Heat and mass transfer at rotary heat exchangers in consideration of the condensation potential

In many industrial processes as well as in air conditioning systems heat and moisture is transferred by rotary heat exchangers from the warm exhaust air flow to the cold supply air flow. Rotary heat exchangers are classified as sorption rotors, hygroscopic rotors and condensation rotors. Basic mechanisms of heat and moisture transfer are presented. By means of the condensation potential as the difference between the moisture content of the warm air flow and the moisture content of the cold air flow at saturation the humidity transfer at the different rotor types is investigated. The condensation potential as a reference parameter provides the possibility to describe the influence of various air conditions in exhaust air and supply air flow on the humidity transfer of different rotary heat exchangers and to compare these rotors with each other. In order to give an overview of relevant design parameters, the influence of the speed of turning, the flute height of the rotor matrix and the velocity of the air flow regarding the heat and mass transfer is considered.     

Condensation and isothermal water transfer in cement mortar: Part II - transient condensation of water vapor

The transient wetting of a mortar sample swept by a flow of humid air is experimentally studied at temperatures of 30 and 55°C. The water content profile shape and evolution are found to be very different from those which were observed during imbibition. The boundary condition on the exposed wall of the sample is examined. A convenient evolution of the coefficient of diffusion with water content is explored. This coefficient is interpreted in terms of pure vapor diffusion, even at relatively high water contents. But its values at low water content and its temperature dependence are inconsistent. Additional explanations are then considered with the assumption that the vapor condensation in the medium is not an equilibrium process between vapor and liquid phases. The physical origin of such a nonequilibrium process is discussed. A tentative set of transfer and phase change coefficients is proposed in order to describe the experimental data by means of numerical simulation. Then, some aspects of the imbibition processes are re-examined, taking into account the consequences of a nonequilibrium condensation.Nomenclature volumic rate of phase change - D ₀ coefficient of free diffusion of the water vapor in air - D _hv vapor diffusion coefficient of the medium - E, E equivalent air thickness - h relative humidity of gaseous phase - h _c relative humidity at the capillary condensation threshold - h _a relative humidity of the flowing air - h ₀ relative humidity at the air-material interface - h _E equilibrium relative humidity at a given water content - J global massic flux - M molar mass of water - R gas constant - T temperature - t time - x distance from the interface - ₀ total porosity - volumetric water content - _h condensation coefficient (see Equation (8)) - _L mass density of liquid water - vs mass density of saturated water vapor     

Analyzing and modeling the dynamic thermal behaviors of direct contact condensers packed with PCM spheres

Condensers serve as important components for humidification–dehumidification (HDH) desalination plants. Based on the interpenetration continua approach with volume averaging technique, a mathematical dynamic model for analyzing the heat and mass transfer within direct contact condensers with co-current or countercurrent flow arrangement was developed. It was validated against the experimental data from a small scale HDH desalination system. Comparisons including the productivities and the temperature profiles of gas, liquid, and solid phases show good agreement with the measurements. Phase change material (PCM) melting processes have little effect on water production rate for co-current flow arrangement, but the condenser packed with PCM capsules have higher water production rates than that packed with air capsules packed under given conditions. The relative humidity profile of the bulk gas shows contrary trend with the gas temperature profile. The direct contact condenser with countercurrent flow arrangement can provide much better heat and mass transfer between gas and water and produce about 16.3% more fresh water than the same condenser with co-current flow arrangement in 4 h under given conditions.     

