Relative Contribution Of Convection Research Articles

Transient convective heat transfer in a steam-assisted gravity drainage (SAGD) process

Viscosity reduction through heat transport from steam to bitumen is one of the most important recovery mechanisms of a steam-assisted gravity drainage (SAGD) process. Both heat convection and conduction contribute to the heat transport. Although conduction is considered as dominant through most of a SAGD process, an understanding of heat convection, especially accurate modeling of a condensate convection velocity, is still limited in the literature. This paper develops a mathematical model for the transient heat transfer beyond a steam chamber boundary in SAGD. A convection velocity is clearly formulated, which requires the coupling of heat transport and pressure diffusion. Calculation results show that in SAGD, convection plays a minor role than conduction. In addition, the relative contribution of convection can be influenced by reservoir formation compressibility, steam chamber boundary advancing velocity, and particularly by a difference between steam injection pressure and reservoir initial pressure. Correlations are regressed to estimate the relative contribution of heat convection (ratio) in the overall heat transfer process during a stabilized production period of SAGD.

An Experimental Study of the Effects of Solid-to-Coolant Thermal Conductivity Ratio in Helium-Cooled Divertor Modules

As part of the ARIES study, the Georgia Tech group has experimentally studied the thermal performance of a helium-cooled ‘finger-type’ tungsten divertor design that uses jet impingement and a fin array to cool the plasma-facing surface. These studies were performed using air at Reynolds numbers Re, spanning those for prototypical operating conditions. A brass test section heated with an oxy-acetylene torch at incident heat fluxes up to 2 MW/m2 was used. Recently, data obtained with room-temperature helium suggests that dynamic similarity between the air and helium experiments cannot be achieved by only matching Re because of the difference in the relative contributions of convection and conduction through the annular side walls of the divertor. Numerical simulations suggest that achieving dynamic similarity requires matching the ratio of the thermal conductivity of the divertor module material to that of the coolant under operating conditions, as well as Re.Studies were performed to verify that experiments at the prototypical Re and thermal conductivity ratio using helium at room temperature give Nusselt numbers Nu that are dynamically similar to those at prototypical operating conditions. Given that the thermal conductivity of helium decreases as temperature decreases, matching of the thermal conductivity ratio required a carbon steel test section with a thermal conductivity much lower than that of the brass alloy previously used. The resulting ratio of the test section to coolant thermal conductivities is similar to that of the tungsten alloy and helium at prototypical conditions. The data were used to verify generalized correlations for Nu, as a function of Re and the thermal conductivity ratio. The correlations can be used to determine the maximum heat flux that can be accommodated by the divertor at prototypical conditions.

Verification of Thermal Performance Predictions of Prototypical Multi-Jet Impingement Helium-Cooled Divertor Module

An experimental investigation of the thermal performance of the Helium-Cooled Multi-Jet (HEMJ) modular divertor design developed by the Karlsruhe Research Center (FZK) was previously performed at Georgia Tech using air at Reynolds numbers (Re) spanning those at which the actual He-cooled divertor is to be operated. More recently, another experimental investigation was performed by the Georgia Tech group for a similar finger-type divertor module using both air and He as coolants. The results of these experiments suggest that, in addition to matching Re, dynamic similarity between the air and He experiments requires that a correction be made to account for the differences in the relative contributions of convection and conduction (through the divertor walls) to the overall heat removal rate by the module. This correction factor depends on the thermal conductivity ratio of the solid to the coolant. Experiments similar to those previously conducted have therefore been performed using air, argon, or He as coolant for test sections constructed of brass or steel thus covering a wide range of thermal conductivity ratio. The resultant correlation between Re, the heat removal rate, and the thermal conductivity ratio from these experiments can be used to predict the thermal performance of HEMJlike divertors at prototypical operating conditions.

Conjugate Convection with Surface Radiation from a Square-Shaped Electronic Device with Multiple Identical Discrete Heat Sources

Numerical simulation studies on combined conduction–convection–radiation from a square-shaped electronic device with multiple identical flush-mounted discrete heat sources have been performed and the prominent results are reported here. The problem geometry comprises a square shaped slab with four symmetrically located flush mounted identical discrete heat sources. The heat generated in the heat sources gets conducted through the slab and subsequently gets dissipated from its boundaries by the combined modes of convection and radiation. Air, a radiatively transparent medium is considered to be the cooling agent. The governing equations for temperature distribution in the entire computational domain are obtained by appropriate energy balance between the heat generated, conducted, convected and radiated. The resulting partial differential equations are solved using finite difference method in conjunction with Gauss–Seidel iterative technique. A computer code is prepared for the purpose. Exhaustive numerical studies are performed to elucidate the effects of parameters like volumetric heat generation, thermal conductivity, surface emissivity and convection heat transfer coefficient on local temperature distribution, peak device temperature and relative contributions of convection and radiation in heat dissipation.

Thermal effects of tissue optics in symbiont‐bearing reef‐building corals

Reflectance spectroscopy and microscale temperature measurements were used to investigate links between optical and thermal properties of corals. Coral tissue heating showed a species‐specific linear correlation to the absorptance of incident irradiance. Heat budgets estimated from absorptance and thermal boundary layer measurements indicated differences in the relative contribution of convection and conduction to heat loss in Porites lobata and Stylophora pistillata, and a higher heat conduction into the skeleton of the thin‐tissued branching S. pistillata as compared to the massive thick‐tissued P. lobata. Decreasing absorptance associated with bleaching resulted in decreased surface warming of coral tissue. Action spectra of coral tissue heating showed elevated efficiency of heating at wavelengths corresponding to absorption maxima of major zooxanthellae photopigments. Generally, energy‐rich radiation (< 500 nm) showed the highest heating efficiency. Species‐specific relationships between coral tissue heating and absorptance can be strongly affected by differences in the thermal properties of the skeleton and/or tissue arrangement within the skeletal matrix, indicating a yet unresolved potential for coral shape, size, and tissue thickness to affect heat dissipation and especially the conduction of heat into the coral skeleton.

