Search

This paper discusses the effect of temperature polarization in Vacuum Membrane Distillation (VMD). The main motivation for using VMD in this work is that this module configuration is much simpler and more suitable for this kind of investigation than the other MD configurations such as Direct Contact Membrane Distillation (DCMD). The coupling between heat and mass transfer mechanisms at the feed-membrane interface is presented from a theoretical point of view. In addition, a new simple graphical method and a mathematical model for determining VMD flux are presented. The two methods used in evaluating the extent of temperature polarization effect on water vapor flux (flux sensitivity factors and temperature polarization coefficient (TPC)) are also analyzed and compared. The effect of integrating a heat recovery system in a large scale module on the TPC coefficient has also been studied and presented in this paper. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a new frequency domain method to model the heat transfer between the injected/extracted heat and the temperature of the fluid exiting a borehole heat exchanger. The method is based on in situ measurements and focuses particularly on the short-term borehole heat transfer. It uses a rational function of the Warburg variable in the Laplace domain to model the borehole heat transfer. The rational model is transformed to a time domain model using inverse Laplace transformation. This time domain model makes it possible to calculate the temperature response on a random heat variation signal. The paper also demonstrates a new way to perform the classical thermal response test. Instead of injecting a constant amount of heat, the experiments have been performed using multiple short-duration heat injections. In this way, the obtained rational heat transfer model contains information about both the short- and the long-term heat transfer. The results obtained using the proposed modeling method are compared with those obtained from a state-of-the-art analytical method. The time domain model can be used to design a controller to optimize the performance of a Ground Source Heat Pump system, the efficiency of which depends strongly on the temperature of the fluid exiting the borehole. [Copyright &y& Elsevier]

Abstract: In this paper, a transient numerical model for a liquid-to-air membrane energy exchanger (LAMEE) is developed and the results are compared to the experimental data. The data for the sensible, latent and total effectiveness are compared with transient and steady-state simulations for various values of NTU and operating parameters (Cr∗, inlet temperature and humidity) while the exchanger undergoes a step change in inlet conditions and reaches a new steady-state. The transient and steady-state performances are shown to be a function of the design and operating parameters. The paper also shows that buoyancy forces become important and affect the LAMEE performance under certain operating conditions. The verified numerical model is used to investigate the impact of outdoor air conditions on the LAMEE performance. [Copyright &y& Elsevier]

Abstract: With the aim of dynamic simulation and control, a new non-linear state-space dynamic non-isothermal polymer electrolyte membrane fuel cell (PEMFC) model is developed in this paper. This mathematical model is developed based on mass and energy equation. The present model takes in account subsequent factors as the effects of charge double layer capacitance, the geometrical capacity and the effect of temperature gradient. In this paper, the authors propose a combination of several dynamic equations to study the effect of suddenly variation of some operating parameters like load resistance, gas pressure and gas temperature input. The results are compared to those of an isothermal model. This model will be extremely functional for the best possible design and real-time control of PEMFC systems. The present model is executed in MATHCAD software and the fuel cell is symbolized by an equivalent circuit which incorporates gas diffusion layer, membrane and electrodes. The analysis results show that the main elements that influence the performance of the cell are load resistance and functioning temperature. [Copyright &y& Elsevier]

Abstract: This paper discusses results of simulations conducted for non-contacting face seals showing the influence of the operational parameters on the distributions of temperature in the structural elements and the fluid film separating the sealing rings. The mathematical model developed for noncontacting face seals consists of cross-coupled differential equations with partial derivatives. We solved it analytically, taking into account various boundary conditions. The heat equations were solved using the method of separation of variables. The derived relationships were based on Bessel functions of the first and second kinds. By defining the relationships describing the distributions of temperature in the sealing rings and the fluid film separating them, we could determine the effect of the operational parameters on the variability of temperature fields. Confirmation of the simulation results with experimental data is essential to better recognize the phenomena taking place in the sealing assembly and improve the design of non-contacting seals so that they can be used in various industrial applications. In this paper, we also describe a specially designed test stand for compensation of the misalignment of the flexibly mounted ring and automatic control of the radial clearance height. The test stand can also be used to verify the results of analytical studies. [Copyright &y& Elsevier]

