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The evolution of mesoscale structures of particle packing in binary composite packed beds and their effects on flow characteristics and wall effects were investigated using the discrete element method (DEM) and computational fluid dynamics (CFD). The DEM model was used to build a series of randomly mixed packing structures of particles in accordance with the dynamic change of mass ratio between particles in two size ranges, which were then confirmed by the findings of an X-ray tomography (CT) scan. The results show that the packing structure of b25s75 was conducive to reducing the influence of wall effect in packed bed reactors. For b25s75, the dimensionless distance of radial porosity fluctuation from the wall is 0.3705, which is the smallest among the five packing models, indicating that this structure plays a suppressive role on the wall effect. In addition, the uniformity of velocity and temperature distributions in both the radial and axial directions of different packing structures were compared. The standard deviations of radial relative velocity distributions in the packed beds of b100, b75s25, b25s75 and s100 are 0.28, 0.178, 0.139 and 0.156, respectively, indicating that the stacking mode of b25s75 can make the fluid flow and the gas–solid interactions more uniform. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on the combustion behavior of lateral continuous leakage of heptane and gasoline on a steel substrate. The flame shape characteristics of the two fuels at different leakage rates were recorded and described. Variation trends in burning area and flame height with leakage rate were analyzed, and variations in flame width against the flame height were formulated. The mass loss rate was lower than that of the pool fire at the same equivalent diameter. Based on the fuel layer heat feedback and heat loss analysis, the mass loss rate for thin fuel layer combustion was calculated and verified to be about 0.013 kg/(m2‧s). Then, a multi-layer inclined cuboid flame model was established. Compared with the classical point source model and the cuboid flame model, the new model has better accuracy in predicting the external radiation of the combustion of thin-layer fuel. We hope to gain insights into the dynamics of such fires to inform fire prevention and control in initial spill scenarios. • The variation of thin-layer fuel flame shape with height was characterized. • Burning rate model for pool fires was not applicable to thin-layer fuel combustion. • The constant mass loss rate was verified based on heat transfer calculations. • A multi-layer cuboid flame model was developed to calculate the external radiation. [ABSTRACT FROM AUTHOR]

In this paper, a photovoltaic direct-driven ice storage air-conditioning (PDISAC) system is proposed and performance of the system is experimentally and theoretically investigated. The proposed system is a battery or inverter less photovoltaic direct-driven system where the DC compressor is directly connected to the PV array. Through the test, it has been found that the proposed system was able to lower the test room temperature below 298.15 K, while the reference room temperature was 306.15 K. Moreover, the experimental refrigeration efficiency and solar-energy utilization efficiency were reported 1.028 and 7.1% respectively. Then, a mathematical model for the progress of ice making and cold storage is presented and verified by the experimental results. Besides, the factors such as the ambient temperature, the initial water temperature and volume in the ice storage tank, and the thickness of the ice layer surrounding the coil have been studied. The simulation results indicate that the initial heat transfer rate at the condensation side drops by 10.8% and refrigeration efficiency drops by 32.7% with the ambient temperature increasing from 298.15 K to 308.15 K. The increment of initial water temperature and initial water volume will lead to an increase in refrigerating capacity and refrigeration efficiency. • A novel PDISAC system without battery or inverter is proposed. • The system can keep the experimental room below 25 °C for two hours. • The experimental ηref_sys and ηsolar can reach 1.028 and 0.071 • A mathematical model is proposed and validated using experimental data. • Ambient temperature and ice thickness have great impacts on system performance. [ABSTRACT FROM AUTHOR]

This paper is a re-examination to quantify the flame tilt angle of fires induced by stack effect from a heat and mass transfer perspective. In previous studies, the critical transition and length of the deflection flame were formulated. In the current study, with different kinds of fuels, the flame tilt angles under different strength of stack effect are compared. Results show that under stack effect, the flame tilt angles are quite close at the quasi-steady state within a narrow range regardless of the heat release rate of fire and the position of the open window in a stairwell, which indicates that the inertial force of air flow induced by the stack effect equilibrates dynamically with the thermal buoyancy induced by fire plume. However, the velocity of air flow drawn into the fire compartment is directly related to the strength of stack effect, which is mainly influenced by the heat release rate of fire. A new correlation for the flame tilt angle under the influence of stack effect is developed by taking the dynamic equilibrium into account. The tangent value of the flame tilt angle is linearly proportional to the 4/15 power of the heat release rate of fire. Additionally, a correlation to predict the heat release rate with the titled flame length and flame tilt angle is formulated. [ABSTRACT FROM AUTHOR]

Using a compartment with adjustable oxygen concentration and internal pressure, the shrinkage and cracking of isotropic char layer during pyrolysis were studied by pyrolyzing the medium density fibreboard (MDF) in inert atmosphere with different ambient pressures. The experimental results have shown that the ambient pressure has insignificant effect on shrinkage although some trends have been identified. Therefore, the reduction of tensile strength of material dominates the char cracking under low pressures, leading to noticeable increase in the number of char fissures. The char shrinkage could expose the raw material beneath the char layer and enhance the radiative heat transfer at the sample surface (by fissure width instead of number), which is modeled using a modified thermal conductivity as the typical simplification. It is found that the mass loss rate at the early pyrolysis stage increases up to 20% with increasing width of char fissures. However, the external radiation can only affect a limited depth in the near surface zone which is found less than 3 mm in the current experiments. The thermal conductivity under near regular pressure could be over 2 times higher than its value under low pressure (30 kPa) and with which the differences in the first peaks of mass loss rate under different pressures can be well predicted. [ABSTRACT FROM AUTHOR]

The development of coal-based fuels is the best way to supplement the petroleum shortage. During the storage and transportation of coal-based fuels, fuel leakage often forms spill fires, posing a serious threat to energy utilization. A series of spill fire experiments were conducted on a vertical plate to improve the understanding of the combustion behavior of spill fires. Methanol was used as the fuel and the leakage rate was 0.32 mL/s∼3.07 mL/s. When the fuel leakage rate is small (≤2.17 mL/s), the fuel burning area gradually shrinks until it reaches a stable value. While the fuel leakage rate is large (≥2.46 mL/s), the flame keeps covering the whole plane below the leakage outlet and the burning area does not shrink. Gravity-dominated diffusion combustion on the vertical plane will result in a thinner fuel thickness, with a calculated average methanol thickness of 0.11 times that of the fuel burning on the horizontal plane. A heat transfer equilibrium model of the steady-state combustion process is further performed, which show that the combustion process of methanol in these cases is dominated by convective heat transfer, accounting for about 90 % of the total heat feedback. • The various stages of the flame shape during fuel combustion are shown. • The fuel thickness during methanol combustion is simply calculated. • The combustion characteristics and heat transfer process of methanol spill fire are analyzed. [ABSTRACT FROM AUTHOR]

