Prediction of Natural Convection Heat Transfer in Layered Porous Cavities by Homogeneous Anisotropic Model

2008 ◽  
Author(s):  
R. L. Marvel ◽  
F. C. Lai
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Transfer Coefficient ◽  
System Model ◽  
Porous System ◽  
Anisotropic Model ◽  
Wide Range ◽  
Sublayer Thickness ◽  
The Heat Transfer Coefficient

Steady-state heat transfer by natural convection in a layered porous cavity is examined by using homogeneous anisotropic model. The geometry considered is a two-dimensional square enclosure comprising of three or four porous layers with non-uniform thickness and distinct permeability. The cavity is subjected to differential heating from the vertical walls. The results, which include the flow patterns and temperature profiles as well as the heat transfer coefficients, are presented for a wide range of permeability ratio, sublayer thickness ratio, and Rayleigh number. Particularly, the heat transfer results obtained are compared with those reported from a rigorous numerical model for layered porous media. In addition, the results are compared with the lumped system model that was proposed recently. It has been found that homogeneous anisotropic model predicts the heat transfer coefficient reasonably well within the conductive flow regime. However, beyond this regime, the model fails to represent the layered case for the effective permeabilities and sublayer thickness ratios considered. On the other hand, it is observed that the lumped system model offers better agreement to the heat transfer coefficient of the actual layered porous system over a wider range of parameters and which also significantly reduces computational efforts.

A Natural Convection Fin with a Solution-Determined Nonmonotonically Varying Heat Transfer Coefficient

1981 ◽  
Vol 103 (2) ◽  
pp. 218-225 ◽  
Author(s):  
E. M. Sparrow ◽  
S. Acharya
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Transfer Coefficient ◽  
First Principles ◽  
Numerical Solutions ◽  
Flow Direction ◽  
Operating Conditions ◽  
Fluid Temperature ◽  
Wide Range

A conjugate conduction-convection analysis has been made for a vertical plate fin which exchanges heat with its fluid environment by natural convection. The analysis is based on a first-principles approach whereby the heat conduction equation for the fin is solved simultaneously with the conservation equations for mass, momentum, and energy in the fluid boundary layer adjacent to the fin. The natural convection heat transfer coefficient is not specified in advance but is one of the results of the numerical solutions. For a wide range of operating conditions, the local heat transfer coefficients were found not to decrease monotonically in the flow direction, as is usual. Rather, the coefficient decreased at first, attained a minimum, and then increased with increasing downstream distance. This behavior was attributed to an enhanced buoyancy resulting from an increase in the wall-to-fluid temperature difference along the streamwise direction. To supplement the first-principles analysis, results were also obtained from a simple adaptation of the conventional fin model.

Predicting Heat Transfer From Unsteady Synthetic Jets

2012 ◽  
Vol 134 (8) ◽  
Author(s):  
Mehmet Arik ◽  
Tunc Icoz
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Jet Impingement ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Operating Conditions ◽  
Small Scale ◽  
Wide Range ◽  
The Heat Transfer Coefficient

Synthetic jets are piezo-driven, small-scale, pulsating devices capable of producing highly turbulent jets formed by periodic entrainment and expulsion of the fluid in which they are embedded. The compactness of these devices accompanied by high air velocities provides an exciting opportunity to significantly reduce the size of thermal management systems in electronic packages. A number of researchers have shown the implementations of synthetic jets on heat transfer applications; however, there exists no correlation to analytically predict the heat transfer coefficient for such applications. A closed form correlation was developed to predict the heat transfer coefficient as a function of jet geometry, position, and operating conditions for impinging flow based on experimental data. The proposed correlation was shown to predict the synthetic jet impingement heat transfer within 25% accuracy for a wide range of operating conditions and geometrical variables.

scholarly journals Heat Exchange Modeling in Cooling Fins

2020 ◽  
pp. 669-676
Author(s):  
Evgeniy N. Vasil'ev
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Heat Exchange ◽  
Transfer Coefficient ◽  
Heat Conduction Problem ◽  
Rectangular Cross Section ◽  
One Dimensional ◽  
Wide Range ◽  
Simulation Results ◽  
The Heat Transfer Coefficient

The article discusses the process of heat exchange of a finned wall with a coolant. The temperature field in the wall volume was determined on the basis of a numerical solution of the two-dimensional heat conduction problem, and the analysis of the characteristics of temperature distributions was carried out according to the simulation results. The values of the heat transfer coefficient of cooling fins with rectangular cross section were calculated for two variants of heat transfer conditions at the end of the fins in a wide range of dimensionless parameters. The error in calculating the heat transfer coefficient in the approximation of a thin fin was determined by means of a one-dimensional computational model

Experimental Analysis of Fluid Flow and Surface Heat Transfer in a Three-Pass Trapezoidal Serpentine Smooth Passage