In this paper, a novel model is presented for predicting thermophysical properties of superheated steam (SHS) in offshore superheated steam injection wells (OSSIW). Firstly, a mathematical model comprised of mass, energy and momentum balance equations is proposed for OSSIW. Secondly, the distribution of temperature, pressure and superheat degree along the wellbores are obtained by finite difference method on space and solved by using the iteration technique. Then, model validation and sensitivity analysis are conducted. The results show that (1). The superheat degree increases at first but then turns to decrease with the increase of injection rate. (3). The superheat degree increases with the increase of injection temperature. (4). The superheat degree at well bottom increases at first but then turns to decrease with the increase of injection pressure. (5). The superheat degree in the wellbore has a drop when seawater starts to flow. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an implantable biomedical device for the localized killing of cancer cells through hyperthermia. Heating, accomplished via resistive heating, is modeled using numerical heat transfer techniques, which are tested under experimental conditions. The effect of temperature in the therapeutic domain of 37 to 45°C as studied on breast cancer cell line MDA-MB-231 is also reported. The results show the predicted temperature variations are consistent with temperature measurements obtained from the experimental set-ups. The paper also examines the effects of isothermal heating on the cell morphology. Isothermal heating is shown to cause significant physical changes in the cell cytoskeleton. Finally, the paper explores the effects of hyperthermia on cell growth and cell death under isothermal and cyclic conditions. The underlying effects of heat shock protein expression are elucidated before discussing the implications of the results for cancer treatment via localized hyperthermia. [Copyright &y& Elsevier]

Abstract: This paper aims at developing an analytical model for the coupled heat and mass transfer processes in a counterflow Reversibly Used Cooling Tower (RUCT) based on operating conditions, which is more realistic than most conventionally adopted Merkel approximations. Temperature and moisture content differences are chosen as the driving forces of heat and mass transfer correspondingly and a system of specific difference equations is developed to solve the model more efficiently. The model is investigated by using an iterative algorithm, which is validated with the experimental data reported. The analytical model also accommodates the direct and quick calculation of air and water temperature profiles, moisture content of air and the water mass flow rate change along the vertical length of the RUCT. With the aid of the developed model, the thermal behavior of the counterflow RUCT under various operating and environmental conditions is also studied in this paper. The results reveal that the proposed model can provide a theoretical foundation for practical design and performance evaluation of counterflow RUCT. [Copyright &y& Elsevier]

Abstract: This paper presents a numerical study on laminar forced convection of water in offset strip-fin microchannels network heat sinks for microelectronic cooling. A 3-dimensional mathematical model, consisting of N–S equations and energy conservation equation, with the conjugate heat transfer between the heat sink base and liquid coolant taken into consideration is solved numerically. The heat transfer and fluid flow characteristics in offset strip-fin microchannels heat sinks are analyzed and the heat transfer enhancement mechanism is discussed. Effects of geometric size of strip-fin on the heat sink performance are investigated. It is found that there is an optimal strip-fin size to minimize the pressure drop or pumping power on the constraint condition of maximum wall temperature, and this optimal size depends on the input heat flux and the maximum wall temperature. The results of this paper are helpful to the design and optimization of offset strip-fin microchannel heat sinks for microelectronic cooling. [Copyright &y& Elsevier]

Abstract: In this paper, a magnesium (Mg) reduction model considering both heat transfer and chemical reaction kinetics was firstly developed. The reduction model was carried out on a single retort to investigate the impact of different constant heating temperature on the magnesium reduction process in the retort. And then, the reduction model was incorporated into the conventional coal combustion model to describe the process of magnesium production in a coal-fired furnace. The comprehensive model was solved according to the actual industrial operation. The characteristics of magnesium production in the coal-fired furnace, such as thermal-flow field around the retorts in the furnace, the distribution of magnesium reduction extent in the retorts and the total extent of magnesium reduction with time were in detail studied. Especially, the impact of external thermal-flow around the retorts on the extent of magnesium reduction was investigated and discussed. [Copyright &y& Elsevier]

