Analysis of temperature and pressure changes in liquefied natural gas (LNG) cryogenic tanks

Liquefied natural gas (LNG) is being developed as a transportation fuel for heavy vehicles such as trucks and transit buses, to lessen the dependency on oil and to reduce greenhouse gas emissions. The LNG stations are properly designed to prevent the venting of natural gas (NG) from LNG tanks, which can cause evaporative greenhouse gas emissions and result in fluctuations of fuel flow and changes of fuel composition. Boil-off is caused by the heat added into the LNG fuel during the storage and fueling. Heat can leak into the LNG fuel through the shell of tank during the storage and through hoses and dispensers during the fueling. Gas from tanks onboard vehicles, when returned to LNG tanks, can add additional heat into the LNG fuel. A thermodynamic and heat transfer model has been developed to analyze different mechanisms of heat leak into the LNG fuel. The evolving of properties and compositions of LNG fuel inside LNG tanks is simulated. The effect of a number of buses fueled each day on the possible total fuel loss rate has been analyzed. It is found that by increasing the number of buses, fueled each day, the total fuel loss rate can be reduced significantly. It is proposed that an electric generator be used to consume the boil-off gas or a liquefier be used to re-liquefy the boil-off gas to reduce the tank pressure and eliminate fuel losses. These approaches can prevent boil-off of natural gas emissions, and reduce the costs of LNG as transportation fuel.     

Convective heat transfer coefficients for near-horizontal two-phase flow of nitrogen and hydrogen at low mass and heat flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.     

Experimental study on the flow and heat transfer characteristics of a tetra-n-butyl ammonium bromide hydrate slurry (second report: Heat transfer characteristics)

The heat transfer characteristics of a tetra-n-butyl ammonium bromide hydrate slurry were investigated where the Reynolds number, tube diameters and solid fraction were varied as experimental parameters. For laminar flow, it was found that the ratio of Nusselt numbers increased with solid fraction. An approximation of Nusselt number could be derived using the Graetz number on the basis of the apparent Reynolds number, the solid fraction and the ratio of the average diameter of the hydrate particles to the test tube diameter. For turbulent flow conditions, the ratio of Nusselt numbers had a value of one for each condition at low solid fractions. The ratio of Nusselt numbers increased with solid fraction in the high solid fraction region. Moreover, the apparent Reynolds number, which can be derived by treating the hydrate slurry as a pseudoplastic fluid, can be used to determine the condition under which hydrate slurry heat transfer characteristics vary under turbulent flow.     

Pressure drop data and prediction method for enhanced external boiling tube bundles with R-134a and R-236fa

The pressure drops of external flow over enhanced tube bundles were experimentally obtained at both adiabatic and diabatic conditions using R-134a and R-236fa as test fluids. The tests were carried out at saturation temperatures of 5 and 15 °C, mass fluxes from 4 to 40 kg m⁻² s⁻¹, heat fluxes from 15 to 70 kW m⁻² and inlet vapour qualities ranging from 10% to 90%. The frictional pressure drop was found to be primarily a function of mass flux and vapour quality. After comparisons were made with prediction methods in literature a new pressure drop prediction method was proposed for adiabatic and diabatic conditions. The proposed method is based on local measurements (4 and 8 tube rows) and flow conditions (evaluated per tube pitch) and the prediction method is well adapted to local incremental implementation for flooded evaporator design.     

Heat transfer characteristics of cryogenic helium gas through a miniature tube with a large temperature difference

Heat transfer with a large temperature difference between the inlet and outlet flows is often encountered in many cryogenic systems. The heat transfer characteristics of cryogenic gas with temperature-dependent thermophysical properties (TDTP) are different from those in the ambient condition with constant thermophysical properties. In this paper, a combined experimental and numerical study has been conducted to determine the heat transfer characteristics of cryogenic helium gas with temperature-dependent thermophysical properties in a miniature tube. In order to validate the theoretical model, an experimental investigation was conducted. Utilizing the validated model with the FLUENT software, the temperature distribution and velocity profile in a miniature tube have been calculated, and a correlation predicting the temperature effect on the Nusselt number determined.     

