Prevent Heat Transfer Research Articles

Research on the Indoor Thermal Environment of Attached Sunspace Passive Solar Heating System Based on Zero-State Response Control Strategy

The application of attached sunspace passive solar heating systems (ASPSHS) for farmhouses can improve building performance, reduce heating energy consumption and carbon dioxide emissions. In order to take better use of the attached sunspace to prevent heat transfer or promote natural ventilation, this paper presented a zero-state response control strategy for the opening and closing time of active interior window in the ASPSHS. In order to verify the application of this strategy, an attached sunspace was built in an actual farmhouse. A natural ventilation heat exchange model was built based on the farmhouse with attached sunspace. The proposed zero-state response control strategy was implemented in TRNSYS software. Field measurement in living lab was carried out to inspect the distribution of the thermal environment in the farmhouse with attached sunspace under a zero-state response control strategy in the cold region of northern China. The experimental results show that, even under −5.0–2.5 °C ambient temperature, the application of zero-state response control strategy effectively increases the internal temperature to an average of 25.45 °C higher than the outside, with 23% indoor discernible temperature differential in the sample daytime. The whole-season heating performance was evaluated by simulating the model for the heating season in 2020–2021. The simulation demonstrates that the ASPSHS under zero-state response control strategy can maintain a basic indoor temperature of 14 °C for 1094 h during the heating season, with a daytime heating guarantee rate of 73.33%, thus ensuring higher indoor heating comfort during the day. When compared to a farmhouse with an attached sunspace under the zero-state response control strategy, the energy savings rate can be enhanced by 20.88%, and carbon emissions can be reduced by 51.73%. Overall, the attached sunspace with the zero-state response control strategy can effectively increase the indoor temperature when the solar radiation is intensive and create a suitable thermal environment for the farmhouse in the cold region of northern China.

Study on thermal-mechanical coupling of the ceramic layer in diesel engine piston with thermal barrier coating

Piston performance has an important on vehicle reliability, efficiency, and exhaust emissions. Application of the thermal barrier coating is an effective method for preventing heat transfer from combustion chamber to the substrate. The numerical model of thermal barrier coating piston is established by using finite element method, and a comprehensive thermal-mechanical result is given to determine the influence of ceramic thickness on ceramic layer and substrate. Compared with uncoated piston, the maximum temperature of substrate decreased by 3.34%, 4.09%, 5.19%, 5.95%, and 6.69%, corresponding to ceramic thicknesses of 0.15 mm, 0.2 mm, 0.25 mm, 0.3 mm, and 0.35 mm. The maximum thermal stress decreases from 78 MPa to 73 MPa. For ceramic layer of the thermal barrier coating piston, the maximum temperature appears at the top surface of the ceramic layer, while the maximum thermal stress occurs at the bottom of the ceramic layer. As the ceramics thickness increases from 0.15 mm to 0.35 mm, the maximum temperature of the ceramic layer increases from 322 ?C to 377 ?C, while the maximum thermal stress decreases from 95 MPa to 89 MPa. Thermal-mechanical coupled stress analysis shows that the maximum coupling stress occurs at the pinhole and its value does not change significantly. The thickness of the ceramic layer has little effect on the pinhole, but has a great influence on the ceramic layer.

Preparation and Characterization of Electrosprayed Aerogel/Polytetrafluoroethylene Microporous Materials.

This paper presents the preparation of aerogel/polytetrafluoroethylene (PTFE) microporous materials via needleless electrospray technique, by using an aqueous dispersion of polytetrafluoroethylene as the basic spinning liquid. Different contents of aerogel powders were applied to the spinning liquid for electrospraying to investigate the effect on the structural characteristics and various properties of the materials. Cross-section, surface morphology, and particle size distribution of the electrosprayed materials were examined. Surface roughness, hydrophobicity, and thermal conductivity were evaluated and discussed. The results showed that the electrosprayed aerogel/PTFE layers were compact and disordered stacking structures composed of spherical particles with a rough surface. As the aerogel content increased, the electrosprayed materials demonstrated increased surface roughness and improved surface hydrophobicity with a contact angle up to 147.88°. In addition, the successful achievement of thermal conductivity as low as 0.024 (W m−1 K−1) indicated a superior ability of the prepared aerogel/PTFE composites to prevent heat transfer. This study contributes to the field of development of aerogel/PTFE composites via electrospray technique, providing enhanced final performance for potential use as thermal and moisture barriers in textiles or electronic devices.

