Performance of radiator by using SiO2 nano fluids

ABSTRACT

This article reports a study on heat transfer and flow characteristics of nano fluid consisting of water and ethylene glycol as base fluid with different concentrations of SiO₂as nanoparticles (0.1% and 0.15%) are flowing in a shell and tube exchanger. The SiO₂ nanoparticles of about 15 nm diameter are used in this study. The results showed that the heat transfer coefficient of nano fluid was slightly higher than that of the base fluid at the same inlet temperature and at the same mass flow rate. It was observed that heat transfer coefficient of nano fluid was increased with an increased mass flow rate. Heat transfer coefficient was noticed to increase with the increase of the volumetric concentration of the SiO₂. But increasing the volume concentration caused the increase in the viscosity of the nano fluid resulted in an increase in friction factor.     

Upslope plume migration and implications for geological CO2 sequestration in deep,saline aquifers

Recent investigations regarding CO₂ sequestration in deep saline aquifers have focused on characterization of the injected plume, its migration within the aquifer over time, and possible leakage out of the aquifer. To study these complex flow systems, simplified models are sometimes used to describe both plume evolution and the amount of leakage. Simplifications may include an assumption of perfectly horizontal geological formations, negligible capillary pressure, and symmetry of the injection plume. In this study, we explicitly test the limits of the assumption of a horizontal aquifer through numerical simulation of typical injection scenarios in continental sedimentary basins. Our approach is to simulate injection of CO₂ into a confined saline aquifer for an extended period (we have used 15 years) and examine the effect of different degrees of slope, as well as other system parameters, on plume asymmetry using measures such as the location of the centroid of the CO₂ plume. Dimensional analysis of this system shows that the centroid migrates upslope in proportion with buoyancy, aquifer permeability, and slope, whereas increased porosity and CO₂ viscosity mitigate upslope migration of the centroid. The results of this study show that the effect of slope can be ignored for many aquifers likely to become CO₂ sequestration sites in North America. However, slope will be more important for higher permeability aquifers, such as the site used in the Sleipner sequestration project in the North Sea.     

过热度对翅片管蒸发器性能影响的模拟研究

利用EVAP-COND软件对三排管翅片管蒸发器进行模拟计算。选取3种制冷剂R22、R134a和R410A,4种不同的流路布置方式,分别计算不同制冷剂出口过热度下蒸发器的总换热量,对模拟结果进行分析对比,研究出口过热度对翅片管蒸发器换热性能的影响,得出有利于指导蒸发器优化设计的结论。蒸发器出口制冷剂过热度越大,制冷量越小;设计的4种流路布置方案中第二种方案最为合理;制冷剂R410A换热性能最佳,可以替代R22。     

Embodied and operational carbon dioxide emissions from housing: A case study on the effects of thermal mass and climate change

A 100-year lifecycle carbon dioxide (CO₂) emissions analysis is reported for a two-bedroom, 65 m² floor area, semi-detached house in south-east England. How the balance between the embodied (ECO₂) and operational CO₂ emissions of the building are affected by the inclusion of thermal mass and the impacts of climate change is quantified. Four ‘weights’ of thermal mass were considered, ranging from lightweight timber frame to very heavyweight concrete construction. For each case, total ECO₂ quantities were calculated and predictions for operational CO₂ emissions obtained from a 100-year dynamic thermal modelling simulation under a medium-high emissions climate change scenario for south-east England. At the start of the lifecycle, the dwellings were passively cooled in summer, but air conditioning was installed when overheating reached a certain threshold. The inclusion of thermal mass delayed the year in the lifecycle when this occurred, due to the better passive control of summertime overheating. Operational heating and cooling energy needs were also found to decrease with increasing thermal mass due to the beneficial effects of fabric energy storage. The calculated initial ECO₂ was higher in the heavier weight cases, by up to 15% (4.93 t) of the lightweight case value, but these difference were offset early in the lifecycle due to the savings in operational CO₂ emissions, with total savings of up to 17% (35.7 t) in lifecycle CO₂ found for the heaviest weight case.     

