Laminar-transitional micropipe flows: energy and exergy mechanisms based on Reynolds number, pipe diameter, surface roughness and wall heat flux

2011 ◽  
Vol 48 (1) ◽  
pp. 17-34
Author(s):  
A. Alper Ozalp
Keyword(s):  
Heat Flux ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Wall Heat Flux ◽  
Pipe Diameter ◽  
Energy And Exergy

scholarly journals Optimal heat transfer enhancement in plane Couette flow

2017 ◽  
Vol 835 ◽  
pp. 1157-1198 ◽  
Author(s):  
Shingo Motoki ◽  
Genta Kawahara ◽  
Masaki Shimizu
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Reynolds Number ◽  
Heat Transfer Enhancement ◽  
Three Dimensional ◽  
Wall Heat Flux ◽  
Scalar Dissipation ◽  
Two Dimensional ◽  
Transfer Enhancement ◽  
Optimal State

Optimal heat transfer enhancement has been explored theoretically in plane Couette flow. The vector field (referred to as the ‘velocity’) to be optimised is time independent and divergence free, and temperature is determined in terms of the velocity as a solution to an advection-diffusion equation. The Prandtl number is set to unity, and consistent boundary conditions are imposed on the velocity and the temperature fields. The excess of a wall heat flux (or equivalently total scalar dissipation) over total energy dissipation is taken as an objective functional, and by using a variational method the Euler–Lagrange equations are derived, which are solved numerically to obtain the optimal states in the sense of maximisation of the functional. The laminar conductive field is an optimal state at low Reynolds number $Re\sim 10^{0}$. At higher Reynolds number $Re\sim 10^{1}$, however, the optimal state exhibits a streamwise-independent two-dimensional velocity field. The two-dimensional field consists of large-scale circulation rolls that play a role in heat transfer enhancement with respect to the conductive state as in thermal convection. A further increase of the Reynolds number leads to a three-dimensional optimal state at $Re\gtrsim 10^{2}$. In the three-dimensional velocity field there appear smaller-scale hierarchical quasi-streamwise vortex tubes near the walls in addition to the large-scale rolls. The streamwise vortices are tilted in the spanwise direction so that they may produce the anticyclonic vorticity antiparallel to the mean-shear vorticity, bringing about significant three-dimensionality. The isotherms wrapped around the tilted anticyclonic vortices undergo the cross-axial shear of the mean flow, so that the spacing of the wrapped isotherms is narrower and so the temperature gradient is steeper than those around a purely streamwise (two-dimensional) vortex tube, intensifying scalar dissipation and so a wall heat flux. Moreover, the tilted anticyclonic vortices induce the flow towards the wall to push low- (or high-) temperature fluids on the hot (or cold) wall, enhancing a wall heat flux. The optimised three-dimensional velocity fields achieve a much higher wall heat flux and much lower energy dissipation than those of plane Couette turbulence.

Heat Transfer to Laminar Flow in Tapered Passages

1965 ◽  
Vol 32 (3) ◽  
pp. 684-689 ◽  
Author(s):  
E. M. Sparrow ◽  
J. B. Starr
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Wall Temperature ◽  
Parallel Plate ◽  
Laminar Flows ◽  
Wall Heat Flux ◽  
Heat Transfer Analysis ◽  
Plate Channel

Consideration is given to the fully developed heat-transfer characteristics of laminar flows in converging and diverging plane-walled passages. The analysis is carried out for the two fundamental thermal boundary conditions of prescribed wall heat flux and prescribed wall temperature. As a prelude to the heat-transfer analysis, a new solution for the velocity distribution is derived on the basis of a linearized momentum equation. The Nusselt number for flow in tapered passages is found to depend on the Reynolds number; this is in contrast to the situation for passages of longitudinally unchanging cross section wherein the Nusselt number is independent of the Reynolds number. In general, the Nusselt number for flow in a plane-walled diverging passage falls below that for the parallel-plate channel, while the Nusselt number for a converging flow is usually higher than that for a parallel-plate channel. Moreover, the fully developed Nusselt numbers for prescribed wall heat flux exceed those for prescribed wall temperature.

