Measurement of ventilation and aerosol particles in buildings

The work described in the paper is concerned with the measurement of the flow of tracer gas and aerosol particles in rooms. Measurements were carried out in single- and two-zone systems using SF₆ tracer gas and oil-smoke particles. Initial tests were carried out in a single-zone system using different arrangements of window opening. Results indicated that tracer-gas exchange rates were generally higher than particle exchange rates. This was due to the fact that the ventilation air entering the zone contained a significant concentration of particles but a negligible quantity of tracer gas. The exchange rates of tracer gas and smoke particles through a doorway were also measured in a two-zone system. The doorway coefficients of discharge were calculated using a theory based on the Bernoulli equation. The coefficients of discharge for the opening were in the range 0.4–0.96 for tracer gas measurements and 0.15–0.42 for smoke-particle measurements.     

Characterization of building infiltration by the tracer-dilution method

Air infiltration is an important factor in the total energy budget of a structure. It is also a significant parameter in indoor-outdoor air pollution relationships. Air infiltration cannot be reliably calculated but must be measured in a structure of interest. The tracer-dilution method is a useful technique to determine infiltration rates. This technique entails measurement of the logarithmic dilution rate of a tracer gas concentration with respect to time.     

Tracer gas technology for airflow measurements in HVAC systems

This paper describes the use of tracer gas techniques for airflow measurement in ducts. The author had carried out extensive work in both the laboratory and the field to test the accuracy and viability of these techniques. Preliminary measurements were carried out in the laboratory to examine the accuracy of these techniques. The mixing of tracer gas (e.g. sulphur hexafluoride, SF₆) in ducts of various shapes and sizes was examined using different types of tracer injectors. Measurements of airflow estimated using tracer gas techniques (e.g. constant-injection, pulse-injection) were compared with measurements made using traditional instrumentation such as pitot tubes. Field testing on a large-scale heating, ventilation and air-conditioning system was carried out and tracer gas techniques were used to determine airflow rates in air handling units serving a library building.     

Measurement of heat and mass transfer between the lower and upper floors of a house

The work is concerned with measuring heat and mass transfer between the lower and upper floors of a conventional house via a doorway. Air flows between the two floors were measured using a single tracer gas technique, and the temperatures at various points in each floor were measured using thermocouples. The lower floor of the house was heated to various temperatures in the range 18–35 °C using thermostatically controlled heaters. The upper floor was unheated. Two portable SF₆ systems fitted with electron capture detectors were employed for measurement of the interzonal air flow. The mass and heat flow rates between the two floors were calculated from the tracer gas concentrations, and temperature differences and results were compared with the values predicted by the existing algorithms for the two zone enclosures. The mass flow rate and coefficient of discharge for the doorway were found to be functions of the temperature difference between the floors of the house.     

适用于宽温度和压力范围的湿空气热力性质分段计算方法

根据计算范围的不同,提出适合宽温度、压力范围,涵盖多个湿空气热物理性质参数的分段计算方法：在低温、低压范围内将湿空气看作理想混合气体,采用理想气体状态方程建立模型计算湿空气的热物理性质参数;在高温、高压范围内将湿空气看作干空气和水蒸气的实际二元混合气体,采用维里方程建立湿空气的半经验模型计算湿空气的热物理性质参数.分析确定了分段计算的分界点,并说明了分段计算的连续性.结果表明：与参考文献的计算结果相比,采用该文提出的湿空气热力性质分段方法得出的计算结果的最大误差仅为4.5%,且计算过程简单快捷,易于程序实现,可用于绘制焓湿图.     

Modelling and analyses of heat exchangers in a biomass boiler plant

A boiler plant is presented, in which the fuel is dried before combustion in a silo with air. The drying air is heated in a recuperative heat exchanger by the heat of flue gases. Hot air is then blown through the bed of fuel in the drying silo, while the fuel dries and the air cools down and becomes humidified. Heat of the moist exhaust air of the silo is recovered for the drying air and combustion air by a recuperative heat exchanger. Modelling of the thermal behaviour of the plant helps in understanding complex interdependencies of the two heat exchangers, the boiler and the dryer. The models of the heat exchangers and applications in analysing the boiler system are described in this paper. Calculating the combinations of extreme operational conditions gives the input data needed in comparing different types of heat exchangers, dimensioning the heat transfer area, choosing the control strategy and selecting the operating parameters and set‐values of the control system. Results of verification measurements and practical operation at a 40 kW_th pilot plant and a 500 kW_th demonstration plant are also discussed. Using engineering correlation formulas for heat and mass transfer, an adequate accuracy between the model and the measurements was achieved. Fouling was detected to be a major problem with the flue gas heat exchanger. However, in absence of condensation, the increase of a fouling layer with respect to time was observed to be low. Fouling was also a problem with the drying exhaust gas heat exchanger, but after the installation of a simple dust collector, a reasonable cleaning period was achieved. A mixed‐flow configuration was found to be the most appropriate for the flue gas heat exchanger. In order to avoid condensation of the flue gas the drying exhaust gas heat exchanger is indispensable in Finnish climate in the considered system. In addition to this, it decreases the need of fuel. A parallel‐flow type was found the most appropriate as the drying exhaust gas heat exchanger. Copyright © 2004 John Wiley & Sons, Ltd.     

