Numerical Analysis of Conjugated Convection-Conduction Heat Transfer in a Microtube Gas Flow

2018 ◽  
Vol 11 (1) ◽  
Author(s):  
K. M. Ramadan
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Nusselt Number ◽  
Viscous Dissipation ◽  
Wall Thickness ◽  
Gas Flow ◽  
Thermal Conductivity Ratio ◽  
Conductivity Ratio ◽  
Axial Conduction ◽  
Transfer Parameters

Abstract Numerical solutions for conjugate heat transfer of a hydro-dynamically fully developed, thermally developing, steady, incompressible laminar gas flow in a microtube with uniform wall heat flux boundary condition are presented. The mathematical model takes into account effects of rarefaction, viscous dissipation, flow work, shear work, and axial conduction in both the wall and the fluid. The effect of the tube wall thickness, the wall-to-fluid thermal conductivity ratio, as well as other factors on heat transfer parameters is investigated, and comparisons with the case of zero wall thickness are presented as appropriate. The results illustrate the significance of heat conduction in the tube wall on convective heat transfer and disclose the significant deviation from those with no conjugated effects. Increasing the wall thickness lowers the local Nusselt number. Increasing the wall-to-fluid thermal conductivity ratio also results in lower Nusselt number. In relatively long and thick microtubes with high wall-to-fluid thermal conductivity ratio, the local Nusselt number exhibits minimum values in the entrance regions and at the end sections due to axial conduction effects. The analysis presented also demonstrate the significance of rarefaction, shear work, axial conduction, as well as the combined viscous dissipation and flow work effects on heat transfer parameters in a microtube gas flow. The combined flow work and viscous dissipation effects on heat transfer parameters are significant and result in a reduction in the Nusselt number. The shear work lowers the Nusselt number when heat is added to the fluid.

Analysis of Single Phase Convective Heat Transfer in Microchannels With Variable Thermal Conductivity and Viscosity

2010 ◽  
Author(s):  
Arif Cem Go¨zu¨kara ◽  
Almıla G. Yazıcıoglu ◽  
Sadık Kakac¸
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Viscous Dissipation ◽  
Gas Flow ◽  
Single Phase ◽  
Variable Thermal Conductivity ◽  
Parallel Plates ◽  
Scaling Effects ◽  
Axial Conduction ◽  
Temperature Variable

The need for maximizing the performance of micro-mechanical systems and electronic components has resulted in a trend of minimization. Minimized sizes and dimensions have come along with a complex heat transfer and fluid problem within these devices and components. For a variety of fields in which these devices are used, such as; biomedicine, micro fabrication, and optics, fluid flow and heat transfer at the microscale needs to be understood and modeled with an acceptable reliability. In general, models are prepared by making some extensions to the conventional theories by including the scaling effects that become important for microscale. Studies performed in the last decade have shown that, some of the effects that are thought to become significant for a microscale gas flow are; axial conduction, viscous dissipation, and rarefaction. In addition to these effects, the temperature variable thermal conductivity and viscosity may become important in microscale gas flow due to the high temperature gradients that may exist in the fluid. Therefore, effects of variable thermal conductivity and viscosity in microscale gas flow and convection heat transfer are investigated in this study. For this purpose, simultaneously developing, single phase, laminar and incompressible air flow in a micro gap between parallel plates is numerically analyzed. In the analyses, scaling effects such as rarefaction, viscous dissipation, and axial conduction are taken into account in addition to the temperature variable thermal conductivity and viscosity.

scholarly journals Conjugate Heat Transfer in Rayleigh-Bénard Convection in a Square Enclosure

2014 ◽  
Vol 2014 ◽  
pp. 1-8 ◽  
Author(s):  
Habibis Saleh ◽  
Ishak Hashim
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Rayleigh Number ◽  
Wall Thickness ◽  
Staggered Grid ◽  
Thermal Conductivity Ratio ◽  
Conductivity Ratio ◽  
Square Enclosure ◽  
Conjugate Natural Convection ◽  
Rayleigh Benard

Conjugate natural convection-conduction heat transfer in a square enclosure with a finite wall thickness is studied numerically in the present paper. The governing parameters considered are the Rayleigh number5×103≤Ra≤106, the wall-to-fluid thermal conductivity ratio0.5≤Kr≤10, and the ratio of wall thickness to its height0.2≤D≤0.4. The staggered grid arrangement together with MAC method was employed to solve the governing equations. It is found that the fluid flow and the heat transfer can be controlled by the thickness of the bottom wall, the thermal conductivity ratio, and the Rayleigh number.

