Increase Of Temperature Ratio Research Articles

A brief flow analysis of Williamson fluid near a radiated irregular vertical surface with binary chemical reaction

This analysis deals with the effect of mixed convection and thermal radiation on steady three-dimensional (3D) flow of Williamson fluid near an irregular surface. The phenomenon is examined in semi-infinite domain and the flow is produced by means of non-linear stretching. The mass transfer analysis is carried out by means of activation energy and binary chemical reaction. The governing equations are initially reformed into ODES by applying applicable variables and then solutions are calculated for the flow field, mass transfer rate, skin frictions, temperature, heat transfer rate and concentration via shooting. New characteristics of dimensionless variables are illustrated by means of graphs and tables. The detailed analysis anticipates that velocity declines by the elevation of Weissenberg number and accelerates with the increase of mixed convection parameters. Further, the temperature enhances with the increase of temperature ratio and radiation parameters and behaved oppositely analogous to development in Prandtl number.

Investigation of pressure feedback technique to control ramp based SWBLI

Shock wave boundary layer interaction (SWBLI) and associated boundary layer separation create undesirable phenomena such as enhanced aero-thermodynamic loads, localized turbulent spots, unsteadiness, drag enhancement, pressure recovery losses in high-speed flow applications. It necessitates an investigation on the control of shock-induced boundary layer separation. Therefore, a pressure feedback technique (PFT) to control the boundary layer separation has been studied numerically on a two-dimensional flat plate and ramp configuration. PFT consists of a channel that drains fluid from the separated region and reintroduces it in the core flow under the influence of pressure difference. The effect of various freestream, wall, and geometric conditions on the separation controllability of the PFT is examined. It has been observed that PFT with injection located near the upstream influence point and suction at the ramp foot reduces the length of the separation bubble by 12.15%. Variation of injection location marginally affects the separation reduction capability of PFT, while the performance of PFT is greatly dependent on suction location. The parametric study noted that the optimum suction location, for which the separation size is minimum, shifts downstream with increasing freestream Mach number, wall temperature, and ramp angle. Again the optimum suction location shifts upstream as freestream stagnation temperature to wall temperature ratio increases. Reynolds number is seen to have no impact on the optimum suction location. Moreover, freestream stagnation temperature to wall temperature ratio, which is a governing parameter in SWBLI studies, is also a relevant factor in the mechanism of PFT. Finally, the present limited investigation revealed that the suction end of the PFT positioned between Xsu/L = 1.12 to 1.194 provides a minimum size of shock-induced separation for varying freestream and wall conditions.

Heat Transfer Characteristics of Mixed Convective Magnetohydrodynamic Counter-current Two-Phase Flow in Rotating Inclined Microporous Channel With Slip Velocity and Asymmetric Thermal Boundary Conditions Using LTNE Model

Abstract The heat transfer characteristics of a mixed convective two-phase flow in an inclined rotating microporous channel kept in a transverse magnetic field are investigated numerically. The counterflow arrangement is assumed within the channel. Slip velocity and asymmetric thermal boundary conditions are assumed. The governing energy equation involves the local thermal non-equilibrium (LTNE) between the two phases. The LTNE implications of the control parameters on the flow field variables and the average Nusselt number, Nu, are highlighted, and pertinent observations are documented. When confined to a few specific cases, the current results are consistent with previous research work. The effect of inclination angle on fluid velocity is determined by the wall temperature difference ratio. According to the findings, for certain values of the wall temperature differential ratio, the velocity increases with the angle; however, it takes on a dual character for other values. The Nusselt number (Nu) is expected to increase with the Biot number, Hartmann number, and rotation parameter, while Nu decreases as the Knudsen number increases. The results also show that as the wall temperature ratio increases, the Nu converges to a common minimum value. This research combines computational fluid dynamics (CFD) simulation and artificial neural network (ANN) analysis. The database was generated from the validated CFD model covering a range of control parameters arising in the system. The multilayer perceptron (MLP) networks were trained using this CFD data set to predict Nu. It is observed that the predicted data given by the ANN model is in good accordance with the estimated values of Nu. The average relative error in Nu's prediction is found to be ±2%.

