Showing papers in "Applied Thermal Engineering in 2009"

Fluid selection for a low-temperature solar organic Rankine cycle

Abstract: Theoretical performances as well as thermodynamic and environmental properties of few fluids have been comparatively assessed for use in low-temperature solar organic Rankine cycle systems. Efficiencies, volume flow rate, mass flow rate, pressure ratio, toxicity, flammability, ODP and GWP were used for comparison. Of 20 fluids investigated, R134a appears as the most suitable for small scale solar applications. R152a, R600a, R600 and R290 offer attractive performances but need safety precautions, owing to their flammability.

Biodiesel from cotton seed oil and its effect on engine performance and exhaust emissions

Abstract: The use of biodiesel is rapidly expanding around the world, making it imperative to fully understand the impacts of biodiesel on the diesel engine combustion process and pollutant formation. Biodiesel is known as the mono-alkyl-esters of long chain fatty acids derived from renewable feedstocks, such as, vegetable oils or animal fats, for use in compression ignition engines. Different parameters for the optimization of biodiesel production were investigated in the first phase of this study, while in the next phase of the study performance test of a diesel engine with neat diesel fuel and biodiesel mixtures were carried out. Biodiesel was made by the well known transesterification process. Cottonseed oil (CSO) was selected for biodiesel production. Cottonseed is non-edible oil, thus food versus fuel conflict will not arise if this is used for biodiesel production. The transesterification results showed that with the variation of catalyst, methanol or ethanol, variation of biodiesel production was realized. However, the optimum conditions for biodiesel production are suggested in this paper. A maximum of 77% biodiesel was produced with 20% methanol in presence of 0.5% sodium hydroxide. The engine experimental results showed that exhaust emissions including carbon monoxide (CO) particulate matter (PM) and smoke emissions were reduced for all biodiesel mixtures. However, a slight increase in oxides of nitrogen (NOx) emission was experienced for biodiesel mixtures.

Testing and modeling a scroll expander integrated into an Organic Rankine Cycle

Abstract: Organic Rankine Cycles (ORC’s) are particularly suitable for recovering energy from low-grade heat sources. This paper first presents the results of an experimental study carried out on a prototype of an open-drive oil-free scroll expander integrated into an ORC working with refrigerant HCFC-123. By exploiting the overall expander performance measurements, the eight parameters of a scroll expander semi-empirical model are then identified. The model is able to compute variables of first importance such as the mass flow rate, the delivered shaft power and the discharge temperature, and secondary variables such as the supply heating-up, the exhaust cooling-down, the ambient losses, the internal leakage and the mechanical losses. The maximum deviation between the predictions by the model and the measurements is 2% for the mass flow rate, 5% for the shaft power and 3 K for the discharge temperature. The validated model of the expander is finally used to quantify the different losses and to indicate how the design of the expander might be altered to achieve better performances. This analysis pointed out that the internal leakages and, to a lesser extent, the supply pressure drop and the mechanical losses are the main losses affecting the performance of the expander.

Energetic and economic investigation of Organic Rankine Cycle applications

Abstract: The use of organic working fluids for the realization of the so called Organic Rankine Cycle (ORC) has been proven to be a promising solution for decentralized combined heat and power production (CHP). The process allows the use of low temperature heat sources, offering an advantageous efficiency in small-scale applications. This is the reason why the number of geothermal and biomass fired power plants based on this technology have been increased within the last years. The favourable characteristics of ORC make them suitable for being integrated in applications like solar desalination with reverse osmosis system, waste heat recovery from biogas digestion plants or micro-CHP systems. In this paper, the state of the art of ORC applications will be presented together with innovative systems which have been simulated in a process simulation environment using experimental data. The results of the simulation like efficiencies, water production rates or achievable electricity production cost will be presented and discussed.

