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Transfer Characteristics (transfer + characteristic)
Kinds of Transfer Characteristics Selected AbstractsOpen-loop Analysis of Transfer Characteristics from Blood Pressure to Heart Rate Using an Effectively Total Artificial HeartARTIFICIAL ORGANS, Issue 1 2004Akira Tanaka Abstract:, ,It is desirable for the dynamic behavior of the drive rate of the artificial heart to be as similar as possible to that of the recipient's heart rate (HR) before implantation. This requires a model which can simulate the behavior of HR on the basis of only the information measured with the limited number of approvable implanted sensors. This article provides a linear time series model for explaining the behavior of HR only with aortic pressure and right atrial pressure. This could be obtained from open-loop analysis using a total artificial heart, which was introduced for measuring HR in vivo and for eliminating its effect on blood pressure. The model was identified in a goat equipped with a special biventricular assist device called the effectively total artificial heart (ETAH). The ETAH was introduced to make an open loop and awake situation in the animal with almost intact autonomic nerves, which could enhance the accuracy and reliability of the identification of the model. The adequacy of the proposed model was ascertained in several data sets measured in two goats, which were different from the data set used for identification. Most of the mean estimation errors were less than 3 beats/min and auto-correlation analysis showed approvable statistical appropriateness. However, it was clarified through comparison with the 1/R control method that the proposed model has a few problems still to be solved before its future implementation as an automatic controller of the TAH. [source] Extracting Parameters from the Current,Voltage Characteristics of Organic Field-Effect TransistorsADVANCED FUNCTIONAL MATERIALS, Issue 11 2004G. Horowitz Abstract Organic field-effect transistors were fabricated with vapor-deposited pentacene on aluminum oxide insulating layers. Several methods are used in order to extract the mobility and threshold voltage from the transfer characteristic of the devices. In all cases, the mobility is found to depend on the gate voltage. The first method consists of deriving the drain current as a function of gate voltage (transconductance), leading to the so-called field-effect mobility. In the second method, we assume a power-law dependence of the mobility with gate voltage together with a constant contact resistance. The third method is the so-called transfer line method, in which several devices with various channel length are used. It is shown that the mobility is significantly enhanced by modifying the aluminum oxide layer with carboxylic acid self-assembled monolayers prior to pentacene deposition. The methods used to extract parameters yield threshold voltages with an absolute value of less than 2 V. It is also shown that there is a shift of the threshold voltage after modification of the aluminum oxide layer. These features seem to confirm the validity of the parameter-extraction methods. [source] Fabrication and evaluation of complementary logic circuits using zinc oxide and pentacene thin film transistorELECTRONICS & COMMUNICATIONS IN JAPAN, Issue 9 2009Hiroyuki Iechi Abstract We fabricated hybrid complementary inverters with n-channel zinc oxide (ZnO) transistors as the n-type inorganic material and p-channel organic transistors using pentacene as the p-type organic material. The complementary inverter exhibited a large voltage gain of 10 to 12 and a cutoff frequency of 0.5 kHz. ZnO thin film transistors show n-type semiconducting properties having field-effect mobility of 2.1×10,3 cm2/Vs. On the other hand, pentacene thin film transistors show p-type semiconducting properties having field-effect mobility of 3.2×10,2 cm2/Vs. We describe basic charge transfer characteristics of ZnO thin films. The results obtained here demonstrate that it is important for the transistor using ZnO to be injected charge from electrode to semiconducting material effectively. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 92(9): 36,42, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10085 [source] Wireless signal-preamble assisted Mach,Zehnder modulator bias stabilisation in wireless signal transmission over optical fibreEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 6 2008Debashis Chanda Lithium niobate based Mach,Zehnder electro-optic modulators are increasingly being used in high-speed digital as well as analog optical links. Depending on the application, digital or analog, the bias point of such a modulator is held constant at a particular point on the sinusoidal electrical to optical power transfer characteristics of the modulator. Bias point drift is one of the major limitations of lithium niobate based Mach,Zehnder electro-optic modulators. This increases