Thermal and electrical management of the PV systems integrated with Phase Change Materials has been discussed in this article. The main aim of this review article is to provide the current status of PV-PCM technology along with the research gaps and challenges being faced by this technology. A comprehensive literature review elaborating different aspects of this technology, such as system development, performance evaluation, PCM selection, heat transfer enhancement, simulation, and application in practice is presented. Cost incurred due to inclusion of PCM into PV system is also discussed in detail in this article. Major findings of the current review are that PV panel peak temperature can be reduced up to 20 °C with an increase in electrical conversion efficiency up to 5% by using PCM. At an average, ~ 2.6 kg of PCM is needed per meter square of the PV panel area to reduce the one-degree temperature of the PV panel during peak hours. The current review concludes that PV-PCM cooling system is not yet commercialized because of technological challenges, high system cost, and availability of appropriate PCMs. However, PV–PCM systems are still in the research phase, with great scope for practical applications. Suggestions for the future work along with current challenges are also presented. [ABSTRACT FROM AUTHOR]

Thermal-driven smoke flow is the greatest threat to the trapped people in tunnels, and the basic understanding of the smoke transport characteristics is necessary for the engineering applications of smoke control and exhausting system. However, most of the previous studies only focused on the smoke transport at normal pressure. As a matter of fact, the ambient pressure decreases with the increasing of the altitude, and the influence of different ambient pressures on the transport characteristics of thermal-driven smoke has been rarely considered before. This paper presents an investigation on the effect of ambient pressure on the smoke transport characteristics in a tunnel. A series of fire simulations were conducted in a full scale road tunnel with ambient pressure ranged from 100 kPa to 50 kPa and the heat release rate (HRR) varied from 3 MW to 15 MW. The smoke mass transport along the tunnel for different conditions is analyzed, and results show that for a certain HRR, the smoke mass flow rate decreases with reduced ambient pressure due to the decreased air density and the weakening of air entrainment strength. Based on the dimensional analysis, a quantitative model to predict the average smoke mass flow rate in the tunnel is proposed considering both ambient pressure and HRR. Moreover, the credibility of the predictive model is validated by comparing with a wide range of experimental results from both full scale and reduced scale experiments at different pressures. [ABSTRACT FROM AUTHOR]

Effect of Phase Change Material (PCM) on electrical efficiency and operating temperature of PV panel is studied in this article. PCM is filled in rotatable shutters attached to the back side of PV panel. PCM filled rotatable shutters are used to ensure the complete solidification of PCM during non-sunshine hours. During sunshine hours PCM filled shutters are closed and absorb excess heat generated by PV panel. The absorbed heat is discharged by rotating the PCM filled shutters to ambient during non-sunshine hours. The modeling results revealed that conventional PV panel temperature without PCM can be as high as 64 °C that can be reduced to 42 °C using PCM filled shutters with melting temperature of 35 °C. Efficiency can be improved up to 9% during peak summer season for the PV panel working under hot humid climatic conditions. Maximum benefits of PCM in terms of temperature reduction and are observed during the summer season compared to the winter season. The study also suggested that states if the PCM melting temperature is approximately equal to the average of the maximum ambient temperature of all summer months, then maximum benefits in terms of efficiency improvement and temperature reduction can be achieved for the whole year. [ABSTRACT FROM AUTHOR]

Heat pipe is a high efficient tool in solar energy applications. In this paper, a novel solar concentrating thermoelectric generation using micro-channel heat pipe array (STEG-MCHP) was presented. The flat-plate micro-channel heat pipe array not only has a higher heat transfer performance than the common heat pipe, but also can be placed on the surface of TEG closely, which can further reduce the thermal resistance between the heat pipe and the TEG. A preliminary comparison experiment was also conducted to indicate the advantages of the STEG-MCHP. The optimization based on the model verified by the experiment was demonstrated, and the concentration ratio and selective absorbing coating area were also discussed. In addition, the cost analysis was also performed to compare between the STEG-MCHP and the common solar concentrating TEGs in series. The outcome showed that the solar concentrating thermoelectric generation using micro-channel heat pipe array has the higher electrical efficiency and lower cost, which may provide a suitable way for solar TEG applications. [ABSTRACT FROM AUTHOR]

A dynamic combustion period occurs after the liquid fuel ignition, but the combustion mechanism is complicated and unclear due to dynamic heat transfer between the fuel and the substrate. This work uses a method of experimental research and theoretical analysis to study the dynamic combustion behaviour and heat transfer of pool fire with burning time (t) under thin fuel thickness (δ). Research contents include the relation of flame height (Z), burning rate (m ˙) , heat transfer between fuel and substrate with t , and the evolution mechanism of the output composition of total heat feedback with δ. The research found Z and m ˙ increase with t until maintaining constant, whereas δ is different. Heat convection loss increases rapidly, then increases slowly, and finally keeps constant because thin-layer fuel is quickly heated. Heat radiation loss firstly surges, then fluctuates due to the combined effect of the flame shape and necking effect. Total heat loss tends from radiation to convection. The heat for heating fuel and the heat radiation reflected from the substrate are considered in the dynamic combustion stage. The fractions of heating fuel, vaporization heat, heat loss and heat reflection were calculated and studied. This work adds theoretical knowledge of fire dynamics • Dynamic combustion behavior and heat transfer of thin-layer pool fire are studied. • Total heat loss tends from heat radiation to heat convection with burning time. • Heat of heating fuel and reflected heat are considered into the energy equation. • Fractions of heating fuel, heat loss, heat reflection, fuel vaporization are showed. [ABSTRACT FROM AUTHOR]

