and then the CHTC of the tube bundle will be significantly increased by up to 177%. When the two gases act together, they will weaken each other's influence on the flow field, thus reducing the CHTC of the tube bundle. [ABSTRACT FROM AUTHOR]

The Passive Residual Heat Removal Heat Exchanger (PRHR HX) is the key equipment of the Passive Residual Heat Removal System (PRHRS). In the Swimming Pool-type Low-Temperature Heating Reactor (SPLTHR), the reactor core pressure and outlet/inlet temperature are 0.32 MPa and 100/80 °C, respectively. In this case, the convection temperature difference of natural convection heat transfer outside PRHR HX is small, which affects the operation of PRHRS, so it is necessary to carry out related research. In present work, numerical and experimental study on the outside heat transfer characteristics of the tube bundle under the small convection temperature difference has been carried out. The results show that the high Reynold number k-ε turbulence model with enhanced wall treatment is suitable for simulating the outside natural convection of the tube bundle, and the simulation results are consistent with the experimental data. For fluid-solid-fluid coupled heat transfer process, the heat transfer temperature difference should be used as the parameter of Gr number. Within the boundary conditions of this study, the Nu numbers predicted by traditional correlations are 1.4–2 times lower than the experimental and the simulation results. According to the results of simulation and experiment, an outside natural convection heat transfer correlation of the tube bundle is proposed, and the error between the predicted value and the experimental data is within −3%–10%. The results of this study can provide a basis and reference for engineering design of the PRHRS. • Carried numerical and experimental researches on natural convection heat transfer outside the tube bundle under small temperature difference. • The calculation method for the Gr number outside the tube bundle in the process of fluid-solid-fluid heat transfer is given. • The experimental and simulation results of natural convection heat transfer outside the tube bundle under the small temperature difference are compared with the traditional correlation. • The correlation for predicting the natural convection heat transfer outside the tube bundle under the condition of small temperature difference is given. [ABSTRACT FROM AUTHOR]

• An experimental study on SHP PRHRS under sub-atmospheric has been carried out. • The effect of boundary conditions on natural circulation has been studied. • The process and causes of system instability have been analyzed. • The thermal stratification in the pool has been analyzed. The use of separate heat pipe in Passive Residual Heat Removal System (PRHRS) for Swimming Pool-type Low-Temperature Heating Reactor (SPLTHR) has a good application prospect. An experimental study on miniaturized separate heat pipe-type PRHRS has been carried out. The heat transfer medium in the separate heat pipe is water and is at sub-atmospheric. The boundary conditions of the experiment refer to the operating range of SPLTHR. The two-phase natural circulation characteristics of the separate heat pipe-type PRHRS and the influence of boundary conditions on the system heat transfer capacity, flow instability and heat stratification of pool water are studied. Within the experimental range, the heat transfer rate is from 72.0 kW to 459.2 kW and the steam mass flowrate is from 0.031 kg/s to 0.197 kg/s. During the operation of the system, there is long-term flow instability. The process and causes of system instability are analyzed from the perspective of heat transfer. It is believed that the natural convective heat transfer outside the tube bundle hinders the heat transfer, resulting in instability. The heat transfer capacity of the system under three filling ratios of 30%, 50% and 70% was studied, and the results showed that the filling ratio within this variation range had little effect on the heat transfer capacity of the system. Furthermore, the thermal stratification in the pool has been analyzed. [ABSTRACT FROM AUTHOR]

The high efficiency heat extraction from the containment in severe accident is of utmost importance for nuclear power plants, and the Passive Containment Heat Removal System (PCS) is utilized for removing heat efficiently in third generation nuclear units. In order to enhance external steam-air condensation heat transfer, previous studies have primarily focused on specific structures, rib tubes, S-shaped tubes, or inclined tubes bundles, for instance. In the present study, a novel structure of helical coiled tube was designed and its external heat transfer characteristics were obtained through numerical simulation. The effects of coiled pitch, radial helical pitch, tube diameters and heights, two tube arrangements of vertical and horizontal on heat transfer characteristics were elucidated. It can be concluded that radial helical pitch and coiled pitch jointly impact the heat transfer, a proper large radial helical pitch and narrow coiled pitch can result in an approximate 9% enhancement in condensation heat transfer coefficient (h a), and the utilization of narrowed pipe diameters and longer tube heights facilitates the augmentation of heat transfer. Moreover, for vertical arrangement, h a can be enhanced over 15% by rising tube numbers, for horizontal arrangement, h a primarily influenced by tube pitch, the heat transfer is deteriorated by narrow tube pitch, but improved by widening them. • A novel effect that influences the external condensation heat transfer was proposed. • Derived condensation behavior during steam-air external condensation on helical coiled tube. • Effects of various parameters on heat transfer characteristics outside helical coiled tube were elucidated. • A helical tube bundle structure exhibiting exceptional heat transfer performance was obtained. [ABSTRACT FROM AUTHOR]