A Numerical Study of Conjugate Heat Transfer by Natural Convection and Surface Radiation in a Square Enclosure With Thick Adiabatic Walls

2013 ◽  
Author(s):  
H. Hadim ◽  
K. Blecker
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Nusselt Number ◽  
Natural Convection ◽  
Average Nusselt Number ◽  
Surface Radiation ◽  
Thick Wall ◽  
Thermal Conductivity Ratio ◽  
Conductivity Ratio ◽  
Square Enclosure

A numerical solution of heat transfer by combined natural convection and surface radiation in a square enclosure with thick adiabatic top and bottom walls and isothermal vertical walls is presented. The present model was used to obtain new results with the addition of thermal conduction at the thick top and bottom walls for a thermal conductivity ratio, K = ksolid/kfluid, that ranges from 0 to 10, emissivity of the adiabatic walls that ranges from 0 to 1, and the Rayleigh Number that ranges from 103 to 106. The model was validated by comparing the results to a benchmark solution and other solutions found in the literature. The results showed that with an increase in thermal conductivity ratio, the flow circulation decreases while the average Nusselt Number increases indicating increased heat transfer across the thick walls and the fluid in the corners. The results indicate that while past studies have shown negligible impact of the emissivity of the adiabatic walls on characteristics of the flow and heat transfer within the cavity, when a wall with moderate heat capacity and conductivity is considered, the resulting flow velocity and temperature distribution within the cavity are found to be significantly influenced by the thick wall emissivity. As the conductivity ratio increases this discrepancy between thin and thick walls becomes greater, there is further need for a more complex and accurate model including the thick walls. The results also showed that an increase in the emissivity of the adiabatic walls results in a slight decrease in the average Nusselt Number.

Conjugate Heat Transfer Using Liquid Metals and Alloys for Enclosures With Solid Conducting Blocks

2010 ◽  
Vol 2 (1) ◽  
Author(s):  
M. McGarry ◽  
C. Bonilla ◽  
I. Metzger
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Nusselt Number ◽  
Rayleigh Number ◽  
Conjugate Heat Transfer ◽  
Liquid Metals ◽  
Thermal Conductivity Ratio ◽  
Temperature Sensitive ◽  
Conductivity Ratio ◽  
Prandtl Numbers

A validated computational model was created to simulate the heat transfer from a heated surface using liquid metals and alloys during conjugate heat transfer. This model explores the effect of the Rayleigh number, Prandtl number, thermal conductivity ratio, and aspect ratio on the Nusselt number along the hot surface. The data will show how to keep the temperature sensitive components along the hot wall cool by maximizing the amount of heat removed from the hot wall. The data show three distinct regions that occur as a function of the Rayleigh number for a fixed k∗ and d∗. The data also show that the thermal conductivity ratio between the fluid and the solid conducting block has little effect on the Nusselt number at a fixed Rayleigh number. However, when examining the effect of the aspect ratio on the Nusselt number, two distinct regions can be seen. The results demonstrate that in order to keep the temperature sensitive components cool along the hot wall, one would want to have large Rayleigh and Prandtl numbers. The easiest way to achieve large Rayleigh numbers is by increasing the height of the enclosure. Large Prandtl numbers can be achieved by choosing a fluid that is highly conductive. In addition, the choice of material for the center solid conducting block does not impact the amount of heat removed from the hot wall. However, increased cooling can be achieved by decreasing the spacing between the hot and the cold wall.

scholarly journals Effects of the Wall Properties on the Cooling Efficiency in a Thermosyphon Containing PCM Suspensions

