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The paper investigates the effect of a magnetic field on the forced convection of a nanofluid in a channel with an embedded cavity. The nanofluid is described by the Buongiorno’s model, and a fixed temperature boundary condition is prescribed. The governing equations are written in a dimensionless form and solved numerically by employing COMSOL Multiphysics. The obtained velocity, temperature and concentration fields are presented as function of the governing parameters, and the Nusselt number on the upper boundary is evaluated as well. Particular attention is paid to the effect of the inclination angle of the magnetic field, showing that variations up to 40% may affect the Nusselt number.

In this work, we investigate the replacement of a gas boiler with an electric air-to-water heat pump (AWHP) as a heating generation system for a detached single-family house. Attention is paid to the recent increase in energy prices in Italy. The evaluation of the energy consumption is done via a dynamic analysis performed by employing Trnsys. The investment results are economically advantageous only if the thermal energy demand is quite high, and the recent energy price increase (for both natural gas and electricity) leads to disadvantages in replacing the heat generation system in all the considered cases.

Parallel mixed convection in a vertical porous channel filled with a saturated porous medium is analysed. The case where one boundary wall is subject to a uniform heat flux and the other wall is thermally insulated is considered. The stability analysis versus small-amplitude disturbances is carried out.

In this paper, natural convection is studied in a 2D-cavity with two vertical isothermal walls, kept at different temperatures, and two adiabatic walls which are either straight (rectangular cavity) or elliptic (modified rectangular cavity). The local mass, momentum and energy balance equations are written in a dimensionless form and solved numerically, by means of two different software packages based on Galerkin finite element methods. With reference to a Prandtl number of 0.71, two rectangular cavities are studied: a square one and a cavity with height double than width. Then, for each value of the ratio between height and width, two cavities with elliptic boundaries are investigated. The numerical solution shows that the elliptic boundaries enhance the mean Nusselt number and the dimensionless mean kinetic energy of the fluid.

An investigation of the effect of axial heat conduction in the fluid on laminar forced convection is performed in a cylindrical duct with circular cross section. Two different kinds of boundary conditions are analyzed: (a) a wall temperature distribution which undergoes a sinusoidal axial variation; (b) a wall heat flux distribution which undergoes a sinusoidal axial variation and such that its mean value is equal to the modulus of the amplitude. Reference is made to the hydrodynamically and thermally developed region, where the temperature distribution is given in case (a) by an axially periodic function and in case (b) by the sum of an axially periodic function and a linear function of the axial coordinate. The temperature distribution within the fluid and the local and mean Nusselt number have been evaluated analytically.

Mixed convection flow in an inclined rectangular duct is studied by releasing the Boussinesq approximation. The fully-developed regime with a parallel velocity is considered. The boundary conditions are such that two facing walls are adiabatic, while the other two facing walls are isothermal with different temperatures. Temperature-independent values of the dynamic viscosity, of the thermal conductivity and of the thermal expansion coefficient are assumed. The balance equations are solved analytically by employing double finite Fourier transforms. The friction factor, the dimensionless velocity distribution and the dimensionless pressure distribution in a duct cross-section are determined. Comparisons with the results available in the literature are performed either for the limiting case of Boussinesq flow or for the case of forced convection.

The optimization of energy consumption in buildings’ HVAC systems plays a crucial role in reducing greenhouse gas emissions worldwide. In new and deeply-renovated buildings, characterized by modulating terminal units and aiming at the maximum exploitation of renewables, an accurate hydraulic balance of the distribution network may become critical, resulting in a significant increase of pumping energy consumptions and a deterioration of indoor thermal comfort conditions. In the present paper, we compare the performance of traditional manual balancing valves and new pressure independent control valves (PICVs), used to balance the hydronic loop of HVAC systems. To perform this analysis, a new MATLAB-Simulink model has been specifically developed to simulate the behavior of PICVs and has been used in an application case study. The efficiency of manual and pressure independent control valves is evaluated numerically by simulating a multi-zone distribution network under variable operating conditions and considering different control strategies of the circulating pump. Results show that in off-design conditions, when some of the branches of the hydraulic network are closed, traditional manual balancing valves are not able to guarantee the nominal mass flow rate in the remaining loops. On the contrary, no significant variations of water mass flow rate are observed when PICVs are adopted, even in partial load conditions. Despite their additional cost, PICVs allow to ensure better indoor thermal comfort sensations for individuals, avoiding under/over-heating of rooms and, moreover, yield a decrease of the pump electric consumption with respect to traditional manual balancing valves. In addition, based on the obtained results, general rules concerning the adoption of PICVs are provided, depending on the extension of the hydraulic loop and the adopted pump control logic. The reported results highlight the role that PICVs can play to ensure energy savings in HVAC systems without penalizing users’ indoor comfort conditions.

This data article includes an elaboration of carbon dioxide data available from three different online sources in the years from 2016 to 2019. The data article refers to the paper "Interpolating functions for CO2 emission factors in dynamic simulations: the special case of a heat pump" by the same authors. The data are provided on an hourly basis and are useful to determine the carbon dioxide emission of an electric heat pump or other devices connected to the Italian grid. The importance of the provided data is related to the possibility of having an accurate estimation of the CO2 emission when the device works for only a limited period of time during the year or day. Moreover, since the given data are provided in electronic format (.txt file or .xlsx spreadsheet) they are very useful to perform dynamic simulation using self-made or commercial software such as Trnsys, Energy Plus etc.

