Convective instability in a horizontal porous channel with permeable and conducting side boundaries

The stability analysis of the motionless state of a horizontal porous channel with rectangular cross-section and saturated by a fluid is developed. The heating from below is modelled by a uniform flux, while the top wall is assumed to be isothermal. The side boundaries are considered as permeable and perfectly conducting. The linear stability of the basic state is studied for the normal mode perturbations. The principle of exchange of stabilities is proved, so that only stationary normalmodes need to be considered in the stability analysis.The eigenvalue problem for the neutral stability condition is solved analytically, and a closed-form dispersion relation is obtained for the neutral stability. The Darcy–Rayleigh number is expressed as an implicit function of the longitudinalwave number and of the aspect ratio. The critical wave number and the critical Darcy–Rayleigh number are evaluated for different aspect ratios. The preferredmodes under critical conditions are detected. It is found that the selected patterns of instability at the critical Rayleigh number are two-dimensional, for slender or square cross-sections of the channel. On the other hand, instability is three dimensional when the critical width-to-height ratio, 1.350517, is exceeded. Eventually, the effects of a finite longitudinal length of the channel are discussed

How the Energy Price Variability in Italy Affects the Cost of Building Heating: A Trnsys-Guided Comparison between Air-Source Heat Pumps and Gas Boilers

The paper investigates the variation in building thermal energy demand for different indoor air set-point temperature and presents an economic analysis comparing the cost of a heating generation system based on an air-to-water heat pump and a gas boiler. Dynamic simulations were performed considering three different residential building characteristics of the Italian building stock placed in different Italian municipalities: Milan, Rome and Naples. An economic analysis was carried out considering the gas and electricity prices related to the years 2019–2022 provided by Italian Regulatory Authority for Energy, Networks and Environment (ARERA). The analysis showed the competitiveness of the heat pump compared with the gas boiler as a heating generation system in terms of annual costs for heating in almost all the scenarios considered and also showed an important reduction in building thermal energy demand if the set-point temperature was reduced, even by 1 ◦C

A New Approach for the Modelization of Water and Steam Mixing at High Pressure Conditions

Abstract In the present paper a zero-dimensional thermodynamic model of liquid water and steam mixing is presented. The model provides the temperature distribution during the mixing process, as an exponential function of time. A comparison with experimental data is performed, showing an excellent agreement. In the experimental set up, steam from an adiabatic high pressure storage is mixed, in a mixing chamber, together with liquid water. Measures of the temperature both on the inner and outer surface of the mixing chamber are taken. A comparison between the experimental data and the thermodynamic model proposed shows that the heat transfer between the mixing chamber and the environment is negligible. Moreover, a discussion on the time dependency of the experimental pressure and of the theoretical internal energy of the steam is presented

The bin method to investigate the effect of climate conditions on the SCOP of air source heat pumps: the Italian case

The paper investigates the seasonal performances of electric air-to-water heat pumps specifically related to climatic conditions of the place where the heat pump is installed. The analysis is carried out using easily available weather data for some Italian towns differently located, and using the bin-method proposed by UNI/TS 11300-4. Two different types of heat pumps (on-off and inverter-driven variable speed compressor) are considered and comparisons between different types of heat pumps and different places of installation are performed. The analysis shows that the climate of the installation place is the most important factor that affects seasonal indexes of heat pumps; moreover, as expected, inverter heat pumps better perform than on-off ones

Effect of real temperature data on the seasonal coefficient of performance of air source heat pumps

In this paper, a transient analysis is performed in order to evaluate how effective climate data affect the determination of the seasonal coefficient of performance of ASHPs. Three Italian cities, characterized by different climates, are considered as reference case studies and the influence of real meteorological data, collected from different years, on the heat pump SCOP is performed. The analysis is carried out by employing the dynamic software TRNSYS. Numerical results show small variations in SCOP, while a significant influence on annual thermal energy demand can be observed. The analysis also underlines the absence of a general correlation between annual thermal energy supplied by the heat pump and SCOP and between HDD and SCOP

Convective Roll Instabilities of Vertical Throughflow with Viscous Dissipation in a Horizontal Porous Layer

Published version of an article from the journal: Transport in Porous Media. The original publication is available at Spingerlink. http://dx.doi.org/10.1007/s11242-009-9417-yThe vertical throughflow with viscous dissipation in a horizontal porous layer is studied. The horizontal plane boundaries are assumed to be isothermal with unequal temperatures and bottom heating. A basic stationary solution of the governing equations with a uniform vertical velocity field (throughflow) is determined. The temperature field in the basic solution depends only on the vertical coordinate. Departures from the linear heat conduction profile are displayed by the temperature distribution due to the forced convection effect and to the viscous dissipation effect. A linear stability analysis of the basic solution is carried out in order to determine the conditions for the onset of convective rolls. The critical values of the wave number and of the Darcy-Rayleigh number are determined numerically by the fourth-order Runge-Kutta method. It is shown that, although generally weak, the effect of viscous dissipation yields an increase of the critical value of the Darcy-Rayleigh number for downward throughflow and a decrease in the case of upward throughflow. Finally, the limiting case of a vanishing boundary temperature difference is discussed

Theoretical and Numerical Study on Buongiorno’s Model with a Couette Flow of a Nanofluid in a Channel with an Embedded Cavity

In the present paper, the fluid flow and heat transfer of a nanofluid are numerically investigated. More specifically, reference is made to a nanofluid, described by means of Buongiorno’s model, subjected to Couette flow. The considered domain consists of a channel that displays a cavity shortly after the inlet section. The transport model for the nanofluid, that is the mass conservation, momentum, and nanoparticles equation, is written in a dimensionless form and solved by employing the software package Comsol Multiphysics. Many ideas emerged from this work: the visualization of the velocity stream function, the dimensionless temperature, and nanoparticle concentration fields are provided, as a function of the governing parameters: Reynolds, Peclet, Lewis, Brownian diffusivity number, and thermophoretic diffusivity number. Concerning the nanofluid typical effects, the thermophoretic diffusion seems to affect the solution much more than the Brownian diffusion. The Nusselt number on the upper wall is calculated as well, and the results show that it proves to be, in most of the considered cases, an increasing function of the Reynolds number. Moreover, concerning the Nusselt number, the Brownian diffusion effects are shown to be negligible