bluuebunny/crossref_metadata_2025 huggingface dataset:
Reduced Order Modeling Applied to the Discrete Ordinates Method for Radiation Heat Transfer in Participating Media
Authors: Tencer John , Carlberg Kevin , Hogan Roy , Larsen Marvin
Published: November 2016

Radiation heat transfer is an important phenomenon in many physical systems of practical interest. When participating media is important, the radiative transfer equation (RTE) must be solved for the radiative intensity as a function of location, time, direction, and wavelength. In many heat transfer applications, a quasi-steady assumption is valid. The dependence on wavelength is often treated through a weighted sum of gray gases type approach. The discrete ordinates method is the most common method for approximating the angular dependence. In the discrete ordinates method, the intensity is solved exactly for a finite number of discrete directions, and integrals over the angular space are accomplished through a quadrature rule. In this work, a projection-based model reduction approach is applied to the discrete ordinates method. A small number or ordinate directions are used to construct the reduced basis. The reduced model is then queried at the quadrature points for a high order quadrature in order to inexpensively approximate this highly accurate solution. This results in a much more accurate solution than can be achieved by the low-order quadrature alone. One-, two-, and three-dimensional test problems are presented.
Effect of Entrainment on Liquid Film Dryout in Vertical Upward Annular Flow
Authors: Wu Zan , Wadekar Vishwas , Sundén Bengt
Published: November 2016

This work aims to investigate the effect of liquid entrainment on liquid film dryout in annular flow for flow boiling. Entrainment and deposition rates of droplets were included in mass conservation equations to estimate the local liquid film mass flux in annular flow, and the critical vapor quality at dryout conditions. Different entrainment rate correlations were evaluated using flow boiling data of n-pentane, iso-octane and water. Effect of the initial entrained fraction (IEF) at the churn-annular transition was also investigated. A transition Boiling number was proposed to separate the IEF-sensitive region at high Boiling numbers and the IEF-insensitive region low Boiling numbers. Besides, the diameter effect on dryout vapor quality was studied. The dryout vapor quality increases with decreasing tube diameter. A possible reason is that there is less droplet entrainment in smaller tubes, as evidenced in numerous flow pattern visualization studies. It needs to be pointed out that the dryout characteristics of submillimeter channels to be different because of different mechanisms of dryout, i.e., drying of liquid film underneath long vapor slugs and flow boiling instabilities.
Improving the Resolution of Steady-State, Infrared-Based Thermal Interface Resistance Measurements Using High-Precision Metrology to Determine In-Situ TIM Thickness
Authors: Warzoha Ronald J. , Smith Andrew N. , Harris Maurice
Published: November 2016

The performance characteristics of thermal interface materials (TIMs) are quickly outpacing our ability to measure them using steady-state techniques. In fact, scientists have turned to photothermal techniques like Time-domain Thermoreflectance (TDTR) to measure the impedance to heat flow across TIMs, namely due to their relatively low measurement uncertainties. However, such techniques are costly, require significant sample preparation, only measure local thermal impedances and are not yet equipped to measure thermal resistance as a function of pressure. Instead, it is desirable to maximize the resolution of traditional steady-state equipment for these types of measurements. In this work, we develop a more robust and accurate methodology to determine the temperature difference across the junction of a traditional steady-state apparatus using high accuracy measurements of in-situ TIM thickness in tandem with infrared thermography. This methodology eliminates a significant fraction of the uncertainty associated with the measurement of thermal interface resistance. Importantly, the use of this method improves the accuracy of the measurement device by an order of magnitude at interfacial thermal resistance values on the order of 1·10−6m2·K/W when compared to state-of-the-art, thermal probe-based measurement systems.
Numerical Simulation of Heat Transfer and Pressure Drop Characteristics of Internal Microfin Tubes
Authors: Zhang Jingzhi , Lin Jinpin , Li Wei
Published: November 2016

Heat transfer and pressure drop characteristics of mini smooth and microfin tubes were studied numerically using water as working fluid at Reynolds number ranging from 7500 to 17500. Seven microfin tubes were used with the same inner diameters of 4.6 mm and 18° helix angle and with number of fins ranging from 30 to 50, fin apex angle ranging from 10° to 40°, and fin height ranging from 0.1 to 0.15 mm. The numerical results fit well with the empirical correlations for heat transfer coefficients and pressure drops. The results indicate that the j-factor of the microfin tubes is approximately 1.2∼1.4 times of that in smooth tubes at the same Re. The j-factor increases with increasing number of microfin and the microfin height and with decreasing fin apex angle. The f-factor of the microfin tubes is approximately 1.05∼1.25 times of that in the smooth tube at the same Re, and the difference between the factors increases with the Re rising. The performance evaluation criterions (PEC) of the seven microfin tubes ranges from 1.15 to 1.35, indicating that microfin tubes exhibit better comprehensive performance compared with smooth tubes. The fluid at the center has a strong tendency to move towards the heated wall along the radial direction due to the directing effect of the microfins. The distinctive flow pattern in the radial direction can sufficiently enhance the turbulent flow near the wall and strengthen the mixing between the cold fluid at the center and hot water at the wall, leading to the enhancement of heat transfer in the near-wall region.
A Numerical Study on 2-D Flow and Heat Transfer in a Natural Gas Heater
Authors: Guo Yun , Guo Zhixiong
Published: November 2016

