Showing papers on "Nusselt number published in 2005"

Impingement Heat Transfer: Correlations and Numerical Modeling

Abstract: Uses of impinging jet devices for heat transfer are described, with a focus on cooling applications within turbine systems. Numerical simulation techniques and results are described, and the relative strengths and drawbacks of the κ-e, κ-ω, Reynolds stress model, algebraic stress models, shear stress transport, and ν2f turbulence models for impinging jet flow and heat transfer are compared.

Effects Of Dimple Depth on Channel Nusselt Numbers and Friction Factors

Abstract: Experimental results, measured on dimpled test surfaces placed on one wall of different rectangular channels, are given for a ratio of air inlet stagnation temperature to surface temperature of approximately 0.94, and Reynolds numbers based on channel height from 9940 to 74,800. The data presented include friction factors, local Nusselt numbers, spatially averaged Nusselt numbers, and globally averaged Nusselt numbers. The ratios of dimple depth to dimple print diameter δ/D are 0.1, 0.2, and 0.3 to provide information on the influences of dimple depth. The ratio of channel height to dimple print diameter is 1.00

Heat transport by turbulent Rayleigh-Bénard convection in cylindrical cells with aspect ratio one and less

Abstract: We present high-precision measurements of the Nusselt number N as a function of the Rayleigh number R for cylindrical samples of water (Prandtl number σ=4.38) with diameters D = 49.7, 24.8, and 9.2 cm, all with aspect ratio F≡D/L≃1 (L is the sample height). In addition, we present data for D=49.7 and r=1.5,2,3, and 6. For each sample the data cover a range of a little over a decade of R. For Γ≃1 they jointly span the range 10 7 ≤ R ≤ 10 11 . Where needed, the data were corrected for the influence of the finite conductivity of the top and bottom plates and of the sidewalls on the heat transport in the fluid to obtain estimates of N∞ for plates with infinite conductivity and sidewalls of zero conductivity

A Two-Temperature Model for Solid-Liquid Phase Change in Metal Foams

Abstract: Transient solid-liquid phase change occurring in a phase-change material (PCM) embedded in a metal foam is investigated. Natural convection in the melt is considered. Volume-averaged mass and momentum equations are employed, with the Brinkman-Forchheimer extension to the Darcy law to model the porous resistance. Owing to the difference in the thermal diffusivities between the metal foam and the PCM, local thermal equilibrium between the two is not assured. Assuming equilibrium melting at the pore scale, separate volume-averaged energy equations are written for the solid metal foam and the PCM and are closed using an interstitial heat transfer coefficient. The enthalpy method is employed to account for phase change. The governing equations are solved implicitly using the finite volume method on a fixed grid. The influence of Rayleigh, Stefan, and interstitial Nusselt numbers on the temporal evolution of the melt front location, wall Nusselt number, temperature differentials between the solid and fluid, and the melting rate is documented and discussed. The merits of incorporating metal foam for improving the effective thermal conductivity of thermal storage systems are discussed.

Flow and Forced Convection Heat Transfer in Crossflow of Non-Newtonian Fluids over a Circular Cylinder

Abstract: The steady and incompressible flow of power-law type non-Newtonian fluids across an unconfined, heated circular cylinder is investigated numerically to determine the dependence of the individual drag components and of the heat transfer characteristics on power-law index (0.5 ≤ n ≤ 1.4), Prandtl number (1 ≤ Pr ≤ 100), and Reynolds number (5 ≤ Re ≤ 40). The momentum and energy equations are expressed in the stream function/vorticity formulation and are solved using a second-order accurate finite difference method to determine the pressure drag and frictional drag as well as the local and surface-averaged Nusselt numbers and to map the temperature field near the cylinder. The accuracy of the numerical procedure is established using previously available numerical and analytical results for momentum and heat transfer in Newtonian and power-law fluids. The results reported herein provide fundamental knowledge of the flow and heat transfer behavior for the flow of non-Newtonian fluids over a circular cylinder; t...

Effect of particle migration on heat transfer in suspensions of nanoparticles flowing through minichannels

Abstract: Recent work has shown that suspensions of highly thermally conducting nanoparticles with a size considerably smaller than 100 nm have great potential as a high-energy carrier for small channel systems. However, it is also known that particles in a suspension under certain conditions may migrate. This indicates that the efficiency of heat transfer in the small channels may not be as superior as expected, which bears significance to the system design and operation. This work aims at addressing this issue by examining the effect of particle migration on heat transfer under a fully developed laminar flow regime in small channels. This involves the development of both flow and heat transfer models, and a numerical solution to the models. The flow model takes into account the effects of the shear-induced and viscosity-gradient-induced particle migration, as well as self-diffusion due to Brownian motion, which is coupled with an energy equation. The results suggest a significant non-uniformity in particle concentration and, hence, thermal conductivity over the tube cross-section due to particle migration, particularly for large particles at high concentrations. Compared with the constant thermal conductivity assumption, the non-uniform distribution due to particle migration leads to a higher Nusselt number, which depends on the Peclet number and the mean particle concentration. Further improvement of the model is needed to take into account other factors such as entrance effects, as well as the dynamics of particles and particle–wall interactions.

