Temperature evolution on Rh/Al2O3 catalyst during partial oxidation of methane in a reverse flow reactor

Catalytic partial oxidation of methane was investigated in a reverse flow reactor with commercial Rh/Al₂O₃ catalyst in pellets. The process is carried out in a catalytic fixed bed reactor and switching of feed flow direction is obtained through four electrovalves synchronized in pairs. Temperature profile along the catalyst bed was measured by fast IR thermography and product composition was measured with a continuous gas analyzer.Feed direction switching time, water to methane ratio and inert section length were investigated as process parameters.Data of catalyst bed temperature evolution during the flow cycle are presented, discussed and related to reactor performance as a function of reverse flow switching period.The effect of water addition to the reacting mixture on the dynamics of catalyst bed temperature evolution is also presented.     

Measurement and visualization of impingement cooling in narrow channels

Experimental measurement techniques such as naphthalene sublimation, liquid crystal thermography and real-time holographic interferometry are standard. Their application in narrow channels causes problems and is therefore limited. The channel width must not change too much because the naphthalene sublimation and the liquid crystal coating necessary for the thermography may cause non-negotiable variations. The interferometry fails in turbulent flow area. The diffraction along the channel edges is an additional difficulty. A comparison of the results obtained from the application of all three techniques, which has not been considered in earlier publications, is made here. The methods were used to measure and visualize the heat transfer characteristics of an array of 1.2 mm diameter impinging jets in an enclosed channel (≥2.2 mm) with single-sided flow-off at Reynolds numbers of about Re _z  ≈ 20,000. Scale-up ratios as low as 2.4 have been used in order to maintain similarity as it has not been previously reported. The naphthalene technique provided a high spatially resolved measurement of the Sherwood number along a downstream line. The liquid crystal thermography technique provided 2D contours of the Nusselt number. The temperature distribution within dead water zones was visualized with holographic interferometry. The cross-flow effects caused a shift in the stagnation point and a monotone decrease in the Nusselt number in the downstream direction. Received: 21 April 2000/Accepted: 6 July 2000     

Heat transfer to air–water annular flow in a horizontal pipe

Two-phase air–water flow and heat transfer in a 25 mm internal diameter horizontal pipe were investigated experimentally. The water superficial velocity varied from 24.2 m/s to 41.5 m/s and the air superficial velocity varied from 0.02 m/s to 0.09 m/s. The aim of the study was to determine the heat transfer coefficient and its connection to flow pattern and liquid film thickness. The flow patterns were visualized using a high speed video camera, and the film thickness was measured by the conductive tomography technique. The heat transfer coefficient was calculated from the temperature measurements using the infrared thermography method. It was found that the heat transfer coefficient at the bottom of the pipe is up to three times higher than that at the top, and becomes more uniform around the pipe for higher air flow-rates. Correlations on local and average Nusselt number were obtained and compared to results reported in the literature. The behavior of local heat transfer coefficient was analyzed and the role of film thickness and flow pattern was clarified.     

Tracking of coherent thermal structures on a heated wall by means of infrared thermography

This paper deals with measurements of convective velocity of large-scale thermal structures, using the thin foil technique and infrared thermography to visualize the thermal pattern on the wall. An image correlation method is proposed to track the displacement of the observed thermal pattern. The idea of the method is similar to that of particle image velocimetry, but the thermal patterns on the heated wall are used, rather than tracing particles. On this basis, the thermal patterns created by the coherent structures of turbulent channel flow are examined. Particular attention is paid to the determination of the optimal parameters of image acquisition, including spatial and temporal separation. An attempt is made to relate momentum and scalar transport analyses by considering the propagation velocity of large-scale temperature structures. The proposed technique appears to be an attractive alternative for non-intrusive analysis of turbulent flow, especially, where opaqueness of channel walls excludes the use of optical methods. Received: 18 January 2000/Accepted: 20 May 2000     