Mode transition in a standing-wave thermoacoustic engine: A numerical study

This study investigates the mode transition phenomenon in a standing-wave thermoacoustic engine (TAE) by means of computational fluid dynamics (CFD). Firstly, the steady-state responses of the TAE at selected temperature ratios are examined via continuous wavelet transform. The bifurcation diagram and spectral map indicate that, as the temperature ratio increases, the TAE experiences a series of bifurcations, through which first-mode periodic oscillations, quasiperiodic oscillations and second-mode periodic oscillations occur. Secondly, the TAE performances in the initial decay/build-up, nonlinear saturation and steady states are studied. The onset of the first and/or second acoustic mode is identified via dynamic mode decomposition. The oscillation frequencies and growth/attenuation rates from CFD agree well with those from the reduced-order network model. Nonlinear mode competition takes place during saturation in which the growth of one acoustic mode is affected or even totally inhibited by the growth of the other. At steady state, periodic oscillations exhibit a closed loop in the phase space whilst quasiperiodic oscillations generate a torus. The time-averaged acoustic energy density, acoustic intensity and efficiency increase with increasing temperature ratio. Finally, parametric studies are conducted to investigate the effects of the gap between stack plates and stack position on mode transition. It is found that the TAE will exhibit second-mode oscillations if the stack is near the closed end or the gap is small. Results in this study indicate that mode transition could become a novel approach to match the TAE with external loads for higher electric power outputs.

Numerical investigation on the flow and heat transfer in swirl chambers with distributed multi exit slots and dimple/protrusion structure

Swirl cooling is an excellent candidate for internal cooling of turbine blade leading edge. In this paper, a novel swirl chamber with distributed multi exit slots is proposed, for which different outlet configurations are considered. Flow and heat transfer characteristics are numerically investigated and compared with those of conventional chamber. Then dimples/protrusions are innovatively introduced into the swirl chamber, with various arrangements. The influence of temperature ratio is further discussed. The results show that, a typical flow structure of “inner vortex + outer vortex” is formed in the jet regions. In novel swirl chambers, the extraction effect of exit slots avoids the formation of crossflow, thus the friction can be effectively reduced while the heat transfer is strengthened, for which a 46.9% improvement of overall thermal performance (TP) is achieved. The dimple/protrusion can realize the local flow control, and produce a 54.8% improvement in TP. The nozzle-side arrangement of dimple/protrusion is better than the slot-side arrangement, and the protrusion is superior to the dimple. With the increase of temperature ratio, the TP shows the variation trend of increasing first and then decreasing. This research will provide important data support for the internal cooling design of turbine blade leading edge.

The nonlinear longitudinal lattice waves in a one-dimensional dusty chain with the plasma absorption effect

The nonlinear longitudinal dust lattice wave of a one-dimensional dust chain under the influence of the plasma absorption effect was investigated. A modified KdV-Burgers’ equation with the plasma absorption effect is derived under the continuum limit by the reductive perturbation method. Based on solutions of the Burgers’ equation, the effects of dynamic damping, absorption coefficient, characteristic length and shielding parameter on the shock structure are investigated. It is found that shock wave polarity is determined by hydrodynamical damping. Furthermore, as the electron-to-ion temperature ratio increases, shock amplitude increases correspondingly, but its width remains basically unchanged. Therefore, the shock structure of nonlinear longitudinal dust lattice wave with plasma absorption potential is different from the one under the conventional Debye–Hückel potential.

Stability of three-dimensional dust acoustic waves in a strongly coupled dusty plasma including kappa distributed superthermal ions and electrons

A rigorous theoretical investigation of multi-dimensional instability of dust acoustic (DA) solitary waves in strongly coupled dusty plasma (SCDP) composed of superthermal distributed ions and electrons is presented. The dynamics of linear and nonlinear electrostatic excitations are studied. The dispersion properties of linear DA solitary waves are discussed in details. A nonlinear Zakharov-Kuznetsov (ZK) equation adequate for describing DA solitary waves is also derived. At the vicinity of the critical phase velocity, an extended ZK (EZK) equation is derived. The effects of the electrons and ions superthermality, the strongly and weakly coupling cases, and the ion-to-electron temperature ratio are discussed. The growth rate of the produced waves is calculated. It is found that the DA phase velocity decreases as the ion-to-electron temperature ratio increases. The contrary is occurred for strongly coupled system. The instability growth rate increases (decreases) by increasing the ion-to-electron temperature ratio (the superthermal parameters). The present investigation could be helpful in understanding the propagation and the instability of the detected DA solitary waves in laboratory, space and astrophysics mediums where superthermal particles are present.