Interaction of surface radiation with combined conduction and convection from a discretely heated L-corner

Prominent results pertaining to the problem of multi-mode heat transfer from an L-corner equipped with three identical flush-mounted discrete heat sources in its left leg are given here. The heat generated in the heat sources is conducted along the two legs of the device before being dissipated by combined convection and radiation into air that is considered to be the cooling agent. The governing equations for temperature distribution along the L-corner are obtained by making appropriate energy balance between the heat generated, conducted, convected and radiated. The non-linear partial differential equations thus obtained are converted into algebraic form using a finite-difference formulation. The resulting equations are solved simultaneously by Gauss–Seidel iterative solver. A computer code is specifically written to solve the problem. The computational domain is discretised using 101 grids along the left leg, with 15 grids taken per heat source, and 21 grids along the bottom leg. The effects of surface emissivity, convection heat transfer coefficient, thermal conductivity and aspect ratio on local temperature distribution, peak device temperature and relative contributions of convection and radiation to heat dissipation from the L-corner are studied in detail. The point that one cannot overlook radiation in problems of this class has been clearly elucidated.

On efficient numerical solution of one-dimensional convection diffusion equations in modelling atmospheric processes

A new approach is proposed to the numerical solution of one-dimensional convection-diffusion equations that arise in modelling atmospheric processes and air pollution modelling. The technique is based on upstream-type difference approximations for first-order derivatives and non-standard difference approximations for second-order derivatives of convection-diffusion equations. This approach leads to the significant qualitative improvements in the numerical solutions behaviour. The relative contribution of convection and diffusion is directly incorporated into the corresponding numerical scheme in such a way that large spatial grids can be taken without affecting solution stability. The method is compared with the contemporary computational schemes for solving problems with severe internal and boundary gradients and is shown to be stable and computationally efficient. The results of a numerical experiment are given.

Interaction of Surface Radiation with Conduction and Convection from a Vertical Channel with Multiple Discrete Heat Sources in the Left Wall

This article reports the results of a numerical study of the problem of interaction of surface radiation with conduction and convection from a vertical channel equipped with three flush-mounted discrete heat sources in the left wall. The governing equations for temperature distribution along each of the two channel walls have been deduced based on appropriate energy balance between various energy interactions in which the channel is involved. Radiosity-irradiation formulation is employed to tackle radiation-related calculations, with the view factors involved therein obtained using Hottel's crossed-string method. The governing nonlinear partial differential equations are converted into algebraic form through a finite-volume-based finite-difference method, and are subsequently solved using Gauss-Seidel iterative technique. An optimum grid system, with 26 grids along each of the three discrete heat sources and 11 grids along each of the two non-heat source portions of the board, is used for discretization of the computational domain. The effects of various pertinent parameters, viz., aspect ratio, volumetric heat generation, thermal conductivity, surface emissivity, and convection heat transfer coefficient, on temperature distribution along the left board, maximum temperature in the channel, and relative contributions of convection and surface radiation to heat dissipation from the channel are probed.

Simulation studies on multi-mode heat transfer from an open cavity with a flush-mounted discrete heat source

Prominent results of a simulation study on conjugate convection with surface radiation from an open cavity with a traversable flush mounted discrete heat source in the left wall are presented in this paper. The open cavity is considered to be of fixed height but with varying spacing between the legs. The position of the heat source is varied along the left leg of the cavity. The governing equations for temperature distribution along the cavity are obtained by making energy balance between heat generated, conducted, convected and radiated. Radiation terms are tackled using radiosity-irradiation formulation, while the view factors, therein, are evaluated using the crossed-string method of Hottel. The resulting non-linear partial differential equations are converted into algebraic form using finite difference formulation and are subsequently solved by Gauss–Seidel iterative technique. An optimum grid system comprising 111 grids along the legs of the cavity, with 30 grids in the heat source and 31 grids across the cavity has been used. The effects of various parameters, such as surface emissivity, convection heat transfer coefficient, aspect ratio and thermal conductivity on the important results, including local temperature distribution along the cavity, peak temperature in the left and right legs of the cavity and relative contributions of convection and radiation to heat dissipation in the cavity, are studied in great detail.

Simulation Studies on Multimode Heat Transfer from a Square-Shaped Electronic Device with Multiple Discrete Heat Sources

ABSTRACT The results of a numerical study of the problem of multimode heat transfer from a square-shaped electronic device provided with three identical flush-mounted discrete heat sources are presented here. Air, a radiatively nonparticipating fluid, is taken to be the cooling medium. The heat generated in the discrete heat sources is first conducted through the device, before ultimately being dissipated by convection and surface radiation. The governing partial differential equations for temperature distribution are converted into algebraic form using a finite-volume based finite difference method, and the resulting algebraic equations are subsequently solved using Gauss-Seidel iterative procedure. A grid size of 151 × 91 is used for discretizing the computational domain. The effects of all relevant parameters, including volumetric heat generation, thermal conductivity, convection heat transfer coefficient, and surface emissivity, on various important results, such as the local temperature distribution, the peak temperature of the device, and the relative contributions of convection and surface radiation to heat dissipation from the device, are studied in sufficient detail. The exclusive effect of surface radiation on pertinent results of the present problem is also brought out.