Abstract: In this paper a two-dimensional model of an annular cylindrical reactor filled with metal hydride suitable for hydrogen storage is presented. Comparison of the computed bed temperatures with published experimental data shows a reasonably good agreement except for the initial period. Effects of hydrogen pressure and external fluid temperatures on heat transfer and entropy generation are obtained. Results show that the time required for hydrogen charging and discharging is higher when the thermal capacity of the reactor wall is considered. The time required for absorption and desorption can be reduced significantly by varying the hydrogen gas pressure and external fluid temperatures. However, along with reduction in time the entropy generated during hydrogen storage and discharge increases significantly. Results also show that for the given input conditions, heat transfer between the external fluid and hydride bed is the main source of entropy generation. [Copyright &y& Elsevier]

Abstract: Sufficient downhole steam equality is important for the success of steam injection performance. In this paper, a system model is developed to predict steam quality with variable (T, P) fields in steam injection wells. In particular, the formation temperature, which changes over time, was examined. A hybrid algorithm for solving the model is used, along with the fourth-order Runge–Kutta method and the finite difference method. Data from a Chinese X Well is used as a case history to verify the calculations and a trend and sensitivity analysis was also done for the model. The effect of the process variables such as injection rate, injection pressure, pipe diameter and heat insulation thickness were put into test for the sensitivity of the model. The results provide technical reliability when designing well tests in deviated wells. [Copyright &y& Elsevier]

Abstract: Thermoelectric devices (TEs) can achieve direct conversion of heat and electricity by semiconductor materials, coupling of heat transfer and electric conduction is important to accurately predict the performance of TEs. This paper develops a general, three-dimensional numerical model of TEs with consideration of coupling of temperature field and electric potential field. The model is used to figure out the performance of thermoelectric coolers (TECs) with the temperature-dependent thermal conductivity, electric conductivity, and Seebeck coefficient of semiconductor materials. A miniature TEC is considered and Bi2(Te0.94Se0.06)3 and (Bi0.25Sb0.75)Te3 are selected as the n-type and p-type thermoelectric materials, respectively. The effect of parameters such as the temperature difference and the current is investigated under conditions of variable material properties as well as radiation and convection heat transfer occurred between the TEC and the ambient gas. The results show that the variable properties and the heat losses to the ambient gas have significant effects on the cooling capacity and the coefficient of performance (COP) of the TEC. Three-dimensional temperature distributions within the semiconductors is observed under convective boundary condition and it becomes remarkable at large temperature differences and high currents. [Copyright &y& Elsevier]

Abstract: This paper reports a study of steady-state heat transfer of gas and liquid flowing through a packed bed with spherical particles of 2.06mm and constant wall temperature with Computational Fluid Dynamics (CFDs). The effect of gas and liquid flow rates on heat transfer in packed beds are discussed. The effective radial bed conductivity (λ er ) is calculated based on the steady-state two dimensional model. The results of the CFD simulations have been validated with experiments and they are in a good agreement. For most of the cases the results of the CFD simulations deviate from the experimental results not more than 10%. The results show that the effective radial bed conductivity increases with increase of the velocity of gas and liquid. In trickle flow regime the effective radial bed conductivity is much bigger than in a dry gas flow regime. [Copyright &y& Elsevier]

Abstract: An experimental validation for a computational fluid dynamics (CFD) and an effectiveness-number of transfer units (ε-NTU) model for tubes in a large phase change material (PCM) tank has been conducted. The inlet and outlet heat transfer fluid (HTF) temperatures as well as twelve temperature locations in the PCM tank were compared with the CFD results. The average effectiveness of the phase change process of each experimental point was also compared with results from the CFD as well as the ε-NTU models. From this study, it was concluded that the CFD model and the ε-NTU model developed can accurately predict the behaviour of the thermal storage system during the freezing process. There are however, discrepancies in the melting process due to the exclusion of the effect of natural convection in the models. Using the experimental results, an effective thermal conductivity has been determined to account for buoyancy for various distances of tubes. The paper gives details of the CFD model of the phase change thermal storage system, and presents results from the CFD model, experiments and ε-NTU model. [Copyright &y& Elsevier]

