Dynamic Temperature Changes Research Articles

The dynamic effect of supply water flow regulation on surface temperature changes of radiant ceiling panel for cooling operation

Radiant ceiling cooling panels are limited in applications due to surface condensation. Surface temperature is a key factor affecting condensation problems, and supply water flow regulation is closely related to surface temperature fluctuation. In this paper, the dynamic change of surface temperature in regulating the supply water flow is investigated. It is found that when the chilled water supply is closed, the condensation on the radiant ceiling panel surface can be effectively prevented. Using simulation results and statistical analysis, the three main factors affecting the surface temperature for fixed supply water flow are determined as the average temperature of the non-cooling surface, the indoor air temperature, and the internal surface temperature of the outer window. The contribution rates of these factors are 47.6%, 39.2% and 10.1%, respectively. On the basis of this research, the dynamic changes of surface temperature of radiant ceilings for the closed water supply are simulated and verified, and the results are in agreement with the experimental results.

Effects of dynamic changes in inlet temperature on proton exchange membrane fuel cell

To examine the effects of the inlet temperature on a proton exchange membrane fuel cell (PEMFC), this study proposes a dynamic temperature (DT) model by considering the dynamic variation in temperature that influences the water content of the cell and its performance. A three-dimensional model featuring the geometry and nonconformal mesh of the PEMFC is first formulated. An experiment was then conducted featuring a 34-cm2 single-cell and test system. Following this, a simulation and another experiment were carried out at a working temperature of 333 K with 100% relative humidity (RH) at inlet temperatures of 316 K, 323 K, and 328 K (at 50%, 60%, and 80% RH, respectively). The contours, which included hydrogen in the channels and water in the membrane, were then described. The results show that the proposed DT model can more accurately predict the performance of the PEMFC than the FLUENT model. In particular, when the inlet temperature was 328 K (80% RH), the maximum relative difference between the DT model and FLUENT was 13.8% compared to the results of the experiment. The hydrogen content in the channels decreased and the water content in the membrane increased as the reaction continued. The proposed model provides a new means for studying the effects of dynamic temperature on the PEMFC.

Design of a test rig for the simulation of startup procedures in main heat exchangers of air separation plants

Within the Kopernikus project “SynErgie”, the sub-project “FlexASU” investigates an enhancement of the load flexibility of air separation units with the goal of synchronizing their power consumption with the availability of renewable energy sources. The main heat exchanger of these plants was identified as a key component for a dynamic plant operation, because startup, shutdown and load-change procedures can lead to thermal stress and an increased lifetime consumption of the apparatus. Aluminum-brazed plate-fin heat exchangers (PFHE) are used in these plants because of their high process integration, low production cost and compact design.The main focus of this work, which is an extended version of Haider et al. (2018a), is the transient behavior of PFHE. The temperature profiles within a PFHE can be calculated, and a FEM-based model is available for investigation of the thermal stress induced by dynamic temperature changes in the apparatus. Hence, the reduction in lifetime due to certain dynamic plant operating scenarios can be estimated, and critical operating modes can be identified.For the further development of the model, a PFHE test rig is designed which is representative of extreme operating conditions in air separation units. A test scenario matching the conditions of plant startup after extended downtime in a cold state has been designed. The scenario leads to thermal stress that is expected to damage the PFHE within a few weeks of testing. The location of maximum stress in the FEM-model matches typical cracks observed in PFHE which have been exposed to severe thermal stress.