Thermofluidynamic analysis of the flow in a sharp 180° turn channel

The aim of the present study is to obtain surface flow visualisation, as well as local and spanwise averaged heat transfer measurements near a 180° sharp turn in a rectangular channel. The channel aspect ratio (width to height ratio) varies from 1 to 5 and the ratio between the width of the channel and that of the partition wall is always equal to 5. Heat transfer measurements are performed by means of the heated-thin-foil technique, which practically corresponds to a constant heat flux boundary condition, and by using infrared (IR) thermography. Two different heating conditions, in particular heating from one side (asymmetrical), or from two sides (symmetrical), are implemented. The convective heat transfer coefficient is evaluated from the measured temperature maps and the local bulk temperature of the flow which is obtained by making a one-dimensional balance along the channel. Results are presented in terms of local, or averaged, Nusselt number which is normalised with the classical Dittus and Boelter correlation. The fluid used during the test is air and the Reynolds number, based on the flow average velocity and channel hydraulic diameter, is varied between 16,000 and 60,000.     

Analysis of dryout in horizontal and inclined tubes

The infrared thermography technique was used to study the thermal and hydrodynamic phenomena in intermittent two-phase air–water flow in horizontal and inclined tubes at atmospheric pressure. The study was aimed at elucidating the relationship between the hydrodynamic parameters and dryout phenomena. It focuses on the empirical evaluation of the wall temperature distribution in a uniformly heated pipe. The results reveal the existence of dryout phenomena in horizontal pipe flow only. The flow parameter based on the frequency, length and velocity of elongated bubble is presented for the prediction of dryout.     

IR measurements of the thermodynamic effects in cavitating flow

The understanding of the thermodynamic effects of cavitating flow is crucial for applications like turbopumps for liquid hydrogen LH2 and oxygen LOx in space launcher engines. Experimental studies of this phenomenon are rare as most of them were performed in the 1960s and 1970s. The present study presents time resolved IR (Infra-Red) measurements of thermodynamic effects of cavitating flow in a Venturi nozzle.Developed cavitating flow of hot water (95 °C) was observed at different operating conditions – both conventional high speed visualization and high speed IR thermography were used to evaluate the flow parameters.Both the mean features of the temperature distributions and the dynamics of the temperature field were investigated. As a result of evaporation and consequent latent heat flow in the vicinity of the throat a temperature depression of approximately 0.4 K was measured. In the region of pressure recuperation, where the cavitation structures collapse, the temperature rise of up to 1.4 K was recorded. It was found that the temperature dynamics closely follows the dynamics of cavitation structures.Finally experimental results were compared against a simple model based on the Rayleigh–Plesset equation and the thermal delay theory and plausible agreement was achieved.Experimental data is most valuable for further development of numerical models which are, due to poor ensemble of existing experimental results, still at a very rudimentary level.     

Application of PIV in a Mach 7 double-ramp flow

The flow over a two-dimensional double compression ramp configuration is investigated by means of schlieren visualization, quantitative infrared thermography and particle image velocimetry (PIV) in a short-duration facility producing a free-stream flow at Mach 7. The study focuses upon the accuracy assessment of PIV in the hypersonic flow regime including flow facility effects such as repeatability of test conditions. The solid tracer particles are characterized by means of electron microscopy as well as by measuring the dynamic response across a planar oblique shock wave with PIV. The experiments display a strong variation in the light scattering intensity of the seeded flow over the flow field, due to the large flow compressibility. The mean velocity spatial distribution allows to clearly identify the shock pattern and the main features of the flow downstream of the shocks. However, the spatial resolution is insufficient to determine the wall flow properties. Furthermore the velocity data obtained with the PIV technique allow the determination of the spatial distribution of the Mach number under the hypothesis of adiabatic flow. The double ramp configuration with a variable second compression angle exhibits shock–shock interactions of Edney type VI or V for the lowest and highest ramp angle, respectively. A single heat transfer peak is detected with infrared thermography on the second ramp in case of a type VI interaction while for the type V shock interaction a double heat transfer peak is found. Shock wave angles measured with PIV are in good agreement with theory and the overall flow topology is consistent with schlieren visualization. Also in this respect the results are in agreement with compressible flow theory.     

