bluuebunny/crossref_metadata_2025 huggingface dataset:
The Study of Calcium Carbonate Scaling on Low Energy Surfaces
Authors: Si Xiuhua , Xi Jinxiang , Tao Xihai
Published: March 2011

Scale deposition on heat transfer surfaces from water containing dissolved salts reduces the efficiency and performance of heat transfer equipments considerably. Scale deposition could be reduced through physical or chemical methods. In some cases, chemical methods are unacceptable, due to cost, contamination issues, etc. In these cases, physical methods are the only acceptable choices. Surface energy of the heat exchanger has been thought to be one important factor affecting the growth of fouling. Applying low energy surfaces to reduce scaling deposition is one of the effective physical methods. The formation and the characteristics of the calcium carbonate scaling on low energy surfaces have been studied in this paper. Copper and stainless steel surfaces were modified by micro-scale (μm thickness) PTFE (Poly-Tetrofluorethylene) films and nano-scale (nm thickness) thiolate SAMs (Self-Assembly Monolayers). The resulting surface energy of PTFE films and SAMs layers based on copper and stainless steel were significantly reduced compared with the original metal surfaces. To study the formation of the calcium carbonate scale, a recirculation cooling water system was used. The formation of the calcium carbonate scale on PTFE surfaces, SAMs surfaces, polished copper surfaces, and polished stainless steel surfaces were investigated respectively. The rate of calcium carbonate scale formation was decreased and the induction period was prolonged with the decrease of the heat transfer surface energy. The characteristics of the calcium carbonate scale formed on heat transfer surfaces with different surface energies was analyzed with fractal theory after taking photos with SEM (Scanning Electron Microscope). The fractal dimension values of the calcium carbonate scale on different heat transfer surfaces with different surface energies were calculated. The results showed that the fractal dimension values of calcium carbonate scale formed on lower energy PTFE and Cu-SAMs surfaces were greater than those that formed on higher energy Cu and stainless steel surfaces. Results of this study clearly indicated that the formation of calcium carbonate scaling on lower energy heat transfer surfaces is reduced.
Wetting Front Tracking During Metal Quenching Using Array of Jets
Authors: Abdalrahman Khalid H. M. , Alam Umair , Specht Eckehard
Published: March 2011

Metal quenching is a commonly used heat treatment technique, e.g. Direct Chill aluminum casting, quenching of steel for obtaining desired micro-structures. Film boiling, transition boiling, nucleate boiling and forced convection are the mechanisms of heat transfer during quenching. When the coolant strikes the hot metal surface during quenching, the surface can be divided into two distinct zones which are dry and wet zones. Heat transfer in dry zone is dominated by film boiling and the wet zone is influenced by transition boiling, nucleate boiling and forced convection. Wetting front is the boundary zone which separates the dry and wet regions. Wetting front is a thin region of coolant in which the transition and nucleate boiling occurs. Within a wetting front, the heat flux leaving from the hot surface reaches it global maximum. The speed of the wetting front indicates the quench ability of the hot surface for the corresponding flow conditions and the coolant. Wetting front tracking is more important for the prediction of surface temperature during quenching. This research works presents the combined numerical and experimental aspects of the heat flux estimation during the quenching process. At any instant, the position of the wetting front is simply assumed as the location of maximum heat flux. This assumption implicitly treats the wetting front as a line instead of area. The location of wetting front and its velocity at every instant are determined by using the experimental temperature data and the inverse algorithm. Experimental setup and temperature measurement technique are explained in detail. The developed inverse algorithm predicts the quenched side temperature and heat flux from the measured side temperature. A two-dimensional Inverse Heat Conduction Problem (IHCP) is solved through the non iterative Finite Element Method (FEM). The considered quenching technique for the study, based on the method of coolant supplied which is array of water jets. One kind of coolant used in this study is tap water. Aluminum 2024, Inconel, and Nickel are the three different materials considered for the analysis. A rectangular plate made of Nickel with dimension 140 × 70 × 2 mm, using the same dimensions of the Inconel. As in the case of the use of Aluminum, the thickness is the only change to 3 mm, the plate quenched by array of water jets with velocities 0.9 m/s, 1.2 m/s, 1.5 m/s and 1.8 m/s. The measured temperature data are further processed through the inverse finite element technique for the estimation of heat flux leaving from the quenched surface. The position of maximum heat flux changes with time which indicates the movement of wetting front. In this work, four different coolant velocities are employed, and the change in coolant velocity strongly affects the heat flux and wetting front movement.
Numerical and Experimental Study on Soot Accumulation on the Wall of Falling Fuel Film Micro-Combustor
Authors: Mei Ning , Wang Xiaoyan , Zhao Hongming , Li Yan , Si Hongyu
Published: March 2011

