Compressibility and Rarefaction Effect on Heat Transfer in Rough Microchannels

2007 ◽  
Author(s):  
Giulio Croce ◽  
Paola D’Agaro
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Mach Number ◽  
Pressure Drop ◽  
Fluid Dynamic ◽  
Slip Flow ◽  
Flow Conditions ◽  
Rarefied Flow ◽  
Wide Range ◽  
Poiseuille Number

High pressure drop and high length to hydraulic diameter ratios yield significant compressibility effects in microchannel flows, which compete with rarefaction phenomena at the smaller scale. In such regimes, flow field and temperature field are no longer decoupled. In presence of significant heat transfer, and combined with the effect of viscous dissipation, this yields to a quite complex thermo-fluid dynamic problem. A finite volume compressible solver, including generalized Maxwell slip flow and temperature jump boundary conditions suitable for arbitrary geometries, is adopted. Roughness geometry is modeled as a series of triangular shaped obstructions, and relative roughness from 0% to 2.65% were considered. The chosen geometry allows for direct comparison with pressure drop computations carried out, in a previous paper, under adiabatic conditions. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 300 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to around 0.05, and the competing effects of rarefaction, compressibility and roughness are investigated in detail. Compressibility is found to be the most dominant effect at high Mach number, yielding even inversion of heat flux, while roughness has a strong effect in the case of rarefied flow. Furthermore, the mutual interaction between heat transfer and pressure drop is highlighted, comparing Poiseuille number values for both cooled and heated flows with previous adiabatic computations.

Compressibility and Rarefaction Effects on Pressure Drop in Rough Microchannels

2006 ◽  
Author(s):  
Giulio Croce ◽  
Paola D’Agaro ◽  
Alessandro Filippo
Keyword(s):  
Reynolds Number ◽  
Mach Number ◽  
Pressure Drop ◽  
Slip Flow ◽  
Major Effect ◽  
Flow Conditions ◽  
Fully Developed Flow ◽  
Compressibility Effect ◽  
Wide Range ◽  
Poiseuille Number

A numerical analysis of the flow field in rough microchannel is carried out with a finite volume compressible solver, including generalized Maxwell slip flow boundary conditions suitable for arbitrary geometries. Roughness geometry is modeled as a series of triangular shaped obstructions. Relative roughness from 0% to 2.65% were considered. Since for truly compressible flow we have no fully developed flow condition, the simulation is performed over the whole length of the channel. A wide range of Mach number is considered, from nearly incompressible to chocked flow conditions. Flow conditions with Reynolds number up to around 200 were computed. The outlet Knudsen number corresponding to the chosen range of Mach and Reynolds number ranges from very low value to 0.0249. Performance charts are presented in terms of both average and local Poiseuille number as a function of local Kn, Ma and Re. In particular, it appears that roughness strongly decreases the reduction in pressure loss due to rarefaction. Thus, roughness effect is stronger at high Kn. Furthermore, compressibility effect has a major effect on pressure drop, as soon as local Mach number exceed 0.3.

Friction Factor Correlations of Slip Flow in Micro-Tubes

2007 ◽  
Author(s):  
Chungpyo Hong ◽  
Yutaka Asako ◽  
Mohammad Faghri
Keyword(s):  
Mach Number ◽  
Friction Factor ◽  
Slip Flow ◽  
Constant Heat Flux ◽  
Arbitrary Lagrangian Eulerian ◽  
Constant Wall Temperature ◽  
Wide Range ◽  
Exit Mach Number ◽  
Poiseuille Number ◽  
Energy Equations

Poiseuille number, the product of friction factor and Reynolds number (f·Re) for quasi-fully developed flow in a micro-tube was obtained in slip flow regime. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. Two-dimensional compressible momentum and energy equations were solved for a wide range of Reynolds and Mach numbers with two thermal boundary conditions: CWT (constant wall temperature) and CHF (constant heat flux), respectively. The tube diameter ranges from 3 to 10μm and the tube aspect ratio is 200. The stagnation pressure, pstg is chosen in such away that the exit Mach number ranges from 0.1 to 1.0. The outlet pressure is fixed at the atmospheric pressure. In slip flow, Mach and Knudsen numbers are systematically varied to determine their effects on f·Re. The correlation for f·Re is obtained from numerical results. It was found that f·Re is mainly a function of Mach number and Knudsen number and is different from the values obtained by 64/(1+8Kn) for slow flow. The obtained f·Re correlations are applicable to both no-slip and slip flow regimes.

