Showing papers in "International Journal of Heat and Technology in 2015"

The local media radiant temperature for the calculation of comfort in areas characterized by radiant surfaces

Abstract: Thermo-hygrometric indoor conditions, as well known in literature, depend on the indices directly measurable in the place (Tba, Tbu, φa, va) and analytical solutions (Tmr, Top) and also on subjective assessments, metabolic activity carried out, and thermal resistance of clothing. They are tied to the subject occupied position within the work environment, substantially. In air conditioned rooms designed by systems with average parameters, it may happen that an individual, in some places or times, may feel sensations of thermal discomfort.These phenomena can be exacerbated by heat exchanges occurring in the presence of sources of radiation, such as radiant floors or ceilings or large glass surfaces sunny; these surfaces accentuate the radiative heat exchange with the body surfaces of the exposed individual. This paper proposes a \"local mean radiant temperature\" valid for the regular calculation of comfort in environments characterized by the presence of radiating surfaces. The present study compares the values of the parameters representing the hygrothermal comfort punctual, PMV, in environments characterized by radiating surfaces located in specific positions (ceiling, floor or walls), calculated with the local radiant temperature suggested in this study, with the corresponding values obtained with the mean radiant temperature proposed by the literature.

Thermal performance of nanofluid filled solar flat plate collector

Abstract: The flat-plate solar collector is commonly used today for the collection of low temperature solar thermal energy. It is used for solar water-heating systems in homes and solar space heating. Because of the desirable environmental and safety aspects it is widely believed that solar energy should be utilized instead of other alternative energy forms, even when the costs involved are slightly higher. Solar collectors are key elements in many applications, such as building heating systems, solar drying devices, etc. Solar energy has the greatest potential of all the sources of renewable energy especially when other sources in the country have depleted. The fluids with solid-sized nanoparticles suspended in them are called “nanofluids.” Applications of nanoparticles in thermal field are to enhance heat transfer from solar collectors to storage tanks, to improve efficiency of coolants in transformers. Lund [1] analyzed general thermal behavior of parallelflow flat-plate solar collector absorbers. Nag et al. [2] analyzed parametric study of parallel flow flat plate solar collector using finite element method. Piao et al. [3] studied forced convective heat transfer in cross-corrugated solar air heaters. Kolb et al. [4] experimentally studied solar air collector with metal matrix absorber. Tripanagnostopoulos et al. [5] investigated solar collectors with colored absorbers. Kazeminejad [6] numerically analyzed two dimensional parallel flow flat-plate solar collectors. Temperature distribution over the absorber plate of a parallel flow flatplate solar collector was analyzed with oneand twodimensional steady-state conduction equations with heat generations. Lambert et al. [7] conducted Enhanced heat transfer using oscillatory flows in solar collectors. They proposed the use of oscillatory laminar flows to enhance the transfer of heat from solar collectors. The idea was to explore the possibility of transferring the heat collected from a solar device to a storage tank by means of a zeromean oscillating fluid contained in a tube. Selected nanofluids might improve the efficiency of direct absorption solar thermal collectors. To determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known. That is, their absorption of the solar spectrum must be established. Struckmann [8] analyzed flat-plate solar collector where efforts had been made to combine a number of the most important factors into a single equation and thus formulate a mathematical model which would describe the thermal performance of the collector in a computationally efficient manner. Azad [9] investigated interconnected heat pipe solar collector. Performance of a prototype of the heat pipe solar collector was experimentally examined and the results were compared with those obtained through theoretical analysis. Tyagi et al. [10] investigated Predicted efficiency of a low-temperature nanofluidbased direct absorption solar collector. It was observed that the presence of nanoparticles increased the absorption of incident radiation by more than nine times over that of pure water. According to the results obtained from this study, under ABSTRACT

3d numerical analysis on the effect of rounding off edge radius on thermal separation inside a vortex tube

