Maximum Excess Temperature Research Articles

Experimental investigation on maximum ceiling temperature and longitudinal attenuation in a closed tunnel with an inclined shaft

For tunnel fires with open and closed tunnel ends, the smoke flow pattern and air supply conditions vary considerably, resulting in different maximum excess ceiling temperature (ΔTmax) and its longitudinal distribution (ΔTx). However, few studies have focused on the tunnel closed at both ends with an inclined shaft (typical tunnel structure during construction). Hence, tunnel model 1 (tunnel closed at both ends with an inclined shaft) and tunnel model 2 (tunnel open at both ends) are built for comparison and analysis. The results show that tunnel model 1 has higher ceiling temperatures and slower attenuation than tunnel model 2. Therefore, the ΔTx in tunnel model 1 needs to be re-analyzed in detail. Different inclined shaft slopes, fire heat release rates, longitudinal and vertical fire positions are considered. In tunnel model 1, for h (lifting height of burner surface) = 0, the ΔTmax value tends to be constant for the same heat release rate, irrespective of the longitudinal fire position. For h = 5 cm, the ΔTmax value takes on a rising trend with the fire moving downstream. Moreover, the inclined shaft slope within this study has little influence on ΔTx. As the fire moves downstream, the downstream ΔTx/ΔTmax value decreases more rapidly. From h = 0 to h = 5 cm, the value of ΔTx/ΔTmax drops slightly faster, but the gap is small and can be ignored. Finally, the experimental correlations are proposed to estimate the longitudinal ceiling temperature distribution in a closed tunnel with an inclined shaft.

Adding branched fin angle as degree-of-freedom improves performance of a T-Y-shaped fin

A variable-angle T-Y-shaped fin model is built herein. Constructal design of the variable-angle T-Y-shaped fin model is studied by introducing branched fin angle as an additional design variable. Firstly, with the goal of minimizing maximum dimensionless excess temperature (MDET), the branched fin angle of T-Y-shaped fin model is optimized. Secondly, heat transfer performance of T-Y-shaped fin is further optimized with 2-degree-of-freedom (DOF) and 3-DOF optimizations including branched fin angle. According to the results, in the 1-DOF optimization, when the branched fin angle reaches 17.2°, the MDET reaches the minimum at the value of 30.78, which is 19.32 % lower than that when the branched fin angle is 90°. Furthermore, the MDET of the T-Y-shaped fin under 3-DOF optimization is 27.20, which is 28.70 % lower than the result when the branched fin angle is 90°, and 11.63 % lower than that of the 1-DOF optimization. It is necessary to add the branched fin angle as an optimization variable. Moreover, the optimization effect of 2-DOF is significant, while that of 3-DOF is not obvious. The outcomes obtained from this study may serve as theoretical counsels for heat dissipation in the designs of fins for various thermal systems.

Impact of ventilated tunnels on smoke peculiarities in train carriage fires with multiple lateral openings

This research investigates the dynamics of smoke dispersion in a subway train carriage with multiple lateral openings within a ventilated subway tunnel. Numerous extensive full-scale numerical simulations were conducted to analyze the thermal plume distribution within multiple lateral openings of a train carriage, and the longitudinal and transverse maximum excess ceiling temperature. The results reveal that the heat release rates have a greater impact on the airflow in the carriage than velocities ,even when the longitudinal ventilation velocity reaches 8 m/s or the heat release rate reaches 15 MW. The multiple lateral openings cause significant differences in smoke flow characteristics compared to tunnels with both ends open. Additionally, the transverse maximum ceiling excess temperature inclines towards the lateral opening side due to asymmetrical air entrainment. The analysis of longitudinal maximum ceiling excess temperature identified two regions based on the dimensionless heat release rate, showing a linear increase followed by a constant phase dependent on the heat release rate, with a maximum longitudinal excess gas temperature of 1242 K. Additionally, a relationship between the dimensionless heat release rate and the transverse dimensionless maximum ceiling excess temperature was established. This study can be used as a guide for smoke exhaust mitigation and fire safety measures in the event of subway train carriage fires.

