Experiments were performed using an Isentropic Light Piston Tunnel, a transient facility which enables conditions representative of those in engines to be attained. The results were interpreted using a superposition model, which is shown to be a valuable and concise method of characterising the effects of injection.

Film cooling by means of holes is an essential cooling technique in modern gas turbine engines. This cooling technique is employed over endwalls, as well as on the surface of blades. Thus, there is a need for film cooling predictions in a three-dimensional setting. Currently only boundary layer codes are available for design purposes and they are difficult to apply to the three-dimensional case with secondary flows. Present advanced computation prediction methods are capable of solving the complete flow field in three dimensions with coolant flow. However, the spatial resolution that these methods require eliminate them as suitable options for design tools This study introduces a simpler description of the film cooling process which may be implemented in a code for design purposes. The parameters of turbulence enhancement, turbulence decay, and the coolant distribution at injection were optimized using existing experimental data. Finally, the code was employed in a three-dimensional setting with film cooling present. An experimental study of the flow through cooling holes was also undertaken. Two unique geometries were later developed where a row of cooling holes exited into a vortex region where the flow was mixed before being injected from a slot. The cooling benefits of these geometries is apparent.

No description available.

Doctor of Philosophy / The main contribution of this work is to answer several important questions relating to natural draft wet cooling tower (NDWCT) modelling, design and optimisation. Specifically, the work aims to conduct a detailed analysis of the heat and mass transfer processes in a NDWCT, to determine how significant the radial non-uniformity of heat and mass transfer across a NDWCT is, what the underlying causes of the non-uniformity are and how these influence tower performance. Secondly, the work aims to determine what are the consequences of this non-uniformity for the traditional one dimensional design methods, which neglect any two-dimensional air flow or heat transfer effects. Finally, in the context of radial non-uniformity of heat and mass transfer, this work aims to determine the optimal arrangement of fill depth and water distribution across a NDWCT and to quantify the improvement in tower performance using this non-uniform distribution. To this end, an axisymmetric numerical model of a NDWCT has been developed. A study was conducted testing the influence of key design and operating parameters. The results show that in most cases the air flow is quite uniform across the tower due to the significant flow restriction through the fill and spray zone regions. There can be considerable radial non-uniformity of heat transfer and water outlet temperature in spite of this. This is largely due to the cooling load in the rain zone and the radial air flow there. High radial non-uniformity of heat transfer can be expected when the cooling load in the rain zone is high. Such a situation can arise with small droplet sizes, low fill depths, high water flow rates. The results show that the effect of tower inlet height on radial non-uniformity is surprisingly very small. Of the parameters considered the water mass flow rate and droplet size and droplet distribution in the rain zone have the most influence on radial noniv uniformity of heat transfer. The predictions of the axisymmetric numerical model have been compared with a one dimensional NDWCT model. The difference between the predictions of tower cooling range is very low, generally around 1-2%. This extraordinarily close comparison supports the assumptions of one dimensional flow and bulk averaged heat transfer implicit in these models. Under the range of parameters tested here the difference between the CFD models predictions and those of the one dimensional models remained fairly constant suggesting that there is no particular area where the flow/heat transfer becomes so skewed or non-uniform that the one dimensional model predictions begin to fail. An extended one dimensional model, with semi-two dimensional capability, has been developed for use with an evolutionary optimisation algorithm. The two dimensional characteristics are represented through a radial profile of the air enthalpy at the fill inlet which has been derived from the CFD results. The resulting optimal shape redistributes the fill volume from the tower centre to the outer regions near the tower inlet. The water flow rate is also increased here as expected, to balance the cooling load across the tower, making use of the cooler air near the inlet. The improvement has been shown to be very small however. The work demonstrates that, contrary to common belief, the potential improvement from multi-dimensional optimisation is actually quite small.