the bit error rate of the system and affects adjacent channel performances. In one of the most popular methods of bias control, a pilot tone is used to track the bias point drift. However, pilot tone based bias tracking reduces overall intermodulation free dynamic range of the link. In this paper we propose a method where Mach,Zehnder modulator bias drift is tracked and maintained at the desired point by tracking the power variation of the preamble of wireless signal data frames. The method has no detrimental effects on system performances as no external signal is exclusively injected into the system for bias tracking purposes. Copyright © 2007 John Wiley & Sons, Ltd. [source] Study of the Catalytic Layer in Polybenzimidazole-based High Temperature PEMFC: Effect of Platinum Content on the Carbon SupportFUEL CELLS, Issue 2 2010J. Lobato Abstract In this work, the effect of platinum percentage on the carbon support of commercial catalyst for electrodes to be used in a Polybenzimidazole (PBI)-based PEMFC has been studied. Three percentages were studied (20, 40 and 60%). In all cases, the same quantity of PBI in the catalyst layer was added, which is required as a ,binder'. From Hg porosimetry analyses, pore size distribution, porosity, mean pore size and tortuosity of all electrodes were obtained. The amount of mesopores gets larger as the platinum percentage in the catalytic layer decreases, which reduces the overall porosity and the mean pore size and increases the tortuosity. The electrochemical characterisation was performed by voltamperometric studies, assessing the effective electrochemical surface area (ESA) of the electrodes, by impedance spectroscopy (IS), determining the polarisation resistance, and by the corresponding fuel cell measurements. The best results were obtained for the electrodes with a content of 40% Pt on carbon, as a result of an adequate combination of catalytic activity and mass transfer characteristics of the electrode. It has been demonstrated that the temperature favours the fuel cell performance, and the humidification does not have remarkable effects on the performance of a PBI-based polymer electrolyte membrane fuel cell (PEMFC). [source] Threshold Voltage Shifts in Organic Thin-Film Transistors Due to Self-Assembled Monolayers at the Dielectric SurfaceADVANCED FUNCTIONAL MATERIALS, Issue 6 2009Stefan K. Possanner Abstract Recently, it has been shown by several groups that the electrical characteristics of organic thin-film transistors (OTFTs) can be significantly influenced by depositing self-assembled monolayers (SAMs) at the organic semiconductor/dielectric interface. In this work, the effect of such SAMs on the transfer characteristics and especially on the threshold voltage of OTFTs is investigated by means of two-dimensional drift-diffusion simulations. The impact of the SAM is modeled either by a permanent space charge layer that can result from chemical reactions with the active material, or by a dipole layer representing an array of ordered dipolar molecules. It is demonstrated that, in both model cases, the presence of the SAM significantly changes the transfer characteristics. In particular, it gives rise to a modified, effective gate voltage Veff that results in a rigid shift of the threshold voltage, ,Vth, relative to a SAM-free OTFT. The achievable amount of threshold voltage shift, however, strongly depends on the actual role of the SAM. While for the investigated device dimensions, an organic SAM acting as a dipole layer can realistically shift the threshold voltage only by a few volts, the changes in the threshold voltage can be more than an order of magnitude larger when the SAM leads to charges at the interface. Based on the analysis of the different cases, a route to experimentally discriminate between SAM-induced space charges and interface dipoles is proposed. The developed model allows for qualitative description of the behavior of organic transistors containing reactive interfacial layers; when incorporating rechargeable carrier trap states and a carrier density-dependent mobility, even a quantitative agreement between theory and recent experiments can be achieved. [source] Heat and fluid flow characteristics inside differentially heated square enclosures with single and multiple sliding wallsHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 7 2009E.M. Wahba Abstract Fluid flow and heat transfer characteristics of differentially heated lid driven cavities are numerically modeled and analyzed in the present study. One-, two-, and four-sided lid driven cavity configurations are considered with the vertical walls being maintained at different temperatures and the horizontal walls being thermally insulated. Eight different cavity configurations are considered depending on the direction of wall motion. The Prandtl number Pr is taken to be 0.7, the Grashof number is taken to be 104, while two values for the Richardson number Ri are considered, 0.1 and 10. It is found that both the Richardson number and the cavity configuration affect the heat and fluid flow characteristics in the cavity. It is concluded that for Ri=0.1, a four-sided driven cavity configuration with all walls rotating in the same direction would triple the value of the average Nusselt number at the cold wall when compared to a one-sided driven cavity configuration. However, for Ri=10, the cavity configuration has minimal effect and all eight cases result in an average Nusselt number value at the cold wall ranging between 1.3 and 1.9. