R. Letan

Numerical and Experimental Study of Solidification in a Spherical Shell

The present study explores numerically and experimentally the process of a phase-change material (PCM) solidification in a spherical shell. At the initial state, the PCM liquid occupies 98.5% of the shell. The upper segment of 1.5% contains air, which flows in as the solidification progresses. In the experiments, a commercially available paraffin wax is used. Its properties are engaged in the numerical simulations. The investigation is performed for solidification in spherical shells of 20 mm, 40 mm, 60 mm, and 80 mm in diameter at the wall uniform temperature, which varied from 10°C to 40°C below the mean solidification temperature of the phase-change material. Transient numerical simulations are performed using the FLUENT 6.2 software and incorporate such phenomena as flow in the liquid phase, volumetric shrinkage due to solidification, and irregular boundary between the PCM and air. The numerical model is validated versus the experimental results. Shrinkage patterns and void formation are demonstrated. Dimensional analysis of the results is performed and presented as the PCM melt fractions versus the product of the Fourier and Stefan numbers. This analysis leads to a generalization that encompasses the cases considered herein.

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.

Natural convection inside ventilated enclosure heated by downward-facing plate: experiments and numerical simulations

The present paper deals with heat transfer inside a ventilated enclosure. The enclosure is heated by a horizontal downward-facing constant-temperature hot plate. In order to provide a comprehensive picture of the problem, three main approaches are used in parallel: temperature measurement, flow visualization, and numerical simulation. The measurements are done by thermocouples distributed uniformly at the vertical mid-plane of the enclosure. The visualization is performed using the smoke of incense sticks, with video recording and consequent image processing. The computer simulations of the flow and temperature fields are performed both for steady and transient ventilation and compared with the results of the measurements and visualization. Applicability to ventilation is 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.

Temperature moderation in a real-size room by PCM-based units

The objective of this work is to study the feasibility of temperature moderation inside a room using a phase-change material (PCM) which is stored in storage units. A real-size room which is at temperature conditions typical in a desert region in summer is considered. The idea is to use a phase change material which could melt during the day hours, absorbing heat from the room, while at night it solidifies due to a low night temperature. The heat from the room air to a PCM unit is free or forced-convected. The numerical model includes the transient heat conduction inside the walls/ceiling, free/forced convection of air, and radiation inside the room. The processes inside the PCM are modeled by the effective heat capacity (EHC) method. The PCM is assumed to melt and solidify within a certain temperature range, which represents the true situation for most commercial-grade phase-change materials. The numerical calculations are performed for the transient temperature fields inside the three-dimensional room, including PCM in the units, walls/ceiling, and the interior of the room. The boundary conditions for the room are chosen according to the experimental data which were obtained in previous works. The basic conservation equations of continuity, momentum, and energy are solved numerically, using the FLUENT 6.1 software. The numerical simulations are performed for at least one full 24-h cycle. Effect of different parameters on the behavior of the system is discussed, including the mass of the PCM and radiation effects inside the room. The night cooling by free and forced convection is analyzed. It is shown that a complete 24-h cycle is feasible in a properly designed configuration with a suitable PCM.

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

Experimental and Numerical Study of Mixing in a Hot-Water Storage Tank

Thermal mixing and stratification are explored numerically and experimentally in a cylindrical tank, which simulates a storage of water heated by a solar collector. The tank is 70 cm in height and 24 cm in diameter. The inlet and outlet are vertical and located off the centerline of the tank. The study is conducted in a transient mode, namely, the tank is filled with hot water, and as the hot water is being withdrawn, the tap water replaces it in a stratified way or by mixing. The flowrates of 2 l/min, 3 l/min, 5 l/min and 7 l/min, which correspond to superficial velocities of 4.35 cm/min, 6.52 cm/min, 10.87 cm/min, and 15.2 cm/min, are explored. Temperature of hot water ranges within 40-50 °C, while the tap water is about 25-27°C. Installation of one and two horizontal baffles above the inlet is examined. Simultaneous experimental and numerical investigations are performed. In the experiment, both flow visualization and temperature measurements are used. Three-dimensional transient numerical simulations are done using the FLUENT 6 software. Validation of the numerical model is achieved by comparison with the experimental results. Then, the numerical model is applied to a study of various possible changes in the system. The results show that at low flowrates, up to a superficial velocity of about 11 cm/min through the tank, the baffles have no effect on tap water mixing with the stored hot water. At higher flowrates, a single horizontal baffle prevents the mixing and preserves the desired stratified temperature distribution in the storage tank.

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