Low Temperature Airflow Research Articles

Vegetables Treated before Drying with Natural Antioxidants plus UV-C Improve Colour and Bioactive Compounds

The quality of minimally processed fruits and vegetables is essential for consumers’ health and effective marketing. This study investigates the effects of UV-C irradiation, applied alone and combined with various natural antioxidants, on the preservation of bioactive compounds and the colour of dehydrated fruits and vegetables. To achieve this, red peppers, yellow peppers, tomatoes, zucchini, eggplants, and melons were subjected to pre-treatments with natural antioxidants and UV-C before processing using low-temperature airflow (50 °C). The drying kinetics showed typical curves of hygroscopic materials, while the drying time was high due to the low temperature applied. The higher drying rate was found for eggplant, due to its porosity, thus allowing a faster moisture removal. The application of antioxidants and UV-C treatments effectively preserved the colour parameters L*, a*, and b*, while in the case of untreated dried vegetables, a significant worsening of colour parameters was noted. However, most applied pre-treatments had positive effects on bioactive compound losses. The best results were obtained using a combination of UV-C with one antioxidant mix that was composed of vanillin, rosemary, and citrus extracts, and combined with a mixture of olive, onion, garlic, and citric acid extracts, which was highly effective in preserving the colour and bioactive compounds of most dried vegetables.

Optimizing airflow in spiral blast freezers

Spiral blast freezing is a common unit operation used in food processing facilities for rapidly freezing a variety of foodstuffs. The purpose of a blast freezer is to generate high velocity, low temperature air flow over food products being conveyed within refrigerated enclosures to accomplish the freezing process. However, air flow patterns observed within field operating blast freezers are often suboptimal, resulting in diminished system performance. This paper applies a Monte Carlo simulation technique to a food product freezing simulation in order to identify velocity profiles that optimize the freezing process. A one-dimensional food product model is used to evaluate the interplay between the time variation in the magnitude of the air velocity over food products conveyed through the freezing system and the resulting dwell time needed to achieve a target product core temperature at the blast freezer exit. Temporal heat transfer coefficients derived from field measurements made in a newly installed spiral blast freezer serve as a basis to calibrate the one-dimensional product model.The results of the Monte Carlo analysis show freezing system performance is improved when high and stable air velocities over the product are achieved early in the freezing process dwell time. Air flow patterns within a freezing system that result in high air velocity later in the freezing process dwell time are suboptimal. Field-measured data on a newly installed spiral blast freezer showed this suboptimal air flow pattern and the use of baffling within the spiral enables improved airflow leading to an estimated 10 % increase in production throughput.

Effects of airflow direction on the thermal-hydraulic performance of louvered fin-common flow up vortex generator

To improve the thermal-hydraulic performance of the louvered fin and flat tube heat exchangers (LFHEs), this paper innovatively proposes the louvered fin-common flow up vortex generator (LF-CFUVG). Then, numerical simulations are conducted to study the effects of airflow direction γ on the thermal-hydraulic performance of the LF-CFUVG. The corresponding results indicate that the flow resistance of the LF-CFUVG is always greater than the Baseline (the LFHE without CFUVGs), as the CFUVGs reduce the minimum free flow area while forming low-speed wake zones behind them. The heat transfer capacity of the LF-CFUVG exceeds the Baseline as well, this is attributed to the longitudinal vortices induced by the CFUVGs, which is helpful to homogenize the temperature field and transport the high-speed and low-temperature airflow. And the thermal-hydraulic performance of LF-CFUVG also outperforms the Baseline. In comparison to the working condition of γ is 0°, the pressure drop Δp for the LF-CFUVG declines with increase of γ from 0° to 30°, thus a larger angle helps to reduce flow resistance. However, the heat transfer coefficients hLF and performance evaluation criteria (PEC) of the LF-CFUVG deteriorate with the increasing γ, so a smaller angle is recommended to obtain an intenser heat transfer and better PEC.

Ignition and combustion of a group of waste-derived fuel slurry droplets: Mutual influence upon low-temperature convective heating

