Convective Heat Transfer in the Impingement Region of a Buoyant Plume

Fires of hazardous scale generate turbulent plumes within which convective heat transfer to surfaces can be important. Relatively little work has been done on developing reliable convective heat transfer correlations applicable to such large-scale flows. The present study, confined to heat transfer rates within the plume impingement region on a ceiling, achieves plume Reynolds numbers an order of magnitude beyond those of previous work by performing laboratory-scale experiments at elevated ambient pressures. Flow disturbances which normally cause scatter in plume heat transfer data are reduced as a consequence of this technique. It is shown that impingement zone Nusselt number depends on the 0.61 power of plume Reynolds numbers in the range of 104 to 105. This result is between the 1/2 power dependence expected for strain rate control (forced jet impingement) and the 2/3 power expected for buoyancy control of turbulent heat transfer rates.

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Enhancement of Turbulent Convective Heat Transfer using a Microparticle Multiphase Flow

Energies ◽

10.3390/en13051282 ◽

2020 ◽

Vol 13 (5) ◽

pp. 1282

Author(s):

Tao Wang ◽

Zengliang Gao ◽

Weiya Jin

Keyword(s):

Heat Transfer ◽

Particle Size ◽

Reynolds Number ◽

Convective Heat Transfer ◽

Convective Heat ◽

Thermal Performance ◽

Volume Fraction ◽

Turbulent Heat Transfer ◽

Turbulent Heat ◽

Better Than

The turbulent heat transfer enhancement of microfluid as a heat transfer medium in a tube was investigated. Within the Reynolds number ranging from 7000 to 23,000, heat transfer, friction loss and thermal performance characteristics of graphite, Al2O3 and CuO microfluid with the particle volume fraction of 0.25%–1.0% and particle size of 5 μm have been respectively tested. The results showed that the thermal performance of microfluids was better than water. In addition, the graphite microfluid had the best turbulent convective heat transfer effect among several microfluids. To further investigate the effect of graphite particle size on thermal performance, the heat transfer characteristics of the graphite microfluid with the size of 1 μm was also tested. The results showed that the thermal performance of the particle size of 1 μm was better than that of 5 μm. Within the investigated range, the maximum value of the thermal performance of graphite microfluid was found at a 1.0% volume fraction, a Reynolds number around 7500 and a size of 1 μm. In addition, the simulation results showed that the increase of equivalent thermal conductivity of the microfluid and the turbulent kinetic energy near the tube wall, by adding the microparticles, caused the enhancement of heat transfer; therefore, the microfluid can be potentially used to enhance turbulent convective heat transfer.

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FRACTAL-STRUCTURAL ANALYSIS OF CONVECTION HEAT TRANSFER IN A TURBULENT MEDIUM

Eurasian Physical Technical Journal ◽

10.31489/2020no2/61-68 ◽

2020 ◽

Vol 17 (2) ◽

pp. 61-68

Author(s):

A.Zh. Turmukhambetov ◽

Keyword(s):

Heat Transfer ◽

Experimental Data ◽

Structural Analysis ◽

Convective Heat Transfer ◽

Convective Heat ◽

Convection Heat Transfer ◽

Convective Heat Exchange ◽

Spherical Body ◽

Turbulent Heat Transfer ◽

Turbulent Heat

The features of convective heat transfer of bodies in a turbulent environment are considered. The results of experimental research by one of the authors are discussed. Experimental data show that the heat transfer of a spherical body is affected by natural convection, the thermo-physical properties of the medium, the tightness of the flow, the turbulent flow regime, etc. Due to these factors, the formula for calculating convective heat transfer, which includes many experimental constants, becomes cumbersome and inconvenient for practical application. The paper presents the results of applying fractal-structural analysis methods to describe experimental data on convective heat exchange of badly streamlined (cylinder and sphere) bodies in a channel. Quantitative relations are obtained that link the intensity of turbulent heat transfer with the criteria for the degree of self-organization.