The cylinder type natural gas heater with heat transfer medium is most widely used in gas application systems. But the conventional symmetrical arrangement of heat-exchange surfaces in cylinder is not conducive to the formation of an effective heat flow field. An effective measure of rotating the symmetrical layout structure at a certain angle is put forward creatively, which can optimize the overall heat transfer effect of the heater. The combined natural convection and radiation heat transfer model for both heating and cooling surfaces in the heat-exchanging cylinder is built. The finite volume method with unstructured body-fitted grids is employed. Analyses and comparisons of the flow conditions and temperature distributions of the cylindrical natural gas heater indifferent clockwise rotation angles (10°, 20°, 30°, 35°) are carried out. With increasing of the angle, the natural convection and radiation heat transfer will be correspondingly enhanced. When the angle exceeds than 30°, however, the natural convective circulation is destroyed instead. The numerical simulation shows the best rotation angle is 20∼30°.
Hybrid Ballistic-Diffusive Solution of the Frequency-Dependent Phonon Boltzmann Transport Equation
Authors: Allu Pareekshith , Mazumder Sandip
Published: November 2016

The phonon Boltzmann Transport Equation (BTE) is difficult to solve on account of the directional and spectral nature of the phonon intensity, which necessitates angular and spectral discretization, and ultimately results in a large number (typically few hundreds) of four-dimensional partial differential equations. In the ballistic (large Knudsen number) regime, the phonon intensity is highly anisotropic, and therefore, angular resolution is desirable. However, in the diffusive (small Knudsen number) regime, the intensity is fairly isotropic, and hence, angular discretization is wasteful. In such scenarios, the method of spherical harmonics may be effectively used to reduce the large number of directional BTEs to a few partial differential equations. Since the Knudsen number is frequency dependent, the decision to preserve or eliminate angular discretization may be made frequency by frequency based on whether the spectral Knudsen number is large or small. In this article, a hybrid method is proposed in which for some frequency intervals (bands), full angular discretization is used, while for others, the first order spherical harmonics (P1) is invoked to reduce the number of directional BTEs. The accuracy and efficiency of the hybrid method is tested by solving several steady state and transient nanoscale heat conduction problems in two and three-dimensional geometries. Silicon is used as the candidate material. It is found that hybridization is effective in significantly improving the efficiency of solution of the BTE — sometimes by a factor of three — without significant penalty on the accuracy.
Turbulent Double-Diffusive Free Convection in a Porous Square Cavity Simulated With the Two Temperature Approach
Authors: de Lemos Marcelo J. S. , Carvalho Paulo H. S.
Published: November 2016

This work investigates the influence of thermal conductivity ratio on energy and mass transport across a porous square cavity. Modeling of heat transfer from side to side of the enclosure assumed the hypothesis of thermal non-equilibrium between the solid matrix and the fluid phase. Transport equations were discretized using the control-volume method and the system of algebraic equations obtained was relaxed via the SIMPLE algorithm. Results showed that Shw, mass flux of chemical species and heat flux in the solid phase are strongly dependent of ks/kf, significantly increasing their values as such ratio increases.
A Numerical Study of Radiative Heat Transfer in a Cylindrical Furnace by Using Finite Volume Method
Authors: Wang Zhenhua , Sunden Bengt , Dong Shikui , He Zhihong , Yang Weihua , Wang Lei
Published: November 2016

In designing industrial cylindrical furnaces, it is important to predict the radiative heat flux on the wall with high accuracy. In this study, we consider CO2 and H2O which have strong absorption in the infrared range. The absorption coefficients of the gases are calculated by using the statistical narrow band (SNB) model. The spectrum is divided into 15 bands to cover all the absorption regions of the two non-gray gases. The radiative transfer equation is solved by the finite volume method (FVM) in cylindrical coordinates. To make the FVM more accurate, we discretize the solid angle into 80 directions with the S8 approximation which is found to be both efficient and less time consuming. Based on the existing species and temperature fields, which were modeled by the FLUENT commercial code, the radiative heat transfer in a cylinder combustor is simulated by an in-house code. The results show that the radiative heat flux plays a dominant part of the heat flux to the wall. Meanwhile, when the gas is considered as nongray, the computational time is very huge. Therefore, a parallel algorithm is also applied to speed up the computing process.
Energy Saving Effect of Different Wall Structures Based on Intermittent Energy Use Characteristics in a Hot Summer and Cold Winter Zone
Authors: Zhu Yanyan , Li Wei , Zhou Bin , Kukulka David J.
Published: November 2016