In Swimming Pool-type Low-Temperature Heating Reactor (SPLTHR), the Passive Residual Heat Removal System (PRHRS) is an indispensable part of the reactor safety system. In the heat transfer process of the PRHRS system, the natural convection heat transfer outside the Passive Residual Heat Removal Heat Exchanger (PRHR HX) in the reactor pool has a great influence on the flow and heat transfer characteristics of the system. In this study, an experimental facility was built to study the natural convection heat transfer outside the tube bundle under low temperature difference. According to the experimental results, a PRHRS for SPLTHR was designed. And a double-coupled natural circulation system analysis code was developed to investigate the start-up characteristics of the system under different initial conditions. The research results showed that the correlation of Yang predicts the experimental heat transfer results well. The calculation results indicated that the designed system met the heat removal requirements after reactor shutdown. If the temperature in the pipeline between the Passive Residual Heat Removal Heat Exchanger (PRHR HX) and the air-cooler was linearly distributed, the system would quickly establish a natural circulation and reduced the number of oscillations before the system reached stability. At the air-cooler, when the temperature of the heat transfer fluid in the water side changed, the temperature and mass flow rate of the air loop would quickly follow the change. The designed system and calculation results can provide a design basis for engineering applications. [ABSTRACT FROM AUTHOR]

Highlights • A new kind of packing arrangement is developed with radial layered configuration. • A CFD analysis is conducted to study the characteristics of the new packed bed. • Outstanding pressure drop reduction and heat transfer intensification are observed. • The new packing has much higher energy efficiency than the conventional. Abstract The channeling effect at the container wall results in difficulties for heat transfer in the central part of packed beds with small tube-to-particle diameter ratios. To overcome this shortcoming, a new kind of packed bed is developed with radial layered configuration: a fluid-penetrable wire mesh is used to divide the container into two parts in the radial direction, which allows particles of different sizes for regional loading. Therefore, following the general rule of fluid flow distribution in packed beds, small particles could be filled in the zone at the container wall to restrain the channeling effect and large particles in the center to promote local heat transfer capacity. In the present work, to facilitate the proper functionality of new equipment using the novel method, a CFD simulation combined with the DEM method for random packing generation is conducted to study the characteristics of the new packed bed on the packing structure, fluid flow, and particles-to-fluid convective heat transfer under a high fluid velocity regime (Re ∼ 104). It finds that the new packing method could exhibit a much regular structure than the conventional packed beds. And outstanding performance is observed in pressure drop reduction and heat transfer efficiency improvement. Moreover, the well-known equations for pressure drop and convective heat transfer predictions are found not applicable to the new bed and distinctive features of fluid velocity and temperature distribution are identified. These findings are helpful for the design of high efficiency reactors. [ABSTRACT FROM AUTHOR]

Abstract Previous investigations on steam condensation in the presence of air have revealed general heat transfer characteristics in vertical plates and single tubes. However, there remains limited knowledge regarding the thermal-hydraulic phenomena and heat transfer properties of tube bundles. In order to evaluate steam condensation on the external surfaces of tube bundles, STAR-CCM + code with a diffusion boundary layer steam condensation model was employed in this work. In the assessments, 3 × 3 tube bundles with various tube pitches (from 1.5d to 5d) were investigated, and the results were compared to those of a single tube. The data analyses mainly focused on three aspects: (i) global and local condensation phenomena in tube bundles; (ii) heat transfer characteristics in tube bundles; and (iii) influences of tube pitches and global parameters (such as pressure and air mass fraction) on the condensation heat transfer. The phenomena analyses indicate that the axial field profiles are mainly determined by the development of a concentration boundary layer, while the radial field profiles are mostly affected by the tube pitches. Local heat flux analyses indicate that the condensation heat flux exhibits a significant reduction downwards along the vertical tube. In comparison, the condensation heat flux fluctuates in the tube peripheral direction. In the latter case, heat transfer enhancement and inhibition regions are identified and classified. For the tube average heat transfer, only the central tube is significantly affected by the tube pitches, and the degradation may be more than 30% at a tube pitch of 1.5d. The tube pitch effect is almost independent of the bulk air mass fraction and changes slightly with an increase in the wall sub-cooling and bulk pressure. [ABSTRACT FROM AUTHOR]

