bluuebunny/crossref_metadata_2025 huggingface dataset:
Sudden and Smooth Transitions to Weak Turbulence in Porous Media Convection
Authors: Vadasz Johnathan J. , Roy-Aikins Joseph E. A.
Published: December 2008

The fundamental understanding of the transition from laminar to turbulent convection in porous media is far from being conclusive. In isothermal flow in porous media no experiments identifying the three dimensional nature of a transition from the Darcy regime, via an inertia dominated regime, towards turbulence are available. In particular this detailed description of turbulence is missing in the problem of porous media convection where an additional non-linear interaction appears as a result of the coupling between the equations governing the fluid flow and the energy equation. The latter can typically cause a transition to a non-steady and non-periodic regime (referred to as weak turbulent) at much lower values of the parameter controlling the flow, when compared to the corresponding isothermal system. The present paper identifies the conditions for sudden and smooth transitions. In addition it attempts to address the question related to the reason for the subcritical transition to weak turbulence and the existence of a range of values of the porous media Rayleigh number over which the transition occurs, i.e. the Lorenz attractor.
Finite Amplitude Convection in Rotating Porous Media
Authors: Vadasz Johnathan J. , Govender Saneshan
Published: December 2008

The linear stability of centrifugally induced convection in rotating porous layers suggests a wide range of possible convective solutions. While the linear solutions indicate possible convective regimes and flow details, it is the non-linear effect that eventually establishes the detailed nature of the convection patterns. The latter is accounted for in a finite amplitude analysis. The results of the finite amplitude analysis via the weak non-linear theory are presented here with the aim to assist in selecting between these solutions and providing further analytical detail on the nature of convection.
Heat Transfer Processes in the Curing Tube During the Production of XLPE Insulated Cables
Authors: Shugai Galina , Yakubenko P. A.
Published: December 2008

A high voltage cable consists of a conductor (typically made of either aluminium or copper) which is covered by several layers of other materials with different properties and functions (Thue 1999).
Mesoscopic Heat Conduction and Onset of Periodicity
Authors: Sharma Kal Renganathan
Published: December 2008

Mesoscopic approach deals with study that considers temporal fluctuations which is often averaged out in a macroscopic approach without going into the molecular or microscopic approach. Transient heat conduction cannot be fully described by Fourier representation. The non-Fourier effects or finite speed of heat propagation effect is accounted for by some investigators using the Cattaneo and Vernotte non-Fourier heat conduction equation: q=−k∂T/∂x−τr∂q/∂t(1) A generalized expression to account for the non-Fourier or thermal inertia effects suggested by Sharma (5) as: q=−k∂T/∂x−τr∂q/∂t−τr2/2!∂2q/∂t2−τr3/3!∂3q/∂t3−…(2) This was obtained by a Taylor series expansion in time domain. Manifestation of higher order terms in the modified Fourier’w law as periodicity in the time domain is considered in this study. When a CWT is maintained at one end of a medium of length L where L is the distance from the isothermal wall beyond which there is no appreciable temperature change from the initial condition during the duration of the study the transient temperature profile is obtained by the method of Laplace transforms. The space averaged heat flux is obtained and upon inversion from Laplace domain found to be a constant for the the case obeying Fourier’s law; 1 − exp(−τ) using the Cattaneo and Vernotte non-Fourier heat conduction equation, and upon introduction of the second derivative in time of the heat flux the expression becomes, 1 − exp(−τ)(Sin(τ) + Cos(τ)). Thus the periodicity in time domain is lost when the higher order terms in the generalized Fourier expression is neglected.
Critical Thickness of High Temperature Barrier Coatings of Magnesium Oxychloride Sorrel Cement
Authors: Sharma Kal Renganathan
Published: December 2008

The critical thickness of high temperature barrier coating is derived to avoid cycling of temperature from the finite speed heat conduction equations. When a cylinder is subject to a step change in temperature at the surface of the cylinder the transient temperature profile is obtained by the method of separation of variables. The finite speed of heat propagation is accounted for by using the modified Fourier’s law of conduction with a heat velocity of √α/τr. In order to avoid pulsations of temperature with respect to time the cylinder has to be maintained at a radius no less than 4.8096√ατr. In the asymptotic limit of infinite heat velocity the governing equation becomes parabolic diffusion equation. In the limit of zero velocity of heat and infinite relaxation time the wave equation result and solution can be obtained by a relativistic coordinate transformation. In the asymptote of zero velocity of heat and zero thermal diffusivity the solution for the dimensionless temperature is a decaying exponential in time. The average temperature of the naval warhead as indicated by UL 1709 test was estimated by using a idealized finite slab, and Leibnitz rule and an analytical expression for the average temperature was obtained using convective boundary condition. The solution is: For1/2>=Bi,=exp(−τ(1/2+sqrt(1/4−Bi*)))ForBi>1/2,=exp(−τ/2)Cos(τsqrt(−1/4+Bi*))) The average temperature is damped oscillatory in time domain. Further the transient temperature profile is represented by an infinite series of decaying exponential in time and Bessel function of the first kind and 0th order. The constant can be obtained from the principle of orthogonality. The bifurcated nature of the exact solution gives rise to the lower limit on the radius to avoid cycling of temperature with respect to time. The exact solution is thus, u=Σ0∝cnJ0(λnX)exp(−τ(1/2−sqrt(1/4−λn2))) and when λn > 1/2 u=Σ0∝cnJ0(λnX)exp(−τ/2Cos(τsqrt(−1/4+λn2)) where, λn=(2.4048+(n−1)π)(√α/τr/R) cn is given by equation (53). The term in the infinite series onward where the contribution is oscillatory is identified.
A Molecular Dynamics Study on Thermophysical Properties and Stability of Nanoscale Liquid Thread
Authors: Kim Byeong Geun , Lee Joon Sik , Park Seungho , Choi Young Ki
Published: December 2008