Fluid flow contributes much to fuel-air mixture formation in a micro-combustor, the RNG k-ε turbulence model was used to simulate the cold flow field of a falling fuel film microcombustor, and comparison was made between numerical result and experimental results. It is shown that the RNG k-ε turbulence model translated the flow field of a complex structure micro-combustor and the soot accumulation on the wall of combustion chamber. The experimental results showed that soot accumulation occurs in vortex backflow area near the wall of combustion chamber and the numerical methods is helpful for understanding the way of soot accumulation in the wall of combustion chamber. Therefore, modifications on the flow field with different diameters and entrance direction of the air flow into the primary combustion chamber were made. The numerical simulation of flow distribution showed that the flow field of micro-combustor could be ideal for eliminated soot accumulation.
Performance of Plate Heat Exchangers Used as Refrigerant Liquid-Overfeed Evaporators
Authors: Huang Jianchang , Sheer Thomas J. , Bailey-McEwan Michael
Published: March 2011

The heat transfer and pressure drop characteristics of plate heat exchangers were measured, when used as refrigerant liquid over-feed evaporators. The three units all had 24 plates but with different chevron-angle combinations of 28°/28°, 28°/60°, and 60°/60°. R134a flowing upwards was used as the refrigerant, in a counter-current arrangement with water flowing on the other side. Heat transfer and pressure drop measurements were made over a range of mass flux, heat flux and corresponding outlet vapour fractions. The effect of system pressure on the evaporator performance was not evaluated due to the small range of evaporating temperature. Experimental data were reduced to obtain the refrigerant-side heat transfer coefficient and frictional pressure drop. The results for heat transfer showed a strong dependence on heat flux and weak dependence on mass flux and vapour fraction. Furthermore, the chevron angle had a small influence on heat transfer but a large influence on frictional pressure drops. Along with observations that were obtained previously on large ammonia and R12 plate evaporators, it is concluded that the dominating heat transfer mechanism in this type of evaporator is nucleate-boiling rather than forced convection. For the two-phase friction factor, various established methods were evaluated; the homogeneous treatment gives good agreement.
An Efficient Approach for Performance Evaluation and Parameter Design of Heat Transfer Enhancing Structure
Authors: Zhou Wen-Jing , Fan Ju-Fang , Wu Zhi-Geng , He Ya-Ling , Tao Wen-Quan
Published: March 2011

The effects of main geometrical parameters of the longitudinal vortex generator (LVG) called “common flow up” on heat transfer enhancement and pressure loss are numerically investigated. Taguchi method is used to guide the numerical simulations. Based on the results of the Taguchi method, a new fin with the combination of different vortex generators is proposed to substitute the original wavy fin-and-tube surface. The results show that the new fin with LVGs can save 9.08% of pumping power while reaching the same amount of heat transfer rate as the wavy fin at inlet velocity of 2m/s, and it can also save 33% of heat transfer area.
Optimization and Development for Fin-and-Tube Heat Exchangers With Vortex Generators
Authors: He Ya-Ling , Chu Pan , Tao Wen-Quan
Published: March 2011

In this paper, heat transfer enhancement and pressure loss penalty for fin-and-tube heat exchangers with rectangular winglet pairs (RWPs) were numerically investigated in a relatively low Reynolds number flow. The purpose of this study was to explore the fundamental mechanism between the local flow structure and the heat transfer augmentation. The RWPs were placed with a special orientation for the purpose of enhancement of heat transfer. The numerical study involved three-dimensional flow and conjugate heat transfer in the computational domain, which was set up to model the entire flow channel in the air flow direction. The effects of attack-angle of RWPs, row-number of RWPs and placement of RWPs on the heat transfer characteristics and flow structure were examined in detail. It was observed that the longitudinal vortices caused by RWPs and the impingement of RWPs-directed flow on the downstream tube were important reasons of heat transfer enhancement for fin-and-tube heat exchangers with RWPs. It was interesting to find that the pressure loss penalty of the fin-and-tube heat exchangers with RWPs could be reduced by altering the placement of the same number of RWPs from inline array to staggered array and simultaneously maintain the heat transfer enhancement level. The results showed that the rectangular winglet pairs (RWPs) can significantly improve the heat transfer performance of the fin-and-tube heat exchangers with a moderate pressure loss penalty.
Modeling of Heat Transport in a Direct Methanol Fuel Cell With Anisotropic Gas Diffusion Layers
Authors: He Ya-Ling , Miao Zheng , Tao Wen-Quan
Published: March 2011

A non-isothermal two-dimensional two-phase numerical model is developed in this paper to investigate the heat generation and transport processes in a direct methanol fuel cell with anisotropic gas diffusion layers (GDLs). Thermal contact resistances at the GDL/CL (catalyst layer) and GDL/Rib interfaces, and the deformation of GDLs are considered together with the inherent anisotropy of the GDL. Latent heat effects due to condensation/evaporation of water and methanol between liquid and gas phases are also taken into account. Formulation of the two-phase mass transport across the membrane electrode assembly (MEA) is mainly based on the classical multiphase flow theory in the porous media. The numerical results show that the overall heat flux in MEA is mainly contributed to heat generation in anode and cathode CLs. And the three anisotropic factors of the GDLs, including the inherent anisotropy, the spatially varying contact resistances, and the deformation of GDLs, have a strong impact on the heat transport processes in the DMFC by altering the distribution of temperature across the MEA.
Theoretical Analysis on the Thermal Characteristics of Oscillating Heat Pipe
Authors: Xian Haizhen , Yang Yongping , Liu Dengying , Liu Xiaomin , Du Xiaoze
Published: March 2011

