Higher Nusselt Number Research Articles

Experimental Investigation on Mist Flow and Heat Transfer in a Uniformly Heated Vertical Cylinder

The steady, internal, and ultrasonic-generated water mist cooling in a uniformly heated vertical cylinder with evaporating subcooled water film has been investigated experimentally. The goal is to elucidate the effects of different operating parameters, viz. water mist concentration, carrier gas velocity, loaded heat flux, and flow orientation on the heat transfer rate and pressure drop characteristics. The experiments were performed in a turbulent flow regime over a range of Reynolds number (from 6000 to 18000) using air as the carrier fluid. Compared with the cylinder without water mist, the results showed that using water mist as a coolant increases the Nusselt numbers along the cylinder, high Nusselt numbers are obtained by suspending the high-water mist and it was about 224% for heat flux q=1.56 kW/m2. The results also showed that the local Nusselt number rapidly decreases as the heat flux increases, this decreasing in the Nusselt number was the most at high heat flux q=4.81 kW/m2, in which the water film early breakdown and the breakdown point gets closer to the entrance region. In addition, a new empirical correlation of the heat transfer rate was proposed to estimate the real benefits in using water mist of the enhanced cylinder.

Finite element analysis for free convection with in porous trapezoidal cavity

Natural convection analyze in porous trapezoidal enclosures have many important energy-related applications such as solar and geothermal energy sectors. A numerical study of thermal mixing and heat distribution during steady laminar natural convective flows within the porous trapezoidal enclosure undergoing various radiation effects was investigated in this study using the Darcy model and the Finite Element Approach. The lower and upper walls of the trapezoidal enclosure are kept constant temperature at Th and Tc respectively, while the walls adjacent to the vertical are thermally insulated. Simulations have been carried out for the various Rayleigh numbers 101 –103 and the radiation factors 1, 5 and 10. Enhanced convection at high radiation factor significantly affects the distribution of heat energy, which is clearly illustrated by the high local Nusselt numbers at Ra = 103 . Isothermal heating of the walls with the radiation factor was found to increase the thermal mixing and heat distribution. Significant changes in temperature contours and energy streamlines have been observed for increasing Rayleigh numbers.

Multi-Faceted Surface Effect on Heat Transfer Performance and Flow Dynamics at the Onset of Optimum Nusselt Number

A multi-faceted cylinder of half-circle, flat, and triangle surfaces is investigated numerically in this study. The flow characteristics were observed. Their effects on thermal performance were investigated under various cross-flow conditions and blocking ratios. The multi-faceted cylinder's effectiveness compared to the circular cylinder in terms of a heat exchanger is also presented. The multi-faceted cylinder suffers an average of 0.47% drop in effectiveness relative to the circular cylinder. However, despite producing a higher heat transfer coefficient, the circular cylinder has a lower Nusselt number contributed by a higher Strouhal number. A lower Strouhal number in a multi-faceted cylinder enables it to have a higher Nusselt number in the downstream surface than the circular cylinder. This is due to better fluid contact through flow attachment at the flat surface and the sharp edge. The multi-faceted cylinder's asymmetrical surface also creates higher velocity imbalances in the upper and lower half of the channel, which produced a distinct drag coefficient pattern. The multi-faceted cylinder offers enhanced convective heat transfer compared to the circular cylinder but with relatively higher drag under increasing Reynolds number.

Assessment of the magnetic field influence on heat transfer transition of natural convection within a square cavity

The present work investigates numerically the natural convection within a square cavity under effects of inclined magnetic field with focus on heat transfer transition. The streamlines and the corresponding isotherms at various magnetic field intensities and angles are evaluated. The results disclose that enhancing the magnetic field intensity causes significant changes of streamlines and isotherms. Both the average Nusselt number (Numean) and maximum streamline function (Smax) decrease with magnetic field and exhibit sinusoidal oscillations with a period of 180°. In the condition of high Hartmann number (Ha), maximum velocity and Nusselt number appear at the left bottommost corner of the cavity subjected to magnetic angle of θ = 45◦, leading to the largest Smax and the best heat transfer performance. Owing to the interaction between the flow field and magnetic field, heat transfer patterns can be characterized by the variation of dimensionless Nusselt number Nu* over the angle of magnetic field, and named Pattern unimodal, Pattern peak-valley, Pattern transition, and Pattern bimodal. At various conditions of Ha and Rayleigh number (Ra), an empirical correlation is proposed to predict the heat transfer transition, which is useful in further understanding of the heat transfer mechanism within the square cavity.