In recent years, the characteristic behavior of nanoscale liquid thread has drawn much attention because of its scientific interest and industrial applications. Since the experimental investigation of liquid thread properties and its stability is extremely formidable, most studies rely heavily on the theoretical or numerical approach.
Experimental Studies of Shell and Tube Condenser Fouling and Accelerated Particulate Fouling in Internal Helical-Rib Roughness Tubes
Authors: Li Wei
Published: December 2008

This paper addresses fouling in a family of seven 15.54 mm I.D. copper, helically ribbed tubes, which have different ridge heights, helix angles, and number of ridge starts. A series of semi-theoretical linear fouling correlations as a function of the product of area indexes and efficiency indexes for long term combined precipitation and particulate fouling (PPF) in cooling tower systems and a series of semi-theoretical linear fouling correlations as a function of the efficiency indexes for particulate fouling were developed. The correlations can be directly used to assess the fouling potential of enhanced tubes in actual cooling water situations.
Theoretical Study of the Transient Shock Wave Propagation During Laser Ablation
Authors: Zhang Zhaoyan , Gogos George
Published: December 2008

Laser ablation consists of three coupled processes: i) heat conduction within the solid, ii) flow through a discontinuity layer (evaporation wave) attached to the solid surface, and iii) shock wave expansion of the laser induced vapor. In this paper; a one-dimensional solution for all three coupled processes is presented. The heat conduction and the evaporation wave are solved numerically. The shock wave expansion of the laser induced vapor, how ever, is solved analytically for the first time Analytical solutions for the classic Riemann problem have been employed to solve the transient propagation of the strong shock wave. This model provides a sound theoretical basis for the analysis of the laser ablation process. The effects of the laser intensity, back temperature and back pressure are analyzed. The temperature pressure; density and velocity of the laser induced vapor are calculated and the results are discussed.
Thermal Sizing Analysis for a Copper Bonded Steam Generator in LMR
Authors: Kim Eui-Kwang , Kim Yeon-Sik , Eoh Jae-Hyuk , Kim Seong-O
Published: December 2008

In this study, a one-dimensional thermal-hydraulic analysis computer model was developed for the thermal sizing of a copper bonded steam generator. It was assumed that the conduction heat transfer of the copper region between the hot side and cold side tubes is one-dimensional, and the thermal resistance of the copper region was derived as a function of tube pitch to diameter ratio. The flow region of the water/steam sides was divided into four sub-regions, which were sub-cooled, saturated nucleate boiling, film boiling, and super-heated regions. The number of tubes selected was from 280 to 3500 for the parametric study calculation. The ratios of pitch to tube diameter were 1.4, 1.6 and 1.8. The calculation results showed that when the number of tubes is 3000, the required length of heating tube is about 12.9 m and the diameter of the steam generator is about 3.2 m. If the P/D ratio increases, the thermal resistance of the copper region also increases, but the length of the heating tube does not increase so much. The developed program can be used for the examination of a conceptual design for the copper bonded steam generator, which is one of the candidates for an IHTS (Intermediate Heat Transport System) simplification concept.
An Experimental Investigation for Studying the Effect of Heating Element Shape During Pool Boiling
Authors: ElGafy Ahmed , Lafdi Khalid
Published: December 2008

An experimental study is performed to investigate and predict the influence of the heating element shape on nucleate pool boiling heat transfer from vertical cylindrical heating elements. An experimental apparatus test is designed and fabricated for this purpose, while water is utilized as the working fluid. Five vertical cylindrical heating elements with different shapes are used. The average heat transfer coefficient is calculated for each case and a shape factor, which includes the heating element geometry, is introduced for each heating element. An empirical correlation is obtained to relate the mean heat transfer coefficient with the heat flux and shape factor. Another correlation is obtained to relate the mean heat transfer coefficient with the saturation temperature difference and the shape factor. A comparative study is performed between the enhancement ratio and both the area ratio and shape factor ratio.
Single-Side Heated Monoblock, High Heat Flux Removal Using Water Subcooled Flow Boiling
Authors: Boyd Ronald D. , Ekhlassi Ali , Cofie Penrose , Martin Richard , Zhang Hongtao
Published: December 2008

