Thermophysical Properties Of Gases Research Articles

Performance of a supercritical carbon dioxide compressor using a streamline curvature based throughflow method

Supercritical carbon dioxide (sCO2) has emerged as a promising working fluid for the Brayton power cycle, offering enhanced efficiency due to its high density and low viscosity near the critical point. A critical component of the sCO2 Brayton cycle is the turbomachinery, namely the compressor. The pronounced sensitivity of the thermophysical properties of sCO2 in the vicinity of the critical point where the compressor operates can significantly impact the performance characteristics of sCO2 compressors. This study presents a two-dimensional (2D) streamline curvature-based throughflow method, considering real gas thermophysical properties, to analyze a centrifugal compressor operating with sCO2 at near-critical inlet conditions. The method’s validity is established through comparisons of the predicted performance characteristics with those of the Sandia National Laboratories compressor and by comparing the predicted flow field with the pitch-averaged flow field obtained from computational fluid dynamic (CFD) analysis. Moreover, various compressor intake conditions including subcritical vapor, supercritical vapor, supercritical liquid, and near-critical conditions are investigated to explore their impact on compressor performance. The streamline curvature-based throughflow method demonstrates its effectiveness by achieving significant agreement between the flow field and performance characteristics, both at design and off-design conditions where efficiency predictions were within 5.84% near the best efficiency point compared to CFD simulations. At near critical conditions, a maximum of 3201% increase the computational run-time required for CFD based computation was observed compared to superheated conditions while only a maximum of 164% increase in run-time was observed in the throughflow based computations as a result of the numerical under-relaxation required.

Modeling the effect of shape deformation induced by gravity on the evaporation of pendant and sessile drops

Pendant and sessile drops form a spherical cap only in the absence of gravity. The effect of gravity on drop shape is often neglected on the basis of the assumption that the drop size is smaller than the capillary length [Lc=(σ/gρ)1/2], although the deformation may not be fully negligible even in those cases. This paper focuses on evaluation of the effect that deformation due to gravity has on the evaporation characteristics of pendant and sessile drops. The drop shape is described by the Bashforth–Adams equation, a non-linear second order ordinary differential equation, which is solved numerically using a Runge–Kutta method with variable time steps. Under quasi-steady approximation, the species and energy conservation equations in the gas phase have analytical solutions, even for temperature-dependent gas thermophysical properties, once the solution of a basic Laplace problem is known. The Laplace equation is solved in axial symmetric geometry by using COMSOL Multiphysics®, for a wide range of drop sizes and contact angles, yielding vapor distribution, vapor fluxes, and evaporation rates. Comparison with the results from drops of same size in microgravity (i.e., having a spherical cap shape) shows that the effect is also perceptible for drops with a size smaller than the capillary length and that it can become quite important for those with larger sizes. Complementary results are found for sessile and pendant drops with respect to wall wettability, suggesting that the phenomenon can be analyzed using a unitary approach.

Effects of Gas Thermophysical Properties on the Full-Range Endwall Film Cooling of a Turbine Vane

To protect turbine endwall from heat damage of hot exhaust gas, film cooling is the most significant method. The complex vortex structures on the endwall, such as the development of horseshoe vortices and transverse flow, affects cooling coverage on the endwall. In this study, the effects of gas thermophysical properties on full-range endwall film cooling of a turbine vane are investigated. Three kinds of gas thermophysical properties models are considered, i.e., the constant property gas model, ideal gas model, and real gas model, with six full-range endwall film cooling holes patterns based on different distribution principles. From the results, when gas thermophysical properties are considered, the coolant coverage in the pressure side (PS)-vane junction region is improved in Pattern B, Pattern D, Pattern E, and Pattern F, which are respectively designed based on the passage middle gap, limiting streamlines, heat transfer coefficients (HTCs), and four-holes pattern. Endwall η distribution is mainly determined by relative ratio of ejecting velocity and density of the hot gas and the coolant. For the cooling holes on the endwall with an injection angle of 30°, the density ratio is more dominant in determining the coolant coverage. At the injection angle of 45°, i.e., the slot region, the ejecting velocity is more dominant in determining the coolant coverage. When the ejecting velocity Is large enough from the slot, the coolant coverage on the downstream endwall region is also improved.