Ocular Response to Millimeter Wave Exposure Under Different Levels of Humidity

It is known that millimeter waves (MMW) are absorbed by surface tissue such as skin and the cornea. We previously demonstrated that MMW damages the eye by heat accumulation and transfer. To examine the influence of humidity on such damage, we investigated the difference in MMW-induced ocular tissue temperature increase under high (70%) and low (30%) relative humidity. Eyes of pigmented rabbits (Dutch, 11–15-week old) were exposed unilaterally to 40-GHz 200 mW/cm2 MMW for 5 or 30 min at the center of the pupillary area with a spot-focus type lens antenna. Infrared thermography was used to measure corneal surface temperature during exposure. Temperature of corneal stroma and lens nucleus were measured using a fluoroptic thermometer during 0–30-min exposure. To visualize the dynamic change of temperature and its distribution in the anterior chamber, saline containing 0.2% microencapsulated thermochromic liquid crystal (MTLC) was injected into the eye anterior chamber. All three temperature measurements were much higher under high than low humidity at 24 °C room temperature and MTLC results showed the same trend during MMW exposure. These findings indicate that humidity affects MMW-induced ocular temperature elevation. We showed that low humidity during MMW exposure decreased heat accumulation and transfer, whereas high humidity increased the thermal effect. The present data demonstrated that controlling the environmental humidity might have an impact on ocular damage in eyes exposed to MMW.

Using water temperature series and hydraulic heads to quantify hyporheic exchange in the riparian zone

Surface-water/groundwater exchange through the evaluation of riparian-zone temperature data has attracted increasing attention in recent years. The Fox Ditch Canal, Nevada, USA, was chosen for this study on seasonal variations of riparian-zone water exchange. Groundwater temperature and hydraulic head real-time monitoring data were collected from March to November 2012. A calibrated hydro-thermal coupling model was used to characterize the riparian-zone temporal and spatial temperature distribution, thereby providing a standard against which the performances of four analytical solution models for calculated riparian-zone vertical seepage velocity could be assessed. The results indicated that the proposed model provided a simulation that was able to represent dynamic changes in riparian-zone soil temperature. Although small variations in patterns and magnitudes of riparian-zone water exchange were evident at a daily scale, they varied significantly over a seasonal scale. Comparison of the results of the four analytical solutions and numerical computation found that the Hatch solution by the amplitude method provided the highest accuracy for calculating groundwater velocity in this area (2.47 × 10−6 to 3.15 × 10−6 m/s). Global sensitivity analysis of hydro-thermal coupling model parameters showed that porosity had the most significant impact on temperature in the model.

Cost Effective Alternative Solution for the Renovation of Concrete Pavements

The renovation of concrete pavements (typically motorways) is commonly done by deep replacement of the existing damaged concrete layers including underlying basal layers with newly installed layers with a new surfacing. This technology is rather expensive and time consuming. New alternative solution based on including of intermediate mechanically stabilised layer into road structure is presented. Full scale laboratory model of this structure has been realised. Results and observations are presented. Main contribution of new approach is possibility to remain old concrete layers on place and cover them by new pavement. Standard road structures where concrete layers are overlaid by new pavement in combination with long-term intensive dynamic loading and temperature changes are prone to reflective cracking. Proposed technology avoids that problem and enables cost effective reconstruction of damaged road.

Multiplicative On-Column Solute Focusing Using Spatially Dependent Temperature Programming for Capillary HPLC.

The benefits of capillary liquid chromatography columns are truly realized when small, limited sample volumes require signal enhancement, but the available sample volume does not permit on-column focusing during injection onto a larger column. This dilemma is common when samples are naturally small or precious (such as in biological, forensic, art, and archeological investigations) and analyte concentrations are low. Signal enhancement by solvent-based focusing is effective with capillary columns, but it is limited to a single band-compression step and can only be achieved at the inlet. Here we evaluate multiplicative temperature-assisted solute focusing using a linear array of ten independently controlled 1.0 × 1.0 cm thermoelectric cooling elements (TECs) to generate dynamic temperature changes along the length of the column. The evaluation has two prongs: simulation and experimental. Simulation is required to understand the effect of a particular temperature change at a particular place and time on the column to determine optimal timing of temperature changes. Because the accuracy of the simulations is good, as long as the effect of temperature on retention factor is known, experimental conditions required to achieve a particular focusing objective can be estimated. We evaluated the capability of the technique to selectively focus only one of two solutes. This was achieved using three adjacent zones with temperature controlled by (upstream first) four, two, and one TECs. The three focusing steps occurring on column gave a 20-fold increase in peak height without solvent-based focusing for a solute with modest retention enthalpy.