Plasma-facing components for fusion reactors and other high heat flux heat sinks are usually subjected to a peripherally non-uniform heat flux. The configuration under study is related to these applications and consists of a single-side heated monoblock cross-section test section with a circular coolant channel bored through the center. The monoblock test section has a heated length of 180.0 mm and has 10.0 mm and 30.0 mm inside diameter and outside square sides, respectively. It was subjected to a constant heat flux on one side only, and the remaining portion of the outside surfaces is not exposed to a heat flux. The inlet channel water temperature was held near at 26.0°C, the exit pressure was maintained at 0.207 MPa, and the mass velocity was 0.59 Mg/m2s. The results consist of three-dimensional monoblock test section wall temperature distributions and a clear display of both critical heat flux and post-critical heat flux for this single-side heated configuration. These results are very encouraging in that they are among the first full set of truly three-dimensional monoblock test section wall temperature measurements for a one-side heated monoblock flow channel which contains the effects of conjugate heat transfer for turbulent, subcooled flow boiling. Comparisons are made between these results for the monoblock test section and those for a single-side heated circular test section.
Predictions of Radiative Properties of Patterned Silicon Wafers by Solving Maxwell’s Equations in the Time Domain
Authors: Liu J. , Zhang S. J. , Chen Y. S.
Published: December 2008

A rigorous electromagnetic model is developed to predict the radiative properties of patterned silicon wafers. For nonplanar structures with characteristic length close to the wavelength of incident radiation, Maxwell’s equations must be used to describe the associated radiative interaction and they are solved by the finite difference time-domain (FDTD) method. In the die area, only one period of the structure is modeled due to its periodicity in geometry. To truncate a computational domain, both the Mur condition and perfectly matched layer (PML) technique are available to absorb outgoing waves. With the steady state time-harmonic electromagnetic field known, the Poynting vector is used to calculate the radiative properties. Due to its importance, the reflection error is checked at first for two absorbing boundary conditions. As expected, the PML technique yields much lower errors than the Mur condition and it is thus used in this study. To validate the present model, radiative interactions with a planar structure and a nonplanar structure are investigated, and predicted reflectivities are found to match available other solutions very well. To demonstrate the importance of the present study, a patterned wafer consisting of periphery and die area is also investigated. While the thin film theory is accurate for the wafer periphery, the rigorously electromagnetic model described in this study is found to be necessary to accurately predict the radiative properties in the die area.
First Steps Towards Quantitative Validation of the Unsaturated-Flow Theories in Liquid Composite Molding
Authors: Roy Tonmoy , Babu Baiju Z. , Pillai Krishna M.
Published: December 2008

In liquid composite molding technologies such as Resin Transfer Molding (RTM), a thermoset resin is injected into a mold cavity with a pre-placed preform made of fiber mats to create a cured part. In order to improve the physics of resin flow in dual-scale (woven, stitched or braided) fiber mats, the authors carried out many transient 1-D mold-filling experiments to investigate the onset of unsaturated flow through the inlet-pressure history. Their study revealed that the measured pressure history, which droops downwards for dual-scale fiber mats, is at a variance with the linear pressure profile predicted by state-of-the-art Liquid Composite Molding (LCM) mold-filling simulation physics. It was also observed that the drooping of the inlet pressure increases with an increase in the compression of fiber mats. In this paper, the correlation between a previously proposed dimensionless number pore volume ratio and the droop in the inlet pressure history has been sought. Studying the micrographs of composite samples, pore volume ratio is measured for various fiber mat compression. It is observed that the droop in the inlet pressure profiles increase with an increase in the pore volume ratio. This is the first attempt to quantitatively validate the previous theories on the unsaturated flow.
Heat Transfer Testing of Enhanced Finned-Tube Bundles Using the Single-Blow Technique
Authors: O’Brien J. E. , Sohal M. S. , Wallstedt P. C.
Published: December 2008

A single-blow test facility has been developed at the INEEL for pressure-drop and heat transfer performance testing of heat exchanger finned-tube bundles. Initial testing in the facility has been aimed at evaluating several enhanced heat transfer geometries to be used with individually finned tubes in which winglet vortex generators have been punched into the fin surfaces at specific locations. The winglets interact with the air flow to create longitudinal vortices that enhance fin-surface heat transfer performance with minimal increase in pressure drop. Results of two initial studies indicate heat transfer enhancement levels of 20–30% across the tested Reynolds number range (100–3000), with an increase in pressure drop of only 4–12%, over the same range.
An Accuracy Analysis of the Steady-State Thermal Conductivity Measurement Method With Single Constant Temperature Region
Authors: Sun Ming-Tsung , Chang Chin-Hsiang
Published: December 2008

