Ratio Of Specific Heat Capacities Research Articles

Optimization of thermophysical properties of nanofluids using a hybrid procedure based on machine learning, multi-objective optimization, and multi-criteria decision-making

The rheological and thermal behavior of nanofluids in real-world scenarios is significantly affected by their thermophysical properties (TPPs). Therefore, optimizing TPPs can remarkably improve the performance of nanofluids. In this regard, in the present study, a hybrid strategy is proposed that combines machine learning (ML), multi-objective optimization (MOO), and multi-criteria decision-making (MCDM) to select optimal parameters for water-based multi-walled carbon nanotubes (MWCNTs)-oxide hybrid nanofluids. In the first step, four critical TPPs, including density ratio (DR), viscosity ratio (VR), specific heat capacity ratio (SHCR), and thermal conductivity ratio (TCR), are modeled using two efficient ML techniques, the group method of data handling neural network (GMDH-NN) and combinatorial (COMBI) algorithm. In the next step, the superior models are subjected to a four-objective optimization by the well-known non-dominated sorting genetic algorithm II (NSGA-II), which aims to minimize DR/VR and maximize SHCR/TCR. This study considers volume fraction (VF), oxide nanoparticle (NP) type, and system temperature as optimization variables. In the final step, two prominent MCDM techniques, TOPSIS and VIKOR, were used to identify the desirable optimal points from the Pareto fronts generated by the MOO algorithm. ML results reveal the COMBI algorithm's superior reliability in accurately modeling various TPPs. The pattern of Pareto fronts for all oxide-NPs indicated that over one-third of the optimal points have a VF > 1.5 %. On the other hand, the distribution of optimal points across different temperature ranges varied significantly depending on the type of oxide-NPs. For Al2O3-based nanofluid, around 90 % of the optimal points were within 40–50 °C. Conversely, for nanofluids containing CeO2 NPs, only approximately 24 % of the optimal points were found within the same temperature range. Considering diverse scenarios for weighting TPPs in the MCDM process implied that combining CeO2/ZnO oxide-NPs with MWCNTs in water-based nanofluids is highly effective across various real-world applications.

Application of the successive over relaxation method for analyzing the dusty flow over a surface subject to convective boundary condition

Heat transport features of a dusty fluid flow within a resistive porous medium are numerically interpreted under the magnetohydrodynamic environment. Fluid and dust phase characteristics are overlooked against the preeminent parameters. The main concern, in this work, is to analyze how much the resistive porous media affects the dusty flow in the presence of Lorentz force. The use of convection boundary conditions will aid attain the desired heat transfer rate in engineering applications. The nonlinear partial differential equations governing the fluid and dust phase are transmuted into a system of coupled and highly nonlinear ordinary differential equations which are first discretized using central differences, and then tackled numerically using the successive over-relaxation (SOR) method. This technique creates an effective way to solve complex dynamical problems comprising highly nonlinear-coupled differential equations. The flow features of the concerned problem are explored for different parameters such as the magnetic field, the porosity of the medium, the relative density of the two phases, ratio of free stream velocity, specific heat capacity ratio of the fluid to the dust phase as well as Prandtl number. Our results may be helpful to understand the dusty airflow during storms, mud particles in canals, ponds, and rivers.

General Drag Coefficient for Flow over Spherical Particles

A generalized physics-based expression for the drag coefficient of spherical particles moving in a fluid is developed. The proposed correlation incorporates essential rarefied effects, low-speed hydrodynamics, and shock-wave physics to accurately model the particle-drag force for a wide range of Mach and Knudsen numbers (and therefore Reynolds number). Owing to the basis of the derivation in physics-based scaling laws, the proposed correlation embeds gas-specific properties and has explicit dependence on the ratio of specific heat capacities. The correlation is applicable for arbitrary particle relative velocity, particle diameter, gas pressure, gas temperature, and surface temperature. Compared with existing drag models, the correlation is shown to more accurately reproduce a wide range of experimental data. Finally, the new correlation is applied to simulate particle trajectories in high-speed dusty flows, relevant to a spacecraft entering the Martian atmosphere. The enhanced surface heat flux due to particle impact is found to be sensitive to the particle drag model.

