# Convection Heat Transfer

28 Jan 2005-pp 173-239

TL;DR: The Q12-40 density: ρ ((kg/m) specific heat: Cp (J/kg ·K) dynamic viscosity: ν ≡ μ/ρ (m/s) thermal conductivity: k, (W/m ·K), thermal diffusivity: α, ≡ k/(ρ · Cp) (m /s) Prandtl number: Pr, ≡ ν/α (−−) volumetric compressibility: β, (1/K).

Abstract: Geometry: shape, size, aspect ratio and orientation Flow Type: forced, natural, laminar, turbulent, internal, external Boundary: isothermal (Tw = constant) or isoflux (q̇w = constant) Fluid Type: viscous oil, water, gases or liquid metals Properties: all properties determined at film temperature Tf = (Tw + T∞)/2 Note: ρ and ν ∝ 1/Patm ⇒ see Q12-40 density: ρ ((kg/m) specific heat: Cp (J/kg ·K) dynamic viscosity: μ, (N · s/m) kinematic viscosity: ν ≡ μ/ρ (m/s) thermal conductivity: k, (W/m ·K) thermal diffusivity: α, ≡ k/(ρ · Cp) (m/s) Prandtl number: Pr, ≡ ν/α (−−) volumetric compressibility: β, (1/K)

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TL;DR: In this article, a single-and two-phase model with either constant or temperature-dependent properties is employed to develop laminar forced convection flow of a water-Al2O3 nanofluid in a circular tube, submitted to a constant and uniform heat flux at the wall.

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TL;DR: It is shown that animal movement (running, flying, swimming) and fluid eddy movement (turbulent structure) are both forms of optimized intermittent movement.

Abstract: SUMMARY Biologists have treated the view that fundamental differences exist between
running, flying and swimming as evident, because the forms of locomotion and
the animals are so different: limbs and wings vs body undulations,
neutrally buoyant vs weighted bodies, etc. Here we show that all
forms of locomotion can be described by a single physics theory. The theory is
an invocation of the principle that flow systems evolve in such a way that
they destroy minimum useful energy (exergy, food). This optimization approach
delivers in surprisingly direct fashion the observed relations between speed
and body mass ( M b ) raised to 1/6, and between frequency
(stride, flapping) and \(M_{\mathrm{b}}^{-1{/}6}\) , and
shows why these relations hold for running, flying and swimming. Animal
locomotion is an optimized two-step intermittency: an optimal balance is
achieved between the vertical loss of useful energy (lifting the body weight,
which later drops), and the horizontal loss caused by friction against the
surrounding medium. The theory predicts additional features of animal design:
the Strouhal number constant, which holds for running as well as flying and
swimming, the proportionality between force output and mass in animal motors,
and the fact that undulating swimming and flapping flight occur only if the
body Reynolds number exceeds approximately 30. This theory, and the general
body of work known as constructal theory, together now show that animal
movement (running, flying, swimming) and fluid eddy movement (turbulent
structure) are both forms of optimized intermittent movement.

243 citations

### Cites methods from "Convection Heat Transfer"

...…law calls for the minimization of the total destruction of work per distance traveled L: Throughout this paper we use the method of scale analysis (Bejan, 2004), which consists of solving the appropriate conservation equations as algebraic equations, with the additional simplification that…...

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Abstract: Conversion of low-grade waste heat into electricity is an important energy harvesting strategy. However, abundant heat from these low-grade thermal streams cannot be harvested readily because of the absence of efficient, inexpensive devices that can convert the waste heat into electricity. Here we fabricate carbon nanotube aerogel-based thermo-electrochemical cells, which are potentially low-cost and relatively high-efficiency materials for this application. When normalized to the cell cross-sectional area, a maximum power output of 6.6 W m−2 is obtained for a 51 °C inter-electrode temperature difference, with a Carnot-relative efficiency of 3.95%. The importance of electrode purity, engineered porosity and catalytic surfaces in enhancing the thermocell performance is demonstrated. Conversion of low-grade waste heat into electricity is an important energy harvesting strategy. Here, the authors fabricate carbon nanotube aerogel-based thermo-electrochemical cells, which are potentially low-cost and relatively high-efficiency materials for this application.

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### Additional excerpts

...(2) is divided by the Carnot efficiency (ηc = ΔΤ/TH):...

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TL;DR: In this paper, the authors present the results of laboratory experiments measuring particle penetration through surrogates of cracks in building envelopes, using common building materials: aluminum, brick, concrete, plywood, redwood lumber, pine lumber, and strand board.

Abstract: Particle penetration into buildings influences human exposure to particles of ambient origin. In this study, we present the results of laboratory experiments measuring particle penetration through surrogates of cracks in building envelopes. Rectangular slots were prepared, with crack heights of 0.25 and 1 mm and flow-path lengths of 4-10 cm, using common building materials: aluminum, brick, concrete, plywood, redwood lumber, pine lumber, and strand board. Air was drawn through a slot from a well-mixed chamber by applying a pressure difference ( j P) of 4 or 10 Pa. Nonvolatile, electrically neutralized particles were generated and introduced into the chamber. The particle penetration factor was determined, for particle sizes 0.02-7 w m, as the ratio of the particle concentration downstream of the slot to that in the chamber. Particle size and crack height were the two main factors that governed fractional particle penetration. Consistent with prior modeling results, the penetration factor was nearly unity ...

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### Cites methods from "Convection Heat Transfer"

...We used scale analysis (Bejan 1984) to estimate the particle concentration boundary layer thickness within the crack....

