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In multiphase particulate systems, the turbulence of the continuous phase (gas or liquid) is modulated due to interactions between the continuous phase and the suspended particles. Such phenomena are non-trivial in the essence that addition of a dispersed phase to a turbulent flow complicates the existing flow patterns depending on the physical properties of the particles leading to either augmentation or attenuation of continuous phase turbulence. In the present study, this aspect has been comprehensively analysed based on the available experimental data obtained from the well-studied turbulent flow systems such as channel and pipes, free jets and grids. Relevant non-dimensional parameters such as particle diameter to integral length scale ratio, Stokes number, particle volume fraction, particle momentum number, and particle Reynolds number have been utilised to characterise the reported turbulence modulation behavior. Some limitations of these commonly used dimensionless parameters to characterise turbulence modulation are discussed, and possible improvements are suggested. [ABSTRACT FROM AUTHOR]

In the present work, 3-D numerical simulations have been carried out to understand the flow patterns and heat transfer on the jacket side of a stirred vessel under mixed convection conditions. Various parameters have been varied (1 ≤ Uin ≤ 20 m/s, 30 ≤ ΔT ≤ 60 K, 30 ≤ Jg ≤ 150 mm, Di= 1 and 3 m) to understand their effect on the heat transfer and pressure drop on the jacket side. SST k-ω model has been used for the numerical simulations because of its capability to predict natural convection heat transfer and low Re forced convection heat transfer. It was found that, the heat transfer coefficient increases by 40-80% by increasing the inlet fluid velocity from 1 to 5 m/s. The pressure drop increases by 120-200% and heat transfer per unit pumping power decreases by 60-75% as we increase the velocity from 1 to 5 m/s. It has also been observed that the flow inside the jacket is 3-D, nonuniform and has dead zones and hot pockets at various locations. The effects of inlet flow impinging have also been studied by varying inlet diameter and removing inlet pipe. Impinging by inlet flow on jacket wall contributes about 10-20% to heat transfer and 10-25% to pressure drop. Due to the presence of recirculating flow zones, dead zones, and impinging effect, existing correlations are not suitable for the estimations of heat transfer in large jackets. Large differences have been observed between the 3-D CFD predictions and the widely used empirical correlations. Further, the 3-D CFD predictions have been shown to be comparable to the experimental data available in the published literature. Based on simulations performed in the present work and also validated by experimental data, a new correlation has been proposed which is expected to be useful to design engineers, particularly for those, having limited computational facility. [ABSTRACT FROM AUTHOR]

In some nuclear reactors, under accidental conditions, core debris forms a molten pool, which is later located in a core catcher. The core catcher proposed by the authors uses special refractory material to absorb enthalpy of corium so that temperatures are within 1500 K, which is possible to cool with side cooling and top flooding. Since performing a full-scale prototypic experiment is extremely challenging and complex because of the involvement of very high temperatures and the presence of radioactive materials, it is important to develop a Computational Fluid Dynamics (CFD) model capable of simulating coolability of the melt pool with the above cooling strategy. In the present work, a CFD model was developed for the above purpose and was benchmarked with experiments conducted under simulated conditions by the authors. The experiment involved the melting of about 25 L of sodium borosilicate glass at about 1473 K and cooling it in a scaled-down core catcher model. In the presence of decay heat inside the melt pool, turbulent natural convection plays an important role in the temperature distribution inside the melt pool and on the vessel walls. For this, we used different turbulence models. Comparisons among the Standard k-ε, Shear Stress Transport (SST) k-ω, and two-dimensional (2D) Large Eddy Simulation (LES) turbulence models show that SST k-ω and 2D LES turbulences are found to be in good agreement with the experimental results for the temperature distribution in the melt pool, and SST k-ω is found to be computationally less expensive than 2D LES. In general, the CFD model is capable of simulating heat transfer with phase changes inside the heat-generating melt pool. In view of this, the model can be further extended to include cooling of the melt pool in the prototype core catcher. The evolution of crust formation has been investigated in detail using a CFD model. [ABSTRACT FROM AUTHOR]

The objective of the present numerical study is to investigate the thermal-hydraulic characteristics of various circular (plain circular fin, crimped spiral fin, serrated fin) and plate fins [plain plate fin, wavy fin and fin with punched delta winglet pair (DWP)]. The geometrical parameters are, tube outer diameter (OD) = 7 mm, fin spacing (S) = 3–7 mm, fin height ( h f ) = 5 mm, number of tube rows ( N r ) = 2, transverse tube pitch ( P t ) = 23 mm and longitudinal tube pitch ( P l ) = 18 mm. The Reynolds number based on hydraulic diameter has been considered in the range of 2500 ⩽ Re h ⩽ 4000. The circular fins outdo the plate fin in terms of the heat transfer coefficient and the heat transfer per unit pumping power for the circular fins has been found to be 140–170% higher as compared to the plate fins for a fixed Reynolds number. Local Nusselt number around the periphery of the tubes showed that fin patterns affect the wake region formation and flow separation on the tube surface. It has been observed that heat transfer coefficient decreases with an increase in the fin spacing due to increase in bypass flow rate through the heat exchanger at a constant Reynolds number. The heat transfer per unit pumping power showed an increase of more than 100% with an increase in the fin spacing from 3 to 6 mm. The flow patterns have been studied in detail for serrated fin, crimped fin and fin with DWP. The vortices generated by the fins merge with the horseshoe vortices and enhances the heat transfer in the domain. The fin efficiency has been found to be in the range of 77–83% for the circular fins and 55–66% for the plate fins for a range of 2500 ⩽ Re h ⩽ 4000. Only one circular fin design i.e., crimped fin provides higher volume goodness factor as compared to the plate fins. In general it has been observed that more compact heat exchanger can be designed using plate fins but at a higher pumping power cost. [ABSTRACT FROM AUTHOR]

Gas–liquid two phase flows are widely encountered in industry. The design parameters include two phase pressure drop, mixing and axial mixing in both the phases, effective interfacial area, heat and mass transfer coefficients. Currently, there is a high degree of empiricism in the design process of such reactors owing to the complexity of coupled flow and reaction mechanism. Hence, we focus on synthesizing recent advances in computational and experimental techniques that will enable future designs of such reactors in a more rational manner by exploring a large design space with high-fidelity models (computational fluid dynamics) that are validated with high-fidelity measurements (hot film anemometry (HFA), Laser Doppler anemometry (LDA), particle image velocimetry (PIV), etc.) to provide a high degree of rigor. Understanding the spatial distributions of dispersed phases and their interaction during scale up are key challenges that were traditionally addressed through pilot scale experiments, but now can be addressed through advanced modelling. For practically complete knowledge of the fluid mechanical parameters, it is desirable to implement direct numerical simulations (DNS). However, the current computational power does not permit full DNS for real bubble columns. Therefore, we have been using simplified turbulence models (such as large eddy simulation, Reynolds stress, k – ε , etc.) which need the knowledge of turbulence parameters. For the estimation of these parameters, currently semi-empirical procedures are being used pending the knowledge of turbulence. Further, the formulation of governing equations in all the CFD models (except DNS), the knowledge of interface forces (drag, lift, virtual mass, Basset, etc.) is needed and for their estimations empirical correlations are being employed, again pending the knowledge of fluid mechanics under turbulent conditions in bubble columns. In gas–liquid dispersions, the gas is sparged in the form of bubbles. During the bubble rise, the mechanism of wake detachment creates turbulence which can be called as wake generated turbulence. In addition, energy gets transferred from the gas phase to liquid phase. The quantitative amounts are negligible when bubble motion is not hindered and the gas–liquid dispersion is homogenous. The amounts increase with an increase in the extent of hindrance. However, in the homogenous regime, even under extreme conditions, the extent of energy transfer in the bulk gas–liquid dispersions (volume other than wake volume) is fairly limited. On contrast, in the heterogeneous regime, the rates of energy transfer become sizeable. The energy received by the liquid (in both the regimes) also creates turbulent motion and termed as bulk generated turbulence. In turbulent flows a compendium of eddies (flow structures) of different length and time scales contribute towards improved/enhanced mixing, momentum transfer, heat transfer, and mass transfer (transport phenomena). Hence, a proper understanding of the dynamics of these turbulent flow structures, and their role in the transport phenomena, can bring substantial improvement in the scale-up and design procedures. The present paper brings out the current status of knowledge on bubble generated turbulence. All the published literature in experimental measurements and DNS simulations has been critically analysed and coherently presented. [ABSTRACT FROM AUTHOR]