Abstract: Vortex-chamber is a main part of vortex tube which the pressured gas is injected into this part tangentially. An appropriate design of vortex-chamber geometry leads to better efficiency and good vortex tube performance. In this study, the computational fluid dynamics (CFD) model is created on basis of an experimental model and is a three-dimensional (3D) steady compressible model that utilizes the k-ε turbulent model. In this paper the effect of changing radius of rounding off edge at hot tube entrance (r1) on vortex tube performance has been studied for different value of r1 and the optimized radius has been determined. According to numerical results the cold temperature difference has increased when we take into account the effect of the radius of rounding off edge in the range of 0-1.5 mm and when the radius of rounding off edge has located in the range of 1.5-4 mm, the cold temperature difference has decreased. The highest ΔTc is 47.26 K for r1=1.5 mm at a cold mass fraction of 0.3, higher than basic model around 7.5% at the same cold flow fraction. Finally, the results obtained, particularly the temperature values, are compared with some available experimental data, which show good agreement.

Evaluation on the convenience of a citizen service district heating for residential use. A new scenario introduced by high efficiency energy systems

Abstract: The target of the study is to assess, in the light of today’s energy production technologies with high efficiency, the actual energetic advantage, in terms of environmental and economic sustainability, of traditional district heating systems for residential use. It was examined a real case study consisting of a residential complex in Milan served by the district heating system of the city. The study was conducted by comparing costs for heating and hot water supplied by district heating with those of alternative potential systems for energy production present on the market today. Two proposals for the replacement of the district heating system with high efficiency heat generators or with heat pumps were evaluated .Therefore, was carried out an evaluation of technical and economic feasibility with particular reference to the payback period, considering interruption of district heating services currently ongoing. (Presented at the AIGE Conference 2015)

Study on svm temperature compensation of liquid ammonia volumetric flowmeter based on variable weight pso

Abstract: When the quality of liquid ammonia is measured by volumetric flowmeter, the traditional quadratic expression method can't meet the accuracy of temperature compensation in modern coal chemical industry. So the temperature compensation method by support vector machine (SVM) regression is presented, and kernel function parameter σ of SVM is optimized by variable weight particle swarm optimization (PSO). After the performance analysis and comparison in PSO, the suitable linear inertia weight method is selected. Experimental results show that the temperature compensation accuracy of the SVM method based on the variable weight PSO is significantly higher than that of the traditional quadratic expression method.

Numerical simulation of gas-particles two phase flow in pipe of complex geometry: pneumatic conveying of olive cake particles toward a dust burner

Abstract: Two phase CFD calculations, using Eulerian-Lagrangian model with commercial Fluent 6.3 were employed to predict the gas and particle flow in pipe of complex geometry designed for pneumatic conveying of olive cake particles toward a pulverized burner. The numerical calculations were validated against experimental data from the literature. The effects of gas velocity and particles size distribution on the mixture flow behavior were studied. The present results help to understand the phenomena occurring in gas-solid flow and optimizing the conveying system.

Unsteady hydromagnetic forced convective heat transfer flow of a micropolar fluid along a porous wedge with convective surface boundary condition

Abstract: In this paper we analyze unsteady two dimensional hydromagnetic forced convective heat transfer flow of a viscous incompressible micropolar fluid along a permeable wedge with convective surface boundary condition. The potential flow velocity has been taken as a function of the distance x and time t. The governing time dependent non-linear partial differential equations have been reduced to a set of non-linear ordinary differential equations by introducing a new class of similarity transformations. Comparisons with previously published work are performed, and the results are found to be in excellent agreement. The resulting local similarity equations for unsteady flow have been solved numerically by applying NachtsheimSwigert shooting iteration technique along with sixth order Runge-Kutta integration scheme.Numerical results in the form of nondimensional velocity, microrotation and temperature profiles are presented graphically and discussed for different material parameters entering into the analysis. The effects of the pertinent parameters on the local skinfriction coefficient, plate couplestress and the rate of heat transfer are also displayed in tabulated form and discussed them from the physical point of view. The obtained numerical results show that the rate of heat transfer increases with the increase of the unsteadiness parameter and decreases with the increase of the surface convection parameter.