Mapping temperature distributions in flows using radiating high-porosity meshes

A technique to visualise and measure temperature distributions in gas flows is described which places fine, highly-emissive planar meshes in the heated flow and images them with an IR camera. Fine meshes with high porosity are used to minimise the disturbance to the flow field and ensure that the local mesh temperature is close to the local gas temperature. The radiation received by the camera is a function of both the temperature and the emissivity of the body visualised. In the case of a porous mesh, the camera will visualise both the mesh surface and the background through the mesh apertures. An effective emissivity, which combines the relative area fraction and emissivity of the mesh can be obtained via calibration. This effective emissivity is used to reduce the intensity data to temperature distributions. Attention must be paid to the ratio of the size of the projected camera pixel to the mesh opening size to ensure accuracy. The technique is demonstrated on a number of buoyant jet flows and the potential application of the technique to higher temperature flows is discussed.     

Thermography Applied to the Study of Fatigue Crack Propagation in Polycarbonate

Thermography was used to study the propagation of fatigue cracks during cyclic loading of pre-cracked SAE keyhole polycarbonate specimens. A micro-bolometer infrared camera (FLIR A655sc) and a commercially available software program (DeltaTherm2) were employed. The stress intensity factors were determined using a hybrid thermoelastic stress analysis (TSA) technique. The crack growth rate was determined via thermography using two different approaches. The first approach used the output of the crack-tip position from the developed TSA algorithm and the number of cycles between data sets. The second approach used temperature measurement as a new way to determine da/dN (crack growth rate) directly. As a result, da/dN vs ΔK (stress intensity factor range) graphs were plotted and fitted using Paris’ law. A comparison between the resultant da/dN vs ΔK curves and results found in the literature, as well as curves from the finite element method (FEM) simulations showed good agreement. The conclusion was that thermography is a very powerful tool that can detect, measure and monitor fatigue cracks in polycarbonate.     

Rapid evaluation of the fatigue limit in composites using infrared lock-in thermography and acoustic emission

Fatigue limit determination via the conventional Wöhler-curve method is associated with extended experimental times as it requires testing of a large number of specimens. The current paper introduces a methodology for fast, reliable and experimentally economic determination of the fatigue limit in monolithic and composite materials by means of combined usage of two nondestructive inspection methods, namely infrared (IR) lock-in thermography and acoustic emission (AE). IR thermography, as a real-time and non-contact technique, allowed the detection of heat waves generated due to thermo-mechanical coupling as well as of the energy dissipated intrinsically during dynamic loading of the material. AE, on the other hand, was employed to record the transient waves resulting from crack propagation events. Aluminum grade 1050 H16 and cross-ply SiC/BMAS ceramic matrix composites were subjected to fatigue loading at various stress levels and were monitored by an IR camera and AE sensors. The fatigue limit of the monolithic material, obtained by the lock-in infrared thermography technique and supported by acoustic emission was found to be in agreement with measurements obtained by the conventional S–N curve method. The fatigue limit of the ceramic matrix composite was validated with acoustic emission data.     