Effect of Water Saturation on Pressure-Dependent Permeability of Carboniferous Shale of the Qaidam Basin,China

Permeability is the most important parameter that describes gas flow characteristics in shale. Water saturation and effective pressure have a considerable effect on shale permeability. This paper presents the results of a laboratory study of the effects of water saturation and effective pressure on gas permeability in Carboniferous shales of the Qaidam Basin, China. The permeability of shale samples with varying water saturation (0–33 wt%) was measured at effective pressure of 6.9 to 27.59 MPa and at low mean pore pressure (<?6.89 MPa) at room temperature, using a pressure pulse decay permeameter. The results indicate that the water saturation and the effective pressure are the main factors affecting the shale permeability. Permeability of sample C034, which has a high clay content and is dominated by nanoscale slit-shaped pores, shows a large decrease (up to 90%) with increasing water saturation (from 0 to 31.7 wt%), depending on the effective pressure. A much larger permeability reduction with increasing water saturation fraction is associated with the swelling of clay minerals. For each sample with varying water saturation, our analyses revealed a consistent line relationship between log permeability and effective pressure variation. The impact of effective pressure on the measured permeability becomes more significant as water saturation increases. With increasing water saturation, the gas slippage factor decreases and calculated effective pore size increases, and gas–water flow in the shale samples occurs as channel flow. This study provides practical information for further studies of stress-dependent permeability of shale with water and the gas slippage effect in two-phase, gas–water flow.     

Experimental and numerical analysis of two-phase infiltration in a partially saturated soil

The purpose of this study is to analyze the effects of the soil air flow on the process of water infiltration in a 93.5 cm deep vertical column for varied boundary conditions at the surface - positive time constant head; time constant fluxes smaller and greater than saturated soil hydraulic conductivity.Several experiments conducted on a sandy soil column with and without a possible air flow through the wall are presented. Continuous and simultaneous measurements of water content and air and water pressure heads at different depths allow the analysis of the air and water movements within the soil and the determination of the capillary pressure and relative permeability for each phase as functions of the volumetric water content.A numerical solution of the equations describing the simultaneous flow of air and water is compared with the experimental data and with the traditional one-phase flow modeling. The results show that the air movement may significantly affect water flow variables such as infiltration rates, water content profiles, and ponding times.Furthermore, some basic assumptions used in two-phase flow modeling, such as the hydrodynamic stability of the wetting fronts and the pertinence of the relative permeability concept, are discussed in the light of the experimental data.     

Modeling of Coal Fine Migration During CBM Production in High-Rank Coal

During CBM (coalbed methane) production, the interaction of coal fracture surface with water flow commonly generates and starts coal fine flow. Part of flowing coal fines deposit in coal fracture system due to water production reduction and methane production increase. The fine sedimentation results in the reduction of coal permeability and well productivity. Despite the increasing awareness of the importance of fine migration, limited research has been carried out on the flow model of coal fine coupled with water and gas. In this paper, a flow model of coal fine is established coupled with water and gas flow, taking coal fine generation, migration and sedimentation process into consideration. Then, case simulations are conducted to illustrate effects of water production schedule, permeability performance and gas content on production performance in flow model. The simulation results indicate that methane rate with the lowest initial water rate is observed to have the highest production in late production period. This is mainly due to the reason that the low water flow cannot generate and start the flow of coal fine. Further, the case with high initial water production has faster gas and water flow rate, thus higher coal fine generation rates, which can improve well productivity at earlier production period. As water production declines quickly, both permeability and production performance decrease, which leads to the loss of well productivity. Meanwhile, higher gas content will lead to a faster water production decline at late production period. This indicates that a portion of coal fines plugged in the fracture as water production deceases and the CBM reservoir with high gas content should not adopt a high initial water production schedule.     

Experimental study on effect of relative humidity on heat transfer of an evaporating water droplet in air flow

Dispersed water droplets are often seen in environmental air flows in rain, cloud, mist, sea spray and so on. It is therefore of great importance to precisely estimate heat transfer between water droplets and atmospheric air in developing a reliable climate model. The purpose of this study is to fabricate the measurement system for the temperature of a small water droplet in air flow under the controlled relative humidity condition and to investigate the effect of relative humidity on heat transfer across the surface of an evaporating water droplet in air flow. The results show that the droplet temperature decreases in the low-relative-humidity condition, whereas it increases in the high-relative-humidity condition. Nusselt number on the droplet surface is not affected by the relative humidity.     