Long-term continuous in-situ monitoring of tunnel lining surface temperature in cold region and its application

Abstract: In-situ monitoring of tunnel lining surface temperature is the basis for deciding the measures, distribution and parameters of freezing-proof. To overcome the deficiencies in timing frequency of the previous monitoring, an automatic recorder is invented to adapt to low temperature environment and record the lining surface temperature of Hongfu tunnel every hour for nearly two years. With the measured data, main results are obtained as follow: annual temperature changes nearly in a sine form; influenced by the air flow inside the tunnel, temperature changes in a similar way at every monitored section inside. Before tunnel breakthrough, the further the section is from the two ends of the tunnel, the higher the average temperature is in the coldest month. And the average temperature of the coldest month is above 0°C when the distance is over 200m. After tunnel breakthrough, the temperature is below 0°C inside the tunnel. The measured data is taken as the boundary condition to simulate the temperature of tunnel lining and surrounding rock with finite element method. The simulation results show that: tunnel lining may freeze and thaw many times in a year; the temperature of tunnel lining and surrounding rock close to lining is below 0°C in the coldest month, and heat transfers into lining from surrounding rock and its maximum heat flux is 36.08w/m2 on the lining surface. Taking the fitted data by sine function as boundary condition will make some error in tunnel lining temperature.

Evaluation of the performance of a small biomass gasifier and micro-chp plant for agro-industrial firms

Abstract: Biomass is an alternative source to fossil fuels as proven by the increase of the primary energy production of 6.1% between 2012 and 2013. In European countries, the biomasses are 3.5% of renewable energy sources and they have a strategic role to reach the objective established by the European Directive 2009/28/CE. However, it is necessary to increase the use of renewable fuels, as syngas and biogas, to satisfy the objectives of the UE in the field of renewable energies and greenhouse gas emissions. The gasification and pyrolysis processes to produce syngas are promising technologies for a practical use of the biomass. In this context, gasification of biomass from agro-forestry activities is considered among the most effective sustainable technologies to recover waste that otherwise would be difficult to dispose. The micro-CHP electrical and thermal power were calculated through a balance of mass and energy based on an input-output model of the overall system. The results of this study demonstrated that it is possible to produce thermal and electrical energy for small agro-industries through the valorization of the residuals biomass deriving by their activities. (Presented at the AIGE Conference 2015)

Cfd simulation of air-steam condensation on an isothermal vertical plate

Abstract: This article presents a numerical simulation of the condensation of water vapor with air as the non-condensable gas on an isothermal vertical wall and for inlet mass fraction of the air from 0.05 to 0.5 using the Volume of Fluid (VOF)model of Fluent 6.3 software. The condensation model was used for two-phase, two-dimensional and compressible VOF model and it is suitable for the condensation in presence of additional non-condensable gases on an isothermal vertical plate. All the thermal properties of the water vapor and air were assumed to be functions of temperature and were calculated in userdefined functions (UDF). The source terms including mass source term, energy source term and species source term were also calculated using UDF and hooked to their corresponding conservation equations. The results from the condensation simulation are compared with the experimental results of Dehbi, Uchida and Tagamia from the literature. The condensation simulation results match closely with Dehbi’s experimental correlation, and differ with the curve of Dehbi’s 6%, which show that the simulation model is reliable. The condensation simulation results show that, the surface normal velocity has a prominent contribution in enhancing the convection heat transfer process, the wall shape should be changed to increase the normal velocity component. The results also show that the change of the mass fraction of the non-condensable gas directly influence the thickness of the condensate film, and then affect the local heat transfer coefficient.

Dynamic modeling and simulation of buildings energy performance based on different climatic conditions

Abstract: The aim of this work is the analysis, under dynamic conditions, of the energy performance of buildings based on different climatic conditions. Two school buildings, Liceo Classico “E. Duni” and Liceo Scientifico “D. Alighieri”, located in Matera, Italy, are considered. Furthermore, a strategy to improve the energy performance of the two school buildings is proposed by the installation of a co-trigeneration plant integrated with a solar plant. Such a plant is equipped with an absorption chiller to produce chilled fluid. The analysis under dynamic conditions has been performed by using a well-known simulation software, TRNSYS 17, and the results have been compared with those obtained under stationary conditions by employing a numerical solver, MC-11300, which is certified by the Italian Thermotechnical Committee. At first, the results obtained by considering the dynamic and stationary states and the experimental data measured in situ are compared by considering the actual buildings plants. Then, the energy performance of the two buildings is computed by considering three different climatic zones of Italy. Finally, a discussion of the advantages of the proposed requalification solution, which employs the trigeneration plant, is given. (Presented at the AIGE Conference 2015)

