Multiple Flow Patterns and Heat Transfer in Confined Jet Impingement

The flow field of a 2-D laminar confined impinging slot jet is investigated. Numerical results indicate that there exist two different solutions in some range of geometric and flow parameters. The two steady flow patterns are obtained under identical boundary conditions but only with different initial flow fields. Three different exit boundary conditions are investigated to eliminate artificial effects. The different flow patterns are observed to significantly affect the heat transfer. A flow visualization experiment is carried out to verify the computational results and both flow patterns are observed. The bifurcation mechanism is interpreted and discussed.

An Experimental Investigation for Flow Past a Circular Cylinder With Rectangular Strips

This paper reports an experimental investigation for fluid flow past a circular cylinder with two small rectangular strips and single sharp-edge strips on its surface. The experimental results reflected that different arrangements or dimensions of the strips produced significantly different effects on the flow. The forward step caused a stronger disturbance with a small increase in drag. The backward step arrangement softened the disturbance but reduced the drag coefficient by 33%.

Fluid Flow and Lateral Fluid Dispersion in Bounded Granular Beds

Results are presented from a numerical study examining the flow of a viscous, incompressible fluid through random packing of nonoverlapping spheres at moderate Reynolds numbers (based on pore permeability and interstitial fluid velocity), spanning a wide range of flow conditions for porous media. By using a laminar model including inertial terms and assuming rough walls, numerical solutions of the Navier-Stokes equations in three-dimensional porous packed beds resulted in dimensionless pressure drops in excellent agreement with those reported in a previous study (Fand et al., 1987). This observation suggests that no transition to turbulence could occur in the range of Reynolds number studied. For flows in the Forchheimer regime, numerical results are presented of the lateral dispersivity of solute continuously injected into a three-dimensional bounded granular bed at moderate Peclet numbers. Lateral fluid dispersion coefficients are calculated by comparing the concentration profiles obtained from numerical and analytical methods. Comparing the present numerical results with data available in the literature, no evidence has been found to support the speculations by others for a transition from laminar to turbulent regimes in porous media at a critical Reynolds number.

Packing Characteristics and Its Effect on Heat Transfer Characteristics in Melting of Granular Packed Bed Subject to Horizonal Forced Convection

A series of experiments are conducted to investigate the characteristics and its effect on the melting and heat of a packed bed consisting of melting ice particles to horizontal forced convection. The volumes and situations of the melting ganular packed beds are by the visualization observations and measurements digital camcorders within the range of Re = 71 ~ 2291, Gr/Re2 = 1.48×10−5 ~ 17.32, and Ste = 0.0444 ~ 0.385, respectively. The mass of ice particles is measured at the time interval during the melting process. Two types of pattern can be found under the different conditions. The different types of heat transfer characteristics emerge in type of packing pattern. The correlations for each type of pattern are obtained based on the experimental results.

Experimental Study of Bubble Dynamics and Concentration Variation in the Presence of Boron in the Liquid

Accumulation of neutron absorber (boron or boron compounds) within the porous crud layer on the fuel rods in the cores of pressured water reactors (PWR) results in the so-called axial offset anomaly (AOA). There is practically little information on the gradients of neutron absorber concentration near a nucleation site on the cladding surface during the growth of a bubble. The objective of the present work is to study bubble dynamics and associated concentration field of aqueous boron. As a first step in solving the complete problem, dynamics of single bubble was studied under pool boiling conditions. Distilled water and an aqueous solution containing 3,000ppm by weight of boric acid were used as test liquids. Single bubble was generated at a micro-fabricated cavity on a polished silicon wafer. It was found that the growth and departure processes of single bubbles are similar for both the test liquids. A miniature sensor for measurement of concentration was developed and calibrated. Concentration variation near the liquid-vapor interface was detected successfully. The measured concentration variations at different radial locations from the center of cavity have the same trend as given by the numerical simulations but the magnitude is much smaller.

A Non-Source Term Methodology in Simulation of Solidification

In this paper a non-source term method is developed to solve the energy equation. In this new method the discrete form of the energy equation remains the same and no extra source term is introduced. The mushy-zone is treated as a porous media during solidification. This method is incorporated into the existing finite volume based CFD code. Test cases analyzed in this paper include solidification of pure metal, pure metal solidification with natural convection due to the buoyancy, and binary alloy mushy flow problem with variable Cp. The calculated results are in good agreements with available published data. This method can be applied to simulate a wide range of melting/solidification processes.

