Showing papers on "Nusselt number published in 1994"

Natural convection flow in a square cavity revisited: Laminar and turbulent models with wall functions

Abstract: Numerical simulations have been undertaken for the benchmark problem of natural convection flow in a square cavity. The control volume method is used to solve the conservation equations for laminar and turbulent flows for a series of Rayleigh numbers (Ra) reaching values up to 1010. The k-ϵ model has been used for turbulence modelling with and without logarithmic wall functions. Uniform and non-uniform (stretched) grids have been employed with increasing density to guarantee accurate solutions, especially near the walls for high Ra-values. ADI and SIP solvers are implemented to accelerate convergence. Excellent agreement is obtained with previous numerical solutions, while some discrepancies with others for high Ra-values may be due to a possibly different implementation of the wall functions. Comparisons with experimental data for heat transfer (Nusselt number) clearly demonstrates the limitations of the standard k-ϵ model with logarithmic wall functions, which gives significant overpredictions.

Free convection on a vertical stretching surface with suction and blowing

Abstract: The natural convective heat transfer from a vertical stretching sheet with surface mass transfer is analyzed. A three dimensional similarity solution to the governing momentum and energy equations is presented. Numerical data for the friction factor and Nusselt number has been tabulated for a range of surface mass transfer rates and Prandtl numbers. Surface mass transfer has a considerable influence on the heat transfer mechanism.

Heat transfer characteristics of a continuous stretching surface

Abstract: The similarity solutions for the governing ordinary differential equations of the boundary layer corresponding to a stretching surface have been reported. Power law velocity and temperature distribution were assumed for velocity exponent 3≥m≥−0.41176, −1.1≥m≥−3, and for temperature exponent 3≥n≥−3. Solutions have been found forn=0 and allm where heat transferred from the stretching surface to the ambient. The direction and amount of heat flow were found to be dependent on the magnitude ofn andm for the same Prandtl number. Nusselt number increases with increasingm andPr for uniform and variable surface temperature however, for uniform surface heat flux it decreases with increasingm for constantPr.

Heat transfer characteristics of water flowing through microchannels

Abstract: The forced-flow convection of water through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and aspect ratios of H/W = 0.333-1 was investigated experimentally. The flow friction was measured to analyze the heat transfer regimes and to explore the physical aspects of the connective heat transfer. The experimental measurements indicated that the upper bound of the laminar heat transfer regime occurred at a Reynolds number of 200-700, and fully turbulent connective heat transfer was reached at Reynolds numbers of 400-1,500. The transition Reynolds number diminished with the reduction of the microchannel dimension, and the transition range was observed to become smaller in magnitude. For the laminar heat transfer regime, the Nusselt number was found to be proportional to Re0.62while the turbulent heat transfer case exhibited a typical relationship between Nu and Re, but with a different empirical coefficient, CH,t. The geometric parameters were found to be important variables that could...

Heat transfer characteristics of a continuous stretching surface

Abstract: The similarity solutions for the governing ordinary differ- ential equations of the boundary layer corresponding to a stretching surface have been reported, Power law velocity and temperature distribution were assumed for velocity exponent 3 > m > - 041176, - 11 _> m >_ - 3, and for temperature exponent 3 > n >_ - 3 Solu- tions have been found for n = 0 and all m where heat transferred from the stretching surface to the ambient The direction and amount of heat flow were found to be dependent on the magnitude of n and m for the same Prandtl mmaber Nusselt number increases with increasing m and Pr for uniform and variable surface tempera- ture however, for uniform surface heat flux it decreases with increas- ing m for constant Pr

Comparison of Wing-Type Vortex Generators for Heat Transfer Enhancement in Channel Flows

Abstract: Longitudinal vortices can be generated in a channel flow by punching or mounting small triangular or rectangular pieces on the channel wall. Depending on their forms, these vortex generators (VG) are called delta wing, rectangular wing, pair of delta winglets, and pair of rectangular winglets. The heat transfer enhancement and the flow losses incurred by these four basic forms of VGs have been measured and compared in the Reynolds number range of 2000 to 9000 and for angles of attack between 30 and 90 deg. Local heat transfer coefficients on the wall have been measured by liquid crystal thermography. Results show that winglets perform better than wings and a pair of delta winglets can enhance heat transfer by 46 percent at Re=2000 to 120 percent at Re=8000 over the heat transfer on a plate.