An analysis is performed for a typical residential living space with intermittent energy use in a hot summer and cold winter zone. Analysis performed in this study include a dynamic, three dimensional heat transfer model that examine the intermittent heating and cooling loads in a typical living space in Shanghai. Different wall structures (non-insulation structure, exterior insulation structure, or interior insulation structure) can have different influences on the energy efficiency in buildings. Results conclude that the interior insulation structure provides the largest reduction of energy consumption in buildings when compared to the other wall structures in a hot summer and cold winter zone. For the interior insulation structure in this study, the typical thermal insulation thickness is 0.03 m.
Heat Transfer Characteristics of Aviation Kerosene in Vertical Upward High Flux Tubes at Supercritical Pressure
Authors: Chen Jingxiang , Song Yachao , Xu Guoqiang , Wen Jie , Li Haiwang , Tao Zhi , Li Wei
Published: November 2016

An experimental investigation on heat transfer peculiarity of kerosene flowing in vertical upward high flux tubes at supercritical pressure is presented. Three inner-sintered steel powder coating tubes (high flux tubes) and one smooth tube are tested under the different super-critical pressure and different mass flux of kerosene in the experiment. Results are found that all three high flux tubes perform much better than smooth tube at the same parameters of the tube and same working conditions. It can be obviously deduced that the outer wall temperature is reduced by the disturbance in the flow field of the sintered metal coating at the inner tube-side, while the reduced mass flux can increase the wall temperature on the contrary. Heat transfer coefficient is found 2.5 times as the smooth tube, yet both too large and too small particle diameters of metal powder sintered on the tube surface can deteriorate heat transfer. Density and viscosity, thermal conductivity of kerosene at different temperatures and pressures under supercritical pressure can be evaluated by using the extended corresponding state principle, which shows good consistency with the experimental results.
Heat Transfer in Two-Phase Vertical Co-Flow in the Presence of a Mesh-Type Bubble Breaker
Authors: Kalbfleisch Alan , Siddiqui Kamran
Published: November 2016

Bubble breakers have been shown to be effective at reducing bubble size and delaying transition from bubbly to slug and churn flow regimes in the two-phase vertical pipe flow. If used in bubble column reactors, bubble breakers can increase the surface area-volume ratio of the gas-liquid interface allowing for an enhanced mass transfer or chemical reaction rate. Studies have been done showing the effect of bubble breakers on bubbles size and flow regime but none exist to show the effect of a bubble breaker on heat transfer for a two-phase pipe flow. A new method of measuring the heat transfer for a two-phase vertical pipe flow is proposed in the current study. The method uses thermocouples inserted directly into the flow for bulk fluid temperature measurements and a thermal camera for surface temperature measurements of a thin walled stainless steel pipe. Heat transfer measurements, expressed as a Nusselt number, for a single phase laminar liquid flow are compared to accepted values to show the validity of the experimental method. Preliminary results of two-phase gas-liquid heat transfer rates with and without a bubble breaker present in the vertical pipe are compared. The liquid flowrates used in the experiment represented superficial Reynolds numbers of ReI<2000 and the gas flowrates used in the experiment represented superficial Reynolds numbers of Reg<100. Without a bubble breaker, the convective heat transfer coefficient, represented as Nusselt number, was found to decrease with increasing gas flowrate. When a bubble breaker was added, the effect on the heat transfer was dependent on the flow regime. For most cases, the bubble breaker had very little effect on the measured heat transfer rate. In a case where the bubble breaker was able to generate slug flow rather than churn flow that was generated when no bubble breaker was present, the measured Nusselt number was increased.
Modeling Thermal Conductivity of Aligned CNT-Matrix Composites With Increasing Volume Fraction
Authors: Grandio Diana , Fabris Drazen
Published: November 2016

In prior work an effective medium approach (EMA) has been developed to evaluate composite physical properties such as thermal conductivity, dielectric function or elastic modulus (C.-W. Nan, Prog. Mat. Sci. V. 37, 1993). This model combined with the Kapitza interface resistance can predict the effective thermal conductivity of randomly dispersed long fibers for a very low volume fraction (f < 0.01). The present study compares finite-element (FEA) computations and the EMA model for CNT-matrix compositions with low to moderate volume fractions, 0.001 to 0.02. The FEA results obtained show that the EMA model underestimates the effective thermal conductivity of the composite when the particles are very close to each other, even for small particle volume fractions. For aligned fibers the Kaptiza resistance cannot be neglected in the longitudinal direction. This paper proposes a general correction function for the dependence on particle to particle interaction based on the near neighbor distances and the number of near neighbors. This correction function reduces the EMA under prediction to within several percent (< 5%) in most cases.
Mixed Convection Heat Transfer in an Annulus With Rotating Inner Cylinder
Authors: Nair Yadukrishnan , Shakkottai Venkateshan , Prasad B. V. S. S. S.
Published: November 2016

Natural as well as mixed convection heat transfer are studied in an annulus formed by concentric cylinders where the outer cylinder is the enclosure and the inner cylinder is either stationary or rotating about its axis at various angular speeds. The parameters involved in this study are Rayleigh number (Ra), Richardson number (Ri), Rotational Reynolds number (Re) and Nusselt Number (Nu). The present study uses a two Dimensional steady state Finite Volume method with a coupled scheme approach of pressure-velocity coupling. Three cases of Ra with a temperature difference of 5K, 50K and 90K are investigated. For each case, natural convection and mixed convection heat transfer is studied by varying Ri. For mixed convection study, the range of values used for Ri are 10−3–103 in multiples of 10 and hence the rotational speed of inner cylinder is varied suitably. For each case of Ra, for each Ri, local Nu variation on the rotating cylinder as well as the enclosure are presented. Surface averaged values of Nu are plotted against Ri for different values of Ra. Further, radial velocity component variation is also examined along a particular region in the domain.
Experimental Investigation on Cryogenic Line Chill-Down Process by Using Liquid Nitrogen
Authors: Jin Lingxue , Seo Mansu , Cho Hyokjin , Park Changgi , Lee Cheonkyu , Jeong Sangkwon
Published: November 2016