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Abstract: This work is focused on the numerical solution of steady natural convection boundary-layer flow of a nanofluid consisting of a pure fluid with nanoparticles along a permeable vertical plate in the presence of magnetic field, heat generation or absorption, and suction or injection effects The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis The governing boundary-layer equations of the problem are formulated and transformed into a non-similar form The obtained equations are then solved numerically by an efficient, iterative, tri-diagonal, implicit finite-difference method Comparisons with previously published work are performed and are found to be in excellent agreement Representative results for the longitudinal velocity, temperature, and nanoparticle volume fraction profiles as well as the local heat transfer rates for various values of the physical parameters are displayed in both graphical and tabular forms

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##### References

More filters

••

TL;DR: In this article, a single-and two-phase model with either constant or temperature-dependent properties is employed to develop laminar forced convection flow of a water-Al2O3 nanofluid in a circular tube, submitted to a constant and uniform heat flux at the wall.

434 citations

••

TL;DR: It is shown that animal movement (running, flying, swimming) and fluid eddy movement (turbulent structure) are both forms of optimized intermittent movement.

Abstract: SUMMARY Biologists have treated the view that fundamental differences exist between
running, flying and swimming as evident, because the forms of locomotion and
the animals are so different: limbs and wings vs body undulations,
neutrally buoyant vs weighted bodies, etc. Here we show that all
forms of locomotion can be described by a single physics theory. The theory is
an invocation of the principle that flow systems evolve in such a way that
they destroy minimum useful energy (exergy, food). This optimization approach
delivers in surprisingly direct fashion the observed relations between speed
and body mass ( M b ) raised to 1/6, and between frequency
(stride, flapping) and \(M_{\mathrm{b}}^{-1{/}6}\) , and
shows why these relations hold for running, flying and swimming. Animal
locomotion is an optimized two-step intermittency: an optimal balance is
achieved between the vertical loss of useful energy (lifting the body weight,
which later drops), and the horizontal loss caused by friction against the
surrounding medium. The theory predicts additional features of animal design:
the Strouhal number constant, which holds for running as well as flying and
swimming, the proportionality between force output and mass in animal motors,
and the fact that undulating swimming and flapping flight occur only if the
body Reynolds number exceeds approximately 30. This theory, and the general
body of work known as constructal theory, together now show that animal
movement (running, flying, swimming) and fluid eddy movement (turbulent
structure) are both forms of optimized intermittent movement.

243 citations

••

TL;DR: Carbon nanotube aerogel-based thermo-electrochemical cells are fabricated, which are potentially low-cost and relatively high-efficiency materials for this application and the importance of electrode purity, engineered porosity and catalytic surfaces in enhancing the thermocell performance is demonstrated.

Abstract: Conversion of low-grade waste heat into electricity is an important energy harvesting strategy. However, abundant heat from these low-grade thermal streams cannot be harvested readily because of the absence of efficient, inexpensive devices that can convert the waste heat into electricity. Here we fabricate carbon nanotube aerogel-based thermo-electrochemical cells, which are potentially low-cost and relatively high-efficiency materials for this application. When normalized to the cell cross-sectional area, a maximum power output of 6.6 W m−2 is obtained for a 51 °C inter-electrode temperature difference, with a Carnot-relative efficiency of 3.95%. The importance of electrode purity, engineered porosity and catalytic surfaces in enhancing the thermocell performance is demonstrated. Conversion of low-grade waste heat into electricity is an important energy harvesting strategy. Here, the authors fabricate carbon nanotube aerogel-based thermo-electrochemical cells, which are potentially low-cost and relatively high-efficiency materials for this application.

215 citations

••

TL;DR: In this paper, the authors present the results of laboratory experiments measuring particle penetration through surrogates of cracks in building envelopes, using common building materials: aluminum, brick, concrete, plywood, redwood lumber, pine lumber, and strand board.

Abstract: Particle penetration into buildings influences human exposure to particles of ambient origin. In this study, we present the results of laboratory experiments measuring particle penetration through surrogates of cracks in building envelopes. Rectangular slots were prepared, with crack heights of 0.25 and 1 mm and flow-path lengths of 4-10 cm, using common building materials: aluminum, brick, concrete, plywood, redwood lumber, pine lumber, and strand board. Air was drawn through a slot from a well-mixed chamber by applying a pressure difference ( j P) of 4 or 10 Pa. Nonvolatile, electrically neutralized particles were generated and introduced into the chamber. The particle penetration factor was determined, for particle sizes 0.02-7 w m, as the ratio of the particle concentration downstream of the slot to that in the chamber. Particle size and crack height were the two main factors that governed fractional particle penetration. Consistent with prior modeling results, the penetration factor was nearly unity ...

196 citations

••

TL;DR: In this article, the numerical solution of steady natural convection boundary-layer flow of a nanofluid consisting of a pure fluid with nanoparticles along a permeable vertical plate in the presence of magnetic field, heat generation or absorption, and suction or injection effects is focused.

Abstract: This work is focused on the numerical solution of steady natural convection boundary-layer flow of a nanofluid consisting of a pure fluid with nanoparticles along a permeable vertical plate in the presence of magnetic field, heat generation or absorption, and suction or injection effects The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis The governing boundary-layer equations of the problem are formulated and transformed into a non-similar form The obtained equations are then solved numerically by an efficient, iterative, tri-diagonal, implicit finite-difference method Comparisons with previously published work are performed and are found to be in excellent agreement Representative results for the longitudinal velocity, temperature, and nanoparticle volume fraction profiles as well as the local heat transfer rates for various values of the physical parameters are displayed in both graphical and tabular forms

185 citations