Evaporating Rayleigh–Bénard convection: prediction of interface temperature and global heat transfer modulation

We propose an analytical model to estimate the interface temperature $\varTheta _{\varGamma }$ and the Nusselt number $Nu$ for an evaporating two-layer Rayleigh–Bénard configuration in statistically stationary conditions. The model is based on three assumptions: (i) the Oberbeck–Boussinesq approximation can be applied to the liquid phase, while the gas thermophysical properties are generic functions of thermodynamic pressure, local temperature and vapour composition, (ii) the Grossmann–Lohse theory for thermal convection can be applied to the liquid and gas layers separately and (iii) the vapour content in the gas can be taken as the mean value at the gas–liquid interface. We validate this setting using direct numerical simulations in a parameter space composed of the Rayleigh number ( $10^6\leq Ra\leq 10^8$ ) and the temperature differential ( $0.05\leq \varepsilon \leq 0.20$ ), which modulates the variation of state variables in the gas layer. To better disentangle the variable property effects on $\varTheta _\varGamma$ and $Nu$ , simulations are performed in two conditions. First, we consider the case of uniform gas properties except for the gas density and gas–liquid diffusion coefficient. Second, we include the variation of specific heat capacity, dynamic viscosity and thermal conductivity using realistic equations of state. Irrespective of the employed setting, the proposed model agrees very well with the numerical simulations over the entire range of $Ra$ – $\varepsilon$ investigated.

Optimization of an Organic Ranking Cycle Radial Turbine Using a Reduced-Order Model Coupled With Computational Fluid Dynamics

Abstract This paper presents the geometry optimization of a single stage radial turbine for an organic ranking cycle (ORC) system operating over a pressure ratio of 9. The specific fluid used in this investigation is R1233zd (E), but the methodology applies to other organic fluids as well. The ORC system is used to recover excess waste heat from the operation of an offshore oil and gas platform in the gulf of Thailand and its conditions will be replicated at pilot plant level. The geometry is optimized for the highest total-to-static efficiency using nongradient based algorithms to allow for wide design space. Firstly, a one-dimensional meanline geometry is optimized, which is followed by a computational fluid dynamics (cfd) optimization in three-dimensional using a parameterized model. cfd is used to validate and calibrate the meanline model as well as to understand the flow and the sensitivity of the design parameters not captured by the low-order model. Moreover, the flow field of the successful designs is analyzed by cfd to identify the main flow structures that explain the difference in performance among the designs. The nonideal gas thermophysical properties of R1233zd (E) are calculated using equations of state to account for the nonideal gas behavior.

Numerical simulation and parameterization of the heating and evaporation of a titanium (IV) isopropoxide/p-xylene precursor/solvent droplet in hot convective air

Flame spray pyrolysis (FSP) is an excellent method to produce metal-oxide powders in the nano-size range. In this framework, the heating and evaporation of precursor solutions in hot oxidizing environments is investigated. A single spherically symmetric precursor/solvent droplets of titanium (IV) isopropoxide (TTIP) – Ti[OCH(CH3)2]4 in p-xylene – C6H4(CH3)2 at room temperature in convective hot air at atmospheric pressure is considered. Both variable liquid and gas thermophysical properties are incorporated and the non-random two-liquid (NRTL) model is used to describe the real behavior of the mixture. The bi-component droplet interior is not physically resolved and time-dependence is considered through the distillation-limit model for droplet heating and the rapid-mixing model accounts for droplet vaporization. A parameter study of the heating and evaporation characteristics of the single precursor/solvent droplet on the initial droplet size, the initial TTIP mass fraction in the droplet, the ambient air temperature, and the relative gas and droplet velocity is performed and discussed. The results are parameterized and tabulated in terms of polynomial fits of the individual mass evaporation rates of the components, the droplet surface temperature, and the normalized droplet surface area. A computer code in the programming language C is provided that can be incorporated into more complex simulations of FSP.