Development of interfacial velocity correlation for benchmarking porous media based three-dimensional code for steam generator with a square rod array

ABSTRACT A steam generator is one of the essential components in Pressurized Water Reactors. One of its critical design issues is to prevent heat transfer tube damage caused by corrosion and flow-induced vibration. When designing a U-tube bundle of steam generators, a thermal hydraulic simulation code is used to predict velocity and density distribution along tubes, especially porous-media-based code, from the viewpoint of computational cost. The code first calculates a control-volume-averaged velocity. A mass conservation equation converts it to the area-averaged mixture volumetric flux at the smallest gap between two tubes. Then, the area-averaged mixture volumetric flux is converted to the local interfacial velocity at the center of the sub-channel using an interfacial velocity correlation. Comparison between measured and calculated interfacial velocities ensures the validity of the simulation code. The validation process for the code requires an interfacial velocity correlation. This study shows the newly developed interfacial velocity correlation applicable to two-phase flow in a tube bundle with a square array. The prediction uncertainty of the correlation is +6.7% (bias) ± 24.2% (random error). The correlation based on an analytical equation suggests that the correlation has good scalability to apply to a wide range of flow and geometrical parameters.

Facilely produced highly adhered, low thermal conductivity and non-combustible coatings for fire safety

Fire resistant coatings have been proven as an efficient way to improve fire safety in three aspects: reducing the Heat Release Rate (HRR), delaying the ignition time and preventing heat transfer. Herein, a SiO2 based polymeric composite coating with a lower thermal conductivity and brilliant fire resistance was developed. Isocyanate and sodium silicate could form the final Si-O-Si network structure by polymerization. Compared to the wood without coating, the coated wood shows a significantly increase in limit oxygen index (LOI), has reached 48.0 vol% in the test. As for the cone calorimetry test, coated wood shows a 55.3% decrease in the first peak Heat Release Rate (pHRR) and the Total Heat Release (THR) obtains fire-resistant standard. After exposed to butane flame for 30 mins, the coated wood could still maintain its structural integrity with only 180℃ on the non-exposed side. The commercial standard test of the coating was also investigated. To better understand what role does the polyurea play in the system, a theoretical calculation was done during the research to discuss the interaction between the silica and polyurea. As a fast brush-formed coating, it exhibits a great potential in the field of fire-resistant materials, and may broaden the application prospects of wood.

Influence of Density on Foam Collapse under Burning.

The fire behaviour of flexible polyurethane foams was studied using a cone calorimeter, with a special emphasis on the collapse step. Only one peak of heat release rate, ranging from 200 to 450 kW/m2, is observed for thin foams, depending on the foam density and the heat flux. On the contrary, heat release rate (HRR) curves exhibit two peaks for 10 cm-thick foams, the second one corresponding to the pool fire formed after foam collapse. In all cases, the collapse occurs at a constant rate through the whole thickness. The rate of the recession of the front was calculated using digital and infrared cameras. Interestingly, its value is relatively constant whatever the heat flux (especially between 25 and 35 kW/m2), probably because of the very low heat conductivity preventing heat transfer through the thickness. The rate increases for the lightest foam but the fraction of burnt polymer during collapse is constant. Therefore, the pool fire is more intense for the densest foam. A simple macroscopic model taking into account only the heat transfer into the foam leads to much lower front recession rates, evidencing that the collapse is piloted by the cell walls’ rigidity.

Effect of Bamboo Flour on Flame Retardancy and Smoke Suppression of Polypropylene/Ammonium Polyphosphate Composites

In this paper, the flame retardant PP composites were prepared by melt blending. APP and bamboo flour are selected as flame retardant and smoke suppressant of the composite material. Among them, bamboo flour as a synergist can effectively improve the flame retardancy and smoke suppression effect of PP/APP composites. The effects of bamboo flour on the mechanical properties, crystallization behavior, thermal degradation, flame retardancy, and especially the smoke suppression effect of PP/APP composite materials were studied. TGA, LOI, cone calorimetry, SEM and other characterization methods were used to study the thermal stability, flame retardancy and combustion characteristics of the composites and the microscopic morphology of carbon residue. Experimental results show that when the total addition amount of APP/bamboo flour (2:1) in PP55/APP30/BF15 composite is 45%, the residual carbon has more specific surface area and micro-pores, which makes the composite have the best smoke suppression effect. The PHRR of the PP55/APP30/BF15 composite was reduced to 308.2 kW/m2 and the amount of carbon residue is 25%. The continuous carbon layer formed during combustion can effectively protect the matrix material and prevent heat transfer.

