In some industrial air-cooled heat exchangers, such as those in the generating set industry, the flow must turn through 90° after exiting the heat exchanger. In such arrangements, the plenum depths are typically very shallow. Furthermore., the axial fan often operates in the mixed-flow region of the fan characteristic, due to the restrictive nature of the system. These two factors lead to a reduction in the thermal performance of the system. The purpose of this study was to investigate the effect on thermal performance of inclining the heat exchanger relative to the axial fan. It was also important to compare this with simply increasing the plenum depth without inclining the heat exchanger, since inclination itself may increase the mean plenum depth. This was achieved through an isothermal experimental investigation, complemented with a numerical study using CFD. The results showed that as the heat exchanger was inclined, the low velocity core at the centre of the heat exchanger tended to move to one side. The opposite side had increased flow through the heat exchanger due to the inclination. For a mixed-flow fan operating point typical of some industries, it was found that inclination has a negligible effect on the performance of the system, when compared with a baseline case. Increasing the plenum depth also had no significant effect. At the axial fan operating point investigated, it was found that an angle of approximately 30° inclination gave the best performance. Increasing the plenum depth was found to improve the performance more than inclination. The best performing case was the non-inclined case with a plenum depth of 0.65 fan diameters. This gave an increase in flow of2.8% over the baseline case, and a corresponding 1.1 % increase in thermal performance.

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Fouling and corrosion are probably the major engineering problems in process plants and in heat exchanger design and operation. Fouling is the formation of undesired deposits on heat transfer surfaces and corrosion is the deterioration and loss of material. The occurrence of these processes has strong adverse effects on the performance of heat exchangers and operating plants. In the present work, the influence of surface properties (surface free energy & surface roughness) on crystallisation fouling, biological fouling, wax deposition and corrosion has been investigated. In order to carry out the experimental investigations on crystallisation and biological fouling, the surfaces of several pairs of Alfa Laval M3 stainless steel heat exchanger plates have been modified. The experiments for the investigation on paraffin deposition on modified surfaces have been carried out using stainless steel tubes with various surface modifications, to change surface energy and roughness. The corrosion tests have been performed with modified carbon steel specimens. Test samples have been treated using different surface treatment technologies, namely ion beam implantation, magnetron & arc ion sputtering, carbo-nitriding & oxidising, electroplating, chemical vapour deposition, commercial coatings and Ni-P-PTFE coating. These treatment techniques have been used for the reduction of the surface free energy of the test surfaces. Two pairs of plates have been pickled and electropolished in order to achieve rougher and smoother surface conditions compared to the untreated plates. The surface free energy of the treated surfaces was calculated from dynamically measured advancing contact angles using the sessile drop method. For the 'high energy' surfaces (untreated, pickled, sand blasted, sanded and electropolished), a two-liquid method was applied using two different n-alcanes and water as test liquids. In case of 'low energy' surfaces (ion implanted, sputtered, carbo-nitrated & oxidised and Ni-P-PTFE coated), the contact angles were determined with a one-liquid method. Water, benzyl alcohol and ethylene glycol were used as test liquids, and air represented the embedding phase. Flow simulations in 3-dimensional heat exchanger ducts were performed, using a commercial CFD package. The objective was the prediction of local flow patterns; temperature distribution, pressure drop as well as local heat transfer coefficients and wall shear forces. Laminar and turbulent simulations with heat transfer were carried out for the corrugated section of plate heat exchanger ducts. The flow velocity was varied in the range from Re=1000 to Re=3000. The RNG k-E model was chosen for the turbulent simulations, because of its capability to solve complex swirling flows even at low Reynolds numbers. Periodic boundary conditions were applied to produce a fully developed flow in the computational domain. The simulations of turbulent flow in corrugated ducts with two different corrugation angles, namely 30°/30° and 60°/60° have shown that the flow conditions in the wake of the contact point cause vortex formation leading to re-circulation zones. Additionally stagnant zones around the contact point could be seen. Around the contact points high temperatures caused by heat accumulation were predicted. Since high temperature and low flow velocities are two criteria necessary for fouling to occur, the prevailing flow conditions are responsible for the risk of deposition due to CaSO4 crystallisation. Two new designs of plate corrugation were developed. The plates with asymmetrical corrugations have a corrugation angle of 60°/60°. The difference compared with a conventional Alfa Laval M3 duct (60°/60°) is that the sinusoidal wave is a combination of two different wavelengths leading to an asymmetrical shape of the corrugation. The comparison of pressure drop showed that the new design caused a higher pressure drop due to the strong vortex formation.

