Heated Air Jet Research Articles

Effect of non-uniform heating on temperature and velocity profiles of buoyancy driven flow in vertical channel of prismatic modular reactor core

To overcome the lack of experimental results of buoyancy-driven flow fields in vertical core channels of prismatic modular reactors, hot wire anemometry; micro foil sensors; and T-type thermocouples were integrated simultaneously at six axial positions along an electrically heated stainless steel channel of a test facility designed and developed with a representative geometry of prismatic modular gas-cooled reactor. At each axial position, nine radial measurements were obtained from the wall to the centerline of the channel. Fluid measurements display typical temperature and velocity profiles throughout the inlet- and mid-section of the channel. However, wall-to-fluid temperature differences along with axial variation of velocity fields indicate flow instabilities at the top section of the channel. This could be attributed to heat losses by conduction through the thickness of channel’s wall in addition to interactions occurring between heated air jets exiting the core channel and cooled air plumes cooled in the upper plenum which is anticipated to take place more rigorously in real prismatic facilities. Additionally, a new definition of the characteristic temperature difference is proposed to calculate a modified Grashof number that can be used to indicate such flow instabilities. Distortion factors between the developed facility and reference modular gas-cooled reactor are found to range from 18% to 24%. Current results are crucial for guiding and validating the commercial computational fluid dynamics codes.

A new technique using background oriented schlieren for temperature reconstruction of an axisymmetric open reactive flow

A new technique, called 3D ray tracing, for refractive index field reconstruction of axisymmetric flows from displacement fields measured from background oriented schlieren (BOS) experiments is developed and applied to a lean premixed methane/air reactive flow at Reynolds number of 4000 on a 12 mm diameter circular burner. The temperature distribution is then calculated using a species independent direct relationship between refractive index, temperature, and ambient conditions. The error introduced by the approximation to reach this relationship is quantified using simulated flow fields and is found to be 8% within the inner unburnt region of the flow field, decreasing to 2% through the reaction zone, and then quickly reducing to 0% outside the flow field. The effect of random noise and reconstruction resolution on the accuracy of the method is assessed via application to synthetically generated data sets that mimic the characteristics of a heated air jet expelled into ambient. The novel 3D ray tracing allows for accurate temperature reconstructions of open axisymmetric reactive flows where 2D displacement fields are measured, which is shown to be a shortcoming of current direct methods in literature. Additionally, this is done without the need for any prior knowledge of flow field parameters; only ambient conditions to the system must be known. The simple experimental setup and low computational cost make this approach with BOS a good option for application into existing experimental combustion systems with minimal effort.

Transverse wedge-shaped rib roughened solar air heater (SAH) - Exergy based experimental investigation

The major challenge faced in heat transfer augmentation in roughened solar air duct is the formation of viscous sublayer near heat transferring surface. The present investigation uses main air supply flow under roughened duct mounted with transverse wedge shaped roughened absorbers for the heated air jet delivery. This technique is prolific as it provides the ambient cool air to the entire absorber plate surface as well as the roughness element on the heated absorber plate that considerably enhances the heat transfer coefficient. Data of everyday was documented under diverse range of parameters viz., Re 3000–18000, rib wedge angle (φ) of 8°, 10°, 12° and 15°, relative roughness pitch (p/e) of 60.17φ−1.0264<p/e < 12.12 and relative roughness height (e/Dh) of 0.015–0.033. The mass flow rate ranges from 0.0174 to 0.0352 kg/s. Maximum exergetic efficiency (ηII) observed was 4.95 corresponding to (To-Ti)/I of 0.035 at a mass flow rate of 0.0346 kg/s. Also maximum (ηII) was 4.14 at p/e = 7.57, e/Dh = 0.033 and at φ = 10° at a mass flow rate of 0.03 kg/s with respect to varying geometrical configuration. Maximum exergy efficiency of 4.95% accomplished justifies that the use of transverse wedge shaped roughness element is prolific for enhancing heat transfer.