Machine learning-based efficient multi-layered precooler design approach for supercritical CO2 cycle

In the current study, a multi-layered approach, combining 3D Reynolds Averaged Navier–Stokes (RANS) model, Artificial Neural Network (ANN), and in-house precooler design and analysis code (PDAC), is developed to investigate the influence of various parameters on the design of the precooler used in sCO2 cycle. The validated 3D-RANS model is utilized to compute thermal and hydraulic characteristics of a zigzag channeled PCHEs operating under the precooler's conditions. The generated CFD data is used to develop empirical correlations and train a machine learning (ML) model based on Artificial Neural Network (ANN). Finally, both trained ML model and empirical correlation are utilized in the PDAC to design and analyze the precooler under different design conditions. Regression data for the precooler, operating in the pseudocritical zone, show that the prediction accuracy of the developed Nusselt number and pressure drop correlation is relatively poor. At the same time, the trained Artificial Neural Network (ANN) can estimate 99% of the data with 90% confidence in the same operating regime. Further, precooler design and analysis code (PDAC) based on the trained ML model can accurately mimic the CFD prediction at a significantly reduced computational time. It is found that pressure losses on the s C O 2 -side and water pumping power are exceptionally sensitive to desired exit temperature and channel flow rate. Exit temperature lower than the pseudo critical temperature and higher channel flow rate result in pinch point location well inside the precooler, resulting in a massive rise in the heat exchanger’s size and pressure losses. The pinch point can be avoided by choosing the channel mass flow rate ratio of the hot and cold-sides fluid comparable with their ratio of specific heat capacities corresponding to pseudocritical temperature.

Estimation of the specific heat capacity ratio of gasoline vapor based on the relationship between the maximum pressure rise rate and initial pressure of gas explosion

Gasoline is volatile and can form a flammable gas mixture with air easily. Researches on the gas explosions still need to be developed so that more effective explosion prevention techniques and measures can be proposed. The maximum overpressure rise rate and deflagration index are two important parameters for measuring gas explosion damage power. The relationship between these two parameters and the initial pressure is controversial. Razus et al. insisted that it was a linear relationship, however, Faghih et al. declared that it was an exponential relationship. In this paper, the relationship between the maximum pressure rise rate (the deflagration index) and the initial pressure is deduced and discussed. The results show that the relationship is still linear. On this basis, the deduced relationship formula is used to estimate the specific heat capacity of gasoline vapor. Results show that the estimated value is reasonable and acceptable.

Shock-wave propagation in supercritical CO2 induced by nanosecond-pulsed arc plasma

Shock waves generated by arc plasma in supercritical (SC) CO2 have the potential to create novel reaction fields. However, there have been few studies of shock-wave characteristics in SC-CO2. This study provides the results of visualization of shock-wave propagation in SC-CO2 generated by nanosecond-pulsed arc plasma. A propagating cylindrical shock wave originating from a discharge channel was observed using time-resolved shadowgraph imaging. The shock wave separated from the cylindrical dark-region induced by pulsed arc plasma within 64 nanoseconds. The Mach numbers of the shock waves were investigated against the medium density of the CO2 ranging from a high-pressure gas phase to the SC phase. The Mach number reached a local maximum at the critical CO2 density level. The anomaly in the Mach number can be explained by the local maximum of the specific heat capacity ratio of the CO2 at close to the critical condition of SC-CO2.