Numerical investigation of nanofluids forced convection in circular tubes

Abstract: In this paper, developing laminar forced convection flow of a water–Al2O3 nanofluid in a circular tube, submitted to a constant and uniform heat flux at the wall, is numerically investigated. A single- and two-phase model (discrete particles model) is employed with either constant or temperature-dependent properties. The investigation is accomplished for size particles equal to 100 nm. The maximum difference in the average heat transfer coefficient between single- and two-phase models results is about 11%. Convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement increases with the particle volume concentration, but it is accompanied by increasing wall shear stress values. Higher heat transfer coefficients and lower shear stresses are detected in the case of temperature dependents models. The heat transfer always improves, as Reynolds number increases, but it is accompanied by an increase of shear stress too. Moreover a comparison with data present in the literature is carried out.

Energy and exergy analysis of a steam power plant in Jordan

Abstract: In this study, the energy and exergy analysis of Al-Hussein power plant in Jordan is presented. The primary objectives of this paper are to analyze the system components separately and to identify and quantify the sites having largest energy and exergy losses. In addition, the effect of varying the reference environment state on this analysis will also be presented. The performance of the plant was estimated by a component-wise modeling and a detailed break-up of energy and exergy losses for the considered plant has been presented. Energy losses mainly occurred in the condenser where 134 MW is lost to the environment while only 13 MW was lost from the boiler system. The percentage ratio of the exergy destruction to the total exergy destruction was found to be maximum in the boiler system (77%) followed by the turbine (13%), and then the forced draft fan condenser (9%). In addition, the calculated thermal efficiency based on the lower heating value of fuel was 26% while the exergy efficiency of the power cycle was 25%. For a moderate change in the reference environment state temperature, no drastic change was noticed in the performance of major components and the main conclusion remained the same; the boiler is the major source of irreversibilities in the power plant. Chemical reaction is the most significant source of exergy destruction in a boiler system which can be reduced by preheating the combustion air and reducing the air–fuel ratio.

Development of small-scale and micro-scale biomass-fuelled CHP systems – A literature review

Abstract: A review is carried out on the development of small- and micro-scale biomass-fuelled combined heat and power (CHP) systems. Discussions have been concentrated on the current application of Organic Rankine Cycle (ORC) in small- and micro-scale biomass-fuelled CHP systems. Comparisons have been made between ORC and other technologies such as biomass gasification and micro-turbine based biomass-fuelled CHP systems. The advantages and disadvantages of each technology have been discussed. Recommendations have been made on the future development of small- and micro-scale biomass-fuelled CHP.

Thermal management issues in a PEMFC stack – A brief review of current status

Abstract: Understanding the thermal effects is critical in optimizing the performance and durability of proton exchange membrane fuel cells (PEMFCs). A PEMFC produces a similar amount of waste heat to its electric power output and tolerates only a small deviation in temperature from its design point. The balance between the heat production and its removal determines the operating temperature of a PEMFC. These stringent thermal requirements present a significant heat transfer challenge. In this work, the fundamental heat transfer mechanisms at PEMFC component level (including polymer electrolyte, catalyst layers, gas diffusion media and bipolar plates) are briefly reviewed. The current status of PEMFC cooling technology is also reviewed and research needs are identified.

Food transport refrigeration – Approaches to reduce energy consumption and environmental impacts of road transport

Abstract: Food transport refrigeration is a critical link in the food chain not only in terms of maintaining the temperature integrity of the transported products but also its impact on energy consumption and CO 2 emissions. This paper provides a review of (a) current approaches in road food transport refrigeration, (b) estimates of their environmental impacts, and (c) research on the development and application of alternative technologies to vapour compression refrigeration systems that have the potential to reduce the overall energy consumption and environmental impacts. The review and analysis indicate that greenhouse gas emissions from conventional diesel engine driven vapour compression refrigeration systems commonly employed in food transport refrigeration can be as high as 40% of the greenhouse gas emissions from the vehicle’s engine. For articulated vehicles over 33 ton, which are responsible for over 80% of refrigerated food transportation in the UK, the reject heat available form the engine is sufficient to drive sorption refrigeration systems and satisfy most of the refrigeration requirements of the vehicle. Other promising technologies that can lead to a reduction in CO 2 emissions are air cycle refrigeration and hybrid systems in which conventional refrigeration technologies are integrated with thermal energy storage. For these systems, however, to effectively compete with diesel driven vapour compression systems, further research and development work is needed to improve their efficiency and reduce their weight.

