A Numerical Investigation of the Effect of Inlet Velocity Oscillation on Heat Transfer in a Two-Dimensional Laminar Jet Impinging on an Isothermal Surface

2016 ◽  
Author(s):  
Johnny Issa ◽  
Najib Saliba ◽  
Bchara Sidnawi
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Steady State ◽  
Strouhal Number ◽  
Average Nusselt Number ◽  
Inlet Velocity ◽  
Isothermal Surface ◽  
The Right ◽  
State Case

Heat transfer in a laminar confined oscillating slot jet impinging on an isothermal surface is numerically investigated. A uniform inlet velocity profile, oscillating with an angle φ, is used at the jet exit. The angle φ changes in a sinusoidal form. The height-to-jet width ratio is fixed at 5. The working fluid is air with constant physical properties corresponding to Prandtl number, Pr, equal to 0.74 at ambient conditions. Reynolds number, Re, is defined based on the jet hydraulic diameter and is varied in the self-stable range between 100 and 400. Strouhal number, St, is also varied between 0.05 and 0.75. Oscillating the jet at Reynolds number equal to 100 showed no heat transfer improvement over the steady state case, regardless of the used Strouhal number values. The vortices generated by the oscillation were too weak and could barely reach the wall. The flow showed a high vulnerability to severe oscillations which drastically reduced the jet heat removal ability. The vorticity contours showed a perfect symmetry which resulted in instantaneous and average Nusselt number distributions that are symmetric about the center of the isothermal surface at x = 0. The average stagnation Nusselt number, Nu0, decreased by about 1.25% as Strouhal number is increased from 0.4 to 0.625 then dipped by 44.1% as St is further increased to 0.75, a fact that was attributed to reduction in the bulk momentum by the relatively high frequency. With Reynolds number at 250, the lowest two frequencies corresponding to St of 0.05 and 0.1, resulted in a flow field that is more developed to the right side of the channel, a phenomenon that was linked to the direction of the first jet swing. The corresponding average Nusselt number distributions were consequently asymmetric, with a significant shift to the right. This asymmetric behavior gradually disappeared as the frequency is increased. At St of 0.4 and 0.5, the average stagnation Nusselt number Nu0, showed a 2.2% increase over the steady jet case. As Strouhal number is further increased beyond 0.5, the average Nu0 gradually decreased, since the oscillation period became too short for a vortex to be strong enough to reach the wall. For Reynolds number set at 400, the oscillating condition at the inlet engaged the jet into flapping. The jet showed a tendency to a permanent lean towards one side of the channel, for all used frequencies. Flapping was more one-sided which led to a shift in the average Nusselt number distribution at low frequencies. As Strouhal number is increased to 0.75, flapping became more stable and the generated vortices were expectedly weaker due to the higher frequency. Also, at this Strouhal number value, the average Nu distribution showed the best symmetry with a 2.45% improvement of the average stagnation Nusselt number, over that of the steady state case.

Effect of Variation of Pin-fin Height on Jet Impingement Heat Transfer on a Rotating Surface

2021 ◽  
Author(s):  
Pratik S. Bhansali ◽  
Srinath V. Ekkad
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Gas Turbine ◽  
Average Nusselt Number ◽  
Jet Impingement ◽  
Impinging Jet ◽  
Rotating Disc ◽  
Disc Surface ◽  
Pin Fins

Abstract Heat transfer over rotating surfaces is of particular interest in rotating machinery such as gas turbine engines. The rotation of the gas turbine disc creates a radially outward flow on the disc surface, which may lead to ingress of hot gases into the narrow cavity between the disc and the stator. Impingement of cooling jet is an effective way of cooling the disc and countering the ingress of the hot gases. Present study focusses on investigating the effect of introducing pin-fins over the rotating disc on the heat transfer. The jet Reynolds number has been varied from 5000 to 18000, and the rotating Reynolds number has been varied from 5487 to 12803 for an aluminum disc of thickness 6.35mm and diameter 10.16 cm, over which square pins have been arranged in an inline fashion. Steady state temperature measurements have been taken using thermocouples embedded in the disc close to the target surface, and area average Nusselt number has been calculated. The effects of varying the height of the pin-fins, distance between nozzle and the disc surface and the inclination of the impinging jet with the axis of rotation have also been studied. The results have been compared with those for a smooth aluminum disc of equal dimensions and without any pin-fins. The average Nusselt number is significantly enhanced by the presence of pin fins. In the impingement dominant regime, where the effect of disc rotation is minimal for a smooth disc, the heat transfer increases with rotational speed in case of pin fins. The effect of inclination angle of the impinging jet is insignificant in the range explored in this paper (0° to 20°).