The aim of this research is to numerically and experimentally scrutinise the thermal performance of a typical heat exchanger fitted in a domestic condensing boiler. The optimisation process considered the pins' geometry (circular pins and elliptical pins), pins' spacing, pitch distance, the pressure drop across the heat chamber and the occurrence of thermal hot spots. The first part of the study focused on the effect of altering the circular pins spacing and pins pitch distance of the heat exchanger. Computational Fluid Dynamics (CFD) is used to scrutinise the thermal performance and the air flow properties of each model by changing these two parameters. In total, 13 circular pin models were investigated. Numerical modelling was used to analyse the performance of each model in three-dimensional computational domain. For comparison, all models shared similar boundary conditions and maintained the same pin height of 35 mm and pin diameter of 8 mm. The results showed that at a given flow rate, the total heat transfer rate is more sensitive to a change in the pins spacing than a change of the pins pitch. The results also showed that an optimum spacing of circular pins can increase the heat transfer rate by up to 10%.The second part of the study, focused on investigating the thermal performance of elliptical pins. Four elliptical pin setups were created to study the thermal performance and the air flow properties. In comparison with circular pins, the simulation results showed that the optimum use of eccentricity of elliptical pins could increase the total energy transfer by up to 23% and reduce the pressure drop by 55%. To validate the acquired CFD results, a Thermal Wind Tunnel (TWT) was designed, built and commissioned. The experimental results showed that the numerical simulation under predicted the circular pin models' core temperatures, but over predicted the elliptical models core temperatures. This effect is due to the default values of the standard k - E transport equations model used in the numerical study. Both numerical and experimental results showed that the elliptical models performed better compared to its circular pins counter parts. The study also showed that heat exchanger optimisation can be carried out within a fixed physical geometry with the effective use of CFD.

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Achieving the reductions in carbon dioxide emissions which are necessary will require improvements in the way in which domestic space heating is supplied. Air Source Heat Pumps and micro-Combined Heat and Power units both have the potential to reduce emissions while using primary energy resources more efficiently. The performance which these technologies can achieve is fundamental to fulfilling this potential and yet it is still subject to some uncertainty. This thesis analyses the performance of Air Source Heat Pumps and micro-Combined Heat and Power units in terms of their energy and exergy requirements and in terms of the carbon dioxide emissions associated with their operation. A review of the literature identified that it was appropriate to develop a novel modelling approach. Models of many components currently exist and these are adopted and extended wherever possible within this modelling approach. However, it is the unique way in which this research combines these models and adds additional components which delivers performance data relating to a wider range of conditions at a greater level of detail than that which was previously available. The model which was developed can dynamically simulate the heating and power demands in many dwellings simultaneously, facilitating meaningful study of effects which are dependent upon the sum of their power flows. Consideration of the effect of operating conditions includes permutations of climate, control systems (including those which engage with demand side management), grid generation mixes and building properties. Efficient Air Source Heat Pumps units have the potential to make energy and carbon emissions savings at present but their performance is sensitive to the conditions studied. In particular, appropriate control of the units can yield energy savings of around 25%. Additionally, the carbon emissions intensity of the grid is an important consideration which is explored in depth. Currently, energy requirements and carbon emissions can be reduced by the use of micro-Combined Heat and Power units. Their potential to further reduce carbon emissions diminishes if the grid is predominantly decarbonised but units with high electrical efficiencies can still save energy. The effect of the control approach which is adopted is also significant and has different effects on fuel-cell based units compared to combustion-based units. The key contribution of this work is the analysis of performance data for a selection of units operating under a range of conditions, calculated with a consistent, accurate methodology. Comparison is made between the technologies and between the effects of different operating conditions. A second significant contribution of this work is the development of the model which was used to generate the performance results. These advances allow more detailed comparative analysis of performance data in a wider range of conditions than previously possible.