冷却水中重悬浮颗粒影响沸腾传热的试验研究

试验研究了冷却水中添加氧化铁(Fe3O4/Fe2O3)颗粒在高温钢坯冷却过程中影响沸腾传热的性质。系统分析了冷却水(悬浮液)喷淋初速度、加入重颗粒的粒径、质量分数等因素对热钢坯温度下降强度、传热热流密度、沸腾传热系数、过冷沸腾长度等参数的影响,总结了冷却水中引入重悬浮颗粒用以强化沸腾传热的优化方案。     

沸腾对柴油机冷却系统数值模拟的影响

计算对比了不考虑沸腾和考虑沸腾2种冷却系统数值模拟计算,得出结论:考虑沸腾传热对内燃机冷却水腔内流动与压力的分布影响不明显,而对冷却水腔内传热过程的影响是很大的。若只考虑纯对流传热,计算结果可能与实际情况存在很大的差异。因此,在对强化内燃机进行流动与传热问题的研究时必须考虑沸腾传热的因素,以获得更为真实、准确的结果。     

Effect of heated length on the critical heat flux of subcooled flow boiling. Part 1: Observation of bubbles and slug length at atmospheric pressure

The effect of the heated length on the Critical Heat Flux (CHF) of subcooled flow boiling with water was experimentally investigated by means of a direct current heated plate made of stainless steel with various lengths. The motion of bubbles at near burnout was observed by a high‐speed video camera under atmospheric pressure conditions. Consideration was made of the critical wavelength of the Helmholtz instability in several CHF mechanistic models. The slug length at CHF almost coincided with the critical wavelength. It is further confirmed that when the heated length is shorter than the critical wavelength, a large enhancement of the CHF is seen. © 2000 Scripta Technica, Heat Trans Asian Res, 29(2): 132–143, 2000     

Combined heat and power plant effects on CO2 emission: Case study of the University of Perugia

In this work the CO₂ emissions due to the University of Perugia's energy needs have been evaluated. Perugia is a city set in the middle of Italy, in the ‘Umbria’ region, with a population of about 1400 000 people. The University of this city was founded in 1260 and nowadays consists of eleven Faculties with 30 000 students and about 1200 teachers. The energy needs of the University are mainly due to thermal and electrical users. In particular, the thermal consumption is strictly connected to building heating in the autumn–winter months. The reduction of CO₂ emissions is one of the main goals that developed countries have established to be achieved in the next few years, not only related to the Kyoto treated goals, but also for increasing economical and industrial production with a sustainable development concept. Carbon dioxide is not strictly a toxic substance, but makes a large contribution to the ‘greenhouse effect’ with negative repercussions for the environment. For these reasons the possibility of satisfying the University of Perugia's energy needs by the adoption of one, or more, cogeneration power plant fuelled by natural gas, opportunely located at the University, has been evaluated both in terms of energy production and of CO₂ emission reduction. This study also takes into account the experiences of the cogeneration power plant that is in operation at the Engineering Faculty of Perugia University. The results have been compared with the Regional Energetic Development Planning for evaluating the influence of the University's energy needs and emissions on the whole Region. Copyright © 2001 John Wiley & Sons, Ltd.     

进气中氮气的体积分数对柴油机着火与燃烧影响的研究

在一台可以拍摄缸内混合气形成和燃烧过程的发动机上，用高速摄像机研究了在进气加入不同体积分数的N2后柴油机的油束发展，混合气形成，着火以及燃烧过程，同时还采用了三基色法对高速摄像机拍摄出的燃烧火焰的照片进行了温度场分布的计算分析，拍摄的图像和计算结果表明，加入N2后，对燃油束的喷雾及混合气体形成过程没有影响，对着火滞燃期的影响也不大，但对火焰的燃烧最高温度和燃烧室的平均温度影响很大，N2的加入量越大，燃烧温度下降越多，燃烧过程的结束也就越早。     

Experimental study and heat transfer analysis on the boiling of saturated liquid nitrogen under transient pulsed laser irradiation

The boiling behavior of the liquid nitrogen (LN2) under the transient high heat flux urgently needs to be researched systematically. In this paper, the high power short pulse duration laser was used to heat the saturated LN2 rapidly, and the high-speed photography aided by the spark light system was employed to take series of photos which displayed the process of LN2's boiling behavior under such conditions. Also, a special temperature measuring system was applied to record the temperature variation of the heating surface. The experiments indicated that an explosive boiling happened within LN2 by the laser heating, and a conventional boiling followed up after the newly-defined changeover time. By analyzing the temperature variation of the heating surface, it is found that the latent heat released by the crack of the bubbles in the bubble cluster induced by the explosive boiling is an important factor that greatly influences the boiling heat transfer mechanism.     