Finite-size evaporating droplets in weakly compressible homogeneous shear turbulence

We perform interface-resolved simulations of finite-size evaporating droplets in weakly compressible homogeneous shear turbulence. The study is conducted by varying three dimensionless physical parameters: the initial gas temperature over the critical temperature $T_{g,0}/T_c$ , the initial droplet diameter over the Kolmogorov scale $d_0/\eta$ and the surface tension, i.e. the shear-based Weber number, $We_{\mathcal {S}}$ . For the smallest $We_{\mathcal {S}}$ , we first discuss the impact on the evaporation rate of the three thermodynamic models employed to evaluate the gas thermophysical properties: a constant property model and two variable-properties approaches where either the gas density or all the gas properties are allowed to vary. Taking this last approach as reference, the model assuming constant gas properties and evaluated with the ‘1/3’ rule is shown to predict the evaporation rate better than the model where the only variable property is the gas density. Moreover, we observe that the well-known Frössling/Ranz-Marshall correlation underpredicts the Sherwood number at low temperatures, $T_{g,0}/T_c=0.75$ . Next, we show that the ratio between the actual evaporation rate in turbulence and the one computed in stagnant conditions is always much higher than one for weakly deformable droplets: it decreases with $T_{g,0}/T_c$ without approaching unity at the highest $T_{g,0}/T_c$ considered. This suggests an evaporation enhancement due to turbulence also in conditions typical of combustion applications. Finally, we examine the overall evaporation rate and the local interfacial mass flux at higher $We_{\mathcal {S}}$ , showing a positive correlation between evaporation rate and interfacial curvature, especially at the lowest $T_{g,0}/T_c$ .

Gas-Phase Temperature Mapping of Evaporating Microdroplets.

Evaporation is a ubiquitous and complex phenomenon of importance to many natural and industrial systems. Evaporation occurs when molecules near the free interface overcome intermolecular attractions with the bulk liquid. As molecules escape the liquid phase, heat is removed, causing evaporative cooling. The influence of evaporative cooling on inducing a temperature difference with the surrounding atmosphere as well as within the liquid is poorly understood. Here, we develop a technique to overcome past difficulties encountered during the study of heterogeneous droplet evaporation by coupling a piezo-driven droplet generation mechanism to a controlled micro-thermocouple to probe microdroplet evaporation. The technique allowed us to probe the gas-phase temperature distribution using a micro-thermocouple (50 μm) in the vicinity of the liquid-vapor interface with high spatial (±10 μm) and temporal (±100 ms) resolution. We experimentally map the temperature gradient formed surrounding sessile water droplets having varying curvature dictated by the apparent advancing contact angle (100° ≲ θ ≲ 165°). The experiments were carried out at temperatures below and above ambient for a range of fixed droplet radii (130 μm ≲ R ≲ 330 μm). Our results provide a primary validation of the centuries-old theoretical framework underpinning heterogeneous droplet evaporation mediated by the working fluid, substrate, and gas thermophysical properties, droplet apparent contact angle, and droplet size. We show that microscale droplets residing on low-thermal-conductivity substrates such as glass absorb up to 8× more heat from the surrounding gas compared to droplets residing on high-thermal-conductivity substrates such as copper. Our work not only develops an experimental understanding of the heat transfer mechanisms governing droplet evaporation but also presents a powerful platform for the study and characterization of liquid-vapor transport at curved interfaces wetting and nonwetting advanced functional surfaces.

Experimental Studies on Thermal Effect of H2O on the Combustion of Pulverized Coal Char

ABSTRACT The differences in the gas thermophysical properties (including heat capacity, thermal conductivity and diffusivity) between char combustion in wet and dry environments have remarkable influences on the combustion process. We conducted a detailed experimental study on the thermal effects of H2O on the coal char combustion at 1773 K in a high-temperature drop tube furnace (HTDTF). An appropriate portion of Ar was introduced into the bulk gas (O2/H2O/N2) during combustion to eliminate the opposed thermal effect of H2O. Results indicate that the H2O thermal effect initially increased with higher H2O concentrations from 0 to 10 vol.%, but it declined with a further increase in H2O concentrations in the bulk gas. Meanwhile, its effect was dominant in the preliminary and middle stages of char burning at low H2O concentrations. The increased H2O concentration and residence time promoted the gasification reaction and the water-gas shift reaction so as to weaken the negative thermal effect of H2O and improve the carbon conversion. In the 15%O2/5%H2O/80%N2 environment, the relative contribution of thermal effects to carbon conversion ratios ranked up to −36% at the residence time of 0.3 s, but it decreased to −0.3% at 21%O2/30%H2O/49%N2 atmosphere and the residence time of 0.4 s.

