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[pt] Neste trabalho foi feita um análise teórica, da influência da relação densidade-temperatura, nas proximidades da densidade máxima, sobre a transferência de calor por convecção natural. Para este estudo foi considerado o sistema formado por uma esfera isotérmica imersa em água: Usando as simplificações da camada limite e a transformação de similaridade, as equação de conservação de massa, momentum e energia, foram reduzidas a sistemas de duas equações diferencias ordinárias, não lineares, de condições de contorno. Estas equações diferenciais simultâneas as quais descrevem os campos de velocidade e temperatura da superfície da esfera, como da temperatura da água. A solução destas equações dão dois tipos de regimes de fluxo, o primeiro o usual fluxo unidirecional e o segundo o bidirecional no qual existe fluxo inverso. Em ambos regimens a direção do fluxo depende tanto da temperatura da superfície da esfera, como da temperatura da água. O objetivo principal deste estudo foi obter a variação do coeficiente de transmissão de calor, o qual depende tanto da temperatura de superfície da esfera, como da temperatura da água. Tendo em vista que a densidade máxima da água ocorre na temperatura de 3,98 graus Celsius , e a temperatura da esfera de 0 graus Celsius a 35 graus Celsius. Os resultados numéricos foram obtidos com o emprego dos computadores digitais IBM-1130 e /370 do Rio Datacentro da Puc. / [en] In this work a theoretical analysis was made on the influence of the temperature density relationship on natural convetion heat transfer in the region of maximum desity. An Isothermal sphere immersed in water was considered in this study. Using boundary layer simplifications and similarity transformationhs, thecontinity, momentum, and energy equations which are non linear and depend on the boudary conditions. These symultaneous differential equations, which describe the velocity and temperature of the sphere as well as the water. The sotution of these equations gives twotypes of flow regimes; the first, the common unidirectional one., and the second, a bidirectional one, in which there is flow reversal. In both of theases regimes of the sphere and water. The principle objective oh this study was to abtain the variation in the heat transfer coefficient wich is dependent on the velocity field, wich in turn is dependent upo the temperatures of the sphere and water. Since the maximum desity of water occurs at 3,98 Celsius degrees, the temperature of the water in this study was varied between 0 Celsius degrees and 20 Celsius degrees, while the temperature of the sphere was varied between 0 Celsius degrees and 35 Celsius degrees. Numerical results were obtain with the use of the IBM-1130 and 370 computors at Rio Datacentro of PUC.

This study explores natural convection heat transfer and fluid flow from a vertical plate in a radiating gas accounting for real gas spectral behavior. Finite volume techniques are used to solve the coupled nonlinear partial differential equations for mass, momentum, and energy conservation, while radiation transfer is modeled using the Discrete Ordinates finite volume finite angle method. Real gas spectral behavior is accounted for using the Rank Correlated Spectral Line Weighted-sum-of-gray-gases method. It is found that gas temperature and velocity are higher in the boundary layer, thickening the thermal and hydrodynamic boundary layers compared to the limiting case of pure convection. Gas species and concentration significantly impact boundary layer development, affecting radiative heating, temperature, velocity, and wall heat fluxes. Wall radiation transport dominates over convective transport. Increasing the wall temperature for the same wall-quiescent surroundings temperature difference increases local radiative heating, temperature, and velocity, and results in higher wall heat fluxes. As Rayleigh number increases, convection gains importance relative to radiation. Higher total gas pressures moderately increase radiative heating, temperature, and velocity, while reducing wall heat fluxes and convective transport. Increased wall emissivity raises radiative heating, temperature, and velocity, while raising wall heat flux and reducing convective flux. It is concluded that the neglect of participating gas radiation effects can result in significant errors in the predicted flow and thermal behavior, and the total transport. These insights advance understanding of radiation-convection interplay in radiating gas scenarios.

