Case Of Constant Temperature Research Articles

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Hydro-mechanical modelling of swell behaviour of bentonite buffer

Effect of temperature and heat flux boundary conditions on hydrogen production in membrane-integrated steam-methane reformer

In order to operate reforming plants at maximum efficiency it is important to investigate their operation under various conditions. This study aims to perform a comprehensive computational analysis to examine the effect of thermal boundary conditions on SMR and hydrogen recovery and, consequently, enriching the literature on hydrogen generation through steam-methane reforming. The studied thermal boundary conditions include constant and variable inlet temperature, outer wall temperature, and heat flux on outer wall of the reactor. A constant inlet flow rate of 0.001 kg/s (methane mass flow rate = 0.00018 kg/s), steam-to-methane ratio (or steam-to-carbon ratio, i.e., S/C) of 4 is considered in the reformer side for all the cases. Furthermore, the difference of pressure between both faces of the membrane is kept constant. Sweeping flow is incorporated to further raise the hydrogen partial pressure difference. Steam with flow rate of 0.001 kg/s is used to induce the sweeping effect throughout the study. Constant and variable temperatures/heat fluxes along with the effect of radiation in the reformer and transient inlet temperatures are examined in this study in terms of CH4 conversion ( %), H2 recovery ( %), H2 mass flow rates (kg/s), hydrogen mass flux through the membrane (kg.m−2. s−1), and other important parameters. About 1.7 times increase in methane conversion is noted as the reformer temperature is increased from 500 to 700 °C. The hydrogen recovery is found to decrease with the increase in temperature. Around 57 % and 56 % decrease in methane conversion and 42 % and 11.5 % rise in hydrogen recovery is observed with segmented case as compared to the constant heat flux and constant temperature cases, respectively. As compared to the reference 600 °C constant wall temperature, the increasing temperature profile results in 10.6 % rise in methane reforming and 11.2 % drop in the recovery of hydrogen. On the other hand, the decreasing temperature profile demonstrates 9.4 % decrease in conversion and 10.2 % elevation in recovery rate in contrast with the constant temperature case.

Dimensionless Characterization to Estimate Horizontal Groundwater Velocity from Temperature–Depth Profiles in Aquifers

The outcome of a dimensionless characterization study in a two-dimensional porous media domain in which groundwater flows at a constant horizontal velocity is presented in this report. Using spatial discrimination, the dimensionless groups that govern the solution patterns are determined from dimensionless governing equations. As a boundary condition on the surface, the case of constant temperature is studied. From the mathematical deduction of the groups, a characteristic horizontal length emerges. This length determines the region in which temperature–depth profiles are affected by flow. Existing analytical solutions have been shown to be invalid due to the severe assumption that the horizontal thermal gradient has a constant value. Therefore, universal solutions based on pi theorem have been obtained for the characteristic horizontal length, temperature field, temperature–depth profiles and horizontal temperature profiles. Dependencies between dimensionless groups have been depicted by universal curves, abacuses and surfaces. These graphical solutions are used in an easy way to estimate groundwater velocity from experimental temperature measurements in the form of an inverse problem. In addition, an easy and fast protocol for estimating fluid flow velocity and groundwater inlet temperature from temperature profile measurements is proposed. This protocol is applied in a scenario of groundwater discharge from a quaternary aquifer to a salty lagoon located in the southeast of Spain.

Hydrate-based gas separation for working fluid mixtures: Application to composition-adjustable organic Rankine cycle

Composition-adjustable organic Rankine cycle (ORC) is deemed to improve the overall energy efficiency compared to the conventional ORC, bringing more energy saving and emission reduction. Inspired by hydrate-based gas separation, the hydrate-based composition regulation of working fluid was proposed and investigated experimentally using R22/R125 mixtures as feed gas. To improve the hydrate formation kinetics, stirring was employed and boosted the initial hydrate formation rate by about 200% compared with the unstirred cases. Cases with more R22 fraction in the feed gas showed faster initial hydrate formation rate. Partition of components between gas and hydrate phases was observed in a kinetic separation manner, i.e., R22 led the hydrate formation at the early stage while R125 was enclathrated more at the later stage. With proper formation time (45 – 135 min), the R125 fraction in the gas phase could increase by 9.5 – 18.7 mol% from various feed gas compositions (30 – 70 mol%). In the hydrate phase R22 was typically enriched with the best separation factor varied between 2.71 and 10.27. Compared to hydrate formation at a constant temperature (275.5 K), the temperature ramping down mode (from 278.5 K to 275.5 K) slowed down the hydrate formation kinetics but amplified the separation performance, with the best R22 separation factor 2.84 times higher than that of the constant-temperature case achieved at the same time. Visual images revealed a “wall-climbing” hydrate growth pattern for R125/R22 mixed hydrate formation particularly in unstirred conditions. These results support the proposal of a hydrate-based composition-adjustable ORC and could lead to further development of this technology with practical insights.

