Unsteady heat transfer by natural convection in the cavity of a passive heating room (original) (raw)
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Brazilian Journal of Chemical Engineering, 2007
A numerical study was conducted to investigate steady heat transfer and flow phenomena of natural convection of air in enclosures, with three aspect ratios (H/W = 1, 2, and 4), within which there is a local heat source on the bottom wall at three different positions, W h . This heat source occupies 1% of the total volume of the enclosure. The vertical walls in the enclosures are insulated and there is an opening on the right wall. The natural convection is influenced by the difference in temperature between the left and right walls, represented by a Rayleigh number (Ra e ), and by local heat source, represented by a Rayleigh number (Ra i ). Numerical simulations were performed for several values of the Rayleigh number ranging between 10 3 and 10 6 , while the intensity of the two effects -the difference in temperature on the vertical walls and the local heat source -was evaluated based on the Ra i /Ra e ratio in the range between 0 and 2500. The analysis proceeds by observing variations in the streamlines and isotherms with respect to the different Ra e , R ratios, aspect ratios, of the radius and positions of the local heat source. The average Nusselt numbers on the hot and cold walls are influenced by different values of the parameters R, Ra e , W h , and H/W. Results show the presence of different flow patterns in the enclosures studied. Thus, the flow and heat transfer can be controlled by external heating, and local heat source.
Calculation of convective heat transfer coefficients of room surfaces for natural convection
Energy and Buildings, 1998
Convective heat transfer from internal room surfaces has major effect on the thermal comfort, air movement and heating and cooling loads for the room. Recent studies have shown that the values of convective heat transfer coefficient used in building thermal models greatly influence the prediction of the thermal environment and energy consumption in buildings. In computational fluid dynamics (CFJ3) codes for room air movement prediction, accurate boundary conditions are also necessary for a reliable prediction of the air flow. However, most CFD codes use 'wall functions' derived from data relating to the flow in pipes and flat plates which may not be applicable to room surfaces. This paper presents results for natural convection heat transfer coefficients of a heated wall, a heated floor and a heated ceiling which have been calculated using CFD. Two turbulence models have been used to calculate these coefficients: a standard k -E model using 'wall functions' and a low Reynolds number k -E model. The computed results are compared with data obtained from two test chambers. 0 1998 Elsevier Science S.A. All rights reserved.
Natural convection from heated room surfaces
Energy and Buildings, 1999
. Current convective heat transfer coefficients CHTC's for internal room surfaces have, in most cases, been based upon data for small, free-edge heated plates. An extensive survey of CHTC data has shown that a very wide variation exists in CHTC values for vertical and horizontal surfaces. For example, a CHTC value in the range 1-6 W m y2 K y1 has been obtained for walls. Both building thermal and CFD models require accurate CHTC's for calculations of the thermal conditions and the air movement in a room. However, most such models use convective coefficients obtained for free-edge heated plates. This paper presents convective heat transfer coefficients for the heated surfaces of an environmental chamber and a small box measured under controlled conditions. Using uniformally heated plates attached to an internal surface of the chamber or the box and by accurately measuring the surface and air temperatures, the CHTC's were deduced after allowing for conduction and radiation losses from the plates. Data is presented for a heated wall, a floor and a ceiling for natural convection. q 1999 Elsevier Science S.A. All rights reserved. 0378-7788r99r$ -see front matter q 1999 Elsevier Science S.A. All rights reserved.
2019
A numerical study of natural convection of heat transfer in a three dimensional square cavity is considered. The two opposite vertical walls and the bottom wall have been considered cold constant temperature with a source of heat fixed at the corner top surface and the non heated part of the top surface considered insulated. The study is aimed at examining the velocity flow and temperature distribution. The central finite difference method is used in solving the energy and momentum equations. The differential equations are solved by the central difference method and the forward difference method .The solutions are presented at various Reynolds number, Froude number, Eckert number with constant Prandtl number 0.71. When varying Re number, the velocity is seen to decrease, while when the Fr number is varied, the velocity decreases with increase in room depth. The behavior of the flow fields are analyzed by 2D graphs. When the Fr number is varied, the velocity decreases with increase i...