Forced Convection in a Porous Channel With Localized Heat Sources

Abstract: A numerical study is performed to analyze steady laminar forced convection in a channel filled with a fluid-saturated porous medium and containing discrete heat sources on the bottom wall. Hydrodynamic and heat transfer results are reported for two configurations: (1) a fully porous channel, and (2) a partially porous channel, which contains porous layers above the heat sources and is nonporous elsewhere. The flow in the porous medium is modeled using the Brinkman-Forchheimer extended Darcy model. Heat transfer rates and pressure drop are evaluated for wide ranges of Darcy and Reynolds numbers. Detailed results of the evolution of the hydrodynamic and thermal boundary layers are also provided

Analysis of forced convection enhancement in a channel using porous blocks

Abstract: This work presents a detailed investigation of forced convection enhancement in a channel using multiple emplaced porous blocks. The Brinkman-Forchheimer-extended Darcy model is used to characterize the flowfield inside the porous regions in order to account for the inertia effects as well as the viscous effects. Solution of the coupled governing equations for the porous/fluid composite system is obtained using a stream function-vorticity approach. Important fundamental and practical results have been presented and discussed. These results thoroughly document the dependence of the streamline, isotherm, and local Nusselt number distributions on the governing parameters defining the problem, such as the Reynolds number, Darcy number, Prandtl number, inertial parameter, and two pertinent geometric parameters. An in-depth discussion of the formation and variation of the recirculation caused by the porous medium is presented, and the existence of an optimum porous matrix is demonstrated. It is shown that altering some parametric values can have significant and interesting effects on both the flow pattern as well as heat transfer characteristics.

Heat Transfer Measurement and Analysis for Sintered Porous Channels

Abstract: This paper presents the results of heat transfer measurement and analysis for two 5×5×1 cm porous channels. The channels were made of sintered bronze beads with two different mean diameters, d p =0.72 and 1.59 mm. The local wall temperature distribution, inlet and outlet pressure and temperatures, and heat transfer coefficients were measured for heat flux of 0.8, 1.6, 2.4, and 3.2 W/cm 2 with air velocity ranging from 0.16 to 5 m/s and inlet air pressure of 1∼3 atm

3D convection at infinite Prandtl number in Cartesian geometry — a benchmark comparison

Abstract: We describe the results of a benchmark study of numerical codes designed to treat problems of high Prandtl number convection in three-dimensional Cartesian geometry. In addition, quantitative results from laboratory convection experiments are compared with numerical data. The cases of bimodal convection at constant viscosity and of square cell convection for temperature-dependent viscosity have been selected.

Application of a Complex Nusselt Number to Heat Transfer During Compression and Expansion

Abstract: Heat transfer during compression and expansion can be out of phase with bulk gas-wall temperature difference. An ordinary convective heat transfer model is incapable of predicting this phenomenon. Expressions for compression/expansion heat transfer developed from simple conduction models use a complex heat transfer coefficient. Thus, heat flux consists of one part proportional to temperature difference plus a second part proportional to rate of change of temperature. Surface-averaged heat flux was calculated from experimental pressure-volume data for piston-cylinder gas springs over a range of speeds, pressures, gases, and geometries. The complex Nusselt number model proved capable of correlating both magnitude and phase of the measured heat transfer as functions of an oscillation Peclet number.