Experimental parameter identification for elastocaloric air cooling

This contribution provides an experimental analysis of relevant parameters to determine the air cooling potential of elastocaloric shape memory alloys (SMAs) in the form of wires. An essential step consists in developing a scientific test setup, which offers the ability to determine these parameters under varying conditions like wire geometry, material composition, and process parameters.With the help of this test setup the influence of different airflow rates on the heat transfer from SMA wire to air and the coherent heat transfer coefficients as well as the approximate air temperature ΔT values are determined and presented in this contribution. Furthermore, the latent heat of the SMA material is experimentally identified with a novel approach based on comparing dynamic temperature changes achieved by elastocaloric phase transformation with direct caloric heating. Analysing these parameters will greatly support the simulation, development and construction of elastocaloric air cooling devices to provide environmentally friendly alternatives to the prevalent vapour compression based cooling principles.

EFFECT OF DYNAMIC STRESSES ON THE DURABILITY OF QUARTZ OPTICAL FIBERS IN OPTOELECTRONIC MEASUREMENT DEVICES

The method of a theoretical estimation of optical fiber durability used in optical fiber sensors as a sensitive element, depending on the nonstationary longitudinal strain arising at dynamic temperature changes is offered. The developed analytical model takes into account design features of a fiber (diameters of a core and cladding, a metal or polymeric covering), doping types, relative humidity of an environment, as well as the temperature dependence of the activation energy and the Young's modulus of the optical fiber, the linear expansion coefficient of the metallic coating. Numerical modeling of optical fiber service life time at dynamic influences of measured temperature and comparison with experimental data is carried out.

Assessment of climatic trend of air temperature at the earth surface in the context of stable development (case of Gyumri city)

Formulation of the problem. The study and assessment of air temperature fluctuations, especially in conditions of expected climate change, is of particular relevance and haste. At the same time, the assessment of temporary changes in air temperature is important as a guarantee of stable development and a long-term economic development. The purpose of the article. The purpose of this work is to find out, analyze and evaluate the patterns of the dynamic changes in air temperature in Gyumri city. To solve this problem, the results of actual observations of the air temperature of the Gyumri weather station, stored at the Ministry of Emergency Situations of the Republic of Armenia «Service for Hydrometeorology and active influence on atmospheric phenomena» have been collected, refined and analyzed. Methods. In the course of research we analyzed and clarified appropriate literary sources, using mathematical-statistical, extrapolation, complex, analyze and correlation methods. Results. A multifactorial correlation relationship was obtained between the average annual air temperature and its absolute maximum and minimum values of Gyumri city. A close correlation was also obtained between the average annual air temperature and its absolute maximum and minimum values of Gyumri city, between the average annual and extreme values of the surface air temperature and the absolute maximum and minimum values of the soil surface of Gyumri city. This makes it possible to calculate the value of any temperature characteristic, while having the other one. Note that in winter the average air temperature values do not differ much from the average soil surface temperature values. This difference grows and reaches its maximum in summer (July – August). As a result of the research, it turned out that according to the actual data on the air temperature in Gyumri city, there was a tendency for the average annual values and absolute maximum of the air temperature to rise, while the absolute minimum values showed a downward trend (in the absolute sense). That is, in the study area in the warm season, an increase in aridity is expected on climate drainage, and in the cold season – a softening. In the work we tried to define the monthly average values of air temperature in Gyumri in 2030, 2070, 2100, using the extrapolation method. The result shows that if the changing process continues, the annual average values of air temperature will increase with comparison to current normal: in 2030 – 0.09 ºC (1.37 %), in 2050 – 0.27 ºC (4.12 %), in 2070 – 0.48 ºC (7.32 %), in 2100 – 0.85 ºC (13 %). This pattern is also characteristic of other regions of the republic. Therefore, it is necessary to implement integrated measures to adapt to the effects of air temperature changes and take this into account when working out programs for social, environmental and economic development.