Submerged Jet Impingement Boiling of Water Under Subatmospheric Conditions

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
Ruander Cardenas ◽  
Vinod Narayanan
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Roughness ◽  
Reynolds Number ◽  
Strong Dependence ◽  
Jet Impingement ◽  
Nucleate Boiling ◽  
Pressure Surface ◽  
Wall Superheat ◽  
Jet Impingement Boiling

An experimental study of jet impingement boiling is presented for water under saturated and subcooled conditions. Unique to this study is the documentation of boiling curves of a submerged water jet under subatmospheric conditions. Data are reported at a fixed nondimensional nozzle-to-surface distance of H/dj = 6 and for a fixed surface-to-nozzle diameter ratio, dsurf/dj, of 23.8. Saturated jet impingement experiments are performed at three subatmospheric pool pressures of 0.176 bar, 0.276 bar, and 0.478 bar with corresponding saturation temperatures of 57.3 °C, 67.2 °C, and 80.2 °C. At each pressure, jet impingement boiling at varying Reynolds numbers are characterized and compared with pool boiling heat transfer. The effect of surface roughness and fluid subcooling is studied at the lowest pressure of 0.176 bar. Boiling curves indicate a strong dependence of heat flux on jet Reynolds number in the partially developed nucleate boiling region but only a weak dependence in the fully developed nucleate boiling region. At a fixed wall superheat, fluid subcooling is found to shift the boiling curve to the left thereby enhancing heat transfer performance. Critical heat flux is found to increase with increases in pressure, surface roughness, and Reynolds number.

Three-Dimensional Mixed Convection in Plane Symmetric-Sudden Expansion: Bifurcated Flow Regime

2006 ◽  
Vol 129 (7) ◽  
pp. 819-826 ◽  
Author(s):  
M. Thiruvengadam ◽  
B. F. Armaly ◽  
J. A. Drallmeier
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Reynolds Number ◽  
Aspect Ratio ◽  
Flow Regime ◽  
Three Dimensional ◽  
Wall Heat Flux ◽  
The Other ◽  
Sudden Expansion ◽  
Plane Symmetric

Simulations of three-dimensional laminar mixed convection in a vertical duct with plane symmetric sudden expansion are presented to illustrate the effects of the buoyancy-assisting force and the duct’s aspect ratio on flow bifurcation and heat transfer. The stable laminar bifurcated flow regime that develops in this geometry at low buoyancy levels leads to nonsymmetric temperature and heat transfer distributions in the transverse direction, but symmetric distributions with respect to the center width of the duct in the spanwise direction. As the buoyancy force increases, due to increases in wall heat flux, flow bifurcation diminishes and both the flow and the thermal fields become symmetric at a critical wall heat flux. The size of the primary recirculation flow region adjacent to the sudden expansion increases on one of the stepped walls and decreases on the other stepped wall as the wall heat flux increases. The maximum Nusselt number that develops on one of the stepped walls in the bifurcated flow regime is significantly larger than the one that develops on the other stepped wall. The critical wall heat flux increases as the duct’s aspect ratio increases for fixed Reynolds number. The maximum Nusselt number that develops in the bifurcated flow regime increases as the duct’s aspect ratio increases for fixed wall heat flux and Reynolds number.

scholarly journals The effects of longitudinal ribs on entropy generation for laminar forced convection in a microchannel

2016 ◽  
Vol 20 (6) ◽  
pp. 1963-1972 ◽  
Author(s):  
Nader Pourmahmoud ◽  
Hosseinali Soltanipour ◽  
Iraj Mirzaee
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Reynolds Number ◽  
Entropy Generation ◽  
Constant Heat Flux ◽  
Wall Heat Flux ◽  
Entropy Generation Rate ◽  
Total Entropy ◽  
Flux Boundary Condition ◽  
Energy Equations

This paper deals with fluid flow, heat transfer and entropy generation in an internally ribbed microchannel. Mass, momentum and energy equations for constant heat flux boundary condition are solved using the finite volume method. Average Nusselt number and Fanning friction factor are reported as a function of rib height at different Reynolds numbers. The effects of non-dimensional rib height, wall heat flux and the Reynolds number on the entropy generation attributed to friction, heat transfer and total entropy generation are explored. The first law indicates that rib height has the great effect on the flow filed and heat transfer. The second law analysis reveals that for any values of Reynolds number and wall heat flux, as rib height grows; the frictional irreversibility increases while, there is a rib height which provides the minimum heat transfer irreversibility. It is found that the optimum rib height with the minimum total entropy generation rate depends on Reynolds number and wall heat flux.

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