Local Nusselt Number Distribution Research Articles

Jet array impingement heat transfer in a rectangular cavity with effusion holes

Various researchers have studied jet array impingement heat transfer in impingement/effusion cooling systems. However, there is a lack of research on impingement/effusion cooling systems installed within rectangular cavities that focus on the impact of the proximity of the jet hole to the cavity sidewalls on cooling performance. The main objective of this study is to investigate the flow and heat transfer characteristics of jet array impingement with effusion holes in a rectangular cavity, considering various spacings between the cavity sidewalls and the outermost jet hole. The design parameters in this study include the ratio of the jet hole pitch to jet hole diameter of 7.1, 10.0, and 16.7, and the ratio of the distance between the jet and impingement plates to jet hole diameter of 2, 6, and 10, with the Reynolds number based on the jet hole diameter ranging from 2500 to 15,000. Heat transfer characteristics in the stagnation region and wall jet region were examined using local Nusselt number distributions on the impingement surface, measured by liquid crystal thermography. The local Nusselt number was high in the stagnation region and decreased radially from the stagnation region as the wall jet region formed. The closer the outermost jet hole is to the sidewall, the higher the Nusselt number on the impingement surface near the sidewall. Moreover, the flow structure in the rectangular cavity was numerically investigated, and the velocity vectors and streamlines showed that primary and secondary vortices were generated in the middle of two neighboring jets and near the sidewall, respectively. This study also assessed previous average Nusselt number correlations. Based on experimentally determined average Nusselt number data with 54 center unit cells and 1296 side unit cells, new correlations to predict the average Nusselt number on the impingement surface in a rectangular cavity with effusion holes were developed.

Experimental and numerical studies of detailed heat transfer and flow characteristics in the rib turbulated annulus of a double pipe heat exchanger

An enhancement in the overall heat transfer between the fluids in a Double pipe heat exchanger (DPHE) can be achieved through various active or passive techniques. The present study focuses on the passive heat transfer enhancement in the annular region of a DPHE by incorporating circular ribs on the outer surface of the inner pipe. The primary objective is to investigate the influence of the ribs on the local distribution of heat transfer and fluid flow in the annulus of the heat exchanger. The local Nusselt number (Nu) distribution was determined experimentally using a test section specially designed with a provision to measure the wall temperature distribution of the inner pipe with an IR thermal imaging camera. The pressure drop across the channels with the ribs was also measured in the experimental studies. Numerical studies are employed to gain insights into the flow characteristics in the annular region in the presence of ribs. Studies comprise various rib diameters (e/Dh = 0.08, 0.14), rib-to-rib pitches (P/e = 6, 12, 18), and Reynolds number values (Re = 5000, 7500, and 10000). The heat transfer behaviour from the experimental study is discussed based on the flow mechanism observed using numerical techniques. The flow separation and reattachment due to the rib and the induced recirculation zone cause large variations in the Nusselt number distribution between the ribs. The peak Nusselt number value between adjacent ribs is observed at the location of reattachment of the flow to the ribbed wall. Acquired data also shows that the flow and heat transfer characteristics differ significantly with changes in the P/e ratio. The rib configuration with e/Dh = 0.14 and P/e = 12 is identified as optimal, achieving a balanced improvement in heat transfer while showing a relatively lower pressure drop.

Experimental study on the effect of sound waves on the heat transfer characteristics of heated pipes

This study has experimentally investigated the heat transfer characteristics of a heated steel pipe (diameter = 20 mm) placed horizontally in a vertical experimental cylindrical pipe under longitudinal sound waves. The ranges of sound parameters used in this study are as follows: sound frequency f = 210–1000 Hz, and sound pressure level SPL = 125–135 dB. The heat transfer coefficient and Nusselt number of the cross- section of the heated pipe are used to describe the influence of different sound parameters on the heat transfer behavior of the heated pipe. The results show that under the action of low-frequency strong sound wave (f < 230 Hz), the distribution of local Nusselt number of heated pipe decreases (0-90°) at first and then increases (90-180°), while under the action of high-frequency strong sound wave (f > 500 Hz), the distribution of local Nusselt number first increases and then decreases. The relative Nusselt number decreases exponentially with the increase of the Strouhal number. In addition, the oscillating flow induced by sound waves is greatly influenced by the natural convection. The local enhancing or weakening effects of strong sound waves with different parameters on the heat transfer in a hot steel pipe are related to the characteristics of local flow.

