Marco Lorenzini

Several engineering systems currently use microchannel heat sinks. In order to increase the performance of these devices, optimisation according to the first and second law of thermodynamics is employed. One way to achieve the goal is to modify the geometry of the cross-section, as is done in this paper for square ducts, having the walls at a uniform temperature which is higher than that of the bulk fluid at the inlet. The effects of both the thermal entry region of the duct and the heat generation due to viscous dissipation are considered. The resulting Graetz–Brinkman problem is solved numerically to obtain the velocity and temperature fields. It is demonstrated that non-negligible viscous heating eventually causes the heat flux to reverse (from fluid to walls), and that, only after this condition is achieved, can the flow become fully developed, which makes the entry region the only useful stretch for real-life applications. The length after which the direction of the heat flux reverses due to viscous heating in the fluid is obtained as a function of the Brinkman number and of the smoothing radius. Optimisation with performance evaluation criteria and entropy generation minimisation was carried out separately, and the results were combined into a single objective function. A comparison with published models highlights how neglecting the entry region and viscous heating yields misleading results. It turns out that smoothing the corners is always profitable in the case of the constrained heated perimeter or area of the cross-section but seldom when the characteristic length or the hydraulic diameter is fixed. With few exceptions, viscous heating amplifies the trends experienced for zero-Brinkman flows. The results are in non-dimensional form, yet they have been obtained starting from plausible dimensional values and are applicable to real-life devices.

Modelling of evaporative condensers relies heavily on the correct choice of the correlations for transport coefficients to give an accurate estimate of the dissipated thermal load and evaporated water mass flowrate, yet the literature does not cover all flow arrangements, nor operating conditions. In order to avoid expensive, and often impractical, experimental campaigns devoted to the determination of these quantities for a specific piece of equipment, a statistical approach based on the analysis of variance can be adopted: by analysing the sensitivity of the dependent variables on the choice of the transport coefficients, the best set of correlations can be determined, depending on whether a more accurate estimate is desired for the heat flux or evaporated mass flowrate, or if both are equally important.

Electro-osmotic flow, that is, the motion of a polar fluid in microducts induced by an external electric field, is one micro-effect which allows fluid circulation without the use of mechanical pumping. This is of interest in the thermal management of electronic devices, as microchannels with cross sections of almost arbitrary shape can easily be integrated on the chips. It is therefore important to assess how the geometry of the channel influences the heat transfer performance. In this paper, the thermal entry region and the fully developed electro-osmotic flow in a microchannel of rectangular cross section with smoothed corners is investigated for uniform wall temperature. For the fully developed region, correlations for the Poiseuille and Nusselt numbers considering the aspect ratio and nondimensional smoothing radius are given, which can be used for practical design purposes. For thermally developing flow, it is highlighted how smoothing the corners increases the value of the local Nusselt number, with increases up to 18% over sharp corners, but that it also shortens the thermal entry length. It is also found that Joule heating in the fluid may cause a reversal of the heat flux, and that the thermal entry length has a linear dependence on the Reynolds number and the hydraulic diameter and on the logarithm of the nondimensional Joule heating.

Natural convection heat sinks are still important in many industrial applications thanks to their simple design, effectiveness and noise-free operation. In this paper we present some case studies of experimental determination of the thermal characteristics of such devices, carried out using an innovative three-dimensional method of position detection for the temperature transducer (thermocouple). The three-dimensional position detection system uses acoustic waves emitted by a small grid of four loudspeakers, which simultaneously emit pseudo-random noise sequences orthogonal to one another. A small microphone samples the sounds coming from the four loudspeakers and its position in the three-dimensional space is obtained through trilateration calculations and signal analysis. Microphone and thermocouple are mounted on the same movable support, so that by measuring the position of the microphone, the position of the thermocouple is also detected. In this way it is possible to measure the temperature in a cloud of points of known position in space and obtain the three-dimensional thermal field around the heat sink. From these measurements it is possible to evaluate the thermal field around the heat sink and to easily compare different construction geometries. The position detection system has good precision and is inexpensive, allowing this type of measurement to be carried out even in low-budget situations. It also differs from thermal imaging systems such as IR cameras, in that it allows measurement of the temperature in the fluid surrounding the heat sink and not on its surface only.

Cogeneration is an often viable and efficient solution to cover combined demands of electric and thermal power, especially when the demand of the latter remains fairly constant over time. The efficient use of energy and the economic advantages that are associate to it have made cogeneration the solution of choice in many industrial applications, among which paper mills represent a significant share. The heat recovery steam generator often has an auxiliary burner to ensure full coverage of the thermal load in all operating conditions. In order to reduce installation and conduction costs, the combustion chamber may lack an extraction fan, which has the drawback that pressure varies according to changes in the loads, and this affects the stability of the flame adversely. In order to ensure constant pressure in the combustion chamber, application of a gate valve downstream of the heat recovery steam generator is proposed. A lumped-parameter model is set up in Simulink® and the feedback loop tested for to sudden variations in the thermal power required by the paper mill, mainly due to variations in the number of operating machines, and to failure of the internal combustion engine. It is demonstrated that the gate valve is necessary and that the use of a PID feedback control system ensures a constant pressure in the combustion chamber.