Dynamic Heat Research Articles

Various disturbances propagation analysis of district heating system based on the standardized thermal resistance method

AbstractVarious heating disturbances and faults in the heating network are necessary to be controlled and handled by some intelligent heating strategies with the increasing complexity of the heating network. This paper constructed the dynamic modeling of the heating system using standard thermal resistance and obtained a dynamic heat current model of the heating system. On this basis, we analyzed the heat transfer performance of the heating system. Five disturbances are selected, including the behavior of users, indoor heat source, heat exchanger heat transfer performance deterioration, pipe blockage, and pipe leakage. The effects of different disturbances on the overall system and user side water supply temperature were obtained by establishing a dynamic model for segmented heating pipelines. Feasible control methods for the heating network and load side are sorted out, mainly changing the water supply's temperature and the water supply's flow rate to reduce the water supply's fluctuation. Five specific control strategies are proposed for five types of disturbances. Comparing the case without control and the case with control, the results show that the fluctuation of water supply temperature is significantly reduced, and the control strategies can reduce the impact of disturbances on the heat network system and customers and improve the comfort of customers in the presence of disturbances. under the disturbance of pipeline leakage, the method proposed in this article reduces the temperature fluctuation amplitude by 75% and the fluctuation duration by 60%.

Modified shape coefficient of building with Thermal considerations: concept, experiment and Application in solar-enriched areas

ABSTRACT The shape coefficient of building (SCB) characterizes the correlation between building shape and building energy consumption. Strictly restricting the SCB was harmful to reducing energy consumption, neglecting the potential benefits of solar radiation. A novel concept and determining method of modified shape coefficient of building (MSCB) were proposed based on the energy dissipating and solar heat gain. Field measurements of the indoor air temperature of model buildings for solar radiation, building orientations, and U-values under natural and heating conditions were conducted in western Sichuan. Results demonstrated that with identical SCB, the indoor thermal environment was substantially affected by the above parameters, proving the one-sidedness of SCB simultaneously. A simulation method verified via the experiments was employed to forecast the dynamic heating load of a real building. The influence of building orientations on the MSCB under actual climatic conditions from different time scales was discussed. Results illustrated that the SCB could be reduced by 34.6% in the present case. The monthly correction factor trend and the heat consumption were consistent. The restriction on SCB could be liberalized moderately even if the heating demand was tremendous. This research could provide a reference for the energy-saving design of passive buildings.

Combined CFD and Heat Treatment Simulation of High-Pressure Gas Quenching Process

Abstract To improve the process development for high pressure gas quenching a digital quenching simulation model combining the fields of computational fluid dynamics and heat treatment process simulation has been developed. It was found that the gas flow and hence the quenching properties depend on both local (geometry of parts, carrier and chamber) as well as global influencing factors (fan characteristics, system pressure and hydraulic resistances). Therefore, a computational fluid dynamics model that includes all these factors was realized. The approach includes a heat transfer analysis to determine the local heat exchange coefficients on a component level. By connecting the computational fluid dynamics model and heat treatment simulation the local quenching characteristics are used to compute the temperature history of the quenched part. Based on a thermo-metallurgical heat treatment simulation the computed local cooling curves and metallurgical phase compositions are used to accurately predict the part properties like microstructure and hardness. The applicability of the model has been confirmed by hardness measurements. Hardness results for different batch positions, batch setups or tray systems can now be computed enabling an efficient virtual development of the gas quenching process.

Transient model and parameters matching of a segmented solar thermoelectric system

ABSTRACT Solar thermoelectric generators (STEGs) with segmented thermoelectric (TE) materials have higher energy conversion efficiency than traditional STEGs with uniform TE materials. This paper constructs a transient model to investigate the dynamic heat conduction characters of the segmented STEGs. The solutions of temperature field and energy conversion efficiency are obtained. The parameters matching problem of segmented TE materials is also studied based on the developed transient model. Numerical results show that there exists an optimum parameter matching at which the efficiency could attain its local maximum value. In addition, tuning and matching segmented TE material parameters could enhance the efficiency and might increase thermal stress simultaneously. Therefore, in this paper, an optimization purpose function is defined to analyze the effect of parameters matching on both performance and structural safety. Neural network (NN) is also used to find segmented STEGs with high efficiency and low thermal stress. This work may offer useful information for the optimal design of real segmented STEGs and promote their practical applications.