Spatially resolved wall temperature measurements during flow boiling in microchannels

Spatial and temporal variations of channel wall temperature during flow boiling microchannel flows using infrared thermography are presented and analyzed. In particular, the top channel wall temperature in a branching microchannel silicon heat sink is measured non-intrusively. Using this technique, time-averaged temperature measurements, with a spatial resolution of 10 μm, are presented over an 18 mm × 18 mm area of the heat sink. Also presented, within a specific sub-region of the heat sink, are intensity maps that are recorded at a rate of 120 frames per second. Time series data at selected point locations in this sub-region are analyzed for their frequency content, and dominant temperature fluctuations are extracted using proper orthogonal decomposition.Results at low-vapor-quality boiling condition indicate that temperatures can be determined from recorded radiation intensities with a temperature uncertainty varying from 0.9 °C at 25 °C to 1.0 °C at 125 °C. The time series data indicate periodic wall temperature fluctuations of approximately 2 °C that are attributed to the passage of vapor slugs. A dominant band of frequencies around 2–4 Hz is suggested by the frequency analysis. Proper orthogonal decomposition results indicate that first six orthogonal modes account for approximately 90% of the variance in temperature. The first mode reconstruction accounts for temporal variations in the dataset in the sub-region analyzed; however the magnitude of fluctuations and spatial variations in temperature are not accurately captured. A reconstruction using the first 25 modes is considered sufficient to capture both the temporal and spatial variations in the data.     

Flow of Colloid Liquid in Horizontal Cell under Heating from Sidewall

Based on numerical simulation, the influence of sedimentation on the convective flow of colloid liquids filling a horizontal cell heated from the sidewall is studied. The set of nonlinear equations is solved by the finite-difference method using explicit schemes. Three convective patterns differing in spatial structure and behavior in time are distinguished. The transition between the patterns is accompanied by a jump in the dimensionless heat flow. Bifurcation diagrams of the convection patterns (the dependences of the heat flow intensity on the Rayleigh number) are given. It is shown that the weak flow of a colloidal suspension exists at a low temperature gradient, the intensity of which is several orders of magnitude lower than the intensity of the flow of a homogeneous liquid under the same parameters. The concentration in the flow with a weak intensity is redistributed in such a way that the density gradient becomes almost vertical, and the heat flow across the layer is absent at the same time. The transition from a weak to a strong one-vortex flow filling the entire cell proceeds abruptly. It is found the threshold of the transition from a weak to intense flow depends on the Boltzmann number characterizing the degree of gravitational stratification. One more flow, namely, a three-vortex flow with an intermediate intensity is generated upon a decrease in the Rayleigh number. Stream-function and concentration fields are manifested for all the observed types of flows.     

Quantitative measurement of spatio-temporal heat transfer to a turbulent water pipe flow

A technique using high-speed infrared thermography was applied to measure the spatio-temporal heat transfer to a turbulent water flow in a horizontal circular pipe. The instantaneous distribution of the heat transfer coefficient and its temporal fluctuation was evaluated by solving inverse heat conduction equation of the heated thin-test-surface. As a result, it was demonstrated that the quantitative measurement, not only the time-averaged heat transfer, but also the statistics of the spatio-temporal fluctuation, was possible using this technique. In addition, a unique feature of the spatio-temporal heat transfer was clearly visualized for the turbulent pipe flow, which was dominated by the streaky structure similar to that for the turbulent boundary layer and the turbulent channel flow.     

Oblique stagnation-point flow and heat transfer towards a shrinking sheet with thermal radiation

An analysis is made of steady two-dimensional oblique stagnation-point flow and radiative heat transfer of an incompressible viscous fluid towards a shrinking sheet which is shrunk in its own plane with a velocity proportional to the distance from a fixed point. Here the axis of the stagnation flow and that of the shrinking sheet are not aligned. A similarity transformation reduces the Navier-Stokes equations to a set of non-linear ordinary differential equations and are solved numerically using a shooting technique. The analysis of the results obtained shows that multiple solutions exist for a certain range of the ratio of the shrinking velocity to the free stream velocity. The effect of non-alignment for the wall shear stress and the horizontal velocity components are discussed. Streamline patterns are also shown for shrinking at the sheet with aligned and non-aligned cases. It is found that the temperature at a point in the fluid decreases with increase in effective Prandtl number (Pr _eff ). The results pertaining to the present study indicate that as Pr _eff increases, the rate of heat transfer also increases. The reported results are in good agreement with the available published work in the literature.     