Effect of non-parallel mean flow on the acoustic spectrum of heated supersonic jets: Explanation of “jet quietening”

Noise measurements of heated axisymmetric jets at a fixed supersonic acoustic Mach number indicate that the acoustic spectrum reduces when the temperature ratio increases. The “spectral quietening” effect has been observed both experimentally and computationally using large Eddy simulations. It was explained by Afsar, Goldstein, and Fagan [AIAA J. 49, 2522 (2011)] through the cancellation introduced by the enthalpy flux/momentum flux coupling term using the generalized acoustic analogy formulation. But the parallel flow assumption is known to give inaccurate predictions at high jet speeds. In this paper, we therefore extend the nonparallel flow asymptotic theory of Goldstein, Sescu, and Afsar [J. Fluid Mech. 695, 199 (2012)] for the vector Green’s function of the adjoint linearized Euler equations in the analogy. Using the steady Reynolds averaged Navier Stokes calculation for the jet mean flow, we find that the coupling term propagator is positive-definite and asymptotically subdominant at low frequencies corresponding to the peak jet noise when nonparallel flow effects are taken into account and self-consistent approximations for the turbulence structure are made. We then assess the validity of the non-parallel flow-based acoustic analogy model by computing the overall sound pressure level (OASPL) at various observation angles. Interpretation of the latter allows a more rational explanation of the quietening effect. In general, our noise predictions are in very good agreement with acoustic data beyond the peak frequency.

Effects of circumferential nozzle number and temperature ratio on swirl cooling characteristics

In this paper, the reasonable swirl cooling structures are established to study the effects of circumferential nozzle number and temperature ratio on swirl cooling. Numerical simulations are conducted by solving the Reynolds Averaged Navier-Stokes (RANS) equations and the standard k-ω turbulence model. Under the same coolant mass flow rate and nozzle aspect ratio, two swirl cooling models of keeping the nozzle geometry unchanged, and keeping the nozzle inlet total area unchanged are studied, respectively. Besides, effects of inlet to target wall temperature ratio on multiple circumferential jet nozzles configurations are involved in parametric studies. Results indicate that when keeping the nozzle geometry unchanged, the heat transfer intensity and total pressure penalty decrease with the increasing circumferential nozzle number. And the target Nusselt number distribution becomes more uniform. When keeping the nozzle inlet total area unchanged, the heat transfer intensity increases firstly and then decreases with the increase of circumferential nozzle number. Finally, it becomes the largest value at the circumferential nozzle number equaling two. Meanwhile, the total pressure loss increases with the increasing circumferential nozzle number. Overall, the configuration of the circumferential nozzle number equaling two has the optimum cooling efficiency. Additionally, the increase of temperature ratio contributes to a relatively higher Nusselt number and a lower heat flux. Compared with former studies, it suggests that different circumferential and axial jet nozzles may affect the influences of temperature ratio on swirl cooling performance.

Spatial-Temporal Instability of an Inviscid Shear Layer

In this work, we explore the transition of absolute instability and convective instability in a compressible inviscid shear layer, through a linear spatial-temporal instability analysis. From linearized governing equations of the shear layer and the ideal-gas equation of state, the dispersion relation for the pressure perturbation was obtained. The eigenvalue problem for the evolution of two-dimensional perturbation was solved by means of shooting method. The zero group velocity is obtained by a saddle point method. The absolute/convective instability characteristics of the flow are determined by the temporal growth rate at the saddle point. The absolute/convective nature of the flow instability has strong dependence on the values of the temperature ratio, the velocity ratio, the oblique angle, and M number. A parametric study indicates that, for a great value of velocity ratio, the inviscid shear layer can transit to absolute instability. The increase of temperature ratio decreases the absolute growth rate when the temperature ratio is large; the effect of temperature ratio is opposite when the temperature ratio is relatively small. The obliquity of the perturbations would cause the increase of the absolute growth rate. The effect of M number is different when the oblique angle is great and small. Besides, the absolute instability boundary is found in the velocity ratio, temperature ratio, and M number space.