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/htj.20264 [source] Fluid flow and heat transfer characteristics of cone orifice jet (effects of cone angle)HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 4 2009Mizuki Kito Abstract The use of a jet from an orifice nozzle with a saddle-backed-shape velocity profile and a contracted flow at the nozzle exit may improve the heat transfer characteristics on an impingement plate because of its larger centerline velocity. However, it requires more power to operate than a common nozzle because of its higher flow resistance. We therefore initially considered the use of a cone orifice nozzle to obtain better heat transfer performance as well as to decrease the flow resistance. We examined the effects of the cone angle , on the cone orifice free jet flow and heat transfer characteristics of the impinging jet. We compared two nozzles: a pipe nozzle and a quadrant nozzle. The first one provides a velocity profile of a fully developed turbulent pipe flow, and the second has a uniform velocity profile at the nozzle exit. We observed a significant enhancement of the heat transfer characteristics of the cone orifice jets at Re=1.5×104. Using the cone orifice impinging jets enhanced the heat transfer rates as compared to the quadrant jet, even when the jets were supplied with the same operational power as the pipe jet. For instance, a maximum enhancement up to approximately 22% at r/do,0.5 is observed for ,=15°. In addition, an increase of approximately 7% is attained as compared to when the pipe jet was used. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20243 [source] Study on heat transfer characteristics of porous metallic heat sink with conductive pipe under bypass effectHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 3 2009Sheng-Chung Tzeng Abstract The work investigated the forced convection heat transfer of the heat sink situated in a rectangular channel by considering the bypass effect. The fluid medium was air. The relevant parameters were the Reynolds number (Re), the relative top by-pass gap (C/H), and the relative side by-pass gap (S/L). The size of the heat sink was 60 mm (L)×60 mm(W)×24 mm(H). Two heat sinks were employed as test specimens: (A) the 0.9-porosity aluminum foam heat sink and (B) the 0.9-porosity aluminum foam heat sink with a 20 mm diameter copper cylinder. The copper cylinder was used as a conductive pipe of heat sink. The average Nusselt number was examined under various forced convection conditions. Experimental results demonstrate that increasing by-pass space decreased the Nusselt number. Besides, the average Nusselt number of mode B heat sink was higher than that of mode A heat sink by 30% for the case without by-pass flow. The heat transfer enhancement by the copper cylinder would decline as the by-pass space grew. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20247 [source] Analysis of flow and heat transfer in evaporator porous wicking structure of a flat heat pipeHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 2 2009Congxiang Hu Abstract With a specified pressure distribution, an analytical investigation was conducted to explore the flow and heat transfer characteristics in an evaporator porous wicking structure of a flat heat pipe. The boundary effect on the flow rate is more significant than the inertia, and both the boundary and inertia effects exert very little influence on fluid layer thickness and velocity distribution. The bottom of the porous layer is at a quite uniform temperature, and the heat flux is almost normal to the solid boundary. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20237 [source] Fluid flow and heat transfer of natural convection around array of vertical heated platesHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 2 2009Kenzo Kitamura Abstract Natural convective flows around an array of vertical heated plates were investigated experimentally. Main concerns were directed to the influences of plate numbers on the heat transfer characteristics of the plates. Both surfaces of the test plates were heated with constant and equal heat fluxes and their local heat transfer coefficients were measured. The results showed that the coefficients of the surfaces of the array facing outward became higher than those facing inward. The flow fields around the bottom of the plate array were visualized with smoke. The result showed that the ambient flow is directed from the sides to the center of the array and enters the parallel channel obliquely. These flows cause the above difference in the coefficients. While the difference gradually diminished in between the plates placed in the central section of the array, their coefficients asymptotically approach those of the analytical correlation that assumed a uniform velocity at the channel inlet. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20232 [source] Convective heat transfer and pressure drop of annular tubes with three different internal longitudinal finsHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 1 2008Lin Tian Abstract Pressure drop and heat transfer characteristics of air in three annular tubes with different internal longitudinal fins were investigated experimentally at uniform wall heat flux. The tested tubes have a double-pipe structure with the inner blocked tube as an insertion. Three different kinds of fins, plain rectangle fin, plain rectangle fin with periodical ridges and wave-like fin, were located peripherally in the annulus. The friction factor and Nusselt number can be corrected by a power-law correction in the Reynolds number range tested. It was found that the tube with periodical ridges on the plain fin or with wave-like fin could augment heat transfer; however, the pressure drop was increased simultaneously. In order to evaluate the comprehensive heat transfer characteristics of the tested tubes, two criteria for evaluating the comprehensive thermal performance of tested tubes were adopted. They are: 1) evaluating the comprehensive heat transfer performance under three conditions: identical mass flow, identical pumping power, and identical pressure drop; 2) the second law of thermodynamics, i.e., the entropy generation. According to the two different evaluating methods, it was found that the tube with wave-like fins provided the most excellent comprehensive heat transfer performance among the three tubes, especially when it was used under higher Reynolds number conditions. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(1): 29,40, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20186 [source] Study on heat transfer characteristics of reservoir embedded loop heat pipe (1st report, Influence of evaporator orientation against gravity and charged liquid weight on heat transfer characteristics)HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 3 2007Hiroaki Ishikawa Abstract High-powered satellites need larger heat rejection areas. A deployable radiator is one of the key technologies for a high-powered satellite bus. A Reservoir Embedded Loop Heat Pipe (RELHP) is a two-phase heat transfer device that constitutes a deployable radiator. RELHP has an evaporator core which is used as a liquid reservoir to enhance operational reliability. This paper presents the heat transport characteristics of a RELHP under changing evaporator orientation against gravity and charged ammonia weight by experiment and calculation. Liquid slug position in the reservoir has a great influence on heat transport characteristics, caused by changing heat transfer coefficients between returned liquid into the evaporator and vapor in the reservoir. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(3): 143, 157, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20150 [source] The formation of rising liquid thin film on the fluted surface of a horizontal tubeHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 6 2005Li Yan Abstract The purpose of this study is to investigate the mechanism of the formation of the rising liquid thin film and its flow characteristics on the fluted surface of a horizontal tube. By analyzing the wetting behaviors of the fluted tube, which was primarily responsible for the formation of the rising liquid thin film, a numerical model of one-phase fluid was established to analyze the distribution of the velocity and thickness of the rising liquid thin film during its evaporation. The behaviors of the flow characteristics were discussed and the results showed that geometric properties of the fluted surface of a horizontal tube and surface tension of the fluid were essential for the formation of a continuous and uniform liquid thin film. Theoretical analysis suggested that the capillary force created by the fluid surface tension was a key value for the formation of the thin film. The heat and mass transfer characteristics of the formed thin film also had an effect on the formation of the rising film. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 396,406, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20075 [source] Steam absorption process of water/LiBr system inside vertical small bore pipesHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 1 2005Masanori Kiyota Abstract In a previous paper, a numerical model for absorption within vertical pipes was proposed and compared with the experiments. Agreements were good for pipes with an OD 28,15 mm but at 10 mm pipe experiments fell below the predicted values. For smaller diameters, the difference between the surface area of the falling liquid film and that of the outer surface of the pipe is not negligible and the thickness of the liquid film is also not negligible. In this paper a new model is formulated in cylindrical coordinates and experiments using pipes with 9.52 mm and 7 mm OD are done. Smooth pipes and two kinds of internally finned pipes, originally developed and used to enhance the heat transfer characteristics of the evaporator and condenser of a refrigerator using HFC as refrigerant, are tested in the experiments. The absorption performance is enhanced by 30% when compared to the smooth pipes, but the difference between the finned pipes is small. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(1): 18,28, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20040 [source] Reverse computation of forced convection heat transfer for optimal control of thermal boundary conditionsHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 3 2004Kazunari Momose Abstract A reverse computation based on adjoint formulation of forced convection heat transfer is proposed to obtain the optimal thermal boundary conditions for heat transfer characteristics; for example, a total heat transfer rate or a temperature at a specific location. In the reverse analysis via adjoint formulation, the heat flow is reversed in both time and space. Thus, using the numerical solution of the adjoint problem, we can inversely predict the boundary condition effects on the heat transfer characteristics. As a result, we can obtain the optimal thermal boundary conditions in both time and space to control the heat transfer at any given time. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(3): 161,174, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20002 [source] Heat transfer characteristics between inner and outer rings of an angular ball bearingHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 1 2003Keiji Mizuta Abstract Heat transfer between the inner and the outer rings of an angular ball bearing is investigated experimentally and heat transport by balls is analyzed theoretically. The bearing used is lubricated by oil and rotated in the range from 600 to 4000 rpm. Considering heat generation by friction, the net heat flow between the rings is evaluated. The results show that balls are the dominant heat carrier and their conductance depends on rotational speed and thrust force. The other heat transfer route is supposed mainly to be between the rings based on the fact that its heat flow rate depends on the rotational speed. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 42,57, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10070 [source] Augmentation of boiling heat transfer from horizontal cylinder to liquid by movable particlesHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 1 2002Yoshihiro Iida Abstract This paper presents a series of experimental results on a passive augmentation technique of boiling heat transfer by supplying solid particles in liquid. A cylindrical heater 0.88 mm in diameter is placed in saturated water, in which a lot of mobile particles exist, and the nucleate and film boiling heat transfer characteristics are measured. Particle materials used were alumina, glass, and porous alumina, and the diameter ranged from 0.3 mm to 2.5 mm. Particles are fluidized by the occurrence of boiling without any additive power, and the heat transfer is augmented. The maximum augmentation ratio obtained in this experiment reaches about ten times the heat transfer coefficient obtained in liquid alone. The augmentation ratio is mainly affected by the particle material, diameter, and the height of the particle bed set at no boiling condition. The augmentation mechanism is discussed on the basis of the experimental results. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 28,41, 2002 [source] Effect of tabs on impinging heat transferHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 7 2001Munehiko Hiwada Abstract The present work experimentally investigates the effect of vortex generators, in the form of small tabs projecting normally into the flow at the nozzle exit, on the fluid flow and heat transfer characteristics of an axisymmetric impinging air jet in the subcritical Reynolds number range. With this comes the expectation of a large eddy structure variation and the possibility of active control. Local heat transfer and static pressure were measured on a target plate for a round air jet issuing from a circular nozzle with rectangular tabs whose numbers and lengths changed at a constant nozzle-to-plate gap (L/d = 8) and jet Reynolds number (Re = 34,000). The main results are the following: When two tabs were set at the exit of the circular nozzle, Cpw and Nu profiles flatten in the direction of the tab setting. In the case of three tabs, however, among both Cpw and Nu profiles a concentric