Generally, a part of transporting liquids are used as the coolant to chill down the transfer line. Minimizing the line chill-down time and the consumption of the cryogenic propellant during the line chill-down is a mainly concerned issue. In this study, the cryogenic line chill-down experiments are conducted on a 12.7 mm outer diameter, 1.25 mm wall thickness and 7 m long stainless steel horizontal pipe with liquid nitrogen (LN2) injection. The pipe is vacuum jacketed to reduce the heat inleak from room temperature to the cryogenic liquid. The temperature and the pressure profiles of the chill-down line are obtained at the location 6 meters away from the pipe inlet under various mass flow rates. The history of the transient temperature, the pressure and the mass flow rate during the line chill-down process are monitored for various cases. The relationship between the line chill-down time and the mass flow rate is analyzed and presented for one representative pipe line geometry.
Melting in a Rectangular Cavity
Authors: Kozak Y. , Ziskind G.
Published: November 2016

The present study deals with a new numerical approach for solid-liquid phase-change modeling. The new model is based on the enthalpy method and takes into account natural convection in the melt which can be coupled to the solid bulk sinking motion by the force balance on the solid bulk. A basic configuration is investigated, namely a two-dimensional rectangular cavity with a constant wall temperature. The effect of rather low values of the Archimedes and Rayleigh numbers on the sinking motion of the solid bulk, the melting rate and melting patterns, is explored. It is found that the density difference, between the solid and liquid phases, for the studied case does not affect considerably the results. However, it is shown that by natural convection alone the solid sinking motion is established, the melting rate is enhanced and the melting patterns are completely different in comparison with the first studied case.
Using Thermoelectric Cooling With Tourniquets for Nerve Preservation
Authors: Trupiano M. , Aarabi S. , Emery A. F.
Published: November 2016

The use of a tourniquet leads to nerve damage, even if applied for short periods of time. This damage can be minimized if the limb is cooled. Because of the low conductivities of human tissue, core limb cooling is slow unless the surface temperature is very cool. Subzero surface temperatures can lead to skin injury (i.e., frostbite). Ideally one would adjust the limb surface temperatures as a function of time to maximize the cooling rate while avoiding permanent tissue damage. One possible approach is to use a thermoelectric cooler (TEC) in conjunction with a programmable power supply. TEC performance varies strongly with heat absorption rate, a function of limb thermal properties, and hot side temperatures that are strongly affected by the surface conditions on the hot side, i.e., overall heat transfer coefficients and ambient conditions.

The paper describes the use of finite element simulation to predict the usefulness of using thermoelectric coolers applied to the surface of a limb when compared to the standard approach of using ice packs. Since the TEC performance is strongly influenced by its warm side thermal conditions, experimental results are presented for different ambient temperatures, free and forced convection, and evaporation of water from a wickable covering.
Experimental and Numerical Investigation on Mixed Convection in Horizontal Channels Partially Filled With Aluminum Foam and Heated From Below
Authors: Buonomo Bernardo , Cirillo Luca , Manca Oronzio , Nardini Sergio
Published: November 2016

Combined natural convection and forced convection gets a great attention for its importance in practical applications in various modern systems such as electronic cooling, chemical vapor deposition and solar energy systems. Buoyancy force due to the heating of the lower cavity wall induces secondary flows hence the local heat transfer increases. The onset point of the secondary flows is important because it delineates the region after which the two-dimensional laminar flow becomes three-dimensional and a transition motion from laminar to turbulent flow is observed.

In this paper, mixed convection in a horizontal channel partially filled with a porous medium and the lower wall heated at uniform heat flux is studied experimentally and numerically. The heated wall temperature profiles as a function of the Gr/Re2 values are presented. Pictures of flow visualization along longitudinal, transversal and horizontal section are given. Average Nusselt numbers are evaluated.

Results in terms of wall temperature profiles, local and average Nusselt numbers are presented for different Reynolds and Rayleigh number values. Some comparison between experimental and numerical results are accomplished.
Experimental and Numerical Investigation on Natural Convection in Horizontal Channels Partially Filled With Aluminium Foam and Heated From Below
Authors: Buonomo Bernardo , Manca Oronzio , Nardini Sergio , Diana Alessandra
Published: November 2016

