Study Of Heat Transfer Augmentation Research Articles

CFD study of heat transfer augmentation and fluid flow characteristics of turbulent flow inside helically grooved tubes

CFD study of heat transfer augmentation and fluid flow characteristics of turbulent flow inside helically grooved tubes

Numerical study of heat transfer augmentation in an axially rotating pipe equipped with Kenics mixer

This paper reports fluid flow and heat transfer in an axially rotating pipe equipped with Kenics mixer under constant wall heat flux condition. Numerical simulations were processed with the commercial finite volume CFD code, ANSYS Fluent 19.2. The realizable k-ε turbulent model was used to close the Reynolds Average Navier-Stokes (RANS) equations. Buoyancy effects (βΔT) and gravity (g = 9.81 m/s2) were considered in the simulations. In the range of axial Reynolds numbers between 10,000 and 30,000 (10,000 ≤ Rea ≤ 30,000), the Kenics mixer enhanced the Nusselt number of the rotating pipe by 41–50%, 38–47% and 33–42% at rotation rates of 1, 2 and 3 respectively. Enhancement in Nusselt number was linked to additional swirl created by the Kenics mixer insert which improved fluid mixing and boundary layer disturbance. CFD results also showed that increasing the axial Reynold number augmented the Nusselt number but decreased the friction factor.

Simulation of Turbulent Heat Transfer Augmentation with Hybrid Nanofluid

Study of heat transfer augmentation with hybrid nanofluid represents a new class of heat transfer augmentation. The CFD model by using commercial software depending on finite volume technique and adopting SIMPLE algorithm is performed. Mixture of Aluminum Nitride (AlN) and alumina (Al2O3) nanoparticles into water as a basefluid is classified as a new class of hybrid nanofluids that can augment heat transfer. The nanofluid volume fraction and Reynolds number are in the range of (1% to 4%) and (5000 to 17000) respectively. The size diameter of nanoparticles and heat flux around a horizontal straight tube are fixed at 30 nm and 5000 w/m2 respectively. The numerical solution has been successfully validated by using an experimental data available in the literature. Results show that combination of AlN - Al2O3 nanoparticles into water basefluid tends to augment significant heat transfer performance.

Experimental study of heat transfer augmentation in a two-pass channel featuring V-shaped ribs and cylindrical dimples

Higher turbine inlet temperature requirements in modern gas turbine airfoils necessitate the development of high performance internal cooling designs for increasing turbine durability and performance. The internal cooling passages of turbine airfoils feature ribs which promote turbulent mixing, increase in near wall shear and hence lead to heat transfer augmentation. One other method of increasing heat transfer enhancement is by adding depressions (commonly called “dimples”) on the walls of internal cooling passages. Earlier studies have investigated different shapes of rib turbulators and dimples, however, the limitations in manufacturability of dimple in actual turbine airfoils has been a concern in the past. Recent advancements in the field of additive manufacturing, e.g. Direct Metal Laser Sintering (DMLS), opens endless possibilities for the development of novel cooling features which can be incorporated in actual engines. The present study investigates the heat transfer and pressure drop characteristics of a two-pass channel (AR=1) featuring ribs-alone, dimples-alone and combination of ribs and dimples. The ribs are V-shaped with rib-height-to-hydraulic diameter ratio of 0.125 and rib-pitch-to-rib height ratio of 16. Dimples are cylindrical in shape and have a depth-to-print diameter ratio of 0.3. The heat transfer coefficient was measured using transient Liquid Crystal Thermography and detailed Nusselt numbers have been reported and analyzed. Also, static pressure measurements were carried out at several locations to determine the total pressure drop across the two pass channel and the local variation of pressure coefficient. The experiments were carried out for a wide range of Reynolds number (19,500–69,000) to cover the full spectrum typically found in both land-based and air-breathing engines. It has been observed that the combination of ribs and dimples resulted in higher heat transfer augmentation as well as higher thermal hydraulic performance when compared with ribs alone and dimples alone configurations for the range of Reynolds number studied.