Natural convection in differentially heated enclosures subjected to variable temperature boundaries

2019 ◽  
Vol 29 (11) ◽  
pp. 4130-4141 ◽  
Author(s):  
Abdulmajeed Mohamad ◽  
Mikhail A. Sheremet ◽  
Jan Taler ◽  
Paweł Ocłoń
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Natural Convection ◽  
Average Nusselt Number ◽  
Uniform Heating ◽  
Content Type ◽  
Left Hand ◽  
Heating And Cooling ◽  
Right Hand ◽  
The Right

Purpose Natural convection in differentially heated enclosures has been extensively investigated due to its importance in many industrial applications and has been used as a benchmark solution for testing numerical schemes. However, most of the published works considered uniform heating and cooling of the vertical boundaries. This paper aims to examine non-uniform heating and cooling of the mentioned boundaries. The mentioned case is very common in many electronic cooling devices, thermal storage systems, energy managements in buildings, material processing, etc. Design/methodology/approach Four cases are considered, the left-hand wall’s temperature linearly decreases along the wall, while the right-hand wall’s temperature is kept at a constant, cold temperature. In the second case, the left-hand wall’s temperature linearly increases along the wall, while the right-hand wall’s temperature is kept a constant, cold temperature. The third case, the left-hand wall’s temperature linearly decreases along the wall, while the right-hand wall’s temperature linearly increases along the wall. In the fourth case, the left-hand and the right-hand walls’ temperatures decrease along the wall, symmetry condition. Hence, four scenarios of natural convection in enclosures were covered. Findings It has been found that the average Nusselt number of the mentioned cases is less than the average Nusselt number of the uniformly heated and cooled enclosure, which reflects the physics of the problem. The work quantifies the deficiency in the rate of the heat transfer. Interestingly one of the mentioned cases showed two counter-rotating horizontal circulations. Such a flow structure can be considered for passively, highly controlled mechanism for species mixing processes application. Originality/value Previous works assumed that the vertical boundary is subjected to a constant temperature or to a sinusoidal varying temperature. The subject of the work is to examine the effect of non-uniformly heating and/or cooling vertical boundaries on the rate of heat transfer and flow structure for natural convection in a square enclosure. The temperature either linearly increases or decreases along the vertical coordinate at the boundary. Four scenarios are explored.

Effect of Driven Sidewalls on Mixed Convection in an Open Trapezoidal Cavity With a Channel

2020 ◽  
Vol 142 (8) ◽  
Author(s):  
Muneer A. Ismael ◽  
Ahmed Kadhim Hussein ◽  
Fateh Mebarek-Oudina ◽  
Lioua Kolsi
Keyword(s):  
Reynolds Number ◽  
Nusselt Number ◽  
Mixed Convection ◽  
Richardson Number ◽  
Average Nusselt Number ◽  
Airflow Velocity ◽  
Driven Cavity ◽  
Thermal Fields ◽  
Number Ratio ◽  
The Right

Abstract The mixed convection in an open trapezoidal lid-driven cavity connected with a channel is investigated in the present paper. Four different cases were considered depending on the movement of the cavity sidewalls. For case I, the left sidewall moves downward; for case II, the left sidewall moves downward and the right one moves upward; while for case III, only the right sidewall moves upward. A comparative case (case 0) is accounted when both sidewalls are assumed stationary. The base of the cavity is subjected to a localized heat source of constant temperature Th. The effects of Richardson number Ri and Reynolds number ratio Rer on the flow and thermal fields have been investigated. The results indicated that for cases I and II, the average Nusselt number increases with the increase of the Richardson number and Reynolds number ratio. Moreover, it was found that the maximum average Nusselt number occurs with case I. When the lid-driven speed is three times that of the inlet airflow velocity, the augmentations of the average Nusselt number compared with stationary walls are 163%, 158%, and 96% for cases I, II, and III, respectively.