Liquid fluidised beds often operate with particles of different sizes and densities, encountering partial or complete segregation of solid particles at certain operating conditions. In this study, the segregation and dispersion of binary particle species of the same size but different densities in liquid fluidised beds have been investigated based on the analysis of computational fluid dynamics – discrete element method (CFD-DEM) simulation results. The vertical fluid drag force acting on the particles was found to be responsible for the particle segregation. The mechanisms governing the particle dispersion strongly depended upon the solid–liquid two-phase flow regime, which transited from pseudo-homogeneous to heterogeneous when the superficial liquid velocity reached a certain value. In the homogeneous or pseudo-homogeneous flow regime ( Re p ≤40, ∈ L, ave ≤0.74), particle collisions acted as the main mechanism that drove the dispersion of particles. However, after the system became heterogeneous, the magnitude of the vertical collision force decreased towards zero and correspondingly, the magnitude of the vertical fluid drag force was approaching that of the particle net weight force as the superficial liquid velocity increased. Therefore, in the heterogeneous flow regime ( Re p >40, ∈ L, ave >0.74), the local turbulence of the fluid flow and particle collisions (if there were any) were found to be the main mechanisms that drove the dispersion of particles in all directions. The dispersion coefficient of individual particles varied significantly throughout the system in the heterogeneous flow regime. The simulation results reasonably agreed with the experimental data and the prediction results by existing correlations. [ABSTRACT FROM AUTHOR]

Many advanced nuclear reactors adopt methodologies of passive safety systems based on natural forces such as gravity. In one of such system, the decay heat generated from a reactor is removed by isolation condenser (ICs) submerged in a large water pool called the Gravity Driven Water Pool (GDWP). The objective of the present study was to design an IC for the passive decay heat removal system (PDHRS) for advanced nuclear reactor. First, the effect of inclination of IC tube on three dimensional temperature and flow fields was investigated inside a pilot scale (10 L) GDWP. Further, the knowledge of these fields has been used for the quantification of heat transfer and thermal stratification phenomenon. In a next step, the knowledge gained from the pilot scale GDWP has been extended to design an IC for real size GDWP (∼10,000 m 3 ). Single phase CFD simulation using open source CFD code [OpenFOAM-2.2] was performed for different tube inclination angles ( α ) (w.r.t. to vertical direction) in the range 0° ⩽ α ⩽ 90°. The results indicate that the heat transfer coefficient increases with increase in tube inclination angle. The heat transfer was found to be maximum for α = 90° and minimum for α = 15°. This behavior is due to the interaction between the primary flow (due to pressure gradient) and secondary flow (due to buoyancy force). The primary flow enhanced the fluid sliding motion at the tube top whereas the secondary flow resulted in enhancement in fluid motion along the circumference of tube. As the angle of inclination ( α ) of the tube was increased, the secondary flow became dominant and resulted into enhanced heat transfer near the tube bottom. Three different heat transfer regimes were identified in the transient period: conduction (0 < t < 0.4 s), quasi-steady (0.4 s < t < 10 s) and fluctuating period (10 s < t < 100 s). The effect of inclination angle ( α ) was more pronounced in the fluctuating period. The natural convection and heat transfer in the regime of laminar-turbulent transition was studied in the presence of longitudinal vortices. The heat transfer was enhanced in the transition region due to vortices formation and it was reduced in the turbulent regime due to decay of vortices. [ABSTRACT FROM AUTHOR]

The dynamics of a bubble, initially stationary and spherical, rising in a viscous Newtonian liquid have been studied numerically using 3-D Volume-of-Fluid (VOF) method implemented in the Gerris flow solver. The study encompasses 8.7≤ Eo (= Δ ρ g D 2 / σ )≤641 and Re ≤151. Additionally, results published in the literature encompassing bubbles with lower values of E o numbers were also considered, such that the overall dependencies of bubble shape, wake characteristics, and drag coefficient over a large range of Eo and Re values can be identified. While it was found that the deformation of the bubbles as predicted through the numerical study can generally replicate experimental observations presented, several limitations were identified, such as in the representation of skirt formation behind a skirted bubble and the formation of satellite bubbles behind a bubble rising at high Reynolds numbers. The dependency of the bubble aspect ratio on the Weber and Morton numbers was confirmed for cases of spherical and ellipsoidal bubbles; whilst for spherical cap and skirted bubbles the aspect ratio was found to depend largely on the Reynolds and Capillary numbers, respectively. Finally, the expansion and formation of closed/open laminar wakes behind the rising bubble were analysed and was found to correlate well with the bubble Re and Eo numbers. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate thermal–hydraulic characteristics of an air cooled condenser and optimized the design in terms of heat transfer per unit pumping power, area goodness factor, volume goodness factor and material requirement using the 3D numerical simulations. The annular-finned tubes have been considered in this study, which is one of the most common type of fin used for the design of air cooled heat exchangers and known to provide a high heat transfer coefficient per unit pressure drop. The effect of tube shape (elliptical and round), tube diameter [7–24 mm (circular), 30 × 10–30 × 20 mm (elliptical)], fin spacing (2–10 mm), number of rows (2–10), fin height (5–10 mm), air frontal velocity (4.76–6.32 m/s), transverse tube pitch (36.8–44 mm) on the thermal–hydraulic performance has been studied. It is observed that, with an increase in the fin spacing, the heat transfer coefficient increases (by 35–40%) at a constant inlet velocity and the pressure drop decreases (by 60–80%). The frontal area requirement increases by 100–150% with an increase in the fin spacing for the same heat removal capacity. As the row number is increased, the heat transfer coefficient increases initially for N r < 4 by 7–8%, and for N r > 4, the heat transfer coefficient decreases by 23%. An increase in the tube pitch increases the heat transfer coefficient per unit pressure drop by 40%. The fin efficiency is found to decrease with an increase in the Reynolds number, fin spacing and number of tube rows. In the last section optimization of the design has been performed based on Taguchi method and numerical results obtained. It has been observed that the number of tube rows must be kept between 2–4, fin spacing 3–5 mm, tube pitch around 40 mm, and fin height 5 mm for better performance of the condenser. [ABSTRACT FROM AUTHOR]

The objective of this study is to investigate the transient 3D numerical simulations of natural convection of air around a circular finned tube (24.9 mm OD ) kept in a small chimney. The annular plain fins are considered in this study. The effects of fin spacing to fin diameter ratio (0.057 mm ⩽ S / D f ⩽ 0.24 mm), chimney height (400–1000 mm) and ambient to surface temperature difference (10 K ⩽ T s ⩽ 65 K) on the heat transfer and the driving force have been investigated. The results are presented in terms of the temperature contours, velocity vectors, heat transfer and the driving force. It has been found that the heat transfer coefficient increases with an increase in the fin spacing upto an optimum value ( S = 8 mm) for all the fin geometries, and beyond S = 8 mm, the heat transfer coefficient decreases. The separation of the thermal boundary layer with a variation in the fin spacing and its effects on the heat transfer and driving force has been shown. For a fixed fin spacing, the heat transfer rate, heat transfer coefficient and the driving force increases with an increase in the fin diameter, however, for D f > 41 mm, the rate of increase in the heat transfer coefficient reduces. The heat transfer coefficient increases with an increase in the chimney height and it has been found that the effect of chimney height on the heat transfer coefficient is a resultant effect of the air outlet temperature and the driving force generated by the chimney. The base to ambient temperature difference has been varied to observe the temperature sensitivity on the heat transfer coefficient and flow patterns. In the last section, various circular and elliptical tube designs have been investigated, and it is found that, the elliptical tube with minimum ellipticity ( b / a = 0.33) and circular tube with smallest diameter (7 mm) provides better heat transfer coefficient than the other circular and elliptical designs. [ABSTRACT FROM AUTHOR]

