Study on the smoke back‐layering and critical ventilation in the road tunnel fire at high altitude

A series of numerical simulations were conducted in order to investigate the characteristics of smoke back‐layering and critical ventilation in the road tunnel at high altitude with reduced ambient atmospheric pressures. The results indicated that the smoke back‐layering length decreases with the reduction of ambient pressure. Meanwhile, the dimensionless critical longitudinal ventilation velocity decreases with one‐third power of the factor of ambient pressure at high altitude. By modifying the traditional dimensionless fire heat release rate with ambient pressure, new models were deduced to predict the smoke back‐layering length and critical ventilation velocity in the road tunnel at high altitude.     

Effects of passenger blockage on smoke flow properties in longitudinally ventilated tunnel fires

This paper investigates the effects of passenger blockage on smoke flow properties in longitudinally ventilated tunnel fires. A series of numerical simulations were conducted in a 1/5 small-scale tunnel with the different heat release rates (50-100 kW), longitudinal ventilation velocities (0.5-1 m/s), passenger blockage lengths (2-6 m), and ratios (0.17-0.267). The typical smoke flow properties in different tunnel fire scenarios are analyzed, and the results show that under the same heat release rate and longitudinal ventilation velocity, the smoke back-layering length, maximum smoke temperature, and downstream smoke layer height decrease with increasing passenger blockage length or ratio. The Li correlations can well predict the smoke back-layering length and maximum smoke temperature in tunnel fire scenarios without the passenger blockage. When the passenger blockage exists, the modified local ventilation velocity that takes the blockage length and ratio into account has been proposed to correct the Li correlations. The smoke back-layering length and maximum smoke temperature with the different blockage lengths and ratios can be predicted by the modified correlations, which are shown to well reproduce the simulation results.     

Effect of altitude on vertical temperature distribution and longitudinal smoke layer thickness in long tunnel fires

Based on large eddy simulation, a series of long tunnel fire experiments with different heat release rates (HRRs) and altitudes were carried out. The vertical temperature and thickness of fire smoke are studied. The simulation results show that the higher the altitude, the lower the flame temperature rise, while the change of smoke plume temperature rise is opposite. The movement of smoke in the tunnel can be divided into four regions, and the smoke layer thickness in the longitudinal direction of the tunnel corresponds to the latter three regions. The thickness in Region II increases along the longitudinal direction, the thickness in Region III is a constant value, and the thickness in Region IV increases along the longitudinal direction. Besides, the change of altitude only has an effect on the smoke layer thickness in Region IV. Then, by considering the altitude, HRR, and smoke layer thickness, and using dimensional analysis, an empirical formula for predicting the smoke layer thickness under the influence of altitude in Region IV was established.     

Influence of multisource fire on temperature distribution and natural smoke exhaust effect in urban tunnels with a shaft

The combustion characteristics of multisource fire and single-source fire are quite different, and there is little research on the influence of multisource fire on the natural smoke extraction effect of shaft in urban tunnels. Therefore, in this article, the method of numerical simulation was used to study the influence of fire power and distance between two fire sources on the natural smoke extraction effect of shaft and the temperature distribution in tunnel in the case of multisource fire. Typical characteristics of smoke are analyzed, such as mass flow rate, temperature distribution, velocity vector, and CO concentration. The simulation results show that when there is a certain distance between the fire sources, the two flames are inclined and close to each other. The smoke temperature under the ceiling is higher under multiple fire sources than that under single fire source. In addition, when one of the fire sources is located at the downstream of the shaft, the smoke emission in the shaft is relatively high. As the distance between fire sources continues to increase, the smoke exhaust rate basically remains stable, and an empirical relationship between smoke exhaust rate and fire source location is established.     

