bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of Heat Transport in Graphene Nanoribbons Using the Ab-Initio Scattering Operator
Authors: Landon Colin D. , Hadjiconstantinou Nicolas G.
Published: August 2016

We present a deviational Monte Carlo method for simulating phonon transport in graphene using the ab initio 3-phonon scattering operator. This operator replaces the commonly used relaxation-time approximation, which is known to neglect, among other things, coupling between out of equilibrium states that are particularly important in graphene. Phonon dispersion relations and transition rates are obtained from density functional theory calculations. The proposed method provides, for the first time, means for obtaining solutions of the Boltzmann transport equation with ab initio scattering for time- and spatially-dependent problems. The deviational formulation ensures that simulations are computationally feasible for arbitrarily small temperature differences; within this formulation, the ab initio scattering operator is treated using an efficient stochastic algorithm which, in the limit of large number of states, outperforms the more traditional deterministic methods used in solutions of the homogeneous Boltzmann equation. We use the proposed method to study heat transport in graphene ribbons.
Effects of Kitchen Hood System on Thermal Comfort and Carbon Dioxide Gas Emission From an Urban Residential Kitchen in Developing Countries
Authors: Rahman M. Hamidur , Islam A. K. M. Sadrul , Amin M. Ruhul
Published: August 2016

In this study, a typical kitchen having a standard dimension of 213cm × 243cm × 305cm was modelled with single open door exit. Steady state simulations were performed using three dimensional CFD code with appropriate boundary conditions. Two heat sources were used for modelling the kitchen that resembles the double burner gas stoves of urban residential kitchen in developing countries. In earlier works, the predictions were validated at an optimum grid resolution and the results were presented for thermal comfort [1] and carbon dioxide gas emission [2] under natural, forced and no ventilation cases. In the present work, a kitchen hood system is introduced and the results are presented for both thermal comfort and carbon dioxide gas emission. A comparative analysis has also been reported for the kitchen with and without the hood system. It was observed that the carbon dioxide gas concentration reduced significantly within the breathing zone due to use of kitchen hood system. In the breathing zone, the CO2 concentration is only 500 PPM which is 10 times lower than the close vent case. However, accumulation of high concentration gas was seen in the upper region of the breathing zone. Maximum CO2 concentration was seen about 4500 PPM at 1.2 m height from the breathing point near the roof of the kitchen. Analysis of thermal distributions revealed that high temperature zone in the right and frontal region of the cook’s position exists even for the case with kitchen hood system.
Cryoprotective Agent (CPA) Removal With Dilution-Filtration Method and CPA Concentration Monitoring With Electrical Conductivity Measurements
Authors: Shu Zhiquan , Fang Cifeng , Zhou Xiaoming , Gao Dayong
Published: August 2016

In cell/tissue cryopreservation, cryoprotective agents (CPAs) should be added before freezing and removed after thawing. Nowadays people mainly apply centrifugation for CPA removal, which may cause many problems. Meanwhile, a simple and cheap method for real-time monitoring of the residual CPA concentration during processing remains an unfilled need. In this work, a “dilution-filtration” system with hollow fiber dialyzer was implemented and approved to remove dimethyl sulfoxide (Me2SO) effectively. Compared to other methods (centrifugation and microfluidic approach), this one holds advantages of low time and labor consumption, low osmotic injury to the cells, high effectiveness, ease to control the final suspension volume and low risk of contamination. The “dilution-filtration” system can also be easily modified for CPA addition and cell suspension volume control (concentration or dilution of cell suspension). Meanwhile, a method of electrical conductivity (EC) measurement was applied to monitor the residual CPA concentration. The results showed EC measurements of waste solution can convey the CPA concentrations in cell suspension. This validates the feasibility of a safer and easier way to on-line and real-time monitoring of CPA concentration in cell suspension by measuring the EC of waste solution.
Stability Analysis of Non-Newtonian Rotational Flow With Hydromagnetic Effect
Authors: Ashrafi N. , Hazbavi A.
Published: August 2016

Stability of the magnetorheological rotational flow in the presence of a magnetic excitation in the tangential direction is examined. The conservation of mass and momentum equations for an isothermal Carreau fluid between coaxial cylinders are numerically solved while mixed boundary conditions are assumed. In the absence of magnetic excitation, the base flow loses its radial flow stability to the vortex structure at a critical Taylor number. The emergence of the vortices corresponds to the onset of a supercritical bifurcation. The Taylor vortices, in turn, lose their stability as the Taylor number reaches a second critical number corresponding to the onset of a Hopf bifurcation. The tangential magnetic field turns out to be a controlling parameter as it alters the critical points throughout the bifurcation diagram. Also, the effect of the Hartmann number, the Deborah number and the fluid elasticity on the flow parameters were investigated.
A POD Analysis Method for Multi-Media Heat Conduction Problems
Authors: Gao Xiao-Wei , Hu Jin-Xiu , Huang Shi-Zhang
Published: August 2016

