scholarly journals Averaged energy-balance analysis of wavy micro-channels

2021 ◽  
Vol 2116 (1) ◽  
pp. 012080
Author(s):  
Roxana Durantes ◽  
Justin Moon ◽  
J Rafael Pacheco ◽  
Arturo Pacheco-Vega
Keyword(s):  
Heat Transfer ◽  
Energy Balance ◽  
Channel Model ◽  
Heat Rate ◽  
Operating Conditions ◽  
Temperature Fields ◽  
Global Energy ◽  
Single Wave ◽  
Micro Channels ◽  
Energy Balances

Abstract This study presents numerical simulations of the convective heat transfer on wavy micro-channels to investigate heat transfer enhancement in these systems. The goal is to extend the analysis of our previous work [1, 2], by proposing a methodology based on local and global energy balances in the device instead of the commonly used Nusselt number. The analysis is performed on a single-wave baseline micro-channel model that is exposed to a heat influx. The governing equations for an incompressible laminar flow and conjugate heat transfer are first built, and then solved, for representative models, under several operating conditions, by the finite element technique. From computed velocity, pressure and temperature fields, local and global energy balances based on cross-section-averaged velocities and temperatures enable calculating the heat rate at each section. Results from this study show that this so-called averaged energy-balance methodology enables an accurate assessment of the channel performance.

Heat Transfer Enhancement in Wavy Micro-Channels Through Multiharmonic Surfaces

2018 ◽  
Author(s):  
Justin Moon ◽  
J. Rafael Pacheco ◽  
Arturo Pacheco-Vega
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Transfer Enhancement ◽  
Numerical Solutions ◽  
Cold Water ◽  
Bottom Surface ◽  
Transfer Enhancement ◽  
Cross Sectional ◽  
Single Wave ◽  
Micro Channels

In this study, three-dimensional numerical simulations are performed to investigate heat transfer enhancement in multi-harmonic micro-scale wavy channels. The focus is on the influence of channel surface-topography, modeled as multi-harmonic sinusoidal waves of square cross-sectional area, on the enhancing mechanisms. A single-wave device of 0.5 mm × 0.5 mm × 20 mm length, is used as baseline, and new designs are built with harmonic-type surfaces. The channel is enclosed by a solid block, with the bottom surface within the sinusoidal region being exposed to a 47 W/cm2 heat flux. The numerical solutions of the governing equations for an incompressible laminar flow and conjugate heat transfer are obtained via finite elements. By using the ratio of the Nusselt number for wavy to straight channels, a parametric analysis — for a set of cold-water flowrates (Re = 50, 100, and 150) — shows that the addition of harmonic surfaces enhances the transfer of energy and that such ratio achieves the highest value with wave harmonic numbers of n = ±2. Use of a performance factor (PF), defined as the ratio of the Nusselt number to the pressure drop, shows that, surprisingly, the proposed wavy multi-harmonic channels are not as efficient as the single-wave geometries. This outcome is thought to be, primarily, due to the uncertainty associated with the definition of the Nusselt number used in this study, and establishes a direction to investigate the development of a more accurate definition.

The Application of Availability and Energy Balances to a Diesel Engine

1988 ◽  
Vol 110 (3) ◽  
pp. 462-469 ◽  
Author(s):  
A. C. Alkidas
Keyword(s):  
Heat Transfer ◽  
Diesel Engine ◽  
Combustion Process ◽  
Operating Conditions ◽  
Second Law ◽  
Heavy Duty ◽  
Efficiency Ratio ◽  
Brake Power ◽  
Laws Of Thermodynamics ◽  
Energy Balances

The maximum power that can be extracted from an engine operating at a given condition was determined by means of analyses based on the first and second laws of thermodynamics. These analyses were applied to a heavy-duty single-cylinder open-chamber diesel engine operated at constant speed. Over the range of operating conditions investigated, the second-law efficiency (ratio of brake power to maximum extractable power) of the engine, which increased with engine load, was found to vary from 22 to 50 percent. It was concluded that besides heat transfer, the combustion process was the most important source of irreversibility and accounted for 25 to 43 percent of the lost power.

scholarly journals Turbulent Flow and Heat Transfer in Gas Turbine Blade Cooling Passages

1992 ◽  
Author(s):  
F. J. Cunha
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Gas Turbine ◽  
Turbine Blades ◽  
Operating Conditions ◽  
Temperature Fields ◽  
Air Cooling ◽  
Turbine Blade Cooling ◽  
Gas Turbine Blades ◽  
Analysis Methodology

A numerical analysis methodology has been created to predict the heat transfer within the air cooling passages of gas turbine blades. In this paper, the turbulent flow heat convection with developed velocity and temperature fields is studied for cavities with turbulators. The influence of Coriolis forces and rotational buoyancy effects were also included. The k-equation turbulence model was employed over most of the cross section while a modified Van Driest’s version of the mixing length hypothesis is used in the near-wall sublayer. This methodology was successfully benchmarked against experimental results for air cooling passages of turbine blades. Analytical results are presented in terms of the Reynolds, Rossby and rotational Rayleigh numbers for realistic operating conditions.

