Heat Transfer in Gaseous Microflows: Conjugate Heat Transfer, Rarefaction and Compressibility Effects

Development of Conjugate Heat Transfer models to study the behavior of heat exchange in gaseous microflows. At the scales considered, rarefaction effects play a relevant role so that the need to involve slip flow boundary conditions is fundamental. The wide development of MEMS application, the very fast development in microfabrication technologies, and the increasing industrial applications of microfluidic systems, which are all taking place and evolving in the last decades, require a better knowledge of the behavior of microfluidic systems, especially of gases, which haven\u2019t been yet understood as well as liquid ones have. Flows that are involved in this realm have characteristic dimensions of the order of tens of micrometers. The object of this work is essentially the investigation, and evaluation, of the characteristics and performances of forced convection in micro channels and of Micro Heat Exchangers. A general correlation for the local Stagnation Nusselt number is derived, which is in good agreement with all the results obtained in the study, and it has been proven that it works for the convection heat transfer cases with compressibility effects in the Micro Heat Exchange

Asymptotically entropy conservative discretization of convective terms in compressible Euler equations

A new class of Asymptotically Entropy Conservative schemes is proposed for the numerical simulation of compressible (shock-free) turbulent flows. These schemes consist of a suitable spatial discretization of the convective terms in the Euler equations, which retains at the discrete level many important properties of the continuous formulation, resulting in enhanced reliability and robustness of the overall numerical method. In addition to the Kinetic Energy Preserving property, the formulation guarantees the preservation of pressure equilibrium in the case of uniform pressure and velocity distributions, and arbitrarily reduces the spurious production of entropy. The main feature of the proposed schemes is that, in contrast to existing Entropy Conservative schemes, which are based on the evaluation of costly transcendental functions, they are based on the specification of numerical fluxes involving only algebraic operations, resulting in an efficient and economical procedure. Numerical tests on a highly controlled one-dimensional problem, as well as on more realistic turbulent three-dimensional cases, are shown, together with a cost-efficiency study

Asymptotically entropy-conservative and kinetic-energy preserving numerical fluxes for compressible Euler equations

This paper proposes a hierarchy of numerical fluxes for the compressible flow equations which are kinetic-energy and pressure equilibrium preserving and asymptotically entropy conservative, i.e., they are able to arbitrarily reduce the numerical error on entropy production due to the spatial discretization. The fluxes are based on the use of the harmonic mean for internal energy and only use algebraic operations, making them less computationally expensive than the entropy-conserving fluxes based on the logarithmic mean. The use of the geometric mean is also explored and identified to be well-suited to reduce errors on entropy evolution. Results of numerical tests confirmed the theoretical predictions and the entropy-conserving capabilities of a selection of schemes have been compared.Comment: 9 pages, 4 figure

Numerical treatment of the energy equation in compressible flows simulations

We analyze the conservation properties of various discretizations of the system of compressible Euler equations for shock-free flows, with special focus on the treatment of the energy equation and on the induced discrete equations for other thermodynamic quantities. The analysis is conducted both theoretically and numerically and considers two important factors characterizing the various formulations, namely the choice of the energy equation and the splitting used in the discretization of the convective terms. The energy equations analyzed are total and internal energy, total enthalpy, pressure, speed of sound and entropy. In all the cases examined the discretization of the convective terms is made with locally conservative and kinetic-energy preserving schemes. Some important relations between the various formulations are highlighted and the performances of the various schemes are assessed by considering two widely used test cases. Together with some popular formulations from the literature, also new and potentially useful ones are analyzed

Fiorentino in Capitanata. Cronache da un abitato medievale

The archaeological site of Fiorentino is located on a hill on the edge of the Tavoliere plain. Inhabited since ancient times, reactivated in the 11th century by the Byzantine government, the settlement reached its greatest expansion in the Norman and Swabian age. It was the site of one of the Domus Solaciorum of Frederick II of Swabia who died there in 1250. The Italian-French archaeological campaigns (1982-1994) uncovered dwellings, cisterns, churches, the cathedral, the main castle fortified area, moats, walls. The investigations were accompanied by widespread conservation interventions, often incomplete and inadequate though. Today the site is abandoned and the rare valorization activities undertaken fail to strengthen the contact with the local people. A research carried out by the University of Florence (DIDA) monitored for years the conservation conditions of many structures. The most critical aspects concern the poor quality walls, especially if unprotected, that are in continuous degeneration. The architectural structures on the hillside are increasingly at risk of instability. Constant care and maintenance would be needed, that can fit into broader programs of enhancement of the complex. Punctual and occasional interventions in the long run are ineffective and wasteful