Three dimensional computational fluid dynamics (CFD) analysis has been performed for the moderator flow and temperature fields inside a vertical calandria vessel of nuclear reactor under normal operating condition using OpenFOAM CFD code. OpenFOAM is validated by comparing the predicted results with the experimental data available in literature. CFD model includes the calandria vessel, calandria tubes, inlet header and outlet header. Analysis has been performed for the cases of uniform and spatial distribution of volumetric heat generation. Studies show that the maximum temperature in moderator is lower in the case of spatial distribution of heat generation as compared to that in the uniform heat generation in calandria. In addition, the effect of Archimedes number on maximum and average moderator temperature was investigated. [ABSTRACT FROM AUTHOR]

In this paper, a review is presented on the experimental investigations and the numerical simulations performed to analyze the thermal-hydraulic performance of the air-cooled heat exchangers. The air-cooled heat exchangers mostly consist of the finned-tube bundles. The primary role of the extended surfaces (fins) is to provide more heat transfer area to enhance the rate of heat transfer on the air side. The secondary role of the fins is to generate vortices, which help in enhancing the mixing and the heat transfer coefficient. In this study, the annular and plate fins are considered, the annular fins are further divided into four categories: (1) plane annular fins, (2) serrated fins, (3) crimped spiral fins, (4) perforated fins, and similarly for the plate fins, the fin types are: (1) plain plate fins, (2)wavy plate fins, (3) plate fins withDWP, and (4) slit and strip fins. In Section 4, the performance of the various types of fins is presented with respect to the parameters: (1) Reynolds number, (2) fin pitch, (3) fin height, (4) fin thickness, (5) tube diameter, (6) tube pitch, (7) tube type, (8) number of tube rows, and (9) effect of dehumidifying conditions. In Section 5, the conclusions and the recommendations for the future work have been given. [ABSTRACT FROM AUTHOR]

Many advanced designs of nuclear reactors adopt a methodology of passive safety systems in order to avoid the occurence of a severe accident. In one of such systems, the decay heat generated from a reactor is transferred by natural circulation into large pool of water called the Gravity Driven Water Pool (GDWP). Three-dimensional (3D) convection flows develop, which in turn affect the heat transfer process and hence the temperature pattern. The heat transfer process can get compromised by the possible stratification of the temperature. Further, the material of construction of GDWP may have certain temperature limitations and puts bounds on the extent of stratification. The objective of this study is to investigate the 3D natural circulation phenomenon in GDWP. The present work is in continuation of our earlier work Gandhi et al. [1,2] where the natural convection has been analyzed in 0.025 and 0.21 m 3 vessels. Now, we have reported the simulation for 9247 m 3 GDWP tank. Single phase CFD simulations using open source CFD code [OpenFOAM-1.6] have been performed for a geometry (ID = 12 m, OD = 50 m and height (H T ) = 5 m). In order to reduce the thermal stratification, various geometrical modifications have been incorporated on the heat exchanger design, such as (1) distributing the heat transfer area of heat exchanger among single, double and multiple heat sources (2) to optimize the location of heat exchanger inside the GDWP (3) provision of passive elements such as draft tubes (single or concentric multiple) around the heat source at the center, which can act as a chimney. A detailed CFD analysis of three dimensional temperature and velocity distribution in the secondary side of GDWP has confirmed the mitigation of thermal stratification phenomenon by optimizing the distribution and position of heat exchangers. Passive draft tubes also result in significant enhancement in natural circulation and hence reduction in thermal stratification. [ABSTRACT FROM AUTHOR]

The objective of this work is to investigate the transient 3D numerical simulations of natural convection of air around a circular cylinder (76.2 mm OD ) enclosed in a box of 1000 mm × 600 mm × 1200 mm for a Rayleigh number of 1.3 × 10 6 . The 2D numerical simulations have also been performed and the comparison between the 2D and 3D simulations has been presented in terms of the Nusselt number. The effect of clearance between the top wall and the cylinder (0.2 ⩽ H ∗ / D ⩽ 2.3) on the flow pattern has also been investigated for the case of conducting ceiling. The flow becomes 3D, unstable and oscillating when the H ∗ / D ratio is 0.2, and as the H ∗ / D ratio is increased to 0.4, 1 and then to 2.3; the flow becomes 2D and stable. The studies of Cesini et al. (1999) [5] and Newport et al. (2001) [7] have also been analyzed by 2D and 3D numerical simulations. The flow shows 3D, unstable and oscillatory behavior in the case of Cesini et al. (1999) [5] due to the wall-cylinder interaction. However, the flow was found to be 2D and stable for the case of Newport et al. (2001) [7]. A comparison between our results and their numerical and experimental results has been presented. The time varying behavior of the surface averaged Nusselt number has been estimated, and it was found that, the time to reach the steady state for the flow depends on the aspect ratio of the geometry and 3D nature of the natural convection. [ABSTRACT FROM AUTHOR]

[Display omitted] • Experimental study of breakage and classification in an air jet mill, in a semi-batch mode. • Product size, leakage flux and production rate obtained as a function of operating conditions (pressure and holdup). • Optimal operating conditions determined based on specific energy for grinding. • A population balance model including breakage and classification gives a good match with experiments. The breakage and classification of particles in a spiral air jet mill, operated in a semi-batch mode, is studied for varying operating conditions. The particle size distributions of the product leaving the mill and the material inside the mill are measured, along with the mass in the mill. The median particle size of the product is found to be unchanged with inlet jet pressure at about 5 μ m but increases with holdup to about 7.5 μ m. At low pressures and high holdups, the classification is imperfect and a significant fraction of large particles, in the size range 150–350 μ m, exit in the product. The mass flux from the mill increases with both pressure and holdup and an empirical expression is given to calculate it. A population balance model is presented, which predicts size distributions, mean sizes and mass flux from the mill, in agreement with the measurements. [ABSTRACT FROM AUTHOR]

[Display omitted] • Study of the effect of nozzle angle and classifier tube height on breakage and classification in an air jet mill, operated in a semi-batch mode. • Breakage rate, classification efficiency and mass flux increase with increasing nozzle angle but cut size is constant. • Increase in classifier tube height improves classification efficiency but reduces mass flux. • Nozzle angle of 60 and classifier tube height of 10 mm are optimal based on specific energy for grinding. We study the effect of varying air jet mill design parameters, the nozzle angle and the classifier tube height, on the performance of the mill, operated in the semi-batch mode, using quartz and limestone particles as feed. Particle size distributions of the product and the material in the mill are measured along with the mass in the mill. The product size distribution and the specific mass flux are found to be constant with time. The breakage rate is found to increase with nozzle angle. The efficiency of classification, characterized by the fraction of coarse particles larger than the cut size, increases with nozzle angle and classifier tube height. The energy for grinding per unit mass of the product decreases with nozzle angle and increases with classifier tube height and optimal values of the parameters, at which the specific energy is minimized, are obtained. [ABSTRACT FROM AUTHOR]