Thermal effects in packaging high power light emitting diode arrays

Abstract: The package and system level temperature distributions of a high power (>1 W) light emitting diode (LED) array have been investigated using numerical heat flow models. For this analysis, a thermal resistor network model was combined with a 3D finite element submodel of an LED structure to predict system and die level temperatures. The impact of LED array density, LED power density, and active versus passive cooling methods on device operation were calculated. In order to help understand the role of various thermal resistances in cooling such compact arrays, the thermal resistance network was analyzed in order to estimate the contributions from materials as well as active and passive cooling schemes. Finally, an analysis of a ceramic packaging architecture is performed in order to give insight into methods to reduce the packaging resistance for high power LEDs.

Experimental investigation on the performance and emissions of a diesel engine fuelled with ethanol–diesel blends

Abstract: An experimental investigation on the application of the blends of ethanol with diesel to a diesel engine was carried out. First, the solubility of ethanol and diesel was conducted with and without the additive of normal butanol (n-butanol). Furthermore, experimental tests were carried out to study the performance and emissions of the engine fuelled with the blends compared with those fuelled by diesel. The test results show that it is feasible and applicable for the blends with n-butanol to replace pure diesel as the fuel for diesel engine; the thermal efficiencies of the engine fuelled by the blends were comparable with that fuelled by diesel, with some increase of fuel consumptions, which is due to the lower heating value of ethanol. The characteristics of the emissions were also studied. Fuelled by the blends, it is found that the smoke emissions from the engine fuelled by the blends were all lower than that fuelled by diesel; the carbon monoxide (CO) were reduced when the engine ran at and above its half loads, but were increased at low loads and low speed; the hydrocarbon (HC) emissions were all higher except for the top loads at high speed; the nitrogen oxides (NOx) emissions were different for different speeds, loads and blends.

Optimal operation strategies of compressed air energy storage (CAES) on electricity spot markets with fluctuating prices

Abstract: Compressed air energy storage (CAES) technologies can be used for load levelling in the electricity supply and are therefore often considered for future energy systems with a high share of fluctuating renewable energy sources, such as e.g. wind power. In such systems, CAES plants will often operate on electricity spot markets by storing energy when electricity prices are low and producing electricity when prices are high. In order to make a profit on such markets, CAES plant operators have to identify proper strategies to decide when to sell and when to buy electricity. This paper describes three independent computer-based methodologies which may be used for identifying the optimal operation strategy for a given CAES plant, on a given spot market and in a given year. The optimal strategy is identified as the one which provides the best business-economic net earnings for the plant. In practice, CAES plants will not be able to achieve such optimal operation, since the fluctuations of spot market prices in the coming hours and days are not known. Consequently, two simple practical strategies have been identified and compared to the results of the optimal strategy. This comparison shows that, in practice, a CAES plant can be expected to earn 80-90 per cent of the optimal earnings.

Numerical investigation of geometry parameters for design of high performance ejectors

Abstract: In this study, Computational Fluid Dynamics (CFD) technique is employed to investigate the effects of two important ejector geometry parameters: the primary Nozzle Exit Position (NXP) and the mixing section converging angle θ , on its performance. A CFD model is firstly established and calibrated by actual experimental data, and then used to create 95 different ejector geometries and tested under different working conditions. From 210 testing results, it is found that the optimum NXP is not only proportional to the mixing section throat diameter, but also increases as the primary flow pressure rises. On the other hand, the ejector performance is very sensitive to θ especially near the optimum working point. The entrainment ratio can vary as much as 26.6% by changing θ . A relatively bigger θ is required to better maximize the ejector performance when the primary flow pressure rises. The significance of the study is that these findings can be used to guide the adjustment of NXP and θ in order to obtain the best ejector system performance when the operating conditions are different from the on-design conditions.

