Browsing by Author "Kroger, D. G."

Now showing 1 - 8 of 8
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    Cooling tower performance : a critical evaluation of the Merkel assumptions
    (South African Institution of Mechanical Engineering, 2004) Kloppers, J. C.; Kroger, D. G.
    The simpltfying assamptions made by Merkel ure critically evaluated by comparing the Merkel analysis to the more rigoroas Poppe analysis of cooling tower performnnce. It is shown that the uccaracy of the Merkel method can be greatly improved, under certain cooling tower operating conditions, to predict cooling tower performance within very close tolerance of the performance predicted by the Poppe method, It is shown under which tower operating conditions the thermal tower performance, according to the Merkel method, is likely to diffe, from the performance predicted by the Poppe method.
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    The effectiveness of axial flow fans in a-frame plenums
    (Stellenbosch : University of Stellenbosch, 1990-03) Venter, Sarel Jacobus; Kroger, D. G.; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering
    ENGLISH ABSTRACT: The ultimate goal of this project is to ensure a better understanding of the governing mechanisms present when flow distorting components are installed in close proximity of an axial flow fan. The effect of different parameters on the operation of axial flow fans is investigated. These parameters are divided into flow enhancing and flow reduction effects. The performance of an axial flow fan can be enhanced by changing the tip clearance, by adding a solid disc to the hub of the fan or by varying the number of fan blades. Flow reductions are caused by components such as inlet grids, walkways and their supporting structures, heat exchangers and windwalls. The effects of flow enhancing components are measured and compared to the results of other authors. The sensitivity of these effects to parameters such as the type of fan rotor and the specific system in which the rotor is installed is highlighted. The system effect (the interaction between the fan rotor and flow resistances in close proximity of each other) of individual components, as well as the combination of different components, is predicted both theoretically and experimentally. These predictions are compared to measured data relevant to the components in an installation where the system effects are present. The results are correlated to the kinetic energy flux coefficient of the flow at different locations within the installation. Experimental data obtained from a full scale unit (inlet shroud diameter of 9,216 m) are used to compare to scaled data from the model (inlet shroud diameter of 1,542 m). The hub to tip ratio of the axial flow fans investigated is 0,15. The most important conclusions are that the performance of the type of axial flow fan under investigation can be improved by reducing its tip clearance and by installing a solid disc to the downstream side of the rotor. An increase in the number of blades of the fan leads to only marginal improvements in the fan performance. The overall performance of the system can also be improved by removing some of the flow resisting components, or by changing their relative positions. All these conclusions are based on the assumption that the power input to the fan rotor remains constant.
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    Flooding during reflux condensation of steam in an inclined elliptical tube
    (South African Institution of Mechanical Engineering, 2000) Schoenfeld, P. D.; Kroger, D. G.
    In this erperimental inuestigation the pressure drop is measured between the headers located at the ends of an in,clined air-cooled elliptical tube in which refl,ur condensation of steam occurs. The cross-section of the 7 m long tube has a height of g7 mm (major aris) and a width of 16 mm (minor ads). Steam temperatures are in the range of 45o C to 65o C. The pressure drop can be predicted accurately using the Z apke- I(rri g er pressure drop model applicable to refl,ur condensers. At a certain steam fl,ou, rate a sudden sharp increase in the pressure drop occurs. This phenoTne?t,on is lcnowrt, &s fl,ooding. The rneasured aapour uelocities at fl,oodin,g agree well with the ualues predicted by the Zapke-Krciger fl,ooding correlation. The erperimental results also show that flooding has a detrimental effect on the thermal effectiueness of Ihe elliptical tube.
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    Flow distribution, pressure drop, flooding and entrainment in an air-cooled reflux steam condenser
    (South African Institution of Mechanical Engineers, 2002) Smit, L. v. Z.; Kroger, D. G.
    Measurements on air-cooled reflux steam condensers or dephlegmators at different power plants have shown that sections of these units do not transfer heat effectively over a range of operating conditions. The ineffective areas may be due to flooding in the finned tubes although entrainment of the condensate in certain steam inlet manifolds is usually the main reason for the poor performance. Laboratory experiments were conducted to show under which conditions entrainment occurs, as well as the pressure drop and flow characteristics for different types of inlet headers or manifolds. A practical and cost effective solution is proposed to eliminate the reduction in dephlegmator performance due to entrainment in a particular manifold.
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    Heat transfer between a horizontal flat surface and the natural environment
    (South African Institution of Mechanical Engineers, 2002) Lombaard, I. F.; Kroger, D. G.
    The objective of this study was to obtain a correlation for the effective convective heat transfer between a horizontal 1 m × 1 m surface and the environment. Tests were conducted on a suitable surface at a particular location exposed to different solar radiation intensities, ambient temperatures and wind velocities. Based on the experimental data, a correlation for the effective heat transfer coefficient, that takes into consideration natural convection, forced convection (wind) and solar radiation (clear sky), is presented. This coefficient can be employed to predict the heat transfer on the surface of solar collectors having geometries similar to the present test set-up and operating under similar environmental conditions.
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    A numerical investigation of the errors associated with the scaling of axial flow fan performance characteristics
    (Science Africa for the South African Institution of Mechanical Engineers, 2004) Meyer, C. J.; Kroger, D. G.
    The primary purpose of this numerical investigation is to determine the influence of scaling on the accuracy with which the fan performance characteristics of a scaled axial flow fan can be determined from those of a model fan with the use of the fan laws. The fan laws are based on the premise that similarity exists between the model and scaled fan velocity fields, and does not make provision for fan blade Reynolds number and surface roughness differences between the model and scaled fans. The numerical investigation is conducted using a commercially available computational fluid dynamics code. An axial flow fan model based on actuator disc theory is developed and incorporated into the commercial code. The use of the fan model greatly reduces grid complexity and processing power whilst maintaining a high degree of accuracy. The results of the numerical investigation indicate that, depending on the type of axial flow fan used as well as the operating conditions encountered, inaccuracies due to scaling can arise when using the fan laws to determine the scaled fan performance characteristics.
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    Performance characteristics of an air-cooled steam condensor with a hybrid dephlegmator
    (South African Institution of Mechanical Engineering, 2012) Heyns, J. A.; Kroger, D. G.
    This study evaluates the performance characteristics of a power plant incorporating a steam turbine and a direct air-cooled dry/wet condenser operating at different ambient temperatures. The proposed cooling system uses existing A-frame air-cooled condenser (ACC) technology and through the introduction of a hybrid (dry/wet) dephlegmator achieves measurable enhancement in cooling performance when ambient temperatures are high. During these periods the hybrid (dry/wet) condenser operating in a wet mode can achieve the same increased turbine performance as an oversized aircooled condenser or an air-cooled condenser with adiabatic cooling (spray cooling) of the inlet air at a considerably lower cost. For the same turbine power output the water consumed by an air-cooled condenser incorporating a hybrid (dry/wet) dephlegmator is at least 20 % less than an air-cooled condenser with adiabatic cooling of the inlet air.
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    Solar chimney power plant performance characteristics
    (South African Institution of Mechanical Engineers, 2002) Kroger, D. G.; Buys, J. D.
    The performance characteristics of a large solar chimney power plant are evaluated. The reference plant studied has a glass-covered collector and a chimney that is 1 500 m high with a diameter of 160 m. A turbo-generator is located at the base of the chimney. The draught and relevant energy equations applicable to the plant are solved for specified meteorological data at a particular site in South Africa. It is shown that the output of the plant changes measurably during the day and that power is also generated during the night, due to the thermal capacity of the ground under the solar collector. By optimising the shape and height of the solar collector, the annual power output can be increased.