Experimental Investigation on Convective Heat Transfer Enhancement of Laminar Slot Jet Impingement in the Presence of Porous Medium

2016 ◽  
Author(s):  
Sampath Kumar Chinige ◽  
Arvind Pattamatta
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Porous Medium ◽  
Porous Media ◽  
Nusselt Number ◽  
Heat Sink ◽  
Average Nusselt Number ◽  
Jet Impingement ◽  
Transfer Enhancement ◽  
Porous Obstacle

An experimental study using Liquid crystal thermography technique is conducted to study the convective heat transfer enhancement in jet impingement cooling in the presence of porous media. Aluminium porous sample of 10 PPI with permeability 2.48e−7 and porosity 0.95 is used in the present study. Results are presented for two different Reynolds number 400 and 700 with four different configurations of jet impingement (1) without porous foams (2) over porous heat sink (3) with porous obstacle case (4) through porous passage. Jet impingement with porous heat sink showed a deterioration in average Nusselt number by 10.5% and 18.1% for Reynolds number of 400 and 700 respectively when compared with jet impingement without porous heat sink configuration. The results show that for Reynolds number 400, jet impingement through porous passage augments average Nusselt number by 30.73% whereas obstacle configuration enhances the heat transfer by 25.6% over jet impingement without porous medium. Similarly for Reynolds number 700, the porous passage configuration shows average Nusselt number enhancement by 71.09% and porous obstacle by 33.4 % over jet impingement in the absence of porous media respectively.

Effects of Nanoparticle Shape on Slot-Jet Impingement Cooling of a Corrugated Surface With Nanofluids

2017 ◽  
Vol 9 (2) ◽  
Author(s):  
Fatih Selimefendigil ◽  
Hakan F. Öztop
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Average Nusselt Number ◽  
Volume Fraction ◽  
Jet Impingement ◽  
Heat Transfer Characteristics ◽  
Impingement Cooling ◽  
Corrugated Surface ◽  
Nanoparticle Shapes

Numerical study of jet impingement cooling of a corrugated surface with water–SiO2 nanofluid of different nanoparticle shapes was performed. The bottom wall is corrugated and kept at constant surface temperature, while the jet emerges from a rectangular slot with cold uniform temperature. The finite volume method is utilized to solve the governing equations. The effects of Reynolds number (between 100 and 500), corrugation amplitude (between 0 and 0.3), corrugation frequency (between 0 and 20), nanoparticle volume fraction (between 0 and 0.04), and nanoparticle shapes (spherical, blade, brick, and cylindrical) on the fluid flow and heat transfer characteristics were studied. Stagnation point and average Nusselt number enhance with Reynolds number and solid particle volume fraction for both flat and corrugated surface configurations. An optimal value for the corrugation amplitude and frequency was found to maximize the average heat transfer at the highest value of Reynolds number. Among various nanoparticle shapes, cylindrical ones perform the best heat transfer characteristics in terms of stagnation and average Nusselt number values. At the highest solid volume concentration of the nanoparticles, heat transfer values are higher for a corrugated surface when compared to a flat surface case.

