Initial Heating Period Research Articles

Detailed visualization experiments on the start-up process and stable operation of pulsating heat pipes: Effects of dividing boards

To broaden the applicable orientation for the existing closed loop pulsating heat pipe (CLPHP) with a relatively large tube (internal diameter of 6 mm), this work fabricated a novel quartz CLPHP inserted with a dividing board in the straight section of each tube. The dividing board forms two semi-circles with an effective hydraulic diameter (Dh) of 3.09 mm. This method amplifies the surface tension effect with almost no change in the mass inventory of working fluid. Through a series of visualization experiments, the effect of filling ratio (FR) on the pulsation in the boarded model are first revealed by comparing with the original non-boarded CLPHP under the vertical and the horizontal orientations. The four longitudinal corners generated by dividing board are found not only accelerated the drain down of the liquid slugs but beneficial to the onset of nucleate boiling, prompting the threshold of pulsation motion for 10 - 20 W. Also, the visualization results for the vertical orientation show that the sealing between the dividing boards and the inner tube had a great impact on the performance and heat transfer behavior. The thermal resistance can be improved by 22 - 43 % without leakage between the semi-circles. Under the horizontal orientation, the boarded CLPHP with only three number of turns can be incited with pulsation motion successfully. The boarded design provides additional vapor channels and avoids the liquid slug trains being pushed away from the evaporator during the initial heating period. Also, the un-balanced capillary force due to the hydraulic diameter difference can sustain subsequent pulsation motion. However, the success under only FR = 50 % indicates that an appropriate FR was also necessary.

Compressive Strength parallel to grains of Charred Paulownia Wood

Wood is a common flammable in the building fire and the dominant fuel in the wildland fire. In this study, the surface of wood specimens has been pre-heated by various radiative heat fluxes and heating times to achieve varying char layer thicknesses. The compressive strength parallel to grains versus various charring thicknesses has been obtained by an electronic universal testing machine (WDW-20). Experimental results demonstrate that a higher heat flux and longer heating times promote the formation of the char layer, but the compressive strength versus the charring thickness first decreases to its minimum value and subsequently remains stable. For the 7 kW/m2 case, there is a rapid decrease in compressive strength for the 5-7 heating period. For example, the compressive strength is 31.7 MPa after the initial 5-minute period, and 3.69 MPa after the initial 7-minute period. For the radiative heat fluxes from 7 kW/m2 to 10 kW/m2, the reduction rate of compressive strength will increase from 1.32 MPa/min to 3.75 MPa/min for the initial 5-minute period, and increase from 4.95 MPa/min to 5.34 MPa/min for the initial 7-minute heating period. This study helps us understand the influence of charring processes on the mechanical properties of wood and the failure of timber structures.

Influence of the type of woody biomass on energy and environmental characteristics of the thermal preparation processes and ignition of bio-water-coal fuel particles

The article presents the results of studies of ignition and combustion processes of droplets (during the initial period of heating), and then particles (after drying the surface layer) of bio-water-coal fuel (Bio-WCF) under conditions corresponding to typical furnace spaces of boiler units of thermal power plants (TPPs). The ignition delay times for Bio-WCF droplets of typical (for modern spray devices) sizes (diameter d=3 mm) have been established. The component compositions of the combustion products of significantly inhomogeneous bio-water-coal suspensions have also been established in a wide range of variations in the temperature of the oxidizing medium. A large-scale analysis of the influence of the type and concentration of woody biomass in a bio-water-coal suspension on energy (ignition delay time of WCF) and environmental (intensity of formation of anthropogenic combustion products of fuel) characteristics was carried out. It is shown that the addition of woody biomass to the water-coal suspension leads to a significant acceleration (by 10–20%) of the ignition process. An analysis of the component composition of combustion products showed that the introduction of wood biomass into the composition of water-coal fuel leads to a decrease in the intensity of the formation of anthropogenic combustion products. It has been established that an increase in the concentration of the vegetable component of the fuel leads to a significant sequestration of sulfur and nitrogen oxides.