The paper investigates the seasonal performance of electric air-to-water heat pump comparing an on-off heat pump with an inverter-driven variable speed compressor. Reference is made to a heat pump for domestic space heating installed in Bologna, and the bin-method proposed by UNI/TS 11300-4 is employed in order to investigate the effect of external temperature. The analysis shows that the inverter heat pump performs better than the on-off one, displaying higher values of SCOP about at least 10%.

In the present paper, a concentrated-parameter model is presented, in order to simulate the behavior of a climatic chamber suitably designed for testing tracked and wheeled tractors. Reference is made to standard-severe winter conditions (–18°C), i.e., the tractor dissipates thermal power in a climatic chamber that reproduces winter temperature and humidity conditions. The software package Matlab-Simulink is utilized and the system under investigation consists of 5 blocks. The dynamic modeling first covers a 10 hours period to reach the prescribed conditions for the climatic test chamber. Then, after this preparation period, the tractor’s engine is supposed to work. The simulations show that higher values of the adduction coefficient are preferable since they allow obtaining quickly, and with a lower heating power, the desired comfort conditions for the machine’s cabin.

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.

A simple numerical model, implemented through COMSOL Multiphysics 5.2 (© COMSOL AB), that can be usefully adopted to evaluate the effect of greenery in the pollution reduction in urban canyons, is presented. The model can be easily tailored to account for different plant species and greenery shapes. Some preliminary results obtained with reference to green façades and green hedges show that a good reduction in the pollutant dispersion in the canyon is achievable.

In the last decade, a wide research effort from the heat transfer community has been oriented to nanofluid-saturated porous media. The interest in this subject is mainly motivated by the technological value of an improved design of heat exchangers with high thermal performances and a relatively small size. With this aim in mind, an optimal fluid-to-solid heat exchange is devised through a metal foam, and the usual fluid is replaced by a nanofluid. Hence, when nanofluid-saturated porous media are studied, the porous medium is preferably considered as one with a high thermal conductivity. The argument supporting the use of a nanofluid, instead of an ordinary fluid, is grounded on its enhanced features relative to the high thermal conductivity and the high heat transfer rates. These specific fea-tures, widely studied in the existing literature, are likely to go beyond what one could expect from reasonings based on a mere evaluation of the volu-metric concentration of (metal or carbon) nanoparticles or nanotubes. It is what has been called the “magic” power of nanoparticles by Rao (2010). What Rao calls magic is the not yet convincingly explained anomalous thermal conductivity, heat transfer rates, and viscosity experimentally observed for nanofluids.This chapter is organized as follows: first a short review of the proper-ties and modeling of nanofluids is provided. The momentum and energy transfer in saturated porous media are described. Then, the analysis of the literature on heat and fluid flow in nanofluid-saturated porous media is surveyed, by focusing on three main topics: forced convection, natural or mixed convection, and onset of thermal instability.

In this paper, combined forced and free convection is studied in a vertical rectangular duct with a prescribed uniform wall heat flux (H2 boundary condition). A different heat flux value for each plane wall is considered; the condition of a uniform wall heat flux throughout the duct results as a special case. The local momentum and energy balance equations are written in a dimensionless form and solved numerically, by means of a Galerkin finite element method. The numerical solution gives the dimensionless velocity and temperature distributions, together with the values of the Fanning friction factor, of the Nusselt number, of the momentum flux correction factor and of the kinetic energy correction factor. These dimensionless parameters are reported as functions of the aspect ratio and of the ratio between the Grashof number, Gr, and the Reynolds number, Re. The threshold values of Gr/Re for the onset of flow reversal are evaluated. © 2003 Elsevier Inc. All rights reserved.

Stationary and laminar forced convection in a circular duct is analyzed in the case of a sinusoidal axial change of the wall heat flux such that the modulus of its mean value is either zero or equal to the amplitude. The effect of the axial heat conduction in the fluid is taken into account. Reference is made to the thermally developed region where the temperature distribution can be expressed as the sum of a linear function and a periodic function of the axial coordinate. The temperature field as well as the local and mean Nusselt numbers are evaluated analytically. Comparisons with the solution in the absence of axial heat conduction are performed. (C) 2000 Elsevier Science Ltd. All rights reserved.

In this paper, a numerical study of natural convection in a 2D-enclosure is presented. The enclosure is bounded by two vertical isothermal walls, kept at different temperatures, and by two adiabatic walls which are either straight and horizontal (rectangular cavity) or elliptic (modified rectangular cavity). The dimensionless mass, momentum and energy balance equations are solved by means of two different software packages based on Galerkin finite element methods. An excellent agreement between the solutions is found and provides a cross-validation of the results. Two basic geometries are considered: a square geometry and a rectangular one with height double the width. For each basic geometry, three cavities are investigated: a rectangular cavity, and two modified rectangular cavities. Dry air is considered, with several values of the Rayleigh number. The results show that the elliptic boundaries enhance the mean Nusselt number and the dimensionless mean kinetic energy of the fluid.