Fluxes across double-diffusive interfaces: a one-dimensional-turbulence study

2011 ◽  
Vol 677 ◽  
pp. 218-254 ◽  
Author(s):  
ESTEBAN GONZALEZ-JUEZ ◽  
ALAN R. KERSTEIN ◽  
DAVID O. LIGNELL
Keyword(s):  
Heat Flux ◽  
Lewis Number ◽  
Heat Fluxes ◽  
Velocity Difference ◽  
One Dimensional ◽  
The Stability ◽  
Background Shear ◽  
Dimensionless Heat Flux ◽  
Dimensionless Heat ◽  
Double Diffusive

This work is a parametric study of the fluxes of heat and salt across unsheared and sheared double-diffusive interfaces using one-dimensional-turbulence (ODT) simulations. It is motivated by the need to understand how these fluxes scale with parameters related to the fluid molecular properties and background shear. Comparisons are made throughout with previous models and available measurements. In unsheared interfaces, ODT simulations show that the dimensionless heat fluxNuscales with the stability parameterRρ, Rayleigh numberRaand Prandtl numberPrasNu~ (Ra/Rρ)0.37±0.03whenPrvaries from 3 to 100 and asNu~ (Ra/Rρ)0.31Pr0.22±0.04whenPrvaries from 0.01 to 1. HereRa/Rρcan be seen as the ratio of destabilizing and stabilizing effects. The simulation results also indicate that the ratio of salt and heat fluxesRfis independent ofPr, scales with the Lewis numberLeasRf~Le0.41±0.04whenRρis large enough and deviates from this expression for low values ofRρ, when the interface becomes heavily eroded. In sheared interfaces, the simulations show three flow regimes. When the Richardson numberRi≪ 1, shear-induced mixing dominates, the heat flux scales with the horizontal velocity difference across the interface andRf=Rρ. NearRi~ 1 the heat and salt fluxes are seen to increase abruptly as the shear increases. The flow structure and scaling of the fluxes are similar to those of unsheared interfaces whenRi≫ 1.

Numerical Study of the Onset of Double-Diffusive Cellular Convection Due to a Uniform Lateral Heat Flux

1982 ◽  
Vol 104 (4) ◽  
pp. 649-655 ◽  
Author(s):  
S. Takao ◽  
M. Tsuchiya ◽  
U. Narusawa
Keyword(s):  
Heat Flux ◽  
Numerical Study ◽  
Vertical Wall ◽  
Lewis Number ◽  
Double Diffusive Convection ◽  
Physical Experiments ◽  
Diffusive Convection ◽  
The Stability ◽  
Double Diffusive ◽  
Lateral Heat

When a fluid with a vertical solute gradient of (−dS/dy)0 is heated laterally, roll cells start to form at the boundary, developing into a series of convective layers. Numerical experiments were performed to investigate the onset of the abovementioned double-diffusive convection under the application of a uniform lateral heat flux. The paper reports the results and discussion of the following aspects of the stability of double-diffusive convection; (i) the relationship between the critical value, (Ra/Rs)c, above which convection cells form along the vertical wall and the nondimensional slot width, (d/L), (ii) the effect of the Lewis number on (Ra/Rs)c. It was also confirmed that values of (Ra/Rs)c as well as H/L (the nondimensional vertical size of incipient cells) obtained in this numerical experiment for wide slot widths (d/L>∼30), agreed well with those obtained previously by physical experiments.

Stability and Enhancement of Boiling in Microchannels

2011 ◽  
Author(s):  
A. E. Bergles
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
High Heat ◽  
Heat Fluxes ◽  
Transfer Enhancement ◽  
The Past ◽  
The Stability ◽  
Saturated Boiling ◽  
Microelectronic Components

During the past 20 years, there has been intense worldwide interest in microchannel heat exchangers, particularly for cooling of microelectronic components. Saturated boiling of the coolant is usually indicated in order to accommodate high heat fluxes and to have uniformity of temperature. However, boiling is accompanied by several instabilities, the most severe of which can sharply limit the maximum, or critical, heat flux. These stability phenomena are reviewed, and recent studies will be discussed. Elevation of the critical heat flux will be discussed within the context of heat transfer enhancement. Means to improve the stability of boiling and the enhancement of boiling heat transfer, in general, are discussed.

scholarly journals Characteristics of Steady and Transient Superfluid Transport for the Cooling of Superconducting Rotating Machines

2003 ◽  
Vol 9 (6) ◽  
pp. 427-436
Author(s):  
Thomas C. Chuang
Keyword(s):  
Heat Flux ◽  
Laminar Flow ◽  
Generalized Equation ◽  
Analytical Models ◽  
Rotating Machines ◽  
Transient Transport ◽  
Driving Time ◽  
Common Framework ◽  
Dimensionless Heat Flux ◽  
Dimensionless Heat