Effect of unsteady heat source condition on thermal performance for cascaded latent heat storage packed bed

The intermittency and fluctuations of heat sources such as solar energy and industrial waste heat have a significant impact on the thermal properties of thermal energy storage systems. The dynamic heat transfer properties of latent heat storage devices under unsteady heat sources need to be further studied to better understand the impacts of thermal fluctuations. In this study, the concentric dispersion model is used to investigate the melting process of a cascaded packed bed phase change heat storage system under sinusoidal fluctuating heat sources. The effects of period and amplitude variations on the thermal performance of the system are investigated, using several metrics for thermodynamic evaluation. The study shows that the effect of low-frequency fluctuation period on the melting of packed bed thermal energy storage (PBTES) is differentiated by the limitation of the total melting time. The thermal performance of the system is enhanced when the fluctuation period is much larger than the melting time, especially the melting time is significantly reduced by 24 % when the period is 120 min, and the opposite is true when the fluctuation period is approximately equal to the melting time. The effect of high-frequency features is smaller. The influence of the fluctuation amplitude on the melting process lasts throughout the process. When the amplitude is 15 °C, the complete melting time is shortened by 23 % compared with the steady condition. At the same time, the increase in the amplitude of the fluctuations enhances the sensitivity of the thermal properties of the system to the variability of the fluctuation period, further amplifying the effect of the period on the enhancement or reduction of the thermal properties of the system. This work provides a certain reference for the design of PBTES under unsteady heat sources.

Bilayer thermal metadevices that mold transient heat flows

Thermal manipulation has been widely researched due to its potential in novel functions, such as cloaking, illusion and sensing. To avoid the spatially inhomogeneous and anisotropic heat conductivity tensors (introduced by transformation thermodynamics), several thermal cloaks based on scattering cancellation (SC) approach are designed and experimentally demonstrated. However, the current SC cloak is only effective in the steady-state heat transfer condition as the heat capacity terms are neglected by the requirement of constant external fields. In order to expand the SC cloak to the transient regime, a bilayer scheme is introduced to achieve the accurate solutions of the camouflage equations considering time factors. Indeed, there are two equations to solve with two unknown quantities: first the thermal conductivity and second the product of specific heat capacity by the density. We construct thermal camouflage devices and verify their ability to mold dynamic heat flows both by simulations and experiments. Our proposed method paves an efficient avenue to extend SC approach to dynamic heat transfer regime.

3D numerical model for transient simulations of floor cooling capacity and thermal comfort in a thermally heterogeneous room

This study presents the development of a computational model for an analysis of the dynamic heat transfer in a room equipped with the radiant cooling floor as a full, three-dimensional, thermally heterogeneous environment. The model is validated by comparison with the results of reference models and by the full-scale experimental measurements, which prove the good accuracy of the developed model for the simulation of heat gains in a thermally heterogeneous room (mean error 0.23 °C for the air temperature), as well as for the simulation of radiant floor cooling (mean error 0.16 °C for the temperature of floor area). A set of simulations is performed for an exemplary room with solar, long-wave, and convective heat gains to investigate the effects of type and time-variability on the heat gains for the operational characteristics of the cooling floor. Different types of temperature sensors (operative and air) used in the control system with different set-points are examined. Thermal heterogeneity of the room is included in the thermal comfort analysis performed using PMV, PPD, and PD-cold floor comfort indicators. It is found that the operational characteristics of the radiant floor strongly depend on the effects of dynamic heat flow, so it should be considered on the full spatial scale and the control strategy should not be neglected. It is desirable to develop design methods and tools for cooling floors that allow precise prediction of their operational characteristics but are not too computationally complex, which is satisfied by the proposed model.

Performance Study of Booster-Driven Hybrid Cooling Units for Free Cooling in Data Centers

In the data center, using ambient energy cooling technology can effectively reduce the average power use efficiency, and the heat pipe as an effective use of ambient energy device has attracted much attention. For the dynamic heat pipe, reducing the power consumption of the pump effectively is the key to improving the efficiency. In this paper, the rotary booster is selected as the gas phase booster device of the heat pipe unit, the standard unit of the rotary booster is improved, and three types of boosters are obtained, including two improved boosters and one standard unit. Comparative test studies are conducted on three different types of boosters, and the power of the booster shows a downward trend with the increase in indoor and outdoor temperature differences (outdoor temperature decreases). With the increase in indoor and outdoor temperature differences, the cooling capacity increases first and then decreases. When the indoor and outdoor temperature difference is greater than 20 °C, the suction pressure of the booster is greater than the saturated condensing pressure force under outdoor ambient temperature, and the work of the booster decreases. Among the three types of boosters, the medium pressure ratio booster energy efficiency ratio (EER) is the largest. After throttling the standard unit, results show that its cooling capacity unit increases, but the booster power also increases, and the EER is still smaller than that of the improved unit.