The current research work aims to maintain high photovoltaic (PV) efficiency by keeping them at low operating temperature. For this purpose, commercially available phase change material (PCM) RT24 encapsulated in copper tubes was attached at the back side of a PV panel as a heat sink. The main concept of using the copper tube as PCM encapsulation is to produce a fin effect on the back of the PV panel to enhance the rate of heat transfer from the PV panel to ambient air. The experimental results showed that a maximum temperature reduction of 8.5 °C can be achieved at the front surface of the PV panel with 2.5 kg of PCM per square meter of the PV panel area. The PV back surface was also cooled down from 62 °C to 56 °C by the PCM. Enhancement in efficiency up to 3% was observed. The fin effect for the PV panel cooling was observed as the PV panel was maintained at a lower temperature, even after the PCM was completely utilized. The total cost incurred for the current PCM-based thermal regulation of the PV panel was $93/m2 of the PV area or $0.60/W of the PV power. [ABSTRACT FROM AUTHOR]

In China, because of the emergence of a large number of high-rise buildings, the solar hot water heater system often uses the balcony wall-mounted method for installation. The thermoelectric energy converter is proposed as one of the possible technologies to incorporate solar water heater to produce electricity for building application. In this paper, the conceptual development and theoretical analysis of a novel micro-channel heat pipe evacuated tube solar collector-incorporated thermoelectric generation are all proposed. The new system takes into account many advantages, including the high heat transfer, low convective heat loss, and low contact thermal resistance. The exergy analysis method based on the second law of thermodynamics is also introduced to evaluate the performance of this system. The results show that a novel micro-channel heat pipe evacuated tube solar collector-incorporated thermoelectric generation has a high thermal performance with addition of electricity production. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Characteristics of a high concentration photovoltaic/thermal (HCPV/T) module equipped with point-focus Fresnel lens have been investigated in this paper. Both electrical and thermal models of the module are developed by numerical methods. The electrical model is based on the Shockley diode equation, and the thermal model is grounded on a two-dimensional steady-state heat transfer model. Influences of environmental parameters and coolant water are considered in the models. The inputs of the models consist of irradiance, ambient temperature, wind speed, water temperature and mass flow rate. The outputs mainly include electrical efficiency and thermal efficiency. The simulated results are compared with experimental results and a great agreement is obtained. By the virtue of the validated models, influences of different parameters on module performance are analyzed in detail. The results show that an electrical efficiency of 28% and a thermal efficiency of 60% can be obtained by the HCPV/T module. The electrical efficiency is mainly influenced by solar irradiation rather than cell temperature. The thermal efficiency increases with the increment of irradiance, ambient temperature and water mass flow rate. On the contrary, increasing water temperature and wind speed will lower the thermal efficiency. Also, the HCPV/T module can produce hot water as high as 70 °C without decreasing the electrical efficiency seriously. [ABSTRACT FROM AUTHOR]

Most solar water heating systems (SWHS) are forced circulation systems using AC pumps. Controlled by an ON/OFF differential temperature sensing controller, the heat transfer fluid in a SWHS flows at a constant rate, and the controller consumes electricity. The controller and the sensors lead to system malfunctions, and AC pumps cycle between ON and OFF frequently under low solar irradiance. Hence, PV direct-coupled DC pumps are promising in SWHS, since they are simple and reliable. In this study, an optimization study of a PV direct-coupled SWHS (PV-SWHS) which uses a PV direct-coupled DC pump to circulate the heat transfer fluid is presented. A system model is built and validated with experiments. Two different types of PV module designs are investigated. The performance of the PV-SWHS under different real weather conditions is studied and compared with the performance of the conventional SWHS. And an optimum design method of PV-SWHS is proposed. The results show that: (i) the startup solar irradiance of the pump is the major factor which affects the thermal efficiency of PV-SWHS; (ii) a proper PV module design can reduce the requirement of PV cells by more than 25%; (iii) the PV-SWHS can work well without circulation controller; (iv) the optimized PV-SWHS has nearly the same thermal efficiency and better long-term system reliability than the conventional SWHS. Therefore, the PV-SWHS is very promising. [ABSTRACT FROM AUTHOR]

The utilization potentiality of PV/T which laminates PV onto glass cover to improve reliability (G-PV/T) was proved previously, but how to optimize the structure is not comprehensive. This study focuses on its optimization and energy trade-off among two critical structural parameters (i.e., packing factor, air gap thickness). Firstly, mathematical models for G-PV/T are established and validated by the experimental data. Specifically, a special G-PV/T without air gap is also considered, and where its' difference in optical models and heat transfer models are stated. Secondly, the past studies of PV/T used to investigate parametric optimization by discussing only one parameter at a time, but this study points out that the optimization methods for these structural parameters are interrelated, and their coupling relationship is influenced by outside conditions. So a matrix of the two parameters is used to predict the daily performance of G-PV/T and the results are presented in three-dimensional two-independent-variables plots, evaluated by the first and second laws of thermodynamics. Suggestions are given for structural optimization depending on application requirements and energy trade-off. Thirdly, performance under three important outside conditions is predicted to discuss how the system performance is influenced and how the coupling relationship of the two critical parameters moves. • Mathematical models of G-PV/T with/without air gap are established & validated. • A matrix of two parameters is used to predict the daily performance of G-PV/T. • Coupling relationship of two parameters and the turning points are discussed. • Outside conditions' influences on performance and interrelationship are studied. • Suggestions are given for system optimization. [ABSTRACT FROM AUTHOR]

Numerical simulations were carried out to study the smoke behaviors induced by fires in inclined tunnels with different slopes and the upstream maximum temperatures along the tunnel centerline were specifically focused. The simulation results show that the longitudinal centerline peak temperature occurs at the downstream region of fire source rather than right above the fire source. Two typical behaviors were found during the quasi-steady state: the upstream smoke layer interface is almost parallel to horizontal level while the downstream smoke layer interface is parallel to the inclined tunnel ceiling. The upstream maximum temperature under the ceiling remain approximately constant near the fire sources and then drop progressively with increasing distance to fire source due to the existence of vortexes, which is fairly different from the downstream maximum temperature distribution. Hence, an empirical correlation is developed by taking into account the tunnel slope, heat release rate and the upstream maximum temperature and using dimensional analysis. The correlation indicates that the dimensionless upstream maximum temperature decreases as the distance from fire source increases and it is proportional to 0.56 power of the dimensionless heat release rate and its relationship with tunnel slope is nonlinear and non-monotonous. [ABSTRACT FROM AUTHOR]