Natural convection in horizontal rectangular channel without or with aluminum foam is experimentally and numerically investigated. In the case with aluminum foam the channel is partially filled. In both cases, the bottom wall of the channel is heated at a uniform heat flux and the upper wall is unheated and it is not thermally insulated to the external ambient. The experiments are performed with working fluid air. Different values of wall heat flux at lower surface are considered in order to obtain some Grashof numbers and different heated wall temperature distributions. Two different aluminum foams are considered in the experimental investigation, one from “M-pore”, with 10 and 30 pore per inch (PPI), and the other one from “ERG”, with 10, 20 and 40 PPI. The numerical simulation is carried out by a simplified two-dimensional model. It is found that the heat transfer is better when the channel is partially filled and the emissivity is low, whereas the heated wall temperature values are higher when the channel is partially filled and the heated bottom plate has high emissivity. The investigation is achieved also by flow visualization which is carried out to identify the main flow shape and development and the transition region along the channel. The visualization of results, both experimental and numerical, grants the description of secondary motions in the channel.
Analysis of Gas Diffusion-Induced Cassie to Wenzel State Transition on a Structured Surface
Authors: Kadoko Jonah , Karamanis Georgios , Kirk Toby , Hodes Marc
Published: November 2016

We provide an approximate and one-dimensional solution for transient diffusion of gas between parallel ridges into a degassed and quiescent liquid suspended in the Cassie state above a parallel-ridge type structured surface. At time equal to zero, the liquid and gas are at the same pressure; therefore, the meniscus formed between ridges is flat. The analysis provides the transient gas concentration field in the liquid. It also computes the times when the triple contact line begins to move down the ridges and that when the meniscus contacts the bottom substrate compromising the Cassie state.
Numerical Simulation and Optimization of a Carbon Monoxide Boiler
Authors: Tang Guangwu , Wu Bin , Zhou Chenn Q.
Published: November 2016

Carbon monoxide (CO) boilers play an important role in the petroleum refining industry, completing the combustion of CO in the flue gas generated by the regeneration of fluidized cracking catalyst. The heat released by the CO combustion is used to generate steam for use in the refinery. The flue gas flow path can have a significant effect on the thermal efficiency and operation safety of the boiler. In this paper, a CO boiler which had been experiencing low thermal efficiency and high operation risks was studied. A three-dimensional (3D) computational fluid dynamics (CFD) model was developed with detailed description on the combustion process, flow characteristics and heat transfer. The results obtained from the model have good agreement with the plant measurement data. The heat transfer between the tubes and the combustion flue gas was optimized by adding a checker wall.
Modeling of Steel Slab Reheating Process in a Walking Beam Reheating Furnace
Authors: Tang Guangwu , Saavedra Arturo , Okosun Tyamo , Wu Bin , Zhou Chenn Q. , Bai Dengqi , Wang Yufeng , Bodnar Rick
Published: November 2016

Slab reheating is a very important step in steel product manufacturing. A small improvement in reheating efficiency can translate into big savings to steel mills in terms of fuel consumption and productivity. Computational fluid dynamics (CFD) has been employed in conducting numerical simulations of the slab reheating furnace operation. However, a full industrial scale three-dimensional (3D) simulation of a slab reheating furnace, while comprehensive, is not an efficient way to conduct broad studies of the slab heating process. In this paper, a comprehensive two-dimensional (2D) numerical heat transfer model for slab reheating in a walking beam furnace was developed using the finite difference method. The 2D heat transfer model utilizes the heat transfer coefficients derived from a 3D reheating furnace CFD model which was validated by using mill instrumented slab trials. The 2D heat transfer model is capable of predicting slab temperature evolutions during the reheating processes based on the real time furnace conditions and steel physical properties. The 2D model was validated by using mill instrumented slab trials and production data. Good agreement between the model predictions and production data was obtained.
Periodic Natural Convection Inside a Fluid Saturated Porous Medium Made of Disconnected Solid Obstacles: A Continuum Approach
Authors: Mirehei S. M. , Lage J. L.
Published: November 2016

Results of a numerical study considering the periodic natural convection inside a fluid saturated porous medium are presented. The porous medium is obtained by placing four, large and uniformly distributed solid obstacles of regular (square) shape inside the enclosure, a structure that hinders the option of seeking a porous-continuum modeling approach. The periodic heating is achieved by imposing a time-periodic and spatially uniform high temperature condition at one of the walls of the enclosure, while the other wall is maintained at a constant, uniform and low temperature; the horizontal surfaces are set as adiabatic. Heat transfer results are obtained then by following a continuum modeling approach, and reported on a parametric form with the Prandtl number fixed equal to 7, and the Rayleigh number inside the enclosure varying from 103 to 107. The boundary layer interference phenomenon, observed for the case of constant horizontal heating, is also observed in the case of periodic heating. The visualization of the natural convection process via isotherms and streamlines, together with the periodic (time-varying) Nusselt number, allows the identification of a singular dynamic behavior, including the storage of thermal energy inside the enclosure.
Effect of Porous Insert on Heat Transfer in a Backward-Facing Step Flow
Authors: de Lemos Marcelo J. S. , Galuppo Wagner C.
Published: November 2016