Influence of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME)

This paper describes the effects of ambient flow conditions on the droplet atomization characteristics of dimethyl ether (DME) both experimentally and numerically.In this investigation, the droplet atomization of DME fuel affected by ambient flow conditions was studied in terms of droplet mean size and detected droplet percentage under elevated ambient pressures and temperatures. In order to predict the DME spray atomization, the hybrid breakup model combined with KH-RT (Kelvin-Helmholtz and Rayleigh-Taylor) and KH-DDB (Kelvin-Helmholtz and Drop Deformation Breakup) models was applied in this study.It was revealed that the spray arrival time of DME fuel under a high ambient pressure increased in accordance with the increase in ambient pressure in the spray chamber. It can be seen that more small droplets are distributed at high ambient flow pressure conditions than at atmospheric conditions. This is a consequence of enhanced atomization of DME fuel. On the other hand, when the ambient pressure increases to 2 MPa, the Sauter mean diameter (SMD) increases only slightly compared with that at 1 MPa of pressure. The SMD value of droplets is increased as ambient temperature is increased. Under the high temperature condition in the chamber, the small droplets of DME fuel evaporate quickly and mix with the ambient air. As a result, it promotes the air-fuel mixing in a combustion chamber.     

印刷电路板式换热器内超临界甲烷流动换热特性模拟

印刷电路板式换热器（printed circuit heat exchanger,PCHE）作为一种新型高效微通道换热器,将其应用在LNG浮式储存与气化装置（FSRU）上具有非常大的潜力。对超临界甲烷在PCHE通道中的流动和传热特性进行了数值模拟,结果表明：传热系数随温度先增大后减小,并在准临界温度（202～212 K）处达到峰值;压降随温度先保持不变,然后在准临界温度附近急剧上升,之后随温度增大的趋势变缓;当温度在准临界温度附近时,低质流密度下增大热通量会恶化传热;不同压力下传热系数均在准临界温度处达到峰值;温度低于准临界温度时,压力对压降的影响可以忽略,温度高于准临界温度时,压降随压力增大而显著降低;压力由6.4M Pa提高到8.5 MPa时,传热最大降低32.5%,压降最大降低28.5%;开发的换热和压降关联式平均误差分别为5.6%和4.2%。     

Numerical analysis of smoke flow under the effect of longitudinal airflow in a tunnel fire

This study investigated the influence of the longitudinal airflow on the smoke propagation in a tunnel by large-eddy simulation, which is now widely applied to study the turbulent flow. The smoke movement characteristics were studied in detail, with varying the longitudinal airflow in the tunnel. Six fire scenarios have been simulated with Fire Dynamics Simulator (FDS) and the results of the longitudinal distribution of CO concentration, temperature distribution, interface height, stratification, and the efficiency of smoke extraction in the tunnel have been analyzed to evaluate the different fire cases. FDS predicted a CO concentration distribution compared to calculated values using the Hu model. Furthermore, the predicted maximum smoke temperatures are compared to those given by the Kurioka model. A reasonably good agreement has been obtained for both models. The obtained results showed that the increase of the forced airflow velocity has for results a loss of stratification and significant decrease in the efficiency of extraction.     

Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer

Spray atomization and evaporation play extremely important roles in mixture formation and combustion processes of direct injection (DI) gasoline engines. In this study, the fundamental characteristics of a swirl spray injected into a constant volume vessel are investigated by means of several laser diagnostic techniques including the laser diffraction-based method for droplet size distribution, the laser induced fluorescence-particle image velocimetry for velocity distributions of droplets and spray-induced ambient air flow, and the two-wavelength laser absorption-scattering technique for concentration distributions of liquid and vapor phases in the spray. The results show that the droplets at outer zone of the spray exhibit larger diameter than those at inner zone under both ambient pressures 0.1 and 0.4 MPa. While this can be partially attributed to the effect of spray-induced ambient air flow, the strength of ambient air flow become small when increasing the ambient pressure from 0.1 to 0.4 MPa, indicating the strong influence of spray dynamics on the droplet size distribution. In the evaporating spray, there are higher vapor concentrations near the spray axis than at peripheral zones. At 4.0 ms after start of injection, spray droplets almost completely evaporate under ambient temperature 500 K and pressure 1.0 MPa, but there are significantly amount of fuels with equivalence ratio below 0.5 in the spray. Reduction in ambient pressure promotes the air entrainment and droplet evaporation, but lowered ambient pressure results in more fuel vapor of equivalence ratio above 1.3 along the spray axis.     