ПОВЫШЕНИЕ ЭНЕРГЕТИЧЕСКОЙ ЭФФЕКТИВНОСТИ ВАРОЧНОГО КОТЛА ОПТИМИЗАЦИЕЙ РАБОТЫ ГАЗОВОЙ ГОРЕЛКИ

The use of natural gas in the heat treatment of feed reduces the cost of livestock products and increases competitiveness in the foreign market. An analysis of various modifications of gas-using digesters with a wide range of performance and design, offered by the foreign market, showed that their common disadvantage is the lack of automatic gas flow control in proportion to the heat flow used to heat the feed to boiling and during boiling. An energy-saving fodder boiler with an automatic gas flow control system in proportion to the consumed heat flow has been developed. The studies were carried out on an experimental sample of a feed boiler, in which an automated gas flow control system based on semiconductor thermoelectric elements acts on the actuator without intermediate output signal amplifiers. The efficiency of the automated energy supply system was studied by experimental studies of the dynamics of gas consumption in the process of thermal processing of feed in automatic and manual modes. It was found that the total gas consumption for the preparation of 100 kg of feed in automatic mode with gas flow control proportional to the consumed heat flow is 1.2 times less than when the boiler is in manual mode. The most optimal mode for saving fuel is provided when setting the gas burner in the initial period of heating up to a power of 14 kW for a digester with a volume of 100 liters. Gas overrun up to 20% occurs for boilers with more powerful gas burners (22 kW).

Mathematical Modeling of the Evaporation of a Water Drop from a Heated Surface

This paper presents the results of mathematical modeling of the evaporation of a single water drop from the surface of a copper substrate using a new model, which does not require special experiments to close the system of equations and the corresponding boundary conditions with empirical constants. On the basis of the results of mathematical modeling, it was found that convective currents that occur in a small water drop (≤1 mm in diameter) do not significantly affect the characteristics or conditions of heat and mass transfer processes occurring in a liquid drop heated on a copper substrate. The results of numerical simulation showed that during the initial period of droplet heating, the latter undergoes a rapid transformation of the flow field. Five seconds after the beginning of the thermal action, a quasi-stationary regime of flows in the drop sets in. The model is tested on known experimental data. The theoretical analysis of temperatures at the characteristic points of a water drop and the surface on which the drop is located is carried out in ranges of thermal loads quite typical for practice, conditions for transferring heat and water vapor to the environment. According to the results of mathematical modeling, the possibility of using the developed model in the analysis of the state of cooling of surfaces heated to high temperatures, in cases typically used, is substantiated.

Experimental study on dynamic effect of external insulation on indoor thermal environment and energy consumption

In this paper, two identical model buildings were built, one of which adopted 38 mm Expanded Polystyrene (EPS) board as the external insulation. Under the same heating set-point temperature, the dynamic change process of indoor thermal environment and heating energy consumption of EPS insulation building and non-insulation Reference building was compared and monitored. Results show that under the same heating set-point temperature, the indoor temperature of the two buildings can quickly approach the set value in about 20 min. However, the temperature rise of the internal surface of the wall lags behind. After several hours, the advantage of the temperature rise of the internal surface of the EPS building gradually appeared. The energy consumption of the two buildings is similar in the initial heating period of 20 min, and the energy saving rate of EPS building increases slowly with the increase of the heating duration. The energy saving rate is about 1%–26% in the range of 0.5–8 h intermittent heating in hot summer and cold winter zone. The change curve of cumulative temperature rise ratio with heating time is proposed to evaluate the dynamic improvement effect of external insulation on indoor thermal environment. This research provides a reference basis for the development of energy conservation.

Study on the change of indoor and outdoor air quality with height in high-rise buildings during the initial heating period

The indoor and outdoor pollutant concentrations of a high-rise building in Xi’an was tested and analyzed during the initial heating period in this paper. Also the relationship between indoor and outdoor pollutant concentration, building height and outdoor meteorological parameters in different periods was an-alyzed at the same time. Compare and analyze the ratio of indoor and outdoor particle concentration (I/O value) in different time periods. The results showed that during the test period, the concentration of partic-ulate matter reached the highest value at noon and the lowest value after dusk. The I/O value tends to de-crease with the increase of the building height, but at some moments there is a situation where the I/O value is greater than 1. The research results of this paper can provide a theoretical basis for indoor particulate control and architectural design of high-rise buildings.