Computational Modeling with Phantom-Tissue Validation of Gold-Nanorod-Enhanced Laser Ablation of Prostate Cancers

Abstract The purpose of this study was to develop a computational model for the laser ablation (LA) of prostate cancer, enhanced by gold-nanorods (GNRs) in a phantom-tissue system, and to explore the effect of GNRs on the ablation zone. A prostate biomimetic tissue (PBT) was prepared with different volume fractions of GNRs. Specifically, the computational model was built by considering the change of light properties of PBTs with and without GNRs and introducing the dynamic heat source determined by porcine liver carbonization, reported elsewhere. The computational model was then validated by comparing the simulation and the ex vivo LA experiment in terms of three performance indexes, namely, (i) the spatiotemporal temperature distribution, (ii) ablation zone, and (iii) carbonization zone, with the three volume fractions of GNRs in the PBT model, as mentioned above. Except for minor discrepancies found in the carbonization zone, the proposed model agrees with the experimental data. The effect of GNRs on LA was explored with the help of the model, and nine combinations of the laser powers and the volume fractions of GNRs were tested. The result shows that the ablation zone increases with the increase in the volume fraction of GNRs for all three laser powers used. Two conclusions can be drawn: (1) loading GNRs into the tissues may increase the ablation zone of LA, and (2) the proposed computational model is a reliable tool for predicting the spatiotemporal temperature distribution and the ablation zone of the GNR-enhanced LA.

The generalized anisotropic dynamical Wentzell heat equation with nonstandard growth conditions

The generalized anisotropic dynamical Wentzell heat equation with nonstandard growth conditions

Diurnal urban heat risk assessment using extreme air temperatures and real-time population data in Seoul

Previous heat risk assessments have limitations in obtaining accurate heat hazard sources and capturing population distributions, which change over time. This study proposes a diurnal heat risk assessment framework incorporating spatiotemporal air temperature and real-time population data. Daytime and nighttime heat risk maps were generated using hazard, exposure, and vulnerability components in Seoul during the summer of 2018. The hazard was derived from the daily extreme air temperatures obtained using the stacking machine learning model. Exposure was calculated using de facto population density, and vulnerability was assessed using demographic and socioeconomic indicators. The resulting maps revealed distinct diurnal spatial patterns, with high-risk areas in the urban core during the day and dispersed at night. Daytime heat risk was strongly correlated with heat-related illness ratios (R= 0.8) and accurately captured temporal fluctuations in heat-related illness incidence. The proposed framework can guide site-specific adaptation and response plans for dynamic urban heat events.

MC-Injection Molding with Liquid Silicone Rubber (LSR) and Acrylonitrile Butadiene Styrene (ABS) for Medical Technology.

The multicomponent injection molding of liquid silicone rubbers (LSR) with thermoplastics, such as polybutylene terephthalate (PBT) or polyamide (PA), is a state-of-the-art technique and is used in the manufacturing process for many components in the automotive industry and in the field of sanitary engineering. Standard thermoplastics, such as acrylonitrile butadiene styrene (ABS), cannot be bonded with silicone rubbers in injection molding because of their low heat deflection temperature. In this study, we investigated ABS grades approved for medical applications to show how dynamic mold heating and various pretreatment methods for thermoplastic surfaces can be used to produce ABS-LSR test specimens. In addition, such components' sterilization effect on the adhesive bond will be shown.

The Physical Thickness of Stellar Disks to z ∼ 2

In local disk galaxies such as our Milky Way, older stars generally inhabit a thicker disk than their younger counterparts. Two competing models have attempted to explain this result: one in which stars first form in thin disks that gradually thicken with time through dynamical heating, and one in which stars form in thick disks at early times and in progressively thinner disks at later times. We use a direct measure of the thicknesses of stellar disks at high redshift to discriminate between these scenarios. Using legacy Hubble Space Telescope imaging from the CANDELS and GOODS surveys, we measure the rest-optical scale heights of 491 edge-on disk galaxies spanning 0.4 ≤ z ≤ 2.5. We measure a median intrinsic scale height for the full sample of 0.74 ± 0.03 kpc, with little redshift evolution of both the population median and scatter. The median is consistent with the thick disk of the Milky Way today (0.6–1.1 kpc), but it is smaller than the median scale height of local disks (∼1.5 kpc) that are matched to our high-redshift sample by descendant mass. These findings indicate that, while (1) disks as thick as the Milky Way’s thick disk were in place at early times, (2) to explain the full disk galaxy population today, the stellar disks in galaxies need to on average physically thicken after formation.