Abstract The development of advanced packed beds has remarkable applications in the design of new equipment for related industries. Recently, a hybrid of the structured and random packed beds is proposed—the hollow structured packed bed (HSPB), which can be easily formed with ordered ring structures and a free channel along the centerline by the particle self-assembly feature in a random packed bed at 2 < D / d < 3. In this study, the pore scale CFD simulation is performed to investigate the fluid flow and heat transfer characteristics of the HSPB. It can be observed that, among the HSPB tested in the current work, both pressure drop and heat transfer decrease with varying degrees by increasing the D / d ratios and the packed bed with the largest D / d ratio has the highest overall heat transfer efficiency. Compared with random packed beds and a structured packed bed in simple cubic packing, the HSPB reduces the pressure drop effectively and enhanced the heat transfer capacity than the structured packed bed, which make the HSPB superior than the two typical packing forms on the overall heat transfer efficiency. Together with the convenience in construction, the HSPB is very suitable for industrial applications with the rapid heat removal and low pressure drop requirement. Moreover, affected by the packing structure, the fluid flow velocity and temperature distribution in the HSPB have some unique features. The formation is therefore discussed in this study as well. These findings are helpful in the design of new equipment using the HSPB as well as understanding transport phenomena in packed beds. Graphical abstract Unlabelled Image Highlights • Fluid flow and heat transfer in HSPB are studied by pore scale CFD simulation. • Pressure drop and heat transfer coefficient decrease in HSPB when D / d increase. • HSPB has superior overall heat transfer efficiency than structured packed bed. • HSPB is suitable to applications need rapid heat removal and low pressure drop. [ABSTRACT FROM AUTHOR]

Abstract Steam condensation in the presence of air is a relevant phenomenon in various industrial applications. Its heat transfer property has been broadly revealed by experimental and numerical investigations on vertical plates or single tubes. However, these results may not be directly applied to tube bundle cases, since there may exist mutual interactions among adjacent tubes. To have an insight on this problem, the present work conducted numerical simulations on various tube bundles with a tube pitch 1.5 times of the tube diameter. The cases evaluated were classified into three categories including the single row, double row, and triple row. In each category, structures with various tube columns were assessed. The results indicate that tube bundles will thicken the near-wall high concentration air layer, resulting in inhibited condensation heat transfer. This phenomenon is defined as the inhibition effect. On the other hand, a heat transfer enhancement effect caused by a strengthened natural circulation driven by the density difference between the mainstream and the high concentration air region is found and defined as the stack effect. The average condensation heat transfer for tube bundles is determined by the relative magnitude between the inhibition effect and the stack effect. The stack effect becomes much intensive with the increase of tube rows and columns, which can enhance condensation heat transfer. Typically, the results in the triple row categories show that the maximum average heat transfer coefficient can be 14% greater than that of the single tube. [ABSTRACT FROM AUTHOR]

A set of condensation experiments in the steam–air and pure steam conditions has been conducted to evaluate the heat removal capacity of a vertically pin-fin tube under free convection. Condensation heat transfer coefficient and heat transfer rate have been obtained. The bulk temperature, wall temperature and temperature near tube surface are measured in detail. As a reference, a smooth tube is tested under the same condition. For pure steam case, the pin-fin tube generates restrictions on the condensation heat transfer process in low wall subcooling region. With a higher wall subcooling, the pin-fin tube shows a larger condensation heat transfer coefficient than smooth tube does. For steam–air mixtures case, the result of the temperature measurement demonstrates that there is an air rich region near the pin-fin tube. Compared with the smooth tube, the pin-fin tube generates reduction effect on the condensation heat transfer process under low air mass fraction situation. As air mass fraction increasing, the condensation heat transfer coefficient of the pin-fin tube becomes larger than that of the smooth tube. [ABSTRACT FROM AUTHOR]