Dominant heating mechanisms in a surface barrier discharge

In computational models of atmospheric pressure surface barrier discharges (SBDs) the role of heating of the dielectric material and the quiescent gas is often neglected, impacting the accuracy of the calculated chemical kinetics. In this contribution, a two-dimensional fluid model of an SBD was developed and experimentally validated to determine the relative contribution of the dominant heat transfer mechanisms and to quantify the impact of discharge heating on the resultant chemistry. Three heating mechanisms were examined, including electron heating of the background gas due to inelastic collisions, ion bombardment of the dielectric surface and dielectric heating by the time-varying electric field. It was shown that electron heating of the background gas was not significant enough to account for the experimentally observed increase in temperature of the dielectric material, despite being the dominant heating mechanism of the gas close to the electrode. Dielectric heating was ruled out as the frequency response of typical dielectric materials used in SBD devices does not overlap with the experimentally observed power spectrum of an SBD excited at kHz frequencies. The ionic flux heating was found to be the dominant heating mechanism of the dielectric material and the downstream flow driven by the SBD. The largest impact of plasma heating on discharge chemistry was found in reactive nitrogen species (RNS) production, where the densities of RNSs increased when an appropriate treatment of heating was adopted. This had a marked effect on the discharge chemistry, with the concentration of NO2 increasing by almost 50% compared to the idealized constant temperature case.

Effect of radiation on laminar natural convection of nanofluid in a vertical channel with single- and two-phase approaches

In the present study, heat transfer and laminar flow of a nanofluid in a vertical channel by considering the effect of radiation with single- and two-phase approaches with prescribed surface temperature conditions and prescribed surface heat flux conditions were simulated. The main goal of this study is to investigate the effect of variations of Grashof number (Gr), radiation parameter (Nr) and volume fraction of nanoparticles (ϕ) on flow and heat transfer characteristics. For this goal, flow with Gr = 5, 10, 15 and 20, volume fractions of 0, 0.1 and 0.2 and radiation parameters of Nr = 0, 0.5 and 1 were simulated. The results show that by increasing Grashof number in both cases of constant heat flux and temperature, nanofluid velocity increases and in both cases of constant temperature and heat flux by increasing volume fraction, the velocity and temperature of the nanofluid drops. The presence of moving wall (plate boundary condition) induces secondary flows in the flow field, and the flow movement in the channel will experience drift because of temperature variations and buoyancy forces due to inducement of secondary forces and the effect of penetration of moving plate velocity into the fluid close by it which will affect the entire fluid flow field in the end. For fixed plate case, the velocity of nanofluid at the walls is zero because of fixed position of the plate and presence of no-slip boundary condition on the solid walls. By increasing the applied temperature, the value of kinetic and internal energy of the velocity field rises which results in higher density gradients and higher buoyancy forces. For both constant heat flux and temperature, increasing solid nanoparticles volume fraction results in lowering of the velocity contour elevations. The quantitative level of axial velocity curves for constant heat flux condition compared with constant temperature case for Gr = 5 and Nr = 0.5 is about 2–3 times less. For constant temperature boundary condition, for Gr = 5 and Nr = 0.5 and volume fraction of 0.1%, the maximum velocity happens at regions 30–50% of channel height from the bottom.

Unsteady Free Convection Boundary Layer Flows of a Bingham Fluid in Cylindrical Porous Cavities

We consider two unsteady free convection flows of a Bingham fluid when it saturates a porous medium contained within a vertical circular cylinder. The cylinder is initially at a uniform temperature, and such flows are then induced by suddenly applying either a new constant temperature or a nonzero heat flux to the exterior surface. As time progresses, heat conducts inwards and this may or may not overcome the yield threshold for flow. For the constant temperature case, flow begins immediately should the parameter, Rb, which is a nondimensional yield parameter, be sufficiently large. The ultimate fate, though, is full immobility as the cylinder eventually tends towards a new constant temperature. For the constant heat flux case, the fluid remains immobile but will begin to flow eventually should Rb be sufficiently large. The two cases have different critical values for Rb.