In this paper, a new POD-based analysis method is proposed for numerical analysis of thermal mechanical engineering problems consisting of multiple media. After the creation of a heat conduction solution database for each medium, the “snapshot” approach of Proper Orthogonal Decomposition (POD) technique is applied to facilitate reduced-order modeling of the unsteady heat conduction behavior. The snapshot matrix is constructed medium by medium by collecting individual medium solutions at different instants in time through a column-wise manner. By means of expressing physical variables in terms of reduced modes at individual medium level, a system of differential equations about time is formed by utilizing the consistency conditions of the physical variables on interface boundaries. The solutions of the problem can be obtained by solving the system of equations at any desired time steps. A numerical example is given to demonstrate the efficiency of the proposed method.
Thermal Spreading Resistance Characteristics of a High Power Light Emitting Diode Module
Authors: Lee Ming-Tsang , Yang Kai-Shing
Published: August 2016

In this study, effects of the dimensions and the thermal conductivity of the substrate on the heat transfer characteristics of a LED module are investigated. The total thermal resistance corresponding to a LED module operating at different power levels is measured using a thermal resistance testing system (T3Ster®). In addition, a novel graphite composite material with anisotropic thermal conductivities is used as the substrate of the LED module to investigate the effects of directionally dependent thermal conductivities on the thermal spreading resistance. Furthermore, a finite element method numerical simulation is carried out to analyze the heat transfer phenomena in the LED module. It is found that, for the current experimental conditions, the importance of the thermal spreading resistance effect increases with decreasing substrate thickness and/or increasing input power of the LED module, which corresponds to an increase in the total thermal resistance and correspondingly a higher chip temperature. Experimental and numerical results show that the thermal spreading resistance and thus the chip temperature can be reduced by increasing the substrate thickness or by utilizing materials with high lateral thermal conductivities for the substrate. In consequence, for LED modules with the same substrate thickness, using graphite composite to replace aluminum as the substrate material reduces the spreading resistance by nearly fourteen percent in the current study.
Visualization of FC-72 Flow Boiling in Parallel- and Counter-Flow Plate Heat Exchangers
Authors: Koyama Kohei , Nakamura Yuya , Arima Hirofumi
Published: August 2016

This study investigates FC-72 (Perfluorohexane) flow boiling in a plate heat exchanger. A plate heat exchanger which has a transparent cover plate is manufactured to visualize boiling two-phase flow pattern of the working fluid FC-72 heated by hot water. Titanium is used for heat transfer plate, which has micro pin-fin structure on the heat transfer surface to enhance heat transfer. Experiment is conducted for parallel- and counter-flow arrangements to compare thermal and hydraulic performances. Flow boiling is photographed by a digital camera and instantaneous images are processed to classify flow pattern and to measure void fraction in the heat exchanger. Flow rates and temperatures of FC-72 and hot water at inlet and outlet of the heat exchanger are simultaneously measured to obtain overall heat transfer coefficient. Two-phase flow pattern of FC-72 flow boiling and void fraction along flow direction as well as thermal performance are discussed. Experimental results show that bubbly flow, slug flow, and churn flow are observed for the experimental range of this study. Extent of churn flow in the parallel-flow heat exchanger is larger than that of the counter-flow one due to generated bubbles at upstream region in working fluid channel. Void fraction of the parallel-flow plate heat exchanger increases rapidly compared with that of the counter-flow one due to location of onset of nucleate boiling. Overall heat transfer coefficients for the parallel-flow arrangement is larger than that of the counter-flow due to destruction of thermal boundary layer. The experimental results show that flow arrangement of a plate heat exchanger has the potential to improve its thermal performance.
Numerical Study of a Direct Chill Slab Caster Fitted With a Porous Filter for Aluminum Alloy AA-2024
Authors: Hasan Mainul
Published: August 2016

The present study is undertaken to model an industrial-sized vertical Direct Chill (DC) slab caster fitted with a porous filter near the melt entry region. The modeled alloy is a high strength aluminum alloy AA-2024 which is extensively used by the aerospace industry. The model has incorporated the 3-D turbulent aspect of the melt flow and heat transfer in the liquid sump and the mushy region solidification aspect of this long solidification range (136° C) alloy. The verified 3-D turbulent CFD in-house code is used to study the effects of various parameters of this casting process in order to gain some fundamental understanding of the melt flow and solidification behavior of the process. The studied caster consists of a popular ‘hot-top’ mold fitted with a porous filter above which molten aluminum alloy is delivered with a constant flow-rate across the entire hot-top. Because of two-fold symmetry, a quarter of the domain of the caster is modeled to save computational costs and time. A staggered control volume based finite-difference scheme is used to solve the non-dimensional modeled equations and the associated boundary conditions. The turbulent aspect of the flow in the porous filter is modeled using the latest suggested version of the Brinkman-Forcheimer extended form of Darcy equation for a porous media. The turbulent melt flow and solidification heat transfer in the clear fluid region are modeled using a low Reynolds number version of the k–ε eddy viscosity model. Computed results for the steady-state phase of the casting process are presented for four casting speeds, varying from 100 to 220 mm/min, for three metal-mold contact regions, varying from 20 to 50 mm and for three metal-mold convective heat transfer boundary conditions, varying from 1.0 to 4.0 kW/m2K and all for a fixed inlet melt superheat of 64° C. The permeability of the filter is also varied to ascertain its influence on the predicted results. Computed results of the velocity and temperature profiles, the sump depth and mushy region at the centre of the caster as well as the solidification shell thickness at the exit of the mold are provided and discussed. The present work can provide some useful guidelines in designing and optimizing a vertical DC slab caster for producing good quality casts for the common aluminum alloy AA-2024.
Localized Climatisation of Perishable Products: Solutions for Increasing Energy Efficiency
Authors: Wagner Johannes , Schäfer Mirko , Phan Long , Schlüter Alexander , Hesselbach Jens , Rosano Michele , Lin Cheng-Xian
Published: August 2016