Numerical analysis on the thermal performance of microchannel heat sinks with Al2O3 nanofluid and various fins

• A single-phase three-dimensional flow model compares different finned microchannels. • The fin shape has significant effects on microchannel heat transfer characteristics. • A zigzag fin gives the highest rate of heat removal to hydraulic power loss ratio. • The zigzag fin gives a 9 K lower contact temperature and 60% higher Nusselt number. • Higher thermal performance is achieved with the Al 2 O 3 nanofluid compared to water. The hydraulic and thermal performance of microchannel heat sink configurations for high performance electronic cooling applications is investigated by numerical modelling. Conjugate heat transfer simulations are obtained through the silicon walls and the fluid domain of a square base prism heat sink traversed by 50 parallel rectangular cooling ducts, under a 150 W/cm 2 constant heat flux input through the base. Al 2 O 3 nanofluid coolant with a nanoparticle volume fraction ranging from 0 to 3% is supplied at 298 K, over the Reynolds number range 100 to 350, modelled as a single-phase homogeneous medium. Rectangular, twisted, and zig-zag fins are inserted into the plain rectangular duct to enhance the heat transfer rate. The zig-zag fin and 3% Al 2 O 3 nanofluid provide the best thermal performance, with a 6.44 K lower average heated wall contact temperature, 60% higher Nusselt number, and 15% higher second law efficiency than without fins and plain water cooling. Twist in the microchannel fin unexpectedly reduced the microchannel pressure drop by 2% to 15% compared to a straight fin, possibly due to the more evenly distributed axial mass flux across the microchannel.

Multi objective optimization of TiO2/water nanofluid flow within a heat exchanger enhanced with loose-fit delta-wing twisted tape inserts

A hybrid technique by using TiO2/water nanofluid together with loose-fit delta-wing twisted tape (LTT-W) was employed for heat transfer enhancement. Experiments encompassed the TiO2-water nanofluids having concentration (φ) of 0.05%–0.15 vol% and the loose-fit delta-wing twisted tapes having two different wing arrangements (co- and counter arrangement) and three loose-fit ratios (c/D) of 0.0, 0.15, and 0.2. Experimental revealed that the system with combined enhancement technique gave considerably higher heat transfer than the one without enhancement technique. This can be attributed to the combined influences of swirling flow and higher thermal conductivity of the working fluid. Heat transfer rate and thermohydraulic performance rose with the decrease of loose-fit ratio and the rise of nanofluid concentration. The maximum thermohydraulic performance (TPF) of 1.36 was obtained at c/D = 0.0 and φ = 0.15%. The Multi-Objective Optimization (MOO) was also performed to study the optimal thermohydraulic performance by using GMDH models and NSGA II algorithms. The Pareto front, which contains very useful information, were extracted for both co and counter arrangements. The Pareto fronts have recognized very accurately, the best boundary of the experimental data with respect to the lowest friction factor and highest Nusselt number.

Heat Transfer Measurements for Array Jet Impingement With Castellated Wall

Abstract Impingement cooling is one of the powerful cooling methods in high-temperature devices. For the gas turbine applications, impingement cooling is commonly applied in the transition piece of a combustor and in the leading edge, suction and pressure sides of a turbine blade/vane. In the suction side and pressure side, impingement cooling is applied as a form of an array jet. However, due to the small gap between the jet hole and target surface, the wall jet faces a crossflow inside of the gap. This crossflow has an adverse effect on the jets and deteriorates the heat/mass transfer performance. Therefore, several studies have been conducted to minimize the crossflow effect. The present study also investigated the effect of crossflow reduction in the gap by having a castellated hole plate. The heat/mass transfer was measured using the naphthalene sublimation method. Heat/mass transfer data are compared among three different cases. One is the baseline case which is simple array impinging jets. Others are the castellated cases with and without rib structures on the target wall. Jet-to-jet spacing(s/d) and jet-to-target spacing(z/d) are selected as geometrical variables. Also, the experiments were conducted for the Reynolds numbers (based on jet hole diameter) of 5,000, 15,000, and 30,000. The baseline case was named as B case, and the castellated case without rib structure as C case and with rib structure as CR case. Both C and CR cases showed the crossflow reduction effect and resulted high and similar Nusselt number values.