Numerical study on the maximum temperature rise and thermal-smoke front spread length in the upstream of semi-transverse ventilated Underground Traffic Link Tunnels

To fill the gap between the high realistic demand and lack of research basis, the process of smoke spread in Underground Traffic Link Tunnel fires during operation is investigated by numerical modeling. Good agreement between numerical predictions and experimental measurements for the longitudinal temperature has been achieved. Good agreement between numerical predictions and experimental measurements for the longitudinal temperature has been achieved. The characteristics of smoke transportation at key locations (e.g., maximum excess temperature and smoke fronts) were analyzed. A novel division method for the maximum lengths of smoke bays, which depends on thermal-smoke front spread length established from the characteristic number Ri = Q̇fuel∗1/3/V∗ of mixed convection, to utilize the nature of smoke as much as possible during a fire is provided based on dimensional analysis and similarity principle. The margin of error between the final model prediction in this paper and the simulation was within plus or minus 10%. The outcomes of this study are believed to have practical guidance significance for linkage and interlock control of smoke management and passive fire protection design in tunnels during operation with a transverse ventilation system.

Dimensionless prejudgment criterion of coal spontaneous combustion in longwall gobs and its application

Coal spontaneous combustion (CSC) in longwall gobs is the result of the coupling of multiple physical fields, making its development difficult to predict, so the prejudgment indicators need to be refined. In this work, the dimensionless model of CSC in the gob was established, and three prejudgment indicators such as air leakage intensity indicator, longwall advance rate indicator and exothermic intensity indicator were proposed to consist of the dimensionless prejudgment criteria of CSC in gobs. Numerical simulations were performed to obtain the relationship curves between each indicator and the dimensionless maximum excess temperature in the gob. These curves had formed a series of nomograms that can be used to prejudge the CSC. It shows that the risk of the CSC in gobs is positively correlated with the air leakage intensity indicator and the exothermic intensity indicator, but inversely proportional to the longwall advance rate indicator. The field application indicates that the prejudgment criteria and its Nomogram can be used to accurately predict whether the CSC could occur in the gob, and has good prospects for application.

Study on the maximum excess temperature and temperature distribution under the influence of lateral smoke exhaust in tunnel fires

This paper numerically investigates the temperature characteristics of tunnel fire under the ceiling with the effect of lateral smoke exhaust. In the past, analyses of maximum excess temperature under the ceiling and temperature distribution with extraction flows have been conducted. However, the lateral exhaust has a different impact on temperature than the ceiling exhaust. Consequently, several simulations were run in a tunnel (60m long, 9m wide, and 6m high). The lateral exhaust velocity is set by 0 – 10 m/s with a fire heat release rate of 5 – 20 MW. The temperature beneath the tunnel ceiling is measured by 9 rows of thermocouples. The results show that there is a linear relationship between the maximum excess temperature and the lateral exhaust velocity. The longitudinal temperature distributions at different transverse positions are distinct because of the lateral smoke exhaust especially when the hot smoke passes by the vent. The transverse temperature difference expands with the lateral exhaust velocity increases and the heat release rate decreases. Several modified models are proposed to provide references for predicting the temperature characteristics of tunnel fires with lateral smoke exhaust.