Waste-derived slurries can be burned both in a fluidized bed and when injected into a boiler furnace. This paper is the first to analyze the dynamic characteristics of ignition and combustion of a group of three slurry droplets based on coal slime, water, and waste industrial oil in a low-temperature air flow with a temperature of 500–600 °C and velocity of 0.1–1.2 m/s. The distance between the droplet centers varied from 3 to 15 mm. The results of the study can develop innovative technologies for flare and fluidized bed combustion of fuel slurries at the boiler start-up stage. It was found that a distance of 3–5 mm between droplet centers provides almost simultaneous gas-phase ignition of all droplets. A further increase in the distance critically worsened the ignition and combustion and eliminated the synergy. An increase in the air flow velocity contributed to the appearance of a common flame zone but at the same time increased the ignition delay of droplets by a factor of 1.2–1.6. The negative effects of cooling and entrainment of combustion products by a low-temperature flow can be effectively reduced by adding 5 wt% of industrial oil to the slurry. Also, the use of oil reduced the droplet ignition delay time by 15–20%. At the stages of flame and heterogeneous combustion, the differences for droplets in a group increased (the stage durations differed by 1.5–3 times). For the low-temperature start-up of a boiler, it was found advisable to provide a droplet flow with a sufficient density to stabilize their ignition without significant flow deviations (dimensionless distance 1.5–2.5).

Impact of cooling rate on mechanical properties and failure mechanism of sandstone under thermal–mechanical coupling effect

High geo-temperature is one of the inevitable geological disasters in deep engineering such as resource extraction, space development, and energy utilization. One of the key issues is to understand the mechanical properties and failure mechanism of high-temperature rock disturbed by low-temperature airflow after excavation. Therefore, the experimental and numerical investigation were carried out to study the impact of cooling rate on mechanical properties and failure mechanism of high temperature sandstone. First, uniaxial compression experiments of high temperature sandstone at different real-time cooling rates were carried out to study the mechanical properties and failure modes. The experimental results indicate that the cooling rate has a significant effect on the mechanical properties and failure modes of sandstone. The peak strain, peak stress, and elastic modulus decrease with an increase in cooling rate, and the fragmentation degree after failure increases gradually. Moreover, the equivalent numerical model of heterogeneous sandstone was established using particle flow code (PFC) to reveal the failure mechanism. The results indicate that the sandstone is dominated by intragrain failure in the cooling stage, the number of microcracks is exponentially related to the cooling rate, and the higher the cooling rate, the more cracks are concentrated in the exterior region. Under axial loading, the tensile stress is mostly distributed along the radial direction, and the damage in the cooling stage is mostly due to the fracture of the radial bond. In addition, axial loading, temperature gradient and thermal stress mismatch between adjacent minerals are the main reasons for the damage of sandstone in the cooling stage. Moreover, the excessive temperature gradient in the exterior region of the sandstone is the main reason for the damage concentration in this region.

Experimental investigation on spray and ignition characteristics of plasma actuated bluff body flameholder

Stable combustion of aero-engines and ramjet combustors is greatly dependent on the quality of atomization. It is quite difficult to achieve wider ignition boundary and good combustion performance under low pressure and low temperature airflow, because the quality of fuel atomization is comparative bad and there is no enough surface area for fuel droplets to mix with surrounding air and evaporate. Internal mechanism of how plasma assisted ignition on spray performance in model aero-engine combustors and ramjet combustors remains not clear. In this study, a novel plasma-actuated bluff-body flameholder is proposed to help improve the quality of fuel atomization and thus enhance the ignition performance and stable combustion of original bluff-body flameholder. Spray and atomization properties of aviation kerosene (RP-3) were investigated under various operating conditions including inlet pressure, inlet temperature and gliding arc plasma actuation. A rotating arc discharge plasma column could be formed during the high-voltage tungsten electrode and the fins of bluff-body flameholder. Macroscopic as well as microscopic spray properties was analysed through simultaneous LIF/MIE technique along with particle/droplet image analysis (PDIA) system. Flame ignition limit at different inlet temperature was conducted under combustor inlet Mach number of 0.2. Results show that mean drop diameters were decreased with increase of inlet temperature and inlet pressure. Gliding arc plasma actuation help fuel injected from bluff-body flameholder breakup into smaller droplets and improve uniformity of fuel spray through heating and transport effect. With actuation of gliding arc plasma, Sauter mean diameter (SMD) was minished by 13%∼20% at inlet pressure of 30 kPa, while droplet uniformity index was increased by around 21.8%. Due to better fuel atomization quality together with more active components generated by gliding arc plasma, flame ignition limit of bluff-body flameholder was obviously extended by about 13%∼30%. The gliding arc plasma was verified to promote fuel droplets atomization and to achieve homogeneous distribution under low temperature and pressure, which is beneficial to wider flame ignition and stabilization boundary.