Comparison of the numerical and self-similar solutions of Sedov’s problem on a point explosion in gas

Comparative analysis of solutions of Sedov’s problem of a point explosion in gas for the plane case, obtained by the analytical method and using the open software package of computational fluid dynamics OpenFOAM, is carried out. A brief analysis of methods of dimensionality and similarity theory used for the analytical self-similar solution of point explosion problem in a perfect gas (nitrogen) which determined by the density of uncompressed gas, magnitude of released energy, ratio of specific heat capacities and by the index of geometry of the explosion is given. The system of one-dimensional gas dynamics equations for a perfect gas includes the laws of conservation of mass, momentum, and energy is used. It is assumed that at the initial moment of time there is a point explosion with instantaneous release of energy. Analytical self-similar solutions for the Euler and Lagrangian coordinates, mass velocity, pressure, temperature, and density in the case of plane geometry are given. The numerical simulation of considered process in sonicFoam solver of OpenFOAM package built on the PISO algorithm was performed. For numerical modeling the system of differential equations of gas dynamics is used, including the equations of continuity, Navier-Stokes motion for a compressible medium and conservation of internal energy. Initial and boundary conditions were selected in accordance with the obtained analytical solution using the setFieldsDict, blockMeshDict, and uniformFixedValue utilities. The obtained analytical and numerical solutions have a satisfactory agreement.

A five-equation model for the simulation of miscible and viscous compressible fluids

Typical multispecies compressible Navier–Stokes computations employ conservative equations for mass fraction transport. Upwind discretisations of these governing equations produce spurious pressure oscillations at diffuse contact surfaces between gases of differing ratio of specific heat capacities which degrade the convergence rate of the algorithm. Adding quasi-conservative equations for volume fraction can solve this error, however this approach has been derived only for immiscible fluids. Here, a five-equation quasi-conservative model is proposed that includes the effects of species diffusion, viscosity and thermal conductivity. The derivation of the model is presented, along with a numerical method to solve the governing equations at second order accuracy in space and time. Formal convergence studies demonstrate the expected order of accuracy is achieved for three benchmark problems, cross-validated against two standard mass fraction models. In these test cases, the new model has between 2 and 10 times lower error for a given grid size. Simulations of a two-dimensional air-SF6 Richtmyer–Meshkov instability demonstrate that the new model converges to the solution with four times fewer points in each direction when compared to the mass fraction model in an identical numerical framework. This represents an ≈40 times lower computational cost for an equivalent error in two-dimensional computations. The proposed model is thus very suitable for Direct Numerical Simulation and Large Eddy Simulation of compressible mixing.

Rotor Blade Design of an Axial Turbine using Non-Ideal Gases with Low Real-Flow Effects

This study aims to describe a design methodology for supersonic rotor blade geometry, depending on the working fluid, for a low enthalpy Organic Rankine Cycle (ORC) system. Thus, the working fluid is a non-ideal gas with low impact of real flow effects. An innovate algorithm was developed, in order to generate the two-dimensional geometry of the rotor blade, for various working media. A design method, based on the principle of vortex flow field, was used for the blading design and, for the design of supersonic blades, the method of characteristics was selected as the most optimum. The geometry was tested using a commercial simulation software that uses a pressure-based solving algorithm named SIMPLE (Semi-implicit Method for Pressure-Linked Equations). Key advantages of this procedure are both its simplicity and precision of the results.The above procedure was applied for three working fluids, indicatively isobutane (R-600a), tetrafluroethane (R134a) and a mixture of 15% isobutane – 85% isopentane. Considering the ratio of specific heat capacities as constant, which is a realistic assumption for the operating conditions of these systems, the algorithm produces three different blade geometries. Results comparison indicates that every working fluid, for the same operating conditions and for the same design options, has a significantly differentiated geometry of the two-dimensional blade. Finally, the calculated total to total isentropic efficiency, for these rotor blades, is almost 92%.

Gasdynamic acceleration of microparticles and their interaction with a solid body

Based on the physicomathematical model developed before, verified by comparison results of calculations with experimental data, parameters of a gas-dispersed jet, interacting with a solid body, are numerically investigated. In a wide range of temperature and pressure values in a mixing chamber, material density, the size and mass fraction of particles, the nozzle geometry and the streamlined body form, ratio of specific heat capacities, and the Mach number of carrier gas, calculations are made, which allows one to determine the impact of specified factors on the boundary of a perfectly inelastic collision of particles with the body, accompanied by different physical processes (erosion, cold deposition, ultradeep penetration). Since temperature falls below the Debye value at high expansion of flow, the impact of a significant change of the heat capacity and thermal conductivity of particles are taken into account. Radial distributions of density and velocity of the dispersed mass are plotted, which characterize the “quality” of two-phase flow as a tool for the ground modelling of interaction of a flying vehicle with a dust-laden atmosphere (containing, for example, particles of volcanic outbursts or ice crystals) as well as in technologies of surface treatment.