2-D hydro-viscoelastic model for convective drying of deformable and unsaturated porous material

The aim of this work was to simulate in two dimensions the spatio-temporal evolution of the moisture content, the temperature, the solid (dry matter) concentration, the dry product total porosity, the gas porosity, and the mechanical stress within a deformable and unsaturated product during convective drying. The material under study was an elongated cellulose–clay composite sample with a square section placed in hot air flow. Currently, this innovative composite is used in the processing of boxes devoted to the preservation of heritage and precious objects against fire damage and other degradation (moisture, insects, etc.). A comprehensive and rigorous hydrothermal model had been merged with a dynamic linear viscoelasticity model based on Bishop's effective stress theory, assuming that the stress tensor is the sum of solid, liquid, and gas stresses. The material viscoelastic properties were measured by means of stress relaxation tests for different water contents. The viscoelastic behaviour was described by a generalized Maxwell model whose parameters were correlated to the water content. The equations of our model were solved by means of the ‘COMSOL Multiphysics’ software. The hydrothermal part of the model was validated by comparison with experimental drying curves obtained in a laboratory hot-air dryer. The simulations of the spatio-temporal distributions of mechanical stress were performed and interpreted in terms of material potential damage. The sample shape was also predicted all over the drying process.     

A Numerical Model of Tracer Transport in a Non-isothermal Two-Phase Flow System for {\text{ CO}}_2 Geological Storage Characterization

For the purpose of characterizing geologically stored $\text{ CO}_{2}Air sparging is an in situ soil/groundwater remediation technology, which involves the injection of pressurized air through air sparging well below the zone of contamination. To investigate the rate-dependent flow properties during multistep air sparging, a rule-based dynamic two-phase flow model was developed and applied to a 3D pore network which is employed to characterize the void structure of porous media. The simulated dynamic two-phase flow at the pore scale or microscale was translated into functional relationships at the continuum-scale of capillary pressure?Csaturation (P ^c?CS) and relative permeability??saturation (K _r?CS) relationships. A significant contribution from the air injection pressure step and duration time of each air injection pressure on both of the above relationships was observed during the multistep air sparging tests. It is observed from the simulation that at a given matric potential, larger amount of water is retained during transient flow than that during steady flow. Shorter the duration of each air injection pressure step, there is higher fraction of retained water. The relative air/water permeability values are also greatly affected by the pressure step. With large air injection pressure step, the air/water relative permeability is much higher than that with a smaller air injection pressure step at the same water saturation level. However, the impact of pressure step on relative permeability is not consistent for flows with different capillary numbers (N _ca). When compared with relative air permeability, relative water permeability has a higher scatter. It was further observed that the dynamic effects on the relative permeability curve are more apparent for networks with larger pore sizes than that with smaller pore sizes. In addition, the effect of pore size on relative water permeability is higher than that on relative air permeability.     

不同湿度和近爆炸下限条件下甲烷-空气混合物爆炸特征

为了研究不同湿度条件下低浓度甲烷-空气混合物爆炸特征,设计了饱和湿空气发生及储存装置,对管路、气囊和爆炸腔体进行温度控制和流量调节,实现了不同相对湿度的甲烷-空气混合气体的配置。采用20 L球形爆炸测试装置,分析不同相对湿度、甲烷浓度对混合物的最大爆炸压力、最大压力上升速率、爆炸下限及层流燃烧速度的影响。结果表明,随着相对湿度增大,最大爆炸压力和最大爆炸压力上升速率逐渐下降,且呈一定的线性关系。混合气体相对湿度从27.7%增大到80.1%时,甲烷爆炸下限从5.15%上升到5.25%,上升率1.9%,层流燃烧速度随相对湿度的增大也呈线性降低趋势。在本文条件下,相对湿度对甲烷-空气混合物的爆炸影响较小,这主要与常温常压下水蒸气的饱和分压力较低有关,但在高温高压时仍需考虑水蒸气含量的增大对混合气体爆炸特征的影响。     