Effects of aerodynamic parameters on steam vortex cooling behavior for gas turbine blade leading edge

Three-dimensional Reynolds-averaged Navier–Stokes equations are applied to further explore steam vortex cooling mechanism in gas turbine blade leading edge. Grid independence analysis and turbulence model validation are carried out to determine the proper grid dimension and turbulence model for simulations. Influences of Reynolds number and temperature ratio on steam vortex cooling flow and heat transfer behavior for the blade leading edge are investigated. Heat transfer and friction correlations for steam vortex cooling are achieved on the basis of numerical data. Results show that radial convection is generated due to violent rotational motion and uneven density distribution, contributing to heat transfer enhancement. For the sake of increasing steam velocity, the obvious increase in heat transfer intensity and decrease in friction coefficient are observed with the increasing Reynolds number. When the temperature ratio increases, the heat transfer intensity decreases slightly and the friction coefficient decreases significantly. The thermal performance increases with the increasing Reynolds number and the decreasing temperature ratio. Compared with calculating results, the heat transfer and friction correlations can predict steam vortex cooling characteristics accurately.

Nonlinear electrostatic solitary waves in electron–positron plasmas

The generation of nonlinear electrostatic solitary waves (ESWs) is explored in a magnetized four component two-temperature electron–positron plasma. Fluid theory is used to derive a set of nonlinear equations for the ESWs, which propagate obliquely to an external magnetic field. The electric field structures are examined for various plasma parameters and are shown to yield sinusoidal, sawtooth and bipolar waveforms. It is found that an increase in the densities of the electrons and positrons strengthen the nonlinearity while the periodicity and nonlinearity of the wave increases as the cool-to-hot temperature ratio increases. Our results could be useful in understanding nonlinear propagation of waves in astrophysical environments and related laboratory experiments.

Stability of three-dimensional obliquely propagating dust acoustic waves in dusty plasma including the polarization force effect

Propagation of dust acoustic solitary waves (DASWs) in a magnetized dusty plasma consisting of extremely massive, negatively/positively charged dust fluid and Boltzmann distributed electrons and ions is studied. A nonlinear Zakharov-Kuznetsov (ZK) equation adequate for describing the solitary waves is derived by applying a reductive perturbation technique. Moreover, an extended Zakharov Kuznetsov (EZK) equation is derived at the vicinity of the critical phase velocity. The effects of the polarization force are explicitly discussed and the growth rate of the produced waves is calculated. It is found that the physical parameters have strong effects on the instability criterion as well as on the growth rate. It is noted that the phase velocity decreases as the polarization force, the effective-to-ion temperature ratio, and the ion-to-electron temperature ratio increase. Moreover, the nonlinearity coefficient and the critical phase velocity increase by increasing the polarization force. The relevance of these findings to a recent plasma experiment and astrophysical plasma observations is briefly discussed.

Surface-radiation effect on natural convection flow in a fluid-saturated non-Darcy porous medium enclosed by non-isothermal walls