profile is found, as well as in the case without any tabs. © 2001 Scripta Technica, Heat Trans Asian Res, 30(7): 561,570, 2001 [source] Heat transfer characteristics in a two-dimensional channel with an oscillating wallHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 4 2001Masahide Nakamura Abstract Numerical calculations have been carried out for the laminar heat transfer in a two-dimensional channel bounded by a fixed wall and an oscillating wall. In this calculation, the moving boundary problem was transformed into a fixed boundary problem using the coordinate transformation method, and the fully implicit finite difference method was used to solve the mass, momentum, and energy conservation equations. The calculated results are summarized as follows: (i) The wall oscillation has an effect of enhancing the heat transfer and an effect of increasing the additional pressure loss. (ii) An optimum Strouhal number for the enhancement of heat transfer exists, and this optimum value is strongly affected by the amplitude of wall oscillation. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 280,292, 2001 [source] Computation of heat transfer enhancement in a plate-fin heat exchanger with triangular inserts and delta wing vortex generatorINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 9 2010Gulshan Sachdeva Abstract Longitudinal vortices disrupt the growth of the thermal boundary layer, thereby the vortex generators producing the longitudinal vortices are well known for the enhancement of heat transfer in compact heat exchangers. The present investigation determines the heat transfer characteristics with secondary flow analysis in plate fin triangular ducts with delta wing vortex generators. This geometrical configuration is investigated for various angles of attack of the wing i.e. 15°, 20°, 26° and 37° and Reynolds numbers 100 and 200. The constant wall temperature boundary condition is used. The solution of the complete Navier Stokes equation and the energy equation is carried out using the staggered grid arrangement. The performance of the combination of triangular secondary fins and delta wing with stamping on slant surfaces has also been studied. Copyright © 2009 John Wiley & Sons, Ltd. [source] Two-phase flow convective condensation of refrigerant mixtures under gas/liquid injectionINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 15 2005Samuel M. Sami Abstract The influence of gas/liquid injection on two-phase flow condensation heat transfer characteristics of some refrigerant mixtures in horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of the gas/liquid injection on the heat transfer characteristics such as average heat transfer coefficient of R-507, R-404A, R-410A, and R-407C in two-phase flow condensation inside enhanced surface tubing. The data also revealed that gas, liquid and gas/liquid injection is beneficial at certain gas/liquid injection ratios to the heat transfer coefficient depending upon the Reynolds number and the condensation point of the refrigerant mixtures in question. It was also evident that the proposed condensation correlations and the experimental data were applicable to the entire heat and mass flux, investigated in the present study under gas/liquid injection conditions. The deviation between the experimental and predicted under gas/liquid injection were less than ± 10, for the majority of data. Copyright © 2005 John Wiley & Sons, Ltd. [source] Influence of magnetic field on two-phase flow convective boiling of some refrigerant mixturesINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 15 2005Samuel M. Sami Abstract In this paper, an experimental study on the influence of magnetohydrodynamic (MHD) on heat transfer characteristics of two-phase flow boiling of some refrigerant mixtures in air/refrigerant horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of MHD on the heat transfer characteristics such as average heat transfer coefficients, and pressure drops of R-507, R-404A, R-410A, and R-407C in two-phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition. It was also evident that the proposed correlations for predicting the heat transfer characteristics were applicable to the entire heat and mass flux, investigated in the present study. The deviation between the experimental and predicted value using new and improved correlations for the heat transfer coefficient and pressure drop were less than ±20%, for the majority of data. Copyright © 2005 John Wiley & Sons, Ltd. [source] Pool boiling on a superhydrophilic surfaceINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 2 2003Y. Takata Abstract Titanium Dioxide, TiO2, is a photocatalyst with a unique characteristic. A surface coated with TiO2 exhibits an extremely high affinity for water when exposed to UV light and the contact angle decreases nearly to zero. Inversely, the contact angle increases when the surface is shielded from UV. This superhydrophilic nature gives a self-cleaning