Corrosion Strength Analysis of Casing in Offshore Thermal Recovery Wells

Corrosion is a common problem in offshore oil and gas wells, especially in thermal recovery wells, which will greatly affect the residual strength of casing strings. Field data show that the corrosion pattern of casing is mainly pitting, local corrosion and uniform corrosion, of which pitting is the most common. For this reason, based on the engineering practice of thermal recovery well casing corrosion, the dynamic changes of wellbore temperature and pressure during the gas production process of thermal recovery wells are considered, and the numerical model of casing corrosion under transient temperature and pressure coupling is established. The influence of corrosion morphology and corrosion depth on the residual strength of the casing is analyzed. The transient stress variation law of the corroded casing is studied. The results show that: (1) Under the same corrosion depth, both pitting and uniform corrosion will reduce the residual internal pressure and external extrusion strength of the casing string, and the degree of influence is similar. The effect of uniform corrosion on the residual tensile strength of the casing string is larger than pitting and local corrosion; (2) the residual strength of the casing decreases linearly with the increase of the corrosion depth of the casing; (3) the stress of the corrosion part of the casing shows a significant dynamic change, in the initial stage of steam injection, casing stress rises rapidly at the corrosion position, and then the rate at which the casing stress rises is slowed, which tends to cause fatigue damage to the casing. The research results have reference significance for the assessment of the service status of offshore thermal recovery well casings.

Design of a Test Rig for the Simulation of Startup Procedures in Main Heat Exchangers of Air Separation Plants

The Kopernikus project “FlexASU” investigates an enhancement of the load flexibility of air separation units with the goal to synchronize their power consumption with the availability of renewable energy sources. The main heat exchanger of these plants was identified as a key component for a dynamic plant operation because startup, shutdown and load change procedures can lead to thermal stress and an increased lifetime consumption of the apparatus. Aluminium brazed plate fin heat exchangers (PFHE) are used in these plants because of their high process integration, low production cost and compact design. Finding the optimum operating scheme for the rectification columns depends on detailed knowledge of their effect on the lifetime of the PFHE.The transient behavior of temperature profiles within a PFHE can be calculated and a FEM-based model is available to investigate the thermal stress induced by dynamic temperature changes in the apparatus. Hence, the reduction of lifetime due to certain dynamic plant operating scenarios can be estimated and critical operating modes can be identified. For further development of the model, a PFHE test rig is designed which is representative for extreme operating conditions in air separation units. A test scenario matching the conditions of plant startup after extended downtime in cold condition has been designed. To reach a level of thermal stress comparable to industrial sized plants, the PFHE weighing around 1.5 tons is heated up to 50 °C before rapidly being cooled down by a stream of gaseous nitrogen at -173 °C in a cyclic operation scheme. This test scenario leads to thermal stress that is expected to damage the PFHE within a few weeks of testing and the location of maximum thermal stress in the FEM model matches typical cracks observed in PFHE of air separation units. The test rig is designed to gain a clearer understanding of the underlying damage mechanism and to evaluate measures for lowering thermal stress in further test series. The knowledge gained from the experimental investigations is crucial for maximizing the operation flexibility of an ASU plant.

Optimization of food dye (betanin) retention during hot air drying: Design space calculation with consideration of reaction and substrate transfer kinetics

ABSTRACTRetention of food dye (betanin) during hot air drying of a model food system was optimized by developing mechanistic model that took into account the kinetics of degradation and the transfer of the substrate in the model food system. Betanin (red glycosidic food dye extracted from beets) degrades through hydrolysis, and this reaction is temperature dependent and involved with the amount of water in a system. Degradation kinetics of betanin/dextrin mixture was first measured under isothermal and isohumidity conditions in order to obtain kinetic constants for the degradation reaction. A packed bed filled with glass beads (mean diameter ca 200 µm) was wetted by a betanin/dextrin aqueous solution, and contacted with hot air at selected temperature and relative humidity. The resultant betanin retention in the drying systems was estimated by using the kinetic parameters of the degradation reaction. When estimation was made with consideration of dynamic change in temperature and moisture content during drying by assuming the uniformity of temperature and solute concentration in the system, calculation results seriously overestimated the resultant retention. It found experimentally that the dextrin and betanin transferred toward the drying interface and this made the concentrated film layer at the interface. And the kinetics of these mass transfers significantly affected the retention of betanin in the drying systems. The mass transfer rates of the dextrin and betanin were thus experimentally measured and applied to predict the retention values in the drying systems. The calculation results were in good accordance with the experimental values. A design space that relate the betanin retention to the drying temperature and air humidity conditions was calculated by a mechanistic model that took into account the kinetics of the degradation reaction and substrate transfer.