The unique properties of superfluid helium (He II) make it a very efficient cooling agent for superconducting rotating machines. Steady and transient transport characteristics and design formulas for the cooling of superconducting windings are enumerated in this article. Several superfluid transport analytical models and useful design equations are discussed: laminar flow; turbulent flow; and pure superfluid flow under steady-state and transient conditions. An effort was made to consolidate all analytical models and experimental results into a common framework. Under conditions of steady He II transport, a dimensionless heat flux numberNq, a dimensionless driving force numberN∇T, and a characteristic length where used so that a generalized equation could be derived to describe superfluid transport in any geometry. In the case of transient transport of He II, a dimensionless heat flux numberNq∗and a dimensionless driving time numberNtwere used so that a generalized equation could be derived to describe transient superfluid transport in laminar flow and turbulent regimes. Many experimental data were compiled to substantiate the analysis.

Investigation of laminar fluid flow and heat transfer of nanofluid in trapezoidal microchannel with different aspect ratios

2019 ◽  
Vol 29 (5) ◽  
pp. 1680-1698 ◽  
Author(s):  
Hesam Bakhshi ◽  
Erfan Khodabandeh ◽  
Omidali Akbari ◽  
Davood Toghraie ◽  
Mohammad Joshaghani ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Fluid Flow ◽  
Pressure Drop ◽  
Transfer Rate ◽  
Hydraulic Diameter ◽  
Content Type ◽  
Trapezoidal Microchannel ◽  
Dimensionless Heat Flux ◽  
Dimensionless Heat

Purpose In the present study, laminar steady flow of nanofluid through a trapezoidal channel is studied by using of finite volume method. The main aim of this paper is to study the effect of changes in geometric parameters, including internal and external dimensions on the behavior of heat transfer and fluid flow. For each parameter, an optimum ratio will be presented. Design/methodology/approach The results showed that in a channel cell, changing any geometric parameter may affect the temperature and flow field, even though the volume of the channel is kept constant. For a relatively small hydraulic diameter, microchannels with different angles have a similar dimensionless heat flux, while channels with bigger dimensions show various values of dimensionless heat flux. By increasing the angles of trapezoidal microchannels, dimensionless heat flux per unit of volume increases. As a result, the maximum and minimum heat transfer rate occurs in a trapezoidal microchannel with 75° and 30 internal’s, respectively. In the study of dimensionless heat flux rate with hydraulic diameter variations, an optimum hydraulic diameter (Dh) was observed in which the heat transfer rate per unit volume attains maximum value. Findings This optimum state is predicted to happen at a side angle of 75° and hydraulic diameter of 290 µm. In addition, in trapezoidal microchannel with higher aspect ratio, dimensionless heat flux rate is lower. Changing side angles of the channels and pressure drop have the same effect on pressure drop. For a constant pressure drop, if changing the side angles causes an increase in the rectangular area of the channel cross-section and the effect of the sides are not felt by the fluid, then the dimensionless heat flux will increase. By increasing the internal aspect ratio (t_2/t_3), the amount of t_3 decreases, and consequently, the conduction resistance of the hot surface decreases. Originality/value The effects of geometry of the microchannel, including internal and external dimensions on the behavior of heat transfer and fluid flow for pressure ranges between 2 and 8 kPa.

Nongray Particulate Radiation in an Isothermal Cylindrical Medium

1981 ◽  
Vol 103 (1) ◽  
pp. 121-126 ◽  
Author(s):  
J. D. Felske ◽  
K. M. Lee
Keyword(s):  
Heat Flux ◽  
Exact Solution ◽  
Closed Form ◽  
Radiative Heat ◽  
Exact Result ◽  
Approximate Solutions ◽  
Small Particles ◽  
Dimensionless Heat Flux ◽  
Dimensionless Heat ◽  
Good Agreement

The radial radiative heat flux and its divergence are determined both exactly and approximately for homogeneous suspensions of small particles. Scattering is assumed to be small compared to absorption and the absorption coefficient is taken to be inversely proportional to wavelength. The exact solution is reduced to an infinite series of single integrals. The optically thin and the next higher order behavior appear in closed form as the first two terms in the series. Two approximate solutions are also developed. One is in good agreement with the exact solution while the other is not. Finally, a closed form approximate relation is derived for the dimensionless heat flux at the surface. This expression, which also gives the emissivity or absorptivity of the medium, is in excellent agreement with the exact result.

scholarly journals Microstructure Observations of Turbulent Heat Fluxes in a Warm-Core Canada Basin Eddy

2018 ◽  
Vol 48 (10) ◽  
pp. 2397-2418 ◽  
Author(s):  
Elizabeth C. Fine ◽  
Jennifer A. MacKinnon ◽  
Matthew H. Alford ◽  
John B. Mickett
Keyword(s):  
Heat Flux ◽  
Heat Fluxes ◽  
Canada Basin ◽  
Turbulent Heat ◽  
Bottom Edge ◽  
Warm Core ◽  
Turbulent Heat Fluxes ◽  
Thermohaline Intrusions ◽  
Rate Of Dissipation ◽  
Double Diffusive

AbstractAn intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6°C, significantly warmer than the surrounding −1°C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy ε was quite low . However, at the edges of the eddy core, ε was elevated to . Three different processes were associated with elevated ε. Double-diffusive steps were found at the eddy’s top edge and were associated with an upward heat flux of 5 W m−2. At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m−2 downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m−2. Integrating these fluxes over an idealized approximation of the eddy’s shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1–2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin.