Abstract: The present paper proposes a sub-model for two-layer zone models to describe heat and mass transfer across the interface between the upper and lower layers as a result of the temperature difference between the two layers. The sub-model is integrated into a two-layer zone fire growth and smoke movement model, which now can predict both pre- and post-flashover fires. The comparison of the numerical predictions of the improved model and experimental data shows that the improved model produces better temperature and heat release rate predictions for both pre- and post-flashover fires than the previous model. Compared with hybrid models consisting of a two-layer zone model and a single-layer zone model, the improved model has no transitional difficulties in switching from the pre- to the post-flashover state and it produces more reasonable temperature gradients for post-flashover fires. [Copyright &y& Elsevier]

Abstract: In this paper, the authors introduce a robust numerical strategy to estimate the temperature dependent heat capacity, thermal conductivity and porosity of a saturated porous medium, basing on the knowledge of heating curves at selected points in the medium. In order to solve the inverse problem, we use the least squares criterion (in which the sensitivity coefficients appear), leading to a system of ordinary differential equations (ODE). At the stage of numerical computations, we propose a new global approach, based on the method of lines and ordinary differential equations solvers, combined with a modified Newton method to deal with the nonlinearities presented in the system of coupled equations. [Copyright &y& Elsevier]

Abstract: This paper describes a dynamic model of transient heat and mass transfer across a green roof component. The thermal behavior of the green roof layers is modeled and coupled to the water balance in the substrate that is determined accounting for evapotranspiration. The water balance variations over time directly impact the physical properties of the substrate and the evapotranspiration intensity. This thermal and hydric model incorporates wind speed effects within the foliage through a new calculation of the resistance to heat and mass transfer within the leaf canopy. The developed model is validated with experimental data from a one-tenth-scale green roof located at the University of La Rochelle. A comparison between the numerical and the experimental results demonstrates the accuracy of the model for predicting the substrate temperature and water content variations. The heat and mass transfer mechanisms through green roofs are analyzed and explained using the modeled energy balances, and parametric studies of green roof behavior are presented. A surface temperature difference of up to 25°C was found among green roofs with a dry growing medium or a saturated growing medium. Furthermore, the thermal inertia effects, which are usually simplified or neglected, are taken into account and shown to affect the temperature and flux results. This study highlights the importance of a coupled evapotranspiration process model for the accurate assessment of the passive cooling effect of green roofs. [Copyright &y& Elsevier]

Abstract: This paper reports our efforts to engineer a robust, user-friendly, and broadly tunable helium droplet nozzle, and to quantitatively measure its thermal and mass transport performance. In addition to describing the physical design in detail, we report helium throughput measurements for a 6.4μm diameter nozzle over stagnation conditions ranging from 5 to 50K and 10 to 100bar. The measured flow rates were in excellent agreement with those predicted by a simple effusive flow model for nozzle temperatures above 20K, but were systematically lower for both sub-critical and super-critical jets as the temperature was lowered. The helium flow through a 500μm skimmer was also measured, and the skimmed fraction was found to vary by two orders of magnitude over the range of stagnation conditions investigated. These results indicate a substantial narrowing of the total jet angle spread from ∼90° to 5° at temperatures below 10K. Efforts to image the low temperature jet with Schlieren and shadowgraph techniques were unsuccessful. These details combined with previously reported theory and experiments on the droplet size distributions provide the necessary foundation to predict cluster production rates and to customize nozzle/pump designs for specific applications. [Copyright &y& Elsevier]

Abstract: The primary objective of this paper is to quantify the impact of the internal magnetic field and reversibility of the magnetocaloric effect on the numerical evaluation of the thermal performance of a parallel-plate active magnetic regenerator. To this end, direct measurements of the adiabatic temperature change of commercial-grade gadolinium samples were carried out for an applied field change of 1.65 ± 0.03 T (for both magnetization and demagnetization) and temperatures between 283 and 303 K. The tests were carried out in a specially constructed apparatus and the results were in good agreement with the literature. The data were incorporated into a mathematical model for solving the fluid flow and conjugated heat transfer in the parallel-plate regenerator. It was found that if demagnetization phenomena (i.e., the demagnetization factor and the reversibility of the magnetocaloric effect) are not take properly into account, the thermal performance of the regenerator can be severely over-predicted. [Copyright &y& Elsevier]