Evaluation of heated air jet on pulp chamber temperature at different steps of the restorative protocol

Objectives: Evaluate of the heated air jet temperature (T) in the pulp chamber on cavity 5 depths and photoactivation stages for a bulk fill resin restorative protocol is evaluated. Methods: Class I cavity preparations were conducted at different depths (n = 8) through two protocols for adhesive volatilization (23 °C and 40 °C) and cavities were restored with the Filtek Bulk Fill resin. The pulp chamber temperature variation was evaluated at four steps (times) during the restorative protocol: I (initial), V (after volatilization), A (after photoactivation of the adhesive), and C (after photoactivation of the composite resin). To verify the assumptions of the normality of the errors and homoscedasticity, the Shapiro Wilk and Levene tests were conducted. Subsequently, two-way and three-way analysis of variance was carried out, followed by Tukey's post-hoc analysis (α = 0.05). Results: The maximum T at the different restorative steps, regardless of the volatilization protocol and cavity depth, was as follows: I (36.8 °C) = V (36.9 ºC) &lt; A (37.2 °C) = R (37.8 ºC) (p &lt;0.05). During V, a small greater variation was observed in pulp chamber temperature when dentin was volatilized at 40 °C (p &lt;0.05) at very deep cavity depths (0.31 °C). The largest temperature variations (p &lt;0.05) were observed during A (0.17–0.59 °C) and R (0.50–1.06 °C), reaching peak temperatures in the cavities.

Characterization of the Ce,Pr:LuAG phosphor for Co-doped aerosol phosphor thermometry

The Ce,Pr:LuAG phosphor is investigated for use in co-doped aerosol phosphor thermometry (APT). The phosphor was characterized by measuring the emission intensity for each ion as a function of excitation laser fluence and temperature. A simplified three-level model was used to interpret and fit the data. The phosphor was then used for co-doped APT temperature-imaging experiments performed in a heated air jet up to a maximum temperature of 694 K. The three-level model fit was used to analyze sources of bias in the imaging measurements. Both ions’ emission intensities show a nonlinear dependence on excitation fluence that can potentially be explained by temperature-dependent ground- and excited-state absorption processes included in the three-level model. Single-shot temperature precision from the temperature-imaging experiments was measured to be 22, 23, and 32 K for mean temperatures of 497, 603, and 694 K, respectively, at a spatial resolution of ~1.12 lp/mm and average fluence of 33 mJ/cm2. The results extend the measurement range for co-doped APT to at least 700 K. The findings also provide a simple model for phosphor signal non-linearity with excitation fluence for low activator ion concentrations, and provide a method for estimating temperature bias from several sources including laser fluence.

Diesel fuel combustion by spraying in a superheated steam jet

This paper experimentally studies the process of liquid hydrocarbon combustion by spraying a jet of superheated steam using diesel fuel as an example. A laboratory burner sample (with a power of up to 20 kW) is used to assess the effect of the operating parameters on the characteristics of the combustion process in the presence of superheated steam: heat generation, the composition of the combustion, products and the flame temperature. The dependences of the main calorific and environmental indicators of the burner on the temperature and flow rates of superheated steam and fuel are found. The maximal completeness of fuel combustion reaches 98.6%. With an increase in the mass fraction of the steam (up to 50%), a decrease in the content of nitrogen oxides is observed. The minimal contents of carbon monoxide are CO = 5 mg/kWh, while the level of nitrogen oxides is maximal but does not exceed 90 mg/kWh, corresponding to class 3 by EN 267. Compared with heated air jet technology, steam gasification technology has significant environmental advantages. The results are in demand for scientific substantiation of energy efficient and environmentally friendly ways of utilising substandard liquid hydrocarbons with heat production, verification of the mathematical model, and numerical simulation of the process. The results can be used for the development of the energy efficient and environmentally friendly technologies of liquid hydrocarbon waste utilisation with the production of heat energy.

An Experimental Setup for Pulse Heating of Bodies in Crossed Gas Flows

An experimental setup designed to study the processes of unsteady heat exchange in the interaction of a pulsating turbulent heated-air jet with a model object, which is placed in a stationary drifting flow of cold air, was developed. In fact, this is the interaction of a vertical heated jet with a transverse drifting “cold” stream, during which an object is periodically heated and cooled at a given pulsation frequency of the jet. The design of the setup and its main element, a device for creating a pulsating jet, are described in detail. The characteristics of the pulsating heated jet and the drifting flow in the working region of the setup are given. As an example, the results of nonstationary heating of the plate of a pyroelectric transducer by a pulsating heated jet and subsequent cooling when interrupting the flow of the warm gas are presented.