Effect of Linearly Varying Heating Inside a Square Cavity under Natural Convection

Telecommunication devices such as ADSL Modems or Wi-Fi routers are being widely used around the globe. Thermal management of such equipments are of critical importance as the increased power consumptions caused by technological upgrades results in increased heat generation within these systems. The heat transfer process inside such sealed and passively cooled equipments can be simplified as natural convection inside enclosures. Studying actual conditions inside electronic enclosure are necessary for their effective thermal management. This study aims at investigating the effect of non-isothermal heating inside such enclosures with linearly varying temperature distribution on free convection inside square enclosure. The issue of free convection of air interior of a square chamber with linearly varying temperature distributions on the left partition is studied numerically. The effect of change of Rayleigh number, temperature distributions, on flow and temperature field and rate of transfer of heat are analysed. Rayleigh number is chosen to vary in between 103 and 106. Four different cases of linearly varying temperature distributions are considered. The outcomes are presented as stream line plot, isotherm contour and average Nusselt number. The outcomes depicted that case of linearly increasing temperature along the height gives higher Nusselt number than other cases.

Thermo-hydraulic Performance Investigation of Twisted Tapes Having Teardrop-Shaped Dimple-Protrusion Patterns

The teardrop pattern observed to be more efficient in straight tubes was examined for the first time in a twisted tape wall. The thermo-hydraulic performances of twisted tapes having periodically teardrop-shaped dimple-protrusion patterned walls are numerically investigated. The new designs have two different densities (N=30, and 45), three alternative types of layouts (composed of only protrusions, only dimples, and a combination of dimples and protrusions) align in two different flow directions. Designs have a constant twist ratio (TR) of 3, and calculations are conducted for Reynolds numbers (Re) between 3000 and 27000 by Ansys Fluent under a constant heat flux of 50000 W/m2. All cases have higher Nusselt number (Nu) values than traditional twisted tape (TT) in the tested Re region. Besides, all arrangements except the teardrop protruded twisted tape have higher thermal performance factors than the TT in the Re range from 3000 to 5500. All dense versions of the teardrop patterned twisted tapes have higher Nu and friction factor (f) than sparse versions of them. The dense teardrop dimpled-protruded (D-TDP) case reaches TPF of 1.517, the highest value among the tested twisted tapes, at Re = 3000.

Heat transfer augmentation using periodically spherical dimple-protrusion patterned walls of twisted tape

The heat transfer performances of pipes with twisted tape inserts having periodically spherical dimple-protrusion patterned walls are numerically investigated by Ansys Fluent under a constant heat flux of 5 W/cm2. The inserts having three different densities (N = 0, 30, and 45) and three different kinds of arrangement (composed of only dimples, only protrusions, and a combination of dimples and protrusions) with a constant twist ratio of 3 were tested for Reynolds numbers (Re) within the range from 3000 to 27000. Comparing the turbulence models proposed the validated realizable k-ϵ model coupled with enhanced wall function to employ on the numerical simulations. While the highest Nusselt number (Nu) and friction factor (f) values for the presented study were found for the dense protruded twisted tape (D-PT) arrangement at Re values lower than 18000 and for dense dimpled-protruded twisted tape (D-DPT) arrangement for the Re greater than 18000, dimpled-protruded twisted tape (DPT) and dimpled twisted tape (DT) sequences reached the lowest f values of 0.365 and 0.322 at Re = 3000, respectively. D-DPT and D-PT sequences reached the highest thermal performance factor (TPF) values of 1.508 and 1.478 at Re = 3000, respectively. All patterned twisted tapes performed better Nu than conventional twisted tapes in the analyzing range and performed better TPF than typically twisted tapes in the Re region of less than 6000.