Mixed Convection From Simulated Electronic Components at Varying Relative Positions in a Cavity

Abstract: A numerical study of the combined forced and natural convective cooling of heat-dissipating electronic components, located in a rectangular enclosure, and cooled by an external through flow of air is carried out. A conjugate problem is solved, describing the flow and thermal fields in air, as well as the thermal field within the walls of the enclosure and the electronic components themselves. The interaction between the components is of interest here, depending on their relative placement in the enclosure, and different configurations are considered. For Re=100 laminar, steady flow is predicted for up to Gr/Re 2 =10, but R single-frequency oscillatory behavior is observed for most of the configurations studied, at Gr/R 2 =50

Unsteady Wake Over a Linear Turbine Blade Cascade With Air and CO2 Film Injection: Part I—Effect on Heat Transfer Coefficients

Abstract: The effect of unsteady wake flow and air (D.R. = 1.0) or CO 2 (D.R. = 1.52) film injection on blade heat transfer coefficients was experimentally determined. a spoked wheel-type wake generator produced the unsteady wake. Experiments were performed on a five-airfoil linear cascade in a low-speed wind tunnel at the chord Reynolds number of 3 × 10 5 for the no-wake case and at the wake Strouhal numbers of 0.1 and 0.3. Results from a blade with three rows of film holes in the leading edge region and two rows each on the pressure and suction surfaces show that the Nusselt numbers are much higher than those for the blade without film holes. On a large portion of the blade, the Nusselt numbers «without wake but with film injection» are much higher than for «with wake but no film holes»

Convective Heat Transfer to a Confined Impinging Array of Air Jets With Spent Air Exits

Abstract: This investigation has examined the influence of spent air exits located between the jets on the magnitude and uniformity of the local heat transfer coefficient for a confined 3×3 square array of axisymmetric air jets impinging normally to a heated surface. The heat transfer coefficient was measured using a 0.025-mm-thick stainless steel impingement surface coated with liquid crystals. The temperature distribution along the surface was determined by measuring the reflected wavelength of light from the liquid crystal with the use of bandpass filters and an electronic digitizer board. The effect of small noise-to-plate spacings (0.25 and 1.0 diameters) commonly used in material processing applications was also considered

Experimental Investigation of Heat Transfer and Flow Over Baffles of Different Heights

Abstract: The phenomenon of flow separation in ducts with segmented baffles has many engineering applications, for example, shell-and-tube heat exchangers with segmented baffles, labyrinth shaft seals, laser curtain seals, air-cooled solar collectors, and internally cooled turbine blades. In the present work, an experimental investigation has been done to study the characteristics of the turbulent flow and heat transfer inside the periodic cell formed between segmented baffles staggered in a rectangular duct. In particular, flowfield, pressure loss, and local and average heat transfer coefficients were obtained. The experimental runs were carried out for different values of Reynolds numbers and baffle heights (window cuts) at uniform wall heat flux condition along the top and bottom walls

Correlations for predicting the air-side nusselt numbers and friction factors in chilled-water cooling coils

Abstract: An experimental study was conducted to determine Nusselt numbers and friction factors on the air side of wavy-finned, chilled-water cooling coils. General correlations of the dry-surface Nusselt numbers and friction factors were developed from the data obtained from five different cooling coils. A comparison of the Nusselt number correlation to data from the literature revealed that the correlation was generally able to predict the reported Nusselt numbers within 20%, Under wet-surface conditions, the measured Nusselt numbers showed considerable scatter, with some of the results being higher than the corresponding dry-surface values, while others were lower than the dry-surface values. Friction factors were substantially higher under wet-surface conditions. A correlation to predict Ike friction factors for wet surfaces was also developed.

Analytical Solutions of Pennes Bio-Heat Transfer Equation With a Blood Vessel

Abstract: The heat transfer within a perfused tissue in the presence of a vessel is considered. The bio-heat transfer equation is used for the perfused tissue and a lumped capacitance analysis is used for the convection in the vessel with a constant Nusselt number. Analytical solutions are obtained for two cases: (i) the arterial temperature of the perfused blood in the bio-heat transfer equation is equal to the axially varying mixed mean temperature of the blood in the vessel and, (ii) that arterial temperature is assumed to be constant. Dimensionless equilibrium length and temperature expressions are obtained and presented.