Transient Thermal Management Using a Confined Jet Impingement of Liquid Ammonia

2003 ◽  
Author(s):  
Ryan M. Mead ◽  
Muhammad M. Rahman
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Transfer Coefficient ◽  
Steady State ◽  
Average Nusselt Number ◽  
Liquid Ammonia ◽  
Plate Thickness ◽  
Average Heat Transfer Coefficient ◽  
Steady State Condition ◽  
Average Heat

This paper introduces the results of transient heat transfer involving a jet of liquid ammonia perpendicularly on a solid substrate of finite thickness containing discrete electronic sources on the opposite surface. The jet was confined by using a cover plate to prevent any evaporation or loss of ammonia during the heat transfer process. The numerical simulation considered both the solid and fluid regions as a conjugate problem. The equations solved in the liquid region included the conservation of mass, conservation of energy, and conservation of momentum. For the solid region, only the heat conduction equation was solved. Computed results included the temperature distribution, local and average heat transfer coefficient, and local and average Nusselt number at the solid-fluid interface. Some of the parameters such as the jet velocity, plate thickness, and plate material were altered to examine the effect that they had on the problem. It was found that the average heat transfer coefficient and a average Nusselt number were high at the initial stages of the transient process and decreased steadily with time until it reached the steady condition. As the plate thickness decreased, and as the jet velocity increased, it was observed that the time it took to reach the steady state condition declined. The time it took to reach steady state condition did not change significantly for different plate materials. However, it did change noticeably for different plate thickness and different Reynolds number.

scholarly journals Evaluation of Multiple Semi-Twisted Tape Inserts in a Heat Exchanger Pipe Using Al2O3 Nanofluid

Nanomaterials ◽  
2021 ◽  
Vol 11 (6) ◽  
pp. 1570
Author(s):  
Yongfeng Ju ◽  
Tiezhu Zhu ◽  
Ramin Mashayekhi ◽  
Hayder I. Mohammed ◽  
Afrasyab Khan ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Heat Exchanger ◽  
Friction Factor ◽  
Base Fluid ◽  
Average Nusselt Number ◽  
Three Dimensions ◽  
Twisted Tape ◽  
Twisted Tapes

The hydrothermal performance of multiple semi-twisted tape inserts inside a heat exchanger pipe is numerically examined in three-dimensions. This study aims to find the optimum case for having the highest heat transfer enhancement with the lowest friction factor using nanofluid (Al2O3/water). A performance evaluation criterion (PEC) is defined to characterize the performance based on both friction factor and heat transfer. It was found that increasing the number of semi-twisted tapes increases the number of swirl flow streams and leads to an enhancement in the local Nusselt number as well as the friction factor. The average Nusselt number increases from 15.13 to 28.42 and the average friction factor enhances from 0.022 to 0.052 by increasing the number of the semi-twisted tapes from 0 to 4 for the Reynolds number of 1000 for the base fluid. By using four semi-twisted tapes, the average Nusselt number increases from 12.5 to 28.5, while the friction factor reduces from 0.155 to 0.052 when the Reynolds number increases from 250 to 1000 for the base fluid. For the Reynolds number of 1000, the increase in nanofluid concentration from 0 to 3% improves the average Nusselt number and friction factor by 6.41% and 2.29%, respectively. The highest PEC is equal to 1.66 and belongs to the Reynolds number of 750 using four semi-twisted tape inserts with 3% nanoparticles. This work offers instructions to model an advanced design of twisted tape integrated with tubes using multiple semi-twisted tapes, which helps to provide a higher amount of energy demand for solar applications.

scholarly journals Heat transfer from a rotating circular cylinder in the steady regime: Effects of Prandtl number

2012 ◽  
Vol 16 (1) ◽  
pp. 79-91 ◽  
Author(s):  
Varun Sharma ◽  
Kumar Dhiman
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Circular Cylinder ◽  
Prandtl Number ◽  
Rotation Rate ◽  
Average Nusselt Number ◽  
Steady Regime ◽  
Rotating Circular Cylinder ◽  
Prandtl Numbers

In this work, effects of Prandtl number on the heat transfer characteristics of an unconfined rotating circular cylinder are investigated for varying rotation rate (? = 0 - 5) in the Reynolds number range 1 - 35 and Prandtl numbers range 0.7 - 100 in the steady flow regime. The numerical calculations are carried out by using a finite volume method based commercial CFD solver FLUENT. The isotherm patterns are presented for varying values of Prandtl number and rotation rate in the steady regime. The variation of the local and the average Nusselt numbers with Reynolds number, Prandtl number and rotation rate are presented for the above range of conditions. The average Nusselt number is found to decrease with increasing value of the rotation rate for the fixed value of the Reynolds and Prandtl numbers. With increasing value of the Prandtl number, the average Nusselt number increases for the fixed value of the rotation rate and the Reynolds number; however, the larger values of the Prandtl numbers show a large reduction in the value of the average Nusselt number with increasing rotation rate.