Abstract: Thermal energy storage (TES) has shown potential in improving the overall performance in energy systems, through shifting of thermal load demand, and through matching of uneven energy availability in time and in space. Latent heat TESs demonstrate advantages over sensible heat TESs for their high storage density and small temperature swing; however, lack of accurate knowledge in novel material properties and lack in a holistic design protocol often lead to difficulties in reaching technically viable storage design. With the aim of proposing a sound latent heat based TES design-to-validation protocol, this paper covers material property characterization through Temperature-history (T-history) method, heat exchanger design through heat transfer modeling, and model validation through experimental verification. A model for submerged cylindrically finned heat exchanger latent heat storage unit with phase change material was built. The results show that performance of gelled salt-hydrate based TES can be assessed with a pure conduction based model. This material property characterization-to-model verification approach may serve as a standard in providing accurate storage design for performance evaluation. [Copyright &y& Elsevier]

Abstract: Fluid temperature enters into a variety of petroleum production–operations calculations, including well drilling and completions, production facility design, controlling solid deposition, and analyzing pressure-transient test data. In the past, these diverse situations were tackled independently, using empirical correlations with limited generality. In this review paper, we discuss a unified approach for modeling heat transfer in various situations that result in physically sound solutions. This modeling approach depends on many common elements, such as temperature profiles surrounding the wellbore and any series of resistances for the various elements in the wellbore. We show diverse field examples illustrating this unified modeling approach in solving many routine production–operations problems. [Copyright &y& Elsevier]

Abstract: The cooling performance of ceiling radiant cooling panels (CRCP) is different from those measured in the laboratory under standard conditions. This paper explores the actual cooling performance of CRCP without mechanical ventilation. Experiments were conducted in an office room under different working conditions and a heat transfer model was established based on the experimental results. According to the actual performance of CRCP, the influencing factors were determined and correction coefficients were proposed. Comparing with the manufacture''s data, cooling capacity of panels can be enhanced by 17.1% with characteristic temperature difference of 8°C; temperatures of inner surfaces in the room and air velocity around the panel surface are the two main influencing factors, in which air velocity accounts for nearly 75% of the enhancement; the correction coefficients varied from 1.02 to 1.26 under different conditions. [Copyright &y& Elsevier]

Abstract: The design of technological devices and systems, with reduced size but unchanged or increased performances, has been one of the most important scientific challenges faced in the last decade by many fields of the world industry. The capability to remove and exchange heat in more compact components has allowed great advantages in the optimization of all final products as well as in industrial processes. However the never-ending race to miniaturization of appliances has been taking into account problems concerning computational models and design criteria developed in the past for macro-systems. Nowadays most of these methods are no longer reliable for emerging micro-systems because of the substantial differences in transport phenomena and heat exchange, mainly in two-phase flow (scaling effects). This paper aims to provide its contribution to the burgeoning research activity on boiling heat transfer under forced convection in microchannels. The BO.E.MI.A. experimental facility was designed and specifically developed to perform different regimes heat transfer tests on mini and microchannels: a total of 150 tests were performed using a fluorinert fluid FC-72 as coolant. The test section consists of a horizontal 1mm inner diameter stainless steel tube having a heated length (Joule effect) of approximately 60mm. The mass flux range is between 1000 and 2000kgm−2 s−1 while the applied heat flux is between 10 and 150kWm−2. By means of a preheater we are able to change coolant inlet temperature and by setting the operating pressure in a range between 3 and 7bar, a broad spectrum of subcooling degree at inlet test section is achieved. The local heat transfer coefficients were evaluated for both subcooled and saturated flow boiling regimes. The experimental data show an increase of local heat transfer coefficient for increasing values of heat flux, with a weak dependence on the vapour quality. The experimental values obtained were compared with those from the adoption of one of the main heat transfer correlations in the literature. [Copyright &y& Elsevier]

Abstract: With the aim to develop a generalized model for predicting the frost growth on cold flat plate, experiments were conducted in a suction-type open-loop wind tunnel under variant frosting conditions. Based on the Hayashi correlation, a modified correlation of initial frost density was developed by the experimental results. After validating this proposed correlation, dynamic frosting characteristics, such as the frost thickness, density, frost surface temperature and heat flux were predicted under a wide range of frosting conditions. Compared with 25 groups of experimental results taken from this paper or previous open literatures, the average simulating errors were found to be limited within ±10%. The validation results indicate the proposed model is suitable to predict the dynamic heat and mass transfer process of the frost growth under a wide range of frosting conditions. [Copyright &y& Elsevier]