Heat transfer by natural convection in a cavity is encountered in various industrial applications, such as heating and cooling of living spaces, fire in building, solar thermal collector systems, electronic and photovoltaic cooling devices, thermosiphon heat exchangers, passive decay heat removal systems, air condensers, etc. In such systems, the cooler (heavier) fluid naturally moves towards the bottom of the tank while the hotter lighter fluid rises towards the top. These flows result into certain extent of non-uniformity (or stratification) of temperature/concentration depending upon the intensity of flow. We have carried out flow (using particle image velocimetry) and temperature (using thermocouples) measurements in a rectangular tank (0.8×0.6×0.6m3) fitted with a central tube (forming the heat transfer surface). In order to reduce the stratification, the effects of various internals have been examined. These include (a) changing the ratio of area (heat transfer) to volume (aspect ratio), which is achieved by insulating the heating tube with the help of teflon, (b) introduction of draft tube concentric to the heat transfer tube, and (c) provision of non-conducting or highly conducting fins attached to the tube at different axial locations. Additionally, computational fluid dynamic (CFD) simulations of these systems were performed using the commercial software FLUENT-6.3. The extent of stratification has been investigated for a wide range of Rayleigh numbers (4.34×1011≤Ra≤2.59×1014). In addition, the flow information obtained from PIV was analyzed for insights into the dynamics of turbulent flow structures. For this purpose, we have used the signal processing technique of Discrete Wavelet Transform (DWT). From the analysis, we were able to estimate the size, velocity and energy distribution of turbulent structures. This detailed knowledge was employed in surface renewal type of theories for the estimation of rates of heat transfer. A good agreement was observed between the predicted and the experimental values of heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Abstract: Buoyancy induced flow and heat transfer are important phenomena in a wide range of engineering systems including electronics and photovoltaics cooling, thermosiphon and hydrosiphon heat exchangers, solar-thermal heat absorbers, passive decay heat removal systems, etc. Such systems are vulnerable to thermal stratification, which can significantly compromise performance. The objective of the present work is to study different passive heat transfer systems, characterized by an absence of active components such as pumps. We have investigated two phase (boiling) natural convection, in order to find the ways to modify the extent of thermal stratification. We have considered a base case of a cylindrical tank, fitted with a central heating tube constituting the heat transfer surface. In addition, a large number of design modifications have been considered. The flow field and the corresponding stratification have been compared with the base case. We have considered the following design modifications to this system: (a) changing the ratio of heat transfer length (of the central heating tube) to liquid height (L T /H), (b) provision of different draft tube designs which act as a chimney, and (c) attachment of circular horizontal baffles (both non-conducting and conducting). The investigations were made using Computational Fluid Dynamic (CFD) simulations implemented via the commercial software FLUENT-6.3. The CFD predictions have been compared with the experimental measurement of flow field obtained using Particle Image Velocimetry (PIV) and temperature measurements using thermocouples. The extents of thermal stratification and mixing have been investigated for a wide range of Rayleigh numbers (9.37×1010≤Ra≤5.57×1013). A good agreement has been obtained between the CFD simulations and the experimental measurements. The results show that the use of conducting baffles with appropriate size, number, and location can significantly reduce the stratification and concomitantly enhance the extent of mixing through modifications in the flow fields and heat flux. [Copyright &y& Elsevier]

Abstract: Annular centrifugal extractors (ACEs) based on Taylor–Couette flow, have found various applications, some of which include liquid–liquid extraction and as bio and polymerization reactors. The residence time distribution (RTD) was measured in three size extractors having 50, 150 and 250mm rotor diameter. The rotational speed was varied in the range of 10–30r/s so that the power consumption per unit mass in the annular region was in the range of 25–750W/kg. The extent of axial mixing was substantially reduced by providing helical baffles in the annular region. The effect of pitch was also studied. All the geometries were simulated using computational fluid dynamics (CFDs). A good agreement was obtained between the CFD predictions and the experimental measurements. Also, correlations have been proposed for Peclet numbers in the absence and presence of helical baffles. [Copyright &y& Elsevier]

Abstract: Orifice meters are the most common instruments used for fluid flow measurement because of its ruggedness, simple mechanical construction and other known advantages. Orifice coefficients are empirical because of difficulty in accurately predicting the effects of geometrical complicacy and flow separation from the wall on the flow. In the present paper, Computational Fluid Dynamics (CFD) simulation has been used to predict the orifice flow with better accuracy. CFD simulations have been performed using OpenFOAM-1.6 solver and validated with the published experimental data of and . CFD simulations have been validated with pressure drop and energy balance of our experimental data of water as fluid. The outcomes of the CFD simulations in terms of profiles of velocity, pressure, etc. are discussed in detail. A new scheme has been proposed to track vena-contracta with the help of CFD and with a suitable provision in the hardware of orifice meter. The new scheme maintains the existing advantages of orifice meters and provides better accuracy and sensitivity. [Copyright &y& Elsevier]

In the conventional cooking practice, where a pot or a pan is directly placed on a flame, the thermal energy efficiency is in the range of 10–25%. It was thought desirable to increase this efficiency up to 60% or more. The cooking systems can be of various sizes. In the developing world (85% of the world’s population), open pan cooking is largely still practiced at the family level (4–10 people) or at the community level (50–2000 people or more). The latter requirement is encountered in schools, homes for senior citizens, jails, social and/or religious centers (temples, mosques, churches), social and/or educational functions (conferences, marriages, celebrations, etc.), remand homes, etc. For these different types of final application, in the present work, cooking systems have been developed. A systematic work has has been reported regarding the effect of several parameters on thermal efficiency. The parameters include the cooker size, number of pots, size and aspect ratio of the pots, heat flux, flame size, flux–time relationship, insulating alternatives, etc. Local and global optima of the parameters have been obtained, resulting in thermal efficiency of about 70%. [ABSTRACT FROM AUTHOR]

Sections 2–6 of Part I were devoted to the analysis of heat transfer characteristics of cookers. In all the experiments, only water was employed as a working medium. Now, we extend such an analysis to the actual cooking process in order to arrive at an improved cooking device. The major strategies for the optimization of energy utilization is to design appropriate insulation that has been obtained by two cover vessels. In order to select an air gap, the flow and temperature patterns in the air gap have been extensively analyzed using computational fluid dynamics (CFD). The flow pattern and heat transfer in cooking pots have also been analyzed by CFD. This has enabled us to design suitable internals for minimizing the stratification of temperature. The understanding of fluid mechanics has also given basis for selection of heat flux, gap between burner tip and cooker bottom, and temperature of flue gases leaving the cooker. Chemical engineering principles have been used for modeling and optimization. Kinetics have been obtained in batch cookers. The knowledge of kinetics, thermal mixing, axial mixing, and optimum selection of insulation have been employed for the development of continuous cookers. The continuous mode of operation also helps in saving of energy. Systematic data have been collected for the design and scale up of continuous cookers. [ABSTRACT FROM AUTHOR]

The ease with which residence time could be adjusted, the absence of moving parts, higher values of heat and mass transfer coefficient are the attractive features of bubble columns, hence are widely used for conducting several mass transfer operations/chemical reactions. However selection of sparger design and type is crucial and it affects performance of the reactor implicitly and explicitly. The Part I of this work addressed the roles of various parameters to be considered while designing the sparger for bubble column reactors. The second part, deals with the selection of optimum design and type of the sparger for several designs of bubble column, i.e., effect of column diameter, aspect ratio, operating pressure and volumetric flow rate of gas phase. [ABSTRACT FROM AUTHOR]

