Effect of Periodic Flow Structure on Heat Transfer in Milliscale Confined Impinging Newtonian and Non-Newtonian Flow

2018 ◽  
Author(s):  
Ajay Chatterjee ◽  
Drazen Fabris
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Convective Heat Transfer ◽  
Steady Flow ◽  
Convective Heat ◽  
Relative Length ◽  
Periodic Flow ◽  
Transfer Rates ◽  
Flow And Heat Transfer ◽  
Parallel Surfaces

Impinging flows are widely used to enhance convective heat transfer by promoting separation, recirculation and higher rates of local convection. We consider unsteady flow and heat transfer effects in a prototypical T-shaped geometry as an impinging jet. Depending on the relative length scales, the steady laminar flow in this geometry may lose stability and transition to time periodic flow even at a low Reynolds number. A key feature of the periodic structure is the presence of ‘twin’ circulation regions adjacent to the jet column, and separation vortices anchored at the impinging surface in place of the wall jet in steady flow. The separation vortices are located above shear layers lying along the confining plane of the geometry which is flush with the jet exit. Consequently, convective heat transfer is enhanced across this plane. We present calculations to show the effect of the structure of the periodic flow on heat transfer rates across the two parallel surfaces. For a shear thinning fluid the local Nusselt number at the confining surface averaged over a long length scale (∼ 50 times the nozzle width) is more than twice as large compared to that in steady flow, while for the Newtonian fluid the mean Nusselt number increases about 60%. A mild increase in the transport rate across the impinging surface is also observed. Thus flow periodicity due to instability of the steady flow field provides a mechanism to increase the total heat transfer rate across the two surfaces.

Numerical Study of Natural Convective Heat Transfer From a Very Short Isothermal Cylinder Mounted on a Flat Adiabatic Base

2013 ◽  
Author(s):  
Patrick H. Oosthuizen
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Numerical Study ◽  
Cylinder Surface ◽  
Ansys Fluent ◽  
Transfer Rates ◽  
Number Variation ◽  
The Mean

Natural convective heat transfer from a vertical isothermal cylinder mounted on a flat adiabatic base has been numerically studied. The cylinder has an exposed top surface. The cylinder is relatively very short, i.e., has a height that is equal to or less than the cylinder diameter. Both the cases where the cylinder is pointing upward and where it is pointing downward have been considered. The governing equations have been numerically solved using the commercial CFD solver ANSYS FLUENT©. Results have only been obtained for Prandtl number = 0.74. The mean heat transfer rates have been expressed in terms of a Nusselt number, consideration being given both to the heat transfer rate from the entire cylinder surface and to the heat transfer rates from the side and top surfaces of the cylinder. The effect of the dimensionless cylinder height–to–diameter ratio on the Nusselt number variation has been studied in detail.

Free Convective Heat Transfer Across Inclined Air Layers

1976 ◽  
Vol 98 (2) ◽  
pp. 189-193 ◽  
Author(s):  
K. G. T. Hollands ◽  
T. E. Unny ◽  
G. D. Raithby ◽  
L. Konicek
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Horizontal Layer ◽  
Experimental Measurements ◽  
Number Range ◽  
Transfer Rates ◽  
Air Layers ◽  
Free Convective Heat Transfer

This paper presents new experimental measurements on free convective heat transfer rates through inclined air layers of high aspect ratio, heated from below. The Rayleigh number range covered is from subcritical to 105; the range of the angle of inclination, φ measured from the horizontal is: 0 < φ < ∼70 deg. Although it was anticipated that the results might be identical to the results for the horizontal layer if one replaced Ra by Ra cos φ, significant departures from this behavior were observed, particularly in the range 1708 < Ra cos φ < 104, 30 deg ≤ φ < 60 deg. A recommended relationship giving the Nusselt number as a function of Ra cos φ and φ is reported. This relationship fits all data closely.

APPLICATION OF ADSORPTION METHOD FOR DETERMINATION OF LOCAL CONVECTIVE HEAT TRANSFER RATES

1974 ◽  
Author(s):  
S. Koncar-Djurdjevic ◽  
M. Mitrovic ◽  
S. Cvijovic ◽  
G. Popovic ◽  
Dimitrije Voronjec
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Adsorption Method ◽  
Transfer Rates

Analysis of flow and heat transfer over stretching/shrinking and porous surfaces

2021 ◽  
pp. 875608792110258
Author(s):  
Azhar Ali ◽  
Dil Nawaz Khan Marwat ◽  
Aamir Ali
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Current Model ◽  
Uniform Heat Flux ◽  
Similarity Transformations ◽  
Flow And Heat Transfer ◽  
Special Cases ◽  
Porous Surfaces