1998 ◽  
Author(s):  
C. Cravero ◽  
C. Giusto ◽  
A. F. Massardo
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Experimental Analysis ◽  
Fluid Dynamic ◽  
Flow Analysis ◽  
Strong Impact ◽  
Wide Range ◽  
Rectangular Configuration ◽  
The Heat Transfer Coefficient

The fluid-dynamic and heat transfer experimental analysis of a gas turbine internal three-pass blade cooling channel is presented. The passage is composed of three rectilinear channels joined by two sharp 180 degree turns; moreover the channel section is trapezoidal instead of the rectangular configuration already analysed in depth in literature. The trapezoidal section is more representative of the actual geometrical configuration of the blade and, in comparison with the rectangular section, it shows significant aspect ratio and hydraulic diameter variations along the channel. These variations have a strong impact on the flow field and the heat transfer coefficient distributions. The flow analysis experimental results — wall pressure distributions, flow visualisations — are presented and discussed. The heat transfer coefficient distributions, Nusselt enhancement factor, obtained using Thermocromic Liquid Crystals (TLC), have been studied as well. In order to understand the influence of the cooling mass flow rate, a wide range of flow regimes-Reynolds numbers- has been considered.

The Effect of Density Variation on Heat Transfer in the Critical Region

1961 ◽  
Vol 83 (2) ◽  
pp. 176-181 ◽  
Author(s):  
Yih-Yun Hsu ◽  
J. M. Smith
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Turbulent Flow ◽  
Transfer Coefficient ◽  
Critical Point ◽  
Critical Region ◽  
Large Density ◽  
The Heat Transfer Coefficient

The heat-transfer coefficient between fluid and tube wall in turbulent flow depends upon the physical and thermal properties of the fluid. When density changes across the diameter of the tube are large (for example, when the fluid is near the critical point), the variable density can affect the transfer of momentum and heat. Equations are developed for predicting the magnitude of this effect on the heat-transfer coefficient. Deissler’s [5] expressions for the eddy diffusivity are employed in solving the equations for heat and momentum transfer. For flow in vertical tubes large density variations can also affect the heat transfer by inducing natural convection. By considering the influence of body forces on the shear stress, equations are derived to predict the effect of natural convection on the heat-transfer coefficient for turbulent flow. The results indicate that the effect is significant only for relatively high Grashof numbers and low Reynolds numbers. Such conditions may be encountered in flow of a fluid near its thermodynamic critical point. The derived equations are applied for carbon dioxide flow in the critical region under the conditions for which experimental data were measured by Bringer and Smith [2]. Because of the high Reynolds and low Grashof numbers, natural convection is not significant. However, the effect of the large density variations is found to be significant, and the predicted results agree well with the experimental data.

Heat Transfer Augmentation Due to Coolant Extraction on the Cold Side of Active Clearance Control Manifolds

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
Riccardo Da Soghe ◽  
Cosimo Bianchini ◽  
Antonio Andreini ◽  
Bruno Facchini ◽  
Lorenzo Mazzei
Keyword(s):  
Heat Transfer ◽  
Fluid Dynamics ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Gas Turbine ◽  
Wide Range ◽  
Depth Analysis ◽  
Clearance Control ◽  
The Heat Transfer Coefficient

Jet array is an arrangement typically used to cool several gas turbine parts. Some examples of such applications can be found in the impingement cooled region of gas turbine airfoils or in the turbine blade tip clearances control of large aero-engines. In the open literature, several contributions focus on the impingement jets formation and deal with the heat transfer phenomena that take place on the impingement target surface. However, deficiencies of general studies emerge when the internal convective cooling of the impinging system feeding channels is concerned. In this work, an aerothermal analysis of jet arrays for active clearance control (ACC) was performed; the aim was the definition of a correlation for the internal (i.e., within the feeding channel) convective heat transfer coefficient augmentation due to the coolant extraction operated by the bleeding holes. The data were taken from a set of computational fluid-dynamics (CFD) Reynolds-averaged Navier–Stokes (RANS) simulations, in which the behavior of the cooling system was investigated over a wide range of fluid-dynamics conditions. More in detail, several different holes arrangements were investigated with the aim of evaluating the influence of the hole spacing on the heat transfer coefficient distribution. Tests were conducted by varying the feeding channel Reynolds number in a wide range of real engine operative conditions. An in depth analysis of the numerical data set has underlined the opportunity of an efficient reduction through the local suction ratio (SR) of hole and feeding pipe, local Reynolds number, and manifold porosity: the dependence of the heat transfer coefficient enhancement factor (EF) from these parameter is roughly exponential.