Full-scale experimental and numerical study of smoke spread characteristics in a long-closed channel with one lateral opening

This paper aims to study the characteristics of smoke flow in a long-closed channel with one lateral opening at one end. A series of full-scale experiments with different longitudinal positions and heat release rates of fire sources were carried out. And, the corresponding numerical simulations were conducted to help explain the law of smoke flow field. The characteristics of the smoke spread on both sides of the fire source, the thermal plume distribution within the channel, and the maximum ceiling excess temperature were analyzed. Results show that the existence of the end walls in the long channel causes the smoke to accumulate, which further results in the smoke settlement going through three stages. The lateral opening causes a significant difference of smoke flow and other characteristics from the channel with both ends open. When the fire source approaches the closed end without opening, the vertical fire-induced plume inclines to the closed end because of the asymmetrical air supplementation. Due to the limited air supply, the combustion will change to ventilation-controlled as the fire source heat release rate increases. The maximum ceiling excess temperature is higher than that with both ends open and is basically not influenced by fire positions studied in this work. It has been verified that the dimensionless maximum ceiling excess temperature of the smoke is also proportional to the dimensionless heat release rate. This study can provide a reference for fire prevention and smoke exhaust strategies for long channels.

Experimental and numerical investigations on the heat production and thermal storage characteristics of rapid preparation amorphous powder activated coke

• Preparation of rapid preparation amorphous powder activated coke was showed. • The oxidation characteristics of RAC were studied through experimental method. • The Arrhenius equation was used to analyze the oxidation reaction rate of RAC. • The thermal storage characteristics of RAC were studied with CFD simulation. • The temperature fields and concentration fields of coke bin were analyzed. The heat production and thermal storage characteristics of rapid-preparation amorphous powder activated coke (RAC) were investigated. RAC was prepared by using a drop-tube reactor system. The natural oxidation characteristics of RAC were studied through combined TG–FTIR analysis and temperature-programmed experiment. Experimental results showed that CO and CO 2 were the main oxidation products of RAC in air, and that the oxidation reaction was in accordance with the Arrhenius equation and law of mass action. Thermal storage characteristics were studied through computational fluid dynamics simulation. The maximum excess temperature θ max increases linearly with the increase of the initial temperature. The concentration fields of the products show that CO 2 is mainly concentrated in the upper part of the coke bin, and the CO generated by CO 2 at high temperature is mainly concentrated in the central part of the coke bin.

Investigation on the maximum ceiling temperature of the weak plume impingement flow in tunnel fires under longitudinal ventilation

This paper numerically investigated the characteristics of the fire plume behavior and the ceiling maximum temperature with increasing fire source height, in the longitudinal ventilated tunnel fires. Results show that with the continued increase of the longitudinal velocity, the fire plume gradually tilts to the downstream of the tunnel, causing significantly decrease of the ceiling temperature. With the elevation of the fire source, a circulation area forms at the leeward side of the fire source, the pulling effect of the circulation area and the accumulated smoke layer can protect the fire plume from the effect of ventilation airflow, which can weaken the fire plume inclination. To quantify the influence of the longitudinal flow, the dimensionless velocity v* is adopted and divided into three regions, i.e., v*= 0, 0 < v*≤ 0.19 and v*> 0.19. When 0 < v*≤ 0.19, a velocity influence factor f (v) is added to the original equation to represent the effect of ventilation velocity for increasing fire source locations and the ceiling maximum excess temperature varies as the 2/3 power of the dimensionless heat release rate and linear function of dimensionless ventilation velocity. When v* > 0.19, a concept of virtual fire source is proposed to quantify the effects of the inclined fire plume on the maximum temperature and the ceiling maximum excess temperature varies as the 2/3 power of the dimensionless heat release rate and exponential function of dimensionless ventilation velocity. Besides, the applicability and reliability of the predictive correlations are further verified by comparing to the experimental data of different scales.