Abstract: Carbon nanotube is one of the most talked about materials in recent years due to its unique properties and potential applications in different fields. Out of different techniques, catalytic chemical vapour deposition in a fluidized bed is the most promising technique for bulk production of this exotic material. The objectives of this review are to bring out the science and technology behind this process, the present commercial scenario and to draw a future roadmap. We have critically examined the published literature on the nanoparticle fluidization, different forces involved in it and the hydrodynamics of a nano-agglomerate fluidized bed. The importance of use of discrete elemental method and computational fluid dynamics simulation in the fluidization of carbon nanotubes has also been highlighted. Different preparation routes of catalyst materials have been compiled. The reaction and growth mechanisms of nanotubes, including modeling have been discussed. The data on the effects of different process parameters on the quality and quantity of nanotubes have been critically analyzed. It has been indicated that the available data are scattered in nature and further understanding in the mechanism is required in order to find out the rate controlling step(s) and scale-up of the process. The present commercial scenario of carbon nanotube market has been depicted. At the end, based on the present knowledge gaps, future roadmaps have been suggested. [Copyright &y& Elsevier]

Abstract: The objective of this study was to develop a unified correlation for heat transfer coefficient (h) in bubble columns without internals for various gas–liquid systems using support vector regression (SVR)-based modeling technique. From the data published in open literature, 366 data points from 10 open sources spanning the years 1976–2007 for h were collected. Generalized SVR-based models have been developed for the relationship between h and prominent design and operating parameters such as column and sparger geometry, gas–liquid thermo-physical properties, operating temperature, pressure, superficial gas velocity, etc. Further, this model for h has been uploaded on the link http://www.esnips.com/web/UICT-NCL. The proposed generalized SVR-based correlations for h has prediction accuracies of 98.56% and average absolute relative error (AARE) of 7.05%. Also, the SVR-based correlation showed much improved predictions when compared with those estimated by empirical correlations in the literature. [Copyright &y& Elsevier]

Abstract: In the present paper, numerical simulations of the wake generated by a freely falling sphere, under the action of gravity, are performed. Simulations have been carried out in the range of Reynolds numbers from 1 to 210 for understanding the formation, growth and breakup of the axisymmetric wake. The in-house code used is based on a non-Lagrange multiplier fictitious-domain method, which has been developed and validated by . The onset of instability in the wake and its growth along with the dynamic behavior of a settling sphere is examined at Reynolds number (Re) of 210. It is found that at the onset of instability the sphere starts to rotate and gives rise to a lift force due to the break of the axisymmetry in the wake which in turns triggers a lateral migration of the sphere. The lift coefficient of a freely falling sphere is 1.8 times that of a fixed sphere at a given sphere density of 4000kgm−3 and sphere to fluid density ratio of 4. This is attributed to the Robin''s force which arises due to the rotation of the sphere. At this Reynolds number (Re=210) a double threaded wake is observed, which resembles the experimental observations of . [Copyright &y& Elsevier]

Abstract: The flow behavior of a jet reactor (consisting of a gaseous jet submerged in a molten-metal bath) is very complex. These are operated at high temperatures (1500–3000K) and need to be contained within a heavy metal enclosure. The design of such reactors requires a prior knowledge of the jet dimensions, flow pattern and heat transfer characteristics. However, the fuel opaqueness and the high temperature of the jet create difficulties in observing the reaction mass visually and therefore the literature contains a very brief account of the experimental measurements of the flow pattern. Hence, a systematic study has been undertaken with a reaction pair (HCl gas jet submerged in aqueous NH3), which has the potential for simulating the real systems. The present work is concerned with the CFD simulations by employing k–ε turbulence model and large eddy simulations (LES). The measurements and simulations have been carried out over a wide range of gas velocities (53–323m/s) and these have been compared with the CFD simulations. A comprehensive comparison has also been made between the k–ε and the LES for the mean flow, temperature and the turbulent kinetic energy. An attempt has been made to understand the relative performance of these models. Further, complete energy balance has been established between the energy supply rate through the jet and the energy dissipation rate within the reactor. The plume characteristics obtained from CFD simulations have been compared qualitatively with the photographic images. [Copyright &y& Elsevier]

Abstract: CFD simulation of bed expansion of mono size solid–liquid fluidized beds has been performed in creeping, transition and turbulent flow regimes, where Reynolds number has been varied from 0.138 to 1718. It has been observed that the predicted values of bed voidage using the drag law of Joshi [1983. Solid–liquid fluidized beds: some design aspects. Chemical Engineering Research and Design 61, 143–161] and Pandit and Joshi [1998. Pressure drop in packed, expanded and fluidized beds, packed columns and static mixers—a unified approach. Reviews in Chemical Engineering 14, 321–371] (which has been derived from the first principals), exhibited an excellent agreement with the Richardson and Zaki equation. CFD simulations have also been performed for the prediction of segregation and/or intermixing of binary particle systems having the ratio of terminal settling velocity over a range from 3.2 to 1.06. The Reynolds number has also been varied over the range of 0.33 to 2080. It has been observed that the present CFD model explains all the qualitative and quantitative observations reported in the published literature (complete segregation, partial segregation, complete intermixing, etc) and these predictions are in good agreement with the experimental results. The present CFD model also predicts successfully the layer inversion phenomena which occur in the binary particle mixtures of different size as well as density. Further, the critical velocity at which the complete mixing of the two particle species occurs has also been predicted. [Copyright &y& Elsevier]

Abstract: Planar information of velocity from 2D particle image velocimetry (PIV) and large eddy simulation (LES) data have been studied using multiresolution wavelet transform (WT) formalisms, i.e., discrete and continuous WT. Identification of dominant energy containing structures with their characterization in terms of fractal spectra have been carried out for industrially important equipment exhibiting turbulent behavior. These include annular centrifugal contactor, jet loop reactor, ultrasound reactor, channel flow, stirred tank and bubble column reactor. The characterization of their dynamics based on denoising the data and studying the local energy along the WT scales show sensitive variation and this helps in identifying the size and shape of structures. A dependency is seen between mixing time and the higher order moments of length scale distribution, viz., skewness and kurtosis and a generalized correlation has been built up for important types of equipment and associated flow parameters. The correlation is not only based on the knowledge of reactor geometry and operating conditions but also on the flow structures via their statistical parameters. Wavelet transform modulus maxima (WTMM) methodology has been used to study the evolution of structures and their interaction in a reduced dimensionality by evaluating the fractal spectra. Classification studies have been carried out using principal component analysis (PCA) of the fractal spectra. The results obtained show clear classes for the six types of equipments and delineate regimes to obtain benchmark patterns of flow hydrodynamics based on PCA co-ordinates. This methodology offers a generalized way for the optimal design and operation of different types of reactors. [Copyright &y& Elsevier]

Abstract: Snapshot proper orthogonal decomposition (POD) technique has been applied to reveal the dominant flow structures, their dynamics and length scales in six widely used industrial equipments (stirred tank, bubble column, Taylor–Couette flow (annual contactor), ultrasonic reactor, jet reactor, and channel flow). The variation in length scale of structures within an equipment, with change in its operating conditions (Reynolds number and power input) or change in its geometric configuration (sparger and impeller designs), has been brought out in this work. The planar data set for POD analysis was obtained from particle image velocimetry (PIV) and large eddy simulation (LES) studies. The dominant spatial topology was analyzed by using the velocity and vorticity POD modes. The modes have revealed the following flow structures: the ascending streaks and bursts in channel flow, the vortex tube and leading edge vortices in jets, the irregular small chaotic vortices in Taylor–Couette flow, the variation in plume oscillation and flow structures in the vortical region of bubble column resulting from changes in sparger design, the high intensity vortices near the source of ultrasound in the ultrasonic reactor and the effect of impeller designs on dominant flow structures and near blade vortices in the stirred tank. The length scales of structures are obtained by applying image processing on the spatial modes. The dynamics of these flow structures in each of the items of equipment is captured by reconstructing the flow field using appropriate spatial and temporal modes that contribute to these structures. Further, a unique attempt has been made to correlate the length scale distribution with the mixing time. [Copyright &y& Elsevier]