We present numerical results for turbulent heat transfer past a backward-facing-step channel with a porous insert. A non-linear eddy viscosity model was applied to handle turbulence. For a constant Darcy number, the thickness of the porous insert was varied in order to analyze its effects on the flow pattern, particularly the damping of the recirculating bubble past the insert. Further, the reduction of the Nusselt number along the bottom heated surface, when using porous materials inside the channel, was investigated. The numerical technique employed for discretizing the governing equations was the control-volume method. The SIMPLE algorithm was used to correct the pressure field and the classical wall function approach was utilized in order to handle flow calculations near the wall. Comparisons of results simulated with different porous materials were presented.
Experimental Study on Flame Behavior in Under-Ventilated Train Car
Authors: Wong K. L. , Chow W. K. , Ng Y. W.
Published: November 2016

In Hong Kong, the number of parallel traders has increased quickly within these years. Passengers bring not only handbags as planned for the subway system four decades ago. There are heavy luggage put in trolleys carried by parallel traders. Goods include milk powder, electronic devices, food and drinks. The increase in fire load will enhance the fire risk. Although the subway management limits the maximum allowable luggage to 23 kg, the combustible amount is still large in train cars.

Heat release rate (HRR) is the most important parameter in fire hazard assessment. A 1:15 scale modeling test was carried out to find the HRR in a train car under limited ventilation with parallel goods in this paper. The goods were simulated by a propanol pool fire. HRR for six scenarios with all closed door to all opened door were studied. Mass loss rate of fuel, oxygen consumption, air velocity and fire duration were measured. Ventilation was provided by opening different numbers of doors of the train car. The train car fire can then be ventilation-controlled or fuel-controlled. The burning phenomena of fire in the train under limited ventilation including steady burning, oscillating fire and ghosting flame were studied.

It was found that the fire size and duration depended mainly on ventilation when oxygen was very limited. When the amount of oxygen supply reached a critical value (the fire at the decay period with continuous supply of oxygen), the fire tended to be fuel-controlled. Burning characteristics of train fire under limited ventilation were also observed. They were different from fires burning in open area. Oscillating flame, ghosting flame and self-extinction were observed. Results are useful to assess fire hazards associated with parallel trading activities. Three repeated experiments on the captioned subject have been done with average results of all experiments presented.
Melting of a Phase Change Material Filled in a Metal Foam: A Visualized Study at the Pore-Scale Using Infrared Imaging
Authors: Jin Hong-Qing , Fan Li-Wu
Published: November 2016

The thermal imaging technique was applied in this work to measure the transient temperature fields during melting of a phase change material (PCM) in a metal foam. A paraffin wax was used as the PCM that was filled in an open-celled copper foam. Melting of a paraffin wax in the presence of copper foam was studied in a rectangular cavity that was heated from one lateral side wall, while the top surface was exposed to an infrared (IR) camera. A thermocouple (TC) was also employed to validate the accuracy of temperature measurements by IR thermal imaging. The relative deviation of measured temperature by the TC and IR camera was found to be under 2% in steady state and under 4% during the entire course of melting. The resolution of IR thermal imaging with the aid of a macro lens allowed for temperature measurements at pore-scale of the copper foam. Local thermal imaging was captured through a minor window on the top plate of the container. Three points (Sp1–3) inside a selected individual pore were marked to quantify the temperature variations of melting process within metal foam/PCM at pore-scale. The average temperature differences between Sp1 and Sp2, Sp3 were found to be about 1 °C over the entire course of melting, and the maximum value was up to nearly 10 °C around the melting point. These preliminary results clearly highlighted the effect of metal ligaments on the temperature distributions at pore-scale.
A Combined Heat Pipe Solution for Long Distance Thermal Management
Authors: Cai Steve Q. , Bhunia Avijit , Asfia Julie
Published: November 2016

A heat pipe can efficiently transfer heat through it liquid-vapor phase change mechanism. Among different heat pipes, loop heat pipe (LHP) and pulsating heat pipe (PHP) are known to be capable of high heat flux/high load heat transfer. In this article, LHP and PHP heat transfer systems are combined to achieve passive, reliable and long-distance heat transfer for successful thermal management of modern avionic systems. Aiming at this development goal, a 2m long LHP is designed to transport heat from the avionics chassis to the remote heat rejection site. To reduce saturation pressure and ensure structural safety at higher operating temperature, water is used as the operating fluid of LHP. Within the avionic chassis, conduction heat transfer is enhanced by sandwiching and solder-bonding a PHP between two PCBs (printed circuit boards). Each PHP/PCB assembly is 20cm long and 12.5cm wide, with electrical heaters mounted to mimic electronic heat dissipation. Heat transfer demonstration of the LHP and PHP combo system is conducted in a lab environment with input power varying from 100 to 400W. For all three PHP/PCB assemblies set in the avionic chassis, the maximum heat source temperature is less than 130°C, even when input power is twice as much as the state of the art. The enhanced heat transfer consumes less power, increases system reliability, and enables avionic system operation in harsher environments.
The Heat Transfer Behavior of a Suddenly Expanding Viscoplastic Flow Field
Authors: Hammad Khaled J.
Published: November 2016

