Flow and heat transfer in a micro-cylindrical gas–liquid Couette flow

Analytic solutions for the gas and liquid velocity and temperature distribution are determined for steady state one-dimensional microchannel cylindrical Couette flow between a shaft and a concentric cylinder. The solution is based on the continuum model and takes into consideration the velocity slip and temperature jump in the gaseous phase defined by the Knudsen number range of 0.001 < Kn < 0.1. The two fluids are assumed immiscible. The gas layer is adjacent to the shaft which rotates with angular velocity ω_s and is thermally insulated. The outer cylinder rotates with angular velocity ω_o and is maintained at uniform temperature. The governing parameters are identified and the effects of the Knudsen number and accommodation coefficients on the velocity and temperature profiles, reduction in the overall temperature rise due to the gas layer, the Nusselt number and shear reduction are examined. It was found that the required torque to rotate the liquid in the annular space is significantly reduced by introducing a thin gas layer adjacent to the shaft. Also, reduction in shaft temperature is enhanced through a combination of high energy accommodation coefficient and low momentum accommodation coefficients. Results also indicate that the gas layer becomes more effective in reducing the shaft temperature when the housing angular velocity is much larger than the shaft angular velocity.     

Steam condensing – liquid CO2 boiling heat transfer in a steam condenser for a new heat recovery system

In a new waste heat recovery system, waste heat is recovered from steam condensers through cooling by liquid CO₂ instead of seawater, taking advantage of effective boiling heat transfer performance; the heat is subsequently used for local heat supply. The steam condensing – liquid CO₂ boiling heat transfer performance in a steam condenser with a shell and a helical coil non-fin tube was studied both numerically and experimentally. A heat transfer numerical model was constructed from two models developed for steam condensation and for liquid CO₂ boiling. Experiments were performed to verify the model at a steam pressure range of 3.2–5 kPa and a CO₂ saturation pressure range of 5–6 MPa. Overall heat transfer coefficients obtained from the numerical model agree with the experimental data within ±5%. The numerical estimations show that the boiling local heat transfer coefficient reaches a maximum value of 26 kW/m² K. This value is almost one order higher than that of a conventional water-cooled condenser.     

Vapor condensation onto a turbulent liquid—I. The steady condensation rate as a function of liquid-side turbulence

Data are presented for the rate of vapor condensation onto a turbulent liquid, the turbulence being isotropic in the horizontal plane and bulk-flow free, and the interface being shear-free and relatively free of waves. A correlation is proposed for the rate coefficient in terms of the liquid-side turbulence intensity, turbulence macroscale and subcooling.     

Rethinking “BLEVE explosion” after liquid hydrogen storage tank rupture in a fire

The underlying physical mechanisms leading to the generation of blast waves after liquid hydrogen (LH2) storage tank rupture in a fire are not yet fully understood. This makes it difficult to develop predictive models and validate them against a very limited number of experiments. This study aims at the development of a CFD model able to predict maximum pressure in the blast wave after the LH2 storage tank rupture in a fire. The performed critical review of previous works and the thorough numerical analysis of BMW experiments (LH2 storage pressure in the range 2.0–11.3 bar abs) allowed us to conclude that the maximum pressure in the blast wave is generated by gaseous phase starting shock enhanced by combustion reaction of hydrogen at the contact surface with heated by the shock air. The boiling liquid expanding vapour explosion (BLEVE) pressure peak follows the gaseous phase blast and is smaller in amplitude. The CFD model validated recently against high-pressure hydrogen storage tank rupture in fire experiments is essentially updated in this study to account for cryogenic conditions of LH2 storage. The simulation results provided insight into the blast wave and combustion dynamics, demonstrating that combustion at the contact surface contributes significantly to the generated blast wave, increasing the overpressure at 3 m from the tank up to 5 times. The developed CFD model can be used as a contemporary tool for hydrogen safety engineering, e.g. for assessment of hazard distances from LH2 storage.     

How liquid hydrogen production methods affect emissions in liquid hydrogen powered vehicles?