A novel methodology for estimating space air change rates and occupant CO2 generation rates from measurements in mechanically-ventilated buildings

It is useful to know ventilation rates and carbon dioxide (CO₂) generation rates for evaluating indoor air quality and ventilation efficiency in mechanically-ventilated buildings. A strong limitation of the current models is either they focus solely on a whole building or they are too complicated for practical use in studies of individual spaces. This paper develops a new method for accurately quantifying ventilation rates (i.e. space air change rate) and CO₂ generation rates from measured CO₂ concentrations for individual spaces. The proposed method firstly determined space air change rate using Maximum Likelihood Estimation (MLE). Additionally, a novel coupled-method was initiated for further estimating CO₂ generation rates. Both simulated and experimental data were used to validate the model. Experiments were conducted in a school office by measuring indoor CO₂ concentrations and pressure differences between the return air vent and space. Excellent agreement was obtained. At least 0.998 R² values were obtained for fitting measured CO₂ concentrations when conducting MLE for estimating space air change rate, and the corresponding residual plots showed no pattern and trend. The estimated numbers of occupants were same as the actual ones. Furthermore, the predicted space air change rates showed great consistencies with those from CO₂ equilibrium analysis. The model is simple, handy and effective for practical use. Moreover, the model is also capable for dealing with time-varying space air change rates.     

Reducing CO2 emission from exhaust gases using molten carbonate fuel cells: a new approach

The aim of this study is to couple molten carbonate fuel cell (MCFC) stack with integrated gasification combined cycle fed by refinery residues, to remove CO₂ from gas turbine exhaust gases that have CO₂ emission rate of 14,200 ton/year. By applying multi-objective optimisation (MOO) using genetic algorithm, the optimal values of operating load and the corresponding values of objective functions are obtained. The MOO of the MCFC system regarding two scenarios is performed. The first scenario is minimisation of cost of electricity (COE) and CO₂ emission rate. Objective functions of the second scenario are the same as in the first scenario while CO₂ tax is taken into account. Results show that the second scenario has 29.5% lower average optimal COE and 2.5% lower average emission rate in comparison with the first scenario. A sensitivity analysis is also performed to study the effect of fuel price and CO₂ tax variations on optimal solutions.     

A thermodynamic analysis of a transcritical cycle with refrigerant mixture R32/R290 for a small heat pump water heater

In this study, a thermodynamic analysis on the performance of a transcritical cycle using azeotropic refrigerant mixtures of R32/R290 with mass fraction of 70/30 has been performed. The main purpose of this study is to theoretically verify the possibility of applying the chosen refrigerant mixture in small heat pumps for high temperature water heating applications. Performance evaluation has been carried out for a simple azeotropic mixture R32/R290 transcritical cycle by varying evaporator temperature, outlet temperature of gas cooler and compressor discharge pressure. Furthermore, the effects of an internal heat exchanger on the transcritical R32/R290 cycle have been presented at different operating conditions. The results show that high heating coefficient of performance (COP_h) and volumetric heating capacity can be achieved by using this transcritical cycle. It is desirable to apply the chosen refrigerant mixture R32/R290 in small heat pump water heater for high temperature water heating applications, which may produce hot water with temperature up to 90 °C.     

Combined air conditioning and tap water heating plant using CO2 as refrigerant

A combined air conditioning and tap water heating plant using carbon dioxide (CO₂) as refrigerant has been investigated theoretically and experimentally. The system is suitable for countries with year around cooling demand, such as Indonesia or Singapore, and a need for hot tap water. A unique CO₂ transcritical cycle characteristic for heating process can afford an improvement to a CO₂ air conditioning system when rejected heat from the system is recovered. Some parameters affecting performance of the combined system are discussed.     

Fine particle collection of an electrostatic precipitator in CO2-rich gas conditions for oxy-fuel combustion

The collection of particles in CO₂-enriched environments has long been important for the capture of CO₂ in order to clean gases via oxy-fuel combustion. We here report on the collection characteristics of fine and ultrafine particles using an electrostatic precipitator (ESP) in a CO₂-enriched atmosphere. In order to understand the characteristics of particle collection in CO₂-rich gas mixtures, the ionic properties of a CO₂-enriched atmosphere was also investigated. The electrical mobility of the ions in a CO₂-enriched atmosphere was found to be about 0.56 times that found in a conventional air atmosphere, due to the higher mass of CO₂ gas compared to that of air. The low electrical mobility of ions resulted in a low corona current under CO₂-enriched conditions. The collection efficiency of particles in a CO₂-rich atmosphere for a given power consumption was thus somewhat lower than that found in air, due to the low quantity of particle charging in CO₂-enriched air. At the same time, higher temperatures led to the higher electrical mobility of ions, which resulted in a greater collection efficiency for a given power. The presence of a negative corona also led to a greater collection efficiency of particles in an ESP than that achieved for a positive corona.     