Effects of Coolant Density, Specific Heat Capacity, and Biot Number on Turbine Vane Cooling Effectiveness

This paper describes the effects of coolant-to-mainstream density ratio and specific heat capacity flux ratio (the product of blowing ratio and specific heat capacity ratio) on the overall cooling effectiveness of high pressure (HP) turbine vanes. Experimental measurements have been conducted at correct engine-matched conditions of Mach number, Reynolds number, turbulence intensity, and coolant-to-mainstream momentum flux ratio. Vanes tested were fully cooled production parts from an engine currently in service. A foreign gas mixture of SF6 and Ar was selected for injection as coolant in the facility so that density and blowing ratios were also matched to the engine situation. The isentropic exponent of the foreign gas mixture coincides with that of air. Full-coverage surface maps of overall cooling effectiveness were acquired by an infrared (IR) thermography technique at a range of mainstream-to-coolant temperature ratios. Measurements were subsequently scaled to engine conditions by employing a new theory based on the principle of superposition and a recovery and redistribution temperature demonstrated in previous papers. It is shown that the two aerodynamically matched situations of air- and foreign-gas-cooled experiments give virtually the same effectiveness trends and patterns. Actual levels differ, however, on account of specific heat capacity flux ratio differences. The effect is described and quantified by a one-dimensional analytical model of the vane wall. Differences in Biot number with respect to engine conditions are discussed as they also influence the scaling of turbine metal temperatures.

Another view on the optimization of the Brayton cycle with recuperator

Nondimensional expressions for the work output, thermal efficiency, heat input, and entropy generation number in the Brayton cycle with a recuperator are given as functions of seven input parameters: pressure ratio, temperature ratio, recuperator effectiveness, turbine and compressor isentropic efficiencies, nondimensional pressure drop in the combustor and recuperator and the ratio of specific heat capacities. The expressions for pressure ratios which maximize or minimize the considered quantities are derived. Since it is not possible to maximize all the quantities at a unique value of pressure ratio, three optimization criteria: (i) maximum work output, (ii) maximum thermal efficiency, and (iii) maximum of their weighted sum are defined. For each criterion two diagrams from which one can select optimal pressure ratio and read out all basic quantities defining the Brayton cycle are provided. In the case of criterion of maximum thermal efficiency, when the recuperator effectiveness is lower than the certain limit, the Brayton cycle without a recuperator is better than that with it. In the case of other two criteria, the Brayton cycle with a recuperator is always better than that without it, regardless of the value of the recuperator effectiveness.

Energy Distribution Analysis in Boosted HCCI-like / LTGC Engines - Understanding the Trade-Offs to Maximize the Thermal Efficiency

A detailed understanding of the various factors affecting the trends in gross-indicated thermal efficiency with changes in key operating parameters has been carried out, applied to a one-liter displacement single-cylinder boosted Low-Temperature Gasoline Combustion (LTGC) engine. This work systematically investigates how the supplied fuel energy splits into the following four energy pathways: gross-indicated thermal efficiency, combustion inefficiency, heat transfer and exhaust losses, and how this split changes with operating conditions. Additional analysis is performed to determine the influence of variations in the ratio of specific heat capacities (γ) and the effective expansion ratio, related to the combustion-phasing retard (CA50), on the energy split. Heat transfer and exhaust losses are computed using multiple standard cycle analysis techniques. Furthermore, the various methods are evaluated in order to validate the trends.

Universality classes for thermal transport in one-dimensional oscillator systems.