A novel solar collector named dual-function solar collector (DFSC) has been proven feasible for supplying warm air for space heating in winter and hot water during none-heating season by theoretical and experimental studies. While integrated with building, the DFSC becomes component of the envelope. Because complexly coupling of several heat transfer modes and subjecting to random weather condition, the thermal influence of DFSC on a building remains obscure. In order to investigate the effect of DFSC for passive and active heating, a demonstrating office building integrated with DFSC panels on the southern wall and roof has been built in Hefei, in the east region of China. A simulation project of the whole building system in winter was built by the software TRNSYS with the new component developed for DFSC and compiled into the TRNSYS library based on the validated theoretical method and experimental results from previous studies. The simulation results show that the energy supplied by the DFSC can meet the heating requirement at most of the time of a sunny day, except early morning. More than 30 % solar fraction can be provided during the whole heating season in Hefei, the value further increases to 59 % in a region of high solar irradiance, such as Lhasa. While active DFSC panels are operated to provide fresh warm air through ventilation system, the flow rate shows strong influence on the solar fraction of heating load. With appropriate operation scheme of the active DFSC, more energy savings and thermal comfort can be achieved. [ABSTRACT FROM AUTHOR]

Abstract: To explore the effects of altitude and sample width on the burning characteristics of wood, a series of experiments are carried out at two different altitudes with varying widths (W) of thin wood sheets. Flame size and flame spread rate are measured over a range of sample widths from 2cm to 12cm. At both altitudes, the width effects on both the dimensionless flame height (H f /W) and the spread rate are analyzed. The dimensionless flame heights at both altitudes show negative power law relationships with the sample width, and the decline at a low altitude (50m, Hefei) is much smaller than that at a high altitude (3658m, Lhasa). The spread rate curves at both altitudes contain a turning point: Before the point, the spread rate decreases with sample width, and after it, the rate increases. Furthermore, heat transfer theory was applied to explain the altitude and sample width effects on the flame spread rate, and the experimental results agree well with the theoretical analysis. [Copyright &y& Elsevier]

Experiments were conducted on the Tibetan Plateau to investigate the effects of sample width (W) on flame spread over horizontal charring solid surfaces. The results show that the rate of spread is influenced by the combined effects of convection heat transfer and flame radiation. Moreover, two different regimes depending on sample width were found in flame spread behavior. The rate of spread decreased with width in the convective regime (W=3 cm) but increased in the radiative regime (W>3 cm). Similar results were observed in comparative experiments conducted on a plain in Hefei, with the rate of spread reaching its minimum value at W=4 cm rather than 3 cm. [ABSTRACT FROM AUTHOR]

The proposed system mainly consists of flat-plate compound parabolic concentrators (CPCs) integrated with photovoltaic (PV) cells and organic Rankine cycle (ORC). The technologies of CPC, PV cell and ORC are analyzed, and feasibility of the hybrid solar electricity system is demonstrated. Novel configuration for the hybrid electricity generation is carefully designed to react to different operating conditions. Fundamentals of the innovative system are illustrated, and mathematical models are developed to study the heat transfer and energy conversion processes. The results indicate that the low-temperature solar thermal power generation integrated PV cells can produce much more electric per unit surface area than side-by-side PV panels and CPC–ORC modules. [ABSTRACT FROM AUTHOR]

Abstract: In the design of solar air heaters (SAHs), channel depth is a principal variable to be fixed. In this paper, the effect of the channel depth on the energy gain of type I and type III SAHs has been investigated by computational fluid dynamics (CFD) simulations. Laminar model and k–ω turbulence model of Wilcox are used for the prediction of flow and temperature field in SAHs. Our study shows that the heat transfer corresponding to the temperature distribution across the channel in SAH varies greatly with the change of channel depth. Based on the first and second laws of thermodynamics, the optimal channel depths for type I and type III SAHs with black-painted absorber are suggested as 10 mm. It is found that with selective coating, the absorber plate should be further from the cover glazing in order to prevent excessive convective heat loss, the distance is better of no less than 20 mm. In type III SAH, air flows in two channels above and below the absorber plate, the depth ratio of up channel to down channel should be no less than 1. [ABSTRACT FROM AUTHOR]

CdTe cells have many advantages such as high electrical efficiency, and low cost. Few studies on PV windows combined with CdTe cells exist. The PV windows investigated in the past were mainly designed for cooling needs. In this work, a double-skin ventilated window integrated with CdTe cells (CdTe-DSV) was proposed, which could meet seasonal thermal demands. A mathematical model of the proposed window was developed and validated against experimental data. Based on the model, firstly, the thermal comfort in a building with the proposed window was discussed. Secondly, the energy performance (including heat, daylight, PV output) of the proposed window was analyzed and compared with that of the a-Si-DSV window. Finally, the effects of cavity depth, and PV coverage ratio on energy performance were evaluated. The results indicated that the CdTe-DSV window could ensure a relatively neutral thermal sensation. The energy saving of heat transmission in winter and summer was 205.76kwh and 333.09kwh respectively. The CdTe-DSV window had a higher PV output than the a-Si-DSV window by 62%. The increase of PV coverage can decrease the cooling load and increase the heating load. Increasing the air cavity depth is always beneficial to energy saving. • A double-skin ventilated window integrated with CdTe cells is proposed. • A mathematical model is developed and validated against experimental data. • The seasonal thermal comfort and overall energy performance are evaluated. • Parameter analysis and comparison with a-si window are investigated. [ABSTRACT FROM AUTHOR]

This paper investigates the gas temperature rise and flame length of buoyant turbulent diffusion flames under cross wind, which does exist in fuel leakage fire accidents. Two identical propane burners with the same heat release rate were employed as fire sources. The burner spacing, heat release rate and cross wind velocity were changed. The results show that with the increase of the cross wind velocity, the gas temperatures in downwind region are strongly increased, resulting in a strong heat transfer to nearby energy loads. On the other hand, the non-monotonic trend of the flame length is caused by the cross wind effect. In addition, a segmented function predicting the flame length under cross wind is developed for merging and non-merging flames, which involves the heat release rate, burner spacing, pool size and cross wind velocity. The findings of this work have potential practical applications for optimal arrangements of energy loads, which allow predictions to be made regarding the possible threat and the establishment of the necessary safety distance to prevent the damage of multiple fires. • Experiments were conducted for two fire sources aligned parallel to the cross wind. • Heat release rate, burner spacing, and wind velocity were varied. • The effects of wind velocity on the gas temperatures was investigated. • The model for the flame length under cross wind was established. [ABSTRACT FROM AUTHOR]