Assessing the Use of Portable Blood Gas Measurements (iSTAT Instrument) in Emergency Air Transport

Abstract Hyper and hypoventilation of patients on ventilators leads to poor outcomes. Traditionally, mechanical ventilation protocols in air medical transport are guided by pulse oximetry (SpO2) and continuous end tidal CO2 (EtCO2), which can lead to over-ventilation of patients. The use of portable blood gas measurements is a desirable adjunct in the air medical transport setting due to the complexity and duration of management of critical patients with only limited, noninvasive monitoring options. Previously, protocols relied on SpO2 and EtCO2 to control ventilator settings during in-flight patient management. The iSTAT is a hand-held point of care device often used in acute hospital setting to obtain arterial blood gas results within minutes using a disposable test cartridge and small specimen size. However, iSTAT use in air medical transport has been limited due to analytical and durability concerns in the uncontrolled helicopter environment. The purpose of this study was to test these concerns including the effects of vibrations on the microfluidic transport of blood in the cartridge, pressure changes at altitude on the processing of the cartridge, and temperature changes on the iSTAT instrument between readings during in-flight use. To ensure accurate inflight testing, we assessed precision and accuracy of inflight blood gas measurements compared to pre- and post- flight measurements on quality control material. Precision of initial instrument verification met the manufacturer’s coefficient of variation (CV) claims (lactate = 3.59%, pH = 0.059%, pCO2 = 4%, pO2 = 3%). Initial accuracy was assessed by instrument comparison. Bias of iSTAT compared to the laboratory radiometer ABL800 instrument was acceptable for clinical use (lactate = -7.95%, pH = -0.041%, pCO2 = 1%, pO2 = -2%). For the majority of tests (pH, pCO2, pO2, HCO3, BE, and SO2) we found no significant differences between inflight absolute values compared to pre- and post- flight measurements by one-way anova (p>0.05), and no significant difference in precision (CV) between in air and pre- and post- flight measurements for both low and high quality control samples. In measuring lactate levels, we found significant differences between inflight absolute values compared to pre- and post- flight measurements (p<0.0001) but these were determined to be clinically insignificant (mean (mmol/L): preflight = 6.87, inflight = 6.77, postflight = 6.69). Vibration and pressure differences in air compared to on land were therefore considered clinically insignificant. To keep the instruments at an operable temperature in flight between readings, we found utility in using an insulated lunch box with additional styrofoam placed in the bottom to prevent heat transfer. Most importantly, improved clinical outcomes from proper ventilation of patients were achieved. The results of this study demonstrate that the iSTAT instrument provides clinically accurate blood gas measurements in air as compared to standard in-hospital use.

분리형 코어를 적용한 내부가열형 사출 금형의 열전달 및 유동 특성 연구

The purpose of this study was to develop an efficient mold heating technology by an embedded heating unit. To localize the heating effect in the mold core and prevent heat transfer to surrounding mold plates, the core module with embedded heating unit was assembled to a mold plate in a detachable manner. The detachable core module was then separated from the mold plate when the mold was opened, and thus could be rapidly heated by the embedded heater. The heated core contacted with the mold plate when the mold was closed, and could be cooled by heat conduction to the mold plate of which thermal inertia was much larger than that of the core module. To verify thermal efficiency of the proposed structure, heat transfer simulation was performed with an experimental validation. Mold filling simulation was also performed to investigate the effect of mold heating on improving flow characteristics through a thin and narrow channel. Injection molding experiments were also conducted by adopting the proposed embedded heating module.