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Array Jet Impingement Onto High Porosity Thin Metal Foams at Zero Jet-to-Foam Spacing

Volume 8B: Heat Transfer and Thermal Engineering ◽

10.1115/imece2018-87915 ◽

2018 ◽

Cited By ~ 2

Author(s):

Prashant Singh ◽

Mingyang Zhang ◽

Jaideep Pandit ◽

Roop L. Mahajan

Keyword(s):

Heat Transfer ◽

Pressure Drop ◽

Convective Heat Transfer ◽

Convective Heat ◽

Performance Enhancement ◽

Jet Impingement ◽

Metal Foams ◽

Thin Metal ◽

High Porosity ◽

Transfer Rates

Metal foams enhance heat transfer rates by providing significant increase in wetted surface area and by thermal dispersion caused by flow mixing induced by the tortuous flow paths. Further, jet impingement is also an effective method of enhancing local convective heat transfer rates. In the present study, we have carried out an experimental investigation to study the combined effect of the two thermal performance-enhancement mechanisms. To this end, we conducted a set of experiments to determine convective heat transfer rates by impinging an array of jets onto thin metal foams attached on a uniformly heated smooth aluminum plate simulating a high heat-dissipating chip. The metal foams used were high porosity aluminum foams (ε∼0.94–0.96) with pore densities of 5 ppi, 10 ppi and 20 ppi (ppi: pores per inch) with thicknesses of 19 mm, 12.7 mm and 6.35 mm, respectively. With the jet-to-foam distance (z/d) set to zero, we conducted experiments with values of jet-to-jet spacing (x/d = y/d) of 2, 3 and 5. The jet plate featured an array of 5 × 5 cylindrical jet-issuing nozzles. The normalized jet-to-jet distance was varied by changing the jet diameter and keeping the jet center-to-center distance constant. Steady state heat transfer and pressure drop experiments were carried out for Reynolds number (based on jet diameter) ranging from 2500 to 10000. We have found that array impingement on thin foams leads to a significant enhancement in heat transfer compared to normal impingement over smooth surfaces. The gain in heat transfer was greatest for the 20 ppi foam (∼2.3 to 2.8 times that for the plain surface smooth target). However, this enhancement came at a significant increase of about 2.85 times in the plenum static pressure. With the pressure drop penalty taken into consideration, the x/d = 3 jet plate for the 20 ppi foam and x/d = 2 jet plate for the 10 ppi foam were found to be the most efficient cooling designs amongst the 18 cooling designs investigated in the present study.

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Assessment of Heat Transfer Enhancement Using Metallic Porous Foam Configurations in Laminar Slot Jet Impingement: An Experimental Study

Journal of Heat Transfer ◽

10.1115/1.4037540 ◽

2017 ◽

Vol 140 (2) ◽

Cited By ~ 4

Author(s):

Chinige Sampath Kumar ◽

Arvind Pattamatta

Keyword(s):

Heat Transfer ◽

Porous Media ◽

Nusselt Number ◽

Convective Heat Transfer ◽

Convective Heat ◽

Average Nusselt Number ◽

Heat Transfer Performance ◽

Jet Impingement ◽

Reynolds Numbers ◽

Transfer Enhancement

An experimental study using the liquid crystal thermography technique is conducted to investigate the convective heat transfer performance in jet impingement cooling using various porous media configurations. Aluminum porous foams are used in the present study. Four impinging jet configurations are considered: jet impingement (1) without porous media, (2) over the porous heat sink, (3) with porous obstacle case, and (4) through porous passage. These configurations are evaluated on the basis of the convective heat transfer enhancement for two different Reynolds numbers of 400 and 700. Jet impingement with porous heat sink showed deterioration in the average Nusselt number by 9.95% and 18.04% compared to jet impingement without porous media configuration for Reynolds numbers of 400 and 700, respectively. Jet impingement with porous obstacles showed a very negligible enhancement in the average Nusselt number by 3.48% and 2.73% for Reynolds numbers of 400 and 700, respectively. However, jet impingement through porous passage configuration showed a maximum enhancement in the average Nusselt number by 52.71% and 74.68% and stagnation Nusselt numbers by 58.08% and 53.80% compared to the jet impingement without porous medium for Reynolds numbers of 400 and 700, respectively. Within the porous properties considered, it is observed that by decreasing the permeability and porosity the convective heat transfer performance tends to increase.