scholarly journals Conjugate Heat Transfer of an Internally Air-Cooled Nozzle Guide Vane and Shrouds

2014 ◽  
Vol 6 ◽  
pp. 146523 ◽  
Author(s):  
Leiyong Jiang ◽  
Xijia Wu ◽  
Zhong Zhang
Keyword(s):  
Heat Transfer ◽  
Gas Turbine ◽  
Conjugate Heat Transfer ◽  
Guide Vane ◽  
Operating Conditions ◽  
Temperature Fields ◽  
Pressure Side ◽  
Cooling Flow ◽  
Nozzle Guide Vane ◽  
Nozzle Guide

In order to assess the life of gas turbine critical components, it is essential to adequately specify their aerothermodynamic working environments. Steady-state analyses of the flow field and conjugate heat transfer of an internally air-cooled nozzle guide vane (NGV) and shrouds of a gas turbine engine at baseline operating conditions are numerically investigated. A high-fidelity CFD model is generated and the simulations are carried out with properly defined boundary conditions. The features of the complicated flow and temperature fields are revealed. In general, the Mach number is lower and the temperature is higher on the NGV pressure side than those on the suction side. There are two high temperature regions on the pressure side, and the temperature across the middle section is relatively low. These findings are closely related to the locations of the holes and outlets of the cooling flow passage, and consistent with the field observations of damaged NGVs. As a technology demonstration, the results provide required information for the life analysis of the NGV/shrouds assembly and improvement of the cooling flow arrangement.

Analytical and Hybrid Solutions for Heat Transfer in Combined Electroosmotic and Pressure-Driven Flows

2012 ◽  
Author(s):  
L. A. Sphaier
Keyword(s):  
Heat Transfer ◽  
Analytical Solution ◽  
Integral Transform ◽  
Temperature Fields ◽  
Double Layers ◽  
Driving Mechanisms ◽  
Micro Channels ◽  
Hybrid Solutions ◽  
Axial Heat ◽  
Transfer Problems

This paper presents solutions to heat transfer problems that occur in flow micro-channels driven by the combined effect of electroosmosis and a pressure gradient. Fully developed velocity profiles are considered, and the thermal developing region is analyzed. The solution methodology is based on the Generalized Integral Transform Technique, which leads to fully analytical solution for all presented cases. With the solution of the temperature fields, the behavior of the Nusselt number is investigated for different test-cases. The effects of the flow driving mechanisms, viscous dissipation and Joule heating, as well as axial diffusion are analyzed. The approximated solution with thin Electric Double Layers (EDL) is considered, but cases without this restriction are also analyzed. The cases including axial heat conduction are analyzed for a simplified case of purely electroosmotic-driven flow with a thin-EDL, which leads to a simple analytical solution.

CFD Study on Specifics of Flow and Heat Transfer in Vertical Bare Tubes Cooled With Water at Supercritical Pressures

2017 ◽  
Author(s):  
Alexander Zvorykin ◽  
Nataliia Fialko ◽  
Sherenkovskyi Julii ◽  
Sergey Aleshko ◽  
Natalia Meranova ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Mass Flux ◽  
Cfd Simulation ◽  
Operating Conditions ◽  
Temperature Fields ◽  
Coolant Temperature ◽  
Flow And Heat Transfer ◽  
Medium Mass ◽  
Experimental Values

The paper presents results of a study on flow and temperature fields in bare tubes cooled with SuperCritical Water (SCW). This study is based on a Computational Fluid Dynamics (CFD) simulation with the FLUENT code for upward flows in vertical tubes with heated length of 4 m and an inner diameter of 10 mm. Operating conditions were: Mass flux – G ≈ 500 and 1000 kg/m2s; heat flux – q = 189 – 826 kW/m2; and inlet coolant temperature – Tin = 320–360°C. CFD predictions were compared with experimental data in this study. All three heat-transfer regimes: 1) normal heat transfer; 2) improved heat transfer; and 3) deteriorated heat transfer; were considered. The obtained results show that within normal and improved heat transfer CFD predicts experimental values reasonably well. However, within conditions of deteriorated heat transfer CFD predictions are less satisfactory. The CFD outcomes of the heat flux effect on the flow and heat transfer of SCW are presented. Specifics of flow within the pseudocritical region (i.e., approximately ±25°C around a pseudocritical point) are discussed. The buoyancy effect is investigated by axial velocity profiles at the medium mass flux of 500 kg/m2s and heat flux of 287 kW/m2.

