Heated Air Stream Research Articles

AUTOMATED TEMPERATUREAND HUMIDITY CONTROL AND MONITORING SYSTEM FOR IMPROVING THE PERFORMANCE IN DRYING SYSTEM

Temperature and relative humidity are the key control parameters in drying processes for preserving and improving food quality. To achieve this goal, an automatic control system has been designed and built to provide adequate heat and drying streams according to the ambient requirements of various climatic zones and the kind of dried product. The control system combined with sensors allows the temperature and humidity of the drying chamber to be adjusted online by predetermined parameters. When there is little or no radiation present or when rapid drying rates are required, the heated air stream may be produced utilizing an electric motor fan in addition to an electrical backup heater. The fan automatically modifies its speed using the Pulse Width Modulationtechniquefor energy efficiency depending on the required temperature of the drying chamber. The control system based on Arduino Uno board is built within the solar dryer, after which tests are carried out. The test results are displayed on a display. Themanagement system was set up to maintain an ambient temperature between 40 and 60 °C andrelative humidity between 10 and 20 %. The system is a flexible solution for different climatic zones and dried products, according to experimental findings demonstratingits efficiency in managing the drying environment. Finally, this paper can conserve energy because it only works when the temperature around the food is below 60 oC.

Impregnation of different coals and biomass with rapeseed oil for intensifying their ignition in a heated air stream during oil-free boiler start-up

Studies were conducted to develop the scientific basis for the use of low-grade solid hydrocarbons and biomass as starting fuel and an energy additive to the main fuel for coal-fired boilers. Char, softwood sawdust, brown and bituminous coals with a particle size of 140–250 μm were used in an air-dry state and after impregnation with rapeseed oil. Analytical studies included surface texture analysis of solid fuel particles, determination of their wetting properties with respect to water and rapeseed oil, thermogravimetric and calorimetric analyses, FTIR analysis of gaseous combustion products, analysis of the elemental composition and texture of the ash surface. Fuel combustion characteristics were studied in conditions of fine fuel particles movement in a heated air stream: 5 m/s, 500–800 °C. Impregnation of solid fuels with oil led to the moisture displacement from the pores of fine particles (oil content in various solid fuels 5.6–22.4%). It increased the combustion completeness of fine particles in a shorter period of time (the ignition delay time reduced by 15–30%) and the thermal effect of the exothermic reaction (by 2.2–18.1%). Oil vapors first ignited in the vicinity of the particle, thereby intensifying the combustion of solid component in fossil fuels.

Physiochemical and thermal properties of silica Aerogel–Poly vinyl alcohol / Core–Shell structure prepared using fluidized bed coating process for thermal insulation applications

This work proposes a process for the fabrication of a silica aerogel (SA)–polyvinyl alcohol (PVA) core–shell structure using the fluidized bed method. First, nearly spherical hydrophobic SA particles with a diameter between 0.5 mm and 3.0 mm were prepared from rice husk ash via a sodium silicate route and ambient-pressure drying. Subsequently, a specific batch size of the SA particles was suspended in a heated air stream and sprayed with aqueous PVA using a bottom nozzle via the fluidized bed process. Coating of an individual SA particle is possible for large particle sizes and hydrophobization of the SA surface prevents penetration of PVA into its pores. The core–shell structured material showed a combination of high thermal insulation and good mechanical stability, rendering it suitable for use as filler in polymer resin, paints, and coatings. The core–shell structured material had an average density of 0.09 g cm−3 with well-preserved SA pores. The average shell volume fraction was 2.5%; the shell thickness varied below 50 μm, and was mostly approximately 20 ± 5 μm. Thermal analysis using transient plane hot-disk method and thermogravimetric analysis showed that the SA–PVA/core–shell structure exhibited lower thermal conductivity (0.035 W/mK) and higher thermal stability than uncoated SA.

Effect of hydrogen addition on the counterflow ignition of n-butanol at atmospheric and elevated pressures