Many industries have significant requirements regarding temperature control, air humidity and air pollution which must be strictly adhered to avoid bacterial formation and contamination. High refrigeration specifications are only required in certain areas. However, these specifications are often applied across the whole production hall which results in unnecessarily high energy demand and usage. A more energy efficient approach is the localized cooling of the product, which conditions the direct environment of the product only. This leads to the consideration of separating or localizing the products specifically requiring refrigeration in the production hall. In this paper, localized product cooling systems are analyzed in order to identify the savings potential associated with a localized refrigeration system. The study shows the energy savings potential for a manufacturing company located in three different locations: in Germany, Canada and the USA.
A Comparison Between 3-D Thermal Model and 3-D CFD Model for Vertical DC Casting of Rolling Ingots of Aluminum Alloy 7050
Authors: Hasan Mainul , Begum Latifa
Published: August 2016

In this study, first a 3-D thermal model is developed for an open top, vertical direct chill (DC) casting process of rolling slabs (ingots) by taking into account the casting speed in the form of slag flow in the thermal connective-diffusion equation. The mushy region solidification characteristics of the process are accounted for through the implementation of the enthalpy porosity technique. The thermal model is later extended to a 3-D CFD model to account for the coupled turbulent heat transfer and solidification aspect of the process. Both models simulate an industrial-sized, hot-top type vertical Direct Chill (DC) slab caster for high strength aluminum alloy AA-7050. A staggered control volume based finite-difference scheme is used to solve the modeled equations and the associated boundary conditions. In the CFD model, the turbulent aspects of flow and solidification heat transfer are modeled using a low Reynolds number version of the k–ε eddy viscosity approach. Computed results for the steady-state phase of the casting process are presented for four casting speeds varying from 60 to 180 mm/min for a fixed inlet melt superheat of 32°C. Simulation results of the velocity and temperature fields and heat fluxes along the caster surface are presented for the CFD model and the shell thickness and sump depth are compared between the CFD and thermal models.
Development of Lift-Off Diameter Model of Bubbles Generated on Horizontal Tube
Authors: Ryu Sung Uk , Kim Seok , Euh Dong-Jin
Published: August 2016

In this study, the theoretical correlation for the lift-off diameter of bubbles generated on a horizontal tube is proposed. A force balance analysis in the direction normal to the heated surface at the moment of the bubble lift-off was performed to develop the model. According to the developed model, the bubble lift-off diameter depends strongly on the azimuthal position of the horizontal tube, the relative velocity between a bubble and surrounding liquid, the properties of the bubble, and the liquid. The developed model can be applicable to define the sub-model of wall heat flux partitioning for natural and forced convective boiling.
Novel Arrangements of Tube Banks With Enhanced Heat Transfer and Manageable Pressure Drop Requirement
Authors: Bourisli Raed I. , Al-Mutairi Meshal F.
Published: August 2016

The main objective of this study is to investigate the existence of a more efficient arrangement of tubes in tube banks. The famous inline arrangement generally requires manageable pressure drops but results in poor overall heat transfer. On the other hand, the more common staggered arrangement results in better heat transfer yet with added pressure drop requirements. The key idea here is to have a compromise between the two where the tubes are staggered gradually in a way that forms a longitudinal sinusoidal wave. The process is simulated numerically using the finite volume method. After rigorous validation of the 2-D code using a number of published experimental and numerical results, and a number of established correlations, a parametric study is done to determine the optimum period of the sine wave of the tubes, the number of tubes in one period, and the amplitude. Results show that the newly proposed arrangement reduces pressure drop requirement compared to the staggered arrangement while maintaining the same level of heat transfer. In at least one arrangement, both the pressure drop and heat transfer were better than the staggered one, albeit only slightly for the latter.
Low-Temperature Melting of Silver Nanoparticles in Subcooled and Saturated Water
Authors: Lee Soochan , Phelan Patrick E. , Taylor Robert A. , Prasher Ravi , Dai Lenore
Published: August 2016