Investigation of aerosol phosphor thermometry (APT) measurement biases for Eu:BAM

Aerosol phosphor thermometry (APT) utilizes the luminescence intensity ratio (LIR) of emission from two wavelength bands to infer temperature. This work characterizes the impacts of phosphor seeding concentration and laser fluence on the measured emission spectrum and resulting LIR vs. temperature calibration for the phosphor Eu:BAM in both a heated air jet and a tube furnace. Increasing seeding density led to spectral broadening, but this effect had a minimal impact on the LIR over the range investigated. Increasing laser fluence from 5 to 60 mJ/cm2 resulted in a blue shift in the emission spectrum, corresponding to an increase in the particle temperature of approximately 50 K inferred from the LIR, as previously observed in the literature. Heated jet measurements found that the temperature sensitivity of the LIR for Eu:BAM was not significantly affected by increasing laser fluence for the conditions studied. In contrast, the temperature sensitivity of the LIR measured in the tube furnace was significantly lower than in the jet, likely due to the high phosphor particle concentration present in the aggregate powder environment. These measurements highlight the importance of performing LIR calibrations at representative conditions to achieve unbiased APT measurements when relying on the spectral temperature-dependence of thermographic phosphors.

Calibration of a silver thin film gauge for short duration convective step heat load

Thin film gauges (TFGs) are a promising candidate for measuring transient heat fluxes in the applications involving very short duration of heating environment. They are basically resistance temperature detectors (RTDs) having the capability of responding in the range of few microseconds. In the present study, a silver thin film gauge (STFG) is fabricated and calibrated in-house with a view to assess the performance of STFGs in dynamic environment. Convective heat load is supplied by a hot-air gun where the heated air jet strikes the gauge and its response is obtained through voltage signal. Subsequently, the surface heat fluxes are estimated by using one dimensional heat conduction modeling. The similar experimental environment is studied to obtain the flow behavior of hot-air jet emanating into atmosphere by using numerical simulations. The self-similar velocities are plotted as well as the interference of outer domain into the experiment parameters has been studied. Ultimately, the surface heat fluxes obtained from various methods are compared to analyse the performance of this hand-made STFG. This study reveals the ability of STFGs to be used in practical short duration transient situations.

Thermographic laser Doppler velocimetry.

We propose a point measurement technique for simultaneous gas temperature and velocity measurement based on thermographic phosphor particles dispersed in the fluid. The flow velocity is determined from the frequency of light scattered by BaMgAl10O17:Eu2+ phosphor particles traversing the fringes like in conventional laser Doppler velocimetry. Flow temperatures are derived using a two-color ratio method applied to the phosphorescence from the same particles. This combined diagnostic technique is demonstrated with a temperature precision of 4%-10% in a heated air jet during steady operation for flow temperatures up to 624 K. The technique provides correlated vector-scalar data at high spatial and temporal resolution.

Droplet characteristics and local equivalence ratio of reacting mixture in spray counterflow flames

Spray combustion was studied by injecting monodisperse 125 or 200μm ethanol droplets in a premixed natural gas fuel flame flowing against an opposed heated air jet. Phase Doppler anemometry and chemiluminescence measurements allowed to characterise both droplet parameters and the local reacting mixture. Both types of droplets crossed the flow stagnation plane and entered the opposite jet. However, 125μm droplets reversed their motion and oscillated around the stagnation plane, leading to increased droplet residence time in hot regions. The fuel vapour released by droplets close to the stagnation plane, mixed with the surrounding air and led to the ignition of a second fuel vapour flame below the natural gas flame. For 125μm droplets, mean local equivalence ratio of the fuel vapour flame was about 0.8, suggesting lean-premixed combustion; whereas 200μm droplets led to stoichiometric combustion. “Group Combustion” number was estimated from measurements and suggested that 125μm and 200μm droplets burned in different “Group Combustion” regimes.

Pr:YAG temperature imaging in gas-phase flows

In this study, a new thermographic phosphor for planar gas-phase thermometry is investigated. The thermographic phosphor used is composed of trivalent praseodymium (Pr3+) ions doped into a yttrium aluminum garnet (YAG) host. Spectrally-resolved emission data were taken in a furnace for temperatures up to 1,300 K. The emission spectra were used to develop a temperature measurement strategy utilizing a non-equilibrium population ratio. The developed temperature measurement technique was demonstrated in a turbulent heated air jet for exit temperatures ranging from 300 to 750 K. The results demonstrate the promise of the Pr:YAG phosphor for obtaining high-precision single-shot temperature measurements in gas-phase flows.