Study on the Effect of Bridge Deck Spacing on Characteristics of Smoke Temperature Field in a Bridge Fire

The numerical simulation method is used to simulate the distribution characteristics of the smoke temperature field of a double-deck bridge smoke temperature field during tanker fire under natural ventilation. The influence of the distance between double decks on the truss and ceiling temperature field change in the double-deck bridge is investigated. The results show that the range of high-temperature area gradually decreases with the increase in bridge deck spacing. The maximum excess temperature function of the tunnel ceiling is also applicable to the bridge, but the coefficient is smaller than that of the tunnel experimental formula. An equation is proposed to predict the maximum excess temperature of the truss under different bridge deck spacings. As the bridge deck spacing increases, the maximum excess temperature decreases. The excess temperature of the truss increases along the truss, and the maximum excess temperature appears at the top of the truss. Based on the energy equation, an equation for the excess temperature of the truss is established. As the vertical height increases, the excess temperature of the truss above the fire source exponentially increases. The research results will contribute to the fire hazard evaluation and safety design of bridges.

Experimental investigation on ceiling temperature characteristics induced by weak and strong fire plumes in tunnel fires equipped with two-point extraction ventilation using smoke extraction channel

In this paper, maximum excess temperature and longitudinal temperature decay induced by weak and strong fire plumes were discussed in tunnel fires with two-point extraction ventilation using the smoke extraction channel. First, a model for maximum excess temperature was developed based on theoretical analysis and experimental phenomenon. Subsequently, the characteristics of maximum excess temperature and longitudinal temperature decay were analyzed. Experimental results showed that the maximum excess temperature for the weak plume is higher than that predicted by the previous model due to the existence of the smoke extraction channel. The maximum excess temperature for the strong plume is much higher. Moreover, the function of maximum excess temperature for the weak plume was similar to the previous model, while that for the strong plume was different from the previous model when the fire was located at the tunnel center. Besides, exhaust rate and longitudinal fire location far from the vent did not have a considerable impact on the maximum excess temperature and longitudinal temperature decay in the fire section induced by weak plume or strong plume. Finally, new prediction models for maximum excess temperature induced by the weak and strong fire plume were validated according to experimental data. Moreover, a fast calculation model driven by experimental data was established to predict longitudinal temperature decay.

Experimental study on the temperature distribution in a cross-type interchange subway station fire scenario

In order to investigate the temperature distribution beneath the subway ceiling, a series of experiments were carried out in a 1:20 small-scale model of a three-level cross-type interchange subway station. The experimental model contains three floors, including two lines of the metro. The experiment results show that the maximum temperature below the ceiling in the cross-type interchange subway station is slightly higher than the maximum temperature below the ceiling under the unconstrained space burning, because the complex structure of the transfer station affects the flow of the fire-induced smoke. The maximum temperature model proposed by Alpert was modified by analysing the experiment results of the highest temperature below the ceiling in the small-scale cross-type interchange subway station, and the new prediction formula for the maximum temperature excess was proposed. When the distance between fire location and side wall is 1.5 times the tunnel height, the modified coefficient of maximum temperature excess is 1.1348 based on the model proposed by Alpert. This study on this issue may benefit the current design of fire protection measures for the platform ceiling.

Experimental investigation of maximum temperature and longitudinal temperature distribution induced by linear fire in portals-sealed tunnel

A set of experimental tests were conducted to investigate the maximum gas temperature rise and longitudinal temperature distribution beneath the centerline ceiling in a portals-sealed arched tunnel, taking fuel tank central angle location along the sidewall (expressed by effective ceiling height) and linear fire size into account. A linear fuel tank burner, placed on the arched sidewall of the tunnel, was utilized to simulate the fire source. A new empirical formula was proposed to predict the maximum excess temperature beneath the ceiling in a portals-sealed tunnel fire, based on dimensional analysis. The experimental results show that the non-dimensional maximum excess temperature has an exponential decaying relationship with non-dimensional fuel tank length. A predecessor's equation is modified to present the longitudinal temperature distribution beneath the centerline of arced-ceiling in a portals-sealed tunnel fire, which is consists of the sum of two exponential equations. The comparison of longitudinal temperature distribution between fire in portals-sealed tunnel and natural ventilation tunnel was conducted. The study is expected to provide some references for actual tunnel fire-fighting.