Energies ◽  
2021 ◽  
Vol 14 (3) ◽  
pp. 572
Author(s):  
Ching-Jenq Ho ◽  
Shih-Ming Lin ◽  
Chi-Ming Lai
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Wall Thickness ◽  
Thermal Conductivity Ratio ◽  
Heat Transfer Efficiency ◽  
Wall Properties ◽  
Heating Section ◽  
Cooling Section ◽  
Optimum Heat ◽  
Parameter Ranges

This study explores the effects of pipe wall properties (thermal conductivity k and wall thickness tw) on the heat transfer performance of a rectangular thermosyphon with a phase change material (PCM) suspension and a geometric configuration (aspect ratio = 1; dimensionless heating section length = 0.8; dimensionless relative elevation between the cooling and the heating sections = 2) that ensures the optimum heat transfer efficiency in the cooling section. The following parameter ranges are studied: the dimensionless loop wall thickness (0 to 0.5), wall-to-fluid thermal conductivity ratio (0.1 to 100), modified Rayleigh number (1010 to 1011), and volumetric fraction of PCM particles (0 to 10%). The results show that appropriate selection of k and tw can lead to improved heat transfer effectiveness in the cooling section of the PCM suspension-containing rectangular thermosyphon.

Effects of a centered conducting body on natural convection of non-Newtonian fluid in a square enclosure with time-periodic temperature distribution

2020 ◽  
Author(s):  
Peixin Ye ◽  
Dinggen Li ◽  
Zihao Yu ◽  
Haifeng Zhang
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Capacity ◽  
Transient Heat Transfer ◽  
Thermal Conductivity Ratio ◽  
Conductivity Ratio ◽  
Transfer Ratio ◽  
Periodic Temperature ◽  
Power Law Index ◽  
Time Periodic

In this paper, a modified lattice Boltzmann model that incorporates the effect of heat capacity is adopted to study the effects of a centered conducting body on natural convection of non-Newtonian fluid in a square cavity with time-periodic temperature distribution. The effects of power-law index, Rayleigh number, heat capacity ratio, thermal conductivity ratio, body size, temperature pulsating period and the temperature pulsating amplitude on fluid flow and heat transfer are analyzed in detail. The results showed that the increase of Rayleigh number and thermal conductivity ratio as well as the decrease of power-law index can strengthen both transient and global heat transfer, while the increase of heat capacitance of fluid to the solid wall can only enhance the transient heat transfer, and has little effect on the overall heat transfer. Further, the increase of body size will reduce both the transient heat transfer ratio and the overall heat transfer ratio. In addition, the decrease of temperature pulsating period can enhance the transient heat transfer, but it will slightly weaken the overall heat transfer. Finally, the results show that both the transient and the overall heat transfer ratio are increased with the increase of temperature pulsating amplitude.

Roughness Effect on the Heat Transfer Coefficient for Gaseous Flow in Microchannels

2010 ◽  
Author(s):  
Metin B. Turgay ◽  
Almila G. Yazicioglu ◽  
Sadik Kakac
Keyword(s):  
Heat Transfer ◽  
Surface Roughness ◽  
Nusselt Number ◽  
Flow Regime ◽  
Viscous Dissipation ◽  
Slip Flow ◽  
Roughness Height ◽  
Working Fluid ◽  
Axial Conduction ◽  
Slip Flow Regime

Effects of surface roughness, axial conduction, viscous dissipation, and rarefaction on heat transfer in a two–dimensional parallel plate microchannel with constant wall temperature are investigated numerically. Roughness is simulated by adding equilateral triangular obstructions with various heights on one of the plates. Air, with constant thermophysical properties, is chosen as the working fluid, and laminar, single-phase, developing flow in the slip flow regime at steady state is analyzed. Governing equations are solved by finite element method with tangential slip velocity and temperature jump boundary conditions to observe the rarefaction effect in the microchannel. Viscous dissipation effect is analyzed by changing the Brinkman number, and the axial conduction effect is analyzed by neglecting and including the corresponding term in the energy equation separately. Then, the effect of surface roughness on the Nusselt number is observed by comparing with the corresponding smooth channel results. It is found that Nusselt number decreases in the continuum case with the presence of surface roughness, while it increases with increasing roughness height in the slip flow regime, which is also more pronounced at low-rarefied flows (i.e., around Kn = 0.02). Moreover, the presence of axial conduction and viscous dissipation has increasing effects on heat transfer with increasing roughness height. Even in low velocity flows, roughness increases Nusselt number up to 33% when viscous dissipation is considered.