Diagnosis and control of abnormal combustion of hydrogen internal combustion engine based on the hydrogen injection parameters

In order to improve the limitation of evaluating the abnormal combustion problem of hydrogen internal combustion engine by single index, the abnormal combustion risk coefficient is proposed and defined based on AHP(Analytic Hierarchy Process)-entropy method. The abnormal combustion risk of PFI hydrogen internal combustion engine is comprehensively evaluated from multiple indexes such as the uniformity coefficient of the mixture, the temperature of the hot area, the maximum temperature rise rate, the residual amount of hydrogen in the intake port and the cylinder temperature at the end of the exhaust. The influence of hydrogen injection parameters on abnormal combustion was explored. The results show that the temperature and the maximum temperature rise rate in the hot area decrease first and then increase with the increase of hydrogen injection angle and hydrogen injection flow rate. Although large hydrogen injection angle and hydrogen injection flow rate can reduce the cylinder temperature at the end of exhaust, they will increase the residual hydrogen amount in the intake port. Appropriate hydrogen injection angle and hydrogen injection flow scheme can ensure that all parameters are at a better level, so that the risk coefficient of abnormal combustion decreases by 2.1%–5.5%, and the possibility of abnormal combustion is reduced.     

Experimental study of the flow boiling heat transfer enhancement and pressure drop due to the bubble behavior restricted by a screen sheet

A unique method previously proposed by the authors was applied to the heat transfer augmentation in the flow boiling field. In this method a screen sheet was placed on the horizontal heated surface where bubble nucleation occurred. Generated vapor bubbles were trapped between the screen and the wall, became flat, and moved along the surface. This restricted bubble behavior caused the heat transfer enhancement. Three types of screen sheet were tested in the present experiment and the effect of the screen on the heat transfer and two‐phase flow characteristics was investigated. In two of these cases, the screen was displaced upward by the bubble nucleation. Compared with the ordinary flow boiling case, heat transfer was enhanced by a factor of 1.2 to 6 within the present experimental range. Using a simple flow model, it was made clear that the effect of the height of the displaced screen was important in evaluating the increase in pressure drop. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 319–329, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10094     

The effect of hydrogen on the performance and emissions of an SI engine having a high compression ratio fuelled by compressed natural gas

The aim of this paper is investigation of the effect of hydrogen on engine performance and emissions characteristics of an SI engine, having a high compression ratio, fuelled by HCNG (hydrogen enriched compressed natural gas) blend. The experiments were carried out at 1500, 2000 and 2500 rpm under full load conditions of a modified Isuzu 3.9 L engine, having a compression ratio of 12.5. The engine brake power, brake thermal efficiency, combustion analysis and emissions parameters were realized at 5, 10 15 and 20 deg. CA BTDC (crank angle before top dead center) ignition timings and in excess air ratios of 0.9–1.3 fuelled by hydrogen enriched compressed natural gas (100/0, 95/5, 90/10 and 80/20 of % natural gas/hydrogen).The experimental results showed that the maximum power values were generally obtained with HCNG5 (5% hydrogen in natural gas) fuel. The optimum ignition timing that was obtained according to the maximum brake torque was retarded by the addition of hydrogen to CNG (compressed natural gas), while it was advanced by increasing the engine speed. Furthermore, it was observed that the BTE (brake thermal efficiency) generally declined with the hydrogen addition to compressed natural gas and increasing the engine speed. Additionally, the curves of cylinder pressure and ROHR (rate of heat release values) generally closed to top dead center with the increasing of the hydrogen fraction in the blend and a decreasing engine speed. The hydrocarbon and carbon monoxide emissions generally obtained were lower than the Euro-5 and Euro-6 standards.     