Influence of secondary settling tank performance on suspended solids mass balance in activated sludge systems

Secondary settling is the final step of the activated sludge-based biological waste water treatment. Secondary settling tanks (SSTs) are therefore an essential unit of producing a clear effluent. A further important function of SSTs is the sufficient thickening to achieve highly concentrated return sludge and biomass within the biological reactor. In addition, the storage of activated sludge is also needed in case of peak flow events (Ekama et al., 1997). Due to the importance of a high SST performance the problem has long been investigated ( [Larsen, 1977], [Krebs, 1991], [Takács et?al., 1991], [Ekama et?al., 1997], [Freimann, 1999], [Patziger et?al., 2005] and [Bürger et?al., 2011]), however, a lot of questions are still to solve regarding e.g. the geometrical features (inflow, outflow) and operations (return sludge control, scraper mechanism, allowable maximum values of surface overflow rates). In our study we focused on SSTs under dynamic load considering both the overall unsteady behaviour and the features around the peaks, investigating the effect of various sludge return strategies as well as the inlet geometry on SST performance. The main research tool was a FLUENT-based novel mass transport model consisting of two modules, a 2D axisymmetric SST model and a mixed reactor model of the biological reactor (BR). The model was calibrated and verified against detailed measurements of flow and concentration patterns, sludge settling, accompanied with continuous on-line measurement of in- and outflow as well as returned flow rates of total suspended solids (TSS) and water.As to the inlet arrangement a reasonable modification of the geometry could result in the suppression of the large scale flow structures of the sludge-water interface thus providing a significant improvement in the SST performance. Furthermore, a critical value of the overflow rate (q_crit) was found at which a pronounced large scale circulation pattern develops in the vertical plane, the density current in such a way hitting the outer wall of the SST, turning then to the vertical direction accompanied with significant flow velocities. This phenomenon strengthens with the hydraulic load and can entrain part of the sludge thus resulting in unfavourable turbid effluent.As a representative case study an operating circular SST most commonly used in practice was investigated. Focusing on the sludge return strategies, it was found that up to a threshold peak flow rate the most efficient way is to keep the return sludge flow rate constant, at 0.4Q_MAX. However, once the inflow rate exceeds the threshold value the return sludge flow rate should be slowly increased up to 0.6Q_MAX, performed in a delayed manner, about 20-30 min after the threshold value is exceeded. For preserving the methodology outlined in the present paper, other types of SSTs, however, need further individual investigations.     

Analysis of principal gas products during combustion of polyether polyurethane foam at different irradiance levels

This paper studies the release of the principal gas species produced during the combustion of a non-flame-retarded Polyether Polyurethane Foam (PPUF) of density of 20.9 kg m⁻³ in the cone calorimeter. Five irradiance levels are studied: 10, 20, 30, 40 and 50 kW m⁻². Heat release rate, mass-loss rate and bulk gas mass flow are measured. The mass flow and yields of gas species are measured as well. The analysis of release of gas species relative to time allowed the study of the different stages of PPUF kinetics and to quantify the gas composition. Of the twenty-two different gas species that were monitored simultaneously, the principal species found were CO₂, CO, H₂O, NO and total hydrocarbons. According to species release, two decomposition stages for PPUF are identified. In the first stage, the solid structure breaks down carrying the decomposition of isocyanate, and in the second stage the polyol decomposes. These two stages are in agreement with the decomposition mechanism proposed in the literature. However, the data presented here is the first experimental study of burning behavior of PPUF taking into account the release of gas species too. An elemental analysis was performed and the chemical formula of the virgin material was determined. This allows the mass balance of the elements in the virgin foam content with the gaseous product content. The effective heat of combustion and the ratio between heat release rate and CO₂ mass flow are calculated at each of the irradiance levels.     