Upward and downward conjugate mixed convection heat transfer in a partially porous cavity

2016 ◽  
Vol 26 (1) ◽  
pp. 159-188 ◽  
Author(s):  
Abderrahim Bourouis ◽  
Abdeslam Omara ◽  
Said Abboudi
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Mixed Convection ◽  
Transfer Rate ◽  
Vertical Wall ◽  
Thermal Conductivity Ratio ◽  
Conductivity Ratio ◽  
Content Type ◽  
Moving Wall ◽  
The Right

Purpose – The purpose of this paper is to provide a numerical study of conjugate heat transfer by mixed convection and conduction in a lid-driven enclosure with thick vertical porous layer. The effect of the relevant parameters: Richardson number (Ri=0.1, 1, 10) and thermal conductivity ratio (Rk=0.1, 1, 10, 100) are investigated. Design/methodology/approach – The studied system is a two dimensional lid-driven enclosure with thick vertical porous layer. The left vertical wall of the enclosure is allowed to move in its own plane at a constant velocity. The enclosure is heated from the right vertical wall isothermally. The left and the right vertical walls are isothermal but temperature of the outside of the right vertical wall is higher than that of the left vertical wall. Horizontal walls are insulated. The governing equations are solved by finite volume method and the SIMPLE algorithm. Findings – From the finding results, it is observed that: for the two studied cases, heat transfer rate along the hot wall is a decreasing function of thermal conductivity ratio irrespective of Richardson numbers contrary to the heat transfer rate along the fluid-porous layer interface which is an increasing function of thermal conductivity ratio. At forced convection dominant regime, the difference between heat transfer rate for upward and downward moving wall is insensitive to the thermal conductivity ratio. For downward moving wall, average Nusselt number is higher than that of upward moving wall. Practical implications – Some applications: building applications, furnace design, nuclear reactors, air solar collectors. Originality/value – From the bibliographic work and the authors’ knowledge, the conjugate mixed convection in lid-driven partially porous enclosures has not yet been investigated which motivates the present work that represent a continuation of the preceding investigations.

Conjugate heat transfer in porous triangular enclosures with thick bottom wall

2009 ◽  
Vol 19 (5) ◽  
pp. 650-664 ◽  
Author(s):  
Yasin Varol ◽  
Hakan F. Oztop ◽  
Ioan Pop
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Natural Convection ◽  
Conjugate Heat Transfer ◽  
Bottom Wall ◽  
Thick Wall ◽  
Thermal Conductivity Ratio ◽  
Conductivity Ratio ◽  
Governing Equations ◽  
Content Type

PurposeThe purpose of this paper is to study the conjugate heat transfer via natural convection and conduction in a triangular enclosure filled with a porous medium.Design/methodology/approachDarcy flow model was used to write governing equations with Boussinesq approximation. The transformed governing equations are solved numerically using a finite difference technique. It is assumed that the enclosure consists of a conducting bottom wall of finite thickness, an adiabatic (insulated) vertical wall and a cooled inclined wall.FindingsFlow patterns, temperature and heat transfer were presented at different dimensionless thickness of the bottom wall, h, from 0.05 to 0.3, different thermal conductivity ratio between solid material and fluid, k, from 0.44 to 283 and Rayleigh numbers, Ra, from 100 to 1000. It is found that both thermal conductivity ratio and thickness of the bottom wall can be used as control parameters for heat transport and flow field.Originality/valueIt is believed that this is the first paper on conduction‐natural convection in porous media filled triangular enclosures with thick wall. In the last years, most of the researchers focused on regular geometries such as rectangular or square cavity bounded by thick wall.

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