Analysis of mechanical system ventilation performance in an atrium by consolidated model of fire and smoke transport simulation

Abstract: Atrium is popular element in mercantile buildings, covered shopping mall, airport terminals, and sport arenas. In the event of a fire, these buildings overwhelmingly necessitate the use of a mechanical ventilation system to provide conditions to achieve safe escape route for the building occupants. This manuscript deals with mechanical ventilation system, manual method for the hydraulic calculations and fire design scenarios. This research shows an overview of the Consolidated Model of Fire and Smoke Transport (CFAST) modeling that using to predict ventilation performance and smoke movement in an atrium. CFAST software has many uses in a wide variety of buildings and fire scenarios due to fast, reliable and affective accuracy of output data. In addition CFAST approaches provide a link between outside building weather conditions and fire and smoke development. This paper demonstrated the complete design procedure as an example to fire safety engineer. Key

Constructal design applied to the study of the geometry and submergence of an oscillating water column

Abstract: The wave energy conversion into electricity has been increasingly studied in the last years. There are several converters, among them the Oscillating Water Column (OWC) device. Constructal Design and a computational modeling were applied to a geometric optimization of an Oscillating Water Column Wave Energy Converter, device that transforms the energy of incident waves into electrical energy. The aim is to convert maximum electrical power varying and analyzing the influence of the three degrees of freedom (DoFs): H1/L (ratio between the height and length of OWC chamber), H2/l (ratio between height and length of chimney), and H3 (submergence, which are related to the chamber and the chimney of the device, and the location in water depth respectively. Besides there are two constraints (fixed parameters): total area of the OWC chamber (A1) and total area of OWC device (A2). The computational domain consists of an OWC inserted in a tank where regular waves in a real scale are generated. The mesh was developed in ANSYS ICEM. The computational fluid dynamics code ANSYS FLUENT was used to find the numerical solution which is based on Finite Volume Method (FVM). The multiphasic Volume of Fluid (VOF) model was applied to tackle with the water-air interaction. The results led to a theoretical recommendation about the OWC geometry and its submergence which maximizes the device performance, since a redistribution of the OWC geometry and a variation in the value of its submergence could improve the hydropneumatic power from 10.7 W to 190.8 W for ratios H1/L, H2/l and H3 equal 0.135, 6.0 and 9.5 m respectively, and incident waves characterized by a period of 5 s and wave length of 37.6 m.

Cfd approach for modeling high and low combustion in a natural draft residential wood log stove

Abstract: In this work, a comprehensive CFD model was developed for pyrolysis to provide significant emission reductions. In wood stoves with natural draft, the airflow is driven by buoyancy to overcome the resistance to flow within the stove. This combustion model is tested to reproduce at best the physical and chemical phenomena taking place in a natural draft wood log stove for low power (18 kW) residential heating. The RANS approach was used to solve the aerothermodynamic equations. A body fitted hexahedron mesh was adopted and the k-ε turbulence model was used to ensure closure of the Navier-Stokes equations. The chemical reaction for combustion was modeled using species transport and rate of species production formulated as the EDDY DISSIPATION CONCEPT. The key parameters for validation are based on temperature inside the combustion chamber at the center plane and the [CO]/[CO2] ratio for emissions at the exit plane. Therefore, the developed modelling approach can be used for engineering analysis and optimization of existing stoves and for relatively-quick evaluations of new stove designs.

Dimensionless analysis of the temperature field of surrounding rock in coalface with a finite volume method

