Osvair Vidal Trevisan

Combined Heat and Mass Transfer by Natural Convection in a Porous Medium

Publisher Summary The chapter describes the combined heat and mass-transfer natural convection mechanisms, which are considered an important subfield in contemporary heat and mass-transfer research. This subfield essentially brings together the studies concerned with the combined heat and mass-transfer or double-diffusive processes that are driven by buoyancy through porous media saturated with fluid. The density gradients that provide the driving buoyancy effect are induced by the combined effects of temperature and species concentration nonuniformities present in the porous medium. The chapter considers the phenomena of convection through fluid-saturated porous media generally in terms of volume-averaged quantities. There are four conservation principles considered in the study of convection with more than one buoyancy effect. These include conservation of mass, energy, species, and momentum. Heat and mass transfer in the vertical direction and in horizontal direction are discussed in detail. Another category of studies of combined buoyancy effects in porous media deals with the local fields around buried sources of heat and mass. The recent work in this field focuses on the multilayer structure of flows of the boundary-layer or concentrated-source type.

Mass and heat transfer by high Rayleigh number convection in a porous medium heated from below

Abstract This paper outlines a combined theoretical and numerical study of the mass transfer effected by high Rayleigh number Benard convection in a two-dimensional saturated porous layer heated from below. The focus of this study is on the Darcy flow, heat transfer and mass transfer scales of a single cell (roll) that exists in the steady two-dimensional convection regime. The numerical solutions are based on the complete governing equations for two-dimensional flow, and cover the Rayleigh number range 50–2000. The numerical results compare favorably with the theoretical conclusions of a scale analysis that is based on the recognition of 1. (i) two temperature difference scales in the cell, 2. (ii) a flow field without horizontal boundary layers, and 3. (iii) thermal top and bottom end-regions that are not slender enough to be boundary layers. Writing Le for the Lewis number, the overall mass transfer rate or Sherwood number is shown to scale as Le 1 2 Ra 7 8 if Le > Ra 1 4 , as Le 2 Ra 1 2 if Ra − 1 4 1 4 , and as O(1) if Le − 1 4 . The transition from the Darcy flow to the inertia-dominated Forschheimer flow and the scales of the Forschheimer regime are discussed in the closing section.

Dispersion in heat and mass transfer natural convection along vertical boundaries in porous media

Abstract This paper reports an analytical-numerical study on hydrodynamic dispersion in natural convection heat and mass transfer near vertical surfaces embedded in porous media. The study considers the convective flows promoted by the density variation due to the combination of temperature and concentration gradients. Scale analysis is used to determine predominant parameters from a general descriptive form for the diffusive terms in the governing equations. Four classes of flow are possible according to the relative magnitude of the dispersion coefficients. Order of magnitude reasoning is used to obtain the similarity variables and dimensionless parameters, in the search for similarity solutions. An enhanced form of the Runge-Kutta algorithm is applied to solve the system of coupled similarity equations. Results are presented for several cases in each class of flow, covering an extensive range of the governing parameters.

Analytical modeling of multipass welding process with distributed heat source

In the welding process, the most interesting regions for heat transfer analysis are the fusion zone (FZ) and the heat affected zone (HAZ), where high temperatures are reached. These high temperature levels cause phase transformations and alterations in the mechanical properties of the welded metal. The calculations to estimate the temperature distribution in multiple pass welding is more complex than in the single pass processes, due to superimposed thermal effects of one pass over the previous passes. In the present work, a comparison is made between thermal cycles obtained from analytical models regarding point (concentrated) and Gaussian (distributed) heat sources. The use of distributed heat source prevents infinite temperatures values near the fusion zone. The comparison shows that the thermal cycles obtained from the distributed heat source model are more reliable than those obtained from the concentrated heat source model.

Analytical solutions for heat flow in multiple pass welding

Abstract The purpose of this paper is to present analytical solutions for the heat distribution in the base metal during and after serial heating in multiple pass arc welding. Recent literature on welding heat transfer has covered extensively numerical models for different aspects of the process. Most of these aspects are non-linear problems in heat flow, e.g. varying thermal properties, inclusion of radiative transfer in the arc, and/or convective flow in the welding pool. The basic analytical models in use were established long ago in classical papers, but these refer to single heat cycles. The present paper addresses the circumstances of multiple cycles. In these cases, depending on the bead length and the interpass time, as shown by the proposed solution, the thermal effects of each successive pass will accumulate. When modelling the heat transfer in each pass, the effects of previous passes enter into the differential equations as non-homogeneous initial conditions. The complexity introduced by these conditions requires special treatment: an analytical solution employing the Green’s function method was used. The main focus in this work is on welding of thin plates. Simple changes to the models take account of the convective heat loss to the surroundings and enable temperature dependent properties to be calculated. The solutions were checked against experimental results obtained from a specially designed laboratory setup. Thermal cycles provided by the analytical solutions compare well with temperature histories measured at different locations during three pass gas metal arc welding of a 0.5 in (∼1 cm) AISI 304 stainless steel plate. Measured data and model results show very good agreement. The analytical solutions are also extended to the geometries of moderately thick plates.

