Higher Nusselt Number Research Articles

Nanofluid flow and MHD mixed convection inside a vertical annulus with moving walls and transpiration considering the effect of Brownian motion and shape factor

In the present study, the exact solution of a nanofluid flow and mixed convection within a vertical cylindrical annulus with suction/injection, which is adjacent to the radial magnetic field, is presented with regard to the motion of cylinders’ walls. The impact of Brownian motion and shape factor on the thermal state of CuO–water nanofluid is also considered. The influence of such parameters as Hartmann number, mixed convection parameter, suction/injection, volume fraction of nanoparticles and motion of cylinders’ walls on flow and heat transfer is probed. The results show that the shape of the nanoparticles could change the thermal behavior of the nanofluid and when the nanoparticles are used in the shape of a platelet, the highest Nusselt number is obtained (about 2.5% increasement of Nusselt number on internal cylinders’ wall comparison to spherical shape). The results shed light on the fact that if, for example, the external cylinder is stationary and the internal cylinder moves in the direction of z axis, the maximum and minimum heat transfer take place on the walls of internal and external cylinders, respectively (for η = 300, about 15% increasement of Nusselt number on internal cylinders’ wall). Furthermore, the enhancement of radius ratio between two cylinders increases the rate of heat transfer and decreases the shear stress on the internal cylinder’s wall.

Experimental study of the natural convection heat transfer performance for finned oval tubes at different tube tilt angles

The natural convection heat transfer of finned oval tubes was studied for different tube tilt angles (0° to 40°), fin spacing (6mm to 16mm) and Rayleigh numbers (11,000 to 130,000). Fin efficiency was determined by temperature measurements along the fin surface and temperature gradient calculations. Nusselt number and volumetric heat flux density were chosen as assessment parameters for the thermal performance. A comparison of the experimental data with correlations from literature was made and good agreement was found. Furthermore, the uncertainty of the measurements was evaluated. In the horizontal tube orientation (0°) the Nusselt number increases with fin spacing. However, the fin efficiency and the volumetric heat flux density reduce. The tilt angle of the longitudinal tube axis was found to have an essential impact on the thermal performance, in particular when the fin spacing is high. For the higher fin spacing values the horizontal orientation gives highest Nusselt number and volumetric heat flux density. At tube tilt angle of 40° the thermal performance becomes lowest for all fin spacing values. When the fin spacing is low, the effect of tube tilt angle is minor. From the experimental results correlations between Nusselt number, Rayleigh number, fin spacing and tube tilt angle are proposed to assist the future design of heat exchanger with tilted finned oval tubes.

CFD Investigation of Al2O3 Nanoparticles Effect on Heat Transfer Enhancement of Newtonian and Non-Newtonian Fluids in a Helical Coil

Abstract The present study applied computational fluid dynamics (CFD) to investigate the heat transfer of Newtonian (water) and non-Newtonian (0.3 %wt. aqueous solution of carboxymethylcellulose (CMC)) fluids in the presence of Al2O3 nanoparticles. To analyze the heat transfer rate, investigations were performed in a vertical helical coil as essential heat transfer equipment, at different inlet Reynolds numbers. To verify the accuracy of the simulation model, experimental data reported in the literature were employed. Comparisons showed the validity of simulation results. From the results, compared to the aqueous solution of CMC, water had a higher Nusselt number. In addition, it was observed that adding nanoparticles to a base fluid presented different results in which water/Al2O3 nanofluid with nanoparticles’ volume fraction of 5 % was more effective than the same base fluid with a volume fraction of 10 %. In lower ranges of Reynolds number, adding nanoparticles was more effective. For CMC solution (10 %), increasing concentration of nanoparticles caused an increase in the apparent viscosity. Consequently, the Nusselt number was reduced. The findings reveal the important role of fluid type and nanoparticle concentration in the design and development of heat transfer equipment.