Dynamic Calibration of a Coaxial Thermocouples for Short Duration Transient Measurements

2013 ◽  
Vol 135 (12) ◽  
Author(s):  
Rakesh Kumar ◽  
Niranjan Sahoo
Keyword(s):  
Heat Flux ◽  
Finite Element ◽  
Heat Conduction ◽  
Surface Heat Flux ◽  
Surface Heat ◽  
Heat Fluxes ◽  
Dynamic Calibration ◽  
One Dimensional ◽  
Surface Heat Fluxes ◽  
Heat Loads

Coaxial thermocouple sensors are suitable for measuring highly transient surface heat fluxes because the response times of these sensors are very small (∼0.1 ms). These robust sensors have the flexibility of mounting them directly on the surface of any geometry. So, they have been routinely used in ground-based impulse facilities as temperature sensors where rapid changes in heat loads are expected on aerodynamic models. Subsequently, the surface heat fluxes are predicted from the transient temperatures by appropriate one-dimensional heat conduction modeling for semi-infinite body. In this backdrop, the purpose of this work is to design and fabricate K-type coaxial thermocouples in-house and calibrate them under similar nature of heat loads by using simple laboratory instruments. Here, two methods of dynamic calibration of coaxial thermocouples have been discussed, where the known step loads are applied through radiation and conduction modes of heat transfer. Using appropriate one dimensional heat conduction modeling, the surface heat fluxes are predicted from the measured temperature histories and subsequently compared with the input heat loads. The recovery of surface heat flux from laser based calibration experiment under-predicts by 4% from its true input heat load. Similarly, recovery of surface heat flux from the conduction mode calibration experiments under-predicts 6% from its true input value. Further, finite-element based numerical study is performed on the coaxial thermocouple model to obtain surface temperatures with same heat loads as used in the experiments. The recovery of surface temperatures from finite element simulation is achieved within an accuracy of ±0.3% from the experiment.

Alternative Solutions for Longitudinal Fins of Rectangular, Trapezoidal, and Concave Parabolic Profiles

2006 ◽  
Author(s):  
A. Aziz
Keyword(s):  
Thermal Analysis ◽  
Heat Flux ◽  
Boundary Conditions ◽  
Fixed Temperature ◽  
Thermal Boundary Conditions ◽  
Convective Fin ◽  
The Relationship ◽  
Alternative Solutions ◽  
Dimensionless Heat Flux ◽  
Dimensionless Heat

The traditional thermal analysis of fins is based on the assumption of specified thermal boundary conditions at the base and tip of the fin. For situations when the fin base is in contact with a fluid experiencing condensation and the fin is required to remove the energy released by the fluid, the base is subjected to two boundary conditions: a fixed temperature and a fixed heat flux. This paper develops solutions for the temperature distribution in the fins under these conditions. Solutions are provided for rectangular, trapezoidal, and concave parabolic (finite tip thickness). Results illustrating the relationship between the dimensionless heat flux, the fin parameter, and dimensionless tip temperature are provided for all three geometries. The case of convective fin tip is also considered and lead to a relationship between the dimensionless heat flux, the fin parameter, and the Biot number at the tip. The results presented here provide tools that not only complement the traditional analyses but are believed to have more direct relevance for fin designers.

Rewetting of an Infinite Slab With Uniform Heating Under Quasi-Steady Conditions

2002 ◽  
Vol 124 (5) ◽  
pp. 875-880 ◽  
Author(s):  
A. K. Satapathy ◽  
R. K. Sahoo
Keyword(s):  
Heat Flux ◽  
Peclet Number ◽  
Constant Heat Flux ◽  
Model Parameters ◽  
Péclet Number ◽  
Infinite Slab ◽  
Hot Surface ◽  
Minimum Heat ◽  
Dimensionless Heat Flux ◽  
Dimensionless Heat

The two-dimensional quasi-steady conduction equation governing conduction controlled rewetting of an infinite slab, with one side flooded and the other side subjected to a constant heat flux, has been solved by Wiener-Hopf technique. The solution yields the quench front temperature as a function of various model parameters such as Peclet number, Biot number and dimensionless heat flux. Also, the critical (dryout) heat flux is obtained by setting the Peclet number equal to zero, which gives the minimum heat flux required to prevent the hot surface being rewetted.

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