Ribbed Duct Research Articles

Improving inactivation performance of in-duct ultraviolet germicidal irradiation system with ribbed duct walls by optimizing UVC distribution

The in-duct ultraviolet germicidal irradiation system (ID-UVs) with ribs has been revealed to be effective in reducing the risk of biological contamination, and optimization of its UV radiation distribution is expected to further improve the inactivation performance. In this study, the Critical Survival Fraction Probability (CSFP) and Maximal Bearable UVC Dose (MBUD) methods were synergistically employed as evaluation indicators, and Computational Fluid Dynamics (CFD) was used to optimize the lamp array, duct shape, liner reflectivity and number of UV lamps. Results indicated that the optimal lamp array was positioned at 0° to the airflow direction, and square-section ducts were preferred over circular-section ducts. Ribs obstructed the transmission of specular reflected radiation, while diffuse reflection helps to overcome the obstruction. Reducing the number of lamps alone from four to one increased the volume-averaged irradiance by 14.4–25.3 % in the main irradiated region. For renovation projects, ID-UVs equipped with one lamp and galvanized steel ribs proved optimal, exhibiting increased inactivation efficiencies of 39.7 % (CSFP) and 239.4 % (MBUD) against highly UVC-susceptible MS2 Bacteriophage (K = 0.038 J/m2), and up to 341.1 % (MBUD) against low UVC-susceptible MS2 Bacteriophage (K = 0.028 J/m2) compared to the ID-UVs without ribs and featuring four lamps. Moreover, in comparison to conventional conditions that provide equivalent air-cleaning levels, the optimal ID-UVs allowed for a 60–70 % increase in air handling capacity while achieving at least an 83.6 % reduction in annual energy costs. These findings contribute towards advancing practical applications of ID-UVs with ribbed duct walls.

Thermal behaviors and flow profiles in a square channel fitted with X–V modified ribs: A numerical analysis

A novel design of “X–V rib” turbulators is proposed to improve fluid blending and to perturb thermal boundary layer (TBL) in a heat exchanger (HX) duct. Better fluid blending and more highly disturbed TBL are the two important factors for the improvement of HX performance. Effects of X–V rib geometrical parameters including rib height ratios (b/H = 0.05–0.20), rib arrangements (V-tip pointing upstream called “V-Upstream (VU)” and V-tip pointing downstream called “V-Downstream (VD)”) and rib types (I, II and III) on air stream and thermal mechanisms are numerically investigated in a 3D model. The numerical model is solved with the finite volume method (a commercial code) and a commercial program. Laminar air flow (inlet conditions with Re = 100–2000) is a selected range of the present investigation. Firstly, the created numerical model for the ribbed duct is validated with two significant topics: 1. Plain duct validation and 2. Optimum grid elements (grid independence). The validated results show that the ribbed-duct model has great reliability to simulate air stream and thermal characteristics. According to the numerical results, the disturbed TBL is obviously found in all rib types. Moreover, it is observed that the core flow disturbance occurs and improves fluid blending. The outcomes from the present investigation point out that the knowledge about flow structure and thermal mechanism are important guidelines for the development of vortex turbulators and HX improvement. For the thermal assessments, it is found that the best Nusselt number ratio (Nu/Nu0) is 11.80 for the type III X–V rib with b/H = 0.20 in the VD-direction. Additionally, the maximum thermal enhancement factor within our investigated range is 3.48 at Re = 2000, b/H = 0.20, type II X–V rib in the VU-direction.

RANS Modeling of Turbulent Flow and Heat Transfer in a Droplet-Laden Mist Flow through a Ribbed Duct

The local structure, turbulence, and heat transfer in a flat ribbed duct during the evaporation of water droplets in a gas flow were studied numerically using the Eulerian approach. The structure of a turbulent two-phase flow underwent significant changes in comparison with a two-phase flow in a flat duct without ribs. The maximum value of gas-phase turbulence was obtained in the region of the downstream rib, and it was almost twice as high as the value of the kinetic energy of the turbulence between the ribs. Finely dispersed droplets with small Stokes numbers penetrated well into the region of flow separation and were observed over the duct cross section; they could leave the region between the ribs due to their low inertia. Large inertial droplets with large Stokes numbers were present only in the mixing layer and the flow core, and they accumulated close to the duct ribbed wall in the flow towards the downstream rib. An addition of evaporating water droplets caused a significant enhancement in the heat transfer (up to 2.5 times) in comparison with a single-phase flow in a ribbed channel.

