Thermal Measurements in Rectangular Microchannels

2002 ◽  
Author(s):  
Aristotel Popescu ◽  
James R. Welty ◽  
David Pfund ◽  
David Rector
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Single Channel ◽  
Constant Heat Flux ◽  
Working Fluid ◽  
Flux Boundary Condition ◽  
Scale Theory ◽  
Nusselt Numbers ◽  
Micro Channels ◽  
Macro Scale

This paper reports on an experimental study of heat transfer in high aspect ratio (width/depth), rectangular micro-channels. A single channel with width of 10 μm was cut into polycarbonate spacers of various thicknesses, resulting in channel depths of 128 μm, 263 μm and 521 μm. Heat transfer experiments were performed with a constant heat flux boundary condition applied at the surface of the channel. Experiments conducted for refrigerant R-124 working fluid in the range Re = 300 − 900 and Pr = 3.6 − 3.8 showed small or no departure from macro-scale predictions for channels with hydraulic diameters larger than 500 μm. Results for the 80:1 aspect ratio channel showed a significant departure from theoretical predictions. Experimental values of local Nusselt numbers were approximately 25 percent lower than predicted using macro-scale theory.

Effects of Dented Roughness on Laminar Flow and Heat Transfer in Microchannels

2010 ◽  
Author(s):  
Hui Miao ◽  
Yong Huang ◽  
Fa Xie ◽  
Haigang Chen ◽  
Fang Wang
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Working Fluid ◽  
Heat Transfer Characteristics ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
Micro Channels ◽  
Planar Area ◽  
Poiseuille Number ◽  
Energy Equations

Liquid laminar flow and heat transfer characteristics for parallel plate micro-channels consisting of many triangle shape hollows to fit with the etching surfaces are investigated numerically in the present paper. The height of the channel is 50μm, with three different relative depths, three relative spacing, and three oblique angles of the triangle surface, respectively. The 2D N-S and energy equations are solved using a commercial CFD code FLUENT6.3. Water is used as the working fluid, and the Reynolds number ranges from 100 to 1500. The global Poiseuille number and average Nusselt number are obtained. It is shown that the dented shapes cause a modest influence in Poiseuille number, but a greater impact on the Nusselt numbers. In addition, both of Po and Nu increase modestly with Re. The local Nusselt numbers are always lower in dented area and larger in planar area of dented roughness microchannels, than that of conventional smooth value. Finally, geometry parameters have modest impact on heat transfer for dented roughness.

Influence of Thermo-Physical Properties on the Flow and Heat Transfer in Rectangular Micro-Channels

2011 ◽  
Vol 347-353 ◽  
pp. 2640-2644 ◽  
Author(s):  
Xue Tao Duan ◽  
Bin Xu ◽  
Hao Luo
Keyword(s):  
Heat Transfer ◽  
Physical Properties ◽  
Flow Direction ◽  
Three Dimensional ◽  
Working Fluid ◽  
Fluid Temperature ◽  
Flux Boundary Condition ◽  
Flow And Heat Transfer ◽  
Friction Factors ◽  
Micro Channels

This paper investigated the behaviors of flow and heat transfer of single-phase in rectangular micro-channels with three-dimensional numerical analysis. The single micro-channel is 200μm deep, 50μm wide. Deionized water was used as the working fluid. The fluid physical properties varying with temperature and Re number were studied. Comparisons were made among the results obtained from experiments, numerical simulations, and from those in the literature. The results indicated that the friction factors decreasing along the flow direction were ascribed to the fluid temperature rising under the unified heat flux boundary condition. It was found that influence of viscosity variation with temperature and viscous dissipation effect could be too significant to be neglected.

