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No description available.

No description available.

No description available.

No description available.

No description available.

During the past decade research has shown that global warming may have disastrous effects on our planet. In order to limit the damage that the human race seems to be causing, it was acknowledged that substances with a high global warming potential (GWP) should be phased out. In due time, R-134a with a GWP = 1300, may probably be phased out to make way for nature friendly refrigerants with a lower GWP. One of these contenders is carbon dioxide, R-744, with a GWP = 1. Literature revealed that various Nusselt number (Nu) correlations have been developed to predict the convection heat transfer coefficients of supercritical R-744 in cooling. No proof could be found that any of the reported correlations accurately predict Nusselt numbers (Nus) and the subsequent convection heat transfer coefficients of supercritical R-744 in cooling. Although there exist a number of Nu correlations that may be used for R-744, eight different correlations were chosen to be compared in a theoretical simulation program forming the first part of this study. A water-to-transcritical R-744 tube-in-tube heat exchanger was simulated. Although the results emphasise the importance of finding a more suitable Nu correlation for cooling supercritical R-744, no explicit conclusions could be made regarding the accuracy of any of the correlations used in this study. For the second part of this study experimental data found in literature were used to evaluate the accuracy of the different correlations. Convection heat transfer coefficients, temperatures, pressures and tube diameter were employed for the calculation of experimental Nusselt numbers (Nuexp). The theoretical Nu and Nuexp were then plotted against the length of the heat exchanger for different pressures. It was observed that both Nuexp and Nu increase progressively to a maximal value and then decline as the tube length increases. From these results it were possible to group correlations according to the general patterns of their Nu variation over the tube length. Graphs of Nuexp against Nus, calculated according to the Gnielinski correlation, generally followed a linear regression, with R2 > 0.9, when the temperature is equal or above the pseudocritical temperature. From this data a new correlation, Correlation I, based on average gradients and intersects, was formulated. Then a modification on the Haaland friction factor was used with the Gnielinski correlation to yield a second correlation, namely Correlation II. A third and more advanced correlation, Correlation III, was then formulated by employing graphs where gradients and y-intercepts were plotted against pressure. From this data a new parameter, namely the turning point pressure ratio of cooling supercritical R-744, was defined. It was concluded that the employed Nu correlations under predict Nu values (a minimum of 0.3% and a maximum of 81.6%). However, two of the correlations constantly over predicted Nus at greater tube lengths, i.e. below pseudocritical temperatures. It was also concluded that Correlation III proved to be more accurate than both Correlations I and II, as well as the existing correlations found in the literature and employed in this study. Correlation III Nus for cooling supercritical R-744 may only be valid for a diameter in the order of the experimental diameter of 7.73 mm, temperatures that are equal or above the pseudocritical temperature and at pressures ranging from 7.5 to 8.8 MPa. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2010.