Numerical Study on Melting Heat Transfer in Dendritic Heat Exchangers

The dendritic fin was introduced to improve the solid-liquid phase change in heat exchangers. A theoretical model of melting phase change in dendritic heat exchangers was developed and numerically simulated. The solid-liquid phase interface, liquid phase rate and dynamic temperature change in dendritic heat exchanger during melting process are investigated and compared with radial-fin heat exchanger. The results indicate that the dendritic fin is able to enhance the solid-liquid phase change in heat exchanger for latent thermal storage. The presence of dendritic fin leads to the formation of multiple independent PCM zones, so the heat can be quickly diffused from one point to across the surface along the metal fins, thereby making the PCM far away from heat sources melt earlier and faster. In addition, the dendritic structure makes the PCM temperature distribution more uniform over the entire zone inside heat exchangers due to high-efficient heat flow distribution of dendritic fins. As a result, the time required for the complete melting of the PCM in dendritic heat exchanger is shorter than that of the radial-fin heat exchanger.

Transformations of Iron (III) Precursors in a Wave of Flameless RDX Combustion

Flameless combustion of 40% Fe2O3 – 40% RDX – 20% HDI (mix I) and 30% CoCO3 – 15% iron formate – 40% RDX – 15% HDI (mix II) systems was explored by time-resolved X-ray diffraction (TRXRD). In case of mix I, the reaction was found to proceed via the formation of FeO intermediate: Fe2O3 → FeO → Fe3O4. Variation in the extent of iron reduction was associated with dynamic temperature change and a reductant content of the reaction zone. The reduction proceeded as a solid-state reaction, without amorphization of the structure. The process in system II involved the formation of CoO and FeO intermediates. Further reduction – up to metals – takes place behind the combustion front and yields a mixture of nanosized Co, Fe, and Co0.7Fe0.3 particles. Exposure of hot reaction products – nano-sized Co and Fe – to the air leads to their self-ignition and formation of Co3O4 and Fe3O4, respectively.

Integrating a thermal model of ground source heat pumps and solar regeneration within building energy system optimization

The optimal design and operation of an integrated building energy system consisting of the renewable energy technologies such as ground source heat pumps (GSHPs) and solar thermal collectors, etc., is an important problem to be addressed. This paper describes a methodology for the optimization of a building energy system including a detailed thermal model of a borehole heat exchanger based GSHP. The novelty of this model is that it enables the study of dynamic temperature changes within the ground during operation. Furthermore, a model of solar thermal collectors is also included, which enables the study of solar regeneration of the ground in the short and long-term. Additionally, seven scenarios of building envelope retrofit are evaluated alongside optimal system design solutions. The methodology uses a bi-level multi-objective optimization approach, which consists of a genetic algorithm at the design level, and a mixed integer linear program at the operation level, in order to minimise the total costs and CO2 emissions. The methodology is applied to a single-family residential building in Zurich, Switzerland, in order to demonstrate its application and analyse the design and operation of the system, with special attention to the GSHP. The results indicate that in the short-term, the ground temperature reduces considerably, to almost 5 °C as compared to the initial temperature of 11.5 °C. Furthermore, solar regeneration due to excess heat in summer increases the temperature back above initial temperature. However, due to due to insufficient regeneration in the long-term, the ground temperature drops consistently to almost 4 °C at the end of 20 years of operation. On the demand-side, window retrofitting results in a 27.3% reduction in the total CO2 emissions at almost no additional costs. Retrofitting the whole building including windows, walls, roofs, and floors, is a CO2 optimal solution however, performs worst in terms of cost optimality.