A Fractal Model for Gas–Water Relative Permeability in Inorganic Shale with Nanoscale Pores

A reliable gas–water relative permeability model in shale is extremely important for the accurate numerical simulation of gas–water two-phase flow (e.g., fracturing fluid flowback) in gas-shale reservoirs, which has important implication for the economic development of gas-shale reservoir. A gas–water relative permeability model in inorganic shale with nanoscale pores at laboratory condition and reservoir condition was proposed based on the fractal scaling theory and modified non-slip boundary of continuity equation in the nanotube. The model not only considers the gas slippage in the entire Knudsen regime, multilayer sticking (near-wall high-viscosity water) and the quantified thickness of water film, but also combines the real gas effect and stress dependence effect. The presented model has been validated by various experiments data of sandstone with microscale pores and bulk shale with nanoscale pores. The results show that: (1) The Knudsen diffusion and slippage effects enhance the gas relative permeability dramatically; however, it is not obviously affected at high pressure. (2) The multilayer sticking effect and water film should not be neglected: the multilayer sticking would reduce the water relative permeability as well as slightly decrease gas relative permeability, and the film flow has a negative impact on both of the gas and water relative permeability. (3) The increased fractal dimension for pore size distribution or tortuosity would increase gas relative permeability but decrease the water relative permeability for a given saturation; however, the effect on relative permeability is not that notable. (4) The real gas effect is beneficial for the gas relative permeability, and the influence is considerable when the pressure is high enough and when the nanopores of bulk shale are mostly with smaller size. For the stress dependence, not like the intrinsic permeability, none of the gas or water relative permeability is sensitive to the net pressure and it can be ignored completely.     

Effect of Water Content on Dynamic Fracture Initiation of Vinyl Ester

The effects of water content on the dynamic fracture initiation of notched vinyl ester neat resin samples were examined. The samples were subjected to stress pulses generated by the impact of a projectile launched from an air gun. Two sets of samples of samples were used: the first set was conditioned in an 11 % relative humidity (RH) environment using a saturated salt solution (Lithium Chloride), and the second set was immersed in distilled water. Both sets were kept in their respective environments for 43 days. The dynamic loading conditions were kept constant to analyze the effect of water content on the dynamic fracture initiation of both sample sets. It was observed that the fracture toughness and crack-tip speed showed no significant difference despite a water content differential of 0.49 wt% between the sample sets.     

Hygroscopic growth of aerosol scattering coefficient:A comparative analysis between urban and suburban sites at winter in Beijing

A humidity controlled inlet system was developed to measure the hygroscopic growth of aerosol scattering coefficient in conjunction with nephelometry at an urban site of Chinese Academy of Meteorological Sciences (CAMS) in Beijing and a rural site at Shangdianzi Regional Background Air Pollution Monitoring Station (SDZ) outside Beijing during winter, from December 2005 to January 2006. Measurements were carried out at a wavelength of 525 nm with an Ecotech M9003 nephelometer. The hygroscopic growth function (or factor) of the aerosol scattering coefficientf(RH) increased continuously with increasing relative humidity (RH) and showed no obvious "step-like" deliquescent behavior at both sites during the experiment. The average growth factorf(RH) at the SDZ site could reach 1.5 when RH increased from less than 40% to 92%, and to 2.1 at the CAMS site when RH increased from less than 40% to 93%. The average hygroscopic growth factor at a relative humidity of 80%, f(RH=80±1%), was found to be about 1.26±0.15 at CAMS and 1.24±0.11 at SDZ. Further analysis indicated that under relatively polluted conditions, the average hygroscopic growth factor was higher at the CAMS site than that at the SDZ site. However, under relatively clean air conditions, the difference between the two sites was small, showing a hygroscopic growth behavior similar to those of burning biomass or blowing dust. These results reflected the different characteristics of aerosol types at the two sites.     