Experimental and simulation study on the modified shape coefficient of building integrated thermal considerations for heating buildings in solar-enriched areas

The shape coefficient of building (SCB) is a measure of correlating a building’s shape and its energy consumption. However, strictly adhering to restrictions the SCB may not be conducive to reducing energy consumption, as it disregards the potential benefits of solar radiation. To address this, a novel concept and method for determining the modified shape coefficient of building (MSCB) have been introduced in considerations of heat loss and solar heat gain. Field measurements were conducted in western Sichuan to establish a comprehensive understanding of the factors influencing indoor thermal environments, involving collecting indoor air temperature data for model buildings under natural and heating conditions, including solar radiation, building orientations, and U-values. Results demonstrated that maintaining an identical SCB did not consistently ensure similar indoor thermal environments. The indoor thermal environments were significantly influenced by building orientations, U-values, and solar radiation, highlighting the limitations of relying solely on SCB. A simulation method validated through experiments was utilized to predict the dynamic heating load of an actual building. The influence of building orientations on the MSCB across different time scales and actual climatic conditions was investigated. Results illustrated a potential reduction of the SCB by up to 32.4% in the specific case. This reduction correlated with trends observed in the monthly correction factors and heat consumption, suggesting that moderate relaxation of SCB restrictions could be viable even in scenarios with high heating demands. This research contributes valuable insights into the realm of energy-efficient building design.

Experimental study on bubble dynamics and heat transfer of pool boiling at sub-atmospheric pressures

Pool boiling, as an efficient phase-change heat transfer method, has attracted extensive attention in recent years. However, the bubble dynamics behavior (BDB) and heat transfer characteristics of pool boiling at sub-atmospheric pressure (sub-ATM) are significantly different from those at or above standard atmospheric pressure (std-ATM). This paper presents experimental results of BDB and heat transfer characteristics of pool boiling at sub-ATM in the range of 30–120 kPa. High-speed backlight imaging technique was adopted. The results show that with the decrease of boiling pressure, the diameter and near circularity coefficient of bubble departure increase, and the frequency and aspect ratio decrease. With the decline in pressure, the critical heat flux (CHF) of pool boiling on the surface of the electric heater decreases. Specifically, the CHF at 120 kPa is 23.2% higher than that at 40 kPa. At sub-ATM, the boiling heat transfer coefficient (HTC) grows with the increase of liquid height. However, the HTC hardly changes with liquid height at std-ATM and higher.

Near blowout instabilities and identification based on multivariate analysis with high-speed multi-species optical diagnostic techniques

Early prediction and depth analysis of lean blowouts are desirable for both aeroengine and stationary gas turbines. This study used a combination of high-speed multi-species optical diagnostic techniques and multivariate analysis methods to investigate near-blowout instabilities and identify combustion conditions. The flame root dynamic behavior and heat release pulsation characteristics were investigated using advanced modal decomposition methods. Results showed that a near-blowout flame exhibited a characteristic frequency (∼200 Hz) or “fingerprint” related to the temporal and spatial coupling characteristics of CH and OH radicals. Furthermore, flame image feature screening and multivariate fusion for combustion condition identification are reported for the first time. Significant differential features and effective fusion models were investigated and discussed. The identification accuracy of the multivariate fusion feature model compared with that of the geometric feature model improved by more than 24 %. Moreover, simplified fusion models of low-order moment and geometric features were constructed with relatively high identification accuracy (99 %) and low relative standard deviation value (14.1 %). These results demonstrate the significant potential of combining multi-species optical diagnostic techniques and multivariate analysis methods, thereby expanding their application to the field of combustion science.

Heat dissipation models by convection and radiation during the real-time operation of office equipment: A case study of computers

For the reduction of building energy consumption and the higher operation efficiency of air-conditioning systems, dynamic heat dissipation of equipment by convection and radiation need to be precisely quantified, and the reasons are twofold: On the one hand, they accounted for approximately 20% of the cooling demand of office buildings; On the other hand, they are necessary for depicting the heat storage-release process of thermal masses, which is crucial to the description of instantaneous cooling demand. However, existing methods are hard to accurately describe the variation characteristics or time-delay effect of convective/radiative heat dissipation of equipment, resulting in limited applicability. To address this problem, in this study, models of equipment heat dissipation by convection and radiation during the real-time operation were first established as seventh-order transfer functions. Secondly, an experiment was conducted, and computers were selected as samples. Thirdly, experimental results were adopted in parameter identification and order analyses of proposed models. After that, the proposed models were validated under experimental and actual working conditions, with the MAPE (Mean Absolute Percentage Error) values being ≤ 8.51%. Finally, a performance comparison between the proposed and existing models was carried out. The proposed models achieved a time delay of about 1 h and a maximum heat dissipation reduction of 33.89%. With only one easily accessible and nearly free input, the proposed models demonstrated promising applicability and feasibility during the real-time operation. This study facilitates the calculation of dynamic cooling demand and contributes to the energy conservation of office buildings.