Axial Wall Conduction in Pulsating Laminar Flow in a Microtube

2014 ◽  
Author(s):  
Parthasarathi Mishra ◽  
Manoj Kumar Moharana
Keyword(s):  
Heat Transfer ◽  
Laminar Flow ◽  
Constant Heat Flux ◽  
Wall Material ◽  
Working Fluid ◽  
Maximum Effect ◽  
Pulsation Frequency ◽  
Cross Sectional ◽  
Flux Boundary Condition ◽  
Wide Range

A two-dimensional numerical analysis is carried out to understand the effect of pulsation on the axial wall conduction in simultaneously developing single phase laminar flow in a microtube with constant heat flux boundary condition imposed on its outer surface and while the cross-sectional solid faces exposed to the surrounding are kept adiabatic. Water is used as the working fluid and enters the microtube at 300K with a slug velocity that varying with time sinusoidally, thus causing pulsating flows in the microtube. The inlet velocity thus consists of a fixed component and a fluctuating component which varies sinusoidally with time. For this simulation conductivity ratio is considered at a wide range (ksf 2.26–703) while the thickness ratio (δsf), amplitude (A), and flow rate (Re) remain constant. To understand effect of pulsation, frequency of oscillation (f) is changed by taking four different Womersley numbers (1.414, 2, 2.45, and 3.163). Based on the numerical simulation, it is concluded that for a particular pulsation frequency (Wo) there exists an optimum value of ksf at which overall Nusselt number (Nu) is maximum. Effect of pulsation frequency on heat transfer is found to be very small. Heat transfer is found to be increasing at lower thermal conductive microtube wall material (or ksf) while it is decreasing at higher ksf compared to steady flow in microtube. Existing studies do indicate that pulsation (i) increases heat transfer (ii) decreases heat transfer, or (iii) no effect. The researchers actually failed to observe the present overall trend as none of the existing studies considered a widely varying thermal conductive wall material.

Heat Transfer to Turbulent Swirling Flow Through a Sudden Axisymmetric Expansion

1987 ◽  
Vol 109 (3) ◽  
pp. 613-620 ◽  
Author(s):  
P. A. Dellenback ◽  
D. E. Metzger ◽  
G. P. Neitzel
Keyword(s):  
Heat Transfer ◽  
Swirling Flow ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Working Fluid ◽  
Bulk Fluid ◽  
Maximum Heat ◽  
Flux Boundary Condition ◽  
Nusselt Numbers ◽  
Maximum Heat Transfer

Experimental data are presented for local heat transfer rates in the tube downstream of an abrupt 2:1 expansion. Water, with a nominal inlet Prandtl number of 6, was used as the working fluid. In the upstream tube, the Reynolds number was varied from 30,000 to 100,000 and the swirl number was varied from zero to 1.2. A uniform wall heat flux boundary condition was employed, which resulted in wall-to-bulk fluid temperatures ranging from 14° C to 50°C. Plots of local Nusselt numbers show a sharply peaked behavior at the point of maximum heat transfer, with increasing swirl greatly exaggerating the peaking. As swirl incressed from zero to its maximum value, the location of peak Nusselt numbers was observed to shift from 8.0 to 1.5 step heights downstream of the expansion. This upstream movement of the maximum Nusselt number was accompanied by an increase in its magnitude from 3 to 9.5 times larger than fully developed tube flow values. For all cases, the location of maximum heat transfer occurred upstream of the flow reattachment point.

Natural Convection and Conduction in Massive Wall Solar Collectors With Honeycomb and Without Vents

1995 ◽  
Vol 117 (3) ◽  
pp. 173-180 ◽  
Author(s):  
E. K. Lakhal ◽  
E. Bilgen ◽  
P. Vasseur
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Aspect Ratio ◽  
Solar Collector ◽  
Constant Heat Flux ◽  
Honeycomb Structure ◽  
Constant Heat ◽  
Nusselt Numbers ◽  
Fin Thickness ◽  
Steady Heat

Steady heat transfer by natural convection and conduction is numerically studied in passive solar collector systems consisting of a massive wall with honeycomb structure and without vents. The boundary conditions are constant heat flux on the wall and fins, isothermal on the vertical bounding sides, and adiabatic on the horizontal sides. The governing parameters are the Rayleigh number (106 ≤ Ra ≤ 5 × 109), the aspect ratio of the enclosures (0.4 ≤ A = H′/L′ ≤ 1.4), the dimensionless lengths of the fins (0 ≤ B = l′/Ll′ ≤ 1), the aspect ratio of the microcavities (0.05 ≤ C = h′/L′ ≤ 1), the wall thickness (0.008 ≤ w = w′/L′ ≤ 0.033). The fin thickness (e = e′/H′ = 0.06) and the Prandtl number (Pr = 0.72) were constant, and the conductivity ratio was variable (10−4 ≤ kr ≤ 5 × 106). Local and average Nusselt numbers along the long sides are calculated as a function of various parameters. Streamlines and isotherms are produced. Effects of various parameters on the heat transfer are examined and heat transfer correlations are derived.