Acid-catalyzed fractionation of almond shells in γ-valerolactone/water

The fractionation of almond shells, an agro-industry residue available in some Mediterranean climate regions, was investigated using acid-catalyzed hydrolysis in γ-valerolactone (GVL)/water. A set of non-isothermal experiments at nominal temperatures of 120, 140, and 160 °C and sulfuric acid concentrations from 25 to 75 mM were developed using a constant 80% w/w GVL in water concentration and a reaction time of up to 120 min. GVL was an efficient medium and promoted solubilization of both lignin and hemicellulose, even at low temperature during the initial period of reactor heating, while cellulose conversion was limited. A temperature of 160 °C gave the highest extraction of lignin and hemicellulose, but recovery of hemicellulose carbohydrates was better below 140 °C. Sulfuric acid concentrations above 45 mM promoted excessive dehydration of xylose and glucose to furans and humins, which were recovered with lignin. A model was developed to describe the kinetics of lignin and hemicellulose solubilization. It distinguished three fractions of different reactivity in each polymer (lignin or hemicellulose): fast-reacting, slow-reacting, and unreactive. The amount of each fraction was correlated with acid concentration and reaction temperature. Activation energies and the other parameters in the model were obtained numerically by least-squares optimization using the data from the non-isothermal experiments. Activation energies for the fast-reacting and slow-reacting fractions of hemicellulose were 142 and 39.7 kJ mol−1, and for those of lignin 134 and 71.7 kJ mol−1, respectively. Acid concentration had a larger influence than temperature on establishing the amounts of slow-reacting hemicellulose and lignin, whereas temperature was the dominant variable concerning the fractions of non-reacting polymers.

Assembly behavior, structural characterization and rheological properties of legume proteins based amyloid fibrils

The assembly behavior of three selected legume proteins was monitored using thioflavin T (ThT) fluorescence intensity, atomic force microscopy (AFM) and dynamic light scattering (DLS) in this test. It was found that the cowpea protein (CoP) had the highest ability to form amyloid fibrils due to the higher ThT fluorescence intensity of CoP fibrils than chickpea protein (ChP) fibrils and lentil protein (LP) fibrils. These legume proteins were partially denatured during initial heating period, and then the elongation of fibrils was occurred with prolonged heating time. Correspondingly, the particle size of legume proteins decreased firstly, probably resulting from the degraded polypeptides and disrupted structure under the heat treatment and acid hydrolysis. Then the particle size of proteins increased with increasing heating time due to the reorganization of ordered structure. From the results of SDS-PAGE and AFM, vicilin fractions were more beneficial to the formation of fibrils, and the longer and flexible fibrils were observed in the vicilin enriched CoP fibrils, whereas the semiflexible fibrils and rigid fibrils were occurred in LP and ChP fibrils, respectively. In addition, the rheological properties of CoP fibrils were superior to ChP and LP fibrils, owing to the tight entanglement of the longer and flexible CoP fibrils. This work will strengthen the understanding of the formation mechanism of different legume proteins based fibrils and explore their potential application.

Numerical investigation on the flow and heat transfer characteristics of waxy crude oil during the tubular heating