Effect of water injection and spark timing on the nitric oxide emission and combustion parameters of a hydrogen fuelled spark ignition engine

One of the main problems with hydrogen fuelled internal combustion engines is the high NO level due to rapid combustion. Use of diluents with the charge and retardation of the spark ignition timing can reduce NO levels in Hydrogen fuelled engines. In this work a single cylinder hydrogen fuelled engine was run at different equivalence ratios at full throttle. NO levels were found to rise after an equivalence ratio of 0.55, maximum value was about 7500 ppm. High reductions in NO emission were not possible without a significant drop in thermal efficiency with retarded spark ignition timings. Drastic drop in NO levels to even as low as 2490 ppm were seen with water injection. In spite of the reduction in heat release rate (HRR) no loss in brake thermal efficiency (BTE) was observed. There was no significant influence on combustion stability or HC levels.     

The influence of engine speed and load on the heat transfer between gases and in-cylinder walls at fired and motored conditions of an IDI diesel engine

In this study, the heat transfer characteristics between gases and in-cylinder walls at fired and motored conditions in a diesel engine were investigated by using engine data obtained experimentally. For this investigation, a four-cylinder, indirect injection (IDI) diesel engine was tested under different engine speeds and loads. The heat transfer coefficient was calculated by using Woschni expression correlated for the IDI diesel engines, and also using Annand and Hohenberg expressions. The temperature of in-cylinder gases were determined from a basic model based on the first law of thermodynamics after measuring in-cylinder pressure experimentally. The results show that the heat transfer characteristics of the IDI diesel engine strongly depend on the engine speed and load as a function of crank angle at fired and motored conditions.     

Study on the heat transfer and sorption characteristics of a consolidated composite sorbent for solar-powered thermochemical cooling systems

In this paper, a new consolidated composite sorbent made from barium chloride and expanded graphite is presented for solar-powered thermochemical sorption cooling systems. A larger sorption capacity and volume cooling density can be obtained with chemisorption systems when compared with those based on physicosorption. The heat transfer and sorption characteristics of the composite sorbent were investigated. Experimental results showed that the chemical composite sorbent can effectively utilize solar energy or low-grade waste heat sources with temperature ranging from 75 to 90 °C, and it could incorporate 0.61 kg of ammonia per kg of the reactive salt. The temperature evolution in the reactor was strongly influenced by the physicochemical reaction, whereas the transient heat transfer properties in the reactive composite material were different during the decomposition and the synthesis phases owing to the variation of the ammonia content and solid configuration inside the metallic salt complex. The rate of conversion in the reactor was very sensitive to the working temperatures and pressures, and the COP (coefficient of performance) obtained with the consolidated composite sorbent varied between 0.50 and 0.53 when the evaporation temperature ranged from 0 to 15 °C at a generation temperature of 80 °C.     

The effect of Sr-doped zinc oxide nanoadditives on the performance and emission parameters of a VCR engine powered by soybean biodiesel

In the present study, the effects of soybean biodiesel (SB)–diesel blends containing 1% strontium (Sr) doped zinc oxide (ZnO) nanoparticles (NPs) on the performance and emission parameters of a variable compression ratio (VCR) engine were investigated. To make the fuel blends, 25% soybean biodiesel (SB25) was added to the diesel. To improve the blend's stability, Sr/ZnO NP additions were blended with SB25 at 50 and 75 ppm utilizing an ultrasonication method and a surfactant at 2%. Various physicochemical techniques, such as X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and ultraviolet spectroscopy, were used to characterize the produced NPs. These blends improved overall engine characteristics when used with a VCR. In comparison to the absence of nanoadditives, the brake thermal efficiency increased by 10.37% and the brake-specific fuel consumption decreased by 16.76% while using 50 ppm Sr/ZnO NPs additive in SB25 (SB25Sr/ZnO50). In addition, the presence of Sr/ZnO in SB25 results in lower harmful emissions such as hydrocarbon, CO, CO₂, and smoke, which are reduced by 11.20%, 13.81%, 41.43%, and 21.34%, respectively, when compared to SB25 fuel. The Sr/ZnO NPs in the blend are an excellent choice for improving engine emission and performance.     