Parametric study of major factors affecting heat transfer enhancement in a circular tube with vortex generator pairs

This paper deals with the parametric study of major factors influencing thermal and flow behaviors for delta winglet vortex generator (DWVG) pairs in a circular tube. The investigated parameters include six pitch ratios (PR = P/D = 2.4, 3.6, 4.8, 6.4, 9.6, and 19.2), two flow directions (upstream and downstream), and two winglet configurations (common-flow-down (CFD) and common-flow-up (CFU)). The air-flow and heat transfer behaviors in the tube are examined for Reynolds number Re = 5 × 103–2.5 × 104.The experimental results indicate that PR has a significant effect on the heat transfer enhancement. A smaller PR results in a higher Nusselt number and a higher friction factor. The PR = 9.6, corresponding to two winglet rings, yields the largest thermal enhancement factor (TEF) for the Re range examined. The combinations of winglet configuration, flow direction and angle of attack lead to the best TEF performance for α30°-s15-h7.5, α20°-s15-h7.5, and α30°-s20-h10 where α, s and h represent the angle of attack, spacing between VGs and height, respectively. As to winglet configuration, CFD configuration yields better thermal-hydraulic performance than CFU. Furthermore, the downstream flow renders 30–33% higher TEF than the upstream flow.

Effect of droplet characteristics on heat transfer of mist/air cooling in a pin-finned channel

Effects of droplet characteristics of mist/air cooling on heat transfer for three pin-fin structures are investigated. The round-tip pin-fin structure is newly proposed with partial detachment from one endwall with a round-shaped tip structure. A flat-tip pin-fin with partial detachment and a traditional pin-fin with full attachment serve as references. Reynolds-averaged Navier-Stokes equations and the shear-stress-transport turbulence model are applied. Influences of initial mist temperature, initial mist diameter and initial mist velocity are analyzed in the Reynolds number range 15,000 to 50,000. The round-tip pin-finned channel has highest heat transfer coefficient and lowest pressure loss among the structures. Heat transfer enhancement increases first gradually and then decreases sharply with increasing initial mist diameter but an optimal diameter exists for the highest Nusselt numbers. Nusselt number decreases monotonically with increasing initial mist temperature. Droplet movement and heat transfer are nearly independent of initial mist velocity.

Numerical investigation of the magnetic field effect on the heat transfer and fluid flow of ferrofluid inside helical tube

The effect of a magnetic field on heat and fluid flow of ferrofluid in a helical tube is studied numerically. The helical tube is under constant wall temperature boundary condition. Parametric studies are done to investigate the effects of different factors such as the magnetic field gradient value and Reynolds number on heat transfer rate and pressure drop. Results indicate that the magnetic field increases the Nusselt number by about 40%. At high magnetic gradient value, Nusselt number and friction factor rise slightly, while at low magnetic gradient value, the increment of Nusselt number is considerable. Furthermore, the growth of wall shear stress on tube wall results in lower thermal–hydraulic performance at the high magnetic gradient value. There is an optimum case for thermal–hydraulic performance which results in most top performance of helical tube in the presence of the magnetic field.

Heat transfer and flow structure in a rotating duct with detached pin fins

The realizable turbulence model was used to calculate flow and heat transfer characteristics in a rotating rectangular duct with detached pin fins in staggered arrangement. Transverse and longitudinal spacings of the pin fins were 2.5 D and 2.5 D, respectively. Reynolds number was 7,000 and Rotation number (Ro) varied from 0 to 1 while the clearance ranged from 0 to 0.5. Streamlines, vortex structure, wall shear stresses and Nusselt numbers were obtained. Flow phenomenon at the pin fin rear side and a leakage vortex near the pin fin tip were observed. High Nusselt numbers appeared on leading side. The leakage vortex imposed high energy losses in the mainflow. The flow and heat transfer characteristics were associated with Ro number and clearance. Middle values of Ro and clearance induced highest Nusselt numbers on leading side. However, high Ro and small clearance gave highest Nusselt numbers on trailing side.

