In this Thesis, new simulation codes for the evaluation of a heat pump system seasonal performance are presented. The codes apply to electric air-to-water and ground-coupled heat pump systems based on a vapor compression cycle, used for building heating, cooling and domestic hot water production. Numerical models are developed to simulate different kinds of air-to-water heat pumps by means of the bin-method. The models take into account the different operating modes of mono-compressor on-off, multi-compressor and inverter-driven heat pumps. The heat pump system seasonal performance is analyzed in terms of SCOP and SEER in relation to the thermal characteristics of the building, the climate of the location and the kind of heat pump control system. Furthermore, numerical codes for the hourly simulation of air-to-water heat pump systems are developed. The dynamic codes are implemented in the software MATLAB and apply to on-off and inverter-driven heat pumps for building heating, cooling and domestic hot water production, coupled with storage tanks and integrated by a gas boiler or electric heaters. The codes are used, in particular, to evaluate the seasonal performance and the primary energy consumption of the inverter-driven air-to-water heat pump employed in the retrofit of a residential building in Bologna (Italy). A code for the hourly simulation of ground-coupled heat pump systems is developed. The code, implemented in MATLAB, employs g-functions expressed in analytic form and applies to on-off and inverter-driven heat pumps, used for building heating and/or cooling. The whole system, composed by the heat pump and the coupled borehole heat exchanger field, can be simulated for several years. The code is applied to analyze the effects of the inverter and of the total length of the borehole field on the mean seasonal performance of a ground-coupled heat pump system designed for a residential house with dominant heating loads.

This PhD dissertation deals with the use of optical, non-invasive measurement techniques for the investigation of single and two-phase flows in microchannels. Different experimental techniques are presented and the achieved results are critically discussed. Firstly, the inverse use of the micro Particle Image Velocimetry technique for the detection of the real shape of the inner cross-section of an optical accessible microchannel is shown by putting in evidence the capability of this technique to individuate the presence of singularities along the wetted perimeter of the microchannel. Then, the experimental measurement of the local fluid temperature using non-encapsulated Thermochromic Liquid Crystal particles is discussed. A deep analysis of the stability of the color of these particles when exposed to different levels of shear stress has been conducted by demonstrating that these particles can be used for simultaneous measurements of velocity and temperature in water laminar flows characterized by low Reynolds numbers (Re < 10). A preliminary experiment where the TLC thermography is coupled to the APTV method for the simultaneous measurement of the three-dimensional velocity and temperature distribution in a microchannel is shown. Finally, an experimental analysis of the different flow patterns observed for an adiabatic air-water mixture generated by means of a micro T-junction is discussed. The main air-water mixture features have been deeply observed in 195 different experimental conditions in which values of superficial velocity ranging between 0.01 m/s and 0.15 m/s for both the inlet flows (air and water) are imposed. The flow patterns of the air-water mixture are strongly influenced by the value of the water superficial velocity; on the contrary, the air superficial velocity plays a secondary role for the determination of the characteristics of the bubbles (i.e. length).

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The last decade has witnessed very fast development in microfabrication technologies. The increasing industrial applications of microfluidic systems call for more intensive and systematic knowledge on this newly emerging field. Especially for gaseous flow and heat transfer at microscale, the applicability of conventional theories developed at macro scale is not yet completely validated; this is mainly due to scarce experimental data available in literature for gas flows. The objective of this thesis is to investigate these unclear elements by analyzing forced convection for gaseous flows through microtubes and micro heat exchangers. Experimental tests have been performed with microtubes having various inner diameters, namely 750 m, 510 m and 170 m, over a wide range of Reynolds number covering the laminar region, the transitional zone and also the onset region of the turbulent regime. The results show that conventional theory is able to predict the flow friction factor when flow compressibility does not appear and the effect of fluid temperature-dependent properties is insignificant. A double-layered microchannel heat exchanger has been designed in order to study experimentally the efficiency of a gas-to-gas micro heat exchanger. This microdevice contains 133 parallel microchannels machined into polished PEEK plates for both the hot side and the cold side. The microchannels are 200 µm high, 200 µm wide and 39.8 mm long. The design of the micro device has been made in order to be able to test different materials as partition foil with flexible thickness. Experimental tests have been carried out for five different partition foils, with various mass flow rates and flow configurations. The experimental results indicate that the thermal performance of the countercurrent and cross flow micro heat exchanger can be strongly influenced by axial conduction in the partition foil separating the hot gas flow and cold gas flow.

