Axial Temperature Profiles Research Articles

Impact of Variable Transport Properties and Slip Effects on MHD Jeffrey Fluid Flow Through Channel

The present paper aims at investigating the effects of variable transport properties and slip conditions on the MHD peristaltic mechanism of a Jeffrey liquid. The flow is considered to take place in an axisymmetric channel with compliant walls. The nonlinear momentum and energy equations are solved for small values of variable viscosity and thermal conductivity by utilizing the method of perturbation. Closed-form solutions are found for mass transfer equation. MATLAB programming is employed to get the pictorial representation of various parameters on axial velocity, streamlines, temperature and concentration profiles. The current investigation reveals that an increase in the value of the magnetic parameter reduces the velocity as well as temperature. Moreover, the size of the trapped bolus is seen to be a decreasing function of magnetic and Jeffery parameters.

Experimental and numerical study of syngas production during premixed and ultra-rich partial oxidation of methane in a porous reactor

The synthesis gas (syngas) production from the ultra-rich methane/oxygen mixtures via the thermal partial oxidation in an inert porous reactor was investigated numerically and experimentally. Thermodynamic analysis was firstly conducted based on Gibbs free energy minimization method to find the possible optimum routes of operation. Then, the experiments were performed on the constructed test-rig with a non-catalytic porous based reformer. The flame is stabilized within zirconia (ZrO2) sponge, which has shown very high mechanical strength and thermal resistance. The main influencing parameters such as the equivalence ratio and thermal load have been investigated during different experiments. For this purpose, the reactor axial temperature profile and product compositions were determined experimentally. The obtained results reveal that the heat loss abatement; approaching to the adiabatic condition could effectively improve the amounts of syngas (H2+CO) production. The maximum syngas production was obtained 69.5% of the exhaust gas at the equivalence ratio of 2.5 and thermal load of 8 kW. Moreover, the H2/CO ratio was reported above 1.5, which can be suitable for feeding into other chemical processes. Finally, numerical simulation of the process was performed using the premixed and reactor network models. The contribution of heat loss from the reactor was also considered in the model due to its pivotal role observed in the experimental work. The average relative error of the reactor network model with respect to syngas generated from the reformer was found to be 6.72%. Therefore, the predictions obtained from this model are in fairly good agreement with the experimental data.

Influence of flow rate and heat flux on the temperature distribution in long length counter flow cooled cold dielectric HTS cables

The technical advantages of using High Temperature Superconducting (HTS) cables have drawn the attention of most of the researchers around the world. However, the cooling design of these cables nevertheless remains a challenge for the researchers and manufacturers. Therefore, in the present work, an attempt has been made to ease the challenge of cooling of long length High Temperature Superconducting (HTS) cables. Counter flow cooling arrangements of liquid nitrogen (LN2) have been proposed and analysed the longitudinal temperature distribution along the superconducting cable using heat balance equations. Also, in the present work the solution for the temperature distribution profiles of LN2 in counter cooled HTS cable are also obtained assuming one dimensional (1-D) model. In particular, nonlinear axial temperature profiles of LN2 in the counter cooled HTS cable are found. The single phase cold dielectric HTS cable with stainless steel corrugated pipes as flow paths are used for the analysis. Liquid nitrogen with varying volumetric flow rates of 20–35 L/min and total heat loss (A.C. loss and exterior heat load) of 2.3–2.6 W/m reveals interesting facts about cooling of cables with longer lengths. The defined cable length range at constant diameter (inner and annulus pipe) for the analysis was considered as 1.5–3 km. As a result, the maximum possible HTS cable length of 2.5 km can be achieved at the flow rate of 30 L/min and heat fluxes of 2.3–2.6 W/m within the defined operating range.

Flow and heat transfer in narrow fixed beds with axial thermowells

Axial thermowells are used to monitor temperature in lab scale and pilot plant fixed-bed catalytic reactor tubes. Particle-resolved fixed-bed computational fluid dynamics simulations are used to study the local changes in particle location, flow, and temperature that result from using thermowells in narrow packed tubes. The bed structure, velocity, and temperature were compared for randomly packed spheres in heated-wall tubes. Tube-to-particle diameter ratios 3.96, 5.96, and 7.99 were used, each with three different thermowell sizes and with no thermowell. The measured axial temperature profiles differ from those in the un-instrumented beds due to conduction and flow by-passing along the thermowells.