Experimental and numerical research on heat transfer and flow characteristics in two-turn ribbed serpentine channel with lateral outflow

This paper experimentally and numerically investigates the heat transfer and flow characteristics of a two-turn ribbed serpentine channel with lateral outflow. The heat transfer coefficient was measured by a transient liquid crystal technique. Experiments were carried out at Reynolds numbers between 5,000 and 20,000 and rotation numbers of 0 and 0.03. The results indicate that with increasing inlet Reynolds number, the high-Nusselt number regions of the middle and lateral outflow channels gradually move to upstream ribs. The inlet channel shows the highest increase rate of the Nusselt number; whereas, the rate is lowest at downstream turning area. The rotation increases the trailing surface Nusselt number of the inlet and lateral outflow channels, and the increase rate is more prominent for higher values of the Reynolds number. However, the rotation has negative effects on the averaged area Nusselt number of the middle channel in cases of Re ≤ 17,000. The pressure coefficients decrease the in inlet and middle channels along the flow direction; whereas, they slightly increase in the lateral outflow channel. There is a long low-velocity vortex formed in the lateral outflow channel, and the vortex size is considerably reduced by the rotation. Accordingly, the rotation has the most positive effects on the pressure coefficient of the lateral outflow channel.

Effect of variation of cylindrical pin fins height on the overall performance of microchannel heat sink

Numerous researches are performed to take account of the heat transfer in the microchannel heat sink, due to its high efficiency and application in various fields of microelectronics. An attempt has been made to numerically investigate the effects of variation of cylindrical pin fin height, entrenched on bottom wall of microchannel. Three cases of microchannel heat sink are prepared: case 1: plain rectangular channel; case 2 and case 3 with decreasing pin fin height and with increasing pin fin height in the direction of flow, respectively. Also, diameter of pin fin is varied to obtain effects of any underlying flow feature on heat transfer augmentation. The analysis is performed for single-phase fluid deionized ultra-filtered water with temperature-dependent properties, for low Reynolds number range of 150–300. Higher Nusselt number is obtained for case 2, whereas lower pressure drop is obtained in case 3. The overall thermal performance of case 3 with increasing pin fin height outperforms the corresponding case 2 with decreasing pin fin height for the same pin fin diameter, due to the velocity distribution and reduced pressure drop in the downstream region in microchannel, which shows that the downstream region of microchannel heat sink has a significant impact in terms of the overall efficiency while establishing pressure drop as essential characteristics.

Investigation Of Forced Convection Heat Transfer From A Heater Mounted Ina Cavity Wall Using Various Nanofluids

Forced convection heat transfer from heater mounted in a cavity wall is investigated to reveal the relation among nanofluid properties. The base fluid is considered as water. The present study is focused on forced convection heat transfer from square heater subject to inflow and outflow inside a squarecavity. The interesting physics will be reported in connection with volume fraction, Reynolds number and nanomaterial properties. It is found that for a particular Reynolds number when nanomaterial is introduced the local heat transfer is increased. The wall attached vortex attributes a constant Nusselt number. It is also noticed that when the heater wall is subject to combination of vortex and main stream fluid results high Nusselt number than heat transfer due to wall attached vortices. Nanofluid results high Nusselt number for the same Reynolds number.