Heat Transfer Enhancement for Flows in a Channel With Corona Wind Generator

2010 ◽  
Author(s):  
J. Zhang ◽  
F. C. Lai
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Operating Conditions ◽  
Temperature Fields ◽  
Transfer Enhancement ◽  
Power Requirement ◽  
Maximum Enhancement ◽  
Wind Generator ◽  
Wind Generators ◽  
Corona Wind

Heat transfer enhancement using corona wind generators in a square channel has been numerically examined in this study. Three configurations of the corona wind generators (separately with 4, 12, and 28 pins) are investigated for their effectiveness in the enhancement of heat transfer as well as the pumping power requirement. To achieve the maximum enhancement in heat transfer, the emitting electrodes of the corona wind generator are flush mounted on the channel walls so that the corona wind produced directly perturbs the boundary layer. The Reynolds number considered varies in a range between 200 and 2000. The influence of electric field on the flow and temperature fields is examined for a wind range of operating conditions. The results show that EHD technique has a great potential for many engineering applications.

Modeling of Turbulent Flow and Heat Transfer in Graphitic Foams

2010 ◽  
Vol 297-301 ◽  
pp. 739-744
Author(s):  
J. Cepek ◽  
Anthony G. Straatman
Keyword(s):  
Heat Transfer ◽  
Temperature Fields ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Large Eddy ◽  
Order Of Magnitude ◽  
Void Shape ◽  
Energy Balances ◽  
Les Model ◽  
Transfer Mechanisms

We describe a study conducted to examine the influence of turbulence on the flow and heat transfer in graphitic foams. In this study, a Large Eddy Simulation (LES) model was used to characterize turbulence in the momentum and energy balances, and calculations were done to produce transient results of the velocity and temperature fields inside the spherical pores of the foam. The LES simulations enable excellent insight into the pore-level heat transfer mechanisms for the spherical void shape, and show that the heat transfer increases by a factor of 4.7 for an order of magnitude increase in the Reynolds number.

Thermal Effects in Thrust Washer Lubrication

2001 ◽  
Vol 124 (1) ◽  
pp. 166-177 ◽  
Author(s):  
To Him Yu ◽  
Farshid Sadeghi
Keyword(s):  
Heat Transfer ◽  
Numerical Model ◽  
Thermal Effects ◽  
Groove Depth ◽  
Operating Conditions ◽  
Temperature Fields ◽  
Frictional Torque ◽  
Time Step ◽  
Transfer Equations ◽  
Side Flow

A numerical model was developed to study the thermal effects on the lubrication mechanism of radially-grooved thrust washers. A mass-conserving transient thermohydrodynamic (THD) analysis was performed by solving the modified Reynolds and energy equations for the lubricant pressure and temperature distributions. The heat transfer equations were also solved simultaneously to obtain the temperature fields of the solids (thrust washers). Due to different thermal time responses of the lubricant film and the solids, heat transfer equations of the runner and the thrust washer pad were treated as in quasi-steady state at each time step in the transient solution. Elrod cavitation algorithm was implemented to include lubricant cavitation. The results show that thermal effects do not only reduce load carrying capacity and the frictional torque but also increase side flow rate. Moreover, the numerical model also demonstrates that the thermal effects have greater influence on the load support when the groove depth and groove numbers increase. Furthermore, the analytical results also show that there exists certain operating conditions before thermal effects become the dominating factors in influencing the thrust washer performance.

scholarly journals Mathematical models of the process of submlimationand optimization of lyophilization modes

2018 ◽  
Vol 17 (3) ◽  
pp. 20-28
Author(s):  
E. V. Blynskaya ◽  
S. V. Tishkov ◽  
K. V. Alekseyev ◽  
S. V. Minaev
Keyword(s):  
Heat Transfer ◽  
Energy Balance ◽  
Mathematical Models ◽  
Side Wall ◽  
Factors Affecting ◽  
Mass Transfer Processes ◽  
Transient Energy ◽  
Mathematical Formulas ◽  
Adsorption Desorption ◽  
Energy Balances

The purposeof this study is to analyze methods of mathematical modeling for calculating the stage of primary sublimation, as the most important stage in lyophilization technology. Presented are mathematical formulas, equations for the calculation of heat and mass transfer processes, during the removal of 90 % of all frozen ice. A model is considered that takes into account the contribution of all thermal effects, including the transient energy balance, taking into account the heat transfer through the side wall of the vial and radiation, even if they are present in a small amount. The mathematical model can be used to optimize the lyophilization cycle, and also as tools for technological monitoring (using sensors based on models). The model considered in the article is a one-dimensional nonstationary state model in which the correct comprehensive transient energy balance has been introduced to describe the heat transfer through the glass of the vial, and the results are estimated using experimental data. The equations used in the simulation describe the mass and energy balances in the dried layer, taking into account the rate of adsorption/desorption of water at the interface, mass and heat transfer at the sublimation interface, as well as the energy balance of heat transfer in the wall of vials, shelf and other factors affecting the process of sublimation. Conclusions are made on the presented mathematical models and the characteristic of the direction of the process of optimization of primary sublimation in lyophilization technology is given.

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