Abstract: The metal hydride heat transformer shows great potential for the low-grade heat recovery. However, the heat output is variable, and these dynamics have to be taken into account in realistic applications. In this paper, a new index called true temperature boost is introduced together with conventional indices to assess the performance of the heat transformer system. Detailed analyses are carried out using a mathematical model that authors developed, investigating effects of key design and operating factors on the system’s performance. The amount of hydrogen transferred between coupled reactors is found to be crucial to obtain a high coefficient of performance, while specific heating power is very sensitive to the cycle time. On the true temperature boost, the driving force of reaction and the heat transfer conditions exert the most significant effects. [Copyright &y& Elsevier]

Abstract: This paper describes the design, fabrication and performance evaluation of a high efficiency, compact heater that uses the catalytic oxidation of hydrogen to provide heat to a hydrogen storage system. The heater was designed to transfer up to 30kW of heat from the catalytic reaction to the hydrogen storage system via a recirculating heat transfer fluid. The catalytic heater consists of three main parts: 1) the reactor, 2) the gas heat recuperator, and 3) oil and gas flow distribution manifolds. The reactor and recuperator are integrated, compact, finned-plate heat exchangers to maximize heat transfer efficiency and minimize mass and volume. Detailed, three-dimensional, multi-physics computational models were used to design and optimize the system. At full power the heater was able to catalytically combust a 10% hydrogen/air mixture flowing at over 80 cubic feet per minute and transfer 30kW of heat to a 30 gallon per minute flow of oil over a temperature range from 100°C to 220°C. The total efficiency of the catalytic heater, defined as the heat transferred to the oil divided by the inlet hydrogen chemical energy, was determined to exceed the design goal of 80% for oil temperatures from 60°C to 165°C. [Copyright &y& Elsevier]

Abstract: This paper describes the experimental studies carried out to test thermal cycling of a real-scale PCM–air heat exchanger at ambient temperatures. To achieve this goal an experimental setup previously designed and used for testing real-scale prototypes of PCM–air heat exchangers is modified. The PCM used is commercially available, organic, and paraffin based. The total energy exchanged during melting and solidification, as well as the time elapsed until total melting/solidification are determined from the power curves experimentally obtained. The influence of the inlet air temperature and air flow is studied, and results show that the continuous thermal cycling of the unit is a repetitive process: running experiments with similar conditions leads to the same thermal behavior, no degradation in the PCM properties is noticed. Pressure drop is measured for different air flows. Depending on the inlet air temperature, full solidification of the PCM could be achieved in less than 3h for an 8°C temperature difference between the inlet air and the average phase change of the PCM. Average thermal powers of up to 4.5kW and 3.5kW for 1h are obtained for melting and solidification stages, respectively. An empirical model is developed from the experimental results, which could be a useful designing tool for applications that use such technology: green housing, curing and drying processes, plant production, HVAC, and free-cooling. [Copyright &y& Elsevier]

Abstract: This paper presents an extension of a mathematical model for particle attrition inside a fluidized bed by a supersonic air jet and its application to optimize the nozzle design. A new method to calculate grinding efficiency is presented. Also, heat transfer is included in the model because of the large interfacial temperature difference. Numerical simulations are conducted to investigate various nozzle designs, i.e. a range of area ratios (indicative of the jet being over- or under-expanded) and nozzle expansion angles, and different bed fluidization velocities. It is found that the perfectly expanded nozzle (the exit pressure equal to the outside pressure) provides better attrition performance than over- and under-expanded jets. The nozzle expansion angle also has an influence on the grinding efficiency: narrow angled nozzles have higher grinding efficiency. In addition, the analysis of various bed fluidization velocities indicates that increasing the velocity results in a modest improvement of the grinding efficiency. [Copyright &y& Elsevier]