International Journal for Research in Applied Science and Engineering Technology IJRASET, 2020
Numerical simulations were used to investigate natural radiation and convection interactions in small rooms with three-dimensional geometry. The opposite vertical walls are heated and cooled, while the other walls are supposed to be fixed. Governance flow, momentum equations, and radiative conversion are solved using Ansys Fluent CFD. In estimating thermal terminology, a second-degree wind diagram and a dual solution algorithm are used. The cubic box is filled with air and the flow is considered protective. Air characteristics are assumed to be constant, except for the difference in density in which a Bosnic approximation is used. The surface heat model (S2S) is used as the radiation transfer model. The arithmetic field is a square container Length of the edge = 0.225 m In a surrounding environment at T = 293 K. One of the walls of the case is heated up to 473 K. Note that the radiation from the opposite wall is high, with the improved model. Radiant heat transfer is reduced. I. INTRODUCTION Natural convection and heat transfer by heat radiation in fluid-filled cavities have received much attention in recent years due to its relationship to the thermal performance of engineering applications such as cooling electronic components, electrical boxes, assembler designs, solar designs, heat exchangers, etc.. Therefore, the common characteristics of natural convection and heat transfer by heat radiation are more important. The study of heat transfer in the room was the main topic of the researchers due to a wide range of applications in engineering and working life. It finds its application from cooling the nuclear reactor to removing heat from the micro-electronic components as well as for designing the room air conditioning, thermal design for commercial buildings, cold storage, oven and many more. In addition to a wide range of applications, the simultaneous thought of wall conduction and convection in fluid flow remains an exciting area of research in recent decades. Several articles can be found in the literature to study natural convection in the can. There are many boundary conditions that can be taken into consideration, but the most complex and practically applicable area of least study has been found in the literature. Most of the areas already studied are bottom warming or lateral warming. But in this study, we studied a viable limit state with associated heat transfer. Energy has long been a major topic of discussion among researchers. During any development program for any technology, energy consumption has always been of great importance. An important part of the total energy consumed in the useful life of any building is the operating capacity. This involves maintenance and, most importantly, the energy involved in maintaining the building in comfortable thermal and visual conditions. The building energy analysis tool used before starting any project aims to reduce this operating capacity. It is important to understand that this decrease in energy consumption must be achieved without regard to low performance. Therefore, the building should provide a comfortable environment compared to its external environment. Previous studies have shown the fact that the passenger quickly responds to any discomfort to restore his comfort, but this can negatively affect energy consumption. Therefore, accurate prediction of thermal comfort is very important when designing a building to maintain less energy consumption.
Convective heat transfer coefficients in a full-scale room with and without furniture
Building and Environment, 2001
The convective heat transfer coe cient at an outer ambient wall with a window exposed to natural climate was measured in a room with and without furniture. The method used was to estimate the heat ow from measured temperatures and solar radiation. The convective heat transfer was calculated as the di erence between the heat ow through the building element and the calculated long-wave radiation. Even though the accuracy was at best ±15%, the e ect of di erent heating and ventilation strategies could clearly be detected. Local coe cients may be more than 10 times the expected, due to ventilation or position of the radiator.
Numerical Heat Transfer, Part A: Applications, 2011
Combined heat transfer by natural convection-conduction and surface radiation in an open cavity heated by constant flux is studied here. The laminar flow is solved numerically by employing the SIMPLE algorithm with QUICK scheme. The numerical results show that both radiation and solid conduction increase the average total Nusselt number. The average total Nusselt number is a linear increasing function of emissivity when emissivity is larger than 0.2. The heat conduction of a conductive wall increases the total cooling effect, but its effect is close to a limit when the conductivity ratio exceeds 100. The increase due to radiation ranged from 54.1% to 64.0%.
Natural convective heat transfer rates in rectangular enclosures
Energy and Buildings, 1998
An experimental study of buoyancy-driven convection in rectangular enclosures has been made in order to obtain convection coefficients and data correlations which are more accurate for real building situation. Three different flow regimes viz. stably-stratified flow, buoyancy-driven vertical flow and horizontal flow were investigated. The Nusselt number variation with respect to the Rayleigh number has been plotted and compared with existing correlations. In general, the measured data were lower than the data obtained from the existing correlations which are mainly derived from data obtained from experiments involving isolated surfaces.
Numerical Simulation of Heating and Ventilation by Natural Convection in a Square Cavity
Journal of Renewable Energies
Heating and ventilation in the building is essential to establish thermal comfort. The modern means invented to provide us with our heating and ventilation needs are generally expensive and represent sources of pollution. It is for this reason that there are several studies directed towards the research of renewable energies and non-polluting economic systems. The simulation of such a natural system at an almost low cost is the subject of our study in this work. In this work, a numerical study of passive heating and ventilation was made by natural convection in a room containing a heated vertical wall. In the case of heating, the system is closed and two openings are considered in the heated vertical wall allowing the re-circulation of the air in the room and therefore the heating of this latter. In the case of ventilation, the room is open to the outside through two inlet and outlet openings allowing the air circulation from the outside into the room and from the room into the outs...
2010 14th International Heat Transfer Conference, IHTC 14, 2010
Combined heat transfer of natural convection-conduction and surface radiation in an open cavity heated by constant flux is studied in this paper. Flow model is laminar and SIMPLE algorithm and QUICK scheme are employed. The relevant parameters are as follows, Prandtl number is 0.7 and dimensionless solid thickness is 0.2, conductivity ratio rangs from 0 to 1000, Rayleigh number ranges from 10 3 to 10 9 , surface emissivity ranges from 0 to 1. The numerical results shows secondary circular formed as an effect of radiation which increased the average Nusselt number about from 54.1% to 100.3%. INTRODUCTION Natural convection in an open cavity is a topic of significant interest in a range of engineering applications such as building insulation, solar thermal central receivers, geothermal reservoirs, electronic cooling devices, fire spread in rooms, etc. During the past decades, both experimental and computational studies have been conducted into the cavity-flow physics. These studies were mainly focused on flow and heat transfer of different Rayleigh numbers, aspect ratios, and tilt angles. Also, they studied the occurrence of transition to turbulence and turbulence and how the boundary conditions in the aperture are considered. In a realistic case with any open cavity system, conduction of solid walls and surface thermal radiation always exist, and can both strongly interact with natural convection, because of the coupling between the surface temperature and the flow fields in the cavity.