Double-diffusive instabilities at a horizontal boundary after the sudden onset of heating

When a deep body of fluid with a stable salinity gradient is heated from below at a horizontal boundary a destabilizing temperature gradient develops and can lead to instabilities. We will focus on two variants of this problem: the sudden increase in the boundary temperature at the initial time and the sudden turning on of a constant heat flux. These generate time-dependent temperature profiles. We look at the growing phase of the linear instabilities as an initial value problem where the initial time for the instabilities is a parameter to be determined. We determine numerically the optimal initial conditions and the optimal starting time for the instabilities to ensure that the maximum growth occurs at some given later time. The method that is used is an extension of the method developed by Kerr & Gumm (J. Fluid Mech., vol. 825, 2017, pp. 1002–1034) in their investigation of the stability of developing temperature boundary layers at horizontal and vertical boundaries. This requires the use of an appropriate measure of the amplitude of the disturbances which is identified. The effectiveness of this approach is verified by looking at the classic problem of double-diffusive convection in a horizontal layer, where we look at both the salt-finger regime and the diffusive regime. We show that this approach is an effective way of investigating instabilities where the background gradients time dependent. For the problem of heating a salinity gradient from below, as the heat diffuses into the fluid the effective thermal Rayleigh number based on the instantaneous diffusion length scale grows. For the case of a sudden increase in the temperature by a fixed amount the effective thermal Rayleigh number is proportional to $t^{3/2}$, and for a constant heat flux it is proportional to $t^{2}$, where $t$ is the time since the onset of heating. However, the effective salt Rayleigh number also grows as $t^{2}$. We will show that for the constant temperature case the thermal Rayleigh number initially dominates and the instabilities undergo a phase where the convection is essentially thermal, and the onset is essentially instantaneous. As the salt Rayleigh number becomes more significant the instability undergoes a transition to oscillatory double-diffusive convection. For the constant heat flux the ratio of the thermal and salt Rayleigh numbers is constant, and the instabilities are always double diffusive in their nature. These instabilities initially decay. Hence, to achieve the largest growth at some given fixed time, there is an optimal time after the onset of heating for the instabilities to be initiated. These instabilities are essentially double diffusive throughout their growth.

Thermodynamical transcription of density functional theory with minimum Fisher information

Ghosh, Berkowitz and Parr designed a thermodynamical transcription of the ground-state density functional theory and introduced a local temperature that varies from point to point. The theory, however, is not unique because the kinetic energy density is not uniquely defined. Here we derive the expression of the phase-space Fisher information in the GBP theory taking the inverse temperature as the Fisher parameter. It is proved that this Fisher information takes its minimum for the case of constant temperature. This result is consistent with the recently proven theorem that the phase-space Shannon information entropy attains its maximum at constant temperature.

Phase space view of quantum mechanical systems and Fisher information

Pennini and Plastino showed that the form of the Fisher information generated by the canonical distribution function reflects the intrinsic structure of classical mechanics. Now, a quantum mechanical generalization of the Pennini–Plastino theory is presented based on the thermodynamical transcription of the density functional theory. Comparing to the classical case, the phase-space Fisher information contains an extra term due to the position dependence of the temperature. However, for the special case of constant temperature, the expression derived bears resemblance to the classical one. A complete analogy to the classical case is demonstrated for the linear harmonic oscillator.

Energy and exergy analysis and modeling temperature distribution in a membrane distillation feed channel for VMD and DCM

This research is a comparative study between vacuum membrane distillation and direct contact membrane distillation technologies. The first part focuses on an energy and exergy analysis of vacuum membrane distillation and direct contact membrane distillation configurations. The results of the energy analysis show the vacuum membrane distillation has lower energy losses across the membrane compared to direct contact membrane distillation. The results of the exergy analysis show that direct contact membrane distillation requires less work compared to vacuum membrane distillation. The second part of this research focuses on modeling of temperature distribution in feed channel for DCMD and VMD. The results of the numerical solution show that the temperature polarization effort in VMD is smaller than in DCMD. The membrane temperature in VMD approach constant temperature case and this reduces dissipated energy in the membrane distillation process.

Numerical Study of Heat Transfer and Contaminant Transport in an Unsaturated Porous Soil

Penetration of chemicals in the soil ground through irrigation water or rainfall induces important risks for the environment. These risks are badly known and may lead to direct contamination of the environment (atmosphere or ground water) or harmful effects on organisms living at ground level, indirectly affecting men. It is thus necessary to estimate these potential chemical risks on the environment. For that reason, the gradual change of these products (fertilizers, solutions, pollutants, ...) in the ground has been the subject of a lot of recent research works, based in particular on the study of non-saturated porous media in a theoretical, numerical or experimental way. Most of these works are incomplete and, in order to simplify the problem, they don’t take into accounts some process, which may be of prime importance under particular natural conditions. Complexity of such studies results from their multidisciplinary nature. In this communication, we study simultaneous transport of pollutant, the water that provides transport and the heat transfer in a 200 cm long cylindrical column full of sand taken as a non-saturated porous medium. We consider two kinds of conditions on the temperature at the column surface: the case of constant temperature and the case of sinusoidal temperature. We evaluate the influence of this temperature on the transfers. This study is purely numerical. We use the control volume method to determine hydrous, thermal and pollutant concentration profiles.