Flow boiling heat transfer coefficients at cryogenic temperatures for multi-component refrigerant mixtures of nitrogen–hydrocarbons

The recuperative heat exchanger governs the overall performance of the mixed refrigerant Joule–Thomson cryocooler. In these heat exchangers, the non-azeotropic refrigerant mixture of nitrogen–hydrocarbons undergoes boiling and condensation simultaneously at cryogenic temperature. Hence, the design of such heat exchanger is crucial. However, due to lack of empirical correlations to predict two-phase heat transfer coefficients of multi-component mixtures at low temperature, the design of such heat exchanger is difficult.The present study aims to assess the existing methods for prediction of flow boiling heat transfer coefficients. Many correlations are evaluated against available experimental data of flow boiling of refrigerant mixtures. Silver-Bell-Ghaly correlation and Granryd correlation are found to be more suitable to estimate local heat transfer coefficients. A modified Granryd correlation is recommended for further use.     

大型常压LNG贮罐的国产化探讨

大型常压液化天然气贮罐的开发和建造正逐步实现国产化,开发初期的问题值得大家共同探讨。文章对LNG的性质、国内外LNG大型贮罐现状、贮罐结构形式、贮罐流程组织、附件的选择及标准的遵循等方面进行了阐述。     

铝合金液化天然气罐式集装箱的设计

液化天然气罐式集装箱是运输液化天然气的重要设备。文章介绍了铝合金液化天然气罐式集装箱的设计参数,针对罐体内筒主体材料采用铝合金复合材料及焊接结构进行探讨,并分析了内筒采用5083铝合金的效益优势。     

铝合金液化天然气罐式集装箱的设计

液化天然气罐式集装箱是运输液化天然气的重要设备。文章介绍了铝合金液化天然气罐式集装箱的设计参数，针对罐体内筒主体材料采用铝合金复合材料及焊接结构进行探讨，并分析了内筒采用5083铝舍金的效益优势。     

天然气输送能量的利用

阐述利用天然气输送能量的必要性和效益,详细介绍气态天然气和液化天然气输送能量利用的方法和流程,最后提出跨行业利用天然气输送能量的设想。     

CO2流动沸腾换热干涸研究进展

针对CO2在亚临界管内流动沸腾换热过程中所表现出来的干涸现象研究进展进行了综述,描述了在CO2沸腾换热过程中的干涸现象及其产生的影响因素,分析了热流密度、质量流量、饱和温度、管径等因素对干涸产生的影响及机理.提出CO2流动沸腾换热过程中临界热流密度,流态变化,干涸干度的预测以及抑制干涸提前发生的相应措施是今后研究的方向.     

Flow boiling in small diameter channels

Many studies have been carried out on two-phase flow heat transfer in channels with hydraulic diameter bigger than 6 mm, but relatively little work has been done for small diameter channels in the meso and compact range (diameter from 0.1 to 3 mm). The use of exchangers with small channels in refrigeration units, which are very numerous besides, could bring a significant reduction of the internal volume of the exchanger, and therefore diminish the refrigerant charge of the whole refrigerating system. One can imagine the interest to widen knowledge on the flow and the heat transfer in small-diameter tubes. This paper examine the thermal behavior of refrigerants boiling in small pipes. The correlations available for in-tube evaporation heat transfer coefficient are discussed and evaluated, when possible, and new research areas of relevance than can contribute to expand the use of small-diameter channels evaporators in refrigeration units are suggested.     

天然气的全球市场和贮运发展趋势

概述全球天然气市场的发展情况,比较分析了LNG和CNG的生产、贮存和运输方式,分析世界LNG的贸易状况、方式以及发展趋势和意义,最后阐述中国当前LNG贮运设备的技术水平以及气体行业的应对措施。     

The role of consequence modeling in LNG facility siting

Liquefied natural gas (LNG) project modeling focuses on two primary issues, facility siting and the physical layout of element spacing. Modeling often begins with an analysis of these issues, while ensuring code compliance and sound engineering practice. The most commonly performed analysis involves verifying compliance with the siting provisions of NFPA 59A, which primarily concern property-line spacing (offsite hazard impacts). If the facility is located in the US, compliance with 49 CFR 193 is also required. Other consequence modeling is often performed to determine the spacing of elements within the facility (onsite hazard impacts). Often, many issues concerning in-plant spacing are addressed with the guidance provided in Europe's LNG standard, EN-1473. Spacing of plant buildings in relation to process areas is also a concern as analyzed using the approach given in API RP 752. Studies may also include probabilistic analysis, depending on the perceived risk and cost of mitigation.