Theoretical analysis on the thermal characteristics of the looped oscillating heat pipe was given to predict oscillation characteristics affected by filling ratio, inclination angle and heating power. The numerical results show that the alternative expansion and compression of vapors make the working fluid keep an oscillating flow in the heat pipe. This oscillating flow is affected by the inclination angle, filling ratio and heating power.
Numerical Simulation of Cryosurgery as a Potential Treatment for Uterine Fibroids
Authors: McCaffrey Katherine , Rose Karen , Abraham John P.
Published: March 2011

Nearly 80% of all women may suffer from menorrhagia caused by uterine fibroids (leiomyomas) which are benign tumors made up of muscle and fibrous tissue that grow from the muscular wall of the uterus. The vast majority of women whose symptoms are strong enough to require treatment obtain a hysterectomy. Other treatment options which are less invasive than hysterectomy include thermal therapies such as thermal ablation or cryosurgical removal of tissue. This project numerically evaluates the efficacy of a liquid-nitrogen-based cryotherapy for the treatment of uterine fibroids. A bioheat transfer model was utilized which includes both the effects of blood perfusion and the impacts of liquid-to-solid phase change. Changes in all physical properties including thermal conductivity, heat capacity, and perfusion rate were taken into account as the tissue passed through a range of temperatures where it would be transitioning from unfrozen to fully frozen. The numerical model was based on a one-dimensional unsteady bioheat equation. The results show that even for the direct-contact cooling, it is unlikely that intracellular ice would form during the procedure. On the other hand, based on data obtained from previous cell-survival studies, it was found that necrosis would occur when the cooling rates exceeded 30°/min. According to the present numerical results, necrosis would occur within the tissue up to a depth of approximately 5.8 mm, thereby ensuring that sufficient tissue would be cryosurgically destroyed to result in effective treatment.
A Case Study of Energy Saving by Cogeneration System in Hospital
Authors: Okamoto S.
Published: March 2011

This paper describes a study that starts with an analysis of typical energy demand profiles in a hospital setting followed by a case study of a cogeneration system (CGS) under an energy service company (ESCO) project. The CGS idea is of an autonomous system for the combined generation of electrical, heating, and cooling energy in a hospital. The driving units are two high-efficiency gas engines that produce the electrical and heat energy. A gas engine meets the requirement for high electrical and heating energy demands; a natural gas-fuelled reciprocating engine is used to generate 735 kW of power. In our case, the electrical energy will be used only in the hospital. A deficit in electricity can be covered by purchasing power from the public network. Generated steam drives three steam-fired absorption chillers and is delivered to individual heat consumers. This system can provide simultaneous heating and cooling. No technical obstacles were identified for implementing the CGS. The average ratio between electric and thermal loads in the hospital is suitable for CGS system operation. An analysis performed for a non-optimized CGS system predicted a large potential for energy savings.
Exegetic Modeling and Second Law Based Optimization of Cogeneration Heat and Power System Using Evolutionary Algorithm (Genetic Algorithm)
Authors: Khalilarya Shahram , Jafarmadar Samad , Abadi Arzhang
Published: March 2011

A gas-turbine cogeneration system with a regenerative air preheater and a single-pressure exhaust gas boiler serves as an example for application of CHP Plant. This CHP plant which can provide 30 MW of electric power and 14kg/s saturated steam at 20 bars. The plant is comprised of a gas turbine, air compressor, combustion chamber, and air pre-heater as well as a heat recovery steam generator (HRSG). The design Parameters of the plant, were chosen as: compressor pressure ratio (re), compressor isentropic efficiency (ηac), gas turbine isentropic efficiency (ηgt), combustion chamber inlet temperature (T3), and turbine inlet temperature (T4). In order to optimally find the design parameters a thermoeconomic approach has been followed. An objective function, representing the total cost of the plant in terms of dollar per second, was defined as the sum of the operating cost, related to the fuel consumption. Subsequently, different pars of objective function have been expressed in terms of decision variables. Finally, the optimal values of decision variables were obtained by minimizing the objective function using Evolutionary algorithm such as Genetic Algorithm. The influence of changes in the demanded power on the design parameters has been also studied for 30, 40 MW of net power output.
Convective Heat Transfer in a Helical Coil Solar Thermal Collector
Authors: May Allan , O’Donovan Tadhg S.
Published: March 2011