Peculiarities in breakup and transport process of shock-induced ejecta with surrounding gas

The interaction of shock-induced ejecta with gas beyond the free surface is a critical unsolved issue and being investigated broadly. Using models containing micrometer-sized gas environments, we perform molecular dynamics simulations to investigate the coupling interactions of surrounding gases with ejecta from shock-loaded tin surface. Ejected microjets experience progressively aggravated deceleration with increasing gas density, and particle flows ahead of jet tips are suppressed. Despite the drag effect, the primary fragmentation process is yet intrinsically dominated by a velocity gradient. The continuous interaction between ejecta and gas leads to the progressive formation of transmitted shock waves in background gases, which is jointly determined by ejecta velocity and thermophysical properties of gas. Meanwhile, a mixing layer between ejecta and gas is directly observed, leading to discrepant mass distributions of ejecta along shock direction. With increasing gas density, the volume density tends to rise in the mixing zone while the zone thickness decreases. Further, with the presence of gases, the size distribution of ejected particles is altered with an outstanding feature of enhanced formation of atomic particles. It is found that the stripping effect of gas dominates the growth of ejecta clusters in the transport process. The stripped particles strongly couple and flow with compressed gas, accompanied by recombination into subsequent clusters. As the gas density increases, both formation and annihilation of atomic particles are promoted. The revealed peculiarities provide microscopic views of ejecta interaction with ambient gas, which would further the understanding of gas effects on the breakup and transport of ejected particles.

An analytical model of heat and mass transfer from liquid drops with temperature dependence of gas thermo-physical properties

An analytical model for heating and evaporation of a single component liquid drop is developed. The model accounts for the temperature dependence of density, diffusivity, thermal conductivity and specific heat of the gas species. The effect of variable properties on heat and evaporation rates is analysed and a comparison with the prediction of the classical constant property model is reported for six different species and different operating conditions. The model allows to evaluate the effect of the choice of the gas mixture averaging parameter (the relative weight of free stream and drop surface conditions) on the prediction of the constant property model, and a way to optimise the choice is proposed. The results can be extended to non-spherical drop and examples are reported.

Analysis of landfill gas thermo-physical properties for communal services

In this paper, we analyzed the thermo-physical properties of landfill gas, which can be used in the system of communal services of the city from specialized landfills for collecting municipal solid waste. Authors do not yet consider ways of delivering this type of fuel to subscribers, but imply as an option of possible technologies that there is a storage facility and an industrial factory for cleaning and condensation landfill gas for further use in the communal system of the city. There are Graphs of changes in various indicators of landfill gas in this article. This study can be used to predict energy conservation not only of large cities, but also of small settlements from the use of an alternative energy source - landfill gas. Its main characteristic (in terms of thermal physics) is the lower calorific value of the fuel. It depends on the composition and moisture of the gas. The report discusses the methodology for studying the lower calorific value of landfill gas, based on the chemical balance of this fuel. Another important characteristic is the environmental effect. Reducing emissions of landfill gas decrease the greenhouse effect, which has a negative impact on the Earth’s atmosphere, biosphere and hydrosphere. It could be provided for the production of electrical energy, as well as a combination of electric and thermal. There is the possibility of producing hydrogen for a fuel cell. However, this requires independent economic, environmental and energy comparison of various technologies for using of landfill gas in the engineering infrastructure of the city, as well as different methods of transporting and producing energy-efficient condensate compressed landfill gas (the best option when the methane content in this fuel is like natural gas).

Effect of cross-section geometry on the thermohydraulic characteristics of supercritical CO2 in minichannels

Carbon dioxide (CO2) is becoming an important commercial and industrial working fluid as a potential replacement of the non-environmental friendly refrigerants. For refrigeration and power systems, the minichannel heat exchangers are becoming attractive for transcritical CO2 Rankine cycle and supercritical CO2 Brayton cycle, due to their highly compact construction, high heat transfer coefficient, high pressure capability and lower fluid inventory. This paper employs three-dimensional numerical models to investigate the heat transfer and pressure drop characteristics of supercritical CO2 in minichannels. The models consider real gas thermophysical properties and buoyancy effect and investigate the effect of cross-section geometry on the thermohydraulic characteristics. Six minichannel cross-section geometries with the same hydraulic diameter of 1.22 mm are considered. The geometries include circle, semicircle, square, equilateral triangle, rectangle (aspect ratio = 2) and ellipse (aspect ratio = 2). The inlet temperature, outlet pressure and wall heat flux are 35 °C/75 bar/100 kW/m2 and 35 °C/150 bar/300 kW/m2 for heating conditions and 120 °C/75 bar/-100 kW/m2 and 120 °C/150 bar/-300 kW/m2 for cooling conditions. Comparisons of local Nusselt number and friction factor with those employed empirical correlations are made and useful information and guidelines are provided for the design of compact heat exchangers for supercritical CO2 power system applications.