Single-phase heat transfer in microchannels The importance of scaling effects

Abstract: Microscale single-phase heat transfer is widely used in industrial and scientific applications and for this reason, many related papers have been published in the last two decades. Nevertheless, inconsistencies between published results still exist and there is no generally accepted model for the prediction of single-phase heat transfer in microchannels. This paper presents a review of the experimental and numerical results available in the open literature. Heat transfer in microchannels can be suitably described by standard theory and correlations, but scaling effects (entrance effects, conjugate heat transfer, viscous heating, electric double layer (EDL) effects, temperature dependent properties, surface roughness, rarefaction and compressibility effects), often negligible in macro-channels, may now have a significant influence and have to be accounted for. Furthermore, measurement uncertainties may be more important, due to the reduced characteristic dimensions, so have to be accurately checked and, where possible, reduced. Experiments with single channels are more accurate and in good agreement with predictions from published correlations, in contrast to multi-(parallel) channel experiments. The latter are subject to maldistribution, 3D conjugate heat transfer effects and larger measurement uncertainties. Sub-continuum mathematical models for fluid dynamics are briefly reviewed and explained. These models are expected to gain a growing interest in the near future due to the rapid descent of microchannel dimensions down to the nano-scale. The paper concludes with a concise set of recommendations for purposes of performance and design. For single channels, available correlations for macro-channels can also give reliable predictions at the micro-scale, but only if all the scaling effects can be considered negligible. Otherwise, when scaling effects cannot be neglected or for the case of heat exchangers with parallel channels, suitable numerical simulations may be the sole alternative to carefully designed experiments to evaluate the heat transfer rates.

Numerical investigation into the convective heat transfer of TiO2 nanofluids flowing through a straight tube under the laminar flow conditions

Abstract: A numerical study has been performed by using both single phase method and combined Euler and Lagrange method on the convective heat transfer of TiO2 nanofluids flowing through a straight tube under the laminar flow conditions. The effects of nanoparticles concentrations, Reynolds number, and various nanoparticle aggregates sizes are investigated on the flow and the convective heat transfer behaviour. The results show significant enhancement of heat transfer of nanofluids particularly in the entrance region. The numerical results are compared with the experimental data and reasonable good agreement is achieved.

Numerical study of laminar heat transfer and pressure drop characteristics in a water-cooled minichannel heat sink

Abstract: With the rapid development of the information technology (IT) industry, the heat flux in integrated circuit (IC) chips cooled by air has almost reached its limit about 100 W/cm2. Some applications in high technologies require heat fluxes well beyond such a limitation. Therefore the search of a more efficient cooling technology becomes one of the bottleneck problems of the further development of IT industry. The microchannel flow geometry offers large surface area of heat transfer and a high convective heat transfer coefficient. However, it has been hard to implement because of its very high pressure head required to pump the coolant fluid though the channels. A normal channel could not give high heat flux although the pressure drop is very small. A minichannel can be used in heat sink with a quite high heat flux and a mild pressure loss. A minichannel heat sink with bottom size of 20 mm × 20 mm is analyzed numerically for the single-phase laminar flow of water as coolant through small hydraulic diameters and a constant heat flux boundary condition is assumed. The effects of channel dimensions, channel wall thickness, bottom thickness and inlet velocity on the pressure drop, thermal resistance and the maximum allowable heat flux are presented. The results indicate that a narrow and deep channel with thin bottom thickness and relatively thin channel wall thickness results in improved heat transfer performance with a relatively high but acceptable pressure drop. A nearly-optimized configuration of heat sink is found which can cool a chip with heat flux of 256 W/cm2 at the pumping power of 0.205 W. The nearly-optimized configuration is verified by an orthogonal design. The simulated thermal resistance agrees quite well with the result of conventional correlations method with the maximum difference of 12%.