Experimental Study of Impingement Effusion-Cooled Double-Wall Combustor Liners: Thermal Analysis

A new experimental study is presented for a combustor with a double-wall cooling design. The inner wall at the hot gas side features effusion cooling with 7-7-7 laidback fan-shaped holes, and the outer wall at the cold side features an impingement hole pattern with circular holes. Data have been acquired to assess the thermal and aerodynamic behavior of the setup using a new, scaled up, engine-similar test rig. Similarity includes Reynolds, Nusselt, and Biot numbers for hot gas and coolant flow. Different geometrical setups are studied by varying the cavity height between the two walls and the relative alignment of the two hole patterns at several different blowing ratios. This article focuses on the thermal performance of the setup. The temperature data are acquired using two infrared systems on either side of the effusion wall specimen. In addition to cooling effectiveness evaluations, finite element simulations are performed, yielding the locally resolved wall heat fluxes. Results are presented for three cavity heights and two longitudinal specimen alignments. The results show that the hot gas side total cooling effectiveness can achieve values as high as 90% and is mainly influenced by the effusion coverage. Impingement cooling has a small influence on overall effectiveness, and the area of influence is mainly located upstream where effusion cooling is not built up completely. The analyzed geometric variations show a major influence on cavity flow and impingement heat transfer. Small cavities lead to constrained flow and high local Nusselt numbers, while larger cavities show more equalized Nusselt number distributions. A present misalignment shows especially high influence at small cavity heights. The largest cavity height, in general, showed a decrease in heat transfer due to reduced jet momentum.

Thermal Performance of V-Shaped and X-Shaped Ribs in Trapezoidal Cooling Channels

Convective heat transfer enhancement using rib turbulators is effective for turbine blade internal cooling. Detailed heat transfer measurement of X-shaped ribs in a trapezoidal cooling channel was experimentally conducted using infrared thermography. The novel X-shaped ribs were designed by combining two V-shaped ribs, and more secondary flows generated by the X rib delivered higher heat transfer enhancement. The Reynolds numbers in this study were 10,000, 20,000, and 30,000. These ribs were installed on two opposite walls of a trapezoidal channel in a staggered arrangement. The rib pitch-to-height ratios were 10 and 20, and the rib height-to-hydraulic diameter ratio was 0.128. Results indicated that higher heat transfer distribution was observed in the vicinity of the shorter base in the trapezoidal channel. The full X-shaped ribs and the V-shaped ribs demonstrated the highest Nusselt number ratios among all the cases. Although full X-shaped ribs contributed to higher heat transfer improvement due to intensified secondary flows, they also caused significant pressure loss. Therefore, the cutback X-shaped ribs were proposed by removing a segment in the rib at either upstream or downstream region. Consequently, the upstream cutback X-shaped rib and the V-shaped rib produced the highest thermal performance in this trapezoidal channel.

Heat transfer augmentation in a tube with conical wire coils using a mixture of ethylene glycol/water as a fluid

The effects of using convergent, convergent-divergent and divergent conical wire coils in ethylene glycol and water mixture flow region on the augmentation of heat transfer are experimentally analyzed. Three different volumetric ratios (0:100), (20:80) and (40:60) of ethylene glycol and water, convergent, convergent-divergent and divergent conical wire coils with two different pitch ratios 2 and 3 are intended for the experiments. The experiments are conducted under the turbulent flow conditions Reynolds number ranging from 4627 to 25,099 and constant heat flux. The use of conical wire coils enhance both the heat transfer rate and fluid friction, whereas adding the ethylene glycol on pure water decreases the heat transfer rate and friction factor is slightly increased. The highest Nusselt number is obtained as 558 for the divergent conical wire coil with pitch ratio of two at Reynolds number of 22,846 in the case of using a fluid type with a (40:60) volumetric ratio. The highest performance evaluation criteria of 1.62 is achieved for the divergent wire coil insert P/D = 2 is used at lowest Reynolds number for Ethylene glycol:Water-0:100 fluid type. New correlations are modelled to predict the Nusselt number and friction factor. Therefore, making the currently investigated configurations a potential technique to increase the thermohydraulic performance of the engineering applications.