Flow and Heat transfer Experimental Investigation of Nanofluid in a Double pipe Heat Exchanger

This paper presents an experimental study on heat transfer characteristics in a double-pipe heat exchanger using different types of fluid. The nanofluid consists of zirconia was added to the base fluid (deionized water and Ethylene Glycol). The concentration of nanofluid was varied with a range of 0.2, 0.7 and 1.2 % and the flow rates of the base and nanofluid were varied with a range of 1.5, 1.8 and 2.1 L/min. The results indicated that the Ethylene Glycol has the highest Nusselt number (40.70) at inlet temperature (70 °C) and flow rate (2.1 L/min) among the other working fluids. The mixture of ethylene glycol and water has Nusselt number lower than that for number at (φ =1.2 %, temperature=70 °C and flow rate 2.1 L/min) and Reynolds number is (the Ethylene Glycol alone. From the experimental result it is clear that the Nusselt number of nanofluid increases with an increase in the concentration and flow rate. It was found that (24.2) of Nusselt 3679). The maximum heat transfer enhancement is (1.5077) for ethylene glycol at inlet temperature (70 °C) and the minimum enhancement is (0.16844) for ethylene glycol at temperature (40 °C).

Experimental investigation of forced convection heat transfer over horizontal tube with conical fins for different fin spacings and different inclination angles

This research was performed to experimentally study the heat transfer over a horizontal tube with conical fins for different fin spacings and different inclination angles. The air entering the test section first contacts the large surface of the conical fin. Therefore the conical fin both directs the air towards the heating surface and increases the heat transfer surface area. The experiments were conducted for three different inclination angles (45°, 60° and 80°) and three different fin spacings (10, 12, and 15 mm). Water and air were used as hot and cold fluid in these experiments, respectively. The water temperature, which was the heating fluid, was kept fixed at 65 °C. The cold fluid entered the test section at eight different air flow velocities (2–20 m/s). Experimental results indicated that the Nusselt number at each inclination angle for the 10 mm fin spacing was almost the same. Moreover, for the 15 mm fin spacing, the highest Nusselt number was obtained at 80° and lowest at 60° for Re > 5 × 104, while the Nusselt number was almost the same at each inclination angle for Re < 5 × 104. It was determined that the lowest pressure drop was obtained at 80° for both 10 and 15 mm.

Determining optimal formulations and operating conditions for Al2O3/water nanofluid flowing through a microchannel heat sink for cooling system purposes using statistical and optimization tools

Adding the proper nanoparticles to the fluid will result in the enhancement of transfer rate in a microchannel due to enhanced thermal conductivity of the working fluid, which inspired applying nanofluids in the small heat exchangers. This study aims to optimize the heat transfer characteristics of Al2O3/ water nanofluid flowing in a microchannel for cooling system purposes. Three volume fractions of 0.1%–1% and nanofluid flow were considered in Reynolds numbers of 211–461. The goal was to obtain an optimum condition in terms of volume fraction and Reynolds numbers in order to have the lowest friction factor and the highest Nusselt number. Two distinctive mathematical correlations presented for Reynolds number and volume fraction to model the Nusselt number and friction factor of the nanofluid. First, the analysis of variance (ANOVA) introduced followed by determining models for non-dominated sorting genetic algorithm II (NSGA-II) which result in a multi-objective optimization condition. Furthermore, the optimum points for the nanofluid in each operating conditions were calculated. Finally, a couple of correlations among the calculated optimum points were proposed.

Flow structure and heat transfer characteristics in a 90-deg turned pin fined duct with different dimple/protrusion depths

In this study, numerical simulations are performed to investigate the effect of the dimple/protrusion depth on the flow structure and heat transfer characteristics in a 90-deg turned channel with pin fins. The pin fins and dimple/protrusion are arranged on the endwall surface with staggered layout. Five different dimples/protrusions depth-to-diameter ratios are investigated, i.e., δdd=0,0.1,0.2,0.3,0.4. The Reynolds number ranges from 8500 and 60,000. Results of the flow structure, heat transfer on the endwall surface and friction factor are investigated in this study. Results indicate that the dimple/protrusion depth has a remarkable influence on the flow structure and heat transfer. The horseshoe vortex generated by the pin fin is decreased while the impingement effect within the dimple is increased as the dimple depth increased. In a pin fin channel with protrusion, a new horseshoe vortex which causes a high Nusselt number near the endwall is induced after adoption the protrusion. The higher protrusion depth induces a higher Nusselt number. It is also found that the volume goodness factor is increased after adoption the dimple/protrusion compared to pure pin fin channel.