Experimental Investigation on Laser Ignition and Combustion Characteristics of NEPE Propellant

The ignition and combustion property of solid propellant is the main content in internal ballistic research, which has a great significance for propulsion application and combustion mechanism. In this study, the detailed gas‐phase reaction mechanism of Nitrate Ester Plasticized Polyether Propellant (NEPE) was developed. It is helpful to understand the intricate processes of solid‐propellant combustion. The factors which may have influences on ignition delay time and temperature distribution of propellant surface was analyzed by laser ignition experiment. Using high‐speed camera and an infrared thermometer, the ignition and combustion process and the surface temperature distribution of NEPE propellant under laser irradiation were measured. Laser heat flux, ambient pressure and initial temperature of NEPE propellant have an influence on the ignition delay time and the surface temperature. Results show that the ignition delay time decreases with the increase of laser heat flux, ambient pressure and initial temperature of NEPE propellant. At the same time, with the increase of laser heat flux, the influences of ambient pressure and initial temperature on the ignition delay time decrease. Besides, laser irradiation, ambient pressure and initial temperature have significant influences on the surface temperature distribution of the propellant.     

Combustion and emission characteristics of ethanol-biodiesel-water micro-emulsions used in a direct injection compression ignition engine

This work aims on the efficient use of ethanol-biodiesel-water micro-emulsions in a diesel engine. A single cylinder direct injection diesel engine is tested using neat biodiesel and the micro-emulsions as fuels under variable operating conditions. The results indicate that, compared with biodiesel, the peak cylinder pressure of the micro-emulsions is almost identical, and the peak pressure rise rate and peak heat release rate are higher at medium and high engine loads. At low engine loads, those of the micro-emulsions are lower. The start of combustion is later for the micro-emulsions than for biodiesel. For the micro-emulsions, there is slightly higher brake specific fuel consumption (BSFC), while lower brake specific energy consumption (BSEC). Drastic reduction in smoke is observed with the micro-emulsions at high engine loads. Nitrogen oxide (NOx) emissions are found slightly lower under all rang of engine load for the micro-emulsions. But carbon monoxide (CO) and hydrocarbon (HC) emissions are slightly higher for the micro-emulsions than that for biodiesel at low and medium engine loads.     

盐水液滴降压蒸发析盐过程传热传质特性

针对单个盐水(NaCl溶液)液滴在降压环境下蒸发析盐的传热传质过程建立了数学模型。模型考虑了多孔盐壳在液滴表面的形成过程,降压过程引起的气流运动,液核通过多孔介质的传质扩散,以及液滴表面的蒸发换热和对流换热。将实验数据与计算结果对比,验证了模型的有效性。通过模型计算获得了液滴表面温度及液滴质量随时间的变化。结果表明盐水液滴在降压环境下蒸发析盐过程的温度变化分为4个阶段:温度骤降阶段、温度回升阶段、平衡温度阶段和温度上升阶段。平衡温度阶段,盐壳界面运动较慢,随蒸发进行,液核尺寸逐渐减小,盐壳界面运动速度加快。理论分析了环境压力对盐水液滴蒸发析盐过程的影响,环境压力越低,平衡温度越低,盐分完全析出时间越短。     

Macroscopic spray characteristics and breakup performance of dimethyl ether (DME) fuel at high fuel temperatures and ambient conditions

The purpose of this work was to investigate, both experimentally and numerically, the spray behavior and atomization characteristics of dimethyl ether (DME) at high fuel temperatures and under various ambient conditions. In order to compare the theoretical and measured spray characteristics of DME fuel, macroscopic characteristics such as spray tip penetration and spray cone angle were investigated using spray visualization system with a heating system. DME atomization performance was calculated under various conditions from KIVA-3 V code and studied via analysis of the overall Sauter mean diameter (SMD) map, which is related to ambient gas temperature, ambient pressure, and fuel temperature.DME spray was found to exhibit behavior that differs from diesel spray under atmospheric condition. However, at high ambient pressure conditions, DME and diesel sprays display similar behavior. At ambient atmospheric condition, the spray cone angle of DME fuel is larger than that of diesel spray due to the occurrence of flash boiling. Variation in DME fuel temperature had little effect on spray tip penetration and spray cone angle characteristics. An increase in ambient air temperature caused an increase in DME spray cone angle due to an enhancement of the flash boiling effect. However, the DME spray cone angle showed a decreasing trend at high ambient pressure conditions when the ambient air temperature was increased. This was due to the disappearance of flash boiling and the evaporation of droplets at the exterior of the spray cone. In the overall SMD map, the increase of the ambient gas temperature and fuel temperature induced the increase of DME overall droplet size. On the other hand, the ambient gas pressure have slightly influenced on the overall SMD at a low ambient gas temperature and low fuel temperature, but the effect of the ambient gas pressure is significant at high ambient gas temperature and high fuel temperature. At high ambient gas temperature, the increase of the ambient gas pressure causes the increase of the overall SMD. At high DME fuel temperature, the decrease of the ambient gas pressure induces the increase of the overall SMD.     