The aim of this study is computational fluid dynamic (CFD) simulation of the single- phase pressure drop in fixed and expanded beds. A fixed bed with a column to particle diameter ratio (D/dp) of 5 and having 151 particles arranged in 8 layers was taken as a computational geometrical model. In the case of expanded beds, 0.605 voidage bed consisted of 105 particles and 0.783 voidage bed consisted of 55 particles. Simulations were performed in the creeping transition and turbulent flow regimes, where Reynolds number (dpVLPL/μL) was varied from 0.1 to 10,000. The deviations from Ergun's equation due to the wall effects, which are important in D/dp < 10 beds, were well explained by the CFD simulations. Thus, an increase in the pressure drop was observed due to the wall friction in the creeping flow, whereas, in turbulent regime a decrease in the pressure drop was observed due to the channeling near the wall. Energy balance has been established through the CFD predicted values of energy dissipation rates (viscous as well as turbulent). [ABSTRACT FROM AUTHOR]

The objective of this study was to develop a unified data-driven correlation for the overall gas hold-up for various gas–liquid systems using support vector regression (SVR)-based modeling technique. Over the years, researchers have amply quantified the hydrodynamics of bubble column reactors in terms of the overall gas hold-up. In this work, about 1810 experimental points were collected from 40 open sources spanning the years 1965–2007. The model for overall gas hold-up was established as a function of several parameters which include superficial gas velocity, superficial liquid velocity, gas density, molecular weight of gas, sparger type, sparger hole diameter, number of sparger holes, liquid viscosity, liquid density, liquid surface tension, operating temperature, operating pressure and column diameter of the gas–liquid system. For understanding the hold-up behavior, the data used for training the model was grouped into various gas–liquid systems viz., air–water, gas–aqueous viscous liquids, gas–organic liquids, gas–aqueous electrolyte solutions and gas–liquid systems operated over a wide range of pressure. A generalized model established using SVR was evaluated for its performance for various gas–liquid systems. Statistical analysis showed that the proposed generalized SVR-based correlation for overall gas hold-up has prediction accuracy of 97% with average absolute relative error (% AARE) of 12.11%. A comparison of this correlation with the selected system specific correlations in the literature showed that the developed SVR-based correlation significantly gives enhanced prediction of overall gas hold-up. [Copyright &y& Elsevier]

Abstract: Direct contact steam condensation used in steam jet injectors, and direct contact feed water heaters, has been studied using CFD simulations. Fairly good agreement has been obtained, with the available experimental data of plume length and the profiles of axial and radial temperature. Further, critical analyses have been carried out for all the published semi-empirical models and correlations. In addition, CFD analysis has been extended to examine the role of nozzle diameter and geometry, on heat transfer phenomenon which governs the direct contact steam condensation phenomenon. A new hydrodynamic model has been formulated which estimates the interfacial area available for condensation. A rational correlation has been developed for the estimation of interfacial area, expressed in terms of Nusselt number (Nu), Reynolds number (Re), Prandtl number (Pr) and ratio of viscosity of steam and water. [Copyright &y& Elsevier]

Abstract: The flow pattern and power number in a vessel depend on the impeller blade angle, number of blades, blade width, blade twist, blade thickness, pumping direction and interaction of flow with the vessel wall. Measurements of the power consumption and flow pattern have been carried out in a stirred vessel of 0.5m diameter for the range of impellers to study the effect of blade shape on the flow pattern. The comparison of the flow pattern (average velocity, turbulent kinetic energy, maximum energy dissipation rate, average shear rate and turbulent normal stress) has been presented on the basis of equal power consumption to characterize the flow generated by different impeller geometries. Comparisons of LDA measurements and CFD predictions have been presented. The good comparison indicates the validity of the CFD model. [Copyright &y& Elsevier]

Abstract: This paper discusses methods for the estimation of eddy diffusivity using the instantaneous velocity–time data measured in a bubble column reactor. In the first method, the analysis uses the eddy isolation methodology with a correction for the bubble–beam path interruption in the data. The correction is estimated from the observation of the time-varying data acquisition rate in the time series. The method is oblivious to the type of anemometer used for the data acquisition and is useful for all kinds of multiphase measurements. For validation of the results, we have proposed a strategy based on the synergistic combination of energy spectrum and the intermittency models for revealing different stages in the turbulent cascades. The method uses the actual scales in the cascade for the estimation of eddy diffusivity and hence such a combination has resulted in a robust validation tool. The comparison of the estimations based on the standard k– model and integral length scales is also discussed. [Copyright &y& Elsevier]

Abstract: A criterion for the transition from the homogeneous to the heterogeneous regime in a bubble column is developed based on the theory of linear stability. Hydrodynamics of bubble column is described by two-fluid model incorporating the interphase forces like drag force and added mass force. Added mass force affects the hydrodynamics of gas–liquid flows significantly and is formulated by taking into account the bubble deformation. A proper understanding of the nature of gas–liquid interface (clean or contaminated) is desired for the reliable predictions of the added mass coefficient. Data from the literature on the transition in bubble columns is critically analyzed. A good agreement has been obtained between the experimental transition gas hold-up and the predictions of the same obtained by the theory developed in this work. [Copyright &y& Elsevier]

Mass transfer studies were carried out in a bubble column using the chemical method. Catalytic oxidation of sodium sulfite was chosen for the studies and the corresponding specific rates of oxidation were obtained using a stirred cell. Laser Doppler anemometer (LDA) was used to measure the instantaneous velocities in the same stirred cell as well as in bubble columns (100 and 150 mm i.d.). An efficient algorithm based on the multiresolution analysis of the velocity-time data using wavelets was used for the isolation of data belonging to the gas and liquid phases. Eddy isolation model was used for the characterization of the eddy motion including the estimation of the energy dissipation rate. Using the knowledge of eddy motion, a methodology was developed for the prediction of true mass transfer coefficient (kL) in a stirred cell as well as in bubble columns. The predicted values of kL have been compared with the experimental values obtained by the chemical method. [Copyright &y& Elsevier]

• Detailed study of breakage unobscured by the complications of classification of fines. • Measurements of particle mass distributions show that a unimodal feed results in a trimodal product. • Mean size correlates with the specific energy input, independent of operating conditions. • A population balance model based on three mechanisms of breakage is gives a match with experiments. Jet milling is widely used in several industrial applications to produce fine particles with a narrow size distribution. We studied by means of experiments, the influence of grinding pressure, holdup and milling time on the performance of a spiral air jet-mill operating in a batch mode, with respect to the particles. The experimental results indicated that a uni-modal size distribution of feed particles yields trimodal particle size distributions. The data for variation of the mean particle size and specific surface area with energy supplied collapsed to a curve for all the cases studied, indicating that mean size is primarily dependent on the energy supplied. A population balance model for the dynamics of the batch milling process, based on empirical breakage functions is developed, which describes the experimental results quite well. The form of the breakage functions provides an insight into the mechanisms of breakage of particles. [ABSTRACT FROM AUTHOR]

• 3D CFD simulations of dividing manifold system using OpenFoam. • Relationship between turbulent parameters and flow mal-distribution. • Effect of vortex formation on flow distribution and pressure drop. • Design strategies for achieving uniform flow and minimum dissipation. The flow distribution in a manifold system is a critical design parameter which affects the performance of majority of the chemical equipment. The flow mal-distribution may lead to serious issues such as (i) reduced heat or mass transfer (ii) enhanced pressure drop (iii) high energy dissipation and (iv) creation of dead zones or hot spots. The objective of the present investigation is to identify potential design strategies for attaining uniform fluid flow, reduced pressure drops and minimum energy dissipation inside dividing manifold system. 3D CFD simulations using OpenFoam have been performed to study the effect of eight different design strategies on the extent of non-uniformity (ENU) and pressure drop inside manifold system. The results reveal the dominance of momentum effect near the inlet which results in reverse flow through the branched tubes near the inlet whereas maximum discharge occurs through the tubes near the closed end. The turbulent kinetic energy (k) and dissipation rate (ε) are very high near the T junction, decreases as the flow precedes in the downstream direction and vanishes completely near the closed end of manifold. This study for the very first time reveals the roles of turbulent parameters (k and ε) in controlling the flow mal-distribution and pressure drop inside manifold systems. The most effective design strategies for achieving maximum flow uniformity and minimum energy dissipation are (i) inclusion of perforated baffle which reduces the vortex formation and results in 95% reduction in E N U ¯ and (ii) converging header which results in 66% reduction in E N U ¯. [ABSTRACT FROM AUTHOR]

