Computational and Experimental Study of Enhanced Laminar Flow Heat Transfer in Three-Dimensional Sinusoidal Wavy-Plate-Fin Channels

Enhanced heat transfer characteristics of low Reynolds number airflows in three-dimensional sinusoidal wavy plate-fin channels are investigated. For the computational simulation, steady state, constant property, periodically developed, laminar forced convection is considered with the channel surface at the uniform heat flux condition; the wavy-fin is modeled by its two asymptotic limits of 100% and zero fin efficiency. The governing equations are solved numerically using finite-volume techniques for a non-orthogonal, non-staggered grid. Computational results for velocity and temperature distribution, isothermal Fanning friction factor f and Colburn factor j are presented for airflow rates in the range of 10 ≤ Re ≤ 1500. The numerical results are further compared with experimental data, with excellent agreement, for two different wavy-fin geometries. The influence of fin density on the flow behavior and the enhanced convection heat transfer are highlighted. Depending on the flow rate, a complex flow structure is observed, which is characterized by the generation, spatial growth and dissipation of vortices in the trough region of the wavy channel. The thermal boundary layers on the fin surface are periodically disrupted, resulting in high local heat fluxes. The overall heat transfer performance is improved considerably, compared to the straight channel with the same cross-section, with a relatively smaller increase in the associated pressure drop penalty.

Numerical Simulation for Microconvection Around Nano-Particle With Brownian Motion in Liquid

The development of a numerical model for analyzing the effect of the nano-particles’ Brownian motion on the heat transfer is described. By using the Maxwell velocity distribution relations to calculate the most possible velocity of fluid molecules at certain temperature gradient location around the nano-particle, the interaction between fluid molecules and one single nano-particle is analyzed and calculated. Based on this, a syntonic system is proposed and the coupled effect that Brownian motion of nano-particles has on fluid molecules is simulated. This is used to formulate a reasonable analytic method, facilitating laboratory study. The results provide the essential features of the heat transfer process, contributed by micro-convection to be considered.

Transpiration Cooling Using Porous Triple-Laminated Plates

Transpiration cooling using porous triple-laminated plates was numerically investigated to understand the associated flow mechanism and heat transfer characteristics with/without crossflow. The flow structure and heat transfer behavior are very similar in the two laminate gaps, and crossflow has little influence on them. The cooling performance shows very good uniformity and high efficiency. Violent impingement and turbulent flow inside the plate contribute greatly to local heat transfer intensification. The cooling efficiency might be further improved with enhancement of film cooling effect, by enlarging the discharge holes to decrease the local jet-to-crossflow velocity ratio, or by using inclined discharge holes to increase the film attaching ability.

Transmittance and Radiance Computations for Rocket Engine Plume Environments

Rocket engine exhaust plume is generally thermal in character arising from changes in the internal energy of constituent molecules. Radiation from the plume is attenuated in its passage through the atmosphere. In the visible and the infrared region of the spectrum for clear-sky conditions, this is caused mainly through absorption by atmospheric molecular species. The most important combustion-product molecules giving rise to emission in the IR are water vapor, carbon dioxide, and carbon monoxide. In addition, the high temperature plume reacting with the surrounding atmosphere may produce nitrogen oxides, in the boundary layer, all of which are strongly emitting molecules. Important absorbing species in the atmosphere in the engine plume environment are H2O, CO, CO2, CH4, N2O, NO, and NO2. Under normal atmospheric conditions, the concentrations of O3, SO2, and NH3 are too small to produce any significant absorption. Essentially the problem comprises of the propagation of radiation from a hot gas source through a long cool absorbing atmosphere thus combining aspects of atmospheric and combustion gas methods. Since many of the same molecular species are responsible for both emission and absorption, the high degree of line position correlation between the emission and absorption spectra precludes the decoupling of the optical path into isolated emitter and absorber regions and multiplying the source band radiance by the absorber band transmittance in order to arrive at the transmitted radiance spectrum. Also, very strong thermal gradients may be encountered. All this suggests that a layer-by-layer computation is called for. The pathlength through the plume and the atmosphere is assumed to go through a certain number of layers, each of which is considered to have all molecular species in local thermodynamic equilibrium at constant temperature and pressure within the layer. Radiative transfer problems can be visualized as a set of parallel layers orthogonal to the line of sight, each with an input radiance from the previous layer and an output radiance to the subsequent layer. The MODTRAN (MODerate resolution TRANsmission) code is ideally suited for layer-by-layer absorption/emission calculations for atmospheric molecular species. We have utilized MODTRAN 4.0 computer code, implemented on a Power Mac G3, for the radiance and transmittance computations. The MODTRAN code has been adapted for the engine plume radiance computations. If the plume composition and flowfield parameters such as the temperature and pressure values are known along the line of sight by means of the experimental measurements or (more likely) CFD simulations, one can compute the radiance from any plume with high degree of accuracy at any desired point in space. Emission and absorption characteristics of several atmospheric and combustion species have been studied and presented in this paper with reference to the rocket engine plume environments at the Stennis Space Center. In general transmittance losses can not be neglected for any pathlength of 2 m or more. We have also studied the effect of clouds, rain, and fog on the plume radiance/transmittance. The transmittance losses are severe if any of these occur along the line of sight. Preliminary results for the radiance from the exhaust plume of the space shuttle main engine are shown and discussed.

