EFFECT OF UNIFORM FOULING DEPOSIT ON TOTAL EFFICIENCY OF EXTENDED HEAT TRANSFER SURFACES

1978 ◽  
Author(s):  
Norman Epstein ◽  
Ken Sandhu
Keyword(s):  
Heat Transfer ◽  
Total Efficiency

Numerical Simulation for Thermal Design of a Gas Water Heater With Turbulent Combined Convection

2015 ◽  
Author(s):  
Ali Yari ◽  
Siamak Hosseinzadeh ◽  
Ali Akbar Golneshan ◽  
Ramin Ghasemiasl
Keyword(s):  
Heat Transfer ◽  
Control Volume ◽  
Geometric Parameters ◽  
Thermal Design ◽  
Navier Stokes ◽  
Total Efficiency ◽  
Water Heater ◽  
Hydrodynamic Behaviour ◽  
Turbulent Models ◽  
Energy Equations

This article investigates the effects of geometric parameters on a turbulent asymmetrical heat transfer in vertical channels with radiation and blowing from a wall. Hydrodynamic behaviour and heat transfer results are obtained by the solution of the complete Navier–Stokes and energy equations using a control volume finite element method. In this paper, commercial codes were used to solve the equations. The equations involved were numerically solved with three turbulent models including Spalart-Allmaras, R-N-G k-ε with ‘standard wall function’ wall nearby model, R-N-G k-ε with ‘enhanced wall treatment’ wall nearby model and ‘ray tracing’ radiation techniques. Turbulent flow with ‘low Reynolds Spalart-Allmaras turbulence model’ and radiation with ‘discrete transfer radiation method’ was modelled. The results were compared with experimental data and appropriate methods were selected for turbulent modelling. The problems of different Grashof number, Reynolds number, radiation parameters and Prandtl number were solved and the effects of geometric parameters on the fluid flow, radiation-convection-blowing heat transfer and the total efficiency were determined.

Flow Physics and Profiling of Recessed Blade Tips: Impact on Performance and Heat Load

2006 ◽  
Author(s):  
Bob Mischo ◽  
Thomas Behr ◽  
Reza S. Abhari
Keyword(s):  
Heat Transfer ◽  
Rotor Blade ◽  
Suction Side ◽  
Turbine Rotor ◽  
Tip Clearance ◽  
Limiting Factor ◽  
Total Efficiency ◽  
Axial Turbine ◽  
Blade Tip ◽  
Improved Design

In axial turbine the tip clearance flow occurring in rotor blade rows is responsible for about one third of the aerodynamic losses in the blade row and in many cases is the limiting factor for the blade lifetime. The tip leakage vortex forms when the leaking fluid crosses the gap between the rotor blade tip and the casing from pressure to suction side and rolls up into a vortex on the blade suction side. The flow through the tip gap is both of high velocity and high temperature, with the heat transfer to the blade from the hot fluid being very high in the blade tip area. In order to avoid blade tip burnout and a failure of the machine, blade tip cooling is commonly used. This paper presents the physical study and an improved design of a recessed blade tip for a highly loaded axial turbine rotor blade with application in high pressure axial turbines in aero engine or power generation. With use of three-dimensional Computational Fluid Dynamics (CFD), the flow field near the tip of the blade for different shapes of the recess cavities is investigated. Through better understanding and control of cavity vortical structures, an improved design is presented and the differences to the generic flat tip blade are highlighted. It is observed that by an appropriate profiling of the recess shape, the total tip heat transfer Nusselt Number was significantly reduced, being 15% lower than the flat tip and 7% lower than the baseline recess shape. Experimental results also showed an overall improvement of 0.2% in the one-and-1/2-stage turbine total efficiency with the improved recess design compared to the flat tip case. The CFD analysis conducted on single rotor row configurations predicted a 0.38% total efficiency increase for the rotor equipped with the new recess design compared to the flat tip rotor.

scholarly journals Assessment of Surface Roughness Effects on Micro Axial Turbines

2020 ◽  
Author(s):  
Abdelaziz A. A. Gamil ◽  
Theoklis Nikolaidis ◽  
Joao A. Teixeira ◽  
S. H. Madani ◽  
Ali Izadi
Keyword(s):  
Heat Transfer ◽  
Surface Roughness ◽  
Gas Turbine ◽  
Pressure Ratio ◽  
Careful Consideration ◽  
Inlet Temperature ◽  
Total Efficiency ◽  
Roughness Effects ◽  
Micro Gas Turbine ◽  
Micro Turbine