Heat Transfer and Flow Investigation of a Multi-spoke Flameholder for an Annular Combustor

This paper details the experimental techniques and numerical analysis used to investigate the flow field established by a multi-spoke, annular flameholder. The results from this work were used to maximise the Sustained Hypersonic Flight Experiment (SHyFE) ramjet combustion efficiency whilst ensuring that the vehicle wall temperature limits would not be exceeded. Through an overview of flameholder combustion enhancement the reasoning behind the use of pressure loss, three-dimensional air velocity and turbulence intensity profile measurements in conjunction with CFD analysis to optimise the flameholder is explained. The assessment of three flameholder designs within the paper allows a clearer representation of the processing techniques and allows the influence of spoke size and blockage ratio on mixing capability to be presented. The high resolution liquid crystal thermography technique used to investigate the influence of flameholder design on the SHyFE wall heat loads is also detailed and example results presented.     

Numerical analysis of free convective flows in partially open enclosure

 Laminar steady state buoyancy induced flows in a two-dimensional, air filled partial open enclosure with a discrete flush mounted iso-flux heat source on one of its walls is investigated numerically. The transport equations for energy and vorticity are solved with the aid of the ADI finite difference scheme on uniform mesh. Because of the specific application of the present study in the air cooling of electronic equipments, results are obtained only for a Prandtl number of 0.71 with an aspect ratio of 1.0 for a range of Rayleigh numbers, Ra (≤10⁵), heat flux parameter, Q and opening parameter, A ₀ using constant properties and Boussinesq approximation by imposing approximate conditions at the opening. Results of flow and temperature patterns, velocity and temperature profiles shows that the outgoing flow is governed by strong characteristics of the cavity condition whereas the incoming flow influenced by outside conditions. It is observed that Rayleigh number considerably affects the flow and thermal fields within the open enclosure when compared with intensity of heat flux and size of the opening. Received on 22 January 2001 / Published online: 29 November 2001     

Heat Transfer Influenced by Turbulent Airflow Inside an Axially Rotating Diffuser

The paper presents a study of heat transfer between the turbulent airflow and the inner wall surface of an axial diffuser rotating around its longitudinal axis. Heat transfer was assessed through the measurement of a time-dependent temperature field of the diffuser inner wall surface. Measurements of the instantaneous flow velocity components were performed by a laser–Doppler anemometry system, which delivered information on mean velocity components as well as on the turbulence intensity. A significant increase of all three mean velocity components was observed near the rotating diffuser wall in comparison with a non-rotating diffuser. Temperature field measurements were carried out by means of infrared thermography. The experiment showed a significant dependence of the temperature field on the turbulent flowfield induced by diffuser rotation. A strong influence of the flow separation and reattachment on the temperature distribution was observed, while rotation was found to suppress the occurrence of flow separation from the diffuser wall. Properties of the velocity field such as turbulent kinetic energy were directly coupled with the temperature distribution in order to gain the information on how to enhance or reduce heat transfer by changing the integral parameters of the diffuser (e.g. rotation frequency or amount of flow).     

Characterization of patterns in rimming flow

Patterns were generated inside a horizontal cylinder rotating at low speeds. The cylinder was filled with a very low volume liquid fraction of 1.8% of Newtonian fluid and the rotation speed ranged between 0.08 and 5.2 s⁻¹. A novel laser-plane technique was utilized to obtain time series from each pattern. This enabled the characterization of fluid patterns using Fourier spectral (FS) and dynamical-systems (chaotic) techniques such as the recurrence map, correlation dimension (D₂) and Hurst exponent (H). Four patterns were found (fingers, furrows, waterfall and smooth tooth) before annular flow was reached. The results indicate that the FS technique not is suitable for flow pattern characterization; and H only has the ability to indicate a possible pattern change. The best tool for indicating the pattern transitions and the inner coat liquid evolution was found to be recurrence maps and D₂.