Numerical predictions of flow topology and heat transfer in a square duct with staggered V-ribs

Performance augmentation of a square duct heat exchanger by a passive technique is numerically investigated. V-pattern ribs are used as vortex turbulators to enhance heat exchanger performance. The V-rib arrangement is designed with two important factors: 1. to increase the heat transfer coefficient by disturbing thermal boundary layers and to expedite fluid mixing and 2. to remain or decline the friction loss across the ribbed duct when compared with a typical in-line V-rib arrangement. The effects of rib heights (b/H = 0.05–0.20), rib pitch (P/H = 1–2) and flow paths (+x and -x) on fluid structure and thermal characteristics are studied within a laminar flow regime. Numerical analysis using the finite volume method (FVM) is chosen to predict the fluid structure and thermal mechanisms within the ribbed duct. Validating topics: smooth duct validation and grid independence, are firstly investigated. Simulation results are analyzed in forms of fluid structure and thermal behaviors. Performance assessment (thermal enhancement factor, friction factor ratio and Nusselt number ratio) within the tested section are also concluded. The simulation results show that the best TEF is found to be around 4.5, while the maximum Nusselt number ratio is around 19.39.

Particle image velocimetry measurements of turbulent flows in rotating square ribbed ducts

This paper investigates the flow characteristics along a ribbed square duct using time-resolved particle image velocimetry. The Reynolds number, defined by the main flow velocity (1.82 m/s) and the hydraulic diameter (80 mm), is 10 000. The rotation numbers are 0, 0.13, 0.26, 0.39, and 0.52. The blockage and aspect ratios of the ribbed square duct are 0.1 and 1, respectively. As the rotation number increases, the recirculation zone on the suction side becomes larger, and the reattachment point moves downstream. The opposite behavior is observed on the pressure side, and this trend develops along the duct. Furthermore, the flow characteristics between different pairs of ribs along the duct vary greatly at the same rotation number. For the downstream ribs, the main stream velocity pattern is deflected more to the pressure side, where both the recirculation zone and the extreme region of Reynolds shear stress are larger than on the suction side.

Extension of the boundary integral method for different boundary conditions in steady-state Stokes flows

PurposeThe boundary integral method (BIM) provides unparalleled computational efficiency for solving problems wherever it is applicable. For Stokes flows, the BIM in its current form can only be applied to a limited class of problems that generally comprises boundaries with either specified velocity or stress. This study aims to radically extend the applicability by developing a general method within the BIM framework that can handle periodic, symmetry, zero normal-velocity gradient and the specified pressure boundary conditions. This study is limited in scope to steady-state flows.Design/methodology/approachThe proposed method introduces a set of points near the boundary for the symmetry, zero normal-velocity gradient and specified pressure boundary conditions. The formulation for the first two boundary conditions use a spatial discretization procedure within the BIM framework to arrive at a set of equations for the unknowns. The specified pressure boundary condition warrants the decomposition of the unknown traction term into simpler components before the discretization procedure can be executed. Though the new methodology is illustrated in detail for two-dimensional rectangular domains, it can be generalized to more complex three-dimensional cases. This will be the subject for future investigations.FindingsThe current endeavor has successfully demonstrated the incorporation of the above boundary conditions through simple Stokes flow problems like plane channel flow, flow through ribbed duct and plane wall jet. The predicted results matched adequately with either analytical solutions or with available literature data.Originality/valueTo the best of the author’s knowledge, this is the first time that the exit boundary conditions like zero normal-velocity gradient and specified pressure have been formulated within the BIM for Stokes flows. These boundary conditions are extremely powerful and the current research initiative has the potential to dramatically increase the range of applicability of the BIM for Stokes flow simulations.