Flows and heat transfer over stretching/shrinking and porous surfaces are studied in this paper. Unusual and generalized similarity transformations are used for simplifying governing equations. Current model includes all previous cases of stretched/shrunk flows with thermal effects discussed so far. Moreover, we present three different cases of thermal behavior (i) prescribed surface temperature (ii) Variable/uniform convective heat transfer at plat surface and (iii) prescribed variable/uniform heat flux. Stretching/shrinking velocity Uw(x), porosity [Formula: see text], heat transfer [Formula: see text], heat flux [Formula: see text] and convective heat transfer at surface are axial coordinate dependent. Boundary layer equations and boundary conditions are transformed into nonlinear ODEs by introducing unusual and generalized similarity transformations for the variables. These simplified equations are solved numerically. Final ODEs represent suction/injection, stretching/shrinking, temperature, heat flux, convection effects and specific heat. This current problem encompasses all previous models as special cases which come under the scope of above statement (title). The results of classical models are scoped out as a special case by assigning proper values to the parameters. Numerical result shows that the dual solutions can be found for different possible values of the shrinking parameter. A stability analysis is accomplished and apprehended in order to establish a criterion for determining linearly stable and physically compatible solutions. The significant features and diversity of the modeled equations are scrutinized by recovering the previous problems of fluid flow and heat transfer from a uniformly heated sheet of variable (uniform) thickness with variable (uniform) stretching/shrinking and injection/suction velocities.

scholarly journals Convective Heat Transfer of Ethanol/Polyalphaolefin Nanoemulsion in Mini- and Microchannel Heat Exchangers for High Heat Flux Electronics Cooling

2021 ◽  
Author(s):  
Jaime Rios ◽  
Mehdi Kabirnajafi ◽  
Takele Gameda ◽  
Raid Mohammed ◽  
Jiajun Xu
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Convective Heat ◽  
Phase Flow ◽  
Transfer Performance ◽  
Transfer Enhancement ◽  
Heat Transfer Characteristics ◽  
Two Phase ◽  
Flow And Heat Transfer

The present study experimentally and numerically investigates the flow and heat transfer characteristics of a novel nanostructured heat transfer fluid, namely, ethanol/polyalphaolefin nanoemulsion, inside a conventionally manufactured minichannel of circular cross section and a microchannel heat exchanger of rectangular cross section manufactured additively using the Direct Metal Laser Sintering (DMLS) process. The experiments were conducted for single-phase flow of pure polyalphaolefin (PAO) and ethanol/PAO nanoemulsion fluids with two ethanol concentrations of 4 wt% and 8 wt% as well as for two-phase flow boiling of nanoemulsion fluids to study the effect of ethanol nanodroplets on the convective flow and heat transfer characteristics. Furthermore, the effects of flow regime of the working fluids on the heat transfer performance for both the minichannel and microchannel heat exchangers were examined within the laminar and transitional flow regimes. It was found that the ethanol/PAO nanoemulsion fluids can improve convective heat transfer compared to that of the pure PAO base fluid under both single- and two-phase flow regimes. While the concentration of nanoemulsion fluids did not reflect a remarkable distinction in single-phase heat transfer performance within the laminar regime, a significant heat transfer enhancement was observed using the nanoemulsion fluids upon entering the transitional flow regime. The heat transfer enhancement at higher concentrations of nanoemulsion within the transitional regime is mainly attributed to the enhanced interaction and interfacial thermal transport between ethanol nanodroplets and PAO base fluid. For two-phase flow boiling, heat transfer coefficients of ethanol/PAO nanoemulsion fluids were further enhanced when the ethanol nanodroplets underwent phase change. A comparative study on the flow and heat transfer characteristics was also implemented between the traditionally fabricated minichannel and additively manufactured microchannel of similar dimensions using the same working fluid of pure PAO and the same operating conditions. The results revealed that although the DMLS fabricated microchannel posed a higher pressure loss, a substantial heat transfer enhancement was achieved as compared to the minichannel heat exchanger tested under the same conditions. The non-post processed surface of the DMLS manufactured microchannel is likely to be the main contributor to the augmented heat transfer performance. Further studies are required to fully appreciate the possible mechanisms behind this phenomenon as well as the convective heat transfer properties of nanoemulsion fluids.

scholarly journals A numerical study of free convective heat transfer within domed skylight cavities

2021 ◽  
Author(s):  
AmirAbbas Sartipi
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Numerical Study ◽  
Control Volume ◽  
Energy Performance ◽  
Vortex Cell ◽  
Design Elements ◽  
Gap Ratio