The newly developed method for steady-state thermal conductivity measurement with single constant temperature region is experimentally proven accurate within 3% compared with the guarded hot plate method. The method is suitable for in-situ non-destructive evaluation of insulation materials. However, there are factors that affect the accuracy of measurement yet to be closely studied. In this paper, we present a theoretical analysis of the accuracy of the method by varying the relative size of the heating plate, the thickness of the test specimen, the location of the temperature sensors at the lower temperature side, and different thermal convection coefficients for the free convection boundary condition. In the study, the temperature distribution in a homogeneous material is solved numerically. The thermal conductivity is evaluated from the temperature distribution and the heat flux measured according to the method. By comparing the results with that given in the numerical model, the accuracy can be expressed as functions of the three variables in the analysis. The results are consistent with that of the experiment. They are considered beneficial in providing information to the optimal design and the measurement correction of a commercialized apparatus using the method.
Interaction of Ice-Water Interface With Agglomerates of Activated Sludge During Freezing
Authors: Tao T. , Peng X. F. , Chu C. P. , Lee D. J.
Published: December 2008

Visual experiments were conducted to investigate freezing behavior of activated sludge. Experimental observations indicate that there existed strong interactions of advancing ice-liquid interface with sludge agglomerates during freezing treatment. Agglomerates would be pushed to move by advancing ice-liquid interface in the cases of lower advancing speeds while no needle-like ice-crystals forming at the interface. For faster freezing processes, the ice front interface line turned needle-like, while ice-crystals forming/generating at the interface, and crystals pierced into the sludge agglomerates rather than pushing them. The crystals served as roles holding the agglomerates. It seems that there was a critical speed below which repulsion occurred and above which the particles were entrapped without obviously pushing. The transition of these two cases is still needed to identify.
The Effect of Support Grid Features on Local, Single-Phase Heat Transfer Measurements in Rod Bundles
Authors: Holloway Mary V. , McClusky Heather L. , Beasley Donald E. , Conner Michael E.
Published: December 2008

Local, average heat transfer measurements in a rod bundle downstream of support grids with and without flow-enhancing features are investigated for Reynolds numbers of 28,000 and 42,000. Support grids with disc blockage flow-enhancing features and support grids with split-vane pair flow enhancing features are examined. Grid pressure loss coefficients and feature loss coefficients are determined based on pressure drop measurements for each support grid design. Results indicate the highest heat transfer enhancement downstream of the support grid designs with disc blockages. In addition, the local heat transfer downstream of the split-vane pair grid designs indicates a region of decreased heat transfer below that of the hydrodynamically fully-developed value. This decreased region of heat transfer is more pronounced for the lower Reynolds number case. A correlation for the local Nusselt numbers downstream of the standard support grid designs is developed based on the blockage of the support grid. In addition, a correlation for the local Nusselt numbers downstream of support grids with flow-enhancing features is developed based on the blockage ratio of the grid straps and the normalized feature loss coefficients of the support grid designs. The correlations demonstrate the tradeoff between initial heat transfer enhancement downstream of the support grid and the pressure drop created by the support grid.
Interdigitated Heat/Mass Transfer and Chemical/Electrochemical Reactions in a Planar Type Solid Oxide Fuel Cell
Authors: Li Pei-Wen , Schaefer Laura , Chyu Minking K.
Published: December 2008

The details of the heat/mass transfer in a planar type solid oxide fuel cell that controls the energy conversion performance are studied by employing a three-dimensional numerical computation for the fields of velocity, gas mass fractions and temperature. The SOFC under investigation is a unit working in a SOFC stack. It has the tri-layer of anode-electrolyte-cathode and interconnects having multiple channels for fuel and air. Two designs of the tri-layer, anode-supported and electrolyte-supported, are studied. Pre-reformed fuel gas with components of H2, H2O, CO, CO2 and CH4 is arranged in cross-flow direction with airflow. Further reforming and shift reaction in fuel channels were considered at chemical equilibrium. It was found that the consumption and production of gas species are different in the different channels. High current density was located in the upstream area of fuel channels. The operation conditions of current density affected the temperature level significantly.
Computational Method and Design of a Packed Bed Diffusion Tower for the Desalination of Seawater
Authors: Klausner James F. , Darwish Mohamed Y. , Mei Renwei
Published: December 2008

In a recent study, Klausner et al. [1] have described a diffusion driven process for desalinating seawater at low temperatures. The main advantage of the diffusion driven desalination (DDD) process is that low thermodynamic availability waste heat may be used to drive the process. When low pressure condensing steam from a 100 MW power plant supplies the heat to drive the DDD process, a fresh water production rate of 20 million gallons per day is feasible. This paper describes the computational procedure used to size the diffusion tower for a specified throughput.
Investigation of Polymer Optical Fiber Drawing Force and Heat Transfer
Authors: Reeve Hayden M. , Mescher Ann M. , Emery Ashley F.
Published: December 2008