Natural Convective Heat Transfer From an Unconfined or a Confined Heated Surface

2005 ◽  
Author(s):  
L. K. Liu ◽  
T. W. Lin ◽  
M. C. Wu ◽  
C. H. Peng ◽  
Y. H. Hung
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Steady State ◽  
Local Nusselt Number ◽  
Average Nusselt Number ◽  
Heated Surface ◽  
Nusselt Numbers ◽  
Extended Surface ◽  
Extended Surfaces ◽  
State Average

A series of experimental investigations with a stringent measurement method on the natural heat transfer from an unconfined or confined smooth and extended surface have been successfully conducted. From the results, the maximum transient-/steady-state local Nusselt number exists in the region near the edge of the heated smooth or extended surface, and the transient-/ steady-state local Nusselt number decreases along the distance from the surface edge toward the surface center. The transient-/steady-state local and average Nusselt number increases with increasing Grs, H/W or Hes/W. The effects of Grs, H/W and Hes/W on the Nus/Nus,o distribution are not significant; and the Nus/Nus,o distribution can be expressed as a generalized bowl-shaped profile, which is independent of Grs, H/W and Hes/W. By the statistical sensitivity analysis of ANOVA F-test, the steady-state average Nusselt number for unconfined/confined smooth or extended surface is significantly affected by either one of Grs, H/W and Hes/W. Among them, Grs has the most significant effect. Four new correlations of steady-state average Nusselt number in terms of relevant influencing parameters for unconfined/ confined smooth or extended surfaces are proposed, respectively. Furthermore, two normalized steady-state average Nusselt numbers for confined smooth or extended surfaces are proposed, respectively. As compared with the steady-state average Nusselt numbers for unconfined/confined smooth surface, the steady-state heat transfer enhancement for unconfined/confined extended surface can be achieved between 93.99% and 254.65%.

Liquid Single-Phase Flow in an Array of Micro-Pin-Fins—Part I: Heat Transfer Characteristics

2008 ◽  
Vol 130 (12) ◽  
Author(s):  
Weilin Qu ◽  
Abel Siu-Ho
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Average Nusselt Number ◽  
Single Phase ◽  
Phase Flow ◽  
Single Phase Flow ◽  
Pin Fin ◽  
Pin Fins ◽  
Micro Pin Fins

This is Paper I of a two-part study concerning thermal and hydrodynamic characteristics of liquid single-phase flow in an array of micro-pin-fins. This paper reports the heat transfer results of the study. An array of 1950 staggered square micro-pin-fins with 200×200 μm2 cross-section by 670 μm height were fabricated into a copper test section. De-ionized water was used as the cooling liquid. Two coolant inlet temperatures of 30°C and 60°C and six maximum mass velocities for each inlet temperature ranging from 183 to 420 kg/m2 s were tested. The corresponding inlet Reynolds number ranged from 45.9 to 179.6. General characteristics of average and local heat transfer were described. Six previous conventional long and intermediate pin-fin correlations and two micro-pin-fin correlations were examined and were found to overpredict the average Nusselt number data. Two new heat transfer correlations were proposed for the average heat transfer based on the present data, in which the average Nusselt number is correlated with the average Reynolds number by power law. Values of the exponent m of the Reynolds number for the two new correlations are fairly close to those for the two previous micro-pin-fin correlations but substantially higher than those for the previous conventional pin-fin correlations, indicating a stronger dependence of the Nusselt number on the Reynolds number in micro-pin-fin arrays. The correlations developed for the average Nusselt number can adequately predict the local Nusselt number data.

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