Domed skylights are important architectural design elements to deliver daylight and solar heat into buildings and connect buildings' occupants to outdoors. To increase the energy efficiency of skylighted buildings, domed skylights employ a number of glazing layers forming enclosed spaces. The latter are subject to complex buoyancy-induced convection heat transfer. Currently, existing fenestration design computer tools and building energy simulation programs do not, however, cover such skylights to quantify their energy performance when installed in buildings. his work presents a numerical study on natural laminar convection within concentric and vertically eccentric domed cavities. The edges of domed cavities are assumed adiabatic and the temperature of the interior and exterior surfaces are uniform and constant. The concentric and vertically eccentric domed cavities were studied when heated from inside and heated from outside, respectively. A commercial CFD package employing the control volume approach is used to solve the laminar convective heat transfer within the cavity. The obtained results showed steady flow for small Grashof numbers. For moderate and large Grashof numbers, depending on the gap ratio and the cases of heating from inside or outside, the flow may be steady or transient periodic with a single vortex-cell or multi vortex-cells. The Nusselt number for the case of heated from inside is greater than the case of heated from outside. The numerical results show that the changes in the gap ratio have smaller effect on Nusselt number in high profile domed skylights than lower profile domed skylights.

Influence of Short Helical Tape on Local Convective Heat Transfer and Thermal Performance Characteristics in Turbulent Decaying Swirl Flow

2010 ◽  
Vol 8 (1) ◽  
Author(s):  
Smith Eiamsa-ard ◽  
Vichan Kongkaitpaiboon ◽  
Khwanchit Wongcharee
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Thermal Performance ◽  
Swirl Flow ◽  
Transfer Rates ◽  
Friction Factors ◽  
Plain Tube ◽  
Uniform Wall ◽  
Uniform Wall Heat Flux

This paper reports the experimental investigation of local convective heat transfer enhancement, flow friction and thermal performance factor behaviors in the tube fitted with the short helical tapes (SHTs) acting as decaying swirl flow generators. The tapes with three different helical tape angles (? = 90°, 135° and 180°) and three different channel numbers (N = 2, 3 and 4 channels) were tested under the uniform wall heat flux condition. The performance of each tape is compared with the performance of the plain tube subject to the same pumping power. The experimental results show that the heat transfer rates and friction factors of the tube with SHTs are respectively in range of 1.15 to 1.9 and 1.49 to 2.31 times of those in the plain, corresponding to thermal performances between 0.98 and 1.46. The correlations for Nusselt number (Nu) as a function of Reynolds number (Re), Prandtl number (Pr), helical tape angle (?) and the number of channel (N) are also developed.

Computational analysis of convective heat transfer properties of turbulent slot jet impingement

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Anuj Kumar Shukla ◽  
Anupam Dewan
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
High Efficiency ◽  
Stokes Equations ◽  
Turbulence Models ◽  
Jet Impingement ◽  
Complex Flow ◽  
Content Type

Purpose Convective heat transfer features of a turbulent slot jet impingement are comprehensively studied using two different computational approaches, namely, URANS (unsteady Reynolds-averaged Navier–Stokes equations) and SAS (scale-adaptive simulation). Turbulent slot jet impingement heat transfer is used where a considerable heat transfer enhancement is required, and computationally, it is a quite challenging flow configuration. Design/methodology/approach Customized OpenFOAM 4.1, an open-access computational fluid dynamics (CFD) code, is used for SAS (SST-SAS k-ω) and URANS (standard k-ε and SST k-ω) computations. A low-Re version of the standard k-ε model is used, and other models are formulated for good wall-refined calculations. Three turbulence models are formulated in OpenFOAM 4.1 with second-order accurate discretization schemes. Findings It is observed that the profiles of the streamwise turbulence are under-predicted at all the streamwise locations by SST k-ω and SST SAS k-ω models, but follow similar trends as in the reported results. The standard k-ε model shows improvements in the predictions of the streamwise turbulence and mean streamwise velocity profiles in the zone of outer wall jet. Computed profiles of Nusselt number by SST k-ω and SST-SAS k-ω models are nearly identical and match well with the reported experimental results. However, the standard k-ε model does not provide a reasonable profile or quantification of the local Nusselt number. Originality/value Hybrid turbulence model is suitable for efficient CFD computations for the complex flow problems. This paper deals with a detailed comparison of the SAS model with URANS and LES for the first time in the literature. A thorough assessment of the computations is performed against the results reported using experimental and large eddy simulations techniques followed by a detailed discussion on flow physics. The present results are beneficial for scientists working with hybrid turbulence models and in industries working with high-efficiency cooling/heating system computations.

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