Machine Learning for the Development of Data-Driven Turbulence Closures in Coolant Systems

Abstract This work shows the application of Gene Expression Programming to augment RANS turbulence closures for flows though complex geometries. Specifically, an optimized internal cooling channel of a turbine blade, designed for additive manufacturing. One of the challenges in internal cooling design is the heat transfer accuracy of the RANS formulation in comparison to higher fidelity methods, which are still not used in design on account of their computational cost. However, high fidelity data can be extremely valuable for improving current lower fidelity models and this work shows the application of data-driven approaches to develop turbulence closures for an internally ribbed duct. Different approaches are compared and the results of the improved model are illustrated; first on the same geometry, and then for an unseen predictive case. The work shows the potential of using data-driven models for accurate heat transfer predictions even in non-conventional configurations and indicates the ability of closures learnt from complex flow cases to adapt successfully to unseen test cases.

Direct numerical simulation of turbulent duct flow with inclined or V-shaped ribs mounted on one wall

In this research, highly disturbed turbulent flow of distinct three-dimensional characteristics in a square duct with inclined or V-shaped ribs mounted on one wall is investigated using direct numerical simulation. The turbulence field is highly sensitive to not only the rib geometry but also the boundary layers developed over the side and top walls. In a cross-stream plane secondary flows appear as large longitudinal vortices in both inclined and V-shaped rib cases due to the confinement of four sidewalls of the square duct. However, owing to the difference in the pattern of cross-stream secondary flow motions, the flow physics is significantly different in these two ribbed duct cases. It is observed that the mean flow structures in the cross-stream directions are asymmetrical in the inclined rib case but symmetrical in the V-shaped rib case, causing substantial differences in the momentum transfer across the spanwise direction. The impacts of rib geometry on near-wall turbulence structures are investigated using vortex identifiers, joint probability density functions between the streamwise and vertical velocity fluctuations, statistical moments of different orders, spatial two-point autocorrelations and velocity spectra. It is found that near the leeward and windward rib faces, the mean inclination angle of turbulence structures in the V-shaped rib case is greater than that of the inclined rib case, which subsequently enhances momentum transport between the ribbed bottom wall and the smooth top wall.

Large-Eddy Simulation Investigation of Modified Rib Shapes on Heat Transfer in a Ribbed Duct

Abstract Internal cooling of the gas turbine blade is critical for the durability of the blade material. One of the ways to accomplish this is by passing coolant through serpentine passages roughened with surface elements to enhance the heat transfer. In the present study, the traditional square rib (SQ-rib) placed normal to the flow direction is modified to a backward-facing step rib (BS-rib) and a forward-facing step rib (FS-rib). Large-eddy simulation (LES) is carried out for a square duct at Reb = 20000. Results show that the modified rib shapes result in substantial increase in heat transfer over the square rib with only a marginal increase in flow losses. The BS-rib shape produces the highest heat transfer augmentation followed by the FS-rib. The overall heat transfer augmentation for the BS-rib and FS-rib is 18% and 10% larger than the SQ-rib, respectively. Thermal-hydraulic performance is enhanced by 15%.

The effect of rotation on internal cooling of turbine blades with modified ribs using Large Eddy Simulation (LES)

The cooling of the turbine blade plays a vital role in increasing the durability and thermal efficiency of the turbine. Blade cooling is achieved by passing coolant through internal channels roughened with rib turbulators. In this work, the effect of rotation on heat transfer of a square ribbed duct is studied using Large Eddy Simulation (LES) at a Reynolds number based on the average bulk velocity of 20,000. The study compares the heat transfer performance of the square duct roughened with square (SQ) rib, Backward step (BS) rib, and Forward step (FS) rib at three rotation numbers and In the range of rotation numbers investigated, the overall Nusselt number for SQ, BS, and FS ribs is about 14%, 20%, and 16%, respectively, higher than a non-rotating duct. The overall heat transfer augmentation factor between 3.10 and 3.25 for in the duct with BS-ribs is approximately 24% higher than SQ-ribs, while the friction coefficient is higher only by 10–15%.