Wigner dynamics for quantum gases under inhomogeneous gain and loss processes with dephasing

We present a Wigner function-based approach for the particle density evolution in fermionic and bosonic open quantum many-body systems, including the effects of dephasing. In particular, we focus on chains of non-interacting particles coupled to Lindblad baths. The dissipative processes, described by linear and quadratic jump operators, are modulated by inhomogeneous couplings. Following a semi-classical approach, we find the differential equation governing the Wigner function evolution, which can be solved in closed form in some particular cases. We check the accuracy of the Wigner approach in different scenarios (i.e. Gaussian jump rates), describing the density evolution and the transport phenomena in terms of classical quasi-particles.Comment: 12 pages, 8 figure

Combined refrigeration, heating and air conditioning systems in supermarkets: seeking energy efficient solutions

A deep synergy between the refrigeration and the HVAC plants is a viable solution to reduce the energy use of supermarkets. Not only heat recovery can be performed from the refrigerating plant in favour of space heating and hot water production, but also provision of AC capacity. A model based on TRNSYS and in-house types, validated with field data gathered from a fully instrumented plant in an active supermarket, allows to seek the optimal coupled solution. The model has been used to predict its feasibility and energy use at different climate conditions, for a reference supermarket. An energy saving of about 9% was predicted, regardless of the climate conditions, if the energy performance quality of the building envelope is kept constant. The integration of the refrigeration and HVAC systems shows to be effective in terms of energy use. Significant reductions can also be obtained in the investment costs, space occupied by the plants and amount of refrigerant charge

Fast-projection methods for the incompressible navier–stokes equations

An analysis of existing and newly derived fast-projection methods for the numerical integration of incompressible Navier–Stokes equations is proposed. Fast-projection methods are based on the explicit time integration of the semi-discretized Navier–Stokes equations with a Runge–Kutta (RK) method, in which only one Pressure Poisson Equation is solved at each time step. The methods are based on a class of interpolation formulas for the pseudo-pressure computed inside the stages of the RK procedure to enforce the divergence-free constraint on the velocity field. The procedure is independent of the particular multi-stage method, and numerical tests are performed on some of the most commonly employed RK schemes. The proposed methodology includes, as special cases, some fast-projection schemes already presented in the literature. An order-of-accuracy analysis of the family of interpolations here presented reveals that the method generally has second-order accuracy, though it is able to attain third-order accuracy only for specific interpolation schemes. Applications to wall-bounded 2D (driven cavity) and 3D (turbulent channel flow) cases are presented to assess the performances of the schemes in more realistic configurations.Peer ReviewedPostprint (published version

Subcooling with AC and adiabatic gas cooling for energy efficiency improvement: field tests and modelling of CO2 booster systems

In the last decade several plant configurations and components have been proposed to increase the efficiency of CO2 refrigeration systems. Among these, subcooling is considered a simple but effective solution, together with the employment of adiabatic cooling systems at the gas cooler. In this work, a fully instrumented CO2 booster plant installed in a supermarket is considered, to compare parallel compression, subcooling and adiabatic cooling. Subcooling is performed taking advantage of chilled water available from the HVAC system. The experimental data are used to validate a model for the comparison on a yearly basis. Parallel compression and subcooling show to be almost equivalent in terms of yearly energy use, while the adiabatic cooling system gives the best performance. Comparisons reveal that the subcooler cooling capacity should be chosen carefully to avoid oversizing, while the influence of the EER for the chiller appears quite small. Subcooling performed at the expense of an HVAC plant shows to be an interesting solution, while a great benefit was experienced with the employment of an adiabatic gas cooler