Natural ventilation and indoor air quality in educational buildings: experimental assessment and improvement strategies

Indoor environmental conditions in classrooms, in particular temperature and indoor air quality, influence students’ health, attitude and performance. In recent years, several studies regarding indoor environmental quality of classrooms were published and natural ventilation proved to have great potential, particularly in southern European climate. This research aimed to evaluate indoor environmental conditions in eight schools and to assess their improvement potential by simple natural ventilation strategies. Temperature, relative humidity and carbon dioxide concentration were measured in 32 classrooms. Ventilation performance of the classrooms was characterized using two techniques, first by fan pressurization measurements of the envelope airtightness and later by tracer gas measurements of the air change rate assuming different envelope conditions. A total of 110 tracer gas measurements were made and the results validated ventilation protocols that were tested afterward. The results of the ventilation protocol implementation were encouraging and, overall, a decrease on the CO₂ concentration was observed without modifying the comfort conditions. Nevertheless, additional measurements must be performed for winter conditions.     

Formation characteristics of nitric oxide in a three-staged air/LPG flame

Experimental and numerical studies have been done to examine the effects of excess air ratio and tertiary air swirl number on the formation characteristics of NO in a pilot scale combustor adopting a multi-air staged burner. In numerical calculation the mathematical models for turbulence, radiation and nitric oxide chemistry were taken into account. The radiative transfer equation was solved using the discrete ordinates method with the weighted sum of gray gases model. In the NO chemistry model, the chemical reaction rates for thermal and prompt NO were statistically averaged using a probability density function. The results were validated by comparison with measurements. For the experiment, a 0.2 MW pilot multi-staged air burner has been designed and fabricated. Using the numerical simulation developed here, a variation of thermal and prompt NO formation was predicted by changing the excess air ratio and tertiary air swirl number. As the excess air ratio increased up to 1.9, the formation of the total as well as thermal NO at exit increased while the prompt NO decreased. The formation of thermal NO was more affected by concentration of O₂ and N₂ than gas temperature. When the tertiary air swirl number increased, the formation of the total as well as the prompt NO slightly decreased because of enhanced mixing of fuel and oxygen in the upstream reaction zone and reduced gas temperature at exit.     

Mathematical modelling of heat transfer in dedusting plants and comparison to off-gas measurements at electric arc furnaces

A mathematical simulation tool is presented in order to model enthalpy flow rates of off-gas and heat transfer of cooling systems at dedusting plants in electric steel making sites. The flexibility of the simulation tool is based on a user-defined series of modular units that describe elementary units of industrial dedusting systems, e.g. water-cooled hot gas duct, air injector, drop-out box, mixing chamber, post-combustion chamber, filter, etc. Results of simulation were checked with measurements at industrial electric steel making plants in order to validate the models for turbulence, heat transfer and chemical reaction kinetics. Comparison between computed and measured gas temperature and composition yield excellent agreement. The simulation tool is used to calculate off-gas temperature and volume flow rate, where off-gas measurements are very difficult to apply due to high gas temperatures and high dust load. Heat transfer from the off-gas to the cooling system was calculated in detail for a pressurised hot water EAF cooling system in order to investigate the impact of the cooling system and the dedusting plant operation on the energy sinks of the electric arc furnace. It is shown that optimum efficiency of post-combustion of EAF off-gas in the water-cooled hot gas duct requires continuous off-gas analysis. Common operation parameters of EAF dedusting systems do not consider the non-steady-state of the EAF off-gas emission efficiently.     