effect to the coated surface and has already been applied to some construction materials, car coatings and so on. We applied this property to the enhancement of boiling heat transfer. An experiment involving the pool boiling of pure water has been performed to make clear the effect of high wettability on heat transfer characteristics. The heat transfer surface is a vertical copper cylinder of 17 mm in diameter and the measurement has been done at saturated temperature and in a steady state. Both TiO2 -coated and non-coated surfaces were used for comparison. In the case of the TiO2 -coated surface, it is exposed to UV light for a few hours before experiment and it is found that the maximum heat flux (CHF) is about two times larger than that of the uncoated surface. The temperature at minimum heat flux (MHF) for the superhydrophilic surface is higher by 100 K than that for the normal one. The superhydrophilic surface can be an ideal heat transfer surface. Copyright © 2002 John Wiley & Sons, Ltd. [source] Analysis of heat transfer characteristics of an unsaturated soil bed: a simplified numerical methodINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 15 2001Gopal B. Reddy Abstract This paper is a continuation of a study reported in this Journal in February 1999. The paper presents a summary of the two-dimensional macroscopic continuity, momentum and energy equations in a cylindrical co-ordinate system that describe heat and mass transfer through unsaturated soil. The hydrodynamic laws governing flow of water through unsaturated soil are also presented. The explicit numerical procedure and the method to solve the equations are described. Characteristics of the corresponding computer program are also discussed. The results obtained with the current cylindrical governing equations are compared with the previously reported results based upon the Cartesian system of equations. It is observed that the results obtained with cylindrical formulations are in closer agreement with the experimental results. The effects of various heat transfer processes as well as the motion of fluids on heat transfer in a clay bed coupled to a heat pump are discussed. Heat diffusion into the soil by conduction is shown to be predominant through the early stage of heating, while the liquid water motion contributes to heat transfer during the intermediate times and the gas motion is shown to become significant during the last stages of drying. The contribution of the convective transport increases with the temperature and becomes equal to the contribution by conduction at moderately high temperatures. Copyright © 2001 John Wiley & Sons, Ltd. [source] Investigation of multiphase hydrogenation in a catalyst-trap microreactorJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 3 2009S. McGovern BACKGROUND: Multiphase hydrogenation plays a critical role in the pharmaceutical industry. A significant portion of the reaction steps in a typical fine chemical synthesis are catalytic hydrogenations, generally limited by resistances to mass and heat transport. To this end, the small-scale and large surface-to-volume ratios of microreactor technology would greatly benefit chemical processing in the pharmaceutical and other industries. A silicon microreactor has been developed to investigate mass transfer in a catalytic hydrogenation reaction. The reactor design is such that solid catalyst is suspended in the reaction channel by an arrangement of catalyst traps. The design supports the use of commercial catalyst and allows control of pressure drop across the bed by engineering the packing density. RESULTS: This paper discusses the design and operation of the reactor in the context of the liquid-phase hydrogenation of o-nitroanisole to o-anisidine. A two-phase ,flow map' is generated across a range of conditions depicting three flow regimes, termed gas-dominated, liquid-dominated, and transitional, all with distinctly different mass transfer behavior. Conversion is measured across the flow map and then reconciled against the mass transfer characteristics of the prevailing flow regime. The highest conversion is achieved in the transitional flow regime, where competition between phases induces the most favorable gas,liquid mass transfer. CONCLUSION: The results are used to associate a mass transfer coefficient with each flow regime to quantify differences in performance. This reactor architecture may be useful for catalyst evaluation through rapid screening, or in large numbers as an alternative to macro-scale production reactors. Copyright © 2008 Society of Chemical Industry [source] Solid,liquid mass transfer characteristics of an unbaffled agitated vessel with an unsteadily forward,reverse rotating impellerJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 5 2008Shuichi Tezura Abstract To develop an enhanced form of solid-liquid apparatus, an unbaffled agitated vessel has been constructed, fitted with an agitation system using an impeller whose