The present experimental and computational study describes the non-premixed counterflow ignition of nitrogen-diluted pre-vaporized n-butanol/hydrogen mixtures impinging on a heated air stream for pressures of 1–4 atm and hydrogen molar percentages in the binary fuel blends ranging from ξH = 0% (pure n-butanol) to ξH = 100% (pure hydrogen). The experimental data show that hydrogen addition results in a non-linear decrease in ignition temperatures that can be broken into two regimes; a hydrogen-enhanced regime of ξH = 0–40%, where the addition of more hydrogen significantly decreases ignition temperature, and a hydrogen-dominated regime in the range of ξH = 40–100%, where ignition temperatures exhibit minimal sensitivity to further hydrogen addition. The experimental results are then simulated using n-butanol-specific skeletal mechanisms developed from two comprehensive butanol models available in the literature. The ability of these mechanisms to predict the variation of ignition temperatures associated with hydrogen addition to the n-butanol “base” fuel is assessed. Comparison between the experimental and computational results reveals that both chemical kinetic models capture the two-regime behavior associated with hydrogen addition observed in the present experiments, though both models over-predict experimental ignition temperatures. Further chemical kinetic analysis of the mechanisms reveals that the two-regime behavior is controlled by the production of hydroperoxyl radicals, with production via the reaction of formyl radicals and oxygen molecules dominating at low hydrogen addition levels, and production via the three-body H + O2+M = HO2 + M reaction dominating at high hydrogen addition levels.

The role of global and detailed kinetics in the first-stage ignition delay in NTC-affected phenomena

Using n-heptane as a representative fuel exhibiting NTC (negative temperature coefficient) chemistry, a comprehensive computational and mechanism study was conducted on the role and controlling chemistry of the first-stage ignition delay in the superficially dissimilar systems of the auto-ignition of homogeneous mixtures and the nonpremixed counterflow ignition of fuel versus heated air. It is first shown that the first-stage auto-ignition delay time, τ1, possesses a minimum value, τ1,min, with increasing temperature, and that for temperatures below the range corresponding to τ1,min, τ1 is largely insensitive to the equivalence ratio (ϕ) and pressure (p) of the mixture. Furthermore, in this regime the global reaction order was found to be close to unity, hence supporting the notion that the limiting steps in this temperature regime are the RO2 isomerization reactions, which in turn explains the insensitivity of τ1 on ϕ and p in this temperature regime. However, when the temperature approaches that of τ1,min, competition of QOOH decomposition and the β scission reactions of the alkyl radicals with the low-temperature chemistry chain reactions, as well as the equilibrium shift of the oxygen addition reactions, increases τ1 and consequently results in τ1,min. The corresponding global reaction order also increases, to about two, indicating the progressive importance of the oxygen addition reactions. Extracted values of the global activation energy are also close to those of the controlling reactions in these temperature regimes. Results from the counterflow show the same global kinetic responses by identifying the reciprocal of the counterflow strain rate as the relevant ignition delay time and the temperature of the heated air stream as the homogeneous mixture temperature. It is further found that in the temperature range corresponding to τ1,min, the diminished heat release causes the counterflow to lose its characteristic, non-monotonic S-curve response and consequently distinct, abrupt ignition–extinction transition events.

Experimental Study of a Spray Solar Spray Desalination System Using Parabolic trough Concentrators

The present work described the atomized aqueous solution such as seawater and dense water suddenly encounter the high-temperature hot air heated by solar energy, Heat and mass transfer process between the two fluids, a saline solution was pressured between 0.4Mpa and 0.6Mpa, then to force the solution through one non-pneumatic nozzle to produce solution mesoscopic droplets，blowing a heated air stream through first evaporation chamber, dispersing the droplets into the heated air stream，permitting water in the droplets to evaporate hereby separating water vapor from aqueous solution in the heated air stream，then the outlet hot saturated air was condensed to fresh water .The water vapor from dense brine in the heated air stream was evaporated, and so the salt crystals therefrom could be obtained in the second evaporation chamber. Compared with the conventional Humidification-dehumidification desalination (HDD) process method, the heat efficiency is high, when heated air stream temperature is between 100°C and 150°C ,fresh water production can be increased by 50% or more. There is no general problem of scale desalination, can handle high concentrations of salt water without any chemical additives.

ROBUST CONTROL FOR UNCERTAIN DELAY SYSTEM

In this paper, the Generalized Kharitonov Theorem for quasi-polynomials is exploited for the purpose of synthesizing a robust controller. The aim here is to develop a controller to simultaneously stabilize a given interval plant family with unknowing and bounded time delay. Using a constructive procedure based on Hermite–Biehler theorem, we obtain all PI gains that stabilize an uncertain first-order delay system. An application example is presented for the temperature control of a heated air stream, process trainer PT326.