Thunderstorms have been identified as an important component of the hydrological cycle on the Canadian Prairies, a region that is postulated to have the potential to exert a detectable influence on convective precipitation in the summer. However, very little work has been undertaken exploring and elucidating those aspects of biophysical forcing on the Canadian Prairies that affect lightning activity during the summer months, the constraints under which any linkages operate, and the mechanisms by which surface anomalies modify the structure and moisture content of the convective boundary layer (CBL) so as to modulate lightning activity. Evapotranspiration (ET) from the soil and vegetation canopy is known to be important for modulating the moisture content in the CBL, and this in turn has important implications for the initiation and intensity of deep, moist convection. The Second Generation Prairie Agrometeorological Model (PAMII) of Raddatz (1993) has been used extensively for the purpose of quantifying the evolution of soil moisture and ET in response to atmospheric drivers on the Canadian Prairies. However, the ability of PAMII to simulate the evolution of root-zone soil moisture and ET during the growing season has yet to be verified against a comprehensive set of in-situ observations. In this thesis, we address the above knowledge gaps using unique datasets comprising observed lightning flash data, satellite-derived Normalized Difference Vegetation Index (NDVI) data, observed atmospheric soundings, in-situ soil moisture observations and estimates of daily ET from eddy-covariance systems. A thorough quantitative validation of simulations of root-zone soil moisture and ET from PAMII was undertaken against in-situ soil moisture measurements and ET from eddy-covariance systems at sites on the Canadian Prairies. Our analysis demonstrates that PAMII shows skill in simulating the evolution of bulk root-zone soil moisture content and ET during the growing season, and for contrasting summer conditions (i.e., wet versus dry). As part of the soil moisture validation, a novel multi-model pedotransfer function ensemble technique was developed to quantify the uncertainty in soil moisture simulations arising from errors in the specified soil texture and associated soil hydraulic properties. An innovative approach was used to explore linkages between the terrestrial surface and deep, moist convection on the Canadian Prairies, using datasets which avoid many of the problems encountered when studying linkages between soil moisture and thunderstorm activity. This was achieved using lightning flash data in unison with remotely sensed NDVI data. Specifically, statistical analysis of the data over 38 Census Agricultural Regions (CARs) on the Canadian Prairies for 10 summers from 1999 to 2008 provided evidence for a surface-convection feedback on the Canadian Prairies, in which drought tends to perpetuate drought with respect to deep, moist convection. The constraints in which such a feedback operates (e.g., areal extent and magnitude of the NDVI anomalies) were also identified. For example, our data suggest that NDVI anomalies and lightning duration are asymmetric, with the relationship between NDVI and lightning duration strengthening as the area and amplitude of the negative NDVI anomaly (less vegetation vigour) increases. Finally, we focused on how surface anomalies over the Canadian Prairies can condition the CBL so as to inhibit or facilitate thunderstorm activity, while also considering the role of synoptic-scale forcing on modulating summer thunderstorm activity. We focused on a CAR located over central Alberta for which observed lightning flash data, NDVI data, and in-situ sounding data were available for 11 summers from 1999 to 2009. Our analysis suggests that storms over this region are more likely to develop and are longer-lived or more widespread when they develop in an environment in which the surface and upper-air synoptic-scale forcings are synchronized. On days when a surface or upper-air feature is present, storms are more likely to be triggered when NDVI is much above average, compared to when NDVI is much below average. We propose a conceptual model, based almost entirely on observations, which integrates our findings to describe how a reduction in vegetation vigour modulates the partitioning of available energy into sensible and latent heat fluxes at the surface, thereby modulating the lifting condensation level heights, which in turn affect lightning duration.

During the past decade research has shown that global warming may have disastrous effects on our planet. In order to limit the damage that the human race seems to be causing, it was acknowledged that substances with a high global warming potential (GWP) should be phased out. In due time, R-134a with a GWP = 1300, may probably be phased out to make way for nature friendly refrigerants with a lower GWP. One of these contenders is carbon dioxide, R-744, with a GWP = 1. Literature revealed that various Nusselt number (Nu) correlations have been developed to predict the convection heat transfer coefficients of supercritical R-744 in cooling. No proof could be found that any of the reported correlations accurately predict Nusselt numbers (Nus) and the subsequent convection heat transfer coefficients of supercritical R-744 in cooling. Although there exist a number of Nu correlations that may be used for R-744, eight different correlations were chosen to be compared in a theoretical simulation program forming the first part of this study. A water-to-transcritical R-744 tube-in-tube heat exchanger was simulated. Although the results emphasise the importance of finding a more suitable Nu correlation for cooling supercritical R-744, no explicit conclusions could be made regarding the accuracy of any of the correlations used in this study. For the second part of this study experimental data found in literature were used to evaluate the accuracy of the different correlations. Convection heat transfer coefficients, temperatures, pressures and tube diameter were employed for the calculation of experimental Nusselt numbers (Nuexp). The theoretical Nu and Nuexp were then plotted against the length of the heat exchanger for different pressures. It was observed that both Nuexp and Nu increase progressively to a maximal value and then decline as the tube length increases. From these results it were possible to group correlations according to the general patterns of their Nu variation over the tube length. Graphs of Nuexp against Nus, calculated according to the Gnielinski correlation, generally followed a linear regression, with R2 > 0.9, when the temperature is equal or above the pseudocritical temperature. From this data a new correlation, Correlation I, based on average gradients and intersects, was formulated. Then a modification on the Haaland friction factor was used with the Gnielinski correlation to yield a second correlation, namely Correlation II. A third and more advanced correlation, Correlation III, was then formulated by employing graphs where gradients and y-intercepts were plotted against pressure. From this data a new parameter, namely the turning point pressure ratio of cooling supercritical R-744, was defined. It was concluded that the employed Nu correlations under predict Nu values (a minimum of 0.3% and a maximum of 81.6%). However, two of the correlations constantly over predicted Nus at greater tube lengths, i.e. below pseudocritical temperatures. It was also concluded that Correlation III proved to be more accurate than both Correlations I and II, as well as the existing correlations found in the literature and employed in this study. Correlation III Nus for cooling supercritical R-744 may only be valid for a diameter in the order of the experimental diameter of 7.73 mm, temperatures that are equal or above the pseudocritical temperature and at pressures ranging from 7.5 to 8.8 MPa. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2010.

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