An experimental study on the effect of ultrasonication on viscosity and heat transfer performance of multi-wall carbon nanotube-based aqueous nanofluids

Four samples of 1 wt% multi-walled carbon nanotube-based (MWCNT) aqueous nanofluids prepared via ultrasonication were thermally characterized. Direct imaging was done using a newly developed wet-TEM technique to assess the dispersion state of carbon nanotubes (CNT) in suspension. The effect of dispersing energy (ultrasonication) on viscosity, thermal conductivity, and the laminar convective heat transfer was studied. Results indicate that thermal conductivity and heat transfer enhancement increased until an optimum ultrasonication time was reached, and decreased on further ultrasonication. The suspensions exhibited a shear thinning behavior, which followed the Power Law viscosity model. The maximum enhancements in thermal conductivity and convective heat transfer were found to be 20% and 32%, respectively. The thermal conductivity enhancement increased considerably at temperatures greater than 24 °C. The enhancement in convective heat transfer was found to increase with axial distance. A number of mechanisms related to boundary layer thickness, micro-convective effect, particle rearrangement, possible induced convective effects due to temperature and viscosity variations in the radial direction, and the non-Newtonian nature of the samples are discussed.     

Experimental study of the interactions between long and short-term unsteady heat transfer responses on the in-cylinder and exhaust manifold diesel engine surfaces

The paper presents the results from the analysis of an experimental investigation with the aim to provide insight to the cyclic, instantaneous heat transfer phenomena occurring in both the cylinder head and exhaust manifold wall surfaces of a direct injection (DI), air-cooled diesel engine. The mechanism of cyclic heat transfer is investigated during engine transient events, viz. after a sudden change in engine speed and/or load, both for the combustion chamber and exhaust manifold surfaces. These results are then compared with relevant experimental data from steady state operation which in the present case are used as reference helping to reveal any potential influences of each transient event on cyclic heat transfer. The experimental installation allowed both long- and short-term signal types to be recorded on a common time reference base during the transient event. Processing of experimental data was accomplished using a modified version of one-dimensional heat conduction theory with Fourier analysis, capable to cater for the special characteristics of transient engine operation. Based on this model, the evolution of local surface heat flux during a transient event was calculated. Two engine transient events are examined, which present a key difference in the way the load and speed changes are imposed on each one of them. From the analysis of experimental results it is confirmed that each thermal transient event consists of two distinguished phases the “thermodynamic” and the “structural” one which are appropriately configured and analyzed. In the case of a severe variation, in the first 20 cycles after the beginning of the transient event, the wall surface temperature amplitude on cylinder head was almost three times higher than the one observed at the “normal” temperature oscillations occurring during the steady state operation. At the same time, peak pressure values in the same cycles are increased by almost 15% above their corresponding values at the final steady state. The same phenomena are valid for the exhaust manifold surfaces but on a moderated scale.     

Thermodynamic analysis of a new combined cooling, heat and power system driven by solid oxide fuel cell based on ammonia-water mixture

Although a solid oxide fuel cell combined with a gas turbine (SOFC-GT) has good performance, the temperature of exhaust from gas turbine is still relatively high. In order to recover the waste heat of exhaust from the SOFC-GT to enhance energy conversion efficiency as well as to reduce the emissions of greenhouse gases and pollutants, in this study a new combined cooling, heat and power (CCHP) system driven by the SOFC is proposed to perform the trigeneration by using ammonia-water mixture to recover the waste heat of exhaust from the SOFC-GT. The CCHP system, whose main fuel is methane, can generate electricity, cooling effect and heat effect simultaneously. The overall system performance has been evaluated by mathematical models and thermodynamic laws. A parametric analysis is also conducted to examine the effects of some key thermodynamic parameters on the system performance. Results indicate that the overall energy conversion efficiency exceeds 80% under the given conditions, and it is also found that the increasing the fuel flow rate can improve overall energy conversion efficiency, even though both the SOFC efficiency and electricity efficiency decrease. Moreover, with an increased compressor pressure ratio, the SOFC efficiency, electricity efficiency and overall energy conversion efficiency all increase. Ammonia concentration and pressure entering ammonia-water turbine can also affect the CCHP system performance.     

利用废气再循环与米勒循环提高氢内燃机压缩比的研究

建立了氢内燃机一维模型,分别仿真分析了废气再循环(exhaust gas recirculation,EGR)技术和米勒循环技术对抑制爆震及氮氧化物(NOx)排放的效果,最后结合两种技术探索了可达到的最大压缩比和最好的NOx排放性能.结果显示EGR技术对抑制NOx排放更有效,米勒循环对抑制爆震更有效,最终在本研究的工况...