On Numerical Investigation of Non-dimensional Constant Representing the Occurrence of Secondary Peaks in the Nusselt Distribution Curves

The Study of heat transfer augmentation in micro scale and electronic packaging systems are some of the paramount areas of the impending universe. In such systems the cooling of the heat sinks are generally achieved through the impingement of air jet. Assuming the air jet to be continues & incompressible fluid, the heat dissipation rate over the target surface seems to either uniform or well characterized in radial direction. In order to study the characteristic of heat dissipation rate, a graph of Nusselt number versus radial distance over the target surface is plotted. This is done at various injection and geometric parameters. The resulting trend of Nusselt profile manifests a gradual decrement in its magnitude with increase in the radial distance from the stagnation point. This is the property of characterized Nusselt distribution profile. In order to study the Nusselt distribution profile in more accurate sense, the present research takes an effort in numerically simulating the present geometry. A 2-D axis-symmetric geometric model is being proposed which consists of air (fluid) and aluminum (solid) as two vital computational domains. The computational simulations are being carried out using an appropriate turbulence model, in order to correctly predict the most accurate flow regime. This also enables the recording of necessary heat interactions. As far as the transition and intermediacy in the flow structure which occurs at target surface is concerned, prediction of accurate flow profile using commercial turbulence model becomes difficult. In order to predict these vital phenomenon, SST turbulence model along with Gamma – Theta transitional model is solved simultaneously in a commercial CFX solver. At lower nozzle - target spacing and higher impinging velocity, Nusselt distribution curve was observed to posses some unpredicted and localized secondary rise. The occurrence of such secondary peaks increases the magnitude of area average heat transfer rate. As per the survey, very less light is observed in the research areas of determining the exact cause and the intervening range for the occurrence of such peaks. Looking into this issue the current research focuses over the determination of the critical constant and its magnitude within which these peaks exists. The non dimensional constant representing the critical magnitude is defined as a ratio of diameter based Reynolds number with nozzle - target spacing (Z/d). While the corresponding critical magnitude was approximately investigated to be 6000. Not only that the present work takes an initiative in mapping the velocity contours at different values of this non dimensional constant. This enables a physical understanding of the flow profile at different impinging condition. A very contiguous observation of such contours demonstrates the occurrence of a transition region in the flow regime to be responsible for the origin of such peaks

On Numerical Investigation of Heat Transfer Augmentation through Pin Fin Heat Sink by Laterally Impinging Air Jet

Computational study of heat transfer augmentation using air jet impingement is the most contemporary provocation in the micro scale and electronic packaging systems. Since the cooling of heat sink becomes the most prior and conspicuous issue, as far as the efficiency and life of the system is concerned. Hence enhancement in local heat transfer coefficient becomes a most primitive parameter in the looming world of electronic packaging system. Extensive research is carried out in the area of steady air jet impinging over the flat plate by varying geometric as well as the injection parameters. Not only that, plenty amount of research works is also available with pin fin array as the target surface. But the numerical analysis of the current problem in commercial simulating software seems to be done very less. This work is beneficial as far as the visualization of various contours; physical time and effort required in performing experiments are concerned. The present research tries to propose a valid and optimum grid size required by the current computational domain in order to obtain the results which are independent of grid size. Also an appropriate turbulence model capable of accurately predicting the realizable flow regime is being proposed. Since the heat interaction data needs to be well predicted in near wall and jet system; region simultaneously, shear stress transport model is recommended for the present computational study.

Experimental study of heat transfer augmentation in non-circular duct using combined nanofluids and vortex generator

This study presents the effect of nanofluid and vortex generators compound on heat transfer and friction factor in an equilateral triangular duct experimentally. Two different types of nanofluids namely, Al2O3 and SiO2 nanoparticles suspended in distilled water (DW) with two particle concentrations were successfully prepared and tested. A wide range of Reynolds number was covered from 500 to 8000, approximately. The results of smooth triangular duct using water as a working fluid were validated with literatures and a good agreement was obtained. The present results show a good enhancement in heat transfer by using vortex generator with base fluid while a significant enhancement was registered by using the compound of vortex generator and nanofluids accomplished with a moderate increase in the friction factor. The maximum enhancement in the Nusselt number obtained in this study is 44.64% and 41.82% at 1vol.% and Re≈4000 for SiO2–DW and Al2O3–DW nanofluid, respectively.