A hot-flow model analysis of the MSW incinerator

A hot-flow model incinerator was constructed to simulate the combustion chamber of mass-burn municipal solid waste (MSW) incinerator. Experiments using the reduced scale simplified model were designed to reproduce the physical processes. The Current study focuses on the gas phase mixing and the subsequent oxidative destruction of incomplete combustion products. Excessive carbon monoxide is generated by operating the gas burner under fuel-rich conditions, and is supplied into the furnace chamber as a hot gas stream, where fresh air is provided by the secondary air injection. Mixing is quantified by measuring the local gas concentrations (CO and O₂) and temperatures since the destruction rate of CO is controlled by oxygen availability and chemical kinetic rates. The degree of mixing is monitored while the design alternatives of the secondary air injection pattern are systematically adjusted. This effect of the secondary air injection schemes on the degree of mixing are observed, and the measurements of temperature fluctuation with fine-wire thermocouples are performed for the quantitative evaluation of mixing. The results agree well with the plausible scenario of mixing and provide a better understanding of the mixing process. The hot flow accompanying chemical reactions in the model incinerator is also analysed numerically using the computational fluid dynamics codes to bolster the understanding of the experimental results. The modelling results compare reasonably well with experimental data. This comparison helps to cross-check the results. © 1997 John Wiley & Sons, Ltd.     

Environmental exposure of 2LiBH4+MgH2 using empirical and theoretical thermodynamics

It has been shown that the consequence of environmental exposure can be qualitatively predicted by modeling the heat generated as a result of environmental exposure of reactive hydrides along with heat loss associated with conduction and convection with the ambient surroundings. To this end, an idealized finite volume model was developed to represent the behavior of dispersed hydride from a breached system. Semi-empirical thermodynamic calculations and substantiating calorimetric experiments were performed in order to quantify the energy released, energy release rates and to quantify the reaction products resulting from water and air exposure of a lithium borohydride and magnesium hydride combination. The hydrides, LiBH₄ and MgH₂, were studied in a 2:1 “destabilized” mixture which has been demonstrated to be reversible. Liquid water hydrolysis reactions were performed in a Calvet calorimeter equipped with a mixing cell using pH-neutral water. Water vapor and gaseous oxygen reactivity measurements were performed at varying relative humidities and temperatures by modifying the calorimeter and utilizing a gas circulating flow cell apparatus. The results of these calorimetric measurements were used to develop quantitative kinetic expressions for hydrolysis and air oxidation in these systems. Thermodynamic parameters obtained from these tests were then incorporated into a computational fluid dynamics model to predict both the hydrogen generation rates and concentrations along with localized temperature distributions. The results of these numerical simulations can be used to predict ignition events and the resultant conclusions will be discussed.     

Effect of turbulence and devolatilization models on coal gasification simulation in an entrained-flow gasifier

Numerical simulations of the oxygen-blown coal gasification process inside a generic entrained-flow gasifier are carried out. The Eulerian–Lagrangian approach is applied to solve the Navier–Stokes equations and the particle dynamics. Seven species transport equations are solved with three heterogeneous global reactions and two homogeneous reactions. Finite rates are used for the heterogeneous solid-to-gas reactions. Both finite rate and eddy-dissipation combustion models are calculated for each homogeneous gas-to-gas reaction, and the smaller of the two rates is used. Four different devolatilization models are employed and compared. The Kobayashi model produces slower devolatilization rate than the other models. The constant rate model produces the fastest devolatilization rate. The single rate model and the chemical percolation model produce moderate and consistent devolatilization rate. Slower devolatilization rate produces higher exit gas temperature and higher CO and CO₂ mass fractions, but lower H₂ and heating value, and hence, achieves lower gasification efficiency. Combustion of volatiles is modeled with two-stage global reactions with an intermediate stage via benzene.Turbulence models significantly affect the simulated results. Among five turbulence models employed, the standard k–ε and the RSM models give consistent results. The time scale for employing stochastic time tracking of particles also affects simulated result. Caution has to be exerted to select the appropriate time constant value. Smaller particles have a higher surface/volume ratio and react faster than larger particles. However, large particles possessing higher inertia could impinge on the opposing jet and change the thermal-flow filed and the reaction rates.     

Auto-ignition of toluene-doped n-heptane and iso-octane/air mixtures: High-pressure shock-tube experiments and kinetics modeling

Toluene is often used as a fluorescent tracer for fuel concentration measurements, but without considering whether it affects the auto-ignition properties of the base fuel. We investigate the auto-ignition of pure toluene and its influence on the auto-ignition of n-heptane and iso-octane/air mixtures under engine-relevant conditions at typical tracer concentrations. Ignition delay times τ_ign were measured behind reflected shock waves in mixtures with air at φ = 1.0 and 0.5 at p = 40 bar, over a temperature range of T = 700–1200 K and compared to numerical results using two different mechanisms. Based on the models, information is derived about the relative influence of toluene on τ_ign on the base fuels as function of temperature. For typical toluene tracer concentrations ?10%, the ignition delay time τ_ign changes by less than 10% in the relevant pressure and temperature range.     