Multi-objective optimization of the dimple/protrusion channel with pin fins for heat transfer enhancement

PurposeThe purpose of this paper is to investigate the optimal geometry parameters in a dimple/protrusion-pin finned channel with high thermal performance.Design/methodology/approachThe BSL turbulence model is used to calculate the flow structure and heat transfer in a dimple/protrusion-pin finned channel. The optimization algorithm is set as Non-dominated Sorting Genetic Algorithm II (NSGA-II). The high Nusselt number and low friction factor are chosen as the optimization objectives. The pin fin diameter, dimple/protrusion diameter, dimple/protrusion location and dimple/protrusion depth are applied as the optimization variables. An in-house code is used to generate the geometry model and mesh. The commercial software Isight is used to perform the optimization process.FindingsThe results show that the Nusselt number and friction factor are sensitive to the geometry parameters. In a pin finned channel with a dimple, the Nusselt number is high at the rear part of the dimple, while it is low at the upstream of the dimple. A high dissipative function is found near the pin fin. In the protrusion channel, the Nusselt number is high at the leading edge of the protrusion. In addition, the protrusion induces a high pressure drop compared to the dimpled channel.Originality/valueThe originality of this paper is to optimize the geometry parameters in a pin finned channel with dimple/protrusion. This is good application for the heat transfer enhancement at the trailing side for the gas turbine.

Investigation on the Effect of Different Lands on Trailing Edge Slot Film Cooling

Abstract Trailing edge slot film cooling is a widely used method for protecting the trailing edge of turbine blades from hot gas impingement. The structures that separate the slots, known as “lands,” come in a variety of configurations. This paper presents the effects of the trailing edge with different lands on the film cooling performance. Experimental studies are conducted on the film cooling effectiveness and Nusselt number with different lands. Four trailing edge configurations, including the straight lands, the beveling lands, the fillet lands and the tapered lands are considered under four blowing ratios (0.5, 0.7, 1.0 and 1.5). The Reynolds numbers of mainstream is fixed as 375,000. Film cooling effectiveness and Nusselt number performances are measured by transient liquid crystal measurement technique. Reynolds-averaged Navier-Stokes (RANS) simulation with realizable k-ε turbulence model and enhanced wall functions are performed using a commercial code Fluent. In each case, the slot height is kept constant. It is shown that the beveling lands, the fillet lands and the tapered lands have higher cooling effectiveness and lower Nusselt number compared with the straight lands. Under higher blowing ratios, the trailing edges of all four lands have higher cooling effectiveness and higher Nusselt number.

Analysis on heat transfer and pressure drop of fin-and-oval-tube heat exchangers with tear-drop delta vortex generators

Numerical simulations are performed to examine the thermal and flow characteristics of tear-drop delta vortex generators (VGs) on the fin surfaces of fin-and-oval-tube heat exchangers. The VGs deployed in a “common flow up” configuration behind tubes can induce longitudinal vortices and reduce the area of wake region, which results in significant heat transfer enhancement and negligible augmentation of pressure drop compared with plain fins without VGs. The heat transfer performance and pressure loss are analyzed using the dimensionless parameters j/j0, f/f0, and R = (j/j0)/(f/f0)1/3 with ReDc ranging from 255 to 1533. The results indicate that the tear-drop delta VGs have a better thermal-hydraulic performance than plane delta VGs and the value of R reaches as high as 1.06–1.23 at the ratio of division for chord length (l1/l2) being 2/3. Further parameters study reveals that when the height of VGs is 0.6 times of fin pitch, the lateral length is 0.3 times of fin pitch, and the chord length is 1.2–1.4 times of fin pitch, the optimal overall performance can be obtained. The mechanism of heat transfer enhancement is investigated by intensity of secondary flow and field synergy principle. It suggests that for the case with higher Nusselt number, the corresponding secondary flow intensity is higher and the synergy angle is smaller. In addition, for each case, where the Nusselt number is higher, the secondary flow intensity is higher and the synergy angle is smaller.