The primary objective of core catchers is to minimize the potential risk of core meltdown by stabilization and cooling of molten corium within the reactor containment for a prolonged period by strategically cooling it. Quenching of the high‐temperature melt by top flooding strongly depends on the timing of injection of the cooling water, that is, immediate flooding or delayed flooding. To understand this aspect, we conducted experiments in a simulated core catcher of an Indian advanced nuclear reactor. In the experiments, sodium borosilicate glass, as a mixture of corium and sacrificial material simulant, was melted at about 1200°C and cooled by water from the side of a simulated core catcher and then flooding at the top of the melt pool. Two experiments were conducted; the first one was done with immediate flooding at the top of the melt pool; the second was performed with delayed top flooding of 45 min gap. The results show that immediate flooding quenches the melt pool rapidly compared to delayed flooding. A stable crust formation at the top of the melt pool and wall adhesion is observed in the delayed flooding. It was also found that no ingression of the water in the molten pool occurred with delayed flooding. On the other hand, with immediate flooding, water was found to ingress into the melt pool due to gap formations between melt and core catcher sidewall, causing the melt to break up through eruption by ingressed water. This phenomenon resulted in the formation of sand‐type debris and in faster melt cooling. [ABSTRACT FROM AUTHOR]

The paper discusses alternative designs of receivers for parabolic trough collectors with smaller rim angles. The Half Insulation Filled Receiver (HIFR) and Linear Cavity Receiver (LCR) designs have been studied and optimized for minimum heat loss. Different combinations of insulations and annulus gasses have been evaluated. From this study, the best combination of insulation material and annulus gas and its pressure has been obtained for geometry optimization of the receiver. Configuration of SCHOTT PTR 70 receiver has been considered as the base case for this study. Heat losses from the HIFR and LCR for the microtherm – air (at 0.1 atm pressure) combination are the least being 255 W/m and 246 W/m respectively. The radius ratio (RR = D G /D P ) value of HIFR for this is 2.5 and insulation thickness for LCR receiver is 20 mm. HIFR and LCR show maximum optical efficiency at rim angle ψ = 45° whereas, for a conventional cylindrical receiver the same is true at rim angle ψ = 90°. Therefore, both the proposed receiver designs are expected to be suitable alternatives of evacuated receivers for parabolic trough collectors with smaller rim angle (around 45°). [ABSTRACT FROM AUTHOR]

Though bubble column reactors are widely used in industry, the present design practice is still closer to an art than a desired state of science because of the complexity of its fluid mechanics. The empiricism can be reduced by understanding detailed flow pattern, turbulence characteristics and turbulent structures and their effects on the performance such as mixing and axial mixing in both the phases and rate of heat and mass transfer. For this purpose, in the present work, CFD simulations have been undertaken by using standard k-ε, RSM and LES turbulence models. The cylindrical column having a computational height of H D = 900 mm with inside diameter of D = 150 mm was employed as a bubble column operated at three superficial gas velocities (20, 40 and 100 mm/s). The instantaneous three dimensional velocity field is obtained by means of two phase Eulerian-Eulerian Large Eddy Simulations (LES). The conservation equations for turbulent kinetic energy (k) and the turbulent energy dissipation rate (ε) have been derived from the two fluid governing equations using the Reynolds averaging procedure. This enabled accurate estimation of convective transport, diffusive transport, turbulent transport, production and dissipation of k and ε. These estimations have been compared with the modelled terms of the standard k-ε and Reynolds stress models. The difference in values gives an idea about the severity of assumptions made in these models. An attempt has been made to bring out the implications of simplifying assumptions. [ABSTRACT FROM AUTHOR]

Biomass is the most versatile feedstock for renewable energy and chemical production. Biochemical techniques such as fermentation and biomethanation have been developed extensively for converting biomass into bioethanol, biogas, and high‐value platform chemicals. However, the techno‐economic feasibility of the various biochemical techniques for the production of a range of biofuels and chemicals has not been fully consolidated in a review. This paper reviews the techno‐economic studies of biochemical conversion of biomass in a comparative fashion between feedstocks, treatment methods, and product types. The review starts with an overview of various biomass treatment approaches and the need for pre‐treatment for processing second‐generation feedstocks. This is followed by a review of the main biochemical conversion processes, offering insights into process stages, product yields and quality, as well as commercialization prospects and challenges. The various techno‐economic aspects of biomass conversion via biochemical techniques, such as conversion efficiency, production capacity, minimum selling price, capital cost, unit production cost, and profitability metrics, are critically reviewed. It was found that bioethanol and biogas are the most commercially viable products from the biochemical processing of biomass. The production of other biofuels and chemicals such as biobutanol, biohydrogen, furfural, volatile fatty acids, succinate, levulinic acid, and sugar alcohols via biochemical techniques is still largely limited by low conversion, frail microbial strains, cost of enzymes, and separation, and refining challenges. Overcoming these technical bottlenecks, and addressing the issues of feedstock price and supply security, are crucial for enhancing the overall techno‐economic attractiveness of biochemical processes for fuels and chemical production from biomass resources. © 2023 Society of Industrial Chemistry and John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Shell-and-tube heat exchanger has been extensively used in industrial and research fronts for more than a century. However, most of its design procedures are based on empirical correlations extracted from experimental data of long length shell and tube heat exchanger. In this paper, an attempt has been made to investigate the complex flow and temperature pattern in such a short shell and tube type heat exchanger, with and without baffles in the shell side. Heat exchangers of length by hydraulic diameter ratio between 7 < L / D h < 21 ( 0.15 m < D S < 0.6 m ) for unbaffled and L / D h < 7 ( D S = 0.09 m ) for baffled heat exchangers are analysed using CFD code OpenFOAM-2.2.0 for different mass flow rates. It was observed that the cross flow near the nozzle region has a significant contribution towards the heat transfer, hence the conventional heat transfer correlations do not apply to these short heat exchangers. Furthermore, a sensitivity study of turbulence models was performed and it was observed that the standard k – ε model gives best results for the velocity profile as well as heat transfer, provided average y + of the first node adjacent to the heat transfer surface is maintained greater than 15. The commonly used boundary conditions at the exit are not realistic, as it tends to give either incorrect flow and temperature fields, or the solution was found to diverge. Through a sensitivity study of the exit length, it was found that exit length to shell side velocity ratio of 2.5 is required for proper convergence. Finally the effect of flow field on shell side heat transfer coefficient and a comparison with analytical methods are presented. [ABSTRACT FROM AUTHOR]