Study of UV Rayleigh scattering thermometry for flame temperature field measurement

This paper explores Rayleigh scattering thermometry via a wavelength of 355 nm through a unique measurement scheme. In this context, the p-polarization and s-polarization Rayleigh scattering of flame and air (as the temperature calibration reference) are measured. Subtraction of p-polarization Rayleigh scattering intensity from that of s-polarization is proposed to eliminate the background noise and fluorescence interference influence to reduce the temperature measurement uncertainties. To validate this method, the temperature field of CH4/N2/O2 premixed flame at ϕ=0.78 on a McKenna burner is detected by this Rayleigh scattering thermometry, and the axial temperature profile is validated with the literature data. Within the region of interest domain (−5mm to 5 mm in the radial direction), an overall temperature measurement system precision of ±46.5K is reported. The influence of both p-polarization Rayleigh scattering and laser sheet inhomogeneity on the temperature measurement is further quantitatively studied. The measurement uncertainties relevant to laser energy variation and flame Rayleigh scattering cross-section variation due to temperature increase are specified as 1.4% and 2%–8%, respectively. Eventually, temperature measurements of single-shot images are attempted, and the large signal dynamic range (100–1000 [a.u.]) indicates a promising potential for temperature field interpretation of turbulence combustion.

Heat and peristaltic propagation of water based nanoparticles with variable fluid features

In present article a peristaltic analysis has been carried out to examine the nanofluid flow through the symmetric vertical channel. Three types of nanoparticles, copper oxide (), silver (Ag) and silicon dioxide (SiO2), are incorporated with base fluid as pure water. For this persistence the aspects of mixed convection and heat generation/absorption are accounted to observe the modifications in flow field as well as in fluid temperature. Viscosity is also considered which is determined by temperature. The large wavelength approximation with small Reynolds number is used to simplify the involved nonlinear terms in constitutive equations. The arising expressions for axial velocity, temperature profile, pumping characteristics and phenomenon of trapping are evaluated analytically under established physical conditions at boundaries. The solutions thus acquire is utilized to describe the impacts of various emerging parameters on important phenomena related to peristaltic motion. Results found that axial velocity depicts opposite variations in central and around the walls. Moreover, impact of heat generation parameter has shown increasing trend. It is also noticed that in pumping region, the pressure rise per wavelength increases (decreases) by increasing Grashof number, viscosity parameter and nanoparticles volume fraction. A comparative study is also explored for CuO, SiO2 and Ag nanofluids and pure water.

Modeling and simulation to determine the thermal efficiency of a parabolic solar trough collector system

This study presents a reduced order mathematical model to calculate the heat transfer in steady state in a parabolic trough collector, in which the radial and axial temperature profile of the system is obtained. To solve the model an iterative calculation sequence is used and implemented in Python software, in additional OpenGL is used to generate a schematic visualization of the system. Next, results are validated with data from two different heat-carrier fluids published in the literature, obtaining a maximum relative error less than 10%. The model is used to determine the thermal efficiency using water, thermal oil and nanofluids as heat-carrier fluids. Results show that the thermal efficiency of the parabolic trough collector is higher with nanofluids containing a higher volume fraction of nanoparticles: with a volume fraction of 0.04 and 0.02, the thermal efficiency is of 80% and 79%, respectively. The thermal oil has the lowest efficiency with a maximum efficiency of 76%. The nanofluids allow working at low-pressure levels in the parabolic trough collector compared to pressurized water.