Purpose – The purpose of this work is to study the effect of surface-radiation on the phenomenon of natural convection flow of a Newtonian fluid in a non-Darcian porous media cavity. The study is mainly focused on the interaction between the inertial resistance of the fluid layers and the surface radiation. Design/methodology/approach – For numerical simulation of transient vorticity transport and energy equations, the paper uses the alternate direct implicit method. Forward Time Central Space descretization is used for the transient and diffusion terms in the alternate direct implicit method, whereas for the convective terms, the method is modified using second upwind differencing technique. ADI method is adopted here, since this technique is unconditionally stable as a complete sweep and is second-order accurate in time for low velocity changes. The stream function equation is solved using the successive over relaxation technique with residual tolerance of order 10-5. Findings – It was found that despite the reduction of flow, the heat transfer increases as the Forschheimer resistance is increased. Further, with the increase in the Planck number, the heat transfer from the bottom radiating wall increases. Darcy drag parameter did not have a significant impact on flow properties except a slight reduction in the flow. Nevertheless, the increase in temperature ratio has a significant impact on flow properties. Research limitations/implications – The analysis is valid for unsteady, two-dimensional natural convection flow in a fluid-saturated non-Darcy porous medium enclosed by non-isothermal walls. As a first case, the study is conducted for square cavity. An extension to three-dimensional flow case and the study of Darcy-Forschheimer medium with effect of viscous dissipation is left as a part of future work. Practical implications – The approach is applicable to the modeling of geothermal systems where the inertial resistance to flow also comes into act with the non-uniform temperature distribution. The method is very useful to analyze solar receiver systems, fire research, electronic cooling, brake housing of an aircraft and many environmental geothermal processes. Originality/value – The study may be of some interest to engineers interested in heat transfer in ventilated rooms or enclosures, the industrial waste, water and atmospheric pollution.

Ion-acoustic double layers in electron–positron–ion plasmas with finite ion temperature

AbstractThe large-amplitude ion-acoustic double layers in a collisionless plasma consisting of isothermal positrons, warm adiabatic ions and two-temperature distribution of electrons are investigated. Using the pseudo-potential approach, an energy-integral equation for the system has been derived which encompasses complete nonlinearity for the plasma system. The existence region of the double layers is analyzed numerically. It is found that for a selected set of physical parameters, the rarefactive double layer exists in the electron–positron–ion plasma. It is found that the existence regime of the double layer is very sensitive to the plasma parameters, e.g. cold electron concentration (μ) and temperature ratio of two electron species (β). An increase in the finite ion temperature ratio increases the amplitude of the rarefactive double layer. To study small-amplitude double layers, we have expanded the Sagdeev potential. In the case of small amplitude, it is found that the amplitude of the double layer increases with increase in ion temperature ratio (σ) and cold electron concentration (μ). However increase in positron concentration (α) and temperature ratio of positrons to electrons (γ) decreases the amplitude of the double layer. The effect of various plasma parameters on the characteristics of the double layers is discussed in detail. The results of the investigation may be helpful to understanding basic plasma characteristics in space.

Experimental studies on heat and mass transfer in tubular generator for R134a-DMF absorption refrigeration system

Experimental investigations have been carried out to study heat and mass transfer during desorption of Tetrafluoro ethane (R134a) from R134a and Dimethyl formamide (DMF) solution in a tubular generator. Absorption refrigeration system has been built with brazed plate heat exchangers as condenser, absorber, evaporator and solution heat exchanger and with stainless steel concentric tubes as the generator. Effects of operational parameters viz., solution two phase Reynolds number, driving temperature ratio, driving pressure ratio, solution initial concentration on generator performance are analyzed. Desorption ratio, Sherwood number and Nusselt number increase as the solution Reynolds number, solution initial concentration, driving temperature ratio increase whereas these parameters decrease as the driving pressure ratio increases. Finally a correlation for Nusselt number and Sherwood number are proposed based on the experimental studies.

Heat and mass transfer studies on compact generator of R134a/DMF vapour absorption refrigeration system

Experimental investigation on the performance of compact generator of R134a/DMF vapour absorption refrigeration system is presented. The system uses brazed plate heat exchangers as generator, condenser, absorber, evaporator and solution heat exchanger and has a rated cooling capacity of 1 kW. The influence of driving temperature ratio, driving pressure ratio, R134a mass fraction and solution two phase Reynolds number is studied. This kind of system can be operated with low grade thermal energy such as solar energy, waste heat, etc. The generator temperature is chosen accordingly between 353 K and 368 K. Desorption ratio and Sherwood number increase as the solution Reynolds number, solution initial mass fraction, and driving temperature ratio increase whereas they decrease as the driving pressure ratio increases. The performance of the compact generator at different operating conditions is presented. Based on the present data, empirical correlations for Nusselt number and Sherwood number are proposed.