This paper presents experimental and theoretical research on ceiling jet, burn-through, and flame spread behaviors of the combustible ceiling with the impingement of an incipient fire source. Medium-density fiberboard was utilized as the combustible ceiling in experiments, and the ceiling height above the fire source was varied. The flame radial length (r f), ceiling surface temperature, flame shape and spread rate after burn-through, and radiative heat flux were measured. Results showed that r f beneath the combustible ceiling was significantly longer than that of the non-combustible ceiling. Theoretical analysis was conducted for r f based on energy distribution, and an expression of r f beneath combustible ceiling was proposed. The burn-through behavior of the combustible ceiling was explored by combining the ceiling temperature, small-sized experiment and Fourier law. The advancement of char front along the ceiling thickness was calculated, which was the key point during burn-through. Upper surface flame spread in a circular shape after burn-through, and an expression of spread rate was obtained. A three-dimensional radiation model was adopted to calculate the radiative heat flux received by the unburned ceiling surface during the dynamic flame spread process, the calculated results agreed well with the experimental data. • Ceiling jet, burn-through, flame spread behaviors of timber ceiling are researched. • The mechanism of the flame extension beneath the combustible ceiling is revealed. • Burn-through process and char front advancement are explored. • The flame spread behavior and heat radiation after burn-through are clarified. • Expressions of the flame radial length and flame spread rate are proposed. [ABSTRACT FROM AUTHOR]

Abstract A series of experiments were conducted to study the flame spread and radiation characteristics for diesel tray fires against a sidewall. The tray width and the height of the sidewall were varied. The flame spread appearances and rate, subsurface flow and transverse fuel surface temperature were analyzed. Results showed that the sidewall has a great influence on the flame spread behavior due to its restriction on air entrainment. The flame distribution was uneven along the length of the tray, and the front propagating flame inclines to the tray outboard side edge (i.e. the tray's long side that is not attaching to the sidewall) for more oxygen, then more heat transferred from the flame through tray wall to the fuel results in uneven surface temperature profiles. The main flame being one short distance away from the flame spread front is against the sidewall. When the ratio of the sidewall height to the tray width, H / W , is less than 1, both the flame spread rate and subsurface flow increase with the increasing H / W , due to enhanced heat transfer. For H / W > 1, both of these parameters decrease with the increasing H / W , indicating a strong restriction of air entrainment by the sidewall. To estimate the flame radiation to the fuel surface during propagation, a multi-cylinder radiation model was proposed. By dividing the whole flame into several cylindrical sub-flames, the radiation blockage and sidewall effects are considered in the model. The comparison of calculated and measured radiation heat fluxes shows that the proposed model is reliable with a maximum error of 20%. These findings will provide scientific supports to fire monitoring and contribute to estimating radiative heat flux of spreading flame. [ABSTRACT FROM AUTHOR]

Abstract When an uncontrollable energy along with enormous pollutant emissions and high temperatures induced by a diffusion flame is released to a building, the ceiling-mounted thermal and smoke detectors are required to be actuated timely to achieve quick energy detecting, controlling and cooling. The information of ceiling flame length is important in estimating the energy release rate, radiative heat transfer from flames to surroundings and resulting safety distances of structures and people. In large scale confined spaces, the temperature measurement is provided as a feasible way to determine the ceiling flame length. This work aims to study how the heat release rate (HRR) influences the gas temperature at a given height below the ceiling representing the flame front. Two identical heptane pools were used as the fire sources and the pool size and edge spacing were varied to produce a wide range HRRs. The optimum flame front temperature was determined by comparing the results from the temperature measurement method and the video processing method. The experimental results showed that the characteristic flame temperature increases with increasing the HRR. A vertical temperature distribution model within ceiling jet was established to provide as a quantified verification of the experimental results. Highlights • Ceiling flame length is important in heat detecting, cooling and controlling. • The temperature measurement method is used to determine the ceiling flame length. • The flame front temperature at a same height below the ceiling increases with HRR. • A model for vertical temperature profile is proposed to verify the experimental results. • Different alarm temperatures of ceiling-mounted detectors are suggested in buildings. [ABSTRACT FROM AUTHOR]

Combing PV with Thermoelectric (TE) would be dominant because it can employ the solar fully spectrum to produce electricity. But the TE efficiency is significantly lower than PV efficiency and the coupling effect between them will limit the performance of PV and TE. The analyze and comparison on the different characteristics among the hybrid module, the PV alone and TE alone is significant to obtain the highest the electrical efficiency. In this paper, the attention was paid to the inconsistent phenomenon of thermoelectric load resistance for photovoltaic–thermoelectric modules. The model of PV-TE was built and verified based on two types of PV cells. The load resistance of TE for the maximum power output was also analyzed under different working conditions for the TE alone, TE in the PV-TE and PV-TE. The results showed that the load resistance of TE for the maximum power output of the TE alone, TE in the PV-TE and PV-TE are all different. For example, the PV-TE module based on the c-Si cell attains its peak value at the load electrical resistance of TE of 0.75 Ω, while the internal electrical resistance of the TE is 0.47 Ω. The PV-TE module based on the GaAs cell shows a maximum efficiency of PV-TE with a load resistance of approximately 1.6 Ω, while the internal electrical resistance of the TE is 2.0 Ω. Referring to the load resistance of TE alone is not suitable for PV-TE maximum power output. In addition, the TE maximum power output does not correspond to the PV-TE maximum power output since the TE load resistances in these two conditions are also different. The study will provide the reference for attaining the correct load resistance for the actual maximum power output of PV-TE module. [ABSTRACT FROM AUTHOR]

Heat pipe is a high-efficiency heat transfer device being widely used in solar applications. In this study, an innovative building integrated heat pipe photovoltaic/thermal (BiHP-PVT) system, which offers electricity generation, services water pre-heating, and space air-conditioning load reduction, was investigated. Firstly, a dynamic model was developed for a heat pipe photovoltaic/thermal (HP-PVT) system, and the numerical accuracy was confirmed by experimental validation. Then the annual performance of the BiHP-PVT system was evaluated through a case study in Hong Kong based on the typical weather conditions. The simulation results show that the annual overall heat transmission through the external wall can be reduced to less than a quarter of the normal practice. The annual water heating efficiency and electricity generation efficiency are around 35% and 10% respectively. The overall electricity saving is 315 kW h/year per unit façade surface area. In addition to its attractive energy performance, the BiHP-PVT system presents larger climate adaptability. [ABSTRACT FROM AUTHOR]