Emissions variations of liquid hydrogen (LH₂) production methods in liquid hydrogen powered vehicles are investigated in this study. Volatile organic compounds (VOC), carbon monoxide (CO), nitrogen oxides (NO_x), particulate matters (PM₁₀ & PM_2.5), sulfur oxides (SO_x), and carbon dioxide (CO₂) emissions, which are on well-to-wheel (WTW) basis, are evaluated for 2013 model year's cars in the target year of 2018. GREET software is utilized for the emissions. When the average values of all emissions are compared, hydrogen production by the solar power, nuclear, and electrolysis methods have the lowest emissions, respectively, and hydrogen production by coal and electricity methods have the highest emissions, respectively. On the other hand, it is found that in all emission types and hydrogen production methods, fuel cell vehicles (FCV) emit less emission than spark ignition hybrid electric vehicles (SI HEV) and SI HEVs emit less emission than spark ignition internal combustion engine vehicles (SI ICEV). Emissions decrease by 22.4% in SI HEVs compared to SI ICEVs, 35.1% in FCVs compared to SI HEVs, and 49.6% in FCVs compared to SI ICEVs for average of all emissions.     

Vapor condensation onto a turbulent liquid—II. Condensation burst instability at high turbulence intensities

The process of vapor condensation onto a turbulent, subcooled liquid is shown to become unstable when the liquid-side turbulence intensity exceeds a threshold value which depends on liquid subcooling. Above the threshold, very short, high-intensity bursts of condensation occur intermittently. Data are presented on the nature of the bursts and the conditions for their onset.     

Research on ratio selection of a mixed liquid desiccant: Mixed LiCl– CaCl2 solution

Liquid desiccant cooling system is a new type of air-conditioning system capable of saving energy. The dehumidification process dominates the performance of this system, while the thermal properties of the liquid desiccant play a key role in improving dehumidification effect. However, there is little work about how to choose a proper liquid desiccant that has a better performance. To settle this problem, a novel method is proposed to search an ideal liquid desiccant by applying the nonrandom two-liquid equation (NRTL equation). This idea is further applied to mixed LiCl and CaCl₂ solution to work out the right mixture ratio with a better dehumidification effect under certain working conditions. Moreover, the related experiments were carried out. The results show that: compared to single LiCl solution, the dehumidification effect could be raised by more than 20% with mixed LiCl and CaCl₂ solution.     

Design of a compact absorber with a hydrophobic membrane contactor at the liquid–vapor interface for lithium bromide–water absorption chillers

In this study, design of a compact plates-and-frames absorber possessing a hydrophobic microporous membrane contactor at the aqueous solution–water vapor interface is performed analytically. The absorber is a component of a 5 kW cooling capacity single-effect lithium bromide–water absorption chiller that incorporates a hot water thermally driven generator and a water-cooled absorber and condenser. Good agreement prevailed for the analytically evaluated water vapor mass transfer flux and aqueous solution outlet temperature when compared with measured values at similar operating conditions. At design point conditions, the main design parameters obtained are a membrane contactor area of 6.06 m², a ratio of the mass transfer area to absorber net volume (_Am/_Vnet$A_m/V_{\mathit{net}}$) of 130.1(m²/m³), and ratio of the membrane area (mass transfer area) in this design configuration to the area required for heat transfer is 1.162, respectively. The results clearly indicate that the aqueous solution channel thickness is the most significant design parameter that affects the absorber size compactness; the thinner the thickness of the solution channel, the higher the ratio (_Am/_Vnet$A_m/V_{\mathit{net}}$). The results also show the countercurrent refrigerant flow with the aqueous solution has positive effects on the absorber size compactness.     

Experimental study on desulfurization efficiency and gas–liquid mass transfer in a new liquid-screen desulfurization system

This paper presents a new liquid-screen gas–liquid two-phase flow pattern with discarded carbide slag as the liquid sorbent of sulfur dioxide (SO₂) in a wet flue gas desulfurization (WFGD) system. On the basis of experimental data, the correlations of the desulfurization efficiency with flue gas flow rate, slurry flow rate, pH value of slurry and liquid–gas ratio were investigated. A non-dimensional empirical model was developed which correlates the mass transfer coefficient with the liquid Reynolds number, gas Reynolds number and liquid–gas ratio (L/G) based on the available experimental data. The kinetic reaction between the SO₂ and the carbide slag depends on the pressure distribution in this desulfurizing tower, gas liquid flow field, flue gas component, pH value of slurry and liquid–gas ratio mainly. The transient gas–liquid mass transfer involving with chemical reaction was quantified by measuring the inlet and outlet SO₂ concentrations of flue gas as well as the characteristics of the liquid-screen two-phase flow. The mass transfer model provides a necessary quantitative understanding of the hydration kinetics of sulfur dioxide in the liquid-screen flue gas desulfurization system using discarded carbide slag which is essential for the practical application.     