Fully integrated unit for thermal treatment of gas wastes

A range of methods is used for the treatment of gas or liquid organic compounds. The thermal treatment of wastes can be considered as one of the most frequently used and most efficient processes. This method is based on a high temperature oxidising organic compounds by oxygen from air when inert (i.e. not harmful) gases like CO₂ and H₂O are generated.A new equipment for the thermal treatment of gas wastes i.e. incineration of volatile organic compounds and carbon monoxide contained in polluted air has been developed. This is in fact a process for the thermal treatment of this type of wastes placed into one compact equipment consisting of a combustion chamber (incinerator) combined with heat exchanger (polluted air preheater). This equipment can be used in various branches of industry (for the treatment of polluted air from paint shops, chemical cleaning systems, degreasing processes, printing machines, gas wastes from various chemical processes, for the thermal treatment of off-gases originating in refineries, in production of plastics, pharmaceutical industry, food industry, laboratories, etc.).Principal steps in the research and development are briefly described as follows: idea and basic conception, design geometry, using simulation for setting basic process parameters, developing a computer program for thermal and hydraulic calculation of this equipment, design of a research facility in an industrial scale, experimental research and measurements, a procedure for verification mathematical models including computational fluid dynamics simulation, recommendation for industrial application.     

Carbon support oxidation in PEM fuel cell cathodes

Oxidation of the cathode carbon catalyst support in polymer electrolyte fuel cells (PEMFC) has been examined. For this purpose platinum supported electrodes and pure carbon electrodes were fabricated and tested in membrane-electrode-assemblies (MEAs) in air and nitrogen atmosphere. The in situ experiments account for the fuel cell environment characterized by the presence of a solid electrolyte and water in the gas and liquid phases. Cell potential transients occurring during automotive fuel cell operation were simulated by dynamic measurements. Corrosion rates were calculated from CO₂ and CO concentrations in the cathode exhaust measured by non-dispersive infrared spectroscopy (NDIR). Results from these potentiodynamic measurements indicate that different potential regimes relevant for carbon oxidation can be distinguished. Carbon corrosion rates were found to be higher under dynamic operation and to strongly depend on electrode history. These characteristics make it difficult to predict corrosion rates accurately in an automotive drive cycle.     

Hydrogen production via biomass gasification,and modeling by supervised machine learning algorithms

Prediction of clean hydrogen production via biomass gasification by supervised machine learning algorithms was studied. Lab-scale gasification studies were performed in a steel fixed bed updraft gasifier having a cyclone separator. Pure oxygen, and dried air with varying flow rates (0.05–0.3 L/min) were applied to produce syngas (H₂, CH₄, CO). Gas compositions were monitored via on-line gas analyzer. Various regression models were created by using different Machine Learning (ML) algorithms which are Linear Regression (LR), K Nearest Neighbors (KNN) Regression, Support Vector Machine Regression (SVMR) and Decision Tree Regression (DTR) algorithms to predict the value of H₂ concentration based on the other parameters that are time, temperature, CO, CO₂, CH₄, O₂ and heating value. The highest hydrogen value in syngas was found around 35% vol. after gasification experiments with higher heating value (HHV) of approximately 3400 kcal/m³0.05 L/min and 0.015 L/min were the optimum flow rates for dried air and pure oxygen, respectively. In modeling section, it was observed that H₂ concentrations were being reflected effectively by the concentrations estimated through the proposed model structures, and by having r² values of 0.99 which were ascertained between actual and model results.     

Account for variations in the H2O to CO2 molar ratio when modelling gaseous radiative heat transfer with the weighted-sum-of-grey-gases model

This work focuses on models suitable for taking into account the spectral properties of combustion gases in computationally demanding applications, such as computational fluid dynamics. One such model, which is often applied in combustion modelling, is the weighted-sum-of-grey-gases (WSGG) model. The standard formulation of this model uses parameters fitted to a wide range of temperatures, but only for specific ratios of H₂O to CO₂. Then, the model is limited to gases from fuels with a given composition of hydrogen and carbon, unless several sets of fitted parameters are used. Here, the WSGG model is modified to account for various ratios of H₂O to CO₂ concentrations. The range of molar ratios covers both oxy-fuel combustion of coal, with dry- or wet flue gas recycling, as well as combustion of natural gas. The non-grey formulation of the modified WSGG model is tested by comparing predictions of the radiative source term and wall fluxes in a gaseous domain between two infinite plates with predictions by a statistical narrow-band model. Two grey approximations are also included in the comparison, since such models are frequently used for calculation of gas radiation in comprehensive combustion computations. It is shown that the modified WSGG model significantly improves the estimation of the radiative source term compared to the grey models, while the accuracy of wall fluxes is similar to that of the grey models or better.     