Laminar flame speeds, non-premixed stagnation ignition, and reduced mechanisms in the oxidation of iso-octane

Experimental data on the laminar flame speeds of iso-octane/air mixtures at atmospheric and elevated pressures were acquired using the counterflow flame and the outwardly expanding flame, while the non-premixed ignition temperatures were determined for an iso-octane pool in the stagnation flow of a heated air jet at atmospheric and slightly reduced/elevated pressures. These experimental measurements were compared with calculations based on the mechanisms of Curran et al. and Chaos et al., with the former mechanism systematically and substantially reduced, using directed relation graph and computational singular perturbation, to facilitate the calculation. It was found that the Curran mechanism yielded substantially higher laminar flames speeds as compared to the present experimental results while results from the Chaos mechanism agree well with the present measurements. These trends are in agreement with previous results on the laminar flame speeds for n-heptane. Both mechanisms yield acceptable comparison with the observed non-premixed stagnation ignition temperature.

S0505-1-2 加熱空気円形噴流の風洞実験 : サイドジェットの観察([S0505-1]乱流遷移(1))

Coherent structures were experimentally investigated in a heated air jet vertically ejected from a round nozzle of 30mm in diameter. Some experiments of the present jet, mean velocity at the exit U_a=1.5〜5.0m/s and temperature difference ΔT=0〜100K, were conducted by smoke wire method and smoke method. Coherent structures, vortex pairing and side jet were observed. The side jet occurred in the jet with ΔT=75, 100K and ρ/ρ_∞=0.74〜0.8.

Effect of Pulsating Circular Hot Air Jet Frequencies on Local and Average Nusselt Number

The study was carried out to determine the effect of pulsating frequencies on the local and average heat transfer characteristic of a heated circular air jet. The velocity profile of a heated circular pulsating air jet was measured in the first part of the study. The same set-up was used to measure the heat flux of the pulsating jet impinging on a wall. The heat flux of the heated air jet impinging on the plate was measured using a heat flux microsensor at different radial positions. Measurement of the heat flux was used to calculate the average and local Nusselt Number for different pulsating frequencies and at different flow Reynolds Number. The pulsating frequencies were between 10-80 Hz and the Reynolds number used were 16 000, 23 300 and 32 000. Results obtained show that the local Nusselt number calculated were higher at all radial position away from the stagnation point. The pulsed jet Nusselt number was higher than the average steady jet Nusselt number for all values of frequencies due to the higher localised heat transfer. The higher Nusselt number obtained at localized radial positions can be due to the higher instantaneous velocity as was shown from the velocity profile plotted in the first part of the experiment. Enhanced turbulence intensity found was due to the pulsed jet.

Heterodyne-detected electrostrictive laser-induced gratings for gas-flow diagnostics

The signal of heterodyne-detected electrostrictive laser-induced gratings (LIGs) originates from the interference of a reference laser beam with the laser light diffracted at the counterpropagating sound-wave packets, which are generated in the overlap volume of crossed laser beams. The frequencies of the sound waves, which contain the information on the sound velocity and on the motion of the medium, can approximately be extracted from the frequencies at the maxima of the two peaks, which dominate the power spectrum of the heterodyne LIG signal intensity. In free-air jets behind a slot nozzle, flow velocities up to 60 m s-1 were determined by quick fits from the power spectrum and by fitting the time-dependent signal intensity data to model functions. Results agree well, the standard deviations being about one-half in the latter case (1.6–2.6 m s-1, for positions close to the nozzle). Problems arising from the sampling and from the finite observation time of the signal intensity are discussed. Furthermore, the results of the LIG measurements were compared with data provided by laser Doppler anemometry. As an application of the instantaneous and unseeded LIG measurement technique, temperatures in heated air jets were determined simultaneously with the flow velocity by quick fits from the power spectrum.