Heterotrophic microorganism Rhodotorula mucilaginosa R30 improves tannery sludge bioleaching through elevating dissolved CO2 and extracellular polymeric substances levels in bioleach solution as well as scavenging toxic DOM to Acidithiobacillus species

The introduction of acid-tolerant heterotrophic microorganisms into sludge bioleaching systems has been proven effective in improving sludge bioleaching processes, and such positive effect is mainly attributed to the biodegradation of low molecular weight organic acids or sludge dissolved organic matter (DOM) toxic to Acidithiobacillus species by the heterotrophic microorganisms introduced. Here we report that elevated dissolved CO₂ concentration and resulting extracellular polymeric substances (EPS) in bioleach solution due to the incorporation of heterotrophic microorganisms also play important roles in improving sludge bioleaching. It was found that in tannery sludge bioleaching system coinoculated with Rhodotorula mucilaginosa R30 and Acidithiobacillus species, dissolved CO₂ concentration in bioleach solution can be elevated from 0.23-0.54 mg/L to 0.76-1.01 mg/L compared to the control inoculated only with Acidithiobacillus species. Correspondingly, the distinct degradation of sludge DOM was also observed in this experiment. It was experimentally demonstrated that the accumulation of CO₂ did greatly enhance the growth of Acidithiobacillus thiooxidans and the decrease rate of pH in the medium. In addition, EPS derived from R. mucilaginosa R30 could bind readily Fe³⁺ in bioleach solution with maximum binding capacity (MBC) of 0.82 mg Fe³⁺ by per mg DOC of EPS secreted and the oxidization activity of EPS-bound Fe³⁺ was decreased but not totally inhibited, indicating that the formation of soluble EPS-Fe(III) complexes enhances, to a certain extent, bioleaching efficiency due to maintaining Fe³⁺ level in solution by inhibiting Fe precipitation occurrence.     

Performance analysis of a transcritical N2O refrigeration cycle with vortex tube

In the present study, vortex tube is used in transcritical vapour compression cycle as expansion device to improve the coefficient of performance (COP). The thermodynamic analysis has been performed using nitrous oxide in transcritical cycle with vortex tube (TCVT) and its results are compared with those of a transcritical cycle with expansion valve (TCEV). The evaporator and the gas cooler temperatures have been varied between ?55°C and 5°C and between 35°C and 60°C, respectively, for the analysis. The COP of the TCVT improves by 1.72–27.01% compared to TCEV. A decrease in evaporator temperature and an increase in gas cooler exit temperature result in a decrease in COP. The increase in cold mass fraction brings a negligible increase in maximum COP. The performance comparison of N₂O and CO₂ in TCVT shows that maximum cooling COP for N₂O is higher than for CO₂, but the optimum pressure required for N₂O is lower than for CO₂.     

Effect of green roof on ambient CO2 concentration

Plants can improve air quality by removing pollutants. The air purification capability of plants has been receiving increasing attention because of the rapid deterioration of the environment. However, research on evaluating quantitatively the effect of plants on the environmental pollutant concentrations is still scarce. This paper studies the effect of a green roof on the ambient CO₂ concentration as an example to assess the benefit of urban greening. The study comprises three parts: (1) Field measurement of the difference of CO₂ concentration at a location in the middle of the plants in a small plot of green roof and one in the surrounding area, (2) Experiments to measure the plant’s CO₂ absorption velocity and emission rate using a sealed glass chamber; and (3) Computer simulation of the CO₂ concentration distribution around a green roof using the measured CO₂ absorption velocity and emission rate to quantify the effects of the green roof on the ambient CO₂ concentration.     

Performance characteristics of a two-stage transcritical N2O refrigeration cycle with vortex tube

ABSTRACT

The thermodynamic analysis has been presented in this article using nitrous oxide as the refrigerant in a two-stage transcritical cycle with the vortex tube (TSTCVT) instead of the expansion valve and its results are compared with the two-stage transcritical cycle with the expansion valve (TSTCEV). The evaporator and the gas cooler temperature ranges in both the cycles have been considered between ?55°C to 5°C and 35°C to 60°C for the analysis. Gas cooler and intercooler pressures are simultaneously optimised to obtain the maximum cooling coefficient of performance (COP). The COP of the TSTCVT improves by 1.97–27.19% in comparison to TSTCEV. A decrease in evaporator temperature and an increase in gas cooler exit temperature reduce the COP of TSTCVT. The comparison of refrigerants N₂O and CO₂ in TSTCVT shows that N₂O exhibits higher cooling COP, higher second law of efficiency and lower optimum gas cooler pressure under the considered operating conditions.     