In this study, the force required to draw a polymer preform into optical fiber is predicted and measured, along with the resultant free surface shape of the polymer, as it is heated in an enclosed cylindrical furnace. The applied drawing force affects the degree of chain alignment within the polymer. Chain alignment causes orientational birefringence, an unwanted property that attenuates any propagating optical signal. The draw force is a function of the highly temperature dependent polymer viscosity. Therefore accurate prediction of the drawing force requires a detailed investigation of the heat transfer within the furnace. In this investigation, the full axi-symmetric conjugate problem (including both natural convection and thermal radiation) was solved using the commercial finite element package FIDAP. In addition, the location of the polymer/air interface was solved for as part of the problem and was not prescribed beforehand. Results show that thermal radiation accounts for approximately 70% of the total heating experienced by the deforming polymer, but only 15% of the cooling. The draw force is very sensitive to both the furnace wall temperature and to the feed rate of the polymer. Numerical results compared well with the experimentally measured draw tension and neck-down profiles for several preform diameters, draw speeds, and furnace temperatures. The predicted draw forces were typically within 20% of the experimentally measured values.
Thermal Property Measurement of Semiconductor Melt Using Modified Laser Flash Method
Authors: Lin Bochuan , Zhu Shen , Ban Heng , Li Chao , Scripa Rosalia N. , Su Ching-Hua , Lehoczky Sandor L.
Published: December 2008

This study further develops the standard laser flash method for the measurement of multiple thermal properties of semiconductor melts. The standard laser flash method is widely used to measure thermal diffusivity of solids. Our modified procedure allows thermal diffusivity, thermal conductivity, and specific heat capacity of molten semiconductor material to be determined simultaneously. The transient heat transfer process in the melt and its quartz container was computationally studied in detail. A fitting procedure based on the numerical result and the least root-mean-square error fitting to the experimental data was used to extract thermal diffusivity, specific heat capacity, and thermal conductivity. The results for tellurium (Te) at 873 K: specific heat capacity 300.2 J/kg K, thermal conductivity 3.50 W/m K, thermal diffusivity 2.04×10−6 m2/s, are in good agreement with data published in the literature. Furthermore, uncertainty analysis showed quantitatively the effect of sample geometry, transient temperature measured, and the energy of the laser pulse on the results.
Heat Transfer From a Moving and Evaporating Meniscus on a Heated Surface
Authors: Kandlikar Satish G. , Kuan Wai Keat
Published: December 2008

A stable meniscus is formed between a needle dispensing water over a heated circular face of a rotating copper block. The needle is offset from the axis of rotation and thus forms a moving meniscus. The water flow rate, heater surface temperature and the speed of rotation are controlled to provide a stable meniscus with complete evaporation of water without any meniscus break-up. The experimental heat transfer rate is compared with the transient heat conduction model developed here. The results indicate that the transient heat conduction plays a major role in the heat transfer process from a moving meniscus. The study provides an important insight on the role of transient conduction around a nucleating bubble in pool boiling.
Analysis for a Generalized CHF Correlation in Uniformly Heated Vertical Round Tubes
Authors: Shim W. Jaewoo , Park Joo-Yong , Kim Ohyoung
Published: December 2008

For empirical models based on the local condition hypothesis, few important parameters give significant correlations on the prediction of CHF (Critical Heat Flux). This work is a preliminary study to develop a generalized CHF correlation in uniformly heated vertical round tubes for water. For this analysis, a total of 8,912 CHF data points from 12 different published sources were used. This database consisted of following parameter ranges: 0.101 ≤ P (pressure) ≤ 20.679 MPa, 9.92 ≤ G (mass flux) ≤ 18,619.39 kg/m2s, 0.00102 ≤ D (diameter) ≤ 0.04468 m, 0.03 ≤ L (length) ≤ 4.97 m, 8.5 ≤ L/D ≤ 792.26, −609.33 ≤ Inlet subcooling ≤ 1,655.34 kJ/kg, 0.11 ≤ qc (CHF) ≤ 21.41 MW/m2, and −0.85 ≤ Xe (exit qualities) ≤ 1.58. Five representative CHF data sets at pressure conditions of 0.101, 5.001, 10, 16 and 20 MPa were selected, analyzed, and compared to evaluate the effects of parameters on the CHF. It has revealed that the major variables which influenced the CHF, other than the system pressure (P), were tube diameter (D), mass flux of water (G), and local true mass fraction of vapor (Xt). Square root of GXt and square root of D were the significant parameters that showed strong parametric trends of the data sets. The results of this study have reaffirmed the feasibility that an advanced generalized CHF correlation for uniformly heated vertical round tubes can be found.
Modeling Under Hood Cooling Using a Coupled-Code Approach
Authors: Bella Gino , Rotondi Rossella
Published: December 2008