Eco-friendly flame retardant coating deposited on cotton fabrics from bio-based chitosan, phytic acid and divalent metal ions

With the increasing awareness of environmental protection, the greening of flame retardants has become an inevitable choice for flame retardant technology. Bio-based materials have the advantages of wide source, recyclability and green environmental protection, and have received extensive attention. In this work, chitosan and phytic acid as intumescent flame retardant system and metal ion as a synergist were built on cotton fabrics to achieve efficient flame retardancy by facile dip-pad-dry process. Microscale combustion calorimetry results manifested that the heat release rate values of coated samples were lower than that of uncoated samples. The peak heat release rate and total heat release of uncoated samples were 333.1 W/g and 13.1 kJ/g, while these values of samples coated CH/PA/Ba/PA were 129.1 W/g and 5.4 kJ/g respectively and char residues also were increased sharply owing to the phosphate groups promote the formation of char layer covering the surface of the cotton fabrics, which can prevent heat transfer. Scanning electron microscopy confirmed that there were much more bubbles on the surface of fibers for the coated CH/PA/Ba/PA after horizontal flame tests. The results indicated that the synergy of the intumescent flame retardant system was achieved by the addition of metal ions which could accelerate the produce of a large number of nonflammable gases. Horizontal flame test and limit oxygen index (LOI) test showed that the burning rate and LOI values of samples coated CH/PA/Ba/PA, CH/PA /CH/PA, PA/Ba/PA/Ba were 1.5 mm/s and 22.0, 2.0 mm/s and 20.2 and 2.5 mm/s and 18.0 respectively, compared to 3.3 mm/s and 16.2 of uncoated samples. The samples with intumescent flame retardant system and metal ions (CH/PA/Ba/PA) had best flame retardant performance. Moreover its weight gain (5.2%) was less than that (10.7%) of the sample only with intumescent flame retardant system (CH/PA /CH/PA). The less weight gain had less effect on the softness of cotton fabrics. As a result, the coating is expected to be applied in practice.

Microwave open-ended waveguide for detection and characterisation of FBHs in coated GFRP pipes

Accessing to glass fibre reinforcement plastic (GFRP) pipes in the on-line process is often limited to one side. To detect an internal defect of coated GFRP pipe, common non-destructive testing (NDT) methods such as eddy current testing and thermography are not effective because GFRP pipes are not conductive and their coating prevents heat transfer to the inner section of the material. Moreover, the curvature of GFRP pipes introduces lift-off variation, which is the primary challenge for NDT methods using C-scan technique; e.g., ultrasonic. This work proposes a novel microwave NDT application to detect and characterise internal defect; i.e., flat-bottom hole (FBH) in coated GFRP pipes. Initially, responses of four FBH areas obtained by microwave open-ended waveguide probe are analysed using principal component analysis (PCA) to reveal FBH defect and eliminate the pipe curvature influence. Following that, synthetic aperture radar (SAR) tomography images are produced to select the corresponding principal component and verify the location of FBHs. Finally, FBHs are evaluated using PC feature and SAR tomography including time-of-flight feature. The results show that the proposed method can clearly reveal the area of FBHs. Moreover, depths of FBHs are characterised by analysing the time-of-flight feature of background-subtracted responses.

Year-round performance analysis of a photovoltaic panel coupled with phase change material

Phase change material (PCM) is in employed in photovoltaic (PV) system for thermal regulation and efficiency improvement. The hybrid system is named as a PV-PCM system. While progress is achieved in understanding system performance in laboratory and daily simulation, few results are reported in a seasonal and yearly basis, which requires substantial computation time to solve Navier-Stokes equations. In this study, an enhanced conductivity method is applied to simulate the convective effect without solving N-S equations. The model is developed based on 1-D thermal resistance model and validated with experimental data, possessing a good balance between accuracy and simplicity. The model is, therefore, adopted to examine and optimize seasonal and yearly PV-PCM performance. Within five systems simulated, the result suggests that systems perform diversely under different weather conditions. For instance, PV-PCM system with high melting temperature usually performs well in summer while might prevent heat transfer in winter since the PCM cannot be melt in cold days. Finally, the highest year-round improvement in electricity production 2.46% compared to reference PV system, which is lower than the value reported in most studies conducted only on a sunny day or in laboratory in a short-time scale. Moreover, the economic analysis indicates that, without a significant improvement in PCM performance or exploiting electricity-and-heat cogeneration, PV-PCM system may not be viable for real application at this stage.