Joule Heating Effect in Constant Voltage Mode Isotachophoresis in a Microchannel

Abstract Microchip ITP (isotachophoresis) is getting popularity as a preparative technique for preconcentration and separation of chemical species and/or ions in liquid phase. In constant voltage mode ITP, generally a high electric potential difference is applied to a discontinuous buffer for faster and higher resolution separation. However, the higher current from the applied electric field induces Joule heating in the buffer which modifies the mobility and diffusion coefficient of analytes in the system. This change in mobility and diffusion coefficient strongly influences the transient separation process in ITP. In this study the effect of Joule heating on separation behavior of analyte compounds has been presented in a constant voltage mode ITP where two chemical species are separated from an initial mixture. The model is based on mass, energy, and charge conservation and electroneutrality condition in the system. A set of nonlinear governing equations are solved numerically for temperature dependent properties such as diffusion coefficient, effective electrophoretic mobility, and thermal conductivity using a finite volume based model. Numerical results suggest that for temperature dependent properties of control parameters, the separation speed of analytes is significantly different from that of constant temperature case. In constant voltage mode ITP, the temperature peak forms at the location of trailing electrolyte, and its influence propagates toward the direction of band movement as separation proceeds.

Biodegradation of Toluene Under Seasonal and Diurnal Fluctuations of Soil-Water Temperature

An increasing interest in bioremediation of hydrocarbon polluted sites raises the question of the influence of seasonal and diurnal changes on soil-water temperature on biodegradation of BTEX, a widespread group of (sub)-surface contaminants. Therefore, we investigated the impact of a wide range of varying soil-water temperature on biodegradation of toluene under aerobic conditions. To see the seasonal impact of temperature, three sets of batch experiments were conducted at three different constant temperatures: 10°C, 21°C, and 30°C. These conditions were considered to represent (1) winter, (2) spring and/or autumn, and (3) summer seasons, respectively, at many polluted sites. Three additional sets of batch experiments were performed under fluctuating soil-water temperature cases (21<>10°C, 30<>21°C, and 10<>30°C) to mimic the day–night temperature patterns expected during the year. The batches were put at two different temperatures alternatively to represent the day (high-temperature) and night (low-temperature) times. The results of constant- and fluctuating-temperature experiments show that toluene degradation is strongly dependent on soil-water temperature level. An almost two-fold increase in toluene degradation time was observed for every 10°C decrease in temperature for constant-temperature cases. Under fluctuating-temperature conditions, toluene degraders were able to overcome the temperature stress and continued thriving during all considered weather scenarios. However, a slightly longer time was taken compared to the corresponding time at daily mean temperature conditions. The findings of this study are directly useful for bioremediation of hydrocarbon-polluted sites having significant diurnal and seasonal variations of soil-water temperature.

Modeling of uranium bioleaching by Acidithiobacillus ferrooxidans

In this paper, a mathematical model for the mesophilic bioleaching of uraninite is developed. The case of constant temperature, pH, and initial ore concentration is considered. The model is validated by comparing the calculated and measured values of uranium extraction, ferric and ferrous iron in solution, and cell concentration. Good fits of the model were obtained, while the values of the parameters were within the range expected.New rate expressions were used for the iron precipitation and uranium leaching rates. The rates of chemical leaching and ferric precipitation are related to the ratio of ferric to ferrous in solution.The fitted parameters can be considered applicable only to this study. In contrast, the model equation is general and can be applied to bioleaching under the same conditions.

The stiffness estimation of steel semi-rigid beam–column moment connections in a fire

This study investigates the estimation of the stiffness of steel semi-rigid beam–column moment connections in a fire. The general purpose finite element software ANSYS is adopted. The parameters, such as material properties and geometric conditions, are fully studied to investigate their effect on stiffness. The formula for the stiffness of steel semi-rigid beam–column moment connections, which are made with the H-beam and H-column, for the case of constant temperature with increasing vertical beam load is proposed and verified by the results of ANSYS analysis. The stiffness of steel beam–column connections for the case of increasing temperature with constant load estimated by the proposed formula is found to be conservative.

