Local heat transfer in a vertical gas-cooled tube with turbulent mixed convection and different heat fluxes

Measured local heat transfer data and the results of flow visualization studies are reported for cross-flow mixed convection in a rectangular enclosure with restricted inlet and outlet openings at high Rayleigh number. In this study, experiments using water as the test fluid were conducted in a small-scale test section with uniformly heated vertical side walls and an adiabatic top and bottom. As the flow rate through the enclosure increased, the enhancement of heat transfer, above that for natural convection alone, also increased. The variation of the local heat transfer coefficient over the heated surface was found to be strongly affected by the recirculation of portions of the forced flow within the enclosure. Mean heat transfer coefficients are also presented which were calculated by averaging the measured local values over the heated surface. A correlation for the mean heat transfer coefficient is also proposed which agrees very well with the experimentally determined values. A method of predicting the flow regime in this geometry for specified heating and flow conditions is also discussed.

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Flow Boiling Heat Transfer on Micro Pin Fins Entrenched in a Microchannel

Journal of Heat Transfer ◽

10.1115/1.4000878 ◽

2010 ◽

Vol 132 (4) ◽

Cited By ~ 41

Author(s):

Santosh Krishnamurthy ◽

Yoav Peles

Keyword(s):

Heat Transfer ◽

Heat Transfer Coefficient ◽

Transfer Coefficient ◽

Flow Boiling ◽

Local Heat Transfer ◽

Local Heat ◽

Heat Fluxes ◽

Axial Distance ◽

Mass Fluxes ◽

Pin Fins

Flow boiling of 1-methoxyheptafluoropropane (HFE 7000) in 222 μm hydraulic diameter channels containing a single row of 24 inline 100 μm pin fins was studied for mass fluxes from 350 kg/m2 s to 827 kg/m2 s and wall heat fluxes from 10 W/cm2 to 110 W/cm2. Flow visualization revealed the existence of isolated bubbles, bubbles interacting, multiple flow, and annular flow. The observed flow patterns were mapped as a function of the boiling number and the normalized axial distance. The local heat transfer coefficient during subcooled boiling was measured and found to be considerably higher than the corresponding single-phase flow. Furthermore, a thermal performance evaluation comparison with a plain microchannel revealed that the presence of pin fins considerably enhanced the heat transfer coefficient.

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Can Natural Convection on Smooth Vertical Plates in the Laminar Regime Be Improved by Adding Forward Facing Triangular Roughness Elements?

Volume 1: Fluid Mechanics ◽

10.1115/ajkfluids2019-5441 ◽

2019 ◽

Author(s):

Jakob Hærvig ◽

Anna Lyhne Jensen ◽

Henrik Sørensen

Keyword(s):

Heat Transfer ◽

Natural Convection ◽

Aspect Ratio ◽

Dead Zone ◽

Local Heat Transfer ◽

Local Heat ◽

Heat Fluxes ◽

Smooth Surfaces ◽

Roughness Elements ◽

Triangular Elements

Abstract Vertical smooth surfaces are commonly used for transferring heat by natural convection. Many studies have tried altering smooth surfaces in various ways to increase heat transfer. Many of these studies fail to increase global heat transfer. The problem commonly reported is dead zones appearing just upstream and downstream obstructions that effectively decrease wall temperature gradients normal to the surface. In this study, we simulate how changes geometry of forward facing triangular roughness elements affect local and global heat transfer for isothermal plates. We change the aspect ratio of the triangular elements from L/h = 5 to L/h = 40 at Grashof numbers of GrL = 8.0 · 104 and GrL = 6.4 · 105. In all cases the flow remains laminar. Even when accounting for the increase in surface area, we keep observing a decrease in global heat transfer compared to the smooth vertical plate. However, the results show by carefully selecting the aspect ratio and pitch distance of the triangular elements based on the Grashof number, the dead zone behind the horizontal part can be eliminated thereby significantly increasing local heat transfer. This observation could help to improve cooling of electronics with high localised heat fluxes.

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Local Heat Transfer Coefficient of Flow Boiling in Microchannels With Artificial Reentrant Cavities

ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 1 ◽

10.1115/ht2007-32159 ◽

2007 ◽

Author(s):

Chih-Jung Kuo ◽

Yoav Peles

Keyword(s):

Heat Transfer ◽

Heat Transfer Coefficient ◽

Transfer Coefficient ◽

Flow Boiling ◽

Local Heat Transfer ◽

Local Heat ◽

Heat Fluxes ◽

Transfer Coefficients ◽

Bubble Generation ◽

Parallel Microchannels

Flow boiling in parallel microchannels with structured reentrant cavities was experimental studied. Flow patterns, boiling inceptions and heat transfer coefficients were obtained and studied for G = 83 kg/m2-s to G = 303 kg/m2-s and heat fluxes up to 643 W/cm2. The heat transfer coefficient-mass velocity and quality relations had been analyzed to identify boiling mechanism. Comparisons of the performance of the enhanced and plain-wall microchannels had also been made. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation.