A Study on the Application Possibility of the Vehicle Air Conditioning System Using Vortex Tube

Since refrigerants applied to vehicle air conditioning systems exacerbate global warming, many studies have been conducted to supplement them. However, most studies have attempted to maximize the efficiency and minimize the environmental impact of the refrigerant, and thus, an air conditioning system without refrigerant is required. The vortex tube is a temperature separation system capable of separating air at low and high temperatures using compressed air. When applied to an air conditioning system, it is possible to construct an eco-friendly system that does not use a refrigerant. In this paper, various temperature changes and characteristics of a vortex tube were identified and applied to an air conditioning system simulation device. Additionally, an air conditioning system simulation device using indirect heat exchange and direct heat exchange methods was constructed to test the low-temperature air flow rate (yc), according to the temperature and pressure. As a result of the experiment, the temperature of the indirect heat exchange method was found to be higher than the direct heat exchange method, but the direct heat exchange method had low flow resistance. As a result, the direct heat exchange method can easily control the temperature according to the pressure and the low-temperature air flow rate (yc). Therefore, it was judged that the direct heat exchange method is more feasible for use in air conditioning systems than the indirect heat exchange method.

The heat transfer enhancement of low temperature airflow foam effect in the ice storage charging process

Phase change cold storage, as an energy-saving application, was widely used in central air-conditioning system of commercial building to fit the intermittent and the changing cooling load. Ice storage with different container structures was developed, but the low charging efficiency and non-linear energy storage rate were difficult to match the dynamic change of cooling load and the energy-saving strategy of the chiller system, and increased the energy consumption and the complexity of the ice storage air-conditioning system control. Low temperature airflow (LTA) was introduced into water in the low temperature environment. Gas, liquid and solid coexisted in the phase transformation process, and the process of heat and mass transfer underwent complex changes. The effects of these changes on phase transformation were studied by experiment. It was found that porous frozen body was generated with a large number of microchannels inside. The porous structure dispersed the airflow into isolated bubbles, and foam generated. Foam enlarged the heat transfer area and prolonged the contact time between LTA and phase change material (PCM), sharply improving the heat transfer efficiency, and strengthening the heat transfer between PCM and its geometry boundary. When the air flow rate was 25 L/min, the freezing rate increased by 4 times than that of freezing without LTA. This study concluded that it was simple, practical and efficient to introduce LTA into PCM considering the heat transfer enhancement of the foam effect. It will have a prosperous application in ice storage system.

Research on Energy-Saving of New Type Low-Temperature Air Flow Dehumidification System Based on Exergy Analysis Method

Currently two humidity control methods, namely liquid desiccant and rotary wheel dehumidification, are commonly used to handle humid fresh air. For these two methods, a large amount of heat would be consumed in the processes of solution regeneration and adsorbent regeneration, which results in a serious waste of energy. This paper deals with a new type low-temperature airflow system, which was combined with an ice-storage device. Based on exergy balance model, its performance parameters, such as the cooling load, energy consumption and exergy efficiency, were calculated and compared with those of a rotary wheel dehumidification system and a liquid desiccant system under the same conditions according to experimental data of a practical project in Guangzhou. The calculation results show that the exergy efficiency of new type low-temperature airflow system is higher than that of the rotary wheel dehumidification system by 96.14% and higher than the liquid desiccant system by 40.31%, which conditionally means the proposed low-temperature airflow dehumidification system has much better performances.

Ignition of a coal particle at the low temperature of gas flow

Regularities of physical and chemical processes occurring during the heating of the coal dust particles by low-temperature air flow are investigated by means of thermogravimetric analyzer TA SDT Q600 and experimental setup of optical diagnostics of multiphase flows based on PIV method. Qualitative and quantitative characteristics were established for the processes of the coal particle inert heating, moisture evaporation, thermal decomposition, combustible gas mixture formation, oxidation of volatiles and carbon. It was revealed that the temperature of the oxidizer required for the coal particle ignition is higher than 500 ◦ C. The experimental data can be used to develop predictive mathematical models of technological processes fire hazard in pulverized coal systems of thermal power plants.

A222 高熱流束ヒートシンク実現のためのマイクロ超音速ノズル形状の評価(OS-7: 電子機器・デバイスの熱工学的課題と熱流動現象(4))

In this study, the effect of the geometry of the micro asymmetric converging-diverging nozzle was evaluated based on the numerical simulation for the supersonic air flow inside the micro-channel. This nozzle was designed to generate low temperature air flow for the high heat flux heat sink and configured by simple arc curvature. The channel length was 10 mm. The nozzle width was varied from 20 to 400 μm. The inlet pressure, outlet pressure, and wall temperature were 0.7 MPaA, 0.1 MPaA, and 350 K, respectively. Then, in order to evaluate the effect of back pressure, outlet pressure was varied from 0.1 to 0.4 MPa for the case of 200 μm of nozzle width. From calculation results, the pressure and temperature distributions inside the each channel were compared and discussed.