Continuous, laser-heated boiling experiments with silver nanofluids were conducted to identify the non-equilibrium melting behavior of silver nanoparticles in de-ionized (DI) water. Experimental results with Transmission Electron Microscopy (TEM) and Dynamic Light Scattering (DLS) suggest that surface melting of silver nanoparticles (which have a bulk melting point of 961°C) can occur at ambient pressure when particles are suspended in saturated, and even subcooled (e.g. < 100 °C) water due to the localized (volumetric) heat absorption. These findings are supported by calculating a temperature-dependent Hamaker constant of silver nanofluid — i.e. the interaction between interfaces (Ag-melt-water) at the melting temperature.
Total Temperature Measurement of Micro Gas Jet
Authors: Yamada M. , Hong C. , Asako Y.
Published: August 2016

Total temperature of the nitrogen micro-gas jet from a micro-tube outlet was measured for various flow rates. A thermally insulated tube of foamed polystyrene with six baffles where the gas velocity reduces and the kinetic energy is converted into the thermal energy, was attached to the outlet of the micro-tube. The gas temperature was measured by thermocouples at locations of baffles. The inner diameter of a micro-tube used is 519μm and the tube length is 56.3 mm. The inner diameter of the polystyrene tube is 22 mm. The baffles are equally spaced and the interval of the baffles tested are 5, 10, 20 and 30 mm to investigate the effect of the interval of the baffle on the reduction of the gas velocity. Measured total temperatures are compared with the theoretically obtained total temperatures for a perfect adiabatic micro-tube. The measured total temperatures are slightly higher than the theoretically obtained total temperatures.
A New Method for Direct Exchange Area Calculation in Zonal Method of Radiant Heat Transfer Modeling in Combustion Furnaces
Authors: Malikov G. , Lisienko V. , Titaev A. , Viskanta R.
Published: August 2016

A new method based on the discrete transfer modeling technique for calculating the direct exchange areas (DEA) in zonal methods of radiation heat transfer is presented. The key feature of this method is a fast DEA matrix evaluation. The computational time was found to be short in comparison to other methods for direct exchange areas calculation based on numerical quadrature integration. The accuracy of the procedure is assessed by comparing the predictions with those based on the numerical integration for a test case (IFRF furnace).
The Thermodynamic Performance of Liquid-Vapor Separation Air-Cooled Condenser in ORC System at Low Ambient Temperature
Authors: Chen Ying , Zheng Wenxian , Zhong Tianming , Hua Nan
Published: August 2016

This paper investigated the thermodynamic performance of a novel condenser, liquid-vapor separation condenser (LSC), under the ORC conditions with extreme ambient air temperature. By contrast, a common parallel flow condenser (PFC) with the identical structure of tube and fin, together with the heat transfer area was measured under the same condition. The average condensing temperature was chosen as 35°C, R134a was chosen as the working fluid. The experimental results announced that the in-tube average heat transfer coefficients (AHTCs) of the LSC were 96.7% to 109.1% of the PFC when the initial air temperature varied from −10°C to 10°C, at the R134a inlet mass flux from 437kg/(m2s) to 750kg/(m2s), and heat flux from 3kW/m2 to 5 kW/m2. Specially, the pressure drop was only 35.1% to 53.2% of the PFC under the experiment conditions. The tube wall temperatures of the LSC decreased slower than the PFC. The thermodynamic performance of the LSC was superior to the PFC under the ORC conditions. The result indicates the LSC is a promising condenser in ORC system.
CFD Study of a Fire Whirl of Huge Oil Tank: Burning Rate, Flame Length, Distributions of n-Heptane and Oxygen in a Fire Whirl
Authors: Satoh Koyu , Liu Naian , Xie Xiaodong , Gao Wei
Published: August 2016

The number of huge oil storage tanks is increasing in the world. If a fire occurs in one of these tanks, it is very difficult to suppress. Additionally, if a fire whirl occurs in an oil tank fire, it is extremely dangerous for firefighters to extinguish the fire. The authors have numerically studied huge fire whirls in a large oil tank depot and predicted the generation of those fire whirls. Here, another study is attempted to clarify the details of huge fire whirl in a large oil tank, using two kinds of fire whirl generation channels in CFD simulations using the software, FDS by NIST. Details of burning rates, velocities of whirling flames, radiative heat flux, heat release rates and whirling cycles are examined, using oil tanks with the diameters of 0.2 to 80 m. In oil tanks with a diameter of 80 m, a tall fire whirl is generated. The height is about 1000 m. In this study of oil tanks fires with small to large diameters, it has been found that fire whirl lengths are about 8 to 11 times of the oil tank diameter. The maximum radiative heat flux due to a fire whirl in 80 m diameter oil tanks exceeds 100 kW/m2. Since the maximum radiation is found at twice the distance of oil tank diameters from the tank centers, adjacent oil tanks may be ignited. This study has also examined a method used to prevent fire whirl generation in huge oil tanks.
One Dimensional Numerical Study for Thermal Performance of Intermediate Fluid Vaporizer for Liquefied Natural Gas
Authors: Qu Z. G.
Published: August 2016