The experimental study focuses on the behaviors of two-dimensional flame propagation over diesel and diesel-wetted sand beds. Effects of the diesel depth, ignition position, fuel ratio of diesel volume to sand bed weight and sand diameter on the flame spread are analyzed. The results show that for wetted sand beds, the flame spread rate increases with increasing fuel ratio and decreasing sand diameter. The capillary rise effect still plays a significant role with different fuel ratios. Considering the flame spread rate, flame height, temperature distributions near fuel surface and heat fluxes around the fire, the controlling mechanisms of heat transfer are discussed. For the low fuel ratio, the flame spread over wetted sand beds is dominated by the capillary rise effect and heat conduction of sand beds. With increasing fuel ratio, the main controlling mechanism changes from flame radiation to the combined liquid convection and flame radiation. The spread rate for diesel ignited from pan edge is greater than that ignited from pan center for fuel depths (10–20 mm). The flame spread rate increases due to the effect of flame radiation to the unburned fuel surface. For diesel cases ignited from pan center, the dominant mode of heat transfer to the unburned surface is by flame radiation for 2 mm depth, while the controlling mechanism is liquid convection for deep layers (5–20 mm). For diesel cases ignited from pan edge, flame radiation becomes the dominant heat transfer mode for the very thin and thick depths (2 mm and 20 mm), while the controlling mechanism is liquid convection for 5 mm depth. For deeper fuel layers, the flame radiation generates its effect primarily by raising the fuel surface temperature through extensive preheating region. The present experimental results can provide practical guide for the combustion hazard of accidental fuel spills. [ABSTRACT FROM AUTHOR]

In actual fire, a solid wall is commonly established using nearby noncombustible solid materials to resist flame propagation. However, there are always some narrow channels owing to uneven surface of solid materials. There are inherent differences between liquid flame spread in narrow channels and infinite width fuel, including dynamic shear and heat dissipation. The experiments of flame spread over n -butanol fuel in narrow channels with lengths of 30, 50, 100, 200 and 300 mm and widths of 3, 4, 5, 6, 7, 8, 9 and 10 mm are carried out. The flame spread is divided into initial stage, restriction stage and jet flow stage. The velocity of subsurface flow decreases strongly once the convective flow enters narrow channel. Based on the assumption that surface tension is balanced with sum of viscous force and shear force, the velocity of subsurface flow at restriction stage is calculated which overestimates ∼10% of measurement. The flame cannot pass through the channel when heat loss accounts for more than 43% of total heat flux of subsurface flow. The heat absorption by sidewalls always accounts for more than 80% of total heat loss. The current finding provides a theoretical guidance for actual liquid firefighting. [ABSTRACT FROM AUTHOR]

In order for a solar heating system to provide heat immediately after sunrise, a fast response is needed to the heat demand of a serviced space. The majority of existing solar heating systems have a slow response time due to the large volume of water stored in the heat storage/exchanger unit (HSEU). This leads to a slow heat delivery cycle, which results in discomfort for the occupants and thus creates a huge barrier to the wide deployment of solar heating systems. To overcome this critical issue, a novel interactive heat storage/exchanging unit (HSEU) employing a double-tank configuration was developed. Unlike conventional HSEUs, which have a single tank acting as the heat storage and exchanging unit, the new HSEU is comprised of a small tank for heat exchange combined with a large tank for heat storage. The small tank enables fast transfer of solar heat to the heating loop fluid without having to heat up the large volume of water in the entire HSEU tank, whilst the large tank is used to store and exchange heat between itself and the small tank using a temperature-oriented control mechanism. To test the proposed design, the heat transfer between the first (solar loop) and second (heating loop) fluids, the heat and mass transfer between the small and large tank and the associated operational strategy were investigated experimentally and theoretically for comparison. A conventional single tank HSEU requires around 120 min to deliver heat to a served space, whilst the new interactive double-tank HSEU can provide heat to the served space in around 20 min, thus creating a heating system which can respond significantly faster than traditional systems. The investigation of the heat exchange effect between the solar and heating loop fluids showed that the new HSEU achieved a convective heat transfer coefficient of as high as 391 W/m2·K, which is 551% higher than that of a conventional tank. As a result, the solar thermal efficiency of the solar panel-array in the new HSEU based system was increased by 7.5% compared to conventional HSEU based systems. • A novel fast-responsive heat storage/exchanging unit (HSEU) is proposed. • It is the first time a small tank for heat transfer is used in a large storage tank. • Experimental tests on the prototype have been conducted. • The HSEU can save 140 min for a temperature increment of 10 °C. • The concept of fast-responsive storage for solar heating systems is demonstrated. [ABSTRACT FROM AUTHOR]

As the initial stage of liquid fuel leakage fire, flame spread process determines the development of fire and the severity of energy damage, which is a matter of great concern. Flame spread experiments were conducted in trays with the length of 130 cm and different widths, i.e., 15, 30, 60 and 90 cm. The fuel thicknesses were 0.2, 0.5, 1.0 and 1.5 cm. Results show that in the 15 cm wide tray, the flame height gradually becomes stable, flame spread behavior meets the assumption that radiation is neglectable. However, in trays wider than 15 cm, the assumption no longer applies. The flame height keeps increasing, and the heating effect of radiation weakens the convection between hot fuel and cold unburnt fuel. In the 90 cm trays, the flame front overlaps with the subsurface flow front. A heat transfer equilibrium model is further established, and it is found that the ratio of flame radiation to total heat input (Q r a d / Q i n ) increases with tray width for 1.5 cm thick fuel, Q r a d / Q i n are 4.4%, 23.2%, 39.0% and 41.1% in 15, 30, 60 and 90 cm wide trays. Besides, for 90 cm wide trays, Q r a d / Q i n generally shows a decreasing trend with the increase of fuel thickness. • Flame spread over n-butanol fuel in trays of different widths was experimentally studied. • The impact of flame radiation on liquid convection and temperature distribution was presented. • Variation of the ratio of flame radiation to the total heat input with tray width was revealed. • The influence of fuel thickness on heat transfer in 90 cm wide trays was quantitatively analyzed. [ABSTRACT FROM AUTHOR]