Incompressible Criterion and Pressure Drop for Gaseous Slip Flow in Circular and Noncircular Microchannels

2011 ◽  
Vol 133 (7) ◽  
Author(s):  
Zhipeng Duan
Keyword(s):  
Mach Number ◽  
Simple Model ◽  
Pressure Drop ◽  
Compressible Flows ◽  
Gas Flow ◽  
Slip Flow ◽  
Accommodation Coefficient ◽  
Gaseous Slip Flow ◽  
Practical Engineering ◽  
Poiseuille Number

Slip flow in various noncircular microchannels has been further examined, and a simple model for a normalized Poiseuille number is proposed. As for slip flow, no solutions or graphical and tabulated data exist for most geometries; the developed simple model fills this void and can be used to predict the Poiseuille number, mass flow rate, tangential momentum accommodation coefficient, pressure distribution, and pressure drop of slip flow in noncircular microchannels by the research community for the practical engineering design of microchannels. The incompressible flow criterion for gas flow in microchannels is given. A Mach number less than 0.3 is not sufficient to ensure that the flow is incompressible. Compressibility depends on the product of two dimensionless parameters: L/L(DRe)(DRe) and Ma (Arkilic et al., 1997, “Gaseous Slip Flow in Long Microchannels,” J. Microelectromech. Syst., 6(2), pp. 167–178). Some flows where Ma < 0.3 are low speed compressible flows. A fresh general pressure drop model for isothermal low Mach number compressible flow in microchannels is proposed. If the pressure drop is less than 10% of the outlet pressure, the flow can be considered as incompressible for practical engineering applications. This paper improves and extends previous studies on slip flow in noncircular microchannels.

Heat Transfer and Pressure Drop Characteristics for a Turbine Casing Impingement Cooling System

2010 ◽  
Author(s):  
F. Ben Ahmed ◽  
B. Weigand ◽  
K. Meier
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Mach Number ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Cooling System ◽  
Impinging Jets ◽  
Turbine Casing ◽  
The Heat Transfer Coefficient

Flow mechanisms, heat transfer and discharge coefficient characteristics for a representative part of a turbine casing cooling system, consisting of an array of 20 impinging jets, were numerically investigated. The study focused on the influence of the jet Mach number while maintaining the Reynolds number constant at Re = 7,500. Therefore, the orifice bore diameter or the fluid density had to be varied. The objectives of the current CFD simulations have not been adressed before in literature, not only because heat transfer characteristics and pressure drop are given for impingement jet Mach numbers up to 0.72 at a constant relatively low Reynolds number, but also because fundamental understanding of physical phenomena of the flow in the cylindrical plenum and in the small sharp-edged orifices at the bottom side of the tube is provided. Increasing the Mach number by simultaneously reducing the orifice diameters led to slightly decreasing Nusselt numbers, with average deviations of the order of 14%. However, the heat transfer coefficient increased considerably with increasing Mach number. On the contrary, the variation of the Mach number by varying the density showed only a slight influence on the heat transfer coefficient. The predicted discharge coefficients increased significantly by augmenting the Mach number.

scholarly journals Heat transfer in a triangular wavy channel with CuO-water nanofluids under pulsating flow

2019 ◽  
Vol 23 (1) ◽  
pp. 191-205 ◽  
Author(s):  
Unal Akdag ◽  
Selma Akcay ◽  
Dogan Demiral
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Pressure Drop ◽  
Performance Enhancement ◽  
Volume Fraction ◽  
Pulsating Flow ◽  
Flow Conditions ◽  
Wavy Channel ◽  
Inlet Conditions ◽  
Wavy Channels

In this paper, heat transfer and pressure drop characteristics of CuO-water nanofluid flow in a isothermally heated triangular-wavy channel under pulsating inlet conditions are numerically investigated. A numerical simulation is conducted by solving the governing continuity, momentum, and energy equations for laminar flow using the finite volume approach. In the studies, the main parameters including the Reynolds number, pulsating amplitude and frequency, are changed while the nanoparticle volume fraction and the other parameters are kept constant for all cases. Numerical results are compared with the steady flow conditions, which showed that heat transfer performance significantly increases due to improve thermal conductivity and the use of nanoparticles in the pulsating flow conditions. The results indicate that there is a high potential for promoting the thermal performance enhancement by using nanoparticles under pulsating flow in wavy channels. It is found that the heat transfer enhancement increases with increasing pulsating amplitude and Reynolds number, and there is a slight increase in pressure drop. The obtained results are given as a function of dimensionless parameters.