Simulation on the Flow and Heat Transfer Characteristics of Confined Bubbles in Micro-Channels

2012 ◽  
Author(s):  
Qingming Liu ◽  
Björn Palm ◽  
Henryk Anglart
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Mass Flux ◽  
Phase Interface ◽  
Working Fluid ◽  
Initial Shape ◽  
Thin Film Evaporation ◽  
Flow And Heat Transfer ◽  
Vof Model ◽  
Micro Channels

3D simulations on confined bubbles in micro-channels with diameter of 1.24 mm were conducted. The working fluid is R134a with a mass flux range from 125kg/m2s to 375kg/m2s. The VOF model is chosen to capture the 2 phase interface while the geo-construction method was used to re-construct the 2-phase interface. A heated boundary wall with heat flux varying from 15kW/m2 to 102kW/m2 is supplied. The wall temperature was calculated. The effects of mass flux and heat flux are studied. The shape of the bubble was predicted by the simulation successfully and the results show that they are independent of the initial shape. Both thin film evaporation and micro convection enhance the heat transfer. However, the micro convection which is caused by bubble motion has greater contribution to the total heat transfer at the stage of bubble growth studied.

EXPERIMENTAL STUDY ON THERMAL TRANSPORT PHENOMENON OF NANOFLUIDS AS WORKING FLUID IN HEAT EXCHANGER

2014 ◽  
Vol 22 (01) ◽  
pp. 1450005 ◽  
Author(s):  
SHUICHI TORII
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Performance ◽  
Pure Water ◽  
Horizontal Tube ◽  
Constant Heat Flux ◽  
Working Fluid ◽  
Transfer Performance ◽  
Nanoparticles Concentration ◽  
Transfer Behavior

This paper aims to study the convective heat transfer behavior of aqueous suspensions of nanoparticles flowing through a horizontal tube heated under constant heat flux condition. Consideration is given to the effects of particle concentration and Reynolds number on heat transfer enhancement and the possibility of nanofluids as the working fluid in various heat exchangers. It is found that (i) significant enhancement of heat transfer performance due to suspension of nanoparticles in the circular tube flow is observed in comparison with pure water as the working fluid, (ii) enhancement is intensified with an increase in the Reynolds number and the nanoparticles concentration, and (iii) substantial amplification of heat transfer performance is not attributed purely to the enhancement of thermal conductivity due to suspension of nanoparticles.

Local Swirl Chamber Heat Transfer and Flow Structure at Different Reynolds Numbers

1999 ◽  
Vol 122 (2) ◽  
pp. 375-385 ◽  
Author(s):  
C. R. Hedlund ◽  
P. M. Ligrani
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Flow Structure ◽  
Flow Behavior ◽  
Turbine Blades ◽  
Reynolds Numbers ◽  
Local Flow ◽  
Flux Boundary Condition ◽  
Nusselt Numbers ◽  
Swirl Chamber

Local flow behavior and heat transfer results are presented from two swirl chambers, which model passages used to cool the leading edges of turbine blades in gas turbine engines. Flow results are obtained in an isothermal swirl chamber. Surface Nusselt number distributions are measured in a second swirl chamber (with a constant wall heat flux boundary condition) using infrared thermography in conjunction with thermocouples, energy balances, and in situ calibration procedures. In both cases, Reynolds numbers Re based on inlet duct characteristics range from 6000 to about 20,000. Bulk helical flow is produced in each chamber by two inlets, which are tangent to the swirl chamber circumference. Important changes to local and globally averaged surface Nusselt numbers, instantaneous flow structure from flow visualizations, and distributions of static pressure, total pressure, and circumferential velocity are observed throughout the swirl chambers as the Reynolds number increases. Of particular importance are increases of local surface Nusselt numbers (as well as ones globally averaged over the entire swirl chamber surface) with increasing Reynolds number. These are tied to increased advection, as well as important changes to vortex characteristics near the concave surfaces of the swirl chambers. Higher Re also give larger axial components of velocity, and increased turning of the flow from each inlet, which gives Go¨rtler vortex pair trajectories greater skewness as they are advected downstream of each inlet. [S0889-504X(00)00502-X]