Fire accidents caused by liquid fuel leaks occur at times. In this paper, the combustion behavior of thin layers formed by the free spread of methanol at six continuous leakage rates was studied. The results show that methanol combustion with continuous leakage can be divided into four stages according to the change of burning area. The quasi-steady burning rate of thin-layer fuel is lower than that of pool fire at the same scale, due to the rapid heat exchange between the fuel layer and the substrate. As measured and calculated, the average fuel layer thickness gradually decreases in the spread burning stage, rises in the shrink burning stage, and finally reaches stability. The average fuel layer thickness of the quasi-steady burning stage of each case is used to calculate the heat loss by substrate reflection. Based on the law of energy conservation, a heat equilibrium model of the quasi-steady burning stage of a thin layer combined with fuel flow heat transfer was established. This model explains the uneven substrate temperature found by experimental measurements. The calculation results show that the effect of flow heat transfer is more significant within 20% of the burning area near the leakage port. [ABSTRACT FROM AUTHOR]

This paper presents an analytical study of the impact of a wick’s fractal geometrical parameters on the heat transfer capacity of a novel micro-channel loop heat pipe (MCLHP) which is applicable to solar heating system. By treating the wick of the micro-channel evaporator of the MCLHP as a thin porous layer, i.e. a combination of random/tortuous pores and water-containing skeletons, the impact of the fractal geometrical parameters of the wick on the heat transport capacity of the MCLHP was investigated. Based on the classical heat transfer limits and fractal equations, a dedicated computerised analytical model was developed by using the Newton-Raphson method; this model was then applied to analyze a few macro parameters of the wick (i.e., effective thermal conductivity and permeability) and heat transfer limits of the MCLHP, including capillary, viscous, entrainment, sonic and boiling ones. Comparison among these five limits was made using the minimum value searching approach, leading to the determination of the final heat transfer constraint of the MCLHP, which is identified as the capillary limit. A higher effective porosity and a larger pore diameter lead to an increased wick fractal dimension and thus a higher capillary limit. An increased height difference between the evaporator and the condenser also increases the heat transfer (i.e. capillary) limit of the MCLHP. Decreased effective porosity (ε), pores portion, and increased tortuosity of capillaries help enhance the heat transfer (boiling) limit of the MCLHP. Overall, fractal theory is thought to be an ideal method to address the impact of an irregular porous wick on the heat transfer performance of a MCLHP. [ABSTRACT FROM AUTHOR]

The objective of this paper is to study the thermal characteristic of low-voltage alternating current (AC) arc faults. Two series of experiments were designed, including polyvinyl chloride (PVC) wire ignitions experiments and electrode arcing experiments (non-ignitions) subjected to arc faults at different effective currents. The ignition time of the wire and the temperature of the electrode were investigated to characterize the arcing hazard. Results show that the ignition time of the wire decreases with the increasing arc current, and the wire with a larger cross-section area requires more time to be ignited at the same current. For non-ignition experiments, the electrode temperature increases first and then varies little. A heat equilibrium stage can be reached, where the heat transfer of the arc to the electrode is equal to the heat dissipation of the electrode. Furthermore, a heat transfer model for the arc fault is established to obtain the electrode temperature. The temperature of the arc-electrode interface is used to characterize the arc energy, and it is found the temperature showed a logarithmic increase with the current. The results of this study may be useful in the safety design of electrical wires, and can be exploited as hazard indicators for the risk assessment of electrical fires. [ABSTRACT FROM AUTHOR]

An indicator, namely equivalent hot side temperature (T EHST ) is proposed for the organic Rankine cycle (ORC). T EHST is derived from ideal thermodynamic process, but can denote the efficiency of irreversible ORC. Study on 27 fluids shows that given the operating conditions, fluid of higher T EHST generally offers higher ORC efficiency. This relationship is stronger and more universal than those established with respect to the critical temperature, boiling point temperature, Jacobs number and Figure of Merit. An ORC model by the method of error transfer and compensation is further built, in which the efficiency is quantitatively correlated with T EHST . Unlike the conventional ORC efficiency model, this one consists of thermodynamic parameters on the liquid/vapor curve and is independent on fluid properties at superheated state, and hence is more convenient. It has high accuracy especially for basic ORC and the relative deviation of the estimated efficiency from that calculated by the conventional model is from −0.7% to 3.4%. The novel model is applied for the thermodynamic performance prediction of a recently developed fluid of HFO1336mzzZ based on the phase equilibrium data. The results indicate HFO1336mzzZ is more efficient than R245fa on the conditions of high evaporation temperature and low pump efficiency. [ABSTRACT FROM AUTHOR]

This paper aims to present a CFD investigation into the heat transfer and friction characteristic of a Trombe wall equipped with venetian blinds. This involved (1) development of a CFD model; (2) dedicated experiment for verification of the model; (3) modelling result analyses and conclusion. Based on experimental data, the model was validated and it was able to yield satisfactory predictions. The characteristics of the Trombe wall with venetian blinds inclined from 5° to 85° were investigated. The Nusselt number and friction factor for Reynolds number within the range of 4173–16,693 were also achieved in the simulation. Analyses of the research results indicated that smaller slat angle helps enhance the outlet air temperature and solar thermal efficiency, while increasing the Reynolds number leads to decrease in outlet air temperature and increase in solar thermal efficiency. The empirical equations of heat transfer and flow resistance were proposed. The Nusselt number value of the venetian blind increases with the increase of the Reynolds number value and decrease of the slat angle. In addition, the Nusselt number is proportional to the range of 0.422–0.461 power of the Reynolds number. The friction factor value decreases with the increase of the Reynolds number value and the slat angle. [ABSTRACT FROM AUTHOR]

A complete three-dimensional CFD model was developed to investigate the flow and thermal transport in a novel Trombe wall equipped with a venetian blind. The model was involved in flow and heat transfer in the Trombe wall, which was conjugated with heat conduction in the venetian blind. An experimental rig was constructed and utilized to validate the CFD prediction. It was found that the established model is able to predict the operational performance of the system at a reasonable accuracy. Effect of structural parameters of the Trombe wall was investigated in details under the specific operational condition. The present study indicates that the position of the venetian blind, the width of the air duct and the area of the inlet and outlet vents influence the thermal performance of the system. The results predicted that the optimum of the distance between the glass and the venetian blind is 0.09 m for an air duct of 0.14 m width, the width of the air duct is 0.14 m and the area of each vent is 0.60 m width × 0.10 m height. The research results would assist in developing a high efficient solar air heating system and thus help reduce fossil fuel consumption in the building sector. [ABSTRACT FROM AUTHOR]