Multijet Impingement on Pin Fin Heat Sink With Different Crossflow Schemes

2011 ◽  
Author(s):  
Nagesh K. Chougule ◽  
Gajanan V. Parishwad ◽  
Sachin Pagnis ◽  
Prashant R. Gore
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Sink ◽  
Cross Flow ◽  
Flow Conditions ◽  
Maximum Heat ◽  
Pin Fin ◽  
Flow Scheme ◽  
Air Jet ◽  
Wide Range

The fluid flow and heat transfer characteristics of multi air jet array impinging on a 4×4 pin fin heat sink with 3×3 nozzle arrays are investigated both numerically and experimentally. The results for multi jet impingement with wide range of parameters are not readily available in the literature. Different exit flow conditions such as minimum, semi and maximum cross flow conditions are simulated using shear stress transport (SST) k-ω turbulence model to study the combined effects of Reynolds number (Re) and spacing between nozzle exit and target surface (Z/d) on heat transfer coefficient (havg). The jet Reynolds number is varied from 7000 to 50000 and Z/d is varied from 6 to 10. For Re ≤ 18000, it is noticed that the minimum cross flow scheme gives maximum heat transfer, than semi cross flow and the maximum cross flow schemes at all Z/d considered here. Semi cross flow scheme works better for Re ≥ 18000. At Re = 11000 the minimum cross flow scheme indicates that Nua decreases from 50.1 to 36.41 with increase in Z/d from 6 to 10. It is also observed that the symmetry of the heat transfer patterns occur in the minimum and semi cross flow schemes as the sidewalls restrict the flow in opposite direction. However, in the maximum cross flow scheme, the stagnation peaks shifted and reduced in the stream wise direction by the strong cross flow degradation.

scholarly journals Parametric optimization of a stamped longitudinal vortex generator inside a circular tube of a solar water heater at low Reynolds numbers

2021 ◽  
Vol 3 (8) ◽  
Author(s):  
Felipe A. S. Silva ◽  
Luis Júnior ◽  
José Silva ◽  
Sandilya Kambampati ◽  
Leandro Salviano
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Pressure Drop ◽  
Reynolds Numbers ◽  
Geometric Parameters ◽  
Vortex Generators ◽  
Longitudinal Vortex ◽  
Solar Water Heater ◽  
Water Heater ◽  
Solar Water

AbstractSolar Water Heater (SWH) has low efficiency and the performance of this type of device needs to be improved to provide useful and ecological sources of energy. The passive techniques of augmentation heat transfer are an effective strategy to increase the convective heat transfer coefficient without external equipment. In this way, recent investigations have been done to study the potential applications of different inserts including wire coils, vortex generators, and twisted tapes for several solar thermal applications. However, few researchers have investigated inserts in SWH which is useful in many sectors where the working fluid operates at moderate temperatures. The longitudinal vortex generators (LVG) have been applied to promote heat transfer enhancement with a low/moderate pressure drop penalty. Therefore, the present work investigated optimal geometric parameters of LVG to enhance the heat transfer for a SWH at low Reynolds number and laminar flow, using a 3D periodical numerical simulation based on the Finite Volume Method coupled to the Genetic Algorithm optimization method (NSGA-II). The LVG was stamped over a flat plate inserted inside a smooth tube operating under a typical residential application corresponding to Reynolds numbers of 300, 600, and 900. The geometric parameters of LGV were submitted to the optimization procedure which can find traditional LVG such as rectangular-winglet and delta-winglet or a mix of them. The results showed that the application of LGVs to enhance heat transfer is an effective passive technique. The different optimal shapes of the LVG for all Reynolds numbers evaluated improved more than 50% of heat transfer. The highest augmentation heat transfer of 62% is found for the Reynolds number 900. However, the best thermo-hydraulic efficiency value is found for the Reynolds number of 600 in which the heat transfer intensification represents 55% of the pressure drop penalty.

scholarly journals Simulation approach on turbulent thermal performance factor of Al2O3-Cu/water hybrid nanofluid in circular and non-circular ducts

2021 ◽  
Vol 3 (3) ◽  
Author(s):  
Ing Jiat Kendrick Wong ◽  
Ngieng Tze Angnes Tiong
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Pressure Drop ◽  
Thermal Performance ◽  
Heat Transfer Performance ◽  
Pure Water ◽  
Hybrid Nanofluid ◽  
Performance Factor ◽  
Thermal Performance Factor ◽  
The Heat Transfer Coefficient

