Highest Heat Transfer Augmentation Research Articles

Inhomogeneous flow model of natural convection, entropy generation, and heatlines visualization in wavy I‐shaped enclosure with rectangular heater

AbstractThe present investigation is on examination of the natural convection and entropy generation considering the heatlines visualization of nanofluid I‐shaped enclosure with two corrugated walls considering inner rectangular heater of three different heights. The influence of Brownian motion along with thermophoresis had been implemented using Inhomogeneous two‐phase model of nanofluid. The governing equations were solved numerically using COMSOL software. Influence of Rayleigh number , Buoyancy ratio number , Lewis number , heater length . The results indicate that the influence of Lewis number on heat transfer bettering is stronger at high Rayleigh number while its impact is negligible at a lower value of Rayleigh number (conduction mode). In addition, the total entropy generation gets its highest value at Lewis number . Bejan number, fluid flow strength and heat rate increase as the rectangular heater height increases. Also, higher heat transfer augmentation is taken when the heater height is while increasing the heater height to leads to more total entropy generation. The impact of heater height on total entropy generation is highly affected by Rayleigh number as increasing the heater height from into , total entropy generation increases by at while it increases by at .

Numerical study on significance of wave amplitude, wavelength and aspect ratio on heat transfer and fluid flow characteristics in circular wavy MC heat sink design

In this study, hydrothermal characteristics in a circular wavy microchannel (CWMC) design under laminar flow conditions with uniform heat flux is numerically studied. Parametric studies in an innovative CWMC design were carried out at various wave amplitudes, wavelengths and aspect ratios. Three dimensional numerical study was performed in the Reynolds number (Re) range from 100 to 300 with uniform heat flux (50 W/cm2) applied at bottom of the channel, treating copper as channel material and water as working fluid. The obtained results were compared to sinusoidal wavy microchannel (SWMC).The results showed that heat transfer and fluid flow characteristics were significantly influenced by wave amplitude, wavelength and aspect ratio. Velocity vectors and contours were presented to understand the heat transfer and fluid flow characteristics. Stream-wise local Nusselt number, overall performance factor, span-wise velocity and temperature variation are also presented. It is concluded that CWMC with higher wave amplitude, smaller wave length and smaller aspect ratio gives higher heat transfer augmentation with corresponding pressure drop penalty.

Laminar convection heat transfer and flow performance of Al2O3–water nanofluids in a multichannel-flat aluminum tube

The heat transfer experiments of Al2O3–water nanofluids were carried out at fixed Reynold number and velocity in a multichannel-flat aluminum tube. The studies we have performed revealed that the convection heat transfer coefficient increased about 5.9% at 0.5vol.% Al2O3 concentration at Re=1732. While at a constant velocity, higher addition of nanoparticles had no evident influence on the heat transfer performance. Compared with theoretical correlations, the data obtained were accorded with Shah equation (deviation <10%) except for those in the entrance. Then a comparative study of Al2O3–water nanofluids and the base fluid was made on the local heat transfer coefficient. The results indicated a higher heat transfer augmentation (11.1%) in the entrance region for 0.5vol.% Al2O3-DW nanofluid, which seems to have a bearing on the thermal conductivity enhancement and the thermal boundary layer thickness reduction. The friction factor and the pressure drop were also studied and the results revealed that the pressure drop slightly increased with the volume concentration of nanoparticles. For 0.5vol.% Al2O3-DW, the average pressure drop increased about 4.4%.

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.

Characterization of heat transfer enhancement and frictional losses in a two-pass square duct featuring unique combinations of rib turbulators and cylindrical dimples

Transient heat transfer experiments using liquid crystal thermography have been carried out on a two-pass square channel for testing several unique combinations of ribs and cylindrical dimples. Four different rib shapes, viz. 45° angled, V, W and M have been studied. In compound channels, the rib pitch accommodates cylindrical dimples arranged in the shape of the rib, hence following the direction of rib induced secondary flows. For each rib shape, three different configurations – rib alone, dimple alone and rib–dimpled compound cases were studied. The rib-height-to-channel hydraulic diameter ratio was 0.125 and rib-pitch-to-rib-height ratio was 16. The dimple-depth-to-print diameter ratio was 0.3. The experiments were carried out for a wide range of Reynolds number (19,500–69,000), covering a spectrum typically found in both land-based and air-breathing engines. A total of 52 experiments were carried out to measure detailed heat transfer coefficient on the bottom wall of the two-pass channel. A transient liquid crystal thermography technique was used for heat transfer measurement. Static pressure measurements were carried out to measure the overall pressure drop in the two pass channel. From globally averaged Nusselt number and overall pressure drop, thermal-hydraulic performance of the 13 configurations were determined, compared and analyzed. It has been observed that 45° angled and V compound configurations resulted in higher heat transfer augmentation as well as higher thermal hydraulic performance when compared with their corresponding ribs alone and dimples alone configurations.