Direct numerical simulation of turbulent heat transfer in a square duct with transverse ribs mounted on one wall

Turbulent heat transfer in a ribbed square duct of three different blockage ratios are investigated using direct numerical simulation (DNS). The results of ribbed duct cases are compared with those of a heated smooth duct flow. It is observed that owing to the existence of the ribs and confinement of the duct, organized secondary flows appear as large streamwise-elongated vortices, which intensely interact with the rib elements and four sidewalls and have profound influences on the transport of momentum and thermal energy. This study also shows that the drag and heat transfer coefficients are highly sensitive to the rib height. It is observed that as the rib height increases, the impinging effect of the flow on the windward face of the rib strengthens, leading to enhanced rates of turbulent mixing and heat transfer. The influence of sidewalls and rib height on the turbulence structures associated with temperature fluctuations are analyzed based on multiple tools such as vortex swirling strengths, temporal auto-correlations, spatial two-point cross-correlations, joint probability density functions (JPDF) between the temperature and velocity fluctuations, statistical moments of different orders, and temperature spectra.

Fully Coupled Large Eddy Simulation-Conjugate Heat Transfer Analysis of a Ribbed Cooling Passage Using the Immersed Boundary Method

Abstract The study focuses on evaluating fully coupled conjugate heat transfer (CHT) simulation in a ribbed cooling passage with a fully developed flow assumption using large eddy simulation (LES) with the immersed boundary method (IBM-LES-CHT). The IBM-LES and the IBM-CHT frameworks are validated by simulating purely convective heat transfer in the ribbed duct, and a laminar boundary layer flow over a 2D flat plate with heat conduction, respectively. For the main conjugate simulations, a ribbed duct geometry with a blockage ratio of 0.3 is simulated at a bulk Reynolds number of 10,000 with a conjugate boundary condition applied to the rib surface. The nominal Biot number is kept at 1, which is similar to the comparative experiment. It is shown that the time scale disparity between turbulent fluid flow and heat conduction in solid can be overcome by using an artificially high solid thermal diffusivity. While the diffusivity impacts the instantaneous fluctuations in temperature and heat transfer, it has an insignificant effect on the predicted Nusselt number. Comparison between IBM-LES-CHT and iso-flux heat transfer simulations shows that the iso-flux case predicts higher local Nusselt numbers at the back face of the rib. Furthermore, the local Nusselt number augmentation ratio (EF) predicted by IBM-LES-CHT is compared with experiment and another LES conjugate simulation. The present LES calculations predict higher EFs on the leading face of the rib and show a different trend at the trailing face when CHT is activated.

Experimental and Numerical Investigation of Flow and Heat Transfer in Stationary Two-Pass Rectangular Duct (AR = 1:2) With Continuous and Broken V-Shaped Ribs

Abstract The combined experimental and large eddy simulations (LES) were performed in the stationary two-pass duct of aspect ratio (AR) 1:2. The experiments were conducted with three different rib arrangements, namely, 60 deg V, 60 deg V–IV, and broken 60 deg V–IV ribs, and the analysis was carried out with Reynolds numbers of 45,000, 60,000, and 75,000. The infrared thermography (IRT) technique is employed to obtain the local temperature distribution on heated smooth and ribbed surfaces. In all ribbed cases, the copper ribs are glued to the heated surface with a fixed rib height-to-hydraulic diameter (e/Dh) ratio of 0.125 and the rib pitch-to-height ratio (P/e) of 10 and 5 for continuous and broken ribs, respectively. In addition, the LES turbulence model was adopted for carrying out a simulation to understand the flow and heat transfer behavior in ducts populated with all three V-shaped ribs. The comparison of the time-averaged thermal fields generated using computations has been made with experimentally measured Nusselt numbers, friction factors, thermal performance factor (TPF), and Reynolds analogy performance parameter (RAPP) for all cases. The overall thermal performance factor was found to be quantitatively within 8.0–10.66% between experimental and numerical results. Among all the cases, the 60 deg V–IV ribbed duct provides the best TPF and RAPP than the other two ribbed ducts, whereas the smooth duct shows poor TPF.