Abstract: In this paper, the shortage effect of neglecting heat and mass transfer phenomena in mathematical descriptions of pressure drop through venturi scrubbers is studied. For this purpose, a descriptive model is presented with particular emphasis on the mass and energy equations. There is a correction factor introduced, for the first time, and correlated to improve the model predictions in the divergent section. The results indicate that pressure drop prediction is greatly affected by the simplified assumptions of the existing models. In fact, the occurrence of heat and mass transfer phenomena between two phases decreases the pressure drop intensively. Therefore, neglecting the undeniable effects of these phenomena will lead to some large errors. These errors are enhanced in higher gas temperatures, liquid, and gas flow rates, but the gas humidity has a negligible effect. [Copyright &y& Elsevier]

Abstract: In this paper, design and fabrication of a micro direct methanol fuel cell based on stainless steel in two kind of active and passive with cross strip and parallel flow field are presented. Dropping pressure, velocity and heat transfer of the cell is simulated by a 3D modeling in fluent software. An optimum operating condition of the cell is obtained by the mass flow and temperature limitation. The cell was tested in different temperatures and mass flow rate and the cell performance is investigated in different condition. Maximum power density of the cell in active condition and at 60°C was 170mW/cm2 and in passive condition was 85mW/cm2. A μDMFC with different flow field is investigated. Cross strip flow field has shown better performance and the effect of cell orientation on performance is also investigated. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the two main drawbacks of the heat balance integral methods are examined. Firstly we investigate the choice of approximating function. For a standard polynomial form it is shown that combining the heat balance and refined integral methods to determine the power of the highest order term will either lead to the same, or more often, greatly improved accuracy on standard methods. Secondly we examine thermal problems with a time-dependent boundary condition. In doing so we develop a logarithmic approximating function. This new function allows us to model moving peaks in the temperature profile, a feature that previous heat balance methods cannot capture. If the boundary temperature varies so that at some time t >0 it equals the far-field temperature, then standard methods predict that the temperature is everywhere at this constant value. The new method predicts the correct behaviour. It is also shown that this function provides even more accurate results, when coupled with the new CIM, than the polynomial profile. Analysis primarily focuses on a specified constant boundary temperature and is then extended to constant flux, Newton cooling and time dependent boundary conditions. [Copyright &y& Elsevier]

Abstract: This paper deals with the development of methods for non steady state test of solar thermal collectors. Our goal is to infer performances in steady-state conditions in terms of the efficiency curve when measures in transient conditions are the only ones available. We take into consideration the method of identification of a system in dynamic conditions by applying a Graybox Identification Model and a Dynamic Adaptative Linear Neural Network (ALNN) model. The study targets the solar collector with evacuated pipes, such as Dewar pipes. The mathematical description that supervises the functioning of the solar collector in transient conditions is developed using the equation of the energy balance, with the aim of determining the order and architecture of the two models. The input and output vectors of the two models are constructed, considering the measures of 4 days of solar radiation, flow mass, environment and heat-transfer fluid temperature in the inlet and outlet from the thermal solar collector. The efficiency curves derived from the two models are detected in correspondence to the test and validation points. The two synthetic simulated efficiency curves are compared with the actual efficiency curve certified by the Swiss Institute Solartechnik Pufung Forschung which tested the solar collector performance in steady-state conditions according to the UNI-EN 12975 standard. An acquisition set of measurements of only 4 days in the transient condition was enough to trace through a Graybox State Space Model the efficiency curve of the tested solar thermal collector, with a relative error of synthetic values with respect to efficiency certified by SPF, lower than 0.5%, while with the ALNN model the error is lower than 2.2% with respect to certified one. [Copyright &y& Elsevier]

Abstract: In this paper, some models of comparative combustion characteristics for gasoline and hydrogen fuelled spark ignition internal combustion engines were developed and discussed from a thermodynamic and heat transfer perspective. The geometry used was that of a 3.4L GM V6 engine with a compression ratio of 9.5:1. Models for mass fraction burned, pressure, temperature, and gas speed were developed according to the literature survey and graphed over the cycle range. Furthermore, Pressure–Volume and Temperature–Entropy models were developed for both gasoline and hydrogen fuelled engines. Analysis of these models indicated approximately a 6.42% increase thermal efficiency for the hydrogen fuelled engine due to less exhaust blow down, less heat rejection during the exhaust stroke, and its shorter combustion duration closer to TDC. However, it was found that the hydrogen fuelled engine had approximately a 35.0% decrease in power output at an equivalence ratio of 1.0 due to the decrease in MEP and a greater amount of heat transfer to the cooling system due to the increased combustion temperatures, shorter quenching distance associated with H2 combustion and greater flame speed. Finally, an increase in cycle temperatures and pressures was observed from increasing the equivalence ratio from 0.4 to 1.0 to 1.2. [Copyright &y& Elsevier]