Improved Correlation for Natural Convection Heat Transfer From Inclined Cylinders Having Different Emissivities

1999 ◽  
Author(s):  
Jeffrey C. Stewart ◽  
William S. Janna
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Transfer Coefficient ◽  
Convection Heat Transfer ◽  
Natural Convection Heat Transfer ◽  
Convection Heat ◽  
Transfer Coefficients ◽  
The Heat Transfer Coefficient

Abstract The purpose of this study was to develop an improved correlation for natural convection heat transfer from inclined cylinders having different emissivities. The angle of cylinder inclination varied from horizontal to vertical in 15° increments. The heat transfer coefficient was obtained experimentally with the cylinder in a state of constant heat flux. Three surface finishes were used in the experiment, which consisted of polished copper, black paint, and aluminum paint. The heat transfer coefficients in all cases varied from 1.21 to 1.65 BTU/(hr·ft2·R) [6.87 to 9.37 W/(m2·K)]. Rayeigh numbers for all experiments varied from 1.31 × 103 to 2.23 × 103. The heat transfer coefficient decreased for each cylinder with an increasing angle of inclination (from horizontal to vertical). The goal of this study was to produce Nusselt-Rayleigh number correlations for each cylinder, and then ultimately produce a single equation that can be applied for all emissivities. The Rayleigh number included a geometry term to account for the inclination of the cylinder. The form of the equation that best represented the data was a power law equation.

Thermal-Hydraulic Analytical Models of Split-Flow Microchannel Liquid-Cooled Cold Plates With Flow Impingement

2021 ◽  
Author(s):  
Deogratius Kisitu ◽  
Alfonso Ortega
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Experimental Studies ◽  
Base Plate ◽  
Analytical Models ◽  
Split Flow ◽  
Wide Range ◽  
Cold Plates ◽  
The Heat Transfer Coefficient

Abstract Impingement split flow liquid-cooled microchannel cold plates are one of several flow configurations used for single-phase liquid cooling. Split flow or top-in/side-exit (TISE) cold plates divide the flow into two branches thus resulting in halved or reduced flow rates and flow lengths, compared to traditional side-in /side-exit (SISE) or parallel flow cold plates. This has the effect of reducing the pressure drop because of the shorter flow length and lower flow rate and increasing the heat transfer coefficient due to thermally developing as opposed to fully developed flow. It is also claimed that the impinging flow increases the heat transfer coefficient on the base plate in the region of impingement. Because of the downward impinging and turning flow, there are no exact analytical models for this flow configuration. Computational and experimental studies have been performed, but there are no useful compact analytical models in the literature that can be used to predict the performance of these impingement cold plates. Results are presented for novel physics-based laminar flow models for a TISE microchannel cold plate based on an equivalent parallel channel flow approach. We show that the new models accurately predict the thermal-hydraulic performance over a wide range of parameters.

Forced Convective Heat Transfer From Helicoidal Pipes

2001 ◽  
Author(s):  
Ahmad Fakheri ◽  
Abdelrahman H. A. Alnaeim
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Characteristic Length ◽  
Convection Heat Transfer ◽  
Reynolds Numbers ◽  
Operating Conditions ◽  
Straight Pipe ◽  
Wide Range ◽  
The Heat Transfer Coefficient

Abstract Forced convection heat transfer from helicoidal pipes is experimentally investigated over a wide range of operating conditions. Based on the experimental results, a characteristic length incorporating the tube diameter, the coil diameter, and the coil spacing, is proposed as the relevant scale for defining Nusselt and Reynolds numbers. Based on this characteristic length, Nusselt number for helicoidal pipes can be predicated from the correlations available for cylinders in the range of available experimental data. It is shown that the performance of the coils depends on the Reynolds number. At high Reynolds numbers, the heat transfer coefficient is essentially equal to that of the straight pipe and the coil pitch has little influence on the heat transfer rate. On the other hand, at low Reynolds numbers, the heat transfer coefficient is lower than that of a straight pipe and its value is a strong function of the coil spacing.

Local Heat Transfer Coefficient During Condensation in a 0.8 mm Diameter Pipe

2006 ◽  
Author(s):  
Alberto Cavallini ◽  
Davide Del Col ◽  
Marko Matkovic ◽  
Luisa Rossetto
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Operating Conditions ◽  
Local Heat Transfer Coefficient ◽  
Wide Range ◽  
The Heat Transfer Coefficient

The first preliminary tests carried on a new experimental rig for measurement of the local heat transfer coefficient inside a circular 0.8 mm diameter minichannel are presented in this paper. The heat transfer coefficient is measured during condensation of R134a and is obtained from the measurement of the heat flux and the direct gauge of the saturation and wall temperatures. The heat flux is derived from the water temperature profile along the channel, in order to get local values for the heat transfer coefficient. The test section has been designed so as to reduce thermal disturbances and experimental uncertainty. A brief insight into the design and the construction of the test rig is reported in the paper. The apparatus has been designed for experimental tests both in condensation and vaporization, in a wide range of operating conditions and for a wide selection of refrigerants.

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