The need to shorten the development time for new engine and vehicles is leading to the increasing use of computational design and simulation methods in the automotive industry. In the last years 3D computational models have been used successfully in vehicle and engine development. It is clear that in this kind of simulation, the input complexity, the output data management and the computational time increase. On the other hand 3D simulations increase the details of the results and their link with the analyzed geometry. During a vehicle design several numerical techniques can be used (finite difference, finite volume, spectral methods, boundary elements, etc.) Often, in a complex simulation, that involves several different physical phenomena such as fluid flow and heat transfer only the use of different simulation techniques allows to obtain good results in a acceptable time. In several industrial applications the use of coupled codes, with different features (1D, 3D or different numerical schemes) could provide an optimal solution for the simulation approach. In this paper an example of a complex simulation of an Under Hood Cooling (UHC) of a vehicle is carried out using two different 3D codes with different numerical approaches with the objective to reduce the simulation time [1].
Condensation of Steam on Integral-Fin Tubes in a Bank
Authors: Briggs Adrian , Sabaratnam Sritharan
Published: December 2008

Accurate repeatable data are presented for condensation of atmospheric pressure steam on three rows of integral-fin tubes situated in a bank of plain tubes. The data cover a wide range of vapor velocities and heat fluxes. Unlike previously reported data for plain tubes using the same test bank and apparatus, the heat-transfer coefficients for the finned tubes were unaffected by vapor velocity. When compared to a plain tube of fin-tip diameter and at the same vapor side temperature difference, enhancement ratios between 3.7 and 4.9 were found for the finned tubes compared to a plain tube in quiescent vapor conditions, while values between 1.9 and 3.9 were found when compared to a plain tube at the same vapor velocity.
Microscopic Activation Phenomena in Heterogeneous Nucleation
Authors: Lu J. F. , Peng X. F.
Published: December 2008

The energy property in liquid near the wall was theoretically investigated to understand the effects of wall surface on inception process of nucleation or embryo bubble formation in boiling systems. Analyses indicate that the liquid near heating wall has higher pressure than in bulk region owing to existence of strong attractive forces, and this pressure could maintain a stable liquid microlayer and cause a steady energy peak near the wall. So a vapor embryo is likely to occur beyond the stable microlayer instead of exactly at the solid surface. The stable liquid layer may also be the inception structure of the ultrathin film before nucleation occurs. Fluctuations enhance the phenomenon of energy peak until the nucleation occurs, while energy peak promotes nucleation. Employing the concept of energy peak, the inception phenomena of the microlayer and the formation of embryo bubbles near solid surface were described.
Description of Dynamic Contact Angle on a Rough Solid Surface
Authors: Peng X. F. , Wang X. D. , Lee D. J.
Published: December 2008

An investigation was conducted to understand the contact line movement and associated contact angle phenomena. Contact line was supposed to move on a thin precursor film caused by molecular interaction between solid and liquid and asperity of solid surface. It is expected that contact line has a velocity and is subject to viscous stress on the film or geometrically on the solid surface. With the introduction of a characteristic parameter, λ′, the movement of contact line and contact angle phenomena were very well described in both physics and mathematics. The viscous shearing stress exerted by liquid on solid surface was derived, and the behavior of dynamic contact angle was recognized on rough solid surfaces. The analyses indicate that characteristic parameter, λ′, is dependent upon solid wall intrinsic property and mechanical performance, not liquid property. The comparison of theoretical predictions with available experimental data in open literature showed a quite good agreement with each other.
Effects of Gravity and Surface Tension and Interfacial-Waves and Heat-Transfer Rates in Internal Condensing Flows
Authors: Liang Q. , Wang X. , Barve A. S. , Narain A.
Published: December 2008

The paper presents accurate numerical solutions of the full 2D governing equations for steady and unsteady laminar/laminar internal condensing flows. The chosen geometry allows for film condensation on the bottom wall of a tilted (from vertical to horizontal) channel. It is found that it is important to know whether the exit conditions are constrained or unconstrained because incompressible vapor flows occur only for exit conditions that are unconstrained. For the incompressible vapor flow situations, a method for computationally obtaining the stable steady/quasi-steady solutions is given here and the resulting solutions are shown to be in good agreement with some relevant experimental data for horizontal channels. These solutions are shown to be sensitive to the frequency-content and strength of ever-present minuscule transverse vibrations of the condensing surface. The effects of noise-sensitivity, gravity (terrestrial to zero-gravity), and surface tension on the attainability of stable steady/quasi-steady solutions, structure of superposed waves, and heat-transfer rates are discussed. It is shown that significant enhancement in wave-energy and heat-transfer rates are possible by designing the condensing surface noise to be in resonance with the intrinsic waves.
Experimental Study of Miscible and Immiscible Oil Effects on Heat Transfer Coefficients and Pressure Drop in Microchannel Gas Cooling of Supercritical CO2
Authors: Kuang Guohua , Ohadi Michael , Zhao Yuan
Published: December 2008