Abstract: A single-blow transient test technique based on axial dispersion model is proposed for the determination of both heat transfer coefficient and axial dispersion coefficient in plate heat exchangers, characterized by NTU and dispersive Peclet number respectively. The present experimental analysis deals with the effect of flow maldistribution on the transient temperature response for U-type plate heat exchangers. The experiments are carried out with uniform and non-uniform flow distributions for various flow rates and two different numbers of plates. Special effort has been made to differentiate the deviation from plug flow due to flow maldistribution and fluid backmixing. The fluid axial dispersion is used to characterize the backmixing and other deviations from plug flow. Due to unequal distribution of the fluid, the velocity of the fluid varies from channel to channel and hence the heat transfer coefficient variations are also taken into consideration. The computed outlet fluid temperatures are compared with experimental outlet temperatures, and the values of dispersive Peclet numbers are estimated. The results indicate that in order to get parameters independent of the number of plate used in single-blow experiment, it is essential to isolate flow maldistribution from backmixing. This paper has brought out a practical way in which this isolation can be done in the process of data reduction through suitable computational model. [Copyright &y& Elsevier]

Abstract: This paper presents a heat and mass transfer simulation of an adsorber, taking into consideration the geometry of the heat exchanger and the diffusion in the adsorbent medium. In this model, an increment in the direction of heating/cooling fluid containing one (or more) fin pitch was considered. In the domain of this increment, the distribution of the adsorbent temperature was evaluated in the radial and axial directions. The model is validated by experimental data of different adsorbents, volume flows and temperatures. All data were collected in an adsorber test-bench at Fraunhofer ISE. As a result it was found a coefficient of multiple determination of around 0.94 and an error of 20% during the transient. Finally, the model was run on typical operation for cooling application and compared with data from literature. A numerical energy balance was also evaluated. In summary, the results obtained by a independent group shows the same range of confidence of the model developed here, in terms of the tendency of the curves and even the absolute values. [Copyright &y& Elsevier]

Abstract: In the present study, a 3D numerical simulation of a whole heat exchanger with middle-overlapped helical baffles is carried out by using commercial codes of GAMBIT 2.3 and FLEUNT 6.3. At first, the computational model and numerical method of the whole heat exchanger with middle-overlapped helical baffles is presented in detail, and parallel computation mode is adopted for the simulation of a whole heat exchanger with six cycles of the middle-overlapped helical baffles of 40° helical angle on a grid system of 13.5-million cells; second, the validation of the computational model is performed by comparing the total pressure drop and average Nusselt number of the whole heat exchanger with experimental data. Reasonably good agreement is obtained, and the reasons causing to the discrepancy are analyzed. The shell-side fluid pressure and temperature fields of the whole area are then presented. Finally the cycle average Nusselt number of different cycle in the heat exchanger are compared and it is found that within the accuracy allowed in engineering computation, periodic model for one cycle can be used to investigate the heat transfer and pressure drop characteristics for different heat exchanger to save computational source. The companion paper will provide details of simulation results using the periodic model for different helical angles and different structures of helical baffles. [Copyright &y& Elsevier]

Abstract: Our 1985 paper (JQSRT 1985; 33: 533–549) reported the result of the research we conducted back then to better understand heat transfer processes in large-scale combustion chambers, especially in pulverized coal-fired furnaces. It was one of the first works exploring radiative transfer in three-dimensional enclosures where absorption and scattering coefficients due to combustion particles and gases were allowed to vary within the medium. This flexibility of the mathematical model made it useful for applications to realistic furnaces and different types of high-temperature systems. This note briefly discusses the motivation behind the paper and the immediate extension of the idea to different systems. [Copyright &y& Elsevier]