Three dimensional solar concentrators can achieve concentration ratios in excess of 100 and optical efficiencies in excess of 95% throughout the day without the need for tracking. A helical coil heat exchanger has been designed and investigated numerically as the receiver for this solar thermal application. A computational fluid dynamics (CFD) model of a laminar flow in the heat exchanger was developed in ANSYS CFX and a uniform wall flux boundary condition applied to the outer surface. Due to the curvature of the pipe, Dean Vortices were setup within the flow that substantially increased the overall heat transfer to the solar receiver without significantly increasing the pressure drop across the heat exchanger. A full parametric study is conducted to investigate the effects of geometric properties (dimensionless pitch, coil radius etc) and flow variables (Reynolds number, Dean number, Helical number). The variation of the circumferentially averaged heat transfer coefficient with distance along the heat exchanger is reported. It has been shown that the flow is fully developed after approximately 3.5 turns of the heat exchanger coil and will remain stable throughout the remainder of the heat exchanger.
Molecular Dynamics Simulation of Bubble Nucleation in Superheated Liquid
Authors: Liu Chao , Wu Xiaobo , Zhang Hualing
Published: March 2011

The bubble homogeneous nucleation in superheated liquid argon is studied by molecular dynamics simulation in NVT ensemble. L-J potential is adopted for the interaction of argon atom. The simulated particle numbers of argon atom is 10976. The non-dimensional size of simulated box is 27.8×27.8×27.8. The initial non-dimensional temperature and density are 0.4 and 0.51 separately. The results show that the bubble homogeneous nucleation is divided into the waiting process, the appearing process of numerous small bubble nucleuses and the aggregation process of small bubble nucleuses. By fitting simulated data, we find that the bubble nucleation rate is eight orders of magnitudes bigger than the result of classic nucleation theory. The bubble nucleation rate increases along with the increasing of density and superheated temperature, which agrees well with one of classic nucleation theory.
Experimental Study on the Origin of Dropwise Condensation of Steam on Ion Implanted Metallic Surfaces
Authors: Rausch Michael H. , Leipertz Alfred , Fro¨ba Andreas P.
Published: March 2011

The presented work provides new perspectives on the origin of dropwise condensation of steam on ion implanted metallic surfaces. For the experimental study metallic samples made of different materials implanted with various ion species were used. Condensation experiments with saturated steam and surface analysis via contact angle and surface free energy measurements, atomic force microscopy and scanning electron microscopy were performed. It could be shown that available explanation approaches for the appearance of dropwise condensation based on a reduction of the metal surface free energy by ion implantation have to be discarded. Our results rather suggest a nucleation mechanism proceeding on surfaces featuring nano-scale surface roughness and chemical inhomogeneity. These characteristics seem to be induced by nano-particles precipitated due to supersaturation of the metal substrate with the doping elements. On unimplanted high-alloyed materials like stainless steel, dropwise condensation was found to develop spontaneously because of strong oxidation effects. Obviously the latter naturally produce surface modifications which are similar to those obtained artificially by ion implantation.
Heat Transfer in Separated Flows at High Levels of Free-Stream Turbulence
Authors: Terekhov V. I. , Yarygina N. I.
Published: March 2011

In the present paper, a comparative analysis of the influence of free-stream turbulence on the separated flows past obstacles is given. As the obstacles, a downward-facing step, a flat rib installed at different orientations to the free-stream direction, a system of several ribs, and a cross-flow trench with vertical or inclined walls are considered. The experimental results obtained in the present study are compared to data previously reported by other workers. The structure of the separated flow at an enhanced level of free-stream turbulence is compared to the flow under low-turbulence conditions in terms of the characteristic length of the separation zone, mixing-layer parameters, and pressure distributions. The emphasis is on the thermal characteristics, including the profiles of temperature across the shear layer, the distributions of temperature over the streamlined surface, and the local and mean heat-transfer coefficients. It is shown that the effect of enhanced free-stream turbulence on the separated flow is much more pronounced than that on the boundary-layer flow over a flat surface. For separated flow, this effect is manifested more clearly behind rib than behind step. The largest heat-transfer intensification ratios due to external turbulence were found in the cross-flow trench and in the system of ribs.
Critical Flow of Water Under Supercritical Pressures
Authors: Chen Yuzhou , Zhao Minfu , Yang Chunsheng , Bi Keming , Du Kaiwen , Zhang Shuming
Published: March 2011

An experiment on critical flow of water was conducted in two nozzles of 1.41 mm in diameter and 4.35 mm in length with rounded-edge and sharp-edge respectively, covering the ranges of inlet pressure of 22.1–29.1 MPa and inlet temperature of 38–474 °C. More than 200 data points were obtained. The characteristics and the parametric trends were investigated. In the region of near and beyond pseudo-critical temperature the thermal equilibrium is dominant, and the flow rate can be represented by the modified homogeneous equilibrium model reasonably. In the below pseudo-critical region the results exhibit scattered feature as a result of hysteresis effect in the onset of vaporization, characterizing a bifurcation behavior. This effect is more significant in the nozzle with sharp-edge, especially at higher pressure. For the temperature well below the pseudo-critical point the flow is not at critical condition and is represented by the Bernoulli equation.
Numerical Study of Flow and Heat Transfer in Confined and Unconfined Round Mist Impinging Jet
Authors: Terekhov Viktor I. , Pakhomov Maksim A.
Published: March 2011