Numerical and Analytical Investigation of an Unsteady Thin Film Nanofluid Flow over an Angular Surface

In the present study, we examine three-dimensional thin film flow over an angular rotating disk plane in the presence of nanoparticles. The governing basic equations are transformed into ordinary differential equations by using similarity variables. The series solution has been obtained by the homotopy asymptotic method (HAM) for axial velocity, radial velocity, darning flow, induced flow, and temperature and concentration profiles. For the sake of accuracy, the results are also clarified numerically with the help of the BVPh2- midpoint method. The effect of embedded parameters such as the Brownian motion parameter Nb, Schmidt number Sc, thermophoretic parameter and Prandtl number Pr are explored on velocity, temperature and concentration profiles. It is observed that with the increase in the unsteadiness factor S, the thickness of the momentum boundary layer increases, while the Sherwood number Sc, with the association of heat flow from sheet to fluid, reduces with the rise in S and results in a cooling effect. It is also remarkable to note that the thermal boundary layer increases with the increase of the Brownian motion parameter Nb and Prandtl number Pr, hindering the cooling process resulting from heat transfer.

Modeling of Biomass Gasification in a Downdraft Gasifier with Integrated Tar Adsorption

AbstractThermal gasification of biomass is known for its capabilities in flexible and decentral power station applications for cogeneration. However, the product gas contains tar compounds adversely for a stable operation. Integrated tar adsorption in a subsequent cooled section is therefore an option to reduce the tar pollution. The char coal, used here as adsorption agent, is formed by biomass pyrolysis in the gasifier. A kinetic model is proposed, considering the kinetics of all main reactions as well as heat and mass transport phenomena. Results are presented for axial temperature profiles, gas compositions, and the gas purity at different air‐to‐fuel ratios. The resulting output mass flows could indicate a requirement on the adsorption capacity of at least 0.3 g g−1 for the activated char coal.

Efficient CO2 capture using NH2–MIL–101/CA composite cryogenic packed bed column

CO2 capture using cryogenic packed bed with spherical glass packing material has great potential for applications in the natural gas industry. However, the influence of packing material on their performance has been rarely studied. In the present work, some novel packing materials, including Cellulose Acetate and CA/NH2–MIL–101(Al) were used to enhance the performance of the cryogenic packed bed. Pressure drop was determined as a function of specific surface area and module filling fraction experimentally. The CO2 capture efficiency of the system, axial temperature profile study during cooling and CO2 recovery steps for the spherical glass beads, CA hollow fibres and composite CA/NH2–MIL–101(Al) hollow fibres were also investigated. It was found that the hollow fibres reduce the pressure drop by a factor of 61% and 33% compared with the pressure drop caused by the spherical glass beads and monofilament fibres, respectively. The specific surface area provided by the hollow fibres was 230% and 122% more than that offered by the glass beads and monofilament fibres respectively. It was also observed that the CO2 capture efficiency of composite hollow fibres was 141.9% more than spherical glass beads and 9.5% greater than the pure CA hollow fibres. The temperature profile study reveals that pure CA and the composite CA/NH2–MIL–101(Al) hollow fibres require less energy for cooling than glass beads and provides higher bed saturation time. It was concluded that the NH2–MIL–101(Al) hollow fibre reduces the pressure drop and capital cost along with increasing the CO2 capture efficiency.

On the quench of a debris bed in the lower head of a Nordic BWR by coolant injection through control rod guide tubes

Since the reactor pressure vessel (RPV) of a typical BWR features a lower head that is penetrated by a forest of control rod guide tubes (CRGTs), coolability of the debris bed formed in the lower head during a severe accident can be realized by coolant injection through the CRGTs (so-called “CRGT cooling”). This paper is concerned with performance assessment of such CRGT cooling system, whose heat removal capacity is determined by two mechanisms: (i) heat-up and boiling of coolant inside the CRGTs; and (ii) evaporation of coolant which reached the top of the debris bed from CRGTs (top flooding). For this purpose, analyses were accomplished by coupling the COCOMO and RELAP5 codes, which simulate the quenching process of the debris bed and the coolant flow inside the CRGTs, respectively. An analysis was first carried out for a unit cell with a single CRGT, whose decay heat removal was limited by heat conduction from debris to the CRGT wall. The simulation indicated that without top flooding, though the temperature of the unit cell was eventually stabilized by the cooling of the CRGT wall, remelting of metallic debris (Zr) in the peripheral region was unavoidable due to low conductivity of corium. Boiling in the CRGT was not only beneficial to heat transfer, but also contributing to a flat axial temperature profile. Given the nominal flowrate of the CRGT cooling, the coolant was not completely boiled off in the CRGT, and therefore the remaining liquid water at the outlet of the CRGT was available for top flooding of the debris bed. The subsequent simulation including the top flooding showed that the debris bed was rapidly quenched without any remelting. However, the top flooding may have a side effect which was Zr oxidation risk at high temperature, leading to production of reaction heat and H2. Finally analyses were performed for prototypical cases for a reference Nordic BWR, and the results implied that the CRGT cooling could be used as a promising strategy for severe accident mitigation. It is critical that the debris bed is sufficiently cooled down during its formation so that the oxidation risk is eliminated when the CRGT cooling is applied.