Assessing Wall-Modeled Large-Eddy Simulation for Low-Speed Flows with Heat Transfer

This paper reports wall-modeled large-eddy simulation (WMLES) results of low-speed turbulent flows in a plane channel, in ribbed ducts, and around a film cooling jet. We compare our WMLESs to Pirozzoli et al.’s direct numerical simulations (DNSs) of low-speed plane channel flow (Pirozzoli, S., Bernardini, M., and Orlandi, P., “Passive Scalars in Turbulent Channel Flow at High Reynolds Number,” Journal of Fluid Mechanics, Vol. 788, Feb. 2016, pp. 614–639), our own DNSs of ribbed ducts with various pitch-to-height ratios, and Milani et al.’s WRLES and water-tunnel experiment of film cooling (Milani, P. M., Gunady, I. E., Ching, D. S., Banko, A. J., Elkins, C. J., and Eaton, J. K., “Enriching MRI Mean Flow Data of Inclined Jets in Crossflow with Large Eddy Simulations,” International Journal of Heat and Fluid Flow, Vol. 80, Dec. 2019, Paper 108472). We consider Mach number effects below the often-quoted low-Mach-number limit of . The results show that the Mach number has significant effects on the normalized mean temperature profile, even below the often-quoted low-Mach-number limit of , due to the associated viscous heating. In addition, we compare the first-grid point implementation (FGI) and the third-grid point implementation (TGI) of the equilibrium wall model. We show that, by placing the large-eddy-simulation/wall-model matching location away from the wall, TGI practically reduces the near-wall resolution seen by the wall model, which in turn leads to underperformance of the wall model. By considering three types of flows with increasing levels of complexities, the objective of this study is to systematically assess WMLES in terms of its ability to predict heat transfer for low-speed flows. For the flows considered here (that is, plane channel, ribbed duct, and film cooling), we show that WMLES with FGI is able to accurately model heat transfer at a much more reduced cost than WRLES and DNS.

Measurements of Heat Transfer and Fluid Flow in A Rectangular Duct with Different Types of Rib-Turbulators.(Dept.M)

Experiments are conducted to study the heat transfer and friction in a rectangular duct roughened by arrays of alternate ribs (square-, triangle-. and circle-shaped ribs). The Reynolds number Re based on duct hydraulic diameter rages from 12.000 to 60.000. whereas the rib pitch-to-height ratio (P/h) varies from 6 to 18. To understand the mechanism of heat transfer enhancement the measurements of streamwise mean velocities are also conducted in the square-ribbed duct. The results show that the flow of air through the ribbed duct becomes periodically fully developed after the third rib from the duct inlet. The data indicate also that the triangular type ribs have a substantially higher heat transfer performance than any other ribs in the studied range. The experimental results show that the heat transfer enhancement factors are greater than the friction factor ratios associated with the use of rib-turbulators in the rectangular ducts. Also, the rib conductivity affects significantly on the heat transfer enhancement factor. Correlations for friction and heat transfer in square-ribbed ducts are developed to account for rib spacing to height ratio and flow Reynolds number.

Computational and experimental studies on the development of an energy-efficient drier using ribbed triangular duct solar air heater

Triangular duct cross-section is introduced for solar air heater (SAH) of an indirect type of solar dryer (ITSD). Using computational study, the thermo-hydraulic performance of triangular duct SAH with inclined ribs for varying rib inclination (30° <α < 75°) in the turbulent flow regime (5000 < Re < 17500) is studied. With the rib configuration providing maximum thermos-hydraulic performance, a ribbed rectangular duct SAH is designed, and the performance of the same is compared to the former for similar heat input. Results show that the ribbed (α = 45°) triangular duct has 17% higher effectiveness compared to the latter and 79% when compared to smooth SAH. Ribs in triangular duct solar air heater facilitate the increase in temperature even in the core of the duct, delivering the air at 6 K additional temperature relative to a rectangular ribbed duct for same heat input and flow Re. The superiority of the ribbed triangular SAH is further confirmed by studying the drying characteristics of Okra and two variants of banana, namely Nendran and Robusta for the maximum temperature obtained at the outlet of the respective SAH. Various thin layer drying models available in the literature were analyzed, and Modified page model represented the drying behaviour with R2 = 0.99. For ITSD, ribbed triangular duct SAH exhibits a maximum of 60.3% reduction in moisture ratio with a maximum increase of 97.9% increase in average values of diffusivity coefficient confirming that it is an energy-efficient design for an ITSD.