Highlights: [•] A novel solar loop-heat-pipe façade based heat pump water heating system is described. [•] A dedicated theoretical investigation into the thermal performance of the system is presented. [•] R600a is suggested to be the favourite working fluid filled in the loop heat pipes compared to water, R134a and R22. [•] The impacts of the system configuration and external parameters are explored. [ABSTRACT FROM AUTHOR]

The energetic and exergetic performance of an updated ORC (organic Rankine cycle) is investigated. The thermal efficiencies of the ORC at different heat source temperatures of about 100, 90, 80, and 70 °C are explored. The thermodynamic irreversibility that takes place in the evaporator, condenser, turbine, pump, and separator is revealed. The ORC feasibility for low-temperature applications is demonstrated. With a hot side temperature of around 80 °C, a thermal efficiency of 7.4% and a turbine isentropic efficiency of 0.68 can be achieved. The present research further indicates that exergy destruction caused by heat transfer through an appreciable temperature difference in the evaporator is the largest in the energy conversion process, followed by that in the condenser. The exergy destroyed in the heat exchangers amounts to 74% of the overall exergy loss. The total system exergy efficiency is approximately 40%; thus, ways to improve exergy efficiency are required. HCFC-123, a dry fluid, is experimentally confirmed to be highly superheated after expansion in this study. A regenerator should be used to preheat HCFC-123 prior to entering the evaporator. Meanwhile the heat-transfer configuration with two oil cycles can be a good solution to overcome the thermodynamic disadvantage of a one-stage evaporator. [Copyright &y& Elsevier]

Abstract: A series of comparative laboratory-scale experiments were carried out in the Lhasa plateau and the Hefei plain respectively to investigate the characteristics of flame spread over the extruded polystyrene (XPS) foam, a typical thermal insulation material. Flame shape and the temperature profile in solid phase were monitored, and the effects of altitude on the heat transfer process were analyzed. The results show that the temperature rise with time undergoes three stages: the preheating stage, the melting stage and the pyrolysis stage. The durations of the melting and pyrolysis stages on plateau are longer than that in plain, which sequentially results in a lower flame spread rate on plateau. Comparing of the temperature change rate curve on plateau with that in plain, it is found that the peak characteristics of the curves in the pyrolysis stage changed from single peak to multi-peaks, which suggests that the altitude difference might change the pyrolysis mechanisms of XPS material. Moreover, the sample scale effects on flame spread are also explored. Two different regimes are found in flame spread behavior with sample scale at the both altitudes. The spread rate drops with sample scale in convection regime and rises in radiation regime. [Copyright &y& Elsevier]

Abstract: The fundamentals of a newly constructed kW-scale Organic Rankine Cycle (ORC) system on the use of R123 were illustrated. A specially designed and manufactured turbine was innovatively applied to this system. Formulations were developed to examine the heat transfer and power conversion processes of the ORC. Unlike water pumping, the vapor pressure of the pumped fluid in the ORC system was much higher, and cavitation was more easily facilitated. A technology was introduced to address this issue without a large height difference between the tank and the pump. A preliminary test of the constructed system under varying conditions was conducted. The mass flow rate through the pump was found to be unequal to that through the turbine during the converter frequency adjustment process. The two mass flow rates were influenced in different ways by the evaporator pressure. The experiment results show that a turbine isentropic efficiency of 0.65 and an ORC efficiency of 6.8% can be obtained with a temperature difference of about 70 °C between the hot and the cold sides. Overall, the turbine has demonstrated adequate performance by operating at off-design conditions, which underscores its potential for application in small-scale ORCs. [Copyright &y& Elsevier]

This paper investigates the effects of ullage height (distance between the fuel surface and the pool upper rim) on heat feedback and combustion emission mechanisms of heptane pool fires. Results showed that ullage height significantly influences the flame structure and fuel mass loss rate. The incident radiative heat flux to the fuel surface and its heat feedback fraction first increase and then decrease with ullage height. Correspondingly, the convective heat feedback fraction presents a reverse trend. This attributes to flame base suspension effect and the evolution of soot volume fraction of the flame due to incomplete combustion. While for the conductive heat feedback fraction, it increases with ullage height and becomes the dominant heat feedback mode when the ullage height increases to a certain value. The profiles of the generation rate of the smoke particulates well explain the evolutions of the incident radiative heat flux. The combustion completeness is characterized by the ratio of CO/CO 2. Furthermore, it is calculated that the maximum combustion completeness decreases by 18.4% and 14.3% for pool diameters of 10 cm and 15 cm, respectively. These findings will help to develop and establish more general heat transfer and heat release rate models for pool fires. [ABSTRACT FROM AUTHOR]

A novel micro-channel loop-heat-pipe solar photovoltaic/thermal (LHP- PV/T) system is developed employing the co-axial tubular heat exchanger as the condenser and upper-end liquid header with tiny holes as the liquid feeder. The design facilitates an easier connection among the solar modules. It creates the improved condensation and separate evaporation effects within the LHP. A reduced evaporator area will thereby have a minor impact on the overall heat transfer performance, leading to significant potential for cost reduction. A distributed parameter model is established and validated by experimental data. The model is then applied to analyse the cost reduction potential of the LHP- PV/T via the optimization of geometrical and structural parameters. The impact of the area reduction on the LHP evaporator differs from that on the traditional integral heat pipe PV/T. The decrements in thermal and electrical efficiencies of the LHP- PV/T are 2.47% and 0.03% respectively when the width of heat pipes in the evaporator decreases from 26 to 10 mm. When the number of heat pipes decreases from 30 to 6, the decrements in thermal and electrical efficiencies are 4.63% and 0.12%, whilst the overall system cost drops by 28.58%, thus the cost-effectiveness of the system can be improved. •. A novel loop heat pipe solar photovoltaic/thermal system is built. •. An accurate distributed parameter model is developed. •. Cost reduction potential of the system is investigated. •. Influences of a reduced evaporator area on LHP differ from those on IHP. •. The system cost is reduced by 28.58% with a minor decrement in the performance. [ABSTRACT FROM AUTHOR]