Flow Visualization and Air Temperature Measurements in Symmetrically Heated Vertical Channels With Adiabatic Extensions

Air natural convection in a vertical channel-chimney system with the channel walls symmetrically heated at a uniform heat flux has been experimentally investigated. Flow visualization photographs and average air temperatures are presented. Some profiles of air temperature fluctuations are reported, which point out the fluid flow interactions in the chinmey. The flow visualization showed that the cold air inflow penetrating into the chimney affects the thermal performance of the channel. The improvement in the thermal performance of the channel determined by the chimney effect, for various values of the process parameters, has also been pointed out. In all investigated configurations and ranges of the process parameters the air flow in the channel was laminar. The flow in the chimney is strongly affected by the aspect ratio. Moreover, at the lower values of the expansion ratio the flow was laminar in the chimney and in its lower corner a stable vortex was noticed whereas at larger values of the expansion ratio a cold ambient air downflow worsened the thermal performance of the system. Interactions between the thermal plume arising from the channel, the vortex in the comer in the inlet chimney region and the cold air inflow yield fluctuations in the air temperature in the system. The distribution of time averaged air temperature in the cross sections validates indications given by the flow visualization in the chimney.

A Wind-Tunnel Study of Thermally Stratified Boundary Layers

The objective of this work is to develop, in a wind tunnel environment, boundary layers with different states of development that simulate the structure present in the atmospheric boundary layer. The work analyses the dymamic and thermal characteristics of different types of thick, artificially-generated, turbulent boundary layers. The thermal boundary layer is obtained by two methods: wall surface heating, made through electrical resistance, can furnish an increase in wall temperature of up to 100 °C above the ambient temparatures and can be applied over a 5000 mm long surface with a controlled variation of 2 °C. The main flow heating is obtained by forcing the flow pass through an array of copper wires whose elements can be heated individually. The main flow can be heated up to 100 °C. The whole system can then be used to produce unstable, neutral and stable boundary layers. The parameters of the thermal boundary layers are qualified according to the following parameters: growth, structure, equilibrium, turbulent transport of heat and energy spectrum. The paper describes in detail the experimental arrangements, including the geometry of the wind tunnel and the instrumentation.

Single-Phase Flow Characteristics and Effect of Dissolved Gases on Heat Transfer Near Saturation Conditions in Microchannels

An experimental investigation is carried out to study the heat transfer and pressure drop in the single-phase flow of water in a microchannel. The effect of dissolved gases on heat transfer and pressure drop is studied as the wall temperature approaches the saturation temperature of water, causing air and water vapor mixture to form bubbles on the heater surface. A set of six parallel microchannels, each approximately 200 micrometers square in cross section and fabricated in copper, with a hydraulic diameter of 207 micrometers, is used as the test section. Starting with air-saturated water at atmospheric pressure and temperature, the air content in the water is varied by vigorously boiling the water at elevated saturation pressures to provide different levels of dissolved air concentrations. The single-phase friction factor and heat transfer results are presented and compared with the available theoretical values. The friction factors for adiabatic cases match closely with the laminar single-phase friction factor predictions available for conventional-sized channels. The diabatic friction factor, after applying the correction for temperature dependent properties, also agrees well with the theoretical predictions. The Nusselt numbers, after applying the property corrections, are found to be below the theoretical values available in literature for constant temperature heating on all four sides. The disagreement is believed to be due to the three-sided heating employed in the current experiments. The effect of gas content on the heat transfer for the three gas concentrations is investigated. Nucleation was observed at a surface temperature of 90.5°C, for the reference case of 8.0 ppm. For the degassed cases (5.4 ppm and 1.8 ppm), nucleation is not observed until the surface temperature reached close to 100°C. An increase in heat transfer coefficient for surface temperatures above saturation is observed. However, a slight reduction in heat transfer is noted as the bubbles begin to nucleate. The presence of an attached bubble layer on the heating surface is believed to be responsible for this effect.

Control of Convective Heat Transfer in a Confined Laminar Impinging Jet by Low Amplitude Forcing

A numerical finite-difference model, derived using a control-volume approach, was used to compute the flow and heat transfer characteristics in a two-dimensional confined laminar air jet impinging on an isothermal surface. Several cases were considered with Re=650, 750, and nozzle to plate spacing, H/W=5. The behavior of the jet and the attendant heat transfer from the target wall were investigated when the jet was forced by fluidic excitation at the nozzle exit. At Re between 585 and 610, the unforced jet exhibits a transition to an unsteady regime leading to asymmetric vortex shedding and jet flapping [1, 2]. Investigation of the velocity spectra indicate three distinct dominant modes; the lowest frequency is associated with the jet flapping while the highest frequency is associated with the asymmetric vortex formation which causes buckling of the jet column. As a result of the two combined modes, the peak heat transfer is enhanced and the lateral cooling extent is broadened. The jet was subjected to forcing by introduction of numerical excitation at each side of the jet that modeled fluidic excitation. The jet was forced on both opposing sides at its exit, both with in-phase and out-of-phase modes. Under some conditions, out of phase forcing at Re=650 at the highest frequency leads to stabilization of the normally separated flow on one side only. This unusual asymmetric flow field is unsteady but repeatable, and results in an enhancement of the heat transfer. At Reynolds number of 750, forcing with an out of phase mode at the highest frequency leads to a complete stabilization of the jet. The forcing suppresses the high-amplitude low frequency flapping mode leaving only a high frequency vortex formation mode. The suppression of the jet flapping leads to a decrease in the peak heat transfer, but because separation is suppressed, the average wall heat transfer is enhanced.