Experimental investigation of liquid jet outflow into a plane ventilated channel in self-oscillatory regimes

The results of an experimental investigation of self-oscillatory jet outflow into a plane channel with air injection in its dead-ended part are presented. It is found that at fixed channel geometry and water supply system the main flow parameters depend on the air injection rate and the relative cavity volume. It is shown that with increase in the gas injection at a fixed cavity volume the self-oscillation intensity monotonically increases and the oscillations considerably change in nature, whereas the Strouhal number varies only slightly. At a fixed air injection coefficient the self-oscillation amplitude decreases with increase in the cavity volume and the self-oscillations stop at a certain threshold value. The flow pattern evolution with increase in the injection coefficient is studied in detail using high-speed filming. The developed surge flow pattern is described in detail.     

Permeability identification of a weakly permeable partially saturated porous rock

The present paper deals with the determination of permeability in partially saturated conditions for weakly permeable porous continua such as argillites or deep clayey formations. The permeability can be deduced from measurements of transient weight loss of a sample submitted to a laboratory drying test: a decrease of relative humidity is imposed by saline solution in an hermetic chamber. Assumptions of constant gas pressure equal to atmospheric pressure and of negligible Fickean diffusive transport of vapour are adopted. The only transport phenomenon taken into account inside the sample is the Darcean advective transport of the water liquid. The forward problem is solved by following two modelling approaches: a linear one and a nonlinear one. The parameter identification procedure is based upon the solution of corresponding inverse problems. In the two cases, the Levenberg–Marquardt algorithm has been used for the minimization problem. In the linear approach, the solution of the forward problem is explicit. In the non linear approach, finite volume method for the spatial discretization combined with a Newton–Raphson algorithm has been used to solve the non linear forward problem. The identification method enables variations of permeability and capillary capacity to be estimated. Comparisons between linear and non linear approaches show that the first one is useful to give mean values and order of magnitude of permeability and capacity. A more complete information is deduced from the non linear approach as variations of equivalent capacity and permeability during a test are significant in most cases. The analysis of the obtained results shows that the basic modelling assumption of constant gas pressure inside the sample would not be relevant for lower range of relative humidities and liquid permeability than those investigated.     

Relative Permeability of Gas for Unconventional Reservoirs

Relative permeability of gas gains great significance in exploring unconventional gas. This paper developed a universal relative permeability model of gas, which is applicable for unconventional gas reservoirs such as coal, tight sandstone and shale. The model consists of the absolute relative permeability of gas and the gas slippage permeability. In the proposed model, the effects of water saturation and mean pore pressure on gas slippage permeability are taken into account. Subsequently, the evaluation of the model with existing model is done and then the validation of the model is made with data of tight sandstones, coals and shales from published literatures. The modeling results illustrate that a strong power-law relationship between relative permeability of gas and water saturation and the contribution of gas slippage permeability to relative permeability is determined by water saturation and mean pore pressure simultaneously. Furthermore, a sensitivity analysis of the impact of the parameters in the model is conducted and their effects are discussed.     

Numerical Analysis of Imbibition Front Evolution in Fractured Sandstone under Capillary-Dominated Conditions

Fractures serve as primary conduits having a great impact on the migration of injected fluid into fractured permeable media. Appropriate transport properties such as relative permeability and capillary pressure are essential for successful simulation and prediction of multi-phase flow in such systems. However, the lack of a thorough understanding of the dynamics governing immiscible displacement in fractured media, limits our ability to properly represent their macroscopic transport properties. Previous experimental observations of imbibition front evolution in fractured rocks are examined in the present study using an automated history-matching approach to obtain representative relative permeability and capillary pressure curves. Predicted imbibition front evolution under different flow conditions resulted in an excellent agreement with experimental observations. Sensitivity analyses, in combination with direct experimental observation, allowed exploring the competing effects of relative permeability and capillary pressure on the development of saturation distribution and imbibing front evolution in fractured porous media. Results show that residual saturations are most sensitive to matrix relative permeability to oil, while the ratio of oil and water relative permeability, rock heterogeneity, boundary condition, and matrix–fracture capillary pressure contrast, affect displacement shape, speed, and geometry of the imbibing front.     