Insight in thermal and fluid-dynamic properties of ribbed ducts by means of a novel clustering method

The analysis of experimental results on heat transfer by forced convection in diverse ribbed ducts showed that different geometries lead to comparable thermal and fluid-dynamic performances. Moreover, no evident layout has been observed in data, and therefore a statistical clustering analysis is performed to detect the rationale, if any, underlying experimental results. A novel, ad-hoc developed technique is used to disengage the clustering from the data scaling and to account for the measurement uncertainty, consisting of an agglomerative procedure, based on the definition of dynamically-changing bounding boxes, whose size depends on the Nusselt number and the pumping power. Additional informations, such as the the relevance of the diverse geometric parameters and the persistence of similarity among configurations over a range of operating conditions, can be retrieved by means of the developed technique. The described method is applied to a large dataset, obtained during an experimental campaign carried on at ThermALab of Politecnico di Milano, aimed at identifying the Nusselt number and the friction factor for diverse-rib configurations in a large-aspect ratio channel with low-Reynolds flows. The considerations originated from of the results of the clustering analysis suggest the existence of an underlying structure, pointing to a possible unique parameter, termed “generalized blockage”, which is possibly able to describe the global effect of the ribs geometry on forced convection.

Numerical Simulations of the flow field within a 1:10 Aspect Ratio duct at intermediate Re

Numerical simulations are carried on to study the fluid-dynamical features of a smooth duct with aspect ratio of 10. The duct is operated with an incompressible, newtonian fluid, whose Reynolds number, computed over the hydraulic diameter and bulk velocity, ranges from 470 to 14500, encompassing laminar and turbulent flow. To capture the details of all the flow scales, Direct Numerical Simulations are performed, by means of a code developed at Politecnico di Milano. The adopted code is a finite-difference, structured grid solver, that includes a mass flow rate correction. The latter guarantees high accuracy in the calculation of unsteady flows, or during the transition to turbulent regime, and it allows to check the consistency of numerical results.Both global parameters –including the friction factor and the identification of the laminar-to-turbulent transition and local flow features, e.g., corner vortexes, are investigated and presented in this work. Preliminary analyses agree fairly well with literature data and with experimental results obtained at ThermALab of Politecnico di Milano.The final goal of this work, including a deep integration between the numerical and the experimental setup, is to carry on detailed investigations of the fluid-dynamical and thermal characteristics of ribbed ducts, in the perspective of heat transfer enhancement and pressure drop reduction.

A STUDY OF PARTICLE DEPOSITION ON A RIBBED DUCT WALL WITH THE EFFECT OF PARTICLE CONCENTRATION

A STUDY OF PARTICLE DEPOSITION ON A RIBBED DUCT WALL WITH THE EFFECT OF PARTICLE CONCENTRATION

Analysis of heat transfer on turbulence-generating ribs using dynamic mode decomposition

Ducts with turbulence-promoting ribs are common in heat transfer applications and can often found in the form of serpentine channels in gas turbine blades. This study uses a recent modal extraction technique called Dynamic Mode Decomposition (DMD) to determine mode shapes of the spatially and temporally complex flowfield inside a square ribbed (45 degree) duct at Reynolds number 32,400. One subject missing from current literature is a method of directly linking a mode to a certain engineering quantity of interest. Presented is a generalized methodology for producing such a link utilizing the data from the DMD analysis. Exciting the modes which are identified may cause the flow to change in such a way to promote the quantity of interest, in this case, heat transfer. This theory is tested by contouring the walls of the duct using the extracted mode shapes.An initial, unmodified geometry provides a baseline for comparison to later contoured models. The initial case is run as a steady-state RANS model. LES generates the necessary data for the DMD analysis. Several mode shapes extracted from the flow are applied to the duct walls and run again in the RANS model, then compared to the baseline, and their relative performance examined.