Sergio Nardini

EFFECT OF HEATED WALL POSITION ON MIXED CONVECTION IN A CHANNEL WITH AN OPEN CAVITY

Mixed convection in an open cavity with a heated wall bounded by a horizontally insulated plate is studied numerically. Three basic heating modes are considered: (a) the heated wall is on the inflow side (assisting flow); (b) the heated wall is on the outflow side (opposing flow); and (c) the heated wall is the horizontal surface of the cavity (heating from below). Mixed convection fluid flow and heat transfer within the cavity is governed by the buoyancy parameter, Richardson number (Ri), and Reynolds number (Re). The results are reported in terms of streamlines, isotherms, wall temperature, and the velocity profiles in the cavity for Ri=0.1 and 100, Re=100 and 1000, and the ratio between the channel and cavity heights (H/D) is in the range 0.1-1.5. The present results show that the maximum temperature values decrease as the Reynolds and the Richardson numbers increase. The effect of the H/D ratio is found to play a significant role on streamline and isotherm patterns for differentheating configurations. The present investigation shows that the opposing forced flow configuration has the highest thermal performance in terms of both maximum temperature and average Nusselt number.

Advanced approaches of modeling and measurement for turbulence and heat transfer

Turbulent flow and heat transfer are key issues in nature and numerous engineering applications such as human body, fluid machinery, machining facilities, and refrigeration systems. Insight into relevant transient and steady-state thermo-fluidic physics in such scenarios acts as constructive guidance for both engineers and scientists to improve the performance and reliability of facilities, analyze engineering failures, and understand complicated natural phenomena. Unfortunately, such complex fluid flow and heat transfer problems can rarely be solved analytically due to the strong nonlinearity of the Navier–Stokes equations and the complex nature of turbulence with heat transfer. Instead, advanced modeling and measurement techniques become compensatory yet powerful tools and have been widely used. Especially in recent years, a large number of advanced analytical, computational, and experimental techniques have been developed, which greatly contribute to the exploration of turbulence and heat transfer mechanisms. The main objective of this Special Issue is to bring important information on advanced modeling and measurement techniques together. Their feasibility and performance in investigating various engineering problems are evaluated. In this Special Issue, five original research papers were accepted for publication based on critical peer review by qualified reviewers. We hope that such a frontier of turbulence and heat transfer could be continued to track the updated trend year by year. An introductory review of the accepted papers is presented here. In the paper entitled ‘‘The impact research of control modes in steam turbine control system (digital electric hydraulic) to the low frequency oscillation of grid,’’ theoretical models for frequency domain analysis were developed to investigate the effects of steam turbine control modes on low-frequency oscillation of grid. The effectiveness of such theoretical analysis was well validated by simulation using the control system’s toolbox in MATLAB. In the paper entitled ‘‘Research on the aerodynamic characteristics of a lift drag hybrid vertical axis wind turbine,’’ the effects of various parameters on unsteady aerodynamic and starting performances of a newly designed lift drag hybrid vertical axis wind turbine were numerically investigated. The performances of various turbulence models were evaluated based on experimental data. Fruitful guidance for engineering design was also obtained. In the paper entitled ‘‘Pressure fluctuation prediction in pump mode using large eddy simulation and unsteady Reynolds-averaged Navier–Stokes in a pump-turbine,’’ both large eddy simulation and unsteady Reynolds-averaged Navier– Stokes equations in conjunction with a two-equation turbulence model were adopted to predict pressure fluctuation in pump mode of a pump-turbine. By comparisons between experimental and numerical results, the performances of these two different modeling methods were evaluated. In the paper entitled ‘‘Temperature field measurement of spindle ball bearing under radial force based on fiber Bragg grating sensors,’’ fiber Bragg grating temperature sensors were proven to be an effective method in the measurement of temperature distribution in outer ring of a spindle ball bearing. Such a temperature field is physically beneficial for the reduction of spindle thermal error. Finally, in the paper entitled ‘‘Experimental investigation of flow boiling heat transfer and pressure drops characteristic of R1234ze(E), R600a, and a mixture of R1234ze(E)/R32 in a horizontal smooth tube,’’ the effects of mass flux, heat flux, and quality of several refrigerants on flow boiling and pressure drop characteristics in a horizontal smooth tube was experimentally investigated. The corresponding experimental methods and findings from this study are useful for the design of evaporators.

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.An entropy generation analysis is also proposed in order to find the optimal working condition for the given geometry under given boundary conditions. A simple analytical procedure is proposed to evaluate the entropy generation and its results are compared with the numerical calculations, showing a very good agreement.A comparison of the resulting Nusselt numbers with experimental correlations available in literature is accomplished. To minimize entropy generation, the optimal Reynolds number is determined.