Ethanol-to-ethylene dehydration on acid-modified ring-shaped alumina catalyst in a tubular reactor

New results of endothermic dehydration of undiluted ethanol on alumina catalyst in pilot wall-heated tubular reactor are presented. Activity and physicochemical characteristics of the proprietary acid-modified alumina (AMA) prepared by gibbsite flash calcination in TSEFLAR™ reactor were compared to the properties of commercially available alumina samples prepared by precipitation (CS1) or flash calcination of gibbsite in a flue gas (CS2). The samples differed in phase composition, which corresponds to γ-Al2O3 in CS1, mixed γ- and χ-Al2O3 in AMA and CS2; sodium content and concentration of Lewis acid sites (LAS) were different as well. Acid modification resulted in AMA catalyst with improved acidic properties and catalytic activity higher than that of CS2, despite the lower sodium content in the latter. Ethanol-to-ethylene dehydration in the wall-heated tubular reactor proceeded at the catalyst bed temperatures mainly below 385 °C, where AMA catalyst is the most active due to the highest content of strong LAS. The heat-agent temperature favors yield of ethylene rather than that of by-products. Higher linear velocity and porosity contribute to a flatter axial temperature profile in the bed; a higher average integral temperature gives a quantitative estimate. This ensures higher ethylene yield with a lower ethanol consumption even at a reduced heat-agent temperature. On the ring-shaped AMA catalyst, the greater ethylene yield, catalyst productivity and sharply reduced hydraulic resistance are reached compared to CS2 cylinders. For the first time we showed that use of AMA ring-shaped catalyst in wall-heated tubular reactor in contrast to the SynDol alumina based catalyst in adiabatic reactor can improve the catalyst productivity by 5–8 times and WHSV by 5–12 times.

Kinetics-free transformation from non-isothermal discontinuous to continuous tubular reactors

Focusing on tubular reactors with continuously distributed side injections, a general procedure to evaluate the operating mode able to reproduce the performance of a given semi-batch reactor is worked out. Namely, such operating mode is expressed as axial profiles of feed flowrate and temperature of the cooling/heating medium inside the reactor jacket. This transformation procedure, previously limited to isothermal reactors, is extended here to non-isothermal systems. The process performance (selectivity and conversion) of the original discontinuous reactor are fully reproduced using the continuous intensified reactor, while its productivity remains a degree of freedom. Notably, like in the isothermal case, the transformation procedure is kinetics-free, i.e. the knowledge of the reaction kinetics is not a precondition.As case study, a copolymerisation reaction is considered to demonstrate the potential of the method. Even though any SBR feed policy could be considered when applying this methodology, the optimal feed policy of the semi-batch reactor evaluated according to the so-called “power feed” procedure is examined considering the reactor non-isothermal. Afterwards, the proposed transformation method is applied and the performance of the two systems, discontinuous and continuous, are comparatively evaluated. Finally, given the practical difficulties associated with continuously distributed side injections, a discretisation approach is proposed based on the use of discrete lateral feeds and more realistic reactor configurations.

