Enhancement of heat transfer and entropy generation analysis of nanofluids turbulent convection flow in square section tubes

In this article, developing turbulent forced convection flow of a water-Al2O3 nanofluid in a square tube, subjected to constant and uniform wall heat flux, is numerically investigated. The mixture model is employed to simulate the nanofluid flow and the investigation is accomplished for particles size equal to 38 nm

Second Law Analysis of Al2O3-Water Nanofluid Turbulent Forced Convection in a Circular Cross Section Tube with Constant Wall Temperature:

The present paper proposes an analysis based on the second principle of thermodynamics applied to a water-Al 2 O 3 nanofluid. The nanofluid flows inside a circular section tube subjected to constant wall temperature. The aim of the investigation is to understand, by means of an analytical model, how entropy generation within the tube varies if inlet conditions, particles concentration, and dimensions are changed. To gather these information is of fundamental importance, in order to optimize the nanofluid flow. The results show that according to the inlet condition, there is a substantial variation of the entropy generation, particularly when Reynolds number is kept constant there is an increase of entropy generation, whereas when mass flow rate or velocity are taken constant, entropy generation decreases

Numerical simulation of thermal energy storage with phase change material and aluminum foam

A Latent Heat thermal energy storage system (LHTESS) is employed as a thermal buffer, since it avoids the intermittent supply of thermal energy due to the behaviour of the thermal source, in particular the renewable thermal source like the solar radiation. Therefore a LHTESS allows supplying the thermal energy in continuous way. The principal material of a LHTESS is the phase change material (PCM) given that it storages a high quantity of thermal energy during its phase change process thanks to the high value of latent heat. Moreover the thermal energy is stored at quasi-constant temperature because during the phase change process the heat is engaged to change phase and not to raise the temperature. Please download the full abstract below

Thermal behavior evaluation of ventilated roof under variable solar radiation

A ventilated roof has a good configuration for energy purposes, in order to respect the European Directive priority for building performance requirement to reduce energy consumption. In Mediterranean regions, with high level of solar radiation, the roof design should respect comfort and energy saving, considering that climatic conditions change depending on seasons and territories. This paper illustrates a numerical investigation on a prototypal ventilated roof for residential use, in order to evaluate its thermofluidodynamic behaviors as a function of the solar radiation applied on the top wall of the roof simulating summer and winter conditions. The roof is modeled as a single side and it is analyzed as two-dimensional, in air flow, thanks to the commercial code Ansys-Fluent. Results are given in terms of temperature and pressure distributions, air velocity and temperature profiles along longitudinal and cross sections of the ventilated layer, in order to estimate the differences between the various conditions. Ventilated roof configuration results significant to reach optimal thermal and hygrometric conditions in summer and winter conditions

An experimental investigation on effect of pores per inch in compact heat exchanger with aluminum foam

Metal foams are a new class of materials with low densities and novel thermal and mechanical properties. Aluminum foams combine low weight with good rigidity, strength, damping of vibrations and noise, shock resistance and low thermal conductivity [1]. An experimental investigation on a single row of aluminum tubes, covered with layers of aluminum foams, was carried out by T’joen et al. [2]. A range of foam layer thickness, Reynolds number tube spacing and different type of foam were considered and compared with compact helically finned tube heat exchangers. An experimental investigation was carried out by Sertkaya et al. [3] to compare three metal foam heat exchangers (10, 20 and 30 PPI) to three finned heat exchangers with the same tube layout and overall dimensions. Please download the full abstract below

Numerical study of a confined slot impinging jet with nanofluids

<p>Abstract</p> <p>Background</p> <p>Heat transfer enhancement technology concerns with the aim of developing more efficient systems to satisfy the increasing demands of many applications in the fields of automotive, aerospace, electronic and process industry. A solution for obtaining efficient cooling systems is represented by the use of confined or unconfined impinging jets. Moreover, the possibility of increasing the thermal performances of the working fluids can be taken into account, and the introduction of nanoparticles in a base fluid can be considered.</p> <p>Results</p> <p>In this article, a numerical investigation on confined impinging slot jet working with a mixture of water and Al₂O₃nanoparticles is described. The flow is turbulent and a constant temperature is applied on the impinging. A single-phase model approach has been adopted. Different geometric ratios, particle volume concentrations and Reynolds number have been considered to study the behavior of the system in terms of average and local Nusselt number, convective heat transfer coefficient and required pumping power profiles, temperature fields and stream function contours.</p> <p>Conclusions</p> <p>The dimensionless stream function contours show that the intensity and size of the vortex structures depend on the confining effects, given by <it>H/</it>W ratio, Reynolds number and particle concentrations. Furthermore, for increasing concentrations, nanofluids realize increasing fluid bulk temperature, as a result of the elevated thermal conductivity of mixtures. The local Nusselt number profiles show the highest values at the stagnation point, and the lowest at the end of the heated plate. The average Nusselt number increases for increasing particle concentrations and Reynolds numbers; moreover, the highest values are observed for <it>H/W </it>= 10, and a maximum increase of 18% is detected at a concentration equal to 6%. The required pumping power as well as Reynolds number increases and particle concentrations grow, which is almost 4.8 times greater than the values calculated in the case of base fluid.</p> <p> <b>List of symbols</b> </p