AbstractThis paper presents the numerical study of thermal performance factor of Al2O3-Cu/water hybrid nanofluid in circular and non-circular ducts (square and rectangular). Turbulent regime is studied with the Reynolds number ranges from 10000 to 100000. The heat transfer performance and flow behaviour of hybrid nanofluid are investigated, considering the nanofluid volume concentration between 0.1 and 2%. The thermal performance factor of hybrid nanofluid is evaluated in terms of performance evaluation criteria (PEC). This present numerical results are successfully validated with the data from the literature. The results indicate that the heat transfer coefficient and Nusselt number of Al2O3-Cu/water hybrid nanofluid are higher than those of Al2O3/water nanofluid and pure water. However, this heat transfer enhancement is achieved at the expense of an increased pressure drop. The heat transfer coefficient of 2% hybrid nanofluid is approximately 58.6% larger than the value of pure water at the Reynolds number of 10000. For the same concentration and Reynolds number, the pressure drop of hybrid nanofluid is 4.79 times higher than the pressure drop of water. The heat transfer performance is the best in the circular pipe compared to the non-circular ducts, but its pressure drop increment is also the largest. The hybrid nanofluid helps to improve the problem of low heat transfer characteristic in the non-circular ducts. In overall, the hybrid nanofluid flow in circular and non-circular ducts are reported to possess better thermal performance factor than that of water. The maximum attainable PEC is obtained by 2% hybrid nanofluid in the square duct at the Reynolds Number of 60000. This study can help to determine which geometry is efficient for the heat transfer application of hybrid nanofluid.

The Effect of Pulsed Blowing on the Boundary Layer of a Highly Loaded Low Pressure Turbine Blade

2013 ◽  
Author(s):  
Marion Mack ◽  
Roland Brachmanski ◽  
Reinhard Niehuis
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Mach Number ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Trailing Edge ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Highly Loaded

The performance of the low pressure turbine (LPT) can vary appreciably, because this component operates under a wide range of Reynolds numbers. At higher Reynolds numbers, mid and aft loaded profiles have the advantage that transition of suction side boundary layer happens further downstream than at front loaded profiles, resulting in lower profile loss. At lower Reynolds numbers, aft loading of the blade can mean that if a suction side separation exists, it may remain open up to the trailing edge. This is especially the case when blade lift is increased via increased pitch to chord ratio. There is a trend in research towards exploring the effect of coupling boundary layer control with highly loaded turbine blades, in order to maximize performance over the full relevant Reynolds number range. In an earlier work, pulsed blowing with fluidic oscillators was shown to be effective in reducing the extent of the separated flow region and to significantly decrease the profile losses caused by separation over a wide range of Reynolds numbers. These experiments were carried out in the High-Speed Cascade Wind Tunnel of the German Federal Armed Forces University Munich, Germany, which allows to capture the effects of pulsed blowing at engine relevant conditions. The assumed control mechanism was the triggering of boundary layer transition by excitation of the Tollmien-Schlichting waves. The current work aims to gain further insight into the effects of pulsed blowing. It investigates the effect of a highly efficient configuration of pulsed blowing at a frequency of 9.5 kHz on the boundary layer at a Reynolds number of 70000 and exit Mach number of 0.6. The boundary layer profiles were measured at five positions between peak Mach number and the trailing edge with hot wire anemometry and pneumatic probes. Experiments were conducted with and without actuation under steady as well as periodically unsteady inflow conditions. The results show the development of the boundary layer and its interaction with incoming wakes. It is shown that pulsed blowing accelerates transition over the separation bubble and drastically reduces the boundary layer thickness.

Enhancement of Heat Transfer of Rectangular Saw Tooth Shaped Micro Channel Heat Sink with Water Nano Fluid/Aluminium Oxide

2015 ◽  
Vol 813-814 ◽  
pp. 685-689
Author(s):  
M. Vijay Anand Marimuthu ◽  
B. Venkatraman ◽  
S. Kandhasamy
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Pressure Drop ◽  
Heat Sink ◽  
Micro Channel ◽  
Performance Curve ◽  
Theoretical Level ◽  
Enhancement Of Heat Transfer ◽  
Nano Fluid

This paper investigates the performance and characteristics of saw tooth shape micro channel in the theoretical level. If the conduct area of the nano fluid increases the heat transfer also increases. The performance curve has drawn Reynolds number against nusselt number, heat transfer co efficient. Pressure drop plays an important role in this device. If pressure drop is high the heat transfer increases. The result in this experiment shows clearly that the heat transfer is optimized.

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