Experiments On Natural Ventilation In A Room And Real Source-Sink Pairs
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The present work consists of two parts: experimental study of natural ventilation in a model room and the flow associated with a source-sink pair. The first part describes the natural ventilation of a model room of size 300mmx 300mm x 300mm with water as the fluid medium. It is insulated by air gaps on the four sides and at the top. A constant heat flux of 3148W/m2 has been maintained on the bottom surface of the room. This ‘room’ is surrounded by a large exterior tank containing water. The changes in temperature of the bottom, the interior and the exterior have been measured using T-type thermocouples. There are three openings each on two opposing sides of the model room. For any experiment, only one opening on each side is kept open. Fluid enters or leaves these openings and the flow is driven entirely by buoyancy forces. Three configurations have been studied: (1) The bottom opening at the inlet side and the top opening at the outlet side are open, (2) the bottom opening at the inlet side and the middle opening at the outlet side are open, and (3) the middle opening at the inlet side and the top opening at the outlet side are open. Shadowgraph technique is used for visualization. The buoyancy causes flow to enter through the bottom opening and leave through the top opening. At the openings, buoyant jets are observed and which have higher or lower relative densities. The buoyant jet at the inlet interacts with the plumes on the heated bottom plate. From these visualizations, it appears that free convection at bottom plate will be affected by the buoyant jets at the openings and the degree to which it is affected depends on the position and size of openings and distance between inlet and outlet. The flow rate due to the natural ventilation depends on the bottom surface heat flux and the height difference between the openings. The temperatures of the ﬂoor, the interior and the exterior are calculated using a simple mathematical model (Hunt and Linden ). The mathematical model assumes well mixed conditions within the room and accounts for losses at the openings. The values of temperatures obtained in the experiments are reasonably well predicted by the mathematical model. The second part of the work is concerned with the interaction of a source -sink pair. The source consists of fluid issuing out of a nozzle in the form of a jet and the sink is a pipe that is kept some distance from the source pipe. Such source -sink pairs are observed in many situations including data centers, and collection of fresh water from a large reservoir that has also a discharge of pollutants. The main parameters of the problem are source and sink flow rates, the axial and lateral separations of the source and sink, and the angle between the axes of source and sink. Of concern is the percentage of source ﬂuid that enters the sink as a function of these parameters. The experiments have been carried in a large glass water tank. The source nozzle diameter is 6mm and the sink pipe diameter is either 10mm or 20mm. The horizontal and vertical separations and angles between these source and sink pipes are adjustable. The Reynolds numbers of the source jet is about 3200. Experiments were done with the sink flow rate equal to, lower or higher than the source flow rate. The flow was visualized using KMnO4 dye and planar Laser Induced Fluorescence (LIF). The velocity fields for some cases were obtained using Particle Image Velocitymetry (PIV). To obtain the efficiency (that is percentage of source fluid entering the sink pipe), titration method is used. A small amount of hydrochloric acid (HCL) is added in the jet fluid through the overhead tank and the fluid collected at the sink is titrated with the Sodium hydroxide (NaOH) as base and Phenolphthalein as the pH indicator. The main characteristics of the jet, without a sink, were measured using PIV. The velocity profiles, jet widths and volume flow rates at various axial locations were obtained and compared with results reported in the literature for similar Reynolds number jets. For 100%, 70%, 50% and 25% efficiencies or removals and for zero lateral separations, the sink flow rate is about 1.5 times the flow rate predicted on the basis of jet properties at that point in the absence of a sink. The sink flow rate to obtain a certain efficiency increase dramatically with lateral separation; for example, when the lateral separation is about one half jet width, the required sink flow rate to obtain a certain efficiency increases by about five times. The sink diameter and the angle between source and the sink axes don’t influence efficiencies as much as the lateral separation. Data from our all experiments have been consolidated in the form of correlations that can be used for design of appropriate sinks for removal of heat and pollutants.
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