RADIATION EFFECT ON TRANSIENT NATURAL CONVECTION IN VENTILATED ROOFS

This paper illustrates a numerical investigation on a prototypal ventilated roof for residential use, under summer and winter conditions. The roof is modeled as a single flap, due to its geometric and thermal symmetry, and it is analyzed as two-dimensional, in air flow, thanks to the commercial code Ansys-Fluent. The governing equations are given in terms of k-\uf065 turbulence model taking into account the radiation effect inside the channel. The analysis is performed in order to evaluate thermofluidodynamic behaviours of the ventilated roof, in transient regime with radiative heat transfer presence, as a function of the solar radiation applied on the top wall of the ventilated roof. The discrete transfer radiation model (DTRM) is chosen. Typical summer and winter conditions with heat transfer from the channel top wall toward the external ambient are examined. The bottom wall of the ventilated channel is simulated as isothermal, considering optimal temperature values for the internal ambient in summer and winter regimes. Results are given in terms of temperature and pressure distributions, air velocity and temperature profiles along longitudinal and cross sections of the ventilated layer, in order to estimate the differences between the various conditions. Ventilated roof configuration results significant to reach optimal thermal and fluid dynamic conditions in summer and winter regimes. In summer period, when the effect of solar radiation is more evident thanks to the convective effect within the channel, temperature values are higher and the effect of the ventilated channel is significative to reach comfort conditions. In winter, the effect of the ventilated layer is very important to reach optimal thermal and hygrometric conditions

Thermal behavior evaluation of ventilated roof under summer and winter conditions

One of the European Directive priorities is the development of new strategies for \u201cvery low energy buildings\u201d. In regions with high level of solar radiation, ventilation allows the cooling load during summer period and contributes to the reduction of the energy needs of buildings. The most important advantages are the reduction of the heat fluxes transmitted by the structures exposed to solar radiation, thanks to the combined effect of shading surfaces and heat removed by the air flow rate within the ventilated air gap. This paper illustrates a numerical investigation on a prototypal ventilated roof for residential use. The investigation is performed in order to evaluate thermofluidodynamic behaviors of the ventilated roof as a function of the different conditions applied on the top wall and the bottom wall of the ventilated cavity in summer and winter regimes. Different values of heat fluxes are applied on the top wall of the ventilated cavity to simulate typical summer and winter days conditions, whereas the bottom wall is assumed isothermal and different values of wall temperature are considered. The problem is solved by means of the commercial code Ansys-Fluent. Results are given in terms of temperature and velocity distributions, air velocity and temperature profiles along different longitudinal and cross sections of the ventilated layer in order to estimate differences between analyzed conditions

Local Thermal Non-Equilibrium Investigation on Natural Convection in Horizontal Channel Heated from Above and Partially Filled with Aluminum Foam

Abstract The configuration of two horizontal parallel walls, with heated upper plate and open cavities, gets considerable attention in many thermal engineering applications. In this work, a numerical investigation on steady state natural convection in a horizontal channel partially filled with a porous medium and heated at uniform heat flux from above is carried out. The local thermal non-equilibrium (LTNE) hypothesis is invoked. A three-dimensional model is realized and solved by means of the ANSYS-FLUENT code. Results are presented in terms of velocity and temperature fields and profiles, and they show that the use of porous medium improves the heat transfer in the channel due to the aluminum foam high conductivity

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

In this paper an experimental investigation on mixed convection in air in a heated channel partially filled with an aluminum foam is carried out. The aluminum foam layer is sets on the lower heated wall of the channel. The channel upper plate has a heat transfer toward the external ambient. The investigation allows to evaluate the effect of the aluminum 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, form 10 to 300, wall heat flux and for aluminum foam with 10 and 20 pore per inch. Lower wall temperature values are detected for the channel with foam with respect to the channel clean without foam. Foams with higher pore per inch shows lower average wall temperature values whereas the local wall temperatures showed different behaviors for the different pore per inch values. The presence of foam in heated channel determined weaker secondary motions with respect to the clean cases. The effect of the foams seems more significant for the high Reynolds number values and average Nusselt number increases with the foam presence in the heated channel