Experimental study on temperature profile in a branched tunnel fire under natural ventilation considering different fire locations

Reduced scale experiments in a branched tunnel were carried out to study the variation of the maximum gas excess temperature beneath the ceiling and temperature profile under natural ventilation considering different fire locations. Flame behaviors and ceiling temperature profile were recorded and analyzed. The results show that due to unbalanced air entrainment, the fire located at the intersection of the tunnel tilts towards the sidewall. The maximum temperature beneath the ceiling when fire locates at the intersection is slightly lower than that when fire locates at the two other locations because the accumulated high temperature smoke is less. Based on experimental data, a model predicting the maximum temperature beneath the ceiling was proposed. It correlates the experimental data well and also agrees reasonably with the temperature measured in full scale tests from other's work. Besides, dimensionless expression for longitudinal temperature decay in a branched tunnel was proposed. The predicted longitudinal ceiling temperature was close to the measured data for the main tunnel but slightly higher for the branch due to the rough surface of the joint. The influence of branch on the temperature profile in the main tunnel was also discussed.

Full-scale experimental study on fire-induced smoke movement and control in an underground double-island subway station

The research carried out full-scale fire experiments for smoke movement and control in an underground double-island subway station. Four kinds of fire scenarios were designed in the station hall and platform respectively, and series of thermocouples were arranged in the public space. Several vital fire parameters were analyzed, including the vertical temperature distribution, the longitudinal temperature distribution, smoke layer height and smoke front arrival time. Besides, the effectiveness of exhaust system was evaluated for fire emergency. The results showed that the smoke flow characteristics under different fire scenarios exhibited diversely. Under natural ventilation, the maximum temperature excess near fire source was 18 ℃ and 21℃ in the station hall and platform fire scenarios separately, and the smoke temperature attenuated gradually with the longitudinal distance from fire. Besides, the smoke layer exceeded the height of 4.0 m and 2.8 m when fire in the station hall and platform. After activating the exhaust system in the B-end, it was obvious that the smoke layer height increased and the temperature excess decreased. Additionally, the smoke front arrival time was influenced by the fire source locations and ventilation conditions. In the above scenarios, the smoke diffusion velocity was empirically determined as 0.11–0.18 m/s and 0.21–0.35 m/s when a fire in the station hall and platform respectively. The conclusions from the on-site fire experiment could provide data support for the fire accident emergency in an underground double-island subway station.

A combined numerical simulation and optimization model for the cooling of IC chips under forced convection

Three-dimensional steady state numerical simulations are carried out from seven protruding IC chips (Aluminium) of different sizes mounted at various positions on a high-power SMPS board (Bakelite) cooled under forced convection ([Formula: see text][Formula: see text]W/cm2, [Formula: see text][Formula: see text]m/s). The objective is to predict the temperature distribution of these IC chips and to determine their optimal arrangement on the SMPS board; that will have the lowest maximum temperature excess among all the possible configurations. To achieve the same, a numerical analysis is carried out using the commercial software ANSYS-Icepak, and then the hybrid optimization strategy (Genetic Algorithm (GA) driven by Artificial Neural Network (ANN)) is employed to obtain the global optimal configuration of these IC chips. Preliminary analysis is performed using a dimensionless position parameter ([Formula: see text]) which is found to be a strong function of the IC chip size, and their positioning on the substrate board. It is observed that the combined ANN-GA strategy is able to predict the optimal behavior of the IC chips more accurately. The larger size IC chips generating higher power should be placed at the substrate bottom for better cooling.