A one-dimensional heat transfer model was established for the three typical parts of Intermediate Fluid Vaporizer (IFV), namely, evaporator, condenser and thermolator used to vaporize liquefied natural gas (LNG). Seawater and propane were applied as heat sources and intermediate working fluid respectively for regasification and cold energy recovery of LNG in an IFV system. Based on the energy balance among the evaporator, the condenser and the thermolator, the heat transfer and thermodynamic model was established and the distribution of temperature of all fluids and heat transfer coefficients were predicted. The effects of several parameters, including the inlet temperature of seawater and LNG, the mass flow of seawater and LNG, the pressure of LNG, on the temperature distribution and heat transfer coefficients were conducted. The results show that the heat transfer capacity of evaporator was enhanced greatly by increasing inlet temperature and inlet mass flow rate of seawater and the thermal resistances in the two sides of evaporator were proportionate, and show that the heat transfer coefficient of condenser increases gradually along flow path and the curve exists convex firstly and another concave with the turning point, and also show that the heat transfer capacity of thermolator decreases gradually with the heat transfer coefficient inside tube keeping consistent along with the flow path and the heat transfer coefficient inside tube was much bigger than heat transfer coefficient outside tube. The results show that the main thermal resistance is the heat transfer of nature gas flowing across the outside tube banks for the thermolator and the propane condensing outside tube for the condenser, respectively.
Innovative Realizations of High Heat-Flux Boiling and Condensing Flows for Milli-Meter and Micro-Meter Scale Applications
Authors: Kivisalu Michael , Narain Amitabh , Gorgitrattanagul Patcharapol , Naik Ranjeeth
Published: August 2016

For shear driven mm-scale flows, the traditional boiler and condenser operations pose serious problems of degraded performance (low heat-flux values, high pressure drops, and device-and-system level instabilities). The innovative devices are introduced for functionality and high heat load capabilities needed for shear dominated electronic cooling situations that arise in milli-meter scale operations, certain gravity-insensitive avionics-cooling and zero-gravity applications.
Nonlinear Analysis in a Circle With an Internal Concentric Slotted Round With Different Structure Sizes
Authors: Shen C. Y. , Yang M. , Zhang Y. W.
Published: August 2016

Numerically simulate natural convection heat transfer in the slotted circle within a round in different structure sizes, and study the natural convection heat transfer and nonlinear characteristics according to the numerical results. Verified by numerical calculation, the numerical results coincide with the same working conditions experimental results of the existing literature. Studying natural convection heat transfer in the horizontal slotted round within the circle, with the increase of Ra, it is possible to obtain a stable solution, periodically oscillating solutions, non-periodic oscillation solution, ultimately chaos; on the same slotted direction, the average equivalent thermal conductivity Keqs with the increase of Ra increases, and Keqs is maximum in the vertical direction, so the convective heat transfer is the strongest. when Ra exceeds a critical value, numerical results begin to oscillate, such as the static bifurcation and periodic oscillation phenomenon, and this critical value is associated with slotted direction, The largest in vertical direction, and get the minimum in 45°. For numerical analysis of slotted circle within closed circle under the different radius ratios. The annular space pace is narrower, thermal fluid is more in slotted circle, internal isotherm closer to the horizontal, the worse of heat transfer. But once the radius ratio greater than 2.0, and then increase the radius ratio Keq is not increased too much.
Numerical Investigation of Porous Medium Heat Transfer
Authors: Hossain Mohammad A. , Hossain Sarzina
Published: August 2016

This work deals with numerical investigation of porous medium heat transfer for square tube cross flow heat exchanger. The simulation is done for 2D turbulent flow. In order to resolve the flow field, Reynolds Average Navier-stokes (RANS) model is used. Standard k-epsilon model is considered among different RANS model in order to resolve the mean velocity field, pressure drop and temperature distribution. Both average velocity, temperature distribution and pressure drop are calculated for three different cases having three different porosity (ϕ), permeability (K) and inertial coefficient (β). The results of three different cases are compared and presented. First the mesh independence test is done by different mesh size. ANSYS Fluent is used for the simulation. A pressured based solver is used with SIMPLE solution scheme, in order to find different flow variables. Finally the Nusselt number (Nu) and surface heat transfer coefficient (h) are determined and presented.
Study on Drop Wise Condensation by Using Functionalized Heat Transfer Surface
Authors: Koizumi Yasuo , Yoshizawa Shota
Published: August 2016