Exergy analysis, parametric analysis and optimization for a novel combined power and ejector refrigeration cycle

Abstract: A new combined power and refrigeration cycle is proposed, which combines the Rankine cycle and the ejector refrigeration cycle. This combined cycle produces both power output and refrigeration output simultaneously. It can be driven by the flue gas of gas turbine or engine, solar energy, geothermal energy and industrial waste heats. An exergy analysis is performed to guide the thermodynamic improvement for this cycle. And a parametric analysis is conducted to evaluate the effects of the key thermodynamic parameters on the performance of the combined cycle. In addition, a parameter optimization is achieved by means of genetic algorithm to reach the maximum exergy efficiency. The results show that the biggest exergy loss due to the irreversibility occurs in heat addition processes, and the ejector causes the next largest exergy loss. It is also shown that the turbine inlet pressure, the turbine back pressure, the condenser temperature and the evaporator temperature have significant effects on the turbine power output, refrigeration output and exergy efficiency of the combined cycle. The optimized exergy efficiency is 27.10% under the given condition.

Performance, emission and combustion characteristics of a DI diesel engine using waste plastic oil

Abstract: Increase in energy demand, stringent emission norms and depletion of oil resources have led the researchers to find alternative fuels for internal combustion engines. On the other hand waste plastic pose a very serious environment challenge because of their disposal problems all over the world. Plastics have now become indispensable materials in the modern world and application in the industrial field is continually increasing. In this context, waste plastic solid is currently receiving renewed interest. The properties of the oil derived from waste plastics were analyzed and compared with the petroleum products and found that it has properties similar to that of diesel. In the present work, waste plastic oil was used as an alternate fuel in a DI diesel engine without any modification. The present investigation was to study the performance, emission and combustion characteristics of a single cylinder, four-stroke, air-cooled DI diesel engine run with waste plastic oil. The experimental results have showed a stable performance with brake thermal efficiency similar to that of diesel. Carbon dioxide and unburned hydrocarbon were marginally higher than that of the diesel baseline. The toxic gas carbon monoxide emission of waste plastic oil was higher than diesel. Smoke reduced by about 40% to 50% in waste plastic oil at all loads.

A combined model for the effective thermal conductivity of nanofluids

Abstract: This paper presents a combined static and dynamic mechanisms-based model for predicting the effective thermal conductivity of nanofluids. The model includes the effects of particle size, nanolayer, Brownian motion, and particle surface chemistry and interaction potential which are the static and dynamic mechanisms responsible for the enhanced effective thermal conductivity of nanofluids. Present model shows reasonably good agreement with the experimental results of several types of nanofluids and gives better predictions compared to the existing models.

Integration of Kalina cycle in a combined heat and power plant, a case study

Abstract: This paper presents the integration of the Kalina cycle process in a combined heat and power plant for improvement of efficiency. In combined heat and power plants, the heat of flue gases is often available at low temperatures. This low-grade waste heat cannot be used for steam production and therefore power generation by a conventional steam cycle. Moreover, the steam supply for the purpose of heating is mostly exhausted, and therefore the waste heat at a low-grade temperature is not usable for heating. If other measures to increase the efficiency of a power plant process, like feed-water heating or combustion air heating, have been exhausted, alternative ways to generate electricity like the Kalina cycle process offer an interesting option. This process maximizes the generated electricity with recovery of heat and without demand of additional fuels by integration in existing plants. The calculations show that the net efficiency of an integrated Kalina plant is between 12.3% and 17.1% depending on the cooling water temperature and the ammonia content in the basic solution. The gross electricity power is between 320 and 440 kW for 2.3 MW of heat input to the process. The gross efficiency is between 13.5% and 18.8%.

Design optimization of shell and tube heat exchangers using global sensitivity analysis and harmony search algorithm

Abstract: This study explores the use of global sensitivity analysis (GSA) and harmony search algorithm (HSA) for design optimization of shell and tube heat exchangers (STHXs) from the economic viewpoint. To reduce the size of the optimization problem, non-influential geometrical parameters which have the least effect on total cost of STHXs are identified using GSA. The HSA which is a meta-heuristic based algorithm is then applied to optimize the influential geometrical parameters. To demonstrate the effectiveness and accuracy of the proposed algorithm, an illustrative example is studied. Comparing the HSA results with those obtained using genetic algorithm (GA) reveals that the HSA can converge to optimum solution with higher accuracy.