Investigation of effect of the circular ring turbulators on heat transfer augmentation and fluid flow characteristic of solar air heater

This study presented effects of the circular ring turbulators (CRTs) having different pitch ratio and hole number on heat transfer augmentation in a new type solar air heater (SAH). The experiments have been performed by varying the parameters, pitch ratio (PR), hole number (N) and Reynolds number (Re) in a range of between 3000 and 7500 under solar radiation heat flux (I). The results obtained using CRTs compared with conventional plain tube. The experimental results have shown a significant enhancement in heat transfer on SAHs with CTRs. According to the experimental results, the higher heat transfer augmentation with CRTs inserted inside the tube in SAHs have been obtained for PR=2 and N=2. The best experimental results were found with ∼229% heat transfer enhancement, ∼5.8 times friction factor compared to a conventional plain tube and the thermal performance factor (η) was computed as ∼1.83. In addition, statistical correlations for Nu, f and η were developed. These correlation results illustrated a good agreement with experimental results with ±10% deviation.

Ultrasonic enhancement of subcooled pool boiling of freely oscillated wires

New methods for cooling of microelectronic elements have been recently developed, including application of ultrasonic fields, which can enhance the heat transfer in two-phase cooling. Here we deal with ultrasonic enhancement of heat transfer from wires in subcooled water pool boiling at subcooling of 80K. The main purpose is to find the effect of wire diameter on the heat transfer coefficient. The wire heaters were used as a source of constant heat flux, and as a thermometers. The experiments were carried out using wires of different diameters: 20, 50, 90, 200 and 0.250μm, submerged in a bath of water. The frequency of the ultrasonic field was 40kHz and the intensity was 0.5W/cm2. The wire wall temperature was measured as a function of wire surface heat flux. The effect of acoustic field on heat transfer was measured by the change in the average surface temperature of the heater. When the ultrasonic field was applied, the wall temperature decreased depending on wire diameter and heat flux. Video images of vapor bubble dynamics within the sound field aided in the analysis, and are presented here. The highest heat transfer augmentation was registered for the heated wire of diameter 200μm during pool boiling in the ultrasound field.

Numerical simulation of laminar to turbulent nanofluid flow and heat transfer over a backward-facing step

This paper presents a numerical study of heat transfer to turbulent and laminar Cu/water flow over a backward-facing step. Mathematical model based on finite volume method with a FORTRAN code is used to solve the continuity, momentum, energy and turbulence equations. Turbulence was modeled by the shear stress transport (SST) K–ω Model. In this simulation, three volume fractions of nanofluid (0%, 2% and 4%), a varying Reynolds number from 50 to 200 for the laminar range and 5000 to 20,000 for the turbulent range, an expansion ratio of 2 and constant heat flux of 4000W/m2 were considered. The results show the effect of nanofluid volume fraction on enhancing the Nusselt number in the laminar and turbulent ranges. The effect of expansion ratio was clearly observed at the downstream inlet region where the peak of the Nusselt number profile was referred to as enhanced heat transfer due to the generated recirculation flow. An increase of pressure drop was evident with an increasing Reynolds number and decreasing nanofluid volume fraction, while the maximum pressure drop was detected in the downstream inlet region. A rising Reynolds number caused an increasing Nusselt number, and the highest heat transfer augmentation in the present investigation was about 26% and 36% for turbulent and laminar range, respectively compared with pure water.