The effect of dynamic correlated colour temperature changes on alertness and performance

Light can have acute effects on human performance, including task performance, alertness and circadian phase shift. Most studies have investigated these effects using static light. This study investigates the effects of dynamic light with different cycle times and different ranges of correlated colour temperature on human alertness and task performance. Ten participants took part in the experiment using six conditions of dynamic light with each observing session lasting 4.5 hours. An electroencephelogram, measurements of critical flicker frequency, performance on various cognitive tasks and alertness and sleepiness questionnaires were used to evaluate the human responses. The results showed that participants appeared more alert and performed better under lighting of higher correlated colour temperature range but different correlated colour temperature cycle times had little effect.

Subsurface shallow depth soil layers thermal potential for ground heat pumps in Poland

Subsurface shallow depth soil layer (SSDSL) is characterized by dynamic temperature changes, which may be eligible to constitute as a heat source for ground heat pumps. This paper aims to present the regime of heat flow in SSDSL and its climatic conditions in central Europe, taking into consideration the agriculture periphery characteristics of Wroclaw (Poland). Emphasis was laid on the relation between the radiative properties and heat balance of two contrasting types of analyzed active surfaces (bare soil and grassy surface). The obtained experimental results gathered across a span nine years (August 2007–July 2016) and the conducted thermal analysis aim to evaluate the utilization of SSDSL as a heat source for ground heat pumps. The heat fluxes were measured between the active surface of the soils and their lower layers on an 8 cm depth. The soil temperature data between 5 cm to 10 cm under the ground and the temperatures of the active soil surfaces are represented. The results showed that the biggest monthly sums of ground positive fluxes (G > 0) reached about 80 MJ/m2 (22.2 kWh/m2) for the bare soil from May to July and the adequate average monthly sums reached maximum 55 MJ/m2 (15.3 kWh/m2) in July. The sums for the grassy surface were nearly twice lower, with the monthly maximum equals to 42.5 MJ/m2 (11.7 kWh/m2) in June and average value of 32 MJ/m2 (8.9 kWh/m2) in July. The nine-year average annual sums for G > 0 reached 337.6 MJ/m2 (93.8 kWh/m2) for the bare soil and 170.8 MJ/m2 (59.9 kWh/m2) for the grassy surface. The heat fluxes of the 5 cm depth soil were higher than for the 8 cm and 10 cm soil. Thus, the conclusion of the study suggests that the thermal performance of SSDSL is efficient enough to be used as an environment friendly heat source for interseasonal heat pump operation.

Active Packaging Techniques to Reduce Post-Harvest Loss in Perishables with Special Reference to Mango (cv. Dushari)

All horticultural produce continue their metabolic activities soon after harvest and during storage period. Modified atmospheric packaging is a common practice to minimize postharvest losses and extend shelf life of the produce. Even under modified atmosphere the control of respiration rate of the produce is limited. Optimal packaging micro environment can be adversely affected by dynamic changes in temperature and relative humidity throughout the storage period and under transportation. As an alternative, active packaging technologies provide interactive controls between the produce, package and surrounding environment to achieve and retain optimal atmospheric conditions inside the packages. Various active packaging technologies have been developed and are commercially available for a range of food products including horticultural produce and the combination of these with other postharvest management strategies offers benefits to extend shelf life. This paper reviews the recent active packaging technologies and their applications focused on horticultural produce such as Mango.

Modelling for reactor-style aerobic composting based on coupling theory of mass-heat-momentum transport and Contois equation

This study establishes an optimal mathematical modelling to rationally describe the dynamic changes and spatial distribution of temperature and oxygen concentration in the aerobic composting process using coupling mass-heat-momentum transfer based on the microbial mechanism. Two different conditional composting experiments, namely continuous aeration and intermittent aeration, were performed to verify the proposed model. The results show that the model accurately predicted the dynamic changes in temperature (case I: R2 = 0.93, RMSE = 1.95 K; case II: R2 = 0.86, RMSE = 4.69 K) and oxygen concentration (case I: R2 = 0.90, RMSE = 1.26%; case II: R2 = 0.75, RMSE = 2.93%) in the central point of compost substrates. It also systematically simulated fluctuations in oxygen concentration caused by boundary conditions and the spatial distribution of the actual temperature and oxygen concentration. The proposed model exhibits good applicability in simulating the actual working conditions of aerobic composting process.