Performance of a double-pipe heat exchanger with different met-al foam arrangements

This paper contributes to the field of improving the performance of heat exchangers using metal foam (MF) full-filled and partially/periodically-filled within the gap between the two pipes. The effect of configuration and arrangement of copper MF (15PPI and porosity of 0.95) installed on the outer surface of the inner pipe of a counter-flow double-pipe heat exchanger on the thermal and hydraulic performance was studied experimentally. The test section consisted of concentric two pipes; the inner pipe which was made of copper while the outer pipe was a Polyvinyl chlo-ride. Air was used as a working fluid in both hot and cold sides. A wide cold air flow rate range was covered from 3 to 36 m3/h which corresponds to Reynolds number (Re) range from 2811 to 31,335. The hot air flow rate was kept constant at 3m3/h. The temperature difference (ΔT) be-tween the inlet hot air and inlet cold air was adopted to be (20°C, 30°C, 40°C, and 50°C). The re-sults revealed that the higher Nusselt number (Nu) was at ΔT= 50°C and the thermal performance of the heat exchanger with the MF for all the arrangements was greater than the smooth heat exchanger. The highest and lowest friction factor was 1.033 and 0.0833 for the case 1 and 8, re-spectively, and the optimal performance evaluation criteria (PEC) was 1.62 for case 7 at Re = 2800. The Nu would be increased with a moderate increase in the friction factor by optimizing the arrangement of the MF. The two essential parameters that played an important role for in-creasing the PEC were the MF diameter and the MF arrangement along the axial length of the cold air stream.

Flow and heat transfer in obstacled twisted tubes

The thermal and hydraulic performance of a twisted circular tube with an obstacle attached to the internal surface along the tube length (L = 1000 mm) is numerically analyzed in this work. The considered configuration is investigated by varying obstacle height ratio (OHR = h/D) which is the obstacle height (h) to the tube diameter (D = 20 mm) while obstacle thickness is kept constant under turbulent flow regime, i.e. Reynolds number 4000 ≤ Re ≤ 10,000 for air as working fluid. Various cases of OHR are triggered 0.0, 0.125, 0.25, 0.375, 0.5 and 0.625 respectively. The results show that the larger OHR gives a higher Nusselt number and vice versa due to increasing swirling power to the air in the tube. However, a higher friction factor is the penalty associated with this large obstacle. Recorded thermal performance factor TPF is beyond unity for all cases. The optimal value of TPF is observed at OHR = 0.625 which is recommended for high heat transfer rate requirement. Corresponding TPF = 2.07 at Re = 4000 and TPF = 1.69 at Re = 10,000. The results are validated against the findings of the previous works and the comparisons show good agreements.

Analysis of vortex cooling fluid-structure interaction under different vortex chamber curvature

The vortex cooling with cascade channel and blade leading edge solid region is established. The flow and heat transfer properties of vortex cooling region, and the stress and displacement properties of blade solid region are analyzed by the fluid-structure interaction method. The ellipses edges with vertical axis to horizontal axis ratio a/b of 0.48, 0.72, 1.00, 1.31 and 1.73 are utilized to form the vortex chamber with different curvature. Influences of vortex chamber curvature on the vortex cooling fluid-structure interaction characteristics are researched to reveal the vortex cooling high heat transfer mechanism deeply. Results show that, for a/b decreasing from 1.73 to 0.48, the overall average Nusselt number and comprehensive heat transfer factor will first increase and then decrease, and the drag coefficient will gradually decrease. As a/b decreases, the high Nusselt number region moves from the target wall next to the nozzle to the target wall under the nozzle and at the vortex chamber bottom. With decreasing a/b, the thermal stress and thermal displacement on the blade pressure surface gradually decrease, while on the blade suction surface the thermal stress first decreases and then increases, and the thermal displacement first increases, then decreases and finally increases. In this study, compared with a/b = 1.73, the heat transfer performance for a/b = 0.72 has been enhanced by 21.00% and can provide the best protection for the blade.