rotation alternates unsteadily in direction, i.e. a forward-reverse rotating impeller. In this vessel, solid-liquid mass transfer was studied using a disc turbine impeller with six flat blades. The effect of impeller rotation rate as an operating variable on the mass transfer coefficient was evaluated experimentally using various geometrical conditions of the apparatus, such as impeller diameter and height, in relation to the impeller power consumption. Mixing of gas above the free surface into the bulk liquid, i.e. surface aeration, which accompanied the solid-liquid agitation, was also investigated. Comparison of the mass transfer characteristics between this type of vessel and a baffled vessel with a unidirectional rotating impeller underscored the sufficient solid-liquid contact for prevention of gas mixing in the forward-reverse rotation mode of the impeller. Copyright © 2008 Society of Chemical Industry [source] Flow and mass transfer in aerated viscous Newtonian liquids in an unbaffled agitated vessel having alternating forward,reverse rotating impellersJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 11 2001Masanori Yoshida Abstract Flow and mass transfer characteristics in aerated viscous Newtonian liquids were studied for an unbaffled aerated agitated vessel with alternating rotating impellers (AAVAI), ie with multiple forward,reverse rotating impellers having four delta blades. The effects of operating conditions such as gas sparging rate, agitation rate and the number of impeller stages, and the liquid physical properties (viscosity) on the gas hold-up, ,gD, and volumetric oxygen transfer coefficient, kLaD were evaluated experimentally. The dependences of ,gD and kLaD on the specific total power input and superficial gas velocity differed, depending on the ranges of liquid viscosity. Empirical relationships are presented for each viscosity range to predict ,gD and kLaD as a function of the specific total power input, superficial gas velocity and viscosity of liquid. Based on a comparative investigation of the volumetric coefficient in terms of the specific total power input between the AAVAI and conventional aerated agitated vessels (CAAVs) having unidirectionally rotating impellers, the usefulness of AAVAI as a gas,liquid agitator treating viscous Newtonian liquids is also discussed. © 2001 Society of Chemical Industry [source] Flow distribution and mass transfer in a parallel microchannel contactor integrated with constructal distributorsAICHE JOURNAL, Issue 2 2010Jun Yue Abstract Flow distribution and mass transfer characteristics during CO2 -water flow through a parallel microchannel contactor integrated with two constructal distributors have been investigated numerically and experimentally. Each distributor comprises a dichotomic tree structure that feeds 16 microchannels with hydraulic diameters of 667 ,m. It was found that constructal distributors could ensure a nearly uniform gas,liquid distribution at high gas flow rates where the ideal flow pattern was slug-annular flow. Nevertheless, at small gas flow rates where the ideal flow pattern was slug flow, a significant flow maldistribution occurred primarily due to the lack of large pressure barrier inside each distributor, indicating that dynamic pressure fluctuation in parallel microchannels greatly disturbed an otherwise good flow distribution therein. It was further shown that the present parallel microchannel contactor could realize the desired mass transfer performance previously achieved in one single microchannel under relatively wide operational ranges due to the integration of constructal distributors. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Kinetics of the thermal dissociation of ZnO exposed to concentrated solar irradiation using a solar-driven thermogravimeter in the 1800,2100 K rangeAICHE JOURNAL, Issue 6 2009Lothar O. Schunk Abstract The two-step H2O-splitting thermochemical cycle based on the Zn/ZnO redox reactions is considered for solar H2 production, comprising the endothermal dissociation of ZnO followed by the exothermal hydrolysis of Zn. A solar-driven thermogravimeter, in which a packed-bed of ZnO particles is directly exposed to concentrated solar radiation at a peak solar concentration ratio of 2400 suns while its weight loss is continuously monitored, was applied to measure the thermal dissociation rate in a set-up closely approximating the heat and mass transfer characteristics of solar reactors. Isothermal thermogravimetric runs were performed in the range 1834,2109 K and fitted to a zero-order Arrhenius rate law with apparent activation energy 361 ± 53 kJ mol,1 K,1 and frequency factor 14.03 × 106 ± 2.73 × 106 kg m,2 s,1. Application of L,vov's kinetic expression for solid decomposition along with a convective mass transport correlation yielded kinetic parameters in close agreement with those derived from experimental data. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] | |||||||||||||||||||