The work presents results of numerical investigation of flow structure and heat transfer of unconfined and confined impact mist jets with low mass fraction of droplets (ML1≤1%). The downward gas-droplets jet is issued from a pipe and strikes into the center of the circular target wall. Mathematical model is based on the solution to RANS equations for the two-phase flow in Euler approximation. For the calculation of the fluctuation characteristics of the dispersed phase model equations of Derevich and Zaichik [1] and Zaichik et al. [2] were applied. Predictions were performed for the distances between the nozzle and the target plate x/(2R) = 0.5–10 and the initial droplets size (d1 = 5–100 μm) at the fixed Reynolds number based on the nozzle diameter, Re = 26600. Addition of droplets causes significant increase of heat transfer intensity in the vicinity of the jet stagnation point compared with the one-phase air impact jet.
Weak Turbulence in Small Prandtl Number Convection in Porous Media
Authors: Vadasz Peter
Published: March 2011

The dynamics of weak turbulence in small Prandtl number convection in porous media is substantially distinct than the corresponding dynamics for moderate and large Prandtl numbers. Linear stability analysis is performed and its results compared with numerical computations to reveal the underlying phenomena.
Enhancement of Gas Enthalpy-Radiation Conversion by Utilizing Multilayer Open-Cellular Porous Materials
Authors: Khantikomol Preecha , Saito Shinichi , Yokomine Takehiko
Published: March 2011

Multilayer porous gas enthalpy-radiation converter consisting of two porous plates divided by free space is proposed to enhance the performance of single layer porous converter. The energy equations of the gas and the solid phases are employed by using separate energy equations model, and the convective heat transfer between the two phases based on an empirical volumetric heat transfer coefficient proposed by Kamiuto and San San Yee is estimated. The P1 approximation method is employed to evaluate the radiation of transfer equation. Results show that high porosity and PPI (pores per inch) are effective to raise the gas enthalpy-radiation conversion. The upstream porous layer should be lower in porosity and PPI than the downstream porous layer. To increase the temperature drop across the porous gas enthalpy-radiation converter, the inlet gas temperature should be high and the upstream radiation temperature should be low. To obtain maximum temperature drop across the multilayer porous converter, both layers should be made of pure absorbing porous materials. In case the upstream radiation temperature exceeds an inlet gas temperature, however, use of pure scattering porous plate on the upstream side and pure absorbing porous plate on the downstream side will obtain the maximum temperature drop across the converter. The validity of the predicted numerical model also has been confirmed by the experimental data.
Modeling on Transport Performance of Integrated-Planar Solid Oxide Fuel Cell
Authors: Zhang Chao , Du Xiaoze , Yang Lijun , Yang Yongping , Hao Yazhen
Published: March 2011

The three dimension physico-mathematical model was established for the integrated planar solid oxide fuel cell (IP-SOFC) with the couples of multi components flow of reacting gas, heat transfer and electro-chemical process in order to reveal the inherent multi-scale effect of gas distributing duct and the porous support layer, and also, the microscale effect on the transport process in fuel cell. The mutual influences between heat transfer and chemical components transport were included in the model. In addition, the thermal effect of chemical reactions and its influences on polarizations of fuel cell were considered. And also, besides the Darcy diffusion, the Knudsen diffusion in the sub-microscale structure of the porous support is taken into consideration. Numerical simulation was employed to solve the model, by which, the output performance and polarization characteristics of a single cell were analyzed and compared for electrolyte-supported, anode-supported and cathode-supported SOFC, respectively. The present model was also validated comparing with the experimental data.
The Importance of Drainage in Dropwise Condensation From Flowing Air-Steam Mixtures
Authors: Grooten Mart H. , van der Geld Cees W.
Published: March 2011

In this study, the importance of drainage on dropwise condensation of a flowing air-steam mixture is investigated. The initial phase of drop growth, when diffusion is not limiting, is artificially made more important to separate the diffusion resistance to heat transfer. An apparatus with controlled removal of condensate droplets from the condenser plates is designed and tested. The dropwise condensation process is frequently interrupted upon which nucleation restarts each time. Non-artificial drainage occurs at low frequencies of typically 4 large drops per second per dm2. Condensate removal at such a frequency that does not allow formation of large drops artificially makes the initial phase of drop growth more important. The results are believed to be important for the explaining of differences between filmwise and dropwise condensation heat transfer. It is found that the total heat transfer resistance decreases with increasing droplet removal frequency, f. When f is increased, the ratio of condensate mass flow rate to gas mass flow rate increased as well, by 11% for a droplet removal frequency of 0.8 Hz. With increasing f, the relative importance of convective heat transfer decreases.
Experimental Investigation of Steam Condensation in a Silicon Microchannel
Authors: Odaymet A. , Louahlia-Gualous H.
Published: March 2011

Experimental investigations of a two-phase flow were conducted to study heat transfer and various flow patterns of steam condensation in two different microchannels. Microchannels have a rectangular cross-section with hydraulic diameter of 305μm (depth of 310μm and width of 300μm) and 410.5μm (depth of 312μm and width of 600μm). The length of each microchannel is of 50 mm. The silicon microchannel is covered with a transparent thin Pyrex plate to view different flow patterns. Microthermocouples (K-type, 20μm) were placed in rectangular silicon grooves. Measurements are carried out for different inlet pressures and flow rates of steam while the outlet pressure of the microchannel is kept at atmospheric pressure. Plug/slug flow patterns are observed in the microchannel for different mass fluxes. Local surface temperatures along the microchannel corresponding of each two-phase flow structure are measured and analyzed.
Condensate Drop Movement on Heat Transfer Surface With Bulk Temperature Gradient in Marangoni Dropwise Condensation
Authors: Chen Zhihao , Utaka Yoshio
Published: March 2011