Heat Pipe Thermal Management at Hypersonic Vehicle Leading Edges: A Low-Temperature Model Study

The intense thermal fluxes and aero-thermomechanical loads generated at sharp leading edges of atmospheric hypersonic vehicles traveling above Mach 5 have motivated an interest in novel thermal management strategies. Here, we use a low-temperature stainless steel-water system to experimentally investigate the feasibility of metallic leading edge heat pipe concepts for thermal management in an efficient load supporting structure. The concept is based upon a two-phase, high thermal conductance “heat pipe” which redistributes the localized thermal flux created at the leading edge stagnation point over a larger surface for effective removal. Structural efficiency is achieved by configuring the system as a wedge-shaped sandwich panel with an I-cell core that simultaneously permits axial vapor and returns liquid flow. The measured axial temperature profiles resulting from a localized thermal flux applied to the tip are consistent with effective thermal spreading that lowered the peak leading edge temperature and reduced the temperature gradients when compared with an equivalent structure containing no working fluid. A simple finite element model that treated the vapor as an equivalent, high thermal conductivity material was in good agreement with these experiments. The model is then used to design a niobium alloy-lithium system that is shown to be suitable for enthalpy conditions representative of Mach 7 scramjet-powered flight. The study indicates that the surface temperature reductions of heat pipe-based leading edges may be sufficient to permit the use of nonablative, refractory metal leading edges with oxidation protection in hypersonic environments.

Experimental investigation of in-tube condensation of low GWP refrigerant R450A using a fiber optic distributed temperature sensor

Heat transfer coefficients and frictional pressure drop values are experimentally measured during condensation of R134a and its proposed low global warming potential (GWP) replacement, R450A. R450A is a non-flammable zeotropic mixture of R134a and R1234ze (42/58% by mass). Experiments were conducted in a horizontal tube (ID = 4.7 mm) for a range of mass fluxes (100 kg m−2 s−1 to 550 kg m−2 s−1) and saturation conditions (45 ∘C and 55 ∘C). The measured pressure drop values and heat transfer coefficients were compared with established literature correlations. Good agreement indicates that existing correlations can be used for design of condensers with R450A as the working fluid. Additionally, this study evaluates the viability of using distributed fiber optic temperature sensors during refrigerant condensation experiments. The fiber sensor is installed in the annulus of a tube-in-tube heat exchanger and is used to obtain the axial temperature profile of the cooling water. The measurements from the distributed temperature sensors are compared with conventional RTD measurements co-located within the test section. The results (Average Deviation = ±0.26 ∘C, Maximum Deviation = ±1.11 ∘C) suggest that fiber optic measurement techniques can provide temperature data with accuracy and high spatial resolution (equal to 0.65 mm).

Numerical analysis of microwave heating cavity: Combining electromagnetic energy, heat transfer and fluid dynamics for a NaY zeolite fixed-bed

Three-dimensional mathematical model was developed for a rectangular TE10n microwave heating cavity system, working at 2.45 GHz. Energy/heat, momentum equations were solved together with Maxwell’s electromagnetic field equations using Comsol Multiphysics® simulation environment. The dielectric properties, ε' and ε'', of NaY zeolite (Si/Al = 2.5) were evaluated as a function of temperature. Considering these values, the microwave heating of a porous fixed-bed made of dry NaY zeolite was simulated. Electric field distribution, axial and radial temperature profiles and temperature evolution with time were obtained. The zeolite fixed bed was heated up to 180 °C in 5 min, with 30 W power. The fixed-bed temperature evolution under non-steady state conditions showed the same trend as the one observed experimentally with only an average deviation of 10.3%. The model was used to predict microwave heating of other materials improving energy efficiency of the microwave cavity. Furthermore, the developed model was able to predict thermal runaway for zeolites.