Carbon Dioxide (CO2) is being investigated as an alternative refrigerant for vapor compression systems. In addition to its environmental benefits, Carbon Dioxide offers certain attractive thermal characteristics such as small surface tension, small liquid viscosity and large refrigerant capacity. Furthermore, combination with microchannels provides CO2 heat exchangers that have low weight, high compaction and high heat transfer coefficient. But certain oil (e.g., lubricate oil for compressor) will be carried into the vapor compression system, which usually has negative effect on heat transfer and pressure drop. The objective of the present paper is to study the effect of oil addition on heat transfer coefficient and pressure drop in supercritical gas cooling process in microchannels. Experiments addressed effect of three different types of oil (two immiscible and one miscible) at various oil concentrations ranging from 0% (no oil) to 5% by weight. As expected, oil addition has significant negative effect on heat transfer coefficients. At higher oil concentrations the heat transfer coefficients are substantially lower and the pressure drops are higher. As far the type of oil is concerned, the immiscible oil demonstrated more negative influence on the heat transfer and pressure drops than the miscible oil.
Numerical Simulation of Gas Phase Reaction Chemistry in Methane-Hydrogen Mixtures
Authors: Garg R. K. , Gore J. P. , Fisher T. S.
Published: December 2008

In the present work, gas-phase reactions between opposing streams of mixtures of hydrogen (H2) and methane (CH4) in the presence of volumetric energy input were simulated. The goal of the simulations is to estimate the concentrations of precursors responsible for the formation of carbon nanotubes (CNTs). These estimates are expected to help in understanding fundamental mechanisms of CNT formation and in controlling the synthesis process through parameters such as inlet composition and temperature, reactor pressure and absorbed energy. The simulation employs gas-phase kinetics of the GRI-2.11 mechanism with only reactions involving molecules that contain C and H atoms. The results indicate that the concentrations of H radicals, C2H2 and C atoms increase significantly with increases in volumetric energy deposition rate beyond a threshold.
Forced Convective Boiling Heat Transfer and Pressure Drop Characteristics of Two-Phase Flow Inside a Small Horizontal Helically Coiled Tubing Once-Through Steam Generator
Authors: Zhao Liang , Guo Liejin , Bai Bofeng , Hou Yucheng , Zhang Ximin
Published: December 2008

The pressure drop and boiling heat transfer of steam water two-phase flow were studied in a small horizontal helically coiled tube once-through steam generator of 9-mm inside diameter with 292-mm coil diameter and 30-mm pitch. Experiments were performed at a range of qualities up to 0.95, a system pressure range of 0.5∼3.5MPa, a mass flux range of 236∼943kg/m2s and a heat flux range of 0∼900kW/m2. Based on the experimental results, a new two-phase frictional pressure drop correlation was developed on the basis of Chisholm’s B-coefficient method. In the present experimental range, boiling heat transfer was found to be dependent not only on mass flux but also on heat flux. This result implies that both the nucleation mechanism and the convection mechanism have the same importance to forced convective boiling heat transfer in small horizontal helically coiled tube over the full range of qualities (pre-critical heat flux qualities of 0.1∼0.9) which is contrary to situations in larger helically coiled tube where the convection mechanism dominates at qualities typically > 0.1. Traditional single parameter Lockhart-Martinelli type correlations failed to satisfactorily predict present experimental data and in this paper a new flow boiling heat transfer correlation was put forward to better predict the experimental data of the present study.
Calculation of Direct Exchange Areas for Non-Uniform Zones
Authors: Tian Weixue , Chiu Wilson K. S.
Published: December 2008

This paper presents a special transformation of variables to reduce a double integral into three single integrals and its use for calculating Direct Exchange Areas (DEA) in Zonal method. This technique was originally presented for calculation of DEA using a uniform zone system in a cylindrical enclosure. However, non-uniform zones are needed for applications with large thermal gradients. Thus we extended this technique to calculate the DEA for non-uniform zones in an axisymmetrical cylinder system. At least six times of saving in computational time was observed in calculating DEA compared with cases without transforming of variables. It is shown that accuracy and efficiency of estimation of radiation heat flux is improved when using a non-uniform zone system. Reasonable accuracy of all DEA are calculated without resorting to the conservative equations. Results compared well with analytical solutions and numerical results of previous researchers. A brief discussion of its application in calculating DEA in a 3-D rectangular enclosure is also provided.
Prediction of Instabilities of Spherically Propagating Flames in Laminar Premixture
Authors: Haq M. Z.
Published: December 2008