真空制冰过程中水滴动态特性

为研究真空制冰水滴温度影响因素并进行分析,搭建了真空制冰动态特性研究实验台,进行相关实验,采集了相关图像和实验数据。对采集的图像进行了定性分析。采集的实验数据主要是在不同环境温度、环境压力、供水水温、水质、粒径及水滴下落初速度等情况下水滴温度随时间的变化情况,并与模拟计算值一并进行了对比分析。分析得出环境温度、供水水温、下落初速度对其影响较小,而环境压力、水滴粒径对其影响较为明显,供水水质对其影响比较特殊,主要表现在液滴的最大过冷度上。     

高温高压水在垂直下降管内的传热特性

在压力11.5~28 MPa,质量流速450~1550 kg·m^-2·s^-1,内壁热负荷50~585 kW·m^-2的范围内,对水在垂直下降管内的传热特性进行了实验研究,得到了垂直下降管内水的传热特性,分析了热负荷对内壁温度和传热系数的影响,给出了能用于工程实际的传热实验关联式,并且对垂直下降流动和垂直上升流动的传热特性进行了比较。实验结果表明:在亚临界和近临界压力区,垂直下降管会发生第一类传热恶化--膜态沸腾和第二类传热恶化--干涸。热负荷的增大,会导致传热恶化的提前发生和传热恶化发生后的壁温飞升值增大。在超临界压力区,壁温在低焓值区随焓值平缓增加,而在高焓值区随焓值明显升高,表明在拟临界点附近发生了传热强化现象。     

Experimental investigation on the combustion and exhaust emission characteristics of biogas-biodiesel dual-fuel combustion in a CI engine

An experimental investigation was performed to study the influence of dual-fuel combustion characteristics on the exhaust emissions and combustion performance in a diesel engine fueled with biogas-biodiesel dual-fuel. In this work, the combustion pressure and the rate of heat release were evaluated under various conditions in order to analyze the combustion and emission characteristics for single-fuel (diesel and biodiesel) and dual-fuel (biogas-diesel and biogas-biodiesel) combustion modes in a diesel engine. In addition, to compare the engine performances and exhaust emission characteristics with combustion mode, fuel consumption, exhaust gas temperature, efficiency, and exhaust emissions were also investigated under various test conditions. For the dual-fuel system, the intake system of the test engine was modified to convert into biogas and biodiesel of a dual-fueled combustion engine. Biogas was injected during the intake process by two electronically controlled gas injectors, which were installed in the intake pipe.The results of this study showed that the combustion characteristics of single-fuel combustion for biodiesel and diesel indicated the similar patterns at various engine loads. In dual-fuel mode, the peak pressure and heat release for biogas-biodiesel were slightly lower compared to biogas-diesel at low load. At 60% load, biogas-biodiesel combustion exhibited the slightly higher peak pressure, rate of heat release (ROHR) and indicated mean effective pressure (IMEP) than those of diesel. Also, the ignition delay for biogas-biodiesel indicated shortened trends compared to ULSD dual-fueling due to the higher cetane number (CN) of biodiesel. Significantly lower NO_x emissions were emitted under dual-fuel operation for both cases of pilot fuels compared to single-fuel mode at all engine load conditions. Also, biogas-biodiesel provided superior performance in reductions of soot emissions due to the absence of aromatics, the low sulfur, and oxygen contents for biodiesel.     