Long-term continuous monitoring of the dissolved CO2 performed by using a new device in groundwater of the Mt. Etna (southern Italy)

We present a new device for continuous monitoring of the concentration of CO₂ dissolved in water. The device consists of a tube made of a polymeric semi-permeable membrane connected to an infrared gas analyser (IRGA) and a pump. Several laboratory experiments were performed to set the best operating condition and test the accuracy of measurements. We used the device for performing 20 months of continuous monitoring of dissolved CO₂ concentration (DCC) in groundwater within a drainage gallery at Mt. Etna. The monitored groundwater intercepts the Pernicana Fault, along which degassing is observed related to volcano-tectonic activity. The acquired data were compared with continuous and discrete data obtained using existing methods. The measurements of DCC resulted in some period of the year well correlated with air temperature. We also found that long-term trends, as well as short-term variations, are probably linked to the dynamics of volcanic activity and/or perturbations in the local or regional stress fields.     

Methods for the limit state design of triangular steel gusset plates

This paper compares the design methods of Salmon et al. [2], the draft British Steel Code[3] and the author, with 15 experimental results by Salmon et al.[2] and 50 experimental results by the author.The draft British Steel Code[3] method is shown to be based on the wrong assumptions, is unsafe and inaccurate.The Salmon et al.[2] method based on the correct mode of failure is conservative and not very accurate. The ratio of _u experimental/P_u theory = 1.92 with a coefficient of variation of 0.64 for 22 experimental results for non slender gussets. Other experimental results for slender gussets are compared graphically.A method developed by the author is less conservative and more accurate. The ratio of P_u experiment/P_u theory = 1.45 with a coefficient of variation of 0.14 for 22 experimental results for non-slender gussets. Other experimental results for slender gussets are compared graphically. An alternative approximate method by the author for gussets where the slenderness ratio of the free edge is less than 185 is shown to be of comparable accuracy.The gusset plate aspect ratio H/L required to produce optimum weight design is shown to be approximately one.     

Effects of flow and water chemistry on lead release rates from pipe scales

Lead release from pipe scales was investigated under different water compositions, stagnation times, and flow regimes. Pipe scales containing PbO₂ and hydrocerussite (Pb₃(OH)₂(CO₃)₂) were developed on lead pipes by conditioning the pipes with water containing free chlorine for eight months. Water chemistry and the composition of the pipe scales are two key factors affecting lead release from pipe scales. The water rarely reached equilibrium with pipe scales within one day, which makes solid-water contact time and corrosion product dissolution rates the controlling factors of lead concentrations for the conditions tested. Among five water compositions studied, a solution with orthophosphate had the lowest dissolved lead release rate and highest particulate lead release rate. Free chlorine also decreased the dissolved lead release rate at stagnant conditions. Water flow increased rates of release of both dissolved and particulate lead by accelerating the mass transfer of lead out of the porous pipe scales and by physically destabilizing pipe scales. Dissolved lead comprised the majority of the lead released at both stagnant and laminar flow conditions.     

Field Evaluation of CO2 Detector Tubes for Measuring Outdoor Air Supply Rate in the Indoor Environment