Local Heat Transfer Distribution On Thin Metal Foil Impinged by Array of Free Surface Jets

Abstract The present study focuses on the measurement of the local heat transfer distribution of a smooth flat plate impinged by an array of free surface jets on a thin metal foil. Local Nusselt number distributions are measured for a fixed jet diameter (d = 3 mm). The jet arrays consist of perfectly round apertures arranged in a square pattern, with a uniform spacing of 4d between adjacent jets in both the streamwise and spanwise direction. A wide range of Reynolds numbers varying from 1000 to 12500 are covered in this study. The nozzle to plate spacing (z/d) is varied between 1 to 10. The effect of the Reynolds number and nozzle to plate spacing on the local and spanwise average Nusselt number distributions are studied. The local Nusselt number exhibits an increase with nozzle-to-plate spacing within the low Reynolds number range (Re = 1000 to 2500). However, for Reynolds numbers exceeding 2500, the influence of nozzle-to-plate spacing on Nusselt number distributions remains negligible up to a nozzle-to-plate distance (z/d) of 5. Beyond this point, there is a gradual decrease in the Nusselt number value. The Nusselt number value gradually decreases beyond z/d of 5. At a Reynolds number of 1500, the Nusselt number increases by 71% for z/d = 10 in comparison to z/d = 1. Empirical correlations for local and spanwise average Nusselt number are proposed which satisfactory predict the local as well as spanwise average Nusselt number distributions.

Experimental Investigation of Local Nusselt Profile Dissemination to Augment Heat Transfer under Air Jet Infringements for Industrial Applications

An experimental study is presented in this paper to evaluate the enhancement of heat transfer characteristics. This includes the study of a steady air jet impinging on a planar aluminum plate with constant heat flux. Extensive industrial applications of heat transfer include electronic heat sinks, the food industry, automobiles, and heat exchangers. The magnitude of the local Nusselt number along the streamwise direction is determined through detailed experimental computation using the infrared radiometry technique. The range of Reynolds number was selected between 6500 ≤ Re ≤ 15000 and the air jet outlet-to-target surface spacing 2 ≤ Y/D ≤ 6. The objective is to develop a semi-empirical correlation that can be used to determine the distribution of local Nusselt numbers over a flat plate. Considering the objective function of the spacing between the air jet outlet and target surface spacing (Y/D), Reynolds number (Re), and non-dimensional number from stagnation point in the radial direction of the plate (r’/D), it was observed that the heat transfer rates are higher for the configuration Y/D = 4 for varying Reynolds numbers, a 5 percent increase was observed. The stagnation Nusselt number shows an increasing trend up to Y/D = 4, for Reynolds numbers 10,000 and 12,000 respectively, the corresponding stagnation Nusselt numbers are 110 and 115. In comparison to the stagnation Nusselt number, which is the same Reynolds number corresponds to, the transition field Nusselt number had heat transfer values that were 12% lower.

Experimental investigations on the heat transfer characteristic of impingement/swirl cooling structures inside turbine blade leading edge

Experimental investigations were performed to study the heat transfer characteristics of impingement/swirl cooling structures inside a turbine leading edge (LE) as measured using the transient thermochromic liquid crystal (TLC) technique. The considered Reynolds numbers (Re) were 10,000, 20,000, and 30,000, and the effects of Re and the ratio of the impingement cooling hole offset distance to the impingement cooling hole diameter (e/d) on the heat transfer performance of the impingement/swirl cooling structures inside the blade LE were analyzed. The effects of the coolant outflow mode were also considered. The results show that the heat transfer intensity of the high heat transfer region of impingement/swirl cooling structures increases with Re. The e/d on the local Nusselt number (Nu) distributions for the two shooting areas are similar under different Re conditions. Changes in e/d significantly influence the heat transfer characteristics. As e/d increases from −1.5 to 1.5, the heat transfer effect and thermal uniformity of the LE inner wall chamber improved. Comparative analyses with the experimental results of the LE model without film cooling holes show that the thermal uniformity of the LE model significantly improves when including film cooling holes.