Finite time thermodynamic modelling and analysis for irreversible IFGT cycles

<title/>Indirectly or externally fired gas turbines (IFGTs or EFGTs) are interesting technologies under development for small and medium scale combined heat and power supplies in combination with micro gas turbine technologies. The emphasis is primarily on the utilisation of the waste heat from the turbine in a recuperative process. The possibility also exists for burning biomass even ‘dirty’ fuel by employing a high temperature heat exchanger (HTHE) to avoid the combustion gases passing through the turbine. In this paper, the theory of finite time thermodynamics is used in the performance analysis of a class of irreversible closed IFGT cycles coupled to variable temperature heat reservoirs. The analytical formulae for the dimensionless power output and efficiency, as functions of the total pressure ratio, component (HTHE, hot and cold side heat exchangers) effectiveness, compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficient, the heat reservoir inlet temperature ratio, are derived. Analyses are carried out based on the formulation. Indirectly fired gas turbine cycles are most efficient under low compression ratio ranges (2·0-5·0) and fit for low power output circumstances for integration with micro gas turbine technology. Optimal total pressure ratio π c under maximum power output is always higher than that under maximum cycle thermal efficiency. When either of the heat transfer effectiveness of the hot or the cold side heat exchanger, the pressure recovery coefficient, isentropic efficiencies of the gas turbine and the compressor and the heat reservoir inlet temperature ratio increase, the dimensionless power output, cycle efficiency and their corresponding optimal total pressure ratios increase. It must be noted that the optimal total pressure ratio π c under the maximum cycle thermal efficiency decreases with increase in heat transfer effectiveness of the HTHE. The model derived can be further used to optimise the operational parameters and forecast performance of practical IFGT configurations and choices.

Thermodynamic modelling and performance analysis of a closed endoreversible indirectly-fired gas turbine cycle

SYNOPSIS Indirectly or externally-fired gas-turbines (IFGT or EFGT) are interesting technologies under development for small and medium scale combined heat and power (CHP) supplies in combination with micro-gas-turbine technologies. The emphasis is primarily on the utilisation of the waste heat from the turbine in a recuperative process and the possibility of burning biomass (even “dirty” fuel) by employing a high temperature heat exchanger (HTHE) to avoid the combustion gases passing through the turbine. In this paper, the theory of finite time thermodynamics is used in the performance analysis of a class of endoreversible closed IFGT cycles coupled to variable temperature heat reservoirs. The analytical formulae for the dimensionless power output and efficiency, as functions of the total pressure ratio, component (HTHE, hot- and cold-side heat exchangers) effectivenesses, compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficient and the heat reservoir inlet temperature ratio, are derived and analysed based on illustrations. IFGT cycles are most efficient under low compression ratio ranges (2.0–5.0) and fit for low power output circumstances for integrating with micro-gas-turbine technology. Optimal total pressure ratio πc under maximum power output is always higher than that under maximum cycle thermal efficiency. When either of the heat transfer effectivenesses of the hot or the cold-side heat exchanger, the pressure recovery coefficient and the heat reservoir inlet temperature ratio increases, the dimensionless power output and efficiency and their corresponding optimal total pressure ratios increase. It must be noted that the optimal total pressure ratio πc under the maximum cycle thermal efficiency decreases with the increase of heat transfer effectiveness of the HTHE. The model derived can be further used to optimise the operational parameters and forecast performance of practical IFGT configurations and choices.

Effect of Temperature Ratio on Jet Array Impingement Heat Transfer

This paper consider the effects of temperature ratio on the heat transfer from an array of jets impinging on a flat plate. At a constant Reynolds number of 18,000 and a constant Mach number of 0.2, different ratios of target plate temperature to jet temperature are employed. The spacing between holes in the streamwise direction X is 8D, and the spanwise spacing between holes in a given streamwise row Y is also 8D. The target plate is located 3D away from the impingement hole exits. Experimental results show that local, line-averaged, and spatially averaged Nusselt numbers decrease as the Twa∕Tj temperature ratio increases. This is believed to be due to the effects of temperature-dependent fluid properties, as they affect local and global turbulent transport in the flow field created by the array of impinging jets. The effect of temperature ratio on crossflow-to-jet mass velocity ratio and discharge coefficients is also examined.