V. Dubovsky

Chimney-enhanced natural convection from a vertical plate: experiments and numerical simulations

Abstract This study deals with natural-convection heat transfer from a vertical electrically heated plate, which is symmetrically placed in a chimney of variable height. The heated plate serves as a thermal pump for ventilation of a symmetrical enclosure beneath the chimney. In order to provide a comprehensive picture of the phenomena, three main approaches are used in parallel: temperature and velocity measurements, flow visualization, and numerical simulation. Temperature measurements are done by thermocouples distributed inside the plate and through the chimney. Velocity measurements are performed by means of a precise anemometer. Visualization is performed using smoke of incense sticks, with video recording and consequent image processing. Computer simulations of unsteady flow and temperature fields are performed in 3D and compared with measurements and visualization, with special attention paid to velocity fluctuations. Analysis is presented on the dependence of the temperature distribution on the flow field. The air flow rate on the heating plate in the chimney increases with the chimney height and is adequately predicted by the numerical simulation of the system.

Ventilation by natural convection of a one-story building

Abstract The objective of the present paper is to study passive ventilation of a one-story detached building. The flow of air is induced by a hot element of the building heated by solar energy. The hot element could be a part of a roof or a wall of the building, or a chimney through which the air is sucked from the building. The method does not require electrical power or mechanical installations, thus it can be applied in remote areas and buildings that are not connected to electric power, like desert-located buildings. The method may be used also for removal of toxic gases, like radon, from the ground floor of the building, without additional expenses. Experiments and simulations have been performed, in steady and transient states, in a scaled-down laboratory model. The results obtained from the simulations and fully supported by measurements and visualization, indicate that it is possible to obtain effective ventilation by the proposed method. Numerical simulations for steady and transient ventilation in real-size buildings are presented and discussed.

Passive ventilation and heating by natural convection in a multi-storey building

Abstract Passive ventilation and heating in a multi-storey structure, by natural convection in a heated vertical duct were studied. Experimental study and computer simulations were first performed in a scaled-down laboratory model, divided into three levels, and connected by a duct, in which an electrically heated plate was used. The experiments included temperature and velocity measurements at each inner space, and inside the duct. The results obtained from the simulations and supported by the measurements, indicated that effective ventilation and heating, by the proposed method, were achievable in the laboratory structure. For a real-size structure of a five-storey building, which has a duct heated by solar irradiation, computer simulations were performed. Temperature fields and average temperatures were obtained at all levels of the building. The results have shown that even at low solar irradiation fluxes ventilation was achieved in summer, and heating in winter. The study has demonstrated that the proposed method is operable and feasible.

Simulation of PCM Melting and Solidification in a Partitioned Storage Unit

The present study explores numerically the processes of melting and solidification of a phase change material (PCM). The material used was a commercially available paraffin wax, which is non-toxic, recyclable, chemically inert, non-corrosive and can withstand an unlimited number of cycles. The phase-change material was stored in a rectangular box, open at the top. The bottom of the box could be heated or cooled. The inner space of the box was partitioned by vertical conducting plates attached to the bottom. Thus, heat was transferred to and from the PCM both through its melted/solidified layer and by conduction through the vertical plates. Transient two-dimensional numerical simulations were performed using the Fluent 6.0 software. The melting temperature of the wax, 23–25°C was incorporated in the simulations along with its other properties, including the latent and sensible specific heat, thermal conductivity and density in solid and liquid states. The simulations provided detailed temperature and phase fields inside the system as functions of time, showing evolution of the heat transfer in the system as the phase change material melts/solidifies. The dependence of the heat transfer rate on the properties of the system and on the PCM phase composition at various time instants is presented and discussed.Copyright

Study of PCM-Based Pin-Fin Heat Sinks

This study deals with heat transfer from pin-fin aluminum heat sinks to a phase-change material (PCM) which fills the inter-fin space. The sinks have a horizontal base and accordingly their fins are vertical. The sink base dimensions are 100 mm by 100 mm, with fin height of 10 mm, 20 mm or 30 mm, and cross section of 4 mm×4 mm. The number of fins varies, e.g. 49, 64, 81, etc. The applied power is between 50 W to 250 W, corresponding to the heat fluxes of 5–25 kW/m2 . The present paper reports mostly numerical results, but the numerical model is validated using the findings from an ongoing experimental investigation, in which a commercially available paraffin wax RT-35 is used as the PCM, with the melting temperature of about 35 °C. The simulations reflect the material properties, geometry, and other features of the experimental set-up, including heating with an electrical foil heater. Accordingly, the base temperature serves as the dependent parameter. Numerical simulations, performed using the Fluent 6.2 software, serve to obtain detailed melting patterns and explain the effect of fin size and number on sink performance.© 2009 ASME