A parametric study is implemented to study the impact of inflow conditions, rheology, and thermo-physical properties on the flow and heat transfer behavior of suddenly expanding, recirculating, and non-recirculating, viscoplastic non-Newtonian flows. The governing mass and fully-elliptic partial differential equations of motion and energy along with the Bingham constitutive equations are numerically solved to provide accurate predictions of the velocity and temperature fields. The expansion ratio was fixed at 2. Inflow conditions, yield stress presence, and transitioning from a non-recirculating to a recirculating viscoplastic flow regime are found to have a strong influence on the overall heat transfer characteristics downstream the expansion plane.
Highly Conductive Thermal Paste of Liquid Metal Alloy Dispersed With Copper Particles
Authors: Li Gen , Ji Yulong , Wu Mengke , Ma Hongbin
Published: November 2016

In this paper, a kind of highly conductive thermal paste is investigated, which consists of liquid metal alloy (LMA) and copper particles. The LMA used in the current research is a gallium-indium-tin eutectic alloy (Ga62.5In21.5Sn16). The copper particles dispersing into LMA have an average diameter of 9 μm. During the dispersing process, a degassing process was conducted in order to reduce air bubbles and increase the thermal conductivity of the investigated paste. A new method based on laser flash (LFA) was used to test the total thermal conductivities of the samples. Three types of thermal pastes were prepared and tested, i.e., LMA, oxidized liquid metal alloy (OLMA), and OLMA mixed with copper particles. Results show that when LMA, OLMA, and OLMA mixed with copper particles at a ratio of 5wt%, the resulting thermal conductivities of the investigated thermal pastes can achieve 44.48 W/mK, 13.55 W/mK, and 24.34 W/mK, which result in the corresponding thermal contact resistances of 4.044 mm2K/W, 5.638 mm2K/W, and 4.075 mm2K/W, respectively. In addition, the effect of the copper particle ratio on the thermal performance was investigated. Results show that when the ratio of copper particles increased from 5wt% to 10wt%, the thermal conductivity of investigated thermal paste increased from 24.34 W/mK to 29.07 W/mK, and the thermal contact resistance decreased from 4.075 mm2K/W to 3.37 mm2K/W.
A Roadmap for Building Thermal Models for AlGaN/GaN HEMTs: Simplifications and Beyond
Authors: Azarifar Mohammad , Donmezer Nazli
Published: November 2016

AlGaN/GaN based high electron mobility transistors (HEMTs) have been intensively used due to their high-efficiency power switching and large current handling capabilities. However, the high power densities and localized heating in these devices form small, high temperature regions called hotspots. Analysis of heat removal from hotspots and temperature control of the entire device is necessary for the reliable design of HEMT devices. For accurate analysis of heat transfer using thermal simulations in such devices with heat transfer occurring at different length scales, a roadmap is needed. For this purpose relative importance of different heat transfer modes in removing heat from devices with different substrate materials, operating at different power densities while different boundary conditions are analyzed using two and three-dimensional COMSOL Multiphysics simulations. Results give the relative importance of different parameters on the heat removal mechanism from devices and provide a roadmap for building simpler yet still accurate thermal models for AlGaN/GaN HEMTs and similar devices.
Bubble Behavior at an Uneven Wall
Authors: Shmueli H. , Ziskind G. , Letan R.
Published: November 2016

The present study deals with single bubble growth on an uneven wall. A model problem is defined and solved using a three-dimensional numerical simulation. The wall has the shape of a triangular cavity and feature vortices. The equations solved in the present study are based on macro region modelling of the bubble alone and describe its growth from the initial state to detachment from the surface and consequent motion. The model includes a simultaneous solution of conservation equations for the liquid and gaseous phases, in conjunction with three-dimensional interface tracking. The latter is achieved using the level-set method. The numerical modeling includes the multi-grid method. The complete three-dimensional model is discretized using an original in-house numerical code realized in MATLAB.

Different cases of bubble growth on the triangular cavity walls are investigated. The main conclusion from the calculations is that the bubble shape and its growth rate strongly depend on its location and on the channel orientation. New features, not possible for flat walls and special for this case, are revealed and discussed. It is demonstrated that under certain conditions, the bubble is obstructed by the surface geometry. It is also shown how a growing bubble affects the flow field inside a cavity, interacting with the vortex structure.
Lattice Boltzmann Method Simulation of 3-D Melting Using Double MRT Model With Interfacial Tracking Method
Authors: Li Zheng , Yang Mo , Zhang Yuwen
Published: November 2016

Three-dimensional melting problems are investigated numerically with Lattice Boltzmann method (LBM). Regarding algorithm’s accuracy and stability, Multiple-Relaxation-Time (MRT) models are employed to simplify the collision term in LBM. Temperature and velocity fields are solved with double distribution functions, respectively. 3-D melting problems are solved with double MRT models for the first time in this article. The key point for the numerical simulation of a melting problem is the methods to obtain the location of the melting front and this article uses interfacial tracking method. The interfacial tracking method combines advantages of both deforming and fixed grid approaches. The location of the melting front was obtained by calculating the energy balance at the solid-liquid interface. Various 3-D conduction controlled melting problems are solved firstly to verify the numerical method. Liquid fraction tendency and temperature distribution obtained from numerical methods agree with the analytical results well. The proposed double MRT model with interfacial tracking method is valid to solve 3-D melting problems. Different 3-D convection controlled melting problems are then solved with the proposed numerical method. Various locations of the heat surface have different melting front moving velocities, due to the natural convection effects. Rayleigh number’s effects to the 3-D melting process is discussed.
Numerical Investigation of the Melting of a Phase Change Material in a Thermal Storage Device With Embedded Air Flow Channels
Authors: Koz Mustafa , Erden Hamza S. , Khalifa H. Ezzat
Published: November 2016