Numerical Investigation of Fluid Flow and Heat Transfer Characteristics over Double Backward-Facing Step with Obstacles

Many engineering device, including nuclear reactors, heat exchangers, and electronic circuit boards, exhibit flow separation and reattachment. To reduce thermal stress impacts, the components of these devices, which resemble stepped channel designs and are susceptible to high heat flux, must be properly cooled. The present work numerically investigates the fluid flow and heat transfer characteristics through a double backward-facing step with elliptic obstacles located after each step. For this purpose, equations governing fluid flow and heat transfer are solved in a Cartesian framework using an in-house code based on streamline upwind/Petrov-Galerkin finite element method. The effect of various parameters (axis ratios (0.25, 0.5, 0.75, 1), vertical location of obstacles (0.38, 0.577, 0.769), Reynolds number (300, 500, 800, 1000)) on fluid flow and heat transfer characteristics in the channel with and without obstacles are compared and quantified. The recirculation region length of the double backward-facing stepped channel with obstacles decreases after the first and second steps when compared with no obstacles. The local Nusselt number distribution along the corner regions of the stepped wall is enhanced due to obstacles. The axis ratio of obstacles has less influence on the convective heat transfer enhancement than the vertical location of the obstacles.

Numerical analysis of Magnetohydrodynamic convection heat flow in an enclosure

This article investigates the modeling and numerical simulation of Magnetohydrodynamic (MHD) buoyancy-driven convection flow in a differentially heated, square enclosure. Left vertical side is given a high temperature and the right vertical side is sustained at a low temperature. Horizontal sides of the enclosure are insulated. A constant magnetic field is presumed horizontally. Findings of the governing differential equations are explored numerically considering the impact of Magneto-hydrodynamic (MHD). Problem is deciphered by Galerkin finite element approach in COMSOL Multiphysics. Numerical solutions are computed for different values of Rayleigh number ranging 103≤Ra≤107 and Hartmann number ranging 0≤Ha≤40. Rate of heat that passes from the heated side is affected by increasing Rayleigh and Hartmann numbers. Comportment of MHD free convection heat flow from transient to steady state is numerically examined for a period of 0 to 1 s. The numerical solutions are discussed in respect of streamlines, iso-contours, and isotherms. In addition, physical quantities such as velocity and Nusselt number are studied. It is seen that with increasing values of Rayleigh number there is increase in local Nusselt number distribution on heated side of the cavity. Velocity distribution in the flow domain decreases in variations with increasing Hartmann number.

Heat transfer augmentation by plate motion in a wall jet flow over a heated plate: A conjugate heat transfer technique

The present study is an initiation to investigate the effect of plate movement on heat transfer characteristics in the solid plate and the fluid domain due to the turbulent wall jet flow over the plate in a quiescent surrounding. A conjugate technique is used to carry out the heat transfer solution that involves conduction in solid and convection in fluid coupled at the interface surface. The plate is heated continuously with a constant flux at its lower surface. A low-Reynolds number k − ε turbulence model developed by Yang and Shih (YS) is applied to simulate the turbulent flow. The parameters considered for the present study are the plate-jet velocity ratio ( 0 − 2 ) , the plate thickness ratio ( 0.5 − 1.5 ) , the conduction ratio ( 500 − 2000 ) , and the Reynolds number ( 7700 − 25 , 000 ) . The temperature distribution diagrams for various plate velocities, the interface temperature profile, and the averaged Nusselt number show a higher cooling rate at high plate-jet velocity ratios. The effect of plate thickness and conductivity ratios is visible in the temperature distribution diagrams. A better heat transfer is achieved at a higher Reynolds number, evident from the averaged and local Nusselt number distribution. A correlation is developed for average Nusselt number with the Reynolds number and plate-jet velocity ratios as the varying parameters.