Heat transfer is a critical part of engineering design, from the cooling of rocket engines to the thermal management of the increasingly dense packaging of electronic circuits. Even for the most fundamental modes of heat transfer, a topic of research is devoted to finding novel ways to improve it. In recent decades, investigators experimented with the idea of exposing systems to acoustic waves with the hope of enhancing thermal transfer at the surface of a body. Ultrasound has been applied with some success to systems undergoing nucleate boiling and in single-phase forced and free convection heat transfer in water. However, little research has been done into the use of sound waves to improve heat transfer in air. In this thesis the impact of acoustic waves on natural convection heat transfer from a horizontal cylinder in air is explored. An experimental apparatus was constructed to measure natural convection from a heated horizontal cylinder. Verification tests were conducted to confirm that the heat transfer could be described using traditional free convection heat transfer theory. The design and verification testing of the apparatus is presented in this work. Using the apparatus, experiments were conducted to identify if the addition of acoustic waves affected the heat transfer. For the first set of experiments, a 40 kHz standing wave was created along the length of the heated horizontal cylinder. While our expectation was that our results would mirror those found in the literature related to cooling enhancement using ultrasound in water (cited in the body of this thesis), they did not. When a 40 kHz signal was used to actuate the air surrounding the heated cylinder assembly, no measurable enhancement of heat transfer was detected. Experiments were also performed in the audible range using a loudspeaker at 200 Hz, 300 Hz, 400 Hz, 500 Hz, and 2,000 Hz. Interestingly, we found that a 200 Hz acoustic wave causes a significant, measurable impact on natural convection heat transfer in air from a horizontal cylinder. The steady-state surface temperature of the cylinder dropped by approximately 12℃ when a 200 Hz wave was applied to the system.

No description available.

<p>Solar thermosiphons integrated into the thermal envelop of buildings has been studied for their potential to take advantage of solar energy in heating buildings. The annual performance of solar thermosiphons cannot currently be predicted with the correlations from previous research. Also, no work has been done on the supply mode of a solar thermosiphon even though it has the potential to provide heating and fresh ventilation air. An investigation was done with the goal of developing a numerical model that could predict the performance of the supply mode of a solar thermosiphon. The numerical model included infrared thermal radiation and conduction through the glass, phenomenon which had not been used in previous numerical models. To validate the numerical model a novel steady state experiment was developed. This experiment included radiation as the heat source and the ability to vary geometric lengths. The performance parameters of mass flow rate and thermal efficiency were comparable between the numerical predictions and experimental results. However, due to uncertainties in the current experimental setup, full validation of the numerical model was not possible. These uncertainties would have to be addressed before the numerical model that was developed can be fully validated and used for generating correlations. After consideration of practical implementation constrains, it was shown that it was easier to implement the indoor air curtain mode of a solar thermosiphon than the supply mode. The indoor air curtain mode provides the same amount of energy from solar radiation to heat a building as the supply mode of a solar thermosiphon.</p> / Master of Applied Science (MASc)

no / The coupling between natural convection and conduction within rectangular enclosure was investigated numerically. Three separate heat sources were flush mounted on a vertical wall and an isoflux condition was applied at the back of heat sources. The governing equations were solved using control volume formulation. A modified Rayleigh number and a substrate/fluid thermal conductivity ratio were used in the range 10^4 −10^7 and 10−10^3 respectively. The investigation was extended to examine high thermal conductivity ratio values. The results illustrated that, when Rayleigh number increased the dimensionless heat flux and local Nusselt number increased and the boundary layers along hot, cold and horizontal walls were reduced significantly. An opposite behaviour for the thermal spreading in the substrate and the dimensionless temperature, were decreased for higher Rayleigh number. Moreover, the thermal spreading in the substrate increased for higher substrate conductivity, which affected the temperature level. However the effect of the substrate is negligible when the thermal conductivity ratio higher than 1,500. / The full text of book chapters are not available for self deposit under the Publisher's copyright restrictions.