In permanent mold casting, die design for cast aluminium alloy and magnesium alloy products includes a number of high conductivity material cooling blocks (also called channels or cooling circuits) that are aimed to extract heat away from molten metal through direct conduction heat transfer and freeze the casting as quickly as possible in a directional manner. One of the biggest problems during this solidification process occurs when the molten metal naturally shrinks away from the mould as it solidifies. This makes it increasingly difficult to efficiently and effectively cool targeted areas in the casting through conduction, since the direct contact between the solidifying casting and the cooling block is significantly reduced or even lost. A typical cooling block (termed in this thesis as a “chill”) is a cooling circuit that is embedded in a permanent mold (or die) and positioned to enable high heat transfer (effective cooling) to a targeted large section in the casting. If a large volume section in a casting does not cool efficiently and in the correct sequence in the overall product (i.e. solidification first in the furthest part from the sprue inlet followed by successive and ordered solidification towards the sprue inlet, until finally the sprue inlet itself), then it will create a “hot spot” which will create macro-shrinkage in the casting. This can create millions of dollars of waste in terms of casting rejects, lost productivity, and reworks for a given manufacturing company. When the molten metal solidifies, it shrinks by about 6.6 % for aluminium alloys and 4.0 % for magnesium alloys. This creates an air gap at the casting and mold interface. This air gap causes inefficient, random and isolated pockets of heat transfer from the casting to a contacting chill, which in turn causes a significant variation in the temperature distribution in the casting and die during solidification. A die operating in an incorrect and unstable temperature band will very likely produce adverse secondary effects in the final product such as macro shrinkage, micro shrinkage, hot tearing, gas porosity, or even misruns. This aim of this study is to theoretically understand and experimentally develop a cooling technique that can offset or close up the growing air gap and maintain high heat transfer between the casting and contacting chill, by ensuring that the chill is pushed closer into the casting at specific times during the solidification (and shrinking) process. A movable copper chill was designed and built to push forward into an insulated mold. The experiments were carried out using commercially available A356 aluminium alloy. The chill was pushed into the casting as it solidified in the mold. Studies were carried out to understand the effect of a movable chill with different cooling conditions compared to a fixed chill. Numerical simulations were conducted using developed boundary conditions in a commercial casting solidification package(ProCASTTM). The boundary condition used to emulate the air gap is a temporally distributed interfacial heat transfer coefficient function between the casting and chill and this is manually calculated using inverse modelling in an in-house developed optimisation package (OPTCASTTM) to compare and validate with experimental data. The resulting sensitivities of the casting due to different chill conditions (i.e. fixed vs. moving) are described through physical phenomenon, metallographic analysis and computational modelling. Results show that the effective cooling can be increased by 39.2 % by using movable chill with cooling compared to fixed chill with cooling. The percentage of complete contact between the casting and chill has been increased from 10 % in case of fixed chill with cooling to 76% in case of movable chill with cooling. Apart from effectiveness of cooling, the quality of casting produced with new cooling technique has significantly improved. The secondary dendrite arm spacing (SDAS) of the casting produced under the movable chill have be reduced by 26 % compared to fixed chill.

Doctor of Philosophy / The main contribution of this work is to answer several important questions relating to natural draft wet cooling tower (NDWCT) modelling, design and optimisation. Specifically, the work aims to conduct a detailed analysis of the heat and mass transfer processes in a NDWCT, to determine how significant the radial non-uniformity of heat and mass transfer across a NDWCT is, what the underlying causes of the non-uniformity are and how these influence tower performance. Secondly, the work aims to determine what are the consequences of this non-uniformity for the traditional one dimensional design methods, which neglect any two-dimensional air flow or heat transfer effects. Finally, in the context of radial non-uniformity of heat and mass transfer, this work aims to determine the optimal arrangement of fill depth and water distribution across a NDWCT and to quantify the improvement in tower performance using this non-uniform distribution. To this end, an axisymmetric numerical model of a NDWCT has been developed. A study was conducted testing the influence of key design and operating parameters. The results show that in most cases the air flow is quite uniform across the tower due to the significant flow restriction through the fill and spray zone regions. There can be considerable radial non-uniformity of heat transfer and water outlet temperature in spite of this. This is largely due to the cooling load in the rain zone and the radial air flow there. High radial non-uniformity of heat transfer can be expected when the cooling load in the rain zone is high. Such a situation can arise with small droplet sizes, low fill depths, high water flow rates. The results show that the effect of tower inlet height on radial non-uniformity is surprisingly very small. Of the parameters considered the water mass flow rate and droplet size and droplet distribution in the rain zone have the most influence on radial noniv uniformity of heat transfer. The predictions of the axisymmetric numerical model have been compared with a one dimensional NDWCT model. The difference between the predictions of tower cooling range is very low, generally around 1-2%. This extraordinarily close comparison supports the assumptions of one dimensional flow and bulk averaged heat transfer implicit in these models. Under the range of parameters tested here the difference between the CFD models predictions and those of the one dimensional models remained fairly constant suggesting that there is no particular area where the flow/heat transfer becomes so skewed or non-uniform that the one dimensional model predictions begin to fail. An extended one dimensional model, with semi-two dimensional capability, has been developed for use with an evolutionary optimisation algorithm. The two dimensional characteristics are represented through a radial profile of the air enthalpy at the fill inlet which has been derived from the CFD results. The resulting optimal shape redistributes the fill volume from the tower centre to the outer regions near the tower inlet. The water flow rate is also increased here as expected, to balance the cooling load across the tower, making use of the cooler air near the inlet. The improvement has been shown to be very small however. The work demonstrates that, contrary to common belief, the potential improvement from multi-dimensional optimisation is actually quite small.

Thesis (M.S.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 124-133). Also available on the World Wide Web.

Thesis (Ph.D.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references: leaf 156-164.

Thesis (M.S.)--Ohio University, August, 1998. / Title from PDF t.p.

Thesis(M.S.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.