Numerical study of the thermohydraulic performance of printed circuit heat exchangers for supercritical CO2 Brayton cycle applications

The printed circuit heat exchanger is currently the preferred type of recuperative heat exchanger for the supercritical CO2 Brayton cycle due to its highly compact construction, high heat transfer coefficients and its ability to withstand high pressures and temperatures. This paper employs a three-dimensional numerical model to investigate the thermohydraulic performance of supercritical CO2 flow in a printed circuit heat exchanger. This numerical model considers entrance effects, conjugate heat transfer, real gas thermophysical properties and buoyancy effects. The inlet temperature and pressure are 100 °C/150 bar on the cold side and 400 °C/75 bar on the hot side while the mass flux is varied from 254.6 to 1273.2 kg/(m2·s). The overall performance of the heat exchanger and comparisons of local heat transfer and friction pressure drop with predictions from the empirical correlations are presented and discussed. Overall, this paper provides useful information that can be employed in the design of recuperators for supercritical CO2 Brayton cycle applications.

Investigating the effect of hydrogen injection on natural gas thermo-physical properties with various compositions

Blending of produced hydrogen from renewable energy resources into the natural gas (NG) grid has been promoted as a viable means of energy transportation and long-time storage in large-scales. However, variation of thermo-physical properties of hydrogen-NG admixture can impact the performance of the measuring devices, accessories, and end-use appliances. Therefore, present work investigates the effect of the hydrogen addition on the thermo-physical properties of NG from gas fields with different compositions. The permissible hydrogen concentration, which obviates the need for alteration or modification of the equipment, is considered to be between 1 and 10 vol%. The investigations are conducted on the most important characteristics of the mixture, and it is observed that hydrogen injection into NG increases the upper and lower flammability limits and compressibility factor, while both lower and higher heating values, lower and higher Wobbe indices, along with the relative density decrease with hydrogen concentration increase. Results show that although injecting up to 10 vol% hydrogen into NG causes a reduction of about 7% in lower heating value for investigated compositions, the NG consumption with the NG + H2 admixture is reduced about 3.5%. Energy consumption of the compressors increases by around 9–14%, depending on the composition of the NG.

Adaptation of Radiative Properties of the End Products of Fuels Combustion within the Temperature Range of 1,000…2,000 K

.

Analysis of thermodynamic scope engine simulation model empirical coefficients: Suitability assessment and tuning of conventional hydrocarbon fuel coefficients for bio syngas

The sensitivity of empirical correlations and coefficients of thermodynamic quasi and zero dimensional SI engine models to mixture thermo-chemistry and engine thermo-kinematic response variations is addressed. Mixture thermo-chemistry alterations, as in case engine operation with non-regular alternatives, and accompanying in-cylinder fluid flow and combustion dynamics perturbations induce variations in full cycle thermodynamic and process thermo-kinematic response with potential influence on empirical coefficients. As an example, producer gas and natural gas thermo-physical properties differ significantly, extending to thermo-kinematic response over a range of operating conditions. Literature indicates monotonous adoptions of conventional fuel coefficients for simulation studies with non-regular fuels and tuning efforts are directed at initial and boundary conditions, which are many times thermodynamically untenable. The current work evaluates the suitability of empirical correlations and coefficients tuned for conventional hydrocarbon fuels, for numerical simulation of producer gas fuelled operation. Producer gas specific coefficients are evolved, following a generic approach, wherever necessary.Assessment of producer gas thermo-physical properties indicates four times higher thermal conductivity of producer gas (40.0 mW/mK) compared to gasoline (11.2 mW/mK), attributed to the presence of hydrogen. The influence is on convective cooling which is reflected in about 32% cooling load as against 25% for spark ignited engines. In response, the Reynolds number power coefficient of convective heat transfer correlation nearly doubles from 0.35 to 0.76. Analysing laminar flame speed coefficients, the inability of CHEMKIN-III to accurately predict laminar flame speed of hydrogen containing gases requiring adoption of CHEMKIN-II is a key observation. Substantial deviations are also observed for apparent heat release profiles with extended terminal stage combustion duration for producer gas, attributed to enhanced convective cooling. Curve fit analysis evolves producer gas specific shape and efficiency coefficients of 0.71 and 2.23 as against 2.0 and 5.0 for conventional fuels. The terminal combustion duration also changes from 1.22 to 2.12 ms. Using tuned coefficients permits near concurrent evolution of simulation and experimental pressure and heat release profiles establishing the need for fuel specific coefficients.