When a bulk temperature gradient was applied to a horizontal condensing surface in Marangoni dropwise condensation, the spontaneous movement of condensate drops occurred. The characteristics of the condensate drop movement in a condensate system of water and ethanol binary vapor mixture were experimentally investigated for a wide range of bulk temperature gradients and for various mass fractions. Drops moved from the low-temperature side to the high-temperature side of the heat transfer surface. When the initial drop distance was adopted as a parameter for the Marangoni force acting on the condensate drop together with the surface tension gradient corresponding to the surface temperature of the condensing surface, the drop moving velocity correlated well as a function of both the surface tension gradient and the initial drop distance. In the range of larger initial drop distances, the condensate drop velocity increases as the initial drop distance is reduced and it subsequently decreases after the velocity reaches its maximum value under an almost constant bulk surface tension gradient.
Atomization of Superheated Water
Authors: Marinichev Dmitrii V. , Nizovskii Lev V. , Nizovskii Vladimir L. , Zalkind Valerii I. , Zeigarnik Yurii A.
Published: March 2011

An experimental study of disperse composition of an air-droplet flow under atomization of water superheated relative to the saturation temperature is conducted. Atomization was accomplished with the use of centrifugal (with a nozzle diameter of 0.6 mm) and once-through jet (with a nozzle diameter of 0.3 mm) atomizers. Water pressure at the inlet to the atomizer was 4 and 8 MPa, with a temperature being varied from 20 to 270°C. The disperse composition of the atomized water was determined from a distribution of the intensity of the scattered laser beam with a wavelength of 532 nm over a wide range of the scattering angles. This technique was combined with measurements of light extinction, filming of the droplet spray, and temperature measurements in the flow to determine a degree of droplets evaporation. The experiments showed that at a water temperature of more than 170°C flashing of water discharging from the atomizers takes place, due to which considerably finer atomization of water is obtained as compared to that with the use of mechanical and pneumatic atomizers. At a temperature of injected water of 220–240°C, 70% of droplets (by mass) have a value of d32 diameter of 0.2 to 3 μm. While atomizing superheated water, the size distribution of droplets, has a bimodal pattern, i.e., along with the existence of fine droplets, certain amount of droplets of 6–15 μm in diameter (up to 30%) is retained in the flow. When atomizing superheated water, centrifugal and jet atomizers provide approximately the same distribution of droplet sizes. This attests that flashing plays the determining role in the atomization process. Saturation of water to be atomized by air or carbon dioxide does not noticeably change disperse composition of the air-droplet mixture, while atomizing water at a temperature of more than 170°C. Due to the impact of the air flow, fine droplets form a spatially localized spray extended along the flow with a semi-angle of expansion of 10–12 deg. This fact should be accounted for while arranging water injection systems. The fine atomization achieved significantly increases a rate of droplet evaporation in the carrying flow, prevents their deposition on the articles of equipment, and provides more efficient cooling of the working medium.
Experimental Study of the Vapor Film Behavior on a Highly Heated Surface Immersed Into Subcooled Water
Authors: Belov Kirill I. , Ivochkin Yury P. , Kubrikov Konstantin G. , Oksman Alexandr A. , Vavilov Sergei N. , Zeigarnik Yury A. , Zhilin Vyacheslav G.
Published: March 2011

Results of the experimental studies of the vapor film behavior on a highly heated hemispherical surface immersed into water subcooled relative to the saturation temperature are presented. Transition from film to nucleate boiling was studied with the use of high-speed videocamera. Temperature characteristics of the vapor-film collapse, pressure pulses, acoustic effects, and vapor-film thickness were measured, as well. The decisive influence of the material and condition of the heating surface and the degree of water subcooling on the mode of transition (explosion-like, quiet, or intermediate one) was confirmed. The explosion-like mode of the vapor-film collapse is accompanied by ejection of vapor jets (single or multiple) and pressure pulses with an amplitude of up to 1 MPa. A structure of the pulse packs under multiple jets ejection was investigated. Synchronous measurements of the pressure pulses and an area of the direct cold liquid contact with a hot surface made it possible to determine a time delay between an instant of the contact and the pressure pulse. Typical value of this time lag was several tens microseconds. The dependence of the pulse frequency and the number of pulses on the hemisphere temperature was obtained.
Active Upstream Components for Enhanced Heat Transfer of Longitudinally Finned Heat Sinks
Authors: Daly John
Published: March 2011