Effects of the pocket cavity on heat transfer and fluid flow of the downstream outlet guide vane at different flow attacking angles

A pocket cavity is generated at the junction position of the low pressure turbine (LPT) and the outlet guide vane (OGV) in the rear part of a modern gas turbine jet engine. In the present study, a triangular pocket cavity is placed upstream of an OGV at different distances. The effects of the pocket cavity on heat transfer and fluid flow of the downstream OGV with different flow attack angles are investigated numerically with well validated turbulence models. The flow attack angles are varied as –30°, 0°, and +30° at a constant Reynolds number =160,000. The turbulent flow details are provided by numerical calculations using two turbulence models, the unsteady DES model and the steady k-ω SST model. For different flow attack angles, the high Nusselt number regions around the OGV are changed. The high heat transfer region is really drawn back at a flow attack angle = +30° (Case 2b) compared with Case 2a with a flow attack angle =0°. As the flow attack angle is changed to –30° (Case 2c), the high Nusselt number regions are greatly enlarged not only on the suction side also on the pressure side because of the strengthened flow impingement on the vane surfaces. The pocket cavity weakens the flow impingement on the vane surfaces and the effect is more obvious when the pocket cavity is placed close to the vane. In addition, the heat transfer distribution over the pocket surface is also affected by the location of the vane. When the vane is placed close to the pocket cavity (Case 1), the heat transfer on the pocket edge is increased. In the case with a flow attack angle =0°, the high turbulent kinetic energy region is mainly located near the vane and wake region downstream the vane and recirculating flows can hardly be found.

Numerical simulation for external magnetic field influence on Fe3O4-water nanofluid forced convection

PurposeThe purpose of this paper is to simulate nanofluid laminar steady flow in a lid-driven porous cavity under the impact of Lorentz forces.Design/methodology/approachShape effect of nanoparticles and magnetic field impact on nanofluid properties are taken into consideration. The solutions of final equations are obtained by control volume based finite element method (CVFEM).FindingsGraphs are depicted for different values of Darcy number, Fe3O4-water volume fraction, Reynolds and Hartmann numbers.Originality/valueResults illustrated that using Platelet-shaped nanoparticles results in the highest Nusselt number. Nusselt number augments with rise of Darcy and Reynolds number.

Numerical comparison for thermo-hydraulic performance of pin fin heat sink with micro channel pin fin heat sink

Cooling of miniature size electronic components has become a challenge for designer in the development of integrated circuits. Micro pin fin heat sink and Micro channel pin fin heat sink are thermal management techniques for effective cooling. The paper presents comparison of fluid flow and heat transfer characteristics for micro pin fin heat sink and micro channel pin fin heat sink with unfinned micro channel heat sink. A three-dimensional heat sink with water as coolant subjected to constant heat flux 10 W/cm2, for Reynolds number ranging between 100 and 900 was considered for the study. Extended surfaces of different shapes namely, square and circular with staggered arrangement was considered for both micro pin fin heat sink and micro channel pin fin heat sink. Two non-dimensional parameters namely Nusselt number and thermal performance index were employed to access the performance of heat sink. Results indicate that the microchannel pin fin heat sink has highest nusselt number and friction factor over the whole Reynolds number range. Results also revealed that formation of secondary vortices enhances heat transfer in micro channel heat sink with square pin fin compared to micro channel heat sink with circular pin fin. However, pin fin heat sink has better thermal performance index compared to Micro channel pin fin heat sink and is more preferable when heat dissipation is compared with pressure drop penalty. The Governing equations for fluid and solid domain were solved using FLUENT to study flow and heat transfer characteristics.