Transient method for measuring thermal properties of saturated porous media

Abstract This paper describes the development of a transient technique to measure thermal diffusivily and conductivity of porous samples. The method uses the film heat sensor to probe heat flux. Temperature and heat flux are measured dynamically allowing conditions to vary in time at the point of measurement. The data are then treated by a deconvolution algorithm, rendering results proper to simpler models for the same geometry. The numerical treatment in the deconvolution procedure was verified for a hypothetical case. The method was finally tested in the laboratory, with experiments made on samples of natural rock.

Oil recovery and wettability alteration in carbonates due to carbonate water injection

Enhanced oil recovery in carbonates due to wettability alteration has received much attention recently. In contrast to sandstones reservoirs, carbonates are neutral-wet or preferentially oil-wet. The use of CO2 dissolved into water has emerged as an alternative method for EOR operations in carbonates reservoirs. In the present paper, oil recovery from two carbonate outcrop rocks was evaluated with different brines containing CO2. The evaluation included a qualitative approach by spontaneous imbibition tests and a quantitative assessment by the Amott–Harvey index. The rocks tested were limestone and dolomite samples known as analogous to Brazilian pre-salt reservoirs. The testing fluids were a medium gravity crude oil, seawater concentration brine, formation equivalent brine and the carbonated version of these brines. Results shown additional oil recovery directly associated with wettability alteration driven by brine concentration switches. Oil recovery increases were observed independently if the brine concentration decreased or increased over the water replacement process. Wettability alteration took place in both carbonate types starting at oil-wet to neutral-wet conditions. Similar tests carried out with equivalent carbonated brines showed similar alterations in the wetting properties, trending no dependence on salt concentration. However, CO2 and its derived ions dissolved in the brines seem to inhibit the wettability alteration mechanism.

Evaluation of Permeability Changes in a Carbonate Rock under Carbonate Water Flow

Carbon dioxide (CO2) injection in reservoirs promotes reactions which depend on rock nature, brine composition, partial pressure of CO2, reservoir temperature and pressure among other conditions. The reactions may cause changes in the petrophysics properties, including porosity and permeability, that are important parameters to the fluid flow. The present study focus on the effects of carbonated brine injection in carbonate rocks similar to pre salt reservoirs. The effects are evaluated through the changes of the rock absolute permeability provoked by the acidic action of the injected fluid. Experiments were designed to detail permeability changes along the length of a long carbonate core using using a coreholder equipped with multiple pressure taps. The experiments were conducted in dynamic regime, at the temperature of 22°C and at the mean pressure of 2,000 psi, at flow rates of 0.5; 1 and 2 cc/min. The results show significant permeability alterations at the different segments of the sample, which are also highly dependent on the injection rate.

Wettability Alteration in Limestone and Dolomite With Brines and CO2

Wettability test is one of the most used tools for evaluating rock/fluid interaction in oils reservoir. In the present paper two carbonate outcrop rocks are evaluated for wettability alterations when subjected to brine injections of varying salinities and content of dissolved CO2. The evaluation included a qualitative appraisal via spontaneous imbibition tests and a quantitative assessment by the Amott-Harvey procedure. Rocks refer to limestone and dolomite samples with petro-physical properties similar to the Brazilian pre-salt reservoirs. Testing fluids are a medium gravity crude oil, seawater concentration brine, formation equivalent brine and the carbonated version of these brines. Results show additional oil recovery directly associated with wettability alteration provoked by brine concentration changes. Increments in recovery were observed independently if the brine concentration decreased or increased in the replacement process. For dolomites and limestone wettability changed in the direction of turning the rock from oil-wet to neutral wet. Tests carried out with equivalent carbonated brines show that similar alteration in the wetting properties also occur. Alterations were as well independent of the increase or decrease of the salt concentration in the brine changed. However, CO2 or its derived ions dissolved in the brines seem to inhibit the mechanism of wettability change when rocks are subject to changes in brine salt concentrations.Copyright