Condensation heat transfer experiments were performed by using steam at 0.1 MPa The enhancement of drop wise condensation heat transfer by functionalizing a heat transfer surface was examined. A gold-plated surface was used to produce the drop wise condensation. Grooved heat transfer surfaces were adopted to functionalize the heat transfer surface. The shape and the size of the grooves were rectangular and 2 mm × 2 mm × 2 mm, 3 mm × 3 mm × 3 mm and 2 mm × 3 mm × 2 mm (groove depth × groove top part width × groove bottom width), respectively. The heat flux of the grooved surface was larger than that of the plain gold-plated surface. When the groove size was 2 mm × 2 mm × 2 mm and the top parts and the walls of grooves were plated with gold, the heat transfer rate augmentation was highest; the augmentation rate was 1.53. Since to increase the width of the top part of the grooves tends to bring the quality of the surface structure close to the plain surface, to increase the top width of the grooves is not right direction. It was also implied that to make summits and troughs on the surface tends to expose the summit part to steam more, which might result in the heat transfer augmentation.
Experimental Investigation on the Performance of Vortex Tube With Non-Freeze Enhancement
Authors: Duan Ran , Eri Qitai , Li Kexin
Published: August 2016

The vortex tube is a temperature separating device. In some of its applications (e.g., pressure reduction of natural gas), low gas temperature in the cold end is required, which may freeze the impure gas. To solve this problem, a vortex tube with non-freeze enhancement was designed and an experimental system was built. The non-freeze design enabled a reverse flow ejection in the pipe of the cold end. Moist air was used as the working fluid and the performance parameters of five similar vortex tubes were compared in this experiment. The characteristics of freeze based on the experiment were presented. The results indicated that thermal conductivity and mixing process played the most important role to avoid freeze when this vortex tube worked in low level of cold fraction. A feasible way to extend the range of working cold fraction for vortex tube is proposed accordingly.
Combustion Characteristics of Advanced Vortex Combustor Burning H2 Fuel
Authors: Deng Yangbo , Jiang Xi , Su Fengmin
Published: August 2016

The combustion characteristics of the advanced vortex combustor (AVC) burning H2 fuel are studied numerically. First, using the 19-step reaction mechanism, the flame morphology of the pre-mixed H2/Air combustion under the different conditions, is computed. The calculation results are in agreement with experimental data from the literature. Second, a numerical model of a lean premixed H2/Air combustion is set up, based on the 19-step reaction mechanism. A numerical simulation is carried out to study the combustion characteristics of the AVC. The results show that the combustion can be steadily maintained, with the equivalence ratio of the H2/Air main flow kept at 0.45. At the same time, the total pressure loss coefficient is 2.77% and the combustion efficiency is 99.8%. The total pressure loss, vortex configuration and stability, combustion efficiency of the AVC are influenced by the equivalence ratio, total pressure and static pressure of main flow in the AVC.
Heat Transfer Enhancement for Turbulent Flows in Corrugated Tubes
Authors: Yao Zhi-Min , Feng Zhi-Gang , Qian Zuo-Qin , Chen Zhi-Zhe
Published: August 2016

Heat transfer rate and pressure drop of turbulent flows of water in a smooth-wall tube and five corrugated tubes at Reynolds numbers between 7,500 and 50,000 are studied using the commercial software FLUENT. The corrugated tube is constructed by placing protruded ridges evenly along a tube. Depending on the different design of corrugated tubes, our numerical simulation results show that the use of corrugated tubes can improve heat transfer rate by a factor of 1.5 to 2 at Reynolds numbers between 7,500 and 12,000 when compared to a smooth-wall tube. However, the rate of enhancement gradually decreases to a factor of 1.1 to 1.5 as flow Reynolds number increases to 50,000. We further studied the pressure drop and friction factors of the corrugated tube. For the corrugated tube with the highest heat transfer enhancement, we found the pressure drop increases by a factor of 3 to 4 compared to a smooth-wall tube, while the friction factor increases by a factor of 3.5 to 4.4. These findings can be very useful in the design of more efficient heat exchangers.
Smoothed Particle Method for Studying Heat and Mass Transfer Between Fluid and Solid
Authors: Feng Zhi-Gang , Ponton Miguel Cortina
Published: August 2016

Smoothed Particle Hydrodynamics (SPH) is a meshless Lagrangian method in which the domain is represented by particles. Each particle is assigned properties such as mass, pressure, density, temperature, and velocity. These properties are then evaluated at the particle positions using a smoothing kernel that integrates over the values of the surrounding particles. In the present study, the SPH method is first used to study the flow between infinite plates at different temperatures; velocity and temperature profiles are compared to the analytical solution and good agreements are found. Then, the SPH is further extended to study the heat transfer of an isothermal cylinder in a periodic flow. The velocity profile is found to be in good agreement with results provided in the literature, and the temperature field obtained seems reasonable. Finally, the natural convection of a heated cylinder in a square enclosure is studied. For this, the discretized version of the heat equation is used in conjunction with the Boussinesq approximation. The results between the SPH with and without a tensile instability are compared.
Effect of the Wavy Walls on the Turbulent Flow and Heat Transfer in Rotating Two-Pass Rectangular Channels
Authors: Mohammed Aounallah , Abdelkrim Miloud , Mustapha Belkadi , Lahouari Adjlout , Omar Imine
Published: August 2016