A comparative study on thermo-fluid characteristics of free and wall-bounded cross-flow heat exchangers

In recent years, wall-bounded cross-flow heat exchangers have gained significant attention for battery cooling applications. Due to similarities in geometry, these systems are often evaluated based on the heat and flow knowledge of free cross-flow heat exchangers. To determine the reliability of this assumption, this study performs a numerical comparison of the thermo-fluid behavior of wall-bounded and free cross-flow heat exchangers. Both heat exchangers have similar dimensions, with transverse and longitudinal pitch ratios of 2.074 and 1.037, respectively, and are investigated at a Reynolds number of 40000 using the Unsteady Reynolds-Averaged Navier–Stokes (URANS) method. It is observed that the k−kl−ω transition model provides the most accurate predictions of the flow field when compared to available experimental data. The results suggest that for wall-bounded heat exchangers with an aspect ratio of 2 or larger, the flow behavior in the central flow region resembles that of a free heat exchanger, but with varying magnitudes due to the increase in velocity in the core region to counterbalance the reduction near the walls. The area-averaged mean Nusselt number from 2D and 3D models for free heat exchangers shows no significant difference compared to wall-bounded heat exchangers. However, there are considerable differences in the local Nusselt number distributions in the angular and spanwise directions. Overall, it is determined that certain conditions must be satisfied to ensure that applying the thermo-fluid characteristics of a free cross-flow heat exchanger to wall-bounded cross-flow heat exchangers in battery thermal management systems is accurate.

Effect of jet plate thickness on the local heat transfer coefficient with multiple air jet impingement

For designing a multiple jet impingement cooling system, the thickness of jet plate (a passage through which jet comes out) is one of the important parameters. Considering the thermal point of view, the thickness of the plate should be chosen so that it would enhance the heat transfer rate. In this study, the effect of jet plate thickness on the local heat transfer coefficient on a flat plate with multiple impinging air jets is investigated. The thickness ratios (t/d) of 0.5, 1.0 and 2.67 are studied. Reynolds number is varied from 5000 to 20,000 and jet-to-plate distance (z/d) is varied from 1 to 5. Jet diameter (d) is chosen to be 6 mm. The jet-to-jet distance (p/d) is kept constant at 3.Steady-state thin metal foil technique, along with infrared thermography, is used to obtain the temperature distribution and thereby the heat transfer coefficient. Coefficient of discharge for each jet plate, central line velocity decay and pressure distribution on the target surface are measured. These measurements provided new insights to better understand the heat transfer results in this study. For lower jet-to-plate distances (t/d = 0.5 to 1), heat transfer decreases with the increase in the jet plate thickness ratio. However, heat transfer is not affected with the further increase in the jet plate thickness ratio i.e., t/d = 1 to 2.67. Therefore, smaller jet plate thickness ratio is suitable to obtained high heat transfer rate. This study presents a new correlation for the local Nusselt number distribution or different jet plate thickness.

How do the Intravenously Injection of Nanoparticle and Oxytatic Bacteria Affect Through Circular Cylinder Cell for Non-Newtonian Micropolar Fluid: Mathematical Approach

In this paper, we have researched the conduct of non-Newtonian micropolar nanofluid flow through horizontal circular under the impacts of nanoparticles, oxytatic bacterial and Hall current effects. The utilizations of the current investigation are exceptionally powerful in biomedical therapies, for example, obliteration of malignant growth over biological cells by utilizing drug conveyance of nanoparticles and oxytatic microscopic organisms. If there should arise an occurrence of biological nanofluid it’s accepted to concern variable physical parameters which rely on the nanofluid temperature. Implicit Chebyshev pseudospectral (ICPS) technique by helping MATHEMATICA software has been applied to governing nonlinear system of dimensionless partial differential equations (PDEs). The nanofluid velocity, microrotation angular velocity, temperature, motile bacterial density distributions, oxygen concentration, local skin friction coefficient, Nusselt number, and wall motile density gradient distributions are delineated graphically for various variable physical parameters, likewise comparison between certain results in literature and our current output is introduced and great arrangement is found.