In this work, effect of salt on cellulose pyrolysis behaviour using chemical percolation devolatilization (CPD) model has been investigated. The distributed activation energy model (DAEM) has been used to determine the kinetics of pure and NaCl-loaded cellulose pyrolysis in a thermogravimetric analyser (TGA). A number of these experiments were performed for a range of heating rates (5–100 °C min−1) and for different concentrations of NaCl in cellulose (0.25–2 mass/%) which showed a pronounced effect of salt on the primary pyrolysis reactions of cellulose. These data were then used in a two-component DAEM to calculate the kinetic parameters for the chemical percolation devolatilization model (CPD) for both pure and NaCl-loaded cellulose. The optimized kinetic parameters for the pure and NaCl-loaded cellulose were used in the CPD model, and the product yield was predicted. The CPD predictions showed that the fraction of gases and char in the pyrolysis products increased in the presence of salt. The amount of char for pure cellulose was 6.97% which increased to 16.4% in the presence of 0.25% NaCl. The amount of gases produced as a result of pure cellulose pyrolysis was 9.63% and this amount increased with addition of NaCl to 22.85% and then further increases with an increase in NaCl concentration. Amount of tar produced for pure cellulose was 83.4%, and this amount reduced to 60.7% with addition of 0.25% NaCl. The presence of NaCl accelerates cellulose decomposition and favours the formation of gases and char. [ABSTRACT FROM AUTHOR]

Absorption column occupies a dominant place in the manufacturing of nitric acid. A mathematical model has been developed for the absorption of NO x gases using air and enriched oxygen in packed columns. For the first time, this paper considers the kinetics of nitrous acid decomposition and oxidation in the liquid phase. Till date, the published literature considers the instantaneous decomposition of nitrous acid in the liquid phase. The same case has also been considered in this paper as the base case. In addition, three more cases have been considered: (1) decomposition and oxidation kinetics of nitrous acid with a typical concentration of oxygen which is usually employed in nitric acid plants (2) complete decomposition and oxidation with enriched oxygen (3) complete conversion of nitrous to nitric acid by liquid phase nitrous acid oxidation. Enriched oxygen results in substantial reduction in column volume and/or the operating pressure. The enriched oxygen enhances the rate of nitrous acid oxidation (to nitric acid) and hence the nitrous acid available for decomposition reduces throughout the column. This implies that lesser quantity of nitric oxide gets desorbed to the gas phase. This feature has crucial implication because the need for gas phase oxidation of NO reduces and hence the absorption column becomes compact due to reduction in oxidation volume. Thus, the enriched oxygen plays a significant role in process intensification and equipment miniaturization. Additionally, the liquid phase oxidation of nitrous acid carried out in a suitable oxidizer was found to give substantial additional reduction in the column volume. For packed column absorber, the effects of design parameters viz. column diameter, heights of packed and empty sections, and operating parameters such as pressure and temperature have been examined. [ABSTRACT FROM AUTHOR]

Passive systems are being examined for the future Advanced Nuclear Reactor designs. One of such concepts is the Passive Moderator Cooling System (PMCS), which is designed to remove heat from the moderator in the Calandria vessel passively in case of an extended Station Black Out condition. The heated heavy-water moderator (due to heat transferred from the Main Heat Transport System (MHTS) and thermalization of neutrons and gamma from radioactive decay of fuel) rises upward due to buoyancy, gets cooled down in a heat exchanger and returns back to Calandria, completing a natural circulation loop. The natural circulation should provide sufficient cooling to prevent the increase of moderator temperature and pressure beyond safe limits. In an earlier study, a full-scale 1D transient simulation was performed for the reactor including the MHTS and the PMCS, in the event of a station blackout scenario ( Kumar et al., 2013 ). The results indicate that the systems remain within the safe limits for 7 days. However, the flow inside a geometry like Calandria is quite complex due to its large size and inner complexities of dense fuel channel matrix, which was simplified as a 1D pipe flow in the aforesaid analysis. In the current work, CFD simulations are performed to study the temperature distributions and flow distribution of moderator inside the Calandria vessel using a three-dimensional CFD code, OpenFoam 2.2.0. First, a set of steady state simulation was carried out for a band of inlet mass flow rates, which gives the minimum mass flow rate required for removing the maximum heat load, by virtue of prediction of hot spots inside the Calandria. Next, a pseudo transient simulation was carried out by taking the power and mass flow rate at different time instants to the Calandria from the 1D analysis, as an input to the 3D steady state simulations. The results indicate that temperature hotspots are located at the top center of the fuel channel matrix depending on the operating regime. Furthermore, it was observed that for a period of 7 days the temperatures are well within the safe limits. [ABSTRACT FROM AUTHOR]

In designing a gas-liquid bubble column, an accurate estimation of the gas hold-up is important. A generalized Machine learning-based data-driven methodology is presented to predict the gas hold-up, with the industrial-scale application for designing a bubble column. The predictions by machine learning methods such as support vector regression (SVR), random forest (RF), extra trees (EXT), and artificial neural network (ANN) have been compared. The methodology adopted for the prediction of gas hold-up with the help of independent parameters such as column diameter, column height, sparger design, sparger location, percentage free area, superficial gas, liquid velocity, pressure, temperature, the density of gas & liquid, viscosity of gas & liquid, surface tension has been presented. An extensive set of experimental data (4042 data points) has been extracted from the literature covering various design and operating parameters. The performance of the machine learning methods has been compared using mean absolute percentage error (MAPE), mean square error (MSE), and determination coefficient/prediction accuracy (R-square). Based on these statistical parameters, with 97% of prediction accuracy (R-square), MSE = 0.00031, and MAPE = 7.9, the performance of the extra tree is found to be most suitable for the prediction of the gas hold-up. [Display omitted] • Developed a data-driven methodology for prediction of gas holdup in bubble column. • Predictions by machine learning methods: SVR, RF, EXT and ANN have been compared. • Statistical parameters: MSE, MAPE and R-square have been used for comparison. • Performance of data-driven extra tree regressor was found to be best. • Developed methodology can be applied for prediction of gas holdup in the industry. [ABSTRACT FROM AUTHOR]

Concentration solar power (CSP) systems convert solar radiation to heat and use heat engines to convert the heat to electricity. The solar receiver over which the solar radiation is concentrated and converted to heat is the most important part of the CSP. To attain maximum efficiency, the receiver in the CSP systems needs to be coated with an efficient selective solar absorber coating. In recent years, a lot of research has been focused on solar selective coatings. This has resulted in the synthesis of novel coatings that have high thermal and chemical stability, long term durability, and excellent solar selectivity making them suitable for solar thermal applications. This report reviews various solar selective coatings based on transition metals and their compounds. Various failure mechanisms are discussed in detail along with suggested prevention methods. Several thermal stability and durability tests are reported with their benefits and limitations. The effect of long-term durability on the levelized cost of coating is also discussed. Finally, we list some excellent systems and explore different ways of improving the thermal stability for SSCs, thus providing a reference for the design and optimization of new SSCs. [ABSTRACT FROM AUTHOR]

Stirred tank reactor systems often employ jackets/limpets, single/multiple internal coils for heat transfer. Internal coils are used for heat removal in the case of exothermic reaction systems. The desired heat transfer area can be accurately estimated based on the overall heat transfer coefficient (HTC) considering the process side, utility side, and wall conduction resistance. For estimation of process side HTC, a large number of empirical/semi‐empirical correlations are available in the published literature. These correlations account for impeller Reynolds number, fluid Prandtl number, viscosity ratio, dimensionless geometric factors, and so forth. However, all the factors over the entire range of industrial operation do not get covered in a single study, hence the predictions may involve significant errors for industrial cases. The advanced artificial intelligence‐based techniques can provide unified correlation based on the experimental data set extracted from various investigations and the predictions can be accurate and reliable. In the present work, three techniques random forests (RF), artificial neural networks with Bayesian optimization (ANN‐BO), and support vector regression with grid search optimization (SVR‐GS) have been used with a data set of 1297 points with R2 values of 0.995, 0.972, 0.990 on the training data set and 0.962, 0.945, 0.905 respectively on the testing data set. Furthermore, the statistical measures show that RF provides a better fit as compared with other AI models, so RF is used for parametric sensitivity analysis. The permutation feature importance shows that the agitation speed, impeller dimensions, and positioning has a significant impact on the HTC. [ABSTRACT FROM AUTHOR]