yes / A numerical study of natural convection heat transfer in water filled cavity has been examined in 3-D for single phase liquid cooling system by using an array of parallel plate fins mounted to one wall of a cavity. The heat generated by a heat source represents a computer CPU with dimensions of 37.5∗37.5mm mounted on substrate. A cold plate is used as a heat sink installed on the opposite vertical end of the enclosure. The air flow inside the computer case is created by an exhaust fan. A turbulent air flow is assumed and k-ε model is applied. The fins are installed on the substrate to enhance the heat transfer. The applied power energy range used is between 15 - 40W. In order to determine the thermal behaviour of the cooling system, the effect of the heat input and the number of the parallel plate fins are investigated. The results illustrate that as the fin number increases the maximum heat source temperature decreases. However, when the fin number increases to critical value the temperature start to increase due to the fins are too closely spaced and that cause the obstruction of water flow. The introduction of parallel plate fins reduces the maximum heat source temperature by 10% compared to the case without fins. The cooling system maintains the maximum chip temperature at 64.68°C when the heat input was at 40W that is much lower than the recommended computer chips limit temperature of no more than 85°C and hence the performance of the CPU is enhanced.

Computational fluid dynamics was used to simulate solar chimney power plants and investigate modeling techniques and expected energy output from the system. The solar chimney consists of three primary parts: a collector made of a transparent material such as glass, a tower made of concrete located at the center of the collector, and a turbine that is typically placed at the bottom of the tower. The collector absorbs solar radiation and heats the air below, whereby air flows inward towards the tower. As air exits at the top of the tower, more air is drawn below the collector repeating the process. The turbine converts pressure within the flow into power. The study investigated three validation cases to numerically model the system properly. Modeling the turbine as a pressure drop allows for the turbine power output to be calculated while not physically modeling the turbine. The numerical model was used to investigate air properties, such as velocity, temperature, and pressure. The results supported the claim that increasing the energy into the system increased both the velocities and temperatures. Also, increasing the turbine pressure drop decreases the velocities and increases the temperatures within the system. In addition to the numerical model, analytical models representing the vertical velocity without the turbine and the maximum power output from a specific chimney were used to investigate the effects on the flow when varying the geometry. Increasing the height of the tower increased the vertical velocity and power output, and increasing the diameter increased the power output. Dimensionless variables were used in a regression analysis to develop a predictive equation for power output. The predictive equation was tested with new simulations and was shown to be in very good agreement. / Master of Science

Experimental observations indicate the presence of attached, gravity induced, horizontal buoyant currents above large area fires. Their driving mechanism is indirect and resembles the one observed above heated horizontal plates. Classic plume modelling is satisfactory for providing information for the flow far from the source. In dealing with large areas and directing attention to the flow close to the source, the classic plume theory should fail because the radial pressure gradient that is responsible for the driving of the flow is squeezed in the long and thin classic plume assumption. For this we propose a new plume structure for the description of the buoyant flow above a circular region of large radius L as “The flow field must be divided into three regions. A region where the flow is predominantly horizontal and attached to the surface, a transition region from horizontal to vertical where separation of the attached current takes place, and a region where vertical flow is established and classic plume theory can be applied”. A model for the description of the gross properties of the horizontal currents is developed under the term “horizontal plume”. The modified Richardson number for the horizontal plume a, being analogous to the radius of the large area, is studied asymptotically in the limit a → ∞ and second order uniformly valid semi-analytical solutions are obtained. The hot plate experiment was set up in order to test the model and facilitate its improvement. A chapter is dedicated to the data analysis coming from thermocouple readings and visualisation of the flow using particle image velocimetry.In the remainder of this thesis two classic problems of laminar natural convection are revisited. That of the first order laminar boundary layer above an isothermal circular plate of radius a and the first order laminar boundary layer above the semi- infinite plate inclined to horizontal. In both cases allowances to variable property effects were made through the introduction of a nondimensional parameter λT, with its value set to zero implying the assumption of the Boussinesq approximation. For the circular plate, fourth order series solutions were obtained valid at the edge of the plate where the effects of λT and Prandtl number Pr are studied. Furthermore a finite difference scheme for the numerical solution of the nonsimilar partial integro- differential equation was developed using the Keller Box method and compared with results obtained from the commercial finite element software COMSOL Multiphysics 3.5a. For the semi-infinite plate, fourth order series approximations valid at the edge of the plate were obtained, while an extensive analysis for the effect of λT, Pr and inclination parameter σ was performed on the flow. Positions of the separation points when the inclination is negative (σ < 0) as a function of Pr and λT were recovered.