Bernardo Buonomo

NUMERICAL STUDY OF NATURAL CONVECTION IN VERTICAL CHANNELS WITH ADIABATIC EXTENSIONS DOWNSTREAM

ABSTRACT Recent trends in natural-convection research are the finding of new configurations to improve heat transfer parameters or the analysis of standard configurations to determine optimal geometric parameters in order to achieve a better heat transfer rate. In this article a numerical simulation of natural convection in air in a channel–chimney system heated symmetrically at uniform heat flux is carried out. The regime analyzed is two-dimensional, laminar, and steady-state. Wall temperature profiles, air velocity and temperature profiles, and centerline pressure profiles are presented. A comparison with experimental data given in [4] is accomplished and some differences are observed, but thermal and dynamic behavior for different expansion ratios are the same. Results analysis explains how and why the “chimney effect” worsens. The effect is connected to the cold inflow at the outlet section, and this effect is more marked at higher Rayleigh number, Ra. Consequently, optimal thermal configuration for assigned extension ratio presents a larger value of expansion ratio at the lower Ra. values.

Transient Natural Convection in Vertical Channels Symmetrically Heated at Uniform Heat Flux

A numerical transient analysis of natural convection in air between two vertical parallel plates, heated at uniform heat flux, is carried out. The problem is two-dimensional and laminar and the full Navier-Stokes and energy equations are employed. The control volume method is used to discretize the equations on a uniform grid. Results are given at different aspect ratio values and Rayleigh number values. The simulation allows detection of complex structures of the flow inside and outside the channel. Temperature profiles as a function of time show an overshoot and undershoot increase at the lowest aspect ratio and highest Rayleigh number. Inside the channel conductive and convective regimes as well as an inverse fluid motion are observed. Transient average Nusselt number presents oscillations before the steady-state.

Thermal and fluid dynamic behaviors in symmetrical heated channel‐chimney systems

Purpose – The purpose of this paper is to evaluate the thermal and fluid dynamic behaviors of natural convection in a vertical channel‐chimney system heated symmetrically at uniform heat flux in order to detect the different fluid motion structures inside the chimney, such as the cold inflow from the outlet section of the chimney and the reattachment due to the hot jet from the channel, for different extension and expansion ratios of the adiabatic extensions.Design/methodology/approach – The model is constituted by two‐dimensional steady‐state fully elliptic conservation equations which are solved numerically in a composite three‐part computational domain by means of the finite‐volume method.Findings – Stream function and temperature fields in the system are presented in order to detect the different fluid motion structures inside the chimney, for different extension and expansion ratios of the adiabatic extensions. The analysis allows to evaluate the effect of the channel aspect ratio on the thermal and ...

Numerical investigation on sensible thermal energy storage with porous media for high temperature solar systems

In this paper different high temperature TES components are numerically analyzed. The difference is defined by the different type of porous medium employed in the storage. Two different porous media are considered: spheres or foams. In all cases a ceramic material is considered. In the formulation of the model it is assumed that the system geometry is cylindrical, the fluid and the solid thermophysical properties are temperature independents, the radiation heat transfer mechanism is taken into account. The commercial CFD Fluent code is used to solve the governing equations in transient regime and in local thermal non-equilibrium (LTNE). Numerical simulations are carried out at different mass flow rates of the heat-carrying fluid. The results show the effects of the porosity and of the working fluid mass flow rate on the stored thermal energy and on the storage time.

Numerical investigation of convective–radiative heat transfer in a building-integrated solar chimney

Solar chimney can be a new technology to generate electrical power. The chimney consists of a converging channel with one vertical wall and one inclined wall. The analysis is carried out on a three-dimensional turbulent model by the commercial code Ansys-Fluent. The numerical analysis is intended to examine the effect of the glass wall inclination. Results are given in terms of wall temperature distributions, air velocity and temperature fields for a uniform wall heat flux on the vertical wall equal to 300 W/m2. Some numerical simulations are performed by considering solar irradiation in Naples (Italy). Thermal and fluid dynamic behaviours are evaluated in order to have some indications to improve the energy efficiency of the system. The better thermal performances for the same imposed heat flux are in correspondence with an inclination of 2°, whereas when solar irradiation is considered, better performances are obtained at the greatest investigated inclination angle. Solar chimney is also convenient as regards additional heat gains into the building in winter. A heat gain in the range 8–12% of solar radiation entering the chimney for a thermal conductance equal to 2.0 W/m2 K can be estimated.

Enhancement of Heat Transfer in Partially Heated Vertical Channel Under Mixed Convection by Using Al2O3 Nanoparticles

ABSTRACT Laminar mixed convection in a two-dimensional symmetrically and partially heated vertical channel is investigated. The heaters are located on both walls and uniform temperature is applied on the heated sections. The number of heaters is considered as 1, 4, 8, and 10. Aluminum oxide/water nanofluid is considered as working fluid and the inlet velocity is uniform. The continuity, momentum and energy equations with appropriate boundary conditions are solved in dimensionless form, numerically. The study is performed for Richardson number of 0.01 and 10, Reynolds number of 100 and 500, and nanofluid volume fraction of 0% and 5%. Based on the obtained velocity and temperature distributions, the local and mean Nusselt number is calculated and plotted for different cases. The variation of the mean Nusselt number with the number of the heated portions is also discussed. It is found that the addition of nanoparticles into the base fluid increases mean Nusselt number but the rate of increase depends on Reynolds, Richardson numbers and number of heated portions. It is possible to increase mean Nusselt number 138% by increasing Reynolds number from 100 to 500, Richardson number from 0.01 to 10 and number of heated portions from 1 to 10 when volume fraction value is 5%.