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Flow Boiling in Mini and Microchannels

ASME 2nd International Conference on Microchannels and Minichannels ◽

10.1115/icmm2004-2367 ◽

2004 ◽

Cited By ~ 2

Author(s):

M. Cortina Di´az ◽

H. Boye ◽

I. Hapke ◽

J. Schmidt ◽

Y. Staate ◽

...

Keyword(s):

Heat Transfer ◽

Flow Boiling ◽

Local Heat Transfer ◽

Outer Wall ◽

Local Heat ◽

Constant Heat Flux ◽

Heat Fluxes ◽

Transfer Coefficients ◽

Convective Boiling ◽

Inconel 600

Flow boiling heat transfer characteristics of water and hydrocarbons in mini and microchannels are experimentally studied. Two different test section geometries are employed; a circular channel with a hydraulic diameter of 1500 μm, and rectangular channels with height values of 300–700 μm and a width of 10mm. In both facilities the fluid flows upwards and the test sections, made of the nickel alloy Inconel 600, are directly electrically heated. Thus the evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 25 and 350 kg/(m2s) and heat fluxes from 20 to 350 kW/m2 at an inlet pressure of 0.3 MPa are examined. Infrared thermography is applied to scan the outer wall temperatures. These allow the identification of different boiling regions, boiling mechanisms and the determination of the local heat transfer coefficients. Measurements are carried out in initial, saturated and post-dryout boiling regions. The experimental results in the region of saturated boiling are compared with available correlations and with a physically founded model developed for convective boiling.

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An Investigation of Thermal and Velocity Fields for a Confined Jet Over the RE Range of 1,000-24,000

Heat Transfer: Volume 2 ◽

10.1115/ht2008-56220 ◽

2008 ◽

Cited By ~ 1

Author(s):

N. Jeffers ◽

J. Punch ◽

E. Walsh

Keyword(s):

Heat Transfer ◽

Heat Transfer Rate ◽

Transfer Rate ◽

Local Heat Transfer ◽

Local Heat ◽

High Heat ◽

Heat Fluxes ◽

Hydrodynamic Characteristics ◽

Plate Spacing ◽

High Heat Transfer

Contemporary electronic systems currently generate high heat fluxes at component level. Impingement cooling is an effective way to generate high heat transfer coefficients in order to meet thermal constraints. This paper investigates the heat transfer and hydrodynamic characteristics of a confined impinging liquid jet with a nozzle-to-plate spacing (H/D) ratio of 0.5. A custom measurement facility was created to infer local heat transfer rates from infra-red images of a jet impinging on a 12.5μm thick stainless steel foil configured to generate uniform heat flux. Particle-Image Velocimetry (PIV) was performed in order to obtain quantitative velocity data within the jet. A series of experiments were run for Reynolds numbers (Re) in the range of 1,000–24,000 for a jet of 8 mm diameter (D). For Re > 4,000, the local heat transfer rate — in terms of Nusselt number (Nu) as a function of dimensionless radius (r/D) — had a plateau section between 0 < r/D < 0.6 followed by a peak at r/D ∼ 1.35. For higher Re the Nu peak exceeds that of the plateau section. For Re < 4,000, a plateau section exists between 0 < r/D < 0.4 followed by a shoulder located between 1 < r/D < 1.4. The PIV data for Re > 4,000 showed a strong vortex in the area of the secondary peak in Nu which was not present in the lower Re range. This phenomenon — the local peaks of heat transfer rate — has been previously reported in the literature with a degree of uncertainty as to the related fluid mechanics. This paper contributes to an understanding of the fluidic phenomenon responsible for the distribution of heat transfer rate in confined jets.