Two time-dependent mathematical and numerical models with different levels of complexity and fidelity were developed to investigate the melting of a phase change material (PCM) configured as a number of aluminum-encased, PCM-filled slabs with embedded micro-channel aluminum tubes, and with parallel air-flow passages interposed between the slabs. Melting was first analyzed with the COMSOL Multiphysics® finite-element model (FEM) in a 2-D domain representing a full-size slab. The melting process is simulated via the apparent heat capacity method. The model captures the effect of natural convection in the PCM melt as well as the conjugate heat transfer through the aluminum tubes. A fast-executing quasi 2-D reduced-order model (ROM) was developed for repetitive design optimization studies. The ROM relies on a time-dependent 1-D closed-form solution of the heat conduction equation in a melting PCM, coupled with variations of the air temperature and heat transfer coefficient. Consequently, the FEM results were employed to develop corrections to the ROM. The corrected ROM was then utilized to study the melting process in a multi-slab thermal storage device that is designed to freeze the PCM at night and release 500 W-h of cooling over a span of ∼10 h during the day.
Effect of Particle Size and Aggregation on Thermal Conductivity of Metal-Polymer Nanocomposite
Authors: Li Xiangyu , Park Wonjun , Chen Yong P. , Ruan Xiulin
Published: November 2016

Metal nanoparticle has been a promising option for fillers in thermal interface materials due to its low cost and ease of fabrication. However, nanoparticle aggregation effect is not well understood because of its complexity. Theoretical models, like effective medium approximation model, barely cover aggregation effect. In this work, we have fabricated nickel-epoxy nanocomposites and observed higher thermal conductivity than effective medium theory predicts. Smaller particles are also found to show higher thermal conductivity, contrary to classical models indicate. A two-level EMA model is developed to account for aggregation effect and to explain the size-dependent enhancement of thermal conductivity by introducing local concentration in aggregation structures.
Schlieren 3D-CT Reconstruction of Instantaneous Density Distributions of Spark-Ignited Flame Kernels of Fuel-Rich Propane-Air Premixture
Authors: Ishino Yojiro , Hayashi Naoki , Ishiko Yuta , Nazari Ahmad Zaid , Nagase Kimihiro , Kakimoto Kazuma , Saiki Yu
Published: November 2016

For 3D observation of high speed flames, non-scanning 3D-CT technique using a multi-directional quantitative schlieren system with flash light source, is proposed for instantaneous density distribution of unsteady premixed flames. This “Schlieren 3D-CT” is based on (i) simultaneous acquisition of flash-light schlieren images taken from numerous directions, and (ii) 3D-CT reconstruction of the images by an appropriate CT algorithm. In this technique, for simultaneous schlieren photography, the custom-made 20-directional schlieren camera has been constructed and used. This camera consists of 20 optical systems of single-directional quantitative schlieren system. Each system is composed of two convex achromatic lenses of 50 mm in diameter and 300 mm in focal length, a light source unit, a schlieren stop of a vertical knife edge and a digital camera. The light unit has a flash (9 micro-sec duration) light source of a uniform luminance rectangular area of 1 mm × 1 mm. Both of the uniformity of the luminosity and the definite shape are essential for a quantitative schlieren observation. Sensitivity of the digital cameras are calibrated with a stepped neutral density filter. Target flames are located at the center of the camera. The image set of 20 directional schlieren images are processed as follows. First the schlieren picture brightness is shifted by no-flame-schlieren picture brightness in order to obtain the real schlieren brightness images. Second, brightness of these images is scaled by Gladstone-Dale constant of air. Finally, the scaled brightness is horizontally integrated to form “density thickness images”, which can be used for CT reconstruction of density distribution. The density thickness images are used for CT reconstruction by MLEM (maximum likelihood-expectation maximization) CT-algorithm to obtain the 3D reconstruction of instantaneous density distribution. In this investigation, the “density thickness” projection images of 400(H) × 500(V) pixel (32.0 mm × 40.0 mm) are used for 3D-CT reconstruction to produce 3D data of 400(x) × 400(y) × 500(z) pixel (32.0 mm × 32.0 mm × 40.0 mm). The voxel size is 0.08 mm each direction. In this investigation, the target flame is spark-ignited flame kernels. The flame kernels are made by spark ignition for a fuel-rich propane-air premixed gas. First, laminar flow is selected as the premixed gas flow to establish the spherically expanding laminar flame. The CT reconstruction result show the spherical shape of flame kernel with a pair of deep wrinkles. The wrinkle is considered to be caused by spark electrodes. Next turbulent flows behind turbulence promoting grid is selected. The corrugated shape flame kernel is obtained. The schlieren 3D-CT measurements are made for the complicated kernels. CT results expresses the instantaneous 3D turbulent flame kernel shapes.