Constructal design of tree shaped cavities inserted into a cylindrical body with heat generation

The present work is guided by the Constructal theory to identify the best shapes for the cavity inserted in a cylindrical solid body with a constant heat generation rate. From the assertion that a system must evolve to persist in time, an evolution of the elemental (I-shaped) cavity to a tree-shaped cavity with two branches added to a single branch is proposed. With these modifications, two restrictions (area occupied by the trunk and area occupied by the branches of the cavity) and seven degrees of freedom are used to represent the geometry. The study focused on investigating the effect of the degrees of freedom associated with the cavity branches and the number of cavities. Results show that the optimization of each new degree reduces the maximum excess of temperature. However, the results obtained with a more complex cavity were similar to those obtained by the elemental cavity in reducing the maximum excess temperature. In practical terms, the results are similar. Despite the similarity between the present results and those achieved with a most simple shape. The increase of performance with the increase of degrees of freedom studied here indicates that for complete optimization, the performance of complex shapes can surpass that of the simplest cavity.

Numerical investigation on the effect of tunnel width and slope on ceiling gas temperature in inclined tunnels

A series of simulations are conducted in full-scale inclined road tunnels to explore the effect of tunnel width and slope on ceiling gas temperature pattern in fire. Firstly, characteristic temperature pattern in inclined tunnel fires due to the presence of stack effect are indicated. The buoyancy induced airflow velocity at the inlet increases with tunnel slope and stays almost invariable with tunnel width, and the location of impinging point is blown downstream with the increasing tunnel slope and changes little with tunnel width correspondingly. However, the maximum ceiling gas temperature area stays almost unmovable with the change of tunnel width and slope because of the effect of the buoyancy induced airflow at the inlet on the upstream gas back-layering. Then a correlation of the maximum gas excess temperature with tunnel width and slope is proposed by dimensional analysis. Finally, taking the position and value of maximum ceiling gas temperature as references, downstream ceiling gas temperature data can be correlated well by an exponential expression. In addition, the correlation is compared with previous experimental results and simulated results in the tunnels with longer length and different fire source locations.

Experimental study on temperature distribution beneath an arced tunnel ceiling with various fire locations

This paper attempts to study experimentally the impact of fire location on the temperature distribution of thermal plume induced by fire under an arced ceiling. A series of experiments considering different pool sizes and fire locations were carried out in an arced tunnel-like structure. Two fuels (n-heptane and ethanol) were used to generate pool fires with various fire sizes. Results demonstrated that the flame was deviated from the vertical direction when the fire was close to the sidewall, meanwhile, the heat release rate (HRR) increased significantly. Consequently, the flame propagation length was enhanced due to the constraint effect, while its variation was not significant when the HRR was not large enough. The transverse ceiling temperature distribution was predicted using correlating the HRR and the vertical height above the pool for a classical model. Also, a correlation was given to calculate the maximum excess temperature beneath the centerline of the arced ceiling, and it gave a relatively good prediction for the maximum temperatures of the weak plume. Note that this correlation is only available for the ethanol fires. Nonetheless, it is necessary to further study the temperature distribution of strong plumes beneath arced ceilings with constraint effect.

Effects of thermal and electrical contact resistances on the performance of a multi-couple thermoelectric cooler with non-ideal heat dissipation

The influence of adverse working conditions on the cooling performance of thermoelectric cooling chips is numerically studied in this work. A three-dimensional steady-state physical and mathematical model that considers thermoelectric coupling is developed for a commercial thermoelectric cooler (TEC) of bismuth telluride (Bi2Te3). An imaginary non-uniform temperature distribution with the variable maximum excess temperature (θm) and scale of high temperature region (ω) are used as boundary conditions for the hot end of the chip to simulate the effect of a poor heat dissipation. The electrical contact resistance Re,c (=M×re,c) and the thermal contact resistance Rk,c (=N×rk,c) are set at the interface layer of different materials with two magnifying factors (M and N) to simulate varying degrees of interfacial effects from defective contacts. The numerical results show that of all adverse factors, electrical contact resistance may cause the most severe reduction in a TEC's performance. Further studies demonstrated that M has a far greater effect on the performance curve (Qcvs.I and COPvs.I) than the other three parameters (N, θm, ω) in both Qcmax and COPmax modes. In addition, the phenomenon of drift of the optimal current is found and discussed.