Experimental and analytical study on local heat transfer distribution between smooth flat plate and free surface impinging jet from a circular straight pipe nozzle

The present study describes the heat transfer characteristics of free surface single-phase circular liquid jet impinging on a thin smooth flat surface subjected to uniform heat flux. The experimental setup is developed to measure the local temperature of a thin stainless steel metal foil using the infrared thermography technique. The effects of nozzle diameter (d = 2.5 to 10.8 mm), nozzle to plate spacing (z/d = 2 to 10), and Reynolds number (Re = 5000 to 24,000) on the local Nusselt number (Nu) distributions are studied. The local Nu is maximum in the stagnation region, and it decreases in the downstream flow following the stagnation region. The nozzle with a larger diameter shows maximum Nu compared to other nozzles. For a given z/d of 2 and a Reynolds number of 15,000, a maximum increment of 67% in local Nu is observed for a 10.8 mm nozzle in comparison to a 2.5 mm nozzle. The local Nu in the viscous boundary layer is found to be a function of the Reynolds number, Prandtl number, dimensionless r/d, and Weber number. The Nu increases with an increase in Reynolds number, however, the effect of the nozzle to plate spacing is negligible. The off-stagnation peak is observed for the 10.8 mm nozzle in the region of 0≤r/d≤0.7. The existing correlations reported in the literature are not able to capture the measured local Nusselt number distribution. Therefore, in the present study, an effort has been made to propose semi-empirical correlations for local Nu in the stagnation and viscous boundary layer region which will incorporate the diameter effect of the nozzle. The proposed correlations are able to predict the local Nusselt number distribution within the maximum deviation of 20%. The comparison between the free surface and submerged jet has been made which suggests that the local Nu for the submerged jet is higher compared to the free surface jet.

Numerical investigation of the influence of hybrid nano-fluid on heat transfer in semi-annular channel

The influence of hybrid nano-fluid on heat transport in a semi-annular channel is investigated numerically. The hybrid nano-fluid is a liquid water with suspension of SWCNT and MWCNT. Constant heat fluxes are applied to the walls of channel. The finite volume approach is used to solve the governing mathematical equations. The volume percent of SWCNT is fixed at 0.01% and the volume fraction of MWCNT is varied from 0.01% to 0.1%. Heat transport is shown to diminish as the volume fraction of MWCNT increases. The heat transferred from the wall to the near fluid is influenced by the curvature of the walls. The results are displayed as velocity contours and isotherms. For certain volume fractions of nanoparticles, local Nusselt number distributions are given. It has been discovered that walls with a smaller curvature have a stronger convection heat transfer. Pressure is increased when the volume percentage of nanoparticles increases.

Vortex-induced vibration and heat dissipation of multiple cylinders under opposed thermal buoyancy

The vortex-induced vibration of three cylinders in an equilateral triangular arrangement with opposed thermal buoyancy are studied by numerical method. The cylinders are uniformly elastic and can freely vibrate in the transverse flow direction. The temperature of cylinders is higher than free stream. The incoming flow is in the same direction as gravity and opposite to thermal buoyancy. The ranges of Richardson number and reduced velocity considered in this work are −1≤Ri ≤ 0 and 3 ≤ U*≤12. The maximum amplitude of each cylinder rises significantly under opposed thermal buoyancy, and the maximum amplitude of cylinder 1, cylinder 2, and cylinder 3 increased by 206%, 174%, and 172%, respectively. When U*>8, the vibrations of cylinder 2 and cylinder 3 are close to the same phase. The opposed thermal buoyancy hinders the separation of flow boundary layer, resulting in thickening of thermal boundary layer and deterioration of heat transfer. The deterioration of heat transfer can be alleviated at high reduced velocity (U*≥10). The increase of Richardson number will lead to the weakening of heat transfer at up-flow surface of cylinder and enhancement of heat dissipation near the back stagnation point. The local Nusselt number distribution around cylinder is affected by vortex-induced vibration, vortex shedding, and the interaction of shear layers from each cylinder.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF HEAT TRANSFER ENHANCEMENT WITH TWISTED TAPE INSERT