Indoor carbon dioxide (CO₂) concentration can be used to estimate the degree of air recirculation and outdoor air supply rate. Three types of CO₂ detector tubes were evaluated by using Fourier Transform Infra-Red (FTIR) Spectroscopy as a reference method. Two types of detector tubes (Draeger CH 30801 and Kitagawa 126 B) showed a good correlation with the reference method (r = 0.98), the 95% confidence interval of the slope being 0.89-1.06 and 0.80-0.95, respectively in linear regression analysis. The third type (Gastec 2LL.) showed lower correlation (r = 0.91) and a wider 95% confidence interval (0.52-0.80) of the slope. A t CO₂ concentrations in the range 800-1000 5 l/l(ppm), control values suggested for the indoor environment, the Draeger and the Gastec tubes underestimated the CO₂ concentration, while the Kitagawa tube showed a correct value. The difference in reading between observers was similar for all three brands of detector tubes (5-7%), expressed as relative standard error No significant influence of the air humidity or temperature on the readings could be demonstrated. It is concluded that some brands of CO₂ detector tubes can be used to measure indoor carbon dioxide concentration with sufficient precision and accuracy. Since the relative error is relatively large at lower CO₂ concentrations, the use of such tubes for the determination of air recirculation in ventilation systems should be avoided. As a crude estimate of the outdoor air supply rate, however, CO₂ detector tubes may be used. In order to minimize the error in reading, the type of detector tube and the need for recalibration should be considered. When using CO₂ measurements as an estimate of outdoor air supply rate, the influence of age and work-load on the individual's emission of CO₂ and the time needed to reach equilibrium, should also be taken into consideration.     

Reacteur a cellules nitrifiantes immobilisees sur garnissage aere a co- ou contre-courant. Etude experimentale et modelisationNitrification by attached-cell reactors aerated at co- or counter-current. Experimental data and modelling

Attached-cell reactors using a bed of granular material for wastewater treatment develop a high biomass concentration which allows an important reduction of the required residence time (Jeris et al., 1977; Elmaleh, 1982). In nitrification of ammonia containing wastewater, oxygen is currently the limiting substrate; in theory, 4.18 g of oxygen are required per 1 g of nitrogen (Painter, 1970). Oxygen can be added with hydrogen peroxide (Grigoropolou, 1980; Seropian, 1980; Yahi et al., 1982) which is nevertheless expensive and it seems better to transfer oxygen from a gas phase, i.e. air, to the liquid phase through a fixed bed (Charpentier, 1976).Two attached-cell reactors (Fig. 1) were operated in parallel for nitrification of ammonia containing synthetic wastewater (Table 2). Air was upflowed through a granular packing (Table 1) maintained in fixed bed while the liquid influent was injected at co- or counter-current.

1. (1) Owing to the high oxygen transfer properties of the system and to the fact that the thickness of biofilm is always less than 100 μm, the whole process was not limited by oxygen concentration of which remained larger than 7 mg l⁻¹ (Fig. 2a) (Bungay et al., 1969). Oxidised nitrogen ammonia is completely converted into nitrate (Fig. 2b). Experimental conditions are given in Table 3.

2. (2) The plot of ammonia conversion against air superficial velocity shows a maximum (Fig. 3) after which conversion decreases rapidly by overloading of the packing (Prost, 1965). Experimental conditions are given in Table 4.

3. (3) Process efficiency decreases when superficial upflow velocity is increased (Fig. 4).

4. (4) Complete abatement of inlet pollution is reached when nitrogen concentration is less than 25 mg l⁻¹ (Fig. 5) which corresponds to a volumetric loading up to 0.6 kg N (NH₄⁺) m⁻³ day⁻¹.

Moreover, the experimental data were fitted to a model based on classical assumptions (Roques, 1980; Grady, 1982; Atkinson and Fowler, 1974; Grasmick et al., 1979; Grasmick, 1982; Harremoes, 1976, 1978; Jennings et al., 1976; Williamson and MacCarty, 1976); i.e. zero order intrinsic kinetics and diffusion transport (Table 5), and recently developed (Grasmick, 1982; Rodrigues et al., 1984). This model provides, particularly, a very easy method to check its own use—in reaction regime and in diffusion regime—when time spans or inlet concentration are changed; experimental results can indeed be plotted in such a way that straight lines are obtained (Table 6). Figures 6 and 7 show the data obtained with the counter-current nitrification reactor when respectively inlet concentration and time spans are varied. The plotted straight lines show that the overall reaction is zero order and that, therefore, the biofilm is fully penetrated. A critical time span θ_c and a critical inlet concentration C_c, for which complete conversion is achieved, are then calculated, θ_c is theoretically proportional to C₁ which is verified in Fig. 8. The straight line θ_c vs C₁ can then be used in reactor design.