In industrial practice, tubular heating is known as an effective technology to ensure the security storage of waxy crude oil in the tank. Insights of the thermal and flow behaviors during the tubular heating is a key aspect to optimize the strategy of this heating technology, in order to reduce the cost. In this study the physical and mathematical models representing the heat transfer and flow of waxy crude oil during tubular heating are established. The additional specific heat capacity and momentum source terms methods are employed to address the changing physical properties related to the paraffin crystallization and dissolution. The FVM, PISO and fully implicit first order temporal differentiation algorithms are used to perform the numerical simulation. Our outcomes show that the thermal process during tubular heating can be divided into four phases: ①Local thermal response phase, ②Thermal diffusion phase, ③Global thermal response phase, ④Elimination of gelled oil phase. The initial thermal influence range is inhibited by the thermal condition and solid-like gelled waxy crude oil formed after cooling. The plume flow takes great effects of transferring the heat from the tubes to crude oil and accelerating the phase transition, and finally facilitates the oil temperature uniform increasing in most parts of the tank. Whereas the gelled crude oils in the corner between the sidewall and base wall have the strongest solid-like character and lowest temperature. A low temperature layer keeps covering the base wall. At the top wall, the center region between the sidewall and center of the tank is long time occupied by gelled oil. The proportion of solid-like gelled crude oil keeps decreasing with a small slope value following a linear relationship with heating time. A pronounced phase change of waxy crude oil from the transition state into the pure liquid is observed. The minimum temperature value decreases in the initial heating period, and then steeply increases to an approximate plateau value. The fluctuations of the heating powers are observed indicating the transient thermal convections around the tubes. The heat loss power is lowest at the base wall and nearly unchanged. The heat loss power at top wall and sidewall are significantly influenced by heating tubes and depend on the wall temperature.

The investigation of evaporation behavior of lubricating oil droplet at high ambient temperature

The formation and vaporization of combustible mixture was crucial for the auto-ignition of lubricating oil droplets. In this study, a quartz glass tube was used as the evaporation chamber, and a micro-syringe was used to extract a specific volume of oil from the cross-fiber. The evaporation process of lubricating oil droplets at atmospheric pressure and different ambient temperatures was investigated by using the cross-fiber technique. The experimental results showed that the evaporation lifetime of lubricating oil droplets could be divided into three phases: initial transient heating period, equilibrium evaporation period, and stable evaporation period. The bubble generation and puffing behavior were observed during the evaporation phase. The equilibrium evaporation rate (Kave) was proportional to the ambient temperature. Compared to Kave, the stable evaporation rate (Ksta) was slightly influenced by the ambient temperature. The absolute rate of equilibrium evaporation (kave) and the absolute rate of stable evaporation (ksta) increased with an increase in initial diameter of the lubricating oil droplets. In order to describe the evaporation characteristics of lubricating oil droplets, a new correlation between evaporation absolute rate, initial diameter, and ambient temperature was obtained.

Performance analysis and optimization of a solar-air source heat pump heating system in Tibet, China

Owing to its great potential for solar energy and uniquely low relative humidity, Tibet is an ideal region for the application of solar-air source heat pump (SASHP) heating systems. This study aimed to evaluate the performance and optimize the key design parameters of a SASHP in Tibetan climatic conditions. A SASHP parallel system was designed for providing space heating in office buildings, and was modeled and simulated in transient system simulation (TRNSYS) software. The simulation results showed that for a reference SASHP system, the solar collection efficiency, with a daily average value of 0.27 and a maximum value of 0.44, is low during the entire heating season. The solar collectors could afford a high percentage of heating (even 100%) at every initial heating period. However, the overall solar fraction for heating was only 42.79% for the entire heating season. The SASHP operated with a high efficiency for most of the heating period, with a maximum coefficient of performance (COP) of 2.68 and an average COP of 1.95. Furthermore, the hot water temperature of the water tank generally varied near 45 °C, indicating that the water temperature satisfied most of the heating requirements with regard to the heating time. In the performance optimization, the optimal values for the area of the solar collector, angle of the solar collector, and volume of the water tank were found to be 225 m2, 45°, and 30 m3, respectively. The average solar fraction increased to 0.793, an improvement of 85.3% over the reference system. Furthermore, orthogonal experiment results indicated that the solar collector area and water tank volume were the priority factors for the design of a SASHP system in Tibet.

Semisolid microstructural evolution of (CNTs + Sip)/AZ91D powder compacts prepared from powders by cold pressing and remelting

The evolution of semisolid microstructure during partial remelting of (CNTs + Sip)/AZ91D powder compacts prepared by cold pressing was studied. The results indicate that rapid grain coarsening is driven by the dissolution of eutectic β phase material during the initial heating period of 0–10 min, so the AZ91D powders with fine equiaxed grains surrounded by intergranular eutectic phases evolve into compact particles. As the heating time proceeds, α-Mg particles were gradually separated by liquid due to the phase transformations of α-Mg + β → L and α-Mg → L. The primary particles coarsened rather slowly and the as-received Mg powder evolved into nearly spheroidal particles surrounded by liquid phase after partial remelting. The in situ synthesized Mg2Sip were distributed homogeneously around the CNTs with maintained structural integrity during partial remelting. Moreover, this microstructural evolution was accompanied by densification through pore filling. An ideal semisolid ingot suitable for thixoforming can therefore be obtained by partially remelting a (CNTs + Sip)/Mg powder compact.