Numerical predictions of three-dimensional turbulent flow and heat transfer are presented for rotating two-pass rectangular channel with an aspect ratio (AR = 1:4). The aim of this study is to investigate the effect of the wavy walls of the heated part on the cooling enhancement. The influence of the wave length is also explored. Several calculations have been performed with Reynolds number (Re = 10 000), rotation number (R0 = 0.0 to 0.14) and coolant-to-wall density ratio (1-(T0 /Tw) is limited to 0.13. The commercial code ANSYS FLUENT is used to solve the steady 3D Reynolds–Averaged Navier–Stokes (RANS) equations. The performances of the k-omega SST and the second moment closure RSM models are evaluated by comparing their numerical results against the available experimental data. Detailed predictions of mean temperature, secondary flow and Nusselt number distributions are presented.
Numerical Modeling of the Frosting Process on a Cold Finned Surface With Variable Fin Spacing
Authors: El Zaabalawy Assem , Diab Aya , Ghoneim Zakaria
Published: August 2016

Frost formation can incur damage to agricultural crops, roads, railways, electrical transmission lines, transport of oil and natural gas in cold climates, as well as refrigeration and air-conditioning equipment. Modeling of the frost formation process involves coupled heat and mass transport phenomena which are not only function of time but also of space. To model such complex interactions, a transient two dimensional mathematical model has been developed using a control volume approach, discretizing Navier-Stokes equations over a fixed Cartesian staggered grid coupling the pressure and velocity via the SIMPLE algorithm. To identify the interface between the air and the frost sub-domains, a cut cell approach has been used. The validated numerical model is used to investigate the impact of the heat exchanger configuration on the frost layer distribution. Specifically, a variable fin spacing (converging channel) is compared to the regular configuration (with straight fins) and is shown to have a positive impact on the frost layer distribution.
Vortex-Induced Vibration for Heat Transfer Enhancement
Authors: Shi Junxiang , Schafer Steven R. , Chen Chung-Lung (C. L. )
Published: August 2016

A passive, self-agitating method which takes advantage of vortex-induced vibration (VIV) is presented to disrupt the thermal boundary layer and thereby enhance the convective heat transfer performance of a channel. A flexible cylinder is placed at centerline of a channel. The vortex shedding due to the presence of the cylinder generates a periodic lift force and the consequent vibration of the cylinder. The fluid-structure-interaction (FSI) due to the vibration strengthens the disruption of the thermal boundary layer by reinforcing vortex interaction with the walls, and improves the mixing process. This novel concept is demonstrated by a three-dimensional modeling study in different channels. The fluid dynamics and thermal performance are discussed in terms of the vortex dynamics, disruption of the thermal boundary layer, local and average Nusselt numbers (Nu), and pressure loss. At different conditions (Reynolds numbers, channel geometries, material properties), the channel with the VIV is seen to significantly increase the convective heat transfer coefficient. When the Reynolds number is 168, the channel with the VIV improves the average Nu by 234.8% and 51.4% in comparison with a clean channel and a channel with a stationary cylinder, respectively. The cylinder with the natural frequency close to the vortex shedding frequency is proved to have the maximum heat transfer enhancement. When the natural frequency is different from the vortex shedding frequency, the lower natural frequency shows a higher heat transfer rate and lower pressure loss than the larger one.
Exergetic Analysis of a Cross-Flow Microchannel Heat Exchanger for Bleed Air Cooling in Aircraft Gas Turbine Engines
Authors: Rivera Matthew B. , Manteufel Randall D.
Published: August 2016

A current issue with high-pressure-ratio compressors found in aircraft engines is the temperature of the air exiting the compressor. The exiting air is used as coolant for engine components found in later stages of the engine such as first-stage turbine blades, and afterburner walls. A viable option for reducing outlet temperature of high-pressure-ratio compressors is to “bleed-off” a fraction of the air which is cooled in a heat exchanger by rejecting heat into the liquid fuel stream and then use the air for cooling critical components downstream. Bleeding off air from the outlet of the compressor has two benefits: (1) air temperature is reduced, and (2) fuel temperature is elevated. Along with reduced air temperatures, the fuel will ultimately receive the heat lost from the air, making the fuel more ideal for combustion purposes. The higher temperature the fuel is received in the combustion process, the greater the work output will be according to the basics of thermodynamic combustion. The objective of this case study is to optimize the efficiency of the cross-flow micro channel heat exchanger, with respect to (1) volume (1.75–2.75 mm3) and heat transfer, and (2) weight (0.15–.25 N) and heat transfer. The optimization of the heat exchanger will be evaluated within the bounds of the 2nd law of thermodynamics (exergy). The only effective way to measure the 2nd law of thermodynamics is through exergy destruction or its equivalent form: entropy generation as a factor of dead state temperature. With relations and equations obtained to design an optimal heat exchanger, applications to high performance aircraft gas turbine engines is considered through exergy. The importance of developing an exergetic analysis for a thermal system is highly effective for identifying area’s within the system that have the path of highest resistance to work potential through various modes of heat transfer and pressure loss. Thus, optimization to reduce exergy destruction is sought after through this design method alongside verifying other heat exchanger methods through effectiveness.
Effects of Solid Shield and Shroud on Plasma Jet Flame in APS Process
Authors: Liu Ting , Zheng Lili , Zhang Guanzhong , Zhang Hui
Published: August 2016