Does Klinkenberg's Law Survive Upscaling?

This work deals with the large-scale mathematical modelling of flow of gas at low pressure in porous media. At the pore scale, this type of flow is characterised by a wall-slip effect, which at the sample scale entails a dependence of permeability upon gas pressure. This latter property is described by Klinkenberg's law. The goal of the present work is to examine the robustness of this law, by determining whether it is still verified on a large-scale: upscaling is thus applied, starting with Klinkenberg's law at the local scale. A Klinkenberg's flow of gas in a two-constituent composite porous medium is considered, and the constituents are firstly assumed to be homogeneous. The cases of low and of high permeability contrast are successively examined. Upscaling is performed using the homogenisation method of multiple scale expansions. In both cases, the large-scale permeability tensor differs from its liquid counterpart. Except in the particular case of equal Klinkenberg factors, Klinkenberg's law is not verified at low permeability contrast. At high permeability contrast, the large-scale gas permeability verifies Klinkenberg's law. The case of heterogeneous constituents is then examined. It is shown that the large-scale permeability differs from its liquid counterpart, but it does not verify Klinkenberg's law.     

Simulation of Gas Dipole Tests in Fractures at the Intermediate Scale Using a New Upscaling Method

A high-level radioactive waste disposal site may lead to gas generation by different physical mechanisms. As these sites are to be located in areas with low water flow, any small amount of gas can lead to relative high gas pressures, so that multiphase flow analysis becomes relevant. The movement of gas and water through the system has two important implications. Firstly, water flow takes place in unsaturated conditions, and thus travel times of the radioactive particles transported are affected; and secondly, gas can also carry radioactive particles. Therefore, one of the key points in such studies is the time when gas would break through the biosphere under a number of different flow conditions. In fractured zones, gas would flow preferentially through the most conductive features. We consider a two-dimensional system representing an isolated fracture. In each point we assign a local porosity and permeability and a local pressure-saturation relationship. A dipole (injector-producer) gas flow system is generated and the variation in water saturation is studied. A simple method is proposed for obtaining upscaled values for several parameters involved in two-phase flow. It is based on numerical simulation on a block scale assuming steady-state conditions and absence of capillary pressure gradients. The proposed method of upscaling is applied to simulate a dipole test using a coarser grid than that of the reference field. The comparison between the results in both scales shows an encouraging agreement.     

Prediction of Porosity and Permeability of Caved Zone in Longwall Gobs

The porosity and permeability of the caved zone (gob) in a longwall operation impact many ventilation and methane control related issues, such as air leakage into the gob, the onset of spontaneous combustion, methane and air flow patterns in the gob, and the interaction of gob gas ventholes with the mining environment. Despite its importance, the gob is typically inaccessible for performing direct measurements of porosity and permeability. Thus, there has always been debate on the likely values of porosity and permeability of the caved zone and how these values can be predicted. This study demonstrates a predictive approach that combines fractal scaling in porous medium with principles of fluid flow. The approach allows the calculation of porosity and permeability from the size distribution of broken rock material in the gob, which can be determined from image analyzes of gob material using the theories on a completely fragmented porous medium. The virtual fragmented fractal porous medium so generated is exposed to various uniaxial stresses to simulate gob compaction and porosity and permeability changes during this process. The results suggest that the gob porosity and permeability values can be predicted by this approach and the presented models are capable to produce values close to values documented by other researchers.