Study of Horizontal-Base Pin-Fin Heat Sinks in Natural Convection

The present paper deals with horizontal-base pin fin heat sinks in free convection. The sinks have the same base dimensions and variable fin pitch. They are made of aluminum, and there is no contact resistance between the base and the fins. The fins have a constant square cross-section. The effect of fin pitch on the performance of the sink is studied experimentally and numerically. In the experiments, the heat sinks are heated using foil electrical heaters. The heat input is set, and temperatures of the base and fins are measured. In the corresponding numerical study, the sinks are modeled using the Fluent 6 software. The results show that heat transfer enhancement due to the fins is not monotonic. The differences between sparsely and densely populated sinks are analyzed. Also analyzed are the effects of the sink edges on the total heat transfer. A relative contribution of outer and inner fin rows in the sink is assessed, together with the effect of fin location in the array on the heat transfer rate from an individual fin. Dimensional analysis of the results is attempted, and a correlation presenting the Nusselt number vs. the Rayleigh number is suggested, where the inter-fin spacing serves as the characteristic length.Copyright

Experimental Investigation of Solid-Liquid Phase Change in Cylindrical Geometry

The present study explores experimentally the process of melting of a phase change material (PCM) in cylindrical geometry. The study is performed with a commercially available paraffin-type material with the melting point of about 28 degrees Celsius. The experiments are conducted using vertical tubes of four different diameters, filled with the PCM and immersed in a water bath. In each tube the experiments are performed at the water bath temperatures of 10, 20 and 30°C above the melting point of the paraffin. The tubes are transparent, and the melting process is monitored and recorded by a digital camera. Each tube is thermally insulated at the bottom, and at its top open to atmosphere, to allow free expansion of the melt liquid. The digital pictures of the melting process were analyzed, and the results were graphically presented as melt fraction vs. time, showing for the plain tubes the effects of tube diameter and temperature difference. Numerical simulations are performed in order to provide an insight into the mechanisms governing the process. Generalization of the results is attempted based on the dimensionless groups, including the Fourier, Stefan, and Rayleigh numbers. A correlation connecting the melt fraction with these dimensionless groups is suggested.Copyright

Solar-Assisted Induced Ventilation of Small Field Structures

Induced passive ventilation was studied in a small field structure, about 60 cm X30 cm X85 cm in size, heated by solar irradiation. The structure has three stories, connected by a vertical duct, which in turn is connected to a horizontal duct above the upper level of the structure. The outer walls of these two ducts are metal sheets painted on the outside with black matte paint and covered by glass sheets. The other outer walls and inner partitions of the structure are made of cardboard attached to a metal frame. Additional elements included in some experiments were a water tank, used for heat storage, and a chimney for enhancing air flow. The structure orientation was with its vertical metal sheet facing south-west. Experimental study, based on temperature and velocity measurement, and computer simulations, using the FLUENT software, were performed. During a typical experiment, the structure has been exposed to the sun for a full day in July through November. The ports of the system were either opened in the morning, or kept closed until about 13:45-14:00 and then opened. The results of the study indicate that effective ventilation has been achieved: the calculated rates of air change inside the stories were rather high, and the mean air temperatures were only about 1-2°C above the ambient in its lower stories and 2-3 °C above the ambient in the upper story. Detailed comparison of the experimental and numerical results is presented and discussed.

Air-Water Transient Heat Transfer in a Bubble Column

This work focuses on a direct contact heat transfer between water and air bubbles. A tube 0.2m in outer diameter and 1.4m high was filled with water. Low-humidity air was continuously dispersed into bubbles at the bottom of the tube. The water inside the tube could be heated initially or continuously during the experimental runs, using controllable electrical strip heaters. Temperature measurements were simultaneously performed at several locations inside the column. The results show that the water temperature was almost uniform due to perfect mixing induced by the bubbles movement. The column of water was not insulated, and the cooling by convection and radiation at the walls was accounted for. The heat-transfer rates between the water and the rising air bubbles were estimated for various modes encountered in the experimental study, and mathematically modeled. The model simulations for the batchwise cooled water compare well with the experimental results.Copyright

An Analytical Technique of Transient Phase-Change Material Melting Calculation for Cylindrical and Tubular Containers

ABSTRACT In this study, an analytical model for a class of heat storage that utilizes latent heat of a phase-change material (PCM) is developed. Two basic shell-and-tube configurations are considered, one in which the PCM melts inside the tubes while the heat transfer fluid (HTF) flows in the shell along it, and the other in which HTF flows inside the tubes while PCM melts outside. A system of partial differential equations, which describes heat transfer and melting of the PCM and heat transfer in the HTF, is derived with some simplifying assumptions, while still capturing and preserving the essential features of the processes involved. These equations are solved analytically, yielding the overall heat exchange parameters, like instantaneous heat transfer rate, stored energy, and overall operation time of the system. The present work shows that the use of the proposed analytical technique and its modifications for the practical PCM arrangements is beneficial. Proper application of the model makes it possible to obtain the parameters of a real PCM melting process in the form of algebraic formulas, both for the transient values of variables over time, and for the overall process characteristics. A comparison with the results of numerical calculations of transient melting, made using computational fluid dynamics, confirms the validity of analytical findings and allows to assess the degree of accuracy of the results of our analytical method in various practical cases.