With the ever increasing heat flux from next-generation chips forced convection cooling is beginning to reach its limits within current standard heat sink capabilities. Methods of extending the air cooling capabilities prior to a transition to liquid or refrigerant-based cooling which is seen as costly and complex, have become more critical. This paper investigates the enhanced heat transfer by the addition of active components upstream of a longitudinally finned heat sink. This paper addresses piezoelectric fans for natural and forced convection environments. Experimental measurements are taken for a low powered DC fan operating at a frequency of 114Hz. For the forced convection experiments a fully ducted flow was used. The main thrust of the paper is to determine the effects of piezoelectrics in augmenting forced convection systems at hot component locations. The effects on pressure drop, thermal resistance and pumping power with the addition of the technology are presented. The paper concludes by reporting on the performance enhancement and limitations of the piezoelectric fans compared to the conventional longitudinally finned heat sink geometry.
Numerical Determination of the Temperature Dependent Thermophysical Properties in Solid Materials: Experimental Instrumentation
Authors: Zueco Joaquin , Be´g O. Anwar
Published: March 2011

An iterative approach is presented to determine the temperature-dependent thermo-physical properties from the temperature measurements taken at various points of the medium. Network Simulation Method (NSM) is used as the numerical tool in the form of an inverse problem to obtain a stable and exact numerical solution for simultaneously estimating thermal conductivity and specific heat. The approach provides estimations of the functions k(T) and ce(T), regardless of the waveform of those functions even without a priori information on the kind of dependence concerned. The solutions are reached by a piece-wise functions, whose number of stretches may be specified. The input data (temperature measurements) are obtained from an experimental installation has been designed and using various points of measurement in different solid materials. The sensitivity of the functional versus the slope of the line, at each step, is acceptable and the complete piece-wise solution is very close to the exact thermo-physical properties in all the cases studied. The proposed general procedure may be applied regardless of the kind of temperature dependence for k(T) and ce(T) as long as these are continuous temperature functions given by an explicit mathematical function or by a finite stretches piece-wise function. The estimations of k(T) and ce(T) are piece-wise functions with a number of stretches that may be specified to approximate the inverse solutions to the exact values as much as required. The typical functional of these problems contains the simulated measured data taken at three points of the solid. These measurements are compared in the functional with the solution of the partial inverse problem by applying NSM as the numerical technique in each iteration, in order to estimate each of the stretches that conform the whole piece-wise estimation. NSM has been successfully applied before to solve both DHCP and IHCP. Among the advantages of this method is the fact that no mathematical manipulations or convergence criteria are needed to solve the finite difference equations resulting from the discretization of the partial difference equations of the mathematical model. Both tasks are carried out by the powerful software used to solve the network model. The close agreement between the exact solutions and the estimated results shows the potential of the proposed method in finding the accurate value of the temperature-dependent thermo-physical properties in inverse heat conduction problem.
Numerical Simulation of Turbulent Flow in a Rectangular Channel With Periodically Mounted Longitudinal Vortex Generators
Authors: Saha Pankaj , Biswas Gautam
Published: March 2011

Detailed flow structures in turbulent flow through a rectangular channel containing built-in winglet type vortex generators have been analyzed by means of solutions of the full Navier-Stokes equations using a Large-Eddy Simulation (LES) technique. The Reynolds numbers of investigation is 6000. The geometry of interest consists of a rectangular channel with a built-in winglet pair on the bottom wall with common-flow-down arrangement. The winglet pair induces streamwise longitudinal vortices behind it. The vortices swirl the flow around the axis parallel to the mainstream direction and disrupt the growth of thermal boundary layer entailing enhancement of heat transfer. The influence of the longitudinal vortices persists far downstream of the location of the winglet-pair. Since the structure of the turbulence is strongly affected by the streamline curvature, the flow of interest, despite the simplicity of its geometry, turns out to be extremely complex. Therefore it calls for more accurate calculation of the turbulence quantities. In the present study, flow structures are studied by using time-averaged quantities, such as the iso-contours of velocity components, vortices and turbulent stresses. The simulation shows that the secondary flow is stronger in the regions where the longitudinal vortices are more active. The wake like structures of streamwise velocity occurs due to strong distortion of the boundary layer by vortices. The spanwise distributions of turbulent kinetic energy and Reynolds stresses show the evidence of strong secondary flow. The computational results compare well with the experimental data qualitatively.
Experimental and Numerical Analysis of Droplet Cooling
Authors: Tartarini Paolo , Corticelli Mauro A. , Santangelo Paolo E.
Published: March 2011

Dropwise cooling represents a major subject of interest for both academic and industrial researches. The present work is focused on investigating the thermal transient occurring as two water droplets are gently released (We < 30) onto a heated solid surface. This latter has been kept at initial temperature lower than 373.15 K to analyze the single-phase-evaporation regime. To the purpose, both an experimental and a numerical approach have conveniently been employed. Infrared thermography has been used to evaluate the temperature trend at the solid-liquid interface: an experimental facility has been built to carry out measurements from below, thus realizing a fully non-intrusive approach. A transparent-crystal disk has been inserted to serve as the solid substrate; its upper surface has been painted by a black coating, thus providing a black-body surface as the solid-liquid interface. The infrared thermocamera has been placed below and perpendicular to that surface; temperature has been thereby measured, being emissivity a known parameter. A numerical code has been developed to predict the involved physical phenomena: temperature trend, evaporation time and evaporated flux result from discretizing the three-dimensional energy-diffusion equation by the finite-volume method. Moreover, the model is based on structured non-uniform mesh to adapt to the occurring temperature gradients. Very good agreement is shown between experimental and numerical outcomes in terms of thermal transient and recovery.
Experimental Study on the Heat Transfer Characteristics of Cryogenic Micro Channel Heat Exchanger
Authors: Yoon Seok Ho , Kim Duckjong , Choi Jun Seok
Published: March 2011