Two universality classes for thermal transport in one-dimensional oscillator systems are proposed. In class A the asymptotic behavior of the frequency dependent thermal conductivity is κ(ω)∼ω-1/2, whereas the bulk viscosity is finite. In class B the asymptotic behavior of the thermal conductivity is κ∼ω-α, where α<0.4, and the frequency dependent bulk viscosity has the same asymptotic behavior as the thermal conductivity. It is further proposed that the criterion for membership in class A is that the ratio of specific heat capacities γ≡cP/cV=1. A one-dimensional cubic-plus-quartic coupled oscillator is examined at conditions for which γ=1 but P≠0. It is found that the system belongs to class A, in agreement with the proposed criterion. Additionally, it is proposed that examination of whether a system has a well-defined bulk Prandtl number is a more reliable way of determining whether a system is in class A or class B.

Natural gas fueled compression ignition engine performance and emissions maps with diesel and RME pilot fuels

When natural gas is port/manifold injected into a compression ignition engine, the mixture of air and the natural gas is compressed during the compression stroke of the engine. Due to the difference in the values of specific heat capacity ratio between air and natural gas, the temperature and pressure at the time of pilot fuel injection are different when compared to a case where only air is compressed. Also, the presence of natural gas affects the peak in-cylinder (adiabatic flame) temperature. This significantly affects the performance as well as emissions characteristics of natural gas based dual fueling in CI engine. Natural gas has been extensively tested in a single cylinder compression ignition engine to obtain performance and emissions maps.Two pilot fuels, diesel and RME, have been used to pilot natural gas combustion. The performance of the two liquid fuels used as pilots has also been assessed and compared. Tests were conducted at 48 different operating conditions (six different speeds and eight different power output conditions for each speed) for single fueling cases. Both the diesel and RME based single fueling cases were used as baselines to compare the natural gas based dual fueling where data was collected at 36 operating conditions (six different speeds and six different power output conditions for each speed). Performance and emissions characteristics were mapped on speed vs brake power plots. The thermal efficiency values of the natural gas dual fueling were lower when compared to the respective pilot fuel based single fueling apart from the highest powers. The effect of engine speed on volumetric efficiency in case of the natural gas based dual fueling was significantly different from what was observed with the single fueling. Contours of specific NOX for diesel and RME based single fueling differ significantly when these fuels were used to pilot natural gas combustion. For both of the single fueling cases, maximum specific NOX were centered at the intersection of medium speeds and medium powers and they decrease in all directions from this region of maximum values. On the other hand, an opposite trend was observed with dual fueling cases where minimum specific NOX were observed at the center of the map and they increase in all direction from this region of minimum NOX. RME piloted specific NOX at the highest speeds were the only exception to this trend. Higher specific HC and lower specific CO2 emissions were observed in case of natural gas based dual fueling. The emissions were measured in g/MJ of engine power.

Modeling Acceleration Effects on Ram Accelerator Thrust at High Pressure

A = tube section a = acceleration cp = isobaric heat capacity F = thrust h = specific enthalpy I = nondimensional thrust, F/(p A) L = length M = Mach number m = mass P = pressure ratio p = pressure Q = nondimensional heat release, q/cpT R = gas constant T = temperature u = velocity v = specific volume α = intermediate calculation parameter, LCVap = adiabatic specific heat capacity ratio γ = specific heat capacity ratio q = heat release η = calorific imperfection, h/cpT σ = compressibility factor

Mathematical model for predicting the molecular intensity of a solenoid-actuated pulsed beam source

A mathematical model for evaluating the molecular intensity produced by a solenoid-actuated pulsed beam source is described. The particular source studied here is used to introduce reactant gas pulses at various user-specified intervals into a fixed-bed microreactor system for transient catalysis studies where quantification of the inlet pulse intensity is required for data analysis. The model equations which describe the beam source performance utilize relationships from one-dimensional isentropic gas expansion, gas effusion through a small hole, and macroscopic mass balances around the valve orifice from which a closed-form algebraic expression for the pulse intensity is obtained. The parameters in this expression represent observables which include the ideal gas specific heat capacity ratio, the stagnation properties of the gas in the supply chamber of the beam source, and the beam valve driver settings. Comparisons between experimental and predicted values for the pulse intensities, where the latter are of the order 1017 molecules per pulse, show excellent agreement.