Abstract Surface roughness significantly affects the aerodynamics and heat transfer within micro-scale turbine stages. It results in a considerable increment in the blade profile loss and leads consequently to sizeable performance reductions. The provision of low roughness surfaces in micro gas turbine stages presents challenges on account of the small (mm scale) sizes, manufacturing complexity and associated costs. The axial turbine investigated in this study is fitted to Samad Power’s TwinGen domestic micro combined heat and power unit. The micro gas turbine has a compressor pressure ratio of 3, 1200K turbine inlet temperature and a rotational speed of 170,000 rpm. This paper presents a numerical assessment of the effects of varying the surface roughness on the performance and heat transfer of the micro turbine. The surface roughness was uniformly distributed on the NGV and rotor blades. The results showed that increasing the surface roughness from 3 microns to 6, 20, and 100 microns resulted in a reduction in stage total efficiency of 0.8%, 4% and 12% respectively as well as a comparable decrease in output power (0.7%, 3.6%, and 11% respectively). The turbine temperature was also observed to be very sensitive to surface roughness and a temperature increase of some 5% at the rotor hub and over 4% increment in the blade tip surface was observed for 100 microns when compared to the 3 microns surface roughness case. The findings of this paper highlight the adverse effects of the surface roughness on the micro-turbine performance and temperature distribution as well as the importance of careful consideration of wall roughness during the design and manufacturing stages.

Flow Physics and Profiling of Recessed Blade Tips: Impact on Performance and Heat Load

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Bob Mischo ◽  
Thomas Behr ◽  
Reza S. Abhari
Keyword(s):  
Heat Transfer ◽  
Rotor Blade ◽  
Suction Side ◽  
Tip Clearance ◽  
Limiting Factor ◽  
Total Efficiency ◽  
Base Line ◽  
Axial Turbine ◽  
Blade Tip ◽  
Improved Design

In axial turbine, the tip clearance flow occurring in rotor blade rows is responsible for about one-third of the aerodynamic losses in the blade row and in many cases is the limiting factor for the blade lifetime. The tip leakage vortex forms when the leaking fluid crosses the gap between the rotor blade tip and the casing from pressure to suction side and rolls up into a vortex on the blade suction side. The flow through the tip gap is both of high velocity and of high temperature, with the heat transfer to the blade from the hot fluid being very high in the blade tip area. In order to avoid blade tip burnout and a failure of the machine, blade tip cooling is commonly used. This paper presents the physical study and an improved design of a recessed blade tip for a highly loaded axial turbine rotor blade with application in high pressure axial turbines in aero engine or power generation. With use of three-dimensional computational fluid dynamics (CFD), the flow field near the tip of the blade for different shapes of the recess cavities is investigated. Through better understanding and control of cavity vortical structures, an improved design is presented and its differences from the generic flat tip blade are highlighted. It is observed that by an appropriate profiling of the recess shape, the total tip heat transfer Nusselt number was significantly reduced, being 15% lower than the flat tip and 7% lower than the base line recess shape. Experimental results also showed an overall improvement of 0.2% in the one-and-a-half-stage turbine total efficiency with the improved recess design compared to the flat tip case. The CFD analysis conducted on single rotor row configurations predicted a 0.38% total efficiency increase for the rotor equipped with the new recess design compared to the flat tip rotor.

Numerical Simulation for Thermal Design of a Gas Water Heater With Turbulent Combined Convection

2015 ◽  
Author(s):  
Ali Yari ◽  
Siamak Hosseinzadeh ◽  
Ali Akbar Golneshan ◽  
Ramin Ghasemiasl
Keyword(s):  
Heat Transfer ◽  
Control Volume ◽  
Geometric Parameters ◽  
Thermal Design ◽  
Navier Stokes ◽  
Total Efficiency ◽  
Water Heater ◽  
Hydrodynamic Behaviour ◽  
Turbulent Models ◽  
Energy Equations

This article investigates the effects of geometric parameters on a turbulent asymmetrical heat transfer in vertical channels with radiation and blowing from a wall. Hydrodynamic behaviour and heat transfer results are obtained by the solution of the complete Navier–Stokes and energy equations using a control volume finite element method. In this paper, commercial codes were used to solve the equations. The equations involved were numerically solved with three turbulent models including Spalart-Allmaras, R-N-G k-ε with ‘standard wall function’ wall nearby model, R-N-G k-ε with ‘enhanced wall treatment’ wall nearby model and ‘ray tracing’ radiation techniques. Turbulent flow with ‘low Reynolds Spalart-Allmaras turbulence model’ and radiation with ‘discrete transfer radiation method’ was modelled. The results were compared with experimental data and appropriate methods were selected for turbulent modelling. The problems of different Grashof number, Reynolds number, radiation parameters and Prandtl number were solved and the effects of geometric parameters on the fluid flow, radiation-convection-blowing heat transfer and the total efficiency were determined.

scholarly journals Numerical Analysis for Thermal Performance of a Photovoltaic Thermal Solar Collector with SiO2-Water Nanofluid

2018 ◽  
Vol 8 (11) ◽  
pp. 2223 ◽  
Author(s):  
Ali Chamkha ◽  
Fatih Selimefendigil
Keyword(s):  
Heat Transfer ◽  
Numerical Analysis ◽  
Basis Function ◽  
Volume Fraction ◽  
Particle Volume Fraction ◽  
Solar Irradiation ◽  
Inlet Temperature ◽  
Total Efficiency ◽  
Cylindrical Shape ◽  
Particle Volume