Mathematical model for gas–liquid two-phase flow and biodegradation of a low concentration volatile organic compound (VOC) in a trickling biofilter

A theoretical model is established for predicting the biodegradation of a low concentration volatile organic compound (VOC) in a trickling biofilter. To facilitate the analysis, the packed bed is simplified to a series of straight capillary tubes covered by the biofilm in which the liquid film flow on the surface of biofilm and the gas core flow in the center of tube. The theoretical formulas to calculate liquid film thickness in the capillary tube are obtained by simultaneously solving a set of hydrodynamic equations representing the momentum transport behaviors of the gas–liquid two-phase flow under co-current flow and counter-current flow. Subsequently, the mass transport equations are respectively established for the gas core, liquid film, and biofilm with considering the mass transport resistance in the liquid film and biofilm, the biochemical reaction in the biofilm, and the limitation of oxygen to biochemical reaction. Meanwhile, the surface area of mass transport in the capillary tube is modified by introducing the active biofilm surface area, namely the specific wetted surface area available for biofilm formation. The predicted purification efficiencies of VOC waste gas are found to be in good agreement with the experimental data for the trickling biofilters packed with ?8 mm, ?18 mm, and ?25 mm ceramic spheres under the gas–liquid co-current flow mode and counter-current flow mode. It has been revealed that for a fixed inlet concentration of toluene, the purification efficiency of VOC waste gas decreases with the increase in the gas and liquid flow rate, and increases with the increase in the specific area of packed materials and the height of packed bed. Additionally, it is found that there is an optimal porosity of packed bed corresponding to the maximal purification efficiency.     

Comparison of the renewable transportation fuels,liquid hydrogen and methanol,with gasoline—Energetic and economic aspects

In this paper, the renewable energy vectors liquid hydrogen (LH₂) and methanol generated from atmospheric CO₂ are compared with the conventional crude oil-gasoline system. Both renewable concepts, liquid hydrogen and methanol, lead to a drastic CO₂ reduction compared to the fossil-based system. The comparison between the LH₂ and methanol vector for the transport sector shows nearly the same fuel cost and energy efficiency but strong infrastructure advantages for methanol.     

Nitrogen oxides reduction by liquid petroleum gas over Co–Ce–Ti catalysts in a simulated rotary reactor

Hydrocarbons could be used as the reductant for elimination of NO_x emissions. Liquid petroleum gas, with higher carbon hydrocarbons and cheaper costs, may be of practical value as reducing agents. Due to the consumption of hydrocarbons by oxygen, the NO_x reduction efficiency is significantly inhibited by oxygen in the flue gas. In this research, a novel rotary reactor, realizing the alternating cycle of adsorption zone and reduction zone, was proposed to overcome this negative effect. Co–Ce–Ti mixed oxide catalysts synthesized by a sol–gel method were tested in a simulated rotary reactor for NO_x removal by liquid petroleum gas and characterized by SEM, BET, XRD and XPS. The results showed that catalysts exhibited better NO conversion efficiency at higher temperature but were highly susceptible to oxygen. Catalysts achieved nearly full removal of NO_x from flue gas at 300 °C in a simulated rotary reactor, and 300 °C is considered to be the most optimum temperature with lower energy consumption and excellent flue gas purification performance.     

Analysis of pinch point in liquid–vapour heat exchanger of R134a–DMAC vapour absorption refrigeration system

R134a (1,1,1,2 tetrafluro ethane)–DMAC (N,N Dimethyl Acetamide) vapour absorption refrigeration system can be used for sub-zero temperature applications and in industries where ammonia is forbidden. But it needs rectification of vapour from generator and draining of residual R134a–DMAC liquid from evaporator. As such, owing to the comparatively low ratio of latent heat of vapourisation to vapour specific heat of R134a, liquid vapour heat exchanger (LVHX) is required and the residual liquid further enhances its prominence in sub-cooling the incoming condensate to improve COP. In this paper LVHX is analyzed in detail by varying operating parameters like rectifier efficiency and evaporation and generator temperatures. Heat capacity rate of the cold stream (vapour and residual liquid) changes continuously due to the progressive phase change of the residual liquid. Depending on the rectifier efficiency, the maximum temperature difference shifts from one end of LVHX to the other, while at certain efficiencies it occurs within the heat exchanger indicating that normal design procedure would lead to its design oversize. The importance of LVHX increases with a decrease in both rectifier efficiency and evaporator temperature.     