A spherically expanding flame in a quiescent premixture is a bifurcation phenomenon, in which the flame becomes unstable at a radius, greater than some critical value, while remaining stable below that critical radius. Beyond this critical radius, developing instabilities are initiated by propagating cracks to form a coherent structure covering the entire flame surface and flame accelerates. The present paper reports a schlieren photographic study of spherical flame propagation in methane–air, iso-octane–air and n-heptane–air premixtures at different initial conditions where the onset of instability and the flame acceleration are clearly perceived. Hence, elapsed time and flame radius for onset of instability are correlated with the flame parameters. Predicted critical time and flame radius for the onset of instability are in good agreement with available experimental data obtained from a large-scale unconfined explosions in methane-air premixture.
Multiphase Cooling of High Frequency Conductors Encased in a Porous Medium
Authors: Morgan N. R. , Ammerman C. N. , Sims J. R. , Konecni S. , Wilson J. C.
Published: December 2008

This study presents a new method for cooling high frequency conductors requiring substantial structural support. This new method of cooling involves boiling a dielectric, fluorinert™ refrigerant that is in direct contact with the supported conductor. A porous, dielectric medium was created for structural support with enough interconnected voids to sustain liquid-vapor flow. A pool boiling test chamber was constructed to investigate the heat transfer characteristics of a conductor encased in a porous medium. The conductor-porous media assembly was mounted horizontally on two conductor posts and submerged into the liquid. The tests were performed using support structures with different interconnected void sizes. A DC power supply was used to apply heat to the conductor and all tests were performed at the saturation temperature. Critical heat flux was captured and each assembly evaluated for best performance in transferring heat. The results from these experiments will enhance the cooling technology used in high frequency conductor applications.
Integration of IR Video to Enhance Measurements of the Thermal Diffusivity Tensor
Authors: Huang Xin , Davis Sean E. , Wright Neil T.
Published: December 2008

Proper positioning of temperature sensors is required to minimize error in measurements of thermal diffusivity. Calculating the proper position of these sensors a priori, however, requires knowing the very thermophysical properties of the material one is testing. Presented here are calculations illustrating how IR video might be used to optimize a biaxial, pulsed thermal diffusivity measurement system. Results will be shown for representative elastomers.
Towards a Mechanistic Model for Aluminum Particle Combustion
Authors: George Prasanth , DesJardin Paul E.
Published: December 2008

A relatively simple mechanistic model for the combustion of an aluminum particle in air is presented. The model assumes combustion to occur in two stages. In the first stage, phase transition and heterogeneous surface reactions take place until the melting temperature of the oxide is reached. In the second stage, a quasi-steady state diffusion flame is established allowing for the use of commonly employed flame sheet approximations. Modified Ranz-Marshall and standard drag correlations for a sphere are used to describe the unsteady heating, mass loss rate, and drag of the particle, with the surrounding gas. A system of non-linear ordinary differential equations are formulated and numerically integrated in time for predictions of particle mass, temperature and velocity with, and without, the effects of heterogeneous combustion. Results indicate that, within the assumptions of the current model, the effects of heterogeneous combustion have a significant impact on the overall particle burn time and temperature history for gas temperatures ranging from 1500 to 2500 K. At higher particle Reynolds number, and for temperatures greater than 2500 K, the effects of heterogeneous combustion are not as important and an ignition criterion based on the oxide melting temperature may be sufficient.
Temperature Distributions in Microscale Fractal-Like Branching Channel Networks
Authors: Alharbi Ali Y. , Pence Deborah V. , Cullion Rebecca N.
Published: December 2008

Heat transfer to liquid flow through fractal-like branching flow networks is investigated using a three-dimensional computational fluid dynamics approach. Results are used to assess the validity of, and provide insight for improving, assumptions imposed in a previously developed one-dimensional model to predict wall temperature distributions along a fractal-like flow network. Assumptions in the one-dimensional model include (1) reinitiating thermal and hydrodynamic boundary layers following each bifurcation, (2) negligible minor losses at the bifurcations, and (3) constant thermo-physical fluid properties. It is concluded that temperature varying fluid properties and minor losses should be incorporated in the one-dimensional model to improve its predictive capabilities. No changes to the redevelopment of the boundary layers at each wall following a bifurcation are recommended. Surface temperature distributions along heat sinks with parallel and fractal-like branching flow networks are also investigated and compared. For the same observed maximum surface temperature between the two heat sinks, considerably lower temperature variations and pressure drops, greater than 50 percent, are noted for the fractal-like heat sink.
Thermal Process Maps for Controlling Microstructure in Laser Deposited Materials
Authors: Klingbeil Nathan W. , Bontha Srikanth
Published: December 2008

The ability to predict and control microstructure in laser deposited materials requires an understanding of the thermal conditions at the onset of solidification. To this end, the focus of this work is the development of thermal process maps relating solidification cooling rate and thermal gradient (the key parameters controlling microstructure) to laser deposition process variables (e.g., laser power and velocity). Results presented herein are based on the Rosenthal solution for a moving point heat source traversing an infinite substrate. Ongoing work includes the effect of a distributed laser power through superposition of the Rosenthal solution, while the effects of finite geometry, temperature dependent properties and latent heat of transformation are included through thermal finite element modeling of the laser deposition process.