Effect of pitch distance of rotational twisted tape on the heat transfer and fluid flow characteristics

Numerical simulation is performed to evaluate the privilege of inserting stationary or rotating twisted tape inside a tube as a heat transfer enhancement technique. The main emphasis of this work is placed on understanding the effect of pitch distance on the hydrothermal performance of such a system. The obtained results are validated with published experimental findings. The results of numerical study shows that the heat transfer, pressure drop and total energy consumption are increased by inserting a twisted tape inside a tube. Under the stationary condition, decreasing the pitch distance results in higher Nusselt number together with more friction factor. As the twisted tape starts to rotate (RTT1 case), both Nusselt number and friction factor are further increased. However, decreasing the pitch distance from L/2 to L/6 exhibits a marginal impact on the Nusselt number and friction factor. Further increase in the angular velocity (RTT3 case), although improved the average Nusselt number, comes at the expense of much more friction factor as well as energy consumption. Considering performance evaluation criterion (PEC) as a metric to assess the interaction between heat transfer and energy consumption, it is found that rotating twisted tape is beneficial at lower Reynolds number. Among the design parameters analysed in this research, the highest PEC number of 1.5 corresponds to the case of stationary twisted tape at Reynolds number of 1000 and twisted tape pitch of L/6.

Optimizing thermal performance and exergy efficiency in hydrogen-fueled meso-combustors by applying a bluff-body

Recognizing the importance of a bluff-body on the flow separation and its role in enhancing the fuel mixing, the effect of applying a bluff-body on the thermal performance and exergy efficiency in hydrogen-fueled meso-scale combustors are discussed using 3D numerical simulations involving a detailed kinetics mechanism. Key parameters including the bluff-body dimensionless axial location l and combustor configuration are evaluated to obtain the optimal geometry. It is found that the bottom wall mean temperature (BWMT) and its uniformity are significantly enhanced with a bluff-body applied, generally accompanied with a high average Nusselt number. This is mainly due to the fact that the bluff-body can create longitudinal vortices and disrupts the growth of the thermal boundary layer, leading to intensified heat transfer. There is a non-monotonic relationship between BWMT and l with the optimum position being l = 2/5. Further, the proposed Combustor A is involved with a higher and more uniform BWMT. Finally, the exergy efficiency in the bluff-body combustor is found to be higher compared to the conventional combustor, although the elevated pressure loss. This work demonstrates the feasibility of implementing a bluff-body to enhance the thermal performance, and some of the interesting findings can be also applied to power and propulsion systems utilizing the combustor wall temperature.

CFD Analysis and Taguchi-Based Optimization of the Thermohydraulic Performance of a Solar Air Heater Duct Baffled on a Back Plate

In this paper, a solar air collector duct equipped with baffles on a back plate was numerically investigated. The Reynolds number (Re) was varied from 5000 to 20,000, the angle baffle (a) from 30° to 120°, the baffle spacing ratio (Pr) from 2 to 8, and the baffle blockage ratio (Br) from 0.375 to 0.75 to examine their effects on the Nusselt number (Nu), the friction factor (f), and the thermohydraulic performance parameter (η). The 2D numerical simulation used the standard k-ε turbulence model with enhanced wall treatment. The Taguchi method was used to design the experiment, generating an orthogonal array consisting of four factors each at four levels. The optimization results from the Taguchi method and CFD analysis showed that the optimal geometry of a = 90°, Pr = 6, and Br = 0.375 achieved the maximum η. The influence of Br on all investigated parameters was considerable because as Br increased, a larger primary vortex region was formed downstream of the baffle. At Re = 5000 and the optimal geometry parameters, a maximum η of 1.01 was reached. A baffle angle between 60° and 90° achieved a high Nusselt number due to the impingement heat transfer.

Heat transfer augmentation in a circular tube fitted with tri-partition flow splitters

This paper focuses on the numerical and experimental study of a heated tubes heat transfer and flow characteristics with a novel tri-partition flow splitter under forced convection. A low thermal conducting fluid like air is considered to be a working fluid. Tri-partition flow splitters were manufactured with different thickness and arranged alternatively with 180° rotations to change the direction of airflow. Seven Reynolds number in the range of 5000–15,000 were considered as an operating parameter. The results of the plain tube are validated with the standard Dittus–Boelter and Gnielinski equations available in literature. In addition, the experimental results with tri-partition flow splitters are used to validate the computational fluid dynamics model used to examine the flow characteristics in the heated tube. The overall thermal performance enhancement is evaluated by the performance evaluation criteria (PEC). The results showed that the Nusselt number (Nu) is increased with an increase in Reynolds number. The maximum PEC 1.98 is obtained for 2 mm thickness splitters. The highest Nusselt number (Nu = 271.99) is found for 10 mm thickness with a higher friction drop. Friction factor values are found dropped by reducing the thickness of the splitters.