Experimental study on effective microwave heating/fracturing of coal with various dielectric property and water saturation

Microwave heating is a promising technology in coal processing and coal seam permeability enhancement. It is vital to investigate the influencing factors for microwave heating of coal to ensure the optimal heating effect and the best energy efficiency. To address this, different types of coal samples with various water saturation were treated with various microwave powers and irradiation times. Thermocouple and infrared thermal image system were used to measure the temperature of coal samples during and after microwave treatment. Through analysing the temperature, the effects of dielectric property and water saturation of coal, microwave power and treatment time on microwave heating were investigated. It was found the heating rate of coal samples increases with the loss factor at the initial heating period, which then changes as the composition changes under microwave irradiation. It was also found that coal samples with low water saturation have much better microwave heating effects. Moisture not only impedes microwave heating but also facilitates uniformed heating, which impedes the formation of thermal fractures. Additionally, the experiment results suggest the average temperature increases with microwave power and irradiation time. However, extending the microwave irradiation time is more effective when the microwave power increases to a certain extent.

Transient temperature fields and thermal stress fields in glazing of different thicknesses exposed to heat radiation

Thermal performance analysis and cracking prediction of glazing elements are essential for fire safety evaluation of a building. This work investigated the transient temperature fields and the corresponding stress fields in glazing of different thicknesses with one side exposed to heat radiation. Based on the results of investigation, the criterion of critical temperature difference in cracking prediction was formulated. Glass samples of different thicknesses were tested under the same radiation exposure. Besides, a transient two-dimensional heat transfer model was presented to obtain the transient temperature fields in the glass samples. In addition, the temperature fields were loaded to ANSYS to obtain the thermal stress fields. It is found that in the initial heating period, the maximum stress was mainly caused by the temperature gradient component across thickness and the stress value was larger for thicker glazing; as time evolved, the maximum stress was gradually dominated by the temperature gradient component along the planar direction. Thus, for thick glazing under strong heat, thermal bending stress caused by temperature gradient across thickness should be calculated first to evaluate the cracking time, and the criterion of critical temperature difference is appropriate only if the thermal bending stress cannot reach the breaking value. For other conditions, cracks will result from the stress caused by the temperature gradient along the planar direction, and the criterion of critical temperature difference is applicable for cracking prediction of glazing in any thickness.

Conductive thin film drying kinetics relevant to drum drying

Direct assessment of the kinetics of drum drying operation has been a difficult task as the mass and temperature profiles are hard to monitor. Still, developing better understanding of conductive drying would help to identify new operating windows for this technology. The drying kinetics was investigated by drying maltodextrin and starch suspensions with a novel custom-built laboratory-scale apparatus, which allows on-line monitoring of mass and temperatures. During drying, three separate periods were identified: the heating, the boiling and the conductive drying (declining rate) periods. The duration of the initial heating period was proportional to the film thickness and was responsible for a relatively small amount of water evaporated due to natural convection. During the boiling period, the drying rate kept constant while bubble formation impeded the heat transfer. Larger bubbles were observed for starch suspensions due to its viscoelastic properties. Thus, large temperature gradients between the heating pan and the film were observed for starch suspensions. During the conductive drying period, the initial amount of dry solids per surface area determines the drying rate as it determines the thickness of the semi-moist layer subjected to conductive drying. Application of a thin film is preferred to avoid boiling, especially at increasing solids content. This situation also better approaches double drum drying processes, where boiling occurs in the pool and conductive drying occurs on the drum.