CFD analysis of a supersonic air ejector. Part I: Experimental validation of single-phase and two-phase operation

Abstract: This paper deals with comparisons between CFD and experiments for a supersonic ejector. Good results are presented in terms of entrainment rate compared to home-made experimental data for an air ejector. Over the whole range of operating conditions, the overall deviation is below 10% for the k-epsilon model, while the results for the k-omega-sst model are in a less agreement. Furthermore, it is demonstrated that the validation of a global performance parameter, such as the entrainment rate, is not sufficient for a correct assessment even though a wide range of operation is tried out. Indeed, for exactly the same prediction both models may give very different local flow features. Finally, a first attempt to tackle two-phase aspects was also performed experimentally and by simulations. (C) 2008 Elsevier Ltd. All rights reserved.

Thermal conductivity enhancement of carbon fiber composites

Abstract: The effective thermal conductivity enhancement of carbon fiber composites is investigated in this contribution using a three-dimensional numerical method. First a more realistic three-dimensional distribution of fibers dispersed in a matrix phase is reproduced by a developed random generation-growth method to eliminate the overrated inter-fiber contacts by the two-dimensional simulations. The energy transport governing equations are then solved through the three-dimensional structures using a high-efficiency lattice Boltzmann scheme. The resultant predictions agree well with the available experimental data. Compared with the existing theoretical models, the present method does not depend upon empirical parameters which have to be determined case by case, so that it is useful for design and optimization for new materials, beyond prediction and analysis just for existing composites.

The investigation of groove geometry effect on heat transfer for internally grooved tubes

Abstract: An experimental study of surface heat transfer and friction characteristics of a fully developed turbulent air flow in different grooved tubes is reported Tests were performed for Reynolds number range 10,000–38,000 and for different geometric groove shapes (circular, trapezoidal and rectangular) The ratio of tube length-to-diameter is 33 Among the grooved tubes, heat transfer enhancement is obtained up to 63% for circular groove, 58% for trapezoidal groove and 47% for rectangular groove, in comparison with the smooth tube at the highest Reynolds number (Re = 38,000) Correlations of heat transfer and friction coefficient were obtained for different grooved tubes In evaluation of thermal performance, it is seen that the grooved tubes are thermodynamically advantageous (Ns, a

Experimental investigation of nanofluids on sintered heat pipe thermal performance

Abstract: Dilute dispersion of silver nano-particles in pure water was employed as the working fluid for conventional 1 mm wick-thickness sintered circular heat pipe. The nanofluid used in present study is an aqueous solution of 10 and 35 nm diameter silver nano-particles. The experiment was performed to measure the temperature distribution and compare the heat pipe temperature difference using nanofluid and DI-water. The tested nano-particle concentrations ranged from 1, 10 and 100 mg/l. The condenser section of the heat pipe was attached to a heat sink that was cooled by water supplied from a constant temperature bath maintained at 40 °C. At a same charge volume, the measured nanofluids filled heat pipe temperature distribution demonstrated that the temperature difference decreased 0.56–0.65 °C compared to DI-water at an input power of 30–50 W. In addition, the nanofluid as working medium in heat pipe can up to 70 W and is higher than pure water about 20 W.

Experimental studies on heat transfer and friction factor characteristics of thermosyphon solar water heater system fitted with spacer at the trailing edge of twisted tapes

Abstract: Experimental investigation of heat transfer and friction factor characteristics of thermosyphon solar water heater system with full- length twist, twist fitted with rod and spacer fitted at the trailing edge for lengths of 100, 200 and 300 mm for twist ratio 3 and 5 has been carried out and compared with plain tube collector for the same operating conditions. The experimental data for Nusselt number and friction factor for plain tube collector is verified with fundamental equations. Empirical correlations are developed for Nusselt number and friction factor for twist ratio 3 and 5. Results conclude that the decrease in Nusselt number for full length helical twist compared to twist fitted with rod is minimum and is quite significant for twist with spacer. But the decrease in friction factor is maximum in twist fitted with spacer compared to twist fitted with rod. The over all performance for twist fitted with rod is found to be better than twist fitted with spacer.