In recent years, many researchers investigated micro channel heat exchangers because of its high efficiency and compactness. However, few experimental studies about micro channel heat exchanger in cryogenic environment have been conducted. In this study, micro channel was fabricated by chemical etching and heat exchanger core was made by diffusion bonding method for cryogenic reliability. Performance test was conducted in cryogenic test rig. Working fluids are liquid nitrogen and methane gas. Methane gas was condensed in the micro channel heat exchanger. Heat transfer coefficients and pressure drop were measured and the heat transfer characteristics were investigated. These results can be used to design the heat exchanger of gas liquefaction plant.
The Effect of Surface Radiation and Conjugate Conduction on Laminar Free Convection From a Vertical Flat Plate
Authors: Prasanna S. , Venkateshan S. P.
Published: March 2011

The role of conduction and surface radiation on laminar free convection heat transfer from a heated vertical flat plate has been studied. Steady state experiments have been conducted on vertical flat plates, of different thermal conductivities and surface emissivities, with an embedded heater and the results have been reported in [1]. The plate dimensions were held fixed in all the experiments. An effort is made here to identify important parameters that are involved in wall conduction - free convection - radiation interaction phenomena. The convective heat transfer from a vertical surface is affected by the surface temperature of the plate and its variations which is influenced by two other modes of heat transfer, conduction within the plate and surface radiation. Hence, the present paper attempts to understand the interaction phenomenon between the three modes of heat transfer and explain the temperature distributions within the plate, observed both experimentally and in numerical simulations. It is found that radiation is very important as it significantly affects the temperature distribution along the plate.
First and Second Law Analysis of Fluid Flow in the Regenerator of Pulse Tube Refrigerators
Authors: Roshanghalb F. , Saidi M. H. , Jafarian A. , Imanimehr F. , Asadi M.
Published: March 2011

The objective of the present work is to analyze the performance of the regenerator of pulse tube refrigerators. Hydrodynamic and thermal behavior of the regenerator is investigated in this respect. To consider the system performance, a system of conservation equations including two energy equations for the regenerator as a porous media is employed. The present model considers one dimensional periodic unsteady compressible flow in the regenerator. The conservation equations are transformed by implementing the volumetric average scheme. Method of harmonic approximation is employed to derive an analytical solution. To explore the system performance, net energy flow and entropy generation minimization technique is applied in order to calculate the regenerator first and second law efficiencies. The effect of geometry and operating key parameters on the regenerator performance are considered as well.
Geometric Optimisation of Forced Convection in Cooling Channels With Internal Heat Generation
Authors: Olakoyejo Olabode T. , Bello-Ochende Tunde , Meyer Josua P.
Published: March 2011

This paper presents a three dimensional geometric optimization of cooling channels in forced convection with internal heat generation within the solid. Three configurations were studied, circular channels, square channels and rectangular channels with different porosities. The configurations were optimized in such a way that the peak temperature is minimum. The optimization is subject to the constraint of fixed volume and solid material. The fluid is forced through the channels by the pressure difference across the channels. The structure has two degrees of freedom as design variables: channel hydraulic diameter and channel-to-channel spacing. The results obtained show the behaviour of the applied pressure drop on the optimized geometry. Results also show that as pressure drop increases the minimized peak temperature decreases.
An Experimental Study on the Effect of Configuration of Multiple Microchannels on Heat Removal for Electronic Cooling
Authors: Zhang Jingru , Prakash Shaurya , Jaluria Yogesh , Lin Lei
Published: March 2011

This paper presents the design, fabrication, and characterization of three different configurations of multiple microchannel heat sink devices to improve their overall cooling efficiency for potential applications in electronic cooling. A fabrication and packaging process based on standard UV-lithography, wet etching, and bonding was developed to allow a rapid parametric study. An anisotropic chemical etch with potassium hydroxide, water, and isopropanol is used to fabricate microchannels on (110)-oriented silicon wafers. PDMS (Polydimethylsiloxane) was tested as the cover of microchannels due to its mechanical flexibility. It is transparent so that the microchannel flow can be visualized using a microscope. An open flow loop, which consists of syringe pump and a power supply, was designed to test the heat sinks with different configurations. Temperature data were collected at different locations by a Data Acquisition (DAQ) system and recorded by Labview software to investigate the heat transfer characteristics of the heat sink. Three heat sinks, with different configurations, were tested. They all included microchannels of width 50 μm, depth 60 μm, and fin width 200 μm. Some Typical results on heat transfer are presented, along with discussion on the efficiency for heat removal.