Kinetics of Quality Changes of Shrimp (Litopenaeus setiferus) During Pasteurization

Effect of cooking between 1 and 80 min at 60 to 100 °C on several quality attributes of whole peeled shrimp (Litopenaeus setiferus) (80–90 counts/kg) was studied using an isothermal heating method. Cook loss, area shrinkage, and hardness of shrimp increased with increasing heating time and temperature, following a fractional first-order kinetic model with activation energies (E a ) of 71.0, 53.3, and 29.9 kJ/mol, respectively. Cook loss, area shrinkage, and hardness were positively correlated. The toughness of shrimp muscle increased in the initial period of heating, then decreased in the later period during the treatments. The overall color change (ΔE) increased with increasing treatment time and temperature, and followed a zero kinetic model with an E a of 37.2 kJ/mol. The kinetic parameters obtained from this study can be applied toward understanding and predicting shrimp-quality changes during pasteurization treatments, and further provides insight into the pasteurization conditions required to achieve safe and high-quality shrimp products and potentially other crustacean shellfish and seafood products.

Emissivity Model of Aluminum 7075 During the Growth of Oxide Layer Over the Temperature Range from 800 to 910 K at a Wavelength of 1.5 μm

The emissivity models of aluminum 7075 were studied during the growth of oxide layer over the temperature range from 800 to 910 K at a wavelength of 1.5 μm. In the experiment, the samples were heated to a certain temperature and kept at that temperature for approximately 6 h. To determine the accurate emissivity, the surface temperature of specimens was measured by the two thermocouples, which were symmetrically welded onto the front surface of samples. The average of their readings was regarded as the true temperature. Eleven models were employed to evaluate the variation of emissivity with temperature or growth of oxide film on the specimen surface at a certain condition. The effect of the number of parameters used in the models on the fitting quality was studied at a certain temperature. The conclusion was obtained as more the number of parameters used in the models, the better the fitting quality became on the whole. The variation of emissivity with temperature was investigated at a certain thickness of oxide layer. The three approximate models of variation in emissivity with temperature and thickness of oxide film were proposed. The strong oscillations of emissivity were observed during the initial heating period, which were affirmed to arise from the interference effect between the two radiations stemming from the oxide layer and coming from the substrate.

The determination of the gradient and thermal stresses for the heating of type 18-10 stainless steel ingots

The aim of this paper is to determine the gradient and thermal stresses depending on the heating rates, based on parameters resulted from industrial studies, for the purpose of optimizing the heating of type 18-10 stainless steel ingots, considering the cases where the heating is required starting from the cold state. If the heating for the plastic deformation processing is to begin from the cold state, the determination of the heating rate up to the temperature of 700 – 800 ºC must be done with extreme care, as this is in fact the most conducive period to the formation of defects. Having analysed the existing domestic and foreign situation, the heating process for this type of steel raises difficult issues and differs from one company to another, relying mostly on their own accumulated experience. Despite the fact that, practically, the possibility of increasing the heating rate has been demonstrated, it is restrained by the fear of achieving an insufficient section heating and of the destruction of steel's tightness during the initial heating period. For these reasons we have taken the step of the analytical review of the gradient variation, as well as of the thermal stresses during the preheating period.

A 3rd-order polynomial temperature profile model for the heating and evaporation of moving droplets

Droplet heating and evaporation is of fundamental importance in various engineering applications. Based on the assumption that droplets’ internal temperature follows a third-order polynomial distribution, a new liquid phase model is proposed, taking into account both temperature gradient and internal circulation inside the droplet in terms of Hill’s spherical vortex. The new liquid model and other commonly used liquid models are compared with the experimental data outlined in the literature. The internal temperature predicted by the new model matches the experimental data of an n-Decane droplet heating best, whereby the enhancement of heat transport by internal circulation is correctly reflected. In comparison, other models always predict the monotonic variation of internal temperature during the initial heating period, which deviates widely from the experimental data. The new model displays the best performance with regard to predicting droplet evaporation in the later vaporizing period, whereas the parabolic temperature profile model and conduction limit model significantly underestimate and overestimate the droplet evaporation rate, respectively. The CPU requirement for the new model is far less than that for the conduction limit model and the effective thermal conductivity model with an order of magnitude, which shows its potential for implementation into CFD codes.