Mathematical modeling of thermophilic bioleaching of chalcopyrite

Previous studies have shown that the different preferences of thermophiles to oxidize S 0 or Fe 2+ is reflected by different [Fe 3+]/[Fe 2+] levels in solution. In those studies it was concluded that [Fe 3+]/[Fe 2+] governs the thermophilic bioleaching of chalcopyrite rather than temperature or pH. Therefore, the proposed model is mainly based on the finding that thermophilic bioleaching of chalcopyrite is governed by [Fe 3+]/[Fe 2+] that result from the activity of thermophiles. A direct interaction between chalcopyrite and thermophiles is neglected because it has been reported that this is not a general behavior for all thermophiles. The case of constant temperature, initial pH 1.5–2.5, and chalcopyrite concentrates is considered. The main assumption is that chalcopyrite can be anodically oxidized or cathodically reduced depending on [Fe 3+]/[Fe 2+] in solution. When chalcopyrite is oxidized at high [Fe 3+]/[Fe 2+] levels, Cu 2+ is formed directly at low rates: CuFeS 2 + 4Fe 3+ → Cu 2+ + 5Fe 2+ + S 0. Whereas, when chalcopyrite is reduced at low [Fe 3+]/[Fe 2+] levels, an intermediate (Cu 2S) is formed at higher rates: CuFeS 2 + Fe 2+ + Cu 2+ + 2H + → Cu 2S + 2Fe 3+ + H 2S. Because the oxidation of Cu 2S is relatively fast: Cu 2S + 4Fe 3+ → 2Cu 2+ + S 0 + 4Fe 2+, its accumulation is assumed to be negligible. To take into account the possibility of chalcopyrite being oxidized or reduced depending on [Fe 3+]/[Fe 2+] in solution, the principle of mixed potentials is used. The model is validated by comparing the calculated and measured values of copper extraction, total iron in solution, and pH.

Thermal Stress in a Finitely Deformed Incompressible Elastic Medium Containing a Penny-Shaped Crack

ABSTRACTThe thermal stress for a penny-shaped crack contained in an infinite isotropic elastic solid initially subjected to an axisymmetrical tension of any amount at infinity is investigated using the techniques of Hankel transforms and multiplying factors. The effect that the lateral normal stress has on the thermal stresses is studied on the basis of the theory of small deformations superposed on finite deformation. Symmetrical thermal loadings are applied over the crack surfaces. For the case of constant temperature over the crack surfaces, expressions for the crack shape and thermal stresses in the crack plane are obtained in closed forms. The stress intensity factor is also obtained and shown to be dependent on the lateral stress.

A modified theoretical Σ − D relation for supernova remnants: I. The case of constant temperature within the supernova remnant

We present a modification of the theoretical ? ? D relation for supernova remnants (SNRs) in the adiabatic expansion phase. This modification is based on the convolution of the relation first derived by Shklovsky with the ? ? D relation derived in this paper for thermal bremsstrahlung radiation from the ionized gas cloud. We adopt McKee &amp; Ostriker?s model for the components of the interstellar medium as part of our derivation. The modified Shklovsky theory agrees well with empirical results. Kesteven?s modified theoretical relation gives the best agreement with the updated Galactic empirical ? ? D relation.

Transport of a passive scalar at a shear-free boundary in fully developed turbulent open channel flow

Direct numerical simulations of fully developed turbulence in an open channel geometry were performed in which a passive scalar was introduced. The simulations were intended to explore transport at free surfaces in two cases for which (1) the free surface was maintained at constant temperature and (2) the interfacial flux was fixed. These cases can be considered models for mass and evaporative heat transport where buoyancy and surface deformation effects are negligible. Significant differences were found in the thermal fields in these two cases. The turbulent statistics reveal that the surface flux in the constant temperature case was significantly more intermittent compared to the surface temperature field in the constant flux case. The surface temperature field in the latter case formed large patches of warm fluid, reminiscent of the so-called fish scale patterns revealed in recent infrared imagery of the air–water interface. The wake-like structure of the patches was evident despite the absence of surface shear. A model of surface renewal based on the existence of two disparate time scales (a fast hydrodynamic scale, and a slow, diffusional scale) was introduced to explain these differences in a heuristic manner. The model appears successful in explaining, in a qualitative sense, the surface thermal structure in each case. Correlations between the surface thermal fields (flux or temperature) and the subsurface hydrodynamics were also computed. A model based on the hypothesis that hairpin eddies are the dominant kinematic structure responsible for surface renewal is consistent with the correlations. However, these results cannot rule out the importance of other turbulent structures in free surface heat and mass transport processes.