Diffusion Driven Desalination for Simultaneous Fresh Water Production and Desulfurization

The diffusion driven desalination (DDD) process has been previously introduced as a process for distilling water using low-grade waste heat. Here, a configuration of the DDD process is introduced for simultaneously distilling water and scrubbing sulfur dioxide (SO2) out of heated air streams, which is also known as flue gas desulfurization (FGD). This novel DDD/FGD process utilizes the low-grade waste heat carried in industrial discharge air streams. There are many applications, where the industrial air discharge also contains SO2, and in order to utilize the waste heat for the DDD process, the SO2 must be scrubbed out of the air stream. The two major components of the DDD process are the diffusion tower and the direct contact condenser. In the present work, a thermal fluid transport model for the DDD/FGD process, that includes SO2 scrubbing, is developed. It is an extension of the heat and mass transport model previously reported for the DDD process. An existing laboratory scale DDD facility was modified and tested with SO2 in the air stream and with seawater as the feed water to the diffusion tower. The experimental investigation has been completed to evaluate the fresh water production and SO2 scrubbing potential for the DDD/FGD process. The experimental results compare favorably with the model predictions. Chemical analysis on the condenser water demonstrates the capability of the DDD/FGD process to produce high quality fresh water using seawater as the input feed water to the process.

Nanofibers from gas-assisted polymer melt electrospinning

The concept of a gas-assisted polymer melt electrospinning process is presented. This technique allows for reduced quenching of the melt jet in the spinning region, and thus increasing the jet attenuation rate and resulting in production of sub-micron scale fibers. A comprehensive melt electrospinning model was used to analyze the effects of the heated air stream on the polymer jet. It was found that under the investigated conditions in electrospinning of polylactic acid (PLA) melt, air drag produced an additional 10% thinning compared to the un-assisted melt electrospinning process, and the heating provided by the air stream resulted in an additional 20-fold jet thinning.

Modeling of the Front of Melting and Destruction of a Melt Film During Gas-Laser Cutting of Metals

The products of laser cutting of metals on an automated laser setup are investigated. Results of model experiments are presented, where soft wax was used instead of metal transforming into the melt; soft wax filled a narrow flat slot between two glass plates and was removed by a heated air stream. The physical processes of melting of the liquid‐wax film, its destruction, and entrainment by the gas jet being assumed to be analogous to the processes of metal‐melt spraying inside the cut in full‐scale experiments, the characteristic size of drops formed thereby is evaluated. The modeling results are in qualitative agreement with the results of full‐scale experiments. It is shown that the quality of laser cutting of metals directly depends on the character of spraying of the liquid melt and the process of its removal.

Study of interaction in spray between evaporating droplets and turbulence using second order turbulence RANS modelling and a Lagrangian approach

The objective of this work is to analyse the interaction between evaporating droplets and the turbulent flow of the carrier gas using a second order turbulence RANS modelling in an Euler-Lagrangian approach including two-way coupling. The analysis is performed in a vertical isopropanol polydispersed spray issuing into a co-flowing, heated air stream. The results mainly display the turbulence modulation effects besides the turbulent dispersion of evaporating droplets: 1) The influence of droplet diameter and interface transport on the gas phase turbulence as well as 2) The effect of the droplet evaporation on the mass and heat transfer processes are compared with experimental data. 3) The reciprocal influence of the modification of interface transport by turbulence, i.e., the influence of the turbulent fluctuations of different quantities (such as velocity, temperature, etc.) on the droplet evaporation (through the mass and heat evaporation rates, the droplets diameter distribution) are computed and discussed.

Analysis of isothermal and evaporating turbulent sprays by phase-Doppler anemometry and numerical calculations

Abstract The present paper summarises detailed experimental and numerical studies on stationary turbulent sprays performed by the authors research group over the past 5 years. The main objective of these studies was related to the improvement and validation of the Euler/Lagrange approach for the numerical prediction of spraying systems. For this purpose three different spray configurations were considered, a hollow cone spray issuing into a low speed air stream, a spray interacting with a solid cone, and a hollow cone spray being injected into a heated air stream. The first case focuses on the effect of droplet collisions and coalescence and evaporation, the second case concentrates on the modelling of secondary atomisation associated with a spray-wall interaction and the third case is related to spray evaporation in a moderately heated turbulent air stream. For all these cases detailed measurements by phase-Doppler anemometry were performed in order to provide appropriate inlet conditions and data for the validation of the predictions.