Numerical investigation of bubble dynamics and heat transfer in subcooling pool boiling under low gravity

Abstract The numerical simulation of a single vapor bubble growth in subcooling liquid under different gravity conditions has been carried out. In the numerical model, a thin superheated layer and the thermocapillary convection caused by the surface tension variation along the surface are considered. The continuity equation and energy equation are modified to allow for the phase change. In addition, the thermocapillary convection effect has been included in the momentum equation. The vapor-liquid interface is captured by the phase field method. The results show that the bubble behavior in the numerical model agrees well with previous experiments conducted in high subcooling liquid under microgravity. The effects of gravity level, contact angle and wall superheat on the bubble growth, critical subcooling (the liquid subcooling under the condition that the evaporation rate of a bubble is equal to its condensation rate), together with heat transfer have been investigated. The growth period and departure radius both reduce with the increase in gravity level, while the critical subcooling increases slightly. Large contact angle at the three-phase contact line augments the departure radius. However, the critical subcooling decreases as contact angle increases. With the wall superheat increasing, the growth period reduces rapidly, while the departure radius and the critical subcooling increase. What’s more, the non-departing bubble adhering to the surface would prevent heat transfer with a dry spot, which may damage the heating element in application.

Problems of Autoclaved Aerated Concrete Treatment

The problems of autoclave c of cellular concrete are considered. It was shown that the key problem of autoclaving is the appearance of areas of concrete with different dark colors and a reduced strength. The main reason for their occurrence is the presence of air in the porous space of cellular concrete, which prevents heat transfer. Cold air does not allow the concrete to warm up to a temperature of 170-175 °C, which hinders the flow of physical-chemical processes of hardening. The proposed activities that improve the quality of autoclave processing and precluding the formation of «non-steamings» in the concrete: ensure the temperature inside cellular concrete mass of at least 80 °C before curing, the vacuum (P=‒0.5 Bar) before pressure rise in the autoclave, preventing the ingress of air into the autoclave at the beginning of the process.

Influence of liquid–solid intermolecular force on levitation of impacting nanodroplet

When droplets impact on a heated wall, they can levitate owing to the vapor stream generated by the droplet evaporation. This phenomenon is called the Leidenfrost effect, and the vapor layer prevents heat transfer between the droplet and heated wall. In this study, we investigated the influence of the intermolecular force between liquid and solid molecules on the levitating phenomenon, which is caused by heat transfer, for nanodroplets. We used a molecular dynamics (MD) simulation to evaluate the detailed behavior of droplet levitation and investigated the temperature field of the impacting droplet. We found that the droplet levitation was likely to occur at lower temperature when the intermolecular force was stronger. In addition, when the intermolecular force was strong enough, the liquid molecules stayed on the heated wall and an adsorption layer was formed. This adsorption layer exceeded the critical temperature of the liquid molecules, and the existence of the adsorption layer significantly affected the onset of the droplet levitation.

Predicción de la reducción del impacto térmico en un edificio con doble pared

En la ciudad de Santa Fe de la Vera Cruz, Argentina, se está construyendo un edificio de altura que incluye elementos de arquitectura sustentable, eficiencia energética y confort logrado en base a la utilización de recursos naturales. Particularmente, un diseño de doble fachada en los frentes que dan al exterior para lograr una cámara de aire que impida la transferencia térmica desde el exterior al interior en verano y al revés en invierno. Este trabajo presenta un estudio numérico de la evaluación de la performance térmica de la cavidad interpuesta en la doble fachada del edificio, para distintas condiciones climáticas, considerando dos alternativas de diseño: cámara de aire cerrada y cámara de aire conectada con el exterior. Ambos casos se comparan con la situación de inexistencia de la cámara, cuya transferencia de energía térmica se constituye en el caso patrón. Los resultados muestran que para las condiciones de verano, la cavidad con conexión al exterior sería la más recomendable, mientras que para el invierno, la cavidad cerrada es más apta para el ahorro de energía.