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 Flow and Convective Heat Transfer in a Simulated Scaled Up Low Emission Annular Combustor

Modern dry low emissions (DLE) combustors are characterized by highly swirling and expanding flows that makes the convective heat load on the gas side difficult to predict and estimate. A coupled experimental–numerical study of swirling flow inside a DLE annular combustor model is used to determine the distribution of heat transfer on the liner walls. Three different Reynolds numbers are investigated in the range of 210,000–840,000 with a characteristic swirl number of 0.98. The maximum heat transfer coefficient enhancement ratio decreased from 6 to 3.6 as the flow Reynolds number increased from 210,000 to 840,000. This is attributed to a reduction in the normalized turbulent kinetic energy in the impinging shear layer, which is strongly dependent on the swirl number that remains constant at 0.98 for the Reynolds number range investigated. The location of peak heat transfer did not change with the increase in Reynolds number since the flow structures in the combustors did not change with Reynolds number. Results also showed that the heat transfer distributions in the annulus have slightly different characteristics for the concave and convex walls. A modified swirl number accounting for the step expansion ratio is defined to facilitate comparison between the heat transfer characteristics in the annular combustor with previous work in a can combustor. A higher modified swirl number in the annular combustor resulted in higher heat transfer augmentation and a slower decay with Reynolds number.

Effect of Coriolis forces in a rotating channel with dimples and protrusions

Abstract Large-eddy simulations are used to investigate the effect of Coriolis forces and dimple depth on heat transfer and friction in a channel with dimples and protrusions on either side. Two geometries with two different dimple–protrusion depths, δ = 0.2 and 0.3 of channel height are investigated over a wide range of rotation numbers, Ro b = −0.77 to 1.10 based on mean velocity and channel height. It is found that the dimple side of the channel is much more sensitive to destabilizing rotational Coriolis forces than the protrusion side of the channel, although both dimples and protrusions react to the stabilizing effects of Coriolis forces on the leading side. The dimpled surface on the trailing side experiences a large increase in heat transfer coefficient from an augmentation ratio of 1.9 for stationary flow to 3.5 at Ro b = 0.77 for δ = 0.2, and from 2.3 to a maximum of 3.8 for δ = 0.3. Placing protrusion on the trailing side, however, only increases the augmentation ratio to between 3.25 and 3.7 from the stationary values of 3.0 and 3.4 for δ = 0.2 and 0.3, respectively. The dimpled leading side experiences a large drop in heat transfer to between augmentation ratios of 1.1 and 1.4 for the two dimple depths. The protrusion surface on the leading side also experiences a large drop in augmentation from 3.0 for a stationary channel to 1.3 at Ro b = 0.77 for δ = 0.2 and from 3.4 to 1.8 at Ro b = 1.1 for δ = 0.3. The results lead to the conclusion that for low rotation numbers | Ro b | Ro b | > 0.2, dimples on the trailing side of the duct give better overall performance. Between the two depths, the deeper dimple/protrusion ( δ = 0.3) gives higher heat transfer augmentation at the price of more frictional losses ranging from 6 to 10 versus 3 to 5 for depth δ = 0.2.

Terrestrial and microgravity pool boiling heat transfer from a wire in an acoustic field

Pool boiling heat transfer experiments from a platinum wire heater in FC-72 liquid were conducted under terrestrial and microgravity conditions, both with and without the presence of a high intensity acoustic standing wave within the fluid. The purpose of this research was to determine the effects of an acoustic sound field on nucleate pool boiling in both terrestrial gravity and microgravity. The platinum wire heater was used as a constant heat flux source and as a thermometer. The effects of acoustics on heat transfer were measured by the change in the average surface temperature of the heater. In terrestrial gravity a partial boiling curve was established with and without acoustics. In general, it was found that both the natural convection and boiling heat transfer rates were increased with the presence of an acoustic standing wave in terrestrial gravity. The highest heat transfer augmentation was registered when the heater wire was placed at the acoustic pressure antinode. In microgravity, the acoustic force was shown to be a promising means in maintaining nucleate boiling.

High performance heat transfer ducts with parallel broken and V-shaped broken ribs

The effect of the broken rib orientation on the local heat transfer distributions and pressure drop in a square channel with two opposite in-line ribbed walls is investigated for Reynolds numbers from 15 000 to 90 000. The square channel is composed of ten isolated copper sections and has a length-to-hydraulic diameter ratio of 20. The rib height-to-hydraulic diameter ratio is 0.0625, and the rib pitch-to-height ratio equals 10. The results show that the 60° parallel broken rib or 60° V-shaped broken rib provides a higher heat transfer augmentation than the 45° parallel broken rib or 45° V-shaped broken rib and, subsequently, higher than the 90° broken rib. The parallel ‘broken rib’ or V-shaped ‘broken rib’ has 2.5–4 times heat transfer augmentation compared with the previous parallel ‘continuous rib’ or V-shaped ‘continuous rib’ with 2–3 times heat transfer augmentation for the same amount of 7–8 times pressure drop penalty.