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Mist/Steam Cooling in a Heated Horizontal Tube—Part 2: Results and Modeling

Journal of Turbomachinery ◽

10.1115/1.555451 ◽

1999 ◽

Vol 122 (2) ◽

pp. 366-374 ◽

Cited By ~ 34

Author(s):

Tao Guo ◽

Ting Wang ◽

J. Leo Gaddis

Keyword(s):

Heat Transfer ◽

Wall Temperature ◽

Experimental Studies ◽

Local Heat Transfer ◽

Horizontal Tube ◽

Local Heat ◽

Heat Fluxes ◽

Steam Flow ◽

Transfer Model ◽

Steam Cooling

Experimental studies on mist/steam cooling in a heated horizontal tube have been performed. Wall temperature distributions have been measured under various main steam flow rates, droplet mass ratios, and wall heat fluxes. Generally, the heat transfer performance of steam can be significantly improved by adding mist into the main flow. An average enhancement of 100 percent with the highest local heat transfer enhancement of 200 percent is achieved with 5 percent mist. When the test section is mildly heated, an interesting wall temperature distribution is observed: The wall temperature increases first, then decreases, and finally increases again. A three-stage heat transfer model with transition boiling, unstable liquid fragment evaporation, and dry-wall mist cooling has been proposed and has shown some success in predicting the wall temperature of the mist/steam flow. The PDPA measurements have facilitated better understanding and interpreting of the droplet dynamics and heat transfer mechanisms. Furthermore, this study has shed light on how to generate appropriate droplet sizes to achieve effective droplet transportation, and has shown that it is promising to extend present results to a higher temperature and higher pressure environment. [S0889-504X(00)02502-2]

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Heat transfer of ribbed surfaces. Local heat transfer coefficient with different heat fluxes for rib and basic surfaces.

TRANSACTIONS OF THE JAPAN SOCIETY OF MECHANICAL ENGINEERS Series B ◽

10.1299/kikaib.55.3166 ◽

1989 ◽

Vol 55 (518) ◽

pp. 3166-3171

Author(s):

Makoto HISHIDA

Keyword(s):

Heat Transfer ◽

Heat Transfer Coefficient ◽

Transfer Coefficient ◽

Local Heat Transfer ◽

Local Heat ◽

Heat Fluxes ◽

Local Heat Transfer Coefficient

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Impairment of Local Heat Transfer of the Turbulent Mixed Convection in a Vertical Flat Plate

Volume 6B: Thermal-Hydraulics and Safety Analyses ◽

10.1115/icone26-82010 ◽

2018 ◽

Author(s):

Myeong-Seon Chae ◽

Bum-Jin Chung

Keyword(s):

Heat Transfer ◽

Mass Transfer ◽

Mixed Convection ◽

Flat Plate ◽

Forced Convection ◽

Convection Heat Transfer ◽

Local Heat Transfer ◽

Local Heat ◽

Convection Heat ◽

Vertical Flat Plate

The heat transfer of the buoyancy-aided turbulent mixed convective flow in a vertical flat plate was investigated experimentally. Mass transfer experiments were carried out based on the heat and mass transfer analogy. The Rayleigh numbers ranged from 1.69 × 108 to 2.11 × 1013, depending on the height of the vertical flat plate. The Reynolds numbers varied from 4,585 to 17,320 for turbulent regimes. The test results for turbulent forced convections agreed well with the forced convection correlations established by Petukhov et al. The local heat transfer rates of the turbulent mixed flow exhibited the impairment of heat transfer compared to the forced convection and non-monotonous behavior along the axial position due to buoyancy effect. The local minimum heat transfer was 38.6% lower than the forced convection heat transfer. The turbulent mixed convection heat transfer is affected by the height of vertical plate.

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Effects of Conjugate Heat Transfer on Steady MHD Mixed Convective Heat Transfer Flow over a Thin Vertical Plate Embedded in a Porous Medium with High Porosity

Mathematical Problems in Engineering ◽

10.1155/2012/261871 ◽

2012 ◽

Vol 2012 ◽

pp. 1-19 ◽

Cited By ~ 4

Author(s):

Ahmet Kaya

Keyword(s):

Heat Transfer ◽

Porous Medium ◽

Mixed Convection ◽

Conjugate Heat Transfer ◽

Vertical Plate ◽

Convection Heat Transfer ◽

Local Heat Transfer ◽

Local Heat ◽

High Porosity ◽

Local Skin

This study investigates mixed convection heat transfer about a thin vertical plate in the presence of magneto and conjugate heat transfer effects in the porous medium with high porosity. The fluid is assumed to be incompressible and dense. The nonlinear coupled parabolic partial differential equations governing the flow are transformed into the nonsimilar boundary layer equations, which are then solved numerically using the Keller box method. The effects of the conjugate heat transfer parameterp, the porous medium parameterk1, the Forchheimer parameterF*, the mixed convection parameter Ri, the magnetic parameter Mn, and the electric field parameterE1on the velocity and temperature profiles as well as on the local skin friction and local heat transfer are presented and analyzed. The validity of the methodology and analysis is checked by comparing the results obtained for some specific cases with those available in the literature.

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