Numerical analysis of a photovoltaic-thermal (PV/T) unit with SiO 2 -water nanofluid was performed. The coupled heat conduction equations within the layers and convective heat transfer equations within the channel of the module were solved by using the finite volume method. Effects of various particle shapes, solid volume fractions, water inlet temperature, solar irradiation and wind speed on the thermal and PV efficiency of the unit were analyzed. Correlation for the efficiencies were obtained by using radial basis function neural networks. Cylindrical shape particles were found to give best performance in terms of efficiency enhancements. Total efficiency enhances by about 7.39% at the highest volume fraction with cylindrical shape particles. Cylindrical shape particle gives 3.95% more enhancement as compared to spherical ones for the highest value of solid particle volume fraction. Thermal and total efficiency enhance for higher values of solid particle volume fraction, solar irradiation and lower values of convective heat transfer coefficient and inlet temperature. The performance characteristics of solar PV-thermal unit with radial basis function artificial neural network are found to be in excellent agreement with the results obtained from computational fluid dynamics modeling.

Numerical and Experimental Characterization of a Plate Compact Multipass Counter Flow and Locally Cross-Flow Recuperator

2006 ◽  
Author(s):  
Paolo M. Congedo ◽  
Giuseppe Starace
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Pressure Drop ◽  
Cross Flow ◽  
Operating Conditions ◽  
Geometrical Parameters ◽  
Total Efficiency ◽  
Operating Pressure ◽  
Counter Flow ◽  
The Real

A compact and efficient heat exchanger for exhaust gas recovery energy was needed to raise the total efficiency of a thermo-photovoltaic system TPV (Thermo-Photo-Voltaic) for automotive applications (see [1]). In order to respect the strict condition of a high heat transfer surface to volume ratio, a heat exchanger configuration with a plate compact multi-pass counter flow and locally cross-flow recuperator has been chosen. The goal of this work is the understanding of the behaviour of the heat exchanger with numerical and experimental analysis for different geometrical and operating conditions. A high number of dimensions and manufacturing constraints was evaluated before reaching a definite design of a compact and efficient heat exchanger to be tested in the lab for initial experiments. The experimental work was needed in order to validate the numerical model. As the material needed for the real application could not be easily manufactured and instrumented in a workshop, a simplified real model, made of brass, was built, in order to compare numerical results and experimental findings. It was supposed that results obtained in this way would be sufficient to be considered valid when extrapolated in the real heat exchanger high temperature operating conditions and manufacturing material. The experimental results have been successfully compared with numerical ones obtained with the Fluent CFD code (release 6.2.16) Curves of performance (ε-NTU diagram plotted as a function of the ratio between the minimum and the maximum thermal capacities of the flows and pressure drop -mass flowrate diagram as a function of the average temperature) have been obtained and were useful to choose the adequate configuration for different applications, depending on the requested heat transfer and maximum allowable pressure drop. The output of the investigation was: heat transfer, outlet temperatures for both air flows, heat exchanger efficiency, differential pressure drop for both hot and cold sides. After this validation final numerical simulations have been carried out in order to understand the dependence of the heat exchanger efficiency on other geometrical parameters and operating conditions such as plates dimensions, numbers and height of vanes, operating pressure and so on.

scholarly journals Performance of Graphite-Dispersed Li2CO3-K2CO3 Molten Salt Nanofluid for a Direct Absorption Solar Collector System

Molecules ◽  
2020 ◽  
Vol 25 (2) ◽  
pp. 375 ◽  
Author(s):  
M. A. Karim ◽  
Majedul Islam ◽  
Owen Arthur ◽  
Prasad KDV Yarlagadda
Keyword(s):  
Heat Transfer ◽  
Molten Salt ◽  
Solar Collector ◽  
Volume Fraction ◽  
Heat Transfer Fluid ◽  
Total Efficiency ◽  
Nanoparticle Volume Fraction ◽  
Direct Absorption ◽  
Solar Salt ◽  
Direct Absorption Solar Collector

Considered to be the next generation of heat transfer fluids (HTFs), nanofluids have been receiving a growing interest over the past decade. Molten salt nanofluids have been shown to have great potential as an HTF for use in high temperature applications such as direct absorption solar collector (DAC) system. Very few studies using molten salt nanofluids as the HTF in a DAC receiver can be found in the open literature. This study aimed to develop a 3D computational fluid dynamics model of the receiver of a DAC using graphite-nanoparticle-dispersed Li2CO3-K2CO3 molten salt nanofluid to investigate the effects of design and operation parameters on receiver performance. Receiver total efficiency using Li2CO3-K2CO3 salt was compared with that using solar salt nanofluid. Spectral properties of the base fluid and nanoparticles were modeled as wavelength-dependent and the absorption of the solar radiation was modeled as a volumetric heat release in the flowing heat transfer fluid. Initial results show that the receiver efficiency increases with increasing solar concentration, decreasing nanoparticle volume fraction, and decreasing receiver length. It was also found that the Carnot efficiency increases with increasing receiver length and nanoparticle volume fraction, and decreasing solar concentration and inlet velocity. Comparative study shows that solar salt HTF could provide higher total efficiency. However, a higher operating temperature of Li2CO3-K2CO3 will allow for a greater amount of thermal energy storage for a smaller volume of liquid.

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