Embedding twisted tape (TT) inserts are a well-known technique widely used in industry because of their easiness in manufacturing and installation. The current work investigates the distribution of local wall temperature and Nusselt number with twisted tape inserts using an infrared camera. The thermohydraulic performance of the twisted tape is evaluated for various geometrical parameters. A numerical and experimental analysis is performed for twist ratios of 2.5, 3.5, 5, and 10 with varying tape widths of 10, 14, 16, and 18 mm. The analysis is carried out for a Reynolds number ranging from 7500 to 30,000. The results obtained indicate an increase in the Nusselt number (Nu) and friction factor (&lt;i&gt;f&lt;/i&gt;) with the increment in tape width and decrement in the tape pitch. The maximum rise in Nu and &lt;i&gt;f&lt;/i&gt; is observed to be 1.72 and 2.19 times in the plain tube for full width and minimum twist ratio. Empirical correlations, incorporating geometrical parameters of TT, such as twist ratio and normalized width (&lt;i&gt;W/d&lt;/i&gt;), are developed for Nu and &lt;i&gt;f&lt;/i&gt; for an internal flow inside a circular tube with twisted tape inserts, using water as working fluid. The correlations predict all the data for f and Nu with a mean absolute deviation of 5.47 and 7.18&amp;#37;, respectively.

Study on convective heat transfer characteristics of inclined jet impinging cylindrical target surface in the confined space

• Heat transfer of cylinder surface with flow confinement is experimentally tested. • The effects of jet angle and Reynolds number on heat transfer have been studied. • The mechanism of heat transfer for different jet angles is analyzed in detail. In this study, the convective heat transfer characteristics of a cylinder with flow confinement under the impingement of a circumferentially arranged turbulent inclined circular air jet array are experimentally studied. This jet configuration can be effectively applied in the cooling/heating of rotating machinery, such as cooling of the steam turbine shaft, cement rotary kiln cooling, and textile drying. The purpose of the study is to elucidate the effect of the jet angle ( θ ) and the jet Reynolds number ( Re d ) on the heat transfer behaviors of the cylindrical target surface. The jet Reynolds number varies from 20,000 to 35,000, and three different jet angles ( θ =20°,30°,45°) are adopted. The local Nusselt number distribution of the region of interest on the cylindrical target surface is experimentally measured by applying the transient thermochromic liquid crystal technique. Numerical studies are conducted to obtain details of flow patterns in the confined space, so as to analyze the convective heat transfer mechanisms. The results show that for small jet angles ( θ =20°,30°), due to the intense circumferential cross-flow, the overall distribution of the Nusselt number is relatively uniform. As the jet angle increases to 45 °, the Nusselt number in the middle of the target surface increases significantly because of the formation of the stagnation region. As the jet angle increases from 30 ° to 45 °, the heat transfer uniformity on the cylindrical surface decreases significantly. The maximum difference of the variation coefficient of the Nusselt number is 26.6% at Re d = 35 , 000 . As the jet Reynolds number increases, the local Nusselt number on the cylindrical target surface increases as a whole. The average Nusselt number with Re d = 35 , 000 increases by 64.3%-74.9% compared with the cases of Re d = 20 , 000 .

Turbulent thermal-hydraulic characteristics in a spiral corrugated waste heat recovery heat exchanger with perforated multiple twisted tapes

