An emerging trend in the building services industry is the installation of passive deaerators on the flow line of domestic wet central heating systems. To date, no data and theoretical models predicting the two-phase flow characteristics in domestic wet central heating systems are available in the open literature. This gap in literature has prevented essential design improvements to passive deaerators thus impeding the efficiency enhancement of such devices. Hence, the current study is aimed at assisting designers of deaeration devices by providing fundamental data and model correlations with respect to the two-phase flow characteristics typical in a wet domestic central heating system. For this purpose an experimental research project was adopted and several studies were carried out, including; (1) a comprehensive review to understand the background of the phenomena, (2) the design and construction of an experimental test rig to conduct the necessary investigations into the phenomenon of two-phase flow in domestic wet central heating systems, (3) the development of a reliable image capture and analysis technique, (4) the completion of a number of experiments to investigate typical bubble sizes, volumetric void fractions, bubble distributions and nucleation and dissolution rates and (5) the correlation of the data gathered as part of the present study with existing bubble size, nucleation and dissolution prediction models. This research has, for the first time, provided an in depth analysis into two-phase flow characteristics in wet domestic central heating systems through the use of a high speed camera and image analysis techniques. The two-phase phenomenon finds its origins in high dissolved gas concentrations present in the water flowing through the closed loop system, thus resulting in super saturation conditions at the primary heat exchange wall conditions. Bubble sizes at the boiler flow line were found to be dependent on the bulk fluid velocity, heat flux and pressure, with a measured mean diameter in the range of 0.13 mm to 0.39 mm. The Winterton (1972a) force balance model for bubble size prediction was in reasonable agreement with the experimental results. This model was further improved through the correlation of our data with the inclusion of dimensionless groups. Bubble nucleation rates have been calculated in the range of 0.3 to 4 bubbles / cm2 s with total system bubble production rates measured in the range of 784 to 6920 bubbles per second. Bubble nucleation rates have been calculated through the consideration of the heat exchanger surface under super saturation conditions. A correlation for the model by Hepworth et al. (2003) for nonclassical heterogeneous nucleation is proposed based on the experimental data gathered during the present study. Experimental results have shown dissolution rates for the bubble size ratio in the range of 0.4 to 12 % per second with system conditions. A modification of the model developed by Epstein and Plesset (1950) for stationary bubble dissolution is proposed with the inclusion of the Sherwood number to capture the effects of turbulent diffusion. The volumetric void fraction distribution in vertical pipes was found to be quasi-homogenous across the pipe section while being strongly dependent on gravitational and turbulence effects in horizontal pipe bubbly flow. A CFD simulation predicted the volumetric void fraction distribution with reasonable accuracy.

This thesis develops and evaluates a method to simulate energy consumption and water production for a novel sorption dehumidication pro-cess. The system consists of a chamber comprising a hygroscopic materialand a heating device. The process consists of an adsorption phase anda regeneration phase. For both the regeneration phase and the adsorp-tion phase the model considers the heat distribution by thermal diusionand convection and the water transport by diusion and convection. Forthe regeneration phase the radiation is also considered since the radia-tive power increases with temperature to the power of four. Further, amodel for the condensation process is implemented and a model for thecondensation is suggested. To model the properties of the hygroscopicmaterials, the adsorption curves for SiO2 and AlO2 are investigated. Themodel were evaluated by comparing the simulated values with experimen-tal measurements.The results from the the simulation of the regeneration phase showsa good agreement with experimental data for the power and the energyconsumption even though the simulated values are a bit underestimated,about 10%. The water production is simulated to be about 25% higherthan the measured values. This discrepancy could be explained by aleakage of water vapour that was found in the experimental set up, whichis not considered in the model. This could also explain the underestimatedenergy consumption since the condensation energy in the system is toogreat. To improve the accuracy for the model the water leakage wouldneed to be implemented. The overestimation of water seemed to be thesame for the measurements from the same apparatus.For the adsorption phase a developed model, from an article for ad-sorption in silica, was implemented and tuned for the specic system. Thesimulations are in good agreement with the measurements but could betested further for more certainty.

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