Experimental analysis of a Flat Plate Pulsating Heat Pipe with Self-ReWetting Fluids during a parabolic flight campaign

A Flat Plate Pulsating Heat Pipe (FPPHP) filled with an ordinary liquid (water) and a self-rewetting mixture (dilutes aqueous solutions of long-chain alcohols with unusual surface tension behavior) is investigated under variable gravity conditions on board a ‘Zero-g’ plane during the 65th Parabolic Flight Campaign of the European Space Agency. The FPPHP thermal performance in terms of evaporator and condenser temperatures, start-up levels and flow regimes is characterized for the two working fluids and a power input ranging from 0 to 200 W (up to 17 W/cm2 at the heater/evaporator wall interface). The experimental set-up also includes a transparent plate enabling the visualization of the oscillating flow patterns during the experiments. For a low power input (4 W/cm2), the pulsating heat pipe filled with pure water is not able to work under low-g conditions, because the evaporator immediately exhibits dry-out conditions and the fluid oscillations stops, preventing heat transfer between the hot and cold side and resulting in a global increase of the temperatures. On the other hand, the FPPHP filled with the self-rewetting fluid runs also during the microgravity phase. The liquid rewets several times the evaporator zone triggering the oscillatory regime. The self-rewetting fluid helps both the start-up and the thermal performance of the FPPHP in microgravity conditions.

THE EFFECTS OF PID PARAMETERS ON TEMPERATURE CONTROL OF UNINSULATED ENVIRONMENT

In this study, temperature of an uninsulated environment was controlled by PID for on-off and various parameter values ​​using Arduino micro controller. A medium with high heat transmission and with no insulation materials on the junctions and the surface to prevent heat transfer into the external environment, is heated by an AC voltage-operated resistor. Temperature of the heated environment is taken with LM35 temperature sensor and converted to 0-10V industrial standards. Temperature information converted to voltage value was applied as an analogue input to the Arduino Due micro controller and resistance voltage was set via semiconductor switching path as a result of PID operation. Despite deteriorating experimental results such as heat loss, it is seen that the reference temperature is captured by the PID.

Development and on-orbit operation of loop heat pipes on chinese circumlunar return and reentry spacecraft

This paper describes the development and result of ground experiment and analyzes the on-orbit operation of Loop heat pipes (LHP) for the Chinese circumlunar return and reentry spacecraft. A novel LHP was developed to satisfy the thermal control requirements of Inertial measurement units (IMU). The LHP consisted of an integrated evaporator subassembly, a cambered condenser, and an electric explosion valve. Ground experiments were conducted to investigate the startup characteristics, and heat transfer and heat insulation performance of the LHPs. During a flight mission, the LHPs work as a thermal switch and perform both heat transfer and insulation functions. Before returning to the earth’s atmosphere, the valves of the reentry capsule are opened to discharge working fluid and prevent heat transfer from the capsule wall to the devices. In the flight mission, the LHPs performed all the required functions, and the IMU temperatures were maintained within an allowable range.

Late Pleistocene and mid-Holocene climate change derived from a Florida speleothem

A stalagmite collected from Brooksville Ridge Cave in west-central Florida was deposited from ∼30 to 20 kyr BP, encompassing Heinrich event 2 (H2), and from ∼5 to 4 kyr BP, the later part of the mid-Holocene. The H2 event in the speleothem occurs at ∼24 kyr BP which is temporally similar to its timing in other paleoclimate reconstructions. However, the oxygen and carbon isotope values indicate the climate in Florida was relatively warm and wet for a glacial period rather than extremely cool and dry, as seen in other regions. The higher temperatures and increased precipitation, compared to the preceding climate state, produced a shift towards enhanced vegetation growth (dense forest) during this period. One possible cause is the faltering of the North Atlantic Thermohaline Circulation, due to increased glacial meltwater input, thereby preventing heat transfer via the Gulf Stream from the tropics to the northerly latitudes. This change in the circulation would lead to warmer Gulf of Mexico and subtropical Atlantic sea surface temperatures (SSTs). These higher SSTs would promote an increase in convective thunderstorm activity due to higher evaporation rates. However, to give this shift some paleoclimate perspective, the average mid-Holocene speleothem oxygen isotope values were more depleted by ∼1‰ indicating even warmer temperatures and higher precipitation amounts compared to the glacial period. Additionally, the carbon isotopes show a ∼3‰ shift towards more negative values, indicating a more heavily forested west-central Florida. The variability within both the mid-Holocene and Late-Pleistocene data support the possibility of atmospheric teleconnections between the tropics/subtropics and northerly latitudes contributing to shorter-term shifts in precipitation amount superimposed on the larger-scale glacial-interglacial isotopic composition of the speleothem.