Abstract: The thermal conductive mathematical model of surrounding rock was established in a moving coordinate in order to determine the temperature field of surrounding rock around coalface with a moving boundary. It was made possible to have a non-dimensional mathematical model by introducing dimensionless parameters such as Biot coefficient and Peclet coefficient. The dimensionless mathematical model was dispersed with a finite volume method, and then a computer code was developed for determining the temperature field of surrounding rock in coalface. Under the condition where the ratio of face width and semi mining height i s 8/3 for example, the dimensionless temperature distribution of surrounding rock is obtained by assuming both the Biot coefficient and Peclet coefficient to be parallel. When the Biot coefficient and Peclet coefficient are assumed to be variants, the results r eveal that the temperature increases with the increase of Peclet coefficient at a constant Biot coefficient, while the temperature decreases with the increase of Biot coefficient at a constant Peclet coefficient. Five kinds of working face are chosen for calculation with typical ratio of face width and semi mining height, and each variation curves of unstable heat transfer criterion with increasing Peclet coefficient are obtained with different values of the Biot coefficient, which reveals that the unstable heat transfer criterion increases with the increase of Biot coefficient and Peclet coefficient. The complex calculation of heat dissipating capacity is greatly simplified and the results are given universal significance. The unstable heat transfer criterion can be determined by a consulting graph, and this method provides critical parameters for forecasting air temperature and calculating chilling requirements in coalface.

Thermo-fluid dynamics analysis and air quality for different ventilation patterns in an operating theatre

Abstract: At present there is a great deal of research on ventilation system efficiency for providing the most effective airflow pattern for contaminant removal (microorganisms i.e. fuWngi, bacteria, viruses; chemical i.e. waste anesthetic gases, CO2 and particulate matter) and assuring the control of surgical site infection risk. Computational fluid dynamics (CFD) modelling and simulation, compared with experimental monitoring results and literature evidences, are widely applied for providing useful indications on proper indoor microclimate conditions and IAQ [1]. Usually, threedimensional time-dependent CFD models are developed to assess the airflow patterns, air velocity and temperature distribution due to laminar unidirectional (vertical downward) air flows [2], in other cases to turbulent air flows and also to the air curtains so as to achieve IAQ and optimal air asepsis [3]. Some authors have used CFD for a three dimensional analysis of thermal comfort and contaminant removal inside hospitals: checking model validity by experimental literature evidence and data comparison, they calculated the predicted mean vote (PMV), contaminant removal effectiveness (CRE) and mean contaminant concentration in the breathing zone by transient simulation results, in order to assess HVAC system efficiency. They investigated the effect of horizontal location of supply and exhaust grilles also for maintaining adequate comfort conditions for patients and surgical staff [4]. Many literature has used CFD simulation to assess colony forming unit (cfu) dispersion and evaluate air flow distribution effectiveness in real OTs [. In a more specific research the influence of periodic bending movement of the medical staff combined with bacteria carrying particle distribution on the airflow field and temperature distribution have been investigated [5]. Most of these studies consider correct use, on-design operational conditions of OTs. Some new analyses have been proposed concerning moving object effects on the airflow inside OTs under correct and incorrect use conditions [6] [7]. In the most recent literature, the assessment of ventilation efficiency in OTs, including air flow distribution under different configurations and different positions of diffuser supply velocity, is studied by applying CFD simulation and full-scale experimental investigations [8]. A recent study based on experimental measurements and CFD simulation shows the efficacy of different ventilation systems in removing bacteria and particles using adducted return strategy to induce air flow from clean to less clean zones [9]. In the literature there is not much that addresses the analysis of the efficiency of a ventilation system in an OT, equipped with ante-chamber and/or air locking system for guaranteeing air pressure difference, indoor microclimatic conditions and IAQ, minimizing the air flow inlet from non-clean zones to the OT. A recent research work of the present authors has dealt with ABSTRACT

Experimental analysis of heat transfer enhancement in a circular tube with different twist ratio of twisted tape inserts

Abstract: An experimental investigation was carried for measuring heat transfer, Reynolds number, and friction factor fitted in concentric tube with twisted tape inserts. The different twist ratio y=2.52, 3.00 and 3.20 were studied for the laminar flow using computational fluid dynamics package. An aluminium inner tube of 18 mm internal diameter and 20mm outer diameter and 220mm length was used for experimental study. The outer tube material is mild steel of 28mm inner diameter and 32mm outer diameter was used for the experimental record. A copper twisted tape of different twist ratio of twist was inserted and the obtained plain tube data were verified with twisted tape inserts and ensure the validation of simulation results. The results are analyzed that the twisted tape accessible high heat transfer with increases in friction factor. In additionally the experimental value of twisted tape as compared with numerical simulation. A CFD package of commercial Ansys Fluent were utilized for simulate the twisted tape.