A Study of Heat Transfer Augmentation for Recuperative Heat Exchangers: Comparison Between Three Dimple Geometries

This study presents an investigation of the heat transfer augmentation for the purpose of obtaining high effectiveness recuperative heat exchangers for waste heat recovery. The focus of the present work is in the fully developed portion of a 2:1 aspect ratio rectangular channel characterized by dimples applied to one wall at channel Reynolds numbers of 10,000, 18,000, 28,000, and 36,000. The dimples are applied in a staggered-row, racetrack configuration. In this study, a segmented copper test section was embedded with insulated dimples in order to isolate the heat transfer within the dimpled feature. The insulated material used to create a dimpled geometry isolates the heat transfer within the dimple cavity from the heat transfer augmentation on the surrounding smooth walls promoted by the flow disturbances induced by the dimple. Results for three different geometries are presented, a small dimple feature, a large dimple, and a double dimple. The results of this study indicate that there is significant heat transfer augmentation even on the nonfeatured portion of the channel wall resulting from the secondary flows created by the features. Overall heat transfer augmentations for the small dimples are between 13–27%, large dimples between 33–54%, and double dimples between 22–39%, with the highest heat transfer augmentation at the lowest Reynolds number for all three dimple geometries tested. Heat transfer within the dimple was shown to be less than that of the surrounding flat regions at low Reynolds numbers. Results for each dimple geometry show that dimples are capable of promoting heat transfer over the entire bottom wall surface as well as the side walls; thus the effects are not confined to within the dimple cavity.

Study of heat transfer augmentation in a differentially heated square cavity using copper–water nanofluid

Effect of copper–water nanofluid as a cooling medium has been studied to simulate the behavior of heat transfer due to laminar natural convection in a differentially heated square cavity. The transport equations for a non-Newtonian fluid have been solved numerically following finite volume approach using SIMPLER algorithm. The shear stresses have calculated using Ostwald–de Waele model for an incompressible non-Newtonian fluid. The thermal conductivity of the nanofluid has been calculated from the proposed model by Patel et al. Study has been conducted for Rayleigh number ( Ra) 10 4 to 10 7 while solid volume fraction ( ϕ ) of copper particles in water varied from 0.05% to 5%. It has been observed that the heat transfer decreases with increase in ϕ for a particular Ra, while it increases with Ra for a particular ϕ. The copper nanoparticle diameter has been taken as 100 nm for all of the studies.

LES study of heat transfer augmentation and wake instabilities of a rotating disk in a planar stream of air

Large-eddy-simulations are performed for the heat transfer and the wake flow of a thin rotating disk subjected to an outer parallel passing stream of air. Above a critical value for the angular velocity of the disk, heat transfer augmentation sets on. This is strongly related to a flow instability that leads to a periodic vortex generation at the counter-moving disk side. The resulting phenomena are captured by the classical Landau model. For higher angular velocities the wake becomes fully turbulent, and here the transition to turbulence seems to be very abrupt. In this regime, a periodic vortex generation is observable at the co-moving disk side, too.

Numerical Study of Heat Transfer Augmentation of Viscous Flow in Corrugated Channels

The convective heat transport characteristics of subcritical and supercritical flows in periodic two-dimensional corrugated channels are investigated numerically using the spectral element method. Computations were performed in the Reynolds number range of 50-800 and Prandtl number of 1. for corrugation angles of γ = 20° and γ = 30°, and for wall spacing-to-periodicity length ratio of b/L = 0.15-0.4. The critical Reynolds number for onset of time-periodic, self-sustained oscillatory state decreases with increasing corrugation angle and with decreasing b/L ratio. The natural frequency of these oscillations is computed, and it corresponds to the least stable subcritical mode (Tollmien-Schlichting frequency) compatible with the given periodic geometry. The transport of heat and momentum due to the supercritical steady periodic states in the corrugated channel are investigated, and the relevant time- and space-averaged Nusselt number and friction factor are calculated