柜式空调微通道蒸发器换热性能测试

设计并搭建了柜式空调用微通道蒸发器的性能实验测试平台,测试了微通道蒸发器扁管进出口端温度分布及蒸发器进出口温差、压差、输入功率、制冷量和系统能效比随环境舱温度(18~23℃)升高的变化,并与常规管翅式蒸发器进行了对比。结果表明,微通道蒸发器具有较好的制冷剂流量分配特性,提高了空调出风口温度分布均匀性;由于微通道蒸发器制冷剂充注量低于管翅式蒸发器,且流程也相对缩短,相同工况下,微通道蒸发器进出口压差比管翅式蒸发器降低了33.9%,输入功率降低了4.12%,制冷量提升了2.95%,系统能效比最高提高了6.69%。     

亚大气压下AP/HTPB微尺度稳态燃烧的数值模拟

为研究亚大气压下高氯酸铵/端羟基聚丁二烯(AP/HTPB)的燃烧特性,采用三步反应动力学机理,建立二维三明治模型,耦合气固两相;对20~80kPa下AP/HTPB的微尺度燃烧进行模拟,并与高压下(4MPa)AP/HTPB燃烧特性差异进行对比。结果表明,亚大气压下BDP模型中第一步反应靠近燃面,放热量较大,在AP/HTPB推进剂燃烧过程中占主导地位;燃烧环境压强不同,导致火焰的特性不同,亚大气压下火焰中扩散与混合过程共存,高压下为扩散火焰;相比于高压,亚大气压火焰离燃面远,面积大;由于高低压下放热区域及放热率差异导致气固相温度分布不同,从而影响燃面形状,亚大气压下AP与HTPB交界处相对于整个燃面突出,而高压下交界处相对于整个燃面凹陷。     

垂直上升内螺纹管内流动沸腾传热特性

在压力9～22 MPa,质量流速450～2000 kg·m^-2·s^-1,内壁热负荷200～700 kW·m^-2的参数范围内,试验研究了用于1000 MW超超临界锅炉φ28.6 mm×5.8 mm垂直上升内螺纹水冷壁管内汽水流动沸腾传热。研究表明：内螺纹管内壁螺纹的漩流作用可抑制偏离核态沸腾（DNB）传热恶化,内螺纹管在高干度区发生蒸干型（DO）传热恶化。增大质量流速可推迟壁温飞升,壁温飞升幅度随质量流速增大而降低。热负荷越大管壁温越高,随热负荷增大管壁壁温飞升提前,且传热恶化后壁温飞升值增大。随着压力增加,壁温飞升发生干度值减小。内螺纹管汽水流动沸腾传热系数呈π形分布,传热系数峰值出现在汽水沸腾区。文中还给出了亚临界压力区内螺纹管单相区和汽水沸腾区的传热系数试验关联式。     

Study on the influence of air supply and smoke exhaust on full transverse exhaust of long highway tunnel

This study investigated the efficacy of the full transverse exhaust method for smoke extraction in tunnel fires. It examines factors such as the number and layout of air supply and exhaust outlets, analyzing their impact on smoke spread, tunnel temperature, visibility, and airflow. The results demonstrate that the full transverse exhaust method effectively controls smoke emissions in raised highway tunnels. It limits smoke spread, reduces tunnel temperature, and effectively controls the fire-affected area. The number and layout of outlets significantly influence smoke dispersion, with fewer exhaust outlets providing better smoke control and optimizing the tunnel environment. However, insufficient outlets disrupt gas flow stability. The position of exhaust outlets affects smoke distribution, and caution is advised to prevent directing fresh air flow toward the fire. Opening an equal number of exhaust outlets on one side of the fire source yields superior smoke extraction results, reducing tunnel ceiling temperatures and minimizing risks to personnel and structures. Though stabilization may take longer, this configuration proves advantageous. The study offers valuable insights and practical guidelines for implementing the full transverse smoke control method in real-world scenarios.     

CO2跨临界热泵系统特性再分析

在原有研究的基础上,结合实际系统,本文对CO₂跨临界热泵系统的特性进行再分析,通过参数计算,分析回热温度、气体冷却器出口温度、运行压力三种因素如何影响系统性能,提出提高CO₂热泵运行效率的方法。分析结果表明：回热器并不总有效,而是与气体冷却器出口温度有关,当温度小于某临界值时回热会降低系统运行制热性能系数COP_h,当温度大于此临界值时回热则有助于提高COP_h;对应气体冷却器出口温度存在最优压力,但实际压缩机的可承受压力是有限的,导致系统在某些气体冷却器出口温度下不能在最优压力下运行,同时在不同的排气压力下,存在气体冷却器出口温度最高限定值,否则COP_h不合理也不可接受;热泵出水温度以及气体冷却器出口温度共同影响系统排气压力的选择。