Performance prediction of PEM fuel cell with wavy serpentine flow channel by using artificial neural network

Effects of serpentine flow channel having sinusoidal wave at the rib surface on performance of PEMFC having 25 cm² active area are investigated at different flow rates, three different amplitudes changing from 0.25 mm to 0.75 mm and three different cell operation temperatures. A proton exchange membrane fuel cell (PEMFC) is modeled for the prediction of the output current by using artificial neural network (ANN) that is utilized the aforementioned experimental parameters. Effect of hydrogen and air flow rate, the fuel cell temperature, amplitude of channel is tested. The results indicated that model C1 having lowest amplitude is enhanced maximum power output up to 20.15% as compared to indicated conventional serpentine channel (model C4) for 0.7 SLPM H₂ and 1.5 SLPM air and also model C1 has better performance than C2, C3 and C4 models. The maximum power output is augmented with increasing the cell temperature due to raising the fuel and oxidant diffusion ratio. Cell temperature, amplitude, H₂ and air flow rate and input voltage is used as input variables in train and test of the developing ANN model. MAPE of training and testing is determined as 2.89 and 2.059, respectively. Prediction results of developed ANN model including two hidden layer shows similar trend with experimental results. Developed ANN model can be used to both decrease the number of required experiments and find the optimum operation condition within the range of input parameters.     

Coal-fired oxy-fuel power unit – Process and system analysis

As oxy-fuel power generation is currently on the pre-demonstration stage of development many studies dealing with optimization and simulation aspects are still in progress. This paper is focused on the development of a simulation model of the integrated oxy-fuel system and cumulative energy analysis of an oxy-unit operation in separated national economy. The analyzed oxy-fuel system consists of a steam boiler, steam cycle, air separation unit, as well as a CO₂ purification and compression island. The built model is based on physical relations. It has been developed as a set of modules modelling isolated devices (e.g. combustion chamber or flue gas dehumidifier). Interconnections between these devices can be easily changed, which permits to analyse different oxy-fuel structures and operating parameters. The simulation model has been partially verified on the basis of literature data. Results of oxy fuel energy analysis have been presented for one selected structure and compared with an air combustion power unit, assuming the same steam cycle parameters. For both cases indices of cumulative primary energy consumption have been calculated. The obtained results show that the increase of oxy-fuel primary energy consumption (compared with air-based combustion) can be significantly reduced if by-produced nitrogen will be used for external applications.     

Sensitivity analysis of a biomass gasification model in fixed bed downdraft reactors: Effect of model and process parameters on reaction front

A detailed sensitivity analysis is performed on a one-dimensional fixed bed downdraft biomass gasification model. The aim of this work is to analyze how the heat transfer mechanisms and rates are affected as reaction front progresses along the bed with its main reactive stages (drying, pyrolysis, combustion and reduction) under auto-thermal conditions. To this end, a batch type fixed-bed gasifier was simulated and used to study process propagation velocity of biomass gasification. The previously proposed model was validated with experimental data as a function of particle size. The model was capable of predicting coherently the physicochemical processes of gasification allowing an agreement between experimental and calculated data with an average error of 8%. Model sensitivity to parametric changes in several model and process parameters was evaluated by analyzing their effect on heat transfer mechanisms of reaction front (solid–gas, bed–wall and radiative in the solid phase) and key response variables (temperature field, maximum solid and gas temperatures inside the bed, flame front velocity, biomass consumption and fuel/air ratio). The model coefficients analyzed were the solid–gas heat transfer, radiation absorption, bed–wall heat transfer, pyrolysis kinetic rates and reactor-environment heat transfer. On the other hand, particle size, bed void fraction, air intake temperature, gasifying agent composition and gasifier wall material were analyzed as process parameters. The solid–gas heat transfer coefficient (0.02 < correction factor < 1.0) and particle size (4 < diameter < 30 mm) were the most significant parameters affecting process behavior. They led to variations of 88% and 68% in process velocity, respectively.