Nano-PCMs for passive electronic cooling applications

The present work aims at investigating a new challenging use of oxide (TiO2, Al2O3, etc.) nanoparticles to enhance the thermal properties: thermal conductivity, specific heat, and latent heat of pure paraffin waxes to obtain a new class of Phase Change Materials (PCMs), the so-called nano-PCMs. The nano-PCMs were obtained by seeding different amounts of oxide nanoparticles in a paraffin wax having a melting temperature of 45°C. The thermophysical properties such as latent heat and thermal conductivity were then measured to understand the effects of the nanoparticles on the thermal properties of both the solid and liquid PCM. Finally, a numerical comparison between the use of the pure paraffin wax and the nano-PCM in a typical electronics passive cooling device was implemented. Numerical simulations were carried out using the Ansys-Fluent 15.0 code. Results in terms of solid and liquid phase temperatures, melting time and junction temperature were reported. Moreover, a comparison with experimental results was also performed.

A comparison of nanofluid thermal conductivity measurements by flash and hot disk techniques

The conversion into nanofluids is considered a suitable solution to increase the heat transfer efficiency of such fluids. Several theories with an emphasis on different thermal nanofluid mechanisms have appeared to predict enhanced conductivity measurements. There are many ways to measure the thermal conductivity of fluids. Some researchers argued that the anomalous k enhancement data are caused by inaccuracies of thermal measurement methods. In this paper, measurements on thermal conductivities of nanofluid mixtures (alumina/water) by means of two different methods are accomplished, i.e. the flash and the hot disk technique. In the first method, a NETZSCH model LFA 447 NanoFlash is employed, while in the second one a Hot Disk model TPS 2500 S is used. A comparison between the results obtained from the different measurement techniques is done. Two-step method is used to prepare nanofluids with a nanoparticles volumetric concentration from 0.1% to 4%. Each mixture, at assigned volumetric concentration, is treated with a sonicator for different times and thermal conductivity is measured in the range of temperature from 20°C to 50°C. Moreover, for assigned volumetric concentration and sonication, the stability analysis is performed and thermal conductivity measurements are carried out to determine the effect of sonication time. Results show the thermal conductivity dependence on sonication time, and an asymptotic value is evaluated for each volumetric concentration.

Numerical Simulation of Convective-Radiative Heat Transfer in a Solar Chimney

Solar chimney is a new method to produce electrical power. It employs solar radiation to raise the temperature of the air and the buoyancy of warm air to accelerate the air stream flowing through the system. By converting thermal energy into the kinetic energy of air movement, solar chimneys have a number of different applications such as ventilation, passive solar heating and cooling of buildings, solar-energy drying, and power generation. Moreover, it can be employed as an energy conversion system from solar to mechanical. A component, such as a turbine or piezoelectric component, set in the path of the air current, converts the kinetic energy of the flowing air into electricity.In this paper, a numerical investigation on a prototypal solar chimney system integrated in a south facade of a building is presented. The chimney is 4.0 m high, 1.5 m wide whereas the thickness is 0.20 m for the vertical parallel walls configuration and at the inlet 0.34 m and at the outlet 0.20 m for convergent configuration. The chimney consists of a converging channel with one vertical wall and one inclined of 2°. The analysis is carried out on a three-dimensional model in airflow and the governing equations are given in terms of k-e turbulence model. The problem is solved by means of the commercial code Ansys-Fluent. The numerical analysis was intended to examine the effect of the solar chimney’s height and spacing. Further, comparison between radiative and non-radiative model is examined and discussed. Results are given in terms of wall temperature distributions, air velocity and temperature fields and transversal profiles for a uniform wall heat flux on the vertical wall equal to 300 W/m2. Thermal and fluid dynamics behaviors are evaluated in order to have some indications to improve the energy efficiency of the system.Copyright

Mixed convection in horizontal channels partially filled with aluminium foam heated from below and with external heat losses on upper plate

In this work mixed convection in a horizontal channel partially filled with a porous medium and the lower wall heated at uniform heat flux is studied both experimentally and numerically. The experimental test section is made of a horizontal wall and a parallel wall with heat losses toward the external ambient. A simplified two dimensional numerical model is considered to evaluate the dynamic and thermal fields inside the heated channel. The investigation allows to evaluate the effect of the aluminium foam on the mixed convection in the heated channel by wall temperature measurements and flow visualization. Results are given for heated channel without and with foam in terms of wall temperature profiles for different Reynolds number value, from 100 to 300, wall heat flux and for aluminium foam with 10 and 20 pore per inch. Experimental and numerical results are given in terms of wall temperature profiles and both allow to show that the foam determines lower wall temperature profiles along the fluid flow than the ones in the clean channel cases. Moreover, the foams with different pores per inch present different local thermal behaviours. The simplified numerical model allows to estimate the possible heat losses toward the external ambient.