Theoretical Aspects on Spatial Thickness Variations in Laser-Deposited Thin Films.

AbstractWe have studied predicted thickness uniformity of thin films deposited by the pulsed laser ablation technique. Unlike a conventional evaporation process, the spatial thickness characteristics of laser-deposited films is controlled by a number of laser and other deposition variables including laser wavelength, pulse energy density, substrate-target distance, area and shape of the laser irradiated spot, etc. The effect of these parameters on the spatial thickness variations have been analyzed by using a model proposed by Singh et. al. [1]. This model is based on the anisotropic expansion of the laser-generated high-temperature and high-pressure plasma, which is initially confined to small dimensions and then expands anisotropically in vacuum. The results show that spatial film uniformity can be improved by decreasing plasma temperature and specific heat capacity ratio of the plasma gas. For large laser irradiated spot diameters (&gt; 8 mm), the film uniformity was found to decrease with decreasing spot diameters, but this trend reversed at smaller: irradiated spot sizes. Depending on the laser irradiated conditions and substrate-target geometry, the thickness variations have been found to vary from (cos θ)2.5 to (cos θ)12. Optimization of various parameters to decrease the spatial-thickness inhomogeneities across the film are also discussed.

Parametric dependence of the density of specularly reflected ions at quasiperpendicular collisionless shocks

We present and use two simple analytical models, based on the Rankine—Hugoniot conservation laws, to investigate the dependence of the number density of ions specularly reflected at quasi-perpendicular shocks upon four different upstream parameters. The fraction of incident ions reflected is found to increase with the angle between the direction of the upstream magnetic field and the shock normal, θ Bn, and with the upstream plasma beta, the increase being faster at the lowest values of θ Bn and beta. The effect of changing the plasma beta is less significant at higher Mach numbers. The fraction reflected is strongly dependent on the magnitude of the ratio of specific heat capacities, γ, chosen in the Rankine-Hugoniot relations. More importantly, we find that the number density of reflected ions reaches and remains at a maximum value in the high Mach number limit. Ion reflection can therefore always provide the amount of dissipation required by the Rankine-Hugoniot relations for quasi-perpendicular collisionless shocks.

Recent Developments in Rocket Nozzle Configurations

T THRUST of a rocket engine is primarily dependent upon the momentum imparted to the products of combustion by discharging them through an exhaust nozzle. During their passage through the nozzle, the exhaust gases are continuously accelerated from low subsonic velocities to high supersonic velocities at the nozzle exit. For the purpose of understanding the flow process, the nozzle can be divided into three portions: the convergent subsonic section, the throat section, and the divergent supersonic section. This division is appropriate because of the different effects each part has in determining the thrust developed by the rocket, and because different methods of analysis have to be used in determining the flow field in these three regions. The mass flow rate through a rocket nozzle is determined by the area of the throat section and operating conditions of the combustion chamber. Design changes in the configuration of the convergent portion of the nozzle would influence the mass flow of the exhaust gases and also, to some extent, the combustion efficiency achieved in the chamber. The sonic velocity attained by the exhaust gases is fixed by the combustion chamber conditions and can be further increased by the expansion occurring in the divergent supersonic portion of the nozzle. This additional velocity, leading to thrust increment, is dependent only on the configuration of the diverging nozzle walls and the exit area. In recent years the design of the divergent supersonic portion of the rocket nozzle has received considerable attention. The scope of this review will be limited to this portion of the nozzle and improvement thereof. Analysis of rocket nozzle flows in any real case should include radiative heat loss, chemical reactions due to incomplete combustion, and chemical properties of the exhaust gases. However, gross performance comparisons can be made by assuming that the exhaust gases expand adiabatically and behave like an ideal gas with a constant ratio of specific heat capacities. The appropriate choice of nozzle configuration for a specific rocket engine may, to a great extent, depend on the fabrication methods of nozzle walls, cooling requirements, allowable limits on dimensions, influence of nozzle weight on overall rocket performance, etc. Detailed examination of all these features involves various special engineering fields aside from the knowledge of supersonic expansion process of the