Gas entrainment occurs in atmospheric plasma spray (APS) after plasma jet flows into the cold surroundings, resulting in rapid decaying of speed and unwanted oxidation. Installing a solid shield or inserting a gas shroud at the end of the nozzle may be helpful to reduce air entrainment. Effects of solid shield and inert gas shroud on plasma jet are investigated numerically. It is found that solid shield and gas shroud will expand high temperature field of plasma jet in both axial and radial direction, consequently, enabling a thorough reaction of oxygen content for virtually oxygen free plasma jet. The mass fraction of the O2 gas in the plasma flow outside the shield decreases with the increase of the inert gas velocity, which maintains nearly zero in the solid shield. Inert gas velocity and its injection angle turn out to be important factors to offset the negative effects of solid shield on the main flow. It is demonstrated that the flow field is less affected when the inert gas is axially injected with the inlet velocity of 150m/s. Ar is more appropriate than N2 as an inert gas, which is mainly attributed to its physical properties.
Two Dimensional-Steady State-Natural Convection During the Buoyancy-Induced Flow of CuO-Water Nanofluid Along a Vertical Channel
Authors: Koronaki I. P. , Nitsas M. T. , Vallianos Ch.
Published: August 2016

In many engineering applications, heat and mass transfer is of vital importance. Therefore a lot of research has been done trying to maximize the heat transfer rate. It is proved, mostly through experimental processes that nanofluids i.e., liquid suspensions of nanometer size particles, have the required capability to augment heat transfer since their efficacy is based on their improved properties compared to those of the base fluid. The present paper examines the two dimensional-steady state-natural convection during the buoyancy-induced flow of the incompressible CuO-water nanofluid along a vertical channel whose walls are uniformly heated. The available literature suggests static and dynamic models for calculating the effective conductivity and viscosity of nanofluids. In this work, both models are assumed so as the Brownian motion of nanoparticles to be considered. The governing equations of continuity, momentum and energy have been solved numerically with the Finite Difference Method (FDM) by using suitable dimensionless variables. The results of the aforementioned analysis prove that the convection coefficient is enhanced due to the presence of nanofluids and it increases further by changing the volume concentration of the nanoparticles. Finally, the effect of the nanoparticles size on heat transfer and the type of the base fluid is investigated.
Natural Ventilation of a Solar House in Hot and Humid Climate: A Study Using Building Energy Simulation Method
Authors: Belleil Elise , Phan Long , Lin Cheng-Xian , Schäfer Mirko , Wagner Johannes
Published: August 2016

The solar powered house at the Engineering Center of Florida International University is out of the U.S. Solar Decathlon 2005 competition. A computational simulation using EnergyPlus is conducted to study different ventilation strategies in this solar house model, with consideration of the hot and humid climate in Miami, Florida. Several modes of ventilation including mechanical cooling systems, natural ventilation utilization, and hybrid systems were considered to seek the best possible option for ventilation in such extreme climate. While the need for a mechanical ventilation system is always present, a resort to natural ventilation could significantly reduce energy consumption. As for natural ventilation utilization, a few methods including earth tubes (ET), thermal chimneys (TC), cooling towers (CT), and openings have been simulated and compared with the mechanical cooling system of the original house. However, as the simulation results suggested, relying on only natural ventilation could cause a dramatic impact to the human thermal comfort. Therefore, a coupling strategy between mechanical systems and natural ventilation was extensively investigated in hope for a better solution in terms of both energy consumption and thermal comfort. In fact, the hybrid system has proved to tremendously reduce energy consumption while complying with the minimum requirements for thermal comfort recommended by ASHRAE standards.
Numerical Simulation and Parametric Analysis of Multistage Ionic Wind Pump for Enhanced Air-Cooling
Authors: Kiflemariam Robel , Zevallos Francisco , Lin Cheng-Xian
Published: August 2016

Unlike multistage rotary fans which benefit from decades of basic research in heat transfer and optimization studies, there is lack of comprehensive study on heat transfer and fluid dynamics characteristics of multistage ionic wind pumps and the optimization of system parameters to enhance heat transfer. Particularly, there are only few reports of systematic study of stacking characteristics of velocity and efficiency, and the possibility of using a hybrid series and parallel stages has not been explored. In this paper, we investigate numerically the use of multi-stage configuration for ionic wind pumps. The study focuses on multiple wire/plate and multiple wire/cylinder configurations. The integration of multistage ionic pumps with local hot spots for better cooling performance and heat transfer enhancement is also studied. Results show that multiple stage ionic wind pumps have generally better performance in terms of velocity and heat transfer enhancement than single stage ionic wind pumps, but the rate of increase in performance decrease with increased number of stages for series stacking of stages.