Thermal, structural, and chemical characteristics evaluation of wood/phase change material composites for applying as a finishing material in floor radiant heating systems

Application of wood to floor finishing materials is increasing. Phase change materials (PCMs) was utilized to improve thermal performance of wood. Appropriateness of PCM impregnation was evaluated by analyzing morphological properties of composites. Compatibility as flooring material was evaluated by analyzing structural and chemical properties of composites. Their thermal performances were verified by measuring latent heat, surface temperature, thermal conductivity, and dynamic heat transfer test. Composite showed a latent heat of 86.20 J/g, which was 32.53% of thermal performance of PCM. Consequently, composites have prospective applications as finishing materials for floor radiant heating systems.Application of wood to building materials is increasing. In this study, phase change materials (PCMs) which possess latent heat was utilized to improve thermal performance of wood. Appropriateness of PCM impregnation was evaluated by analyzing morphological properties of composites. Compatibility as flooring material was evaluated by analyzing structural and chemical properties of composites. Their thermal performances were verified by measuring latent heat, surface temperature, and thermal conductivity. Moreover, dynamic heat transfer test demonstrates potential of composites for stabilizing the indoor temperature. Consequently, composites manufactured in this study have prospective applications as finishing materials for floor radiant heating systems.

Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface

To provide further insight into humid air condensation on hybrid surfaces, an experiment was conducted to visually investigate the condensation process on wettability-interval grooved surfaces, which had hydrophobic ridges and hydrophilic grooves. The droplet dynamic behavior and heat transfer performance of condensation on a wettability-interval grooved surface were explored and compared with four other functional surfaces, including the plain hydrophilic surface, plain hydrophobic surface, hydrophilic grooved surface, and hydrophobic grooved surface. The presence of hydrophobic ridges perpendicular to the groove direction and hydrophilic grooves allowed for the exclusion and easy spreading of droplets, respectively. Compared with the other four functional surfaces, the coupling phenomena during condensation, i.e., the spontaneous suction and directional drainage via hydrophilic grooves, were only found on the wettability-interval grooved surface. These could not only remove condensate quickly but also suppress the formation of the flooded liquid film, which was beneficial to the enhancement of heat transfer performance. It was proven by the experimental results that at subcooling 12 K, the condensation heat flux on the wettability-interval grooved surface reached 1280 W/m2, which was 1.25 times that of the plain hydrophobic surface (1030 W/m2), and 15% higher than that of the hydrophobic grooved surface (1110 W/m2). This indicated that the wettability-interval microgrooves could effectively enhance humid air condensation heat transfer performance.

Superhydrophobic Microchannel Heat Exchanger for Electric Vehicle Heat Pump Performance Enhancement

Battery-powered electric vehicles (EVs) have emerged as an environmentally friendly and efficient alternative to traditional internal combustion engine vehicles, while their single-charge driving distances under cold conditions are significantly limited due to the high energy consumption of their heating systems. Heat pumps can provide an effective heating solution for EVs, but their coefficient of performance (COP) is hampered by heat transfer deterioration due to frost accumulation. This study proposes a solution to this issue by introducing a microchannel heat exchanger (MHE) with superhydrophobic surface treatment (SHST) as a heat pump evaporator. A computational fluid dynamics MHE model and a dynamic heat pump model are developed and rigorously validated to examine the detrimental impact of frost accumulation on heat transfer, airflow resistance, and heat pump performance. When the frost layer thickness is 0.8 mm at a given air-side velocity of 1.0 m/s, the air-side heat transfer coefficient can be reduced by about 75%, and the air-side pressure drop sharply increases by 28.4 times. As frost thickness increases from 0 to 0.8 mm, the heating capacity drops from 3.97 to 1.82 kW, and the system COP declines from 3.17 to 2.30. Experimental results show that the frost thickness of the MHE with SHST reaches approximately 0.4 mm after 30 min, compared to that of 0.8 mm of the MHE without SHST, illustrating the defrosting capability of the superhydrophobic coating. The study concludes by comparing the performance of various heating methods in EVs to highlight the advantages of SHST technology. As compared to traditional heat pumps, the heating power consumption of the proposed system is reduced by 48.7% due to the defrosting effect of the SHST. Moreover, the single-charge driving distance is extended to 327.27 km, an improvement of 8.99% over the heat pump without SHST.