Using Reconfigurable Tooling and Surface Heating for Incremental Forming of Composite Aircraft Parts

The application of composites in the aircraft industry has increased significantly over the past few decades. With traditional composite laminate shaping, each layer is made to conform to the mold surface by hand before subsequent layers are added. This is a very labor- and time-intensive process. There is a great deal of interest in developing an automated process for forming composite parts with compound curvatures. The proposed composite forming process utilizes a computer-controlled, reconfigurable discrete element mold to incrementally form a compound curvature part shape from a flat lay-up, thereby facilitating process automation. An elastomeric interpolating layer, called an interpolator, is placed on top of the hemispherical forming ends of the die elements to prevent dimpling of the composite lay-up. The process employs vacuum to pull a single diaphragm (top), composite, and interpolator into contact with the mold surface. Through an experimental investigation, this new composites forming process with “active” tooling has been successfully demonstrated. Heating of the composite is accomplished by uncontained, forced convection using a matrix of heated air jets mounted above the composite. However, low-powered conduction is shown to be the best heating method in terms of both composite heating time and minimization of through-thickness temperature. Using vacuum to conform both the composite and the interpolator to the mold, and choosing sufficiently stiff diaphragm and interpolator materials, undimpled and wrinkle-free composite parts have been formed in an incremental fashion.

Temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence for PLIF applications

The temperature and excitation wavelength dependencies of 3-pentanone absorption and fluorescence were studied in support of planar laser-induced fluorescence (PLIF) imaging of temperature and mixture fraction in flows of practical interest. The temperature dependencies (300–875 K) of absorption and fluorescence were measured for gaseous 3-pentanoneat atmospheric pressure in a nitrogen bath gas using 248, 266, and 308 nm excitation. The results indicate that the fluorescence signal per unit mole fraction using 248 nm excitation is highly temperature-sensitive below 600 K, while the signal from 308 nm excitation is not temperature sensitive below 500 K. For quantitative measurements over a broad range of temperatures, one must choose excitation schemes carefully to balance the trade-off between measurement sensitivity and the amount of signal at the expected conditions. As an example of such a choice and to show the capabilities of ketone PLIF techniques, we include temperature and mixture fraction images of a 300–650 K heated air jet using near-simultaneous 308 and 266 nm excitation.

Effects of turbulence on nonpremixed ignition of hydrogen in heated counterflow

Experiments were conducted to determine the effects of turbulence on the temperature of a heated air jet required to ignite a counterflowing cold hydrogen/nitrogen jet. In contrast to pseudo-turbulent flows, where turbulence was generated by only a perforated plate on the fuel side, resulting in little effect on ignition in a hydrogen system, fully turbulent flows with perforated plates on both sides of the flow were found to produce noticeable effects. The difference was attributed to the fact that in fully turbulent flows, a significantly larger range of turbulent eddies extend to smaller scales than in pseudo-turbulent flows. At atmospheric pressure, the lowest turbulence intensity studied had ignition temperatures notably lower than laminar ones, while further increases in turbulence intensity resulted in rising ignition temperatures. As a result, optimal conditions for nonpremixed hydrogen ignition exist in weakly turbulent flows where the ignition temperature is lower than can be obtained in other laminar or turbulent flows at the same pressure. Similar trends were seen for all fuel concentrations and at all pressures in the second ignition limit (below 3–4 atm). At higher pressures, turbulent flows caused the ignition temperatures to continue to follow the second limit resulting in ignition temperatures higher than the laminar values. The extension of the second limit ends at the highest pressures (7 to 8 atm) where evidence of third limit behavior appears. Three mechanisms were noted to explain the experimental results. First, turbulent eddies similar in size to the ignition kernel can promote discrete mixing of otherwise isolated pockets of gas. Second, this mixing can promote HO 2 chain branching pathways, which can account for the enhanced ignition noted in the second limit where reaction is governed by crossover temperature chemistry. Third, turbulence limits the excursion times available for reaction, inordinately affecting the slower HO 2 reactions. This is responsible for the increasing ignition temperature with turbulence intensity and pressure.

Humidity change and its effects on baking in an electrically heated air jet impingement oven

A high temperature humidity sensor was used to monitor the absolute humidity inside an electrically heated conveyorized, food service-scale, two-zone, air-jet impingement tunnel oven as commonly used by pizza chain restaurants. The oven was first operated empty, then loaded with dummy loads consisting of pans of water in perlite while baking layer cakes, and finally when baking cakes with added dummy loads. Temperature and loading affected humidity, and humidity affected final product quality. High humidity lightened cake crust color, increased volume, and raised the final moisture content of the cake. There was no significant effect of humidity on the heat transfer coefficient as determined by a standardized metal target plate. In this study, one pan of perlite approximated three pans of cake. Adding perlite dummy loads was shown to be a useful technique to quantitatively measure and to demonstrate the effect of humidity during baking. It is also a convenient way to accelerate oven load-testing without producing and discarding a large number of samples.