Mesoporous Silica Synthesized by Solvent Evaporation: Spun Fibers and Spray-Dried Hollow Spheres

Simple synthesis methods for mesoporous fibers and powders by rapid evaporation of hydrolyzed silicon alkoxide−surfactant solutions are described. Mesoporous fibers are prepared by dry spinning, and mesoporous powders by spray drying of alkoxide−surfactant solutions. Precursor solutions, containing fully hydrolyzed tetraethoxysilane, cetyltrimethylammonium chloride surfactant, and, in the case of the fibers, a fiber-forming polymer, in an acidic alcohol/water mixture, are drawn into continuous filaments or atomized into droplets in a heated air stream. During solvent drying, silica and surfactant self-assemble to form the hexagonally ordered mesophase structure, and all of the nonvolatile components (silica, polymer, and surfactant) are incorporated into the mesophase. The pore diameter and surface area of calcined fibers were 20 A and 1100 m2/g, respectively. For the powders, pore sizes of 25 A and surface areas as high as 1770 m2/g were measured. Spray-dried powders consisting of hollow spherical partic...

Experimental determination of the volumetric heat transfer coefficient between stream of air and ceramic foam

An experimental investigation is described that characterizes heat transfer between a heated air stream and ceramic foam. An apparatus is designed to determine the volumetric heat transfer coefficient between the foam and a stream of air using a single-blow transient technique. Experiments are reported on different mean pore diameter specimens. The governing conservation equations of energy for both the gas and the solid phases with appropriate boundary and initial conditions are solved using a finite-difference procedure. A nonlinear, least-square fit of predicted and measured gas temperatures is used to determine the volumetric heat transfer coefficient between air and ceramic foams. Two different types of ceramic foams are investigated, and the results obtained are compared. Heat transfer coefficient correlations are developed for each different mean pore diameter of ceramic foam and for the range of Reynolds numbers between 12 and 563 covered in the experiments.

Transient heat and mass transfer of interacting vaporizing droplets in a linear array

A validated computer simulation model, based on an extended finite element software package, has been developed for the detailed analysis of colinear interacting vaporizing droplets in a heated air stream. The complete transport equations describing transient laminar axisymmetric fluid-particle flow with phase change have been solved for three closely-spaced n-octane fuel droplets and a free-stream ambient with Re 0 = 100, T ∞ = 1000 K and p ∞ = 10 atm. Of interest are the gas-phase temperatures jmd vapor concentrations with time, the transient interfacial phenomena and integral properties, such as Nu g , Sh g and c D , for each droplet. Comparisons with approximate analyses of three colinear monodisperse droplets and empirical studies for a solitary blowing sphere are included.

Experimental Study of Self-Ignition and Smoldering of Moist Cellulosic Materials

The self-ignition and smoldering characteristics of cellulosic materials, as exemplified by cotton fibers, when subjected to convective low-velocity heated air stream were studied experimentally at atmospheric pressure. The effects of the content of moisture in the raw cotton on its ignition delay and smolder front propagation rates were established. Some comparative tests were also conducted on dry cotton samples containing instead of moisture, various concentrations of n-hexane or an alcohol mixture.

Volatilization and Ignition of Oil Sand Samples During Intermittent Exposure to Hot Low-Velocity Air Streams

The behavior of small oil sand samples was monitored experimentally when subjected repeatedly to low-velocity heated air streams, with either prompt quenching or slow cooling of the samples to their initial temperatures between these exposures. The stream temperature was either in the range of 300°C to 500°C in air or at higher temperatures of up to 760°C, while being exposed to the products of combustion of very lean hydrogen-air mixtures. This was done in relation to understanding better the associated processes in some in-situ recovery trials where stoppage of the combustion front and/or the flow of the injected fluids can occur. A variety of multi-exposure-cooling schemes was considered and their effects on the extent of volatilization and ignition established. Comparison to the corresponding behavior of similar samples under uninterrupted exposure to the heated streams was made throughout.

An Experimental Examination of the Spontaneous Ignition of Fibrous Materials in Heated Air Streams

The behavior of a bed of fibrous combustile material, exemplified by pure cotton (ASTM, 1982), when subjected to a controlled stream of heated air, was examined experimentally. The effects of some controlling basic physical parameters, such as sample compactness and ventilation, air stream temperature, and air stream velocity on the spontaneous ignition and combustion characteristics were established. Moreover, the observed spontaneous ignition delay time was correlated in terms of these variables. The effective activation energy during the ignition reactions was derived and shown to be dependent on the compactness of the sample.

An Investigation of the Ignition and Combustion of Loosely Compacted Commercial Steel Wool

The ignition and combustion characteristics of loosely compacted commercial steel wool samples when introduced promptly into low velocity heated air streams and the morphology of the quenched residues were investigated. The effects of the important factors of sample compactness, stream temperature and velocity, and exposure time were established. Comparative tests were also made under similar conditions involving similarly prepared samples of cotton, sheep wool, fiber glass wool, and processed cigarette tobacco.

Sonic spray of H2SO4 in a swirling heated air stream

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