Numerical study of a confined slot impinging jet with nanofluids

AbstractBackgroundHeat 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.ResultsIn this article, a numerical investigation on confined impinging slot jet working with a mixture of water and Al2O3 nanoparticles 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.ConclusionsThe dimensionless stream function contours show that the intensity and size of the vortex structures depend on the confining effects, given by H/ 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 H/W = 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. List of symbols

Experimental investigation of mixed convection in a channel with an open cavity

Mixed convection in an open cavity with a heated wall bounded by a horizontally unheated plate is investigated experimentally. The cavity has the heated wall on the inflow side. Mixed convection fluid flow and heat transfer within the cavity is governed by the buoyancy parameter, Richardson number (Ri), and Reynolds number (Re). The results are reported in terms of wall temperature profiles of the heated wall and flow visualization for Re = 100 and 1000, Ri in the range 30–110 (for Re = 1000) and 2800–8700 (for Re = 100), the ratio of the length to the height of cavity (L/D) is in the range 0.5–1.5, and the ratio of the channel height to cavity height (H/D) is in the range of 0.5 and 1.0. The present results show that the maximum dimensional temperature rise values decrease as the Reynolds and the Richardson numbers decrease. The flow visualization points out that for Re = 1000 there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100 the effect of a stronger buoyancy determines a penetration of thermal plume from the heated plate wall into the upper channel. Nusselt numbers increase when L/D increase in the considered range of Richardson numbers.

Numerical Simulation of Water/Al2O3 Nanofluid Turbulent Convection

Turbulent forced convection flow of a water-Al2O3 nanofluid in a circular tube subjected to a constant and uniform temperature at the wall is numerically analyzed. The two-phase mixture model is employed to simulate the nanofluid convection, taking into account appropriate thermophysical properties. Particles are assumed spherical with a diameter equal to 38 nm. It is found that convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement is increasing with the particle volume concentration and Reynolds number. Comparisons with correlations present in the literature are accomplished and a very good agreement is found with Pak and Cho (1998). As for the friction factor, it shows a good agreement with the classical correlation used for normal fluid, such as Blasius formula.

Linear Regression Models to Forecast Electricity Consumption in Italy

Abstract The influence of economic and demographic variables on the annual electricity consumption in Italy has been investigated in order to develop a simple and data light electricity consumption forecasting model, to be used as part of more complex planning tools. The time period considered for the historical data is from 1970–2007. Multiple and single regression models are developed using historical electricity consumption, gross domestic product (GDP), GDP per capita, and population. Annual electricity consumption was strongly related to the selected variables, with adjusted regression coefficients, adj. R2, equal to 0.990 for residential consumption, 0.961 for non-residential consumption, and 0.981 for total consumption. Comparisons with national forecasts showed that the developed regressions are congruent with the official projections, with ±5% error considered acceptable in relation to the considered time span.

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-Al2O3 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 investigation of air forced convection in channels with differently shaped transverse ribs

Purpose – The purpose of this paper is to investigate the flow and the heat transfer characteristics of a two‐dimensional rib‐roughned rectangular duct with the two principal walls subjected to uniform heat flux. In particular, the main goal is to generate friction and heat transfer data, for different values of p/e with square, rectangular, trapezoidal and triangular shape ribs for Reynolds numbers in the range between 20,000 and 60,000 and different heights and to describe the temperature and fluid‐dynamic fields around the ribs.Design/methodology/approach – The model is constituted by a two‐dimensional duct. On the duct wall square, rectangular, triangular and trapezoidal ribs are introduced by changing different geometry ratios. Governing equations are solved numerically by means of the finite‐volume method.Findings – Simulations show that maximum Nusselt numbers are detected in correspondence with dimensionless pitch equal to 12 and 10 for the square, trapezoidal and rectangular ribs, and triangular ...

Numerical Analysis of Water Forced Convection in Channels with Differently Shaped Transverse Ribs

Heat transfer enhancement technology has the aim of developing more efficient systems as demanded in many applications. An available passive method is represented by the employ of rough surfaces. Transversal turbulators enhance the heat transfer rate by reducing the thermal resistance near surfaces, because of the improved local turbulence; on the other hand, higher losses are expected. In this paper, a numerical investigation is carried out on turbulent water forced convection in a ribbed channel. Its external walls are heated by a constant heat flux. Several arrangements of ribs in terms of height, width, and shape are analyzed. The aim is to find the optimal configuration in terms of high heat transfer coefficients and low losses. The maximum average Nusselt numbers are evaluated for dimensionless pitches of 6, 8, and 10 according to the shape while the maximum friction factors are in the range of pitches from 8 to 10.