Automated experimental facility to investigate a complex of thermophysical properties of liquids and gases

The process of investigation of a complex of liquid and gas thermophysical properties (heat capacity, PVT) by means of the Kh.I. Amirkhanov’s high-temperature adiabatic calorimeter is described. The technique of automation of that experimental facility by means of measuring and controlling devices and a personal computer is described as well.

(Outstanding Achievement Award of the Sensor Division) From Antibody to X-ray 25 Years in MEMS Sensors

Micromachining silicon with anisotropic etching solutions has been a core technology in MEMS for over twenty years and resulted in some novel structures including high emissivity surfaces to increase heat transfer and X-ray focusing devices built for Argonne National Laboratories. A key challenge in biosensing is achieving selectivity which is often provided by immobilizing antibodies at an electrode surface or on a magnetic bead for preconcentration of a target. Impedance based sensing for low power, and direct detection of microbial toxins was studied at porous platinum electrodes, at the University of Illinois at Chicago and Illinois Institute of Technology. This method can be compared with energy generation by enzymes using thermopiles, such as glucose sensors, and redox recycling at interdigitated electrode arrays for the detection of cytokines in a sandwich assay. We are currently investigating methods to improve sampling and pre-concentration through efficient bead-based manipulation using rotating magnetic fields. The use of a CD platform with centrifugal micro-fluidic pumping system will also provide a high throughput approach for target capture using magnetic beads. Manipulation of fluid transport using magnetic bi-directional latching microvalves was investigated for direct methanol fuel cells and miniature gas chromatography systems. The control of sample injection is a critical component of GC systems. Miniature GC columns with integrated heaters have now been developed to improved separation efficiency. These systems show great promise for low power portable chemical analysis systems. Low power sensors are needed, which can include cantilever sensors possessing exquisite sensitivity for chemical detection. We have investigated metal organic framework films, a nanoporous coating, in collaboration with Sandia National Laboratories, on cantilever sensor for detection of VOC’s. This mechanical based detection approach shows improved reproducibility and stability while using minimal power. MEMS accelerometers and gyroscopes are now in most cell phones, while ultra-low power chemical sensors are yet to be integrated. However sensors based upon changes in the thermophysical properties of gases, under development in collaboration with KWJ Engineering, may soon provide gas detection to monitor the environment, improve safety, and even provide breath analysis for heath care.

Three-dimensional analysis of gas flow and heat transfer in a regenerator with alumina balls

A three-dimensional (3D) unsteady mathematical model for regenerator with alumina balls has been developed based on the assumption of porous media and solved by a commercial computational fluid dynamics (CFD) software, FLUENT. The standard k–ɛ turbulence model combined with standard wall functions is used for modeling gas flow. Momentum equation is revised to consider the impact of porous media on fluid flow. Radiation from combustion gas to the storage materials is considered in the model. User-defined functions (UDFs) program has been developed in C language and linked to FLUENT to define user-defined scalar (UDS) transport equation of energy conservation for solid phase, and to calculate interphase heat transfer as well as thermophysical properties of gas and solid phases, which are dependent on temperature. The calculated results were compared with test data, and the maximum relative error is 3.73%. Results of the model calculation showed that after 25 times of alternate changes in heating and cooling cycles, the heat absorption of regenerative balls is equal to heat released to air, implying the operation of regenerator reaches steady state. As the ball diameter decreases, the pressure gradients become steep and fluid velocity decreases. To reduce temperature difference in the horizontal plane, the geometry of the regenerator has been optimized by the model.