scholarly journals Local Swirl Chamber Heat Transfer and Flow Structure at Different Reynolds Numbers

1999 ◽  
Author(s):  
C. R. Hedlund ◽  
P. M. Ligrani
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Flow Structure ◽  
Flow Behavior ◽  
Turbine Blades ◽  
Reynolds Numbers ◽  
Local Flow ◽  
Flux Boundary Condition ◽  
Nusselt Numbers ◽  
Swirl Chamber

Local flow behavior and heat transfer results are presented from two swirl chambers, which model passages used to cool the leading edges of turbine blades in gas turbine engines. Flow results are obtained in an isothermal swirl chamber. Surface Nusselt number distributions are measured in a second swirl chamber (with a constant wall beat flux boundary condition) using infrared thermography, in conjunction with thermocouples, energy balances, and in situ calibration procedures. In both cases, Reynolds numbers Re based on inlet duct characteristics range from 6000 to about 20000. Bulk helical flow is produced in each chamber by two inlets which ore tangent to the swirl chamber circumference. Important changes to local and globally-averaged surface Nusselt numbers, instantaneous flow structure from flow visualizations, and distributions of static pressure, total pressure, and circumferential velocity are observed throughout the swirl chambers as the Reynolds number increases. Of particular importance are increases of local surface Nusselt numbers (as well as ones globally-averaged over the entire swirl chamber surface) with increasing Reynolds number. These are tiad to increased advection, as well as important changes to vortex characteristics near the concave surfaces of the swirl chambers. Higher Re also give larger axial components of velocity, and increased turning of the flow from each inlet, which gives Görtler vnrtex pair trajectories greater skewness as they are advected downstream of each inlet.

scholarly journals Nusselt numbers for Poiseuille flow over isoflux parallel ridges accounting for meniscus curvature

2016 ◽  
Vol 811 ◽  
pp. 315-349 ◽  
Author(s):  
Toby L. Kirk ◽  
Marc Hodes ◽  
Demetrios T. Papageorgiou
Keyword(s):  
Nusselt Number ◽  
Poiseuille Flow ◽  
Numerical Solutions ◽  
Surface Deformation ◽  
Slip Length ◽  
Constant Heat Flux ◽  
Flux Boundary Condition ◽  
Nusselt Numbers ◽  
Dual Series Equations ◽  
Hydrodynamic Slip

We investigate forced convection in a parallel-plate-geometry microchannel with superhydrophobic walls consisting of a periodic array of ridges aligned parallel to the direction of a Poiseuille flow. In the dewetted (Cassie) state, the liquid contacts the channel walls only at the tips of the ridges, where we apply a constant-heat-flux boundary condition. The subsequent hydrodynamic and thermal problems within the liquid are then analysed accounting for curvature of the liquid–gas interface (meniscus) using boundary perturbation, assuming a small deflection from flat. The effects of this surface deformation on both the effective hydrodynamic slip length and the Nusselt number are computed analytically in the form of eigenfunction expansions, reducing the problem to a set of dual series equations for the expansion coefficients which must, in general, be solved numerically. The Nusselt number quantifies the convective heat transfer, the results for which are completely captured in a single figure, presented as a function of channel geometry at each order in the perturbation. Asymptotic solutions for channel heights large compared with the ridge period are compared with numerical solutions of the dual series equations. The asymptotic slip length expressions are shown to consist of only two terms, with all other terms exponentially small. As a result, these expressions are accurate even for heights as low as half the ridge period, and hence are useful for engineering applications.

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