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In the present work, the multi-objective optimization by genetic algorithms is investigated and applied to heat transfer problems. Firstly, the work aims to compare different reproduction processes employed by genetic algorithms and two new promising processes are suggested. Secondly, in this work two heat transfer problems are studied under the multi-objective point of view. Specifically, the two cases studied are the wavy fins and the corrugated wall channel. Both these cases have already been studied by a single objective optimizer. Therefore, this work aims to extend the previous works in a more comprehensive study.

This work focuses on magnetohydrodynamic (MHD) mixed convection flow of electrically conducting fluids enclosed in simple 1D and 2D geometries in steady periodic regime. In particular, in Chapter one a short overview is given about the history of MHD, with reference to papers available in literature, and a listing of some of its most common technological applications, whereas Chapter two deals with the analytical formulation of the MHD problem, starting from the fluid dynamic and energy equations and adding the effects of an external imposed magnetic field using the Ohm's law and the definition of the Lorentz force. Moreover a description of the various kinds of boundary conditions is given, with particular emphasis given to their practical realization. Chapter three, four and five describe the solution procedure of mixed convective flows with MHD effects. In all cases a uniform parallel magnetic field is supposed to be present in the whole fluid domain transverse with respect to the velocity field. The steady-periodic regime will be analyzed, where the periodicity is induced by wall temperature boundary conditions, which vary in time with a sinusoidal law. Local balance equations of momentum, energy and charge will be solved analytically and numerically using as parameters either geometrical ratios or material properties. In particular, in Chapter three the solution method for the mixed convective flow in a 1D vertical parallel channel with MHD effects is illustrated. The influence of a transverse magnetic field will be studied in the steady periodic regime induced by an oscillating wall temperature. Analytical and numerical solutions will be provided in terms of velocity and temperature profiles, wall friction factors and average heat fluxes for several values of the governing parameters. In Chapter four the 2D problem of the mixed convective flow in a vertical round pipe with MHD effects is analyzed. Again, a transverse magnetic field influences the steady periodic regime induced by the oscillating wall temperature of the wall. A numerical solution is presented, obtained using a finite element approach, and as a result velocity and temperature profiles, wall friction factors and average heat fluxes are derived for several values of the Hartmann and Prandtl numbers. In Chapter five the 2D problem of the mixed convective flow in a vertical rectangular duct with MHD effects is discussed. As seen in the previous chapters, a transverse magnetic field influences the steady periodic regime induced by the oscillating wall temperature of the four walls. The numerical solution obtained using a finite element approach is presented, and a collection of results, including velocity and temperature profiles, wall friction factors and average heat fluxes, is provided for several values of, among other parameters, the duct aspect ratio. A comparison with analytical solutions is also provided, as a proof of the validity of the numerical method. Chapter six is the concluding chapter, where some reflections on the MHD effects on mixed convection flow will be made, in agreement with the experience and the results gathered in the analyses presented in the previous chapters. In the appendices special auxiliary functions and FORTRAN program listings are reported, to support the formulations used in the solution chapters.

In this thesis, the field of study related to the stability analysis of fluid saturated porous media is investigated. In particular the contribution of the viscous heating to the onset of convective instability in the flow through ducts is analysed. In order to evaluate the contribution of the viscous dissipation, different geometries, different models describing the balance equations and different boundary conditions are used. Moreover, the local thermal non-equilibrium model is used to study the evolution of the temperature differences between the fluid and the solid matrix in a thermal boundary layer problem. On studying the onset of instability, different techniques for eigenvalue problems has been used. Analytical solutions, asymptotic analyses and numerical solutions by means of original and commercial codes are carried out.

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