• The effect of CO 2 on the steam condensation outside a vertical tube is numerically analyzed. • A CFD model to simulate the steam condensation in presence of CO 2 /air is validated by the experiments carried out on the COAST facility. • The steam concentration will influence of the effect of CO 2 on steam condensation heat transfer coefficient. Under severe accident conditions of a pressurized water reactor nuclear power plant, the reactor core may melt, and the molten corium even reacts with concrete in the containment, producing a large amount of CO 2 into the entire containment gas space. This may have an obvious impact on the heat transfer characteristics of steam condensation in the containment. However, there are few studies focused on this phenomenon. In this paper, a commercial CFD software, STAR-CCM+, is used to study the effect of CO 2 on steam condensation heat transfer. The calculations are based on 0.4 MPa gas pressure, 40 K wall sub-cooling, 0.66 or 0.2 steam mole fraction, and the CO 2 mole fraction varies from 0 to 0.34 or 0.8. The results show that the CO 2 demonstrates different influence laws at high and low steam concentrations, respectively. When the steam mole fraction is 0.66, as the proportion of CO 2 in the non-condensable gas increases, the condensation heat transfer coefficient (HTC) will increase, and the enhancement can reach 30%. When the steam mole fraction is 0.2, the condensation HTC will first decrease and then increase, as the proportion of CO 2 in the non-condensable gas increases. The effects of gas pressure and wall sub-cooling on the condensation HTC are also analyzed. The results show that the condensation HTC increases with the increase of the gas pressure and decreases with the increase of wall sub-cooling. [ABSTRACT FROM AUTHOR]

Steam condensation consisting of noncondensable gas has been broadly investigated on external surfaces of various single tube and plates. Previous numerical analyses revealed that the high concentration air layer (HCAL) is the main thermal resistance for condensation. Comparing with the single tube/plate, condensation for bundle can be more complex. The HCAL near the adjacent tube surfaces may interfere and then influencing flow and heat transfer characteristics in the bundle. To evaluate the bundle effect, this study conducted experiments based on a 3 × 3 tube bundle at a wider pressure range (0.15–1.6 MPa). The results indicate that the bundle effect can be divided into two sub-effects: the enhancement effect and the inhibition effect, and the enhancement effect can make the average heat transfer performance of the bundle better than that of the single tube. The parameter thresholds for the two sub-effects were determined according to the broaden parameter scope. Generally, at small subcooling and low pressures, the inhibition effect dominants, and the bundle condensation heat transfer coefficient (CHTC) can be 22% less than that of the single tube; at large subcooling and high pressures, the enhancement effect dominants, and the bundle CHTC can be 71% greater. • The 3 × 3 bundle condensation experiment was conducted at pressures up to 1.6 MPa. • The tube bundle both has enhancement and inhibition effects on condensation. • The enhancement effect will exceed the inhibition effect with pressure increases. • The bundle average condensation performance can be greater than that of single tube. [ABSTRACT FROM AUTHOR]

• The pore-scale CFD is performed to study fluid flow in packed beds at D / d < 10. • The quasi-regular packed bed is identified at very low D / d ratios. • The typical confining wall effect is found in packed beds with large D / d ratios. • The regularity from the wall dominates the packing and flow field at D / d < 4. • The simulations results are compared with well-known empirical correlations. The presence of the confining wall exerts critical influences on packing structure, as well as transport properties of packed beds with small D / d ratios. In the present work, the variations of pressure drop and particle-to-fluid convective heat transfer in the packed beds of strong wall interference were evaluated under the high Reynolds number regime (Re m ∼ 104) by using the CFD simulation combined with the DEM method for random packing generation. The variations upon the D / d ratios were discussed, and the applicability of empirical correlations was examined. It found that the lower D / d ratios, the lower pressure drop and heat transfer performance, and the Carman correlation and the Wakao correlation provided predictions of better accuracy for the two parameters respectively than the other empirical correlations. Then, the discussion was extended to the packing structure and fluid flow distribution. A packed bed with distinct features—the quasi-regular packed bed—was encountered at D / d = 4, in which the packing structure regularity from the wall extents over the entire bed. Accordingly, much uniform flow velocity and temperature distributions were observed in the bed. Finally, some literature results in the range of 1 < D / d < 4 were revisited in conjunction with the present findings to reach a complete picture of the confining wall influence on the packing structure and related transport properties. It suggests that the studies for packed beds in the range of D / d < ∼ 4 should be isolated from the other packed beds of higher D / d ratios and the related analysis should be developed on a case-by-case basis. These findings may provide a new perspective to address the confining wall influence in packed beds with small D / d ratios. [ABSTRACT FROM AUTHOR]