Effect of Compressibility on Gaseous Flows in a Micro-Tube

The product of friction factor and Reynolds number (f·Re) of gaseous flow in the quasi-fully developed region of a micro-tube was obtained experimentally and numerically. The tube cutting method was adopted to obtain the pressure distribution along the tube. The fused silica tubes whose nominal diameters were 100 and 150 μm, were used. Two-dimensional compressible momentum and energy equations were solved to obtain the flow characteristics in micro-tubes. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The both results agree well and it was found that (f·Re) is a function of Mach number.

Heat Flux Sensor With Minimal Impact on Boundary Conditions

A technique for determining the heat transfer on the far surface of a wall based on measuring the heat transfer and temperature on the near wall is presented. Although heat transfer measurements have previously been used to augment temperature measurements in inverse heat conduction methods, the sensors used alter the heat flow through the surface, disturbing the very quantity that is desired to be measured. The ideal sensor would not alter the boundary condition that would exist were the sensor not present. The innovation of this technique in that it has minimal impact on the wall boundary condition. Since the sensor is placed on the surface of the wall, no alteration of the wall is needed. The theoretical basis for the experimental technique as well as experimental results showing the heat flux sensor performance is presented.

A Comparison Between 2-D and 3-D Conduction Shape Factors

Conduction shape factors are frequently used in a variety of heat transfer applications to evaluate heat transfer from one three-dimensional body to another three-dimensional body. Previous investigators have used conduction shape factors derived using the 2-D cross-section of the 3-D geometries for non-heating conditions as approximations to 3-D conduction shape factors with heating and no-heating present. This paper investigates the suitability of neglecting the axial conduction and power deposition in deriving expressions for conduction shape factors for the case of a single, cylindrical vessel imbedded concentrically in a cylindrical, uniformly heated tissue matrix. It is shown that 1) conduction shape factors are functions of the deposited power and the temperature distribution and 2) the magnitudes of conduction shape factors are affected significantly by axial conduction.

Modeling of Localized Heating Effect in Sub-Micron Silicon Transistors

The performance and reliability of sub-micron semiconductor transistors demands accurate modeling of electron and phonon transport at nanoscales. The continued downscaling of the critical dimensions, introduces hotspots, inside transistors, with dimensions much smaller than phonon mean free path. This phenomenon, known as localized heating effect, results in a relatively high temperature at the hotspot that cannot be predicted using heat diffusion equation. While the contribution of the localized heating effect to the total device thermal resistance is significant during the normal operation of transistors, it has even greater implications for the thermoelectrical behavior of the device during an electrostatic discharge (ESD) event. The Boltzmann transport equation (BTE) can be used to capture the ballistic phonon transport in the vicinity of a hot spot but many of the existing solutions are limited to the one-dimensional and simple geometry configurations. We report our initial progress in solving the two dimensional Boltzmann transport equation for a hot spot in an infinite media (silicon) with constant temperature boundary condition and uniform heat generation configuration.

Application of a Macroscopic Turbulence Model to Simulation of Flow in a Channel With Equally Spaced Porous Fins

In this work, numerical solutions are presented for turbulent flow in a channel containing fins made with porous material. The condition of spatially periodic cell is applied longitudinally along the channel. A macroscopic two-equation turbulence model is employed in both the porous region and the clear fluid. The equations of mass continuity, momentum and turbulence transport equations are written for an elementary representative volume yielding a set of equations valid for the entire computational domain. Results are presented for the velocity field as a function of Reynolds, porosity and permeability of the fins. Pressure drop along the channel is compared with the case of solid material.

A Simple Heat Transfer Correlation for Three Inlet Configurations Using Artificial Neural Network in the Complex Transition Flow Regime

Local forced and mixed heat transfer coefficients were measured by Ghajar and Tam (1994) along a stainless steel horizontal circular tube fitted with reentrant, square-edged, and bell-mouth inlets under uniform wall heat flux condition. For the experiments the Reynolds, Prandtl, and Grashof numbers varied from about 280 to 49000, 4 to 158, and 1000 to 2.5×105, respectively. The heat transfer transition regions were established by observing the change in the heat transfer behavior. The data in the transition region were correlated by using the traditional least squares method. The correlation predicted the transitional data with an average absolute deviation of about 8%. However, 30% of the data were predicted with 10 to 20% deviation. The reason is due to the abrupt change in the heat transfer characteristic and its intermittent behavior. Since the value of heat transfer coefficient has a direct impact on the size of the heat exchanger, a more accurate correlation has been developed using the artificial neural network (ANN). A total of 1290 data points (441 for reentrant, 416 for square-edged, and 433 for bell mouth) were used. The accuracy of the new correlation is excellent with the majority of the data points predicted with less than 10% deviation.