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An Semi-Empirical Model of Turbulent Convective Heat Transfer to Fluids at Supercritical Pressure

Volume 3: Thermal Hydraulics; Instrumentation and Controls ◽

10.1115/icone16-48914 ◽

2008 ◽

Cited By ~ 11

Author(s):

J. Derek Jackson

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Convective Heat Transfer ◽

Convective Heat ◽

Critical Pressure ◽

Supercritical Pressure ◽

Turbulent Heat Transfer ◽

Turbulent Heat ◽

Governing Equations ◽

Semi Empirical

Recently, there has been a renewed interest in heat transfer to fluids at supercritical pressure because of the consideration now being given to the Supercritical Pressure Water-cooled Reactor (SPWR). This will supply high temperature ‘steam’ to turbines at pressures well above the critical value. The particular feature of fluids at pressures just above the critical pressure which makes them of special interest is that as they change from being liquid-like to gaseous the transition occurs in a continuous manner over a narrow band of temperature without the discontinuous behaviour encountered when phase occurs in fluids at sub-critical pressure. However, when heat takes place within fluids at supercritical pressure, extreme non-uniformities of physical and transport properties can be present. The governing equations for flow and convective heat transfer have to be written in a form which takes account of the temperature dependence of the properties. They are complicated, highly non-linear and strongly inter-dependent. The proportionality between heat flux and temperature difference found in constant property forced convection no longer exists. Also, the effectiveness of heat transfer can be very sensitive to imposed heat flux. Particular problems arise due to the non-uniformity of density by virtue of the fluid being caused to accelerate where the bulk density is falling or as a consequence of the flow field and turbulence being modified by the influence of buoyancy. Severe impairment on heat transfer can be encountered due to such effects. The requirements for achieving similarity and the approach to the correlation of data on heat transfer to fluids at supercritical pressure are matters that need to be carefully considered and soundly based. This necessitates representing the general form of the governing equations and the boundary conditions in non-dimensional form to identify the parameters that are involved. In this paper, an extended model of turbulent heat transfer to fluids at supercritical pressure is presented which utilises a semi-empirical multiplier to account for the combined effects of flow acceleration and buoyancy.

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Effect of Surface Curvature on Heat Transfer and Hydrodynamics Within a Single Hemispherical Dimple

Journal of Turbomachinery ◽

10.1115/1.1348020 ◽

2000 ◽

Vol 123 (3) ◽

pp. 609-613 ◽

Cited By ~ 22

Author(s):

N. Syred ◽

A. Khalatov ◽

A. Kozlov ◽

A. Shchukin ◽

R. Agachev

Keyword(s):

Heat Transfer ◽

Large Scale ◽

Reynolds Numbers ◽

Surface Curvature ◽

Turbulent Heat Transfer ◽

Turbulent Heat ◽

Remarkable Effect ◽

Convex Wall ◽

Flow Curvature ◽

Curvature Parameter

Turbulent heat transfer and hydrodynamics have been studied in concavely and convexly curved dimples with Reynolds numbers ranging from 1.3×105 to 3.1×105. The large-scale single hemispherical dimple 50 mm in diameter and 25 mm in depth was arranged on the smooth concave or convex wall of a curved rectangular-shaped passage. The fluid flow and heat transfer measurements, and surface streamline observations were performed within the flow curvature parameter δ**/R ranged from 0.002 to 0.007. The “tornado-like” oscillating vortex bursting periodically out of the dimple was registered in the experiments with a “curved” dimple. This vortex structure is similar to that earlier observed in a “flat” dimple. The surface curvature considerably influences the dimple heat transfer rate in both cases. It enhances heat transfer in a “concave” dimple and reduces it in a “convex” one; however, the more remarkable effect occurred in a concavely curved dimple. The correction factors describing the effect of curvature on average heat transfer in a “curved” dimple have been obtained as a result of experimental study.

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