95/01862 Study of heat transfer augmentation in a high-temperature field by a radiation promoter generating a secondary flow

Solid electrolytes can be used as active catalyst supports to dramatically and reversibly alter the catalytic properties of porous metal films by electrically polarizing the metal-solid electrolyte interface. The common features of this effect, termed Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA), are summarized in the present work, together with the origin of the effect, which is due to the controlled change in catalyst work function and to the concomitant changes in the strength of chemisorptive bonds

Turbulent Heat Transfer Augmentation Using Microscale Disturbances Inside the Viscous Sublayer

We report here on an experimental study of heat transfer augmentation in turbulent flow. Enhancement strategies employed in this investigation are based on the near-wall mixing processes induced in the sublayer through appropriate wall and near-wall streamwise-periodic disturbances. Experiments are performed in a low-turbulence wind-tunnel with a high-aspect-ratio rectangular channel having either (a) two-dimensional periodic microgrooves on the wall, or (b) two-dimensional microcylinders placed in the immediate vicinity of the wall. It is found that micro-disturbances placed inside the sublayer induce favorable heat-transport augmentation with respect to the smooth-wall case, in that near-analogous momentum and heat transfer behavior are preserved; a roughly commensurate increase in heat and momentum transport is termed favorable in that it leads to a reduction in the pumping power penalty at fixed heat removal rate. The study shows that this favorable performance of microcylinder-equipped channel flows is achieved for microcylinders placed inside y+ ≃20, implying a dependence of the optimal position and size on Reynolds number. For microgrooved channel flows, favorable augmentation is obtained for a wider range of Reynolds numbers; however, optimal enhancement still requires a matching of geometric perturbation with the sublayer scale.

Experimental study of heat transfer augmentation versus pumping power in a horizontal R12 evaporator

An experimental investigation has been carried out to study the effect of ‘swirl generators’ inside a horizontal R12 evaporator. The swirl generators consisted of twisted stainless-steels strips with ratios ( y) of 3.76, 5.58, 7.37 and 10.15. The variation of swirl flow heat transfer coefficients with pumping power has been studied. The ratio of swirl flow to plain heat transfer coefficient per unit of pumping power has been found to be more than one in several test runs. The data cover a heat flux range of 8000–13 600 W m −2, a mass velocity range of 7.0 × 10 5−14.0 × 10 5 kgh −1 m −2 and a vapour quality range of 0.30 to 0.75.

Fundamental study of heat transfer augmentation of tube inside surface by cascade smooth surface-turbulence promoters

Arc-shaped turbulence promoters along the circular tube inside surface which have capability of cleaning down tube fouling by sponge balls are proposed for enhancing the heat transfer performance. First, flow characteristics and heat transfer performance of air flow in a rectangular channel for several promoters were investigated. The arc-shaped promoter was found to have the lowest pressure loss. Secondly, turbulence structures of water flow by the arc-shaped promoters in a circular tube were measured by use of a newly developed two-color LDV. The local heat transfer coefficient was shown to be well correlated to the Reynolds stress and the existence of the optimum height of the promoter as elucidated. Lastly, performances of thermoelectric power generation by using cascade arc-shaped promoters for OTEC were measured and the optimum condition was investigated.

Fundamental study of heat transfer augmentation by smooth turbulence surface promotors.

海洋温度差熱電発電の熱交換器に要求されるスケール除去可能な高性能伝熱面の基礎的研究として,正方形をはじめ円弧形,正弦波形などの滑らかなカスケード突起後流の乱流構造をX形熱線プローブ,スプリットフィルムプローブを用い解明すると共に,局所熱伝率を測定し,平均熱伝達率には壁近傍の乱流構造同様,突起形状による差異があらわれず,圧力損失を考慮した伝熱促進の程度は,円弧形突起が最適であることが示された.