Numerical Simulation on Interfacial Characteristics in Supersonic Steam–water Injector Using Particle Model Method

Steam–water injectors have been widely applied in various industrial fields because of their compact and passive features. Despite its straightforward mechanical design, the internal two-phase condensing flow phenomena are extremely complicated. In present study, a numerical model has been developed to simulate steam–water interfacial characteristics in the injectors based on Eulerian–Eulerian multiphase model in ANSYS CFX software. A particle model is available for the interphase transfer between steam and water, in which a thermal phase change model was inserted into the model as a CFX Expression Language (CEL) to calculate interphase heat and mass transfer. The developed model is validated against a test case under a typical operating condition. The numerical results are consistent with experimental data both in terms of axial pressure and temperature profiles, which preliminarily demonstrates the feasibility and accuracy of particle model on simulation of gas–liquid interfacial characteristics in the mixing chamber of injector. Based on the dynamic equilibrium of steam supply and its condensation, interfacial characteristics including the variation of steam plume penetration length and steam–water interface have been discussed under different operating conditions. The numerical results show that steam plume expands with steam inlet mass flow rate and water inlet temperature increasing, while it contracts with the increase of water inlet mass flow rate and backpressure. Besides this, the condensation shock position moves upstream with the backpressure increasing.

Experimental and Numerical Investigations of Structure and Stability of Premixed Swirl-Stabilized CH4/O2/CO2 Flames in a Model Gas Turbine Combustor

Premixed CH4/O2/CO2 flames were studied experimentally and numerically in a swirl-stabilized combustor. The bulk throat velocity of the incoming combustible mixture was maintained constant at 5.2 m/s. The oxygen fraction of the O2/CO2 oxidizer was kept constant at 60 vol %, while the effect of the equivalence ratio was examined from the blowout limit to the flashback one. The LES computational model was successfully validated through comparisons with experimental data in terms of axial and radial temperature profiles, as well as predicted OH* concentration maps versus visual flame appearance. Three different flame structures were observed while varying the equivalence ratio, namely, (I) double conical flame, (II) corner-stabilized flame, and (III) swirl-stabilized (V-shaped) flame. The double conical flame is observed at low equivalence ratios near the blowout limit. This flame stabilizes within both the inner and outer shear layers and has two distinct reaction zones with two inner recirculation zones (I...

Steady-state and transient modelling of a microchannel reactor for coupled ammonia decomposition and oxidation

A computational fluid dynamic (CFD) model was developed to describe the transient and steady-state behavior of NH3 decomposition in an experimental autothermal microchannel reactor. The effects of NH3 decomposition and NH3 oxidation flows on the axial temperature profile and product gas composition were investigated. During reactor start-up, the NH3 oxidation reaction was initiated at 300 °C, and attained a steady state value of approximately 650 °C after 6 h. Thus modelling of the reactor start-up regime enabled an accurate interpretation of temperature increases and H2 production in this dynamic period. Appropriate governing equations were used to account for mass, momentum, energy, and species transport. As a result of the different time scales associated with the chemical reaction and the heat transfer respectively, it was necessary to develop a new solution method to solve the model, especially in the transient period. This numerical method and results are described in detail in this paper. The experimental and simulated temperature and H2 production performance are found to be within reasonable accuracy during the transient period, while the steady-state results compare very well. Indications are that CFD can be used in future reactor designs to minimize time loss and accelerate H2 production during start-up.

High throughput production of single-wall carbon nanotube fibres independent of sulfur-source.

Floating catalyst chemical vapor deposition (FC-CVD) methods offer a highly scalable strategy for single-step synthesis and assembly of carbon nanotubes (CNTs) into macroscopic textiles. However, the non-uniform axial temperature profile of a typical reactor, and differing precursor breakdown temperatures, result in a broad distribution of catalyst particle sizes. Spun CNT fibres therefore contain nanotubes with varying diameters and wall numbers. Herein, we describe a general FC-CVD approach to obtain relatively large yields of predominantly single-wall CNT fibres, irrespective of the growth promoter (usually a sulfur compound). By increasing carrier gas (hydrogen) flow rate beyond a threshold whilst maintaining a constant C : H2 mole ratio, CNTs with narrower diameters, a high degree of graphitization (G : D ratio ∼100) and a large throughput are produced, provided S : Fe ratio is sufficiently low. Analysis of the intense Raman radial breathing modes and asymmetric G bands, and a shift in the main nanotube population from thermogravimetric data, show that with increasing flow rate, the fibres are enriched with small diameter, metallic CNTs. Transmission electron microscopy corraborates our primary observation from Raman spectroscopy that with high total flow rates, the fibres produced consist of predominantly small diameter SWCNTs.