Luiz Fernando Milanez

Natural convection in rectangular enclosures heated from below and symmetrically cooled from the sides

Steady natural convection in an enclosure heated from below and symmetrically cooled from the sides is studied numerically, using a streamfunction-vorticity formulation. The Allen discretization scheme is adopted and the discretized equations were solved in a line by line basis. The Rayleigh number based on the cavity height is varied from 103 to 107. Values of 0.7 and 7.0 for the Prandtl number are considered. The aspect ratio LH (length to height of the enclosure) is varied from 1 to 9. Boundary conditions are uniform wall temperature and uniform heat flux. For the range of the parameters studied, a single cell is observed to represent the flow pattern. Numerical values of the Nusselt number as a function of the Rayleigh number are reported, and the Prandtl number is found to have little influence on the Nusselt number. A scale analysis is presented in order to better understand the phenomenon.

Choice of a reference state for exergetic analysis

The concepts of thermomechanical exergy, chemical exergy, restricted dead state, and unrestricted dead state are described. A detailed discussion is presented on identification of the reference state. The influence of commonly-used methods on evaluation of the overall exergy is analysed. The effect of using a standard reference state is compared with that for a reference state in which the composition is defined according to the problem under study. Finally the second-law efficiency is defined and discussed for general control volumes and closed systems.

Thermodynamic analysis of a coke dry quenching unit

A thermodynamic analysis (first and second laws) was performed for a coke dry quenching (CDQ) unit with all data obtained in the site and under the unit normal operating conditions. Expressions to evaluate the coke specific heat, enthalpy, entropy and physical and chemical exergies, as well as particulate material (dust) formation indices, and the loss of coke mass through chemical reactions, were developed and compared with the literature. The analysis was extended to a coke wet quenching (CWQ) unit, a process still largely used across the world. The procedure is fully described. The comparison between the performances of the CDQ and CWQ processes supports that such analysis can be a useful and accessible tool in the decision making process.

Effect of soybean oil ethyl ester/diesel fuel blends on engine efficiency

Variation of engine thermal efficiency in response to the addition of soybean oil ethyl ester to diesel fuel was analysed in terms of the in-cylinder phenomena and fuel properties. Blends with concentrations of up to 30% of soybean oil ethyl ester in volume were used in steady-state experiments conducted in a high speed turbocharged direct injection engine. Engine internal processes were studied by means of exergy balances based on engine indicating data, which provided information about the impact of biodiesel blending on the amount of fuel exergy exchanged through heat, work and mass transfer. The data obtained from this exergy analysis served to identify the causes of the alterations in brake thermal efficiency measured in dynamometric bench tests. Operating in full load conditions and fuelled with the B20 blend, the engine showed an average increase of 4.16% in brake thermal efficiency and a gross increase of 4.24% in indicated thermal efficiency in relation to its performance with mineral diesel fuel.

Thermally Developing Forced Convection of Non-Newtonian Fluids Inside Elliptical Ducts

Laminar-forced convection inside tubes of various cross-section shapes is of interest in the design of a low Reynolds number heat exchanger apparatus. Heat transfer to thermally developing, hydrodynamically developed forced convection inside tubes of simple geometries such as a circular tube, parallel plate, or annular duct has been well studied in the literature and documented in various books, but for elliptical duct there are not much work done. The main assumptions used in this work are a non-Newtonian fluid, laminar flow, constant physical properties, and negligible axial heat diffusion (high Peclet number). Most of the previous research in elliptical ducts deal mainly with aspects of fully developed laminar flow forced convection, such as velocity profile, maximum velocity, pressure drop, and heat transfer quantities. In this work, we examine heat transfer in a hydrodynamically developed, thermally developing laminar forced convection flow of fluid inside an elliptical tube under a second kind of a boundary condition. To solve the thermally developing problem, we use the generalized integral transform technique (GITT), also known as Sturm-Liouville transform. Actually, such an integral transform is a generalization of the finite Fourier transform, where the sine and cosine functions are replaced by more general sets of orthogonal functions. The axes are algebraically transformed from the Cartesian coordinate system to the elliptical coordinate system in order to avoid the irregular shape of the elliptical duct wall. The GITT is then applied to transform and solve the problem and to obtain the once unknown temperature field. Afterward, it is possible to compute and present the quantities of practical interest, such as the bulk fluid temperature, the local Nusselt number, and the average Nusselt number for various cross-section aspect ratios.