The peculiarities of sprayed liquid’s thermal state change,as droplets are heated by conduction

The change of thermal state and phase transformation intensity of sprayed water, n-hexane, n-heptane and n-decane is numerically modelled in the case, as droplets are heated by conduction; the influence of the Knudsen layer is neglected; warming and evaporation of the droplets has no influence on the state of the carrying air flow. The research results prove that a peculiar change of the thermal state of sprayed liquid, irrespective of droplet’s dispersivity, exists in the time scale, expressed by Fourier number. The above-mentioned change can be conveniently defined by the characteristic curves, representing the change of a droplet surface, centre, and mean mass temperatures, which are sensibly influenced by temperature of gas mixture and partial pressure of liquid vapour in it. As these characteristic curves were expressed in regards to the initial and equilibrium evaporation temperatures of liquid, the universal curves, representing the change of thermal states of the examined liquids, were obtained in the time scale, expressed by Fourier number. It is shown that liquid evaporation rate and the change of a droplet dimension can also be described by characteristic curves.     

Continuous liquid–vapor phase transition in microspace

Molecular dynamics and Monte Carlo methods are adopted to predict thermodynamic parameters of small argon systems. Simulation results of isotherms are similar to the van der Waals loop in the P–V diagram, which implies the continuity between the liquid and gas states. Both simulation and experiment of argon system indicate that the density variation inside the liquid–vapor interface is gradual and the interface may become considerably thick as the saturation temperature is close to the critical point. The temperature in the interface region is not uniform and higher than the saturation temperature, which is another example of the continuous phase transition between liquid and vapor.     

“Investments and public finance in a green,low carbon,economy”

The paper evaluates the impacts on investments and public finance of a transition to a green, low carbon, economy induced by carbon taxation. Four global tax scenarios are examined using the integrated assessment model WITCH. Taxes are levied on all greenhouse gases (GHGs) and lead to global GHG concentrations equal to 680, 560, 500 and 460 ppm CO₂-eq in 2100. Investments in the power sector increase with respect to the Reference scenario only with the two highest taxes. Investments in energy-related R&D increase in all tax scenarios, but they are a small fraction of GDP. Investments in oil upstream decline in all scenarios. As a result, total investments decline with respect to the Reference scenario. Carbon tax revenues are high in absolute terms and as share of GDP. With high carbon taxes, tax revenues follow a “carbon Laffer” curve. The model assumes that tax revenues are flawlessly recycled lump-sum into the economy. In all scenarios, the power sector becomes a net recipient of subsidies to support the absorption of GHGs. In some regions, with high carbon taxes, subsidies to GHG removal are higher than tax revenues at the end of the century.     

Effectiveness–NTU relation for packed bed liquid desiccant–air contact systems with a double film model for heat and mass transfer

One-dimensional models were usually utilized to describe the coupled heat and mass transfer processes in packed bed liquid desiccant–air contact systems. In this paper, a double film model was utilized for both parallel and countercurrent flow configurations. The model considered the effects of non-unity values of Lewis factor, unequal effective heat and mass transfer areas, liquid phase heat and mass transfer resistances, changes in solution mass flow rate and concentration. Within the relatively narrow range of operating conditions usually encountered in a specified application, a linear approximation was made to find out the dependence of equilibrium humidity ratio on solution temperature and concentration. Constant approximations of some properties and coefficients were further made to render the coupled equations linear. The original differential equations were rearranged and an analytical solution was developed for a set of newly defined parameters. Analytical expressions for the tower efficiency and other effectiveness values were further developed based on the analytical solution. Comparisons were made between analytical results and numerical integration of the original differential equations and the agreement was found to be quite satisfactory.     

Performance analysis of a trigeneration system based on a micro gas turbine and an air-cooled,indirect fired,ammonia–water absorption chiller

The objective of this paper is to experimentally determine the efficiency and viability of the performance of an advanced trigeneration system that consists of a micro gas turbine in which the exhaust gases heat hot thermal oil to produce cooling with an air cooled absorption chiller and hot water for heating and DHW. The micro gas turbine with a net power of 28 kW produces around 60 kW of heat to drive an ammonia/water air-cooled absorption chiller with a rated capacity of 17 kW. The trigeneration system was tested in different operating conditions by varying the output power of the micro gas turbine, the ambient temperature for the absorption unit, the chilled water outlet temperature and the thermal oil inlet temperature. The modelling performance of the trigeneration system and the electrical modelling of the micro gas turbine are presented and compared with experimental results. Finally, the primary energy saving and the economic analysis show the advantages and drawbacks of this trigeneration configuration.