Waste heat recovery technology can undoubtedly improve energy saving and reduce carbon emissions, as one of the key equipment, heat exchangers always determine the waste heat recovery efficiency. In this study, to improve the heat transfer potentiality of conventional plain tube heat exchangers, a synergistic thermal enhancement technology of a spiral corrugated tube (SCT) with multiple twisted tapes (TTs) was devised by taking liquid-gas heat exchanging as a typical waste heat recovery scenario. The impacts of the TTs numbers, transverse space ratios (S/D), and longitudinal perforation length ratios (L/D) on tubular turbulent heat transfer-fluid flow characteristics were numerically investigated, consisting of flow fields, temperature contours, local Nusselt number (Nu) distributions, vortex structures, and turbulent kinetic energy contours, as well as average Nu, friction factor (f), and overall thermal performance evaluation index (η). The findings proved that the fitting of multiple TTs into the SCT greatly homogenized the flow fields and temperature distributions, and increased the Nu and f, in which both of them also raised with the increase of the number of TTs. The η dropped and subsequently augmented as the quantity of TTs increased. Furthermore, perforations on the surface of the quadruple TTs could attenuate the f with a relative less reduction in the Nu, which caused higher η values than the ones without perforations. Notably, as demonstrated by the vortex structures, this could be ascribed to the fact that the perforations reduced excessive longitudinal vortex interactions in the mainstream regime without reducing the impingement to the thermal boundary layer too much. Both the Nu and f generally declined with rising S/D and L/D. Overall, by means of simple mounting perforations on the surface of the quadruple TTs, the maximum η could be further enhanced up to around 7.9% and a maximum η of 1.23 was obtained by the SCT with S/D = 0.152 and L/D = 0.348.

Modeling of heat transfer in a narrow rectangular channel with partial swelling blockage

In plate-fuel-assembly reactor cores, owing to the cladding swelling caused by irradiation damage under high burnup conditions, narrow rectangular channels may be partially blocked. The partial blockage will alter the flow channel shape and affect the heat transfer characteristics. Thus, it is important to focus on the flow and heat transfer characteristics under partial blockage conditions. Accordingly, in this study, the heat transfer at the entrance region of a narrow rectangular channel with protrusions is numerically investigated. Further, variations in the local Nusselt number and fluid field distribution are studied. Results reveal that the protrusions lead to a wake effect that influences the local heat transfer in the blockage region and wake zones. At the upwind face of the protrusions, the local heat transfer increases to a crest value over the hydraulic diameter. A low-speed recirculation region is also formed over a short distance at the intermediate leeward face of the protrusion, resulting in local deterioration of the heat transfer. Moreover, in the wake zone, heat transfer is enhanced owing to the disturbance of the flow streams with different velocities. The effects of the blockage ratio, number, and interval of protrusions on the local heat transfer are also analyzed. This augmentation of the heat transfer decreases with an increase in the Reynolds number, whereas it increases with the height ratio of the protrusions. Additionally, a multiple regression analysis method is employed to analyze the numerical simulation data. Based on these numerical investigations, dimensionless correlations are proposed to estimate the local Nusselt number for rectangular channels with and without partial swelling blockages. The predicted results are in good agreement with the experimental data and reference correlations. For Eqs. (20) and (22), the major points are located within the relative deviation limit of ± 15%, while for Eq. (21), all the data points fall within the relative deviation limit of ± 20%. Comparison results indicate that the correlations can be used to understand the heat transfer upstream and downstream of partial blockages. Furthermore, numerical investigations and qualitative analyses of the effect of spare protrusions on the local heat transfer in the wake area are performed. These spare protrusions lead to local heat transfer deterioration in the wake zone; notably, the spacing between these protrusions does not have a significant influence on the augmentation of the heat transfer downstream.

Investigation on heat transfer and flow-induced vibration of three cylinders in equilateral triangle arrangement

Flow-induced vibration (FIV) and heat transfer of three circular cylinders were numerically investigated in the reduced velocity range of 3 ≤ U⁎ ≤ 15. To obtain the vibration and heat transfer characters, the influence of FIV and Richardson number (0 ≤ Ri ≤ 1) on the heat transfer performance are studied. It's found that the rise of Ri can lead to the suppression of FIV and narrow the reduced velocity (U⁎) range of lock-in. The amplitude and lift coefficient of two downstream cylinders are higher than that of 1st cylinder (upstream one) for most of U⁎. The FIV can significantly enhance the heat transfer of 2nd and 3rd cylinders (downstream cylinders), the highest increments of averaged Nusselt number are 9.74% (2nd cylinder) and 9.78% (3rd cylinder), which can be obtained when Ri = 0.5 and U⁎ = 5. Meanwhile, the distribution of local Nusselt number shows that the heat transfer mainly occurs at the upstream side of each oscillator. The heat transfer characteristics on the surface of each cylinder are closely influenced by the thickness of thermal boundary, which determines the gradient of temperature around cylinders.