Heat Transfer Analysis of Digital Transmission Equipment with Horizontally Arranged Printed Circuit Boards

Printed circuit boards (PCBs) may be arranged horizontally in many situations, as for example in some digital transmission equipment being used in Brazil. Such pieces of equipment, known as slim racks, have a high aspect ratio (height/width) and are assembled side by side and back to back. The present work deals with the steady-state thermal dissipation in the so-called thermal unit of the slim rack made of the PCB, the magnetic shield above it and the bounding walls constituting an enclosure. The heat transfer in each thermal unit was investigated by assuming isothermal enclosure surfaces with uniform radiosities. Once the thermal paths are identified, an electric circuit analogy is employed to obtain the overall thermal resistance network. Most of the thermal resistances are temperature-dependent, so the associated nonlinear algebraic equations can be solved only by an iterative method. An experimental investigation was carried out in a module of the slim rack with simplified boundary conditions in orde...

Internal Combustion Engine Indicating Measurements

In engine combustion diagnosis, the apparent heat release rate and the combustion reaction extent are the most useful quantities obtainable from engine indicating data. The apparent heat release rate is calculated by computing the amount of fuel chemical energy release necessary to obtain the experimentally observed pressure, while the combustion reaction extent is evaluated through the released fraction of the total fuel chemical energy. Heat release analysis is often complemented using optical techniques and its utilization as a diagnostic tool covers a wide range of objectives, including the development of new combustion systems, the analysis of alternative fuel burning, the validation of mathematical models for engine simulation, the investigation of combustion chamber insulation effects and the study of new injection strategies.

Numerical Analysis of the Thermal Dissipation in Horizontal Isothermal Eccentrically Insulated Cylinders by Free Convection

Heat transfer in horizontal concentrically insulated cylinders exposed to free convection is of importance in industry. However, the NuD and the surface temperature are not constant around the cylinder, raising the point that the use of concentric insulation may not be the best way to arrange the insulating layer. Thus, if an eccentric layer is used, the surface temperature should have a larger variation, changing the flow around the cylinder and the heat dissipation. A numerical analysis of the heat dissipation in horizontal isothermal eccentrically insulated cylinders exposed to free convection (Pr ≅ 0.715) is presented. The conduction through the insulating layer was solved analytically (using the bicylindrical coordinate system) and integrated numerically, while the free convection was solved by the PHOENICS software. The parameters analyzed were the ratio between the outer and inner radius of the insulating layer, the ratio between the insulation and air thermal conductivities, the Rayleigh number and the eccentricity of the insulation. An equation is suggested to calculate the total heat of an eccentric arrangement in terms of the total heat of the corresponding concentric arrangement and the ratio between the convective and conductive thermal resistances, for a given ratio of radius and eccentricity.Copyright

Heat Transfer in a Horizontal Cylinder With Eccentric Surfaces

Heat transfer in horizontal cylinders exposed to free convection and radiation is of importance in many industries. Usually this problem is treated by adopting a concentric geometry, disregarding that the external surface temperature is not uniform. If an eccentric geometry is used, the external surface temperature should have a larger variation, changing the flow around the cylinder and the heat transfer coefficient, either improving or reducing the heat transfer. A numerical analysis is presented of the heat transfer in a horizontal cylinder with an internal isothermal surface eccentric to the external surface that is exposed to air free convection and radiation. The conduction problem was solved analytically and integrated numerically, while the free convection was solved by the PHOENICS software. The parameters analyzed were the ratio of radius, the ratio between the material and air thermal conductivities, the Rayleigh number, the emissivity of the outer surface, and the eccentricity between the external and inner surfaces. The parameters of a proposed equation to estimate the total heat of an eccentric arrangement in terms of the total heat of the corresponding concentric arrangement and the ratio between the convective and conductive thermal resistances were determined for given ratios of radius and eccentricities.

Natural Convection in Two-Dimensional Horizontal Channel With Heat Sources

Laminar natural convection in a two-dimensional horizontal channel is very important in laptop design, since optimizing the utilization of the cooler saves energy from the battery. In this work, this configuration has been numerically studied. Three cases were studied according to the position of the heat sources in the lower wall, upper wall and both. The computational domain consisted of two adiabatic walls where the heat sources were positioned, and two open boundaries, where the manometric pressure and normal gradient of velocity were zero. Ambient temperature was prescribed for the entering fluid and zero normal gradient for the exiting fluid. Fluid properties were assumed constant except for the density change with temperature on the buoyancy term. The influence of the modified Rayleigh number, position of the heat sources and heat flux ratio between the sources were analyzed for Prandtl number of 0.7. The maximum temperature excess on the heat source is lower for the case with two heat sources and Ra = 104 . This preliminary study showed the existence of a minimum value of the excess temperature for the studies aspect ratio (0.1).Copyright