Theoretical analysis on heat and mass transfer in a direct evaporative cooler

Abstract: The heat and mass transfer between air and water film in the direct evaporative cooler is theoretically analyzed in present paper. A simplified cooling efficiency correlation is proposed based on the energy balance analysis of air. The correlation may be applied to the water-drip cross-flow direct evaporative cooler, in which the wet special durable papers with different wave angles form the air channel. The Influences of the air frontal velocity and the thickness of pad module on the cooling efficiency of a direct evaporative cooler are discussed. An optimum frontal velocity of 2.5 m/s is recommended to decide the frontal area of pad module in the given air flow. The simplified correlation of cooling efficiency is validated by the test results of a direct evaporative cooler.

Analysis of chimney height for solar chimney power plant

Abstract: Current in solar chimney power plant that drives turbine generators to generate electricity is driven by buoyancy resulting from higher temperature than the surroundings at different heights. In this paper, the maximum chimney height for convection avoiding negative buoyancy at the latter chimney and the optimal chimney height for maximum power output are presented and analyzed using a theoretical model validated with the measurements of the only one prototype in Manzanares. The results based on the Manzanares prototype show that as standard lapse rate of atmospheric temperature is used, the maximum power output of 102.2 kW is obtained for the optimal chimney height of 615 m, which is lower than the maximum chimney height with a power output of 92.3 kW. Sensitivity analyses are also performed to examine the influence of various lapse rates of atmospheric temperatures and collector radii on maximum height of chimney. The results show that maximum height gradually increases with the lapse rate increasing and go to infinity at a value of around 0.0098 K m � 1 , and that the maximum height for convection and optimal height for maximum power output increase with larger collector

Development of a numerical model for the simulation of vertical U-tube ground heat exchangers

Abstract: Vertical U-tube ground heat exchangers are a key component in geothermal energy utilization systems like ground source heat pumps (GSHPs). This paper presents a three-dimensional unstructured finite volume model for them. The model uses Delaunay triangulation method to mesh the cross-section domain of the borefield (borehole field), and consequently may intactly retain the geometric structure in the borehole. To further improve the computational accuracy, the soil is divided into many layers in the vertical direction in order to account for the effect of changing fluid temperature with depth on the thermal process in the borefield. The inlet temperature of the ground heat exchanger (GHE) is used as a boundary condition, and the inside and outside surfaces of the U-tube pipes are treated as the conjugated interfaces in the domain. Thus, the conjugate thermal processes between the fluid in the pipes and the soil around it and between the two pipe legs may be accounted fully. A comparison of the model predictions and experimental data shows that the model has good prediction accuracy.

Reducing cold-start emission from internal combustion engines by means of thermal energy storage system

Abstract: Increasing environmental pollutions is an important problem appearing at cold start of internal combustion engines. Developments of new devices that solve this problem are an extremely urgent need especially for cold regions. In this study, a developed experimental sample of thermal energy storage system (TESS) for pre-heating of internal combustion engines has been designed and tested. The development thermal energy storage device (TESD) works on the effect of absorption and rejection of heat during the solid–liquid phase change of heat storage material (Na2SO4 · 10H2O). The TESS has been applied to a gasoline engine at 2 °C temperature and 1 atm pressure. Charging and discharging time of the TESD are about 500 and 600 s, respectively and temperature of engine is increased 17.4 °C averagely with pre-heating. Maximum thermal efficiency of the TESS system is 57.5 % after 12 h waiting duration. CO and HC emissions decrease about 64% and 15%, respectively, with effect of pre-heating engine at cold start and warming-up period.