Similarity solutions for MHD thermosolutal Marangoni convection over a flat surface in the presence of heat generation or absorption effects

Abstract: The problem of steady, laminar, thermosolutal Marangoni convection flow of an electrically-conducting fluid along a vertical permeable surface in the presence of a magnetic field, heat generation or absorption and a first-order chemical reaction effects is studied numerically. The general governing partial differential equations are converted into a set of self-similar equations using unique similarity transformations. Numerical solution of the similarity equations is performed using an implicit, iterative, tri-diagonal finite-difference method. Comparisons with previously published work is performed and the results are found to be in excellent agreement. Approximate analytical results for the temperature and concentration profiles as well as the local Nusselt and sherwood numbers are obtained for the conditions of small and large Prandtl and Schmidt numbers are obtained and favorably compared with the numerical solutions. The effects of Hartmann number, heat generation or absorption coefficient, the suction or injection parameter, the thermo-solutal surface tension ratio and the chemical reaction coefficient on the velocity, temperature and concentration profiles as well as quantitites related to the wall velocity, boundary-layer mass flow rate and the Nusselt and Sherwood numbers are presented in graphical and tabular form and discussed. It is found that a first-order chemical reaction increases all of the wall velocity, Nusselt and Sherwood numbers while it decreases the mass flow rate in the boundary layer. Also, as the thermo-solutal surface tension ratio is increased, all of the wall velocity, boundary-layer mass flow rate and the Nusselt and Sherwood numbers are predicted to increase. However, the exact opposite behavior is predicted as the magnetic field strength is increased.

Optimized Heat Transfer for High Power Electronic Cooling Using Arrays of Microjets

Abstract: Electronic cooling has become a subject of interest in recent years due to the rapidly decreasing size of microchips while increasing the amount of heat flux that they must dissipate. Conventional forced air cooling techniques cannot satisfy the cooling requirements and new methods have to be sought. Jet cooling has been used in other industrial fields and has demonstrated the capability of sustaining high heat transfer rates. In this work the heat transfer under arrays of microjets is investigated. Ten different arrays have been tested using deionized water and FC40 as test fluids. The jet diameters employed ranged between 69 and 250μm and the jet Reynolds number varied from 73 to 3813. A maximum surface heat flux of 310W∕cm2 was achieved using water jets of 173.6μm diameter and 3mm spacing, impinging at 12.5m∕s on a circular 19.3mm diameter copper surface. The impinging water temperature was 23.1°C and the surface temperature was 73.9°C. The heat transfer results, consistent with those reported in the literature, have been correlated using only three independent dimensionless parameters. With the use of the correlation developed, an optimal configuration of the main geometrical parameters can be established once the cooling requirements of the electronic component are specified.

Natural convection near a vertical plate with ramped wall temperature

Abstract: The unsteady natural convective flow of an incompressible viscous fluid near a vertical plate has been considered. It is assumed that the bounding plate has a ramped temperature profile. The exact solutions of the energy and momentum equations, under the usual Boussinesq approximation, have been obtained in closed form. There are two different solutions for the fluid velocity—one valid for the fluids of Prandtl numbers different from unity, and the other for which the Prandtl number is unity. The variations of the fluid temperature, velocity as well as the Nusselt number and wall skin friction have been presented graphically. The natural convection near a ramped temperature plate has also been compared with the flow near a plate with constant temperature.

Theoretical and experimental studies on mesoscale flame propagation and extinction

Abstract: Mesoscale flame propagation and extinction of premixed flames in channels are investigated theoretically and experimentally. Emphasis is placed on the effect of wall heat loss and the wall–flame interaction via heat recirculation. At first, an analytical solution of flame speed in mesoscale channels is obtained. The results showed that channel width, flow velocity, and wall thermal properties have dramatic effects on the flame propagation and lead to multiple flame regimes and extinction limits. With the decrease in channel width, there exist two distinct flame regimes, a fast burning regime and a slow burning regime. The existence of the new flame regime and its extended flammability limit render the classical quenching diameter inapplicable. Furthermore, the results showed that at optimum conditions of flow velocity and wall thermal properties, mesoscale flames can propagate faster than the adiabatic flame. Second, numerical simulation with detailed chemistry demonstrated the existence of multiple flame regimes. The results also showed that there is a non-linear dependence of the flame speed on equivalence ratio. Moreover, it is shown that the Nusselt number has a significant impact on this non-linear dependence. Finally, the non-linear dependence of flame speed on equivalence ratio for both flame regimes is measured using a C3H8–air mixture. The results are in good agreement with the theory and numerical simulation.